Triplet Excitons in Pentacene Are Intrinsically Difficult to Dissociate via Charge Transfer

Singlet fission (SF) has the potential to bypass the Shockley-Queisser limit for solar cell efficiency through the production of two electron-hole pairs per photon. However, in polycrystalline pentacene this goal is hindered by slow charge transfer from triplets (≤107 s-1) after SF. In this paper we show that slow charge transfer is an intrinsic property of triplet states in this material and most likely not connected with the triplet pair states that may result from singlet fission. We compare two perylene diimide/pentacene charge transfer systems that differ only by triplet generation mechanism: SF versus intersystem crossing (ISC), sensitized by using a soluble lead phthalocyanine derivative. We use time-resolved microwave conductivity (TRMC) to measure the charge yield in each system and transient absorption (TA) to follow the triplet population dynamics. These experiments are described by a single global kinetic model, with most of its parameters fixed via control experiments. While we observe modest differences in the charge-transfer rate constants between each sample, 4-10× as a function of triplet generation mechanism, all samples remain far below the predicted diffusion-limited rate constant of 107-108 s-1.