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Title: Copper pseudohalides as solution-processable hole-transport materials for opto/electronic applications
Author: Wijeyasinghe, Nilushi
ISNI:       0000 0004 7655 6371
Awarding Body: University of London
Current Institution: Imperial College London
Date of Award: 2018
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This thesis presents the development of novel copper pseudohalide hole-transport layers (HTLs) for thin-film transistors (TFTs), organic photovoltaic (OPV) cells, perovskite solar cells (PSCs), and organic light-emitting diodes (OLEDs). Their impact on device performance is assessed relative to two reference HTLs: a conventional polymer HTL, and copper(I) thiocyanate (CuSCN) deposited via an n-alkyl sulphide solvent (diethyl sulphide, DES). The first experimental chapter demonstrates aqueous ammonia (NH3 (aq)) as a novel processing solvent for CuSCN, which produces HTLs with greatly enhanced electronic and structural properties. CuSCN/NH3 HTLs exhibit exceptional anode planarisation properties and mean field-effect hole mobility (μ) of 0.05 cm2 V-1 s-1. OPV cells and PSCs employing a CuSCN/NH3 HTL consistently outperform devices utilising a reference HTL by achieving maximum power conversion efficiency of 10.7% (OPV) and 17.5% (PSC). Next, a fluorinated fullerene (C60F48) is utilised as a p-dopant for CuSCN/DES. Analysis of material and device characterisation data reveal strong evidence of a successful p-doping process. Mean μ of 0.12 cm2 V-1 s-1 is measured in TFTs based on CuSCN:C60F48 (0.5 mol%), which is a twelvefold increase relative to pristine CuSCN. Additional advantages include an order of magnitude reduction in contact resistance, a dramatic increase in bias stability, and a change in the dominant hole-transport mechanism from trap limited conduction to percolation conduction. Optimised CuSCN:C60F48 HTLs also outperform reference HTLs in OPV applications; substantial increases in fill factor and device yield are observed. Finally, the third experimental chapter reports on a novel wide-bandgap (≥3.1 eV) p-type semiconductor, copper(I) selenocyanate (CuSeCN). Its electronic, structural and optical properties are predicted using density functional theory calculations and verified using numerous experimental techniques. CuSeCN/DES layers annealed at 140 ̊C exhibit excellent performance in TFTs, OPV cells, and OLEDs. Hence, this thesis demonstrates the tremendous potential of copper pseudohalides as universal HTLs for opto/electronics.
Supervisor: Anthopoulos, Thomas Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral