In this paper, a new D-A copolymer, PFDBCPDT, which consists of benzo-2,1,3-thiadiazole as acceptor units and cyclopentadithiophene and fluorene as donor units, was synthesized. The thermal, electrochemical, photophysical and photovoltaic properties of PFDBCPDT were studied. PFDBCPDT showed a low optical band gap of 1.84 eV, and relatively low HOMO level of 5.69 eV. The best device performance was obtained by PFDBCPDT/PC61 BM (1:3) with 0.5 vol% DIO. The device exhibited a power conversion efficiency of 3.06%, with a relatively high open circuit voltage of 0.87 eV.
A selective solvent vapor, i.e., cyclohexanone or isopropyl benzene, which is a poor solvent for poly(3-hexylthiophene-2,5-diyl) (P3HT) and a good solvent for fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), was employed to reduce the size of PCBM aggregates and prolong the formation time of big PCBM aggregates in P3HT/PCBM film. PCBM nucleates and aggregates of 10-20 nm scale form in the first few minutes annealing. Then the size of PCBM aggregates kept unchanged until annealing for 60 min. Finally, larger PCBM aggregates of micron-size formed hours later. On the contrary, the growth rate of PCBM aggregates was faster and their size was larger when treated with a good solvent vapor for both components. The P3HT crystallinity was the same with different types of annealing solvents, although the rate of P3HT self-organization was decreased after a selective solvent vapor annealing. Because of the smaller size of phase separation, the device annealed in a selective solvent vapor for 30 min had a higher PCE than that annealed in a good solvent vapor.
The preparation of large area coverage of films with uniaxially aligned poly(3-hexylthiophene)(P3HT) nanofibers by using zone-casting approach is reported.The length and the orientation of the nanofibers are defined by the solubility of the solvent,the P3HT molecular weight and the substrate temperature.The length of the oriented nanofibers could be increased from 1 μm to more than 10 μm by adding poor solvent into the P3HT solution.It is found that for P3HT of relatively low molecular weight,a solvent with relatively low solubility has to be chosen to get the oriented film.While for the high molecular weight P3HT,the solvent with a relatively high solubility has to be used.The well-aligned film could be obtained because of the solute concentration gradient in the region where the critical concentration is reached during the zone-casting process.Particularly,the solvent evaporation rate and crystallization rate must be chosen properly to satisfy the stationary conditions above,which were controlled by an appropriate choice of solvent and substrate temperature.The film prepared by zone-casting approach had microcrystalline P3HT domains with more inter-chain order than spin-coating film.Meanwhile,the P3HT π-π stacking direction was parallel to the alignment direction of the nanofibers.
We report on the effects of aggregation of P3HT with ordered conformation in solution on improving the uniaxial alignment of the P3HT nanofibers by zone casting.Two approaches were employed to change the aggregation of P3HT:P3HT blending with coil insulating polymer and ultrasonic oscillating.The insulator polymer(i.e.PS) which has good solubility in the solution would disturb the aggregation of P3HT to prevent the chains entanglement.The ultrasonic oscillation can further improve the P3HT aggregation with ordered conformation in the solution.As a result,the P3HT nanofibers in the film grew much orientedly by zone casting the ultrasonic oscillating P3HT/PS polymer blends solution than the same solvent P3HT solution without ultrasonic oscillating and blending.The P3HT tt-tt stacking direction is parallel to the alignment direction of the nanofibers.Meanwhile,the P3HT/PS blend ratio and PS molecular weight have influence on the uniaxial alignment of P3HT nanofibers.Only P3HT/PS is 1:1, the P3HT nanofibers oriented well.The low molecular weight PS can make the P3HT nanofibers orient better than that of the high molecular weight.
Xiang GaoJian-Gang LiuYue SunRu-Bo XingYan-Chun Han
