The influence of grain boundary (GB) properties on device parameters of polycrystalline silicon (poly-Si) thin film solar cells is investigated by two-dimensional device simulation. A realistic poly-Si thin film model cell composed of antireflection layer, (n + )-type emitter, 1.5 μm thick p-type absorber, and (p + )-type back surface field was created. The absorber consists of a low-defect crystalline Si grain with an adjacent highly defective grain boundary layer. The performances of a reference cell without GB, one with n-type and one with p-type GB, respectively, are compared. The doping concentration and defect density at the GB are varied. It is shown that the impact of the grain boundary on the poly-Si cell is twofold: a local potential barrier is created at the GB, and a part of the photogenerated current flows within the GB. Regarding the cell performance, a highly doped n-type GB is less critical in terms of the cell's short circuit current than a highly doped p-type GB, but more detrimental in terms of the cell's open circuit voltage and fill factor.
The analysis of injection-dependent charge carrier lifetimes is a well-established method to determine material and interface quality in crystalline silicon wafer-based device structures such as solar cells. However, for thin films, this method has rarely been used. One reason is that the physical interpretation of experimental data must rely on advanced theoretical models. In this study, we show by numerical simulations and analytical approximations that the effective charge carrier lifetime in thin films is heavily affected by space charge regions (SCR) over a wide range of injection levels. By analysis of the characteristic features in the injection-dependent effective charge carrier lifetime curves, qualitative information about SCRs that occur at grain boundaries or interfaces can be obtained. In contrast, information about the defect density can only be extracted in a very limited range of injection levels and the relationship between effective charge carrier lifetime and the quasi-Fermi level splitting, which is limiting the open circuit voltage of wafer-based solar cells, is not valid in thin films. On the basis of this theoretical study, we analyze measurements of effective charge carrier lifetime in 1.5 μm thin and 2 μm fine-grained polycrystalline silicon films with lifetimes of up to 100 μs and find experimental evidence for grain boundary potential barriers. Finally, we present guidelines for optimized photoconductance measurements and the evaluation of charge carrier lifetimes in thin films, in general.
The charge carrier lifetime and accordingly the diffusion length in polycrystalline semiconductor materials is known to be detrimentally influenced by disordered interfaces like grain boundaries (GBs). The GB light beam induced current (GB‐LBIC) technique is suitable for the extraction of the minority charge carrier diffusion length in unprocessed polycrystalline materials. This measurement method is based on the GB itself acting as a charge collector. A spatially‐resolved light beam induced current can thus be measured even without a collecting p–n junction or Schottky contact, and without biasing the sample. In this contribution we present a simulation based analysis of measured GB‐LBIC line scans on bonded silicon wafers and polycrystalline silicon thin films with different laser wavelengths and intensities.
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