Comparison of photophysics across bulk heterojunction organic photovoltaic cells containing different classes of electron acceptors

Access to energy has been a key driver of global progress and growth, with this growth comes increased global energy demand; currently at around 18 TW on average and projected to increase. An accessible and distributed source of power is needed to fill this growing need. Solar power fits this need, and the future of this technology lies in reducing the energy cost to manufacture photovoltaic (PV) systems, retain or improve performance, and tailor their properties for niche applications.

Since the design of new PV materials depends on optimising the response of materials to light, optical spectroscopy is a critical experimental tool. Of the different spectroscopic tools, transient absorption spectroscopy is ideal for understanding the photophysics of solar cells because PV materials always have visible absorption signatures that evolve over time. Being a pump–probe technique, the advent of amplified femtosecond laser systems allows femto– to micro– second timescales to be routinely measured, making it possible to track the evolution of the excited state species from exciton formation to relaxation or recombination back to the ground state.

This thesis characterises the photophysics of multiple different bulk–heterojunction (BHJ) organic photovoltaic (OPV) systems containing different electron acceptors. Using transient absorption spectroscopy, we have developed an understanding of how changes to the organic materials and morphology affect the photophysics and subsequent solar power conversion efficiency.

The thesis is separated into four main sections; the first, Chapter 2 looks at improvements made to the experimental setup to allow the characterisation of organic–photovoltaics over a low excitation power that can provide information relevant to operating conditions, and in the following three chapters, this improved technique is used to look at the various classes of electron acceptors in BHJ–OPVs: fullerenes, polymers, and small molecule acceptors (SMA).

Chapter 3 covers fullerene acceptor BHJ–OPVs looking at two different systems. First, the weight percentage of PC71BM is varied, resulting in changes to the photophysics consistent with variation in domain size and purity; this interplay between photophysics and morphology highlights how understanding both is crucial to optimising BHJ–OPVs. The second looks at changing the functionality of the fullerene, looking at three common fullerene derivatives PC61BM, PC71BM, and ICBA. Here, the differences are more subtle, showing that while ICBA and PC61BM blends generate charges promptly, a combination of poor quenching in the ICBA blend and shorter lifetimes in the PC61BM blend explain the higher power conversion efficiency of the PC71BM system.

In Chapter 4, blends with a polymer used as the electron donor and a different polymer used as the electron acceptor are studied. In this chapter, we see that the photophysics of a system can have little to no correlation to the overall power conversion efficiency. The increased fluorination of the polymer electron acceptor, PNDITPhT, results in no observable changes in the measured photophysics, while the power conversion efficiency (PCE) changes from 3.1 % to 4.3 % (improves by more than 30 %). Also, in this chapter, the effect of the donor polymers (PTB7–Th and PTB7) blended with P(NDI2OD–T2) is investigated; showing a doubling of power conversion efficiency from 2.1 % to 4.4 %; attributed to longer–lived and more mobile charges.

Finally, we investigate at small molecule acceptors in Chapter 5. Here, we look at one of the first small–molecule acceptor systems to have better efficiency than fullerene:polymer blends developed at this time. By comparing several polymer acceptors, it is found that for the small–molecule acceptor IDIC, the photophysics of an efficient blend are similar to those observed when using a fullerene acceptor. And, when matching the SMA with low–bandgap polymers, the materials do not suffer from the triplet loss mechanism present in low–bandgap BHJ–OPVs.

Together, this thesis provides insight into the photophysics of several different material systems, and improves our understanding of how the power conversion efficiency is improved, whether it be by morphology, charge lifetimes, mobility, or increased absorption. We ultimately find that while many of the key photophysical requirements remain unchanged between fullerene and SMA–based BHJ–OPVs, the active role that SMAs play in absorbing light and generating photocurrent provides a new avenue to increase efficiency, with more work still to be done in understanding the heterojunction and how it could be tuned to optimise generation from electron –donor and –acceptor phases.