High‐Efficiency Fullerene Solar Cells Enabled by a Spontaneously Formed Mesostructured CuSCN‐Nanowire Heterointerface

Abstract

Fullerenes and their derivatives are widely used as electron acceptors in bulk‐heterojunction organic solar cells as they combine high electron mobility with good solubility and miscibility with relevant semiconducting polymers. However, studies on the use of fullerenes as the sole photogeneration and charge‐carrier material are scarce. Here, a new type of solution‐processed small‐molecule solar cell based on the two most commonly used methanofullerenes, namely [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC60BM) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC70BM), as the light absorbing materials, is reported. First, it is shown that both fullerene derivatives exhibit excellent ambipolar charge transport with balanced hole and electron mobilities. When the two derivatives are spin‐coated over the wide bandgap p‐type semiconductor copper (I) thiocyanate (CuSCN), cells with power conversion efficiency (PCE) of ≈1%, are obtained. Blending the CuSCN with PC70BM is shown to increase the performance further yielding cells with an open‐circuit voltage of ≈0.93 V and a PCE of 5.4%. Microstructural analysis reveals that the key to this success is the spontaneous formation of a unique mesostructured p–n‐like heterointerface between CuSCN and PC70BM. The findings pave the way to an exciting new class of single photoactive material based solar cells.

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

“…[3,22,23] In the region of charge-induced absorption (1.25 eV), more than 90% of the signal (at 1.25 eV) decays within 500 ns. These results together with other recent studies [3,9,12] suggest that fullerene and its derivatives are capable of producing substantial photocurrent and this needs to be taken into account when discussing the device physics of fullerene-based solar cells. This is surprising and we assign it to the larger fraction of long-lived triplet excitons observed at higher fluences in this spectral region.…”

“…[4] Due to the high electron-hole (exciton) binding energy, neat fullerene solar cells exhibit very low external quantum efficiency (EQE). [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. [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.…”

“…[9] However, recently it has been shown that blending fullerene molecules with optically transparent high bandgap inorganic compounds in a hybrid heterointerface improves substantially the power conversion efficiencies of such cells. [12] Subsequently, Firdaus et al demonstrated that the substantial increase in PCE in hybrid PC 70 BM:CuSCN solar cells is caused by improved charge carrier generation and transport, facilitated by the unique mesostructure of CuSCN nanowires penetrating the PC 70 BM domain. [12] Subsequently, Firdaus et al demonstrated that the substantial increase in PCE in hybrid PC 70 BM:CuSCN solar cells is caused by improved charge carrier generation and transport, facilitated by the unique mesostructure of CuSCN nanowires penetrating the PC 70 BM domain.…”

“…[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.…”