Recombination mechanisms in highly efficient thin film Zn O,S Cu In,Ga S2 based solar cells

Progress in fabricating Cu In,Ga S2 based solar cells with Zn O,S buffer is presented. An efficiency of 12.9 was achieved. Using spectral response, current voltage and temperature dependent current voltage measurements, current transport in this junction was studied and compared to that of a highly efficient CdS Cu In,Ga S2 solar cell with a special focus on recombination mechanisms. Independently of the buffer type and despite the difference in band alignment of the two junctions, interface recombination is found to be the main recombination channel in both cases. This was unexpected since it is generally assumed that a cliff facilitates interface recombination while a spike suppresses it

Current Transport in Cu In,Ga S 2 Based Solar Cells With High Open Circuit Voltage Bulk Vs. Interface

Cu In,Ga S2 thin films prepared by rapid thermal processing of metallic precursors yielded solar cells with efficiencies reaching 12.9 [1]. A good short circuit current density was observed together with open circuit voltages up to 850 mV. However, the fill factor was close, but typically not exceeding, 70 [2]. The dark jV curve shows a distinct two diode behavior which was not previously observed in CuIn Ga S2 based cells. It is conceivable that this contributes to the non optimum fill factor. Furthermore, results suggest that the photocurrent is voltage dependent, i.e. non optimum transport of the photocurrent. Process control of the buffer layer preparation suggests different absorber surface properties, compared to the Ga free reference, and the need to re adjust cell preparation parameters in order to fully exploit the efficiency potential of the absorber layers. This implies that the interface plays a significant role for photocurrent transport. In this contribution, in addition to the straightforward approach of quantifying the effects of CdS deposition parameters, the problem is addressed using device modeling and careful surface and interface analyses. Cu In,Ga S2 surfaces are analyzed using near edge X ray absorption fine structure NEXAFS for the conduction band edge CBE and ultraviolet photoelectron spectroscopy UPS for the valence band edge VBE . The correlation between absorber structure, buffer layer properties and cell performance is discussed. [1] S. Merdes, R. Kaigawa, J. Klaer, R. Klenk, R. Mainz, A. Meeder, N. Papathanasiou, D. Abou Ras, S. Schmidt, Proc. 23rd European Photovoltaic Solar Energy Conference, Valencia 2008 . [2] R. Mainz, J. Klaer, R. Klenk, N. Papathanasiou, Proc. 22nd European Photovoltaic Solar Energy Conference, Milan 2007 2429. Keywords Chalcopyrite, Cu In,Ga S2, Rapid Thermal Processing, Cd

Spray ILGAR ZnS nanodots In2S3 as defect passivation point contact bilayer buffer for Cu In,Ga S,Se 2 solar cells

The sequential and cyclic spray ion layer gas reaction Spray ILGAR technique allows the controllable deposition of metal chalcogenide films from monodispersed nanodots to homogeneous compact layer. With access to this technique, a structured buffer layer for Cu In,Ga S,Se 2 cells, named defect passivation point contact bilayer buffer is introduced at the heterogeneous interfaces to replace the conventional CdS buffer material and to improve the pure In2S3 buffer. Here, the Spray ILGAR ZnS nanodots serve as passivation layer with a reduced interface recombination, while the compact Spray ILGAR In2S3 film on top and in between the nanodots acts as point contact layer for the charge carrier transport. The optimal ZnS dot density, In2S3 thickness and process temperature are determined and discussed in detail. As yet, the solar cell efficiency with ZnS In2S3 buffer layer could be improved by about 1.5 absolutely as compared to a pure In2S3 buffered cell. Apart from the electronic properties of the absorber buffer interface, also the chemical and diffusion processes during junction formation, which may influence the properties of the completed solar cell, are investigated and discussed