Nowadays almost all of the high-performance polymer solar cells are solution-processed by a spin coating method in conjunction with the expensive and brittle indium tin oxide as the transparent electrode, which are totally ill-suited to a cheap large-area roll-to-roll process in accompany with the great solution waste problem.
The doctor-blading technique, which is fully suitable to a continuous roll-to-roll process, can perfectly solve these problems by using cheap conducting materials like PEDOT:PSS as the top transparent anode.
It is confirmed that the insertion of the interfacial buffer layer between the photoactive layer and the cathode is highly necessary in polymer solar cells. However, the addition of the interfacial buffer layer certainly increases the fabrication complexity in the large-area roll-to-roll processing due to the requirements of rigorous alignment of multiple layers, accurate thicknesses and limited mutual solubility, which are needed to be improved.
Researchers led by Professor Lintao Hou from Jinan University in Guangzhou of China investigated an easy spontaneous vertical separation self-assembled technique by doping interfacial buffer material directly into the bulk photoactive layer. An interfacial buffer layer can be formed in the bottom and a photoactive layer on the top via a single doctor-blading step, which has not yet been investigated. This work is published in the Journal of Materials Chemistry A.
The authors discovered that the performance of one-step doctor-blading ITO-free inverted polymer solar cells is primarily influenced by the inherent interfacial buffer layer stratification purity rather than the fine donor/acceptor phase separation for the rigid backbone PTB7 system, which is significantly different from that of the conventional two-step doctor blading devices.
The surface energy results strongly demonstrate that the formation of the interfacial layer between the ITO-free cathode and the photoactive layer is significantly controlled by the in situ solvent drying time, which determines the self-assembly quality and can be greatly manipulated from 2700 to 1200 s by different substrate temperatures. The pure interfacial layer formed at low substrate temperatures improves charge separation and transport, whereas high substrate temperatures limit its growth, leading to the decrease of device performance.
Results from impedance spectroscopy also showed that the self-assembly interfacial layer has a big effect on the internal resistance and capacitance of devices. The invariation of resistance and capacitance is perfectly in accordance with the device performance, confirming that the performance of one-step doctor-blading ITO-free inverted polymer solar cells is primarily influenced by the inherent interfacial buffer layer stratification other than the photoactive donor/acceptor phase separation.
The authors also discovered the spatial and local distribution of photocurrent is uniform over the one-step doctor-blading device at a low substrate temperature.
For comparison, bulk heterojunction morphology of another highly crystalline donor polymer, synthesized by Ergang Wang group from Chalmers University of Technology in Sweden, seems to play a bigger role in improving the doctor-blading device performance than that in the homogeneous donor polymer, indicating the different donor systems should be dealt with each case on its merits for the one-step doctor-blading technique.
Encouraging power conversion efﬁciency of 6.56% is achieved from environment-friendly simple one-step doctor-blading ITO-free polymer solar cells at a very low substrate temperature, which is energy saving and appropriate to industrialized roll-to-roll production. In contrast, the highest power conversion efficiency of 7.11% ever reported for two-step doctor-blading ITO-free inverted polymer solar cells was obtained at a high substrate temperature for achieving a fine morphology without regard to the vertical delamination.
Yuanbao Lin1, Chaosheng Cai1, Yangdong Zhang1, Wenhao Zheng1, Junyu Yang1, Ergang Wang2, Lintao Hou1,*. Study of ITO-free roll-to-roll compatible polymer solar cells using the one-step doctor blading technique. Journal of Materials Chemistry A, 2017,5, 4093-4102.Show Affiliations
1 Siyuan Laboratory, Guangzhou Key Laboratory of Vacuum Coating Technologies and New Energy Materials, Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong Higher Education Institutes, Department of Physics, Jinan University, Guangzhou 510632, PR China, Email: [email protected]
2 Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96, Göteborg, Sweden.
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