Low-resistivity p-type a-Si:H/AZO hole contact in high-efficiency silicon heterojunction solar cells

Decreasing the contact resistance between hydrogenated amorphous silicon (a-Si:H) and transparent conductive oxide film (TCO) is beneficial for achieving high efficiency silicon heterojunction (SHJ) solar cells. This study reports the implementation of trimethyl boron B(CH3)3 (TMB) doped p-type a-Si:H (a-Si:H(p)) film as hole transport layer contacting with indium-free aluminum doped zinc oxide (AZO) in SHJ solar cells. The influence of doping concentration on the nanostructure of a-Si:H(p), TCO/a-Si:H(p) contact resistivity as well as the resultant cell performance was systematically investigated. It was found that excessive TMB doping results in more carbon and voids inside the films and reduces the doping efficiency, lowering the conductivity and increasing the contact resistivity. a-Si:H(p) film with low defect density and high doping level was obtained at a moderate doping concentration, which facilitates tunneling transport for holes to overcome the high energy barrier at the a-Si:H(p)/AZO interface and results in a low contact resistivity down to 0.14 Ωcm2. The optimized low-resistivity a-Si:H(p)/AZO contact enables a fill factor above 81% and efficiency of 23.6% for M2 SHJ solar cells, which is comparable with 23.7%-efficient cells using traditional tin doped indium oxide (ITO). To our knowledge, this is the highest efficiency for AZO-implemented SHJ cells without double anti-reflection layer and silver back reflector. This work provides design principles on how to achieve high-efficiency SHJ cells with low resistive loss at the hole contact side via doping engineering.