Low-band gap copolymers as well as their compounds with fullerenes have received a great deal of research attention owing to the fact that they can be used as active layers on organic photovoltaic gadgets. Photovoltaic systems with bulk heterojunctions as well as donor-acceptor blends appear to be the most promising devices.
Photovoltaic gadgets with bulk heterojunctions composed of blends of donor-acceptor copolymers as well as fullerene derivative, phenyl C61-butyric acid methyl ester have shown varying power conversion efficiencies dictated by side chain nature as well as the behavior of the donor units. Blend film quality is an important parameter for photovoltaic device performance. Raman spectroscopy appears as a useful tool for monitoring polymer order, and has enabled the study of chemical composition of blends normally used in bulk-heterojunction structures.
Researchers led by Věra Cimrová from the Academy of Sciences of the Czech Republic studied thin films with three low band gap donor-acceptor copolymers and their blends of varying ratios with the soluble fullerene derivative as heterojunction solar cell layers. They used UV-vis absorption spectroscopy as well as Raman microspectroscopy for the study. In particular, they used Raman measurements with varying excitation wavelengths to differentiate the low-wavelength photoluminescence of the two components in the blends. Their work is published in peer-reviewed journal, Organic Electronics.
The authors prepared three donor-acceptor copolymers, CDTF, CDTDOP, and CDTDP, composed of 4,6-bis(3′-dodecylthiophen-2′-yl)thieno[3,4-c][1,2,5]thiadiazole-5[Symbol],5[Symbol]-diyl as an electron-acceptor structural unit and electron-donor structural units 9,9-bis(2-ethylhexyl)fluorene-2,7-diyl, 2,5-didodecyloxy-1,4-phenylene and 2,5-didodecyl-1,4-phenylene, respectively. Recent research works of authors have shown that copolymers composed of electron- accepting thienothiadiazole-based components are promising for photovoltaic applications owing to their high electron affinity, low band gap and reversible redox attributes.They prepared polymer and polymers-blend thin films through spin coating onto fused silica substrates from 1,2-dichlorobenzene solutions.
The researchers observed that the maxima of the thin film spectra were red shifted as opposed to the solution spectra indicating strong intermolecular interactions in the solid state. The long-wavelength intra-chain charge transfer absorption of the CDTDOP was observed to be located at lower wavelengths compared to the other two copolymers owing to strong electron donor character of the 2,5-didodecyloxy-1,4-phenylene components as well as stronger aggregation that was also evident in its solution absorption.
The fullerene derivative absorbed in the UV spectral region, and for this reason, in the blend films, the UV absorption was observed to increase with the increase in the fullerene derivative. The absorption in the visible region decreased as compared with the absorption of the thin films. However, there were no shifts observed in the maxima positions of the CDTF and CDTDOP copolymers. This was an indication that blending the copolymers with the fullerene derivative did not hinder planarization of the main chain.
From the Raman spectroscopic studies of the thin films of the three copolymers and their blends with fullerene derivative, the authors observed different behavior in the CDTDP blends from that of CDTF and CDTDOP blends.
The authors found separate regions (in-homogeneities) in the blend films of the copolymers with fullerene. Photoluminescence of the two components determined in the Raman spectra indicated that the in-homogeneities were composed of more fullerene and a small copolymer amount as opposed to homogeneous blend.
The results of their study showed that Raman microscopic and optical absorption approaches are important in getting additional information about polymer molecule planarity as well as interactions in the polymer thin films.
Věra Cimrová, Zuzana Morávková, Veronika Pokorná, Drahomír Výprachtický. Investigation of donor-acceptor copolymer films and their blends with fullerene in the active layers of bulk heterojunction solar cells by Raman micro-spectroscopy. Organic Electronics, volume 47 (2017), pages 194-199.Go To Organic Electronics