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.

This, combined with little material consumption (≈1 g organic semiconductor per m 2 ), low-temperature processing and the compatibility with flexible substrates enables light-weight devices made in roll-toroll production and a large versatility in applications. This could make OSC the cheapest source of electricity in the world.The main difference in operation between silicon solar cells and OSC, and the reason that OSCs lag silicon solar in their commercialization, is that light absorption in organic semiconductor thin films does not lead to efficient generation of free charge carriers, but to the generation of strongly bound excitons having limited diffusion length. A solution to the former was published by Tang in 1986: [6,7] efficient exciton separation was achieved at the planar interface between two different organic semiconductors, an electron donor and an electron acceptor, leading to a type II heterojunction. The key concept for overcoming the limited exciton diffusion length was introduced by Hiramoto et al. [8] in 1991 by co-evaporating donor and acceptor molecules, leading to a bulk heterojunction (BHJ) with a distributed donor-acceptor interface. Here, each exciton is generated within its diffusion length of the heterojunction. Such BHJs are at the core of all efficient OSC today, independent whether they are processed in vacuum or made via solution-based processes. A typical OSC stack structure, along with a monolithic series connection of subcells into a module, is shown in Figure 1.Continuous research and development of organic semiconductors tailored for OSC, of processing techniques and stack design, have led to materials with better absorption and donor-acceptor energy offsets, [9,10] optimization of the BHJ microstructure, [11,12] and stack design, [13,14] pushing power conversion efficiencies (PCEs) to around 18% in single-junction solar cells. [15] PCEs of >20% appear to be within reach and module efficiencies are catching up with these values.PCEs of OSC devices and modules are important, but due to the further balance of system cost in a photovoltaic system [16] a high PCE alone is not sufficient for OSC to contribute at scale to solving climate change. For this, sufficient lifetime and scalability to terawatts of installed capacity at competitive cost are required, as well. In the following, we highlight some of the key research challenges, discuss OSC markets, and give an outlook on the transformative potential of OSC in terms of cost and carbon emissions. Research Challenges Voltage LossesOSCs can achieve short circuit current densities [22] and fill factors [23] on par with the ones of GaAs or perovskite-based devices 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 buildi...

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Near-infrared (NIR) light detection is key to an ever-growing demand for technical solutions in applications such as surveillance systems, facial recognition, industrial sorting and inspection, pulse oximetry, optical coherence tomography, and