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Title: Transient optical characterization of hybrid polymer : inorganic nanocomposite films for use in photovoltaic devices
Author: Bansal, Neha
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2013
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Nanocrystals are well suited as light-harvesting agents in solar cells because they are robust, have tuneable effective band gaps, and are easy to process. The research undertaken here is targeted towards advancing the functional understanding of solar cells based upon inorganic semiconducting nanocrystals and polymeric semiconductors. Gaining control of the photoinduced interfacial charge transfer processes occurring at hybrid inorganic-organic semiconductor heterojunctions forms a crucial part of any attempt to optimise the performance of devices based on such materials. The research presented here focuses on the identification of the key parameters influencing charge transfer at such hybrid interfaces. In particular, the studies presented in this thesis reveals how efficient and long-lived charge separation can be achieved in these hybrid solar cell architectures and offers design rules for solar conversion devices in general. In chapter 4, Transient absorption spectroscopy is employed to study the charge photogeneration processes in polymer: CdS nanocrystal photoactive layers. The primary aim of this work is to investigate the parameters influencing charge transfer in hybrid organic-inorganic solar cells using nanocrystals as the electron acceptor and a polymer as the electron donor. Optical spectroscopy is used to study the influence of energetic driving force on the efficiency of charge separation in hybrid films. The results presented in this chapter show that the yield of charge separation is strongly dependent on the driving energy with for example efficient charge transfer only being achieved at high driving energies (> ~ 0.5eV). This observation appears consistent with the need of a large driving energy to minimize or indeed reduce geminate recombination losses. Such behaviour has been previously observed in polymer: fullerene based systems but this is first time it has been demonstrated in polymer: inorganic systems. Chapter 5, considers the influence of crystallinity of the inorganic semiconductor electron acceptor within the hybrid film on the charge separation yield. The results presented in this chapter show that the crystallinity of the CdS acceptor plays a crucial role in determining the overall yield of charge separation in polymer-CdS films. Specifically, it is found efficient charge separation can be achieved at low driving energies (< ~ 0.1eV) when using a more crystalline electron acceptor. This observation is consistent with the larger, more crystalline CdS domains encouraging delocalization of the geminate pair state. This study elucidates an important design rule for the development of more efficient hybrid solar cells. The next two chapters address the development of new inorganic electron acceptor materials for hybrid solar cells. A key focus of this chapter is the realization of new inorganic electron acceptors as alternatives to the most commonly used cadmium based materials. As such, this thesis focuses on alternative Sb2S3 (antimony sulphide) and CuInS2 (copper indium disulfide) nanocrystals for use as electron acceptors and light-harvesting materials in hybrid solar cells. The hybrid photoactive layers are fabrication using an in-situ approach based on the thermal decomposition of single-source metal xanthate precursors in a polymer film. A combination of optical, structural and optoelectronic studies have been used to characterize the Sb2S3:polymer and CuInS2:polymer photoactive layer and devices. Studies on Sb2S3:polymer system indicates that hole transfer plays an important role in the mechanism of charge separation and photocurrent generation in such devices. The data presented in chapters 6 and 7 suggest that both Sb2S3 and CuInS2 are promising light harvesting and electron acceptor materials in solution processes polymer solar cells.
Supervisor: Haque, Saif Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available