5 years ago

Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies

Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies
Fortunato Piersimoni, Dieter Neher, Christian Koerner, Koen Vandewal, Olaf Zeika, Felix Holzmueller, Donato Spoltore, Vasileios C. Nikolis, Johannes Benduhn, Matthias Lau
High photon energy losses limit the open-circuit voltage (VOC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the VOC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha-sexithiophene (α-6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the VOC of an α-6T/SubNc/SubPc fullerene-free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D–A interface. By accurately measuring the optical gap (Eopt) and the energy of the charge-transfer state (ECT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. Eopt – qVOC losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the VOC-optimized devices, the low-energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low-voltage losses can be combined with a high EQE in organic photovoltaic devices. The insertion of a thin interlayer at the D/A interface of a cascade organic solar cell leads to a reduction of the voltage losses by 180 mV, while maintaining peak external quantum efficiencies approaching 80%. A detailed analysis of the organic multilayer device with exceptionally low losses reveals a minimal driving force for charge separation and suppressed nonradiative recombination.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/aenm.201700855

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