Abstract
To study the effect of π-spacers on dye performance in DSSC, five novel phenothiazine dyes have been prepared where the number of successive thiophenes in the π-spacer was increased from zero to four. Additionally, a dye bearing thieno[3,2-b]thiophene was synthesized, and compared to its unfused bithiophene analogue. When the number of thiophenes were two or more the light absorption properties was significantly better in the region 400–550 nm compared to the dye without a π-spacer. DSSC device testing revealed that the reference dye without a π-spacer gave the best overall performance producing a power conversion efficiency of 5.7% (JSC = 10.1 mA cm−2, VOC = 0.83 V, FF = 0.68), attributed to the superior VOC of this dye. The incorporation of one and two thiophenes resulted in a similar performance as the reference, while incorporation of larger oligothiophenes proved to be deleterious to DSSC-performance. Electrochemical impedance spectroscopy indicates that the voltage drops and performance loss associated with these dyes are due to more facile recombination.
Highlights
The nascence of dye-sensitized solar cells (DSSC), brought on by O’Regan and Gratzel in 1991, established a novel photovoltaic tech nology with unique and promising properties [1]. Traits such as their transparency make DSSC interesting for building integrated photovol taics (BIPV) [2], they can be made using flexible materials allowing them to be integrated on cloth and wearable devices [3], and their power conversion efficiency under low-light conditions exceed that of the established GaAs solar cells proving their viability for low-power applications [4]
It has been shown that increasing the electron donor properties raise the energy level of the highest occupied molecular orbital (HOMO), and red-shift the absorption maxima [7,8,9]
It has been shown that stronger electron acceptors red-shift the UV/Vis adsorption [10], as this will ensure an efficient internal charge transfer (ICT)
Summary
The nascence of dye-sensitized solar cells (DSSC), brought on by O’Regan and Gratzel in 1991, established a novel photovoltaic tech nology with unique and promising properties [1]. Ronca et al performed a theoretical study on a set of known sensitizers [27] They discovered that significant charge-rearrangement occurred at the dye-titania interface, and demonstrated that longer π-linkers were associated with less efficient charge transfer, leading to a downshift of the conduction band of TiO2 and a subsequent drop in VOC. Our previous work on π-linkers revealed only marginal performance increase using a furan spacer (AFB-11 in Fig. 1), while the other spacers in the study (AFB-3,-5,-7, and -9), including thiophene, showed lower efficiency than the reference dye without a π-linker [20] In view of these con flicting results, we wished to investigate whether longer oligothiophenes can generate sufficiently high photocurrents to offset the associated loss of photovoltage. One dye was synthesized (DMA-5) with a rigidified thieno[3,2-b]thiophene and tested
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