Conversion efficiency and stability of organic solar cells incorporating titanium dioxide with fullerene-based acceptor

Abstract

It is necessary for organic solar cells (OSCs) to have a high and stable power conversion efficiency (PCE) in ambient temperature and at different environmental conditions before they are commercially available on the market. However, the efficiency and stability of OSCs are limited by the type, structure, and architectural design of their active layers. Therefore, the main focus of the current research is to find an appropriate method to improve the efficiency and stability of OSCs. In this study, five different approaches were used with the aim of improving the efficiency and stability of OSCs based on PTB7:PC71BM blend. The first one is assigned to optimize the PTB7:PC71BM active layer thickness of the OSC devices. The second method explores the impact of TiO2 nanostructures on the physical properties and electrical performance of OSCs based on PTB7:PC71BM bulk heterojunctions. The third approach is devoted to studying the rate of improvement in the overall performance of PTB7:PC71BM-based OSCs by using TiO2 as an electron transport layer (ETL). The fourth method investigates the effect of thermal annealing treatment on stability, reproducibility, and photovoltaic performance. The fifth approach presents the hot substrate coating method as a novel strategy to improve the PTB7:PC71BM-based OSC in terms of stability and photovoltaic performance. For the active layer thickness optimization, results revealed that the OSC based on PTB7:PC71BM with a 70 nm thickness showed the best PCE of 5.63%. After incorporating 10% TiO2 into the PTB7:PC71BM blend and annealing at 100 °C, an unexpectedly low efficiency of 0.08% was obtained. Alternatively, the performance improvement of the PTB7:PC71BM-based OSCs was achieved by using a very thin layer of titanium oxide (TiO) as ETL. Results showed that with the use of 5 nm TiO (ETL), the PCE was improved from 5.6% to 9.63%. Moreover, results demonstrated an improvement in the OSCs efficiency when they were annealed at 50 °C, with a reported PCE of 9.75%, which is considered to be the highest efficiency reported for the single junction OSC based on PTB7:PC71BM. The device annealed at 50 °C exhibited higher stability and better reproducibility than the un-annealed device. The last strategy performed in this research is called hot substrate-coating method. In this method, two different batches of OSCs were fabricated on hot substrate and room temperature (RT) substrate under similar environmental conditions. Results showed that OSCs fabricated via hot substrate-coating presented a PCE of 7.94% compared to 5.6% for the RT-coated device. The hot substrate-coated device retained 83% of its initial PCE after 20 days of operation, considered to be the highest stability achieved for the single junction OSC based on PTB7:PC71BM. The proposed hot substrate-coating approach could be utilized to improve both the efficiency and stability of OSCs.