Indium phosphide based solar cell using ultra-thin ZnO as an electron selective layer

According to the Shockley-Queisser limit, the maximum achievable efficiency for a single junction solar cell is ∼33.2% which corresponds to a bandgap (E _g) of 1.35 eV (InP). However, the maximum reported efficiency for InP solar cells remain at 24.2% ± 0.5%, that is >25% below the standard Shockley-Queisser limit. Through a wide range of simulations, we propose a new device structure, ITO/ ZnO/i-InP/p⁺ InP (p-i-ZnO-ITO) which might be able to fill this efficiency gap. Our simulation shows that the use of a thin ZnO layer improves passivation of the underlying i-InP layer and provides electron selectivity leading to significantly higher efficiency when compared to their n⁺/i/p⁺ homojunction counterpart. As a proof-of-concept, we fabricated ITO/ZnO/i-InP solar cell on a p⁺ InP substrate and achieved an open-circuit voltage (V _oc) and efficiency as high as 819 mV and 18.12%, respectively, along with ∼90% internal quantum efficiency. The entire device fabrication process consists of four simple steps which are highly controllable and reproducible. This work lays the foundation for a new generation of thin film InP solar cells based solely on carrier selective heterojunctions without the requirement of extrinsic doping and can be particularly useful when p- and n-doping are challenging as in the case of III-V nanostructures.