The role of sol–gel chemistry in the low-temperature formation of ZnO buffer layers for polymer solar cells with improved performance

A new approach is proposed in this work to chemically control the low-temperature sol-gel formation of ZnO thin films used as efficient electron transporting layers (ETLs) in inverted polymer solar cells (PSCs). The chemical composition of the ZnO sol-gel precursor was modified by systematically employing different [H2O]/[Zn2+] molar ratios in the starting sol formulation and evaluating their influence on film properties and PSC device performance. A thorough characterization of the obtained ZnO ETLs evidenced the key importance of the [H2O]/[Zn2+] molar ratio to achieve effective control on the sol-gel hydrolysis and condensation processes. Based on these evidences, a mechanism for the formation of the ZnO films at the low processing temperatures used in this work was proposed. PSC devices were fabricated incorporating ZnO ETLs obtained from ZnO sol precursor formulations with increasing [H2O]/[Zn2+] ratios and their photovoltaic characterization revealed the presence of a maximum device efficiency for intermediate [H2O]/[Zn2+] values. Finally, the effect of water in the ZnO sol precursor on the long-term (>1000 h) shelf-life of PSCs fabricated onto flexible PET substrates was investigated and a correlation was found between chemical composition of the ZnO sol precursor and device shelf-life. The results of this study give a clear demonstration of a viable strategy to achieve improved PSC device performance by chemically controlling the formation of the sol-gel based ZnO ETL at processing temperatures compatible with flexible plastic substrates and provide useful guidelines for the development of efficient sol-gel derived metal-oxide buffer layers for highly performing flexible photovoltaics.