Relationship between absorber layer defect density and performance of a‐Si:H and µc‐Si:H solar cells studied over a wide range of defect densities generated by 2 MeV electron bombardment

Abstract

We summarize an extensive study on the impact of absorber layer defect density on the performance of amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon solar cells. To study the effects of the absorber layer defect density we subjected set of a-Si:H and μc-Si:H cells to a 2MeV electron bombardment. Subsequently the cells were stepwise annealed to vary the defect density. The cells have varying thicknesses and are illuminated from either the p- or n-side. For reference we subjected i-layers to the same treatment as the cells. The procedure enabled the reversible increase of the i-layer defect density (NS) with two orders of magnitude according to electron spin resonance measurements (ESR) performed on reference samples. The large variation of NS induces substantial changes in the current–voltage characteristics (J–V) and the external quantum efficiency spectra (EQE). These changes in device characteristics provide a solid reference for analysis and device simulations. It was found that performance of a-Si:H cells degraded weakly upon NS increase up to 1017cm−3 and dropped steeply as defect density was increased further. In contrast, performance of µc-Si:H cells showed continuous reduction as NS raised. By comparing p- and n-side illuminated cells we found that, for NS above 1017cm−3, the p-side illuminated a-Si:H cells outperformed the n-side illuminated ones, however, the difference was barely visible at NS below 1017cm−3. On the contrary, the device performance of n-side illuminated µc-Si:H cells was much more affected by the increase in defect density, as compared to the p-side illuminated cells. EQE results evidenced a significant asymmetry in collection of electrons and holes in µc-Si:H devices, where carrier collection was limited by holes as defect density was increased. Based on the experimental data we speculate that the improvement of absorber material in terms of as-deposited defect density is not of primary importance for the performance of a-Si:H cells, whereas in μc-Si:H based solar cells, the reduction of the absorber layer defect density below the state-of-the-art levels, seems to improve the cell performance.