Solution processed polymer/fullerene blend films are receiving extensive attention as the photoactive layer of organic solar cells. In this paper we report a range of photophysical, electrochemical, physicochemical and structural data which provide evidence that formation of a relatively pure, molecularly ordered phase of the fullerene component, phenyl-C 61 -butyric acid methyl ester (PCBM), may be the key factor driving the spatial separation of photogenerated electrons and holes in many of these devices. PCBM crystallisation is shown to result in an increase in its electron affinity, providing an energetic driving force for spatial separation of electrons and holes. Based upon our observations, we propose a functional model applicable to many organic bulk heterojunction devices based upon charge generation in a finely intermixed polymer/fullerene phase followed by spatial separation of electrons and holes at the interface of this mixed phase with crystalline PCBM domains. This model has significant implications for the design of alternative acceptor materials to PCBM for organic solar cells.

We report the synthesis of a selenophene -diketopyrrolopyrrole monomer and its co-polymerisation with selenophene and thieno [3,2-b]thiophene monomers by Stille coupling. The resulting low band gap polymers exhibit ambipolar charge transport in organic field effect transistors. High and balanced electron and hole mobilities in excess of 0.1 cm 2 V -1 s -1 were observed in bottom gate, bottom contact devices, 10 suggesting that selenophene inclusion is a promising strategy for the development of ambipolar organic semiconductors.

The dependence of photoinduced carrier generation and decay on donor-acceptor nanomorphology is reported as a function of composition for blends of the polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT-C(14)) with two electron-accepting fullerenes: phenyl-C(71)-butyric acid methyl ester (PC(71)BM) or the bisadduct of phenyl-C(61)-butyric acid methyl ester (bis-PC(61)BM). The formation of partially or fully intercalated bimolecular crystals at weight ratios up to 1:1 for pBTTT-C(14):PC(71)BM blends leads to efficient exciton quenching due to a combination of static and dynamic mechanisms. At higher fullerene loadings, pure PC(71)BM domains are formed that result in an enhanced free carrier lifetime, as a consequence of spatial separation of the electron and hole into different phases, and the dominant contribution to the photoconductance comes from the high-frequency electron mobility in the fullerene clusters. In the pBTTT-C(14):bis-PC(61)BM system, phase separation results in a non-intercalated structure, independent of composition, which is characterized by exciton quenching that is dominated by a dynamic process, an enhanced carrier lifetime and a hole-dominated photoconductance signal. The results indicate that intercalation of fullerene into crystalline polymer domains is not detrimental to the density of long-lived carriers, suggesting that efficient organic photovoltaic devices could be fabricated that incorporate intercalated structures, provided that an additional pure fullerene phase is present for charge extraction.

The role of intermixed phases in organic solar cell blends is evaluated through manipulation of their number of phases.

Light-induced charge formation is essential for the generation of photocurrent in organic solar cells. In order to gain a better understanding of this complex process, we have investigated the femtosecond dynamics of charge separation upon selective excitation of either the fullerene or the polymer in different bulk heterojunction blends with well-characterized microstructure. Blends of the pBTTT and PBDTTPD polymers with PCBM gave us access to three different scenarios: either a single intermixed phase, an intermixed phase with additional pure PCBM clusters, or a three-phase microstructure of pure polymer aggregates, pure fullerene clusters and intermixed regions. We found that ultrafast charge separation (by electron or hole transfer) occurs predominantly in intermixed regions, while charges are generated more slowly from excitons in pure domains that require diffusion to a charge generation site.The pure domains are helpful to prevent geminate charge recombination, but they must be sufficiently small not to become exciton traps. By varying the polymer packing, backbone planarity and chain length, we have shown that exciton diffusion out of small polymer aggregates in the highly efficient PBDTTPD:PCBM blend occurs within the same chain and is helped by delocalization.