LIQUID CRYSTAL MIXTURES FOR ORGANIC PHOTOVOLTAICS

Lead Research Organisation: University of Hull
Department Name: Physical Sciences

Abstract

Organic photovoltaics are a very promising approach to achieve efficient and low-cost solar energy conversion. They may also be required to power a new generation of disposable organic electronic devices which are expected to create new consumer markets in intelligent packaging, electronic labelling etc. We propose a novel type of organic solar cell for these applications based on polymerisable liquid crystals. Thin films of these materials are polymerised when exposed to ultraviolet light and so become insoluble. They are usually transparent and insulating and are used to improve the visibility of liquid crystal displays at wide viewing angles. We have recently modified these materials to be visible-light absorbing and semiconducting and they can be designed to conduct either electrons (negative charge) or holes (positive charge). This new class of liquid crystal in solar cells allows two new device configurations which solve problems associated with these organic approaches. They are also amenable to large-area, low-cost processing. Organic solar cells often contain a thin film of an electron transporting material on top of a layer of hole transporting material sandwiched between two electrical contacts. When sunlight is incident, charge is generated, separated at the interface between the layers and transported to the contacts to provide electrical power. Sometimes blends of electron and hole transporting organic materials are used in a single layer structure. In the first of our novel configurations, nanoscale liquid crystal sponges will be used to form a large-area interface between layers of hole and electron transporting materials. This increases the chances of charge separation and so improves the power efficiency of the solar cell. The second configuration uses blends of polymerisable LCs whose nanoscale structure can be controlled and optimised by exposing the films to ultraviolet light. The high viscosity of the materials after polymerisation should ensure that the nanoscale structure has long term stability. This should also improve efficiency. The materials can also be patterned using ultraviolet light to allow integration with other electronic or photonic devices. An interdisciplinary approach will be used to optimise the performance of solar cells based on these two concepts. An organic chemist will design and make new materials which will be analysed and incorporated in devices by a physicist. Rapid feedback from material and device analysis will inform the design and synthesis of improved materials. Two classes of materials which absorb visible light will be made to conduct electron and holes respectively. Molecular engineering will be used so that the materials are liquid crystals at room temperature, which ensures that the nanoscale structure of the blends can be varied during ultraviolet exposure. We will measure the key material and blend parameters, such as absorption, charge mobility and lifetime, molecular energy levels, efficiency of light emission, etc. to understand how material and blend properties affect device performance. A range of strategies will be used to optimise the nanoscale structure in both types of devices. Atomic force and electron microscopy as well as scattering techniques will be used to measure the spatial scale of this structure. Different material, processing and device configurations will be systematically investigated to optimise the device efficiency.
 
Description We have developed new semiconducting liquid crystals suitable for a novel type of organic solar cell, which may be used to power products such as radio frequency identification tags, electronic paper, disposable electronic and intelligent labelling based on plastic electronics. These materials enable new photovoltaic device configurations which are compatible with low cost manufacturing. We have also developed a new way to photopattern organic semiconductors with nanoscale surface structures which may have major applications in photonics as well as solar cells. The results from this grant have resulted in one patent, numerous publications and are being exploited by our new spinout company.
Organic photovoltaics are a very promising approach to achieve efficient and low-cost solar energy conversion. They may also be required to power a new generation of disposable organic electronic devices which are expected to create new consumer markets in intelligent packaging, electronic labelling etc. We proposed a novel type of organic solar cell for these applications based on polymerisable liquid crystals. Thin films of these materials are polymerised when exposed to ultraviolet light and so become insoluble. They are usually transparent and insulating and are used to improve the visibility of liquid crystal displays at wide viewing angles. We have recently modified these materials to be visible-light absorbing and semiconducting and they can be designed to conduct either electrons (negative charge) or holes (positive charge). This new class of liquid crystal in solar cells allows two new device configurations for organic photovoltaics. They are also amenable to large-area, low-cost processing.
Organic solar cells often contain a thin film of an electron transporting material on top of a layer of hole transporting material sandwiched between two electrical contacts. When sunlight is incident, charge is generated, separated at the interface between the layers and transported to the contacts to provide electrical power. In the first of our novel configurations, nanoscale liquid crystal sponges were used to form a large-area interface between layers of hole and electron transporting materials. This increases the chances of charge separation and so improves the power efficiency of the solar cell. We studied a large range of electron and hole transporting materials and showed a correlation between photovoltaic performance and the spatial scale of the interface. The second configuration uses materials which were patterned with ultraviolet light. A nanoscale surface relief grating is spontaneously generated on irradiation of a light-emitting liquid crystalline semiconducting thin film through a phase mask. No post-annealing or wet etching step is required bringing the potential for high-throughput fabrication. Gratings deeper than the original film thickness are made with periods as small as 265 nm. Grating formation is attributed to mass transfer, enhanced by co-operative effects, from dark to illuminated regions. We show that the performance of a bilayer organic photovoltaic is improved by incorporation of the grating.
An interdisciplinary approach was used to optimise the performance of solar cells based on these two concepts. An organic chemist designed and made new materials which were analysed and incorporated in devices by a physicist. Rapid feedback from material and device analysis informed the design and synthesis of improved materials. Some of the novel materials were light-emitting and were tested in OLEDs. We measured the key material and blend parameters. Some unexpected properties were found which can be exploited to improve device performance. For example, electron mobility is much higher in blends of our materials rather than in the pure material. We also show by experiment and theoretical modelling that intermolecular interactions significantly change the absorption spectra of the materials.
Exploitation Route Licensing of patents or use of Hull synthesised materials in solar cells
Sectors Chemicals,Digital/Communication/Information Technologies (including Software),Electronics,Energy

 
Description Higher Education Innovation funding
Amount £206,148 (GBP)
Organisation Higher Education Funding Council for England 
Sector Public
Country United Kingdom
Start 05/2012 
End 06/2014
 
Description Research Project grants
Amount £217,972 (GBP)
Funding ID F/00 181/S 
Organisation The Leverhulme Trust 
Sector Academic/University
Country United Kingdom
Start 09/2010 
End 11/2012
 
Description MartaFellowship 
Organisation University of Zaragoza
Country Spain 
Sector Academic/University 
PI Contribution Hosted Spanish government funded fellow from University of Zaragoza, Shared know-how
Collaborator Contribution Fellow contributed to research activities at Hull
Impact Joint papers
Start Year 2008
 
Description Solution processed p-i-n doped OLEDs 
Organisation University of Malaya
Country Malaysia 
Sector Academic/University 
PI Contribution It is difficult to produce multi-layered OLEDs using solution processing because of the intermixing of adjacent layers unless incompatible solvents are used. The Organophotonics group at Hull has recently pioneered a liquid crystal approach to organic semiconductors, whereby thin, uniform films of light-emitting and charge-transporting nematic materials are formed as insoluble polymer networks by photopolymerisation using ultraviolet light. This enables the solution processing of multi-layers devices. Additional advantages are polarised emission and that the films can be patterned photolithographically. This technology is transferred to the University of Malaya.
Collaborator Contribution Dr Woon at Malaya has recently obtained research funding to develop p and n doped solution processed OLEDs and so has expertise in the technology. He also has specialised equipment unavailable at Hull. This expertise is transferred to the University of Hull who also access the equipment at Malaya.
Impact Joint papers in preparation
Start Year 2013
 
Title Liquid Crystalline Interpenetrating Polymer Networks 
Description A photovoltaic cell is provided. The photovoltaic cell can be an interconnecting liquid crystalline polymer network. Reactive mesogens of the formula B-S-A-S-B wherein A is a chromophore, S is a spacer and B is an end group susceptible to polymerization are used in the manufacture of the interconnecting liquid crystalline polymer network. 
IP Reference US 11/547,324 
Protection Patent application published
Year Protection Granted
Licensed No
Impact This patent provides part of the know-how leading to the formation of a spin-out company, Polar OLED, by the University of Hull.
 
Company Name Polar OLED 
Description Polar OLED Ltd is the unique supplier of crosslinkable, liquid crystal based polymer materials for use as photoalignment layers, insoluble charge transport layers or light emitting polymers in OLED optoelectronic devices. Polar OLED is a spin out company from the University of Hull that has developed novel materials for use in optoelectronic devices. It is working alongside the IP Group plc and with manufacturers to develop and commercialise the patented materials and include them in commercial OLED systems. It is based at the Centre for Process Innovation, Sedgefield. 
Year Established 2008 
Impact N/A
 
Description Alumni talk 2013 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Increased alumni awareness of current research in facullty

N/A
Year(s) Of Engagement Activity 2013
 
Description ChelthenhamFestival 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Talk promoted understanding of applications of liquid crystals research and its current research direction at the University of Hull.

Media interest in Hull connection with liquid crystals
Year(s) Of Engagement Activity 2013
 
Description Christmas Lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Talk sparked interest in liquid crystal science in Hull, its heritage and current research directions.

Local media coverage of Liquid Crystal activities in HUll University
Year(s) Of Engagement Activity 2013
 
Description Faculty Science festival 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Workshop related to current research topics with activities for children and adults

Increased profile of cutting edge research at the University of Hull
Year(s) Of Engagement Activity 2012,2013,2014
 
Description Heritage Open Day 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Improved awarenesss of heritage and current research in liquid crystals at the University of Hull

N/A
Year(s) Of Engagement Activity 2014
 
Description RSCpublic lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? Yes
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Lively discussion followed talk

Increased awareness of cutting edge research into liquid crystals at Hull University
Year(s) Of Engagement Activity 2014
 
Description Royal Society 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Workshop sparked interest in non-traditional applications of liquid crystals

N/A
Year(s) Of Engagement Activity 2010
 
Description University's week2014 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Talk and workshop sparked interest in non-traditional applications of liquid crystals.

N/A
Year(s) Of Engagement Activity 2014