Symposium EN19 : Novel Inorganic Semiconductors for Optoelectronics and Solar Energy

The United States’ Materials Genome Initiative (MGI), announced in 2011, helped launch the era of Materials-by-Design which combines theory, computation, experiment and data sciences to accelerate materials development and deployment. So far, Materials-by-Design research has primarily focused on the intrinsic properties of equilibrium materials. However, many technologically relevant materials are metastable and, further, roughly 1/3 or more of new materials discovered each year are metastable rather than equilibrium materials. Therefore, for new materials to help in addressing energy needs, we must directly address the research challenges of both including metastable materials and moving from Materials-by-Design to Solutions-(or Devices)-by-design. Our approach to and progress towards this challenge will be demonstrated by examples taken from current research on opto-electronic semiconductors and transparent conductors.

Bismuth-based solar absorbers are of interest due to similarities in the chemical properties of bismuth halides and the exceptionally efficient lead halide hybrid perovskites. Both Pb²⁺ and Bi³⁺ possess a similar soft polarisability and form a wide range of compounds with rich structural diversity, such as BiX₆ clusters, 1D ribbons, and layered perovskite type structures. Whilst both are composed of earth-abundant materials and experience the same beneficial relativistic effects acting to increase the width of the conduction band, bismuth is non-toxic and non-bioaccumulating, meaning the impact of environmental contamination is greatly reduced.[1]

Here, we use hybrid density functional theory, with the addition of spin orbit coupling (SOC), to examine a range of bismuth containing V-VI-VII candidate photovoltaic (PV) absorbers.[2-5] We show that BiSI and BiSeI possess electronic structures suitable for photovoltaic applications. Furthermore, we calculate band alignments against commonly used hole transporting and buffer layers, which indicate band misalignments are likely to be the source of the poor efficiencies reported for devices containing these materials. Based on this, we have suggested alternative device architectures expected to result in improved power conversion efficiencies. Lastly, we explore the defect properties of BiSI and suggest ideal growth conditions for optimised film properties.

References

[1] A. M. Ganose, C. N. Savory and D. O. Scanlon, Chem. Commun. 53, 20–44 (2017)
[2] K. T. Butler, J. M. Frost, and A. Walsh, Energy Environ. Sci. 8, 838 (2015)
[3] A. M. Ganose, K. T. Butler, A. Walsh, and D. O. Scanlon, J. Mater. Chem. A 4, 2060 (2016).
[4] A. M. Ganose, M. Cuff, K. T. Butler, A. Walsh and D. O. Scanlon, Chem. Mater. 28, 1980 (2016)
[5] D. S. Bhachu et. al., Chem. Sci. DOI: 10.1039/C6SC00389C (2016)

Given rapid advances in photovoltaics, transparent electronics, and other emerging energy technologies, the development of p-type transparent conductors (TCs) has been a relatively slow-moving front. Both n-type and p-type TCs are conventionally oxides, but there is increasing evidence that chalcogenide (S, Se, Te) semiconductors should have higher hole transport and probability of p-type doping than oxides (in exchange for lower transparency). We use a high throughput computational framework to screen a large database of chalcogenide compounds likely to have a high hole conductivity and high optical transparency. Our ultimate goal is to synthesize promising compounds in the laboratory for use in stable devices. To this effect, we investigate potential metrics for TC performance including various weighting methods for Boltzmann transport calculations, defect formation energies, dopant selection criteria and global thermodynamic stability. From these criteria, we discover over one hundred computationally stable multi-anionic compounds with indirect computed gaps E_G > 1.5 eV (since PBE underestimates the gap) and average effective masses 0 < m_h* < 1 by screening the Materials Project database. Several compounds studied previously as TCs emerge from our screening, including ZnS and sulvanites TaCu₃X₄ (X = S, Se, Te). We further pare down this list for synthesis by selecting only single anionic compounds, removing compounds with toxic and highly reactive elements, and estimating p-type dopability. A refined list of ten top experimentally-favorable candidates emerges and includes spinel ZnAl₂S₄, distorted rocksalt BaSnS₂, and several other rocksalt structures. We will also present our initial attempts to combinatorially synthesize and characterize a few of the candidates, and lay out a roadmap for a future high-throughput screening, synthesis, characterization closed loop to enable new device paradigms using transparent conducting chalcogenides.

The use of advanced machine learning algorithms for design and discovery of novel inorganic semiconductors for optoelectronics energy conversion applications requires large datasets amenable to data mining. Whereas a number of computational materials property databases exists (e.g. materialsproject.org, materials.nrel.gov), the machine learning based on experimental data is limited by the lack of large and diverse data resources. Here, we report on our progress towards a publicly open High Throughput Experimental Materials (HTEM) Database (htem.nrel.gov). Presently, this database contains 130,000 sample entries, characterized by structural (100,000), chemical (70,000), optical (50,000) and electrical (40,000) properties of novel inorganic thin film semiconductor materials, grouped in >4,000 sample entries across >100 materials systems. More than a half of these data are publicly available. In addition to showing how HTEM database may enable scientists to explore materials by browsing web-based user interface, this presentation will discuss the underlying laboratory information management system (LIMS). Also, this presentation will illustrate how advanced machine learning algorithms can be adopted to materials science problems of predicting materials conductivity using random forest methods, and clustering unrelated samples into groups based on composition similarity.

The discovery of earth abundant, functional materials is critical for sustainable technological advancement. There is a concerted global effort to reduce the time it takes to realize such materials via databases, high-throughput screening, informatics, and mapping out the ‘‘materials genome.’’ But what fraction of theoretical chemical space is represented by the number of known compounds that have been thoroughly characterized to date? Forming a four-component compound from the first 103 elements results in excess of 10¹² potential combinations. Such a search space is intractable to high-throughput experiment or first principles calculations.

We present a hierarchical screening approach that is capable of dealing with such a search space, consisting of three key stages: First, we employ an arsenal of simple chemical rules that are the product of centuries of research, in order to filter out chemically implausible element compositions. This is implemented using the open-source SMACT package.¹ Second, we use supervised machine learning and data mining to rapidly filter for target properties and suggest likely structures of leading candidates. Finally, we apply density functional theory (DFT) calculations in order to verify stability, structure and target properties. At each stage, the size of the search space is drastically reduced, ensuring that as computational cost increases, the number of candidate materials remains feasible.

We demonstrate the power of this approach by discovering new quaternary oxide materials for solar energy applications. SMACT is used to reduce the search space of billions to ~1 million chemically sensible compositions. A gradient boosting regression machine learning model is trained on a database² of high-quality bandgap calculations, then used to identify ~20,000 oxide compositions that are most likely to have useful bandgaps. A recent statistics-based approach to structure prediction using a probabilistic model proposed by Hautier et al.³ is employed to suggest ~500,000 likely crystal structures, which are then fed to the AFLOW-ML⁴ model to predict thermodynamic stability via machine-learnt DFT total energies. All of these steps can be carried out in a matter of hours to days, using minimal computing resources. The result is a series of potential new energy materials; our methodology can be applied to materials design in a range of contexts and is an important new tool in the quest for accelerated materials discovery.

1. D. W. Davies, K. T. Butler, A. J. Jackson, A. Morris, J. M. Frost, J. M. Skelton, A. Walsh, Chem, 2016, 1, 617.
2. I. E. Castelli, F. Hüser, M. Pandey, H. Li, K. S. Thygesen, B. Seger, A. Jain, K. A. Persson, G. Ceder, K. W. Jacobsen, Adv. Energy Mater., 2015, 5, 1400915
3. G. Hautier, C. Fischer, V. Ehrlacher, A. Jain, G. Ceder, Inorg. Chem., 2011, 50, 656.
4. O. Isayev, C. Oses, C. Toher, E. Gossett, S. Curtarolo, A. Tropsha, Nat. Commun., 2017, 8, 15679.

Potentially low-cost high-throughput approaches have been demonstrated that form inorganic semiconductor films directly from nanoparticle or molecular inks. The highest efficiency devices have been prepared with CuInGaSe2 utilizing hydrazine as a solvent and complexing agent. Here, we present our progress to develop of a class of solution-phase routes to Cu₂ZnSn(S,Se)₄, Cu(In,Ga)(S,Se)₂ and most recently bismuth halides that do not use suspensions of nanoparticles or hydrazine. We have discovered new effects of alloying and doping using a combinatorial ultrasonic spray coater and high-throughput screening method to map the optoelectronic properties of absorber layers. The presentation will focus on: (i) the formation of films and elimination of deleterious elements, (ii) incorporation of dopants and their effects of absorber properties and grain boundaries, (iii) alloying to form record high open circuit voltage relative to the maximum theoretical open circuit voltage for the bandgap, (iv) a new understanding of grain growth and impurity removal in kesterites, and recent results to yield tandem solar cells with hybrid perovskites.

[1] Xin, H., Vorpahl, S.M., Collord, A.D., Braly, I.L., Uhl, A.R., Krueger, B.W., Ginger, D.S., Hillhouse, H.W., Phys. Chem. Chem. Phys. 17, 23859-23866 (2015).
[2] Uhl, A.R., Katahara, J.K., Hillhouse, H.W., Energy & Environmental Science 9, 130-134 (2016).
[3] Collord, A.D., Hillhouse, H.W., Chem. Mater. 28, 7, 2067–2073 (2016).
[4] Clark, J.A., Uhl, A.R., Martin, T., Hillhouse, H.W., Chem. Mater. ASAP, DOI: 10.1021/acs.chemmater.7b03313, (2017).
[5] Uhl, A.R., Yang, Z., Jen, A.K.-Y, Hillhouse, H.W., J. Mater. Chem. A 5, 3214-3220 (2017).
[6] Williamson, B.W., Eickemeyer, F.T., Hillhouse, H.W., Submitted.

In this work, we present the effects of annealing on the structural and optical properties of polycrystalline ZnGeP₂ films on Si substrates. As structural analogs to III-V materials, II-IV-V₂ materials have the potential for optoelectronic applications beyond the present III-V materials. Currently, III-V optoelectronic devices enable fiber communications, solid-state lasers, light emitting diodes and high efficiency photovoltaics, but they rely on epitaxial heterostructures limited by lattice matching considerations. In contrast, II-IV-V₂ materials can be lattice matched to silicon and have the potential for tunable electronic properties for fixed composition through control of cation ordering. Therefore, implementation of a material with similar properties to the III-Vs and lattice matched with silicon could be transformative for tandem photovoltaics. ZnGeP₂ is one such material with a lattice matching within 1% of silicon and a band gap of 2.1 eV.
In order to control crystallinity and ordering independently of composition, stoichiometric amorphous ZnGeP₂ films were grown and then annealed ex-situ. The crystallinity and ordering of the annealed films was studied with x-ray diffraction and transmission electron microscopy. To gain an understanding of how optical properties change with structure, spectroscopic ellipsometery was used to determine the optical constants and absorption coefficient of the films. Using these characterization techniques, we have confirmed the ability to control crystallinity and ordering of the films as a function of anneal temperature and time. Subsequently, with changes in crystallinity and ordering we observed variations in the optical properties of the films. From our results we can conclude that ZnGeP₂ shows large potential for integration in Si-based devices.

Tunable electrical conductivity – the ability to change carrier concentration over a wide range of useful values while maintaining superior mobility – is arguably the central technological advantage of an Amorphous Oxide Semiconductor (AOS) such as ternary or quaternary oxides of post-transition metals, for example, In-Sn-O, Zn-Sn-O, or In-Ga-Zn-O. Compared to the crystalline counterparts, where the electron mobility is governed primarily by scattering on ionized impurities, phonons, and grain boundaries, the nature of and the relationship between the carrier generation and transport in AOSs are more complex. Although amorphous materials lack grain boundaries, the strong local distortions in the Metal-Oxygen (M-O) polyhedra associated with a weak ionic M-O bonding as well as long-range structural correlations in the disordered system give rise to entangled transport phenomena at different length scales [1-3]. Given the many degrees of freedom in the amorphous structure, the long-range structural characteristics and the electronic properties of the donor defects in AOSs differ fundamentally from those in the crystalline transparent conducting oxides. Therefore, defects in AOSs must be considered along with the structural evolution of the disordered system.

In this work, we report the results of computationally intensive ab-initio molecular dynamics (MD) simulations combined with accurate density functional electronic structure calculations for amorphous In-based and Sn-based oxide semiconductors. Novel approaches for non-stoichiometric-melt cooling and time-dependent statistical analysis not only show a significant improvement over the standard oxygen vacancy models but also allow us to simultaneously address the structural morphology, evolution, and the dynamics of defect formation, thereby providing the necessary integral framework to comprehensively understand the fundamental materials properties of AOSs. We demonstrate that the approach provides a statistically complete defect picture of conducting amorphous oxides by capturing the formation of both shallow defects that produce carriers and localized deep defects that limit carrier mobility via electron trapping or scattering. The scheme also allows us to study the long-range structural reconstruction in AOSs and the defect dynamics during quenching or annealing processes providing the necessary information for optimizing the electronic and optical properties of AOSs toward their application in optoelectronic technologies.

[1] D. Buchholz, Q. Ma, D. Alducin, A. Ponce, M. Jose-Yacaman, R. Khanal, J.E. Medvedeva, and R.P.H. Chang,
Chemistry of Materials, 26, 5401-5411 (2014).
[2] R. Khanal, D.B. Buchholz, R.P.H. Chang, and J.E. Medvedeva, Physical Review B, 91, 205203 (2015).
[3] J.E. Medvedeva, D.B. Buchholz, and R.P.H. Chang, Advanced Electronic Materials, 3, 1700082 (2017).

Ternary oxides represent a large but relatively little-explored class of semiconductors that are of interest for photoelectrochemical energy conversion applications. One of the best performing candidates thus far is BiVO₄, an n-type photoanode with a bandgap of 2.4 eV. One well known ‘trick’ to improve this material is doping. Donor doping with tungsten (W) supposedly improves the photocurrent by increasing the conductivity, but is also found to reduce the magnitude as well as the lifetime of the photoconductivity. Another dopant that was recently found to greatly improve the photocurrent of BiVO₄ is hydrogen [1]. We found that in contrast to tungsten, hydrogen actually increases the carrier lifetime and diffusion length in BiVO₄ [2]. This is due to the passivation of trap states, the possible origins of which will be discussed. A second ‘trick’ to improve the performance of BiVO₄ is by deposition of a cobalt phosphate (CoPi) co-catalyst on the surface. We recently showed that the main role of this ‘co-catalyst’ is not to enhance the charge transfer kinetics, but to passivate surface defects on BiVO₄ [3]. To get a better understanding of the chemical nature of these states, we employed ambient pressure resonant photoemission (AP-resPES) and operando XPS methods. These experiments revealed the presence of two separate states in the bandgap of BiVO₄ and a redistribution of the phosphate species in the electrolyte under illumination [4]. These initial results are the first steps towards a molecular-level understanding of the BiVO₄/electrolyte interface that may eventually help to design efficient solar fuel generators. In the last part of the talk I will discuss recent results on CuBi₂O₄, a photocathode material with a bandgap of ~1.7 eV. We developed a modified solution chemistry that enables us to spray highly homogeneous films that show photocurrent densities up to 2 mA/cm² [5]. To enhance the charge separation in these films, we introduced a gradient of copper vacancies across the film thickness. This results in the formation of a homojunction in CuBi₂O₄ that increases the carrier diffusion length and reduces charge recombination [6]. The resulting films showed AM1.5 photocurrent densities of up to 2.5 mA/cm² at +0.6 V vs. RHE in the presence of a hole scavenger, which is a new benchmark for this material.

References
[1] J. Cooper et al., Chem. Mater. 28, 5761 (2016)
[2] J.W. Jang et al., Adv. Energy Mater. 1701536 (2017)
[3] C. Zachäus et al., Chem. Sci. 8, 3712 (2017)
[4] M. Favaro et al., J. Phys. Chem. B (in press, DOI:10.1021/acs.jpcb.7b06942)
[5] F. Wang et al., J. Mater. Chem. A 5, 12838 (2017)
[6] F. Wang et al., J. Am. Chem. Soc. 139, 15094 (2017)

Improving the efficiency of solar-power oxygen evolution is both critical for development of solar fuels technologies and challenging due to the broad set of properties required of a solar fuels photoanode. Bismuth vanadate, in particular the monoclinic clinobisvanite phase, has received substantial attention and has exhibited the highest radiative efficiency among metal oxides with a band gap in the visible range. Efforts to further improve its photoelectrochemical performance have included alloying one or more metals onto the Bi and/or V sites, with progress on this frontier stymied by the difficulty in computational modelling of substitutional alloys and the high dimensionality of co-alloying composition spaces. Since substituional alloying simultaneously changes multiple materials properties, understanding the underlying cause for performance improvements is also challenging, motivating our application of combinatorial materials science techniques to map photoelectrochemical performance of 948 unique bismuth vanadate alloy compositions comprising 0 to 8% alloys of various p-block, alkalai earth, rare earth, and 4d & 5d elements (e.g Bi-V-A) along with a variety of compositions from each pairwise combination (e.g. Bi-V-A-B) of these elements. Upon identification of substantial improvements in the co-alloying Bi-V-A-B spaces, structural mapping was performed to reveal a remarkable correlation between performance and a lowered monoclinic distortion. First-principles density functional theory calculations indicate that the improvements are due to a lowered hole effective mass and hole polaron formation energy, and collectively, our results identify the monoclinic distortion as a critical parameter in the optimization and understanding of bismuth vanadate-based photoanodes.

Inorganic and hybrid organic–inorganic main-group halides that adopt the perovskite structure combine excellent performance in photovoltaic applications, ease of preparation, and abundant constituent elements, but the origins of their remarkable properties are a matter of debate [1]. Here, we address two unusual aspects of the crystal structure and dynamics of these materials which suggest the primacy of the group IV–halogen sublattice in dictating performance, and apply these insights to discover new optoelectronic materials via high-throughput computational screening.

First, X-ray scattering studies reveal local off-centering of the group 14 cations within their coordination octahedra across the materials class reflecting a preference for lower symmetry coordination than that implied by crystallographic approaches [2,3]. Ab initio calculations, optical measurements, and analogies to existing theory implicate the ns² lone pair electrons as the origin of this phenomenon, which we propose leads to enhanced defect screening, reduced thermal conductivity, and unusual temperature-dependence of the electronic bandgap [2]. We further demonstrate control of the strength of this phenomenon by chemical substitution on all sites of the perovskite [3].

Second, solid state nuclear magnetic resonance and dielectric spectroscopies reveal the full temperature-dependent dynamics of molecular reorientation in the high-performance formamidinium lead iodide [4]. Despite markedly different barriers for molecular rotation compared to those in the homologous methylammonium lead iodide, both systems exhibit similar dynamics at room temperature [4]. Together with the vastly different dipole moments for the two molecules, this result sheds light on emerging hypotheses of polaronic transport and transient Rashba–Dresselhaus effects.

Using design criteria based in part on these findings about the unusual electronic structure and lattice polarizability of the halide perovskites, we screen 54,000 compounds in the Materials Project database to identify candidate optoelectronic materials. Subsequent ab initio calculations and experimental preparation and screening are employed to test the validity of these criteria as predictors of high performance.

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under award number DE-SC-0012541.

1. D. H. Fabini, J. G. Labram, A. J. Lehner, J. S. Bechtel, H. A. Evans, A. Van der Ven, F. Wudl, M. L. Chabinyc, R. Seshadri, Inorg. Chem. 56 (2017).
2. D. H. Fabini, G. Laurita, J. S. Bechtel, C. C. Stoumpos, H. A. Evans, A. G. Kontos, Y. S. Raptis, P. Falaras, A. Van der Ven, M. G. Kanatzidis, R. Seshadri, J. Am. Chem. Soc. 138 (2016).
3. G. Laurita, D. H. Fabini, C. C. Stoumpos, M. G. Kanatzidis, R. Seshadri, Chem. Sci. 8 (2017).
4. D. H. Fabini, T. A. Siaw, C. C. Stoumpos, G. Laurita, D. Olds, K. Page, J. G. Hu, M. G. Kanatzidis, S. Han, R. Seshadri, J. Am. Chem. Soc. 2017, DOI:10.1021/jacs.7b09536.

P-type transparent conducting oxides (TCOs) present many exciting problems for materials scientists due to low conductivity arising from large hole effective masses. The applicability of TCOs to technologies like flat panel displays and photovoltaic cells establishes the need for complementarity between the well documented and commercially available n-type TCOs and p-type TCOs and motivates the search for a means to delocalize the O-2p states that limit shallow acceptors and lead to large hole effective masses. CuAlO₂ and AgAlO₂ (XAO) show promise as p-type TCOs due to the presence of X-3d/4d states, which hybridize with O-2p states and delocalize the valence states. Additionally, p-doping with Mg may further enhance conductivity of pristine XAO.
XAO exists as three polymorphs: the delafossites 2H and 3R, and an orthorhombic polymorph which is the least stable polymorph energetically. This study is restricted to the 2H polymorph since it is the least studied of the two delafossites. In this work, a theoretical study based on first-principles calculations is presented on pure and Mg-doped (replacing Al) 2H-XAO using density functional theory as implemented in the Vienna Ab initio Simulation Package (VASP) code. Projector augmented wavefunction pseudopotentials with a cutoff energy of 400 eV are employed and the exchange correlation energy is treated using the generalized gradient approximation with the addition of a Coulomb interaction energy (Hubbard correction U) for the Cu-3d and Ag-4d states. Results are also obtained using the hybrid functional (HSE06) approach. Pure 2H-XAO is initially modelled using an 8-atom hexagonal primitive cell, which is used to relax the crystal structure. Band structure, density of states, and the complex dielectric function are obtained. By calculating the total energy per atom as a function of V/V₀, where V₀ is the relaxed volume, it was observed that the 2H(3R) polymorphs modelled using hexagonal primitive(unit) cell are equal in total energy down to 1 meV/atom at V=V₀, with the rhombohedral 3R model 90(37) meV higher for CuAlO₂(AgAlO₂). This suggests that the 2H polymorph may have a slightly lower total energy. Hole effective masses are determined using parabolic curve fitting of the lower band gap edge around the high symmetry k points of the first Brillouin zone.
Once the electronic structure of the bulk using the primitive cell is well described, 64-atom hexagonal supercells are used to model pristine and Mg-doped 2H-XAO. The electronic structures, densities of states, optical properties and hole effective masses for these systems are presented and discussed in the context of experimental results from literature. A discussion of the effects of Mg-doping on the optical properties and its effectiveness in reducing hole effective masses and increasing conductivity is also presented.

Indium tin oxide (ITO) is the most commonly used transparent conducting oxide (TCO) due to its high carrier mobility (up to ~100 cm² V⁻¹ s⁻¹) and low absorption of visible-light. However, In-free TCOs have been sought for some time due to the cost and relative scarcity of In. Among these, alloys of ZnO and SnO₂ (commonly referred to as ZTO) offer acceptable electron mobility (>10 cm² V⁻¹ s⁻¹) and high thermodynamic stability [1]. Importantly, the large diameter spherical orbitals of the Zn²⁺ and Sn⁴⁺ cations in ZTO cause the carrier mobility to be insensitive to structural disorder [1]. Amorphous ZTO (a-ZTO), produced with low growth temperatures, therefore exhibits similar carrier mobility to its polycrystalline counterpart.

Here, we investigate the characteristics of a-ZTO deposited energetically using high power impulse magnetron sputtering (HiPIMS) [2]. This technique has proved capable of producing high quality metal oxide layers suitable for electronic devices [3, 4]. Reactive co-deposition from Zn (HiPIMS mode) and Sn (DC magnetron sputtering mode) targets yielded a-ZTO with varying Zn:Sn composition across a 4-inch diameter sapphire substrate. The electrical and optical properties of this film were then studied as a function of composition. As-deposited, the films were amorphous, transparent and semi-insulating. Hydrogen (a known n-type dopant in ZnO and SnO₂) was introduced into the a-ZTO by post-deposition annealing (1 h, 500 °C, 100 mTorr H₂) and resulted in significantly increased conductivity with no measurable structural alterations. After annealing, Hall effect measurements revealed n-type carrier concentrations of ~1 × 10¹⁷ cm⁻³ and mobilities up to 15 cm² V⁻¹ s^–1. These characteristics proved both stable and suitable for device applications. Results from transistors and memristors based on energetically deposited a-ZTO will be presented. These suggest that HiPIMS can produce dense, high quality a-ZTO suitable for electronic applications.

REFERENCES

[1] H. Chiang, J. Wager , R. Hoffman, J. Jeong and D. A. Keszler, High mobility transparent thin-film transistors with amorphous zinc tin oxide channel layers, Appl. Phys. Lett. 86 (2005) 013503
[2] V. Kouznetsov, K. Macák, J. M. Schneider, U. Helmersson and I. Petrov, A novel pulsed magnetron sputter technique utilizing very high target power densities, Surface and Coatings Technology 122 (1999) 290–293
[3] J. G. Partridge, E. L. H. Mayes, N. L. McDougall, M. M. M. Bilek, D. G. McCulloch Characterization and device applications of ZnO films deposited by high power impulse magnetron sputtering (HiPIMS) , Journal of Physics D: Applied Physics 46 (16), 165105 (2013)
[4] B. J. Murdoch, D. G. McCulloch, R. Ganesan, D. R. McKenzie, M. M. M. Bilek, and J. G. Partridge, Memristor and selector devices fabricated from HfO_2-xN_x, Appl. Phys. Lett. 108, 143504 (2016)

Semiconductor compounds are widely used for photocatalytic hydrogen production applications, where photogenerated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr_1-xNbO₃, 0.03 < x < 0.20) were reported to show competitive photocatalytic efficiencies under visible light which was attributed to interband absorption. This discovery expanded the range of materials available for optimized performance as photocatalysts. Here we have studied epitaxial thin films of SrNbO_3+δ and found that their bandgaps are ~4.1 eV. Surprisingly the carrier density of the conducting phase exceeds 10²² cm^-3 and the carrier mobility is only 2.47 cm² V^-1 s^-1. Contrary to earlier reports, the visible light absorption at 1.8 eV (~688nm) is due to the plasmon resonance, arising from the large carrier density. We propose that the hot electron and hole carriers excited via Landau damping (during the plasmon decay) are responsible for the photocatalytic property of this material under visible light irradiation.

Reference:
1. D.Y. Wan, Y.L. Zhao, Y. Cai, et al., Nature Communications 8, 15070 (2017).
2. T.C. Asmara, D.Y. Wan, Y.L. Zhao, et al., Nature Communications 8, 15271 (2017).

The development of flexible solar cells is necessary for achieving market competitiveness through the implementation of low cost solar cells and for applying customized business models, such as Building Integrated Photovoltaics (BIPV). Solar cells with the CZTS-based (Cu₂ZnSnS₄, Cu₂ZnSnSe₄, Cu₂ZnSn(S,Se)₄) absorbers are advantageous for lowing the cost, but the development of solar cells based on flexible substrates has been relatively unexplored. In this work, a flexible CZTSSe solar cell applying the Mo foil substrate was developed. The characteristics of solar cells were examined by applying the sputtering process for four precursor stacking orders (Cu/Sn/Zn/Mo foil, Cu/Zn/Sn/Mo foil, Zn/Sn/Cu/Mo foil, and Sn/Cu/Zn/Mo foil). In addition, NaF was deposited on the Mo foil, and the effects of Na were investigated. The samples were characterized by means of scanning transmission electron microscopy-energy dispersive spectrometry (STEM-EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), a solar simulator, external quantum efficiency (EQE) measurements, depth resolved Raman spectroscopy, and admittance spectroscopy (AS).
Acknowledge: This work was supported by by the DGIST R&D Program of the Ministry of Science and ICT (18-BD-05) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20173010012980).

Photon upconversion is a promising strategy to boost the conversion efficiencies of commercially available photovoltaics beyond the Shockley-Queisser limit by harvesting photons with energy below the band gap of a host cell and converting pairs of such low-energy photons into single high-energy photons that can be returned to and absorbed by the host cell. Existing upconversion materials based on lanthanide-doped nanocrystals and sensitized triplet-triplet annihilation have limited potential benefit for solar energy harvesting applications because of their narrow absorption bandwidth and low quantum efficiency.¹ II-VI based semiconductor nanostructures offer a new paradigm for photon upconversion in which bandgap, relative band alignment, and carrier dynamics can be tailored through material composition, size, and morphology to achieve high quantum yield at desired wavelengths with broad spectral absorption.² Recent advances in the synthetic capabilities of semiconductor nanoparticles has enabled the fabrication of complex heterostructures composed of various materials. One class of new semiconductor heterostructures are colloidal double quantum dots (QDs), where two spatially separated semiconductor nanoparticles are electronically coupled.³ Colloidal synthesis of such semiconductor heterostructures offers a route to solution-compatible, low-cost processing. We report a semiconductor nanostructure upconverting platform consisting of coupled quantum dots (QDs)³ that efficiently upconverts low-energy photons under continuous-wave illumination. The nanostructure consists of a narrower bandgap tellurium-doped cadmium selenide QD absorber and a wider bandgap cadmium selenide emitter spatially separated by cadmium sulfide nanorod. The double quantum dot nanostructure is designed such that electrons promoted by a first photon absorption in the Te-doped CdSe absorber are delocalized over the entire structure but the holes remain confined. A second low-energy photon excites the hole via intraband absorption, allowing it to escape into the cadmium sulfide and ultimately relax into the emitter QD. We will present synthesis conditions, structural characterization, and detailed optical spectroscopy of the upconversion photophysics that collectively demonstrate control over the photon energies absorbed and the corresponding photon energy gain through control over the composition and size of the absorbing QDs.

1. Zhou, J., Liu, Q., Feng, W., Sun, Y. & Li, F. Upconversion Luminescent Materials: Advances and Applications Jing. Chem. Rev. 115, 395–465 (2015).
2. Sellers, D. G. et al. Novel nanostructures for efficient photon upconversion and high-efficiency photovoltaics. Sol. Energy Mater. Sol. Cells 155, 446–453 (2016).
3. Deutsch, Z., Neeman, L. & Oron, D. Luminescence upconversion in colloidal double quantum dots. Nat. Nanotechnol. 8, 649–653 (2013).

Compositionally graded In_xGa_1-xN layers of high structural and optical quality grown on Si have the potential for cheap and efficient solar harvesting systems¹. The theoretical maximum efficiency for tandem photoelectrochemical water-splitting systems could reach over 22.5% with Si as bottom cell and In_xGa_1-xN as top cell with a bandgap between 1.6-1.8eV, an indium content of 0.37-0.44, respectively^1,2. However, the use of In_xGa_1-xN as a water-splitting photoelectrode (PE) for solar hydrogen production has yet to show promising performance as initial demonstrations of In_xGa_1-xN on a GaN substrate have exhibited photocurrents below 0.1mA/cm² at AM1.5G irradiation, i.e. a current even below what has been reported for pure n-GaN³. We developed a numerical model of In_xGa_1-xN water-splitting PEs which aims at identifying and quantifying the losses in the system. The model included electromagnetic wave propagation calculations for the determination of the locally resolved light absorption and charge generation terms, which built the generation terms in the charge transport and conservation equations solved in the semiconductor. The charge transfer at the semiconductor-electrolyte interface, a boundary condition to our model, accounted for Fermi level pinning and a potential drop in the Helmholtz layer due to surface states. The complete numerical model was validated using linear sweep voltammograms of In_xGa_1-xN PEs grown by plasma-assisted molecular beam epitaxy with varying indium content (0.095, 16.5, 23.5, 33.3 and 41.4) and under varying light irradiance (1%, 10%, 50% and 100% of AM1.5G). Parametric analyses were performed on optical and electronic properties to identify key performance parameters and evaluate their impact on the performance of compositionally graded In_xGa_1-xN PEs.

Low cost and large-scale production of semiconducting materials requires development of solution processing routes. While developing such solution processing routes for photovoltaic systems, it is also important to achieve sufficient efficiencies of photovoltaic devices which will ultimately reduce the cost of power generation.
Numerous solution processing routes have been tried for fabricating various semiconducting films for solar energy conversion. Amongst which, Hydrazine has shown highest conversion efficiencies for materials like CIGS and CZTS. However, because of its explosive and carcinogenic nature, it is not feasible to use this chemical in any scale up production. On the other hand, amine-thiol system which is safer than Hydrazine has shown promising results in dissolving array of metals, metal salts, oxides, chalcogenides. Even though devices made with amine-thiol system have demonstrated promising efficiencies, the hydrocarbon chains in amine and thiol species results in formation of carbonaceous fine grain layer in the final film which affects the performance of photovoltaic devices.
Herein, we present a novel approach in utilizing amine-thiol chemistry for film fabrication. Understanding amine-thiol chemistry using various analytical techniques has enabled us in fabricating better quality films. This presentation will exclusively focus on fabrication of such two semiconducting thin film materials viz. CISe and Se-Te alloy. Amongst which, CISe is a low bandgap material and is of great interest for its application in fabrication of tandem solar architectures with high band gap materials like perovskite. We will discuss the effect of various precursor selection and various selenization conditions on CISe film quality and also will demonstrate the fabrication of working device from CISe system with more than 11% efficiency.
Along with novel approach for fabricating conventional material like CISe, we will also discuss our work in synthesizing a novel thin film alloy of Se and Te for photovoltaic application. While people have fabricated working devices with selenium as an absorber material for indoor applications, we will demonstrate an alloying process of Se with Te to tune the bandgap of the absorber material. Our understanding of chalcogen dissolution mechanism in amine-thiol system has made it possible to alloy two chalcogens to form crystalline phase material. This presentation will include synthesis as well as thin film fabrication from Se-Te alloy followed by primary attempt of making a working photovoltaic device with various architectures.
In conclusion, we have come up with better understanding of amine-thiol system which enabled us to develop novel approach for its use in better quality semiconducting films. The results of which are applicable to the synthesis of nanoparticles and films of a vast array of chalcogenides including Cu₂S, WSe₂, SnS, SnSe, FeS₂, CZTS and other Kesterites.

Solar energy is an abundant, clean and free access resource, but it require harvesting and storage for sustainable future. Photovoltaics or photocatalysis technologies dedicated to sun light conversions frequently involve photo-visible-responsive semiconductors [1], such us materials with formula BiMOS (M; Cu [2] or Ag). In this study, we applied a strategy of substitution of Cu by Ag to produce a new family of oxysulfide BiAgOS [3]. We were interested to address how the total substitution of Cu by Ag in BiCuOS system affect its crystal structure, optical and electronic properties by using experimental characterizations and theoretical calculations. Single-phase bismuth silver oxysulfide BiAgOS was prepared by a hydrothermal method. Its structural, morphological, and optoelectronic properties were investigated and compared with those of BiCuOS. Rietveld refinement of the powder X-ray diffraction confirmed that BiAgOS has the same crystal structure as BiCuOS. The diffraction peak positions of BiAgOS, relative to those of BiCuOS, are shifted toward lower angles, indicating an increase in the cell parameters. Combined with experimental measurements, density functional theory calculations employing the range-separated hybrid HSE06 exchange-correlation functional with spin–orbit coupling quantitatively elucidated photophysical properties such as absorption coefficients, effective masses, and dielectric constants. BiCuOS and BiAgOS were found to have indirect bandgaps of 1.1 and 1.5 eV, respectively. The difference in the bandgap results from the difference in the valence band compositions. The hybrid level of the S and Ag orbitals is located at a more positive potential than that of S and Cu leads to widening band gap. Both materials possess high dielectric constants and low electron and hole effective masses. BiAgOS has dielectric constant larger than BiCuOS making it very interesting for photoconversion applications because the material could efficiently screen photogenerated charges. By combining the UV-Vis, Mott-Schottky, and photoelectron spectroscopy in air measurements, the relatively low bandgap of and their p-type character, BiCuOS and BiAgOS can be considered as interesting starting compositions for the development of new semiconductors for photovoltaics or Z-scheme photocatalytic applications. Moreover, this study opened the window toward new oxychalcogenides materials BiAgOCh (Ch; Se, Te) which are not yet synthesis or investigate.

[1] P. Hoffmann, Tomorrow’s Energy; MIT Press: Cambridge, MA, 701, 2004; Vol. 1.
[2] W.C. Sheets, et al., Inorg. Chem., 2007, 46, 10741.
[3] A. BaQais, et al., Chem. Mater., 2017, 29 (20), 8679.

The high diffusivity of Cu⁺ ions in the hexagonal-close-packed structure of Cu_2-xS often leads to interesting and unexpected observations. Herein, we present the etching of oxide and sulfide thin film underlayers during the atomic layer deposition of Cu_2-xS thin films. The infiltration of the underlayers by Cu⁺ ions is an essential step that precedes the etching process. However, it is suspected that the eventual etching of the underlayer, and the etch rate strongly depend on the lattice (bond dissociation energy) of the underlayer material. Thin films of ZnS, ZnO, SnS, and SnO were etched to different degrees during the deposition of Cu_2-xS while SnO₂ exhibited a high resistance to etching. Interestingly, a selective removal of Zn²⁺ was observed when a ternary Zn_1-xSn_xO film was used as underlayer. Based on XPS results and findings from other supplementary experiments, a possible reaction mechanism was proposed for the etching process. Finally, the observation was extended to the synthesis of Cu_2-xS nanowires that can be used as effective absorbers for photovoltaic cells. Details of the findings of this work will be presented at the conference.

Solar cell efficiency could potentially be increased by exceeding the Shockley-Queisser limit through singlet fission. The Shockley-Queisser limit on photovoltaic efficiency is the theoretical maximum efficiency of a p-n junction. It states that nearly two-thirds of the light energy incident on a conventional photovoltaic material is not converted to electrical energy. Some of this lost energy could be harvested through singlet fission. Via the Dexter process, inorganic colloidal PbS nanocrystals are used to harvest the energy from triplet excitons generated by organic 1,6-diphenyl-1,3,5-hexatriene (DPH). Singlet fission (SF) is a spin-allowed process in which a high energy photon creates a singlet state in one chromophore, moiety of organic molecule responsible for absorption and emission of light. The singlet state splits into two triplet states—one in the original chromophore and one in a neighboring chromophore that is correctly orientated for electronic coupling. The Dexter process is a simultaneous, correlated transfer of an excited electron on one molecule (donor) to another molecule (acceptor) via a non-radiative pathway which depends on the wavefunction overlap between donor and acceptor. SF material has previously been made from organics, specifically polyacenes such as tetracene and pentacene. Polyacenes have been observed to have shorter triplet lifetimes and energy triplets compared to DPH. This makes DPH a prominent candidate to produce a more efficient SF material with a higher SF yield. The Wittig Reaction is utilized to synthesize DPH from derivatives with specific functional groups that allow DPH to bind to PbS. Energy transfer from DPH (donor) to PbS (acceptor) is characterized by absorbance and emission measurements.

In past few years, transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten disulfide (WS₂), tungsten diselenide (WSe₂), have attracted great research interests due to their superior electronic, optical, and mechanical properties. In particular, excellent absorbance (5-10 %)^[1] and high quantum efficiency (~10⁴ %)^[2] have allowed TMDs to be exploited as channel materials in high-performance photodetectors with high responsivity and high detectivity. However, most TMD photodetectors operated in the limited detection range from ultraviolet to visible region (10 ~ 780 nm) because TMDs generally have an energy bandgap between 1 and 2 eV. Although it was possible to detect 1064 nm and 980 nm lights by utilizing rhenium diselenide (ReSe₂) and multi-layer MoS₂ with relatively small bandgap (1 and 1.3 eV, respectively)^[3], no further researches to detect lights with longer wavelength by using TMD materials were reported yet.

Here, we demonstrate a highly responsible IR photodetector operating based on the interlayer optical transition phenomenon. This IR photodetector formed on ReS₂/ReSe₂ heterostructure not only extended the detection range up to IR region (1310 nm) that individual ReS₂- and ReSe₂- photodetectors cannot cover, but also maintained high responsivity even under IR region (3.64 × 10⁵ A/W at λ = 980 nm and 1.58 × 10⁵ A/W at λ = 1310 nm). The small interlayer bandgap of 0.62 eV formed at the ReS₂/ReSe₂ heterojunction interface lead to the interlayer optical transition phenomenon, consequently enabling photocarriers to be easily generated with less energy than the band-to-band transition.


References

[1] J. Shim, H.-Y. Park, D.-H. Kang, J.-O. Kim, S.-H. Jo, Y. Park, J.-H. Park, Adv. Mater. 29 39 (2017)
[2] S. Yang, S. Tongay, Q. Yue, Y. Li, B. Li, F. Lu, Sci. Rep. 4 5442 (2014)
[3] S.-H. Jo, H.-Y. Park, D.-H. Kang, J. Shim, J. Jeon, S. Choi, M. Kim, Y. Park, J. Lee, Y. J. Song, S. Lee, J.-H. Park, Adv. Mater. 28 6711 (2016)

Rare earth (RE) elements are frequently employed in many applications, such as catalysis, magnetism, phosphor lighting etc., due to their unique properties stemming from the steady 4f energy levels. Recently, there has been a push to replace these RE elements due to increased economic and national security issues. One proposed family of alternatives; are transition metal (TM) dopants, but are typically avoided because of their field dependent properties. For example, the strong overlap of the TM d orbitals with surrounding ligand is often viewed as an unfavourable phenomenon for luminescent or magnetic applications. In the current project, the field dependent hybridization of TM is utilized to engineer the d energy levels, allowing for controlled optoelectronic properties of TM ion.
In this work, thin films of TiO₂:Ni (~ 50 nm) were deposited onto Si substrates by employing sol-gel chemistry and spin-coating techniques. Weak localized external fields were created by surface functionalization of these films with benzoic acid ligands. Initial structural and optical characterization on TiO₂:Ni was performed to determine the crystal structure and crystal field splitting. X-ray photoelectron and soft X-ray absorption spectroscopy studies were used to investigate the ability to manipulate Ni core and valence levels without affecting the oxidation states or crystal structures. Additionally, the band structure of surface modified TiO₂:Ni, investigated by a combination of ultraviolet photoelectron and X-ray emission spectroscopy techniques, revealed that the interband Ni 3d states suffered a shift towards/away from the valence band depending upon the electron withdrawing/donating nature of the ligand. Furthermore, this phenomenon was theoretically supported by the ligand field multiplet calculations and time-dependent density functional theory simulations on bulk-mimicking TiO₂:Ni²⁺ clusters. These preliminary results on TiO₂:Ni demonstrate that the reversible tuning of TM d energy levels can be exploited to as the first step to the substitution of RE elements in phosphors that can have controlled luminescent properties.

Inorganic semiconductor based pn junctions constitute an integral part of most optoelectronic devices, such as photovoltaic (PV) solar cells and light emitting diodes (LEDs). Although the inorganic materials offer higher stability and superior electro-optic properties relative to organics, they present exorbitant cost and scale-up challenges. This paper presents a radically different approach to circumvent such challenges. It takes advantage of naturally formed nanocrystalline pn junctions, resulting from judicious coupling of semiconductor materials and electrochemical processes. Certain combinations can lead to highly practical inorganic material systems, comprising process-induced bulk pn junction nanostructures. The paper will illustrate an exemplary material system, based on single-step electrodeposited copper-indium-selenide (CISe) ordered defect chalcopyrite compounds. Thermodynamically driven reactions enable the electrodeposition of highly-ordered, interlinked, space-filling CISe films, in a single step. This approach creates a low-cost processing platform to produce nanocrystalline films, with all the attributes necessary for efficient bulk homojunction (BHJ) operation. Surface analytical microscopies and spectroscopies reveal unusual phenomena and extraordinary electro-optical properties that could potentially maximize spectral absorption and reduce recombination loss. They support the fortuitous formation of surprisingly ordered, sharp, abrupt 3-dimensional, nanoscale CISe pn BHJs. The specific BHJ structure enables efficient separation and transport of free carriers and essentially performs the same functions as planar pn junctions. The CISe nanocrystals are very different from colloidal nanocrystals used in state-of-the-art BHJs; they exhibit mixed conductivity and high doping densities. These distinctive innate attributes of CISe films naturally create ordered nanoscale morphology and facilitate interconnections between the nanocrystals to form the BHJ structure. This totality manifests a highly significant advance in semiconductor processing as it creates an accessible, low cost solution-based method to fabricate high quality pn BHJ nanocrystalline material systems that can be directly used in PV or LED devices. Furthermore, with the incorporation of finely band-aligned contact electrode materials, the CISe BHJ film can transition into high performance flexible devices and roll-to-roll processing in simple thin-film form factor for easy scale-up.

Mid-infrared detection devices, which are employed in medical imaging, gas sensing, security surveillance and navigation sensing system applications, are essential components in an internet of things platform. InSb is one of the promising candidates for mid-infrared detection devices. It has a room temperature bandgap energy of 0.17 eV and is capable of detecting photon with a wavelength up to 7.3 µm. Growth of InSb on Si overcomes the restrictions of InSb substrate, which are expansive, small size (< 4 inches in diameter) and low ruggedness [1]. Hetero-epitaxy growth of InSb devices on Si is also one of approaches to realize the integration of mid-infrared InSb with Si-based electronic devices on a single wafer, without requiring any wafer bonding process. However, the growth of InSb on (100) Si substrate is challenging due to their large lattice mismatch (19.3 %) and different lattice structures (zinc blende vs. diamond), resulting in high density of defects. Furthermore, direct growth of InSb on (100) Si surface is prohibited because of the formation of In metallic islands on Sb-terminated Si surface instead of InSb film [2]. Therefore, an intermediate buffer layer between InSb layer and Si substrate is needed. In this report, we demonstrated two different intermediate buffer: Ge/GaAs buffer and AlSb/GaSb buffer.
Growth of InSb on a 6° offcut Si substrate using an AlSb/GaSb intermediate buffer was carried out using a molecular beam epitaxy (MBE) system. A 5nm AlSb island was firstly grown, followed by a 100 nm GaSb layer. Subsequently, a 50 nm AlSb layer was grown. Finally, a 0.8 µm InSb was grown on the AlSb surface using interfacial dislocation array to accommodate lattice mismatch. At the initial growth stage of InSb, a spotty RHEED pattern was observed and changed to a clear (1×3) after ~1 minute of growth. Using this InSb layer, an InSb photoconductor was fabricated and its photo-response was measured.

In Ge/GaAs buffer, growth of Ge on Si substrate was carried using a MOCVD system. Growth of GaAs and InSb was carried using a MBE system. Lattice-mismatch (14.6%) strain between GaAs and InSb was accommodated by an interfacial misfit (IMF) array formed at InSb/GaAs interface, which consisted of uniformly distributed 90° misfit dislocations [3]. TEM observation exhibited a low defect density in InSb layer. An InSb p-i-n photo-detector structure was grown. Spectral response of the InSb photodetector with the detector area of 0.0285 mm² was measured using a Fourier Transform Infrared (FTIR) spectrometer with a KBr beam splitter from 80 K to 200 K.


Reference
[1] J.I. Chyi, D. Biswas, S. Iyer, N. Kumar, H. Morkoc, R. Bean, K. Zanio, H.Y. Lee, H. Chen, Appl. Phys. Lett. 54(11) (1989) 1016-1018.
[2] G. Franklin, D. Rich, H. Hong, T. Miller, T.-C. Chiang, Phys. Rev. B 45(7) (1992) 3426.
[3] Jia, B. W.; Tan, K. H.; Loke, W. K.; Wicaksono, S.; Yoon, S. F., Mater. Lett. 2015, 158, 258-261.

Zinc oxide (ZnO) nanorods (NRs) have attracted lots of attention as a potential material for light-emitting diode (LED) applications, due to its outstanding characteristics such as large direct band gap and high exciton binding energy. However, despite these advantages, ZnO NRs are not used in LEDs due to several problems. Since ZnO NRs have naturally n-type conductivity owing to donor type defects in the lattice, the achievement of p-type conductivity is difficult. As a result, most studies have employed heterojunctions which use alternative p-type materials to fabricate LEDs using ZnO NRs. But, heterojunction LEDs generally exhibit poor luminescent efficiency due to lattice mismatch at the p-n junction interface. Another difficulty in fabricating ZnO NRs-based LEDs is the initial growth control of vertical ZnO NR on conventional electrodes, which is due to different crystal structure between the ZnO NR and the electrodes.
In this report, to overcome the problems of ZnO NRs, an Ag bottom electrode was employed as a multiple role implementing layer, which acted as a dopant source for p-type ZnO NRs and a growth template for ZnO NRs. During growth of ZnO NRs on the Ag bottom electrode without seed layer, Ag⁺ ions are dissolved from the Ag bottom electrode and then incorporated into the ZnO lattice. Then, the n-type ZnO NRs were homoepitaxially grown on these Ag-doped p-type ZnO NRs to fabricate a highly efficient p-n homojunction LED with minimum interface defects. These ZnO NRs p-n homojunction LEDs showed a typical rectifying behavior with a turn-on voltage of 3.5 V and a high rectifying ratio of 1.5 × 10⁵ at 5 V. Furthermore, under a forward bias of 9 V, the LED exhibited a wide yellow EL emission centered at 645 nm, which was attributed to the various emission sites of ZnO deep-level defects.

Keywords: Ag doping, p-type ZnO NRs, Homoepitaxial p-n junction, Solution process, Light-emitting diode.

A typical perovskite solar cell consists of a hole transporting layer (HTL), a perovskite light absorber and an electron transporting layer (ETL). The most commonly studied HTL’s are organic/polymeric materials such as Spiro-OMeTAD(2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene ) or PTAA(poly(triaryl amine)), which are expensive and unstable in the presence of water vapor.

This presentation describes the application to perovskite solar cells of a new kind of inorganic HTL synthesized using atomic layer deposition (ALD). By alloying TiO₂ with IrO_x in a super-cycle ALD process, we found that the electron transporting material TiO₂ becomes an effective HTL. Atomic layer deposition offers the prospect of depositing such hole contact materials over a wide variety of substrate materials and geometries, and with excellent control of layer thickness and composition. Previous research [1] on ALD-grown alloys of TiO₂ and RuO₂ showed that a large work function (~ 5.2 eV) can be achieved as result of a ~ 10 mol% RuO₂ addition. In the current work, a Cs_0.17FA_0.83Pb(Br_0.17I_0.83)₃ PSC including a TiO₂-IrO_x HTL of composition 20mol% IrO_x and ~ 10 nm thickness achieved a high power conversion efficiency of 13.4% with 1.004 V open circuit voltage, which is 100 mV higher than the V_oc of the reference device using a PEDOT:PSS. These results indicate that ALD-grown TiO₂-transition metal oxide alloys are promising HTL’s for perovskite photovoltaics.

References
[1] O.L. Hendricks, A.G. Scheuermann, M. Schmidt, P.K. Hurley, P.C. McIntyre, and C.E.D. Chidsey, “Isolating the Photovoltaic Junction: Atomic Layer Deposited TiO₂–RuO₂ Alloy Schottky Contacts for Silicon Photoanodes,” ACS Appl. Mater. Interfaces 8, 23763-73 (2016).

In the search for low-cost, high efficiency solar cells, researchers have been investigating new materials to use as carrier-selective contact layers. Sub-stoichiometric molybdenum trioxide (MoO_3-x ) is a promising material for interface contacts due to its hole selectivity and low parasitic absorption. It has been investigated as a contact for a number of different photovoltaic devices using different absorber materials, including ZnO/PbS quantum dot, CdS-CdTe nanotubes, CuInSe, and kesterite, as well as organic materials, perovskites, and crystalline silicon.

The performance of these contacts depends on the electrical and chemical properties of MoO_3-x. Of particular interest are the bandgap, electron affinity, and the presence of a defect band, the latter imparting n-type semiconducting properties to the metal oxide. These material properties have been found to be sensitive to the presence of intrinsic defects, especially oxygen vacancies. An understanding of how the defect chemistry of MoO_3-x varies with preparation conditions could allow the material properties of MoO_3-x to be optimised for interfaces with different absorber materials.

This paper reports on the use of density functional theory (DFT) simulations to predict defect concentrations as a function of temperature and oxygen partial pressure for crystalline MoO_3-x, by constructing Brouwer diagrams. Additionally it is shown that samples prepared in contact with silicon may be prone to contamination under common preparation conditions, and this contamination can affect the electronic structure of the material. It is therefore reasonable to assume that contamination of MoO_3-x may also occur for other absorber materials and the implications of this contamination in terms of device performance and durability needs to be considered. We then extend this analysis into the properties of amorphous MoO_3-x by means of reverse Monte Carlo modelling based on experimental thin film diffraction data. This extension is of particular value because most commonly-used preparation methods typically result in an amorphous metal oxide. This study provides a critical theoretical contribution to our understanding of the role of defects in transition metal oxide functionality as a carrier-selective contact for photovoltaic devices.

Thin-film solar cells provide an alternative to conventional silicon-based photovoltaics, and modules based on thin film cadmium telluride (CdTe) are cost-competitive with silicon. Device efficiency has increased from 16% to 21% in the last 5 years. Key factors enabling the improvements have been alterations in the device structure, including replacement of the traditional cadmium sulphide (CdS) window layer with higher band-gap alternatives like magnesium-doped zinc-oxide (MZO), and grading/alloying of the near-interface region in the CdTe absorber layer with selenium. However, research on devices incorporating these changes is limited. Little high resolution microstructural/chemical/electronic characterisation has been published. Improved characterisation, and hence understanding, of these devices, can lead to improved cell design/processing for enhanced performance, taking record cell efficiencies closer to the thermodynamic limit of ~30%.

In this work, devices with three architectures are characterised by high resolution dynamic SIMS (‘NanoSIMS’) giving 3-Dimensional chemical maps of the cells at nanometre resolution. This is supplemented by correlative high resolution EBSD and cathodoluminescence measurements, to establish the effects of observed microstructural and chemical features on performance. The three absorber/buffer layer architectures are: 1) traditional CdS/CdTe; 2) MZO/CdTe; and 3) high efficiency, MZO/CdSeTe/CdTe, selenium-graded devices. This set spans the evolution from more conventional device structures to the newer, high-efficiency structures.

The advantage of NanoSIMS is that it’s high sensitivity, combined with high resolution, can directly detect the behaviour of chlorine in grain interiors. This means that, where present, variations in chlorine concentrations can be observed, both between grains and within grains. For instance, data shows concentration gradients within grains that indicate ingress of chlorine from grain interiors to the grain bulk. These local variations in grain interior chlorine concentration can then be correlated to the local luminescence intensity, to ascertain whether chlorine effects carrier recombination activity in the grain bulk. Another interesting behaviour observed is the segregation of chlorine to linear or ribbon-shaped features within many of the grain interiors in all three device types. The 3D nature of the data enables behaviour of chlorine at the three different interfaces to be tracked.

In addition to chlorine, the chemical maps show the three-dimensional behaviour of sulphur and selenium alloying in each of the device types.

A fundamental understanding of carrier transport is imperative for efficient semiconductor electronics and optoelectronics. Here, we use high speed black phosphorus photodetectors with sub-40 ps response time to elucidate carrier transport dynamics along its armchair and zigzag directions. Here we report a direct observation of carrier transport transition dynamics from phonon scattering transport to multiple trapping and release transport mechanism along the armchair direction, resulting from the relaxation of free carriers above the band edge to the band-tail states. We identified that the suppression of phonon scattering effects, a characteristic by Hall and field effect transistor measurements, is due to carrier transport in band tail states. Along the zigzag direction, only multiple trapping and release transport in band-tail states is observed, which might be due to low carrier mobility.

2D van der Waals heterostructures with different types of band alignment have recently attracted great attention due to their unique optical and electrical properties. Most 2D heterostructures are formed by transfer-stacking two monolayers together, but the interfacial quality and controllable orientation of such artifcial structures are inferior to those epitaxial grown heterostructures. Herein, for the frst time, a direct vapor phase growth of high-quality vertically stacked heterostructure of SnS2/MoS2 monolayers is reported. An extremely Type II band alignment exists in this 2D heterostructure, with band offset larger than any other reported. Consistent with the large band offset, distinctive optical properties including strong photoluminescence quenching in the heterostructure area are observed in the heterostructure. The SnS2/MoS2 heterostructures also exhibit well-aligned lattice orientation between the two layers, forming a periodic Moiré superlattice pattern with high lattice mismatch. Electrical transport and photoresponsive studies demonstrate that the SnS2/MoS2 heterostructures exhibit an obvious photovoltaic effect and possess high on/off ratio (>106), high mobility (27.6 cm2 V-1 s-1) and high photoresponsivity (1.36 A W-1). Effcient synthesis of such vertical heterostructure may open up new realms in 2D functional electronics and optoelectronics.

High quality WS₂ film with the single domain size up to 400 microns was grown on Si/SiO₂ wafer by atmospheric pressure chemical vapor deposition. The effects of some important fabrication parameters on the controlled growth of WS₂ film have been investigated in detail, including the choice of precursors, tube pressure, growing temperature, holding time, the amount of sulfur powder, gas flow rate and so on. By optimizing the growth conditions at one atmospheric pressure, we obtained tungsten disulfide single-domains with an average size over 100 microns. Raman spectra, atomic force microscopy and transmission electron microscopy provided direct evidence that the WS₂ film had an atomic-layer thickness and a single-domain hexagonal structure with a high crystal quality. And the photoluminescence spectra indicated that the tungsten disulfide films showed an evident layer-number-dependent fluorescence efficiency, depending on their energy band structure. Our study provides an important experimental basis for large-area controllable preparation of atomic-thickness tungsten disulfide thin film, and can also expedite the development of scalable high-performance optoelectronic devices based on WS₂ film.

In the present study, two-dimensional (2D) material molybdenum disulfide (MoS₂), deposited using chemical vapour deposition is studied for its interesting electrical, optical and mechanical properties, as a function of number of layers. Raman spectroscopy has been employed to obtain the magnitude of difference between E_2g (~385cm^-1) and A_1g (~404cm^-1) peaks of MoS₂ which has been used as a signature of the number of layers. The 2D MoS₂ is characterized based on the nanoscale variations in the junction properties using Kelvin probe force microscopy (KPFM), conductive atomic force microscopy (CAFM) techniques and macroscale variations in the I-V characteristics, as a function of number of layers. The surface potential values obtained using KPFM technique, gives the effective value of work function for different number of layers of MoS₂. For efficient implementation of 2D materials in electronic and optoelecronic devices, it is imperative to form a good metal-semiconductor contact. The metal-semiconductor junction is studied as a function of applied loading force and number of layers of MoS₂ using CAFM. The I-V characteristics are obtained with two different AFM metal tips in contact mode, namely, Cobalt (Co) and Platinum (Pt), which should form an Ohmic and Schottky contact with MoS₂, theoretically. However, experimental investigation of I-V characteristics using CAFM shows the formation of Schottky barrier and hence a rectifying contact due to Fermi level pinning even for the Co metal electrode contact. The study emphasizes the critical influence of singlelayer nature on the metal contacts in novel 2D materials based devices. Metal – 2D semiconductor contact studied nanoscopically and junction behavior analyzed gives insights for the device formation for photodetector applications. In order to substantiate the layer dependence of MoS₂ samples on the optoelectronic properties, photoresponse measurements (I-t characteristics) have been studied macroscopically using Keithley 2400 sourcemeter with a two-electrode configuration. The photoresponse behavior of thin film MoS₂ is observed at different wavelengths of light. The photoresponsivity changes from 12.48 for monolayer sample to 4.79 for multilayer sample due to the optically generated carriers for the typical white light wavelength. Photoelectrical measurements on layer dependent, poly-crystalline MoS₂ samples show excellent sensitivity, fast photoresponsivity and good reproducibility as a photodetector. The study shows the critical role played by the number of layers on the electronic and optical properties of MoS₂ based devices.

Two-dimensional layered materials such as garphene, MoS₂ and WSe₂ have attracted considerable interest in recent times and becoming an important material platform in condensed matter physics and modern electronics and optoelectronics. The studies to date however generally rely on mechanically exfoliated flakes which always be limtited to simple 2D materals. To fully explore the potential of this new class of materials, it is necessary to develop rational synthetic strategies of two dimensional lateral complicated struture,such as lateral heterostructure,multiheterostructure, superlattice,quantum well,etc.
With a relatively small lattice mismatch (∼4%) between MoS₂ and MoSe₂ or WS₂ and WSe₂, it is possible to produce coherent MoS₂–MoSe₂ and WS₂–WSe₂ heterostructures through a lateral epitaxial process. Our studies indicate that simple sequential growth often fails to produce the desired heterostructures because the edge growth front can be easily passivated after termination of the first growth and exposure to ambient conditions. To this end, we have designed a thermal CVD process that allows in situ switching of the vapour-phase reactants to enable lateral epitaxial growth of single- or few-layer TMD lateral heterostructures. We used this technique to realize the growth of compositionally modulated MoS₂–MoSe₂ and WS₂–WSe₂ lateral heterostructures. The WS₂–WSe₂ lateral heterostuctures with both p- and n-type characteristics can also allow us to construct many other functional devices, for example, a CMOS inverter.
In a typical sequential-growth process for 2D lateral heterostructure, the excessive thermal degradation or uncontrolled nucleation during the temperature swing between sequential growthsteps represents the key obstacle to reliable formation of monolayer r lateral complicated structure.We designed a modified CVD system. A reverse flow from the substrate to the source during the temperature swing between successive growth steps. A forward flow only applied at the exact growth temperature. So, the existing monolayer materials will not exposure to high temperature and chemical vapor source at the tempreture increasing and decreasing steps to minimize thermal degradation and eliminate uncontrolled homogeneous nucleation.We used our approach initially for the general synthesis of a wide range of 2D crystal heterostructures. We also grew more complex compositionally modulated superlattices or multiheterostructures, the number of periods and repeated spacing can be readily varied during growth. HADDF-STEM analysis of the atomic structure of the lateral heterostructures and Multiheterostructures show the atomically sharp interface can be clearly observed.
References
1.Xidong Duan, Anlian Pan,Ruqin Yu, Xiangfeng Duan,et,al, Nature Nanotechnology 9,2014,1024-1030.
2.Zhengwei Zhang,Xidong Duan, Xiangfeng Duan,et al, Science, 357, 2017,788–792.

It is a common problem that growing a given composition in a desired morphology is not always possible and thus limiting the application gamut. In this research, we have demonstrated dual ion exchange processes to grow novel optoelectronic structures that are unlikely to be grown directly. In the literature, separate anion or cation exchange has been demonstrated before, but to the best of our knowledge dual ion exchange processes where both anion and cations are partially replaced has been systematically demonstrated for the first time with this research. The scope of this study is to grow any composition of ZnCdSSe quaternary alloys in nanosheet form. It is demonstrated that growing CdS- and CdSe-rich quaternary ZnCdSSe alloys in nanosheet form is easy, but due to very low vapor pressure of wide bandgap materials such as ZnSe and especially ZnS it is almost impossible. The reason is that only VLS mechanism is active under the low vapor pressure and thus leading to grow those materials in 1D nanowire form. On the other hand, materials in Cd, S and Se-rich are easily grown in 2D nanosheet morphologies and therefore they provide a good platform for the morphology transfer process. Here, we have used CdSe-rich materials as a basis and dual ion exchange process was implemented on those materials in a temperature region higher than the one used to grow CdSe nanosheets. As a result of this process we have grown ZnS- and ZnSe-rich ZnCdSSe quaternary alloys in nanosheet form and this paved the way for variety of optoelectronic applications such as white laser, personalized light emitters, full color photodetectors, and so on. This mechanism is not only applicable for ZnCdSSe, but also for any other material systems in which the desired morphology and composition combinations cannot be obtained directly.

Polymer solar cells (PSCs) have attracted great attention due to their potential applications as low-cost renewable energy sources. In Conventional PSCs, anode degradation caused by the PEDOT diffusion induced etching of ITO can occur in conventional PSCs, resulting in short lifetimes of the device. In order to enhance the stability of PSCs, PSCs of inverted structures, where the nature of charge collection is reversed, have been employed as an alternative to improve the lifetime of solar cells. To further enhance the device stability, inorganic semiconductor oxides inserted between ITO and the active layer has been implemented as a buffer layer in an inverted device structure to selectively collect electrons as well as avoid the contact of ITO film with PEDOT polymer. The buffer layer works as an electron-collecting electrode and a hole-blocking layer, which is essential for achieving high efficiency PSCs.
SrTiO₃ with the perovskite structure is a very attractive material for application in microelectronics due to its high charge storage capacity, chemical stability, good insulating properties and excellent optical transparency in the visible region. It has a similar band structure as ZnO but a higher dielectric constant (~10⁴), which can favor charge transfer and thus be used as electron transport layer in PSCs. Here, we report on the inverted PSCs with pure SrTiO₃ films as cathode buffer layer for the first time. Uniform SrTiO₃ films were fabricated on ITO substrate via the PAD method to work as cathode buffer layer in the inverted PSCs. The results indicate that the power conversion efficiency of the solar cells based on P3HT and PCBM with SrTiO₃ film as cathode buffer layer and MoO₃ as anode interfacial layer is up to 3.5%, comparable to that of PSCs with ZnO as buffer layer reported previously.

Cu₂O is a p-type semiconductor that has promising properties for solar cells, lithium ion batteries, emitting diodes, photo-catalyst for hydrogen production, photo-catalyst for water and air decontamination, sensor for organic molecules, among others. In this work, the conversion of CuO thin films to Cu₂O ones, in less than 30 s via an argon/dry-air microwave plasma, is studied. The process is carried out in a lab-made equipment built with low-cost components, such as a commercial microwave oven . Our simple, but reliable processing, is faster, cheaper and easier than other similar plasma treatments (watch video, https://goo.gl/yS7Y1h), this could allow for its application at mass scale production of Cu2O films.

CuO film targets are easily produced by dip-coating on glass substrates from a homogeneous copper acetate solution. CuO targets are annealed in groups at different temperatures, from 350 °C to 550 °C in increments of 50 °C, in open atmosphere. To obtain Cu₂O, CuO thin films are treated for 15, 20, 25 or 30 s, under an argon/dry-air microwave plasma. The plasma processing is performed inside a quartz chamber at low vacuum (15 mbar), with small gas fluxes of argon (6 sccm) and dry-air (6 sscm), and with a microwave power of 1500 W.

Interestingly, pure Cu₂O is only produced from a metastable form of metallic copper that is obtained after the plasma treatment. The conversion process can take from a couple of minutes, under a controlled flow of oxygen, to some hours, in open atmosphere. This partial oxidation of metallic copper is clearly driven by the oxygen availability right after the plasma treatment, when the sample is still hot. To our knowledge, this phenomenon has not been reported before.

To study the effect of oxygen, argon and dry-air fluxes are shut down immediately after the plasma treatment and a controlled flow of pure oxygen is supplied for 10 minutes. When no oxygen is used, the resulting film looks dark green; XRD patterns show metallic copper as the majority phase. Once pure oxygen is supplied after the treatment, the film acquires a lighter green color; for these films XRD patterns show a mixture of Cu and Cu₂O. Pure Cu₂O is achieved once a certain minimum amount of oxygen is supplied. The transformative role of oxygen has a profound effect not only in the crystalline phase of the film, but also in its optical and electrical properties. This phenomenon is being studied with the aim of tailoring some of the properties of pure Cu₂O.

Depending on different process conditions, like time of plasma, the crystallite size of Cu₂O can be increased and controlled. Nonetheless, there is also an optimal annealing temperature, around 400 °C, for which crystallite size can be maximized. By treating the CuO samples for a longer time, bandgap can be decreased from 2.35 to 2.17 eV. The advantages of our plasma processing lie in the simplicity, short time of treatment and, low cost of the built equipment.

The piezotronic effect and piezo-phototronic effect on materials and devices have been widely studied in binary semiconductors. Wide-band ternary semiconductors are a great class of materials with potential application in nano/microdevices, because of their continuously tunable physical properties with composition. Here, we first demonstrate the piezo-photronics effect of ternary wurtzite structured nanowires (NWs), opening an innovative materials system. Single-crystal ternary CdS_xSe_1–x (x = 0.85, 0.60, and 0.38) NWs were synthesized with site-controlled compositions via a chemical vapor deposition process, and high-performance visible photodetectors (PDs) with fast response speed (<2 ms), high photosensitivity, high responsivity, and broadened photoresponse region (than CdS NW) were fabricated based on these ternary materials. By introducing an external tensile strain, the performance of PDs is enhanced by 76.7% upon 0.2 mW/cm² 442 nm light illumination for CdS_0.85Se_0.15 by the piezo-phototronic effect. The composition effect of materials in ternary materials on light detecting and piezo-phototronics was also first investigated systematically. The results indicate that in the CdS_xSe_1–x system, as the value of x decreases, the photocurrent and responsivity experience an increase, while the enhancement of the piezo-phototronic effect was weakened. The change in piezoelectric coefficient and carrier screening effect are proposed for the observed phenomenon. This study reports a high-quality ternary CdS_xSe_1–x NWs system used for high-performance PDs, broadens the family of piezotronic materials, offers an innovative material for high-performance visible PD, and provides a new pathway to modulate the performance of piezo-phototronic devices by tuning the atomic ratios of ternary wurtzite semiconducting materials. This is essential for developing a full understanding of piezotronics on a broader scope, and it also enables the development of the better performance of optoelectronic devices.

Recently, the ternary material system of Cu-Zn-S has shown significant promise as a p-type transparent conductor (TC) for photovoltaic and optoelectronic applications. Previous studies have found evidence of both Cu doping onto the Zn antisite (Cu_xZn_1-xS) and a solid solution of Cu_yS and ZnS (Cu_yS:ZnS), depending on the thermodynamics and kinetics of the growth process. Here, we investigate this material system using combinatorial sputtering, high-throughput characterization and percolation theory to explore the phase transitions (both in chemical potential and temperature space) and resulting structure-property relations. Samples are grown with copper concentrations across the entire chemical space, with Cu/(Cu+Zn) ranging from 0 to 1, and films are found to crystallize at room temperature with optimized p-type conductivity and transparency within the range of 0.2 < x < 0.4, in agreement with the “TC regime” of previous studies. We find conductivity to increase monotonically as a function of Cu concentration, which is evidence for a solid solution of amorphous Cu_yS and ZnS, yet it increases with distinct jumps by an order of magnitude at two different Cu concentrations. Using high spatial resolution synchrotron x-ray diffraction, we find these jumps in conductivity to correlate with structural changes between the wurtzite and zinc-blende crystal structures. This could indicate either (1) greater Cu incorporation into the wurtzite phase than zinc-blende or (2) higher transport in wurtzite due to lower computed effective mass. Additionally, we find conductivity within the “TC regime” to increase to up to 250 S/cm at growth temperatures of 185 - 200 deg. C with no decrease in transparency. At elevated temperature films are found to be solid solutions of Cu₂S₅ and zinc-blende ZnS, so conductivity is likely due to larger crystal grains and a connected network of Cu₂S₅ regions within a transparent ZnS matrix. We also present initial results from heterojunction solar cells with combinatorially sputtered Cu-Zn-S and discuss this material’s excellent TC figure of merit in the context of both state-of-the-art p-type TCs and the authors’ recent computational screenings.

Sb₂Se₃ possesses a great potential for low-cost and high-efficiency thin film photovoltaic, with a suitable bandgap, high absorption coefficient, benign grain boundaries and earth-abundant constituents. However, the efficiency improvement of Sb₂Se₃ thin film solar cells was severely restricted by the absorber quality and the trap-assisted recombination caused by the deep defects in the absorber. Herein, we developed a vapor transfer deposition (VTD) technique to fabricate Sb₂Se₃ films, which highly enhanced the performance of Sb₂Se₃ solar cells. After the optimization, we significantly enhanced the superstrate CdS/Sb₂Se₃ solar cells with certified power conversion efficiency of 7.6%, a net 2% improvement over previous 5.6% record of the same device configuration. We analyzed the deep defects in the devices and found that the density of the dominant deep defects was reduced by one order of magnitude using VTD process. Furthermore, the VTD fabricated devices showed fewer interface defects, longer carrier lifetime, and then better performance, compared with the devices fabricated by rapid thermal evaporation. These encouraging results promote the development of Sb₂Se₃ solar cells in high-efficiency thin film photovoltaic devices.

Photovoltaic research activities over the past decades have focused on the development of low-cost highly efficient materials for application as absorbers in photovoltaic technologies. Popular material systems under consideration in recent years include metal-halide perovskite, organic-inorganic hybrid perovskite, and copper chalcogenide semiconductors such as CuIn_1-xGa_xSe₂ (CIGS). The large absorption coefficient of these materials coupled to the ability to engineer their bandgap through chemical substitutions enable the realization of solar cells devices with power conversion efficiency exceeding 20%. Despite the promise of these material systems, thermal instability associated with hybrid perovskite, restriction on the use of heavy metals (Cd, Pb etc.) and the limitation in supply for In are roadblocks to large scale deployment of the existing leading perovkite and chalcogenide-based technologies. To address these issues, earth abundant copper chalcogenides such as kesterites, Cu₂SnZn(S,Se)₄ (CZTS), that can be obtained through chemical substitution of In³⁺ atoms in CuIn(S,Se)₂ by Zn²⁺ and Sn⁴⁺, have been investigated. However, the efficiencies of solar cell devices based on these materials remain around 12.6% due to unavoidable anti-site defects such as Cu_Zn and Zn_Cu. It therefore appears that achieving low-cost, earth abundant copper chalcogenide solar cells with high efficiency requires the development of novel compositions with new crystal structure rather than a simple variation of the chemistry of existing structures. In this work, we report for the first time on the Earth-abundant ternary copper titanium selenide, CTSe, as a promising light-absorbing material for the fabrication of ultra-thin low-cost high efficiency solar cell devices. CTSe is a p-type semiconductor featuring indirect (1.15 eV) and direct (1.34 eV) bandgaps, which are both desirable for ideal solar absorber materials. It crystallizes in a new noncentrosymmetric cubic structure (space group F-43c) in which CuSe₄ tetrahedra share edges and corners to form the octahedral anionic clusters, [Cu₄Se₄]^4-, which in turn share corners to build the three dimensional framework, with Ti⁴⁺ ions located at tetrahedral interstices within the channels. This unique structural feature results in large density of states (DOS) and relatively flat bands near the band edges, which are believed to be responsible for the ultra-large absorption coefficients (~10⁵ cm^-1) observed throughout the visible range for CTSe thin-film. These findings point to CTSe as a promising solar absorber material for scalable low-cost high-efficiency thin-film solar cells.

Optimal transparent conductive oxides (TCOs) are essential to reduce parasitic absorption losses in optoelectronic devices. Hydrogenated indium oxides (IO:H, ICO:H, IWO:H) lead the race, as they have electron mobilities > 100 cm²/Vs, low absorption in the visible and near-infrared parts of spectra and they can be easily deposited over a wide range of substrates. During deposition, the introduction of water hampers the material crystallization, hence as deposited films have amorphous microstructure. After annealing at 200°C, these materials experience a phase transition and coalesce in big crystalline domains, which are linked to the high electron mobility. Nonetheless, the presence of water during deposition is a drawback to upscaling humidity sensitive technologies to production.
In this work, we propose an alternative TCO sputtered without the intentional introduction of water during deposition: zirconium-doped indium-oxide (or IO:Zr). Using a combination of optoelectronic characterization, high-end electron microscopy techniques, electron recoil dispersive analysis and Rutherford backscattering, we fully characterize the material and explain the fundamental mechanisms limiting its electron transport. Even without the intentional introduction of water during deposition, these films have an amorphous microstructure and after annealing at 200 °C form crystallites with average size ~ 320 nm. With an electron mobility > 100 cm²/Vs and free carrier density as high as 2.5 × 10²⁰ cm^-3, 100 nm-thick films have a wider bandgap (between 3.5 eV and 3.9 eV) than the afore mentioned In-based TCOs. In addition, we found that the main limiting mechanism of electron transport in IO:Zr is ubiquitous phonon scattering.
Motivated by the high conductivity of the material, we reduced the thickness of the films from 100 nm to 15 nm to further reduce the optical absorptance. The thinnest films have an optical absorptance close to the glass substrate in which they were deposited- while still presenting high electron mobility (50 cm²/Vs), and high free carrier density. Interestingly all films, from 100 nm down to 15 nm-thick films, show the presence of large crystalline grains after annealing at 200 °C.
Finally, to demonstrate the applicability of the material, IO:Zr thin films with different thickness were applied as front electrode in silicon- and perovskite-based solar cells; showing in all cases an improvement in current density thanks to high transparency of IO:Zr as compared to ITO.

Dopants and additives such as alkali-ions can substantially affect the PV performance of thin-film semiconductor PV ¹. For instance, Na has been linked to the passivation of grain boundaries ², while Sb decreases the crystallization temperature of CIGS ³and. In this work, we shed new light on the role of Na and Sb in the performance of Cu₂ZnSnS₄ (CZTS) solar cells employing temperature dependent photoluminescence, impedance spectroscopy photoemission microscopy. CZTS films are generated using a single solution precursor from metal salts and thiourea dissolved in DMF-Isopropanol mixture. The precursor is spin-coated onto Mo-coated glass substrates and finally annealed at 550 ^oC under S atmosphere. The process yields polycrystalline films with a band gap of 1.4 eV consisting of Cu-poor and Zn-rich composition with tetragonal phase. Electron microscopy shows the formation of 1.2 mm films with more crystalline (grain sizes up to 700 nm) and compact morphology upon the inclusion of Sb and Na:Sb. This is due to the lowering of crystallization enthalpy as determined by differential scanning calorimetry. The quantitative analysis of X-ray diffraction reveals decrease in the isotropic thermal parameter, associated with the atomic site disorder, especially for Sn site upon Sb doping.⁴ Solar cells are completed to have a substrate configuration of: glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni−Al with individual cells scribed with a total area of 0.5 cm². Statistical analysis of J-V measurements over 200 devices under AM1.5 G spectrum with 100 mW/cm² demonstrates an overall improvement in fill-factor (FF) and open-circuit voltage (V_OC) with better conformity resulting in a rise of average power conversion efficiency (h) from 3.2 ± 0.6 to 5.2 ± 0.3% on doping. The best performing device with Na:Sb doping yielded an 14.9 mA cm⁻² short-circuit current, 610 mV V_OC, 63% FF and h of 5.7%, rating amongst top reported for pure sulphide kesterite.⁵ Recently, a computational study predicted that depending on concentration, Sb doping may either lead to the appearance of states in the gap detrimental to the performance or may boost the performance through lowering the Sn disorder .⁶ For the first time, we monitor the electronic states due to Sb doping employing low-temperature photoluminescence and surface sensitive energy filtered ultraviolet photoemission spectroscopy which allows the rationalizing the systematic change in device efficiency.

1. Prog. Photovoltaics Res. Appl., 2017, 25, 645–667.
2. Adv. Mater., 1998, 10, 31–36.
3. Chem. Mater., 2010, 22, 285–287.
4. Chem. Mater., 2016, 28, 4991–4997.
5. Prog. Photovoltaics Res. Appl., 2016, 24, 879–898.
6. J. Mater. Chem. A, 2017, 5, 6606–6612.

Realization of sustainable hydrogen production via photoelectrochemical (PEC) water splitting is contingent on developing efficient and low-cost photoelectrode. Sb₂Se₃ recently receives great interest as a promising low-cost light-absorbing material for solar energy conversion. In this talk, we will present a synthetic methodology to produce efficient Sb₂Se₃ nanostructure photocathodes by a simple spin-coating of Sb-Se molecular ink. The Sb-Se molecular ink is prepared by using the solvent mixture of thioglycolic acid (TGA) and ethanolamine (EA) and the aspect ratio of 1-D Sb₂Se₃ nanostructures can be controlled by adjusting the relative mixing ratio of TGA and EA. After the deposition of TiO₂ and Pt, an appropriately oriented Sb₂Se₃ nanostructure array exhibits a significantly enhanced PEC performance; the photocurrent reached 12.5 mA cm^-2 at 0 V versus reversible hydrogen electrode under AM 1.5 G illumination. The role of carboxylate nucleophile enabling the unique 1-D nanostructures will be elucidated by liquid Raman spectroscopy in conjunction with the observation of the morphological evolution. In addition, the strengths and limitations of Sb₂Se₃ based photocathode will be discussed with various analyses including incident photon-to-current conversion efficiency, THz spectroscopy, and time-resolved photoluminescence.

Although several wide-bandgap (1.6-2.0 eV) chalcopyrites (e.g. CuGaSe₂ Cu(In,Ga)S₂, Cu(In,Al)Se₂, (Ag,Cu)GaSe₂) have been thus far studied as top cell absorbers for photoelectrochemical (PEC) water splitting tandem devices, CuGa(S,Se)₂ is a wide-Eg chalcopyrite that has not yet received any attention in this regard. Thus, we present the performance of wide-bandgap chalcopyrite CuGa(S,Se)₂ photocathodes as a top cell for PEC tandem water splitting. To be able to assess the PEC performance of CuGa(S,Se)₂ as well as its optical transmittance, transparent conductive fluorinated tin oxide (FTO) was used as a contact. During our study though we learned that synthesizing CuGa(S,Se)₂ films on an FTO substrate would degrade the optoelectronic properties of the FTO and so we present a synthesis to circumvent this problem and fabricate functioning CuGa(S,Se)₂ PEC photocathodes. We then present the PEC performance of these functioning CuGa(S,Se)₂ PEC photocathodes (J_SAT≈10 mA/cm²) as well as measurements relevant to its performance as a top cell, such as quantum efficiency of a low-Eg Cu(In,Ga)Se₂ shaded by the CuGa(S,Se)₂ PEC photocathodes and sub-bandgap transmittance of the CuGa(S,Se)₂ PEC photocathodes.

The everlasting demanding for high-efficiency, low-cost solar electricity and lighting motivate the research on new earth-abundant and toxicity-free semiconductors. Sb and Bi based materials are relatively less explored partially because they contain s² electron lone pairs which often leads to compounds with low crystal symmetry. In this presentation we will discuss two parts: Sb₂Se₃ for thin film photovoltaics and Bi based halide perovskite for lighting.
For the first part, our efforts and recent progress in Sb₂Se₃ photovoltaics will be presented. Binary chalcogenide Sb₂Se₃ has appropriate band gap and excellent optoelectronic properties, nontoxic and earth-abundant composition, and large vapor pressure enabling easy evaporation, making it a possible green alternative to CdTe. Using sprayed ZnO as the buffer layer and rapid thermal evaporation deposited Sb₂Se₃ as the absorber, ZnO/Sb₂Se₃ solar cells with certified 5.93% efficiency and outstanding stability was demonstrated¹. Through a further Sb₂Se₃ film optimization, a further 7.6% device efficiency was achieved, but sadly using the CdS buffer layer. The new understanding of Sb₂Se₃ material properties and device physics will also be briefly presented in this talk.

For the second part, Bi based hybrid and all inorganic halide perovskites as a potential alternative to Pb perovskite for photoluminescence (PL) application will be presented. We investigated these materials for PL because Bi³⁺ is isoelectronic to Pb²⁺ but toxicity-free, and more importantly because Bi halide perovskites have low-dimensional crystal structure (0D, 1D or 2D) naturally enjoying a large exciton binding energy which is beneficial for lighting application. We will discuss our recent progress in the synthesis, characterization and lighting application of Bi-based perovskite nanocrystals², with the emphasis on the Cs₃Bi₃Br₉ nanocrystals with >50% PL yield and excellent stability.
We conclude that Sb and Bi based semiconductors, if carefully engineered to take advantage of their low crystal structure symmetry, are competitive for some optoelectronic applications.
Reference
J. Tang et. al. Nat. Energy, 2017, 2, 17046.
J. Tang et. al. Adv. Funct. Mater. 2017, in press.

The poor stability of high efficiency photoabsorber materials in aqueous media is one factor holding back the realization of photoelectrochemical (PEC) water splitting for large scale, practical solar fuels generation. Here, we demonstrate that highly efficient thin film Sb₂Se₃–fabricated by a simple, low temperature selenization of electrodeposited Sb–is intrinsically stable towards photocorrosion in strongly acidic media (1 M H₂SO₄). Coupling with a photoelectrodeposited MoS_x hydrogen evolution catalyst gives high photocurrents (5 mA cm^-2 at 0 V vs RHE) and high stability without protective layers. A low temperature sulfurization of the Sb₂Se₃-MoS_x stack dramatically improved the onset potential, resulting in high photocurrent densities up to ~16 mA cm^-2 at 0 V vs RHE. The simplicity with which these photocathodes are fabricated, combined with the high photocurrents and stability, make Sb₂Se₃ a strong candidate for scalable PEC cells.

Antimony selenide solar cells are an emerging thin-film technology of growing interest. They benefit from a direct ~1.17eV bandgap, containing no scarce materials, have a simple phase chemistry and an interesting 1D nanoribbon grain structure. Despite the first respectable device efficiency being reported as recently as 2014 and the relative paucity of research, they have already reached efficiencies of 6.5% in excess of other widely studied binary compounds such as SnS. Due to the nascent nature of the technology the optimal cell structure is still to be determined and as a result number of n-type partner layers such as ZnO, CdS and TiO2 have thus far been reported.
In this work we compare devices based on two Sb₂Se₃ deposition routes, thermal evaporation and close space sublimation, to examine the influence of the n-type partner layer and the interface on device performance. Our results show that while CdS is a suitable partner layer for thermally evaporated Sb₂Se₃ devices (efficiencies >4%) for CSS deposited layers CdS is unsuitable (<2.5% efficiency). In contrast TiO₂ layers are highly effective for CSS material (>5.5% efficiency) but are unsuited to thermally evaporated Sb₂Se₃ devices (<1% efficiency). We will demonstrate this is due to the degree of S/Se interdiffusion at the interface and thereby linked to the Sb₂Se₃ deposition process. Device performance will be linked to XPS analysis of band alignments, cross sectional TEM of the interface and deep level transient spectroscopy (DLTS) analysis of defect composition in complete cell structures.

ZnO nanowire (NW) arrays have emerged as promising building blocks for a wide variety of optoelectronic and photovoltaic devices, including the extremely thin absorber (ETA) solar cells. In this novel architecture, a direct p-type semiconductor absorber is typically deposited on a ZnO NW array to form core-shell p-n heterojunctions following the type-II band alignment. Increasing interest has been dedicated to this radial architecture owing to efficient light trapping and charge carrier management together with the use of a low amount of materials.¹

Antimony trisulfide (Sb₂S₃) is a p-type semiconductor with a 1.7 eV band gap energy and a high absorption coefficient that has been integrated into mesoporous-TiO₂-based dye-sensitized solar cells, showing a power conversion efficiency (PCE) as high as 7.5%.² It is usually grown by low-cost, low-temperature chemical deposition techniques, which still make its combination with ZnO NWs difficult owing to their instability in acidic conditions.

In this work, the crystallization process of Sb₂S₃ thin films is investigated by in situ x-ray diffraction and in situ Raman spectroscopy, revealing the intermediate formation of a metallic antimony phase and showing the optimal annealing temperature of 270°C.³ Furthermore, an 8 nm-thick TiO₂ protective layer is grown by atomic layer deposition onto ZnO NW arrays grown by chemical bath deposition. Sb₂S₃, as an absorbing shell, is subsequently deposited by chemical spray pyrolysis. The Sb₂S₃ 10 nm-conformal shell with high crystalline quality covers the ZnO/TiO₂ NW arrays from the bottom to the top. The photovoltaic performance of the ZnO/TiO₂/Sb₂S₃ core shell NW heterostructures using P3HT as hole transporting material results in a promising PCE of 2.3% with a high J_sc of 7.5 mA/cm²_, when considering that the Sb₂S₃ shell is 10 nm-thick, and a high V_oc of 656 mV.⁴ The use of low-cost, surface scalable chemical deposition techniques for the fabrication of the whole ZnO/TiO₂/Sb₂S₃ structure opens the way for improving the performances of ZnO NW-based ETA solar cells.

¹E.C. Garnett et al. Annual Review of Materials Research 41 (2011), 269-295
²Y.C. Choi et al. Adv. Funct. Mater. 24 (2014), 3587
³R. Parize et al. Materials & Design 121 (2017), 1-10
⁴R. Parize et al. The Journal of Physical Chemistry C 121 (2017), 9672-9680

It is well known that one way of creating an environmentally friendly energy production momentum, which allows mitigating the effects of global warming, is closely linked to the commercial relevance of renewable energy production systems. Photovoltaic (PV) energy can play an important role in this field. Currently dominated by technology based on Si this technology has some drawbacks that prevent a greater market presence. High energy payback time and low industrial production rate, among others, are constrains to a higher PV share in the energy production systems in most countries. Due to monolithic integration, lower energy processes and lower material demand, thin film technology presents good arguments to overcome Si technology. CIGS and CdTe based PV cells are currently the most powerful representatives of thin film technology on the market. However, the solar cells based on these materials present problems related to the scarcity and toxicity of some elements that compose them. Alternative materials are currently been studied, such like Cu₂ZnSn(S,Se)₄, to be applied as absorber layer in the solar cell structure. But due to its complexity and restricted growth conditions some difficulties are been encountered. These facts have prevented the production of devices with efficiencies compatible with their commercialization.

In this work we present a method to grow Sb₂Se₃ thin film which can be used as absorber layer in a solar cell structures. These films were grown on the top of different substrates such as soda-lime glass, Mo coated soda-lime glass and Si. The Sb-Se precursor’s films were deposited by RF magnetron sputtering and then annealed with an H₂Se gas flow. Different annealing temperatures were tested and analyzed. This study also analyses the effects of the use of different substrates on properties of the film. Compositional and morphological analyzes are performed by Energy Dispersive Spectroscopy and Scanning Electron Microscopy, respectively. Two techniques are used to phase identification and structural characterization, namely, X-ray Diffraction and Raman Dispersion Spectroscopy. Special attention is taken to Raman scattering characterization conditions in order to avoid measurement artefacts. Many authors show results with Sb₂O₃ Raman modes identified as Sb₂Se₃. In the work we clearly differentiate these modes from each binary compound and show how to avoid the oxidation of Sb. Spectrophotometry is also performed in order to determine absorption coefficient and the band gap energy of the semiconductor.

The strong absorption and visible spectrum energy bandgaps for the transition metal dichalcogenides (TMDCs) of molybdenum and tungsten render them as attractive candidates for photovoltaics (PV) and optoelectronics. Further, the atomically thin nature is favorable for efficient separation and collection of photo-excited charge carriers. Thus if the three major optoelectronic criteria, i.e., i) sunlight absorption, ii carrier collection and iii) operating voltage can be addressed for TMDC materials, they may be candidates for high efficiency photovoltaics and photocatalysis¹. We recently demonstrated near-unity broad-band absorption of above band-gap photons for < 15 nm TMDC layers², and have also achieved high external quantum efficiency in < 10 nm thick active layer photovoltaic devices in a pn junction of WSe₂/MoS₂ with graphene contacts³. To date, achievement of high open-circuit voltage (Voc) has remained an outstanding challenge for achieving high photovoltaic efficiency. We report here high open-circuit voltages (V_oc) in TMDC absorbers based photovoltaic devices, achieved by tailoring the conduction and valence band-alignments between a single TMDC absorber layer and carrier-selective contact layers for electron collection (titanium oxide) and hole collection (nickel oxide), respectively. The band alignments measured using X-ray photoelectron spectroscopy indicate the asymmetric and selective nature of the metal oxide carrier-selective contacts, and we observe open circuit voltages exceeding 700 mV under AM 1.5 illumination at 1 Sun. We will also discuss TMDC passivation, and architectures for photocatalysts and photoelectrochemical devices that employ these materials as absorbers and catalysts.

References:
1. Jariwala, D.; Wong, J.; Davoyan, A. R.; Atwater, H. A. ACS Photonics 2017, ASAP.
2. Jariwala, D.; Davoyan, A. R.; Tagliabue, G.; Sherrott, M. C.; Wong, J.; Atwater, H. A. Nano Lett. 2016, 16, (9), 5482-5487.
3. Wong, J.; Jariwala, D.; Tagliabue, G.; Tat, K.; Davoyan, A. R.; Sherrott, M. C.; Atwater, H. A. ACS Nano 2017, 11, 7230–7240.

Optoelectronic devices featuring 2D transition metal dichalcogenide (TMDC) semiconductors, such as MoS₂, WS₂, and MoSe₂, hold great promise for the miniaturization of future light emitting and collecting devices. In order for 2D optoelectronic devices to be developed into viable technologies, it is necessary to produce large-area films and vertical device architectures. Vertical devices are expected to increase device performance by reducing the path length of charge carriers by ~100x and by enabling lateral scaling on the order of centimeters. In response, we propose and demonstrate a device architecture that uses a molybdenum film as both the growth substrate for 2D MoX₂ (X = S, Se, Te) and also as the bottom contact for vertical MoX₂-based optoelectronic devices. This architecture allows for TMDC growth directly on top of its bottom contact, simplifying and improving the device fabrication process. Using a Thermal Vapor Sulfurization (TVS) technique developed in previous works, we show that a high-quality 2D film of MoS₂ can be grown on top of molybdenum by reacting the top of the molybdenum layer with sulfur vapor; the remaining molybdenum underneath the MoS₂ is used as a conducting bottom contact. This contact scheme requires no external transfers of the MoS₂ layer and results in an intimate bottom contact interface relatively free of contaminants. Preliminary multilayer MoS₂ photodetector devices on a 150nm molybdenum film were fabricated and characterized. Strong A_1g and E_2g raman peaks, spaced 25cm^-1 apart, indicate a 10-15 layer MoS₂ growth. In addition, diffuse and specular reflection measurements indicate an absorption of up to 85% of visible light, as aided by internal reflection off of the MoS₂/Mo interface. A 30µm × 20µm Ti/Au grid finger array is used as the top contact. When illuminated from 400nm to 700nm using a supercontinuum laser and laser line tunable filter under 6V source-drain bias, the device shows a 3 order of magnitude increase in spectral photocurrent relative to comparable lateral photodetectors. Dark and illuminated IV curves reveal diode-like behavior, with an induced photocurrent of up to 300nA under monochromatic illumination of ~0.05mW. Palladium and other hole-selective contacts will be used to fabricate large-area 2D MoX₂-based Schottky-type photovoltaics with collection areas on the order of cm². Device simulation using the AFORS-HET software package is used to computationally interpret the performance of the devices and to explore the parameter space of device properties. We also emphasize the potential for these devices to be fabricated on micron-thick molybdenum foils, enabling roll-to-roll fabrication on ultra-lightweight and flexible substrates. This presented technique reveals a viable route for industrial-scale synthesis of nm-thick 2D TMDC photovoltaics, with great potential for applications requiring ultra-lightweight photovoltaics, such as spacecraft and vehicle energy collection.

MXenes are a rapidly growing class of 2D transition metal carbides, carbonitrides, and nitrides. Their high electronic conductivities make them promising in many applications including transparent conductive coatings, electromagnetic interference shielding, and energy storage. To date, more than 20 MXenes have been synthesized mostly by selective etching using fluorine-containing etchants, and exfoliated into 2D flakes by employing organic- or inorganic-based intercalants. Using different etchants and intercalants results in variation in surface functionality, flake size, number of defects, and interlayer spacing, all of which may affect the optoelectronic properties of MXenes. In this study, we report the effects of synthesis methods and post-synthesis heat treatment on the optoelectronic properties of titanium carbonitride MXene (Ti₃CN). Transport properties of free-standing MXene ‘paper’, transparent thin films, and monolayer flakes were studied by temperature-dependent resistivity measurements from 10-300 K. The results show that the films prepared using a large organic intercalant are about three orders of magnitude less conductive than films prepared with a smaller inorganic-based intercalant. Furthermore, upon heat treatment, the conductivity can be greatly improved by removing the intercalated molecules. Our results show how the electronic properties of Ti₃CN MXene can be tuned by optimization of synthesis method and post-synthesis heat treatment. A similar approach can be applied to other MXenes to control their optoelectronic properties.

Germanium is an attractive alternative to silicon due to its substantially higher carrier mobility and good lattice match with GaAs that can facilitate materials integration for high-performance electronics and optoelectronics. But its oxides are unstable and have poor electronic properties. Surface passivation by chalcogens could potentially endow Ge surfaces with properties similar to those of 2D metal chalcogenides, in particular a very low chemical reactivity and complete elimination of dangling bonds. For transition metals, direct sulfurization has been reported as a way of producing high quality few- and single-layer MoS₂ and WS₂. In the case of Ge, a similar sulfurization might protect the surface against oxidation by formation of germanium sulfide (GeS), a layered chalcogenide semiconductor with a direct bandgap in the visible range and promising optoelectronic and electronic properties.
Here we report solid-state reactions of Ge with sulfur involving exposure to S vapor at elevated substrate temperatures and near atmospheric pressure to produce Ge-sulfides on extended Ge(100) and (111) surfaces. We analyze the reaction kinetics and derive the activation energy of the rate-limiting step of the sulfurization reaction through X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy on samples exposed to the same S dose at different temperatures. Our results show that the sulfurization process gives rise to a sequence of GeS_x phases, terminating in few nanometer layers of GeS₂. The thickness evolution is thermally activated with a barrier E_A = 0.68 eV, attributed to a reaction-limiting diffusion process through the sulfide layer. GeS_x terminated Ge exhibits excellent long-term stability against oxidation in air, demonstrating the ability of controllably producing Ge-chalcogenide passivation layers via solid-state reactions in S vapor [1].
In addition to these results for flat Ge surfaces, we discuss the sulfurization of Ge nanowires. Semiconductor/chalcogenide core-shell nanowires could become heterostructure materials for photovoltaics, photo-electrochemistry, and optoelectronics. Ge nanowires synthesized by the vapor-liquid-solid (VLS) [2] method and exposed to sulfur vapor at different temperatures and times show the controlled formation of GeS shells. Single nanowire cathodoluminescence measurements establish the optoelectronic properties resulting in these novel one-dimensional core-shell heterostructures [3].

References:
[1] H. Chen, C. Keiser, S. Du, H-J. Gao, E. Sutter, and P. Sutter, “Termination of Ge Surfaces with Ultrathin GeS and GeS₂ Layers via Solid-State Sulfurization”, under review (2017).
[2] E. Sutter, B. Ozturk, and P. Sutter,”Selective Growth of Ge Nanowires by Low-Temperature Thermal Evaporation”, Nanotechnol. 19, 435607 (2008).
[3] C. Keiser, P. Sutter, and E. Sutter, “Controlled Formation of Hybrid Germanium/Germanium Sulfide Core-Shell Nanowire Heterostructure”, in preparation (2017).

There is growing interest in the properties and applications of the family of layered metal dichalcogenides, MX₂ (X=Se, S), which includes transition metal dichalcogenides (TMDs) such as MoS₂ and WSe₂ and group IV dichalcogenides particularly SnSe₂ and SnS₂. While significant scientific advances have been made using monolayer and few-layer flakes exfoliated from bulk chalcogenide crystals, future device development requires the ability to synthesize large area, single crystal films and heterostructures. Our research is aimed at the development of an epitaxial growth technology for layered dichalcogenides, similar to that which exists for III-V and II-VI compound semiconductors, based on gas source chemical vapor deposition (CVD) and metalorganic CVD (MOCVD) in cold-wall reactor geometries. This approach provides excellent control of the precursor partial pressure and reduced pre-deposition upstream of the substrate thereby enabling control over nucleation density, lateral growth rate and film composition for the layer-by-layer growth of 2D films and heterostructures.
Our recent studies have focused on the epitaxial growth of WSe₂ and WS₂ monolayer films and vertical heterostructures using metal hexacarbonyl and hydride chalcogen precursors on substrates including sapphire, SiC, epitaxial graphene and hexagonal boron nitride. A multi-step precursor modulation growth method was developed to control the nucleation density, size, orientation and the lateral growth rate of monolayer domains on the substrate. Using this approach, coalesced monolayer and few-layer TMD films were obtained on sapphire substrates up to 2” in diameter at growth rates on the order of ~ 1 monolayer/hour. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to the sapphire resulting from a merging of predominantly 0^o and 60^o oriented domains. Epitaxial growth of SnS₂, SnSe₂ and Sn(S,Se)₂ alloy films on epitaxial graphene have also been demonstrated using evaporated sources. The gas source CVD method also provides a means to study and quantify surface diffusivities and lateral growth rates of domains as a function of growth conditions providing insight into the fundamental mechanisms of monolayer growth. Applications and challenges of this approach in the growth of 2D heterostructures will also be discussed.

The bulk photovoltaic effect (BPVE) is the generation of photocurrents in the bulk of a single-phase material. It holds advantages over traditional photovoltaics based on p-n junctions, such as above-band gap photovoltages, and current generation in the bulk without the need for interface engineering. Despite numerous theoretical and experimental research efforts into the BPVE, there has been no systematic investigation into its maximum magnitude attainable in solid-state materials. In this talk, we present an upper bound on the dominant microscopic mechanism of BPVE: the shift current response. We show that this bound depends on the band gap, band width, and geometrical properties of the material in question. As a proof of principle, we perform first-principles calculations of the response tensors of a wide variety of materials, finding that the materials in our database do not yet saturate the upper bound. This suggests that new large BPVE materials will likely be discovered by future materials research guided by the factors mentioned in this work.
These results imply that small band gap materials can potentially host large BPVE. As examples, we propose materials which are tuned across a band-gap-closing phase transition from a normal semiconductor into a topological insulating phase. This class includes some inorganic layered semiconductors, such as BiTeI, and inorganic halide perovskites, such as CsPbI₃. We show that this results in a dramatic enhancement of photocurrent as well as an abrupt reversal in its direction. Using first-principles calculations, we show that that this effect is robust across different materials systems as long as such a transition into a topologically insulating phase is present.

Recent developments in photoferroics have led to increased power efficiencies greater than 8%. However, many of the polar materials that have been used in photoferroic devices have bandgaps of 3 eV or more, making the effectiveness of these materials as solar absorbers limited. This has spurred interest in theoretical modeling and experimental realization of new polar materials, with a focus in band gap tuning. In efforts to explore Bi-based perovskite oxides, electronic structure calculations were performed on BiMO₃ perovskite structures to determine band gaps and the electronic dielectric functions (GW approximation), as well as the ionic dielectric constant and the piezoelectric tensors (density functional perturbation theory). BiCoO₃ showed promise as a low band gap semiconductor with a predicted gap of 2.08 eV, a polar P4mm structure, and a predicted relative permittivity of 44.6. To further investigate this material’s potential as the solar absorber in a photoferroic device, BiCoO₃ films were grown via combinatorial pulsed laser deposition. Utilizing a multi-target deposition with Bi₂O₃ and CoO, the substrate, temperature, chemistry and partial pressure of oxygen were varied to achieve a tetragonal P4mm structure. X-ray diffraction was used to track the phase formation was tracked as function of composition and temperature. Additionally, x-ray fluorescence and UV-vis absorption were used to characterize the compostion and band gap. Upon achievement of proper composition, epitaxial strain was used to create high quality films with an aligned internal polarization. From these films metal-semiconductor-metal structure was used for electronic and photoferroic testing.

Despite the potential to exceed the Schockley-Queisser theoretical limit, ferroelectric photovoltaics (FPVs) have performed inefficiently due to their extremely low photocurrents. The reported low photocurrent values are mainly attributed to poor light absorptions. Most ferroelectric materials with sufficiently large polarizations, such as LiNbO₃, BaTiO₃ (BTO), and PZT, have a very wide band-gap energy of >3.0 eV, which places their absorption onset near the ultraviolet (UV). BiFeO₃ is known to have a relatively smaller band gap of 2.7 eV; however, this is still far apart from the optimal band-gap range (1.1-1.5 eV) for PV applications. In this regard, in order to realize the potential of FPVs, it is highly desirable to search for a novel ferroelectric material with both strong polarization and optimal band-gap energy.

We propose a recently discovered material, namely β-CuGaO₂ [T. Omata et al., J. Am. Chem. Soc. 2014, 136, 3378] as a strong candidate material for efficient ferroelectric photovoltaics (FPVs). According to first-principles predictions exploiting hybrid density functional, β-CuGaO₂ is ferroelectric with a remarkably large remanent polarization of 83.80 μC/cm², even exceeding that of the prototypic FPV material, BiFeO₃. Quantitative theoretical analysis further indicates the asymmetric Ga 3-O 2 hybridization as the origin of the Pna2₁ ferroelectricity. In addition to the large displacive polarization, unusually small band gap (1.47 eV) and resultantly strong optical absorptions additionally differentiate β-CuGaO₂ from conventional ferroelectrics; this material is expected to overcome critical limitations of currently available FPVs.

Photocatalytic CO₂ conversion to hydrocarbon fuels, the solar fuels, making possible simultaneous solar energy harvesting and CO₂ reduction, is considered a killing two birds with one stone approach to solving the energy and environmental problems. However, the development of solar fuels has been hampered by the low conversion efficiency and lack of product selectivity of the photocatalysts. Here, we present defect engineering (interstitial, substitutional, and vacancy) in chalcogenides as a viable method towards promising photocatalysts for CO₂ reduction reaction (CO2RR). Three cases will be illustrated: the carbon- and nickel-doped SnS₂ (abbreviated as SnS₂-C and SnS₂-Ni, respectively) nanosheets and single layer MoS_2.
For the first case, the SnS₂-C nanosheets with a typical layer thickness of ~40 nm were synthesized using an L-cysteine-based hydrothermal process. Compared with undoped SnS₂, the interstitial carbon doping induced microstrain in the SnS₂ lattice, resulting in different photophysical properties. Density functional theory calculations were performed for the formation energy, along with the CO₂ adsorption and dissociation on differently configured SnS₂-C for CO2RR. Experimentally, the SnS₂-C exhibited a highly effective photocatalytic activity in gas phase with a photochemical quantum efficiency exceeding 0.7 % under visible light, which is ~250 times higher than that of its undoped counterpart, and also a world-record high value reported for inorganic catalyst. For the second case, substitution doping of Ni is also found to be effective for enhancing the performance of SnS₂. For the third case, the MoS₂ single layers were prepared by chemical vapor deposition, followed by hydrogen plasma post-treatment. With increasing hydrogen plasma treatment time, we observed a trend of blue-shift in the A_1g peak and red-shift in E_2g peak in their Raman spectra, implying creation of sulfur vacancies, of which the resultant stoichiometry ratio of Mo/S was further investigated by X-ray photoelectron spectroscopy. In addition, scanning tunneling microscopic images clearly supported that there were missing atoms in the MoS₂ layers after hydrogen plasma treatment. Productivity and selectivity of CO2RR were found to be strongly dependent with the different Mo/S ratios of the MoS₂ single layers. The role and interplay of the defects and the hosting materials as well as their effects on CO2RR will be discussed in this presentation.

Germanium–tin is a promising alloy system for silicon-compatible optoelectronic elements due to the potential for achieving a direct band gap for efficient light emission¹ and the possibility of low processing temperatures. Lasers, on-chip photonic interconnects, and emitters in the telecommunications wavelengths are examples of potential applications for this material. In particular, core-shell nanowires offer the benefit of decoupling the device from compressive misfit strains imposed by lattice matching to silicon substrates, which inhibits formation of a direct semiconductor gap. In addition, the core-shell structure presents an avenue for optical property engineering through strain manipulation and carrier confinement. Core-shell Ge/GeSn nanowires synthesized using VLS exhibit greatly enhanced light emission properties compared to bare Ge NWs. ^2-3 We hypothesize that core-shell strain plays an important role in the enhancement in emission. In this study, we present core-shell cross-sectional 4D scanning transmission electron microscopy strain mapping in tandem with energy dispersive x-ray spectroscopy elemental mapping to quantify and visualize strains in core-shell Ge/GeSn nanowires. Atom probe tomography is used to confirm the elemental composition of the core-shell structures, which is consistent with collected STEM EDS data. Furthermore, synchrotron microdiffraction is used to obtain detailed strain information from small groups of as-grown nanowires. The strain measured in this way from as-grown nanowires differs significantly from results inferred from TEM cross-section analysis due to elastic strain relaxation in thin TEM samples. By correctly measuring and understanding the interplay of strain and composition in core-shell Ge/GeSn nanowires, improved materials properties for optoelectronic applications can be obtained.

1. Wirths, S.; Geiger, R.; von den Driesch, N.; Mussler, G.; Stoica, T.; Mantl, S.; Ikonic, Z.; Luysberg, M.; Chiussi, S.; Hartmann, J. M.; Sigg, H.; Faist, J.; Buca, D.; Grützmacher, D., Lasing in direct-bandgap GeSn alloy grown on Si. Nat Photon 2015, 9 (2), 88-92.
2. Meng, A. C.; Fenrich, C. S.; Braun, M. R.; McVittie, J. P.; Marshall, A. F.; Harris, J. S.; McIntyre, P. C., Core-Shell Germanium/Germanium–Tin Nanowires Exhibiting Room-Temperature Direct- and Indirect-Gap Photoluminescence. Nano Lett. 2016, 16 (12), 7521-7529.
3. Assali, S.; Dijkstra, A.; Li, A.; Koelling, S.; Verheijen, M. A.; Gagliano, L.; von den Driesch, N.; Buca, D.; Koenraad, P. M.; Haverkort, J. E. M.; Bakkers, E. P. A. M., Growth and Optical Properties of Direct Band Gap Ge/Ge0.87Sn0.13 Core/Shell Nanowire Arrays. Nano Lett. 2017, 17 (3), 1538-1544.

The two-dimensional transition metal dichalcogenides (2D TMDs) are excellent candidates for ultrathin, high-efficiency optoelectronic devices, such as solar cells, due to their direct bandgaps, high luminescence radiative efficiencies upon passivation, and high absorption efficiencies per unit thickness (one to two orders of magnitude higher than conventional semiconductors)[i]. A challenge for large-area optoelectronic or photovoltaic applications of TMDs is to demonstrate high optoelectronic quality in films grown by large-area synthesis methods, such as metalorganic chemical vapor deposition (MOCVD). In this work, we observe improvements in photoluminescence quantum yield in exfoliated molybdenum disulfide (MoS₂) samples and in MOCVD-grown samples by a solution-based passivation with the superacid bis(trifluoromethane)sulfonimide, similar to previous reports[ii]. Our MOCVD-grown MoS₂ films were grown in submonolayer, monolayer, and multilayer form on 1 cm² SiO₂/Si substrates using molybdenum hexacarbonyl and diethylsulfide precursors at T = 600 C. The MoS₂ samples were passivated by immersion in solutions of bis(trifluoromethane)sulfonimide in acetonitrile under atmosphere, as previously reported[iii], and were characterized using photoluminescence mapping, photoluminescence spectroscopy and Raman spectroscopy. We demonstrate a higher photoluminescence quantum yield in MOCVD-grown MoS₂ than in exfoliated MoS₂ layers pre-treatment, and propose mechanisms to explain this surprising observation. Comparative luminescence measurements for exfoliated and MOCVD-grown MoS₂ samples indicated an integrated band-edge intensity for MOCVD-grown layers that was at least 3 times higher than for exfoliated samples. We find that the vapors of the superacid solution can also passivate, and develop a cleaner, residue-free passivation process based on this observation. Since the superacid treatment on its own is unstable under exposure to water and other conditions, we explore the use of hexagonal boron nitride films of varying thicknesses for stable encapsulation of TMDs, as an alternative to encapsulating with a fluoropolymer film[iv]. Finally, we will discuss the role of TMD absorber layer passivation and encapsulation in solving some of the challenges that remain for 2D TMD solar cells including 1) increasing absorption through light-trapping, 2) limiting defects to enable a high radiative efficiency and high open circuit voltage, and 3) enhancing compability with carrier-collecting heterostructures such as p-n homojunctions, TMD heterojunctions or carrier-selective contacts to extract photogenerated charge carriers[v].


[i] Cao, L., MRS Bulletin 40, 592-599 (2015).

[ii] Amani, M. et al, Science 350, 1065-1068 (2015).

[iii] Amani, M. et al, ACS Nano 10, 6535-6541 (2016).

[iv] Kim, H.; Lien, D.; Amani, M.; Ager, J. W.; Javey, A., ACS Nano 11, 5179-5185 (2017).

[v] Jariwala, D.; Davoyan, A. R.; Wong, J.; Atwater, H. A., ACS Photonics (2017).

Near-infrared (NIR) PbS nanocrystal quantum dots (QDs) have immense potential in realizing many technological applications ranging from photovoltaic devices to biological labeling. However, their application in practical devices or in challenging environments, respectively, has been limited by instability in photoluminescence quantum yield (QY) and peak position as a result of surface oxidation. Here, we describe a combination of cation exchange and successive ionic layer adsorption and reaction (SILAR) methods to synthesize stable NIR PbS/CdS/CdS core/shell/shell “giant” QDs (gQDs). The thick shell affords significantly increased stability under ambient conditions as observed in time-dependent absorption and QY studies of ensembles of PbS/CdS/CdS gQDs compared to thinshell variant. Furthermore, we describe the first investigation of PbS QDs at the single dot level using conventional detectors, including blinking statistics as a function of shell thickness. Finally, we show that these newly stabilized PbS QDs can be used to construct efficient solid-state down conversion NIR light emitting devices, for which, like blinking suppression, lifetime operational stability is shell thickness dependent. In this way, we show a clear correlation between nanoscale structure and both single dot properties and device performance.

Proverbially, when light is shone onto a material, it can produce direct current (DC) by converting the energy from the light to electricity by photovoltaic effect. Here we found that when a light (with a wavelength from UV to NIR) illuminated at the silicon based devices with various types of junctions (MS, MIS, PN, PIN, etc.) through rotating chopper, they are also able to produce alternating current (AC). The current energy conversion mechanisms are not able to explain this effect. This is a true effect, not artificial or from the environment, which is verified by superposition rule experiments. The magnitude of the AC signals depends on the light intensity, frequency of the chopper, and the active illumination area. The possible mechanism for the generation of signals might be the shift of the Fermi level of the semiconductor due to the temperature change at the surface caused by absorption of light, resulting a time-varying electric filed. This effect can be utilized as a wireless power source, photo sensor and in communication system, etc., which could be widely used in many fields.

Efficiency and sustainability are two primary considerations for the manufacture of modern day electronics. Silicon is one of the most abundant elements on Earth, and silicon nanocrystals (SiNCs) exhibit potential for applications in light emitting devices (LEDs) and photovoltaic devices (PVs) that could, in theory, rival the efficiencies of the more commonly used (but more expensive, toxic, and rare) cadmium-based chalcogenide NCs. Despite these advantages, SiNCs are not commonly used because the surface defects and oxidization that lower the efficiencies of their optical properties (and which plague most NC materials) are more difficult to eliminate due to the covalent nature of the SiNC surface. The covalent surface prohibits the ligand exchange and shell growth processes that have led to the success of these other materials. Here, we hypothesize that surface defects and oxidization of plasma-produced SiNCs can be mitigated via the “capping” of surface defects and reactive sites by injecting vapor-phase species into the plasma to anneal and protect the surfaces of individual SiNCs in-flight.
The nonthermal plasma reactor has been shown to be a versatile and powerful tool for synthesizing high-quality NCs, including some of the highest-efficiency SiNCs, in terms of photoluminescence. To bring the optical performance of these SiNCs to par with the Group II-VI chalcogenide NCs, we explored multiple plasmas in sequence, each with a different role – synthesis, surface defect mitigation, and shell growth – so that individual SiNCs can be perfected in-flight while still maintaining a continuous, high-yield production scheme. For surface defect mitigation, we explored mixtures of hydrogen gas and other inert vapor-phase constituents such as helium in a secondary plasma for reducing defect densities at the SiNC surfaces. Further, we also used an additional plasma in-line with the first to grow thin SiNx layers around the SiNCs, seeking to stabilize them against ambient oxidation. We found that the SiNx layer reduced the formation of oxygen-related defects at the SiNC surfaces as compared to the core SiNCs, and appeared to reduce oxidation-related shifts in the photoluminescence peaks for these NCs. We are also conducing in-situ plasma characterization to understand the reactions and conditions required for high-quality shell growth on SiNCs. Our future work is devoted to using multi-plasma schemes for discovering new reaction pathways towards stable, high-efficiency photoluminescence from SiNCs.

Photoelectrochemical (PEC) water splitting using nanostructured semiconductor materials has been attracting great attention for production of clean and high efficiency renewable energy. Broad absorption region, effective charge transfer, and high photostability of semiconductor photoanode are the governing features for the improved PEC performance. Vertically aligned one dimensional (1-D) metal oxide semiconductor nanostructure arrays are superior photoelectrodes for high solar energy conversion efficiencies owing to large specific surface area, increased light absorption due to light scattering effect, short diffusion length, ideal geometrical structures for fast electron transport and less charge recombination probability. ZnO-NRs photoanode is emerging as the promising candidate owing to the direct bandgap, higher exciton binding energy, ease of crystallization, facile tailoring of nanostructures, anisotropic growth, and higher electron mobility. CdS is considered to be the most appropriate visible-light sensitizing absorber for ZnO, comprising of direct band gap (~2.4 eV), high absorption coefficient, and similar crystal structure/lattice match, which facilitates better charge transfer between ZnO/CdS nano-architecture (staircase type-II band alignment) required for efficient solar energy conversion applications. In this work, we have fabricated vertically aligned 1-D ZnO/CdS core/shell NRs array by ion-flux controlled two-step chemical bath deposition technique, exhibiting an extended optical absorption from ultraviolet to visible region and enhanced photoresponse. FESEM and HRTEM studies confirm the uniform topotaxial decoration by thin layer (~40 nm) of (002) oriented CdS QDs on the hexagonal prismatic (002) ZnO NRs. The enlarged interface between ZnO and CdS together in close intimacy facilitates effective charge transfer from the excited CdS shell to ZnO NRs core upon visible light illumination. Our optimized ZnO/CdS core/shell NR-array exhibits anodic visible photocurrent density as high as 8.5 mA/cm² at +1.0 V (vs Ag/AgCl) and excellent photostability in 0.1 M Na₂SO₄ aqueous solution. The observed high current density in our core/shell NRs is interpreted in terms of the nucleation controlled growth via ion-by-ion deposition mechanism and subsequently type-II band aligned defect-free ZnO/CdS interface.

β-gallium oxide (β-Ga₂O₃) has received a considerable attention as the material for high power electronic devices due to its wide bandgap of ~4.9 eV and high thermal and chemical stabilities. Although β-Ga₂O₃ is not a van der Waals materials like two-dimensional(2D) materials, it can be mechanically exfoliated into thin flakes in the (100) direction because of its monoclinic structure. However, applications of β-Ga₂O₃ flakes in practical devices could be limited because the thickness of the flakes, which highly affect the electrical performances of devices, is random when obtained by mechanical exfoliation techniques. Therefore, the study of thinning methods to control the thickness of β-Ga₂O₃ flakes is important to enhance the performances β-Ga₂O₃-based devices. Photo-enhanced chemical(PEC) etching is an etching process that uses UV irradiation during wet etching. PEC etching not only has the advantages of wet etching such as simplicity, high selectivity, and physical damage-free process but also increases etch rate at room temperature due to the generation of electron-hole pairs.
In this work, we controlled the thickness of β-Ga₂O₃ flakes using PEC etching process. Unintentionally n-doped flakes were mechanically exfoliated and transferred onto SiO₂/Si substrate. The etch rates of β-Ga₂O₃ flakes depending on the temperature in 85% aqueous H₃PO₄ solution were obtained without and with UV irradiation. The changes in the morphological, optical, electrical properties of thinned β-Ga₂O₃ were measured using atomic force microscope, Raman spectroscopy, and semiconductor parameter analyzer. Furthermore, the activation energy was calculated at each etching condition. The details of our experimental conditions and results will be presented at the meeting.

Recently simple binary and ternary semiconductors such as AgBiS₂, CuSbS₂, CuSbSe₂, PbS, SnS, Sb₂S₃, and Sb₂Se₃ have attracted intensive attention as a low-cost and stable light absorbing materials. Among them, Sb₂Se₃ binary chalcogenide compound is considered to be a promising photovoltaic material due to their relatively low toxicity, long term stability, and earth abundant element availability. Sb₂Se₃ in its crystalline form has a proper bandgap (1.17 eV), high absorption coefficient, good band alignment in combination with various electron transport materials, and intrinsically benign grain boundaries because of its peculiar one-dimensional crystal structure. It can also be easily deposited by different methods such as thermal evaporation, sputtering, and atomic layer deposition (ALD). The most widely used method is evaporation due to its simplicity, but in the case of evaporation process, it is difficult to prevent the formation of Se-vacancy (V_Se) because Se has high vapor pressure during the deposition process of Sb₂Se₃.
In our previous work, we have demonstrated that the ALD method inhibited the formation of defects in Sb₂S₃ and thus planar solar cells using ALD Sb₂S₃ absorber showed high efficiency with good reproducibility. Therefore, in this work, we have tried to deposit Sb₂Se₃ thin films by using plasma enhanced atomic layer deposition (PEALD), which was proceeded by the reaction of tetrakis(dimethylamino)antimony (TDMASb) and diethylselenide (DESe) at low temperature (120 °C). TDMASb is one of the most perspective general precursors for the ALD of antimony chalcogenide compound. DESe is also a commonly used Se precursor to substitute a highly toxic H₂Se precursor. We observed the deposition rate, crystalline structure, morphology, chemical composition, and impurity of Sb₂Se₃ thin films prepared by PEALD. Uniformly deposited ALD Sb₂Se₃ thin films were applied to the superstrate solar cells with the structure of ITO/CdS/ALD Sb₂Se₃/Au and we have investigated the possibility of ALD Sb₂Se₃ as a photovoltaic absorber.
This work was supported by the DGIST R&D Programs of the Ministry of Science, ICT & Future Planning of Korea (17-BD-05).

Processes involving harvesting of near visible and near-infrared photons, followed by re-emission in the green region are relevant for fabrication of modified Si-based high efficiency photodetectors, as well as potentially Si-based solar cells. Requirements for such materials include a) low cost, b) highest possible upconversion PL efficiency to green, which is subsequently detected by a Si detector. The photophysics of doped NaYF₄ materials has been extensively investigated with dopants such as Er³⁺, Tm³⁺, Ho³⁺, along with Yb³⁺ as a sensitizer for upconversion to visible regime at high incident optical power (~100 mW) for colloidal solutions. We report a moderate temperature (~ 320°C) synthesis of NaYF₄:Yb(18%):Er(2%):Gd(15%) using thermal decomposition, resulting in highly crystalline nearly pure phase materials. High-resolution transmission electron microscopy (HRTEM) measurements reveal the formation of nearly spherical nanoparticles with an average diameter of 38 nm. Observed [100] lattice plane spacings (estimated to be 4.4±1.6 Å) in TEM data are in reasonable agreement with standard published X-ray diffraction (XRD) data (~5.17 Å) for comparable NaYF₄-based materials. Powder XRD measurements of the unannealed material suggest the presence of a strongly hexagonal overall phase with lattice constant (~5.17 Å), again in agreement with TEM estimates. Deposited thin films of Gd-doped unannealed material exhibit a broadband (~ 400-1600 nm) near 2-fold enhancement in absorption over baseline well-studied Er³⁺ doped materials. Finally, photoluminescence (PL) upconversion response of the thin films with 785 nm laser excitation appears in green (539 nm) and red (665 nm). Thin films based on this Gd(III) doped material are thus potentially useable as optical antennas for higher efficiency devices based on Si photodetectors.

Transparent conductive oxides (TCOs) are widely used as transparent electrodes for various applications, such as flat-panel displays and IR reflecting glasses. Ta-doped SnO₂ (TTO) films have recently attracted much attention as TCO material due to high thermal stability. As for the sputter deposition of this material, rf magnetron sputtering is adopted because the target is not conductive. In this study, we used dc magnetron sputtering to deposit the TTO film by using the sintered TTO target, where a small amount of ZnO is added into the target to obtain the high packing density and good conductivity. Moreover, in order to decrease the effect of bombardment of highly-energetic particles on the film properties during the deposition, the off-axis dc magnetron sputtering is utilized.[1]

TTO films were deposited on synthesized glass substrates heated at 400 °C using the high-dense target (Ta: 6.8 at.%, AGC Ceramics). The distance between substrate and target was 110 nm. The total gas pressure during deposition was maintained at 0.5 Pa. The sputtering gas was Ar, and the reactive gas was O₂. The sputtering power was kept at 50 W. The film thickness was 400 nm.

XRD patterns showed that TTO films had a high crystallinity at the off-axis positions, which is due to the almost bombardment-free of high-energetic particles during the deposition. The lowest resistivity of the TTO films of 7.9 × 10⁻³ Ω cm was also obtained at the off-axis position, which may be attributed to the improvement of the crystallinity.


[1] J. Jia, et al., Thin Solid Films 559 (2014): 69-77

Cu₂ZnSnS₄ is a potential p-type absorber layer for photovoltaic and PEC water splitting applications. It has optimum band gap energy (1.4-1.5 eV), high absorption coefficient and involves earth abundant compositions. CZTS can be used as photocathode for hydrogen evolution, where its conduction band minima (CBM) edge is more negative than the hydrogen evolution potential. The quaternary semiconductors Cu₂Zn – IV-VI4 (IV- Si, Ge, Sn and VI- S, Se) are widely considered as fourth generation solar cell absorbing materials with tunable band gap of around 1.0–3.0 eV. Two photon absorber layer devices having different band gaps (1.0 eV and 1.7 eV) show better PEC response, however, different properties (e.g. crystal structure) of constituent semiconductor materials and band alignment issue can create hindrance in efficiency enhancement. Polycrystalline Si layer over CIGS is found to improve net efficiency as compared to the bottom cell (CIGS) alone. In present work, Si is doped on CZTS, which can make its surface stable and increase the band gap, which is favorable for PEC application due to reduction in recombination loss of charge carriers. The first principle calculations were performed using DFT simulations for Cu₂Zn(Sn_0.5Si_0.5)S₄ structure. The optical band gap of this kesterite structure was calculated to be ~2.0 eV, which is higher than purely Sn based structure (CZTS). The experimental synthesis of Cu₂Zn(Sn_xSi_1-x)S₄ structure is performed by magnetron co-sputtering method. The metallic precursor Cu, Zn and Sn were deposited on Mo-coated glass and bare glass substrates by co-sputtering for 60 minutes at 10-12 mTorr Ar working pressure. The Si has been deposited over Cu-Zn-Sn films for 5 and 7 minutes of duration followed by sulphurization at 520^οC for 20 minutes giving samples, S0, S1 and S2. The composition ratios Cu/(Zn+Sn), Zn/(Sn+Si), Si/Sn for samples S0 to S2 are observed as ~ 0.97, 1.4, 0; 0.84, 1.04, 0.35; 0.96, 0.84, 0.57, respectively. The XRD analysis for samples shows kesterite single phase, which is also confirmed by micro-Raman analysis carried out using 532 nm laser excitation source. The Raman peaks observed at ~337 and 286 cm^-1 belong to kesterite CZTS phase. The band gap increases from samples S0 to S2 as 1.43, 1.45 and 1.47. Hence, the doping of Si in CZTS results in a slight increase in band gap which can enhance the photocurrent density, because thermodynamic reaction loss decreases due to water reduction potential (~1.23 eV) straddle well with photo-electrode. The photocurrent density for pure CZTS thin film sample (S0) on FTO protected by i-ZnO is found to be ~0.5 mA/cm². In pure CZTS, the crystallites of sizes ~1-3 µm are observed to be lying on the top surface, but the density of these crystallites increases on going from samples S0 to S2. Hence, this new material can be potential photo –electrode material for future solar water splitting applications. The PEC measurements for Cu₂Zn(Sn_xSi_1-x)S₄ samples are in progress.

We have developed a facile method to synthesize metal oxide semiconductors and by utilizing this unique strategy we have fabricated a photovoltaic cells with a few simple steps. Our method relies on electroplating the metal species (Cu and Zn in our case) and then annealing in different atmosphere (air in this case) to grow various semiconductor materials. This method is very promising when especially the large scale production is the particular interest because even most of the very expensive methods are not capable of doing this. Moreover, this method is not only applicable for Metal oxides, but also for other materials. Only difference is that annealing should take place in different ambient (e.g. Sulfur for ZnS, CuS or Selenium for ZnSe or CuSe and etc…).Our solar cell was constructed by sandwiching the photoanode electrode which consists of Glass/ITO/TiO2/ZnO sensitized with N719 dye molecules and counter electrode consisting Glass/TiO2/CuO. Between two electrodes iodide electrolyte was injected and it was distributed uniformly by the capillary force. ZnO on top of TiO2 acts as a barrier layer and minimize the carrier recombination from the conduction band of TiO2 to dye or electrolyte. Since ZnO has much higher carrier mobility than TiO2, electrons created as a result of photon absorption in the dyes move to the TiO2 quickly and eventually reach to the load. On the other hand CuO is also promising and can be used instead of very expensive platinum and our results prove that it has very high catalytic activity. Overall device performance was improved substantially when compared with and without ZnO barrier layer and CuO counter electrode performed as good as Pt due to its high catalytic activity and large surface area.

Wireless communication is essential for the vision of internet of things. A true wireless world calls for wireless power solutions both indoors and out. In this work, we present a detailed study of solar cell performance under indoor lighting conditions. In the past, crystalline solar cells were disregarded due to low performance under low illumination combined with relatively high cost. New solar cell designs now allow crystalline silicon solar cells to reach efficiencies close to 27%, and their cost has fallen 70% in the last 5 years. The conditions and behavior of solar cells under indoor lighting are very different from standard AM 1.5 conditions of full sunlight. At AM 1.5 the spectral range is broad and the power density is 1000 W/m². Most indoor light is in the visible range of 400-750 nm, and the intensities are in the range of 1-10 W/m² (100 to 1000 lux), i.e. 100-1000 times lower power than full sunlight. We show that the poor performance of crystalline silicon solar cells under indoor lightening is due to shunt resistance. Shunt resistivities (R_shunt) of kΩ.cm² have negligible effect on the solar cell performance under outdoor light conditions but it is catastrophic under low light. For R_shunt of few kΩ.cm², the efficiency of a solar cell drops from 22% to less than 5% under indoor lighting conditions. Even for the best commercial cells with R_shunt values of 10-100 kΩ.cm² the efficiency drops below 10% under low light. However, efficiency under low light conditions is significantly improved for R_shunt greater than 1 MΩ.cm². With R_shunt greater than 1 MΩ.cm², a silicon solar cell with an AM1.5 efficiency of 22% shows an efficiency of 17% for illumination of 1W/m². Such high R_shunt are typical on silicon heterojunction solar cells. These solar cells are shown to outperform cells such as amorphous silicon that are often used for indoor light conversion. Typical solar cell illuminated IV measurement systems do not accurately report the R_shunt values over 10 kΩ.cm². In this work, we used a more sensitive source measure unit. These results show the potential of silicon heterojunction solar cells to power indoor devices. Moreover, the narrow and short wavelength range means that indoor silicon cells no longer need light trapping or thick substrates so that only 1-10 µm of material is needed. Thinner substrates lead to flexible solar cell devices. Finally, using crystalline silicon based solar cells provides an opportunity to integrate the logic circuits and electronics in the same substrate.

With its low mineral extraction costs, a reported direct bandgap of 1.35-1.4 eV and high minority carrier mobility, the WSe₂ material system is seeing renewed interest as an absorber in next-generation solar cell material development. When preparing WSe₂ thin films via the selenization of tungsten metal, the lattice expansion required to accommodate WSe₂ formation presents a large activation barrier that requires very high temperatures (> 900 ^oC) and long reaction times (24-72 hrs) to overcome. It is known that tungsten oxide may be reacted with various selenium sources to form WSe₂ but literature reports still include processing steps that involve high temperatures, reducing atmospheres, and/or oxidative pre-treatments of tungsten oxide. In this work, we report a three-step non-vacuum process for the fabrication of compositionally high quality WSe₂ thin films via the selenization of tungsten oxide that does not require a reducing atmosphere, temperatures above 550 ^oC, or any pre-treatment of the tungsten oxide prior to selenization. Hydrated tungsten oxide was prepared via a chemical bath reaction between Na₂WO₄ and diethyl sulfate and deposited onto Corning glass substrates using a centrifuge method. The product phase, monoclinic-WO₃ ● 2 H₂O or orthorhombic-WO₃ ● H₂O, could be controlled by varying the Na₂WO₄ concentration in the bath, and using the latter phase as the tungsten source resulted in more effective conversion to WSe₂. Films of orthorhombic-WO₃ ● H₂O were heated in a graphite susceptor containing elemental selenium using a two-stage heating profile (250 ^oC for 15 minutes and 550 ^oC for 30 minutes) under a static argon atmosphere. Finally, this heating was repeated in a graphite susceptor with no elemental selenium. X-ray diffraction analysis revealed that the product film was exceptionally well ordered with a (002) plane orientation. The Raman spectrum was consistent with previously reported WSe₂ peak frequency assignments and the closely-spaced E¹_2g and A_1g major peaks were very well resolved. X-ray photoelectron spectroscopy and photoluminescence studies are in progress and will be discussed.

Copper aluminum disulfide (CuAlS₂) is one of the promising wide-gap (E_g of 3.6 eV) chalcopyrite-like compounds that can be used in thin film solar cells. The band gap energy of the CuAlS₂-based absorber must be significantly reduced. An obvious approach for such reduction of the CuAlS₂ band gap consists in combining CuAlS₂ in solid solutions with other ternary compounds that have a lower band gap, for instance, CuFeS₂. The goal of the present paper is to study the phase relations and to determine the limits of solubility in the CuAlS₂ - CuFeS₂ system by X-ray powder diffraction (XRPD) and Scanning Electron Microscopy (SEM).

The thermobaric treatment (techniques of high pressure and temperature) was applied to prepare samples of the alloys from previously prepared ternary compounds CuAlS₂ and CuFeS₂. Synthesis of the initial ternary compounds CuAlS₂ and CuFeS₂ was performed in quartz ampules by melting the elements at a temperature that exceeds the melting point of the compound by 20 K. After preparation of the initial compounds, their homogeneous mixtures with molar part of CuFeS₂ (x) equaling to 0.025, 0.05, 0.10, 0.125, 0.20, and 0.30 were prepared and treated at the high pressure of 5.5 GPa and temperatures ranging from 1170 to 1270 K.

The X-ray studies of the CuAlS₂ - CuFeS₂ alloys were carried out using monochromatic Cu K_a-radiation (1.5406 Å, step size 0.01° or 0.04°, counting time 10 s). The Rietveld analysis of the X-ray powder diffraction data was done using the FullProf software.

Phase formation in the (CuAlS₂)_1-x-(CuFeS₂)_x system was investigated and the unit-cell parameters (the lattice constants and the unit-cell volume) were computed as a function of composition. It was found the absence of complete solubility in the (CuAlS₂)_1-x-(CuFeS₂)_x system. The formation of solid solutions with the tetragonal structure of chalcopyrite was detected for the compositions with molar part of CuFeS₂ x<0.20. The system with the compositions with higher content of CuFeS₂ contains two phases with the chalcopyrite-like structure – CuAlS₂-based and CuFeS₂-based phase. The CuFeS₂-based phase is not stable and may decompose into two phases. Both of these phases are Fe-substituted phases. One of the phases is a phase with the structure of chalcopyrite, for which the ratio ([Cu]+[Fe])/[S] is ~1. The second phase is a phase with the structure of talnakhite, for which the ratio ([Cu]+[Fe])/[S] is >1. The phase assemblages of these lamellar intergrowths are different depending on the proportion of Cu and Fe in the high-temperature solid solution. The solid solutions with Cu>Fe form an intergrowth of chalcopyrite (with a composition close to CuFeS₂) and a phase similar to talnakhite (Cu₉Fe₈S₁₆). The solid solutions with Cu<Fe form an intergrowth of chalcopyrite with a phase having Cu<<Fe considered to be the high-temperature cubanite.

Acknowledgment. B. Korzun would like to thank PSC-CUNY for financial support of the studies under project TRADA-48-527.

During the last decade, the quest for a durable, highly efficient, sustainable, nonprecious metal based electrocatalyst for water splitting with minimal environmental impact has intensified. In that regards, chalcogenide electrocatalysts have recently made a promising impact based on their high activity towards oxygen evolution reaction (OER), one of the major roadblocks towards overall water splitting. In this presentation we will discuss the electrocatalytic activity of ternary CoNi₂Se₄ nanoflakes, with special emphasis on the effect of anion coordination, crystal structure and morphology on the OER catalytic activity. Ternary CoNi₂Se₄ was prepared by direct electrodeposition on carbon fiber paper (CFP) and gold coated glass (Au/glass) electrodes. Such electrodeposition directly on the electrodes produces a binder-free film which can utilize full potential of the catalyst. CoNi₂Se₄ crystallizes in a vacancy-ordered spinel structure type. Through detailed XPS analysis it was determined that the as-deposited film contains Ni in the +3 oxidation state. This is very important since it has been known that Ni³⁺ is the actual catalytically active species for OER in alkaline medium, and in Ni-based electrocatalysts, Ni³⁺ is generated through oxidation of Ni²⁺ visible as a pre-oxidation peak. Having Ni³⁺ is the as-synthesized catalyst expectedly can reduce the onset potential for OER activity. Detailed electrochemical studies showed that CoNi₂Se₄ on CFP needs an overpotential of only 160 mV to reach 10 mA cm^-2 for OER, which is one of the lowest overpotentials reported thus far. Moreover, this catalyst was also active for hydrogen evolution reaction (HER) in the same medium, showing an overpotential of only 220 mV to deliver 10 mA cm^-2. This bifunctional catalyst could eventually split water delivering a current density of 10 mA cm^-2 at a very low cell voltage of 1.61 V in 1.0 M KOH. Through detailed SEM, XPS, pxrd, EDS, and TEM studies, we have shown that the composition of the catalyst remains unaltered even after prolonged periods of catalytic activity under conditions of OER. The functional stability of the catalyst was also measured through chronoamperometric studies. In this presentation we will discuss the OER and HER catalytic activity of CoNi₂Se₄ nanoflakes along with establishing the compositional and functional stability under conditions of continuous OER/HER.

Antimony sulfide is a semiconductor material with interesting optoelectronic properties that have been used for its application as an absorber in the development of thin film- and semiconductor sensitized-solar cells, reaching power conversion efficiencies above of 5%. It has an n- or p-type conductivity, depending on the growth conditions and elements used for its doping, of 10-8-10-6 ohm-1 cm-1, an optical band gap of 1.5-1.8 eV, and a strong optical absorption in the UV-Vis region (>104 cm-1). However, it has been recognized that Sb2S3 films suffer from recombination of the photogenerated carriers in its surface as well as in the bulk due to their poor crystallinity and intrinsic defects. Different techniques have been reported for the growth of Sb2S3 thin films such as SILAR, electrodeposition, chemical bath deposition, thermal evaporation, atomic layer deposition, sputtering, and so on. Among these chemical bath deposition (CBD) offers the advantages of low-temperature deposition at atmospheric pressure with inexpensive infrastructure. In this way, different metallic/sulfur sources and complexing agents have been used for the growth of SB2S3 films by CBD. In spite of all these efforts, it has been obtained films with poor adherence and porous morphology that affect the performance of the solar cells. Then it is necessary to find new routes of synthesis and deposition of the Sb2S3 films in order to improve the quality and adherence of the films to obtain better photovoltaic devices.

In the present work, it is demonstrated the deposition of antimony sulfide thin films obtained by chemical bath deposition using tartaric acid as complexing agent. For the deposition, it is used antimony potassium tartrate and thioacetamide as sources of antimony and sulfur ions, respectively. The ph of the solution is adjusted to be in the range of 9-10 by using ammonium hydroxide. The chemical deposition is done at a constant temperature of 80°C during a period of time from 1 to 4 hours. The resulting thin films are homogeneous, compact, and well adherent to substrates coated with cadmium sulfide and zinc sulfide showing a yellow to orange color, depending on the thickness of the film. Furthermore, the as-deposited thin films showed an amorphous feature, which crystallize into the orthorhombic stibnite crystalline phase after they are treated in a nitrogen atmosphere. Analysis and results of SEM, XRD, UV-Vis spectroscopy, RAMAN and XPS are showed and discussed. Finally, it is shown the performance of these Sb2S3 thin films into the superstrate thin film SCs using cadmium sulfide as window layer.

In this study, thin films of Cu₂ZnSnS₄ (CZTS) were synthesized via a low cost, wet chemical technique of chemical bath deposition (CBD) without any requirement of additional sulfurization. The aims of the study were: to achieve kesterite stoichiometry in the films without the need of post-synthesis sulfurization; to control the band structure and work function of the material only through precursor variation and to determine the correlation between Cu/Zn ratio and the photoresponse by the real-time, surface analysis technique of Kelvin probe force microscopy (KPFM).
Films having different sulfur content were deposited by varying the ratio of S/(Cu+Zn+Sn) while keeping the Cu/(Zn + Sn) and Zn/Sn ratios constant. Detailed electrical and optical characterization demonstrated that a controlled variation of the sulfur precursor allowed the formation of CZTS films with S/(Cu+Zn+Sn) ratio of ~1.1. As sulfur incorporation increased within the lattice, unwanted secondary phases decreased, band gap decreased, work function increased and the Fermi level moved closer to the valence band maxima; all of which point towards the material becoming more p-type in nature. To further study the effect of the Cu and Zn content on the band structure and photoresponse, the Cu/Zn ratio was varied keeping all other precursor ratios constant. Analysis showed that as the films transitioned from Cu-rich to Zn-rich, the band gap increased, work function decreased and the Fermi level moved away from the valence band maxima indicating a decrease in the hole population due to a decrease in Cu_Zn type acceptor defects. At the same time, a significant improvement in photoresponse was observed in the KPFM mapping of the samples as Cu/Zn decreased and this was attributed to removal of secondary phases and atomic substitutions within the existing kesterite structure due to the Zn-rich environment. This mapping of the shifting surface potential (and therefore work function) of CZTS is unique and demonstrates an extremely simple method for mapping the changing band structure in a material without the requirement of any complex technique or equipment.
Thus, in addition to removing the need for post-synthesis sulfurization, the findings reveal the capability of significantly improving the photoresponse of CZTS by carefully controlling the Cu and Zn precursors which in turn has a direct correlation to the open-circuit voltage when used as an absorber layer in a device. This study demonstrates that optimizing the deposition conditions and the ability to control the band structure of the CZTS films prepared, allows them to be useful in a wide range of photovoltaic and photoelectrochemical devices.

CZTSSe is one of the most promising PV materials in terms of being constituted of low-cost, earth abundant and non-toxic elements. Though intense research has been carried out on it, device efficiencies have not been able to surpass those of the more established material CIGS. Even then, the CZTSSe based devices reported to have the best efficiencies are those made by sputtering which in itself is an expensive process requiring high vacuum equipment. Therefore it is essential to find a solution to this problem (of low device efficiencies), which doesn’t compromise on the synthesis technique being simple, scalable and economically viable.
In this work, a low-cost, wet chemical technique (by CBD) to grow CZTSSe films has been optimized. The effect of variation of elemental ratios on the band structure has been studied and used to established a direct correlation between the starting chemical precursor ratios and important material properties (band gap, work function and valence band edge position). The process gives repeatable CZTSSe films with minimal secondary phases. Following this basic optimization procedure, zinc and tin have been partially replaced by novel elements (Cd, Ni, Sb) and detailed material characterization carried out to study the effect of this elemental replacement on the film's properties.
It has been found that there are clear changes in the band gap, x-ray diffraction pattern and Raman spectroscopy data. Aditionally, x-ray photoelectron spectroscopy has also been carried out to analyze the band structure which together with the basic characterization data, has provided a complete picture of the changing material properties as elemetal doping is carried out.

In this talk we will discuss the state of the art and potential future directions in paper-based electronics with special emphasis to the work developed at CENIMAT|i3N, covering electronic devices, smart displays, printed electronics, sensors and diagnostic tests.
We have been observing a rapid and growing interest concerning the utilization of biological materials for a wide range of applications. One of the most representative example is cellulose, not only in the form of raw material mainly for pulp and paper production, but also in the development of advanced materials/products with tailor-made properties, especially the ones based on nanostructures.
5 years ago paper electronics was pure science fiction, but today we have already several paper-based electronics like integrated circuits, supercapacitors, batteries, fuel cells, solar cells, transistors, microwave electronics, digital logic/computation, displays, force-sensing MEMS, user interfaces, transparent substrates, substrates with high strength, wearable devices, and new rapid diagnostic test sensors. These devices with their associated physics and processing will play an important and relevant to our society ongoing efforts to in environmental sustainability, safety, communication, health, and performance.

Thin film solar cells based on Cu (In,Ga)Se₂ (CIGS) and earth abundant Cu₂ZnSnS₄ (CZTS) photo-absorbers utilize chemical bath deposited (CBD) CdS as heterojunction layer to attain high efficiency. Toxicity of CdS is a deterrent both in deployment and in production. As CBD is indispensable for high efficient cells, circumventing visible absorbance by thinner CdS deposition has homogeneity issue. This led to search for alternate sulfides, Zn and In with O and OH species (In_X(OH,S)_Y , ZnS(O,OH)) which proved less efficient than CdS. Though SnO₂ as indium-tin oxide (ITO) is used in solar cells, Sn-sulfides have received little attention. Difficulty lies in concurrent deposition of sulfides with Sn(II) and Sn(IV) valence states as SnS is p-type (Eg=1.2-1.4 eV) and SnS₂ is an n-type (Eg =2.44 eV) semiconductor. We investigated the CBD method and modified it to suppress SnS formation and also achieved controlled oxygen induction to form a new Sn disulfide-oxide (SnS_2-XO_X) buffer layer. Addition of oxygen have provided phase stability and also increased visible transmission as SnO₂ has wider 3.6 eV bandgap. Using x-ray photoelectron spectroscopy (XPS), we investigated the bonding state of O and by Raman spectroscopy ascertained the effect of deposition conditions on SnS and SnS₂ phase evolution. Combined with the optical band gap analysis, this study led to evaluation of SnS_2-XO_X as a heterojunction layer for solar cells.
The CBD growth of SnS₂ films was accomplished with SnCl₂ and thioacetamide as precursors for Sn and S, respectively in varied constituent ratio and forming Sn complex with tartaric acid to control Sn to S reaction. With added organic constituent having OH affinity groups, selective formation of Sn(OH)₄ was initiated to add oxide phase by decomposition in creating new SnS_2-XO_X buffer layer. Such compound formation was confirmed by deconvolution of Sn 3d_5/2 XPS line consistent with the binding energies at 486.6 and 487.3 eV corresponding to SnS₂ and SnO₂ phases and further affirmed by oxygen O1s XPS line analysis. Consistency of the characteristic Raman peak at 312±2 cm^-1 from A_1g vibration modes of SnS₂ with decrease and or absence of 94 cm^-1 Raman line from SnS with reduction in S-precursor indicated that it was possible to obtain single phase SnS₂ in preparation of final SnS_2-XO_X phase in which x-variation is dependent on organic additive concentration. Optical analysis is consistent with direct band gap energy indicating possible band edges at 2.43 and 3.1 eV which is attributed to oxygen incorporation and thus higher transmittance for visible photons reaching absorber- junction. The fibrous and porous morphology of SnS₂ buffer layer in contrast with compact morphology of the SnS_2-XO_X phase is highly suited for a conformal coating over p-absorber for increased surface coverage without limiting the transparency, a desired attribute for a heterojunction layer. This paper will report result of these investigations.

Three sets of solar cell quality CdTe thin film samples were prepared: two sets were grown by rf magnetron sputtering whereas the other was grown by closed space sublimation. Each set was subsequently divided into 3 subsets: one subset was left as-grown, one was treated with CdCl₂, and one was treated with CdCl₂ and further etched with Br₂/MeOH. Grazing incidence X-ray diffraction and photothermal deflection spectroscopic measurements were performed on all samples in order to study the effects of the treatments on the structural and optical properties of the resultant polycrystalline thin films. For sputtered CdTe, grazing incidence X-ray diffraction shows that the as-grown preferred surface orientation is the <111> direction. After CdCl₂ treatment, however, the intensity of the <111> peak is greatly reduced, and other peaks appear, representing a random orientation. Further treatment with Br₂/MeOH leaves the sample unchanged from this polycrystalline form. The samples grown by closed space sublimation do not have a preferred <111> orientation and are structurally insensitive to post-growth treatments. Photothermal deflection spectroscopy was performed on all samples to determine the shape of the optical absorption spectrum of the samples. For the sputtered films, the optical absorption edge becomes sharper when the films are treated with CdCl₂, with an additional small sharpness increase when the sample is further etched in a Br₂/MeOH solution. For the sputtered films, the Urbach parameter, E_0, which is inversely proportional to the sharpness, decreasing after CdCl₂ treatment and showed little to no change after etching. A direct comparison of the optical absorption curves shows that the absorption from the gap states increases after CdCl₂ treatment and further increases after the Br₂/MeOH etching.

Transition metal dichalcogenides (TMDC) monolayers are being investigated intensively due to their unique optoelectronic properties as two-dimensional materials beyond graphene. Here, we interface and modify TMDC monolayers with various chirality carbon nanotubes and dopants to modulate their optical and electronic properties. We synthesize and employ large area TMDC (MoS₂, MoSe₂, WS₂, or WSe₂) monolayers via chemical vapor deposition, where the TMDCs are laterally grown to tens of microns. For this study, we are observing changes in the photoluminescence (PL) of the TMDCs when interfaced with carbon nanotubes or doped with n- and p-type dopants (e.g. F4-TCNQ). When the TMDC monolayers are interfaced with carbon nanotubes, we observe changes in the PL that is potentially due to charge transfer between the TMDC and carbon nanotubes. We confirm the adsorption of carbon nanotubes on TMDC monolayers via the confocal Raman spectroscopy and X-ray photoelectron spectroscopy. Similarly, we see changes to the PL when the TMDC monolayers are modified with dopants, which is likely due to quenching of carriers. We observe that the photoluminescence blue shifts when modified by p-type dopants, which can be contributed to trions converting to excitons. Interestingly, such doping effects vary from the edge to the center of the monolayer. We also examine the electronic properties such as time resolved microwave conductivity to investigate the carrier mobility. Our findings and hypotheses would help the further understanding of two-dimensional materials and facility their potential applications in solar fuel and photovoltaic.

The ordering of the group III elements in In_xGa_1-xP (x=0.5) have been observed to vary during the growth of GaAs quantum well and InAs quantum dot structures grown on InGaP by metalorganic vapor phase epitaxy. The effect of crystal polarity on ordering has been established using high angle annular dark imaging in the scanning transmission electron microscopy. Ordering is observed along <110> projections when the dumbbell configurations show the group V element on top of the group III element. For GaAs quantum wells on InGaP, ordering is maintained throughout the growth process. However, when InAs quantum dots are grown and then covered with a thin GaAs layer, ordering is not observed in the InGaP on top of the GaAs layer. It is observed that after a few nanometers of growth, ordering resumes. We attribute this to compositional pulling effects, where some excess indium in the gas phase is necessary to grow the InAs quantum dot. The excess indium is eventually consumed in the InGaP and ordering resumes when the intended composition is achieved. Cathodoluminescence has been used to correlate the electronic properties with the microstructure of our thin film structures.

It has been previously reported that use of a CdSe_xTe_1-x window layer increases the short-circuit current density (J_sc) of a CdTe solar cell by increasing the cell photo-response for both short- and long wavelength photons. However, open-circuit voltage (V_OC) and fill factor (FF) are frequently lower than for traditional CdS/CdTe solar cells. One possible reason behind lower V_OC and FF relates to increased recombination losses within or at the interfaces of the CdSe_xTe_1-x region of the device. Because the use of CdSe_xTe_1-x is relatively new for the CdTe solar cell technology, we focus here on characterization of native and impurity defects as a basis for understanding the performance in CdSe_xTe_1-x/CdTe solar cells. Low temperature photoluminescence (PL) spectra have been measured for CdSe_xTe_1-x (x = 0.00, 0.08, 0.14, 0.21, 0.29, 0.36 and 0.44) films. CdSe_xTe_1-x films used in this study were prepared via a co-sputtering process using CdSe and CdTe targets. Tauc plots and room temperature steady-state PL measurement of these films confirm the band gap bowing in CdSe_xTe_1-x films with bowing parameter, b = 0.81 ± 0.03. The lowest band gap energy of ~1.394 eV is achieved for at x ≈ 0.36. X-ray diffraction measurements show that the lattice constant of the CdSe_xTe_1-x film decreases linearly with Se content. Time resolved PL measurement reveals that photogenerated carrier lifetime increase monotonically with Se content, indicating that the narrowed bandgap region of the absorber may, unlike CdS, contribute to the cell’s photocurrent as has been observed experimentally. Temperature dependent and laser power dependent PL together with time resolved PL will be carried out to explore the sources of observed defects. In addition, PL and TRPL analyses will be reported for CdTe deposited by close-space sublimation onto sputtered CdSe thin films.

S. Mohsin, D. Quispe, A. Leilaeioun, Z.J. Yu, and Z.C. Holman
Transparent conductive oxide (TCO) electrodes are commonly found at the front—and sometimes the rear—of thin-film solar cells and modules, including those composed of CdTe, CIGS, and perovskites. In this role, the TCOs are responsible for transporting charge laterally to the nearest scribe line, which are spaced 5–10 mm apart, and for transmitting photons into the cell that are shorter than the bandgap wavelength, which is usually 700–1100 nm for the cells of interest. Indium tin oxide, which can have carrier densities of over 1021 cm-3 and usually has a mobility of 20–30 cm2/Vs, is most commonly used in these cells, and typical layers have sheet resistances of 10–20 Ω/sq.
Tandem solar cells that have a narrower-bandgap bottom cell, such as silicon, also require TCOs—both at the front surface and possibly between the sub-cells, to act as a recombination junction—but the optimal TCO has considerably different properties. In particular, the TCO must remain transparent out to 1200 nm, which means that the carrier-density-to-mobility ratio must be small according to the Drude model, and the sheet resistance can be as large as 100 Ω/sq if closely spaced metal fingers are employed, as in silicon solar cells.
In this contribution, we deposit and characterize indium zinc oxide (IZO) layers and compare these to ITO layers. The IZO layers are sputtered in atmospheres with varying oxygen gas partial pressures, leading to varying carrier density. This, in turn, tunes the sheet resistance of approximately 100-nm-thick layers from 30 Ω/sq to over 10,000 Ω/sq, but the mobility remains nearly constant for all layers at the high value of 45–65 cm2/Vs. Absorption measurements of films on glass confirm what Hall measurements reveal: for identical thicknesses and sheet resistances, an IZO layer can have more than 5% less parasitic absorption than its ITO counterpart.

Silicon Germanium Tin (SiGeSn) is an emerging class of group IV alloy that is opening new pathways for the development of integrated photonics on microelectronics platform. SiGeSn has several appealing features such as higher mobility compared to silicon, the ability to alter the band gap along with indirect-direct transitions and the ability to engineer the lattice constant independent of the band gap and grow it on various substrates. Thus, SiGeSn can lead towards promising near- and mid-infrared optoelectronic devices which will create plenty of opportunities in the fields of high performance computing networks and energy trapping and conversion. Semiconductor-on-insulator technology is of interest as its development can lead to devices such as high mobility transistors and higher frequency applications. Sapphire is one good choice for an insulator substrate as it has a transparency window up to 6 µm and it offers high-frequency parasitic capacitance, radiation hardness and reduction of latch-up in CMOS structures. Silicon dioxide is another popular choice for insulator because of its low dielectric loss and low loss tangent.
In this work, SiGeSn films were deposited on three different substrates i.e c-plane sapphire, 100 nm silicon dioxide coated silicon (100) and silicon (100). The films were deposited using plasma enhanced chemical vapor deposition (PECVD) technique, as it offers a cost effective and low temperature route for deposition of the films. The composition of the films was varied by varying the input precursor flow rates of Si, Ge and Sn. RBS analysis of the films revealed that Sn and Si concentrations through 10% and 25% respectively were realized. From XRD analysis, it was observed that the deposited films were polycrystalline in nature irrespective of the substrate used. Singular peaks were observed corresponding to the standard (111), (220), (311) and (004) Ge peaks which indicate that the deposited films were cubic in nature. It was observed that films deposited on sapphire mostly grew in the (111) orientation. For films deposited on silicon dioxide, growth took place majorly in the (111) and (220) orientation. For films deposited on silicon (100) substrate, the films adopted the (111) and (004) orientation substantially. It was also observed that the position of the XRD peaks change to a lower or higher angle depending on the Sn and Si concentrations present in the films. However, the peak position did not change for films with same composition grown on different substrates. This corroborates with the RBS analysis that the composition does not change with change in substrate. From Raman spectroscopy, peaks correlating with Ge-Ge, Ge-Sn, Si-Sn and Si-Ge bonds were observed for the films deposited on the various substrates. This confirms the formation of the ternary alloy in the deposited films. The peak positions either red shifted or blue shifted depending upon the film composition.

Electrolyte-gated transistors hold promise for applications in printable and flexible electronics. Metal oxide semiconductors are particularly interesting as electrolyte-gated channel materials for their abundance, thermodynamic stability and ease of process in ambient conditions.
In this work, we synthesized by sol-gel and hydrothermal methods different types of tungsten oxide to be used as channel materials in ion gel-gated transistors. X-ray diffraction and scanning and transmission electron microscopy revealed that the differently processed oxides show a different structure (hexagonal and monoclinic) and morphology (granular, nanofiber and nanoplate). We studied the electrochemical and transistor properties of the oxides using, as the gating media, two different ion gels prepared from the same ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM]TFSI]), and two different block copolymers.
We propose that, for sufficiently high values of the gate-source bias, the doping results from chemical and electrochemical contributions (the chemical contribution being attributed to the reduction of the oxide by the molecular hydrogen formed by reduction of the acidic proton of the cation of the ionic liquid). Our work sheds light on the complexity of the electrolyte gating phenomenon, to fully exploit its technology potential.

A typical thin film silicon solar cell has the core structure of a thin film silicon absorber, such as hydrogenated amorphous silicon (a-Si:H), that is sandwiched between two wider band gap contact layers (CLs), which facilitate the separate collection of photogenerated electrons and holes from the absorber. Commonly, doped thin film silicon or its alloys (DTF-Si/A) are employed as CLs. DTF-Si/A layers are typically deposited by a chemical vapour deposition (CVD) from expensive, toxic and flammable gases that contain the dopant atoms. To prevent dopant contamination of the absorber, which is detrimental to power conversion efficiency, a multi-chamber CVD system is employed for depositing thin film silicon based layers. This, however, increases the cell processing complexity, and costs. Therefore, to advance thin film silicon solar cell technology, it is desirable to replace DTF-Si/A layers with alternative materials that are cheap, safe, abundant, and, that can be deposited by simple, low temperature physical deposition (PVD) methods. Here, dopant free a-Si:H solar cells with the structure of Asahi U substrate / MoO_x / a-Si:H / LiF / Al are processed, where the MoO_x, LiF and Al layers are deposited by PVD. The best dopant free a-Si:H solar cell generated 8.02 mW/cm² under standar test conditions, demonstrating the promising potential for the dopant free thin film silicon cell concept.

In the field of solar cells, harvesting the energy of UV and IR photons is essential to achieve higher solar conversion efficiency (SCE) and decreasing the negative impacts of UV and IR photons such as the generation of extra heat and the degradation of solar cell materials. The commercial Si-solar module can reach high photon conversion efficiency in the visible and NIR region, however, the photon energy in the UV and IR region are mostly not utilized. Several approaches have been proposed to combine multiple dyes and quantum dots to achieve a broad wavelength coverage, but the added complexity in manufacturing the solar cells creates an economic barrier to use these approaches. In most the studies, wavelength shifting materials were directly applied to the top or bottom of the solar cells [1, 2]. Considering the simplicity of applying retrofitted coatings to the cover glass of solar modules in contrast to changing the structure and the fabrication processes of solar cells, we developed a multifunctional thin film that can be directly applied to the solar modules to increase the power output. This multifunctional thin film is capable of wavelength shifting and reflection reduction at the same time. These thin films were fabricated using functional nanoparticles from scalable nanomanufacturing processes [3]. The relation between the enhanced SCE of mc-Si solar cells and the micro- and nanostructures of the thin films was investigated experimentally along with optical modeling in this study.
Reference
1. B.S. Richards, “Luminescent layers for enhanced silicon solar cell performance: Down-conversion.” Solar Energy Materials & Solar Cells, vol. 90, 2006, pp. 1189-1207.
2. A. Shalav, et al. “Luminescent layers for enhanced silicon solar cell performance: Up-conversion.” Solar Energy Materials and Solar Cells, vol. 91, 2007, pp. 829-842.
3. He, Yujuan, et al. “Continuous, Size and Shape-Control Synthesis of Hollow Silica Nanoparticles Enabled by a Microreactor-Assisted Rapid Mixing Process.” Nanotechnology, vol. 28, no. 23, 2017, p. 235602.

The solar photoelectrochemical generation of hydrogen and carbon-containing fuels comprises a critical energy technology for establishing sustainable energy resources. The limited number of known photoelectrocatalytic materials with a visible band gap poses a substantial materials discovery challenge that has not been sufficiently addressed by either experimental or computational campaigns. By evaluating the suite of theory and experiment screening techniques for identifying photoanode materials, choosing the techniques that provide robust and complimentary screening, and integrating these techniques into a multi-tiered screening pipeline, we have identified not only a variety of new photoanodes but also trends in their underlying electronic structure and thermodynamic stability that yield new design mechanisms. In total, 17 ternary oxides in the composition systems M-V-O where M = Cr, Fe, Co, Ni, Ag, Bi and M-Mn-O where M = Sr, Mg, Ni, Ba, Ca have been identified as photoelectrocatalysts for the oxygen evolution reaction with band gap below 2.6 eV, comprising the largest collection of discovered materials by combined theory-experiment pipelines in any domain and indicating that the collection of photoanode materials may be much broader than suggested by the historical literature.

Development of scalable semiconductor photoanodes with a suitable bandgap (1.8 - 2.4 eV), low resistivity, and long-term chemical stability is a primary challenge in photocatalyst and photoelectrochemical (PEC) research. The complex oxide semiconductor, pseudobrookite Fe₂TiO₅, is a prospective candidate consisting of earth-abundant elements Ti and Fe with an ideal optical bandgap of 2.18 eV [1]. Despite a few studies focusing on the application of Fe₂TiO₅ as photoanodes in polycrystalline thin films and in heterojunctions [2], the intrinsic PEC properties of Fe₂TiO₅ have not been clarified, fundamentally limiting further evaluation and development of this materials system.
Recently, we successfully fabricated epitaxial Fe₂TiO₅ thin films on single crystalline LaAlO₃ (001) substrates using pulsed laser deposition, and thoroughly characterized their optical and transport properties, finding that the intrinsic resistivity (10 - 100 Ωcm) at room temperature is substantially lower than that of un-doped α-Fe₂O₃, despite similar optical gaps [3]. Based on these results, here we present our investigation of the intrinsic PEC properties of epitaxial Fe₂TiO₅ thin film photoanodes for solar-water splitting. A photocurrent density of 0.08 mA/cm² was achieved at 2 V vs. RHE under 1 Sun, and a flatband potential of 0.40 vs. RHE was obtained in pH = 13. These results indicate that Fe₂TiO₅ has high potential as an ideal photoanode in solar-fuel conversion. Details of the photocurrent measurement, Mott-Schottky analysis, and long-term stability in solution will be discussed in the presentation.

[1] D. S. Ginley and M. A. Butler, J. Appl. Phys. 48, 2019 (1977).
[2] Q. Liu et al., Nat. Commun. 5, 5122 (2014).
[3] M. Osada, et al., 2017 MRS Spring Meeting & Exhibit ES7.18.03.

Semiconductor compounds are widely used for water splitting applications, where photo-generated electron-hole pairs are exploited to induce catalysis. Recently, powders of a metallic oxide (Sr_1-xNbO₃, 0.03 < x < 0.20) have shown competitive photocatalytic efficiency, opening up the material space available for finding optimum performance in water-splitting applications¹. In one of our previous report, the hot electron and hole carriers excited via Landau damping (during the plasmon decay) are responsible for the photocatalytic property of this material under visible light irradiation².
To better understand the photocatalytic mechanism of such materials and to design high performance photocatalyst, we prepared high quality single crystal thin films of MNbO₃ (M=Ca, Sr, Ba). We found that all MNbO₃ are metallic oxides and have a very large bandgap of ~4.0 eV. Surprisingly the carrier densities can exceed 10²² cm^-3, which is only one order smaller than that of elemental metals and the carrier mobility is only 2.47 cm²/(V×s). Contrary to earlier reports, the visible light absorption at 1.8 eV (~688 nm) is due to the bulk plasmon resonance, arising from the large carrier density, instead of an interband transition. By fitting the reflection spectra with Drude-Lorentz model, we show that the peaks of plasmon resonance in CaNbO₃, SrNbO₃ and BaNbO₃ were at 693nm, 715nm and 682nm, respectively. The fit can also reproduce the experimentally observed optical absorption peak in the visible for all M values. Excitation of the plasmon resonance results in a multifold enhancement of the lifetime of charge carriers. Thus, we propose that the hot charge carriers generated from decay of plasmons produced by optical absorption is responsible for the water splitting efficiency of this material under visible light irradiation.² The relative performance of the photocatalytic activity for different M (where the water splitting efficiency of CaNbO₃ > SrNbO₃ > BaNbO₃) showed a strong correlation with the product of the hot carrier lifetime, solar energy absorption and surface area. This work lays the foundation for a novel class of photocatalysts driven by intrinsic plasmonic absorption with significant future promise for the field of sustainable energy sources.

Many present-day investigations of water splitting photoelectrodes are based on buried p–n junctions, which usually offer an improved photovoltage and therefore a higher solar-to-hydrogen efficiency in tandem photoelectrochemical cells. In this work, we demonstrate that the dual working electrode (DWE) technique enables the measurement of the surface potential of water splitting buried-junction photocathodes under operation, enabling the deconvolution of the photovoltaic and electrocatalytic performance in operando. Consequently, we can access properties of the buried p–n junction independent of the surface kinetics, and gain information related to the charge transfer through the electrode/electrolyte interface independent of the photovoltaic properties. Moreover, the DWE technique provides a clearer understanding of the photocathode degradation mechanism during stability tests. Two p–n junction-based photocathodes are investigated in this work: a pn⁺-Si/TiO₂ photocathode as model system, and the application of the developed method to the emerging material system Cu₂O/Ga₂O₃/TiO₂.

Sustainable electrochemical reduction of CO₂ into chemical products, in particular hydrocarbons and oxygenates, could provide an alternative to mankind’s current use of fossil fuels. A complete solar-driven system based on photovoltaic cells coupled to a CO₂ electrolysis cell will be discussed. Optimizing the efficiency of CO₂ electrolysis requires minimizing potential losses in all aspects of the device including the cathode, anode, electrolyte, and membrane. Selective production of hydrocarbon and oxygenate products requires management of multi-electron transfer reactions. Finally, the efficiency under illumination critically depends on the coupling of the PV and electrolysis elements.
Optimization of the components, as well as the overall design, of an aqueous phase CO₂ electrolyzer cell enables an energy conversion efficiency for hydrocarbons and oxygenates in the 20-30% range to be achieved (overall efficiency to all products can be >50%). A nanostructured IrOx anode was synthesized by electrodepositing IrO₂ nanoparticles on ZnO nanorods template and a nanotube IrOx anode was obtained by removing ZnO template. IrOx nanotube anode is employed here to drive the oxygen evolution reaction in the neutral pH range of CO2-saturated bicarbonate buffer (6.8 – 7.4). The IrO_x anode has only a 300 mV overpotential at 10 mA cm^-2 and can be used for months. High rate electrodeposition of Cu on Ag is used to product a dendritic bimetallic “nanocoral” cathode which maintains high selectivity to hydrocarbons and oxygenates (~60%) and has multi-day stability. Importantly, the high selectivity to hydrocarbons and oxygenates is retained at high concentrations of bicarbonate electrolyte, which enables ohmic losses in the overall cell to be decreased.
Use of power matching electronics enables coupling to series-connected solar cells and to high efficiency 4-terminal tandem cells, and overall solar to hydrocarbon and oxygenate efficiencies of 3-4% (0.3-1 sun illumination) and >5% (1 sun) are achieved, respectively [1]. Notably, these values exceed that of natural photosynthesis (<1%) and approach those of devices, which make simpler and potentially less valuable C₁ products such as CO and formic acid. Prospects for further improvements through the use of cathodes with lower overpotentials and by integrating more efficient solar cells will be discussed.
This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.
Gurudayal et al., Energy. Env. Sci. 2017 10, 222-2230.

This talk will follow the emergence of several promising thin-film solar energy conversion materials based on earth-abundant Cu₂ZnSn(S,Se)₄ (CZTS), Cu₂BaSn(S,Se)₄ (CBTS) and related systems, having a I₂–II–IV–VI₄ stoichiometry and an interconnected network of metal (I, IV) chalcogenide (VI) tetrahedra. Simple solution- and vacuum-based film deposition approaches have enabled fabrication of relatively high-performance absorber layers, with resulting photovoltaic (PV) sunlight-to-electricity power conversion efficiencies exceeding 12% for CZTS [1] and 5% for CBTS [2]. CBTS has also recently been incorporated into photoelectrochemical (PEC) cells, yielding a stable (over 10 hr) 12 mA/cm² photocurrent at 0 V/RHE [3]. In CBTS, the much larger Ba ion occupies a site that has 8-fold coordination rather than 4-fold (as for Cu, Zn and Sn), reducing the probability of anti-site disorder and associated band tailing (relative to CZTS), which can limit device performance. A similar arrangement can be found within the broader family of I₂–II–IV–VI₄ (I = Cu, Ag; II = Sr, Ba; IV = Ge, Sn; VI = S, Se) compounds, which fall into five distinct structural types [4]. Computational and preliminary experimental assessment reveals several family members, beyond CBTS, that may be interesting for PV/PEC. Although at an early stage of development, CBTS and the broader concept of employing atomic size and coordinatlion discrepancy for limiting anti-site disorder may offer a pathway for overcoming performance issues encountered within complex multinary chalcogenide semiconductors.

This work was supported by NSF CBET-1511737.
[1] W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu, D. B. Mitzi, Adv. Energy Mater. 4, 1301465 (2014).
[2] D. Shin, T. Zhu, X. Huang, O. Gunawan, V. Blum, D. B. Mitzi, Adv. Mater. 29, 1606945 (2017).
[3] Y. Zhou, D. Shin, E. Ngaboyamahina, Q. Han, C. Parker, D. B. Mitzi, J. T. Glass, submitted (2017).
[4] T. Zhu, W. P. Huhn, G. C. Wessler, D. Shin, B. Saparov, D. B. Mitzi, V. Blum, Chem. Mater. 29, 7868 (2017).

Cu₂ZnSn(S,Se)₄ (CZTSSe) is one of the earth-abundant/non-toxic alternative compounds for the commercialized metal chalcogenides (i.e., CdTe and Cu(In,Ga)(S,Se)₂) thin-film photovoltaics (PVs). To raise the performance of CZTSSe based solar cells, much effort has been applied to improving the quality of absorbers, band alignments/passivation at p-n junction, front and back interfaces/contacts. [1,2,3]
In this work, we demonstrated the enhanced open circuit voltage (V_oc) of CZTSSe PVs by introducing an interfacial alkaline earth fluoride (several nm MgF₂) between the absorber (i.e., CZTSSe) and the buffer layer (i.e., CdS) after sulfo-selenization processes without post-annealing. In statistical studies (10 cells), the alkaline earth fluoride increases power conversion efficiency from 7.5% to 8.8%, short circuit current density (J_sc) from 30.7 mA/cm² to 32.6 mA/cm² and Voc from 440 mV to 470 mV, possibly resulting from the MgF₂ induced electric field passivation at grain boundaries. [2]. Finally, a 9.4 % efficient CZTSSe solar cell with V_oc of 470 mV, J_sc of 32.96 mA/cm² and fill factor (FF) of 60.7 % was obtained.
The morphology, elemental composition, and distribution of the absorber layers are being examined by scanning Kelvin probe microscopy measurement, X-ray diffraction (XRD), X-ray fluorescence spectrometry (XRF), scanning electron microscopy (SEM), Raman spectroscopy, and nano Auger electron spectroscopy (AES).

References
[1] V. Tunuguntla, W.C. Chen, P.H. Shih, I. Shown, Y.R. Lin, C.H. Lee, J.S. Hwang, L.C. Chen and K.H. Chen, J. Mater. Chem. A, 2015,3, 15324-15330
[2] Y.R. Lin, V. Tunuguntla, S.Y. Wei, W.C. Chen, D. Wong, C.H. Lai, L.K. Liu, L.C. Chen and K.H. Chen, Nano Energy, 2015, 16, 438
[3] W.C. Chen, C.Y. Chen, V. Tunuguntla, S.H. Lu, C. Su, C.H. Lee, K.H. Chen and L.C. Chen, Nano Energy, 2016, 30, 762-770

One of the current key challenges in solar photovoltaics and solar-driven water splitting is to identify an efficient, stable, and inexpensive material to be used as a high-band gap (1.6-2.0 eV) photabsorber in tandem device configurations. To identify promising candidates that have not been considered before, we have computationally screened 705 compounds with the ABS₃ formula (A,B = metals; S = sulfur). Only 15 compounds pass all the screening rounds, which include criteria such as phase stability, suitable band gap, low effective mass, and defect tolerance. The list of 15 compounds includes the previously synthesized materials BaZrS₃ and SrZrS₃ [1], as well as a number of other compounds that have not yet been reported experimentally. We have therefore synthesized and characterized a few of such novel ABS₃ compounds in thin film form by means of a two-step process. First, sputter deposition of metallic (AB) or oxide (ABO₃) precursors; and subsequently, sulfurization of those precursors in H₂S gas.
In this contribution, we will show that LaYS₃ is a particularly attractive high-band gap photoabsorber. It features a direct band gap of 2.0 eV, which is optimal for a top photoabsorber in tandem water splitting devices [2]. Furthermore, the offset between its band gap and its photoluminescence peak is only 0.1 eV, implying that band tailing problems are not too severe in this first generation of films. The photoluminescence intensity of LaYS₃ also compares well to that of high-quality Cu₂ZnSnS₄ films. We will present the back- and front contact materials targeted for a single-junction LaYS₃ solar cell, the corresponding device architecture, and the challenges encountered in our first attempts at fabricating a LaYS₃ solar cell. Issues in the synthesis of other novel ABS₃ compounds will briefly be mentioned.

[1] S. Niu, H. Huyan, Y. Liu, M. Yeung, K. Ye, L. Blankemeier, T. Orvis, D. Sarkar, D.J. Singh, R. Kapadia, and J. Ravichandran, Adv. Mater. 29, 1604733 (2017).
[2] J.H. Montoya, L.C. Seitz, P. Chakthranont, A. Vojvodic, T.F. Jaramillo, and J.K. Nørskov, Nat. Mater. 16, 70 (2016).

As conventional solar cells use the incident light through only one side of the cell, it is important to arrange the solar cells toward the direction of the sunlight, which varies with time. Bifacial solar cells can use the incident light from both sides, which makes the cell efficiency less dependent on the angle of the incident light. Therefore, bifacial solar cells produce higher power per unit area than conventional monofacial solar cells. Among the various solar cells, thin-film solar cells have attracted attention due to their advantages of light weight and low manufacturing cost. One type of the thin-film solar cells, Cadmium Telluride (CdTe) thin-film solar cells, have been commercialized successfully and investigated with great interest owing to their high theoretical efficiency(~29%). However, only a few researches on the bifacial configuration of CdTe thin-film solar cells have been reported.
In this work, copper nanowires/indium tin oxide (CuNWs/ITO) are used as transparent back contact material. CuNWs have high optical transmittance and conductivity, which are essential properties for transparent back contact. In addition, Cu can act as a p-type dopant in CdTe. Therefore, CuNWs are expected to act as a dopant and enhance the contact properties with CdTe layer. CuNWs were spray-coated onto CdTe and followed by the ITO deposition. ITO improves the electrical properties of NWs network and protects the CuNWs from oxidation without significant loss of optical transmittance. Optical, electrical and photovoltaic properties of the CdTe thin-film solar cells with CuNWs/ITO back contact were investigated and high total efficiency of ~10% was obtained by optimizing the amount of CuNWs deposited. The details of our work will be presented in the meeting

Kesterite based absorbers (Cu₂ZnSn(S,Se)₄ – CZTSSe) has become one of the most relevant thin film photovoltaic emerging technologies based on earth abundant elements. Nevertheless, and in spite of the impressive progresses achieved in the last years, this technology still needs to demonstrate the possibility to achieve efficiencies higher than 15% in the near future, in order to be attractive for its industrialization. The main technological problem of kesterites lies on the high voltage deficit typically exhibited by the solar cell devices, which is markedly higher than those obtained in commercial thin film technologies like CdTe and Cu(In,Ga)Se₂. This can be linked to different origins, all of them related to the complex structure of these materials, including the presence of secondary phases presence, Cu/Zn disorder, Sn volatility and multivalence, macro and micro inhomogeneities, etc., that negatively impact on the electrical and transport charge properties of the material. To solve this very relevant issue, doping and alloying strategies have been revealed as the most promising ways to reduce the current voltage deficit of kesterites.
In this work, the main doping (alkali doping, Ge doping, etc.) and alloying (with Ag, Cd and Ge, etc.) strategies reported in the literature will be reviewed and presented. In the first part, the effect of different doping elements (Li, Na, K, Rb, Cs, Ge, Sb, Bi, etc.) will be analyzed and discussed. In particular, the observed impact onto the optoelectronic properties of solar cell devices, and the possible consequences for the reduction of the voltage deficit will be deeply analyzed, showing that Li and Ge are positioned as the most interesting doping candidates.
In the second part of the talk, different attempts for kesterite alloying by cation substitution will be reviewed, in particular the substitution of Cu by Ag, Zn by Ba, Cd or Mg, and Sn by Ge or Si. The potential of kesterite alloying for the solution of the different problems identified in this technology such as Cu/Zn disorder, Sn multivalence, Sn volatility etc., will be discussed, in regards of their effect onto the solar cell devices properties. Finally, the possible application of these alloying elements for the formation of graded band-gap concepts in kesterites, as well as the most promising strategies to be followed in the future for the improvement of the conversion efficiency of kesterite based solar cells through the reduction of the voltage deficit will be analyzed.

Given their established robustness and favorable optoelectronic properties, the semiconducting transition metal dichalcogenides (TMDs, e.g. MoS₂ and WSe₂) are attractive for solar energy application.^[1] Recent advances in the solvent-assisted exfoliated of TMDs into 2D nanoflakes suggests that inexpensive roll-to-roll processing can be used to prepare TMD devices inexpensively over large area.^[2] However, the high concentration of edge defects in these materials act as recombination sites for photogenerated carriers. In this presentation the challenges with charge transport, separation, recombination and interfacial transfer in these systems will be discussed with respect to the 2D flake size and defect passivation/charge extraction treatments.^[3] Our results give insight into the roles of edge and bulk defects and suggest routes for improvement. Overall it is shown that TMDs can achieve internal quantum efficiency for photon harvesting similar to bulk single crystal samples. Moreover, promising performance is demonstrated for the direct solar to fuel coversion using WSe₂ as a photocathode for water reduction.

[1] X. Yu, K. Sivula, ACS Energy Letters 2016, 1, 315.
[2] X. Yu, M. S. Prevot, N. Guijarro, K. Sivula, Nat Commun 2015, 6, 7596.
[3] X. Yu, A. Rahmanudin, X. A. Jeanbourquin, D. Tsokkou, N. Guijarro, N. Banerji, K. Sivula, ACS Energy Letters 2017, 2, 524.

Transparent conducting oxides (TCOs) are critical components in many devices including solar cells and touchscreens. The search and development of new TCOs combining high conductivity and transparency is a major endeavor of modern Materials Science. Novel p-type TCOs are especially greatly sought for as they lie much behind their n-type counterpart and the discovery of a high performance p-type TCO would enable important technological breakthroughs.
There are two types of transport mechanism for carriers in materials: band transport involves delocalized, almost free, carriers while small-polaron transport involves carriers trapped in the crystalline lattice. Materials exhibiting transport through small polarons have been traditionally disregarded for TCO applications as they offer small mobilities. In this work, we use well-established physical models to compare the performances of TCOs based on band- and small-polaron transports. Surprisingly, we demonstrate that small-polaron TCOs can outperform band TCOs in terms of transparency and conductivity, especially p-type. We link this unexpected observation to the absence of collective, Drude-like, optical absorption and reflection in small-polaron materials.
Using our analysis, we outline what materials properties are necessary for high performance small-polaron TCOs, leading to a series of design principles. We also further analyze the recently proposed Sr-doped LaCrO₃ p-type TCO presenting small-polaron transport [1]. Using first-principles computations as well as experimental data, we rationalize the good performances of Sr-doped LaCrO₃ in view of these design principles and suggest avenues for its improvement. We explain how to obtain the small-polaron properties of a material from first principles and motivate the search for new efficient small-polaron p-type TCOs through our outlined design principles.

[1] Adv. Mater. 27, 5191–5195 (2015).

Earth-abundant and non-toxic metal oxides are attractive catalysts for energy conversion and storage applications. While extensive searches for efficient and selective catalysts have been enthusiastically carried out mostly for n-type semiconductors, p-type photocathode materials have been the subject of a much fewer studies with very few noticeable exceptions. One example is the fascinating delafossites with an AMO₂-type general composition, where ‘A’ is monovalent metals of Cu, Ag, etc. and ‘M’ is trivalent metals of Al, Ga, In, Fe, etc. Some recent studies on delafossites have demonstrated that they could be used as electrocatalysts and photocatalysts and are invaluable to rational design and optimization for various catalytic applications. Therefore, it is emergent to improve their optical and electrochemical activity and explore their potential electrochemical (EC) and photoelectrochemical (PEC) catalytic applications. Based on a widely used strategy to increase the surface area of functional materials, here using CuGaO₂ as an example, we synthesized delafossite nanoflakes by a hydrothermal method. Their EC and PEC performance for oxygen evolution reaction (OER) in 0.5 M KOH electrolyte versus Ag/AgCl along with their stability were studied for cost-effective and active electrode material and compared with CuGaO₂ microplates synthesized under the same conditions without SDS. It was demonstrated that the delafossite CuGaO₂ nanoflakes synthesized in the presence of SDS showed better EC and PEC performance and stability for OER than those synthesized without surfactant SDS. Furthermore, PEC performance was better than the EC performance for the same samples under identical measurement conditions. Thus, this hydrothermal method in combination of surfactant was an efficient approach towards the synthesis of delafossite CuGaO₂ nanostructures with reduced size as cost-effective and high performance (photo)electrocatalysts.

The family of heterovalent ternary nitride semiconductors offers an intriguing opportunity to expand the range of materials properties and potential for innovative device designs beyond those available solely with the binary nitrides and their homovalent alloys. The heterostructures formed by III-nitride and II-IV-nitride can potentially address key issues that are currently faced by the III-nitride community. One example is the close values predicted for the spontaneous polarization coefficients for the Zn-IV-nitrides (IV=Si,Ge,Sn), which leads to much smaller built-in polarizations for heterostructures grown along the c-axis. Other possibilities arise with the large band offsets predicted for GaN-ZnGeN₂ heterointerfaces, which allow for the design of novel LED structures that are calculated to improve electron-hole wavefunction overlap by more than a factor of 2 compared to conventional InGaN-GaN device structures. The large band offsets, coupled with the close lattice match and similar optimal growth temperatures for GaN and ZnGeN₂, are also advantageous for the design of quantum cascade laser structures for emission in the near-infrared intersubband transition wavelength range. Furthermore, several of the heterovalent ternary nitrides are composed entirely of abundant, nontoxic, inexpensive elements. One of these, ZnSnN₂, has garnered considerable interest as a potential solar photovoltaic material. The increased complexity of the heterovalent ternary nitride lattice, compared to the binary nitride lattice, allows for octet-rule-preserving polytypes and random stacking of two orthorhombic phases, as well as for cation exchange defects. This situation is especially pertinent to ZnSnN₂, as the two orthorhombic phases are predicted to have very close energies of formation and band gaps. Octet-rule-preserving polytypes are also possible for mixtures of the binary and heterovalent ternary nitrides. While the proposed LED and quantum cascade structures noted above do not require p-doping of the ZnGeN₂ layer, p-doping will be necessary for may applications. Ab initio calculations of defect energies and concentrations indicate that p-doping by substitutional elements or by adjustments in cation stoichiometry may be problematic for the heterovalent ternary nitrides, and successful doping strategies to achieve high p-doping may have to rely on the formation of defect complexes.

This work was supported by the NSF DMREF SusChEM award 1533957 and by the NSF SusChEM award 1409346.

ZnSnN₂ (ZTN) has been proposed as a new earth abundant absorber material for PV applications. While carrier concentration has been reduced to values suitable for device implementation, other properties such as ionization potential, electron affinity and work function are not known. Here, we experimentally determine the value of ionization potential (5.6 eV), electron affinity (4.1 eV) and work function (4.4 eV) for ZTN thin film samples with Zn cation composition Zn/(Zn+Sn) = 0.56 and carrier concentration n = 2x10¹⁹ cm^-3 [1]. Using both experimental and theoretical results, we build a model to simulate the device performance of a ZTN/Mg:CuCrO₂ solar cell, showing a potential efficiency of 23% in the limit of no defects present. We also investigate the role of band tails and recombination centers on the cell performance [1]. In particular, device simulations show that band tails are highly detrimental to the cell efficiency, and recombination centers are a major limitation if present in concentration comparable to the net carrier density. The effect of the position of the band edges of the p-type junction partner was assessed too. Through this study, we determine the major bottlenecks for the development of ZTN-based solar cells and identify avenues to mitigate them [1].
Devices’ fabrication is currently ongoing, and preliminary results would be reported in this presentation

[1] Elisabetta Arca, et al. IEEE Journal of Photovolaitcs, accepted.

The electronic properties of multinary semiconductors often depend strongly on order/disorder effects on the cation sublattice. In its pure, stoichiometric, and fully ordered form, ZnSnN₂ (ZTN) is a 1.4 eV gap semiconductor [1] with remarkable electronic properties similar to those of successful photovoltaic materials like CdTe or Cu(In,Ga)Se₂. In practice, ZnSnN₂ incorporates considerable amounts of oxygen, accommodates variable cation stoichiometries, and can exhibit various degrees of disorder. The computational task is to predict the photovoltaic properties of the real material as function of controllable process parameters. The optoelectronic properties of interest include band gap, effective masses, absorption coefficient, carrier localization, as well as the net doping. In order to obtain a comprehensive computational picture we calculate the non-equilibrium defect phase diagram, and perform Monte-Carlo simulations for oxygen containing, non-stoichiometric, and disordered ZTN.
[1] S. Lany et al., Phys. Rev. Mater. 1, 035401 (2017).

In this work, we present a combinatorial study of the structural and optical properties of sputtered ZnGeN₂ thin films, with cross-cutting applications in both fundamental materials science and novel device development. While III-N materials such as GaN have revolutionized modern optoelectronics, device structures rely on stringent lattice-matching to avoid deleterious defects. The II-IV-N₂ materials, which are structural analogs to III-N materials, offer the possibility of controlled disorder of the cation sublattice which would allow tunable properties at fixed composition. ZnGeN₂ is analog and closely lattice-matched to GaN and exhibits a direct bandgap with predicted strong absorption. However, there is disagreement in the literature regarding optical properties, as experimental works to date measure an absorption onset of ~3 eV, which is smaller than the predicted bandgap for ordered material. Additionally, little work to date has attempted to explore properties as a function of cation composition, which has been shown to greatly impact properties in other II-IV-N₂ materials such as ZnSnN₂. In this work, we present a study of combinatorial ZnGeN₂ grown by RF co-sputtering. Spatially resolved characterization was performed in order to correlate structure with properties. X-ray fluorescence reveals cation compositions (given by x = Zn/(Zn+Ge) ) ranging from x=0.40 to x=0.65 and X-ray diffraction shows that films are phase pure and crystallize in the expected cation-disordered wurtzite structure. Pawley refinement of XRD data demonstrates that lattice constants shift up to 3% with cation composition. UV-visible spectroscopy was performed to determine absorption coefficient as a function of incident energy, and reveals an absorption onset shift from 2.8 eV to 2.1 eV with increasing Zn cation composition. These results suggest that large changes in properties are possible within the Zn-Ge-N materials space. While it remains to be determined how these changes will impact applications, this study re-affirms the complexity and potential of thin film ZnGeN₂ as a direct- and wide- bandgap optoelectronic material.

Nitride materials possess significant technological and industrial relevance owing both to their optoelectronic (e.g. GaN) and mechanical (e.g. TiN) properties. Due to the low chemical potential of diatomic nitrogen, many of the transition metal nitrides are found with metals in low oxidation states (Zr³⁺ in ZrN), and exhibit metallic behavior. Recent computational work in our group has revealed the stability of previously unreported semiconducting ternary nitrides with Mg-TM-N₂ (TM=Ti, Zr, Hf) stoichiometry. Electronic properties were predicted to change with the transition metal period, with indirect 0.9-1.8 eV bandgaps, direct 2.1-2.9 eV bandgaps, and light effective masses for both electrons (0.6-1.4 m_e) and holes (1.4-1.6 m_e). Synthesis of these compounds in thin film form was achieved by magnetron co-sputtering from metallic targets in the presence of nitrogen gas. Good agreement in structure and properties are observed between experiment and calculation. Furthermore, isostructural alloying of ternary Mg-TM-N towards the binary endmembers is possible by varying relative amounts of Mg and TM, as verified by shifts in lattice constant. This allows for tunable optoelectronic properties when moving from the stoichiometric compound - either increasing the bandgap and reducing the conductivity when made Mg-rich, or transitioning to metallic when made transition metal rich. These ternary nitride semiconductors are an exciting class of new materials with optical and electronic properties well aligned with the solar spectrum for optoelectronic energy conversion applications.

Development of p-type transparent conducting oxides (TCOs) is required for fabricating transparent devices based on p-n junction. However, it is difficult to realize the p-type TCOs with high hole mobility, because their valence band maximum (VBM) is composed of localized O 2p orbital. It is essential for high hole mobility to reduce the effective mass of holes by delocalizing VBM. SnO₂ is a typical n-type TCO where conduction band minimum (CBM) is composed of delocalized Sn 5s orbital. Considering high electron mobility of SnO₂, Sn²⁺ oxides with their VBM composed of Sn 5s orbital is expected to have high hole mobility. This approach had been demonstrated by SnO epitaxial thin films with the hole mobility of 2.4 cm²/Vs. But SnO has indirect band gap of 0.7 eV, showing non-negligible absorption of visible light. Moreover, high mobility is incompatible with high carrier density because the hole carriers are generated by Sn vacancies, which leads to destroy the carrier pathway. Thus, we consider the other Sn²⁺ oxides as new p-type TCOs, and succeeded in preparation of p-type Sn₂(Nb_2-xTa_x)O₇ (x=0-2) by solid state reaction. The band gap of Sn₂(Nb_2-xTa_x)O₇ is variable in the range from 2.4 eV to 3.0 eV by changing x. The mobility and density of hole carriers were estimated to be 0.28 to 1.9 [× 10^-1 cm²/Vs], and 0.20 to 1.40 [× 10¹⁸ /cm³], respectively. The lower mobility of our samples compared with SnO epitaxial thin film was considered to be due to the low relative density (~60 %) of the samples. Since no report on the p-type Sn₂M₂O₇ (M=Nb ,Ta) was found so far, it is considered that generation of hole carriers is difficult. Therefore, we also discussed the mechanism of hole carriers. Sn₂M₂O₇ has its crystal structure of pyrochlore, which is composed of Sn₂O tetrahedra and M₂O₆ ochtahedra. It is well known to have three kinds of defect; the vacancy of Sn (V_Sn^-2), the substitution of Sn⁴⁺ in M⁵⁺ site (Sn_M^-1), and the vacancy of O (V_O⁺²). From mutual relationship between the defect content and the electrical properties, we concluded that the hole carriers were generated by Sn_M^-1 in the M₂O₆ octahedra. Sn_M^-1 acting as the carrier generation center in M₂O₆ was separated spatially from Sn₂O tetrahedra acting as the hole carrier pathway. The separation is effective for reducing ionized impurity scattering, which is the advantage for realizing high hole mobility. Sn₂(Nb_2-xTa_x)O₇ was considered to be a new candidate for p-type TCOs.

Tin monoxide (SnO_x) has attracted much attention as a promising p-type oxide material. Its behavior is in contrast to SnO_2-x (4+), which is highly degenerated n-type semiconductor with high visible transparency by doping F, Sb, or Ta [1,2]. The p-n junctions can be easily realized between SnO_x and SnO_2-x by reactive sputtering, resulting in novel optoelectronic applications. Because p-type SnO_x films still has an obviously low conductivity compared with n-type SnO_2-x films, it is crucial to find new way to improve the conductivity of the p-type SnO_x films. However, the origin of p-type conductivity of SnO_x is still a subject of debate [3, 4], and lack of the direct experimental demonstration. In this study, we firstly deposited various SnO_x film with the highly precise control on the stoichiometric compositions, and then investigated the local structure of SnO_x film to determine the possible defect structure using the extend X-ray absorption fine structure (EXAFS).

In this study, p-type SnO_x films were deposited by reactive sputtering (RM400, FEP) using a Sn metal target, in which noble gases (Ar or Ne) were used as the sputtering gas and O₂ was used as the reactive gas. An impedance control system with plasma control unit was used to precisely adjust the inlet of oxygen, and hence control the oxidation state of target surface. With the impedance control system, each cathode voltage corresponded to only one value of the O₂ flow ratio, and the typical transition region in reactive sputtering disappeared. X-ray diffraction patterns showed that the p-type SnO films were successfully fabricated using such system. The best hall mobility and hall density of the film was 3.38 cm²/Vs and 1.12×10¹⁸ cm^-3, respectively, when Ar gas was used as the sputtering gas. Whereas, the best hall mobility and hall density of the films were 1.9 cm²/Vs and 1.05×10¹⁸ cm^-3, respectively, when Ne gas was used as the sputtering gas.

It is found that SnO_x films have the high hole mobility in a relatively reduced state. Such reduced state is considered to exist a lot of defects. In order to understand the relationship between the defect structure and the conduction mechanism in SnO_x, we deposited the p-type SnO_x films with the different carrier densities, and studied their defect structure by investigating the local structure around Sn atoms using EXAFS measurements. We discussed the relationship between the defect structure and the conduction mechanism, and determined the orign of p-type conductivity in SnO_x films.

Reference
[1] Y.Muto, et al., Thin Solid Films 520 (2011) 1178.
[2] Y.Muto, et al., Thin Solid Films 520 (2012) 3746.
[3] D. Granato, et al., Appl. Phys. Lett. 102 (2013) 212105.
[4] E. Fortunato, et al., Appl. Phys. Lett. 97 (2010) 052105.

The optoelectronic properties of transparent conducting oxides (TCOs) are linked to the presence of defects in the films. These defects may range from the nanometer to the atomic scale and can lead to reduction of the carrier mobility and optical transparency. Here we present an experimental and computational study of both the nature and influence on the optoelectronic properties of subgap defects in state-of-the-art amorphous zinc tin oxide (ZTO) and tin oxide (SnO₂) films deposited by sputtering. Based on this, passivation strategies are proposed to improve the optoelectronic properties of the TCOs.
The optoelectronic properties and microstructure of amorphous ZTO thin films were investigated as a function annealing temperature and atmosphere (oxygen-rich, or reducing) up to 500 °C. We demonstrate that the detrimental visible absorption centers in ZTO after deposition are suppressed by thermal treatments at temperatures > 400 °C in O-rich atmospheres. Defect suppression by oxygen intake results in an improvement of the mobility by 75%, up to 35 cm²/Vs as the number of ionized scattering centers is decreased. Conversely, annealing in a H₂ atmosphere > 400 °C increases both subgap absorption and free carrier concentration but decreases the mobility. Overall, the films retain their amorphous dense microstructure throughout these experiments, indicating that the changes in optoelectronic properties are linked to a change in atomic defect population rather than microstructure.
To investigate the influence of point defects on the optoelectronic properties, amorphous atomic structures having the same composition as the sputtered thin films were generated using molecular dynamics. The subsequent analysis of these structures using density functional theory , revealed that the presence of oxygen deficiencies (V_o) and atomic hydrogen within the amorphous network plays a crucial role in the resulting properties of ZTO. Undercoordinated Sn atoms that result from the presence of V_o may provide free carriers but generate absorptance centers in the visible spectra and limit the electron mobility. In accordance with experimental data, defect passivation by adding oxygen atoms in the simulated structure removes these absorption centers. Furthermore, the addition of hydrogen in the vicinity of V_o shifts the associated states deeper into the bandgap.
To reduce the temperature at which defects are passivated and match the thermal requirement of several devices, we demonstrate that co-sputtering either ZTO or SnO₂ together with SiO₂ also decreases subgap absorption. Interestingly, for films that contain 2.0 wt% SiO₂, the decrease in absorptance does not affect the electrical properties of the TCO. Co-sputtering Sn-based TCOs with SiO₂ is hence an effective strategy to passivate subgap defects even at temperatures < 200°C, an effect that could not be reached by tuning the oxygen flow during deposition or by annealing in an O-rich atmosphere at these temperatures.

P-type transparent conductors are desirable for applications in optoelectronic devices like solar cells and light emitting displays. However, semiconductors that combines high transparency and good p-type conductivity are limited to a few categories, most of which are prepared at above 500 °C. Previously BaCu₂S₂ has been proven to be a potential p-type transparent conductor that can be prepared at relatively low temperatures. However, little is known about the influence of chemical composition and processing temperature on the BaCu₂S₂ structure and properties. Also, not much is known about the properties of other barium copper sulfides, like BaCu₄S₃, and about the corresponding Sr-based materials. Here, we investigated the structural, optical and electrical properties of Barium Copper Sulfide compounds using combinatorial method [1]. It turns out that phase-pure BaCu₂S₂ thin films can be prepared from 200 to 420 °C, while BaCu₄S₃ thin films can be prepared from 200 to 250 °C substrate temperature by co-sputtering from BaS and Cu₂S targets. These barium copper sulfide compounds show a phase transition from BaCu₂S₂ to BaCu₄S₃ with the increase of Cu/(Ba+Cu) ratio. The optimized transparency (corrected by reflectance) reaches 90% from 600 to 1000 nm for BaCu₂S₂ and and 90% from 650 to 1000 nm for BaCu₄S₃. BaCu₂S₂ shows a wider bandgap than BaCu₄S₃ under the same processing temperatures. The conductivity of sputtered BaCu₂S₂ and BaCu₄S₃ thin films is improved to 53 S/cm (at 250 °C substrate temperature) and 74 S/cm (at 200 °C substrate temperature), compared to their solution/bulk counterparts reported in literature. The conductivity of BaCu₄S₃ is not sensitive to the processing temperatures, but the conductivity of BaCu₂S₂ drops down sharply at above 380 °C. At the same time, the Cu/(Ba+Cu) ratio also strongly influences the conductivity near the BaCu₂S₂ composition at higher substrate temperatures. The synthesis attempts of analogous strontium copper sulfides resulted in phases-separated material, and these results were used to put an upper limit on their formation enthalpies. In summary, the excellent optical and electrical performance and reasonably low thermal budget pave the path for Barium Copper Sulfides as p-type transparent conductors in solar cells and other optoelectronic devices.

[1] Y. Han, S. Siol, Q. Zhang, A. Zakutayev, Chemistry of Materials (2017), DOI: 10.1021/acs.chemmater.7b02475

The energy positions of the valence and conduction electronic states with respect to the vacuum level are essential parameters to evaluate how the band gaps of semiconductors or Fermi-levels of metals line up with respect to each other. Such electronic structures of materials can be determined using photoemission spectroscopy (PES). PES measurements, however, remain challenging for inhomogeneous materials with nano- to micrometer lateral dimensions due to its mesoscopic probing area, typically no less than several microns. Photoemission electron microscopy (PEEM) is a cathode lens electron microscopy technique that combines photoemission imaging with spectroscopic modes of operation to provide photoemission spectra from areas less than one micron in size. Here, we present PEEM studies of the electronic structure of polycrystalline cadmium telluride (CdTe) thin films, a test case to examine the applicability of this new microscopic approach to photovoltaic materials. Post-deposition CdCl₂ treatment of polycrystalline CdTe is known to increase photovoltaic efficiency. However, the precise chemical, structural, and electronic changes that underpin this improvement are still debated. In this study, spectroscopic PEEM was used to spatially map the vacuum level and ionization energy of CdTe films, enabling the identification of electronic structure variations between grains and grain boundaries. In vacuo preparation and inert transfer of oxide-free CdTe surfaces isolated the separate effects of CdCl₂ treatment and ambient oxygen exposure. Qualitatively, grain boundaries displayed lower work function and downward band bending relative to grain interiors, but only after air exposure of CdCl₂-treated CdTe. This study highlights the importance of probing the spatially varying electronic structure, elucidating the concurrent impacts of processing steps (CdCl₂ treatment and oxygen exposure) to develop a comprehensive picture of local electronic structure in this inhomogeneous semiconductor.

The PEEM work was performed at the Center for Integrated Nanotechnologies, an Office of Science User Facility (DE-AC04-94AL85000). M. B. and T. O. was supported by the CINT user program and Sandia LDRD. M. B. and C. C. were supported by a U.S. DOE-EERE SunShot BRIDGE award (DE-FOA-0000654 CPS25859). Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

The search of new materials and semiconductors for optoelectronic applications is a challenging task and a growing activity exists in that field. Among the various criteria to be fulfilled for new materials, environmental aspects are very important as well as the use of non-toxic and highly recyclable elements.
For terawatt-scale energy needs the electrical energy supply will certainly go toward an energy mix. Photovoltaics could then be the backbone of such a renewable energy system. To fulfil the challenging price targets and to develop new markets the design and production of solar cells working at highest efficiencies is essential all by fulfil the green and sustainable aspects. Different ways exist for the PV devices improvement such as tandem cells. Nevertheless, those technologies require the use of scarce materials such as indium and gallium. Moreover, III-V tandem cells mainly require up to now the use of epitaxial growth that remain both expensive and limiting for large scale realization. Other technologies are also studied such as new quaternary thin films but making cells can require the use of toxic elements (such as cadmium) so their use and acceptance for large scale applications is therefore limited.
In the field of lighting devices, LEDs are will replace the classic sources of light because of their good efficiency. Here again, the material used contain indium and the current technology must be improved to overcome the "green gap" restriction.
I will describe few results of study of new materials that could be used for both PV and lighting technologies. It could be for example nitride materials of the Zn-IV-N2 family or oxides and nanomaterial used as new absorber for solar cells or as emitting materials.
For example, ZnSnN2 contain abundant and non-toxic elements. Very recently, it has been shown that ZnSnN2 is a promising n-type material for photovoltaic applications [1]. Since this nitride has been scarcely studied, it is necessary to continue research activities to optimize its functional properties, more especially on silicon. Moreover, adding germanium could also be of great interest for LED applications. Other results about (nano)materials such as oxide will also be described.
1] A. N. Fioretti at al., Advanced Electron. Mater. 1600544 (2017)

Research highlighted in this abstract aims to provide a clean, cost-effective, and locally produced solar fuel by using electroactive, two-dimensional (2D) layered transition metal nitrides (MXenes) photoelectrodes. Theoretical reports have shown that these materials, although inherently metallic, can become semiconductors via surface functionalization, and can be used as photoelectrodes for solar fuel production. Results highlighted in this presentation revealed the physical and photoelectrochemical properties of Ti₄N₃T_x and Ti₂NT_x nitrides with surface functional groups (T_x = O, OH, F). The materials showed excellent photoelectrochemical properties (i.e. solar-to-hydrogen efficiency) and good stability in aqueous media. Ti₄N₃T_x and Ti₂NT_x nitrides were synthesized via molten salt fluoride treatment of their corresponding Ti₄AlN₃ and Ti₂AlN precursors. The phase purity, crystal structure, and removal of Al from the precursors were confirmed by X-ray diffraction and X-ray photoelectron analyses. Scanning electron microscopy and high-resolution transmission electron microscopy were used to access the morphologies of the layers (multilayers and single layers). Both materials showed an order of magnitude increase in surface area, and more importantly, exhibited semiconductor behavior as indicated by the absorption and emission data. Ti₄N₃T_x exhibited two bang gaps located at ~1.9 eV and ~3.1 eV. These were attributed to different layer thicknesses. We hypothesized that the higher energy band gap derived from single or few layers, while the lower band gap is due to multiple layers. This hypothesis was further validated with Ti₂NT_x material. When delaminated with dimethyl sulfoxide for half and hour, the energy band gap in Ti₂NT_x was increased from ~3.1 eV to ~3.6 eV as corroborated by a blue shift in the absorption spectrum to lower wavelength. The change in the properties of Ti₄N₃T_x and Ti₂NT_x from metal to semiconductor was attributed to surface functional groups (T_x = O, OH, F) as supported by the Fourier-transform infrared spectroscopy results.

ZnO nanowire (NWs) based core-shell heterostructures have been attracting considerable attention for optoelectronic devices owing to efficient light trapping and charge carrier management.¹ These type-II heterostructures have recently been integrated into novel self-powered nanoscale UV photodetectors² by combining wide band gap p-type semiconducting shells, such as CuSCN³, with ZnO NWs. These devices benefit from the photovoltaic effect in the UV region to operate at 0 V bias. Among the delafossite group, CuCrO₂ is a promising direct wide band gap p-type semiconductor^4,5 (2.95-3.30 eV) reaching conductivities as high as 217 S.cm^-1 through Mg doping.⁵ It has previously been integrated into p-n diodes with ZnO thin films, but with relatively low rectifying behavior.⁶

In this work, we present the fabrication of an original ZnO / CuCrO₂ core-shell NW heterostructure and its integration into efficient self-powered UV photodetectors, using scalable chemical approaches. In particular, the ZnO NW arrays are grown by the low-cost, low temperature chemical bath deposition technique on commercial ITO/glass substrates, while the CuCrO₂ shell is deposited by aerosol-assisted chemical vapor deposition at 400°C.

A 35 nm-thick CuCrO₂ shell with high conformity and uniformity has been deposited on the ZnO NWs. The structural morphology of the CuCrO₂ grains and their composition have been characterized by automated crystal phase and orientation mapping with precession (ASTAR) in a transmission electron microscope as well as by energy-dispersive x-ray spectroscopy, revealing a columnar grain growth at the top of the ZnO NWs, while smaller nano-grains are located on their vertical sidewalls.

The ZnO / CuCrO₂ core-shell NW heterostructure device shows a high rectification ratio up to 5500 at ±1 V, and high absorption above 85% in the UV region. The attractive responsivity, response (rise/decay) times and photovoltaic performances of the device, performing as a UV photodetector at 0 V bias, are also measured. The fabrication of devices using ZnO NWs covered with a semiconducting copper-based compound as a shell with low-cost surface scalable chemical fabrication routes at moderate temperatures establishes the ZnO / CuCrO₂ core-shell NW heterostructures as a promising cost-efficient, all oxide self-powered UV photodetector.

¹E.C. Garnett et al. Annual Review of Materials Research 41 (2011), 269-295
²L. Su et al. Small DOI: 10.1002/smll.201701687 (2017), 1701687
³J. Garnier et al. ACS Appl. Mater. Interfaces 7 (2015), 5820−5829
⁴D.O. Scanlon and G.W. Watson J. Mater. Chem. 21 (2011), 3655
⁵T.S. Tripathi and M. Karppinen, Adv. Electron. Mater. 3 (2017), 1600341
⁶L. F. Chen et al. Jpn. J. Appl. Phys. 52 (2013), 05EC02

Achieving efficient solar capture and optical emission requires the ability to distinguish between desired and unwanted wavelengths, which is known as spectral selectivity. Particularly for semiconductor materials operating above ambient, many of these properties can be temperature dependent. At high enough temperatures, spectral selectivity can be degraded greatly through multiple mechanisms. This gives rise to a need for developing new materials less susceptible to spectral degradation. At the same time, it is preferred that they be earth-abundant to provide scalable solutions to solve energy generation and energy efficiency challenges at a global scale.

To improve spectral selectivity, an earth-abundant thin-film semiconductor-metal tandem structure based on thin-film single-crystalline silicon and silicon nitride (Si₃N₄) is designed and tested here. It is first modeled, studied experimentally at high temperatures in a custom-designed test chamber, and then evaluated for applications in selective solar absorption and efficient optical sources. The thin-film Si with 10-20 μm thickness is fabricated through wet etching on commercial single-crystalline Si wafers. Earth abundant Si₃N₄ is then sputtered on top as an anti-reflection coating. Characterization in visible-near infrared (IR) shows that Si₃N₄ can strongly enhance above-bandgap absorption and therefore the spectral selectivity. At high temperatures, the emittance spectra measured by FTIR show significantly lower parasitic emission compared with a wafer based absorber. For a Si wafer based absorber at 535°C, the spectral-average solar absorptance α_ave is 0.72 and the spectral-average emittance ε_ave is 0.60. At a higher temperature (595°C), the thin-film selective absorber has a similar α_ave of 0.75 and a lower ε_ave of only 0.24. The physical basis of the improvement is that the reduced thickness of Si lowers the intrinsic carrier concentration per unit area. Furthermore, these structures can survive repeated thermal cycling with negligible loss of performance. In addition, single-crystalline Si based thin films exhibit strong mechanical flexibility, allowing conformal coating on various surfaces in real applications. Such properties make these earth-abundant material structures attractive candidates for high-temperature solar thermal, efficient incandescent lighting, infrared sources, and other applications.

The intermediate band (IB) photovoltaic concept has the potential for large efficiency gains beyond single gap solar cells, but the development of suitable materials has so far proved challenging. One route to form a mid-gap band for sub-band gap absorption is to dope a wide gap semiconductor with a suitable dopant at high concentration. A proposed strategy to limit non-radiative recombination is to co-dope with both n-type and p-type species, effectively passivating the recombination sites. Additionally, incorporation of both types of dopant is thought to increase the solubility limits.

TiO₂ is earth abundant and non-toxic, and anatase in particular has a wide band gap and high photocatalytic response, making it a promising candidate for an IB host. Here we explore chromium nitrogen co-doping of TiO₂ using a computational approach to evaluate the electronic and optical properties. Density functional theory is used to calculate the effect of Cr and N substitution on the anatase and rutile phases of TiO₂, using hybrid functionals to replicate experimental band gaps. Transition level diagrams are constructed to give an overview of the defect physics. The density of states and band structure are calculated across a range of dopant concentrations. Optical properties are examined by calculation of the dielectric function and absorption spectra, with local field effects included beyond the random phase approximation (RPA). Finally, the relative stabilities of the doped anatase and rutile phases are considered by including high-order correlations in the adiabatic connection fluctuation–dissipation theory with RPA.

This work provides a fundamental understanding of the effect of Cr-N co-doping on the optoelectronic properties of TiO₂ and further to the viability of similar co-doping schemes to deliver an IB material.