Symposium F.EL06—Contacting Materials and Interfaces for Optoelectronic Devices

Native defects in semiconductors are known to play an important role in determining their optoelectronic properties. Nickel oxide (NiO) is one of the few wide gap oxide semiconductors broadly applied as the hole transport layer in many optoelectronic devices. NiO exhibits p-type conductivity and its conductivity is believed to be controlled by Ni vacancy acceptors. Herein, we present a systematic study comparing the optical and electronic properties of p-type oxygen-rich NiO with Ni vacancies (NiO:V_Ni) and insulating Ni-rich NiO with O vacancies (NiO:V_O) and stoichiometric NiO. The optical properties were obtained by spectroscopic ellipsometry while valence band spectra were probed by high-resolution X-ray photoelectron spectroscopy. The experimental results are directly compared to first-principles DFT + U calculations. The absorption spectra for the NiO:V_Ni sample show significant defect-induced features below 3.0 eV compared to NiO, and NiO:V_O samples. The increase in sub-gap absorptions in the NiO:V_Ni can be attributed to gap states observed in the electronic density of states. Computational results confirm that gap states in NiO:Vo are predominantly Ni 3d states while those of NiO:V_Ni is composed of both the Ni 3d and O 2p states. This work reveals the relations between native vacancy defects and optical properties of NiO and provides insights that would aid in designing optoelectronic devices utilizing NiO with native defects.

Organohalide lead (hybrid) perovskites have emerged as competitive semiconducting materials for photovoltaic devices due to their high performance and low cost. To further the understanding and optimization of these materials, solution-based methods for interrogating and modifying perovskite thin films are needed. In this work, we report a hydrofluoroether (HFE) solvent-based electrolyte for electrochemical processing and characterization of organic–inorganic trihalide lead perovskite thin films. Organic perovskite films are soluble in most of the polar organic solvents, and thus until now, they were not considered suitable for electrochemical processing. We have enabled electrochemical characterization and demonstrated a processing toolset for these materials utilizing highly fluorinated electrolytes based on a HFE solvent. Our results show that chemically orthogonal electrolytes based on HFE solvents do not dissolve organic perovskite films and thus allow electrochemical characterization of the electronic structure, investigation of charge transport properties, and potential electrochemical doping of the films with in situ diagnostic capabilities

NiO_x is a transparent conductive oxide (TCO) that has attracted a lot of attention because of its electrical and optical properties. Most of the available TCOs are n-type semiconductors, while the development of efficient p-type TCOs remains a challenge. NiO_x is one of the promising p-type candidates, because of its superior chemical stability and optical transparency. Some potential optoelectronic applications of NiO_x are as a p-type channel in transparent thin film transistors (TFTs) and as a hole transport layer in perovskite solar cells. In this work thin NiO_x films were obtained by Pulsed Laser Deposition (PLD), d.c. sputtering, e-beam evaporation and sol-gel techniques.
All films were deposited on glass substrates with a size of 2×2 cm. The PLD films were deposited from a NiO target at room temperature. D.C sputtering and e-beam evaporation were used to deposit pure Ni films, which were then oxidized in air at various temperatures. The sol-gel NiO_x was obtained by depositing chemical precursors and subsequent thermal treatment. Various parameters were varied in the experiments and it was found that the structural, optical and electrical properties of the PLD films depend strongly on the oxygen pressure. The most important parameter for the other three types of oxides was the annealing temperature.
All samples were characterized by UV-Vis spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy. Results for the properties of NiO_x films deposited by the above techniques will be presented and a discussion for their possible application in TFTs and solar cells will be given.

Colloidal quantum dots (CQDs) are promising semiconducting materials to engineer photovoltaic and optoelectronic devices due to tunable size-dependent absorption and emission properties. Great efforts have been made to find suitable surface ligand to reduce spacing between the adjacent quantum dots to ease charge transport but there has been negligible attention given to purification strategies and systematic efforts to find the ideal ligand populations to optimize device performance. Charge transport could be hindered in the presence of excess free ligands forming interparticle barriers; on the contrary, an under-passivated QD can have high density of trap states and could also undermine the nanocrystal’s stability toward coalescence. Precipitation and redissolution (PR) methods are widely used purification techniques that must typically be repeated to get clean CQDs with appropriate populations of surface-bound molecules (ligands). PR often results in loss of native ligands bound to the surface which induces more surface traps and/or compromise the chemical stability of these nanocrystals.
Gel permeation chromatography (GPC) has been proven to be an effective technique to remove the byproducts and impurities from CQDs without damaging the QDs surface. In this study, we demonstrate the effectiveness of GPC in removing byproducts and unbound ligands from pre and post ligand exchanged PbS CQDs.
NMR and UV-Vis-NIR spectroscopy of oleate-capped PbS quantum dots shows that despite removal of unbound surfactants, PbS QDs remain stable in solution following GPC. These purified nanocrystals were used to made CQDs photodetectors and photovoltaic devices.
We show that films fabricated using GPC purified PbS QD solutions exhibit higher charge transport, and low contact resistance, as investigated through in-plane conductivity and photoresponse. The charge carrier mobility was extracted from the mobility lifetime product and recombination lifetime determined by the spatial profiles of the photocurrent obtained through scanning photocurrent microscopy (SPCM), and transient photocurrent measurements, respectively. Furthermore, our findings show that post synthetic purification and optimum ligand population can significantly improve physical characteristics including reduced surface roughness, suggesting that impurities and optimum ligand coverage play an important role in film morphology and device performance.

Dislocations in III-Nitrides constitute a long standing controversial topic. The lattice and thermal mismatch associated with the heteroepitaxy of GaN results in high dislocation densities in the order of 10⁸ to 10¹⁰ cm^-2. Although III-Nitride optoelectronic devices can operate even at these high dislocation densities, recent investigations indicate that c-type screw dislocations may have a detrimental effect on the properties and reliability of high-power electronic as well as of optoelectronic devices: For example, the observed high reverse (forward) leakage current in electronic (optolectronic) devices is attributed to screw type dislocations [1]. Recently, it has been proposed that these defects are associated with open core structures, thus forming nanopipes with diameters of a few nms [2]. However, imaging screw dislocations at atomic resolution by electron microscopy to validate this model is not trivial. Moreover, first principles calculations that can accurately describe the energetics and atomic and electronic structure of these defects, have been restricted to MBE growth and/or did not explicitly consider the highly strained core region of these defects [3].

In the present study we combine density functional theory (DFT) with large scale empirical potential calculations within an Implicit Boundaries Mutliscale Scheme calculations [4] to investigate the energetics, atomic structure and electronic properties of c-type screw dislocations in GaN. We explicitly consider both closed and open core screw dislocations as well as more than 100 different core reconstructions. Based on these calculations we construct a c-type screw dislocation phase diagram which describes the energetically most favorable core structures as function of the Ga, N and H chemical potentials. Our calculations indicate, in agreement with previous DFT studies, that growth under typical MBE growth conditions, i.e. metal rich growth, that these defects stabilize a Ga filled core. Moreover, under N rich conditions the core is stoichiometric, though a column of Ga-N dimers is removed form the core. However, the energy of the free m-plane surfaces under MBE conditions cannot be compensated by the core and strain energy and hence nanopipe formation is highly unfavorable. In contrast, under typical MOCVD or MOVPE growth conditions the Ga and N dangling bonds at the open core inner surfaces are passivated by NH_x molecules and H atoms, respectively. This reduces considerably the surface energy under these conditions enabling the formation of nanopipes with hexagonal cross section and diameters ranging for ≈1 to ≈2 nm. Our calculations show that these nanopipes obey electron counting rule and hence are semiconducting. Nevertheless, they have important implications on the efficient diffusion and/or incorporation of impurities and/or dopants at the side facets and at the neighborhood of the open core, which will be discussed in detail. We will also discuss in detail the energetics and structure of superscrew dislocations and their implications on the diameter of the nanopipes as well as the potential implications of the nanopipes on the properties and the efficiency of the electronic devices.

[1] K. Nomoto et al., IEEE Electron Device Lett. 37, 161 (2016).
[2] S. Usami et al., Appl. Phys. Lett. 112, 182106 (2018).
[3] J. E. Northrup, Appl. Phys. Lett. 78, 2288 (2001).
[4] L. Lymperakis et al., Phys. Rev. Lett. Phys. Rev. Lett. 93, 196401-1 (2004).

This project has received funding from the ECSEL Joint Undertaking (JU) project UltimateGaN under grant agreement No 826392. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Austria, Belgium, Germany, Italy, Slovakia, Spain, Sweden, Norway, Switzerland.

Transparent electronics and optoelectronics have been garnering increasing attention for use in high performance devices such as next generation displays, photovoltaics, electrochromic devices and sensors. Metal oxides are considered to be necessary components in these applications due to their excellent optical transparency ranging from visible to near infrared regime as well as pseudo-metallic electrical conductivity[1, 2]. These metal oxides have been implemented in devices, depending on their electrical conductivity, as transparent electrodes by degenerately-doped oxides such as InSnO and doped ZnO; semiconducting active layers based on indium oxides, ZnO or SnO₂; and insulators (e.g., SiO₂, Al₂O₃, HfO₂) for dielectrics or encapsulations.

Thin film transistors (TFTs) are one of the key elements in displays as a pixel driving switch of which the required properties include high carrier mobility, low thermal budget processing, structural phase stability and reliable device performance in the presence of thermal and bias stress conditions. In recent years, amorphous oxide semiconductors based on indium oxides have been considerably investigated due to their promising carrier mobility, >5-25 cm²/Vs, compared to conventional amorphous Si (<1 cm²/Vs); low temperature processing; mechanical flexibility compared to the crystalline counterparts; and large area process-ability. Indium oxide-based binary and ternary cation material systems have shown tremendous promises for next generation displays since these materials meet the property and process needs that are described above. A binary cation material of indium zinc oxides (IZO) has been considered since the addition of Zn into In₂O₃ stabilizes the amorphous phase of In₂O₃ while the amorphous phase of undoped In₂O₃ is rapidly crystallized at very low homologous temperatures (T/T_m<0.19) at 150 °C [3, 4]. The reported carrier mobility of IZO is as high as 20-50 cm²/Vs for both Hall and TFT field effect mobilities. A ternary cation system of InGaZnO (IGZO) [5, 6] has been more popular than binary systems since the addition of Ga in IGZO suppresses the channel carrier density in TFT application, which is preferred for TFT devices to obtain low device off-state current. Other third cation species such as Hf, Si and Zr were implemented. However, the carrier mobility of those ternary cation systems is significantly lower, 3-10 cm²/Vs, which is 3-10 fold lower than that of In₂O₃ or IZO.

Therefore, securing strategies to develop a material system that maintains high carrier mobility (>20 cm²/Vs) and suppresses carrier generation for TFT channel application is of significant importance, which is expected to expedite the realization of next-generation transparent displays exhibiting reliable performance, fast switching speed and, consequently ultra high definition resolution.

Here, we report on a ternary cation material system of InAlZnO (IAZO) where the optimized carrier mobility is as high as 25-45 cm²/Vs. Further, the IAZO demonstrates an enhanced capability of stabilizing the amorphous phase during thermal stresses compared to InZnO. The effect of Al incorporation on the structural, electrical, optoelectronic and chemical properties of IAZO is systemically investigated and compared to those of IZO. The carrier transport behaviors (ionized impurity scattering; neutral scattering; charge screening) as well as the dominant mechanisms for carrier density and resistivity are discussed. The high mobility and greater phase stability may lead to high performance and sustainable device applications in next generation electronic and optoelectronic devices such as transparent displays.

References
[1]Lewis et al., MRS Bull., 25 (2000) 22-27.
[2] Coutts et al., MRS Bull., 25 (2000) 58-65.
[3] Paine et al., 85 (1999) 8445-8450.
[4] Lee et al., Journal of Materials Research, 27 (2012) 2299-2308.
[5] Nomura et al.,Nature, 432 (2004) 488-492.
[6] Hosono, Journal of Non-Crystalline Solids, 352 (2006) 851-858.

Indium nitride (InN) was reported to have a large optical bleaching effect ^[1], which provides potential application as high-speed and high-frequency optical switches in the optical communications. Due to its narrow direct bandgap (~0.7 eV), the bandgap engineering with GaN allows the ternary InGaN system to cover a range of bandgap energies from the infrared of InN to the ultraviolent of GaN. Such wide bandgap tuning provides spectral flexibility for InGaN as optical switches. In order to materialize the potential of InGaN for optical device applications, we investigated the optical bleaching behavior and ultrafast carrier dynamics in epitaxial InN and InGaN films, and proposed a phenomenological model to explain the carrier relaxation behavior.

InN and InGaN films with the thickness of 300 nm were deposited on c-sapphire substrates by molecular beam epitaxy. The optical bleaching effect and ultrafast carrier dynamics in InN-based epitaxial films was investigated by using femtosecond time-resolved pump-probe transmittance/reflectance measurements with a photon energy of 1.50 eV, which is based on open-aperture Z-scan technique.

Both epitaxial InN and InGaN films show large optical bleaching effect from Z-scan measurements, where a nearly 3 times increase in transmittance was observed at 800 nm without the dependence of the film composition. The time-resolved pump-probe transmittance results indicate that hot carriers are relaxed within a time scale of subpicosecond. The carrier lifetime was found to be independent of the film composition and the fabrication condition. The maximum value in the transient transmittance shows a nonlinear increase with increasing the pump-beam power, which may be associated to a change in the nonlinear refractive index. The detailed mechanism on the carrier relaxation behavior will be given in the conference.

This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) (Grant No. 20K05368).

[1] V. Pacebutas, G. Aleksejenko, and A. Krotkus, Appl. Phys. Lett. 88, 191109 (2006).

The low open-circuit voltage V_OC of the earth-abundant kesterite Cu₂ZnSnS(Se)₄ (CZTSSe) solar absorbers is often associated with the large band tails, the origin of which has been tentatively assigned to the Cu-Zn disorder. Such an assignment has recently been questioned by some experiments. Here, by using the cluster expansion method and first-principles calculations, we investigate the role of cation disorder in the band tailing of CZTS. Our results show that the formation of local Cu-Cu-Cu-Sn tetrahedral motif as a result of the disorder is responsible for the band-gap narrowing, leading to a redshift of optical absorption onset by up to 0.2 eV. The Urbach energy, however, is hardly modified by the cation disorder, thereby excluding Cu-Zn disorder as a main source of the large band tails observed in stoichiometric CZTS. On the other hand, we find that extensive Cu-Zn disorder could stabilize the formation of point defects to a large extent. In particular, the defect concentration of Sn_Cu can be as high as 10¹² cm^-3 under typical Cu-poor growth conditions. The defect level is found at 0.2 eV above the VBM, suggesting that Sn_Cu could be a hole trap. Sn_Zn remains the predominant deep donor with defect levels near the midgap and hence is responsible for nonradiative recombination. We show that Sn antisites could be the main culprit of the low V_OC and the persistence of the two donors in the presence of cation disorder presents a major challenge towards a high-efficiency CZTS absorber.

Tin(IV) oxide (SnO₂) is an n-type wide bandgap semiconductor, which is widely used as an electron transport layer (ETL) in organometal perovskite solar cells (PSCs) due to its promising properties such as high transmittance, high electron mobility, good photostability, good energy alignment and easy processing. In this work, SnO₂ nanostructures were synthesized by a solvothermal technique. The impacts of each growth parameter for the SnO₂ nanostructures were demonstrated. By investigating the relations among different growth conditions, optimized recipes were developed for SnO₂ nanostructured ETLs employing in PSCs. Furthermore, the interfaces between the FTO/ETL and ETL/perovskite are optimized in order to enhance the charge extraction efficiency of the devices. Detailed characterizations such as scanning electron microscopy, electrochemical impedance spectroscopy, atomic force microscopy spectroscopy, X-ray diffraction, I-V characteristics etc. have been performed on the SnO₂ ETL alone_, material interfaces and the SnO₂ nanostructures-based PSCs. The obtained results provide a valuable insight into the community for preparing desired PSCs with nanostructured ETL.

Since their introduction in the early 1990s, subwavelength grating guided-mode resonance (GMR) filters have attracted considerable attention due to their simple and compact structures and versatile spectral characteristics. These outstanding features allow miniaturization of filters to pixel scale and high-density of integration for visible and short-wave infrared (SWIR) image sensors where a mosaic of transmission filters of different resonances can produce selective spectral signals to individual pixels of focal plane arrays. However, the conventional GMR-based transmission filter design, essentially unchanged for more than two decades, remains as a limiting factor for practical implementation of such an array of filters on the same substrate. The conventional design based on a single-period grating necessarily involves a homogeneous layer (of subwavelength thickness) underneath a subwavelength grating of a dielectric material to achieve a narrow transmission peak and a wide stop-band. Maintaining the other spectral characteristics, the transmission peak can then be tuned to the desired wavelength by scaling all the dimensions including the grating and homogeneous layer heights. However, to integrate different filters into a compact mosaic, this conventional design requires either different substrates of corresponding thicknesses or a very complicated non-planar fabrication process to use a single wafer leading to low throughput. In this work, we demonstrate a dual-period design to eliminate this issue by fetching additional design flexibilities. First, the dual-period filters possess two fill-fractions and thus unlike their single-period counterparts allow more degrees of freedom to vary the lateral profile parameters (e.g., fill-fractions and periods) for producing desired resonances at different wavelengths as well as to tune a resonance while keeping the grating height constant. Second, to produce a narrow passband with a wide stopband, these filters, unlike the single-period counterparts, do not require a homogeneous layer, allowing a planar fabrication process for integration of different filters on the same substrate. Moreover, dual-period filters show less sensitivity to the fabrication deviations such as sidewall roughness and to those related to grating and/or homogeneous layer heights where the latter, especially, remains as another limiting problem for the conventional narrow passband single-period based filters. For example, a small variation in grating and/or homogeneous layer thickness (while other parameters remain constant) causes the resonance peaks to disappear or be suppressed significantly and thus the fabrication requires very high precision. In this work, we report dual-period GMR filters of dimensions smaller than on silicon-on-quartz (SOQ) wafers using conventional electron beam lithography (EBL) and reactive ion etching (RIE). The filters demonstrate efficiencies as high as 80% at the peak transmission wavelength, e.g. 1550 nm. The passbands are as small as 15 nm with over 350-nm-wide stopband. The spectra of the fabricated filters are in a good agreement with the results of simulations using the finite element method (FEM) in COMSOL Multiphysics® software. Tunability of narrow passband resonances has also been achieved by only varying the fill-factors and/or period while keeping the substrate thickness constant. An array of four filters (all based on dual-period) have been demonstrated on same substrate to generate well-separated passbands in the SWIR region (1.2-1.6 μm ) to work as a pixel. While smaller filter size would allow integration into compact chips, filters should be sufficiently spaced to eliminate the potential cross-talk. In order to produce an optimum pixel size to work in the concerned SWIR region, any potential cross-talk along with filter size effects are also analyzed theoretically and experimentally.

III-V compound solar cells have been considered as the leading photovoltaic devices due to their superior properties including the high carrier mobility, direct bandgap, and controllable bandgap against the solar spectrum. The extremely high solar-to-electric power conversion efficiency can be achieved with III-V solar cells by forming monolithic multi-junction structures through series processes of the epitaxial growth. Nevertheless, their prohibitively high materials and process (i.e. epitaxial growth) cost has limited the application to terrestrial photovoltaic systems. To reduce the cost per watts of III-V solar cells, several approaches have been suggested, such as transfer-printing-assisted assembly for re-using the expensive III-V substrate and solar light concentrator for boosting the power generation in a given cell area. A format of III-V solar cell arrays transfer-printed within the luminescent solar concentrator (LSC) has been particularly of interest due to the potential to mitigate the materials cost and simultaneously to augment the power generation. In spite of the successful demonstration with single-junction (e.g. GaAs) solar cells, the effect of power generation improvement by the LSC system cannot be simply achieved with multi-junction solar cells, because the balanced enhancement of photon absorption at each subcell is required for increasing overall photocurrent of series-connected multi-junction cells. In this regard, here we present a facile route to integrate the transfer-printed multi-junction III-V solar cells with the LSC medium for significantly raising their power generation. Monolithically grown indium gallium phosphide (InGaP)/gallium arsenide (GaAs) double-junction solar cells are completed implanted into a LSC printing medium by the transfer-printing-based fabrication assembly, where luminescent quantum dots (QDs) with the 610 nm emission wavelength are non-uniformly distributed in the LSC. Since the short-circuit current of InGaP/GaAs double-junction cells is limited by a level of photocurrent produced at the InGaP subcell, the LSC is designed for the emitted 610 nm photons of QDs to be preferentially guided into the InGaP subcell. At the optimal LSC configuration, the power generation of 26%-efficient InGaP/GaAs solar cells can be considerably improved by ~50% compared to the identical cells but without add of the LSC.

There has been a recent trend to use molecular dipole layers as a part of a novel
strategy to fabricate selective contacts. Conceptually, a thin film of oriented
molecular dipoles mimics a 2D capacitor, and the build-in voltage within the layer
affects the final contact equilibrium state. When the dipoles are oriented in a
certain direction one achieves larger accumulation of either holes or electrons by
modifying the workfunction of the metal. Amongst many of the dipols used amino
acids have particularly interesiting properties, for instance easy deposition by PVD
methods such as spincoating and thermal evaporation and strong molecular dipole
moment due to the presence within the molecule of a negatively charged
carboxylic group and a positively charged amino group.
We tested the effect on the contact resistance of a Silicon/dipole/Aluminium stack
using Transfer Length Method and testing different amino acids as the dipole
layers (e.g. Cysteine, Glycine, Histidine...).
Preliminary results show an enhanced contact resistivity of orders of magnitude
when dipoles are used on both p and n type silicon wafers, circumventing the
problematic Fermi level pinning in Silicon aluminium as deposited contacts.
Finally, experiments on Histidine using different passivating layers show a clear
difference in contact resistance with the change of the substrate dopant. Probably
indicating that macroscopic orientation of the dipole layer can be promoted when
using proper seed layers.

For many of their applications, 2D materials must be connected to the macroscale world, implying interfaces between the 2D atomic layer and a 3D bulk material. The structure and defects at the interface between a 2D material and a 3D thin film influence (opto-)electronic properties of nanoscale devices such as contact resistance, photo-response and high frequency performance [1]. If the interface is between a 2D material and a 3D nanostructure, the ability to control the nanostructure geometry and lateral positioning is important in enabling nano-optoelectronic applications such as plasmonic arrays or arrays of wires or lines in individual circuits interfaced with 2D materials. When 3D nanoislands are grown on conventional, bulk substrates, “directed self-assembly” is a useful approach where growth takes place spontaneously but is favored at particular nucleation sites. The control of nucleation sites involves control of surface topography, geometry and local interactions [2]. Here we focus on extending this approach to the case of 3D nanoislands on 2D surfaces. We create specific defect locations on 2D materials (graphene, MoS2, WSe2) that act as heterogeneous nucleation sites for 3D metals (Au, Ti), and we tune the 3D metal deposition conditions to produce specific island shapes by self-assembly.

Since directed self-assembly requires avoidance of extraneous nucleation, as well as reliable nucleation at the patterned sites, we first discuss the control of metal island nucleation on unpatterned 2D materials. In particular, the nucleation density is expected to depend on the adatom diffusion distance on the 2D material surface. On 2D materials in the form of suspended layers, we use post-growth imaging by transmission electron microscopy (TEM) to measure the shape and distance between metal islands deposited in an ultra-high vacuum (UHV) evaporation system. We find that nucleation density and island shape depend on factors such as the substrate temperature, topography, background pressure, time, and partial pressure of the deposited material. We achieve the largest island diffusion distance (>2 μm) by removing polymeric residues in the 2D material transfer process, suspending the 2D material samples on TEM grids, using medium (100-200 ^oC) temperature deposition, and minimising defect sites such as wrinkles. We next induce locally reduced adatom diffusion distances using He-Ion beam patterning to create periodic defects in the 2D material. The He-Ion dose rates are calibrated and resulting defects characterised using high resolution scanning transmission electron microscopy. We show that these defects act as heterogeneous sites and we discuss the extent to which they can be exploited to produce patterned, periodic Au nanoisland arrays. We develop automated image analysis techniques and perform statistical analysis of the nucleation site density and location. We believe that this templating and nucleation control opens routes towards self-assembled and tailored interface design. In particular, we discuss the possibilities of interface structures built from self-assembled 3D nanostructures for development of plasmonic nanoarrays with tunable and well-defined resonance frequencies.

[1] Wang, Y. et al. Van der Waals contacts between three-dimensional metals and two- dimensional semiconductors. Nature 568, 70–74 (2019).
[2] Gherasimova, M., Hull, R., Reuter, M. C. & Ross, F. M. Pattern-level assembly of Ge quantum dots on Si with focused ion beam templating. Appl. Phys. Lett. 93, 023106 (2008).

The nano-morphology of bulk heterojunction (BHJ) blends based on poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) blended with [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) is systematically varied by changing the volume fraction of the solvent additive 1,8-diiodooctane (DIO) from 0 vol% to 20 vol% in the casting solution to prepare organic solar cells (OSCs). With increasing addition of DIO, the photoluminescence (PL) from the blend is reduced; however, a relative increase in PL from 750 nm onwards is observed for blends with 20 vol% DIO. As quenching of the blend PL is related to the donor/acceptor (D/A) interface, structural characterizations in real-space (microscopy) and k-space (scattering) are performed to unravel the morphology of blend systems to correlate nano-morphology with photophysical and charge transport processes. Blends prepared with 0 vol% DIO form large phase separated domains of PC₇₁BM, hundreds of nanometer in diameter. With the addition of 3 vol% DIO the size of PC₇₁BM domains is supressed resulting in a more mixed morphology due to the selective dissolution of DIO in PC₇₁BM. On the addition of up to 20 vol% of DIO, the film becomes rougher with a finer interconnected morphology due to polymer aggregation, which contrasts with previous reports. Electron-transport length scales measured by scanning photocurrent microscopy (SPM) are found to increase with the addition of up to 3 vol% DIO associated with the break-up of the large PC₇₁BM aggregates, while the hole-transport length is found to increase on adding DIO up to 20 vol% due to aggregation of polymer chains. Finite-difference time-domain (FDTD) simulations are performed to take into account for optical beam spread due to associated roughness of BHJ blend films which further strengthen the experimental findings to take into account for waveguiding/scattering components of excitation beam.The structural results are found to be in good agreement with the PL quenching and the effective transport lengths. Hence, this work represents a complete set of results systematically examining the effect of nanomorphology on structural and opto-electronic properties of BHJ blends, which can have a direct implication on better understanding of the emerging high efficiency OSC blend systems.

Diamond, by far, is the most thermally conductive material at room temperature and a great candidate for thermal management applications in microelectronic devices. Yet, its application has been discouragingly hindered due to its large thermal barrier at the interface when paired with other CMOS materials. Beryllium oxide (BeO) is a potential material candidate to bridge this thermal barrier between diamond and other wide bandgap semiconductors such as gallium nitride (GaN) and Silicon carbide (SiC) owing to its large bandgap, high thermal conductivity, and large spectral phonon overlap with diamond. In this study, using time-domain thermoreflectance we report the thermal conductivity and interfacial thermal resistance at BeO/diamond and Beo/Si interface across various temperatures. We measure the thermal conductivity of amorphous BeO for thicknesses below 30 nm to be ~10 +- 0.7 W/mK, which is significantly lower than its bulk crystalline value. Our results suggest that the interfacial thermal resistance at BeO/diamond interface is significantly lower than that of GaN/diamond and BeO/Si. This observation paves the way towards more thermally efficient integrated circuits.

Transparent electrodes are a necessary component of many optoelectronic devices. Networks of silver nanowires have emerged as a promising alternative to commonly-used metal oxides such as indium tin oxide (ITO). Their transparency and conductivity values are as high as ITO, while being mechanically flexible, less expensive and can be fabricated using high-throughput roll-to-roll methods. In this talk I will outline an additional advantage that has thus far received less attention: the high transparency of silver nanowire networks in the near-infrared region (NIR). This property is highly desirable for some applications such as window coatings and solar cells. I will show that for electrodes that are 96% transparent in the visible, ones made from ITO are only 35% transparent at a wavelength of 2500 nm, whereas those made from silver nanowires maintain a transparency as a high as 94%. I will present results that show that the nanowires in the network should be sparse and larger in diameter to maximize NIR transparency. Modelling of the nanowire networks will also be shown and help demonstrate the physical reasons this is the case.
At the end of my talk I will present my results on passivating silver nanowire electrodes for capacitive-type devices such as smart windows and displays. To fully realize the advantages of silver nanowire electrodes, candidate passivation materials must be optically transparent (including in the NIR range), inexpensive, mechanically flexible and simple to deposit using high-throughput methods. I will present our findings of the passivation effectiveness of polyurethane, PMMA, a commercial optical adhesive and cyclotene passivation layers on silver nanowire networks.

Charge transport layers play a fundamental role for efficient charge carrier injection and extraction in opto-electronic devices. Using interfacial layers, the open circuit voltage (V_OC) and fill factor (FF) of organic solar cells can improve significantly. Here we show experimentally that the extent of contact between a Molybdenum Oxide (MoO₃) hole extraction layer and the donor molecules in the photoactive layer of organic solar cells causes a proportional increase in non-radiative recombination losses. We quantify this effect by using several planar and bulk heterojunction solar cells device architectures, assigning it to surface recombination. We demonstrate that such losses can be suppressed by inserting a thin fullerenic anodic interlayer. This interfacial layer enhances the built-in potential and reduces the presence of minority charge carriers at the electrodes: a new perspective on the principle of selective charge extraction layers. The improvement to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. These findings are relevant to different branches of organic electronics, given the popularity of MoO₃ as an efficient hole extraction and injection layer, providing insights for future device design.

The electronic properties of interfaces in (opto-) electronic devices play a key role to reach the desired functionality and high efficiency. A wide range of organic and inorganic materials to tune interfacial properties is available today, and a comprehensive understanding of how they interact at interfaces is a prerequisite for realizing their full potential, particularly motivating their use in novel settings. This is exemplified here for the widely used interface modification agent polyethylenimine (PEI), which helps reducing electron injection barriers between electrodes and organic semiconductors. It is shown that PEI mixed with a polymer semiconductor in solution in fact serves two beneficial purposes simultaneously. PEI phase-separates from the semiconductor upon spin-coating and forms low work function bottom and top layers, which enhance electron injection. At the same time, minute amounts of PEI slightly n-dope poly{[N,N’-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} [P(NDI2OD-T2)] in the bulk film, thereby also increasing the electron mobility and boosting the conductivity overall by four orders of magnitude. The full capability of PEI interlayers for optimized electron injection at contacts, however, can only be reached by carefully excluding water during processing and applying mild annealing for improved surface coverage. With this, PEI interlayers reduce the work function of ZnO and graphite electrodes to a record low value of 2.3 eV.
Also the combination of organic conductive polymers and inorganic semiconductors provides a rich playground for interface manipulation. It has been demonstrated before that n-type Si coated with a layer of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) actually returns fully functional solar cells, despite the absence of a pn-junction in Si. We could now provide direct evidence that PEDOT:PSS induces indeed a large upward band bending in n-Si, even reaching inversion conditions. From a variation of processing conditions, suggestions on further improvements for this attractive approach to surface-dope inorganic semiconductors to the extreme can be derived.
It will further be noted how molecular photochromic switches, in the form of a monolayer at the interface between electrodes and an organic semiconductor, can be employed to facilitate multifunctionality of a device. As an example, an organic light emitting diode (OLED) will be discussed, whose brightness can be controlled reversibly during operation by both applied bias and external short-time light irradiation.

Organic semiconductors (OSC) have a low intrinsic carrier density. Devices based on OSC thinfilms require carriers to be doped by chemical, optical, or electrical means. Many types of devices such as light emitting diodes or transistors consist of metal electrodes which inject the carriers that are conducted across the thinfilm. For hole transporting OSC devices, electrodes of metals with high work function are needed for efficient injection. However, at any metal-OSC interface, mismatch in energy levels creates an energy barrier to hole injection. Energy barrier at interface introduces an additional resistance at interface, hampering device performance. Self Assembled Monolayers (SAM) of polar molecules are used as interlayers at metal-OSC interface to reduce the energy barrier.^[1] We show that SAM can act as an agent for interfacial doping along with for work function tuning, thus having dual mechanism for reducing device resistance.

We show this with the case of bottom contact field effect transistor (FET) of the OSC pentacene. Copper electrodes are used for source-drain, and are coated with SAM of different fluorine containing molecules perfluorobenzenethiol (PFBT) or perfluorodecanethiol (PFDT). We find that work function of copper increases from ~4.82 eV to ~5.2 eV for PFBT/Cu and ~-5.4 eV for PFDT/Cu. Drain current was found to be higher with SAM/Cu electrodes as compared with unmodified Cu electrodes, however the trends were in contradictory to those of work function. We show that the drain current increase can be explained on the basis of interfacial carrier doping of pentacene by SAM/Cu. During interface formation, holes are transported from high work function electrode to OSC, accompanied by bending of energy levels, which increases its the equilibrium hole density. We obtain doping profiles by applying a solution to Poisson’s equation, and find that the excess hole concentration decreases exponentially away from interface. PFBT/Cu leads to larger average hole concentration (10¹⁶ cm^-3) as compared to PFDT/Cu (10¹⁴ cm^-3) which is in accordance with trends of drain current.

We thus show, by experimental device characteristics and applying analytical models, that interfacial SAM at electrodes can influence characteristics of OSC based devices by a two-fold mechanism of doping along with electrode work function tuning. Depending on the molecules SAM comprise of, doping amount and profile can be controlled. SAM thus has the twin advantage of enabling us to enhance device characteristics due to doping, as well as control device behaviour by doping amount.

References
[1] Dario Natali, Mario Caironi, Adv. Mater. 2012, 24, 1357–1387

Since the seminal report on the 9.7% efficient and 500 h-stable solid-state perovskite solar cell (PSC) in 2012, following two seed reports on perovskite-sensitized liquid junction solar cells in 2009 and 2011, perovskite photovoltaics has been surged swiftly due to high power conversion efficiency (PCE) obtainable via facile fabrication procedure. As a result, a PCE of 25.2% was recorded in 2019. According to Web of Science, number of publications on PSCs increases exponentially since 2012, leading to the accumulated publications of more than 16,500, which indicates that PSC, needless to say, is promising next-generation photovoltaics. High photovoltaic performance was realized by compositional engineering, device architecture and fabrication methodologies for the past 10 years. Toward theoretical efficiency over 30% and commercialization of PSC, further studies on removal of recombination and scalable technologies are required for next 10 years. In this talk, importance of interface engineering is emphasized to reach the theoretical efficiency with voltage over 1.3 V and fill factor over 0.9. Material selection rule and interface engineering methodologies will be introduced. For commercialization, materials may be issued because the current precursor mixture is problematic due to the underlying aging effect. We developed cost-effective materials based on delta FAPbI₃ powder for reproducibly high efficiency PSC. Large-area uniform perovskite coating is also important in upscaling PSC. Acetonitrile-based and 2 methoxy ethanol-based coating solutions were developed for large-area perovskite films. In addition, additive engineering and bifacial stamping technique were found to be effective ways for the upscaling PSC. A paradoxical approach is discussed for improving stability of PSCs. PCEs over 23% could be achieved by the interface engineering, the additive engineering and the topographical approach.

To improve the performance of existing perovskite solar cells (PSCs), a detailed understanding of the underlying device physics during their operation is essential. We have developed and validated a numerical device model that describes the operation of PSCs and quantitatively explains the role of contacts and of (doped) transport layers (TLs), ionic movement, carrier generation, drift and diffusion of carriers and recombination. We fit the simulation to experimental data of various perovskite solar cells to identify the main losses and routes to optimization.

The performance of PSCs can be limited by losses due to the poor transport properties of the commonly used TLs. State-of-the-art TLs suffer from rather low mobilities when compared to the mobilities of perovskites. We introduce a conductivity criterion to guarantee the design of a good TL through a combination of experimental and modelling data.

We use large-scale drift-diffusion simulations to get a better understanding of the light intensity dependence of the open-circuit voltage and how it correlates to the dominant recombination process. We introduce an automated identification tool using machine learning methods to pinpoint the dominant loss using the light intensity-dependent performances as an input. The machine learning was trained using over 150,000 simulations and gives an accuracy of the prediction up to 80%.

Space-charge limited current (SCLC) measurements have been widely used to study the mobility and trap density in semiconductors. While such measurements have proven to be valuable to investigate the properties of organic semiconductors, their applicability to perovskite materials is not straightforward.

We present a description of the pitfalls of SCLC measurements when considering perovskite semiconductors with an emphasis on the effect of mobile ions, inherent properties of these materials, on the measurements. We show using drift-diffusion simulations that the presence of ions strongly affects the measurement and that the usual analysis and interpretation of the values extracted from SCLC needs to be refined. In particular, the trap density and mobility cannot be directly quantified using classical methods. We also discuss the advantages of pulsed SCLC measurements to get reliable data that are less influenced by ionic motion and degradation of the perovskite under voltage stress. Fitting the pulsed SCLC measurement using drift-diffusion modelling appears to be a reliable method to extract simultaneously mobility, trap and ion densities. We were able to obtain a trap density of 10¹³ cm^-3, ion density of 10¹³ cm^-3 and mobility of 13 cm²/Vs for MAPbBr₃ single-crystal.

The long-term viability of perovskite-based solar cells is contingent on the evolution of their performance and the improvement of their stability, which primarily require a deep understanding of their property-function relationships. A comprehensive description of the electronic and chemical properties of the device layers and interfaces is particularly relevant, since these properties ultimately dictate the charge transport across the interfaces and throughout the device. Still, an accurate understanding of the mechanisms governing device operation requires a controlled operando monitoring of the respective layers’ electronic properties, i.e. when indispensable biases such as light and electrical bias are applied.
In our study, in order to establish an X-ray photoelectron spectroscopy (XPS) operando procedure, we primarily designed and investigated perovskite half-devices consisting of a multi-cation mixed halide perovskite layer between a FTO/TiO₂ substrate and a partial Au top electrode. We consistently monitor the electronic and chemical properties across the surface of the device sample, which reveals a change of the iodine and lead species from the Au to perovskite side, consistent with a previously observed underpotential deposition process[1]. Then, we use synchrotron-based photoelectron spectroscopy (PES) that allows investigation under operating conditions. The X-ray beam provided by the synchrotron light source also provides the spatial resolution required to use the designed sample configuration and to monitor the changes of the electronic and chemical properties as a function of applied electrical bias and illumination.. A positive electrical bias applied to the Au anode produces a shift in binding energy of the core levels of the perovskite layer, as expected, although it is consistently smaller than the bias. This effect could be attributed to inhomogeneous electric potential across the surface, as it is notably observable close to the Au/perovskite boundary, highlighting the importance of sample design for operando studies. We further discuss the possible implications of transient and dynamic processes in these changes. Moreover, by varying measurement parameters and monitoring the effects on chemical reactions - that may be at the core of degradation processes in perovskite solar cells -, we attempt to disentangle these effects from those induced by the external biases. Importantly, our results indicate a gradual shift of the core level binding energy towards lower values over the early course of the measurements on the perovskite side, potentially indicating an X-ray-induced photovoltage effect. The results provide first insight into perovskite device electronic properties under operating conditions and guidelines for XPS operando studies.

[1] Kerner et al., APL Materials 7, 041103 (2019)

Understanding the carrier injection/extraction mechanism at organic/inorganic interfaces is of pivotal importance for the future progress of organic optoelectronic devices such as organic light-emitting diodes (OLEDs), organic thin-film transistors (OTFTs), organic solar cells (OSCs), perovskite solar cells (PSCs), and organic semiconductor laser diodes (OSLDs). Since barrier-free contacts for holes and electrons are essential for efficient organic optoelectronic devices, the method of tuning the work function (WF) of electrodes and related phenomena has been intensively studied. A higher WF is ideal for efficient hole injection/extraction, and the surface WF of anodes has been tuned to above 5 eV by using metal oxides such as p-type dopants and molybdenum trioxide, both of which have relatively high electron affinity. On the other hand, a cathode surface WF has been mainly tuned using reactive materials such as alkali elements with low WFs of less than 3 eV. Because such alkali elements are highly chemically reactive and easily oxidize in the presence of ambient oxygen and moisture, there is a strong desire to eliminate reactive materials from devices, especially flexible optoelectronic devices.
As a promising electron injection/extraction technique, the utilization of a coordination reaction between phenanthroline derivatives and metals has been studied. Yoshida [1] reported that the high electron injection and current collection efficiency at the Ag electrode/batocuproin (BCP) interface is due to the formation of Ag–BCP complexes. Very recently, Bin et al. [2] have reported that the electron injection efficiency of Ag-doped 4,7-dimethoxy-1,10-phenanthroline (p-MeO-Phen) films is higher than that of not only Ag-doped BPhen films, but also Cs-doped p-MeO-Phen films. It was also reported that the introduction of an electron-donating dimethoxy substituent in Phen results in greater electron exchange between p-MeO-Phen and Ag than between BCP and Ag, therefore improving the electron injection efficiency. However, the coordination reaction around the cathode is not yet systematically understood, and a feasible alternative material to reactive metals has not yet been found.
Here, we have found a novel phenanthroline derivative that can be a feasible alternative to reactive metals in electron injection through an in-depth elucidation of the coordination reaction [3]. The surface WF of the ITO/ZnO electrode with 4.10 eV can be tuned in the range of 3.29–2.43 eV by forming thin films of the Phen derivative on ITO/ZnO. From density functional theory (DFT) calculations, the change in WF (ΔWF) was found to be strongly correlated with the amount of electron transfer associated with the coordination reaction compared with the other physical properties that likely affect WF, such as the molecular dipole moment of Phen derivatives. The electrode modified with 4,7-bis(1-pyrrolidinyl)-1,10-phenanthroline (p-Pyrrd-Phen) shows a lower WF than the electrode modified with p-MeO-Phen and a higher electron injection efficiency than LiF. Furthermore, the air stability of the p-Pyrrd-Phen-modified electrode is about 10 times higher than that of LiF, and the operational stability of the OLED using the p-Pyrrd-Phen-modified electrode is comparable to that of the OLED using LiF. Since both the air stability and operational stability of this phenanthroline-modified cathode was demonstrated to be high, our findings open the way to the fabrication of electronic systems that do not require air-sensitive materials.

[1] H. Yoshida, J. Phys. Chem. C vol.119, p.24459 (2015).
[2] Z. Bin et al., Nat. Commun. vol.10, p.866 (2019).
[3] H. Fukagawa et al., Nat. Commun. vol.11, p.3700 (2020).

Surface ligand treatment of organic metal halide perovskites is a promising route to passivate surface defect states, improve material and device stability, alter interfacial energetics, and ultimately increase the perovskite solar cell (PSC) performance. In this work surface ligands with different binding groups are investigated to determine their extent of surface coverage, whether they form a surface monolayer or penetrate into MAPbI₃, how they influence MAPbI₃ surface energetics and photoluminescence, and how this combination of factors affects PSC performance. Using angle-dependent X-ray photoelectron spectroscopy (XPS) we show that surface ligands with ammonium and phosphonic acid binding groups often penetrate into MAPbI₃, which can significantly impact surface energetics, as determined through ultraviolet and inverse photoelectron spectroscopy measurements. These changes in surface energetics significantly impact the PSC performance, with solar cell performance decreasing for ligands that create less favorable energy landscapes for electron transfer from MAPbI₃ to the electron transport layer, C₆₀. Through investigating a series of ammonium containing surface ligands, we propose guidelines to select ligands that will either penetrate into MAPbI₃ or remain only on the MAPbI₃ surface.

Scandium nitride (ScN) is a semiconducting nitride that has the rocksalt crystal structure, but with a lattice parameter that provides a close match to GaN; the lattice mismatch is less than 1%. One reason ScN has attracted recent interest is because adding ScN to AlN enhances the piezoelectric properties, and may even lead to ferroelectricity. Here I will focus on interfaces between ScN and GaN. ScN can be highly n-type doped and can thus serve as an almost metallic contact or current spreading layer to GaN. If rocksalt ScN is grown along the [111] direction on top of c-axis-oriented wurtzite GaN, interesting phenomena related to spontaneous polarization occur. GaN is polar with a large spontaneous polarization, while ScN is centrosymmetric. This suggests the possibility of a very large spontaneous polarization difference at the interface. We have examined the properties of such interfaces using first-principles calculations based on density functional theory [1]. We first apply the modern theory of polarization to obtain the bulk properties of the individual materials. The resulting difference in polarization constants yields a polarization discontinuity at the ScN/GaN interface of -1.358 C/m². As a result, we expect a high-density electron gas on the N-polar GaN/ScN interface, and a high-density hole gas on the Ga-polar GaN/ScN interface, with carrier concentrations up to 8.5E14/cm². This value is over an order of magnitude larger than what can be achieved at GaN/AlGaN interfaces. We have confirmed the presence of strong electric fields and bound interface charges in explicit interface calculations. The large polarization difference and small strain makes ScN a promising material for applications in polarization-enhanced p-GaN/ScN/n-GaN tunnel junctions. Current tunnel junctions make use of InGaN interlayers to reduce the effective barrier and increase the polarization field across the p-n junction to reduce the junction width. ScN is a promising alternative, with a higher polarization field, smaller bandgap, and much smaller strain.

Work performed in collaboration with N. L. Adamski and C. E. Dreyer, and supported by AFOSR.

[1] N. L. Adamski, C. E. Dreyer, C. G. Van de Walle, Appl. Phys. Lett. 115, 232103 (2019).

Photo-sensing devices using amorphous-selenium (a-Se) have gained significant attention in medical imaging due to their avalanche phenomena. a-Se photodetectors are wide bandgap (E_g) semiconductor with high fill-factor, offers a high-gain (~10² – 10³) with negligible low excess noise, and has ~90% quantum efficiency to blue wavelength facilitating a scintillator-based conversion detector for gamma rays and X-rays. At electric fields above 70 V/μm, holes in a-Se undergo impact ionization which increases the effective sensitivity of the photodetector. However, at these high electric fields required for avalanche gain, dark current due to charge injection from the metallic electrode can become a significant source of noise, limiting the dynamic range of the photodetector. Hole-blocking layer (HBL)/ electron transport layer (ETL) is an n-type semiconductor typically incorporated at the interface between positive electrode and a-Se to minimize the dark current from charge injection while conducting only electrons. However, the avalanche gain of the a-Se sensor has been limited so far by the use of inefficient hole-blocking layers. Two factors are necessary to achieve efficient a-Se devices with HBL. One is, the device should be fabricated with HBL on top of a-Se (p-i-n fabrication sequence where ‘p’ denotes p-type electron-blocking layer, ‘i’ is for intrinsic a-Se and ‘n’ for n-type HBL) to develop a scalable imager that is compatible with pixelated read-outs. Second, a-Se crystallizes at ~50°C which emphasizes a room-temperature deposition technique of HBL on a-Se. Among previously reported HBLs/ETLs, vacuum-deposited bulk cerium dioxide (CeO₂) has been successfully utilized in avalanche a-Se sensors. Bulk CeO₂ cannot be directly deposited on a-Se, as the high-temperature required for the vacuum deposition crystallizes a-Se and is fabricated in the n-i-p sequence, with a-Se deposited on top of CeO₂. a-Se with bulk CeO₂ suffers from the high dark current due to a lower effective hole barrier of CeO₂ (2.8 eV). In our work, we successfully synthesized colloidal CeO₂ quantum dots (QDs) with a large bandgap (E_g = 3.77 eV), which are electronically passivated by surfactants/ligands and spin-coated directly on a-Se at room temperature without inducing any crystallization. As a large bandgap semiconductor, CeO₂ is extremely transparent at the 450 nm wavelength during photo-response measurement of a-Se. Our best performing device with 40 nm thick CeO₂ HBL achieved an avalanche gain of 42. Also, our prototype device of 150 nm thick CeO₂ achieved an ultra-low dark current of 30 pA/cm² at 70 V/μm which is three orders of magnitude lower than any avalanche a-Se device previously reported and might be the lowest dark current ever recorded for an avalanche device operated at room temperature.

The scope of luminescent materials that can be used for electroluminescent devices has been limited due to material processability and band alignment considerations. These limitations impede the development of electroluminescent devices at extreme wavelengths and hinder the use of electroluminescence (EL) spectroscopy as an analytical technique. Herein we report a device that can excite EL from various materials independent of their physical form and chemical composition. The device produces EL from the long-wave infrared (0.13 eV) to ultraviolet (3.3 eV) range depending on the emitting layer drop casted on top, with onset voltages closely approaching the optical energy gap of the emitting materials. The two-terminal device employs a carbon nanotube network as the source contact and utilizes an AC gate voltage to achieve charge injection through transient band bending. The concept of EL spectroscopy is demonstrated by probing a chemical reaction in a liquid droplet and an EL sensor is developed for detecting organic vapors.

The development of ultraviolet (UV) LEDs has long been a focus due to the wide application space including germicidal disinfection of water, medical sterilization of surfaces, UV polymer curing, DNA sensing, and solar blind detectors. The COVID-19 pandemic has further fueled the need for a time effective and economic solution to disinfect public areas from the coronavirus. Current UV-light disinfection devices use mercury-based gas discharge lamps because high-performance UV LEDs require an electrically conducting electrode material that is highly transparent in the UV range.¹
In this talk we will demonstrate that the correlated metal SrNbO₃ is an ideal transparent electrode for UV applications due to its high optical transmission coupled with a metallic like electrical conductivity superior to indium tin oxide. We will present electrical and optical properties of SrNbO₃ films grown by pulsed laser deposition² and by magnetron sputtering.³ The optical transmission was found to be in excess of 80% throughout the visible range and near ultraviolet down to 270 nm for films with thicknesses smaller than 12 nm while maintaining an electrical resistivity lower than 2×10^-5 Ωcm, giving rise to a sheet resistance of about 68Ω/sq due to its metal-like carrier concentration of ~1×10²² cm^-3, comparing favorably to conventional UV transparent conductor materials such as doped Ga₂O₃ or ZnGa₂O₄. In addition, we will share insights obtained by density functional theory and experiments how sizeable Sr vacancy concentrations affect electrical and optical properties of SrNbO₃ films.

1. Hirayama, H. Recent Progress in AlGaN Deep-UV LEDs. in Light-Emitting Diode - An Outlook On the Empirical Features and Its Recent Technological Advancements (InTech, 2018). doi:10.5772/intechopen.79936.
2. Park, Y. et al. SrNbO₃ as Transparent Conductor in the Visible and Ultraviolet Spectrum. Commun. Phys. 3, 102 (2020).
3. Roth, J. et al. Sputtered Sr_xNbO₃ as a UV transparent conducting film. Accepted to ACS Appl. Mater. Interfaces.

Organic light emitting devices (OLEDs) are one of the fascinating topics in both academic and industrial fields. Certain properties of OLEDs make them exciting prospects for next generation lighting and display applications, such as the large-area emission characteristics not found in other light sources. OLEDs are fabricated in either a conventional configuration or an inverted configuration. In the conventional configuration, the anode is the transparent electrode and the cathode is the metal electrode. For the inverted configuration the cathode and anode materials are reversed (metal as anode, transparent electrode as cathode). OLEDs are also divided based on the direction of light emitted from the device - bottom-emitting (BE) or top-emitting (TE). Most BE OLEDs are fabricated on indium-tin-oxide (ITO) coated glass substrates; however, this device type has some disadvantages. For example, the emitted light has to pass through a high refractive index ITO layer and a thick glass substrate to exit the device, which traps light in these layers. Additionally, ITO is brittle so this limits its application in flexible OLEDs. TE OLEDs are more compatible with n-type active matrix display-driven OLED technologies. In these devices, the top transparent electrode can be very thin (<100 nm) and the bottom metal layer is thicker so that it acts a reflecting surface. Therefore, TE OLEDs have reduced light trapping, and, therefore, increased light-extraction efficiency compared to BE OLEDs.

In this work, the main objective is to compare the performance and stability of both inverted BE and TE OLEDs fabricated using organic light-emitting polymer semiconductor active layers. The inverted BE OLED has the following layers in order from bottom to top: glass, ITO, zinc oxide (ZnO), cesium carbonate (Cs₂CO₃), poly(9, 9-dioctylfluorene-alt-BT) (F8BT), molybdenum trioxide (MoO₃), aluminium (Al). ITO and Al are the cathode and anode respectively; ZnO and Cs₂CO₃ are the electron injection layers; F8BT is the light-emitting polymer where electron and holes combine to emit light; and MoO₃ is the hole injection layer. Cs₂CO₃ is deposited on ZnO because it lowers the barrier for electron injection into thick polymer films [1]. The inverted TE OLED has the following layers in order from bottom to top: glass, silver (Ag), titanium dioxide (TiO₂), F8BT, MoO₃, silver (Ag) [2]. The performance and stability of both devices is compared by studying current-voltage, current-light flux and voltage-light flux variations. In addition, optoelectronic simulations are employed to aid in optimization of the thickness of the device layers and to validate the experimental results.



References

[1] D. Kabra, L. P. Lu, M. H. Song, H. J. Snaith, and R. H. Friend, “Efficient single-layer polymer light-emitting diodes.,” Adv. Mater., vol. 22, no. 29, pp. 3194–3198, 2010, doi: 10.1002/adma.201000317.
[2] C. M. Carter, J. Cho, A. Glanzer, N. Kamcev, and D. M. O’Carroll, “Cost, energy and emissions assessment of organic polymer light-emitting device architectures,” J. Clean. Prod., vol. 137, pp. 1418–1431, 2016, doi: 10.1016/j.jclepro.2016.07.186.

Lead halide perovskites have witnessed in the last few years a major progress as active materials for light-emitting applications. In only four years, efficiencies of perovskite light-emitting diodes (PeLEDs) soared up from less than 1% in 2014¹ to more than 20%^2–4 in 2018. These high efficiencies, together with remarkable properties such as cheap solution processes at low temperatures, efficient charge injection/transport, high color purity and easy wavelength tuning, make perovskite materials a very promising route for future display application. Especially, the use of quasi-2D perovskites forming multi-quantum well structures has enabled to improve radiative recombination probability by increasing the exciton binding energy and by inducing energy transfer within the perovskite material⁵. However, the efficiency and stability of PeLEDs are still lagging behind those of more established technologies like OLEDs. One of the main hurdles for the fabrication of efficient and stable PeLEDs is related to interfacial layer properties, adjacent to the perovskite material. Although the Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is famous as an easy-to-use excellent hole injection and transport layer, it is also known that luminescence quenching in perovskite emitter layers can appear when deposited onto PEDOT:PSS films, which affect performances of the PeLED⁶. Up to now though, exact interactions occurring at the PEDOT:PSS/perovskite interface are not fully understood.

In this contribution, we use a PEA₂(FAPbBr₃)_n-1PbBr₄ quasi-2D perovskite and discover a very significant conductivity doping phenomenon located at this PEDOT:PSS/perovskite interface. It translates into unwanted light emission outside of the delimited active area of the PeLED, which may lead to calculation error of device efficiencies. It is demonstrated that this doping is specific to the association of perovskite and PEDOT:PSS materials through a singular behavior. In addition, a study of different interlayers is proposed in order to eliminate the interactions at the PEDOT:PSS/perovskite interface, with emphasis on deposition issues we encountered such as wettability of the perovskite material on those interlayers, which can drastically affect PeLED performances and the manufacture reproducibility from one batch to another.

References
1. Tan, Z. et al. Bright light-emitting diodes based on organometal halide perovskite. Nat. Nanotechnol. 1–6 (2014). doi:10.1038/nnano.2014.149
2. Zhao, B. et al. High-efficiency perovskite-polymer bulk heterostructure light-emitting diodes. Nat. Photonics 1–10 (2018).
3. Cao, Y. et al. Perovskite light-emitting diodes based on spontaneously formed submicrometre- scale structures. Nature 562, 249–253 (2018).
4. Lin, K. et al. Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent. Nature 562, 245–248 (2018).
5. Yang, X. et al. Efficient green light-emitting diodes based on quasi-two-dimensional composition and phase engineered perovskite with surface passivation. Nat. Commun. 9, 2–9 (2018).
6. Meng, Y. et al. High performance and stable all-inorganic perovskite light emitting diodes by reducing luminescence quenching at PEDOT:PSS/Perovskites interface. Org. Electron. 64, 47– 53 (2019).

Contact materials for electron injection is critical for OLEDs and halide perovskite LEDs. This type of materials requires several properties; low work function, chemically stable, optically transparent and ohmic contact formation with cathode. In this talk, Iwould like to show 3 new transparent amorphous semiconductors which were developed for OLEDs/QLEDs along with materials design concepts. First is amorphous electride C12A7:e with very small work function and chemical stability [1]. Second is X-ray amorphous ZnO-SiO₂ thin films with small work function and easy Ohmic contact formation with various metals and semiconductors [2,3]. Such a feature makes it possible to fabricate tandem-type OLED devices without significant voltage drop [4]. The last is amorphous transparent p-type semiconductor, Cu-Sn-I with large mobility (~10cm2/Vs) which can be fabricated by spin-coating at low temperature [4]. Applying a-ZSO as the electron injection layer [5].led very high green and red light emission from perovskite LEDs, revealing the critical role of exciton confinement and carrier injection.

[1] H.Hosono, J.Kim, Y.Toda, T. Kamiya, and S.Watanabe, Proc. Natl. Acad. Sci. USA, 114(2), 233 (2017).
[2] N.Nakamura, J. Kim, H.Hosono, Adv. Electron. Mater., 4, 1700352, (2018).
[3] N.Nakamura, J. Kim, K.Yamamoto,S. Watanabe, and H. Hosono. Org.Electro. 51, 103-110(2017).
[3] H.Yang, J.Kim, K.Yamamoto, H.Hosono, Org.Elect.46, 133(2017).
[4] T. Jun, J. Kim, M. Sasase, H. Hosono, Adv. Mater., 30, 1706573 (2018).
[5] K. Sim, T. Jun, J.Bang, H. Kamioka, J, Kim,H. Hiramatsu, H. Hosono, Appl.Phys.Rev. 6, 031402 (2019)

Graphene has emerged as a novel material with enhanced electrical and structural properties that can be used for a multitude of applications from super-capacitors to biosensors. In this context, an ultra-sensitive biosensor was developed using a low-cost, simple and mask-free method based on laser-induced graphene technique for electrodes patterning. The graphene was produced on a polyimide substrate, showing a porous multi-layer structure with a resistivity of 102.4 ± 7.3 Ω/square. The biosensor was designed as a 3-electrode system. Auxiliary and working electrodes were made of graphene by laser patterning and the reference electrode was handmade by casting a silver ink. A molecularly-imprinted polymer (MIP) was produced at the working electrode by direct electropolymerization of eriochrome black T (EBT). As proof-of-concept, the MIP film was tailored for chloramphenicol (CAP), a common contaminant in aquaculture.

The present study shows a simple, fast and cost-effective way of converting industrial waste into a lightweight optoelectronic device to detect UV radiation. An innovative recycling process of ZnO-rich (> 95% ZnO) residues from the brass industry has been investigated through the preparation of ceramic targets to be used in the deposition of ZnO thin films. A comparison between optical, structural and morphological properties of ZnO thin films deposited by sputtering technique on glass and polyimide substrates was carried out. The ZnO thin films deposited on polyimide substrates were used as seed layers for an ultrafast growth of ZnO nanorods arrays by hydrothermal method assisted by microwave radiation. Laser-induced graphene electrodes were fabricated on the polyimide substrates prior to the ZnO thin film deposition for UV sensor characterization. The produced UV sensor exhibits a responsivity around 12 μA/W for an UV light of 365 nm under 10 V bias. The photoresponse measurements showed a stable UV sensor without significant variations in the sensor’s responsivity when tested for long periods of time.

New opto-electronic devices from solar cells to transparent transistors require high performance materials. Ab initio high-throughput computations offer an accelerated path towards the discovery of these exceptional materials. We can now screen thousands of materials by their computed properties even before the experiments. This computational paradigm allows experimentalists to focus on the most promising candidates, and enable researchers to efficiently and rapidly explores new chemical spaces.
In this talk, I will present the challenges and opportunities in materials discovery offered by high-throughput ab initio computing in searching for materials with exceptional opto-electronic properties. Recent examples from our work in the fields of transparent conducting materials and solar absorbers will be used to illustrate the approach. I will discuss the materials highlighted by these computational searches but also show how trends and design rules can be extracted from high-throughput data sets. Finally, I will report on our efforts in scaling up to high-throughput new properties from point defects to carrier mobilities.

The most common approach to describe surfaces in density-functional theory is the repeated slab geometry based on periodic boundary conditions. A common strategy to avoid artificial charge transfer from one side of the slab to the other when modelling semiconductor surfaces is to passivate partially filled surface dangling bonds at the backside of the slab. Using the example of wurtzite (0001) surfaces we demonstrate that conventionally used passivation schemes (e.g. pseudo H or surface reconstructions) break down for materials exhibiting spontaneous polarization. We have therefore developed a generalized passivation method that accounts for the effect of spontaneous polarization and correctly describes the asymptotic bulk limit for pyroelectric materials. It is robust and ensures quick convergence of total energies and electronic structure with respect to system size. The performance of our approach will be demonstrated using the example of wurtzite ZnO polar surfaces.

This work is supported by the Cluster of Excellence RESOLV (EXC 2033) funded by the Deutsche Forschungsgemeinschaft. This project has received funding from the ECSEL Joint Undertaking (JU) project UltimateGaN under grant agreement No 826392. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Austria, Belgium, Germany, Italy, Slovakia, Spain, Sweden, Norway, Switzerland.

Among many candidate materials that can overcome the high cost and brittleness of indium tin oxide (ITO), silver nanowire (AgNW) networks appear as a promising substitute. The percolating networks of AgNWs exhibit high flexibility and excellent optoelectronic properties, with sheet resistance of a few Ω/sq and total transmittance of 90% at 550 nm, fulfilling the requirements for many applications such as solar cells or transparent heaters.^1,2 In addition, the fabrication of these electrodes can be achieved by scalable solution-based methods. However, AgNW’s susceptibility to corrosion and thermal instability remain limiting factors to its widespread market adoption. Depending on the application target, bare AgNW networks usually require stabilization by encapsulation layers to increase resilience to chemical or electrical stress.

In this study, we present a scalable and economically viable process involving electrophoretic deposition (EPD) to fabricate a highly stable hybrid transparent electrode with a sandwich-like structure where the AgNW network is covered by GO on both sides.³ The newly developed process allows the conductive transparent film to be transferred to an arbitrary surface after deposition and demonstrates excellent sheet resistance (15 Ω/sq) and transmittance (87% at 550 nm) on glass substrates. Unlike the uncoated AgNW network, the hybrid electrode was found to retain its original conductivity under long-term storage at 80^○C. This chemical resilience was explained by the absence of major silver corrosion products for the AgNW encapsulated by GO as indicated by X-ray photoelectron spectroscopy (XPS).

In situ electrical ramping and resistance measurement up to 20V combined with Infrared Thermography were also conducted in order to assess the electrical stability of our electrodes. The electrical behavior of bare AgNW networks typically undergoes distinct phases comprising “optimization” (Joule-effect-induced sintering of the junctions) at first,⁴ then “degradation” (Joule-effect-induced morphological instabilities of some AgNWs), and finally “breakdown”. It was demonstrated experimentally that the latter breakdown stage involves the formation and propagation of a crack parallel to the contact electrodes.⁵ On the other hand when encapsulating in GO, the results indicate a novel stabilization mechanism enabled by the presence of GO that prevents abrupt divergence of the resistance to the MΩ range.

Finally, we will explain the benefits of using these transferrable GO-stabilized AgNW networks in devices by giving some examples of successful integration into organic solar cells (OSC). Importantly, our film can be used as a transparent top electrode for OSC, paving the way for transparent photovoltaics with longer lifetimes. Our unique double-sided EPD-GO/AgNW/GO design where GO is already integrated with the AgNW network serves a dual purpose of encapsulating the entire device as well as enhancing the contact between AgNW and underlying device stack. This results in an enhanced device performance along with increased lifetime by a factor of 5 compared to an unencapsulated device with sprayed AgNWs as top contact.

(1) T. Sannicolo, M. Lagrange, A. Cabos, C. Celle, J.-P. Simonato, D. Bellet, 2016. Small, 12 (44), 6052–6075
(2) M. Lagrange, T. Sannicolo, D. Muñoz-Rojas, B.G. Lohan, A. Khan, M. Anikin, C. Jiménez, F. Bruckert, Y. Bréchet, D. Bellet, 2017. Nanotechnology, 28 (5), 55709
(3) W.H. Chae, T. Sannicolo, J.C. Grossman, 2020, ACS Applied Materials and Interfaces, 12 (15), 17909-17920
(4) T. Sannicolo, D. Muñoz-Rojas, N.D. Nguyen, S. Moreau, C. Celle, J.-P. Simonato, Y. Bréchet, D. Bellet, 2016. Nano Letters, 16 (11), 7046–7053
(5) T. Sannicolo, N. Charvin, L. Flandin, S. Kraus, D.T. Papanastasiou, C. Celle, J.-P. Simonato, D. Muñoz-Rojas, C. Jiménez, D. Bellet, 2018. ACS Nano, 2018, 12, 4648−4659

A vast array of modern electronic devices require electron transport layers (ETLs), including photovoltaic cells and batteries. In order to be efficient, various constraints need to be fulfilled with respect to material it is contacting, including structural compatibility and band alignment, so a wide range of different possible electron transport layers is needed for current and future technologies. It is also vital that these materials are produced sustainably, to minimise resource depletion and global heating.
Here, BaBi₂O₆ has been synthesised by a solution-based method at ambient temperatures from relatively non-toxic elements. BaBi₂O₆ was shown to crystallise in the layered PbSb₂O₆ structure, which is shared by a number of other materials,¹ some of which have magnetic and catalytic capabilities; and is stable up to around 600^oC.
Hybrid density functional theory (DFT) calculations were then used to analyse its potential as an ETL. The electronic transport properties were calculated from two codes, BoltzTraP,² which solves the Boltzmann transport equations using the relaxation time approximation (RTA); and AMSET, a new code which does not use this approximation. Using AMSET, the rates of polar optical phonon, acoustic deformation potential and ionised impurity scattering were each calculated, leading to a much more accurate approximation of the electrical conductivity compared to BoltzTraP. Via AMSET it is estimated that BaBi₂O₆ possesses an electrical conductivity of around 6,800 S m^-1 with 10^-19 electrons cm^-3 at 300 K in the in-plane direction, or a mobility of approximately 42 cm² V^-1 s^-1. By calculating the full set of intrinsic defects using hybrid DFT, it was then shown that BaBi₂O₆ is intrinsically n-type, and by also calculating extrinsic La and F defects, it was shown La in particular is an effective donor dopant.
Overall, we show to a high level of theory that BaBi₂O₆ is a sustainable material which possesses the high electron mobility and dopability required to be a promising ETL.
[1] Blasse, G. & De Pauw, A. D. M. J. Inorg. Nucl. Chem., 1970, 32, 2533.
[2] Madsen, G. K. H.; Singh, D. J. Comput. Phys. Commun. 2006, 175, 67.

The search for a cheaper and more efficient alternative to transparent conductor tin-doped indium oxide (ITO) in light-emitting devices, displays, and solar cells is still going. Strontium molybdate (SrMoO₃) is known to be a transparent conductor with the lowest electrical resistivity among all known perovskite oxides and might be a better substitute for ITO. Optical transparency and electrical conductivity greatly depend on the concentration of defects; therefore, materials of exceptional quality are desirable. Currently used techniques for the growth of SrMoO₃, such as pulsed laser deposition and electron beam evaporation, are based on the use of high energy species, and it may lead to the unintentional formation of defects and degraded quality of the material. Molecular beam epitaxy enables the growth of single-crystalline thin films of exceptional quality, provides unparalleled control over film stoichiometry and only utilizes low-energy particles; therefore, it is a suitable technique for the growth of SrMoO₃.
However, it is challenging to grow high quality SrMoO₃ thin films via MBE because molybdenum has very low vapor pressure, making it difficult to supply Mo atoms through a conventional effusion cell. One solution might be to use MoO₃ as a source of molybdenum atoms instead of a pure metal. In this talk, I will provide the results of the growth of transparent conductor SrMoO₃ thin films via molecular beam epitaxy (MBE) using MoO₃ pellets as a source of molybdenum atoms. Thermodynamic calculations of vapor pressure and stability of MoO₃ will be presented. I will show the images of custom-built effusion cell we used to prevent the reduction of volatile MoO₃ to non-volatile MoO₂ in vacuum. Additionally, I will present reflection high energy electron diffraction, x-ray diffraction, and atomic force microscopy data, as well as the results of optical and electrical measurements to prove that high-quality transparent conducting SrMoO₃ thin films can be grown via MBE in a self-regulated manner.

Halide perovskites are a chemically and electronically diverse class of compounds. They encompass 3D materials like the hybrid organic-inorganic CH₃NH₃PbI₃ and the all-inorganic double perovskite Cs₂BiAgBr₆, solar absorbers with excellent features for photovoltaic applications and beyond. This diversity of structures and optoelectronic properties is further expanded when reducing the dimensionality of these systems, as exemplified by the Ruddlesden-Popper perovskite (BA)₄AgBiBr₈ (BA=butylammonium), which can be thought of as a double perovskite monolayer with alternating AgBr₆ and BiBr₆ octahedra, separated by an organic spacer layer [1,2].
Accurately predicting the fundamental band gaps and excited state properties of this zoo of materials from first principles is challenging because of the intricate coupling of structural and electronic degrees of freedom [3], complex structural dynamics and strong spin-orbit interactions. In this contribution, I will discuss our recent progress in describing the optoelectronic properties of halide perovskites using density functional theory (DFT) and Green’s function-based many-body perturbation theory [4]. DFT-based molecular dynamics simulations allow us to consider temperature dynamics of halide perovskites and design structural models for our electronic structure calculations. A particular focus of this talk will be our calculations of the electronic structure and optical excitations of the double perovskite series Cs₂AgBX₆ (B=Bi, Sb; X=Br, Cl) and their 2D derivatives – a set of materials with unusual and highly tunable optoelectronic properties.

[1] B. A. Connor, L. Leppert, M. D. Smith, J. B. Neaton, H. I. Karunadasa, J. Am. Chem. Soc. 140, 5235 (2018).
[2] B. A. Connor, R. I. Biega, L. Leppert, H. I. Karunadasa, Chem. Science, Accepted Manuscript (2020).
[3] L. Leppert, S. E. Reyes- Lillo, J. B. Neaton, J. Phys. Chem. Lett. 7, 3683 (2016).
[4] L. Leppert, T. Rangel, J. B. Neaton, Phys. Rev. Materials 3, 103803 (2019).

Transparent conducting electrodes (TCEs) are essential in many optoelectronic devices including solar cells, LEDs, sensors and displays. The most common approach for TCEs is the use of transparent metal oxide layers, in particular Indium tin oxide (ITO). However, ITO comes with numerous drawbacks such as the relatively high cost, fragility, rarity of Indium, strong UV absorption and relatively high sheet resistance. An exciting TCE alternative is the use of interconnected metallic nanowires, where their sub-wavelength cross-sections makes them transparent to visible light [1,2,3].

In this work, we show a promising cost-effective method to fabricate metal nanowire structures using selective-area electrochemical deposition (SAEC). SAEC is based on electrochemical deposition through an insulating template onto a conductive substrate. Next to the low fabrication cost, electrochemical depositions is a bottom-up method with numerous advantages such as scalability, ambient operation conditions, control over nucleation and high purity. While submicron sized features have been already demonstrated by SAEC [4,5], obtaining homogeneous void free filling of features below 100 nm using direct plating is a major challenge because of the terminal effect and the poor control of nucleation of the seed layer.

Here, we have explored the SAEC fabrication of silver nanowire grids over a large area by using soft-conformal imprint lithography as means to fabricate the template on ITO or Si substrates. Numerical simulations of the full electrochemical 3D system have been performed to highlight the influence of ion diffusion, concentration, kinetic parameters and trench aspect ratio on the homogeneous bottom-up filling of the trenches in the template. Finally, we compare the optical and electrical properties of the SAEC-fabricated metal grids with those fabricated using conventional thermal or e-beam evaporation followed by lift-off. This work represents a low-cost and scalable strategy to fabricate sub 100 nm predefined metal structures, enabling the commercial viable fabrication of nanowire based transparent conductive electrodes for optoelectronic devices.

[1]: J. van de Groep, P. Spinelli, A. Polman, Nano Letters, 12, 3138-3144, (2012)
[2]: B. Sciacca, J. van de Groep, A. Polman, E.C. Garnett, Adv. Matt., 28, 905-909, (2016)
[3]: M.W. Knight, J. van de Groep, P.C.P. Bronsveld W.C. Sinke, A. Polman, Nano Energy, 30, 398-406, (2016)
[4]: D. Josell, S. Ambrozik, M.E. Williams, A.E. Hollowell, C.Arrington, S.Muramoto, T.P. Moffat, J. Electrochem., 166, D898-D907, (2019)
[5]: T.P. Moffat, J.E. Bonevich, W.H. Huber, A.Stanishevsky, D.R. Kelly, G.R. Stafford, D. Josell, J. Electrochem., 147, 4524-4535, (2000)

The combination of electrical conductivity and optical transparency in a single material gives transparent conducting oxides (TCOs) an important role in modern optoelectronic applications such as in solar cells, flat panel displays, and smart coatings. The most commercially successful TCO so far is tin doped indium oxide (Indium Tin Oxide – ITO), which has become the industrial standard TCO for many optoelectronics applications; the ITO market share was 93% in 2013. Its widespread use stems from the fact that lower resistivities have been achieved in ITO than in any other TCO; resistivities in ITO have reached as low as 7.2 × 10^-5Ω cm, while retaining >90% visible transparency. In recent years, the demand for ITO has increased considerably, however, indium is quite a rare metal, having an abundance in the Earth’s crust of only 160 ppb by weight, compared with abundances for Zn and Sn of 79000 ppb and 2200 ppb respectively, and is often found in unstable geopolitical areas. The overwhelming demand for ITO has led to large fluctuations in the cost of indium over the past decade. There has thus been a drive in recent years to develop reduced-indium and indium-free materials which can replace ITO as the dominant industrial TCO.[1,2] In this talk I will outline the strategies that we use in the Materials Theory Group to look beyond the current TCO materials, highlighting the interplay of theory and experiment.[3,4]

[1] J. E. N. Swallow et al., Advanced Functional Materials , 28, 1701900 (2018)
[2] M. J. Powell et al., Chemical Science, 9, 7968 (2018)
[3] J. E. N. Swallow et al., Materials Horizons, 7, 236 (2020)
[4] B. A. D. Williamson et al., Chemistry of Materials, 32, 1964 (2020)

Despite long-standing research efforts to develop p-type transparent conductive materials (TCMs), the current generation of optoelectronic devices still relies exclusively on n-type TCM contacts due to their much better trade-off between conductivity and transparency. An important issue in oxide-based p-type TCMs is the deep energy and localized nature of the 2p oxygen states in the valence band, which has negative consequences on both hole dopability and hole mobility in most oxides.

Recent computational work [1,2] has identified several phosphides (particularly BP and CaCuP) that could potentially outperform the existing p-type TCMs. These phosphides have shallow, disperse valence bands that are ideal for high hole dopability and mobility. Although their indirect band gaps are rather low (around 2 eV), their direct band gaps are much wider and could ensure transparency in thin film samples. However, thin films of CaCuP have not been made yet, and previously reported BP thin films have not been thoroughly evaluated as transparent contacts [3].

Using a unique combinatorial sputter chamber with reactive PH₃ gas, we have mapped the compositional and thermal phase space in BP and CaCuP. To evaluate their potential as p-type TCMs, we have characterized their application-relevant properties (optical transmission, carrier concentration, and mobility) with high-throughput methods.

In this contribution, we will discuss the pros and cons of sputter-deposited BP and CaCuP as prospective p-type TCMs. We will also contextualize their performance metrics by considering their thermal budget, air stability, and compatible substrates. Based on our experience, we will propose potential search strategies for new p-type transparent phosphides.

[1] B. Williamson et al., Chem. Mater., 29, 2402–2413 (2017).
[2] J. Varley et al., Chem. Mater., 29, 2568–2573 (2017).
[3] A. Fioretti, M. Morales-Masis, J. Photonics Energy, 10, 042002 (2020)

Transparent conductive electrodes based on metal oxide layers (such as indium tin oxide - ITO and fluorine-doped tin oxide - FTO) are commonly used materials in a variety of optoelectronic devices as they offer adjustable electrical and optical characteristics. However, these conventional transparent conductive oxide (TCO) electrodes possess several drawbacks, such as chemical and/or thermal instability. Such obstacles limit application of TCO’s in advanced device structures, which require suitable optoelectronic characteristics following high temperature or chemical treatment [1].
A broad range of carbon-based materials is currently under development as a promising alternative to conventional TCO electrodes. Particularly, synthetic diamond in the form of thin polycrystalline films doped with boron can be optimized for suitable optical transparency and electrical conductivity, such that it enables favorable chemical and thermal characteristics over conventional TCO electrodes [1,2]. Tunable electrical, optical and chemical properties of boron-doped diamond (BDD) predestine the potential for BDD based electrodes for implementation in optoelectronic and energy conversion devices [2,3].
In this work, we report on the development of optically transparent BDD electrodes synthesized using microwave plasma enhanced chemical vapor deposition. The BDD layers with a nanocrystalline form (thickness below 500 nm) were grown on transparent glass substrates and optimized for high optical transmission and low electrical resistivity. To expand potential applicability of fabricated electrodes in various temperature-sensitive structures, synthesis temperature was varied between 250 and 750 °C. The effect of the BDD synthesis temperature on the structural, morphological and optoelectronic properties of fabricated BDD electrodes is discussed. The BDD electrodes were examined by scanning electron microscopy, Raman spectroscopy, optical transmission measurements, Hall effect measurements and secondary ion mass spectroscopy. Additionally, electrochemical characteristics of synthesized BDD electrodes is presented, to highlight applicability of such electrodes in integrated photoelectrochemical systems as a multi-functional coating [2].

This work has been supported by the Grant Agency of the Czech Republic (GACR) contract 19-09784Y.

[1] T.H. Stuchlikova et al., Phys. Status Solidi B, 2020, 1900247
[2] P. Ashcheulov et al., ACS Appl. Mater. Interfaces 2018, 10, 35, 29552–29564
[3] A. Kovalenko et al., Sol. Energ. Mat. Sol. C., 2015, 134, 73–79.

The prediction of electronic levels introduced by impurities and electronic level alignment at interfaces is critical to the design of optoelectronic materials, particularly for photovoltaic applications, for optimal function. Such prediction is possible using first principles density functional theory (DFT) calculations. Due to both a need to reduce computational cost and a desire to extract design principles, machine learning has increasingly been used to extract relationships between these computed quantities and physical or chemical descriptors of the semiconductor system. In this talk, we will discuss our work on machine learning (ML) of impurity levels in optoelectronic materials including classical II-VI [1], III-V, and group IV semiconductors, as well as hybrid perovskite halides. We will discuss the accuracy and uncertainty quantifications of different approaches for ML. We will also discuss the extension of this approach to prediction of electronic band alignments at interfaces.

[1] Arun Mannodi-Kanakkithodi, Michael Y Toriyama, Fatih G Sen, Michael J Davis, Robert F Klie, Maria KY Chan, “Machine-learned impurity level prediction for semiconductors: the example of Cd-based chalcogenides,” npj Comput Mater 6, 39 (2020). https://doi.org/10.1038/s41524-020-0296-7

During the last decade, metal halide perovskites have revolutionized the area of renewable energy research. However, the performance of a perovskite solar cell (PSC) depends, not only on the perovskite, but also on the combination and alignment of all its comprising layers, such as electrodes and charge transport layers. NiO is a very promising hole transport layer and its electronic levels need to be optimally aligned with a given perovskite for maximum PSC efficiency. Applying surface modifiers is one of the most widespread strategies to tune its energy levels. Here, we investigate the effect of single layer adsorption of twenty different alkali halides on the electronic levels of NiO, using Density Functional Theory (DFT). Our results show that alkali halides can shift the position of the valence band maximum (VBM) of NiO to a surprisingly large extend in both directions, from -3.10 eV to +1.59 eV. Our results indicate that with alkali halide surface modifiers, the electronic levels of NiO can be tuned robustly and potentially match those of many perovskite compositions in perovskite solar cells.

Surfaces and charge-extracting contacts remain major sources of non-radiative recombination loss; limiting the performance of perovskite PV compared to theoretical limits. We investigate mixed-cation mixed-halide perovskites and demonstrate that, with chemical surface passivation, they can achieve >4 μs minority carrier lifetimes, and >20% external PL quantum yields (PLQY), and nearly single-exponential decay kinetics. This high PLQY observed corresponds to ~97% of the Shockley-Queisser theoretical quasi-fermi level splitting, a first for MA-free mixed-cation mixed-halide compositions. The high PLQY and long PL lifetimes allow us to measure average surface recombination velocities (SRV) <10cm/s, with a champion low of ~1cm/s (a conservative estimate). We show how such improvements in SRV should lead to an absolute improvement in PCE of >4% even with the best of contact alignments. We investigate different MA-free high bandgap compositions, including bandgaps relevant for tandem applications (1.7-1.8eV), and demonstrate performance improvements across all these compositions with our surface passivation strategy. Lastly, using a combination of surface characterization techniques, we show Lewis base interactions of the passivating agent with the perovskite surfaces.

Photovoltaic (PV) devices based on metal halide perovskite (MHP) absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 25% for lab cells and with large area devices in excess of 16%. For the solar application stability, the most demanding requirement to assess for PV, remains the outstanding issue. The problem of stability motivates basic science driven work on MHP based PV at NREL. Material and device insight can enable MHP PV stability along with the associated opportunities to further improve efficiency, scalability and realize unique device architectures. This talk will highlight work at NREL to develop understanding of defects and interfaces in MHPs and what these experiments indicate regarding the requirements to enhance stability of HPSCs. Discussion will focus on efforts to more carefully understand the implications of processing on a range of defects and interfaces to provide a foundation for enabling the technological requirements for PV and other devices. Ongoing efforts make, measure and ultimately control defects via material formation and processing in the context of high-volume manufacturing will also be touched upon. Hypothesized links between device architecture choices, questions regarding materials integration and coupling of formation along with resulting defects to stability will be discussed. More importantly approaches to move from hypothesis to insight using different experimental approaches will be presented. Work at NREL along these lines to provide critical insight regarding interface formation and passivation that appears to have direct implications for performance and its evolution over time in the resulting devices will be presented. New approaches to understanding the critical physical, chemical and electronic properties to provide predictive insight will also be discussed.

In solar cells, perovskite films are typically sandwiched in between charge selective layers, either organic or inorganic semiconductors, as well as their combinations. Electron/hole selectivity results from appropriate alignment of the electronic transport levels of these layers with the electronic bands of the perovskite. While transport layers should efficiently extract the photogenerated charge carriers, they should also not quench the perovskite luminescence to avoid a reduction of the quasi-Fermi level splitting and hence the maximum attainable photovoltage. Finally, ohmic charge injection/extraction and a high built-in potential are desirable to maximize fill factor both in single junction and tandem perovskite solar cells. We will review these design rules using fully vacuum deposited solar cells as a platform, as every layer can be fine-tuned or exchanged without substantially alter the device fabrication. The focus will be on wide bandgap, mixed halide perovskite solar cells, which suffer from a higher voltage deficit as compared to narrow bandgap analogous. We will highlight the importance of using non-quenching, energetically-matched hole and electron transport layers in order to reduce non-radiative charge recombination losses. Novel strategies and multilayers structures (including inorganic, organic and ionic compounds) to improve the device functioning will be presented. The implication for the perovskite compositional stability as well as for the device lifetime will also be discussed.

Within a decade, there has been a surge of interest into halide perovskite semiconductors. With tuneable bandgaps, versatile and facile deposition processes and excellent optoelectronic properties, these materials have found applications in photovoltaics (PV), broad-spectrum photodetectors, light-emitting diodes and lasers. While research into using these materials for emission and X-ray detection applications is now beginning to surge; currently, halide perovskites are most well known for their remarkable PV performance, with perovskite solar cells achieving certified power conversion efficiencies exceeding 25% in a remarkably short developmental timescale. Despite the truly impressive device performance, these materials have not yet reached their full potential. The major obstacle to this is an incomplete understanding of crystallisation processes, as well as defects which exist on the surfaces and interfaces of perovskite thin-films. Deficiencies at these interfaces, likely formed during the crystallisation process, are responsible for the major losses in perovskite-based optoelectronic devices; limiting charge extraction, increasing non-radiative recombination, contributing to hysteresis and increasing the voltage-loss of perovskite photovoltaics. Herein, I will present various effective strategies to reduce the defect densities of perovskite films and thus improve the performance of perovskite optoelectronic devices. First, I will discuss how manipulating precursor solutions can have significant impact on the quality of the perovskite films, through enabling reduced defect density. Consequently, this is one approach which can be used to boost device performance. Next, I will focus on a strategy in which we extrinsically modify the bottom interface of perovskite solar cells such that the crystallization of the perovskite yields films with fewer defects. Specifically, we show that ionic liquids can have a dramatic effect on the interfacial trap density in a perovskite solar cell. And finally, I will illustrate an approach in which we manipulate the charge-carrier density in the perovskite thin-film after its fully crystallized. We utilise molecular dopants to significantly alter the interfacial energetics in order to suppress non-radiative recombination losses. As a result, we observe a boost in both performance and stability. Importantly, these interface modification strategies can be used in tandem, resulting in further improvements in device efficiency. The utility of these defect mitigation strategies can readily be applied beyond perovskite PV and is likely to also improve the performance of a range of other perovskite-based optoelectronic devices.

Transparent electrodes are essential components of several kinds of high-efficiency solar cells. The photovoltaic devices working in the broadband spectra, from the ultraviolet (UV) to the near-infrared (NIR), require minimized parasitic absorption and high conductivity for these electrodes. Transparent conductive oxide (TCO) thin films are widely accepted choices for such applications. Among various successful TCOs, recently, Zr-doped indium oxide (IZrO) has been demonstrated as a broadband transparent and conductive material suitable for silicon heterojunction (SHJ) and silicon/perovskite tandem solar cells.^1,2 The main advantage of IZrO is combining specifications of wide bandgap, high carrier density, and high electron mobility in the same material.
Magnetron sputtering is by far the most established technique to fabricate TCO layers. During sputtering target material is removed in a ballistic process that sensitively depends on many interconnected processing parameters. This makes a stoichiometric transfer of target material challenging, while high kinetic energies of the arriving species may damage sensitive functional layers beneath. Conversely, in Pulsed Laser Deposition (PLD), the energy source for material ablation is physically decoupled from the vacuum equipment. This favors the stoichiometric transfer of the target material as well as provides a larger choice for deposition pressures and thermalizing the arriving species.
Despite these advantages, PLD is still considered an exotic fabrication method within the photovoltaic community due to the common concerns about the scalability of the technique reasoned by the small sample size for uniform coating and low deposition rates. In fact, significant progress in the upscaling of PLD³ has already allowed fabrication of high-quality films by on 200 mm circular wafers and fabrication of superconducting tape with deposition rates > 750 nm/min⁴, which is considerably high deposition rate.
In our work, we introduce wafer-scale (4”) room-temperature fabrication of high-mobility IZrO by PLD with deposition rates on par (>3 nm/min) with lab-scale RF sputtering equipment. We studied the optoelectronic properties and microstructure of the films and made a comparison with IZrO films grown by RF sputtering, both fabricated from an IZrO solid target with the same composition.
X-ray diffraction (XRD) reveals that as-deposited IZrO films are amorphous with small embedded nanocrystals. Their orientation is preserved as the preferred one after solid-phase crystallization at 200 °C. IZrO films exhibit excellent optoelectrical properties combining low absorptance (<10%) in the measured range (300-1500 nm) with high mobility (>60 cm²/Vs) leading to impressive sheet resistivity (R_sh) of <20 Ohm/sq for a 100-nm PLD grown film.
When applied as a front electrode in solar cells, IZrO enables high short circuit currents and the possibility to reduce the amount of indium by reducing the thickness of the films compared to standard ITO, which is critical considering the large scale utilization of such materials. On the other hand, higher deposition pressures during PLD fabrication offer a promising way to mitigate the sputter-induced damage for the deposition of rear transparent electrodes on top of sensitive functional layers as in the perovskite solar cells, which opens new opportunities for the development of high-efficiency tandem solar cells.

References
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2. E. Aydin et al., Advanced Functional Materials, vol. 29, no. 25, p. 1901741, 2019.
3. J. A. Greer, Journal of Physics D: Applied Physics, vol. 47, no. 3, p. 034005, 2013.
4. S. Lee et. al, Superconductor Science and Technology, vol. 27, no. 4, p. 044022, 2014.

Metal halide perovskite solar cells have largely fulfilled their potential of combining III-V-like optoelectronic properties with low-cost, solution-processing deposition techniques, achieving single-junction power conversion efficiencies (PCEs) >25%. A semiconducting nickel oxide (NiO_x) thin film is a highly desirable wide bandgap hole harvesting contact for perovskite solar cells and organic solar cells because it is low-cost, stable, and readily scalable.(1)Perovskite solar cells with NiO_x HTLs have delivered some of the best reported stability results under a variety of stressors, including the 85°C/85% relative humidity damp heat test, temperature cycling from -40°C to 85°C, and extended operational stability at elevated illumination and temperature.(2, 3) However, perovskite solar cells made with NiO_x HTLs typically result in lower open-circuit voltages (V_OC) compared to ultra-thin, undoped organic hole transport layers, which are more expensive and present problems for both scalability and stability. Enhancing the V_OC and increasing efficiency of NiO_x-based devices will directly result in a decreased photovoltaic module cost ($/W).
Here, we identify a surface-assisted electron transfer-proton transfer (ET-PT) mechanism at the NiO_x/perovskite interface leading to barriers for hole harvesting and device V_OC loss. Specifically, under-coordinated metal cation sites (Ni^≥3+) act both as a Lewis acid/oxidant towards dissociated lead halide precursors (i.e. I^-) and as a Brønsted base (proton-acceptor) with protonated amine precursors, forming A-site deficient – and therefore PbI_2-xBr_x-rich – perovskite at the perovskite/NiO_x interface. This defective perovskite region both increases recombination at the interface and acts as a hole extraction barrier, resulting in device V_OC loss.
We demonstrate the elimination of the extraction barrier via a simple, easily scaled approach: titration of surface oxide proton-acceptor sites by incorporating 1-5 mol% excess A-site cation to the perovskite precursor solution. This amount is sufficient to counteract the loss of organic cation, preventing formation of a PbI_2-xBr_x layer at the interface, ultimately leading to a V_OC increase >200 mV. We demonstrate that this mechanism applies to all forms of NiO_x, including thin films created from sol-gel precursors, nanoparticles, atomic layer deposition (ALD), and radio frequency (RF) sputtering, and is generalizable across multiple perovskite active layers, including state-of-the-art single junction and tandem relevant compositions. A p-i-n perovskite solar cell with 19.7% PCE at maximum power point (MPP) and V_OC up to 1.15 V is achieved. Devices maintain >90% of their initial performance after 2000 hours of thermal stability testing at 85 °C or >80% of their initial performance after 400 hours of accelerated operational stability testing under illumination and 55 °C in nitrogen. Our results extend the generality of acid/base-coupled oxidation-reduction reactions as the driving force for interface equilibration at metal oxide/perovskite interfaces, suggesting that metal halide perovskites have similar reactions with many types of metal oxide layers.

1. M. D. Irwin, D. B. Buchholz, A. W. Hains, R. P. H. Chang, T. J. Marks, p-Type semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cells. Proc. Natl. Acad. Sci. 105, 2783–2787 (2008).
2. C. C. Boyd, R. Cheacharoen, T. Leijtens, M. D. McGehee, Understanding Degradation Mechanisms and Improving Stability of Perovskite Photovoltaics. Chem. Rev. 119, 3418–3451 (2019).
3. J. Xu, C. C. Boyd, Z. J. Yu, A. F. Palmstrom, D. J. Witter, B. W. Larson, R. M. France, J. Werner, S. P. Harvey, E. J. Wolf, W. Weigand, S. Manzoor, M. F. A. M. van Hest, J. J. Berry, J. M. Luther, Z. C. Holman, M. D. McGehee, Triple-halide wide-band gap perovskites with suppressed phase segregation for efficient tandems. Science. 367, 1097–1104 (2020).

Perovskite solar cells (PSCs) have potential to deliver TW-scale energy via low-cost manufacturing. However, scaling is limited by slow, high temperature annealing of the inorganic transport layers and insufficient understanding of how scalable processing impacts interfacial device physics. This study addresses the NiO, a leading hole transport layer (HTL) for both high-stability single junction architectures and tandem cells. We present a method for scaling ultrathin NiO HTLs by pairing high-speed (60 m/min) flexographic printing with a rapid annealing Cu:NiO sol-gel ink. By engineering precursor rheology for rapid film-leveling, large area (> 50 cm²) NiO HTLs were flexographically printed with high uniformity and photovoltaic performance matching that of spin coated films. Doping and the combustion precursor design enables 30X faster annealing while reducing intrinsic variability of work function and conductivity originating from NiOH surface species.

Integrating these printed transport layers in planar inverted PSCs allows rapid fabrication of high-efficiency (PCE > 16 %) Cs_xFA_1-xPbI_1-yBr_y solar cells with low hysteresis and open circuit voltages (V_OC) of 1.11 V. XPS studies of the transition from Ni-OH to Ni-O bonding for low-temperature annealed NiO clarify the balance required for both effective carrier collection and high photostability. Paired with variable illumination intensity measurements and detailed transient photovoltage characterization, these measurements provide an improved understanding of the intrinsic defects present at the NiO-perovskite interface and the impact of scalable synthesis of interfacial charge transport layers.

Vacuum deposition is a key technique in the field of perovskite photovoltaics due to the advantages it presents with respect to the classical deposition methods such as solution processed perovskites. The lack of solvents and the need of annealing makes it a good candidate for industrialization and upscaling as it has already been demonstrated in the OLED field.
Despite its advantages, tuning the interfaces is essential to reach the high power conversion efficiencies found in solution processed perovskite solar cells. Metal oxides are an interesting choice for extraction layers due to their improved stability and reduced parasitic absorption. In this work we present our recent advances in using metal oxides as extraction layers optimising the interfaces in order to reach record open circuit voltages in CH3NH3PbI3 perovskite and using MoO3 as extraction layer by molecular passivation in both PiN and NiP structure improving the stability and opening new possibilities for tandem applications. We also show how we implemented this stable pinhole-free bottom layers in large area devices and mini modules, which is very promising for future industrial purposes.

The power conversion efficiency (PCE) of metal halide perovskite solar cells has shown rapid improvement since their inception over a decade ago, recently surpassing 25%. Stabilizing these devices for long-term operation is needed for their future commercial success. In addition to good external encapsulation, engineering electron and hole contact layers and their interfaces with the perovskite absorber are a means to achieving this goal. Atomic layer deposition (ALD) has emerged as a powerful tool for the growth of these contact layers due to the nanoscale control it affords over film thickness and composition. In this work, we will describe the significant impact that ALD metal oxide contacts can have on improving the long-term operational stability of perovskite solar cells. We show that unencapsulated, single-junction p-i-n perovskite devices with metal electrodes that incorporate ALD SnO₂ electron contacts exhibit high initial PCEs (up to 18.5%), and maintain on average 82% of their initial PCE following 250 hours of continuous, illuminated operation at maximum power in air at 60 °C. This result is a marked improvement over devices with only organic contacts, which lose all PCE after only 20 hours in the same harsh testing conditions. We also describe the development of ALD VO_x hole contact layers for n-i-p perovskite solar cells. We demonstrate that unencapsulated devices with ALD VO_x, a more stable alternative to the commonly used material of evaporated MoO_x, retain on average 71% of their initial PCE when aged for 1100 hours in ambient atmosphere with constant illumination and bias at 70 °C. The vapor deposition strategies described in this work can be applied to various perovskite solar cell designs, including tandem or non-planar devices, and provide a framework for improving the quality of barrier/encapsulation layers in other optoelectronic technologies.

Inorganic charge transport layers have not been readily adopted as the sole charge transport layer on top of perovskite in inverted P-I-N architectures because processing methods that involve high temperature annealing, particle bombardment, or sol-gels damage the perovskite layer. We report on a scalable, low-temperature TiO_x electron transport layer (ETL) produced directly on top of perovskite in an inverted architecture device by an open-air chemical vapor deposition (OA-CVD) process. It is an attractive low-energy and low-temperature alternative involving simple equipment that can be adopted for roll-to-roll and sheet-to-sheet processing, compatible with industrial manufacturing.
The OA-CVD pyrolysis of a titanium ethoxide precursor forms a fully inorganic, conformal, and compact TiO_x film while preserving the underlying perovskite structure. The resultant film is mechanically robust and exhibits a fracture toughness 30-fold higher than commonly used fullerene-based ETL films. The TiO_x is an effective barrier layer against environmental degradation—unencapsulated devices with the TiO_x ETL retain 60% of their initial efficiency after 1000 hours in ambient aging conditions—compared to devices with a fullerene ETL which were inoperable after 144 hours. TiO_x ETL films on top perovskite demonstrate comparable photoluminescence responses with a commonly used organic C₆₀ ETL counterpart. Inverted architecture devices with an optimized TiO_x ETL thickness of 70nm exhibited high open-circuit voltages of 1.08V. We comment on the photocatalytic properties of TiO_xand the influence of an amorphous film on maximum power point stability.

Perovskite solar cells (PSCs) with transparent electrodes can be integrated with existing solar panels in tandem configurations to increase the power conversion efficiency. A critical layer in semi-transparent PSCs is the inorganic buffer layer, which protects the PSC against damage when the transparent electrode is sputtered on top. The development of n-i-p structured semi-transparent PSCs has been hampered by the lack of suitable p-type buffer layers. In this presentation we develop a p-type CuO_x buffer layer, which can be grown uniformly over the perovskite device without damaging the perovskite or organic charge transport layers, can be grown using industrial scalable techniques and has high hole mobility (4.3 ± 2 cm² V^-1 s^-1), high transmittance (>95%), and a suitable valance band maximum for hole extraction (5.3 ± 0.2 eV). Semi-transparent PSCs with efficiencies up to 16.7% are achieved using the CuO_x buffer layer. Using these as top-cells, we demonstrate four-terminal tandems with two types of silicon bottom cells made using industrially-relevant methods, achieving efficiencies reaching 24.4%. Our work provides a new approach to integrate the most efficient architecture for PSCs in tandems, as well as other applications requiring semi-transparent devices, such as photoanodes for bias-free solar fuel production, transistors with transparent electrodes, or for spectroscopy on devices under operation.

Transparent conductive film (TCF) materials have become a critical area of research in the field of optoelectronics. Although indium tin oxide (ITO) has traditionally excelled as such a material, device specifications have moved towards flexible designs with milder treatments, and with indium a scarce resource, other materials have begun to replace ITO. Electrodes composed of 1D and 2D materials have matched the high performance of ITO, with very mild deposition processes. We utilized a superhydrophilic antireflective surface to investigate the impact of nanocapillary forces on the deposition of 1D and 2D materials using silver nanowires (AgNWs) and graphene oxide (GO), as model 1D and 2D materials, respectively. A layer-by-layer deposition of silica nanoparticles (SiNPs) and poly(allylamine hydrochloride) (PAH) was used to produce the superhydrophilic antireflective surfaces. The influence of the nanocapillary forces on the deposition of AgNWs and GO from their suspensions solution processed films was investigated. It was discovered that the coffee-ring effect during deposition was suppressed by the nanocapillary forces. The performance of our TCFs prepared with AgNWs on the superhydrophilic antireflective coatings was evaluated. A TCF of 25.0 Ω sq^-1 with a transparency of 92.1 % transmittance was obtained without removing the substrate background. This TCF reached a figure of merit of 1.76×10^-2 and retained the wetting properties of the nanoporous coating. The superhydrophilic antireflective surfaces with AgNWs showed an areal root mean square roughness (S_q) 28.41 nm, and the roughness was brought down to 15.67 nm with gradual coating of GO.

Electrical contacts are critical to nearly all semiconductor devices, and the behavior of these contacts can have massive implications to overall device performance. Decades of research into metal-semiconductor contacts continues today because of their complexity and our modern ability to control and probe these interfaces.
Our work, primarily in the context of photovoltaics, examines two classifications of modified metal-semiconductor contacts: Metal-insulator-semiconductor (MIS) Schottky barriers, and selective contacts based on metal oxides. MIS contacts were indeed the foundation of some of the first PV devices, but quickly gave way to p-n junction architectures because of greater stability and power conversion efficiency in the latter architecture. Using device physics simulations, we show that if proper junction properties can be created (Schottky barrier height, interface passivation), MIS-type PV devices can approach efficiencies of p-n homo- and hetero-junction cells. Our work exploring the interface properties of MIS structures for PV makes use of purposeful chemical surface preparation and the deposition of well-controlled oxide insulators using atomic layer deposition (ALD). I will discuss several observations including the role of fixed interface charge and interface layer thickness on Schottky barrier height.
Secondly, so-called “selective contacts” have been examined to boost efficiency of p-n and heterojunction silicon PV, and also to provide PV junctions without the need for traditional p-n junctions, similar to the MIS Schottky barrier case. In both cases, it can be argued that such contacts may reduce processing costs due to the low temperatures for the deposition of these materials. Notable developments include electron and hole-selective thin films that provide selective transport for one carrier while often reducing interfacial recombination rates. Our studies explore the introduction of aluminum oxide tunnel layers sandwiched between crystalline silicon and selective contact materials such as MoO_x. The potential impact of this configuration is to 1) increase in stability of the junction and 2) and increase in performance through minimization of carrier recombination at the contact. We have examined the role of the alumina layer thickness on interface quality and specific contact resistance. We also examined the role of several deposition techniques and explore the different processing conditions that modify the interfacial electronic properties of these passivated contacts.

One-dimensional (1D) nano-arrays can reduce light reflection loss, suppress recombination dynamics, guide charge carrier transport, and relax stress and strain in flexible optoelectronic devices, and thus result in improving optoelectronic function and stability under mechanical bending. However, in-situ fabrication of 1D nano-arrays, especially on polymer-based flexible electrodes, is very challenging, mainly due to degradation of the flexible electrodes at high temperature of the in-situ growth. In this talk, I will present that nanopillar arrays (NaPAs), made of NiO_x (function as hole transport layers)[1] and TiO₂, SnO₂ and metal Ti (serving as electron transport layers), are in-situ deposited onto a rigid/flexible electrode by glancing angle deposition (GLAD), and then are hybridized with perovskite to generate solar cells and photodetectors. The as-grown NaPAs enhance light transmittance, facilitate light harvesting in perovskite, facilitate the formation of large perovskite grains, prohibit exciton recombination dynamics, promote charge carrier transport and collection, and prevent the perovskite decomposition. All these advanced features lead to excellent performance of the hybrid perovskite-based optoelectronic devices with good anti-aging and mechanic stability. We devise an advanced technique of low-substrate-temperature GLAD generally adapted to in-situ deposit charge carrier transport layers made of inorganic NaPAs on flexible electrodes, to significantly enhance optoelectronic performance of flexible devices.

References
[1] S. Cong, G. F. Zou,* Y. H. Lou, H. Yang, Y. Su, J. Zhao,* C. Zhang, P. P. Ma, Z. Lu, H. Y. Fan,* and Z. F. Huang,* Fabrication of Nickel Oxide Nanopillar Arrays on Flexible Electrodes for High-Efficient Perovskite Solar Cells, Nano Lett. 2019, 19, 3676-3683.

I will discuss progress in interface management in, and among, perovskites and quantum dots, especially in relation to photovoltaics.

Silicon heterojunction (SHJ) solar cells coupled with hydrogenated intrinsic amorphous silicon (a-Si:H) layers enable high open-circuit voltages (V_oc) of up to 750 mV, but suffer from short-circuit current density (J_sc) losses due to parasitic absorption in a-Si:H, transparent conductive oxide (TCO) layers and shadowing of the metal fingers. Significant improvement in V_oc is expected only for thinner wafer. Increasing J_sc is a more promising route to further improve cell efficiency. Besides the use of high bandgap materials such as nanocrystalline silicon oxide or low carrier density TCO layers to reduce parasitic absorption, an improved light trapping that enables more light to be absorbed within the silicon wafer represents another important opportunity for J_sc improvement, which becomes even more significant when thinner wafers are used. In this work, a thin intrinsic a-Si:H layer was introduced after metallization of SHJ solar cell, formed a TCO/a-Si:H back reflector. Unlike sophisticated structures such as diffraction gratings, black silicon or other kind of nanophotonics, TCO/a-Si:H back reflector is much easier to be fabricated. Compared with the very often used silver back reflector, a-Si:H is less expensive and can still keep some of the bifaciality of the cells.

The intrinsic a-Si:H layer was deposited by plasma enhanced chemical vapor deposition (PECVD). It has a refractive index of around 4 at infrared (IR) wavelengths extracted from ellipsometry measurement. Sentaurus TCAD simulation was used for the rear optics in terms of optimizing the thickness of TCO/a-Si:H stack. We systematically investigated how the TCO/a-Si:H back reflector increases rear internal reflectance by measuring reflectance and external quantum efficiency before and after deposition of the a-Si:H layer. As predicted from simulation, due to increased IR reflection, J_sc was 0.4 mA/cm² higher with TCO/a-Si:H back reflector when applied on SHJ solar cells using 110 mm thickness wafer, while fill factor (FF) and V_oc remained constant. Combined with an optimized front MgF₂ double antireflection coating, we could demonstrate a 0.9 mA/cm² improvement in J_sc, which boosts the cell efficiency by 0.5%. Ultimately, a M2 size total area cell efficiency of 23.7% has been achieved with V_oc of 745 mV, J_sc of 39.4 mA/cm² and FF of 80.8% on 110 um thin wafer. It’s worth noting that the cell still kept a bifaciality of more than 50%. The reflector concept introduced here has a possibility to be used in advanced double-glass module encapsulation.

Hydrogenated amorphous silicon (a-Si:H) is a key material in realizing efficient passivating contacts for Si solar cells known as the silicon heterojunction (SHJ) technology. However, one drawback of the SHJ solar cells is that the a-Si:H, particularly when doped with boron, gives rise to a parasitic optical absorption loss due to its high absorption coefficients in the visible wavelengths. This adverse effect is pronounced in bifacial solar cells, as this type of device allows illumination from both the front and the rear surfaces. Here we pursued alternative materials focusing on hole-selective passivating contacts: one is the hydrogenated nanocrystalline Si (nc-Si:H) and the other is the novel metal-oxide material. For the nc-Si:H material, it is demonstrated that the replacement of (p)a-Si:H by (p)nc-Si:H provides not only an increase of J_sc, but also improvements in V_oc and FF. This results in an efficiency increase by 0.6% absolute compared to our reference cell with (p)a-Si:H, and a best cell efficiency of 23.5%. The factors that alter the optimum hole contact layer thickness depending on the material and deposition condition will be discussed. For the metal-oxide material, we have developed an atomic-layer-deposited TiO_x nanolayer that functions as an efficient and transparent hole-selective passivating contact, leading to proof-of-concept solar cells with efficiencies above 20%. This new function of TiO_x is opposite from the previous understanding that it acts solely as electron contact.

The use of metal-insulator-semiconductor (MIS) tunnel diodes as photovoltaics (PVs) relies on the Schottky barrier between the metal and semiconductor, which induces band bending in the semiconductor absorber allowing for efficient separation of electrons and holes. In the years following early MIS PV research in the 1970s and 1980s, the advent of atomic layer deposition (ALD) has provided a tool for enhanced control of ultrathin film deposition. One method to potentially improve MIS PV performance is the use of ALD oxide bilayers as tunnel insulators. Oxides with different oxygen areal densities have been demonstrated to form interfacial dipoles, manipulating the Schottky barrier. Previous studies have only explored the impact of dipoles between a single ALD oxide and interfacial SiO_x. The present work combines two ALD oxides, one (AlO_x) with oxygen areal density higher than that of SiO_x and another (LaO_x) with oxygen areal density lower than that of SiO_x according to literature. It has been hypothesized that the greater oxygen areal density difference would result in a greater impact on the Schottky barrier. It is demonstrated that, compared to AlO_x alone, an ALD LaO_x/AlO_x stack increases the average n-Si/Ni Schottky barrier height from 0.63 to 0.70 eV, and the average p-Si/Al barrier height from 0.81 to 0.90 eV. LaO_x was the oxide in direct contact with the Si substrate. The observation that the barrier height increases for both substrate types suggests that the effect is due to the ability of LaO_x to depin the Fermi level rather than an interfacial dipole.

Carrier selective contacts have become one of the leading advancements in photovoltaics with the most efficient structures exceeding 26% conversion efficiency. Selective carrier conduction implies that one type of charge carrier is preferentially collected at an electrical contact. Carrier selectivity increases the efficiency of solar cells by reducing recombination at metal contacts and avoiding highly doped emitter layers. Popular selective contact materials are often transition metal oxides (TMOs) due to high optical transparencies and large work functions that are useful for hole selective contacts to silicon. In contrast to previous articles, recent studies have indicated that the interface of p-type silicon and the molybdenum oxide (MoO_x) exhibits a significant Schottky barrier that decreases the efficiency of hole-selective contacts by impeding majority carrier hole collection. To alleviate this issue, atomic layer deposited (ALD) alumina (Al₂O₃) might be added between Si wafer and MoO_x with the expectation to generate a negative interface fixed charge (N_f) after annealing, decreasing band bending. ALD Al₂O₃ also has the ability to passivate interface traps with diffusion of precursor hydrogen during annealing. In this work, we hypothesize that insertion of a tunneling ALD Al₂O₃ layer between MoO_x and p-type Si will enable Schottky barrier height (Φ_bh) minimization that is tunable with N_f, enabling a high efficiency hole-selective contact.
Since there should not be quantifiable N_f in tunneling Al₂O₃ using capacitance-voltage (C-V) analysis, non-tunneling n-Si|Al₂O₃|Al MOSCAP structures were used to determine ideal processing for maximum negative N_f. The ALD growth of Al₂O₃ utilized trimethylaluminum and H₂O at 4 deposition temperatures. Half of the samples were annealed at 425°C in a nitrogen atmosphere prior to the application of Al contacts. After N_f quantification, p-Si|Al₂O₃|MoO_x|Al tunnel diodes with thin (~1 nm) as-deposited and annealed Al₂O₃ layers were fabricated to determine Φ_bh. Current density-voltage-temperature (J-V-T) and Mott-Schottky 1/C²-V measurements were used to establish a range for Φ_bh. Both methods showed that the as-deposited tunneling Al₂O₃ diodes had a higher Φ_bh compared to the diodes with annealed tunneling Al₂O₃, and therefore also supported the existence of a Φ_bh at the Si|MoO_x interface. Future work will involve using larger barrier height diodes using n-Si to more accurately calculate Φ_bh and confirm or reject the phenomenon observed.

Numerous alternative materials and solar cell device structures have been widely explored in order to reduce the cost of photovoltaic (PV) manufacture. Organic-inorganic component solar cells based on nanostructured semiconductor have built up in past two decades, which may promise the potentially low cost and high performance. However, the device performances are generally lower than its pristine all-inorganic PV devices, resulting from the numerous surface defect and improper organic-inorganic phase segregation. Here, we demonstrate that hybrid PVs based on organic conjugated molecular and nanostructured silicon can achieve a high power conversion efficiency by controlling the interface contact as well as surface passiviation.
An advantage of this type heterojunction devices presents the excellent light harvest capability of nanostructured as well as benefiting likely simple fabrication process. Especially, the composite of conjugated organic materials and nanostructured inorganic materials are potential candidates for cost-effective and efficient solar-energy-harvesting devices¹. This device can be operated by a light-modulated field effect solar cell, which can improve the device performance under accumulation of light illumination². In addition, this type device can be easily integrated with other type energy device, for example, an energy harvesting structure that integrates an organic-silicon heterojunction solar cell and a triboelectric nanogenerator (TENG) device is built to realize power generation from both sunlight and raindrop³. In addition, a self-charging power unit based on an organic-Si heterojunction solar cell and an organic supercapacitor, which simultaneously achieved both photoelectric conversion and energy storage⁴. Finally, we will demonstrate that low temperature processed conductive silver-based ink has wide application usage for low temperature processed solar cell and related plastic electronics.

1. Sun, B.; Shao, M.; Lee, S.-T. Advanced Materials 2016, 28, (47), 10539-10547.
2. Wang, Y.; Xia, Z.; Liu, L.; Xu, W.; Yuan, Z.; Zhang, Y.; Sirringhaus, H.; Lifshitz, Y.; Lee, S. T.; Bao, Q.; Sun, B. Advanced Materials 2017, 29, (18), 1606370.
3. Liu, Y.; Sun, N.; Liu, J.; Wen, Z.; Sun, X.; Lee, S.-T.; Sun, B. ACS Nano 2018, ACS Nano 2018, 12 (3), 2893-2899.
4. Liu, R.; Wang, J.; Sun, T.; Wang, M.; Wu, C.; Zou, H.; Song, T.; Zhang, X.; Lee, S.-T.; Wang, Z. L.; Sun, B. Nano Letters 2017, 17, (7), 4240-4247.

Photo-corrosion of anode participating the oxygen evolution reaction (OER) in photoelectrochemical cell (PEC) for water splitting is one of the obstacles for the long term stability and efficient solar to hydrogen conversion. In order to prevent the phot-corrosion, “electrically leaky” thick TiO₂ film was deposited onto a n-Si photoanode surface. However, the carrier transport mechanism through the thick dielectric layer and the influence of oxygen vacancy-related midgap states on the mechanism are still unclear. In order to verify the carrier transport through the midgap states of thick TiO₂ film, we only modulated the defect density of the protective TiO₂ film with no significant change of optical, physical, and chemical properties. It has been demonstrated that the defect density of the TiO₂ film is proportional to water-splitting activity. This result enables to explain the hole transport mechanism of the previously reported electrically leaky TiO₂ protection layer in n-Si photoanode. For the defect-level optimization, titanium sub-oxide (sub-TiO_x) protection layer with higher defect density compared to the TiO₂ layer was also investigated. The controlled incorporation of defects into TiO_x dramatically enhances the hole transport from the photoanode surface to electrolyte solution. The relationship between band structure and PEC activity of the photoanode system was explored depending on the defect density of the protection layer. Mott-Schottky analysis of this system suggests that the defect level of TiO_x films influences the band bending of n-Si, which governs the accessible density of defect states and the carrier recombination.

We report on improvement in the passivation quality of titanium oxide (TiO_x) by doping with tantalum (Ta). TiO_x has been considered one of the most promising materials for electron-selective contacts (ESCs) for crystalline silicon (c-Si) solar cells. There has been a high efficiency of 23.1% successfully achieved for a c-Si solar cell with a TiO_x rear-side ESC [1]. To realize higher efficiencies, it is necessary to further improve the passivation quality. One possible candidate for realizing such improvements is Ta doping. Ta doping reportedly causes a negative shift of the flat band voltage of TiO_x films [2], and the passivation quality of TiO₂ ESCs can therefore be improved by Ta doping. In addition, tantalum oxide (TaO_x) has long-term stability against UV light irradiation [3,4], and it is possible that Ta doping may also improve the long-term stability. In this work, we focus on an improvement in the passivation quality of TiO_x ESCs by doping with Ta.

We deposited TiO_x:Ta films by atomic-layer deposition on the both-sides of n-type c-Si substrates which were in advance oxidized in 30% hydrogen peroxide solution at 80 °C. Ta doping was realized by inserting a unitcycle of TaO_x monolayer into unitcycles of TiO_x. In this study, Ta doping concentration is characterized by a ratio of the number of TaO_x unitcycles to that of TiO_x unitcycles. The thickness of TiO_x:Ta passivation films was basically set to 3.5 nm. After the deposition, the TiO_x:Ta-passivated c-Si substrates were annealed at 130 °C in air. The effective minority-carrier lifetimes (τ_effs) were acquired from quasi steady-state photoconductance measurements, which were used as an indicator for the passivation qualities of the TiO_x:Ta ESCs.

The All the samples passivated with undoped TiO_x films and with TiO_x:Ta films with unitcycle ratios of 0.05, 0.09, 0.17, and 0.50 showed improvements in the τ_eff by annealing at 130 °C. However, the τ_eff of samples with TiO_x:Ta after annealing was, in most cases, higher than that of samples with undoped TiO_x. This indicates that doping with Ta improves the passivation quality of TiO_x ECSs. In addition, TiO_x:Ta with a relatively low amount of Ta doping tended to have high passivation qualities. The TiO_x:Ta with a unitcycle ratio of 0.05 showed the highest τ_eff of 784 μs in this study, which corresponded to an effective surface recombination quality of 24.9 cm/s. The TiO_x:Ta with a unitcycle ratio of 0.50 exhibited a rather lower passivation quality than that of the undoped TiO_x, which indicates excess Ta doping rather degrades the passivation quality.

The above results imply that TiO_x:Ta is one of the promising materials for ESCs. The results of experiments to elucidate the mechanism of the improvements in the passivation quality will be presented.

[1] J. Bullock et al., Adv. Energy Mater. 9 (2019) 1803367.
[2] X. Feng et al., Angew. Chem. Int. Ed. 48 (2009) 8095.
[3] Y. Wan et al., ACS Energy Lett. 3 (2018) 125.
[4] T. Wang et al., Ind. Eng. Chem. Res. 58 (2019) 5510.

Multijunction all-perovskite solar cells offer a route towards efficiencies of III-V materials at low cost by combining the advantages of low thermalization loss in multijunction architectures with the beneficial properties of perovskites - namely low processing cost, high throughput fabrication, and compatibility with flexible substrates. Here, I will discuss why flexible all-perovskite tandems are a very attractive iteration of the perovskite technology. I will focus on what is needed to make a good perovskite-perovskite tandem on a flexible substrate, and the advances that we have made towards this goal.
I will cover advances including a) the development and understanding of wide bandgap materials that leverage a novel bandgap-tuning strategy (without increasing bromine, hence avoiding significant phase segregation) to reach the ideal bandgaps for all-perovskite tandems, resulting in efficient devices that are stable to the combined stresses of heat and light; b) advances that we have made in interlayer design, working towards more ideal material properties for reducing shunt resistance; and c) the integration of these approaches to develop >23% efficient monolithic all-perovskite tandems on rigid substrates and 21% efficient devices on flexible plastic substrates.

Perovskite solar absorbers have the potential to disrupt the photovoltaic industry predominantly with their simultaneous promise of high efficiency and low cost. Whilst these two properties ensure a steady stream of research and development opportunities, commercial scaling requires assuring that the devices are sufficiently characterized to be confident of their long-term performance. Oxford PV has been addressing the challenges associated with commercially scaling its perovskite on silicon tandem solar cell design. Key practices that have been adopted during the development of highly efficient, reliable, and cost-competitive solar cells are the use of rigorous characterization tools and techniques, and the development of necessary models. This talk will highlight some of the challenges and considerations associated with bringing perovskite on silicon tandem solar cells from a laboratory prototype to a commercial product ready for manufacturing and how characterization has shaped and continues to inform that process.

Single-junction crystalline silicon solar cells dominate the photovoltaics market today, but their power conversion efficiencies (PCEs) are approaching the practical limit for this technology. The emergence of hybrid organic-inorganic metal halide perovskite semiconductors over the past decade has opened a window of opportunity to advance the performance of the market-dominating photovoltaic technologies through the development of tandem solar cells. These tandem solar cells promise high PCEs (>33%). To reach this landmark, a number of scientific and technological challenges related to high-efficiency perovskite-based tandem solar cell architectures, stable and high-quality perovskite semiconductor materials, and light management concepts need to be addressed. In this presentation, our progress on passivation strategies and novel interlayers to reduce the voltage losses in wide-bandgap and low-bandgap perovskite solar cells (PSCs), and in turn to increase the PCE of perovskite-based tandem photovoltaics, will be presented.
The search for efficient passivation interlayers for wide-bandgap perovskite top solar cells in n-i-p architecture directed our research in the past towards solution-processed 2D/3D perovskite heterostructures. Making use of these heterostructures for interface passivation, wide-bandgap PSCs (E_g ~1.72 eV) with record open-circuit voltage for this bandgap and very high PCEs of up to 19.4% were demonstrated [1]. The reduced voltage deficit is a result of the decreased non-radiative recombination losses at the perovskite/hole-transport layer (HTL) interface. The solution-processed 2D/3D perovskite heterostructure is compatible with 3D perovskite absorber layers of a wide range of bandgaps (1.65 eV ≤ E_g ≤ 1.85 eV) [2]. Using this strategy in 4-terminal perovskite/silicon and perovskite/CIGS tandem solar cells, high PCEs > 27% [3], and > 25% [2] were demonstrated, respectively, thereby significantly exceeding the record PCESs reported for the respective single-junction solar cell technologies.
Beyond these past results, at the MRS fall meeting we will further present our latest work on novel interlayers and passivation strategies to minimize the voltage loss in wide-bandgap (1.65 eV ≤ E_g ≤ 1.75 eV) as well as low-bandgap (E_g ~ 1.26 eV) perovskite solar cells in p-i-n architecture. The p-i-n architecture is a key to realize high-performance 2-terminal perovskite/silicon as well as perovskite/perovskite tandem solar cells. Moreover, we will present a novel route to deposit uniform ultra-thin polymer passivation layers onto perovskite thin films by chemical vapor deposition (CVD) [4]. The CVD polymerization promises reliable, scalable, and homogeneous deposition for future large-area processing of perovskite-based tandem solar cells.

References:
[1] Gharibzadeh, S. et al. Record Open-Circuit Voltage Wide-Bandgap Perovskite Solar Cells Utilizing 2D/3D Perovskite Heterostructure. Adv. Energy Mater. 9, 1–10 (2019).
[2] Gharibzadeh, S.; Hossain I. M. et al. 2D/3D Heterostructure for Semitransparent Perovskite Solar Cells with Engineered Bandgap Enables Efficiencies Exceeding 25% in Four-Terminal Tandems with Silicon and CIGS. Adv. Funct. Mater. 30, (2020).
[3] Duong, T. et al. High Efficiency Perovskite-Silicon Tandem Solar Cells: Effect of Surface Coating versus Bulk Incorporation of 2D Perovskite. Adv. Energy Mater. 10, 1903553 (2020).
[4] Malekshahi Byranvand M.; Behboodi-Sadabad F. et al. Chemical Vapor Deposited Polymer Layer for Efficient Passivation of Planar Perovskite Solar Cells. submitted

Integrating metal halide perovskite top cells with crystalline silicon (c Si) bottom cells into monolithic tandem devices has recently attracted increased attention due to the high efficiency potential of these cell architectures. To further increase the tandem device performance to a level well above the best single junctions, optical and electrical optimizations as well as a detailed device understanding for such architectures need to be developed [1]. Here we present our recent results on such monolithic tandem cells in the common 2-terminal (2T) as well as in a novel 3-terminal (3T) contacting scheme.
In the 2T design, we recently demonstrated a certified power conversion efficiency (PCE) of 29.15% [2,3]. I will discuss some key elements to this result: using a silicon heterojunction cell electron contact on the rear side, enabling the integration of a perovskite top cell in the p-i-n configuration; employing self-assembled monolayers (SAMs) as hole selective contacts for further reduction of non-radiative recombination losses, etc. With the latter, efficiencies of up to ~ 21 % were demonstrated for p-i-n perovskite single junctions [3].
In 3T tandems, one of the three contacts is shared by both subcells, allowing for the individual operation of both subcells in a monolithic device with a single substrate, thus combining the characteristic advantages of 2T and 4-terminal (4T) devices: high combined photogeneration currents as in state-of-the-art 2T tandems, and ideally independent electrical operation similar to 4T tandems, making the cell more robust against varying spectra. I will discuss our recent proof-of-concept study [4], demonstrating a 3T perovskite/silicon tandem cell by manufacturing an n-i-p perovskite top cell on an interdigitated back contact (IBC) silicon heterojunction solar cell. In our proof-of-concept device, the combined PCE of ~ 17% is still limited both optically and electrically, and it will be discussed how optimizations of the cell stack can bring this architecture to a PCE level comparable to 2T tandems. Furthermore, I will demonstrate that the subcells can indeed be operated largely independently at their individual maximum power points, allowing to extract the maximum power output for varying spectral conditions similar to 4T cells. A minimized series resistance at the shared electron contact on the cell’s rear side is identified as the crucial parameter for independent operation of the subcells, and I will discuss pathways to approach the simulated efficiency potential by an optimized device design.

References:
[1] Jost, Kegelmann, Korte, Albrecht, Adv. Energy Materials, 2020, doi: 10.1002/aenm.201904102
[2] NREL Best Research-Cell Efficiency Chart, https://www.nrel.gov/pv/cell-efficiency.html
[3] Köhnen, Jost, Stannowski, Albrecht et al., Sustainable Energy and Fuels, doi: 10.1039/C9SE00120D
[3] Al-Ashouri, Magomedov, Korte, Albrecht et al., En. Env. Sci, 2019, doi: 10.1039/C9EE02268F
[4] Tockhorn, Wagner, Korte et al., ACS Appl. En. Mat., 2020, doi: 10.1021/acsaem.9b01800

Among all commercialization routes for perovskite photovoltaics (PV), the perovskite/Si tandem is of particular interest as it capitalizes the mature silicon industry with decades of experience. Here we report low-temperature solution process for efficient two-terminal perovskite/Si tandem solar cells, wherein electrical and optical properties of the tunneling junction are carefully modulated. Moreover, we improve materials stability by introducing Eu²⁺/Eu³⁺ “redox shuttle” to suppress the Pb⁰/I⁰ impurities which inevitably occurred during device operation. In addition, we carefully analyze the cost of tandem devices in terms of levelized cost of energy (LCOE) by adopting the “bottom-up cost model”. It reveals perovskite PV exhibit low materials cost, which reduces the LCOE substantially in tandem devices. We conclude that further efforts are required to simultaneously improve the efficiency and lifetime of perovskite modules, which ultimately endows the competitiveness of perovskite tandem solar cells.

Silicon heterojunction (SHJ) solar cells coupled with hydrogenated intrinsic amorphous silicon (a-Si:H) layers enable high open-circuit voltages (V_oc) of up to 750 mV, but suffer from short-circuit current density (J_sc) losses due to parasitic absorption in a-Si:H, transparent conductive oxide (TCO) layers and shadowing of the metal fingers. Significant improvement in V_oc is expected only for thinner wafer. Increasing J_sc is a more promising route to further improve cell efficiency. Besides the use of high bandgap materials such as nanocrystalline silicon oxide or low carrier density TCO layers to reduce parasitic absorption, an improved light trapping that enables more light to be absorbed within the silicon wafer represents another important opportunity for J_sc improvement, which becomes even more significant when thinner wafers are used. In this work, a thin intrinsic a-Si:H layer was introduced after metallization of SHJ solar cell, formed a TCO/a-Si:H back reflector. Unlike sophisticated structures such as diffraction gratings, black silicon or other kind of nanophotonics, TCO/a-Si:H back reflector is much easier to be fabricated. Compared with the very often used silver back reflector, a-Si:H is less expensive and can still keep some of the bifaciality of the cells.

The intrinsic a-Si:H layer was deposited by plasma enhanced chemical vapor deposition (PECVD). It has a refractive index of around 4 at infrared (IR) wavelengths extracted from ellipsometry measurement. Sentaurus TCAD simulation was used for the rear optics in terms of optimizing the thickness of TCO/a-Si:H stack. We systematically investigated how the TCO/a-Si:H back reflector increases rear internal reflectance by measuring reflectance and external quantum efficiency before and after deposition of the a-Si:H layer. As predicted from simulation, due to increased IR reflection, J_sc was 0.4 mA/cm² higher with TCO/a-Si:H back reflector when applied on SHJ solar cells using 110 mm thickness wafer, while fill factor (FF) and V_oc remained constant. Combined with an optimized front MgF₂ double antireflection coating, we could demonstrate a 0.9 mA/cm² improvement in J_sc, which boosts the cell efficiency by 0.5%. Ultimately, a M2 size total area cell efficiency of 23.7% has been achieved with V_oc of 745 mV, J_sc of 39.4 mA/cm² and FF of 80.8% on 110 um thin wafer. It’s worth noting that the cell still kept a bifaciality of more than 50%. The reflector concept introduced here has a possibility to be used in advanced double-glass module encapsulation.

Hydrogenated amorphous silicon (a-Si:H) is a key material in realizing efficient passivating contacts for Si solar cells known as the silicon heterojunction (SHJ) technology. However, one drawback of the SHJ solar cells is that the a-Si:H, particularly when doped with boron, gives rise to a parasitic optical absorption loss due to its high absorption coefficients in the visible wavelengths. This adverse effect is pronounced in bifacial solar cells, as this type of device allows illumination from both the front and the rear surfaces. Here we pursued alternative materials focusing on hole-selective passivating contacts: one is the hydrogenated nanocrystalline Si (nc-Si:H) and the other is the novel metal-oxide material. For the nc-Si:H material, it is demonstrated that the replacement of (p)a-Si:H by (p)nc-Si:H provides not only an increase of J_sc, but also improvements in V_oc and FF. This results in an efficiency increase by 0.6% absolute compared to our reference cell with (p)a-Si:H, and a best cell efficiency of 23.5%. The factors that alter the optimum hole contact layer thickness depending on the material and deposition condition will be discussed. For the metal-oxide material, we have developed an atomic-layer-deposited TiO_x nanolayer that functions as an efficient and transparent hole-selective passivating contact, leading to proof-of-concept solar cells with efficiencies above 20%. This new function of TiO_x is opposite from the previous understanding that it acts solely as electron contact.

Numerous alternative materials and solar cell device structures have been widely explored in order to reduce the cost of photovoltaic (PV) manufacture. Organic-inorganic component solar cells based on nanostructured semiconductor have built up in past two decades, which may promise the potentially low cost and high performance. However, the device performances are generally lower than its pristine all-inorganic PV devices, resulting from the numerous surface defect and improper organic-inorganic phase segregation. Here, we demonstrate that hybrid PVs based on organic conjugated molecular and nanostructured silicon can achieve a high power conversion efficiency by controlling the interface contact as well as surface passiviation.
An advantage of this type heterojunction devices presents the excellent light harvest capability of nanostructured as well as benefiting likely simple fabrication process. Especially, the composite of conjugated organic materials and nanostructured inorganic materials are potential candidates for cost-effective and efficient solar-energy-harvesting devices¹. This device can be operated by a light-modulated field effect solar cell, which can improve the device performance under accumulation of light illumination². In addition, this type device can be easily integrated with other type energy device, for example, an energy harvesting structure that integrates an organic-silicon heterojunction solar cell and a triboelectric nanogenerator (TENG) device is built to realize power generation from both sunlight and raindrop³. In addition, a self-charging power unit based on an organic-Si heterojunction solar cell and an organic supercapacitor, which simultaneously achieved both photoelectric conversion and energy storage⁴. Finally, we will demonstrate that low temperature processed conductive silver-based ink has wide application usage for low temperature processed solar cell and related plastic electronics.

1. Sun, B.; Shao, M.; Lee, S.-T. Advanced Materials 2016, 28, (47), 10539-10547.
2. Wang, Y.; Xia, Z.; Liu, L.; Xu, W.; Yuan, Z.; Zhang, Y.; Sirringhaus, H.; Lifshitz, Y.; Lee, S. T.; Bao, Q.; Sun, B. Advanced Materials 2017, 29, (18), 1606370.
3. Liu, Y.; Sun, N.; Liu, J.; Wen, Z.; Sun, X.; Lee, S.-T.; Sun, B. ACS Nano 2018, ACS Nano 2018, 12 (3), 2893-2899.
4. Liu, R.; Wang, J.; Sun, T.; Wang, M.; Wu, C.; Zou, H.; Song, T.; Zhang, X.; Lee, S.-T.; Wang, Z. L.; Sun, B. Nano Letters 2017, 17, (7), 4240-4247.

In solar cells, perovskite films are typically sandwiched in between charge selective layers, either organic or inorganic semiconductors, as well as their combinations. Electron/hole selectivity results from appropriate alignment of the electronic transport levels of these layers with the electronic bands of the perovskite. While transport layers should efficiently extract the photogenerated charge carriers, they should also not quench the perovskite luminescence to avoid a reduction of the quasi-Fermi level splitting and hence the maximum attainable photovoltage. Finally, ohmic charge injection/extraction and a high built-in potential are desirable to maximize fill factor both in single junction and tandem perovskite solar cells. We will review these design rules using fully vacuum deposited solar cells as a platform, as every layer can be fine-tuned or exchanged without substantially alter the device fabrication. The focus will be on wide bandgap, mixed halide perovskite solar cells, which suffer from a higher voltage deficit as compared to narrow bandgap analogous. We will highlight the importance of using non-quenching, energetically-matched hole and electron transport layers in order to reduce non-radiative charge recombination losses. Novel strategies and multilayers structures (including inorganic, organic and ionic compounds) to improve the device functioning will be presented. The implication for the perovskite compositional stability as well as for the device lifetime will also be discussed.

Photovoltaic (PV) devices based on metal halide perovskite (MHP) absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 25% for lab cells and with large area devices in excess of 16%. For the solar application stability, the most demanding requirement to assess for PV, remains the outstanding issue. The problem of stability motivates basic science driven work on MHP based PV at NREL. Material and device insight can enable MHP PV stability along with the associated opportunities to further improve efficiency, scalability and realize unique device architectures. This talk will highlight work at NREL to develop understanding of defects and interfaces in MHPs and what these experiments indicate regarding the requirements to enhance stability of HPSCs. Discussion will focus on efforts to more carefully understand the implications of processing on a range of defects and interfaces to provide a foundation for enabling the technological requirements for PV and other devices. Ongoing efforts make, measure and ultimately control defects via material formation and processing in the context of high-volume manufacturing will also be touched upon. Hypothesized links between device architecture choices, questions regarding materials integration and coupling of formation along with resulting defects to stability will be discussed. More importantly approaches to move from hypothesis to insight using different experimental approaches will be presented. Work at NREL along these lines to provide critical insight regarding interface formation and passivation that appears to have direct implications for performance and its evolution over time in the resulting devices will be presented. New approaches to understanding the critical physical, chemical and electronic properties to provide predictive insight will also be discussed.

Within a decade, there has been a surge of interest into halide perovskite semiconductors. With tuneable bandgaps, versatile and facile deposition processes and excellent optoelectronic properties, these materials have found applications in photovoltaics (PV), broad-spectrum photodetectors, light-emitting diodes and lasers. While research into using these materials for emission and X-ray detection applications is now beginning to surge; currently, halide perovskites are most well known for their remarkable PV performance, with perovskite solar cells achieving certified power conversion efficiencies exceeding 25% in a remarkably short developmental timescale. Despite the truly impressive device performance, these materials have not yet reached their full potential. The major obstacle to this is an incomplete understanding of crystallisation processes, as well as defects which exist on the surfaces and interfaces of perovskite thin-films. Deficiencies at these interfaces, likely formed during the crystallisation process, are responsible for the major losses in perovskite-based optoelectronic devices; limiting charge extraction, increasing non-radiative recombination, contributing to hysteresis and increasing the voltage-loss of perovskite photovoltaics. Herein, I will present various effective strategies to reduce the defect densities of perovskite films and thus improve the performance of perovskite optoelectronic devices. First, I will discuss how manipulating precursor solutions can have significant impact on the quality of the perovskite films, through enabling reduced defect density. Consequently, this is one approach which can be used to boost device performance. Next, I will focus on a strategy in which we extrinsically modify the bottom interface of perovskite solar cells such that the crystallization of the perovskite yields films with fewer defects. Specifically, we show that ionic liquids can have a dramatic effect on the interfacial trap density in a perovskite solar cell. And finally, I will illustrate an approach in which we manipulate the charge-carrier density in the perovskite thin-film after its fully crystallized. We utilise molecular dopants to significantly alter the interfacial energetics in order to suppress non-radiative recombination losses. As a result, we observe a boost in both performance and stability. Importantly, these interface modification strategies can be used in tandem, resulting in further improvements in device efficiency. The utility of these defect mitigation strategies can readily be applied beyond perovskite PV and is likely to also improve the performance of a range of other perovskite-based optoelectronic devices.

Surfaces and charge-extracting contacts remain major sources of non-radiative recombination loss; limiting the performance of perovskite PV compared to theoretical limits. We investigate mixed-cation mixed-halide perovskites and demonstrate that, with chemical surface passivation, they can achieve >4 μs minority carrier lifetimes, and >20% external PL quantum yields (PLQY), and nearly single-exponential decay kinetics. This high PLQY observed corresponds to ~97% of the Shockley-Queisser theoretical quasi-fermi level splitting, a first for MA-free mixed-cation mixed-halide compositions. The high PLQY and long PL lifetimes allow us to measure average surface recombination velocities (SRV) <10cm/s, with a champion low of ~1cm/s (a conservative estimate). We show how such improvements in SRV should lead to an absolute improvement in PCE of >4% even with the best of contact alignments. We investigate different MA-free high bandgap compositions, including bandgaps relevant for tandem applications (1.7-1.8eV), and demonstrate performance improvements across all these compositions with our surface passivation strategy. Lastly, using a combination of surface characterization techniques, we show Lewis base interactions of the passivating agent with the perovskite surfaces.

An important area of research on metal halide perovskites (MHP) concerns the presence and role of surface and interface electronic gap states introduced by defects inherent to film processing, degradation under stress (T, irradiation, environment) or chemical reaction with neighboring layers, and the restrictions that these states impose on device performance, i.e., limit in Fermi level split, or aid carrier recombination, leading to reduction of solar cell open circuit voltage. This talk reports results on the determination of the presence and density of electronic states tailing above the valence band maximum of several MHPs, e.g., MAPbI3, CsPbI3, and FA0.85MA0.15Pb(I0.85Br0.15)3. Detailed measurements of the top of the valence band of these materials are performed using high-sensitivity UPS achieved in logarithmic detection. We focus specifically on MAPbI3 films processed with different additives mixed in the precursor-DMF solution: (i) no additives; (ii) DMSO; (iii) NH4++DMA+; and (iv) N-methyl-2-pyrrolidone thione (NMPT). The latter, an S-donor solvent, was investigated for its stronger coordination to Pb2+ as compared to the O-donor solvent DMSO, the additive widely used for improving film morphology. Significant differences are revealed between films processed with DMSO and NMPT. Once carefully treated via (experimental) background and parasitic satellite line contribution subtraction, the DMSO processed film exhibits a clear density of gap states extending from the valence band maximum to the Fermi level, whereas the NMPT processed film does not. The study was then complemented by Kelvin probe-based contact potential difference measurements to address the issue of surface photovoltage (SPV) during UPS. KP-based SPV measurements on these films show a significantly larger SPV (150 mV) and EF-VBM on the DMSO-treated films than on the NMPT-treated ones (SPV= 40 mV), confirming both the presence of the gap states and their impact on the electronic structure of the MHP films. The origin and nature of these gap states are still under investigation.

The electronic properties of interfaces in (opto-) electronic devices play a key role to reach the desired functionality and high efficiency. A wide range of organic and inorganic materials to tune interfacial properties is available today, and a comprehensive understanding of how they interact at interfaces is a prerequisite for realizing their full potential, particularly motivating their use in novel settings. This is exemplified here for the widely used interface modification agent polyethylenimine (PEI), which helps reducing electron injection barriers between electrodes and organic semiconductors. It is shown that PEI mixed with a polymer semiconductor in solution in fact serves two beneficial purposes simultaneously. PEI phase-separates from the semiconductor upon spin-coating and forms low work function bottom and top layers, which enhance electron injection. At the same time, minute amounts of PEI slightly n-dope poly{[N,N’-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} [P(NDI2OD-T2)] in the bulk film, thereby also increasing the electron mobility and boosting the conductivity overall by four orders of magnitude. The full capability of PEI interlayers for optimized electron injection at contacts, however, can only be reached by carefully excluding water during processing and applying mild annealing for improved surface coverage. With this, PEI interlayers reduce the work function of ZnO and graphite electrodes to a record low value of 2.3 eV.
Also the combination of organic conductive polymers and inorganic semiconductors provides a rich playground for interface manipulation. It has been demonstrated before that n-type Si coated with a layer of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) actually returns fully functional solar cells, despite the absence of a pn-junction in Si. We could now provide direct evidence that PEDOT:PSS induces indeed a large upward band bending in n-Si, even reaching inversion conditions. From a variation of processing conditions, suggestions on further improvements for this attractive approach to surface-dope inorganic semiconductors to the extreme can be derived.
It will further be noted how molecular photochromic switches, in the form of a monolayer at the interface between electrodes and an organic semiconductor, can be employed to facilitate multifunctionality of a device. As an example, an organic light emitting diode (OLED) will be discussed, whose brightness can be controlled reversibly during operation by both applied bias and external short-time light irradiation.

Electrical contacts are critical to nearly all semiconductor devices, and the behavior of these contacts can have massive implications to overall device performance. Decades of research into metal-semiconductor contacts continues today because of their complexity and our modern ability to control and probe these interfaces.
Our work, primarily in the context of photovoltaics, examines two classifications of modified metal-semiconductor contacts: Metal-insulator-semiconductor (MIS) Schottky barriers, and selective contacts based on metal oxides. MIS contacts were indeed the foundation of some of the first PV devices, but quickly gave way to p-n junction architectures because of greater stability and power conversion efficiency in the latter architecture. Using device physics simulations, we show that if proper junction properties can be created (Schottky barrier height, interface passivation), MIS-type PV devices can approach efficiencies of p-n homo- and hetero-junction cells. Our work exploring the interface properties of MIS structures for PV makes use of purposeful chemical surface preparation and the deposition of well-controlled oxide insulators using atomic layer deposition (ALD). I will discuss several observations including the role of fixed interface charge and interface layer thickness on Schottky barrier height.
Secondly, so-called “selective contacts” have been examined to boost efficiency of p-n and heterojunction silicon PV, and also to provide PV junctions without the need for traditional p-n junctions, similar to the MIS Schottky barrier case. In both cases, it can be argued that such contacts may reduce processing costs due to the low temperatures for the deposition of these materials. Notable developments include electron and hole-selective thin films that provide selective transport for one carrier while often reducing interfacial recombination rates. Our studies explore the introduction of aluminum oxide tunnel layers sandwiched between crystalline silicon and selective contact materials such as MoO_x. The potential impact of this configuration is to 1) increase in stability of the junction and 2) and increase in performance through minimization of carrier recombination at the contact. We have examined the role of the alumina layer thickness on interface quality and specific contact resistance. We also examined the role of several deposition techniques and explore the different processing conditions that modify the interfacial electronic properties of these passivated contacts.

The prediction of electronic levels introduced by impurities and electronic level alignment at interfaces is critical to the design of optoelectronic materials, particularly for photovoltaic applications, for optimal function. Such prediction is possible using first principles density functional theory (DFT) calculations. Due to both a need to reduce computational cost and a desire to extract design principles, machine learning has increasingly been used to extract relationships between these computed quantities and physical or chemical descriptors of the semiconductor system. In this talk, we will discuss our work on machine learning (ML) of impurity levels in optoelectronic materials including classical II-VI [1], III-V, and group IV semiconductors, as well as hybrid perovskite halides. We will discuss the accuracy and uncertainty quantifications of different approaches for ML. We will also discuss the extension of this approach to prediction of electronic band alignments at interfaces.

[1] Arun Mannodi-Kanakkithodi, Michael Y Toriyama, Fatih G Sen, Michael J Davis, Robert F Klie, Maria KY Chan, “Machine-learned impurity level prediction for semiconductors: the example of Cd-based chalcogenides,” npj Comput Mater 6, 39 (2020). https://doi.org/10.1038/s41524-020-0296-7

Halide perovskites are a chemically and electronically diverse class of compounds. They encompass 3D materials like the hybrid organic-inorganic CH₃NH₃PbI₃ and the all-inorganic double perovskite Cs₂BiAgBr₆, solar absorbers with excellent features for photovoltaic applications and beyond. This diversity of structures and optoelectronic properties is further expanded when reducing the dimensionality of these systems, as exemplified by the Ruddlesden-Popper perovskite (BA)₄AgBiBr₈ (BA=butylammonium), which can be thought of as a double perovskite monolayer with alternating AgBr₆ and BiBr₆ octahedra, separated by an organic spacer layer [1,2].
Accurately predicting the fundamental band gaps and excited state properties of this zoo of materials from first principles is challenging because of the intricate coupling of structural and electronic degrees of freedom [3], complex structural dynamics and strong spin-orbit interactions. In this contribution, I will discuss our recent progress in describing the optoelectronic properties of halide perovskites using density functional theory (DFT) and Green’s function-based many-body perturbation theory [4]. DFT-based molecular dynamics simulations allow us to consider temperature dynamics of halide perovskites and design structural models for our electronic structure calculations. A particular focus of this talk will be our calculations of the electronic structure and optical excitations of the double perovskite series Cs₂AgBX₆ (B=Bi, Sb; X=Br, Cl) and their 2D derivatives – a set of materials with unusual and highly tunable optoelectronic properties.

[1] B. A. Connor, L. Leppert, M. D. Smith, J. B. Neaton, H. I. Karunadasa, J. Am. Chem. Soc. 140, 5235 (2018).
[2] B. A. Connor, R. I. Biega, L. Leppert, H. I. Karunadasa, Chem. Science, Accepted Manuscript (2020).
[3] L. Leppert, S. E. Reyes- Lillo, J. B. Neaton, J. Phys. Chem. Lett. 7, 3683 (2016).
[4] L. Leppert, T. Rangel, J. B. Neaton, Phys. Rev. Materials 3, 103803 (2019).

New opto-electronic devices from solar cells to transparent transistors require high performance materials. Ab initio high-throughput computations offer an accelerated path towards the discovery of these exceptional materials. We can now screen thousands of materials by their computed properties even before the experiments. This computational paradigm allows experimentalists to focus on the most promising candidates, and enable researchers to efficiently and rapidly explores new chemical spaces.
In this talk, I will present the challenges and opportunities in materials discovery offered by high-throughput ab initio computing in searching for materials with exceptional opto-electronic properties. Recent examples from our work in the fields of transparent conducting materials and solar absorbers will be used to illustrate the approach. I will discuss the materials highlighted by these computational searches but also show how trends and design rules can be extracted from high-throughput data sets. Finally, I will report on our efforts in scaling up to high-throughput new properties from point defects to carrier mobilities.

Scandium nitride (ScN) is a semiconducting nitride that has the rocksalt crystal structure, but with a lattice parameter that provides a close match to GaN; the lattice mismatch is less than 1%. One reason ScN has attracted recent interest is because adding ScN to AlN enhances the piezoelectric properties, and may even lead to ferroelectricity. Here I will focus on interfaces between ScN and GaN. ScN can be highly n-type doped and can thus serve as an almost metallic contact or current spreading layer to GaN. If rocksalt ScN is grown along the [111] direction on top of c-axis-oriented wurtzite GaN, interesting phenomena related to spontaneous polarization occur. GaN is polar with a large spontaneous polarization, while ScN is centrosymmetric. This suggests the possibility of a very large spontaneous polarization difference at the interface. We have examined the properties of such interfaces using first-principles calculations based on density functional theory [1]. We first apply the modern theory of polarization to obtain the bulk properties of the individual materials. The resulting difference in polarization constants yields a polarization discontinuity at the ScN/GaN interface of -1.358 C/m². As a result, we expect a high-density electron gas on the N-polar GaN/ScN interface, and a high-density hole gas on the Ga-polar GaN/ScN interface, with carrier concentrations up to 8.5E14/cm². This value is over an order of magnitude larger than what can be achieved at GaN/AlGaN interfaces. We have confirmed the presence of strong electric fields and bound interface charges in explicit interface calculations. The large polarization difference and small strain makes ScN a promising material for applications in polarization-enhanced p-GaN/ScN/n-GaN tunnel junctions. Current tunnel junctions make use of InGaN interlayers to reduce the effective barrier and increase the polarization field across the p-n junction to reduce the junction width. ScN is a promising alternative, with a higher polarization field, smaller bandgap, and much smaller strain.

Work performed in collaboration with N. L. Adamski and C. E. Dreyer, and supported by AFOSR.

[1] N. L. Adamski, C. E. Dreyer, C. G. Van de Walle, Appl. Phys. Lett. 115, 232103 (2019).