Efficiency improvement in P3HT:CdSe quantum dots hybrid solar cells by utilizing novel processing of a dual ligand exchangers

CdSe quantum dots of hexagonal Wurtzite crystal structure with an average diameter of ∼7 nm were synthesized and processed for bulk heterojunction solar cell applications. The UV-Vis absorption spectrum shows an excitonic peak at 625 nm and at 635 nm in synthesized and dual ligand exchanged samples, respectively. The synthesized quantum dots were successively ligand exchanged by pyridine and 2-propanethiol to remove the TOPO ligands on quantum dot surface and then hybrid solar cell devices were fabricated. Initially the weight ratio was optimized by using pyridine capped CdSe blend with P3HT polymer as an active layer in chloroform as a solvent on the patterned ITO glass. Then dual ligand exchanged CdSe was compared with pyridine optimized samples. The maximum solar cell conversion efficiency of 1.21% was achieved with Jsc of 4.1 mA/cm ^-2, V_oc of 0.51 and FF of 44 compared to the optimized pyridine capped CdSe quantum dots where efficiency of 0.74% with Jsc of 2.15 mA/cm"^-2, V_oc of 0.53 was observed. The increase in solar cell efficiency was attributed to the better ligand exchanged and additional treatment with 2-propanethiol at ambient temperature. Such an exchange of organic ligands by successive ligand exchanger will open new domain for hybrid solar cell research. The morphology of QDs and microstructures of the heterojunction active layer (P3HT:CdSe) were examined by using TEM, XRD, UV-Vis spectra, and IV curve techniques.