Device Analysis of Cu(In,Ga)Se₂Heterojunction Solar Cells - Some Open Questions

Abstract

The paper discusses the electronic properties of Cu(In,Ga)Se2–based heterojunction solar cells with a special focus on questions which at present are not satisfactorily understood. First, we discuss an apparent quantitative contradiction between measured concentrations of recombination centers in the Cu(In,Ga)Se2 absorber material and the actual recombination rate in the solar cells. We propose, as a possible explanation for that observation, that the defect concentration in Cu(In,Ga)Se2 is spatially inhomogeneous with a systematic increase towards the heterojunction interface. Second, we address the issue of electronic metastabilities in ZnO/CdS/Cu(In,Ga)Se2 heterojunctions and, especially, in devices that use alternative buffer materials instead of CdS. Starting from a brief review of the experimentally observed types of metastabilities, we demonstrate by thermally stimulated capacitance measurements that a specific type of metastability that severely limits the performance of solar cells with non-CdS buffers is present also in high-efficiency standard devices though it has virtually no influence on the output parameters in the latter case. A possible explanation of this type of metastability points to a metastable defect reaction localized in the close to surface region of Cu(In,Ga)Se2. At the moment we cannot propose conclusive models for both open questions. However, we can localize the answers to both problems in the close-to-surface region of the Cu(In,Ga)Se2 absorber.

The Zn(S,O,OH)/ZnMgO buffer in thin‐film Cu(In,Ga)(Se,S)2‐based solar cells part II: Magnetron sputtering of the ZnMgO buffer layer for in‐line co‐evaporated Cu(In,Ga)Se2 solar cells

“…2Á0 of the minimum value specified by the manufacturer. The degradation/recovery effects (transient effects) can be related to different CIGS-specific metastabilities which are discussed by Rau et al 19 In this section, we describe transient effects on CIGS/ZnS/Zn 1-…”

Understanding Performance Limitations of Cu(In,Ga)Se2 Solar Cells due to Metastable Defects—A Route toward Higher Efficiencies

“…CIGS absorber layers generally show metastable behavior, e.g., the doping density and conductivity increase persistently after minority‐carrier injection. [ 17–21 ] The behavior is often assigned to the V Se –V Cu defect complex. [ 22 ] Although the increased acceptor density (i.e., hole quasi‐Fermi level closer to the valence band) has been studied for more than 25 years, [ 23 ] the influence of these metastabilities on the diode factor of the bare absorber (measured optically) and of the solar cell (measured electrically) has not been considered.…”

Comparison of device performance and measured transport parameters in widely-varying Cu(In,Ga) (Se,S) solar cells

“…Even the studies that have been the most successful at relating materials characterization to device performance have been limited to closely related samples from a single laboratory, using only one deposition method, and with process variations limited to a single variable. [3][4][5][6][7] Prior studies by this TFPPP CIS Team of physical materials properties (via secondary ion mass spectrometry, surface and cross-sectional scanning electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy) have shown no discernable correlation between these and performance that is applicable to CIGSS made by diverse methods. The tenuous link between CIGSS material properties and device performance results in a frustrating lack of robust tools and methods for process feedback and control when navigating uncharted process space in manufacturing.…”