Tunable molecular weights of poly(triphenylamine-2,2′-bithiophene) and their effects on photovoltaic performance as sensitizers for dye-sensitized solar cells

Despite the huge progress achieved over the past decade, the relationship between the molecular weights of dyes and the performance of dye-sensitized solar cells (DSSCs) remains unclear. In this article, we report on the fine control of the number-average molecular weight (Mn) of poly(triphenylamine-2,2′-bithiophene) (PPAT) dyes with cyanoacrylic acid moieties as acceptors. We found a correlation between the Mn and photovoltaic performance of these polymers when they were used for DSSC applications. In this study, three samples (PPAT-01, PPAT-02, and PPAT-03) with different Mn values (Mns = 1700, 2800, and 3500 g/mol) were prepared through the control of the polymerization time and characterized by analytical gel permeation chromatography and NMR. Under the same experimental conditions, the overall cell efficiency of the oligomer dyes showed a nonmonotonic tendency with increasing molecular weight. The power-conversion efficiencies were 2.81% for PPAT-01, 4.72% for PPAT-02, and 1.88% for PPAT-03. UV absorption measurements proved that PPAT-03 formed aggregation, whereas PPAT-01 and PPAT-02 were in the monolayer state adsorbed on TiO2. The larger aggregation decreased charge transfer; thus, poor photoelectric conversion performance was observed. Furthermore, a higher molecular weight reduced the amount of PPAT-03 adsorbed on TiO2, and this had a crucial effect on the performance of the cells because of the reduced photocurrent. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44182.