There has been an intensive search for cost-effective photovoltaics since the development of the first solar cells in the 1950s. [1][2][3] Among all alternative technologies to silicon-based pn-junction solar cells, organic solar cells could lead the most significant cost reduction. [4] The field of organic photovoltaics (OPVs) comprises organic/inorganic nanostructures like dyesensitized solar cells, multilayers of small organic molecules, and phase-separated mixtures of organic materials (the bulkheterojunction solar cell). A review of several OPV technologies has been presented recently. [5] Light absorption in organic solar cells leads to the generation of excited, bound electronhole pairs (often called excitons). To achieve substantial energy-conversion efficiencies, these excited electron-hole pairs need to be dissociated into free charge carriers with a high yield. Excitons can be dissociated at interfaces of materials with different electron affinities or by electric fields, or the dissociation can be trap or impurity assisted. Blending conjugated polymers with high-electron-affinity molecules like C 60 (as in the bulk-heterojunction solar cell) has proven to be an efficient way for rapid exciton dissociation. Conjugated polymer-C 60 interpenetrating networks exhibit ultrafast charge transfer (∼40 fs). [6,7] As there is no competing decay process of the optically excited electron-hole pair located on the polymer in this time regime, an optimized mixture with C 60 converts absorbed photons to electrons with an efficiency close to 100 %. [8] The associated bicontinuous interpenetrating network enables efficient collection of the separated charges at the electrodes. The bulk-heterojunction solar cell has attracted a lot of attention because of its potential to be a true low-cost photovoltaic technology. A simple coating or printing process would enable roll-to-roll manufacturing of flexible, low-weight PV modules, which should permit cost-efficient production and the development of products for new markets, e.g., in the field of portable electronics. One major obstacle for the commercialization of bulk-heterojunction solar cells is the relatively small device efficiencies that have been demonstrated up to now.[5] The best energy-conversion efficiencies published for small-area devices approach 5 %. [9][10][11] A detailed analysis of state-of-the-art bulk-heterojunction solar cells [8] There has long been a controversy about the origin of the V oc in conjugated polymer-fullerene solar cells. Following the classical thin-film solar-cell concept, the metal-insulator-metal (MIM) model was applied to bulk-heterojunction devices.In the MIM picture, V oc is simply equal to the work-function difference of the two metal electrodes. The model had to be modified after the observation of the strong influence of the reduction potential of the fullerene on the open-circuit voltage by introducing the concept of Fermi-level pinning. [12] It has also been shown that the V oc of polymer-fullerene solar cells is affected by t...

Bulk heterojunction solar cells have attracted considerable attention over the past several years due to their potential for low-cost photovoltaic technology. The possibility of manufacturing modules via a standard printing/coating method in a roll-to-roll process in combination with the use of low-cost materials will lead to a watt-peak price of less than 1 US$ within the next few years. [1] Despite the low-cost potential, the power conversion efficiency of bulk heterojunction devices is low compared to inorganic solar cells. Efficiencies in the range of 5-6% have been certified at NREL and AIST usually on devices with small active areas.[2]The current understanding of bulk heterojunction solar cells suggests that the maximum efficiency is in the range of 10-12%.[3] Several reasons for the power conversion efficiency limitation have been identified.[1] Some of the prerequisites for achieving highest efficiencies are donor and acceptor materials with optimized energy levels [highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)], efficient charge transport in the donor-acceptor blend, efficient charge generation and limited recombination losses. Power conversion efficiency is strongly dependent on charge transport and charge generation, which are dominated by the phase behavior of the donor and acceptor molecules. The resulting, and often unfavorable, nanomorphology of this two-component blend limits the power conversion efficiency of bulk heterojunction solar cells. Precise control of the nanomorphology is very difficult and has been achieved only for a few systems. [4][5][6] The relation between the chemical structure of donor and acceptor materials and the nanomorphology that they form when they are blended is currently not well understood, and as will be shown in this paper, minor changes in the chemical structure can cause major changes in the performance of the materials in organic solar cells.

The spectroscopic response of a poly(3‐hexylthiophene)/[6,6]‐phenyl‐C61‐butyric acid methyl ester (P3HT/PCBM)‐based bulk heterojunction solar cell is extended into the near infrared region (NIR) of the spectrum by adding the low bandgap polymer 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] to the blend. The dominant mechanism behind the enhanced photosensitivity of the ternary blend is found to be a two‐step process: first, an ultrafast and efficient photoinduced charge transfer generates positive charges on P3HT and PCPDTBT and a negative charge on PCBM. In a second step, the positive charge on PCPDTBT is transferred to P3HT. Thus, P3HT serves two purposes. On the one hand it is involved in the generation of charge carriers by the photoinduced electron transfer to PCBM, and, on the other hand, it forms the charge transport matrix for the positive carriers transferred from PCPDTBT. Other mechanisms, such as energy transfer or photoinduced charge transfer directly between the two polymers, are found to be absent or negligible.

P3HT is a semirigid and semicrystalline polymer with a strong tendency to postcrystallize in solid films upon thermal annealing. Recent investigations showed that macroscopic, crystalline P3HT fibers can already be precipitated from solution, depending on the combination of solvent systems used. In this paper we investigate the mechanism and the dynamics of gelation in P3HT solutions. Rheological and absorption measurements in solution reveal a distinct two-step mechanism. In a first step, aggregates are formed which, in a second step, link to each other into a thermoreversible gel. The correlation between the gelation dynamics and the molecular weight distribution is discussed in more detail for a nonhazardous, printing friendly solvent system.

The UV/vis spectra were measured with a JASCO V-550 UV-Vis spectrophotometer, the emission spectra with a CARY Eclipse fluorescence spectrophotometer. IR spectra were recorded on a JASCO FT/IR-4200 Fourier Transform spectrometer. NMR spectra ( 1 H and 13 C) were measured with a Bruker ARX 400 spectrometer using solutions in CDCl 3 ; J values are given in Hz. Mass spectra were obtained using a Varian MAT 311A instrument with an electro spray ionization source (ESIMS). The microwave-assisted synthesis was carried out in a Discover reaction unit (CEM) using sealed reaction vials. The temperature inside the vial was monitored by an IR sensor; the pressure by a hydraulic system. The polymerizations were performed under temperature control with a maximum microwave power of 300 W. MaterialsAll reactions were carried out under an argon atmosphere using the usual Schlenk techniques.TLC was carried out on dry silica gel plates. For liquid chromatography, silica gel with a pore size 0.06-0.2 nm was used. All solvents were of reagent grade and used as received, unless otherwise specified. 5,8-Dibromo-2,3-dioctylquinoxaline, 1 4,4-bis(2-ethylhexyl)-2,6bis(trimethylstannyl)-4H-cyclopenta- [2,1-b:3,4-b`]dithiophene 2-4 , 2-(tri-n-butylstannyl)thiophene, 5 and 4,7-dibromobenzo-2,1,3-thiadiazole 6 were prepared according to literature procedures. Synthesis 4,7-Bis(thiophen-2-yl)benzo-2,1,3-thiadiazole 4,7-Dibromobenzo-2,1,3-thiadiazole (17.01 mmol, 5 g), 2-(tri-n-butylstannyl)thiophene (37.4 mmol, 13.96 g), KF (136 mmol, 7.91 g) and PdCl 2 (PPh 3 ) 2 (1.36 mmol, 0.955 g) were placed in a 100 ml-Schlenk tube. After addition of dry THF (100 ml) the reaction mixture was stirred for 48 h at 80 °C. Then, the reaction mixture was poured into chloroform (200 ml). The organic phase was washed with water (2 × 200 ml), dried over anhydrous MgSO 4 and the solvent removed under reduced pressure. Purification by column chromatography (silica gel, toluene/n-hexane 1/4 v/v) gave 2.19 g (7.29 mmol; 42.9 %) of red crystals.

The concept of Hansen solubility parameters (HSP) is applied to organic semiconductors in order to determine and predict their solubility behavior, which is essential for the design of functional and environmentally friendly ink formulations for organic photovoltaics. Two different conjugated polymers, one semicrystalline and one dominantly amorphous, and one fullerene derivative are selected as prototype candidates to evaluate the applicability of the HSP concept for organic semiconductors. The method for determining the solubility parameters is described and the quality of the HSP fits as well as their suitability for designing of organic electronic inks are discussed in detail.magnified image

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The photoresponse of P3HT:PC 61 BM 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:PC 61 BM matrix, in accordance with the measured acceptable fi ll factors. Picosecond time-resolved pump-probe spectroscopy reveals that the sensitization of P3HT:PC 61 BM 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 fi tting 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 PC 61 BM phase, both processes being of diffusive nature. At excitation densities corresponding to steady state conditions under one sun, modelling predicts hole transfer effi ciencies exceeding 90%, in accordance with IQE measurements. At higher pump intensities, bimolecular recombination suppresses the hole transfer process effectively. 950 wileyonlinelibrary.com