Charge and Triplet Exciton Generation in Neat PC70\nBM Films and Hybrid CuSCN:PC70\nBM Solar Cells

Safakath Karuthedath, Julien Gorenflot, Yuliar Firdaus, Wai-Yu Sit, Flurin Eisner, Akmaral Seitkhan, Mahesh Kumar Ravva, Thomas D. Anthopoulos, Frédéric Laquai

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

Organic solar cells that use only fullerenes as the photoactive material exhibit poor exciton-to-charge conversion efficiencies, resulting in low internal quantum efficiencies (IQE). However, the IQE can be greatly improved, when copper(I) thiocyanate (CuSCN) is used as a carrier-selective interlayer between the phenyl-C70-butyric acid methyl ester (PC70BM) layer and the anode. Efficiencies of ≈5.4% have recently been reported for optimized CuSCN:PC70BM (1:3)-mesostructured heterojunctions, yet the reasons causing the efficiency boost remain unclear. Here, transient absorption (TA) spectroscopy is used to demonstrate that CuSCN does not only act as a carrier-selective electrode layer, but also facilitates fullerene exciton dissociation and hole transfer at the interface with PC70BM. While intrinsic charge generation in neat PC70BM films proceeds with low yield, hybrid films exhibit much improved exciton dissociation due to the presence of abundant interfaces. Triplet generation with a rate proportional to the product of singlet and charge concentrations is observed in neat PC70BM films, implying a charge–singlet spin exchange mechanism, while in hybrid films, this mechanism is absent and triplet formation is a consequence of nongeminate recombination of free charges. At low carrier concentrations, the fraction of charges outweighs the population of triplets, leading to respectable device efficiencies under one sun illumination.
Original languageEnglish (US)
Pages (from-to)1802476
JournalAdvanced Energy Materials
Volume9
Issue number1
DOIs
StatePublished - Nov 8 2018

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