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A new alternating copolymer of dithienosilole and thienopyrrole-4,6-dione (PDTSTPD) possesses both a low optical bandgap (1.73 eV) and a deep highest occupied molecular orbital energy level (5.57 eV). The introduction of branched alkyl chains to the dithienosilole unit was found to be critical for the improvement of the polymer solubility. When blended with PC(71)BM, PDTSTPD exhibited a power conversion efficiency of 7.3% on the photovoltaic devices with an active area of 1 cm(2).

The power conversion effi ciency of organic photovoltaic (OPV) cells has increased from 4-5% in 2005 [ 1 , 2 ] to 7.4% [ 3 ] and 8.3% [ 4 ] in 2010. The goal of a 10% single junction OPV device seems attainable [ 5 ] making the commercialization of OPV more realistic. With advances made on the effi ciency front, the lifetime and reliability of OPV devices has come into focus. [ 6 , 7 ] To date there has been considerable work done in understanding and quantifying the lifetime and degradation of bulk heterojunction solar cells (BHJs) based on poly( para -phenylene vinylene) (PPV) [8][9][10][11] and poly(3-hexylthiophene) (P3HT) polymers. [12][13][14][15] A comparison of OPV lifetime experimental results across different research groups has posed challenges due to the lack of standardized testing and reporting procedures; however, great strides were made in this regard during the most recent International Summit on OPV Stability (ISOS-3). Modules based on P3HT/fullerene BHJs have shown lifetimes of 5000 h when state-of-the-art encapsulation with a glass-on-glass architecture is used. [ 16 ] Assuming negligible degradation in the dark and 5.5 h of one-sun intensity per day, 365 days per year, this translates into an operating lifetime approaching three years. More recently P3HT/PCBM devices utilizing an inverted architecture have been shown to retain more than 50% of their initial effi ciency after 4700 h of continuous exposure to one-sun intensity at elevated temperatures [ 17 ] and have exhibited a long shelf-life when stored in the dark in ambient conditions. [ 18 , 19 ] However, results to date have yet to show polymer based OPV lifetimes greater than 3-4 years.Here we present a detailed operating lifetime study of encapsulated solar cells comprising poly[9'-hepta-decanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole) (PCDTBT) in BHJ composites with the fullerene derivative [6,6]-phenyl C 70 -butyric acid methyl ester (PC 70 BM). PCDTBT/ PC 70 BM solar cells achieved an effi ciency greater than 6%, [ 20 ] making this one of a small number of polymers able to achieve this level of performance. We describe an experimental set-up that is capable of testing large numbers of solar cells simultaneously, holding each device at its maximum power point while controlling and monitoring the temperature and light intensity. Using this set-up we were able to compare the PCDTBT/PC 70 BM system with the well-studied P3HT/PCBM system and demonstrate a lifetime for PCDTBT devices that approaches 7 years, which is the longest reported operating lifetime for a polymer-based solar cell. Figure 1 shows a typical effi ciency decay pattern for polymer/ fullerene BHJs employing a standard architecture with an organic hole-transporting layer as the anode (e.g., PEDOT:PSS) and a metal cathode (e.g., Ca/Al). [ 14 , 15 ] One typically observes a burn-in period characterized by an exponential loss in effi ciency whose magnitude and duration can vary by polymer system, followed by a linear decay period that someti...

Direct (hetero)arylation polymerization (DHAP) has recently been established as an environmentally benign method for the preparation of conjugated polymers. This synthetic tool features the formation of C-C bonds between halogenated (hetero)arenes and simple (hetero)arenes with active C-H bonds, thereby circumventing the preparation of organometallic derivatives and decreasing the overall production cost of conjugated polymers. Since its inception, selectivity and reactivity of DHAP procedures have been improved tremendously through the careful scrutinity of polymerization outcomes and the fine-tuning of reaction conditions. A broad range of monomers, from simple arenes to complex functionalized heteroarenes, can now be readily polymerized. The successful application of DHAP now leads to nearly defect-free conjugated polymers possessing comparable, if not slightly better, characteristics than their counterparts prepared using classical cross-coupling methods. This comprehensive review describes the mechanisms involved in this process from experimental and theoretical standpoints, presents an up-to-date compendium of materials obtained by this means, and exposes its current limitations and challenges.

Narrow bandgap n‐type organic semiconductors (n‐OS) have attracted great attention in recent years as acceptors in organic solar cells (OSCs), due to their easily tuned absorption and electronic energy levels in comparison with fullerene acceptors. Herein, a new n‐OS acceptor, Y5, with an electron‐deficient‐core‐based fused structure is designed and synthesized, which exhibits a strong absorption in the 600–900 nm region with an extinction coefficient of 1.24 × 105 cm−1, and an electron mobility of 2.11 × 10−4 cm2 V−1 s−1. By blending Y5 with three types of common medium‐bandgap polymers (J61, PBDB‐T, and TTFQx‐T1) as donors, all devices exhibit high short‐circuit current densities over 20 mA cm−2. As a result, the power conversion efficiency of the Y5‐based OSCs with J61, TTFQx‐T1, and PBDB‐T reaches 11.0%, 13.1%, and 14.1%, respectively. This indicates that Y5 is a universal and highly efficient n‐OS acceptor for applications in organic solar cells.

The ongoing depletion of fossil fuels has led to an intensive search for additional renewable energy sources. Solar-based technologies could provide sufficient energy to satisfy the global economic demands in the near future. Photovoltaic (PV) cells are the most promising man-made devices for direct solar energy utilization. Understanding the charge separation and charge transport in PV materials at a molecular level is crucial for improving the efficiency of the solar cells. Here, we use light-induced EPR spectroscopy combined with DFT calculations to study the electronic structure of charge separated states in blends of polymers (P3HT, PCDTBT, and PTB7) and fullerene derivatives (C60-PCBM and C70-PCBM). Solar cells made with the same composites as active layers show power conversion efficiencies of 3.3% (P3HT), 6.1% (PCDTBT), and 7.3% (PTB7), respectively. Under illumination of these composites, two paramagnetic species are formed due to photo-induced electron transfer between the conjugated polymer and the fullerene. They are the positive, P+, and negative, P-, polarons on the polymer backbone and fullerene cage, respectively, and correspond to radical cations and radical anions. Using the high spectral resolution of high-frequency EPR (130 GHz), the EPR spectra of these species were resolved and principal components of the g-tensors were assigned. Light-induced pulsed ENDOR spectroscopy allowed the determination of 1H hyperfine coupling constants of photogenerated positive and negative polarons. The experimental results obtained for the different polymer-fullerene composites have been compared with DFT calculations, revealing that in all three systems the positive polaron is distributed over distances of 40 - 60 Å on the polymer chain. This corresponds to about 15 thiophene units for P3HT, approximately three units PCDTBT, and about three to four units for PTB7. No spin density delocalization between neighboring fullerene molecules was detected by EPR. Strong delocalization of the positive polaron on the polymer donor is an important reason for the efficient charge separation in bulk heterojunction systems as it minimizes the wasteful process of charge recombination. The combination of advanced EPR spectroscopy and DFT is a powerful approach for investigation of light-induced charge dynamics in organic photovoltaic materials.

A series of low‐bandgap alternating copolymers of dithienosilole and thienopyrrolodione (PDTSTPDs) are prepared to investigate the effects of the polymer molecular weight and the alkyl chain length of the thienopyrrole‐4,6‐dione (TPD) unit on the photovoltaic performance. High‐molecular‐weight PDTSTPD leads to a higher hole mobility, lower device series resistance, a larger fill factor, and a higher photocurrent in PDTSTPD:[6,6]‐phenyl C71 butyric acid methyl ester (PC71BM) bulk‐heterojunction solar cells. Different side‐chain lengths show a significant impact on the interchain packing between polymers and affect the blend film morphology due to different solubilities. A high power conversion efficiency of 7.5% is achieved for a solar cell with a 1.0 cm2 active area, along with a maximum external quantum efficiency (EQE) of 63% in the red region.