Efficiency Improvement in P3HT:CdSe Quantum Dots Hybrid Solar Cells by Utilizing Novel Processing of a Dual Ligand Excha
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Efficiency Improvement in P3HT:CdSe Quantum Dots Hybrid Solar Cells by Utilizing Novel Processing of a Dual Ligand Exchangers M. Alam Khan,1,2 U. Farva,3 Yongseok Jun,1 and Omar Manasreh2 1
Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), 100 Baniyeon-ri, Eonyang-eup, Ulju-gun, Ulsan, 689-798 Republic of Korea. 2 Optoelectronic Laboratory, Dept. of Electrical Engineering, University of Arkansas, Fayetteville 72701, AR, United States 3 Department of Material Science and Engineering, Seoul National University, Seoul, 151-744 Republic of Korea ABSTRACT 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 UVVis 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, VOC 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, VOC 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. INTRODUCTION The scarcity of energy supply have significant implications for sustainable global future, for example, in order to sustain the population of 10 billion people on earth with their current lifestyle and their current energy consumption, the need for a minimum of ten additional terawatts (TWs) of energy which is an equivalent of 150 millions of barrels of oil per day (150 M BOE/Day), until the year 2050 [1]. The energy shortage is further augmented by major concerns on global warming and stringent United Nations norms from burning of greenhouse gases due to the increased use of fossil fuel consumptions [2-4]. So, the photovoltaic technology seems to be the most viable and vibrant choice which can meet our clean energy demands. The sun continuously impinges to the earth 120,000 TW of energy, which dramatically exceeds our current rate of energy needs (13TW) [5]. The nano-structured semiconductors, organic-inorganic
hybrid materials, and new molecular assemblies present new opportunitie
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