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Two pairs of indacenodithiophene (IDT) and quinoxaline‐based copolymers with meta‐ or para‐hexyl‐phenyl side chains on the IDT unit are synthesized. The meta‐substituted polymers offer better solubility, higher molecular weight for both fluorinated and non‐fluorinated copolymers, and a superior photovoltaic performance with a power conversion efficiency of 7.8%. The side‐chain design strategy presented is an efficient way to produce high performance conjugated polymers for organic electronics. [ABSTRACT FROM AUTHOR]

The light collection in thin film polymer solar cells is substantially improved by application of a textured retroreflective foil, which reduces primary reflection and outcoupling of unabsorbed light. Consequently the external quantum efficiency (EQE) improves over the whole sensitivity range and the power conversion efficiency is improved by as much as 19%. [ABSTRACT FROM AUTHOR]

The photoresponse of P3HT:PC61BM based organic solar cells can be enhanced by blending the bulk heterojunction with the low band gap polymer Si- PCPDTBT. Organic solar cells containing the resulting ternary blend as the photoactive layer deliver short circuit currents of up to 15.5 mA cm−2. Morphological studies show modest phase separation without the perturbation of the crystallinity of the P3HT:PC61BM matrix, in accordance with the measured acceptable fill factors. Picosecond time-resolved pump-probe spectroscopy reveals that the sensitization of P3HT:PC61BM with Si-PCPDTBT involves the transfer of photogenerated positive polarons from the low band gap polymer to P3HT within few hundreds of picoseconds. Intensity dependent experiments in combination with global fitting show that the charge transfer from Si-PCPDTBT to P3HT competes with non-geminate charge carrier recombination of the holes in the Si-PCPDTBT phase with electrons in the PC61BM phase, both processes being of diffusive nature. At excitation densities corresponding to steady state conditions under one sun, modelling predicts hole transfer efficiencies exceeding 90%, in accordance with IQE measurements. At higher pump intensities, bimolecular recombination suppresses the hole transfer process effectively. [ABSTRACT FROM AUTHOR]

The performance of polymer solar cells critically depends on the morphology of the interface between the donor- and acceptor materials that are used to create and transport charge carriers. Solar cells based on poly(3-hexylthiophene) and ZnO were fully characterized in terms of their efficiency and three-dimensional (3D) morphology on the nanoscale. Here, we establish a quantitative link between efficiency and morphology by using the experimental 3D morphology as direct input for a 3D optoelectronic device model. This model includes the effects of exciton diffusion and quenching; space-charge; recombination, generation, drift and diffusion of charge carriers; and the injection/extraction of carriers at the contacts. The observed trend in internal quantum efficiency as a function of layer thickness is reproduced with a single set of parameters. Several morphological aspects that determine the internal quantum efficiency are discussed and compared to other organic solar cells. This first direct use of morphological data in an optoelectronic device model highlights the importance of morphology in solar cells. [ABSTRACT FROM AUTHOR]

We compare the opto-electronic and photovoltaic properties of two diketopyrrolopyrrole (DPP) based semiconducting polymers in which the DPP unit alternates along the chain with a conjugated bis(dithienyl)phenylene (4TP) unit. The two polymers differ only in the solubilizing substituents on the thiophene rings which are either alkyl (PDPP4TP) or alkoxy (PDPP4TOP) groups. We show that alkoxy groups lower the optical band gap and increase the ionization potential compared to the alkyl groups. As a result, PDDP4TOP provides a significantly higher charge generation efficiency and concomitant higher short-circuit current, 18.0 mA cm−2 vs. 12.4 mA cm−2, compared to PDPP4TP in optimized devices with [6,6]phenyl-C71-butyric acid methyl ester ([70]PCBM) as acceptor, but a simultaneous decrease in open circuit voltage, 0.51 vs. 0.67 V. The increased current arises from a higher external quantum efficiency and a wider spectral coverage. The net result is a small increase in power conversion efficiency from 5.8% for PDPP4TP to 6.0% for the PDPP4TOP in optimized devices. The optimized processing conditions and bulk heterojunction morphology are virtually identical for both photoactive layers. The study demonstrates that the side chains enable effective method for rationally designing new photoactive semiconducting polymers. [ABSTRACT FROM AUTHOR]

Three different theoretical approaches are presented to identify pathways to organic solar cells with power conversion efficiencies in excess of 20%. A radiation limit for organic solar cells is introduced that elucidates the role of charge-transfer (CT) state absorption. Provided this CT action is sufficiently weak, organic solar cells can be as efficient as their inorganic counterparts. Next, a model based on Marcus theory of electronic transfer that also considers exciton generation in both the electron donor and electron acceptor is used to show how reduction of the reorganization energies can lead to substantial efficiency gains. Finally, the dielectric constant is introduced as a central parameter for efficient solar cells. By using a drift-diffusion model, it is found that efficiencies of more than 20% are within reach. [ABSTRACT FROM AUTHOR]