Active layer limitations and non-geminate recombination in polymer-fullerene bulk heterojunction solar cells

This chapter has focused on the apparent thickness limitations of most high performance donor-acceptor bulk heterojunction solar cells, which pose significant challenges to achieving 15% conversion efficiency. Efficiencies approaching this value require high external quantum efficiencies and high fill factors using thick photoactive layers. An extensive literature review showed that only a handful of high performance donor-acceptor bulk heterojunction solar cells maintain high fill factor at film thicknesses larger than 200 nm, with PBnDT-FTAZ, DT-PDPP2T-TT, PtPD3T and PDTSiTTz being notable exceptions. Apart from difficulties in controlling active layer morphologies with increasing layer thickness and space charge layer formation in (often inadvertently) doped bulk heterojunctions, Langevin-type bimolecular recombination is shown to limit charge extraction efficiency in thick devices, thereby leading to low fill factors. Using time-resolved charge extraction techniques such as photo-CELIV, TOF and time-resolved charge extraction, reduced (non-Langevin) bimolecular recombination is demonstrated for PDTSiTTz:PCBM heterojunctions. A brief review of 12 models offering possible explanations for such reduced recombination is presented. This highlights the fact that developing a onesize- fits-all, unified model of reduced recombination is challenging due to the limited number of reduced recombination donor-acceptor systems and the differences in experimental techniques and sample preparation between various reports. Resolving this rather intriguing, fundamentally important issue by establishing design rules for advanced donor-acceptor bulk heterojunction solar cells will be an important milestone affecting not only the performance of laboratory-based devices, but also the commercialization of this low-cost photovoltaic technology