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Optical pump-probe spectroscopy is a crucial tool to investigate the excited state behaviour of materials and is especially useful to investigate the photoexcitation dynamics of bulk heterojunctions as found in organic solar cells. Most common techniques for such investigations involve pulses with an energy density in the range of several tens of J cm(-2), emitted with kHz repetition rate. Such pulse energy can entail non-linear processes due to the formation of high carrier concentrations and can furthermore severely damage the sample material. Here we introduce a softer approach with pulses of nJ cm(-2) energy density and a photon flux which is more than three orders of magnitude lower than in regular techniques. We show the capability of our low-cost and easy to make set-up by investigating two prototypical donor-acceptor polymers, C-PCPDTBT and its silicon variant Si-PCPDTBT. Given the high pulse repetition rate in the MHz regime, we are readily able to monitor sample changes of 10(-5) and find exciton lifetimes of 108 and 150 ps for C- and Si-PCPDTBT respectively.

International audience; Advances in the synthesis and application of highly efficient polymers and small molecules over the last two decades have enabled the rapid advancement in the development of organic solar cells and photovoltaic technology as a promising alternative to conventional solar cells, based on silicon and other inorganic semiconducting materials. Among the different types of organic semiconducting materials, porphyrins and BODIPY-based small molecules and conjugated polymers attract high interest as efficient semiconducting organic materials for dye sensitized solar cells and bulk heterojunction organic solar cells. The highest power conversion efficiency exceeding 9% has been reported so far for porphyrin small molecules and 8.60% for conjugated polymers based on porphyrins. On the other hand, small molecules and conjugated polymers based on BODIPY moiety have been successfully used as donor materials for solution processed bulk heterojunction organic solar cells, and the resultant devices showed power conversion efficiencies exceeding 5.5%. In this article, the development of molecular design of porphyrins and BODIPY small molecules and polymers for bulk heterojunction organic solar cells are reviewed, and a guideline for the structure-performance relationship is provided.

In recent years, steady improvements in organic photovoltaic (OPV) performance have been reported with power conversion efficiency (PCE) reaching 9%. Despite this rapid improvement, the mechanisms limiting the existing performance are not completely known; therefore a better understanding of the loss processes is necessary. In this respect a crucial role may be played by the charge transfer (CT) state, which is the intermediate state between the excitons and the fully dissociated charges. In this review article we outline the dynamics of this intermediate state in light of their influence on the photovoltaic performance, with central emphasis on highly efficient polymer-fullerene bulk heterojunction solar cells.

We report on the comparison of photophysical and photovoltaic properties of three different soluble alkyl derivatives of the low bandgap poly(2,5-thienylene vinylene) (PTV), synthesized using the dithiocarbamate precursor route. The solubility of the precursor material in dichlorobenzene is enhanced by the addition of hexyl, dihexyl and dodecyl sidegroups to the polymer chain. The materials were characterized in solid state by means of absorption, ellipsometry, and atomic force microscopy of films made from both the pristine alkyl-PTVs and alkyl-PTVs:([6,6]-phenyl C-61-butyric acid methyl ester) (PCBM) mixtures in a 1:1 ratio. The materials showed an optical bandgap below 1.7 eV, derived from the absorption spectrum of the polymers. Field-effect transistors made of these materials showed hole mobilities in the range of 10(-7) to 10(-6) cm(2)/Vs. Bulk heterojunction solar cells made with the polymer:PCBM blend reached efficiencies above 0.6%. (C) 2008 Elsevier B.V. All rights reserved. ispartof: Organic electronics vol:9 issue:5 pages:740-746 status: published

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS applied materials and interfaces, copyright © American Chemical Society, after peer review and technical editing by the publisher and may be found at http://dx.doi.org/10.1021/acsami.5b04537 A 4-terminal architecture is proposed in which two thin active layers (

Organic solar cells now exceed 10% efficiency igniting interest not only in the fundamental molecular design of the photoactive semiconducting materials, but also in overlapping fields such as green chemistry, large-scale processing and thin film stability. For these devices to be commercially useful, they must have lifetimes in excess of 10 years. One source of potential instability, is that the two bicontinuous phases of electron donor and acceptor materials in the photoactive thin film bulk heterojunction, change in dimensions over time. Photocrosslinking of the π-conjugated semiconducting donor polymers allows the thin film morphology to be ‘locked’ affording patterned and stable blends with suppressed fullerene acceptor crystallization. This article reviews the performance of crosslinkable polymers, fullerenes and additives used to-date, identifying the most promising.

We report a thin and robust interconnecting layer (ICL) for polymer tandem solar cells. This ICL shows low absorption, good electrical contacts, large work function contrast and robustness. Its use yields tandem cells with a very high fill factor of 76%, making this ICL a promising component of future highly efficient multijunction organic solar cells.

Copyright 2015 Society of Photo Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this publication for a fee or for commercial purposes, or modification of the contents of the publication are prohibited. Over the last three decades, progress in the organic photovoltaic field has resulted in some device features which make organic cells applicable in electricity generation configurations where the standard silicon-based technology is not suitable, for instance, when a semi-transparent photovoltaic panel is needed. When the thin film solar cell performance is evaluated in terms of the device's visible transparency and power conversion efficiency, organic solar cells offer the most promising solution. During the last three years, research in the field has consolidated several approaches for the fabrication of high performance semi-transparent organic solar cells. We have grouped these approaches under three categories: devices where the absorber layer includes near-infrared absorption polymers, devices incorporating one-dimensional photonic crystals, and devices with a metal cavity light trapping configuration. We herein review these approaches.

The solid-state morphology and photo-generated charge carrier dynamics in low-bandgap polymer: fullerene bulk heterojunction photovoltaic blends using the donor-acceptor type copolymers PCPDTBT or its silicon-substituted analogue PSBTBT as donors are compared by two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) and femto-to microsecond broadband Vis-NIR transient absorption (TA) pump-probe spectroscopy. The 2D solid-state NMR experiments demonstrate that the film morphology of PCPDTBT:PC60BM blends processed with additives such as octanedithiol (ODT) are similar to those of PSBTBT:PC60BM blends in terms of crystallinity, phase segregation, and interfacial contacts. The TA experiments and analysis of the TA data by multivariate curve resolution (MCR) reveal that after exciton dissociation and free charge formation, fast sub-nanosecond non-geminate recombination occurs which leads to a substantial population of the polymer's triplet state. The extent to which triplet states are formed depends on the initial concentration of free charges, which itself is controlled by the microstructure of the blend, especially in case of PCPDTBT:PC60BM. Interestingly,PSBTBT:PC70BM blends show a higher charge generation efficiency, but less triplet state formation at similar free charge carrier concentrations. This indicates that the solid-state morphology and interfacial structures of PSBTBT:PC70BM blends reduces non-geminate recombination, leading to superior device performance compared to optimized PCPDTBT:PC60BM blends.