| Summary: | 碩士 === 國立成功大學 === 光電科學與工程學系 === 101 === In this study, the structural effects of the active layers on the photovoltaic characteristics of polymer-fullerene bulk heterojunction (BHJ) solar cells were investigated. To prepare the active layers of the BHJ solar cells, two kinds of conjugated polymers: poly (3-hexylthiophene) (P3HT) and poly[N-9'-heptadecanyl-2 7-carbazole-alt-5 5-(4' 7'-di-2-thienyl-2' 1' 3'- benzothiadiazole)] (PCDTBT) were used as the electron donor, and three kinds of fullerene derivatives: indene-C60 bisadduct (ICBA), indene-C60 monoadduct (ICMA), and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were used as the electron acceptor. The polymer-fullerene blended active layers were prepared by solution deposition via a spin-coating technique using 1,2-dichlorobenzene and chlorobenzene as solvents, respectively. We studied the correlation between the microstructure of the active layers and the photovoltaic properties of the solar cells. The active layers were characterized using absorption spectroscopy, Raman spectroscopy, atomic force microscopy and x-ray diffraction.
For the photovoltaic properties, we found that the P3HT-fullrene-based solar cells that were made using ICBA as the electron acceptor showed the best power conversion efficiency (PCE) of 3.67% and largest open-circuit voltage of 0.83 V. The devices with ICMA and PCBM as electron acceptor showed similar performance with the PCE of ca. 2.5%. For the PCDTBT-fullerene-based solar cells, however, the devices with ICBA exhibited the lowest photovoltaic performance with a PCE of 1.76% due to low short-circuit current density and inferior fill factor values. In contrast, the devices with ICMA and PCBM showed similar performance with the PCE of ca. 2.8%.
We studied the effects of the structure of fullerene on the microstructure of active layers. For the P3HT-fullerene blending active layers, no significant changes were observed in the vertical orientation of the P3HT. Spectroscopy analysis results revealed the addition of ICBA improved the self-assembling ability of the crystalline P3HT. However, when ICBA was used in the PCDTBT-fullerene systems, the P-type and N-type materials tended to separately aggregate, thereby creating a smaller P-N junction area, thus decreasing the probability of exciton dissociation. This provides a reasonable basis for the lowest PCE of the PCDTBT-ICBA solar cells.
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