It was shown that ESR spectroscopy is a very useful technique for monitoring the photochemical and thermal degradation of conjugated polymers commonly used in organic solar cells. The relative stability of materials can be quantified by comparing the rates of trap accumulation (dC(R)/dt) estimated from their ESR profiles.

In view of a rapid development and increase in efficiency of organic solar cells, reaching their long‐term operational stability represents now one of the main challenges to be addressed on the way toward commercialization of this photovoltaic technology. However, intrinsic degradation pathways occurring in organic solar cells under realistic operational conditions remain poorly understood. The light‐induced dimerization of the fullerene‐based acceptor materials discovered recently is considered to be one of the main causes for burn‐in degradation of organic solar cells. In this work, it is shown that not only the fullerene derivatives but also different types of conjugated polymers and small molecules undergo similar light‐induced crosslinking regardless of their chemical composition and structure. In the case of conjugated polymers, crosslinking of macromolecules leads to a rapid increase in their molecular weight and consequent loss of solubility, which can be revealed in a straightforward way by gel permeation chromatography analysis via a reduction/loss of signal and/or smaller retention times. Results of this work, thus, shift the paradigm of research in the field toward designing a new generation of organic absorbers with enhanced intrinsic photochemical stability in order to reach practically useful operation lifetimes required for successful commercialization of organic photovoltaics.

Here we report the application of the Electron Spin Resonance (ESR) spectroscopy as a highly sensitive analytical technique for assessment of the electronic quality of organic semiconductor materials, particularly conjugated polymers. It has been shown that different batches of the same conjugated polymer might contain substantially different amounts of radical species which were attributed to structural defects and/or impurities behaving as traps for mobile charge carriers. Good correlations between the concentrations of radicals in various batches of conjugated polymers and their performances in organic solar cells have been revealed.

We show that extended TBTBT structure (T = thiophene, B = benzothiadiazole) can be used as an electrondeficient building block for designing conjugated polymers with deeply lying HOMO energy levels and narrow band gaps. The first carbazole−TBTBT copolymer P2 demonstrated power conversion efficiencies exceeding 6% in bulk heterojunction solar cells in combination with advanced operational stability, unlike conventional donor polymers such as PTB7, PBDTTT-CF, etc.

We report the synthesis and comparative study of five conjugated polymers P1-P5 with different arrangements of two solubilizing 2-ethlyhexyl side chains in their repeating units. It has been shown that positioning of the alkyl substituents affects strongly frontier energy levels of the polymers and results in variation of their optical band gaps between 1.65 and 2.0 eV. The highest (2.8×10 -4 cm 2. V -1. s -1 ) and the lowest (2.4×10 -5 cm 2. V -1. s -1 ) charge carrier mobilities determined for the polymers P1 and P5, respectively, using SCLC technique differ by more than one order of magnitude. GIWAXS measurements also revealed very different degrees of molecular ordering in the films of polymers P1-P5 which correlate well with the SCLC mobility data and solid state photoluminescence spectra of these materials. The bulk heterojunction solar cells based on the composites of polymers P1-P5 with [60]PCBM demonstrated power conversion efficiencies ranging from 0.6% (P5) to 5.1% (P1) thus evidencing strong influence of the alkyl side chains on the photovoltaic performance of the designed polymer-based materials. This journal is Electronic Supplementary Information (ESI) available: Optical absorption spectra for thin films, cyclic voltammograms, J 1/2 -V plots for hole-only This journal is ARTICLE Journal Name devices, 2D GIWAXS patterns for thin films of P1-P5, AFM images for composites of P1-P5 with [60]PCBM. See