Evaluation of Bulk and Rear Interface Passivation of Thin Film Photovoltaics by Surface Photovoltage Spectroscopy

Thin film photovoltaics have the potential to dramatically reduce cost and carbon footprint of photovoltaic manufacture. However, current solutions like GaAs are cost prohibitive except for special applications, while more affordable CdTe, SbSe and perovskite devices lag behind silicon in efficiency. To identify and resolve these difficiencies, a fuller understanding of the role of interface and bulk passivation, diffusion length, surface recombination velocity, and band structure is necessary.<br/><br/>We present surface photovoltage (SPV) spectroscopy data on multiple CdTe and SbSe photovoltaic devices, comparing the effects of bulk and surface passivation strategies. The effects of doping, wet etchants, CdCl2 annealing, and surface reconstruction are presented and the results interpretted by advanced modeling which fully describes the device under test.<br/><br/>Traditional methods for interpretting SPV such as the Goodman method, are only valid when diffusion length and absorption depth are both significantly less than the thickness of the sample - a situation which is not applicable to modern devices. In contrast we present analytical and computational modeling which accounts for a full device - including multiple junctions. Analytical models are used to explore the effects of a variety of parameters on SPV in the depetion region, quasi-neutral region and space charge region (SCR). The results are then validated by full device modeling in SCAPS-1D software. The effects of bulk diffusion length, doping, and recombination in each region is explored.<br/><br/>Finally, the understanding of how these effects combine in the final SPV signal allows for SPV to become diagnostic in evaluating the efficacy of new passivation strategies.