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Title: Hybrid bulk-heterojunction photovoltaic cells incorporating ZnO
Author: Han, SeungJin
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2013
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This thesis is concerned with the fabrication of large area solution process hybrid photovoltaic (PV) devices using ZnO inorganic materials, incorporating inorganic nanoparticles in the photoactive layer. We investigate four different fabrication methodologies and architectures to directly relate to device performance to optical properties of the active layer. In the process, electronic properties associated with the interface of active layer/metal electrode with different forms of various thienothiophene polymers as the donor in the hybrid PV cells are examined. The results to support the promise of ZnO being an ideal component for hybrid PV systems and suggests ways to improve the performance of organic PV devices. The effects of spherical nanoparticle (NP) dimension and interfacial modification were investigated on the performance of the hybrid polymer:ZnO PV devices. Structures consisting of regioregular poly-3-hexylthiophene (rr-P3HT) polymer and the fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were compared in contact with different sizes of 20 nm, 200 nm, or 300 nm ZnO NPs. Hybrid solar cells were fabricated by embedding the combination of rr-P3HT with 20 nm or 50 nm ZnO NPs. The device performances relied on the ZnO NP size and its dispersion. The effects of the interfacial layers between the photovoltaic layer and metal cathodes on hybrid P3HT:ZnO NP PVs were also examined. Using the blend of rr-P3HT and ZnO formed via the solution process with diethylzinc precursor, the hybrid PV devices were fabricated and optimised. Factors determining the photovoltaic device performance of blends of rr-P3HT and ZnO nanostructures are reported. A decrease in the crystallinity of rr-P3HT upon the formation of ZnO (through hydrolysis) is observed through optical absorption spectroscopy. Increasing the humidity level for the ZnO formation leads to a decrease in the photoluminescence of the rr-P3HT:ZnO blend, together with improved photovoltaic device performance. This is attributed to a more efficient charge extraction due to a decrease in the effective radiative trap sites on the ZnO surface as a result of decreasing the ZnO surface area with increasing humidity level. The BCP, TiOx, and LiF interfacial layers between photoactive material and metal cathode allows the device performance to be enhanced in hybrid rr-P3HT:ZnO PV cells. The best device performance was obtained in hybrid rr-P3HT:ZnO PVs, with a TiOx interfacial layer. Photoelectron spectroscopy is used to investigate the role of titanium oxide as an interfacial layer. The inspection of chemical bonds through X-ray photoemission spectroscopy core peaks indicates that the inner structure of rr-P3HT:ZnO photo-active layer is preserved (i. e. there is no chemical interaction between the active layer and the TiOx layer), despite the deposition of the TiOx. Furthermore, the formation of dipoles is also observed at the interface which explains the band alignment between rr-P3HT:ZnO/TiOx/Al. This band alignment in turn explains the enhancement in power conversion efficiency from 1. 08% to 1. 22% upon incorporating the TiOx layer in rr-P3HT:ZnO photovoltaic cells. The various thienothiophene copolymers were used as a donor in the new architectures of hybrid PV cells with ZnO, which were fabricated and examined for device performance. The best device performance was obtained in a hybrid PV device using Poly(3,6-dialkylthieno[3,2-b]thiophene-co-bithiophene) (pATBT) donor and ZnO acceptor. Using this hybrid system, the effect of the side chain length of pATBT on the performance of hybrid polymer-metal oxide PVs is investigated. The pATBT attached with a dodecyl side chain (pATBT-C12) in hybrid PVs with ZnO was compared to pATBT with a hexadecyl side chain (pATBT-C16). Atomic force microscopic analysis reveals a smoother surface for the pATBT-C16 photoactive layer compared to the pATBT-C12. For hybrid PVs using pATBT-C16, the relative intensity of the external quantum efficiency (EQE) particularly increased by 39 % at a wavelength of 395 nm associated with the ZnO. Furthermore, the EQE spectrum shows a red shift for pATBT-C 16 indicating better structural ordering compared to hybrid PVs with pATBT-C12. As a result, the hybrid PV utilising pATBT-C16:ZnO blend layer is observed to display a better performance with a power conversion efficiency (PCE) of 1. 02 % compared to 0. 672 % of pATBT-C12:ZnO PV.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available