Transient optoelectronic analysis of evaporated and solution processed organic solar cells
File(s)
Author(s)
Pacalaj, Richard
Type
Thesis
Abstract
This thesis focusses on performance limitations in vapour- and solution-processed organic solar cells (OSCs), particularly on the analysis of non-geminate recombination kinetics and voltage losses by means of transient optoelectronic techniques. Following an assessment of the climate mitigating potential of OSCs, the theoretical background and methodology employed in this thesis are introduced. The results section will be separated into two parts focusing on vapour- and solution-processed OSCs, respectively.
In the first part, the morphology-function-relationship in a state-of-the-art evaporated OSCs is systematically investigated. It is further divided into two sub-chapters focusing on the influence of the active layer morphology on the collection efficiency and strategies to increase the open circuit voltage, respectively, two key bottlenecks of evaporated OSCs. Varying the substrate temperature during deposition can significantly enhance the performance of evaporated OSCs. Morphological, optical and optoelectronic characterisations demonstrate that increased domain size and purity lead to better charge transport explaining the observed performance increases. However, comparison with solution-processed OSCs suggests that transport is a key limitation of evaporated OSCs. The second study on evaporated OSCs focusses on the voltage losses in planar and bulk heterojunctions (PHJs/BHJs) and the trade-off with charge generation and collection.
The second part addresses voltage losses in solution-processed PHJ and BHJ OSCs using different non-fullerene acceptors including sub-chapters on IT-4F and Y6. While IT-4F shows similar qualitative voltage trends to the evaporated BHJs/PHJs, Y6 shows additional orientation dependent electrostatic effects on the photovoltage. Despite the potential for reduced voltage losses, the studied PHJs remained limited by poor charge generation or extraction. In the final sub-chapter, charge generation in neat Y6 is assessed. Bias dependent photoluminescence measurements suggest efficient charge generation but only at reverse bias. Optoelectronic characterisations show that non-geminate recombination is not the limiting factor behind the poor OSC performance neat Y6 contrary to previous reports.
In the first part, the morphology-function-relationship in a state-of-the-art evaporated OSCs is systematically investigated. It is further divided into two sub-chapters focusing on the influence of the active layer morphology on the collection efficiency and strategies to increase the open circuit voltage, respectively, two key bottlenecks of evaporated OSCs. Varying the substrate temperature during deposition can significantly enhance the performance of evaporated OSCs. Morphological, optical and optoelectronic characterisations demonstrate that increased domain size and purity lead to better charge transport explaining the observed performance increases. However, comparison with solution-processed OSCs suggests that transport is a key limitation of evaporated OSCs. The second study on evaporated OSCs focusses on the voltage losses in planar and bulk heterojunctions (PHJs/BHJs) and the trade-off with charge generation and collection.
The second part addresses voltage losses in solution-processed PHJ and BHJ OSCs using different non-fullerene acceptors including sub-chapters on IT-4F and Y6. While IT-4F shows similar qualitative voltage trends to the evaporated BHJs/PHJs, Y6 shows additional orientation dependent electrostatic effects on the photovoltage. Despite the potential for reduced voltage losses, the studied PHJs remained limited by poor charge generation or extraction. In the final sub-chapter, charge generation in neat Y6 is assessed. Bias dependent photoluminescence measurements suggest efficient charge generation but only at reverse bias. Optoelectronic characterisations show that non-geminate recombination is not the limiting factor behind the poor OSC performance neat Y6 contrary to previous reports.
Version
Open Access
Date Issued
2024-02-18
Date Awarded
01/12/2024
License URL
Advisor
Durrant, James Robert
Kim, Ji-Seon
Sponsor
Engineering and Physical Sciences Research Council
Global Research Laboratory (Korea) South
Grant Number
EP/T028513/1
EP/L016702/1
NRF-2017K1A1A2013153
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)