S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Electrical and Computer Engineering (전기·정보공학부) Journal Papers (저널논문_전기·정보공학부)
Origin of the Mixing Ratio Dependence of Power Conversion Efficiency in Bulk Heterojunction Organic Solar Cells with Low Donor Concentration
- Song, Hyung-Jun; Kim, Jun Young; Lee, Donggu; Song, Jiyun; Ko, Youngjun; Kwak, Jeonghun; Lee, Changhee
- Issue Date
- American Scientiﬁc Publishers
- Journal of Nanoscience and Nanotechnology Vol.13 No.12, pp. 7982-7987
- 복합학; Organic Photovoltaic Cells; Small Molecule; Bulk Heterojunction; Donor–Acceptor Mixing Ratio
- We studied the origin of the improvement in device performance of thermally evaporated bulk heterojunction organic photovoltaic devices (OPVs) with low donor concentration. Samples with three different donor-acceptor mixing ratios, 0:10 (C70-only), 1:9 (low-doped) and 3:7 (high-doped), were fabricated with 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC):C70. The power conversion efficiencies (PCEs) of these samples were 1.14%, 2.74% and 0.69%, respectively. To determine why the low-doped device showed a high PCE, we measured various properties of the devices in terms of the effective energy band gap, activation energy, charge carrier mobility and recombination loss. We found that the activation energy for charge carrier transport was increased as we increased the TAPC concentration in the blends whereas the hole and electron mobilities became more balanced as the TAPC concentration was increased. Furthermore, the recombination loss parameter alpha (from the light intensity dependence) remained alpha to approximately 0.9 in the low-doped device, but it decreased to alpha to approximately 0.77 in the high-doped device, indicating a large recombination loss as a result of space charge. Therefore, the improved PCE of low-doped OPVs can be attributed to the balance between carrier mobilities with no increase in recombination loss.
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