Abstract
Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control.
Highlights
Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges
An energy level offset between D and A leads to the formation of intermolecular charge-transfer (CT) states, an electronic state optically coupled to the ground state, in which the electron resides on A and the hole on D, playing a key role in free charge carrier generation[3,4]
The device was fabricated at room temperature by evaporating an α-6T layer between the bottom and the top contact
Summary
Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. 0.00 all other devices do not show any subgap absorption features, implying the absence of low-energy CT states originating from the α-6T/BL interface, which could drive charge generation and recombination (Supplementary Fig. 2).
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