  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.
