The degradation mechanism of flexible a-Si:H/μc-Si:H photovoltaic modules

A flexible a-Si:H/μc-Si:H tandem-junction PV module was produced, in which a thin film of ZnO:B grown by metal organic chemical vapor deposition (MOCVD) served as the TCO. Its stability under varying environmental stresses was assessed by a reliability test based on the MIL-STD-883G standard, revealing that its efficiency is reduced by damp heat (85 °C, 85% relative humidity). The mechanism behind this degradation was investigated in further detail by assessing the ZnO:B thin film's reaction to moisture. For this, flexible a-Si:H/μc-Si:H solar cells were produced with deliberately degraded efficiencies of - 20%, - 50% and - 80%, and then compared against similarly degraded ZnO:B thin films. Amongst the electrical parameters, series resistance (Rs) exhibited a dramatic increase and the short circuit current (Isc) is decreased in relation to the extent of degradation. Moreover, the decreasing trend of efficiency of ZnO:B was found to correlate to its Hall mobility. This indicates that the effect of moisture is to increase the ZnO:B layer's resistance, thus increasing the Rs of the a-Si:H PV. Related changes in the physical and chemical properties of ZnO:B were analyzed by XPS and SIMS; demonstrating that OH^- is increased, whereas Zn^{2 +} and B^{3 +} are reduced in the grain boundaries of the ZnO:B surface. Furthermore, a surface reaction occurs between hydroxide and Zn2 + ions to form Zn(OH)² at the grain boundaries, which is considered to be an important aspect of the observed degradation of Rs and Isc.