Microcrystalline n- and p-layers at the tunnel junction of a-Si:H/a-Si:H tandem cells

Abstract

Completely microcrystalline layers have been incorporated at the tunnel junction of a tandem cell in the configuration SnO 2:F/p-a-SiC:H/i-a-Si:H/n- μc-Si:H/p- μc-Si:H/i-a-Si:H/n-a-Si:H/Ag and an efficiency of 9.89% has been achieved. We present here our experimental findings and our modelling predictions using the computer programme AMPS. A wide band gap a-Si:H buffer layer between p- μc-Si:H and i-layer a-Si:H was necessary for obtaining good performance of the bottom cell. The spectral response in the blue region is reduced with the increase of buffer layer thickness and thickness as small as 1.0 nm was necessary to achieve the best efficiency. Computer prediction shows that recombination in the p- μc-Si:H could be very high when no buffer is used because of the combination of low band gap and high (>10 17 cm −3) defect density. Buffer layer prevents electron back diffusion into the μc-Si:H p-layer. Computed light current voltage characteristics did not change significantly when tunnelling across the thin buffer layer was incorporated in the model. To achieve larger efficiencies in the tandem configuration, insertion of oxide layers between a-Si:H layer and n- μc-Si:H layer of the top cell and between n- μc-Si:H layer and p- μc-Si:H layer was necessary. Our simulations show that, (a) the recombination rate is increased at the junction due to lower mobility gap of the microcrystalline layers, (b) the density of trapped carriers is reduced with microcrystalline junction, not only in the p- and n-layers but also in the i-layers, (c) the recombination rate at the microcrystalline junction is increased with an oxide layer at the p- μc-Si:H and n- μc-Si:H interface showing an improvement of both FF as well as V oc.