Investigation into Device Optimisation of Organic Solar Cells Using Narrow Bandgap Polymer and the Role of Acceptor Material

Organic solar cells (OSCs) based on PBDTTT-EFT copolymer as the electron donor material have shown promising power conversion efficiencies (PCE) when blended with fullerene derivative acceptor materials in bulk heterojunction structures. The properties of blended donor and acceptor material and their compatibility plays a critical role in the device performance of OSCs. Therefore, to optimise OSCs devices based on PBDTTT-EFT, it is necessary to investigate the behaviour of the donor and acceptor materials in their pristine and blended form. Based on the literature, the role of donor: acceptor [D:A] blend ratio is amongst the critical aspects for optimising the active layer morphology. However, for OSC devices based on PBDTTT-EFT: PC71BM, the significant correlation between device performance and device physics for different D:A ratios has not yet been fully understood.
Therefore, in this research project, one of the major case studies investigated, will be on devices fabricated from PBDTTT-EFT: PC71BM blends with different PC71BM percentage weight ratios. Results obtained from combined optical absorption spectroscopy, structural, morphological and electrical characterisation, indicates that the best device performance belongs to D:A ratio of 1:2, achieving an average PCE of 9.56%. This is believed to stem from the change in preferred molecular orientation of PBDTTT-EFT molecules from Face-on to Edge-on, as a result of increased PC71BM content within the blend. For the first time, the existence of PBDTTT-EFT molecules in Edge-on orientation was identified using an OOP GIXRD. Also, further evidence of the GIXRD analysis indicates that the highest vertical stacking of PC71BM molecules occurs at D:A blend ratio of 1:2, which is favourable for charge transport and extraction.
The other major investigation on PBDTTT-EFT based OSC devices is on the impact of different fullerene derivative materials to be used as the acceptor, such as PC61BM and IC61BA. IC61BA, in particular, was chosen, as it has higher levels of LUMO compared to PC71BM. Therefore, the Voc of devices fabricated from PBDTTT-EFT: IC61BA is expected to be higher than PBDTTT-EFT: IC61BA. Although device enhancement of some OSCs has been reported by employing IC61BA instead of PCBM
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[1], using IC61BA as the acceptor material in a blend with BDT-based low bandgap polymers has shown the opposite effect [2]. As an example, it has been reported that devices fabricated using blend of PBDTTT-C-T: IC61BA will show poor device performance compared to PBDTTT-C-T: PC61BM, despite achieving higher Voc values when employing IC61BA as the acceptor [2]. It was suggested that poor ‘network’ formation between the donor and acceptor material within the blend causes the problem in reduced device performance, however very little detail is provided as to what is the exact issue. Therefore, investigating PBDTTT-EFT: IC61BA devices has also been the focus of this PhD project. Devices fabricated from PBDTTT-EFT: IC61BA blends resulted in averaged PCE of 5.86%, with Voc value just under 1V.
However, the Jsc and FF parameters are reduced when compared to devices fabricated from PCBM (C61 or C71). From the analysed GIXRD results, it was evident that IC61BA has a very low vertical segregation and stacking, which is detrimental for electron charge transport and extraction. Also, it has been noticed that IC61BA molecules had the least impact on the change in molecular orientation of the PBDTTT-EFT molecule. The mobility measurements further provide evidence to unbalance charge transport mechanism in PBDTTT-EFT: IC61BA devices compared to PBDTTT-EFT: PC71BM devices.
Photoluminescence spectroscopy indicates that thin films composed of PBDTTT-EFT: IC61BA have a higher recombination rate compared to PBDTTT-EFT: PC71BM blend. Thus, when compiled with the analysis of GIXRD results from the blend films, it is concluded that the morphology between PBDTTT-EFT molecules and IC61BA is poor, and their phase domain separation is significant. The recombination analysis elucidates that the inadequate vertical segregation of IC61BA molecules (due to the nature of the molecule shape) results in weak charge dissociation and transport. Therefore, high recombination rates will take place within the active layer, mainly governed by the trap-assisted mechanism.