Hydrogen-induced dopant neutralisation in p-type AIC poly-Si seed layers functioning as buried emitters in ALICE thin-film solar cells on glass

Abstract

Poly-Si thin-film solar cells fabricated on glass superstrates via the aluminium-induced crystallisation solid-phase epitaxy (ALICE) method, where an aluminium-induced crystallisation (AIC) poly-Si seed layer is thickened via solid-phase epitaxy (SPE) of a-Si:H, are investigated. For comparison, solar cells made by solid-phase crystallisation (SPC) of a-Si:H are also investigated. In both cases, the a-Si:H films are deposited by plasma-enhanced chemical vapour deposition (PECVD) vapour. The structural quality, as determined by Raman and ultraviolet (UV) reflectance measurements, of ALICE diodes is found to be superior to that of SPC diodes. Sheet resistance profiling of n-type ALICE and p-type SPC diodes reveals that the Al doping of the AIC poly-Si layer, which is designed to function as a p + emitter in n-type ALICE diodes, is almost completely neutralised during the high-temperature hydrogenation process. In contrast, the n + (phosphorus) emitter of the p-type SPC diodes is not neutralised appreciably by the same hydrogenation process. To overcome the problem with Al-doped emitters, an ALICE structure with an n + AIC poly-Si emitter and a p-type base (glass/SiN/n +p −p +) is suggested and experimentally investigated. The doping in the n + emitter of ALICE cells is found to be essentially unaffected by the hydrogenation process. Open-circuit voltages of up to 480 mV are achieved for p-type ALICE cells with an n + AIC poly-Si emitter. These voltages seem to be the highest reported so far for poly-Si thin-film solar cells on glass featuring an AIC poly-Si seed layer.