Carrier recombination mechanisms in solar cells fabricated using flash-lamp-crystallized polycrystalline silicon films

Abstract

Flash lamp annealing (FLA), millisecond-order discharge from a Xe lamp array, can form a few µm-thick polycrystalline silicon (poly-Si) films on low-cost glass substrates by crystallizing precursor a-Si films, and flashlamp-crystallized (FLC) poly-Si films can be processed to solar cells that actually show rectifying and photovoltaic properties. The conversion efficiencies of FLC poly-Si solar cells are, however, only about 1% at present. In order to clarify the cause of the low conversion efficiency, we have investigated carrier recombination mechanisms in p + -a-Si/FLC poly-Si heterojunction solar cells from their I-V characteristics. The saturation current densities (J0) of the solar cells fabricated using FLC poly-Si films with surface voids have an activation energy (Ea) of ~0.8 eV, which is close to the energy of effective barrier height, the sum of absorber built-in energy and the energy difference of the bulk Fermi level to the valence band. This means that carrier recombination tends to occur at a p + -a-Si/FLC polySi interface. On the other hand, the use of FLC poly-Si films after etching off surface voids, which are formed during crystallization, leads to higher Ea of J0 of 1.12 eV, accompanied with improvement in conversion efficiency. This value is equivalent to the band gap energy (Eg) of cSi, which means that carrier recombination mainly occurs inside FLC poly-Si. These facts indicate that the removal of surface voids is essential to realize a high-quality heterojunction interface, and more effective defect termination in FLC poly-Si films would result in further improvement in solar cell properties.

Improved Broadband Photoresponse in Schottky‐Like Junction Using Carrier Selective NiO Contact on p‐Si

“…The E a value increases from 0.61 to 0.95 eV with the ultrathin NiO layer. For the reference device, E a is 0.61 eV, which is far from the Si bandgap ( E g = 1.12 eV) and close to ϕ Bp , indicating that carrier recombination is dominant at either the Al/ p ‐Si or p ‐Si/ITO interface . Interestingly, the NiO inserted device has a higher E a of 0.95 eV which is close to E g .…”

“…Interestingly, the NiO inserted device has a higher E a of 0.95 eV which is close to E g . Hence, the dominant region of recombination is shifted from the Al/ p ‐Si interface to within p ‐Si . Figure c depicts the energy band diagram of an Al/NiO/ p ‐Si/ITO device under flat band conditions.…”