| Summary: | 博士 === 國立臺灣大學 === 高分子科學與工程學研究所 === 101 === This thesis consists of three topics concerning solid-state dye-sensitized solar cells (ss-DSCs). In the first part, ss-DSCs are fabricated using Z907 or its thiophene derivative, CYC-B11, as a dye, and poly(3-hexylthiophene) (P3HT) or (2,2'',7,7''-tetrakis-(N,N-di-p-methoxyphenylamine)9,9'')-spirobifluorene (OMeTAD) as a hole transport material (HTM). The effect of the structural compatibility of dye molecules with HTM on device performance is investigated. The CYC-B11/P3HT device has a much higher short-circuit current density than those for Z907/P3HT and CYC-B11/OMeTAD devices. Results from the incident photo-to-electron conversion efficiency and impedance measurements support the use of P3HT, in place of OMeTAD, as HTM markedly increases the photocurrent throughout the absorption spectrum of CYC-B11 and significantly reduces the charge-transfer resistance at the TiO2/dye/HTM interface. As a result, the CYC-B11/P3HT ss-DSC that is fabricated from a thin (0.5 μm) mesoporous TiO2 layer exhibits an outstanding power conversion efficiency (PCE) of 3.66%.
In the second part, we applied P3HT with three different molecular weights, 65000 (P3HT65), 47000 (P3HT47) and 25000 (P3HT25) g/mol, as HTM to fabricate ss-DSCs in which CYC-B11 was used as sensitizer. Results from the space-charge-limited-current, impedance and incident photo-to-electron conversion efficiency measurements indicate that the amorphous phase of P3HT increases its hole mobility and efficiently reduces the charge-transfer resistance at the TiO2/dye/HTM interface, thereby increasing the photocurrent. Moreover, the transient photovoltage experiments show that the CYC-B11/ P3HT47 device has the longest electron lifetime. This result suggests a complete coverage of the dyed-TiO2 surface with P3HT47, resulting in the suppression of charge recombination that causes the increase of open-circuit voltage. Accordingly, the CYC-B11/ P3HT47 device exhibits a striking PCE of 4.72 %, which is ~50 % better than that of the ss-DSCs using P3HT25 as HTM. The replacement of CYC-B11 with CYC-B19 as the sensitizer to improve the light-harvesting capability further increases the PCE up to 4.85 %.
In the third part, An all-conjugated diblock copolymer, poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-hexylthiophene) (PPP-b-P3HT), was synthesized and applied as HTM for the fabrication of ss-DSCs. This copolymer is characterized by an enhanced crystallinity, enabling its P3HT component to self-organize into interpenetrated and long-range ordered crystalline fibrils upon spin-drying and ultimately endowing itself to have a faster hole mobility than that of the parent P3HT homopolymer. Transient photovoltage measurements indicate that the photovoltaic cell based on PPP-b-P3HT as the HTM has a longer electron lifetime than that of the reference device based on P3HT homopolymer. Moreover, comparing the two ss-DSCs in terms of the electrochemical impedance spectra reveals that the transfer of charge carriers across the TiO2/dye/HTM interface is substantially easier in the PPP-b-P3HT device than in the P3HT cell. Above observations suggest that the PPP block facilitates an intimate contact between the copolymer and dye molecules absorbed on the nanoporous TiO2 layer, which significantly enhances the performance of the resulting device. Consequently, the PPP-b-P3HT ss-DSC exhibits a promising power conversion efficiency of 4.65%. This study demonstrates that conjugated block copolymers can function as superior HTMs of highly efficient ss-DSCs.
In the last part, we introduced colored comptibilizers (namely DS1 and DS3) into the ss-DSCs with a squaraine dye (SQ2). The results imply that colored comptibilizers can act not only as interface modifiers between dye and hole transporting material (HTM) but also as co-sensitizers by forster energy transfer. Results from incident photo-to-electron conversion efficiency, surface energy and impedance measurements indicate that the introduction of colored comptibilier increase extra light-harvesting and efficiently reduce the charge-transfer resistance at the TiO2/dye/HTM interface, thereby increasing the photocurrent. Moreover, the transient photovoltage experiments show that the DS modified devices have the longer electron lifetime, leading to the increment of open-circuit voltage compared to the reference cell. This result suggests a complete coverage of the TiO2-SQ2 with OMeTAD through DSs treatment, resulting in the suppression of charge recombination between electrons from the TiO2 conduction band and dye cations. Consequently, the ss-DSC using SQ2 and OMeTAD as dye and HTM with the treatment of DS3 compabilizer, exhibits a promising PCE of 2.68 %, which is ~50 % better than that of the reference cell.
|
|---|
