Optical probes of free charge generation in organic photovoltaics
Authors:   , Barker Alexander J. 
Published: November 2021

<p>Organic photovoltaics (OPVs) show considerable promise as a source of low cost solar energy. Improving our understanding of the processes governing free charge photogeneration in OPVs may unlock the improvements in efficiency required for their widespread implementation. In particular, how do photogenerated charge pairs overcome their mutual columbic attraction, and what governs the branching between bound and free charge pairs that is observed to occur shortly after their creation?  Ultrafast laser techniques such as transient absorption (TA) spectroscopy are the only tools capable of probing the time scales associated with these processes (as short as 10⁻¹⁴ seconds). Challenges include achieving sufficient sensitivity to resolve the tiny signals generated in thin films under solar-equivalent excitation densities, and distinguishing and quantifying overlapping signals due to separate phenomena.  We present the development of a versatile and ultra-sensitive broadband TA spectrometer, along with a comprehensive analysis of the noise sources limiting sensitivity. Through the use of referenced shot-to-shot detection and a novel method exploiting highly chirped broadband probe pulses, we are capable of resolving changes in differential transmission < 3 × 10⁻⁶ over pump-probe delays of 10⁻¹³–10⁻⁴ seconds.  By comparing the absorption due to photogenerated charges to measurements of open-circuit voltage decay in devices under transient excitation, we show that TA is able to quantify the recombination of freely extractable charge pairs over many decades of pump-probe delay. The dependence of this recombination on excitation density can reveal the relative fraction of bound and free charge pairs. We apply this technique to blends of varying efficiency and find that the measured free charge fraction is correlated with published photocharge yields for these materials.  We access a regime at low temperature where thermalized charge pairs are frozen out following the primary charge separation step and recombine monomolecularly via tunneling. The dependence of tunneling rate on distance enabled us to fit recombination dynamics to distributions of recombination rates. We identified populations of charge-transfer states and well-separated charge pairs, the yield of which is strongly correlated with the yield of free charges measured via their intensity dependent recombination. We conclude that populations of free charges are established via long-range charge separation within the thermalization timescale, thus invoking early branching between free and bound charges across an energetic barrier. Subject to assumed values of the electron tunneling attenuation constant, we find critical charge separation distances of ~ 3–4nm in all materials.  TA spectroscopy probes the absorption of excited states, with the signal being proportional to the product of population density and absorption cross-section of the absorbing species. We show that the dependence of signal on probe pulse intensity can decouple these parameters, and apply a numerical model to determine the time-dependent absorption cross-section of photogenerated excitons in thin films of semiconducting polymers.  Collectively, this thesis presents spectroscopic tools and applications thereof that illuminate the process of free charge generation in organic photovoltaics.</p>