Low-temperature a-Si:H/ZnO/Al back contacts for high-efficiency silicon solar cells

The paper analyses the electronic transport of high-efficiency silicon solar cells with high-quality back contacts that use a sequence of amorphous (a-Si) and microcrystalline (μc-Si) silicon layers prepared at a maximum temperature of 220 °C. Our best solar cells having diffused emitters with random texture and full-area a-Si/μc-Si contacts have an independently confirmed efficiency of 21.0%. An alternative concept uses a simplified a-Si layer sequence combined with Al-point contacts and yields a confirmed efficiency of 19.3%. Analysis of the internal quantum efficiency (IQE) shows that both types of back contacts lead to effective diffusion lengths L_eff exceeding the wafer thickness considerably. Fill factor limitations for the full area contacts result from non-ideal diode behavior, possibly due to the injection dependence of the interface recombination velocity.