Organic solar cells demonstrate external quantum efficiencies and fill factors approaching those of conventional photovoltaic technologies. However, as compared to the optical gap of the absorber materials, their open-circuit voltage is much lower, largely due to the presence of significant nonradiative recombination. In this work, we study a large data set of published and new material combinations and find that non-radiative voltage losses decrease with increasing charge-transfer state energies. This observation is explained by considering non-radiative charge-transfer state decay as electron transfer in the Marcus inverted regime, being facilitated by a common skeletal molecular vibrational mode. Our results suggest an intrinsic link between non-radiative voltage losses and electron-vibration coupling, indicating that these losses are unavoidable. Accordingly, the theoretical upper limit for the power conversion efficiency of single junction organic solar cells would be reduced to about 25.5% and the optimal optical gap increases to (1.45-1.65) eV, i.e. (0.2-0.3) eV higher than for technologies with minimized non-radiative voltage losses. Manuscript: "Intrinsic Non-Radiative Voltage Losses in Fullerene-Based OSCs" J. Benduhn et al.

Blending organic electron donors and acceptors yields intermolecular charge-transfer states with additional optical transitions below their optical gaps. In organic photovoltaic devices, such states play a crucial role and limit the operating voltage. Due to its extremely weak nature, direct intermolecular charge-transfer absorption often remains undetected and unused for photocurrent generation. Here, we use an optical microcavity to increase the typically negligible external quantum efficiency in the spectral region of charge-transfer absorption by more than 40 times, yielding values over 20%. We demonstrate narrowband detection with spectral widths down to 36 nm and resonance wavelengths between 810 and 1,550 nm, far below the optical gap of both donor and acceptor. The broad spectral tunability via a simple variation of the cavity thickness makes this innovative, flexible and potentially visibly transparent device principle highly suitable for integrated low-cost spectroscopic near-infrared photodetection.

Spectroscopic photodetection is a powerful tool in disciplines such as medical diagnosis, industrial process monitoring, or agriculture. However, its application in novel fields, including wearable and bio-integrated electronics is hampered by the use of bulky dispersive optics. Here, we employ solution-processed organic donor-acceptor blends in a resonant optical cavity device architecture for wavelength-tunable photodetection. While conventional photodetectors respond to above-gap excitation, the cavity device exploits weak subgap absorption of intermolecular charge-transfer states of the intercalating Poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT):[6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) bimolecular crystal. This enables a highly wavelength selective, nearinfrared photoresponse with a spectral resolution down to 14 nm, as well as dark currents and detectivities comparable with commercial inorganic photodetectors. A miniaturized spectrophotometer, comprising an array of narrowband photo-detectors is fabricated using blade-coated PBTTT:PCBM thin films with a thickness gradient. As application example, we demonstrate water transmittance spectral measured by this device.

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Organic solar cells with an electron donor diluted in a fullerene matrix have a reduced density of donor-fullerene contacts, resulting in decreased free-carrier recombination and increased open-circuit voltages. However, the low donor concentration prevents the formation of percolation pathways for holes. Notwithstanding, high (>75%) external quantum efficiencies can be reached, suggesting an effective hole-transport mechanism. Here, we perform a systematic study of the hole mobilities of 18 donors, diluted at ∼6 mol % in C, with varying frontier energy level offsets and relaxation energies. We find that hole transport between isolated donor molecules occurs by long-range tunneling through several fullerene molecules, with the hole mobilities being correlated to the relaxation energy of the donor. The transport mechanism presented in this study is of general relevance to bulk heterojunction organic solar cells where mixed phases of fullerene containing a small fraction of a donor material or vice versa are present as well.