Symposium : B
Thin Film Chalcogenide Photovoltaic Materials
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| Kesterites growth-2 : S. Siebentritt, J. Palm | ||
| 09:00 | CZTS solar cells processed by rapid thermal processing of stacked elemental layer precursors Authors : R. Lechner (1), S. Jost (1), J. Palm (1), G. Manoharan (2), B. Louis (2), H. Yoo (3), R. A. Wibowo (3), R. Hock (3) Affiliations : (1) AVANCIS GmbH & Co KG, Otto-Hahn-Ring 6, 81739 München, Germany; (2) Saint-Gobain Recherche, 39, Quai Lucien Lefranc, 93303 Aubervilliers Cedex, France; (3) Universität Erlangen-Nürnberg, Institut für Kristallographie und Strukturphysik, Staudtstrasse 3, 91058 Erlangen, Germany Resume : In this contribution we report on the development of a novel two-step process for the formation of Cu2ZnSn(S,Se)4 thin films for solar cells. The two-step formation process of the pentanary kesterite consists of (i) sputter deposition of the metals Cu, Zn and Sn followed by thermal evaporation of chalcogen and (ii) rapid thermal processing of the metal / chalcogen precursors in chalcogen containing ambient. After the absorber formation process, solar cells were processed by deposition of buffer, window layer and metal grid. We evaluated different metal precursor compositions in the ternary Cu-Zn-Sn metal systems regarding their behaviour as appropriate precursors for the crystallization of Cu2ZnSn(S,Se)4 absorbers. Temperature variations were performed during rapid thermal processing for precursors with different composition to find the optimum conditions for the crystallisation of single-phase kesterite thin films. Both precursors and absorbers with different compositions were characterized by a variety of analytical methods. We will report on results of X-ray fluorescence, electron probe microanalyses, scanning electron microscopy, X-ray diffraction, Raman spectroscopy and discuss the consequences of precursor or RTP conditions on material properties and solar cell results. The best solar cell measured so far reached 6.3% efficiency on 1.34cm^2 cell size. | O12 1 |
| 09:15 | Influence of S/Se ratio on series resistance and on dominant recombination pathway in Cu2ZnSn(S,Se)4 thin film solar cells Authors : Alex Redinger, Max Wolter Hilaire, Marina Mousel, Susanne Siebentritt Affiliations : Laboratory for Photovoltaics, University of Luxembourg, 41, rue du Brill, L-4422 Belvaux, Luxembourg Resume : Cu2ZnSn(S,Se)4 semiconductors compounds are promising candidates for the use as absorber layers in thin film solar cells due to a high abundance of Cu, Zn, Sn and S, a tunable direct bandgap and demonstrated power conversion efficiencies in the range of 10%.
Currently the best solar cell absorbers are grown under Cu poor and Zn rich conditions in order to reach high efficiencies. All devices described in literature exhibit a high series resistance RS, which diverges at low temperatures. Moreover an activation energy Ea of the saturation current density lower than the bandgap Eg is found, pointing towards dominant interface recombination.
Here we will show that these findings drastically depend on the S/Se ratio in the absorber layers. Mixed S/Se solar cells show a diverging RS in contrast to pure Se devices which exhibit a low RS down to 100K. In addition RS depends on the final composition of the absorber layers and S-free solar cells show a much lower difference in Ea-Eg than the mixed S/Se devices.
Solar cells with efficiencies exceeding 5% for both mixed S/Se and pure Se absorber are used for temperature dependent current voltage analysis. QE and Photoluminescence measurements are used for bandgap determination and SIMS depth profiles are used to study the film homogeneity. The series resistance appears to be correlated to the occurrence of a ZnS/Se secondary phase. | O12 2 |
| 09:30 | Structural and Optical properties of ZnS, Cu3SnS4 and Cu2ZnSnS4 nanoparticles and related thin film absorbers Authors : Xianzhong Lin*, Jaison Kavalakkatt, Kai Kornhuber, Sergiu Levcenco, Martha Ch. Lux-Steiner, and Ahmed Ennaoui* Affiliations : Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, GERMANY Resume : Cu2ZnSnS4 has been considered as the alternative absorber layer to Cu(In,Ga)Se2 due to its earth abundant and environmental friendly constituents, optimal direct band gap of 1.4-1.6eV and high absorption coefficient in the visible range. As a promising candidate of thin film solar cells absorber layer, Cu2ZnSnS4 has spurred huge attention of the scientific and industrial area. Solar cells with CZTSSe absorber layer have already reached an energy conversion efficiency of 10.1 % [Barkhaouse et. al; Prog. Photovolt.: Res. Appl. (2011)]. The challenging issue in the kesterite Cu2ZnSnS4 processing is the compositional non-uniformity due to the existence of secondary phases especially the highly resistive ZnS (band gap 3.6 eV) [A. Ennaoui et al. / Thin Solid Films 517 (2009) 2511–2514]. In this work we compare optical properties of ZnS, Cu3SnS4 and Cu2ZnSnS4 nanoparticle (NPs) and the resulting thin films obtained by spin coating and annealing in Ar/H2S atmosphere. X-ray diffraction and Raman spectroscopy were used to characterize the structural properties. The chemical composition was determined by energy dispersive X-ray spectroscopy. Optical properties of the Cu2ZnSnS4 thin films, including transmission, reflection and photoluminescence, will be discussed. | O12 3 |
| 09:45 | Improved efficiency of Cu2ZnSnSe4 based solar cells prepared by a two stage process Authors : E. Saucedo1, A. Fairbrother1, X. Fontané1, V. Izquierdo-Roca1, A. Pérez-Rodríguez1,2, J.R. Morante1,2 Affiliations : 1. IREC, Catalonia Institute for Energy Research, C. Jardins de les Dones de Negre 1, 08930 Sant Adrià del Besòs (Barcelona), Spain 2. IN2UB, Departament d’Electrònica, Universitat de Barcelona, C. Martí i Franquès 1, 08028 Barcelona, Spain Resume : Kesterite materials such as those formed by Cu2ZnSn(S,Se)4 (CZTSSe) are becoming a realistic alternative in thin films solar cells technology. In the last two years, very promising results have been obtained for CZTSSe and their parent sulfide compound (CZTS); nevertheless these encouraging results have not been filled at the moment for the selenide compound (CZTSe). Whereas a 10.1% record efficiency has been reported for CZTSSe and efficiencies higher than 6% have been achieved for CZTS, values reported for CZTSe are still below 4%. Trying to improve these results and with the aim of understanding the limitations on the selenium system, we have prepared CZTSe layers by a two stage process depositing a metallic multi-stack precursor by DC-magnetron sputtering, followed by a reactive annealing, varying the Zn/Sn ratio from 0.98 up to 1.64. The effect of the precursor composition on the absorber layer properties was studied using a combination of SEM, Raman spectroscopy and XRD. ZnSe was identified as the main secondary phase at the surface and its impact on the optoelectronic performance of the solar cells was investigated and found to be less dramatic than the effect of ZnS on the CZTS case. As a preliminary result, we have obtained a 4.1% efficiency cell with a Jsc of 20.2 mA/cm2, a Voc of 360 mV and a fill factor of 56.9%. To our knowledge this is the best result obtained for the pure Se compound. The limitations on the efficiency and ways to improve it will be discussed. | O12 4 |
| 10:00 | Coffee break | |
| Device physics-2 : D. Cohen, S. Schorr | ||
| 10:30 | Advanced electrical simulation of thin film solar cells Authors : Marc Burgelman, Koen Decock, Samira Khelifi, Aimi Abass Affiliations : Universiteit Gent, Pietersnieuwstraat 41, B-9000 Gent, Belgium Resume : Electrical and optical numerical simulation of thin film solar cells is becoming common practice. The state-of-the art polycrystalline thin film solar cells (CIGS and CdTe) and the advanced structures under study are however ever more complicated. Also, our new insights in the materials properties of the chalcogenides unveil complicated phenomena and mechanisms, a notorious example being the metastable states of various kinds in CIGS materials. Software tools for numerical simulation of solar cells need to keep pace with these evolutions.
The electrical solar cell simulation programme SCAPS is a development of the University of Gent, and is available to the PV research community. In the past few years until very recently, it’s capabilities have been enhanced as will be illustrated in this contribution: (i) Grading of band gap and all other semiconductor properties; this has become a key feature in modern CIGS cell structures. (ii) Several tunnel mechanisms and defect interlayers are essential to describe bifacial cells and tandem and multijunction cells. (iii) Defects with multiple occupation states; amphoteric states in amorphous Si are well known, but also several important defects in CIGS are multivalent. (iv) Our knowledge on defects that can change their configuration state (donor or acceptor) in a metastable way is by now quite well established; SCAPS can now simulate the effects of these sophisticated materials properties to measured cell characteristics. | O13 1 |
| 11:00 | Red-blue effect in the CIGS devices revisited Authors : M. Igalson1, P. Zabierowski1, A. Urbaniak1, H. Abdel Maxoud1, M. Buffiere2, N. Barreau2, S. Spiering3 Affiliations : 1Warsaw University of Technology, Warszawa, Poland; 2Universite de Nantes, Nantes, France; 3Zentrum fur Sonnenenergie- und Wasserstoff- Forschung (ZSW), Stuttgart, Germany Resume : Well-known features of the current-voltage characteristics of CIGS baseline solar cells are the cross-over of red and white light curves and fill factor improvement under light containing blue photons absorbed in the buffer. Generally accepted explanation attributes them to the photodoping of the CdS buffer. Opposing model of the p layer in the absorber as a source of cross-over and FF losses has also been proposed. Here holes created in the buffer and injected into absorber reduce the p layer. In this work we will present the data which in our opinion provide unambiguous evidence that excessive doping of the close-to interface absorber region resulting from the presence of metastable defects is responsible for FF losses in the absence of blue photons. We show that fill factor improves with the same kinetics as the junction capacitance decreases after switching on the blue light. Thus it is not an increase but a decrease of apparent net doping which is responsible. This effect we demonstrate not only on the CdS buffered cells but also on some cells with Cd-free buffers. Large fill factor improvement together with large cross-over can also be observed in case of Zn(O,S) buffer if only photons of high enough energy are present in the light spectrum. The data indicating correlation of the properties on the near-interface region of the absorber and in particular its Cu content, with the extent of cross-over and fill factor loss under red illumination will also be shown. Simulations showing that the p layer effect might be reinforced or reduced by specific buffer properties will be presented. | O13 2 |
| 11:15 | Phototransistor Effects in Cu(In,Ga)Se2 Solar Cells Authors : Thomas Walter(1), Thomas Ott(1), Thomas Unold(2) Affiliations : University of Applied Sciences Ulm Albert Einstein Allee 55 D 89081 Ulm, Germany (1) Helmholtz-Zentrum Berlin for Materials and Energy Hahn-Meitner-Platz 1 D 14109 Berlin, Germany (2) Resume : Cu(In,Ga)Se2 solar cells show a number of unusual effects in their device behavior that cannot be easily explained using standard pn-junction theories. Among these are
• A blocking behavior of the dark current at low temperatures. Depending on the illumination level the blocking behavior may be cancelled out resulting in a crossover of the dark and illuminated IV-characteristics.
• Voc saturates at low temperatures.
• Voc becomes almost independent of the illumination intensity at low temperatures, indicating a low diode ideality factor < 1.
Recently, the existence of a back diode has been proposed in order to explain admittance measurements, blocking behavior of the dark current, and Voc saturation at low temperatures. We will show that a CIGSe solar cell with a back barrier essentially behaves as a phototransistor with a ZnO-emitter and Schottky collector, and with the photoexcited holes in the field-free region of the absorber providing the base current. In this picture the current-voltage characteristics depends on the width of the field free region between the front and back space charge region and on the electron diffusion length. Comparing SCAPS simulations with measurements it is shown that these features can be quantitatively explained in this model. It will be shown that depending on the barrier height of the back diode, the electron diffusion length and the doping level (affected by metastabilities), these features may be even observed at room temperature. | O13 3 |
| 11:30 | Analysis of generation dependent charge carrier transport in Cu(In,Ga)Se2 solar cells Authors : Melanie Nichterwitz 1, Christian A. Kaufmann 1, Raquel Caballero 2, Hans-Werner Schock 1, Thomas Unold 1 Affiliations : 1 Helmholtz-Zentrum Berlin, Hahn-Meitner Platz 1, D-14109 Berlin 2 Universidad Autónoma de Madrid, Departamento de Física Aplicada, E-28049 Madrid Resume : Cross section electron-beam induced current (EBIC) and current voltage (IV) measurements under different illumination conditions show that charge carrier transport in Cu(In,Ga)Se2 (CIGSe) solar-cells can depend on the generation conditions and often varies for different grains within the same sample. In this work, EBIC and IV results from solar cells with varying absorber properties and numerical simulations are used to develop a consistent model for the electronic band diagram, and the role of defects in the heterojunction region of the device. We show that the generation dependent collection properties in EBIC are most likely caused by the presence of a p+ layer at the CIGSe/CdS interface with a lowered valence band maximum, while the observed “red kink” effect in the low temperature IV characteristics is rather caused by acceptors at the CdS/ZnO interface. Since the occurrence of generation dependent collection in EBIC is found to correlate with the presence of a defect compound in the absorber layer, we conclude that the p+ layer corresponds to a defect layer with a lowered valence band and a high net doping density. We also show that the presence of shallow donor type defect states at the p+ layer/CdS interface of some grains can quantitively describe the fact that the charge-carrier transport properties are found to be grain specific, i.e. inhomogeneous. | O13 4 |
| 11:45 | Transient phenomena in Cu(In,Ga)Se2 solar modules investigated by electroluminescence measurements Authors : T. M. H. Tran 1, M. Schneemann 1, B. E. Pieters 1, C. Ulbrich 1, A. Gerber 1, T. Kirchartz 2 and U. Rau 1 Affiliations : 1 IEK5-Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany; 2 Department of Physics, Imperial College London, South Kensington SW7 2AZ, United Kingdom Resume : Electroluminescence (EL) imaging has evolved as an attractive tool for the characterization of solar cells and modules. As a direct semiconductor, Cu(In,Ga)Se2 (CIGS) is especially suitable for this method and EL imaging was successful applied for the analysis of CIGS modules in the past. The present contribution uses EL imaging for the evaluation of transient phenomena occurring in CIGS devices. We observe metastable transients in a series of successive images recorded during application of forward bias immediately after the CIGS modules were kept in the dark for several hours. Our experiments are conducted either under constant current (monitoring the module voltage) or vice versa. For both situations, the EL intensities increase significantly with time. The change under constant voltage is more pronounced than under constant current. The increased EL intensities go along with a decrease of the voltage or an increase of the current, respectively. We ascribe our observations to persistent photoconductivity (PPC) in the CIGS following the injection of minority carriers. The consequences of PPC on the devices are twofold: Firstly, PPC reduces the local series resistance of the bulk of the CIGS absorber material. Secondly, the increased charge density leads to a reduction of non-radiative recombination. We show that in our experiments the first effect clearly prevails over the second one leading to an increased conductivity of the local diode. | O13 5 |
| 12:00 | Discussion session: Sn loss in CZTS | |
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