With state‐of‐the‐art organic solar cells (OSCs) surpassing 16% efficiency, stability becomes critical for commercialization. In this work, the power of using photoluminescence (PL) measurements on plain films is demonstrated, as well as high‐performance liquid chromatography analysis to reveal the origin of UV instabilities in OSCs based on the most commonly used acceptors PC70BM ([6,6]‐phenyl‐C71‐butyric acid methyl ester), ITIC (3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene), and o‐IDTBR (indacenodithiophene‐based non‐fullerene acceptor). The UV dependent stability tests reveal instabilities in solar cells based on PC70BM and ITIC while devices based on o‐IDTBR are highly stable even under UV illumination. The analysis of solar cell devices based on charge extraction and sub‐bandgap external quantum efficiency only shows the UV‐dependent emergence of traps, while PL spectra of plain films on glass allows the disentanglement and identification of individual instabilities in multi‐component bulk‐heterojunction devices. In particular, the PL analysis demonstrates UV instabilities of PC70BM and ITIC toward the processing additive 1,8 diiodooctane (DIO). The chemical analysis reveals the in‐depth mechanism, by providing direct proof of photochemical reactions of PC70BM and ITIC with UV‐induced radicals of DIO. Based on this scientific understanding, it is shown how to stabilize PBQ‐QF:PC70BM devices.

cells, [1][2][3][4] but also other types based on small molecule:fullerene [5,6] blends as well as solar cells incorporating singlewalled carbon nanotubes (SWCNTs) [7][8][9][10][11][12][13][14] were investigated with increased efforts. All of these new concepts have been introduced to tackle several drawbacks and outstanding issues related to polymer:fullerene solar cells. [15] Concepts such as small molecule:fullerene active layers aim to use simple, well-defined molecules instead of polymers, which always exhibit a distribution in chain length altering their properties. The application of nonfullerene acceptors allows for more freedom to align the energy levels at the donor-acceptor interface. The incorporation of SWCNTs in OSCs or other organic devices [16][17][18][19] (near-infrared (nIR) detectors, nIR lightemitting diodes (LEDs), and field-effect transistors (FETs)) is based on several advantageous key factors that allow to boost device performances. First, SWCNTs have shown to exhibit a high photochemical stability making them superior to conventional polymers. Second, SWCNTs exhibit a very high charge carrier mobility outperforming conventional organic materials by orders of magnitude. [20,21] The high absorption coefficients in the nIR wavelength regime [22,23] make SWCNTs well suited to IR-sensitive OSCs via a ternary concept or in a binary blend with SWCNTs as the main absorber in the IR region.Previously it was shown that SWCNTs work in a type-II heterojunction scheme acting either as an electron acceptor in combination with a polymer or as the electron donor in combination with C 60 or [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). [24] Concerning the combination with C 60 , it was already demonstrated that charge carrier generation and harvesting is highly efficient with internal quantum efficiencies (IQEs) in the range of 85%. [25] The use of nearly monochiral SWCNTs in a bilayer architecture with C 60 facilitated high fill factors (FFs) greater than 60% and a high peak external quantum efficiency (EQE) of 43% at 1050 nm. Employing a bulk heterojunction (BHJ) architecture in combination with multichirality SWCNTs achieved a broad absorption in the nIR with power conversion efficiencies (PCEs) surpassing 3% for all-carbon allotrope absorbers. [24,26] In a recent study performed by Shea et al. [27] on nearly single-chirality (6,5) Current state-of-the-art organic solar cells (OSCs) still suffer from high losses of open-circuit voltage (V OC ). Conventional polymer:fullerene solar cells usually exhibit bandgap to V OC losses greater than 0.8 V. Here a detailed investigation of V OC is presented for solution-processed OSCs based on (6,5) single-walled carbon nanotube (SWCNT): [6,6]-phenyl-C 71 -butyric acid methyl ester active layers. Considering the very small optical bandgap of only 1.22 eV of (6,5) SWCNTs, a high V OC of 0.59 V leading to a low E gap /q − V OC = 0.63 V loss is observed. The low voltage losses are partly due to the lack of a measurable charge transfer state and partly due to th...