Towards Molecular Control of Spin States in Organic Solar Cell Electron Acceptor Materials

Organic photovoltaics (OPVs) have received significant attention in recent years due to society’s growing need for renewable energy sources, the abundance of solar energy, and the inherent advantages of organic materials. In particular, their strong light absorption and chemical tunability means that OPVs can be lightweight, flexible and potentially affordable to produce at large scale. However, the light-to-electricity conversion efficiency has lagged behind solar cells based on silicon and perovskites. Unlike traditional inorganic solar cell materials, organic semiconductors form excitons (Coulombically-bound electron-hole pairs) when they absorb light. Exciton dissociation to charges that can flow as electricity is normally achieved by using two organic materials: a ‘donor’ and an ‘acceptor’ that can undergo an excited state charge transfer reaction. The development of organic materials has led to improvements in OPV performance over time. In particular, the introduction of non-fullerene small molecule acceptors has improved the efficiencies from ∼11% in 2015, to over 19%. Despite this progress, the maximum voltage of OPVs remain low relative to their inorganic counterparts.

To date, OPV design strategies have mainly targeted the short circuit current (JSC)of the device, which is now comparable to silicon/perovskite solar cells. Minimizing voltage losses is now the major target. With this in mind, we noted that some of the recent improvements in materials properties, such as enhanced exciton diffusion lengths leading to longer charge generation lifetimes, may also have introduced new energy loss pathways. Direct conversion of singlet excitons to triplet excitons via intersystem crossing (ISC), which occurs on a timescale of ns-μs, is one of these potential loss pathways. At present, there is a lack of knowledge regarding ISC mechanisms in modern OPV materials. But triplet losses need to be avoided for OPVs to become commercially viable.

In this thesis, we contribute to addressing this knowledge gap by studying ISC rates in a novel series of electron acceptor materials based on the molecular electron acceptor material NIDCS. We report the design and synthesis of this series of structurally related materials in Chapter 2. These acceptors were then studied using a combination of steady state and ultrafast spectroscopic techniques. An introduction to ultrafast transient absorption (TA) spectroscopy and time-resolved photoluminescence (TRPL) spectroscopy is provided in Chapter 3. In Chapter 4 we combine the results of these ultrafast spectroscopic measurements with theoretical studies, to reveal that static conformational disorder in flexible molecules can dramatically accelerate ISC in the solid state. Because the majority of the modern organic materials still possess a degree of conformational flexiblity, the mechanistic model we propose is likely to be valuable for designing better OPV materials.

One of the NIDCS based acceptors (NIDCS-A) was found to undergo intramolecular singlet fission and photoisomersation in solution rather than ISC. Chapter 5 presents a combination of steady-state and time-resolved spectroscopic studies of this compound. Singlet fission (SF) is a process that converts singlet excitons into two triplet excitons, essentially multiplying the effect of a single photon to give two charge pairs, which is attractive for solar cell applications. Molecular photoswitches are materials whose properties (such as optical and chemical reactivity) can be controlled reversibly by light. There is particularly strong interest in visible light photoswitches because of the disadvantages associated with high energy UV photons (such as photodegradation) which limit applications. Both visible light photoswitches and SF materials are rare. This chapter details our efforts to understand the reasons for observing both behaviours in NIDCS-A.

The photophysics of P3HT:NIDCS and TFB:NIDCS donor/acceptor blends are studied in Chapter 6 to verify whether ISC can be observed to compete with charge generation in donor/acceptor blends. Ultrafast transient absorption spectroscopy results on P3HT:NIDCS blend film reveals that ISC in the NIDCS acceptor can indeed compete with both charge generation and excitation energy transfer. These studies use conventional donor polymers that form finely intermixed blends. To study the effects of ISC on charge generation in the future, it will be important to avoid finely intermixed blends because large domain sizes are a key feature of high-performance OPVs.

In summary, this thesis presents valuable insights into molecular design strategies for efficient charge generation in OPVs by minimizing triplet losses due to intersystem crossing. In addition, it also details the unexpected discovery of a singlet fission/photoswitching material and our attempts to understand it.