Klaudia Rejnhard

This project works in collaboration with Pegasus (materials supplier) and SPTS (equipment supplier within the semiconductor industry). The aim is to investigate the use of a new thin film deposition technology known as Molecular Vapour Deposition (MVD) to create functional materials – alongside potentially creating the semiconductors themselves. The project will involve not only the development of deposition processes, but also chemical, optical and structural analysis of the resultant materials. The project will also investigate possible applications across a range of optoelectronic areas. MVD as a stand-alone deposition method, has the potential to be versatile and effective in integrative semiconductor systems.”

 

Drew Riley

Power conversion efficiency depends on a high internal quantum efficiency, indicative of a high short circuit current, as well as a high photovoltage, indicative of a high open circuit voltage (Voc). A near linear increase in Voc with decreasing temperature has been shown experimentally and theorized through detailed balance, with a slope proportional to the external quantum efficiency (EQE). Traditionally the EQE is measured through electro-luminescence, under dark conditions with injection current equal to operational short circuit current or through photoluminescence, with generated densities equal to AM 1.5. Both of these techniques are done far from operational conditions of a solar cell. EMPLQY attempts to bridge this gap by measuring the external quantum efficiency of a cell under operation conditions. By measuring the external quantum efficiency of a cell under operational conditions we can learn valuable information about the origins of the Voc.

 

Nick Burridge

In this project the design, implementation and fabrication of inverted metal grids will be researched in order to find if there are improvements in the efficiencies of transparent conducting electrodes with these metal grids incorporated. This project will involve the optimisation of many fabrication processes such as etching, deposition and solution processing, in order to achieve the desired architecture for the metal grids. In addition, the use of simulations throughout the project will guide the direction of the fabrication efforts, which will enable the ideal grid dimensions for the most highly efficient TCEs possible to be found. Finally, if high transparency, high conductivity TCEs can be produced using these grids, they will be incorporated into OLEDs and solar cells for testing of their properties.

Drew Riley

Frequency mixing has a long tradition in inorganic semiconductors as a method of probing material parameters such as second order recombination or ideality factors. Here this technique is expanded. Applying oscillating carrier injections and extractions while monitoring current signals in the fourier domain one can measure the voltage dependence of various 2nd derivatives such as dJ/dGdV. This can lead to the extraction of material parameters (in certain limits) or the insight into the relation between first and second order recombination. We are using this technique to probe the relationship between true second order recombination (involving one mobile electron and one mobile hole) and pseudo-second order recombination (involving one mobile electron (hole) and a dark injected hole (electron).

 

Christian Osborne

 

In this project we are investigating the relationship between junction thickness and device performance using solution processed bulk hererojunction devices, coupled with electro-optical characterisation techniques and simulations.

Drew Riley

The distance an exciton can travel in a semiconductor (known as diffusion length) is an important quantity in the design and optimization of organic solar cells as there is a linear relationship between diffusion length and short circuit current. The two primary variables that limit the diffusion length are the diffusion coefficient (which describes how far an unimpeded exciton will travel) and the exciton-exciton annihilation coefficient (describing the interaction between excitons). Here we use time-resolved photoluminescence to measure the radiative recombination occurring over time and from this can ascribe the diffusion coefficient as well as the exciton-exciton annihilation strength, giving an estimate of the diffusion length undergone in these films.

Dr. Wei Li

 

Organic photovoltaics are arising as a new generation of film solar cells due to their advantages including low-cost, lightweight, and solution processability. This exhibits a promising prospect in the application of wearable flexible electronics. In this project, our aim is to apply optical and electronic techniques to understand materials and device properties of organic solar cells, providing more basic fundamentals for device physics and fabricating world-record organic solar cells.

 

Nasim Zarrabi

The measurement of exciton diffusion lengths in disordered organic semiconductors represents a significant challenge which is only partially addressed by current methodologies and associated models. A useful probe in this regard is photoluminescence quenching in controlled morphologies and thin film architectures such as bilayers or blends.  With this in mind we present a new method to measure very low photoluminescence quenching yields in the steady state based upon probing the charge generation efficiency. We applied this method to three organic semiconductors (P3HT, PC70BM and PCDTBT) which we minimally doped with exciton quenching molecules. Using appropriate 3D analytical and numerical models we show that there is a combination of two distinct processes involved in exciton quenching: (i) an inefficient quenching pathway which appears only at very low quencher concentrations but corresponds to a quenching volumes as large as 1000 nm3; and (ii) an efficient diffusive quenching pathway that yields 3D exciton diffusion lengths for these materials under continuous illumination conditions Our results are consistent with the standard picture that considers exciton diffusion predominantly responsible for exciton migration in organic semiconductors. However, the inefficient secondary pathway has not been previously reported and may be indicative of exciton delocalisation in the steady state – still a controversial topic in organic semiconductors.

Nasim Zarrabi

 

The photocurrent and open-circuit voltage (VOC) generated by a solar cell is predominantly defined by the bandgap of the used semiconductor through the principle of detailed balance. For donor-acceptor organic solar cells, it has been shown that the intermolecular charge transfer (CT) states define the effective gap energy and, thereby, the VOC. In this work, we employ ultrasensitive photocurrent measurements and detect new sub-bandgap states [1] with energies far below the CT states. The calculated radiative limit of the VOC  with these sub-bandgap states included can be as low as the experimentally measured VOC. We explain this apparent violation from the detailed balance principle by providing evidence that the observed low-energy sub-bandgap states are associated with mid-gap trap states. These states participate in photocurrent generation via a non-linear process of optical release which up-converts these states to CT states.[2] By accounting for these processes, we show that the dark current and VOC of organic photovoltaic devices, including solar cells and photodetectors, can be described by a two-diode model with dark saturation currents extracted from the ultrasensitive external quantum efficiency and well-defined ideality factors. Finally, due to the contribution of the trap states to the reverse bias dark current, the expected detectivities for organic photodetectors is several orders of magnitude lower than what predicted before. [3]

 

[1] S. Zeiske, C. Kaiser, P. Meredith, and A. Armin, ACS Photonics, 7, 1, 256-264 (2020).

[2] O. Sandberg, N. Zarrabi, S. Zeiske, P. Meredith, and A. Armin, Phys. Rev. Lett. In Press (2020).

Dr. Gregory Burwell

 

As solution-processable materials are better understood, optoelectronic devices with real-world applications become a more achievable reality.  At R&D scales, optoelectronic devices such as photovoltaics (PV) and light-emitting diodes (LEDs)  tend to be created at small pixel sizes (< 1cm2). To be practical, these devices must be scaled to larger areas. This scale-up presents new technical challenges, such as the resistance encountered at the transparent conductive electrode (TCE), and the uniform coating of materials over larger areas. In this work, techniques from traditional inorganic semiconductor processing (“hard fab”) are combined with solution-processing techniques (“soft fab”) to overcome these challenges improve large-area device performance.

Dr. Rashid Mohdyusoff

 

In this study, we intend to explore the role of wide bandgap halid perovskite on the device performance. At the same time, we are also trying to understand how the incorporation of halide perovskite significantly improved the open circuit voltage and minimize the non-radiative loss in the system

 

Stefan Zeiske

 

This project focuses on the investigation of charge generation and recombination on a variety of different organic solar cells and thin optoelectronic devices and seeks to get a better, fundamental understanding of these processes. Based on these findings, main loss channels for charge carries in photovoltaic devices can be identified, manufacture processes of solar cells customized, and the power-conversion-efficiency of current solar cells and future photovoltaic systems enhanced.  The project includes the fabrication of thin film solar cells and the use of different electro-optical characterisation techniques including ultra-sensitive external quantum efficiency (EQE) and sensitive intensity dependent photocurrent (IPC).

 

Christina Kaiser

This project studies the influence of sub-gap energy states on the efficiency of organic photodetectors. This includes obtaining a fundamental understanding of the spectral response in the sub-gap spectral range as well as understanding the influence of sub-gap states on the dark and noise current.

 

Robin Kerremans

The optical and electrical properties of semiconductors are fundamental in predicting the response of novel devices that incorporate them. In this project, we take a look at how to best determine these properties through experimental measurement, and how to incorporate them into full device models, using mathematical methods such as transfer matrix and drift-diffusion. The focus lies on new types of opto-electronic devices, in particular next-generation solar cells that use perovskites and non-fullerenes, with the aim to optimize the fabrication and performance of such devices.

Group Foci and Project List

The SAM in Sêr SAM stands for Sustainable Advanced Materials – that is to say future materials with advanced electronic or optoelectronic functionality that can be processed by low embodied energy methods and that contain earth abundant / low toxicity components. These materials are the focus of our primary research interests and include systems such as organic semiconductors, perovskites and new dielectrics that can be deposited at low temperature.

Our research and innovation activities also involve the development of new low energy manufacturing processes, particularly from solution. We are a combined experimental-simulation-theory research group and at a fundamental scientific level seek to establish robust structure-property relationships to both explain materials and device performance and characteristics, and guide rationale design of new systems. We have particular expertise in electro-optics – namely the physics of how light and electrical charge interact in materials particularly semiconductors. We make all of simulation tools such as transfer matrix analysis and optical constant determination available as open source on our Knowledge Sharing Portal.

Our main areas of application and technology interest include: next generation solar cells; photodetectors (both narrow and broad band); bioelectronics (that is the interfacing of electronic read-write systems with biological entities); light emitting diodes; and sensor platforms.

Sêr SAM is now part of a greater collective of cross-platform research activities in the Centre for Integrative Semiconductor Materials (CISM)] to be established in a new £30M device fabrication facility due for completion in early 2022. CISM is a futuristic concept focused on breaking down the silos of different semiconductor systems (silicon, compound, next generation, wide bandgap, low dimensional) to deliver new technology opportunities.

We hope you enjoy reading about our project portfolio.   

Advanced Functional Coatings for Integrative Semiconductor Materials and Devices

 
Klaudia Rejnhard

This project works in collaboration with Pegasus (materials supplier) and SPTS (equipment supplier within the semiconductor industry). The aim is to investigate the use of a new thin film deposition technology known as Molecular Vapour Deposition (MVD) to create functional materials – alongside potentially creating the semiconductors themselves. The project will involve not only the development of deposition processes, but also chemical, optical and structural analysis of the resultant materials. The project will also investigate possible applications across a range of optoelectronic areas. MVD as a stand-alone deposition method, has the potential to be versatile and effective in integrative semiconductor systems.”

Charge Generation and Recombination of Next Generation Thin Film Optoelectronic Devices

 
Stefan Zeiske
 
This project focuses on the investigation of charge generation and recombination on a variety of different organic solar cells and thin optoelectronic devices and seeks to get a better, fundamental understanding of these processes. Based on these findings, main loss channels for charge carries in photovoltaic devices can be identified, manufacture processes of solar cells customized, and the power-conversion-efficiency of current solar cells and future photovoltaic systems enhanced.  The project includes the fabrication of thin film solar cells and the use of different electro-optical characterisation techniques including ultra-sensitive external quantum efficiency (EQE) and sensitive intensity dependent photocurrent (IPC).

Electro-modulated Photoluminescence Quantum Yield (EMPLQY)

 
Drew Riley

Power conversion efficiency depends on a high internal quantum efficiency, indicative of a high short circuit current, as well as a high photovoltage, indicative of a high open circuit voltage (Voc). A near linear increase in Voc with decreasing temperature has been shown experimentally and theorized through detailed balance, with a slope proportional to the external quantum efficiency (EQE). Traditionally the EQE is measured through electro-luminescence, under dark conditions with injection current equal to operational short circuit current or through photoluminescence, with generated densities equal to AM 1.5. Both of these techniques are done far from operational conditions of a solar cell. EMPLQY attempts to bridge this gap by measuring the external quantum efficiency of a cell under operation conditions. By measuring the external quantum efficiency of a cell under operational conditions we can learn valuable information about the origins of the Voc.

High Performance Perovskite Solar Cells with Wide Bandgap Halide Perovskite


Dr. Rashid Mohdyusoff
 
In this study, we intend to explore the role of wide bandgap halid perovskite on the device performance. At the same time, we are also trying to understand how the incorporation of halide perovskite significantly improved the open circuit voltage and minimize the non-radiative loss in the system

Improving the Efficiency of Transparent Conducting Electrodes for Use in Opto-Electronics by Utilisation of an Inverted Metal Grid Architecture

 
Nick Burridge

In this project the design, implementation and fabrication of inverted metal grids will be researched in order to find if there are improvements in the efficiencies of transparent conducting electrodes with these metal grids incorporated. This project will involve the optimisation of many fabrication processes such as etching, deposition and solution processing, in order to achieve the desired architecture for the metal grids. In addition, the use of simulations throughout the project will guide the direction of the fabrication efforts, which will enable the ideal grid dimensions for the most highly efficient TCEs possible to be found. Finally, if high transparency, high conductivity TCEs can be produced using these grids, they will be incorporated into OLEDs and solar cells for testing of their properties.

Investigation of Fundamental Limits of the Specific Detectivity of Organic Photodetectors

 
Christina Kaiser

This project studies the influence of sub-gap energy states on the efficiency of organic photodetectors. This includes obtaining a fundamental understanding of the spectral response in the sub-gap spectral range as well as understanding the influence of sub-gap states on the dark and noise current.

Non-Linear Optical Current Spectroscopy (NLOCS)


Drew Riley

Frequency mixing has a long tradition in inorganic semiconductors as a method of probing material parameters such as second order recombination or ideality factors. Here this technique is expanded. Applying oscillating carrier injections and extractions while monitoring current signals in the fourier domain one can measure the voltage dependence of various 2nd derivatives such as dJ/dGdV. This can lead to the extraction of material parameters (in certain limits) or the insight into the relation between first and second order recombination. We are using this technique to probe the relationship between true second order recombination (involving one mobile electron and one mobile hole) and pseudo-second order recombination (involving one mobile electron (hole) and a dark injected hole (electron).

Making Large Thin Film Solar Cells – Scaling Physics

 
Christian Osborne
 
In this project we are investigating the relationship between junction thickness and device performance using solution processed bulk hererojunction devices, coupled with electro-optical characterisation techniques and simulations.

On the Thermodynamic Limit of Next Generation Photovoltaics


Nasim Zarrabi
 
The photocurrent and open-circuit voltage (VOC) generated by a solar cell is predominantly defined by the bandgap of the used semiconductor through the principle of detailed balance. For donor-acceptor organic solar cells, it has been shown that the intermolecular charge transfer (CT) states define the effective gap energy and, thereby, the VOC. In this work, we employ ultrasensitive photocurrent measurements and detect new sub-bandgap states [1] with energies far below the CT states. The calculated radiative limit of the VOC  with these sub-bandgap states included can be as low as the experimentally measured VOC. We explain this apparent violation from the detailed balance principle by providing evidence that the observed low-energy sub-bandgap states are associated with mid-gap trap states. These states participate in photocurrent generation via a non-linear process of optical release which up-converts these states to CT states.[2] By accounting for these processes, we show that the dark current and VOC of organic photovoltaic devices, including solar cells and photodetectors, can be described by a two-diode model with dark saturation currents extracted from the ultrasensitive external quantum efficiency and well-defined ideality factors. Finally, due to the contribution of the trap states to the reverse bias dark current, the expected detectivities for organic photodetectors is several orders of magnitude lower than what predicted before. [3]
 
[1] S. Zeiske, C. Kaiser, P. Meredith, and A. Armin, ACS Photonics, 7, 1, 256-264 (2020).
[2] O. Sandberg, N. Zarrabi, S. Zeiske, P. Meredith, and A. Armin, Phys. Rev. Lett. In Press (2020).

Optics and Materials of Advanced Organic Solar Cells


Dr. Wei Li
 
Organic photovoltaics are arising as a new generation of film solar cells due to their advantages including low-cost, lightweight, and solution processability. This exhibits a promising prospect in the application of wearable flexible electronics. In this project, our aim is to apply optical and electronic techniques to understand materials and device properties of organic solar cells, providing more basic fundamentals for device physics and fabricating world-record organic solar cells.

Opto-Electrical Interactions in Next Generation Semiconductor Thin Films and Devices

 
Robin Kerremans

The optical and electrical properties of semiconductors are fundamental in predicting the response of novel devices that incorporate them. In this project, we take a look at how to best determine these properties through experimental measurement, and how to incorporate them into full device models, using mathematical methods such as transfer matrix and drift-diffusion. The focus lies on new types of opto-electronic devices, in particular next-generation solar cells that use perovskites and non-fullerenes, with the aim to optimize the fabrication and performance of such devices.

Quantifying Three-Dimensional Exciton Diffusion Lengths in Organic Semiconductors under Operational Photovoltaic Condition

 
Nasim Zarrabi

The measurement of exciton diffusion lengths in disordered organic semiconductors represents a significant challenge which is only partially addressed by current methodologies and associated models. A useful probe in this regard is photoluminescence quenching in controlled morphologies and thin film architectures such as bilayers or blends.  With this in mind we present a new method to measure very low photoluminescence quenching yields in the steady state based upon probing the charge generation efficiency. We applied this method to three organic semiconductors (P3HT, PC70BM and PCDTBT) which we minimally doped with exciton quenching molecules. Using appropriate 3D analytical and numerical models we show that there is a combination of two distinct processes involved in exciton quenching: (i) an inefficient quenching pathway which appears only at very low quencher concentrations but corresponds to a quenching volumes as large as 1000 nm3; and (ii) an efficient diffusive quenching pathway that yields 3D exciton diffusion lengths for these materials under continuous illumination conditions Our results are consistent with the standard picture that considers exciton diffusion predominantly responsible for exciton migration in organic semiconductors. However, the inefficient secondary pathway has not been previously reported and may be indicative of exciton delocalisation in the steady state – still a controversial topic in organic semiconductors.

Scaling up of Solution-Processed Optoelectronic Devices


Dr. Gregory Burwell
 
As solution-processable materials are better understood, optoelectronic devices with real-world applications become a more achievable reality.  At R&D scales, optoelectronic devices such as photovoltaics (PV) and light-emitting diodes (LEDs)  tend to be created at small pixel sizes (< 1cm2). To be practical, these devices must be scaled to larger areas. This scale-up presents new technical challenges, such as the resistance encountered at the transparent conductive electrode (TCE), and the uniform coating of materials over larger areas. In this work, techniques from traditional inorganic semiconductor processing (“hard fab”) are combined with solution-processing techniques (“soft fab”) to overcome these challenges improve large-area device performance.

Time Resolved Photoluminescence and Exciton-Exciton Annihilation


Drew Riley

The distance an exciton can travel in a semiconductor (known as diffusion length) is an important quantity in the design and optimization of organic solar cells as there is a linear relationship between diffusion length and short circuit current. The two primary variables that limit the diffusion length are the diffusion coefficient (which describes how far an unimpeded exciton will travel) and the exciton-exciton annihilation coefficient (describing the interaction between excitons). Here we use time-resolved photoluminescence to measure the radiative recombination occurring over time and from this can ascribe the diffusion coefficient as well as the exciton-exciton annihilation strength, giving an estimate of the diffusion length undergone in these films.