Fabrication Of Hybrid Nanostructures And Interfacial Engineering For Polymer Solar Cells

Polymer solar cells（PSCs） based on bulk heterojunction blend of a semiconducting conjugated polymer donor and a fullerene derivative acceptor are promising candidates for solar energy conversion and have drawn considerable attention, owing to their attractive properties including potential of being inexpensive and sustainable, lightweight and high flexibility, low-temperature solution processing and printable for large scale fabrication by roll-to-roll technical. In recent years, the power conversion efficiency（PCE） of PSCs has been dramatically improved and already reached over 10% for single stack devices by designing new semiconducting light-absorbing conjugated polymer donor materials, controlling the photoactive layer morphology, optimization and tailored of device architectures and interface engineering for devices. Despite the rapidly development of PSCs and significantly improvement in PCE, further improvement of photovoltaic performance and the stability of device is still required for commercialization.In this dissertation, the potential application of poly-3-hexylthiophene（P3HT） based liquid crystalline rod-coil block copolymers in polymer solar cells has been investigated. The solar cells based on the two self-assemble liquid crystalline block copolymers blend with [6,6]-phenyl-C61-butyric acid methyl ester（PC61BM） show poor photovoltaic performance. However, utilization of the liquid crystalline block copolymers as compatibilizers in P3HT:PC61BM blends, the morphology combined with the photovoltaic performance of P3HT:PC61BM solar cells can be significantly improved after annealing from the liquid crystalline states. It is demonstrated that the self-assembly of liquid crystalline block located the donor and acceptor interface can enhance crystallization and ordering of P3 HT chains and guarantee the formation of interpenetrating networks, subsequently resulting in the improvement of efficient exciton separation of the active layer.Two-dimensional graphene-CdS（G-CdS） hybrid nanosheets were in situ synthesized. Incorporation of G-CdS nanosheets into poly[4,8-bis-（2-ethyl-hexylthiophene-5-yl）-benzo[1,2-b:4,5-b]dithiophene-2,6-diyl]-alt-[2-（2-ethyl-hexanoyl）-thieno[3,4-b]thiophen-4,6-diyl]（PBDTTT-C-T） and [6,6]-phenyl C₇₀ butyric acid methyl ester（PC71BM） photoactive film can effectively accelerate exciton dissociation. More importantly, the downhill energy alignment between PBDTTT-C-T, PC₇₀ BM and G-CdS produces versatile heterojunction interfaces of PBDTTT-C-T/PC71 BM, PBDTTT-C-T/G-CdS and PBDTTT-C-T/PC71BM/G-CdS, which can offer the multi-charge-transfer-channels for more efficient charge separation and transfer. As a result, by incorporation of the G-CdS nanosheets into the activelayer, the PCE of inverted solar cell based on PBDTTT-C-T and PC71 BM is improved from 6.0% to 7.5% for the device with 1.5wt% G-CdS nanosheets.CdS nanocrystals were directly synthesized in efficient solid-state thermotropic liquid crystals（LCs） matrix by a straightforward in situ thermal decomposition of metal xanthate precursor strategy. The influence of thermal annealing temperature of LC/CdS precusors upon the nanomorphology, photophysics and potoelectronic properties of the LC/CdS nanocomposites is systematically studied. Specifically, the nanostructure of the LC/CdS hybrid nanocomposites as well as the photophysics and potoelectronic properties were found to strongly depend on the thermal annealing temperature, i.e., annealed at liquid crystal state temperature, resulting in a better organized structure with improved photophysics and potoelectronic properties.Nanostructured three-dimensional hybrid ZnO@CdS nanowalls fabricated by in situ growth were explored as electron transport layer for inverted PSCs. The in situ growth CdS on the surface of ZnO not only can passivate and repair the surface defects of ZnO to offer an imitate contact and the efficient path for electron transport, but also act as a bridge for interfacial charge transfer to enhance the electron selectivity and reduce the recombination probability of electrons and holes, which is favorable for improving the performance of device. Moreover, the imitate contact also can prevent the oxygen and moisture diffusing into the active layers, which dramatically enhance the environmental stability.A novel PEIE-Ag composites by in situ growth silver nanoparticles in poly-（ethyleneimine）-ethoxylated（PEIE） aqueous solution is explored as an efficient interfacial layer for improving inverted PSCs performance. Combination of the advantages of PEIE and Ag nanoparticles, the PEIE-Ag shows enhanced charge transport, electron selective and collection, and improved light-harvesting, mainly due to the surface plasmon resonance（SPR） effect, better energy alignment induced by the formation of ideal dipole layer, as well as the improved conductivity. These distinguished interfacial properties result in the PCE of inverted PSCs based on PBDTTT-C-T:PC71BM substantially improved up to 7.66% from 6.11%. Moreover, the device performance is insensitively with the thickness of PEIE-Ag interfacial layer, which broadening the thicknesses selection window for interfacial materials.A silver nanoparticles decorated graphene oxide（GO-Ag NPs） nanocomposite with SPR effect was inherited to improve the performance of PSCs. The strong coupling between the SPR effect of GO-Ag NPs and incident light offers the probability of improved light absorption and corresponding exciton generation rate with enhanced the charge collection, resulting in significant enhancement in short-circuit current density and PCE. Therefore, the PCE of PSCs based on PBDTTT-C-T:PC71BM has been substantially elevated to 7.54 % from 6.58 % by introducing GO-Ag NPs at the ITO/PEDOT:PSS interface.Heterostructural semiconductor-metal ZnO-Ag nanoparticle composite was constructed via a straightforward photocatalytic strategy by using UV irradiation of ZnO nanoparticles and Ag precursor aqueous solution. The ZnO-Ag nanoparticle composites has been demonstrated to serve as an effective cathode modifying layer in PSCs with increased short-circuit current density owing to the light trapping effect, and improved optical and electrical conductivity properties than pure ZnO nanoparticles. Systematic photoelectron and photophysical investigations confirm that heterostructure ZnO-Ag nanoparticles can significantly improve charge separation, transport, collection and lower charge recombination at the cathode interface, leading to a 14.0% improvement in air processed device power conversion efficiency.