InP Quantum Dots for Hybrid Photovoltaic Devices
Creators: name: Boonkoom, Thitikorn
Publisher: name: Imperial College London
Year: 2012

description: Significant research efforts have been directed towards the development of solar cells comprising blends of conjugated polymers and II-VI inorganic semiconductors (e.g. CdSe and CdS). Despite recent advances in the power conversion efficiency of such devices, the toxicity of Cd-based materials remains a concern with regard to widespread implementation. This thesis focuses on alternative (lower toxicity) InP nanocrystals for use as electron acceptors and light-harvesting materials in solution-processed polymer solar cells. In this thesis a combination of novel materials design/processing, transient absorption spectroscopy (TAS) and time-resolved photoluminescence spectroscopy (TRPL) is used to study the charge generation in InP:polymer photoactive layers. These studies are complimented by morphological characterisation of the photoactive layers as well as device studies. One aim of this thesis is the elucidation of quantitative structure function relationships that can be used to guide the design of new hybrid nanocomposite materials for photovoltaic devices. As such the data presented in this thesis helps to advance the present day understanding how hybrid solar cells work. The first chapter focuses on the synthesis of InP quantum dots (QDs) using an organometallic reaction. The aim of the work in this chapter was to prepare InP QDs with a size that provides an appropriate energy offset relative to the selected the electron donating polymer, poly(3-hexylthiophene) (P3HT). Detailed studies on the growth of InP QDs and how the reaction conditions affect the particle size are provided. The process of ligand exchange from hexadecylamine (HDA) to pyridine prior to blending with P3HT is also described. The second chapter focuses on charge transfer between the P3HT and the InP QDs which is a key process for achieving efficient photovoltaic device operation. Steady state and time-resolved photoluminescence and absorption spectroscopy were used to better understand the parameters influencing charge separation. After the blending and annealing conditions had been optimised to maximise the yield of photogenerated charges, the P3HT:InP blend was found to provide approximately twice yield of standard P3HT:PCBM blends. In addition, the decay lifetime of the polaron in P3HT:InP was found to be longer than that of P3HT:PCBM, suggesting the P3HT:InP blend is a promising active layer material for hybrid solar cells. The third chapter focuses on the fabrication and characterisation of hybrid solar cells. The fabrication conditions were optimised before carrying out detailed studies on the effect of thermal annealing. Although the device performance improved significantly with increasing annealing temperature, the net photocurrent was found to be low, compared to standard P3HT:PCBM devices, suggesting poor charge transport within the device. Nevertheless, if the charge transport can be improved, P3HT:InP still has potential to provide efficient hybrid solar cells. The last result chapter focuses on preliminary studies of quantum dot based light emitting diodes (QDLEDs) using InP QDs as light emitters. ZnO was used as electron transporting and hole blocking layer and poly(9,9-dioctylfluorene) (PFO) as a host medium and a hole transporting layer. The device structure and the PFO:InP blend composition were investigated to obtain QDLEDs with electroluminescence from the InP quantum dots. The findings suggest that ZnO plays a key role in suppressing the electroluminescence of PFO, most likely due to the hole blocking effect of the ZnO layer. Despite the low efficiencies of the InP-based QDLEDs, the results suggest that InP QDs are potential candidates for emitters in QDLEDs.