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Development of novel organic semiconductors and advanced combinatorial characterization methods for high performance, printable polymer solar cells

Subject Area Polymer Materials
Experimental Condensed Matter Physics
Synthesis and Properties of Functional Materials
Organic Molecular Chemistry - Synthesis and Characterisation
Physical Chemistry of Solids and Surfaces, Material Characterisation
Term from 2017 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 322714635
 
Final Report Year 2020

Final Report Abstract

The joint Sino-German research project carried out by the FAU and SCUT teams has focused on the development of advanced organic solar cells (OSCs) towards excellent efficiency and device stability. A certified record efficiency of over 12% has been successfully achieved by the joint research team for OSCs with an active area of 1 cm2. Detailed loss analysis suggested that this type of OSCs has the potential to research higher efficiency up to 14%, when the non-radiative VOC loss can be further minimized while maintaining the high EQE over the entire absorption region. The findings demonstrated in this project have contributed to the recently rapid efficiency evolution for OSCs based on non-fullerene acceptors (NFAs). In addition to the device performance, the microstructure-related device instability has been in-depth characterized and analysed for OSCs based on various acceptors, including fullerene acceptors and NFAs. A universal concept has been developed by adding a solid additive, piperazine, into the polymer:PCBM blend to overcome their microstructural instability. In particular for OSCs based on the PBDB-T:IT-4F system, a very promising operational stability of over 10 years was demonstrated. The residual degradation mechanisms were analysed, quantified and concepts to overcome them were proposed. Compared to the inorganic counterparts, such as silicon and GaAs solar cells, the still lower stability of OSCs could be attributed in many cases to the relatively low miscibility or compatibility between state-of-the-art donor and acceptor materials, which cause demixing or phase separation in the mixed amorphous regions. For instance, due to the diffusion of PCBM within a polymer matrix, we could demonstrate for the first time that demixing in the amorphous regions of PCE11:PCBM reduced donor/acceptor interfaces and hence photocurrent generation. In contrast, for NFA-based OSCs, small‐scale isolated NFA domains were found as a result of polymer aggregation/reorganization and low diffusion coefficient of NFAs. These isolated domains act as morphological traps for electron transport and cause long‐term FF degradation (1st order degradation). In order to achieve stable and highperformance OSCs under operational conditions, it is essential to design all components such that their inevitable mixed regions are stable. This can be achieved by using highly compatible materials, where no demixing occurs, or materials with hindered diffusion, e.g., where a very low diffusion constant delays phase separation.

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