Electrical Characterization of Organic-Inorganic Hybrid Active Layer for Bulk Heterojunction Solar Cell

An organic-inorganic hybrid composite of cadmium selenide (CdSe) quantum dots with conjugated polymer poly (3-hexylthiophene-2,5-diyl) (P3HT) was employed as an active layer for the bulk hetero- junction solar cell. CdSe quantum dots were synthesized by conventional hot injection method with particle size of ~5 nm. Either chloroform/pyridine or chlorobenzene/pyridine mixture was used as a solvent to prepare the solution of CdSe quantum dots and P3HT polymer matrix. Then, the composite active layers were spin-coated onto the patterned indium tin oxide (ITO)-coated glass substrate. The loading amount of CdSe/P3HT mixture in the solution was varied from 5 mg/mL to 20 mg/mL, and the weight concentration of CdSe within the hybrid CdSe/P3HT composite was systematically changed from 10 to 90 wt%.

The composite active layer was characterized by several techniques. Especially, the transmission line method (TLM) was employed to estimate the sheet and contact resistance of active layer. It was found that the sheet resistance of the active layer did not change significantly until the loading amount reached the threshold amount (ex. 15~20 mg/mL for CdSe/P3HT 60/40 wt%) but decreased drastically after that, suggesting the formation of electrical percolation path.&'