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Organic semiconductors are widely investigated for their application in photovoltaics and photodetectors. We show that the efficiency of these devices is strongly influenced by the position of the space charge region, due to unintentional doping, and wavelength-dependent absorption properties in bulk heterojunctions. Spray-coated P3HT:PCBM bulk heterojunction photodiodes with thicknesses up to 4.2 μm and semitransparent top contact enable the characterization of exciton generation and separation in both irradiation directions. A large difference in external quantum efficiency (EQE) is observed for top and bottom illuminated configurations and is explained by a bias dependent arrangement of the space charge region at the two contact electrodes. Numerical drift–diffusion simulations allow to get insight into first order mechanisms behind the spectral features of EQE data in highly-doped organic photodiodes. [ABSTRACT FROM AUTHOR]

Abstract: With the aid of optical modeling, the internal quantum efficiencies of organic Bulk Heterojunction (oBHJ) photovoltaic devices based on low band gap polymer of poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,5′-diyl] (Si-PCPDTBT) blended with the acceptors of 1-(3-Methoxycarbonyl) propyl-1-phenyl [6,6] C61 (PCBM) and bis–adduct (bis–PCBM) are determined. The Si-PCPDTBT:bis–PCBM devices show considerably lower short circuit current density (J sc) as compared to the Si-PCPDTBT:PCBM devices. The results show that 30% of this smaller J sc is due to the lower optical absorption of bis–PCBM, while the major losses originate from the electrical losses. It is found that for the best Si-PCPDTBT:bis–PCBM devices with an active layer thickness in the range of 70–100nm, the inefficient charge generation within the bis–PCBM domains is the major contribution to the whole losses. Increasing the active layer thickness of Si-PCPDTBT:bis–PCBM device significantly enhances recombination losses in polymer/bis–fullerene matrix. [Copyright &y& Elsevier]

We propose a model for short-channel organic thin-film transistors, which accounts for Poole-Frenkel field-dependent mobility and space-charge-limited current effects. The model is developed for devices operating in the linear regime, as well as in depletion and saturation regimes. Super linear output curves for low drain voltages, as well as nonsaturating currents, can be adequately described. Experimental results for short-channel P3HT devices have been fitted, showing good agreement with the proposed model. [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]

Abstract: We have studied organic bulk heterojunction photovoltaic devices based on a bridged-bithiophene donor–acceptor type low-band gap polymer blended with PCBM and bis-PCBM. The impact of the molecular arrangement is discussed in terms of the correlation between the solar-cell performance and the degree of crystallization. Differential scanning calorimetry (DSC) and grazing-incidence X-ray diffraction (GIXRD) prove that films with bis-PCBM typically result in more amorphous blends than comparable films with PCBM. Electron tomography (ET) is used to visualize the three dimensional morphology of photoactive layers, confirming the presence of nanofibers, formed in different scales through the thickness in the blended films with mono and bis-fullerenes. [Copyright &y& Elsevier]

The chemical design of a polymer can be tailored by a random or a block sequence of the comonomers in order to influence the properties of the final material. In this work, two sequences, PCPDTBT and F8BT (F8), were polymerized to form a block or a random copolymer. Differences between the various polymers were examined by exploring the surface topography and charge carrier mobility. A distinct surface texture and a higher charge carrier mobility was found for the block copolymer with respect to the other materials. Solar cells were prepared with polymer:PC71BM blend active layers and the best performance of up to 2% was found for the block copolymer, which was a direct result of the fill factor. Overall, the sequences of different copolymers for solar cell applications were varied and a positive impact on efficiency was found when the block copolymer structure was utilized. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 [ABSTRACT FROM AUTHOR]

The effect of the addition of 1,8-octanedithiol (ODT) during processing on the microstructure of blend films of poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]dithiophene)- alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C71 butyric acid methyl ester ([70]PCBM) is studied. Grazing incidence X-ray diffraction and absorption spectroscopy show that the crystalline order of PCPDTBT increases when ODT is introduced in the solution phase either to neat polymer systems or to blends with [70]PCBM. The increased crystalline order is accompanied by less dispersive hole transport in the polymer, and leads to a more efficient formation of a percolating fullerene network within the blend. This contributes to an increase in photocurrent generation. However, the bimolecular recombination rate as determined from photovoltage transients increases upon addition of ODT, limiting the power conversion efficiency to values well below those expected from the energy levels of PCPDTBT. We propose some explanations for this increase in bimolecular recombination, based also on variable angle spectroscopic ellipsometry measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011 [ABSTRACT FROM AUTHOR]

We designed and synthesized a series of conjugated polymers containing alternating 4H-cyclopenta[2,1-b:3,4-b']dithiophene units and comonomers consisting of 2,2'-bithiophene, 3'', 4'' -dihexyl-α -pentathiophene, 3,4-ethylenedioxythiophene and 5,5' -bis(2-thienyl)-4,4' -dihexyl-2,2' -bithiazole. These polymers possess optical bandgaps in the range of 1.75 to 2.0 eV. The desirable absorption attributes of these materials make then excellent candidates for use in photovoltaic cells. Electrochemical studies indicate desirable HOMO-LUMO levels for use with fullerene derivatives as electron transporters. Field effect transistors made of these materials show hole mobilities in the range of 7.5 × 10-4 cm2/Vs to 2.0 × 10-3 cm2/Vs. Due to the combination of these characteristics, power conversion efficiencies up to 3.1% were achieved on devices made of bulk heterojunction composites of these materials with soluble fullerene derivatives. [ABSTRACT FROM AUTHOR]