Directional Exciton Diffusion, Measured by Subpicosecond Transient Absorption as an Explanation for Squaraine Solar Cell Performance

Abstract

Organic photovoltaic devices have improved dramatically, and small molecular systems with simple synthesis such as squaraines are prime candidates for cost-effective commercialization. Squaraines have a rich set of excited states, which depend substantially on the morphology of a bulk heterojunction (BHJ) device active layer. Therefore, squaraines provide an excellent opportunity to study the influence of active layer mixing, aggregation, and the cascade of exciton diffusion from point-of-photoexcitation to point-of-charge generation, which drives the efficiency of devices. This work provides a focused investigation into the energy transfer properties of photoexcited charge-transfer H-aggregates (CT-HA) in squaraines, which are predominantly formed in squaraine-fullerene BHJ devices. The work explores the relaxation pathway from CT-HA to charge transfer at the BHJ interface, and explains the higher device efficiencies previously observed as squaraine/fullerene mixing increases through chemical modification of the squaraine. Leveraging transient absorption measurements, we scrutinize the excited state spectroscopy and kinetics in both neat and PCBM blended thin films for two squaraines, DBSQ­(OH)2, which mixes well with fullerenes, and DHSQ­(OH)2, which tends to phase separate. Transient absorption spectroscopic features are assigned to populations of monomer, dimer H-aggregate and CT-HA. CT-HA states are targeted through excitation wavelength selection, and subpicosecond spectroscopic changes and kinetics show the depopulation and repopulation of various excited states. From these measurements we infer the energy transfer from the initially photoexcited state across space and through a landscape with an energy-distribution cascade, and with a Dexter mechanism, despite the fluorescence suppression of H-aggregation. For all fullerene BHJ blends, we observe evidence of an excited state complex between fullerene and squaraine. Specifically, for DBSQ­(OH)2 blended films, the energy transfer to these complexes occurs within 100 fs, with apparent charge separation occurring some ten times faster than for DHSQ­(OH)2 films, a result which aligns with an observation of increased efficiency devices as measured in prior work.