Hot electron effect in ultrathin photovoltaic junctions

The open circuit voltage in nanoscopically-thin \textit{p-i-n} amorphous silicon solar cells is found to increase with optical energy (light frequency) [1]. We accredit this increased $V_{oc}$ to the extraction of hot carriers. The ultrathin nature of these junctions also leads to a large electric field, reducing carrier recombination and facilitating anomalously large current in addition to the increased voltage. The large $J_{sc}$ thus indicates improved carrier extraction despite reduced optical absorption for ultrathin absorber layers. The overall power conversion efficiency is $\sim $3{\%} with absorbers less than 1/20$^{th}$ as thick as conventional $a$-Si solar cells ($i$-layer as thin as 5 nm). A simple phenomenological argument provides a semi-quantitative understanding of these effects, and may provide guidance for the design of high-efficiency, hot electron solar cells. MJN, KK and ZFR also at Solasta Inc.\\[4pt] [1] K. Kempa, M.J. Naughton, Z.F. Ren, A. Herczynski, T. Kirkpatrick, J. Rybczynski, Y. Gao, Appl. Phys. Lett. (in press).