Effect of surface recombination on electroluminescence and photoconversion in a-Si:H/c-Si heterojunction solar cells

TL;DR

This study investigates how surface recombination velocity impacts electroluminescence and photoconversion efficiency in large-area a-Si:H/c-Si heterojunction solar cells, revealing that EL is more sensitive to surface conditions than efficiency.

Contribution

The paper provides a quantitative analysis of surface recombination effects on EL and efficiency, introducing a model that explains temperature-dependent behaviors in heterojunction solar cells.

Findings

EL quantum efficiency is more affected by surface recombination velocity than photoconversion efficiency.

Low surface recombination leads to EL efficiency of 2.1%, surpassing known silicon diode values.

EL intensity peaks at 223 K and involves tunneling mechanisms at low temperatures.

Abstract

Surface recombination affects both light-to-electricity and electricity-to-light conversion in solar cells (SCs). Therefore, quantitative analysis and reduction of surface recombination is an important direction in SC research. In this work, electroluminescence (EL) intensity and photoconversion efficiency of a set of 93 large-area (239\,cm$^2$) a-Si:H/c-Si heterojunction SCs (HJSCs) are measured under AM1.5 conditions at 298 K. The HJSC samples differed only in surface recombination velocity, $S$, but otherwise were identical. Variation in $S$ was due to the variation of the chemical conditions under which the samples were treated. It is established that EL quantum efficiency, is affected by $S$ much more strongly than photoconversion efficiency, $\eta$: namely, the reduction of the latter from 20.5\% to 18\% due to an increase of $S$ is accompanied by a decrease of the former by more than an order of magnitude. In HJSCs with well passivated surfaces, i.e. low $S$, EL efficiency reached 2.1\%, which is notably higher than the known values in silicon homojunction diodes. For temperature-dependent measurements of EL and dark I-V curves, one of the samples was cut into small-area (1 cm$^2$) pieces. It was found that EL intensity as a function of temperature develops a maximum at $T$ = 223 K. At low temperatures, the current at weak bias is shown to be due to tunneling mechanism. A theoretical model is developed that explains all these findings quantitatively.