| Summary: | 碩士 === 國立成功大學 === 光電科學與工程研究所 === 98 === We studied the thin-film structures and photovoltaic properties of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) organic bulk-heterojunction solar cells. The P3HT:PCBM blended active layers were prepared by solution deposition via a spin-coating technique using chloroform (CF) and 1,3,5-trichlorobenzene (TCB) cosolvents. The effect of the cosolvent concentration on the correlation between the morphology of the active layer and the photovoltaic characteristics of the solar cells were studied.
For the electrical properties, we found that the solar cells that were created using pure CF as a solvent for depositing the P3HT:PCBM active layers show the best device performance, including a power conversion efficiency (PCE) above 3 %, a short-circuit current density (JSC) as high as 10.73 mA/cm2, and an open-circuit voltage (Voc) of 0.59 V. However, when TCB:CF cosolvents were used to prepare the active layer, all the solar devices exhibited an inferior PCE of below 2%. We observed that the JSC and Voc decreased when the TCB concentration was increased for the cosolvents. The results of our experiment suggest that the cosolvent composition used when preparing P3HT:PCBM active layers played an important role in the development of the photovoltaic characteristics of organic solar cells.
The P3HT:PCBM active layers were characterized using UV-vis absorption spectroscopy, Raman spectroscopy, x-ray diffraction (XRD), and atomic force microscopy (AFM). The results revealed that an increase in the CF concentration for the cosolvents resulted in films with higher absorbance and better P3HT:PCBM blend homogeneity, thus suggesting the formation of a relatively large amount of excitons and an increased probability of exciton dissociation. This provides a reasonable basis for the higher photocurrent of the solar cells. On the contrary, we also observed an increased crystalline size in the P3HT domains with increasing TCB concentrations, thus reducing the contact between P3HT and PCBM, i.e., creating a smaller p-n junction area.
In summary, we observed a good correlation between the morphology of the P3HT:PCBM active layer and the electrical properties of the solar cells when utilizing the cosolvent approach. In spite of the decreasing order of the P3HT chains, the PCE of the device created with the CF solvent is larger than that of the device made with them CF:TCB cosolvents. Hence, we suggest that the enhanced absorbance and improved p-n junction contribute to the enhancement of the photocurrent in organic bulk-heterojunction-type solar cells despite the formation of small crystalline domains that could limit the transport of charge carriers.
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