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Title: Interface modification in organic and hybrid photovoltaics
Author: Schumann, Stefan
ISNI:       0000 0004 2709 4393
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2011
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With the growing importance of organic photovoltaics (OPVs) as an attractive, low cost and sustainable energy source the field has been investigated intensively, showing high potential for commercialisation. To further improve device performance, different routes of development have been explored targeting interfaces that play a crucial role in device performance including the donor (D)/acceptor (A) and electrode/photoactive layer interfaces, as well as incorporation of new materials. Vertical co-deposition of water-soluble small molecule copper(II) phthalocyaninetetrasulfonic acid tetrasodium salt (TSCuPc) and polymeric sodium poly[2-(3- thienyl)ethoxy-4-butylsulfonate] (PTEBS) with polystyrene (PS) nanospheres to template, followed by solvent vapour sphere removal, is shown as an excellent method to generate three-dimensionally ordered macroporous large area thin films of sub-100 nm pore size. After a subsequent infiltration by the electron acceptor phenyl-C61-butyric acid methyl ester (PCBM), three-dimensionally (3D) interdigitated D-A composite structures are generated which are further implemented in complete OPV devices. PTEBS based 3D nanostructured D-A composite devices reached a comparable performance to planar reference devices but did not show the expected photocurrent improvement. This is most likely due to the complexity of this multistep fabrication method and the large probability if impurities in the films. However, it demonstrates a new approach towards nanoengineered 3D interdigitated organic D-A composite OPV devices. For this templating technique monodisperse sub-100 nm PS nanospheres were synthesised by radical initiated surfactant-free emulsion polymerisation controlling different parameters with particular focus on styrene-4-sulfonic acid sodium salt (NaSS) co-monomer concentration. Furthermore, planar heterojunction OPV devices from TSCuPc and PTEBS were studied in detail and optimised for further understanding of the 3D D-A composite devices. A substantial increase in device performance and operational stability in solution processed inverted bulk heterojunction (BHJ) OPVs is demonstrated by introducing a zinc oxide (ZnO) or titanium oxide (TiOx) interlayer between the electron collecting bottom electrode and the photoactive blend of poly(3-hexylthiophene) (P3HT) and PCBM. The introduction of transition metal oxide (TMO) interlayers resulted in a remarkable increase in power conversion efficiency (PCE) with a maximum value of 4.91 %. The structure and morphology of the dense, planar ZnO layers was controlled either by electrodeposition or spray pyrolysis techniques. Organic/inorganic hybrid OPVs combine the advantages of both types of semiconductors and offer an alternative to replace fullerene based electron acceptor materials. The small molecule organic semiconductor, boron subphthalocyanine chloride (SubPc), is a promising donor material for fabrication of inverted planar hybrid solar devices using TiOx as the electron acceptor. The TiOx/SubPc cells demonstrate performance characteristics comparable to the best-reported polymer/TiOx hybrid cells. A relatively high photocurrent and a maximum external quantum efficiency (EQE) of 20 % lead to a PCE of 0.4 % under AM1.5 solar illumination.
Supervisor: Not available Sponsor: BP Solar (Firm)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QD Chemistry ; TK Electrical engineering. Electronics Nuclear engineering