Solar cells based on conjugated polymers are attracting increasing interest due to their potential to enable a renewable energy technology based on simple and low cost manufacture.[1] Devices made from blends of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) have been widely studied and are consistently reported to produce efficiencies over 3%. [2,3] Regioregular P3HT is known to be a good hole transporting polymer with relatively long wavelength absorption, producing films with a highly crystalline morphology and good charge mobility (10 -4 cm 2 V -1 s -1 ). [4,5] According to a model presented by Scharber et al., [6] polymer:PCBM devices could be improved to produce energy conversion efficiencies of up to 11% by replacing P3HT with an electron donating polymer that has better optimised electronic energy levels. In particular, by decreasing the LUMO level of the donor while keeping the HOMO at the same level as P3HT, the reduced band gap would allow a greater light harvesting ability without compromising the open circuit voltage (V oc ).In this paper, we report on a study of a selenium analogue of P3HT, regioregular poly(3-hexylselenophene) (P3HS). The synthesis of this material has recently been reported by Heeney et al. [7] The polymer was shown to have a smaller optical gap (1.6 eV) than regioregular P3HT (1.9 eV) and because both polymers were shown to have the same HOMO level (4.8 eV), this decrease in band gap was assigned to a drop in the P3HS LUMO level. Taking into account the extended absorption range (up to 760 nm) and the similar field-effect transistor (FET) charge mobility to regioregular P3HT, [7] it was anticipated that P3HS might be a good candidate for polymer solar cells. The propensity for P3HT films to form crystalline structures is important for generating the interpenetrating nanomorphology that facilitates charge separation and charge transport in P3HT:PCBM blend devices.[4] X-ray diffraction (XRD) measurements were made of pristine P3HS films, before and after thermal annealing at 150°C, to investigate whether P3HS shares this crystalline nature (Fig. 1a). Since P3HS solutions have a tendency to gel at low temperatures, all P3HS and P3HS blend films reported in this paper were deposited by spin-coating from a hot (80°C) solution on to a substrate of the same temperature. This heating of the substrate and solution allowed for the preparation of much smoother films than if prepared at room temperature. Whilst as spun films showed no XRD peaks, a sharp diffraction peak (diffraction angle 2h = 3.

Time of flight photocurrent transient studies on thin films of bimodal polytriarylamine (PTAA) show two distinct and separate arrival times for hole transport in the same sample at a single field. The corresponding mobilities differ by two orders of magnitude, typically μfast∼10−3 cm2 V−1 s−1 and μslow∼10−5 cm2 V−1 s−1 at room temperature, and are measured parametric in electric field and temperature. The mobility data are analyzed using the correlated disorder model by Novikov, yielding a fitting parameter set. The two conduction paths are believed to come about as a result of phase segregation between the shorter and longer polymer chains with the shorter chains giving rise to the faster conduction pathways (as confirmed by results obtained for monomodal, shorter, and longer chain PTAA, by sample thickness scaling of the photocurrents and by reversal of the illuminated electrode). Separate arrival times are also obtained in a blend of the two short and long chain monomodal polymers. The phase separation within the film is inferred by the appearance of two glass transition temperatures using dynamic mechanical thermal analysis.