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Charge and Triplet Exciton Generation in Neat PC 70 BM Films and Hybrid CuSCN:PC 70 BM Solar Cells
Author(s) -
Karuthedath Safakath,
Gorenflot Julien,
Firdaus Yuliar,
Sit WaiYu,
Eisner Flurin,
Seitkhan Akmaral,
Ravva Mahesh Kumar,
Anthopoulos Thomas D.,
Laquai Frédéric
Publication year - 2019
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201802476
Subject(s) - materials science , exciton , charge carrier , photochemistry , organic solar cell , dissociation (chemistry) , ultrafast laser spectroscopy , hybrid solar cell , heterojunction , analytical chemistry (journal) , optoelectronics , solar cell , polymer solar cell , spectroscopy , chemistry , organic chemistry , condensed matter physics , composite material , physics , quantum mechanics , polymer
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 (PC 70 BM) layer and the anode. Efficiencies of ≈5.4% have recently been reported for optimized CuSCN:PC 70 BM (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 PC 70 BM. While intrinsic charge generation in neat PC 70 BM 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 PC 70 BM 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.