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The dependence of the performance of OC(1)C(10)-PPV:PCBM (poly(2-methoxy-5-(3',7'-dimethyloetyloxy)-p-phenylene vinylene):methanofullerene [6,6]-phenyl C(61)-butyric acid methyl ester)-based bulk heterojunction solar cells on their composition has been investigated. With regard to charge transport, we demonstrate that the electron mobility gradually increases on increasing the PCBM weight ratio, up to 80 wt.-%, and subsequently saturates to its bulk value. Surprisingly, the hole mobility in the PPV phase shows an identical behavior and saturates beyond 67 wt.-% PCBM, a value which is more than two orders of magnitude higher than that of the pure polymer. The experimental electron and hole mobilities were used to study the photocurrent generation of OC(1)C(10)-PPV:PCBM bulk-heterojunction (BHJ) solar cells. From numerical calculations, it is shown that for PCBM concentrations exceeding 80 wt.-% reduced light absorption is responsible for the loss of device performance. From 80 to 67 wt.-%, the decrease in power conversion efficiency is mainly due to a decreased separation efficiency of bound electron-hole (e-h) pairs. Below 67 wt.-%, the performance loss is governed by a combination of a reduced generation rate of e-h pairs and a strong decrease in hole transport.

The performance of bulk-heterojunction solar cells based on a phase-separated mixture of donor and acceptor materials is known to be critically dependent on the morphology of the active layer. Here we use a combination of techniques to resolve the morphology of spin cast films of poly(p-phenylene vinylene)/methanofullerene blends in three dimensions on a nanometer scale and relate the results to the performance of the corresponding solar cells. Atomic force microscopy (AFM), transmission electron microscopy (TEM), and depth profiling using dynamic time-of-flight secondary ion mass spectrometry (TOF-SIMS) clearly show that for the two materials used in this study, 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) and poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV), phase separation is not observed up to 50 wt.-% PCBM. Nanoscale phase separation throughout the film sets in for concentrations of more than 67 wt.-% PCBM, to give domains of rather pure PCBM in a homogenous matrix of 50:50 wt.-% MDMO-PPV/PCBM. Electrical characterization, under illumination and in the dark, of the corresponding photovoltaic devices revealed a strong increase of power conversion efficiency when the phase-separated network develops, with a sharp increase of the photocurrent and fill factor between 50 and 67 wt.-% PCBM. As the phase separation sets in, enhanced electron transport and a reduction of bimolecular charge recombination provide the conditions for improved performance. The results are interpreted in terms of a model that proposes a hierarchical build up of two cooperative interpenetrating networks at different length scales.

A photoinduced electron-transfer reaction has been observed in three fullerene−donor−fullerene triads containing an electron-rich pyrrole ring in the donor moiety. The kinetics of charge separation in solution has been investigated by photoluminescence and transient absorption spectroscopy. The polarity of solvent and the distance between donor and acceptor affect the forward electron-transfer reaction, in qualitative agreement with a semiempirical model for the Gibbs free energy for charge separation. Depending on the conditions, charge separation occurs at a rate of 109−1010 s-1. The charge recombination rate is estimated to be faster than 2 × 1010 s-1. The relatively large contribution of the singlet-excited-state Sn ← S1 absorption of the fullerene moiety to the photoinduced absorption hampered a more accurate determination of the recombination rate.

A donor-donor-acceptor triad, OAn-OPV-C-60, With a redox gradient has been synthesized by covalently linking an oligoaniline (OAn), an oligo(p-phenylene vinylene) (OPV), and a fullerene (C-60) in a nonconjugated linear array. Photoluminescence and femtosecond pump-probe spectroscopy studies reveal that photoexcitation of any of the three chromophores of this triad in a polar solvent results in formation of the OAn(-)OPV(+)-C-60(-) charge-separated state, after an efficient ultrafast ( 1 ns) due to weak electronic coupling between the distant redox sites in the excited state. The stabilization gained is more than I order of magnitude in time. The experimental results are found to be in qualitative agreement with Marcus theory. In thin films, the OAn(+)-OPV-C-60(-) state is formed at a higher rate and in higher quantum yield than in solution.

The synthesis and full characterization of a quadruple bonded fullerene dimer using self-complementary 2-ureido-4[1H]pyrimidinone units with high dimerization constants is described. The chemical integrity of the monomeric moiety in either compound is fully preserved, also with respect to its redox and UV-Vis behavior. Two novel supramolecular dyads consisting of an oligo(p-phenylene vinylene) (OPV) donor and fullerene (C-60) acceptor are created via quadruple hydrogen bonding upon mixing the fullerene dimer and an OPV dimer. In these supramolecular dyads, singlet-energy transfer from the excited OPV unit to the fullerene causes a strong quenching of the OPV fluorescence. The high association constant of the 2-ureido-4[1H]-pyrimidinone quadruple hydrogen-bonding unit results in high quenching factors (Q(max) greater than or equal to 90). The lower limit obtained for the rate constant for energy transfer (k(EN) greater than or equal to 6 x 10(10) s(-1)) is rationalized in terms of the Forster mechanism.