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 polymer solar cells made from a novel low‐bandgap polymer show the highest photocurrent response so far for this class of materials (see figure). Efficiencies up to 3.2 % are realized, but this conjugated polymer has the intrinsic capability to reach 7 % efficiency because of its material properties. Possible loss mechanisms and improvements are discussed.

We designed and synthesized a series of conjugated polymers containing alternating electrondonating and electron-accepting units based on (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene), 4,7-(2,1,3)-benzothiadiazole, and 5,5′-[2,2′]bithiophene. These polymers possess an optical band gap as low as 1.4 eV (i.e., in the case of poly [2,6-(4,4-bis(2-ethylhexyl)), and their absorption characteristics can be tuned by adjusting the ratio of the two electrondonating units: (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene) and 5,5′-[2,2′]bithiophene. The desirable absorption attributes of these materials qualify them as excellent candidates for light-harvesting materials in organic photovoltaic applications allowing for high short-circuit current. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high photovoltaic device open-circuit voltage when fullerene derivatives are used as electron transporters. Field-effect transistors made of these materials show hole mobility in the range of 5 × 10 -4 -3 × 10 -3 cm 2 /(V s), which promises good device fill factor. Because of the combination of these characteristics, power conversion efficiencies up to 3.5% and an external quantum efficiency of at least 25% between 400 and 800 nm with a maximum of 38% around 700 nm were achieved on devices made of bulk heterojunction composites of these materials with soluble fullerene derivatives. Further improvement of the materials will include the modification of both the side chains and the backbone to effect change to the active layer morphology to maintain good charge carrier mobility in the composite.

The design of novel conjugated polymers suitable for use in plastic solar cells is one of today's challenges aiming towards improved key properties like the increase of photocurrent and open circuit voltage of such devices. In this work we present first results on arylene-ethynylene/arylene-vinylene hybrid polymers 3 (poly(-2,5-dioctyloxy-1,4-phenylene-diethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2'-ethyl)hexyloxy-1,4-phenylene-vinylene)) and 5 (poly(2,5-dioctyloxy-1,4-phenylene-ethynylene-9,10-anthracenylene-ethynylene-2,5-dioctyloxy-1,4-phenylene-vinylene-2,5-di(2'-ethyl)hexyloxy-1,4-phenylene-vinylene)), demonstrating photovoltaic action in combination with the soluble C 60 derivative 1-(3-methoxycarbonyl) propyl-1-phenyl [6,6]C 61 (PCBM). Devices with an active layer thickness of about 100 nm yielded power conversion efficiencies of up to 2% under 100 mW cm 22 AM 1.5 white light illumination. The coarse grained morphology of the active layers was identified as the main limitation for the photocurrent, revealed by AFM measurements. The photovoltaic devices were characterized by current-voltage and spectral photocurrent measurements. The results show that the open circuit voltage is weakly dependent on the HOMO (highest occupied molecular orbital) level of the conjugated polymer used as donor.{ Electronic supplementary information (ESI) available: IR spectrum of polymer 5. See

Alkoxy-substituted CN-containing phenylene−vinylene−alt-phenylene−ethynylene hybrid polymers (CN−PPV−PPE), 3a, 3b, and 7a, were obtained from luminophoric dialdehydes 1 by step growth polymerization via Knoevenagel reaction as high molecular-weight materials. Corresponding CN-free polymers 3c and 7b and an ethynylene-free polymer 5 with similar side chains were synthesized for the purpose of comparison. The chemical structures of the polymers were confirmed by IR, 1H and 13C NMR, and elemental analysis. Thermal characterization was conducted by means of thermogravimetric analysis and differential scanning calorimetry. Morphology was investigated by means of optical microscopy and small-angle light scattering. The final morphologies are determined by the molecular characteristics (side chains volume fraction, backbone stiffness) of the studied polymers. All the CN-containing polymers 3b, 5, and 7a exhibit higher fluorescence quantum yield in solid state (50 to 60%), but lower quantum yields (12−40%) in dilute chloroform solution, in total contrast to CN-free polymers 3c, 3d, and 7b. Identical optical, E g opt, and electrochemical band gap energies, E g ec, were obtained for 3b, 3c and 3d with intrinsic self-assembly ability, whereas a discrepancy, ΔE g, was observed in the cases of the fully substituted polymers 5, 7a, and 7b, whose values are dependent on the level of backbone stiffness and length of the side groups combined with the presence or absence of CN units. The incorporation of CN units in 3b and 7a lowers their respective LUMO level by 220 and 350 meV compared to their corresponding CN-free counterparts 3c and 7b, suggesting an improvement of the electron-accepting strength. Polymers 3b and 7a are efficient electron acceptors suitable for photovoltaic application. The experiments indicate that 3b is a better electron acceptor when used together with M3EH−PPV, but transport properties seem to be better for 7a. With 3b, high external quantum efficiencies of up to 23%, an open circuit voltage of up to 1.52 V, and a white light energy efficiency of 0.65% could be realized in bilayer solar cell devices. LED-devices of configuration ITO/PEDOT:PSS/polymer/Ca/Al from 3b, 3c, 7a, and 7b showed low turn-on voltages between 2 and 2.5 V. The CN-free polymers 3c and 7b exhibit far better EL parameters than their corresponding CN containing counterparts 3b and 7a.