Evaluating the efficiency limits of low cost mc Si materials using advanced solar cell processes

Abstract

The evaluation of the efficiency potential of Si materials for solar cell production is one key aspect for strategic decisions in today’s photovoltaic business. In this work a flexible photolithography-based cell process is presented which is in particular well-suited for defect-rich multicrystalline Si material. One decisive feature is the low overall thermal budget of the process since it is based on only one longer high-temperature step (the P diffusion) and a short firing step to obtain a decent hydrogen passivation from a hydrogen-rich PECVD (Plasma-Enhanced Chemical Vapor Deposition) SiNx:H layer. A further MIRHP (Microwave Induced Remote Hydrogen Plasma) step at a temperature below 400°C completes the hydrogen passivation of bulk defects. The process is derived from the standard photolithography based process at the University of Konstanz (UKN) and can easily be adapted to all kinds of dielectric rear side passivation patterns like a-Si, SiO2, SiCx and Al2O3 or stack systems. The rear side contact in this approach is established by Laser Fired Contacts (LFCs). Results presented in this work originate from a process based on an Al2O3 rear side passivation which is deposited at less than 200°C and subsequently annealed at about 400°C. Efficiencies above 18% on EFG and Calisolar polysilicon material, above 14% on RGS and above 20% on FZ reference material are demonstrated on 2 x 2 cm2 solar cells. For all mc-Si materials these efficiencies are very close to the highest efficiencies ever obtained by applying other already established high efficiency processes.