Merdes, S.; Johnson, B.; Saez-Araoz, R.; Ennaoui, A.; Klaer, J.; Lauermann, I.; Mainz, R.; Meeder, A.; Klenk, R.: Current Transport in Cu(In,Ga)S2 Based Solar Cells with High Open Circuit Voltage - Bulk vs. Interface. In: Yamada, A. [u.a.] [Eds.] : Thin-Film Compound Semiconductor Photovoltaics - 2009Warrendale, Pa.: Materials Research Society, 2009 (MRS Symposium Proceedings Vol. 1165), p. 1165-M05-15
Abstract:
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, CdS