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.

Many of the highest-performing polymer photocatalysts for sacrificial hydrogen evolution from water have contained dibenzo [b,d]thiophene sulfone units in their polymer backbones. However, the reasons behind the dominance of this building block are not well understood. We study films, dispersions, and solutions of a new set of solution-processable materials, where the sulfone content is systematically controlled, to understand how the sulfone unit affects the three key processes involved in photocatalytic hydrogen generation in this system: light absorption; transfer of the photogenerated hole to the hole scavenger triethylamine (TEA); and transfer of the photogenerated electron to the palladium metal co-catalyst that remains in the polymer from synthesis. Transient absorption spectroscopy and electrochemical measurements, combined with molecular dynamics and density functional theory simulations, show that the sulfone unit has two primary effects. On the picosecond timescale, it dictates the thermodynamics of hole transfer out of the polymer. The sulfone unit attracts water molecules such that the average permittivity experienced by the solvated polymer is increased. We show that TEA oxidation is only thermodynamically favorable above a certain permittivity threshold. On the microsecond timescale, we present experimental evidence that the sulfone unit acts as the electron transfer site out of the polymer, with the kinetics of electron extraction to palladium dictated by the ratio of photogenerated electrons to the number of sulfone units. For the highest-performing, sulfone-rich material, hydrogen evolution seems to be limited by the photogeneration rate of electrons rather than their extraction from the polymer.

The synthesis and characterization of copper (I) selenocyanate (CuSeCN) and its application as a solution-processable hole-transport layer (HTL) material in transistors, organic light-emitting diodes, and solar cells are reported. Density-functional theory calculations combined with X-ray photoelectron spectroscopy are used to elucidate the electronic band structure, density of states, and microstructure of CuSeCN. Solution-processed layers are found to be nanocrystalline and optically transparent (>94%), due to the large bandgap of ≥3.1 eV, with a valence band maximum located at −5.1 eV. Hole-transport analysis performed using field-effect measurements confirms the p-type character of CuSeCN yielding a hole mobility of 0.002 cm 2 V −1 s −1 . When CuSeCN is incorporated as the HTL material in organic light-emitting diodes and organic solar cells, the resulting devices exhibit comparable or improved performance to control devices based on commercially available poly(3,4-ethy lenedioxythiophene):polystyrene sulfonate as the HTL. This is the first report on the semiconducting character of CuSeCN and it highlights the tremendous potential for further developments in the area of metal pseudohalides.

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.

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.