- Title
- Optical and electrical characterisation of bulk heterojunction and nanoparticulate morphologies for printed large area organic photovoltaics
- Creator
- Al-Mudhaffer, Mohammed Fadhil
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis has investigated the charge generation, transport and recombination processes in organic photovoltaic devices (OPV) composed of three different photoactive layer morphologies and a range of different photoactive layer materials. A systematic toolbox was developed to understand the optoelectronic processes that generate the charge carriers in the photoactive layer. Optical modelling was used to simulate the internal light absorption in the photoactive layer of the OPV devices after accounting for interference effects between various layers in the multilayer systems. Furthermore, to gain a full understanding of the charge generation processes, various electronic techniques were performed to examine these processes further, including photoluminescence (charge dissociation), dark and photoCELIV (charge carrier transport), and transient photovoltage and electrochemical impedance spectroscopy (charge carrier recombination). Optical modelling was performed by measuring the dielectric constants of all interface and active layer materials, then combining them into a Transfer Matrix Method which accounted for interference effects at interfaces using the Fresnel equations. The individual absorption of the photoactive layer alone could then be extracted from this model. The outcome accuracy of the optical modelling was compared with measured data and a good agreement was found between the model and measured data using standard reference devices with well-known absorption characteristics. The optical modelling method was then combined with a suite of steady state and transient electrical measurements to create an investigation toolbox that was applied to examine limiting photophysics in a range of OPV device architectures and active layer materials of inerest. The first system examined was a new low cost acceptor material which is synthesized as a crude mixture of mono, bis and tris indene-adducted fullerene isomers (ICxA), desirable for its suitability to a low cost printing manufacturing process. The light absorption in the active layer was examined and the results confirmed there is no significant loss in the light absorption in P3HT:ICxA active layers in comparison to individual fullerene acceptors PCBM, ICMA, ICBA and ICTA. The internal quantum efficiency (IQE) of the P3HT:ICxA devices was found to decrease with increasing active layer thickness, producing a poor charge extraction at higher thickness where the printing fabrication process operates. From transient electrical measurements for the P3HT:ICxA devices, a high carrier life time and poor carrier mobility was demonstrated, found to arise from an abnormally high trap state density in the P3HT:ICxA devices. This was subsequently shown to arise from a misalignment in the energy level of the individual electron carriers ICMA, ICBA and ICTA, which could be remedied by modifying the synthesis to control the ratio of the three isomers in the ICxA mixture. The characterisation toolbox was then deployed to examine the origin of the low performance of OPV devices prepared with a nanoparticle (NP) active layer morphology in contrast to the highly efficient bulk heterojunction (BHJ) morphology. Optical modelling revealed that the NP active layer structures did not exhibit any discrete optical scattering or plasmon effects. However, the IQE of devices was found to be 24% for NP devices in comparison to 76% for the BHJ structure. Photoluminescence measurements were conducted to analyse the dissociation efficiency in the NP devices, which was found to strongly correlate with the IQE. Transient electrical measurements confirmed that the majority of the generated charge had a higher diffusion length than the active layer thickness at short circuit and could be collected at the electrodes. The origin of low photocurrent at short circuit in nanoparticle devices was thus confirmed to arise from a poor exciton dissociation efficiency in the polymer phase of the devices, an issue arising from the core-shell NP formation morphology. Finally, the concept of ternary blend OPV active layers was examined in order to circumvent the issue of the performance-limiting core shell morphology in the NP OPV devices. Results from optical modelling of planer ternary blend devices proved that energy transfer can occur between the primary donor (P3HT) and the ternary additive (a substituted squaraine molecule, DIBSq) over a distance of ~ 6 nm. When DIBSq was subsequently applied to create ternary blend BHJ structures with varying additive content, only a slight photocurrent increase was detected above the level expected from enhanced light absorption. This improvement was attributed to a very limited amount of energy transfer associated with morphological changes in the active layer. However, when the ternary blend concept was applied to NP devices, the optical modelling results and device performance data analysis confirmed there was an improvement in photocurrent of more than ~70% compared to what is expected from enhanced light absorption from the DIBSq additive. This photocurrent improvement was attributed to energy transfer effects which are uniquely powerful in a ternary blend core-shell structure where the exciton dissociation interface area is substantially improved through energy transfer from the outer donor shell to the intermediate DIBSQ additive and the inner acceptor core.
- Subject
- organic photovoltaics; optical modelling; trensient measurements; large scale orgaic photovoltaics; nanoparticle organic photovoltaics
- Identifier
- http://hdl.handle.net/1959.13/1392650
- Identifier
- uon:33440
- Rights
- Copyright 2018 Mohammed Fadhil Al-Mudhaffer
- Language
- eng
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