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In this paper, the importance of sodium control in ZnO/Cu(In,Ga)Se2 superstrate devices is studied. The superstrate devices were fabricated by the deposition of the Cu(In,Ga)Se2 (CIGSe) absorber material directly onto intrinsic ZnO. Sodium is added to the CIGSe layer as a precursor prior to the absorber deposition or via postdeposition. Capacitance measurements combined with device simulations are presented, which indicate that sodium, if present at the heterointerface, catalyzes the interface reaction between ZnO and CIGSe and induces a high density of deep acceptor states at the heterointerface. This limits the efficiency of the photovoltaic devices. It is shown that only by a very controlled deposition of sodium after the CIGSe deposition, it is possible to achieve devices that allow efficient photocurrent transport across the interfacial GaOx layer. By employing a 10-nm-thick molybdenum buffer layer on top of the absorber's back surface, the diffusion of sodium during the posttreatment can be well controlled in order to achieve efficient and long-time stable devices.

Abstract

Although photovoltaic devices and modules made from crystalline silicon currently dominate the market, many efforts in developing photovoltaics involve the use of alternative materials. Binary and multinary materials with direct band gaps and therefore high absorption coefficients allow for the fabrication of thin-film photovoltaic modules with minimized material use and the possibility for depositing on large areas and alternative substrates such as glass, stainless steel, or polyimide foils. With the great diversity of optoelectronic properties of binary and multinary materials, highly efficient photovoltaic devices fabricated at very low cost are in principle possible. Requirements for efficient photovoltaic devices using nonconventional materials are discussed, and results obtained for photovoltaic devices based on selected binary and multinary materials obtained during the past few decades are summarized.

The systematical study of the admixture of small amounts of group-II elements (<1 at%) with Raman and photoluminescence spectroscopy reveal an increase in the Raman response of the cation-anion vibration modes accompanied with the generation of a new broad emission band at 1.35 eV in the PL spectra of the chalcopyrite system Cu–In–S. Intensity dependent PL measurements indicate that this emission band is strongly influenced by a high degree of compensation in contrast to the commonly found donor-acceptor pair recombination in undoped CuInS2 samples. From temperature dependent conductivity measurements an increased concentration of acceptors and donors in doped CuInS2 thin films is found. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Cu(In,Ga)Se2 (CIGS) solar cells in superstrate configuration promise improved light management and higher stability compared to substrate devices, but they have yet to deliver comparable power conversion efficiencies (PCEs). Chemical reactions between the CIGS layer and the front contact were shown in the past to deteriorate the p‐njunction in superstrate devices, which led to lower efficiencies compared to the substrate‐type devices. This work aims to solve this problem by identifying a buffer layer between the CIGS layer and the front contact, acting as the electron transport layer, with an optimized electron affinity, doping density and chemical stability. Using combinatorial material exploration we identified amorphous gallium oxide (a‐GaOx) as a potentially suitable buffer layer material. The best results were obtained for a‐GaOxwith an electron affinity that was found to be comparable to that of CIGS. Based on the results of device simulations, it is assumed that detrimental interfacial acceptor states are present at the interface between CIGS and a‐GaOx. However, these initial experiments indicate the potential of a‐GaOxin this application, and how to reach performance parity with substrate devices, by further increase of its n‐type doping density.