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Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.

Spectroscopic photodetection plays a key role in many emerging applications such as context-aware optical sensing, wearable biometric monitoring, and biomedical imaging. Photodetectors based on organic semiconductors open many new possibilities in this field. However, ease of processing, tailorable optoelectronic properties, and sensitivity for faint light are still significant challenges. Here, the authors report a novel concept for a tunable spectral detector by combining an innovative transmission cavity structure with organic absorbers to yield narrowband organic photodetection in the wavelength range of 400-1100 nm, fabricated in a full-vacuum process. Benefiting from this strategy, one of the best performed narrowband organic photodetectors is achieved with a finely wavelength-selective photoresponse (full-width-at-half-maximum of approximate to 40 nm), ultrahigh specific detectivity above 10(14) Jones, the maximum response speed of 555 kHz, and a large dynamic range up to 168 dB. Particularly, an array of transmission cavity organic photodetectors is monolithically integrated on a small substrate to showcase a miniaturized spectrometer application, and a true proof-of-concept transmission spectrum measurement is successfully demonstrated. The excellent performance, the simple device fabrication as well as the possibility of high integration of this new concept challenge state-of-the-art low-noise silicon photodetectors and will mature the spectroscopic photodetection into technological realities. China Scholarship CouncilChina Scholarship Council [201706070125, 201706890003, 201706140127]; DFGGerman Research Foundation (DFG)European Commission [VA 1035/5-1]; Sachsische Aufbaubank [100325708]

Efficient charge generation in organic semiconductors usually requires an interface with an energetic gradient between an electron donor and an electron acceptor in order to dissociate the photogenerated excitons. However, single-component organic solar cells based on chloroboron subnaphthalocyanine (SubNc) have been reported to provide considerable photocurrents despite the absence of an energy gradient at the interface with an acceptor. In this work, it is shown that this is not due to direct free carrier generation upon illumination of SubNc, but due to a field-assisted exciton dissociation mechanism specific to the device configuration. Subsequently, the implications of this effect in bilayer organic solar cells with SubNc as the donor are demonstrated, showing that the external and internal quantum efficiencies in such cells are independent of the donor-acceptor interface energetics. This previously unexplored mechanism results in efficient photocurrent generation even though the driving force is minimized and the open-circuit voltage is maximized. V.C.N. and Y.D. contributed equally to this work. The authors would like to thank Prof. Dr. Dieter Neher for providing measurement time on the electroluminescence setup of his lab at University of Potsdam. They would also like to acknowledge the Optoelectronics group in the University of Cambridge for sharing the global analysis codes. This work was supported by the German Federal Ministry of Education and Research (BMBF) through the InnoProfile project "Organische p-i-n Bauelemente2.2" (FKZ 03IPT602X). A.A.B. is a Royal Society university research fellow. Open access funding enabled and organized by Projekt DEAL. Nikolis, VC (corresponding author), Tech Univ Dresden, Dresden Integrated Ctr Appl Phys & Photon Mat IAP, Nothnitzer Str 61, D-01187 Dresden, Germany ; Tech Univ Dresden, Inst Appl Phys, Nothnitzer Str 61, D-01187 Dresden, Germany. Bakulin, AA (corresponding author), Imperial Coll London, Mol Sci Res Hub, London W12 0BE, England. Vandewal, K (corresponding author), Hasselt Univ, Inst Mat Res IMO IMOMEC, Wetenschapspk 1, B-3590 Diepenbeek, Belgium. vasileios_christos.nikolis1@tu-dresden.de; a.bakulin@imperial.ac.uk; koen.vandewal@uhasselt.be

Organic solar cells are a promising technology for a large area conversion of sunlight into electricity. In particular for solar cells based on oligomers (small molecules), efficient donor materials absorbing wavelengths larger than 780 nm are still rare. Here, we investigate three aza-BODIPY dyes absorbing in the infrared. The addition of side groups leads to a red shift of the optical gap from 802 to 818 nm. In optimized devices using these donors in a bulk heterojunction with C-60, we observe a higher charge carrier mobility and a higher power conversion efficiency for the molecules without a methyl or methoxy side group lowering the molecular reorganization energy. Surprisingly, the donor-acceptor blend with the lowest energy loss during the electron transfer to the C-60 yields the highest short circuit current. With increasing size of the attached side chain, the devices exhibit a larger trap density, measured by impedance spectroscopy. Based on the investigation of different blend ratios, we conclude that these traps are mainly present in the donor phase. (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

For an increased lifetime of polymer:fullerene bulk heterojunction (BHJ) solar cells, an understanding of the chemical and morphological degradation phenomena taking place under operational conditions is crucial. Phase separation between polymer and fullerene induced by thermal stress has been pointed out as a major issue to overcome. While often the effect of thermal stress on the morphology of polymer:fullerene BHJ is investigated in the darkness, here we observe that light exposure slows down fullerene crystallization and phase separation induced at elevated temperatures. The observed photo-stabilizing effect on active layer morphology is quite independent on the polymer and is attributed to light-induced dimerization of the fullerene. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1209–1214

The non-geminate recombination of charge carriers in polymer-fullerene solar cells has been modeled in the last few years with a trap-assisted recombination model, which states that the apparent recombination order depends on the concentration of trapped charges tailing into the band gap. Higher concentrations of trapped charges lead to higher apparent recombination orders. In this work, the mass fraction f of highly crystalline nanofibrillar P3HT to the total P3HT content in P3HT:PCBM solar cells is consistently varied, controlling the temperature of a nanofibers-P3HT casting dispersion. A systematic study of the apparent recombination order, measured with a transient photovoltage technique, as a function of f is presented. A correlation is shown between the apparent recombination order, the P3HT crystallinity, and the trap concentration in the band gap measured with an admittance spectroscopy technique