Charge Photogeneration and Recombination in Mesostructured CuSCN‐Nanowire/PC₇₀BM Solar Cells

Abstract

Fullerene-based materials are widely used as electron acceptors in organic bulkheterojunction solar cells; yet, they have rarely been used as the only photoactive component due to their low absorbance and limited charge generation efficiency. However, blending the wide-bandgap p-type material copper (I) thiocyanate (CuSCN) with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 70 BM) leads to the formation of a unique mesostructured p-n like heterointerface between CuSCN and PC 70 BM and solar cells with a power conversion efficiency (PCE) of up to 5.4%. Here, we examine in detail the reasons for the surprisingly good device performance and elucidate the charge photogeneration and recombination mechanisms in CuSCN-based devices with PC 70 BM as the exclusive lightabsorbing material. Our studies clearly demonstrate that a substantial fraction of the photocurrent in the CuSCN-based devices results from improved dissociation of fullerene excitons and efficient charge transfer at the CuSCN:PC 70 BM interface combined with reduced geminate and nongeminate charge recombination losses. Our results have implications beyond the fullerene-based devices studied here, as they demonstrate that careful selection of a mesostructured p-type transparent semiconductor paves the path to a new type of efficient single photoactive material solar cells.

Key Parameters Requirements for Non‐Fullerene‐Based Organic Solar Cells with Power Conversion Efficiency >20%

“…[23,24] Figure 1b shows the TDCF results for the representative NFA-based OPV cells following optical excitation at 532 nm for delay times of 10 ns, and across a range of prebias conditions between −1 to V OC (0.9 V). To either confirm or refute this hypothesis we conducted time-delayed collection field (TDCF) measurements.…”

Charge and Triplet Exciton Generation in Neat PC70BM Films and Hybrid CuSCN:PC70BM Solar Cells

“…[12] In fact, a recent report by Sit et al showed that when copper(I) thiocyanate (CuSCN) was blended with PC 70 BM, a more than fivefold increase in power conversion efficiency (PCE) could be achieved in single junction solar cells using PC 70 BM as the only light absorber. [13] Here, we study the photophysics of neat PC 70 BM films and compare them to CuSCN:PC 70 BM blends to understand what determines the efficiency of such devices. [13] Here, we study the photophysics of neat PC 70 BM films and compare them to CuSCN:PC 70 BM blends to understand what determines the efficiency of such devices.…”

“…[4] However, we note that in line with our observation, Durrant and coworkers recently reported evidence of charge generation in neat PC 60 BM films observed by transient absorption spectroscopy, [3] in accordance with previous reports on free charge generation from loosely bound intramolecular excitons generated from high energy photons. [12,13] However, the exciton-to-charge conversion efficiency is low and thus the PCE of neat fullerene solar cells remains also low. [12,13] However, the exciton-to-charge conversion efficiency is low and thus the PCE of neat fullerene solar cells remains also low.…”

“…[12,13] Figure 5a shows the ps-ns TA spectra of thin-film CuSCN:PC 70 BM blends (1:3) measured in vacuum. This interface is one of few hybrid interfaces, which have been reported to facilitate charge generation.…”

“…This is in stark contrast to previous reports that assigned triplet generation in neat fullerenes to intersystem crossing from singlet excitons. [2,13] To understand this remarkable improvement in short-circuit current density (J SC ) and PCE, we employed picosecond-microsecond transient absorption spectroscopy on neat fullerene films and on CuSCN:PC 70 BM blends. We find that the fraction of charge carriers outweighs largely the population of triplet excitons in both blends and neat films.…”