Influence of boron doping amount on properties of ZnO:B films grown by LPCVD technique and its correlation to a-Si:H/μc-Si:H tandem solar cells

Abstract

Boron-doped ZnO:B (BZO) films with various doping levels have been prepared on large-area substrates by low pressured chemical vapor deposition technique. The influence of doping amount on electrical and optical properties of BZO films has been investigated. It is found that ZnO phase synthesis is hardly affected when the doping gas flow varies from 25 to 100 sccm, but the preferential orientation of grain growth is influenced progressively. It is interesting that there should be a threshold value of doping gas flow of 75 sccm that will cause an abrupt reduction in grain size of BZO and therefore dramatically weakens the light-scattering capacity of the film. It is also noted that the boron atoms doped in BZO films are partly electrically active, and moreover, the heavier doping level, the more inactive B atoms, which not only reduces carrier mobility, but also boosts a stronger light absorption due to enhanced impurity scattering. When the doping gas flow is 75 sccm, the BZO film can achieve a proper comprehensive property with a Rsq of 15.2 Ω/□, an average haze of 21.3% and an average TT of 80.2%. Using this film as the front electrode of a-Si:H/μc-Si:H solar cell, the optimum performance of the solar cell with a Jsc of 12.68 mA/cm2, a Voc of 1.385 mV, and an initial efficiency (η) of 11.83% was obtained.