유기/무기 하이브리드 태양광전지의 모폴로지에 대한 연구

Abstract

Inorganic nanocrystals have been attracting extensive attention for development of various electronic and optoelectronic devices such as light emitting diode (LED), field effect transistor, chemical sensor, diode laser, agents for medical imaging and solar cells owing to their capabilities such as solution processing, low cost, and large area fabrication. Among them, solar cells using inorganic nanocrystals like organic/inorganic hybrid solar cells have received attention due to its unique properties including high absorption coefficients, tunable band gap depending on size and shape, charge transport through elongated nanostructure, and solution processibility. Organic/inorganic hybrid solar cells are comprised of photo-active layer containing conjugated polymers as electron donor and inorganic nanocrystals as electron acceptor, originated from the concept of bulk heterojunction organic solar cells. In general, on the basis of morphological arguments, the performance of hybrid solar cells is tremendously dependent on the morphology of conjugated polymer and inorganic nanocrystals. To improve performance of hybrid solar cells, charge transport toward each electrode should be facilitated by efficient charge transport through elongated inorganic nanorods and percolating pathways of electron and hole generated by vertically and laterally phase-separated conjugated polymers and inorganic nanocrystals. In this thesis, the research is aimed on development of bulk-heterojunction hybrid solar cells based on CdSe nanocrystals and poly(3-hexylthiophene) via investigation and optimization of morphology of P3HT and CdSe for efficient charge transport, leading to an improvement of power conversion efficiency (PCE) of hybrid solar cells. In chapter 2, we introduce ultrahigh density array of CdSe nanorods on indium tin oxide (ITO)-coated glass for P3HT/CdSe hybrid solar cells. This structure exhibit ultrahigh density array (4 x 1011 in-2) of the CdSe nanorods having a length of ~ 100 nm, a diameter of ~ 20 nm, and an inter-distance of neighboring nanorods of ~ 20 nm. This nanostructure exhibits more efficient exciton diffusion toward the interface and facilitated charge transport were achieved through the vertically oriented CdSe nanorods. In chapter 3, we introduce a novel method to improve the device performance of P3HT:CdSe hybrid solar cells by using selenourea for the ligand exchange. Selenourea is easily replaced with long alkyl chain ligands at CdSe surface in P3HT:CdSe solution and decomposed into selenide by thermal annealing in P3HT:CdSe thin film. Consequently, interconnection of CdSe nanorods in nanoscale is obtained without severe aggregations and electron transport toward cathode was greatly enhanced after thermal decomposition of selenourea. The power conversion efficiency (PCE) of the devices with selenourea improved from 1.71 % to 2.63 %. In chapter 4, we have developed an air-stable inverted structure of poly(3-hexylthiophene) (P3HT): cadmium selenide (CdSe) hybrid solar cells using cesium-doped ZnO (ZnO:Cs) electron transport layer. ZnO:Cs layer was simply prepared at low temperature by sol-gel process of ZnO solution containing cesium carbonate (Cs2CO3). With increasing Cs-doping concentration, the conduction band edge of ZnO is decreased, as confirmed by scanning Kelvin probe microscopy. The energy level of ZnO:Cs is effective for electron transport from CdSe. Consequently, the power conversion efficiency (PCE) of the inverted P3HT:CdSe hybrid solar cells using ZnO:Cs electron transport layer is 1.14 %, which is significantly improved over that (0.43 %) of the another device without Cs. By X-ray photoelectron spectroscopy analysis, the amount of CdSe at the substrate (or bottom surface) is larger compared with air (or top) surface regardless of P3HT:CdSe weight ratio. The vertically inhomogeneous distribution of CdSe in the hybrid solar cells gives better charge transport from CdSe to ZnO:Cs in the inverted structure of the device compared with in the normal structure. As a result, the inverted hybrid solar cell consisting of 1:4 (wt/wt) P3HT:CdSe shows the best efficiency, while the best efficiency of a normal hybrid solar cell is achieved at 1:9 (wt/wt) P3HT:CdSe.