Synthesis Of CuInS₂Quantum Dots For Polymer-based Solar Cells

Hybrid solar cells based on conjugated polymers as the electron donor and inorganic semiconductor nanocrystals as the electron acceptor in a bulk heterojunction structure are attractive for low-cost, large area and even flexible solar cells because they combine the particular properties of inorganic semiconductors and conjugated polymers. However, one of the major efficiency-limiting factors in those solar cells is the narrow absorption band of conjugated polymers. One strategy for highly efficient solar cells is to use the inorganic semiconductor quantum dots (QDs) with a small band gap dispersed in organic polymer matrixes, which may offer hybrid materials with complementary absorption spectra in the visible spectrum. Copper indium disulfide (CuInS2) has a small direct band gap of1.5eV that matches well the solar spectrum, a large absorption coefficient and a low toxicity, and is regarded to be a promising light-absorbing material for solar cells. In this dissertation, different crystallographic structures and sizes of CuInS2quantum dots (CuInS2-QDs) are synthesized and the structure-property correlation in the solar cells based on the CuInS2-QDs is systematically studied. The main results are summarized as follows:1. Zincblende CuInS2-QDs are synthesized by solvothermal approach and applied as a potential electron accepting material for polymer-based hybrid solar cells. The CuInS2QDs with a size of2-4nm are synthesized with4-bromothiophenol as both reduction and capping agents, results reveal that the CuInS2-QDs result from the solvothermal decomposition of a soluble organic sodium salt as an intermediate precursor formed by simple reactions among CuCl2, InCl3, HSPh and Na2S at room temperature. Owe to the favorable energy level with respect to MEH-PPV, CuInS2-QDs can quench effectively the luminescence of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene)(MEH-PPV). MEH-PPV/CuInS2-QDs solar cells with a wide spectral response extending from300to900nm are fabricated, in which the CuInS2-QDs act as an effective electron acceptor for the hybrid solar cells, by allowing the efficient charge separation for neutral excited states produced either on the polymer or on the QDs. The MEH-PPV/CuInS2-QDs solar cells exhibit a higher performance after the ligand-exchange on the CuInS2-QDs. 2. Chalcopyrite CuInS2-QDs with tunable sizes are synthesized by a solvothermal method and the size-dependent performance in polymer/CuInS2-QDs solar cells is demonstrated. The CuInS2-QDs of3.2-5.4nm in size are fine tuned by the reaction time in the solvothermal process with a slow supply of In3+ions during crystallization. The size-dependent molecular orbital levels, band gap and efficiency for quenching the MEH-PPV luminescence are revealed in the CuInS2-QDs, which can serve as an effective electron acceptor material for the MEH-PPV/CuInS2-QDs solar cells with a wide spectral response extending from300to900nm. The solar cells exhibit size-dependent short circuit current (Jsc) and open circuit voltage (Voc), with a higher performance in both Jsc and Voc for smaller CuInS2QDs; in particular, it is found that the device Voc is linearly dependent on the energy difference between the the highest occupied molecular orbital (HOMO) level of the polymer and the conduction band edge of the CuInS2-QDs.3. Core-shell structure nanorods with CuInS2as shell and TiO2nanorod as core are synthesized, resulting in TiO2-CuInS2heterostroctured core-shell nanorod array (TiO2-CuInS2-NA); the influence of CuInS2shell on the device performance is demonstrated in MEH-PPV/TiO2-CuInS2-NA solar cells. CuInS2-TiO2-NA displays stronger light-harvesting ability and higher PL quenching efficiency to MEH-PPV in comparison to pristine TiO2-NA, which is resulted from the CuInS2shell as additional light-harvesing materials and the presence of additional MEH-PPV/CuInS2interface. As a result, the MEH-PPV/TiO2-CuInS2-NA solar cells exhibit a higher device performance with a wide absorption spectrum from300to900nm than MEH-PPV/TiO2-NA devices. The device with8nm-thick CuInS2shell ultimately reaches the efficiency of1.79%under AM1.5illumination.4. Polymer-CuInS2hybrid light-harvesting materials are synthesized and the influence of CuInS2on device parameters of polymer solar cells based on orinented metal oxide nanoarray is demonstrated. Light-harvesting hybrids (MEH-PPV-CuInS2) are prepared by blending the CuInS2-QDs with various sizes and crystallographic structures and MEH-PPV in organic solvent, results show that CuInS2-QDs aggregate in the hybrids to form interpenetrating network channels. Using TiO2-NA as straightforward electron transporting channels, device parameters of MEH-PPV-CuInS2/TiO2-NA solar cells consisting larger-sized chalcopyrite CuInS2-QDs enhance remarkably compared to MEH-PPV/TiO2-NA solar cells, which is revealed to mainly originate from increased photocurrent as the results of additional MEH-PPV/CuInS2interfaces in light-harvesting material for exciton dissociation and the effective CuInS2channels for the transport of electrons generated at the MEH-PPV/CuInS2interfaces toward the TiO2-NAs.