A morphologically-stable polymer/fullerene heterojunction has been prepared by minimizing the intermixing between polymer and fullerene via sequential deposition (SqD) of a polymer and a fullerene solution. A low crystalline conjugated polymer of PCPDTBT (poly[2,6-(4,4- bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b0]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]) has been utilized for the polymer layer and PC71BM (phenyl-C71-butyric-acid-methyl ester) for the fullerene layer, respectively. Firstly, a nanostructured PCPDTBT bottom layer was developed by utilizing various additives to increase the surface area of the polymer film. The PC71BM solution was prepared by dissolving it in the 1,2-dichloroethane (DCE), exhibiting a lower vapor pressure and slower diffusion into the polymer layer. The deposition of the PC71BM solution on the nanostructured PCPDTBT layer forms an inter-digitated bulk heterojunction (ID-BHJ) with minimized intermixing. The organic photovoltaic (OPV) device utilizing the ID-BHJ photoactive layer exhibits a highly reproducible solar cell performance. In spite of restricted intermixing between the PC71BM and the PCPDTBT, the efficiency of ID-BHJ OPVs (3.36%) is comparable to that of OPVs (3.87%) prepared by the conventional method (deposition of a blended solution of polymer:fullerene). The thermal stability of the ID-BHJ is superior to the bulk heterojunction (BHJ) prepared by the conventional method. The ID-BHJ OPV maintains 70% of its initial efficiency after thermal stress application for twelve days at 80 °C, whereas the conventional BHJ OPV maintains only 40% of its initial efficiency.
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Acknowledgments: This research was supported by NRF under the program numbers NRF-2015M1A2A2057506 and NRF-2016M1A2A2940914 and by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), who granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20123010010140 and 20163030013900). The authors thank Mr. Sungmin Park and Mr. Yeongsik Kim for assistance with the GIWAXS characterization. The X-ray experiments at PLS-II (Beamline 9A U-SAXS), Korea, were supported in part by MSIP and POSTECH.
© 2017 by the authors.
- Bulk heterojunction
- Organic solar cell
- Sequential deposition