Probing the photophysics of organic photovoltaics from an electron spin perspective

Thesis

The field of organic photovoltaics (OPVs) has undergone a renaissance driven by the advent of non-fullerene acceptor molecules that have propelled efficiencies beyond 20%. Despite this progress, the fundamental processes that drive the conversion of light to electricity remain under debate. Electron paramagnetic resonance (EPR) spectroscopy can help provide answers: by selectively probing electron spin, the key photoinduced states - charge-transfer (CT) states, separated charges, and triplet excitons - can be investigated. A series of donor:acceptor OPV blends were investigated to gain information on the properties and dynamics of the spin states involved in the photovoltaic mechanism.

Initially, the photoinduced separated charges within donor and acceptor domains were identified and characterised through multi-frequency pulse EPR under steady-state conditions. Overlapping spectral contributions were disentangled by exploiting couplings of electron spins to 14N nuclei and leveraging differences in relaxation times. The obtained g-values were verified by quantum chemical calculations validated with electron-nuclear double resonance experiments, and provided the basis for interpreting the more complex spin-correlated CT state signals.

The evolution of CT states at the donor:acceptor interface was probed through time-resolved EPR. Interpretation of the observed spin polarisation, reflective of CT state formation and decay, as well as further separation to free charges, required the development of a purpose-built simulation framework to model the interplay of spin dynamics, kinetics and relaxation. The effects of laser intensity, temperature and film morphology were explored through time-resolved EPR, and extensive modelling revealed two channels of charge separation: a channel slow enough to enable spin-mixing in the CT state, observed for all OPV blends, and an ultrafast charge separation channel, exclusive to the highest-performing materials. This work presents the first observation of both channels for efficient non-fullerene acceptor blends. The observed evolution of spin polarisation additionally confirms the presence of a reversible equilibrium between the CT state and separated charges, previously suggested to explain improved efficiencies of non-fullerene acceptor blends.

A connection between the interfacial CT state and steady-state charges within donor and acceptor domains was established by time-resolved pulse EPR, through a range of pulse sequences aimed at separating contributions from different spin states. This allowed identification of additional efficiency limiting pathways involving quenching of singlet excitons by trapped charges, underscoring the importance of managing trap density in device optimisation.

Authors: Palmer, J

• DOI: 10.5287/ora-d52ryy8mv
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: density operator formalism; photophysics; electron paramagnetic resonance; spin polarisation; organic photovoltaics; donor:acceptor blends; ultrafast charge separation; spin mixing; charge-transfer state; electron spin
• Subjects: Chemistry, Physical and theoretical; Quantum chemistry; Electron paramagnetic resonance spectroscopy; Solar energy