Ultrafast charge transfer at GaAs/P3HT as a model system for hybrid organic-inorganic heterointerfaces
Majid Panahandeh Fard
Date of Issue2015
School of Physical and Mathematical Sciences
Polymer bulk heterojunction solar cells are being widely considered for the development of the future generation of photovoltaic devices, which aim at providing high conversion efficiencies at low cost. These promising devices have been conventionally constructed by blending an organic conjugated polymer donor and a fullerene acceptor. The polymer solar cells have demonstrated a few advantages over their inorganic counterparts: simple and easy processability, high throughput fabrication, and compatibility with low-temperature processes. However, few intrinsic factors are still limiting the overall performance of polymer solar cells, particularly the low carrier mobility, the weak absorbance of thin films and the poor chemical stability. To overcome some of these issues while retaining low production costs and scalability, research in polymer photovoltaics is about to converge towards organic-inorganic hybrid architectures where heterojunction is formed between inorganic acceptors from group IV, III-V, IV-VI and organic compounds (small molecules, oligomers, polymers). Most recently, hybrid photovoltaic based on Group III-V and organic compounds were reported in the literature, which seem rather promising for the achievement of high power conversion efficiencies in a near future. The mainstream III-V compounds have demonstrated several advantages for photovoltaic application. The main advantage in the use of such “traditional” semiconductors lies in their high carrier mobility, optimal staggered band alignment relative to the most common conjugated polymers, and the possibility to finely engineer their band structure and density of states through the conventional methods of alloying, doping, and heterostructure formation. Despite substantial progress in hybrid polymer/III-V photovoltaic devices, the primary processes of exciton dissociation and charge transfer at the heterointerface of these unconventional systems, and the coupling between the excitonic species of the polymer and ii the extended states of the inorganic semiconductor are completely unknown. The understanding of these fundamental issues in polymer/III-V composites will provide a general tool for the engineering of hybrid architectures, with high potential to impact the entire organic photovoltaic research field. In this thesis, we address some of the above issues in a prototype hybrid system based on ntype GaAs and a typical hole transporting conjugated polymer regioregular Poly(3- hexylthiophene-2,5-diyl) (rr-P3HT). Hybrid GaAs/P3HT heterointerface is a very interesting localized/delocalized system where conjugated polymer with discrete and localized orbitals meets inorganic semiconductor with continuous bands of delocalized quantum states. We investigate the interactions of the polymer with GaAs surfaces using model systems, P3HT/GaAs (111B and 110). We approach this problem from both theoretical and experimental points of view by using density functional theory (DFT) calculations and combining various spectroscopy measurements. Our goal is to address some issues of fundamental scientific interest regarding charge transfer processes in organic-inorganic heterointerfaces. In this dissertation, addition to charge photogeneration and charge transfer study of hybrid GaAs/P3HT heterointerfaces, long-lived photoexcitations in pristine P3HT film, transient reflectance spectra of GaAs, the effect of polarity of GaAs surface on charge transfer, and correlation between transient absorption and reflectance spectroscopy are also studied. Our ultrafast spectroscopy provides strong evidence of ambipolar charge transfer of electrons and holes across the GaAs/P3HT heterointerface, where signatures of charge transfer are manifested by presence of long-lived photoinduced absorption and photobleaching by exciting either above or below the polymer optical gap. We demonstrate that ambipolar charge transfer can be regarded as an interesting new concept to optimize photovoltaic power conversion efficiency of hybrid organic-inorganic devices.