The use of nanofibers of P3HT in bulk heterojunction solar cells: the effect of order and morphology on the performance of P3HT:PCBM blends

Abstract

Poly-3-AlkylThiophenes (P3ATs) with an n-alkyl chain length varying from C3 till C9 were synthesized by using the Rieke method. Subsequently, these materials were used to make P3AT/PCBM blends which were investigated in bulk heterojunction (BHJ) solar cells. The phase diagram of a P3H(exyl)T:PCBM blend was measured by means of standard and modulated temperature differential scanning calorimetry (DSC and MTDSC). A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing PCBM concentration: from 12 °C for pure P3HT to 131 °C for pure PCBM. The observed range of Tg's defines the operating window for thermal annealing and explains the long-term instability of both morphology and photovoltaic performance of P3HT:PCBM solar cells. All regioregular P3ATs allow for efficient fiber formation in several solvents. The fibers formed are typically 15 to 25 nm wide and 0.5 to >4 µm long and mainly crystalline. By means of temperature control the fiber content in the casting solution for P3AT:PCBM BHJ solar cells is controlled while keeping the overall molecular weight of the polymer in the blend constant. In this way, fiber isolation and the use of solvent mixtures are avoided and with P3HT nanofibers, a power conversion efficiency of 3.2 % was achieved. P3AT:PCBM BHJ solar cells were also prepared from P3B(utyl)T, P3P(entyl)T and P3HT using the good solvent o-dichlorobenzene and a combination of slow drying and thermal annealing. In this way, power conversion efficiencies of 3.2, 4.3, and 4.6 % were obtained, respectively. P3PT is proved to be a potentially competitive material compared to P3HT.