Symposium Organizers
Daniel Friedman National Renewable Energy Laboratory
Michael Stavola Lehigh University
Wladek Walukiewicz Lawrence Berkeley National Laboratory
Shengbai Zhang Rensselaer Polytechnic Institute
EE1: Oxides, Silicon, and Emerging Materials
Session Chairs
Tuesday PM, April 06, 2010
Room 3007 (Moscone West)
9:30 AM - **EE1.1
Defects in Emerging Photovoltaic Materials: Oxides and Nitrides.
Joel Ager 1
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractConsideration of terrestrial element abundances has led to the consideration of a number of metal oxides and nitrides as components of future-generation solar cells. Driven by both practical and economic constraints, crystalline defects will be an unavoidable feature of these materials. In some cases, they will provide needed conductivity type control, as in the acceptor-type vacancies in p-type oxides such as Cu2O. In other cases, they will be deleterious to charge transport and collection. The affect of defects on the photovoltaic potential of two materials systems will be discussed in detail. (1) The InGaN bandgap can be tuned across nearly the entire solar spectrum (0.7 – 3.4 eV), suggesting this materials system as an ideal absorber for solar cells. It will be shown that native point and extended defects are resonant donors in In-rich material; the implications for pn homojunction and heterojunction formation and for charge transport in solar cells will be discussed. (2) Native defects control the conductivity type of ferroelectric BiFeO3 (Eg = 2.7 eV). Under conditions that encourage Bi vacancy formation, p-type films are produced, and all-oxide solar cells can be fabricated using n-ITO as a top contact. The cells produce open circuit voltages approaching 1 V. Finally, the use of nanostructuring to improve charge collection in materials with poor transport properties will be discussed, using Cu2O nanowires as a model system. This work was performed in the Helios Solar Energy Research Center and is supported by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
10:00 AM - EE1.2
The Behaviour of Hydrogen in Cu2O.
David Scanlon 1 , Graeme Watson 1
1 School of Chemistry, Trinity College Dublin, Dublin, Dublin, Ireland
Show AbstractFor decades, Cu2O has been studied for photovoltaic applications,[1, 2] with much interest centred on developing functional p-n homojunctions made solely of Cu2O. While Cu2O is a native p-type oxide[3], there have been frequent reports of n-type Cu2O samples.[4-7] Oxygen vacancies have been tentatively reported to be the cause of this n-type conductivity[8], but to date an explanation for the reported n-type conductivity in samples of Cu2O has been lacking. One possible unintentional source of n-type conductivity in oxides is hydrogen. Hydrogen is ubiquitous, and has been reported both theoretically and experimentally to be the source of n-type conductivity in ZnO[9,10], SnO2[11, 12] and In2O3[12] among others. Little is known about the behaviour of hydrogen in Cu2O, although a recent Muonics study has indicated that hydrogen causes a transition level ~ 1eV below the CBM of Cu2O. However, this study could not determine whether this represented hole or electron ionization.[13] Using state of the art hybrid density functional theory we report the electronic behaviour of hydrogen defects in Cu2O. We analyse the formation of native n-type and hydrogen defects in Cu2O, and compare them to the formation energies of p-type defects in Cu2O[14] On the basis of these results we discuss the origins of the reported n-type conductivity in Cu2O samples.
[1] D. Trivich, E. Y. Yang, R. J. Comp and F. Ho, Proc. 12th Photovoltaic Specialists Conf, IEEE, p875 (1976)
[2] Z. Yang, C. –K. Chiang and H. –T. Chang, Nanotechnology, 19, 025604 (2008)
[3] H. Raebiger, S. Lany and A. Zunger, Phys. Rev. B, 76, 045209 (2007)
[4] A. Survila, A Surviliene, S. Kanapeckaite, J. Budiene, P. Kalinauskas, G. Stalnionis and A. Sudavieius, J. Electroanal. Chem. 582, 221 (2005)
[5] L. C. Wang and M. Tao, Electrochem, Solid-State Lett. 10, H248 (2007)
[6] C. A. N. Fernando, T. M. W. J. Bandara and S. K. Wethasingha, Sol. Energy Mater. Sol. Cells, 70, 121 (2001)
[7] C. A. N. Fernando, P. H. C. de Silva, S. K. Wethasingha,I. M. Dharmadasa, T. Delsol and M. C. Simmonds, Renewable Energy, 26. 521 (2002)
[8] W. Siripala and J. R. P. Jayakody, Solar Energy Mater. 14, 23 (1986)
[9] C. G. Van de Walle, Phys. Rev. Lett. 85, 1012, (2000)
[10] M.D. McCluskey and S.J. Jokela, Proc. of the NATO Advanced Workshop on Zinc Oxide, 194, 125 (2005)
[11] P. D. C. King, R. L. Lichti, Y. G. Celebi, J. M. Gil, R. C. Vilão, H. V. Alberto, J. Piroto Duarte, D. J. Payne, R. G. Egdell, I. McKenzie, C. F. McConville, S. F. J. Cox, and T. D. Veal, Phys Rev B, 80, 081201 (2009)
[12] A. K. Singh, A. Janotti, M. Scheffler, and C. G. Van de Walle, Phys. Rev. Lett. 101, 055502 (2008).
[13] S F J Cox, J S Lord, S P Cottrell, J M Gil, H V Alberto, A Keren, D Prabhakaran, R Scheuermann and A Stoykov, J. Phys. Condens. Matter, 18, 1061, (2006)
[14] D. O. Scanlon, B. J. Morgan, G. W. Watson and A. Walsh, Phys. Rev. Lett. 103, 096405 (2009)
10:15 AM - EE1.3
Acceptor Levels in p-type Cu2O: Rationalizing Theory and Experiment.
David Scanlon 1 , Benjamin Morgan 1 , Aron Walsh 2 , Graeme Watson 1
1 School of Chemistry, Trinity College Dublin, Dublin, Leinster, Ireland, 2 Department of Chemistry, University College London, London, London, United Kingdom
Show AbstractCuprous oxide (Cu2O) is a prototypical p-type conducting oxide with applications in photovoltaics, dilute magnetic semiconductors, low cost solar cells, gas sensors and catalysis.[1] Cu2O is also the parent compound of many p-type transparent conducting oxides (TCOs) such asCuMO2 delafossites (M = Al, Cr, B, In, Ga, etc.)[2] and SrCu2O2[3], which are thought to retain the valence band features and conduction mechanisms of Cu2O[4]. Understanding conduction in Cu2O is therefore vital to the optimization of Cu-based materials for photovoltaics applications. We have calculated the electronic structure of p-type defects in Cu2O using GGA, GGA+U and Hybrid DFT. We demonstrate that obtaining an accurate description of the polaronic[5-7] nature of p-type defects in Cu2O is not possible using GGA or GGA + U.[8] Using a screened hybrid-density-functional approach we have investigated the formation of p-type defects in Cu2O, giving rise to single-particle levels that are deep in the band gap, consistent with experimentally observed activated, polaronic conduction. Our calculated transition levels for simple and split copper vacancies explain for the first time the source of the two distinct hole states seen in DLTS experiments.[9] The necessity of techniques that go beyond the present generalized-gradient- and local-density-approximation techniques for accurately describing p-type defects in Cu(I)-based oxides is discussed.
[1] S. Kale, S. Ogale, S. Shinde, M. Sahasrabuddhe, V. Kulkarni, R. Greene, and T. Venkatesan, Appl. Phys. Lett. 82, 2100 (2003).
[2] X. Nie, S.-H. Wei, and S. B. Zhang, Phys. Rev. Lett. 88, 066405 (2002).
[3] A. Kudo, H. Yanagi, H. Hosono, and H. Kawazoe, Appl. Phys. Lett. 73, 220 (1998).
[4] A. N. Banerjee and K. K. Chattopadhyay, Prog. Cryst. Growth Char. Mater. 50, 52 (2005).
[5] J.W. Hodby, T. E. Jenkins, C. Schwab, H. Tamura, and D. Trivich, J. Phys. C 9, 1429 (1976).
[6] J. H. Park and K. Natesan, Oxid. Met. 39, 411 (1993).
[7] A. Bose, S. Basu, S. Banerjee, and D. Chakravorty, J. Appl. Phys. 98, 074307 (2005).
[8] D. O. Scanlon, B. J. Morgan, and G.W. Watson, J. Chem. Phys. 131, 124703 (2009).
[9] D. O. Scanlon, B. J. Morgan, G. W. Watson and A. Walsh, Phys. Rev. Lett. 103, 096405 (2009)
10:30 AM - EE1.4
Assessment of Defects in Hematite, Iron Perovskites and Ti-oxynitride With Soft X-ray and Photoelectron Valence Band Spectroscopy.
Artur Braun 1 , Kranthi Akurati 1 , Andri Vital 1 , Debajeet Bora 1 3 , Selma Erat 1 2 , Edwin Constable 3 , Thomas Graule 1 4
1 Laboratory for High Performance Ceramics, EMPA, Dübendorf Switzerland, 3 Department of Chemistry, University of Basel, Basel Switzerland, 2 Nonmetalic Inorganic Materials, ETH Zürich, Zürich Switzerland, 4 , TU Bergakademie Freiberg, Freiberg Germany
Show AbstractInspection of the pre-edge in oxygen near-edge x-ray absorption fine structure spectra of single step, gas phase synthesized titanium oxynitride nanoparticles with 20 nm size reveals an additional eg resonance in the valence band that went unnoticed in previous TiO2 anion doping studies. The relative spectral weight of this Ti(3d)-O(2p) hybridized state with respect to and located between the readily established t2g and eg resonances scales qualitatively with the photocatalytic decomposition power, suggesting that this extra resonance bears co-responsibility for the photocatalytic performance of titanium oxynitrides at visible light wavelengths.A similar observation is made in hematite nanoparticle films that were subject to photoelectrochemical treatment.Heat treatment of photoelectrochemical hematite films under ambient pressure photoelectron spectroscopy shows how paricular spectral features disappear, whereas significant extra spectral intensity appears at the Fermi energy. Our findings confirm that the optical and photoelectrochemical properties are reflected in the valence band structure of the materials.
10:45 AM - EE1.5
Defects in Irradiated ZnO Thin Films Studied by Photoluminescence and Photoconductivity.
Reinhard Schwarz 1 , Rachid Ayouchi 1 , Marta Brandao 1 , Carlos Marques 2 , Eduardo Alves 2 , Melanie Pinnisch 3 , Detlef Hofmann 3 , Bruno Meyer 3
1 Physics Department, Instituto Superior Técnico, Lisbon Portugal, 2 , Instituto Tecnológico e Nuclear, ITN, Lisbon Portugal, 3 I. Physics Department, University of Giessen, Giessen Germany
Show AbstractPulsed-laser-deposited ZnO thin films were exposed to a 2 MeV helium ion beam to study the changes in radiative and non-radiative recombination. We first measured photoluminescence (PL) spectra at 4.2 K excited with the 325 nm line of a HeCd laser. The as-deposited films showed a donor-bound exciton peak at 3.3567 eV at 4.2 K attributed to Zn interstitials. After irradiation the donor-bound-exciton dominated PL spectra shifted to acceptor-bound behaviour with a signal at 3.3519 eV, tentatively attributed to Li or Na acceptors. In contrast to the approximately 30 % decrease of the PL signal near the band edge, we observed a strong enhancement of the “orange” and “green” PL bands near 2.1 eV and 2.8 eV, respectively, by a factor of over 4. Candidates for those transitions are Li impurities and/or O vacancies that increase non-radiative recombination. For comparison, we also scanned the irradiated region by a focused HeCd laser beam and observed an approximately 20 % decrease of the photocurrent (PC) in accordance with the decrease of the radiative PL signal. Time-resolved PC measurements using a fourth-harmonic Nd:YAG laser beam at 266 nm with a 5 ns pulse width showed that the initial PC signal peak height was essentially unaffected by the particle irradiation, while the PC decay at longer times is faster for the degraded films. In terms of detector applications, this result suggests that pulse-height detection is only little affected, while sensors in charge-detection mode would be hampered by increased recombination.
11:30 AM - **EE1.6
Defect Engineering in Earth-abundant Inorganic Photovoltaic Materials.
Tonio Buonassisi 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractIn recent years, many promising directions have emerged to engineer bulk defects in inorganic photovoltaic (PV) materials, towards improving device performance, reducing raw materials constraints, and reducing cost. We will systematically review these advances, using a common framework that emphasizes device performance. Common themes will include the intentional manipulation of impurities and structural defects during growth or cell processing, towards engineering desired electrical and/or optical properties in absorber-layer materials.We being by examining the state-of-the-art in multicrystalline silicon (mc-Si), the material that comprises ~50% of commercial solar panels produced today. Simulations and experimental data indicate that substantial efficiency improvements are possible via “defect engineering,” i.e., intentionally engineering the distributions, chemical states, and electrical activities of bulk microdefects. We focus on two thrust areas of particular technological relevance: engineering of impurities and dislocations.We extend our understanding of bulk defects to evaluate the state-of-the-art in inorganic Earth-abundant thin film materials. Examples of grain boundary and dopant engineering will be provided for model oxide (Cu2O) and sulfide compounds. In closing, we explore the possible *beneficial* roles that bulk defects can play in novel Earth-abundant PV materials, from enhancing optical absorption to improving electrical conductivity.
12:00 PM - EE1.7
Rapid and Highly Efficient Interface Defect Anneal for Amorphous/Crystalline Silicon Heterojunction Solar Cells.
Tim Schulze 1 , Hannes Beushausen 1 , Thomas Hansmann 1 , Lars Korte 1 , Bernd Rech 1
1 Silicon Photovoltaics, Helmholtz-Zentrum Berlin für Materialien und Energie, Berlin Germany
Show AbstractSolar cells based on amorphous/crystalline silicon (a-Si:H/c-Si) heterojunctions have gained much attention due to their high conversion efficiency, lately reaching 23.0%. In order to increase the open circuit voltage Voc of these solar cells, the prime objective is to ‘passivate’ the a-Si:H/c-Si interface, i.e. to suppress interface recombination of photogenerated charge carriers by saturating recombination-active dangling bonds. Thin (~3-10nm) undoped a-Si:H interlayers are commonly used to reduce the density of interface states Dit. Although the potential for improving the passivation by low-temperature post-deposition annealing has been widely explored, the microscopic preconditions for a low Dit (in the as-deposited or annealed state) and the detailed annealing dynamics remain under dispute. Technologically disappointing, thick (not device-relevant) undoped layers were mostly used and extremely long annealing times – several (tens of) minutes – were reported necessary to reach the lowest Dit.The present work elaborates on recently published first results [1] where we demonstrate a dramatic increase of effective minority carrier lifetime τeff upon post-deposition annealing: τeff > 4.5ms (at 1015cm-3 excess carrier density) is reached, corresponding to interface recombination velocities S as low as 2cm/s with 10nm thick undoped a-Si:H layers, in only seconds of annealing time. To our knowledge, these are the lowest values reported so far for a-Si:H layers of device-relevant thickness. In solar cells, they would lead to a Voc > 725mV.It is shown that the potential for a reduction of Dit upon annealing and the annealing dynamics itself are crucially determined by the microscopic configuration of hydrogen (H) in the thin amorphous layers, as probed by Fourier transform infrared spectroscopy (FTIRS). Depending on the a-Si:H deposition parameters (substrate temperature, chamber pressure, hydrogen dilution), distinct H bonding configurations and absolute H content are observed (confirmed by UPS and SIMS), resulting in dramatically different annealing behavior and final Dit values. Using a specific parameter set, S ≈ 2cm/s is obtained after only seconds of pulsed annealing, either by conventional hotplate heating or by a microwave (MW) annealing technique. The analysis of FTIR difference spectra suggests the existence of a thin H-rich interface layer as a prerequisite for the fast annealing dynamics, while H reconfiguration in the a-Si:H bulk seems not to play a dominant role.Pulsed MW heating is further shown to yield accelerated annealing dynamics as compared to hotplate heating. This effect is thought to result from the details of the MW absorption mechanism in a-Si:H, selectively acting on polar Si-H bonds and thus facilitating H bond reconfiguration. We believe that due to the reduced time and energy budget of MW heating, it is promising for industrial application.[1] T.F. Schulze et al., Appl. Phys. Lett. (2009), accepted for publication
12:15 PM - EE1.8
Light Beam Induced Current Mapping of mc-Si Solar Cells. Influence of Grain Boundaries and Intragrain Defects.
Benito Moralejo 1 , Vanessa Hortelano 1 , Miguel Gonzalez 1 , Oscar Martinez 1 , Juan Jimenez 1 , Vicente Parra 2 , Manuel Avella 3
1 GdS Optron lab, Universidad de Valladolid, Valladolid Spain, 2 , Instalaciones Pevafersa, Toro Spain, 3 Unidad de Microscopia, Parque Científico, Universidad de Valladolid, Valladolid Spain
Show AbstractMulticrystalline Silicon (mc-Si) has experienced a noticeable increase in photovoltaic (PV) industry because of the cost-effectiveness; currently accounting for nearly 50% of worldwide production. However, this material is characterized by intrinsic structural heterogeneities (dislocations, grain boundaries (GB), etc.) which are generated by the crystal growth process. In order to improve the conversion efficiency of mc-Si solar cells, increasing the grain size seems to be a key, even if defects and impurities also limit their performance. The minority carrier diffusion length (Ldiff) gives an indication of the material quality and suitability for solar cell use. The laser beam induced current (LBIC) technique allows mapping the local diffusion length from photocurrent contrast data. In order to determine Ldiff one must control the reflected light; however, the reflection of light in mc-Si is highly inhomogeneous because of the random grain orientations. Monitoring reflected light using large numerical aperture objectives reduces the inhomogenity of the reflected light images, revealing that the ligh dispersion at the textured surface is the main source of light reflection.We present here the characterization of commercial mc-Si solar cells using an advanced homemade LBIC apparatus, with three excitation wavelengths supplied by a dual laser diode (Omicron, 639 and 830 nm laser lines) and a second laser diode (785 nm), that highlights the importance of control the laser power excitation and the reflected light signal. The investigation was carried out on two different types of samples: multicrystalline and monocrystalline commercial silicon solar cells from different suppliers . Mc-Si cells have undergone standard texturing treatments.We present several LBIC maps and reflected light measurements obtained with microscope objectives having different numerical apertures. We found that the measurement of the local reflected light required for the calculation of the Ldiff presents a strong dependence on the collecting conditions in the textured mc-Si cells and therefore, a careful analysis of the experimental conditions while measuring the local reflected light is necessary for an accurate quantitative estimation of local Ldiff(x,y).The LBIC maps present an array of dark lines corresponding to regions with high carrier capture rates. Nevertheless, LBIC dark lines generally do not match the grain boundaries (GB) of the mc-Si wafers. Some of these dark lines seem to be related to intragrain defects, acting as effective trapping centers. These results show that there are two types of grain boundaries, i)those electrically active and ii) those not active. We can also conclude that the minority carrier diffusion length (LDiff), estimated by the LBIC measurements, is limited by both grain boundaries and intragrain extended defects.
EE2: Silicon, Tellurides, and Heterojunction Structures
Session Chairs
Tuesday PM, April 06, 2010
Room 3007 (Moscone West)
2:30 PM - **EE2.1
Electrically Active and Electrically Inactive 3d Transition Metal Centers in Si.
Stefan Estreicher 1 , Daniel Backlund 1
1 Physics, Texas Tech University, Lubbock, Texas, United States
Show AbstractTransition metals (TM) from the 3d series are common and unwanted contaminants in Si. Much of what is known about these impurities relates to the isolated interstitial or TM precipitates. Experimental information is also available for a few impurity pairs, such as {FeB} or {NiH}, and small complexes, such as the *Cu defect. Very little is known about other possible interactions involving TM impurities. This talk will summarize the key predictions of systematic first-principles calculations involving three TMs from the beginning, middle and end of the 3d series: Ti, Fe, and Ni. The calculations aim at better understanding the chemistry of TM impurities in Si, including their interactions with vacancies (V), self-interstitials (I) and common impurities such as H, C, or O. Beyond the basic predictions about the properties of the isolated interstitial TMs (equilibrium site, migration energy, electrically-active gap levels, charge and spin states), we examine a range of TM-related defects with electrical properties very different from those of the isolated interstitial TMs. The most interesting and stable of these defects result from the interactions involving vacancies and the formation of substitutional TM. The unpaired 3d electrons of light TM interstitials (Ti and Fe in our calculations) couple with the ‘dangling bonds’ of the vacancy, which results in the formation of covalent TM-Si bonds. The spin multiplicity and the electrical activity are much reduced. Interstitial Ni has a filled 3d shell and no electrical activity. In this case, the interactions with the vacancy result in an increase in the electrical activity. The energy gain in the reaction TMi +V to TMs is less than the formation energy of the vacancy. However, numerous processes are known (or suspected) to inject vacancies into the bulk. A number of trends across the 3d series will be discussed. For example, there are almost no lattice relaxations or distortions for 3d TM impurities at interstitial or substitutional sites; the binding energies of TMi to V are similar; the same holds for the binding energy of H to a substitutional TM; the interactions between a TMi and the A-center (O-V pair) is also very similar for Ti, Fe, and Ni.
3:00 PM - EE2.2
Reaction of H with C in Multicrystalline Si Solar-cell Materials.
Michael Stavola 1 , Chao Peng 1 , Haoxiang Zhang 1 , Vijay Yelundur 2 , Ajeet Rohatgi 2 , Lode Carnel 3 , Mike Seacrist 4 , Juris Kalejs 5
1 Department of Physics, Lehigh University, Bethlehem, Pennsylvania, United States, 2 School of Electrical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 , REC Water AS, Porsgrunn Norway, 4 , MEMC Electronic Materials, St. Peters, Missouri, United States, 5 , American Capital Energy, N. Chelmsford, Massachusetts, United States
Show AbstractHydrogen is commonly introduced into Si solar cells to reduce the deleterious effects of defects and to improve cell performance. When hydrogen is introduced into Si grown by the floating-zone method, H2 molecules are formed. When hydrogen is introduced into multicrystalline Si that is often used for the fabrication of solar cells, the H atoms become trapped by carbon impurities that can be present at high concentration to produce defect structures known at H2*(C) [1-3]. These defects act as both a source and sink for hydrogen in H-related defect reactions. In our experiments, IR spectroscopy has been used to determine what H- and C-related defects are formed in multicrystalline Si when the carbon concentration is varied. An annealing study has also performed to probe the defect reactions that occur.A process that is widely used by industry to introduce hydrogen into Si solar cells is by the post-deposition annealing of a hydrogen-rich SiNx layer. The C-H defects provide a strategy to study the concentration and penetration depth of the hydrogen that is introduced by this method.The work performed at Lehigh University has been supported by the Silicon Solar Research Center SiSoC Members through NCSU Subaward No. 2008-0519-02 and NSF Grant No. DMR 0802278.[1] V. P. Markevich, L. I. Murin, J. Hermansson, M. Kleverman, J. L. Lindström, N. Fukata, M. Suezawa, Physica B 302-303, 220 (2001).[2] B. Hourahine, R. Jones, S. Öberg, P. R. Briddon, V. P. Markevich, R. C. Newman, J. Hermansson, M. Kleverman, J. L. Lindström, L. I. Murin, N. Fukata, and M. Suezawa, Physica B 308-310,197 (2001).[3] J. L. McAfee and S. K. Estreicher, Physica B 340-342, 637 (2003).
3:15 PM - EE2.3
Extrinsic Doping in Silicon Revisited.
Robin Grimes 1 , Udo Schwingenschloegl 2 , Alexander Chroneos 1 , Cosima Schuster 3
1 Materials, Imperial College London, London United Kingdom, 2 PSE Division, KAUST, Thuwal Saudi Arabia, 3 Institut für Physik, Universität Augsburg, Augsburg Germany
Show AbstractThe standard model for n and p type doping of silicon (Si) is at odds with the charge transfer predicted by Pauling eletronegativities. Specifically, phosphorous (P) is n-type and is expected to donate an electron to the conduction band but a P atom is more electronegative than a Si atom - the transfer of charge would therefore be towards the P not away from it. Gallium (Ga) is p-type and yet its electronegativity is lower than that of Si. These incompatible models can only be reconciled if we no longer regarding dopant species as isolated atoms but rather consider them as clusters consisting of the dopant and its four nearest neighbour Si atoms. The process that gives rise to n and p type effects is the charge redistribution that occurs between the dopant and its neighbours, as we illustrate here using electronic structure calculations. The new view point is able to explain why conventional substitutional n-type doping of carbon has been unsuccessful.
3:30 PM - EE2.4
First-principles Study of Back Contact Effects on CdTe Thin Film Solar Cells.
Mao-Hua Du 1
1 , Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractCdTe is an important thin-film solar cell material. It has a direct band gap of 1.5 eV, near the optimum for conversion efficiency in a single-junction solar cell under terrestrial irradiation. Its high light absorption coefficient allows efficient solar energy absorption within a thin film, reducing material cost of the solar cells. These basic material properties of CdTe are excellent for thin film photovoltaic applications, but many other factors also contribute to the overall solar cell performance. The back contact materials affect both efficiency and lifetime of the CdTe solar cells because the contact resistance reduces the carrier collection and the impurity diffusion from the back contact to the CdTe layer causes cell degradation. Therefore, forming a chemically stable low-resistance back contact is extremely important to the CdTe solar cell performance. The currently highest efficiency (16.5%) CdTe solar cells have Cu-containing compounds (such as Cu2Te) as the back contact, because Cu reduces contact resistance. However, Cu can diffuse into CdTe and CdS films, and thus play additional roles, such as accumulating in the grain boundaries (GBs) and the junction region, which may create conductive channels that shunt the solar cell. It has long been suspected that the degradation of the CdTe solar cells is related to the Cu diffusion. The instabililty of the Cu contact prompted the research on the alternative back contact materials. Recent studies show that using a Sb2Te3 back contact without Cu results in an ohmic contact and enhanced solar cell stability but slightly lower initial cell efficiency (15.8%) compared to those with Cu back contacts (16.5%).We report the theoretical study of the effects of Sb2Te3 back contacts on the performance of the CdTe solar cells. We show by first-principles calculations that Sb can diffuse into the p-type CdTe thin film to form low-energy donor defects (on Cd sites). Sb can further bind with oxygen to form a Sb-O complex. The Sb donor impurities in CdTe bulk should have the negative effects of reducing hole concentration and increasing back contact resistance. However, such scenario is inconsistent with the high efficiency observed for the CdTe solar cells with Sb2Te3 back contacts. Thus, we postulate that the majority of the Sb donor impurities may segregate into the GBs. The accumulation of donors at the GBs inside p-type CdTe should cause the band bending at the DBs, which has the benefit of separating photogenerated electrons and holes.
3:45 PM - EE2.5
Detection of Trapped Electric Charges in Photoionized Quantum Dots.
Maxim Dokukin 1 , Nataliia Guz 1 , Igor Sokolov 1
1 , Clarkson University, Potsdam, New York, United States
Show AbstractQuantum dots (QDs) can be used as an effective inorganic photovoltaic material. However, QDs are only a part of a complete photovoltaic device, cell. The interphase between QDs and charge transporting material is very important to attain high efficiency of solar cells. Because of various defects on the interface, electrical charges can be trapped inside QDs. As a result, such quantum dots will not participate in the process of light conversion into electricity. Here we describe a method based on atomic force microscopy, which allows detection of such trapped charges. A method is rather robust, it allows multiple measurements of the same charge without its disturbance. The accuracy of method is sufficient to detect single electron charge with the error down to 10-20%. This method can be used for optimization of interfaces in solar cells, as well as fast screening of novel photovoltaic materials.
4:30 PM - **EE2.6
Gettering of Impurities in Cast Monocrystalline Silicon.
Nathan Stoddard 1 , Rubin Sidhu 1
1 Technology, BP Solar, Frederick, Maryland, United States
Show AbstractMonocrystalline silicon derived from a casting process (BP Solar's Mono2 ™ silicon) has a unique response to the various elements of the solar cell manufacturing process. N-type multicrystalline cast silicon has been shown to have especially high minority carrier lifetime in certain instances. We have cast and processed n-type Mono2 ™ silicon through the normal p-type screen print process. The evolution of wafer lifetime (both average and standard deviation) is presented through diffusion, annealing and co-firing steps, with entire 12.5cm wafers averaging as high as 400 microseconds. With the right lifetime distribution, these high lifetime averages are compatible with back contact cell structures on 180 micron wafers.
5:00 PM - EE2.7
Grain Boundaries and Their Behaviors in Upgraded Metallurgical-grade Silicon for Photovoltaics.
Fude Liu 1 , Chun-Sheng Jiang 2 , Harvey Guthrey 3 2 , Steve Johnston 2 , Manuel Romero 2 , Brian Gorman 3 , Mowafak Al-Jassim 2
1 Mechanical Engineering, The University of Hong Kong, Hong Kong, Hong Kong, China, 2 National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, Colorado, United States, 3 Metallurgical & Materials Engineering, Colorado School of Mines, Golden, Colorado, United States
Show AbstractUsing upgraded metallurgical-grade silicon (UMG-Si) is a cost-effective and energy-efficient approach for the production of solar cells, without complex purification needed as in the case of electronic-grade silicon (EG-Si). Grain boundaries play a very important role in determining the device performance of solar cells made of UMG-Si, because grain boundaries cause carrier recombination and possible shunting paths due to diffusion of dopants along grain boundaries and they are also the preferred sites for extrinsic impurities. In this study, a UMG-Si wafer sliced from the bottom part of an ingot was first investigated with photoluminescence (PL) imaging. Based on the PL results, we selected a few typical grain boundaries according to their different optical response under the illumination of 810-nm light. These grain boundaries were then studied with plan-view electron backscattered diffraction (EBSD) mapping to get information on grain misorientation between two adjacent grains. An excellent correlation was found between the grain misorientation and the corresponding optical response of grain boundaries. The grain boundary structure was further characterized with cross-sectional transmission electron microscopy (TEM) at high resolution. The same procedure was carried out on another UMG-Si wafer sliced from the top part of the same ingot in order to see the impurity impact on the optical response because in general the lower part and the top part of one ingot contain different impurity levels. Based on the findings, we conclude that the optical response at grain boundaries is grain-orientation dependent. The PL features at different grain boundaries also depend on the impurity levels in the wafer, which is closely related to the position where it is sliced from the ingot. We believe that this study will help us with a deeper understanding on grain boundaries and their behaviors not only in UMG Si but also in other polycrystalline materials.Keywords: Upgraded metallurgical-grade silicon (UMG-Si); Photovoltaics; Grain boundaries; Impurities; Characterization
5:15 PM - EE2.8
Interplay Between Stresses, Extended Crystal Defects, Inclusions and Impurities in Intentionally Doped Multicrystalline Silicon Solar Cell Material.
George Sarau 1 , Silke Christiansen 1 2
1 , Institute of Photonic Technology, Jena Germany, 2 , Max-Planck-Institute of Microstructure Physics, Halle Germany
Show AbstractThermal stresses result from inhomogeneous temperature gradients within the ingot during the block casting process as well as from the difference in the coefficients of thermal expansion (CTE) between inclusions and silicon matrix. These stresses relax partially or totally by plastic deformation leading to the formation of extended lattice defects such as dislocations, low-angle grain boundaries, cracks and their combinations. The residual internal stresses combined with an unfavourable configuration of structural defects under external mechanical and thermal stresses applied during further processing may result in additional defects and even unpredictable breakage of silicon wafers/cells. While the stress-related defects are often recombination sites for minority carriers mainly when decorated with impurities reducing the efficiency of solar cells, the breakage affects the process yields. Therefore, understanding and controlling these stresses are important for the mechanical stability and electrical performance of present and future solar cells, both representing key parameters towards lower costs per Watt-peak.We study the interaction between residual stresses, extended crystal defects, inclusions and impurities in standard as well as in intentionally contaminated (only with Ge or Fe & Cu or Fe) multicrystalline silicon block cast solar cell material. We find that the distributions of residual stresses measured by micro-Raman spectroscopy and electrical activity determined by the electron beam induced current technique are inhomogeneous along the same grain boundary (GB) as well as along different GBs of the same type identified by electron backscatter diffraction. These non-uniformities are attributed to the presence of dislocations, their intrinsic structure, impurity decoration and arrangement on GBs. In the case of 2% Ge doped samples, a larger Ge content of up to 5.7% measured by Energy dispersive X-ray spectroscopy (EDX) is found in areas of relatively low dislocation density made visible by defect etching and high residual stresses of up to 850 MPa attributed to an inclusion located below the surface as seen by infrared transmission. Inclusions can act as dislocation generation sources when plasticity is allowed by temperature, while afterwards they can lead to residual stresses that build-up in silicon, both being due to the CTE mismatch of the materials. In the 2 ppma Fe & 20 ppma Cu samples we find Cu rich inclusions as confirmed by EDX that are also accompanied by dislocations and stresses. Their density and distributions depend on the inclusion geometry and crystal orientation. The 2 ppma Fe samples do not show areas of high dislocation density and stresses.We show that stresses on the micrometer scale alongside with details of microstructure and electrical activity have to be taken into account for an advanced physical understanding of the interplay between defects and impurities that may lead to an improved defect engineering.
5:30 PM - EE2.9
Study of Surface Reactions and Defect Reduction by Scalable Ge-on-Si Nanoscale Heterojunction Engineering and GaAs Integration for III-V Photovoltaics.
Darin Leonhardt 1 , Josephine Sheng 1 , Jeffrey Cederberg 2 , Malcolm Carroll 2 , Manuel Romero 3 , Sang Han 1
1 Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe have demonstrated the scalability of a process to substantially reduce threading dislocations in a Ge film grown on 2-inch-diameter chemically oxidized Si substrates. Subsequent integration of GaAs leads to photoluminescence as well as cathodoluminescence intensity comparable to that on GaAs substrates. To mechanistically describe the Ge growth, voids of 3 to 7 nm diameter are created in the oxide at a density greater than 1x1011/cm2 upon exposure to Ge flux. Comparison of data taken from many previous studies, including ours, shows an exponential dependence of oxide thickness on inverse temperature of void formation. Additionally, exposure to a Ge or Si atom flux decreases the temperature at which voids begin to form in the oxide. These results strongly suggest that Ge actively participates in the reaction with SiO2 in the void formation process. Once voids are created in the oxide under a Ge flux, Ge islands selectively nucleate within the void openings on the newly exposed Si. Island nucleation and growth then compete with the void growth reaction. At temperatures between 780 and 550 °C, nanometer size Ge islands that nucleate within the voids continue to grow and coalesce into a continuous film over the remaining oxide. Coalescence of the Ge islands results in the creation of stacking faults in the Ge film with few threading dislocations. Additionally, coalescence results in films of 3 μm thickness having a root-mean-square roughness of 8 to 10 nm. We have found that polishing the films with dilute hydrogen peroxide results in roughness values below 0.5 nm. However, stacking faults originating at the Ge-SiO2 interface and terminating at the Ge surface are polished at a slightly reduced rate, and show up as 1 to 2 nm raised lines on the polished Ge surface at a density of 5x107/cm2. These lines are then transferred into the subsequent growth morphology of GaAs using metal organic chemical vapor deposition. When these stacking faults are not eliminated, room temperature photoluminescence shows that films of GaAs grown on Ge-on-oxidized Si have an intensity that is 20 to 25 percent compared to the intensity from GaAs grown on commercial Ge or GaAs substrates. High resolution cathodoluminescence shows that non-radiative defects occur in the GaAs that spatially correspond to the stacking faults terminating the Ge surface. The exact nature of these non-radiative defects in the GaAs is unknown, however, GaAs grown on annealed samples of Ge-on-oxidized Si, which are free of stacking faults, have photoluminescence intensity that is comparable to GaAs grown on a GaAs substrate.
5:45 PM - EE2.10
Surface Passivation of p-GaTe Layered Crystals for Improved p-GaTe/n-InSe Heterojunction Solar Cells.
Krishna Mandal 1 , Sandip Das 1 , Ramesh Krishna 1 , Peter Muzykov 1 , Shuguo Ma 2 , Feng Zhao 1
1 Department of Electrical Engineering, University of South Carolina, Columbia, South Carolina, United States, 2 Nanocenter, University of South Carolina, Columbia, South Carolina, United States
Show AbstractThe layered chalcogenide semiconductor GaTe single crystals have been grown by a modified vertical Bridgman technique using high purity Ga and Te (7N) precursor materials. X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, low temperature (LT) photoluminescence (PL), LT deep level transient spectroscopy (DLTS) and electrical charge transport property measurements have been carried out to characterize the crystals harvested from ingots of ~15 cm length and ~5.0 cm diameter. The effect of sulphur passivation on the freshly cleaved laminar surfaces of p-GaTe crystals have been investigated for p-GaTe/n-InSe heterojunction solar cells. It was observed that p-GaTe surfaces improved significantly after passivating with (NH4)2S and Na2S pretreatments. The reduced interface state density after surface passivation resulted in significant increase in photoluminescence intensity. The passivated surfaces were thoroughly analyzed by X-ray photoelectron spectroscopy (XPS). Analysis of high resolution XPS spectra shows drastic improvements of the GaTe surfaces. Corresponding improvements in charge carrier transport properties and non-reflecting surface morphologies were studied by electron beam induced current (EBIC) and TEM measurements. Details of numerical crystal growth modeling and simulation, crystal processing, surface passivation, improved heterojunction solar cell fabrication, and characterization will be presented.
Symposium Organizers
Daniel Friedman National Renewable Energy Laboratory
Michael Stavola Lehigh University
Wladek Walukiewicz Lawrence Berkeley National Laboratory
Shengbai Zhang Rensselaer Polytechnic Institute
EE3: II-VI and Oxide Materials and Epitaxy on Glass
Session Chairs
Wednesday AM, April 07, 2010
Room 3007 (Moscone West)
9:30 AM - **EE3.1
Defect Physics of the Quaternary Thin-film Solar Cell Absorber Cu2ZnSnS4.
Su-Huai Wei 1 , Shiyou Chen 2 , Xingao Gong 2 , Aron Walsh 3
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , Surface Science Laboratory and Physics Department, Fudan University, Shanghai China, 3 , University College London, Department of Chemistry, London United Kingdom
Show AbstractCu$_2$ZnSnS$_4$ (CZTS) has become the subject of intense interest because it is considered as an ideal candidate absorber material for thin-film solar cells with an optimal band gap (1.5 eV), high absorption coefficient ($> 10^{4}$ cm$^{-1}$), abundant elemental components, and is adaptable to various growth techniques. The energy conversion efficiency of CZTS based solar cells has increased from 0.66$\%$ in 1996 to close to 7$\%$ recently; however, it is still well below the 32\% single-junction limit. To further improve the efficiency, it is crucial to understand the thermodynamic stability of this quaternary compound and the formation mechanism of the dominant intrinsic defects. However, in contrast to ternary compounds such as CuInSe$_2$, no systematic studies have been carried out for the quaternary systems. In this work, we have performed a series of first-principles electronic structure (density functional theory) calculations, which explain a number of unique features for CZTS that are relevant to its use as a solar cell absorber: (i) The chemical potential region that CZTS can form stoichiometrically is very small. Therefore, it will be very difficult to obtain high quality stoichiometric CZTS samples; (ii) The dominant $p$-type acceptor in CZTS is Cu$_\mathrm{Zn}$, however, the associated acceptor level is relatively high, suggesting that $p$-type doping in CZTS is more difficult than ternary compounds such as CuInSe$_2$; (iii) The formation of the self-compensated defect pair [Cu$_\mathrm{Zn}$+Zn$_\mathrm{Cu}$] will not lead to strong carrier separation, and thus will not contribute the same beneficial effect observed in ternary chalcopyrite compounds; (iv) We predict that to avoid the aforementioned issues in (ii) and (iii), it will be optimal to grow the sample under Cu-poor/Zn-rich conditions, so V$_\mathrm{Cu}$ and Zn$_\mathrm{Cu}$ become the dominant defects in the system. However, in this case, non-equilibrium growth techniques may be required to avoid the formation of secondary phases such as ZnS.
10:00 AM - EE3.2
Identification of Defects and Secondary Phases in Reactively Sputtered Cu2ZnSnS4 Thin Films.
Vardaan Chawla 1 , Stacey Bent 1 , Bruce Clemens 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractWidespread application of photovoltaic power to provide a significant fraction of the world’s energy needs will require a dramatic lowering of photovoltaic cell material cost and thus, the use of inexpensive, abundant materials and low-cost fabrication strategies. One promising candidate material is Cu2ZnSnS4 (CZTS), which has a favorable direct band gap (1.45 ev) for solar cell applications, and which is formed from abundant, not-toxic elements. Thin films of this material for photovoltaic applications are generally grown using a two-step process in which the second step is a high temperature anneal in H2S gas. The diffusion of metal species during this anneal can lead to defects such as Kirkendall voids in the final CZTS film. The focus of this work is to eliminate these defects by using a one step reactive sputtering process. Sulfur is incorporated into the film by introducing H2S gas into the sputtering ambient during deposition. Films grown by this process are textured in the (112) direction and show significant improvements in crystal structure over those in literature. Identification of secondary phases has been shown to be a problem in this material system due to the similarities in crystal structure of competing phases. A detrimental Cu2S phase was identified in these films using a combination of Raman and Auger spectroscopy. The effect of this phase on devices, electronic and optical properties of the material, and removal of the phase using preferential etching will also be discussed.
10:15 AM - EE3.3
Effects of 2nd Phases, Stress, and Na at the Mo/Cu2ZnSnS4 Interface.
Jeffrey Johnson 1 , Ashish Bhatia 2 , Michael Scarpulla 2 1 , Loren Rieth 1
1 Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Materials Science and Engineering, University of Utah, Salt Lake City, Utah, United States
Show AbstractCu2ZnSnS4 (CZTS) is an alternative thin film photovoltaic absorber material to Cu(In,Ga)Se2 composed solely of commodity elements. Thus, if similar material quality and performance can be realized, its use would allow scale-up of terrestrial thin film photovoltaic production unhindered by material price or supply constraints. Here we report on our research on the deposition of CZTS by RF sputtering from a single CZTS target and cosputtering from multiple binary sources on Mo-coated glass. We find that post-deposition annealing in sulfur vapor leads to delamination on some samples, especially those deposited on borosilicate glass; soda-lime glass samples delaminate less frequently. Thus we investigate the influences of the formation of frangible phases such as MoS2 at the interface, residual stress, and sodium on delamination. We also investigate Mo oxide removal by sputter etching before CZTS deposition and its effects on adhesion and series resistance. We implicate fracture in a layer of MoS2 as the mechanism of delamination between the Mo and CZTS layers using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). We evaluate stresses after Mo and CZTS deposition and after annealing using wafer curvature measurements. Significant (~400 MPa) stress may develop for minimally-optimized Mo sputtering; however using a two-step Mo deposition residual stress is reduced to near zero. Deposition of ~700 nm CZTS adds ~100 MPa of stress on borosilicate glass substraqtes. The chemical effects of Na on adhesion are separated from those caused by thermal expansion differences between soda-lime and borosilicate glass by introducing thin Na-containing layers on borosilicate substrates. We report methods of stress balancing and the use of barrier layers to prevent the formation of MoS2 at the interface thus mitigating delamination.
10:30 AM - EE3.4
Synthesis of Optimized CZTS Thin Films for Photovoltaic Absorber Layers by Sputtering from Sulfide Targets and Sulfurization.
Haritha Nukala 1 , Ashish Bhatia 1 , Jeffrey Johnson 2 , Elizabeth Lund 3 , Matt Nowell 4 , Michael Scarpulla 1 2
1 Material Science and Engineering, University of Utah, Salt Lake City , Utah, United States, 2 Electrical and Computer Engineering, University of Utah, Salt Lake City , Utah, United States, 3 Chemical Engineering, University of Utah, Salt Lake City , Utah, United States, 4 , EDAX-TSL, Draper, Utah, United States
Show AbstractCu2ZnSnS4 (CZTS) is a promising alternative for CuInxGa(1-x)Se2 (CIGS) absorber layers in thin film solar cells and is comprised of commodity elements which will enable scale-up of chalcopyrite panel production unhindered by elemental supplies and costs. Various CZTS synthesis methods, especially sulfurization of stacked metal or metal sulfide layers, are being studied and have led to cell efficiencies up to 6.7% [1]. Here we report our studies of CZTS thin film synthesis via room temperature sputtering from a single CZTS target and co-sputtering from Cu2S, ZnS and SnS2 binary targets, both followed by sulfurization between 500 C - 600 C using either elemental sulfur vapor or in-situ generated H2S. Sputtering from sulfur-containing targets is designed to increase the sulfur content in the precursor films to promote stoichiometry. We report on the effects of processing including deposition on soda-lime and borosilicate glasses and deposition of Na-containing layers on film morphology (AFM/SEM), composition (EDS), phase (XRD), grain size (XRD/EBSD), grain boundary structure (EBSD), optical (spectroscopic ellipsometry) and electrical properties. Processing conditions producing desirable Zn-rich/Cu-poor films are identified [1]. The formation of MoSe2 at Mo/CIGS interface is believed to promote Ohmic contacts, but in CZTS we associate excessive formation of frangible MoS2 with film delamination from Mo/borosilicate glass substrates. Strategies for preventing delamination including adhesion layers are investigated and discussed. P-N junctions are formed with CdS/ZnO using chemical bath deposition and sputtering, and I-V characteristics are reported. Schottky junctions are formed and C-V measurements are used to determine the doping in the CZTS absorber layers.[1] H. Katagiri, et al., MRS Symp. Proc. 1165 1165-M04-01 (2009).
10:45 AM - EE3.5
Trap Studies of Nanoparticle CdTe and PbS Solar Cells With Photothermal Deflection Spectroscopy.
Anna Bezryadina 1 2 , Lily Yang 1 , Chris France 1 , Rebekah Graham 1 , Tong Ju 1 , Glenn Alers 1 , Sue Carter 1
1 Physics, UCSC, Santa Cruz, California, United States, 2 , NASA Ames Research Center, Advance Study Laboratories, Moffett Field, California, United States
Show AbstractWe have demonstrated that nanoparticle solar cells have a low intrinsic electronic trap density. Photothermal deflection spectroscopy (PDS) was used to analyze all-inorganic nanoparticle solution-processed solar cell materials consisting of CdTe and PbS. The PDS system is specifically designed to measure mid gap absorption with a resolution of 5 cm-1 and deep level defect transitions inside the band gap. Mid gap trap states can be excited by photons leading to absorption within the bandgap. They can also lead to increased recombination and therefore a low efficiency for the solar cell. Nanoparticle based CdTe solar cells are fabricated by spin coating a solution of CdTe nanoparticles on a glass slide and annealing at 200°C. The film is then treated with CdCl2 in methanol and sintered in air at 400°C. Aluminum contacts were then evaporated to form Shottkey junction solar cells with efficiencies up to 5%. PDS measurements show that prior to sintering the nanoparticle film has a band gap of ~2eV with a very broad band edge. Sintering the CdTe film for only 1 min. resulted in a band gap of 1.5eV with a very sharp band edge. As sintering time increases, we find that the mid-gap trap density increases as the nanoparticles fuse together to form large grains. These results are consistent with charge-pumping measurements that measure the amount of trapped charge in the films through transient current decay. The lowest level of trapped charge is observed in the unsintered films. The low trap density in the nanoparticle films shows that the ligands are effective at electrically passivating the surface of the nanoparticles. The very large surface area of the nanoparticle film does not appear to affect electrically active trap states. After sintering, unpassivated voids in the film and grain boundaries might be responsible for the increase in trap density. The high temperature CdCl2 treatment is necessary to form functional solar cells but also increases the density of deep level trap centers. Very low trap densities have also been observed with unsintered PbS based solar cells. This measurement shows that low cost solution-processed solar cells are stable and could be utilized for solar energy in the near future.
11:30 AM - **EE3.6
Quasi-single Crystal Semiconductors on Glass Substrates Through Biaxially Oriented Buffer Layers.
Toh-Ming Lu 1 , Gwo-Ching Wang 1 , Ishwara Bhat 1 , Shengbai Zhang 1
1 Center for Integrated Electronics, RPI, Troy, New York, United States
Show AbstractHigh efficiency photovoltaic devices are normally fabricated out of single crystalline substrates. In practice, these single crystalline substrates are expensive and volume production for widespread usage has not been realistic. To date, large volume production of solar cells is on less expensive non-crystalline substrates such as glass. Typically the films grown on glass are polycrystalline with less than ideal efficiency. Recently an alternative approach has been suggested. It was proposed that a dramatic gain in the efficiency may be achieved if one uses a biaxially oriented buffer layer on glass to grow biaxial semiconductor solar devices compared to that of films grown directly on glass [1-3]. Biaxial films are not exactly single crystal but have strongly preferred crystallographic orientations in both the out-of-plane and in-plane directions. Typically the misorientation between grains can be small (less than a few degrees) and may possess low carrier recombination rate. Therefore, biaxial films can be considered as “quasi single crystals”. In this talk, we shall discuss techniques that would allow one to produce biaxial buffer layers on glass. A specific strategy using an atomic shadowing mechanism in an oblique angle deposition configuration to grow biaxial buffer layers such as CaF2 on glass substrate will be discussed in detail. Results of heteroepitaxy of semiconductor materials such as CdTe and Ge on these biaxial buffer/glass substrates will be presented.Work supported by NSF CBET 0828401 and Rensselaer.[1] W. Yuan et. al., “Biaxial CdTe/CaF2 films growth on amorphous surface”, Thin Solid Films, on-line version (2009).[2] Alp T. Findikoglu et. al., "Well-oriented silicon thin films with high carrier mobility in polycrystalline substrates", Adv. Mater. 17, 1527 (2005).[3] Charles W. Teplin et. al., "A new approach to thin film crystal silicon on glass: Biaxial-textured silicon on foreign template layers", J. of Non-Crystalline Solids 352, 984 (2006).
12:00 PM - EE3.7
Defects and Recombination Kinetics in Copper Indium Gallium Sulfide Thin Films With Spatially Resolved Luminescence in the µm-scale.
Florian Heidemann 1 , Gottfried H. Bauer 1
1 Institute of Physics, University of Oldenburg, Oldenburg Germany
Show AbstractChalcopyrite Cu(In,Ga)S2 is a technically promising absorber in thin film solar cells [1], although the comparable high band gaps so far do not correspond to equivalent high open circuit voltages. We have performed photoluminescence studies on CdS passivated absorber layers deposited on Mo coated soda lime glass and finally prepared diodes with efficiencies in the range of 9-10%. From spectrally and spatially resolved (≤ 1µm) room temperature photoluminescence measurements we have extracted the local splitting of quasi-Fermi levels (EFn-EFp), local lattice temperature Tlat and local absorption (A(ω)) particularly in the subgap-regime via Planck’s generalized law [2-4]. We observe a substantial negative correlation coefficient between the local subgap/defect absorption and the local (EFn-EFp) which we interpret in terms of and schematically formulate by recombination of photogenerated minorities (here electrons) via subgap states/deep defects. Moreover we have correlated local (EFn-EFp) with corresponding values at neighbor sites versus distance (increment analysis). As we find lateral defect correlation distances in the vicinity of average grain sizes we conclude grains with different minority life times and according different splitting of (EFn-EFp) to be independent from one another and be laterally distributed randomly.[1] Klenk R., Klaer J., Scheer R., Lux-Steiner M.C., Luck I., Meyer N. and Rühle U. 2005 Thin Solid Films 480 509-514.[2] Würfel P. 1982 J. Phys. C: Solid State Phys. 15 3967-3985.[3] Daub E. and Würfel P. 1995 Phys. Rev. Lett. 74 (6) 1020-1023.[4] Gütay L. and Bauer G.H. 2009 Thin Solid Films 517 7, 2222-2225.
12:15 PM - EE3.8
Trap State Photoluminescence of Nanocrystalline and Bulk TiO2,: Implications for Carrier Transport.
Christopher Rich 1 , Fritz Knorr 1 , Kritsa Chindanon 1 , Jeanne McHale 1
1 Chemistry , Washington State University, Pullman, Washington, United States
Show AbstractThe visible photoluminescence (PL) that results from band-gap excitation of nanocrystalline anatase TiO2 has been resolved into contributions from electron and hole traps,1 and the latter are found to be passsivated by TiCl4 surface treatment.2 In this work, we investigate the molecular nature of these traps and the role of carrier transport in limiting the intensity of trap-state photoluminescence. The intensity and shape of the PL spectrum was determined for the (101) and (001) facets of bulk anatase crystals and for nanocrystalline TiO2 preparations containing unusually large percentages of high-energy (001) facets. We also explore the dependence of the PL on the incident photon flux and the presence of adsorbates with variable capacity to scavenge electrons or holes. The competing influences of transport, charge exchange, and surface-trap passivation on the intensity and shape of TiO2 photoluminescence will be discussed and a model for the luminescent electron and hole traps presented. 1. Knorr, F. J.; Mercado, C. C.; McHale, J. L. J. Phys. Chem. C 2008, 112, 12786-12794.2. Knorr, F. J.; Zhang, D.; McHale, J. L. Langmuir 2007, 23, 8686-8690.
12:30 PM - EE3.9
Can Yb3+ Ions Diffuse Into ZnO Lattice?
Nan Jiang 1
1 Physics, Arizona State University, Tempe, Arizona, United States
Show AbstractZnO has been considered as one of the most efficient oxide-based luminescence materials. The luminescent properties of ZnO can be tailored to various wavelengths by doping luminescent centers, amount which the rare-earth ions particularly attracting much attention. Unfortunately, one of the key factors influencing the practical applications of rare-earth ions doped ZnO is the low luminescence efficiency, which is mainly attributed to the low efficient energy transfer from the host to rare earth ions, due to the low solubility of rare-earth ions in ZnO lattice. However, it is controversy whether rare-earth ions can diffuse into ZnO lattice. Different synthesis methods usually gave different conclusions. The lack of direct measurements of rare-earth ions in the ZnO lattice was probably the main reason for the confusion in literature. In this work, we report the direct measurements of Yb distribution in the ZnO particles, using transmission electron microscopy (TEM) combing with electron energy-loss spectroscopy (EELS). The diffusion experiments were carried out by synthesizing Yb2O3 nanoparticles attached to ZnO and then annealing at 1200○C at slightly reduction condition. The spatial distribution of Yb ions in the ZnO lattice was measured using spatially resolved EELS technique. The interpretations of EELS spectra were given with the aid of calculations in various structural models. In summary, Yb3+ ions can indeed diffuse into ZnO lattice, but the diffusion distance is very short, less than several hundred nanometers. The difficulty is due to that the diffusion of Yb3+ ions in the ZnO lattice always accompanies the formation of interstitial O and Zn vacancy, which largely increase the energy of system. Therefore the Yb3+ ions have high concentration near the surface region close to the Yb3+ sources. These results suggests that in order to increase the doping concentration of rare-earth ions in ZnO, one need to shrink the size of ZnO particles, e.g. smaller than 100 nm in diameter, or co-dope other components to reduce the formation energy of O interstitial and Zn vacancy defects.
12:45 PM - EE3.10
Defect Photoluminescence of TiO2 Nanotubes.
Candy Mercado 1 , Jeanne McHale 1
1 Materials Science Program / Chemistry Department, Washington State University, Pullman, Washington, United States
Show AbstractPhotoluminescence (PL) spectroscopy with ultraviolet light excitation describes the defect states within the bandgap of oxide semiconductors such as titanium dioxide. Our study using low power 350nm excitation to observe the PL of TiO2 nanotubes fabricated by anodization of titanium showed a broad visible luminescence from 450 to 700 nm with the peak occurring at 520-550nm or 2.4-2.3eV. Upon comparison, the spectra of these nanotube films are an order of magnitude lower than those of nanoparticulate anatase and P25 films. We consider two inter-related reasons for decreased intensity of PL from TiO2 nanotubes: decreased density of defect states and improved carrier transport. The nature of the defects and the basis for diminished PL in nanotubes compared to nanoparticles is investigated through shifts in the intensity and shape of the PL spectra in hole or electron scavenging environments.
EE4: Nitride and II-VI Materials
Session Chairs
Wednesday PM, April 07, 2010
Room 3007 (Moscone West)
2:30 PM - **EE4.1
Positron Studies of Point Defects in III-nitrides.
Filip Tuomisto 1
1 , Helsinki University of Technology, TKK Espoo Finland
Show AbstractThe direct band gaps of the III-nitrides can be continuously tuned in a very wide energy range from 0.65 eV in InN to 6.2 eV in AlN, covering the whole solar spectrum. These materials have thus great potential for use in photovoltaics, e.g., in tandem solar cells. Point and extended defects present in the semiconductor lattice determine many of the crucial properties, such as the efficiency of electron-hole production through optical excitation and the diffusion length of carriers.We have applied positron annihilation spectroscopy with the aim to identify vacancy defects in AlN, GaN and InN. In a semiconductor material, positrons can get trapped at negative and neutral vacancy defects, and at negatively charged non-open volume defects given the temperature is low enough. The trapping of positrons at these defects is observed as well-defined changes in the positron-electron annihilation radiation. The combination of positron lifetime and Doppler broadening techniques with theoretical calculations provides the means to deduce both the identities and the concentrations of the vacancies in the materials. Performing measurements as a function of temperature gives information on the charge states of the detected defects. The III-sublattice vacancies are identified as common defects in all the III-nitrides, and they compensate donors either by forming vacancy-impurity complexes or by providing deep states for electrons. In some cases also N vacancies can be observed. We will present a comparison of the characteristic features of the vacancy defects in the III-nitride family.
3:00 PM - EE4.2
Influence of Growth Parameters on Mg Doping of GaN by Molecular Beam Epitaxy.
Ruben Lieten 1 2 , Vasyl Motsnyi 1 , Liyang Zhang 1 , Kai Cheng 1 , Maarten Leys 1 , Stefan Degroote 1 , Marianne Germain 1 , Gustaaf Borghs 1
1 , IMEC, Leuven Belgium, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractIII-Nitrides have many excellent properties, allowing a broad range of applications. The ability to tune the direct bandgap by changing the alloy composition holds the promise of solar matched photovoltaic panels. However the use of III-Nitrides has some technological challenges. III-Nitrides have huge structural mismatch with commonly available substrates, leading to highly defective layers when using heteroepitaxial growth. Most III-Nitrides structures for photovoltaic applications are currently grown on sapphire substrates. However silicon is also interesting as substrate as high quality Si wafers are commercial available up to 300 mm and are relatively cheap. Another problem in achieving efficient solar cells with III-Nitrides is inefficient p-type doping. Mg doping is most often used to achieve p-type III-Nitrides. Mg has however a large activation energy and significant influence on the growth process by changing the growth mode, introducing compensating defects, and even changing the polarity of the layer from metal-rich to N-rich.In this present work we investigate the influence of growth conditions on Mg doping of GaN layers by plasma assisted MBE (PAMBE). Semi insulating GaN buffer layers, grown by metal organic vapor phase epitaxy (MOVPE) on Si(111), were used as template. The Mg incorporation and the electrical properties (carrier concentration and mobility) have been investigated in function of growth temperature, Ga to N ratio and different Mg fluxes. It was found that at a fixed Mg flux the dopant incorporation and the electrical properties are highly sensitive to the Ga to N flux ratio. A clear but narrow optimum is observed. This optimum results from a compromise in Mg incorporation and defect formation. A lowest resistivity of 0.97 Ohmcm was obtained for optimized growth conditions. For this layer a hole concentration of 4.3 x 1017 cm-3 and a mobility of 15 cm2/Vs was measured. Our results show that high quality p-type GaN layers can be obtained on GaN templates grown by MOVPE on Si. Besides an optimal Ga to N ratio, also an optimum was observed in Mg flux. High Mg fluxes lead to strongly reduced mobilities and consequent increased sheet resistances by the introduction of crystal defects. Above a certain Mg flux these defects compensate the Mg doping completely, resulting in n-type layers. Besides continuous growth of Mg doped GaN layers we have also investigated different modulated growth methods. Finally we have investigated the influence of atomic hydrogen on the formation of compensating defects. Incorporation of atomic hydrogen has previously been proposed to suppress the formation of N vacancies which act as shallow donors and therefore could improve the electrical properties of Mg doped layers.
3:15 PM - EE4.3
Effects of Stress on Phase Separation in InGaN/GaN Multiple Quantum Wells.
Qinglei Zhang 1 , Fanyu Meng 1 , Peter Crozier 1 , Subhash Mahajan 1
1 , Arizona State University, Tempe, Arizona, United States
Show AbstractPhase separation in InGaN has been the subject of intensive studies due to its relation to strong luminescence of InGaN/GaN multiple quantum wells(MQWs). Most researches focused on the phase separation in InGaN thick layer, without taking into consideration the effect of stresses. In this research, the effect of stress on the occurrence of phase separation was investigated in pseudomorphic InGaN/GaN MQWs with the same indium concentration and different well widths, which is supposed to relate to different compressive stresses. The experiment was designed by making three InGaN/GaN MQW samples with different growth time of InGaN wells. All the samples were grown on sapphire substrates by metalorganic chemical vapor deposition and investigated comprehensively by high resolution X-ray diffraction(HRXRD), high angle angular dark field(HAADF) imaging and electron energy loss spectroscopy(EELS). EELS line scans were carried out at liquid nitrogen temperature. ω-2θ HRXRD scans indicate that indium concentration in the wells is 14% for all the three samples. The InGaN well widths were measured to be 7, 5 and 2.5nm on the HAADF images for the three samples, respectively. Both HRXRD reciprocal space mapping and high resolution HAADF reveal that InGaN wells are pseudomorphic for the three samples. The calculation indicates that compressive stress increases as well width decreases from 7 to 2.5nm for pseudomorphic In0.14Ga0.86N wells. Phase separation was investigated by analyzing the gallium concentration variation in both lateral and vertical directions using EELS line scan. Phase separation, occurs in the lateral direction in 7nm thick well, but is suppressed in 5 and 2.5nm thick wells due to the increased compressive stress. Also, the inhomogeneous gallium distribution in vertical direction was observed for the 7nm thick well, but not observed in 5 and 2.5nm thick wells. The authors gratefully acknowledge support for this research from the National Science Foundation grant DMR-0706631.
3:30 PM - EE4.4
Relation Between Structural Perfection and Luminescence of InxGa1-xN Layers.
Zuzanna Liliental-Weber 1 , Kin Man Yu 1 , M. Hawkridge 1 , S. Bedair 2 , A. Berman 2 , A. Emara 2 , D. Khanal 1 3 , J. Wu 1 3 , J. Domagala 4 , J. Bak-Misiuk 4
1 Materials Science Division, Lawrence Berkeley National Lab, Berkeley , California, United States, 2 Electrical and Computer Engineering Department, North Carolina State University, Raleigh, North Carolina, United States, 3 Department of Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 4 , Institute of Physics Polish Academy of Sciences, Warsaw Poland
Show AbstractInxGa1-xN is used as active material in optical devices and gives the possibility of obtaining light emission with tunable wavelength, depending on In content. However, due to the large size difference between Ga, In and N, a miscibility gap was predicted theoretically and phase separation has been shown experimentally in this alloy system. Hence, the growth of single phase homogeneous InxGa1-xN is still challenging. Despite the general believe that only the growth of InGaN films with In content higher than 20% is difficult and thus hampers the development of long wavelength LEDs, we found that thick layers (>100 nm) of InGaN with low In contents (~10%) also have high density of structural defects that strongly affect their optical properties. Using a variety of analytical techniques (including TEM, x-ray studies, Rutherford backscattering spectrometry, photoluminescence and cathodoluminescence performed on the same samples) we show for 10% In that the layers are sequestrated into sublayers with different In content when the film thickness exceeds a certain critical layer thickness. We find that only samples thinner than 100 nm are almost free of structural defects and these samples give single band edge PL peaks. CL studies on the thin samples confirm PL results with one band edge peak and give multiple peaks when an electron beam is placed in the defective parts of the layers. TEM studies show high density of planar defects in the thicker samples. These defective samples showed multiple PL peaks corresponding to layers with different In content. However, the presence of some low energy PL peaks (λ ~ 4420 Å) required layers with much higher In content that was not detected by TEM, x-ray or RBS in the thick samples. We are suggesting that some of these PL and Cl peaks, especially from the structurally defective samples can originate from defects as well as regions with different atomic arrangements.
3:45 PM - EE4.5
Investigations of Defects in GaN and Its Effects on Pd/n-GaN Schottky Diode Characteristics.
Suresh Sundaram 1 , Ganesh Vattikondala 1 , Prem Kumar Thirugnanam 1 , Balaji Manavaimaran 1 , Baskar Krishnan 1
1 Crystal Growth Centre, Anna University, Chennai, Tamil Nadu, India
Show AbstractGaN and related III-nitrides (III-Ns) have proven itself suitable for applications in the field of optoelectronic devices and high power and high temperature electronic devices [1]. Recently, III-Ns have been explored for use in photovoltaic (PV) devices and related applications [2]. III-Ns alloy systems have many material properties that make it an excellent candidate for high efficiency PV devices due to direct bandgap of the materials covering almost the entire solar spectrum, superior radiation hardness when compared to other III-V systems [3]. This enable the design of multijunction solar cell structures with near ideal bandgaps for maximum efficiency. Still lack of cheaply available defect-free native substrates presents a significant challenge to the device designers. The III-N crystals grown on sapphire contain relatively high densities of threading dislocations. The device uniformity and reproducibility, especially the reverse leakage current of GaN-based devices, are strongly influenced by the dislocations in the GaN film [4]. Degradation of device performance has been associated with trapping centers in the GaN [5]. In the present investigation to improve the understanding and control of the defects in III-N, GaN films were grown on sapphire substrates by MOCVD with different V/III ratio. Variations in defects concentrations were studied by hot-wet chemical etching, atomic force microscopy, positron annihilation spectroscopy, HRXRD, Raman spectroscopy, photoluminescence and Hall measurements. Pd/n-GaN schottky diodes were fabricated on these samples and results of I-V, C-V and I-V-T measurements have been discussed. The effects of compensating centers and deep level centers as a function of MOCVD growth conditions on schottky device characteristics like barrier height, Ideality factor and reverse leakage current are discussedREFERENCES:[1] D.H. Youn, V. Kumar, J.H. Lee, R. Schwindt, W.J. Chang, J.Y. Hong, C.M. Jeon, S.B. Bae, K. S. Lee, J.L. Lee, J.H. Lee, I. Adesida, Electron. Lett. 39, 566 (2003).[2] O. Jani, I. Ferguson, C. Honsberg, and S. Kurtz, Appl. Phys. Lett. 91, 132117 (2007).[3] J. Wu, W. Walukiewicz, K. M. Yu, W. Shan, J. W. Ager III, E. E. Haller, H. Lu, W. J. Schaff, W. K. Metzger, and S. Kurtz, J. Appl. Phys. 94, 6477 (2003).[4] E.J. Miller, D.M. Schaadt, E.T. Yu, C. Poblenz, C. Elsass, and J.S. Speck, J. Appl. Phys. 91, 9821 (2002). [5] P.B. Klein, S.C. Binari, J.A. Freitas Jr., A.E. Wickenden, J. Appl. Phys. 88 (2000) 2843.
4:30 PM - **EE4.6
Effects of Grain Boundaries and Interfaces in Photovoltaic Materials.
Yanfa Yan 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractCarrier recombination is the most critical issue impacting the solar-to-electricity conversion efficiency for solar cells. Interfaces and grain boundaries are the major sources for carrier recombination. Over the past years, we have investigated the atomic structure, chemistry, and electronic properties of interfaces and grain boundaries in various solar cell materials, including wafer Si, poly Si, and thin-film CdTe, and Cu(In,Ga)Se2, and we have correlated these parameters with the device performance using the combination of high-resolution electron microscopy, focused ion beam, and density functional theory. In this presentation, we will overview our results of the effects of grain boundaries and interfaces in photovoltaic materials, including Si, CdTe, and Cu(In,Ga)Se2.
5:00 PM - EE4.7
Characterizing the Grain Boundary Structure in Photovoltaic Materials Using Electron Backscatter Diffraction.
Matt Nowell 1 , Stuart Wright 1
1 , EDAX-TSL, Draper, Utah, United States
Show AbstractIt is well documented that the presence of grain boundaries within a photovoltaic absorber material can affect the efficiency and performance of a solar conversion cell. For silicon based devices the most efficient cells are single crystal materials that do not contain grain boundaries. Polycrystalline silicon cells exhibit lower efficiencies, indicating the grain boundaries have a negative effect on performance. However the cost of producing single crystal cells is significantly higher and techniques to minimize the grain boundary effects in polycrystalline cells can help offset their detrimental properties. In contrast, single crystal CdTe and CIGS based devices have poorer performance than their polycrystalline counterparts. In these materials, grain boundaries are therefore advantageous. Experimental results have shown that not all grain boundaries behave similarly. Certain grain boundary types (i.e. twin boundaries) exhibit preferential electrical properties. Electron Backscatter Diffraction (EBSD) is an ideal analytical technique to measure the grain boundary character of a photovoltaic material. EBSD is a Scanning Electron Microscope (SEM) based technique which can measure the crystallographic orientation on a nanometer scale. By comparing orientations across a grain boundary, the misorientation relationship can be determined. Twin boundaries occur as specific orientation relationships which can be easily identified. As a 2 dimensional analytical technique, EBSD typically does not provide information about the grain boundary plane inclination. However by correlating the known twinning plane with the grain boundary trace, some information can be extracted. As an automated technique, EBSD can analyze thousands of grains in a relatively short period of time, particularly compared to TEM based measurements. In addition to grain boundary character, EBSD can also measure texture and grain size. In this work, EBSD measurements from polycrystalline silicon, CdTe, and CIGS are presented. In particular, the role of twinning in the CdCl2 activation treatment in CdTe is discussed. Examples of how EBSD can be used to measure local strain is also presented.
5:15 PM - EE4.8
Imaging Electron Transport Across Individual Grain Boundaries in Cu(In,Ga)Se2 Thin Films by Complementary Operation of AFM and SEM Techniques.
Manuel Romero 1 , Miguel Contreras 1 , Chun-Sheng Jiang 1 , Mowafak Al-Jassim 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractWe report on recent advances in the development of atomic force microscopy (AFM) measurements of the electron transport and its application to fundamental aspects of Cu(In,Ga)Se2 (CIGS) thin films.It is well accepted that the misorientation of grain boundaries (or relational orientation of adjacent grain interiors) affects critically the conversion efficiencies of CIGS solar cells. In CIGS of <220/204> orientation, grain boundaries are believed to consist of {112}-reconstructed self-assembled surfaces. This spontaneously occurring configuration can accommodate high densities of point defects (intrinsic and extrinsic) without affecting dramatically its electronic activity. This is not the case for grain boundaries with other misorientations that the one described, and the sensitivity to processing conditions (selenium regime, incorporation of impurities) becomes increasingly more important. The impact of boundary misorientation on efficiency is not yet well understood and, because of the locality of the misorientation across boundaries, methods capable of resolving individual grain boundaries are needed.Measuring the electron transport across individual grain boundaries requires a multiprobe setup with high spatial resolution. Our approach is based on integrating in one platform conductive atomic force microscopy (cAFM) and electron-beam-induced current/voltage (EBIC/EBIV) measurements inside a scanning electron microscope. In our case, the conductive ultrasharp tip (sensing) and the highly localized electron beam (excitation) are independent probes that can be controlled independently. The lateral transport across a grain boundary is measured by maintaining the AFM tip over one grain and measuring the current/voltage difference when electrons(holes) are excited within that particular grain and across the boundary. Using this method, we investigate the influence of orientation and selenium regime (from deficiency to excess) on the transport across grain boundaries.
5:30 PM - EE4.9
Native Point Defects and Grain Boundaries in Wide-bandgap p-type Semiconductor BaCuChF (Ch = S, Se, Te).
Andriy Zakutayev 1 , Guenter Schneider 1 , Andreas Klein 2 , Janet Tate 1
1 Department of Physics, Oregon State University, Corvallis, Oregon, United States, 2 Surface Science Division, Darmstadt University of Technology, Darmstadt Germany
Show AbstractDevelopment of p-i-n double heterojunction thin film solar cells based on CdTe and Cu(InGa)Se2 requires a highly-doped wide-bandgap p-type material as an anode. In addition, such a material should form a graded interdiffused interface with the absorber to eliminate the influence of the interface defect states on the open-circuit voltage. Such materials are quite rare, but BaCuChF (Ch=S, Se, Te) family possesses all these properties. Optical absorption measurements and GW density functional theory (DFT) calculations agree that the optical gaps are 3.5 eV in BaCuSF, 3.2 eV in BaCuSeF and 2.9 eV in BaCuTeF. Concentration of holes increases from ~10^18 cm^-3 in BaCuSF through 10^19 cm^-3 in BaCuSeF to 10^20 cm^-3 in BaCuTeF. These parameters may be continuously tuned by formation of BaCuChF thin film solid solutions. Recently it has been shown that clean surfaces of BaCuSeF thin films form graded interface and have no valence band discontinuity with ZnTe, and hence CdTe, so this materials is promising for the photovoltaic applications. To implement BaCuChF in the photovoltaic devices it is important to understand the origin of its p-type conductivity and limiting factors for the charge transport, which will be addressed in this report. DFT calculations and self-consistent thermodynamic simulations suggest that free holes in BaCuChF originate from copper vacancies. Experimental attempts to decrease the carrier concentration by increasing the copper content of the films have had limited success. Different concentration of holes in three chalcogenides is caused by different degree of compensation by donor-like defects related to chalcogen vacancies. Calculated transition levels of donor-like defects agree well with the results of the temperature-dependent absorption spectroscopy and photoluminescence. Comparison of the calculated anisotropic effective masses with experimental mobility in epitaxial and polycrystalline thin films suggest that the main limiting factor of the charge carrier transport in BaCuChF is scattering on grain boundaries. Grain boundaries of BaCuChF pressed pellets are oxidized, according to x-ray photoemission spectroscopy experiments.
5:45 PM - EE4.10
Pulsed Laser Processing of Electrodeposited CuInSe2 Photovoltaic Absorber Thin Films.
Ashish Bhatia 1 , Philip Dale 2 , Matthew Nowell 3 , Michael Scarpulla 1
1 Material Science and Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Laboratoire Photovoltaique, Universite du Luxembourg, Belvaux Luxembourg, 3 , EDAX-TSL, Draper, Utah, United States
Show AbstractWang et al. have demonstrated efficiency increases for sputtered Cu(In,Ga)Se2 (CIGS) thin film solar cells with pulsed laser irradiation.[1] In this work we investigate the effects of laser annealing on as-deposited and on furnace-annealed CuInSe2 (CIS) samples deposited by electrodeposition.A pulsed KrF excimer laser was used to anneal CIS samples in two fluence (J/cm2) regimes: a) high (0.2-0.6 J/cm2) to modify crystalline grain size and orientation distributions and b) low (0-0.2 J/cm2) to modify electronic defect populations. Both types of sample were irradiated with single and multiple (5 & 25) laser shots of varying fluence (J/cm2). Phase, compositional, morphological, and grain structure changes are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and electron backscattered diffraction (EBSD). We find that multiple shots are effective at synthesizing the CIS phase from electrodeposited precursors with higher crystallinity appearing at higher fluence and pulse number. Surface feature size increases and XRD peak width decreases for pre-annealed films in the low fluence regime. In the high fluence regime for pre-annealed samples, cracking appears above a certain threshold depending on number of shots. These laser induced changes were not observed for samples having chemical bath deposited (CBD) CdS buffer layers. Within the limits of EDS detection, we observed some Cu loss with increasing fluence and shot number, however significant Se loss was not observed. C-V measurements on Schottky junctions are used to quantify carrier density and I-V measurements are reported for p-n junctions formed by CBD. [1]Wang et al.,Sol Energ Mat Sol C 88 65 (2005).
EE5: Poster Session
Session Chairs
Thursday AM, April 08, 2010
Salon Level (Marriott)
9:00 PM - EE5.1
Transition Metal – Hydrogen Complexes in Si: First-principles Theory.
Daniel Backlund 1 , Stefan Estreicher 1
1 Physics, Texas Tech University, Lubbock, Texas, United States
Show AbstractHydrogen is commonly used to remove or at least reduce the electrical activity of numerous defects and impurities in Si. Although hydrogenation works quite well in many cases, it has generally been unsuccessful with transition metals (TM) impurities. Numerous TM-H complexes have been detected using optical or electrical measurements. However, they often remain electrically active. The structure of the complex responsible for specific DLTS line is not known. Even the number of H interstitials trapped at the TM is assumed. We have performed systematic first-principles calculations involving several TMs from the 3d series and H. The binding energies of a large number of equilibrium configurations involving one or two H interstitials and a TM (either interstitial or substitutional) are calculated. The electrical activity and vibrational spectra are predicted for the most stable ones. As a general rule, we find that interstitial TM impurities do not react strongly with interstitial H, but that substitutional TM do trap multiple Hs. The binding energies are of the order of 1 eV. Results for Ti, Fe, Ni, and Cu are discussed (the Cu-H and Fe-H results have already been published and the Ti-H and Ni-H ones are new). The binding energies are surprisingly similar across the 3d series, and the stable structures always involve H bound directly to the substitutional TM, which therefore becomes 5-, 6-, or even 7-fold coordinated. We also examine the energy balance of {TM,Hn} complexes vs. {TM,Hn-2} with an H2 molecule nearby.
9:00 PM - EE5.2
Preparation of CuIn0.7Ga0.3Se2 Thin Films Using Sputtered Metallic Cu/In/Ga Films Selenized by Ditert-butylselenide.
Sheng-Yu Hsiao 1 , Jyun-Syong Yang 1 , Jyh-Rong Gong 1 , Kuo-Yi Yen 1 , Dong-Yuan Lyu 2 , Tai-Yuan Lin 2 , Hsin-Lun Su 1 , Shih-Chang Liang 3 , Guo-Yu Ni 3 , Cheng-Han Wu 3 , Far-Wen Jih 3
1 , National Chung Hsing University, Taichung Taiwan, 2 , National Taiwan Ocean University, Keelung Taiwan, 3 , Chung-Shan Institute of Science and Technology, Taoyuan Taiwan
Show Abstract CuIn0.7Ga0.3Se2 thin films were prepared by selenization of Cu/In/Ga metallic films on sodium-lime glass substrates at 600°C using ditert-butylselenide [Se(C4H9)2] under atmospheric pressure. X-ray diffraction (XRD) measurements of the as-grown CuIn0.7Ga0.3Se2 films show prefer-oriented (112), (220/204) and (312/116) peaks. It was found that increase of selenization time interval tended to increase the intensity of (220/204) XRD peak. According to the results of scanning electron microscopy (SEM), the primary part of the Se(C4H9)2-selenized CuIn0.7Ga0.3Se2 film surface exhibits triangle-like terraced structures, which agrees quite well with the dominant (112) XRD peak. A typical optical absorption spectrum of the CuIn0.7Ga0.3Se2 film at room temperature shows a cut-off energy of ~1.05eV. It appears that Se(C4H9)2 can serve as the replacement of Se vapor for the selenization of CuIn0.7Ga0.3Se2 absorbing layer in an open system operated at atmospheric pressure.
9:00 PM - EE5.3
Photographic Imaging of Deep Traps in Poly-crystalline Si Solar Cells by Spectroscopic Electroluminescence.
Takashi Fuyuki 1 , Ayumi Tani 1 , Emi Sugimura 1
1 , Nara Institute of Science and Technology, Takayama,Ikoma, Nara, Japan
Show AbstractCrystalline Si solar cells emit infrared light under the forward bias as so called “Electroluminescence, EL”. We have been developing [1-3] the photographic diagnosis technique of not only the extrinsic deficiencies (substrate breakage and electrode snapping) but also intrinsic defects (crystallographic defects, grain boundaries, etc.). In this study, we have succeeded the direct imaging analysis of deep traps in Si substrates using spectroscopic EL analysis at room temperatures. The photographic imaging of EL intensity gives the spatial information of minority carrier number in a Si base of solar cell with high resolution in less than one second. The EL intensity is proportional to the total number of the minority carriers in Si substrates determined by the minority carrier lifetime. The intrinsic recombination centers such as crystal defects and grain boundaries which reduce the minority carrier density can be clearly detected as dark parts (spots, lines and areas) if we measure the EL caused by the band to band recombination, i.e., 1150 nm emission wavelength. The EL intensity decreased around defects since they are strong drains of minority carriers due to the deep electronic traps. We have pointed out the EL emission by way of those deep defect traps. Typical deep traps in Si locate at 0.3eV below the conduction band, and we could successfully observe the EL emission at the wavelength of around 1500nm (0.8eV). Using an infrared sensitive CCD camera equipped with appropriate band-pass filters (1150 and 1500 nm), the spectroscopic EL images could be obtained at room temperatures. All the defects which reduce the minority carrier number were observed as dark parts by the conventional EL technique detecting 1150nm emission. Among the dark parts, some specific lines and spots showed EL emission at 1500nm wavelength. Those are considered to be originated by so called intrinsic deep traps in the grain boundaries and defects in which oxygen related precipitations occurred. In contrast other dark parts did not emit 1500nm EL emission. Those were considered to be extrinsic defects such as mechanical cracks. The spectroscopic EL technique is a versatile diagnosis tool for imaging characterization method to distinguish the intrinsic and extrinsic defects in Si substrates.References:1.J.Appl.Phys.,106,043717,(2009).2.Appl.Phys. A,DOI:10.1007/s00339-008-4986-0,(2008).3.Appl.Phys.Lett., 86,262108,(2005).
9:00 PM - EE5.4
Copper Vacancy Diffusion in CIGS From Density Functional Theory.
Johan Pohl 1 , Karsten Albe 1
1 Institute for Materials Science, TU Darmstadt, Darmstadt Germany
Show AbstractThe quality of solar cells using absorbers made of Cu(In,Ga)(S,Se)2 (in short: CIGS) is determined by the kinetics of phase formation and subsequent growth of sufficiently large grains during the deposition process. While it is widely believed that concentration gradients in thin CIS films enhance the efficiency of the solar cells, a detailed understanding of the diffusional processes in this material is missing. Among all diffusional processes the mobility of copper is a key parameter.In order to gain a detailed understanding of the copper diffusion mechanisms in CIGS we have calculated diffusion paths and activation barriers of copper vacancies in the boundary phases using density functional theory. We obtain diffusion pathways and activation barriers using the climbing image nudged elastic band method. The implications for the phase formation of CIGS in the absorber deposition process are discussed.
9:00 PM - EE5.5
Electrochemical Synthesis of Cu2ZnSnS4 Thin Films for Photovoltaic Devices.
Byungjun Jeon 1 , Bongyoung Yoo 1
1 Metallurgy and Materials Engineering, Hanyang University, Ansan, Gyeonggi-do, Korea (the Republic of)
Show AbstractThe electrochemical synthesis of Cu2ZnSnS4 (CZTS) thin films, which could be used as an absorber layers with a high absorption coefficient (α =~10^4cm-1) and a direct band gap (Eg=1.45-1.50eV) in photovoltaic devices have been focused. The stannite and kesterite crystal structure of CZTS is similar to chalcogenide crystal structure such as CuInSe2 (CIS) and Cu(In,Ga)Se2 (CIGS). A CZTS unit cell can be obtained from CIS or CIGS by replacing In and Ga with Zn and Sn. A CZTS is particularly attractive because, in comparison to In and Ga, Sn and Zn are naturally abundant materials, and have relatively low-cost and low toxicity. Photovoltaic devices based on CZTS have been achieved with conversion efficiency up to 6.7% by co-sputtering method followed by annealing in sulfurized atmosphere. Compare to vacuum techniques such as evaporation and sputtering, electrodeposition method presents a potential to prepare a CZTS thin films with simplicity and cost-effective route. In addition, synthesis of CZTS thin film by electrodeposition allows controlled composition, desired microstructure, and physical properties by simply changing the deposition parameters. In this work, CZTS thin films were synthesized by electrodeposition from sulfate-ethylenediaminetetraacetic acid (EDTA) complexing baths followed by sulfurization. The morphology of CZTS thin film was examined using scanning electron microscopy (SEM). The composition of CZTS thin film was determined using energy dispersive spectroscopy (EDS) and inductively coupled plasma (ICP). The crystal structure of CZTS thin film was investigated by X-ray diffraction analysis (XRD) with copper Kα radiation. The sulfurized CZTS thin films on Mo coated glass substrate were processed to compose PN junction with following chemical bath deposition (CBD) of CdS. Finally, photoelectrical characteristics of the devices were investigated.
9:00 PM - EE5.6
Characterization of Carrier Lifetime in Ge Films Epitaxially Grown on Si by Nanoscale Heterojunction Engineering.
Josephine Sheng 1 2 , Darin Leonhardt 3 , Malcolm Carroll 2 , Jeffrey Cederberg 2 , Sang Han 3 1
1 NanoScience and MicroSystems, University of New Mexico, Albuquerque, New Mexico, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 3 Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico, United States
Show AbstractHigh-quality Ge-on-Si (GoS) heterostructures are pursued for many applications, including near infrared (NIR) photodetectors and integration with III-V films for multijunction photovoltaics. However, the lattice mismatch between Ge and Si often leads to a high density of threading dislocations. These dislocations, if not reduced, propagate through the subsequently grown GaAs layer, deteriorating its quality. In addition, the thermal expansion mismatch between Ge and Si causes microcracks and eventual delamination of Ge as the thickness of the Ge epilayer continues to increase. Recently, we have demonstrated the scalability of a process to substantially reduce the threading dislocations in Ge epilayers grown on 2-inch-diameter chemically oxidized Si substrates. Subsequent integration of GaAs leads to photoluminescence as well as cathodoluminescence intensity comparable to that on GaAs substrates. Herein, we focus on the characterization of carrier lifetime in the Ge epilayers grown on Si by the said nanoscale heterojunction engineering. Using a photoconductivity decay technique, minority carrier lifetime is measured in the GoS substrates to extract surface recombination velocity as well as carrier lifetime in bulk Ge. The effective surface recombination velocity, representing both Ge-Si interface decorated with chemical SiO2 and Ge surface, is approximately 2x103 cm/sec. We observe that the extracted lifetimes, which vary with the Ge film thickness, correlate well with the dislocation density that varies as a function of distance from the Ge-Si interface.
9:00 PM - EE5.7
Surface and Electrical Characterization of Thin Film Solar Cells Using Atomic Force Microscopy.
Joshua Ford 1 , Chris France 1 , Joyce Zhou 2 , Tong Ju 1 , Sue Carter 1
1 Physics, University of California at Santa Cruz, Santa Cruz, California, United States, 2 Electrical Engineering, University of California at Santa Cruz, Santa Cruz, California, United States
Show AbstractThe properties of thin film solar cells are explored using a conductive-tip atomic force microscopy. Current images taken at a constant bias voltage were compared to topographic images taken concurrently to show how the conductance varies throughout the structure of the film. The film thicknesses and heights of each c-AFM topography image are then used with the current measurements to map the conductivity across the films. Measurements were performed on CdTe and PbS-based nanoparticle thin films as well as InxS deposited by chemical bath deposition. The CdTe nanoparticle thin films were sintered for different lengths of time during their CdCl2 sintering treatment, leading to a loss of quantum confinement and substantial grain growth. Conductive AFM allowed us to study how the higher conductivity along the grain boundaries impacted overall device performance of CdTe nanoparticle Schottky devices. The deterioration of the nanoparticle CdTe film after c-AFM imaging is also shown; this effect has been previously observed in crystalline CdTe thin films [1]. We also discuss how the post-sintering treatment impacts the conductivity along the grains. Quantum confined PbS nanoparticles films were deposited by layer-by-layer deposition and ligand exchange. The PbS quantum dot Schottky devices have similar short-circuit current than the CdTe devices; however, the PbS films showed substantially different topography and current uniformities due to lack of significant grain growth. Preliminary measurements suggest evidence for more filamentary conduction pathways. Finally, an InxS film, useful as a buffer layer, was also studied and showed some of the clearest correlations between film structure and conductivity. [1] Moutinho, H.R.; Dhere, R.G.; Jiang, C.-S.; Al-Jassim, M.M.; Kazmerski, L.L.; “Conductive Atomic Force Microscopy Applied to CdTe/CdS Solar Cells.”; 19th Europoean PV Solar Energy Conference; (2004); NREL/CP-520-36323;
9:00 PM - EE5.8
Defects in Vapor-Liquid-Solid Grown Si Wires for Photovoltaic Applications.
Morgan Putnam 1 , Michael Kelzenberg 1 , Alexander Lapides 1 , Shannon Boettcher 1 , Daniel Turner-Evans 1 , Emily Warren 1 , Ron Grimm 1 , Nathan Lewis 1 , Harry Atwater 1
1 Chemical Engineering, California Institute of Technology, Pasadena, California, United States
Show AbstractThe vapor-liquid-solid (VLS) growth mechanism provides a means for the fabrication of low-cost, large-area arrays of Si wires for microwire solar cells. Inherent to the VLS growth mechanism is the incorporation of metal catalyst into the grown Si wire. As catalyst atoms within a wire may produce deep-level, carrier-recombination centers or cause unintentional doping, the quantification of catalyst concentration and the minority-carrier diffusion lengths in VLS-grown Si wires is critical to the development of high-efficiency microwire solar cells. Our previous work has focused on characterizing Au-catalyzed, Si wires grown from silicon tetrachloride at 1000 °C. Using secondary ion mass spectrometry (SIMS), the Au concentration was found to be 1.6×1016 atoms/cm3 in good agreement with the thermodynamic equilibrium concentration of Au in Si at 1000 °C [1]. Given this bulk Au concentration, bulk minority-carrier diffusion lengths of 1 μm are predicted for both electrons and holes, which is in good agreement with effective hole minority-carrier diffusion lengths of 2-4 μm measured for n-Si wires using scanning photocurrent microscopy (SPCM) [2].This presentation will focus on our recent efforts to characterize the concentrations of Ni and Cu impurities in the wires and their effect on minority-carrier diffusion lengths properties. For both Cu- and Ni-catalyzed Si wires, bulk minority-carrier diffusion lengths are expected to be greater than bulk minority-carrier diffusion lengths for Au-catalyzed Si wires due to the smaller carrier-capture cross-section of the Ni and Cu defects. Using SPCM we have measured an effective electron minority-carrier diffusion length of 10 μm in 2 μm diameter, Cu-catalyzed, p-Si wires with a native oxide surface passivation. While a significant increase over that measured for Au-catalyzed wires, surface passivation studies suggest that this value is limited by recombination at the surface of the wire and not by bulk recombination at metal impurity defects. In addition to further surface passivation studies aimed at elucidating the bulk minority-carrier lifetimes, we will discuss the limits on bulk minority-carrier lifetimes through measurements of the catalyst concentration using inductively coupled plasma mass spectrometry (ICP-MS). Initial ICP-MS measurements have found a Cu concentration of 1018 cm-3 in RCA2 cleaned Si wires. However, this high Cu concentration may arise from a Cu-rich surface phase.[1] M. C. Putnam, M. A. Filler, B. M. Kayes, M. D. Kelzenberg, Y. Guan, N. S. Lewis, John M. Eiler, Nano Lett. (2008)[2] M. D. Kelzenberg, D. B. Turner-Evans, B. M. Kayes, M. A. Filler, M. C. Putnam, N. S. Lewis, and H. A. Atwater, Nano Lett. (2008)
Symposium Organizers
Daniel Friedman National Renewable Energy Laboratory
Michael Stavola Lehigh University
Wladek Walukiewicz Lawrence Berkeley National Laboratory
Shengbai Zhang Rensselaer Polytechnic Institute
EE6: III-V, Oxide, and Emerging Materials
Session Chairs
Thursday AM, April 08, 2010
Room 3007 (Moscone West)
9:30 AM - **EE6.1
Defect Engineering in III-V/Si Photovoltaic Materials Based on GaP/Si Interfaces.
Tyler Grassman 1 , Mark Brenner 1 , Andrew Carlin 1 , Rajagopalan Srinivasan 2 , Raymond Unocic 2 , Ryan Dehoff 2 , Michael Mills 2 , Hamish Fraser 2 , Steven Ringel 1 2
1 Dept. of Electrical and Computer Engineering, The Ohio State University, Columbus, Ohio, United States, 2 Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States
Show AbstractEpitaxy of GaP on Si substrates has been attractive to photovoltaic researchers for decades since the relatively close lattice match would provide a convenient pathway to integrate III-V and Si photovoltaics on a common, inexpensive Si substrate. However, the polar/non-polar nucleation necessary for III-V/IV heterovalent epitaxy is well-known to be a source of numerous electronically detrimental defects, including antiphase domains (APDs), stacking faults (SFs), and microtwins (MTs). It was previously shown in the somewhat analogous GaAs/Ge system that such defects can be mitigated through the proper preparation of the group-IV substrate surface and careful control of the III-V nucleation, enabling the growth of high-quality (In)GaAs and InGaP solar cells on Ge substrates. Unlike GaAs/Ge, numerous additional materials issues have generally impeded progress in the GaP/Si heteroepitaxial system: high chemical reactivity of phosphorus with silicon, difficulty in preparing an ideal Si surface for III-V epitaxy, and the small, but non-negligible, lattice mismatch between GaP and Si, ranging from 0.37% (at room temperature) to ~0.5% (at growth temperature) due to the difference in coefficients of thermal expansion. Despite these addition difficulties, we have recently demonstrated the growth of GaP on Si substrates with full control and elimination of these polar/non-polar nucleation related defects. Using GaAsyP1-y anion sublattice compositionally graded buffers, with GaP as the initial binary starting point, GaAsP photovoltaic materials and devices with bandgaps of 1.6 – 1.8 eV have been grown on Si substrates, exhibiting a complete lack of all extended crystalline defects except for the expected threading dislocations due to the lattice mismatch between GaAsP and Si. Early, unoptimized GaAsP solar cell device results present great promise for this materials system. Work is in progress on the optimization of the GaAs1-yPy/Si buffer system, with significant improvements expected due to the further reduction of threading dislocation density (TDD) in the active device region. We will present here work detailing the relationship of III-V/IV photovoltaic, structural and electronic materials properties, including device characteristics and carrier transport and lifetime, to the presence of crystalline defects, such as antiphase domains and threading dislocations.
10:00 AM - EE6.2
In situ Study of Strain Relaxation Mechanisms During Lattice-mismatched InGaAs/GaAs Growth by X-ray Reciprocal Space Mapping.
Takuo Sasaki 1 , Hidetoshi Suzuki 1 , Akihisa Sai 1 , Masamitu Takahasi 2 , Seiji Fujikawa 2 , Yoshio Ohshita 1 , Masafumi Yamaguchi 1
1 , Toyota Technological Institute, Nagoya Japan, 2 , Japan Atomic Energy Agency, Hyogo Japan
Show AbstractUnderstanding of strain relaxation mechanisms during III-V lattice-mismatched heteroepitaxy is necessary to achieve high-quality optoelectronic devices with long operational lifetime. In mismatched heteroepitaxy, the structure of the buffer layer is particularly important, as it reduces the defect densities in the film and improves the quality of crystal grown on a substrate. By adopting appropriate relaxed buffer layers, the dislocation densities in mismatched semiconductors can be decreased, enabling photovoltaic cells with high performance to be realized. While extensive experimental studies have been performed to investigate the strain relaxation mechanisms, in situ measurements are more promising for understanding details of the strain relaxation process without the influence of the thermal strain due to difference in thermal expansion coefficients of substrate and epitaxial layer. Recently, the residual stain was estimated based on the substrate curvature obtained by using a multibeam optical stress sensor during a molecular beam epitaxy (MBE). From the changes of estimated residual strain in the epitaxial layer, several dislocation behaviors such as nucleation of new dislocations, multiplication, or interaction of dislocations have been suggested. However, the mechanisms involved during the relaxation process have not been fully understood yet.In this study, the strain relaxation mechanisms in the lattice-mismatched In0.12Ga0.88As/GaAs (001) layers are discussed based on in situ real-time measurement of X-ray diffraction during MBE growth. Experiments were performed at synchrotron radiation facility, SPring-8 beamline 11XU, using an MBE system directly coupled to an X-ray diffractometer. The high resolution reciprocal space maps (RSMs) of 004 reflections obtained at interval of 6.2 nm thickness enabled transient behavior of residual strain and crystal quality to be observed simultaneously as a function of InGaAs film thickness. From the evolution of these data, the strain relaxation mechanisms were classified into five phases as a function of layer thickness, and thereby the dominant dislocation behavior in each phase was deduced. Additionally, by applying this technique to the three-dimensional RSMs of 022 reflections, the in-plane asymmetry of the strain relaxation along two orthogonal <110> directions was observed. From the evolution of the in-plane asymmetry, interactions between orthogonal dislocations (α dislocations and β dislocations) were found at the relatively thick InGaAs layer.
10:15 AM - EE6.3
Defect Control in InAlAs/InP Heterostructures for Multijunction Solar Cells.
Marina Leite 1 , Robyn Woo 2 , Koray Aydin 1 , Emily Warmann 1 , Erika Garcia 1 , William Hong 2 , Daniel Law 2 , Harry Atwater 1
1 , CALTECH, Pasadena, California, United States, 2 , Spectrolab, Inc., Sylmar, California, United States
Show AbstractWe articulate an approach for a new multijunction solar cell (MJSC) system based on InAlAs/InGaAsP/InGaAs/InP/Si. Because of the restricted lattice constant layers that can be grown on GaAs substrates, InP substrate is an ideal candidate to fabricate InAlAs/InP and InAlAs/InGaAsP/InP triple junction solar cells. To produce high efficiency cells, high quality InAlAs epitaxial layers are required. In order to minimize defect’s density, we used a direct wafer bonding/layer exfoliation technique. First, high energy H+ ions are implanted 800 nm deep in InP bulk substrates. Both InP and a SiO2/Si(001) substrate (cheap and robust) are brought together and annealed for 1 hour at 350 oC, under uniform pressure (500 mbar). Heating causes micro-cracks in the implanted region of the InP substrate, and a subsequent film exfoliation occurs. The thin InP film is bonded to the SiO2 by covalent bonding. Cross-section Transmission Electron Microscopy (X-TEM) showed an atomically flat and defect-free interface formed by the process described. The layer exfoliation technique allowed us to grow epitaxial, lattice-matched and coherently-strained InxAl1-xAs heterostructures on InP. This procedure was successfully applied to 50 mm InP substrates, and allows InP substrates to be reused several times, significantly decreasing the cell’s overall cost. The method enables the fabrication of MJSC with optimized band gap energies and with lattice mismatch, once crystal dislocations and other defects produced during thin film growth can be eliminated. In order to access a wide band gap region of the solar spectrum (1.5-1.8 eV), 3x1017 cm-3 doped InxAl1-xAs ternary alloys were grown by MOVPE. The structural and optical properties of the alloyed layers were characterized in order to minimize defect, which strongly affect cells performance. Both lattice matched (In0.52Al0.48As) and 0.7% strained (In0.42Al0.58As) 300nm layers with 20 nm windows presented high quality defect-free crystalline layers, as measured by high-resolution X-TEM and X-ray diffraction (XRD). Photoluminescence (PL) measurements showed sharp peaks at 1.36 eV, 1.5 and 1.7 eV, corresponding to InP, In0.52Al0.48As and In0.42Al0.58As, respectively. An additional peak was observed for the lattice matched layer at 1.2 eV, due to a thin InAsP alloyed layer formed at the InP/InAlAs interface. Time-resolved PL measurements showed life times up to 400 picosec for tensile InAlAs layers, indicating that high-efficiency cells can be achieved. In conclusion, the use of the direct wafer bonding was used as an alternate route to fabricate III-V solar cells with wide band gap energy. This detailed characterization and device physics modeling suggest that InxAl1-xAs can be used as a top cell for high efficiency MJSCs.
10:30 AM - EE6.4
MBE Growth of Metamorphic InGaP for Wide-bandgap Photovoltaic Junctions.
John Simon 1 , Stephanie Tomasulo 1 , Paul Simmonds 1 , Yuncheng Song 1 , Minjoo Larry Lee 1
1 Electrical Engineering, Yale University, New Haven, Connecticut, United States
Show AbstractRecently, multijunction solar cells have reached efficiencies of 41.1% by combining absorber materials that have different lattice constants*. To achieve efficiencies beyond 50%, additional junctions may be needed to more efficiently divide the solar spectrum. This will require the development of a wide-bandgap (2-2.2eV) material, such as InyGa1-yP (y=0.27-0.4), to act as the top layer in a 4-, 5-, or even 6-junction solar cell. To enable InGaP to be used for this purpose, metamorphic growth on GaAs and GaP substrates must be developed. In this work we demonstrate wide-bandgap InGaP grown on GaAsP buffers on both GaAs and GaP via solid source molecular beam epitaxy. GaAsxP1-x buffers grown on GaAs exhibited anisotropic strain relaxation due to the disparity in α- and β-dislocation velocity, along with formation of faceted defects (FDs ~100 nm deep) in one of the [110] directions. The density of the FDs was greatly reduced by lowering the grading rate, implying that FD formation is related to the strain relaxation process. Strain relaxation increased from ~55% to ~85% for the top layer of the graded buffer by increasing the growth temperature from 580 to 700°C. Surface roughness as low as 2.45 nm across a 30x30 μm2 area was measured by AFM on buffers graded to x=0.8. The use of high growth temperature and slow grading allowed a threading dislocation density (TDD) of 7.5x105cm-2 to be attained, as measured by plan-view transmission electron microscopy (PVTEM). Graded buffers with x<0.7 had degraded surface morphologies with elevated FD densities that could not be reduced by slower grading. Micro-cracks were also observed by cross-sectional TEM (XTEM) in buffers with x<0.7.In contrast, graded GaAsxP1-x buffers on GaP showed symmetric and nearly-complete strain relaxation of the top layers at substrate temperatures down to 580°C. Surface roughness of ~2.5 nm across a 15x15 μm2 area was measured on buffers graded to x~0.7. No FDs or micro-cracks were observed in these buffers, and a TDD of ~3.5x106cm-2 was obtained by PVTEM on GaAs0.68P0.32/GaP.In0.34Ga0.66P layers were subsequently grown at 460°C on GaAs0.7P0.3 buffers on both GaAs and GaP. Intense room temperature photoluminescence emission at 2.09eV agrees with the composition extracted from X-ray measurements, implying a lack of CuPt ordering. Similar surface morphology and RMS roughness to the underlying buffer were attained. A smooth coherent interface between the GaAs0.7P0.3 and In0.34Ga0.66P was observed by XTEM, with no evidence of phase separation, while selective area diffraction confirmed the lack of CuPt ordering.The low TDD and smooth interfaces of these wide-bandgap In0.34Ga0.66P layers show that they are promising candidates for use as the top junction of a 4- to 6-junction solar cell. Preliminary results towards the demonstration of 2-2.2eV solar cells will also be presented.*Guter, et al, Appl Phys Lett, 94, 223504 (2009).
10:45 AM - EE6.5
Reducing Defect Density in III-V Photovoltaic Films on Flexible Substrates.
Venkat Selvamanickam 1 , Senthil Sambandam 2 , Xuming Xiong 2 , Aarthi Sundaram 1 , Seunghee Lee 1 , Gang Shi 1 , Andrei Rar 2 , Alex Freundlich 3
1 Mechanical Engineering & Texas Center for Superconductivity, University of Houston, Houston, Texas, United States, 2 , SuperPower, Schenectady, New York, United States, 3 Center for Advanced Materials, University of Houston, Houston, Texas, United States
Show AbstractWe are pursuing the goal of integrating III-V photovoltaic materials with flexible metal substrates so as to achieve low-cost, roll-to-roll manufacturing of high-efficiency solar cells. Epitaxial GaAs has been successfully grown on inexpensive, polycrystalline nickel superalloy substrates using an enabling technology of ion beam assisted deposition (IBAD). Essentially, IBAD enables single crystalline-like films with an excellent in-plane and out-of-plane texture even on polycrystalline substrates. Using IBAD templates and intermediate buffer layers, Ge has been epitaxially grown with a good in-plane and out-of-plane texture on flexible metal substrates. All layers including Ge are grown by reel-to-reel ion beam or magnetron sputtering. GaAs grown by molecular beam epitaxy on the Ge film was found to exhibit strong photoluminescence signal. Additionally, an existence of a relatively narrow (FWHM~20 meV) band-edge excitons measured in this film indicated a good optoelectronic quality of deposited GaAs. 2×4 RHEED patterns obtained from GaAs suggest self annihilation of antiphase boundaries and the growth of a single domain GaAs. Data from high resolution X-ray diffraction of GaAs on Ge on IBAD templates indicates a high threading defect/dislocation density of 3 to 5 times 10^8 cm^-2. Cross sectional transmission electron microscopy of the multilayered architecture shows high defect density at the interfaces. Since a high defect density would limit the minority carrier diffusion lengths (<1 μm) leading to a severe performance degradation in a typical 3 - 4 μm-thick III-V solar cell, we are working on a number of approaches to mitigate the defect sources. Buffer layers with graded lattice match as well as methods to modify the Ge layer are being explored to reduce defect density. The latest progress in our approaches to reduce defect densities in epitaxial III-V photovoltaic films on flexible substrates will be presented in this talk.
11:30 AM - **EE6.6
Spin-blockade of Dominant Non-radiative Carrier Recombination Channels via Defects in Ga(In)NAs Alloys.
X. Wang 1 , Y. Puttisong 1 , C. Tu 2 , A. Ptak 3 , V. Kalevich 4 , A. Egorov 4 , L. Geelhaar 5 , H. Riechert 5 , I. Buyanova 1 , Weimin Chen 1
1 Department of Physics, Chemistry and Biology, Linköping University, Linköping Sweden, 2 Department of Electrical and Computer Engineering, University of California, La Jolla, California, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States, 4 , A F Ioffe Physico-Technical Institute, St Petersburg Russian Federation, 5 , Paul-Drude-Institut für Festkörpelektronik, Berlin Germany
Show AbstractGa(In)NAs dilute nitride alloys have in the past years attracted great research interest owing to their potential applications for efficient and low-cost long wavelength telecommunications lasers and highly efficient multi-junction solar cells. Unfortunately, developments of these devices have largely been hindered by introduction of defects facilitated by N incorporation, which provide efficient non-radiative shunt paths for non-equilibrium carriers leading to a severe reduction in radiative recombination efficiency and minority-carrier lifetime. Up to now, the origin of the dominant non-radiative defects and minority-carrier lifetime killers is still largely unknown. Here, we present our recent experimental results that have positively identified Ga interstitial defects as the dominant non-radiative defects in Ga(In)NAs thin films and quantum wells by optical and spin resonance spectroscopy. The defects are found to be common in both MBE and MOCVD grown materials. At least three different types of Ga interstitial defects can be distinguished based on their hyperfine structures, which characterize the interaction between an unpaired electron spin and the nuclear spin of a doubly positively charged Ga interstitial atom. Formation of the defects is shown to strongly depend on growth methods and conditions as well as post-growth rapid thermal annealing. Defect concentrations are found to most profoundly increase with increasing N composition and with decreasing growth temperature. The carrier recombination process via these defects is discovered to be strongly spin-dependent, which opens a door for spin manipulation of the process such as record-high efficient spin filtering at room temperature [1]. In this work we demonstrate that efficient spin-blockade of the non-radiative carrier recombination via these Ga interstitial defects can be achieved once the defect electrons are spin polarized. This spin blockade results in a significant enhancement of light emission efficiency by up to 800%, accompanied by a sizable increase in the non-equilibrium carrier lifetime. As these non-radiative defects are common grown-in defects in Ga(In)NAs, which seems to be unavoidable at least with the current growth technologies, the demonstrated spin blockade of the associated carrier recombination process appears to offer an attractive approach to strongly suppress non-radiative shunt paths that limit efficiency of photonic and photovoltaic devices based on these alloys.[1] X.J. Wang, I.A. Buyanova, F. Zhao, D. Lagarde, A. Balocchi, X. Marie, C.W. Tu, J.C. Harmand and W.M. Chen, Nature Materials 8, 198 (2009).
12:00 PM - EE6.7
Origin of Persistent Photoconductivity Effect in GaAsN Alloys.
Yu Jin 1 , Hailing Cheng 1 , Ryan Jock 1 , Kurdak Cagliyan 1 , Rachel Goldman 2 1 3
1 Department of Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Material Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Department of electrical engineering and computer science, University of Michigan, Ann Arbor, Michigan, United States
Show Abstract(In)GaAsN alloys with a few percent nitrogen have potential applications in long wavelength optoelectronic devices, such as infrared laser diodes, heterojunction bipolar transistors, and high efficiency solar cells. However, due to the large size and electronegativity differences between As and N, the formation of several point defect complexes has been predicted. These point defect complexes are likely the origin of the limited optical emission efficiency and minority carrier transport properties observed in (In)GaAsN alloys. Recently, we identified the atomic structure of Si-N defect complexes and the electronic state associated with N interstitials in GaAsN alloys. The presence of Si-N complexes was revealed by tuning the spatial separation of Si-N during epitaxial growth.[1] The N interstitial concentration was tuned via post growth annealing.[2] Transport measurements revealed a N-interstitial-induced state near the GaAsN conduction band edge.Here, we report a persistent photoconductivity (PPC) effect associated with N interstitial-related defects in GaAsN bulk-like films. PPC refers to a phenomenon where a photo-induced increase in conductivity persists after illumination is terminated, usually on the order of hours or even days. In our GaAsN films, PPC was observed up to 160K, with the photo-capture barrier determined to be 350 - 400 meV. Meanwhile, low temperature transport measurements reveal two distinct T-dependent regimes of the free carrier concentration, n, with a temperature-independent regime above 150K and a strong thermally-activated regime below 150K. These two phenomena are reminiscent of the behavior of n-type AlGaAs due to the presence of DX-center levels,[3] suggesting the presence of similar N-induced defect levels in GaAsN. Furthermore, after rapid thermal annealing (RTA), the PPC effect is suppressed and n increases substantially. Our recent nuclear reaction analysis and Raman spectroscopy data revealed that the interstitial N `concentration is decreased by RTA.[2] Therefore, the RTA-induced suppression of the PPC effect and the increase of n in GaAsN suggest their association with N interstitials.This work is supported by NSF-FRG, grant # DMR-0606406, monitored by L. Hess.[1] Y. Jin, Y. He, H. Cheng, R. M. Jock, T. Dannecker, M. Reason, A. M. Mintairov, C. Kurdak, J. L. Merz, and R. S. Goldman, Appl. Phys. Lett. 95, 092109 (2009).[2] Y. Jin, R. M. Jock, H. Cheng, Y. He, A. M. Mintarov, Y. Wang, C. Kurdak, J. L. Merz, and R. S. Goldman, Appl. Phys. Lett. 95, 062109 (2009).[3] E. F. a. P. Schubert, K, Phys. Rev. B 30, 7021 (1984).
12:15 PM - EE6.8
Diffusion and Doping Properties of Mg Impurities in Crystalline Zn3P2.
Gregory Kimball 1 2 , Nathan Lewis 1 , Harry Atwater 2
1 Chemistry, California Institute of Technology, Pasadena, California, United States, 2 Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractZinc phosphide (Zn3P2) is a promising and earth-abundant alternative to traditional materials (e.g. CdTe, CIGS, a-Si) for thin film photovoltaics. We have demonstrated high purity bulk crystal growth and effective surface passivation of Zn3P2 (G.M. Kimball et al, APL, 2009), allowing for room-temperature photoluminescence to provide a reliable measurement of the band gap Eg,direct at 1.50 eV and Eg,indirect at 1.38 eV. Time-resolved photoluminescence decay rates are consistent with a minority carrier diffusion length of ≥7 μm, significantly greater than the absorption path length for visible light. The record solar energy conversion efficiency for Zn3P2 absorbers reaches 6% (M. Bhushan et al, APL, 1980) using a Mg-Zn3P2 Schottky diode which upon annealing appears to form a pn homojunction. Although Zn3P2 is almost exclusively produced p-type, evidence from this early work has suggested but not definitively shown that Mg interstitials behave as n-type dopants. We have fabricated Mg-Zn3P2 Schottky diodes with VOC’s exceeding 550 mV and after annealing at 200 °C have found the electron concentration in the Mg-diffused layer to be greater than 10^16 cm-3. We have investigated Mg dopant ionization fraction by combined Hall Effect and Secondary Ion Mass Spectroscopy (SIMS), as well as Mg activation energy by temperature dependent Hall Effect measurements. We have also used SIMS to measure the Mg incorporation profile in n-type emitters and the diffusion coefficient of Mg in the temperature range 150-400 °C. Mg-diffused Zn3P2 pn homojunction devices produced in our lab show improved photovoltaic characteristics with respect to the work of previous authors.
12:30 PM - EE6.9
InGaAs Virtual Substrate Templates for Low-defect Epitaxial Growth of Novel InAlAs/InGaAsP/InGaAs Multi-junction Solar Cells.
Emily Warmann 1 , Marina Leite 1 , Dennis Callahan 1 , Koray Aydin 1 , Harry Atwater 1
1 , California Institute of Technology, Pasadena, California, United States
Show AbstractThe three junction cell system InAlAs/InGaAsP/InGaAs enables an optimal combination of energy band gaps and has a theoretical efficiency of 46.79% (series connected) by detailed balance calculations. These alloys are lattice matched to each other, but exhibit this band gap combination at a lattice constant of 5.800 Å, which is not available in any existing wafer substrate for epitaxial growth. Strained epitaxy results in a high defect density and degrades cell performance. Our approach to epitaxial growth of InAlAs/InGaAsP/InGaAs is based on a "Virtual Substrate" template, consisting of a thin, monocrystalline InGaAs film with the desired 5.800 Å lattice constant supported on a planar substrate. The film is InxGa1-xAs grown coherently strained on an InP wafer. It is then removed and its strain elastically relaxed before the film is bonded to a planar mechanical support. To prevent dislocations, the film's thickness is dictated by the critical thickness as determined by lattice mismatch between the InGaAs and InP. At 1.17% mismatch the critical thickness is only ~7 nm as calculated by the Matthews-Blakesly condition. A polymer membrane is used to transfer this InGaAs film to a different substrate and relax its strain. Fabrication experiments for these Virtual Substrates used 40 nm InxGa1-xAs films of varying In content on InP substrates. The mismatch strain ranged from +0.3% to -0.7%, including a sample at zero strain. Because these films are above the critical thickness for the degree of mismatch, they will have some dislocations and a slightly reduced elastic strain. A 600 nm film of polyimide was spin-coated onto the samples and cured at 300°C for 30 minutes. The InP growth substrate was removed by either selective chemical etching or ion implantation induced exfoliation and subsequent etching, leaving the InGaAs film supported by the polyimide membrane and a secondary mechanical support. Scanning electron microscopy (SEM) images show continuous InGaAs film over areas of ~5 mm2. High resolution X-ray diffraction (XRD) measurements of the films as grown on InP will allow characterization of the degree of dislocation-induced relaxation in the epitaxial films, and measurements of the films after InP removal will determine the elastic relaxation of the films on the polyimide. Analytical models of the polyimide/InGaAs film system indicate the modulus mismatch of the two materials compensates for the greater thickness of the polymer film and ~60% of the strain in the InGaAs can be relaxed. Bonded onto a reactor-compatible substrate, this InGaAs film will serve as a Virtual Substrate template for low-defect epitaxial growth of InAlAs/InGaAsP/InGaAs multi-junction solar cells with a lattice constant not available in single-crystal wafer substrates.
12:45 PM - EE6.10
Polaronic Trapping of Electrons and Holes by Native Defects in Anatase TiO2.
Benjamin Morgan 1 , Graeme Watson 1
1 School of Chemistry, Trinity College Dublin, Dublin, Dublin, Ireland
Show AbstractThe primary process in photocatalysis and photovoltaics is the light-induced production of charge carriers.[1] Photocatalysis proceeds by photoexcitation of electrons to unoccupied bands, producting free electron-hole pairs which rapidly undergo trapping followed by diffusion. TiO2has been widely studied as a photocatalyst for the degradation of environmental pollutants,[1] for water splitting,[2] and as a photovoltaic material in solar cells.[3] Nanocrystalline anatase TiO2 is often used, since it is more active than the ground-state polymorph, rutile, and the diffusion pathways of photogenerated charge carriers to the surface are shortened, resulting in increased quantum efficiencies. Following photoexcitation, trapping of charge carriers occurs on a nanosecond timescale.[4] Defects are thought to play a critical role in the trapping process by acting as recombination centers. For example, n-type doping of anatase reduces the photocatalytic activity of experimental samples,[5] and reduced anatase is less efficient than stoichiometric TiO2 in the trapping of photogenerated holes.[6] Understanding the interaction of defects with charge carriers is essential for the optimization of TiO2 samples for photocatalysis. We have investigated the formation of native defects in anatase TiO2 using GGA-DFT modified with onsite Coulomb terms (GGA+U) applied to both Ti d and O p states. Oxygen vacancies and titanium interstitials are found to be deep donors that trap two and four electrons, with transition levels that explain the two features seen in DLTS experiments.[7] Titanium vacancies are deep acceptors accommodating four holes. Self-trapping of both electrons and holes is also predicted. In all cases both donor and acceptor trap states correspond to strongly localized small polarons, in agreement with experimental EPR data.[8-10] Variation in defect formation energies with stoichiometry explains the poor hole-trapping of reduced TiO2.
[1] A. L. Linsebigler, G. Lu, and J. T. Yates, Jr., Chem. Rev. 95, 735 (1995).
[2] A. Fujishima and K. Honda, Nature 238, 37& (1972).
[3] M. Grätzel, J. Photochem. Photobio. C: Photochem. Rev. 4, 145, 2003
[4] D. P. Colombo and R. M. Bowman, J. Phys. Chem. 99, 11752 (1995).
[5] T. Miyagi, M. Kamei, I. Sakaguchi, T. Misuhashi, and A. Yamzaki, Jpn. J. Appl. Phys. 2004, 775 (2004).
[6] T. Berger, M. Sterrer, E. Diwald, O. amd Knözinger, D. Panayotov, T. L. Thompson, and J. T. Yates, Jr., J. Phys. Chem. B 109, 6061 (2005).
[7] T. Miyagi, M. Kamei, T. Mitsuhashi, and A. Yamazaki, Appl. Phys. Lett. 83, 1782 (2003).
[8] R. F. Howe and M. Grätzel, J. Phys. Chem. 91, 3906 (1987).
[9] S. Zhou, A. Cizmar, K. Potzger, M. Krause, G. Talut, M. Helm, J. Fassbender, S. A. Zvyagin, J. Wosnitza, and H. Schmidt, Phys. Rev. B 79, 113201 (2009).
[10] S. Yang, L. E. Halliburton, A. Manivannan, P. H. Bunton, D. B. Baker, M. Klemm, S. Horn, and A. Fujishima, Appl. Phys. Lett. 94, 162114 (2009).
EE7: Emerging Materials, Silicon, II-VI, and Oxides
Session Chairs
Thursday PM, April 08, 2010
Room 3007 (Moscone West)
2:30 PM - **EE7.1
Towards Molecular Control Over Interfaces and Their Defects: New and Renewed Vistas for Solar Cells.
David Cahen 1
1 Materials and Interfaces, Weizmann Institute of Science, Rehovoth Israel
Show AbstractWe explore two main ways to use molecules for hybrid photovoltaics (PV), electrostatically, as dipolar films and electrodynamically, where current passes through them. Incomplete partial dipolar molecular films can control solar cell behavior because the molecules can sufficiently passivate the surface defects so that the molecular and interfacial dipoles affect the electrostatic landscape. We find such effects for single (Si “MIS), poly- and nano-crystalline (ETA and DSSC) cells. Results from others with organic PV fit our findings, even if interpreted differently at times. To have current pass through the molecules, as ideal monolayers as possible are needed. Using alkyl chain monolayers then leads to ideal Metal-Insulator-Semiconductor (MIS) diode behaviour with significant photovoltaic activity. We find, though, that we can actually make MIS solar cells without a separate I(nsulator) layer, because of the above-mentioned combined defect passivation cum dipole actions. That result suggests that what are considered ‘MIS’ effects can be more chemical in nature than conventional wisdom has it. Present efforts focus on combining the static and dynamic effects to use molecular interface passivation and modifications to learn about device operation and optimizations.Work done with Ayelet Vilan, Omer Yaffe, Ariel Biller, Rotem Har Lavan, as well as D. Gal, G. Hodes, L. Kronik, S. Ruehle, A. Salomon, O. Seitz, H. Shpaisman, F. Thieblemont, I. Visoly-Fisher, and with T. Boecking, J. Gooding (UNSW), H. Haick (Technion), Antoine Kahn and group (Princeton U), C. Sukenik, A. Zaban (Bar Ilan U), R. Tung (CUNY), E. Umbach (Wuerzburg/Karlsruhe), N. Ueno (Chiba), H. Zuilhof (Wageningen) and coworkers.
3:00 PM - EE7.2
Defects and Processing in Copper Indium Selenide Single Nanowire Solar Cells.
David Schoen 1 , Hailin Peng 1 , Yi Cui 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractThe surprising tolerance of copper indium selenide (CIS) and related chalcogenide solar cells to internal defects is one of the most fascinating and important questions in thin film photovoltaics. Many hypotheses to explain this behavoir have been proposed, and significant progress towards an answer has been achieved. Most of this progress has focused on the internal nanoscale distribution of copper in these materials, because the conduction type and electronic structure of CIS depends sensitively on the overall copper composition. However, the actual distribution of copper in functioning solar cells has so far resisted experimental investigation, largely due to the significant mobility of copper in these materials and the difficulty of producing cross section samples from thin film devices without altering the material's composition. We report the use of nanowire test structures to study the distribution and movement of Cu through the important processing steps used to fabricate CIS solar cells and also report the successful fabrication of single nanowire CIS solar cells.
3:15 PM - EE7.3
Nanoscale Observation of Carrier Transport on Cu(In,Ga)Se2 Films Using Photo-assisted Scanning Probe Microscopy.
Ran Hee Shin 1 , Ah Reum Jeong 1 , William Jo 1 , Dae-Hwan Kim 2 , Jin Kyu Kang 2 , Dong Ha Lee 2
1 Department of Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 , Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu Korea (the Republic of)
Show AbstractChalcopyrite Cu(In,Ga)Se2 (CIGS) is currently used as an absorber layer for high-efficiency (~ 20%) thin-film polycrystalline solar cells owing to high optical absorption coefficient in the visible region. Classical electronic grain boundary (GB) models cannot explain the remarkable performance of polycrystalline CIGS solar cells. The role of GB is controversial on photovoltaic output parameters. Thus, nanoscale observation of electrical properties is important to determine influence of GB to photovoltaic properties on CIGS materials. It is capable to study local electrical properties using scanning probe microscopy. Especially, local charge transport and surface potential were measured by conductive atomic force microscopy (C-AFM) and Kelvin Probe force microscopy (KPFM) using conductive probe. C-AFM provides topographic and current images of the same area, simultaneously, and local current-voltage measurements. We report local minority-carrier transport at GBs of CIGS film at dark and illuminated states. In current images of dark state, at applied 2V sample bias, most of current were measured at GB that acts minority-carrier accumulation center not recombination center. However, intra-grain transport is dominant conduction process on (112)-preferentially oriented CIGS film at 5V that is enough to overcome intra-grain potential. Local current-voltage characteristics show different junction mechanism between Pt coated tip and CIGS film, which is consistent with current images. In randomly oriented CIGS film, p-n junction was observed, while general metal-semiconductor junction was measured as Schottky-like junction in (112)-preferentially oriented one. The results can explain the minority-carrier transport at GBs depending on crystal orientation. Degree of energy band bending at GBs could be also determined using step bias scanning with different Ga contents, which is confirmed by KPFM. Surface potential change between dark and illuminated states was observed, which can explain carrier transport in CIGS film in an illuminated state.
3:30 PM - EE7.4
Photovoltaic Cu(In,Ga)Se2 Nanoparticles for High Efficient Solar Cells.
Ah Reum Jeong 1 , Nuri Lee 1 , Eunkyoung Kim 1 , William Jo 1 , Dae-Hwan Kim 2 , Jin Kyu Kang 2 , Dong Ha Lee 2
1 Physics, Ewha Womans University, Seoul Korea (the Republic of), 2 , Daegu Gyeongbuk Institute of Science and Technology , Daegu Korea (the Republic of)
Show AbstractCu(In,Ga)Se2 based solar cells have received great attention owing to their highest power conversion efficiency (~20%) among all kinds of thin-film solar cells. Important issues on the microstructure, especially grain size, to understand behaviors of electron-hole pairs are still controversial. For studying grain size effect and achieving low cost, high efficiency devices nanoparticles-based solar cells is considered one of the best candidates. While chemical process for synthesizing CIGS nanoparticles have been studied these days, we have physically synthesized the nanoparticles by pulsed laser ablation which is reliable to match stoichiometry. We already investigated various properties of the nanoparticles and confirmed differences compared with CIGS thin films.According to our previous results, they revealed unique morphology, bandgap shift, and high absorption coefficient. Dark current-voltage characteristics of stacked layers with CIGS nanoprticles on indium tin oxide substrate showed an Ohmic behavior under low treated temperatures but rectifying over 400°C. CIGS nanoparticles were fabricated on different substrates such as molibdenum (Mo) and transparent conductive oxide (TCO) to assess possibility of them applying for the solar cell devices. The best condition was optimized when we made the nanoparticles and devices where general processes were performed for other layers. The conventional structure of CIGS solar cells, but which were composited with an absorber layer of nanoparticles, was manufactured on Mo substrates. Moreover, the possibility to be used top cell in tandem structures was investigated on TCO as one way to improve efficiency. They showed different characteristics when their fabrication conditions were changed. However, both of devices indicated good photovoltaic effect to be applicable for solar cell. Judging from our results, physically synthesized nanoparticles are able to be as an absorber layer in various type CIGS solar cells applying their different properties.
3:45 PM - EE7.5
Optimizing the Heterojunctions of ZnTe/ZnSe Solar Cells: the Effect of ZnSe Surface Treatment and ZnTe Growth Techniques.
Fang Fang 1 , Brian McCandless 2 , Fang Fang 1
1 Materials Science & Engineering, University of Delaware, Newark, Delaware, United States, 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware, United States
Show AbstractAlternative II-VI direct band gap semiconductors are attractive for thin film solar cell (TFSC) applications owing to their potential flexibility in tuning opto-electronic properties of the heterojunctions. n-ZnSe/p-ZnTe as a wide band gap TFSC (EG > 2 eV) is being studied for potential application in tandem cells. Minimization of defect states at the ZnSe/ZnTe interface are crucial in such devices. The tendency to form diffused junctions and surface oxides in the processing of Zn-based II-VI materials results in low open circuit voltage. Also, during growth of these stacking films, the covering ZnTe thin film layer will affect the microstructure of the underlying polycrystalline ZnSe film, resulting in low short circuit currents and fill factors of the cells.The ZnSe films in this study were grown by Close Space Sublimation (CCS), a low-cost deposition method already demonstrated for high efficiency and commercial CdTe TFSC. Surface oxides on these ZnSe films were detected by Al-Kα X-ray photoelectron spectroscopy, and a significant valence band offset induced by such oxide at surface is detected by synchrotron source Ultra-violet photoelectron spectroscopy. Two different deposition techniques were employed for the overlayer ZnTe films: CCS and conventional thermal evaporation. The influence of growth condition on both the microstructure of under-layer ZnSe at the heterojunctions was investigated. Particularly, changes in ZnSe surface morphology during the initial stage of ZnTe film deposition were observed by scanning electron microscopy (SEM). Under certain growth conditions, enlarged pores were observed in ZnSe films in the locations where partially covering ZnTe film was deposited; a degraded device performance is expected. The influence of ZnSe treatment prior to ZnTe film deposition on the microstructures as well as the energy band alignment at the heterojunctions has been determined. Etching processes, annealing temperatures and ambient conditions have been optimized.
4:30 PM - **EE7.6
Defects in Nanostructured and Amorphous Silicon.
P. Taylor 1 , J. Fields 1 , S. Rathi 1 , B. Jariwala 1 , Y. Pai 1 , J. Beach 1 , S. Agarwal 1 , P. Stradins 3 , T. Su 2 , R. Collins 1
1 , Colorado School of Mines, Golden, Colorado, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , United Solar Ovonic, Auburn Hills, Michigan, United States
Show AbstractVarious embodiments of nanostructured silicon are being considered as materials for future generations of photovoltaic devices. Nanostructures are important because they provide the opportunity to dramatically increase the solar cell efficiencies over those that utilize bulk or thin film Si. Silicon is important because it is non-toxic, abundant, and potentially inexpensive. In addition, silicon processing techniques are very well established. In nanostructured Si embedded in a matrix, such as amorphous Si, defects at interfaces become extremely important. Using techniques such as electron paramagnetic resonance (EPR), photoluminescence (PL), infrared absorption (as measured by photothermal deflection spectroscopy, which is a calorimetric technique), and Raman scattering several important defects in mixtures of nanocrystalline Si in a hydrogenated amorphous Si matrix, in Si nanodots, and in Si nanowires will be discussed. Important defects include localized band-tail states and silicon dangling bonds at or near interfaces and impurity-related defects, such as those due to oxygen agglomerates. The latter defects are especially important due to the pervasive, and often deleterious, effect of oxidation of Si nanostructures that are not passivated. Stability is a second major concern in nanostructured materials. Changes in defect concentrations on thermal annealing and optical excitation in nanostructured Si films will be discussed. Research partially supported by the NSF MRSEC program under cooperative agreement No. DMR-0820518 at the Colorado School of Mines, by the DOE under a subcontract (DE-FC36-07 GO 17053) from United Solar Ovonic (PCT), and by a seed grant from the Center for Revolutionary Solar Photoconversion (RTC).
5:00 PM - EE7.7
One-step Process for Treating Photoelectrode Defects for High Throughput Dye-sensitized Solar Cell Fabrication Using Atomic Layer Deposition.
Braden Bills 1 , Mariyappan Shanmugam 1 , Mahdi Farrokh Baroughi 1 , David Galipeau 1
1 Electrical Engineering and Computer Science, South Dakota State University, Brookings, South Dakota, United States
Show AbstractThe performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of electrons from the photoelectrode back into the electrolyte solution, where the photoelectrode is comprised of the porous titanium oxide (TiO2) nanoparticle network and the transparent conducting oxide (TCO). Thus, effectively treating the TiO2/dye/electrolyte and TCO/electrolyte interfaces to reduce electron loss is critically important for improving photovoltaic performance. The conventional way to prevent back-reactions at the TCO/electrolyte interface is with a sol-gel processed TiO2 compact layer [1]. To prevent electrons from being lost at the porous TiO2/electrolyte interface, a wide range of wet chemical processed materials have been deposited onto the porous TiO2 network and were shown to improve the DSSC performance [2-3]. A new approach for depositing an interfacial layer onto the porous TiO2 network is by ALD [4-5], which is advantageous over wet chemical method for treating defective interfaces with metal oxides due to the higher purity of gas precursors, faster diffusion of gas precursors into the porous network, and better conformallity and controllability of metal oxide thickness. The gas precursors are able to diffuse through the pores and uniformly grow on both the TiO2 surface and the uncovered TCO surface.We have used ALD to grow 1, 5 and 10 cycles of hafnium oxide (HfO2) on the entire photoelectrode surface that prevents electrons from reacting with electrolyte. The ALD fabrication is described in greater detail in our previous work [5]. We have used illuminated current-voltage measurements to characterize the three HfO2 layer thicknesses as shown in the table below. A thicker ALD HfO2 layer better prevents the back-reaction of photoexcited electrons as seen by the increasing FF; however, JSC worsens due to the blocking of electrons from dye to TiO2. The tradeoffs between FF and JSC were similar, such that η was the same for all three thicknesses. Thus, one ALD HfO2 cycle can be used to treat the entire photoelectrode surface, which can lead to high throughput, where the surface treatment is very fast(~70 seconds), and lower costs for fabricating DSSCs.References[1] A. Burke, S. Ito, H. Snaith, U. Bach, J. Kwiatkowski, M. Gratzel, Nano Lett. 8, 4, (2008) 977.[2] Kay, A.; Gratzel, M. Chem. Mater. 2002, 14, 2930.[3] Palomares, E. Clifford, J. N.; Haque, S. A.; Lutz, T.; Durrant, J. R. Chem. Commun. 2002, 14, 1464.[4] Hamann, T. W.; Farha, O. K.; Hupp, J. T. J. Phys. Chem. C 2008, 112, 19756.[5] M. Shanmugam, M. F. Baroughi, D. Galipeau, Thin Solid Films 2009 doi:10.1016/j.tsf.2009.08.033.
5:15 PM - EE7.8
Nitrogen Doping Environments in Porous TiO2 Materials Treated by rf Plasma Processing: Elucidating N 1s Binding Environments and Optical Film Properties.
Daniel Pulsipher 1 , Ellen Fisher 1
1 Chemistry, Colorado State University, Fort Collins, Colorado, United States
Show AbstractHighly nitrogen doped (34%) anatase TiO2 materials are accomplished by use of rf inductively coupled plasma processing within a reducing environment. Nitrogen doped titania (N:TiO2)) materials are interesting because of their ability to improve photocatalytic and photovoltaic devices.1 Additionally, ambiguous X-ray photoelectron spectroscopy (XPS) peak assignments are often made for multiple N 1s binding environments. Here, nitrogen binding environments within the films are controlled by plasma processing conditions to elucidate N:TiO2 device improvements. XPS peak assignments for four N 1s binding environments are provided for these materials by utilizing Ti 2p and O 1s XPS peaks, Fourier transform infrared spectroscopy (FTIR) data, and literature N 1s peak assignments. A variety of film colors are observed that parallel with TiNx film compositions of low to high N/Ti ratios. Tauc bandgaps as low as 1.52 ± 0.01 are measured. Three possible origins to these colors will be discussed.2 Photovoltaic devices have been constructed and characterized by I-V and incident photon conversion efficiency curves. Preliminary results suggest film properties are related to changes in film composition.1.Fujishima, A.; Zhang, X. T.; Tryk, D. A., Surf. Sci. Rep. 2008, 63 (12), 515-582.2.Serpone, N., J. Phys. Chem. B 2006, 110 (48), 24287-24293.
5:30 PM - EE7.9
Synergistic Effects on Band Gap Narrowing in Titania by Codoping from First-principles Calculations.
Run Long 1 , Niall English 1
1 , University College Dublin, Dublin Ireland
Show AbstractAbstract: Titania (TiO2)-based photocatalysts have received intense attention as promising photocatalytic materials for the decomposition of organic pollutants present in water or air [1]. However, their universal use is restricted to ultraviolent light (λ < 385 nm) due to the wide band gap of titania (~3.2 eV for anatase). Further, photoexcited electron-hole pairs tend to recombine relatively easily in TiO2. In this study, we have used a passivated approach to modify the band edges of TiO2 by codoping of B with transition metals (TM) to extend the absorption edge to longer visible-light wavelengths. It was found that (B+Fe)-codoped systems exhibit remarkably better narrowing than other of (B+TM)-codoped systems. Our theoretical calculations provide both the theoretical explanation for high photocatalytic efficiency of (B+Fe)-codoped titania and meaningful guides for experiments to develop new powerful visible-light photocatalysts.References1.M. R. Hoffmann, S. T. Martin, WW. Choi, D. W. Bahnemann, Chem. Rev. 95, 69
5:45 PM - EE7.10
Investigation of Charge Trapping at Grain Boundaries in Polycrystalline and Multicrystalline Silicon Solar Cells.
Jennifer Heath 1 , Chun-Sheng Jiang 2 , Helio Moutinho 2 , Mowafak Al-Jassim 2
1 Department of Physics, Linfield College, McMinnville, Oregon, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractThe electronic properties of grain boundaries in polycrystalline and multicrystalline silicon are known to vary significantly depending on the individual grain misorientations, due to differences in dangling bonds, strain, and impurity gettering. By correlating Electron Backscattering Diffraction (EBSD) maps with the Light Beam Induced Current (LBIC) data, we can see, as expected, that certain grain boundary (GB) orientations result in significantly reduced minority carrier diffusion length, while others, including most ∑3 GBs, are relatively benign. Our results also are consistent between both cast multicrystalline silicon samples, which have relatively low impurity levels, and highly contaminated polycrystalline silicon material. Our polycrystalline silicon samples were deposited by hot-wire chemical vapor deposition onto silicon seed layers formed by aluminum-induced crystallization. We have been exploring the properties of grain boundary defects which may be limiting the minority carrier diffusion. We have found that GBs with poor diffusion length also tend to have trapped charge at the GB, resulting in depletion regions along the GB visible in the Scanning Capacitance Microscopy (SCM) data, both in plan-view and cross-sectioned samples. Scans of SCM signal as a function of dc probe voltage allow these regions to be more quantitatively investigated. Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research.