Unravelling the working mechanisms of homoeopathic organic solar cells

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

Organic solar cells (OSC) are a highly active, interdisciplinary field of research drawing together the expertise of chemists, physicists, material scientists and engineers. The research is exciting not only in terms of fundamental science, but also in terms of potential positive impact on the economy and society. OSC have the potential to become a very cost-competitive, large area and versatile photovoltaic technology. Academic and industrial research have produced efficiencies exceeding 10% and brought OSC close to commercialisation.
Until recently, the architecture used for all efficient OSC was based on the bulk heterojunction, a layer consisting of a mixture of donor and acceptor molecules. A mixing ratio between 1:4 and 1:1 (by weight or volume) was thought to be required for an efficient generation of free electron and holes at the interface between donor and acceptor, and for efficient transport to the electrodes. However, in 2011, a novel device architecture was introduced: OSC on the basis of fullerenes, the standard acceptor molecules, as absorbing layer were presented that only have a very small amount (5vol%) of donor molecules, yet worked very well. Up to then, the conventional understanding of OSC was that such OSC should not work at all, or at least not as well as they do; meanwhile they are reaching efficiencies of more than 6%. Their working mechanism is still far from understood. These unexpected results again show that the field of OSCs (and most likely organic electronics in general) holds some surprises and that its full potential is yet hard to estimate. To underpin further long-term technological innovations, fundamental studies are required. Unravelling the working mechanism of this novel architecture for OSC is the core of this project.
To achieve this goal, thin organic films and corresponding OSCs of this novel architecture will be made with systematic variations in the stack and processing conditions. For high control of the device preparation, vacuum processing of purified small molecules will be used. The key difference to other approaches is that this will be combined with the concept of molecular doping. Presently, this method is rarely used in OSCs, despite being the basis of all commercial organic light emitting diodes (OLED) and the current world record OSCs.
Through systematic variations of the OSC hole contact, here realised with doped transport layers, and varying mixing ratios of fullerene and donor and changing substrate temperature, the generation of photovoltage and free charge carriers will be investigated. I will measure the energy of the charge transfer states using Fourier-transform photocurrent spectroscopy (FTPS), quantify the barrier between the hole contact and the organic absorber layer using impedance spectroscopy, FTPS, and current-voltage measurements, as well as determine the microstructure of the mixed films using X-rays, all essential to probe their fascinating interplay. The charge carrier transport, in particular the hole transport, through the absorbing layer and its recombination dynamics will be studied using single-carrier devices and transient measurements. In addition to working efficiently, the solar cells investigated here can be considered of great interest in their own right. The highly diluted nature of the donor molecules is an excellent model system to experimentally study donor-acceptor interactions, something that is central to any OSC and still not fully understood. Discovering the working mechanisms of this novel architecture for OSC will also help to answer the question of why fullerenes are such special and successful acceptor molecules. The results of this project will stimulate the development of novel and better materials, enable researchers to further optimise this promising architecture for efficient and stable solar cells as well as explore new device concepts for other applications of organic electronics.

Planned Impact

Organic electronic materials are already having an economic impact and many people will have used organic electronics without knowing it. Many mobile phone displays are made from organic light emitting diodes (OLED) and this technology is expected to become the leading mobile phone display technology in 2013 (DisplaySearch, 2013). The global market of organic electronics is predicted to reach more than 40 b$ by 2018, implying an annual growth rate larger than 30% (Transparency Market Research, 2012). This will include OLEDs for lighting application, organic solar cells (OSC), and possibly other applications like organic transistors.
OSCs are one of the emerging solar technologies and they are currently close to commercialisation. These solar cells have the potential to become a very cost-competitive, large area and versatile photovoltaic technology. Organic semiconductors are based on abundant and non-toxic raw materials and the manufacturing technologies are in principle capable of coating large areas inexpensively and fast. Hence, OSCs are one of the few renewable energy technologies that could be scalable at low cost to terawatt of installed capacity, which is necessary for any renewable energy technology to make a significant difference in the world's energy system.
The solar cell field is largely driven by power conversion efficiencies. 10%, as shown in the lab for OSC, are good, but to allow a really significant impact, pathways to 15% efficiency in single junction devices have to be found. One central part of this research aims at better understanding of what can be done to reduce energy losses during the conversion from light into electricity. If successful, this research has the potential to yield a wide range of long term economic and social benefits. It may ultimately lead to better OSCs by improved materials for solar energy conversion and/or through more efficient and stable device architectures to fulfil their potential for inexpensive, ubiquitous solar power. This in turn will lead to a significant reduction in CO2 emissions, help the UK to meet its commitment to CO2 reductions, reduce the dependence on energy imports, and eventually reduce electricity prices with huge benefits for both the economy and society. The better understanding of the investigated solar cells developed in this project will have benefits beyond OSCs to organic electronics in general, helping to come up with strategies to further improve devices made from organic semiconductors.
Furthermore, this project will strengthen the UK's activities in the field of vacuum processing organic electronics. The UK has a high-tech industrial cluster for vacuum technology in the area of Hastings that will benefit from my research, both by supplying equipment as well as by being able to further improve their equipment through close collaboration. If any of this work is taken towards commercialisation (licensing, spin-out companies) Isis Innovation, the dedicated technology transfer office of the University of Oxford, will help to maximise the impact.
Last but not least, it will deliver high-level scientific training in preparing, characterising, and simulating OSCs for one PhD student (Ivan Ramirez) with which the department supports starting my new group. He and subsequent PhD students that my group can attract will gain a broad understanding in the field of OSCs and organic electronics in general, benefit from the interdisciplinary environment, hence preparing them for their future careers in science or industry.

Publications

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Ramirez I (2018) Key Tradeoffs Limiting the Performance of Organic Photovoltaics in Advanced Energy Materials

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Vandewal K (2014) Increased open-circuit voltage of organic solar cells by reduced donor-acceptor interface area. in Advanced materials (Deerfield Beach, Fla.)

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Causa' M (2018) Femtosecond Dynamics of Photoexcited C Films. in The journal of physical chemistry letters

 
Description The puzzling properties of homeopathic organic solar cells were found for many different material systems, opening the door to a series of further fundamental studies. One aspect of particular importance is that we found that it is possible to improve the open circuit voltage in such systems without affecting the energy of the charge transfer state. This has serious implications as this turns out to be a promising route in improving the performance of organic solar cells. In collaboration with our partners at the TU Dresden we were able to show this for a series of different materials (two publications, two just accepted, two more planned) determining the intrinsic non-radiative voltage losses in fullerene-based organic solar cells. Furthermore, we found that this system is very suitable to investigate the photo-dimerisation of fullerenes, one degradation pathway for organic solar cells, showing how the exciton lifetime seems to affect the dimerisation process.
Exploitation Route improving the performance of organic solar cells and better understanding of the device stability
Sectors Energy

 
Description G7 Statement with GYA input
Geographic Reach Multiple continents/international 
Policy Influence Type Implementation circular/rapid advice/letter to e.g. Ministry of Health
Impact Some of our recommendations on early stage researchers and women in research to the G7 Science Ministers' meeting made it into the Final Tsukuba Communiqué
URL http://www8.cao.go.jp/cstp/english/others/communique_en.html
 
Description More Open Access Pledge
Geographic Reach Multiple continents/international 
Policy Influence Type Implementation circular/rapid advice/letter to e.g. Ministry of Health
URL https://moreopenaccess.net/
 
Description Position Statement on Open Data by the Young Academies of Europe and the Global Young Academy
Geographic Reach Multiple continents/international 
Policy Influence Type Implementation circular/rapid advice/letter to e.g. Ministry of Health
URL https://globalyoungacademy.net/wp-content/uploads/2016/04/Position-Statement-on-Open-Data-by-the-You...
 
Description Position statement on Open Access by the Young Academies of Europe and the Global Young Academy
Geographic Reach Multiple continents/international 
Policy Influence Type Implementation circular/rapid advice/letter to e.g. Ministry of Health
URL https://globalyoungacademy.net/wp-content/uploads/2016/04/Position-statement-on-Open-Access-by-the-Y...
 
Description The role of Young Academies in achieving the UN SDGs, 10/17
Geographic Reach Multiple continents/international 
Policy Influence Type Implementation circular/rapid advice/letter to e.g. Ministry of Health
Impact taken up and promoted by several senior science academies
URL https://globalyoungacademy.net/wp-content/uploads/2017/10/Statement-RoleYoungAcademies-SDGs-Oct2017....
 
Description Marie Sklodowska-Curie Actions (MSCA) Innovative Training Network
Amount € 3,826,024 (EUR)
Funding ID 722651 
Organisation European Commission 
Department Horizon 2020
Sector Public
Country European Union (EU)
Start 11/2016 
End 10/2020
 
Description STFC GCRF Foundation Awards 2017
Amount £3,600,000 (GBP)
Organisation Science and Technologies Facilities Council (STFC) 
Sector Academic/University
Country United Kingdom
Start 12/2017 
End 03/2021
 
Title Our Group's Github Page 
Description Following Open Science/Innovation approaches, we put all our code, procedures and design up on github 
Type Of Material Improvements to research infrastructure 
Year Produced 2015 
Provided To Others? Yes  
Impact Code, procedures and designs available online 
URL https://github.com/AFMD
 
Description IAPP, TU Dresden 
Organisation University of Dresden
Department Integrated Center for Applied Physics and Photonic Materials
Country Germany 
Sector Academic/University 
PI Contribution discussions and visits
Collaborator Contribution using IAPP's equipment for sample preparation and characterisation due to significant issues (and lack of suitable equipment) in Oxford.
Impact joint publications (papers, presentations, etc.)
Start Year 2013
 
Description Strathclyde, UK 
Organisation University of Strathclyde
Country United Kingdom 
Sector Academic/University 
PI Contribution Discussions, nvestigation of novel molecules
Collaborator Contribution discussions, novel molecules
Impact multi-disciplinary: chemistry & physics
Start Year 2016
 
Description U Fribourg, Switzerland 
Organisation University of Fribourg
Country Switzerland 
Sector Academic/University 
PI Contribution Samples, time for discussions and visits
Collaborator Contribution Discussions, time on ultrafast setup
Impact multi-disciplinary: physics & chemistry
Start Year 2016
 
Description UBC, Canada 
Organisation University of British Columbia
Department Canada's Michael Smith Genome Sciences Centre
Country Canada 
Sector Academic/University 
PI Contribution 3 month visiting professor at UBC
Collaborator Contribution time and resources for low-temperature STM and STS
Impact ongoing collaboration on the investigation of dilute heterojunction organic solar cells
Start Year 2016
 
Title Group Code on Github 
Description Group software, experimental designs, etc. 
Type Of Technology Software 
Year Produced 2015 
Open Source License? Yes  
Impact open science community 
URL https://github.com/orgs/AFMD/dashboard
 
Description Article Chemistry World 06/16 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Mentioning of research
Year(s) Of Engagement Activity 2016
URL https://www.chemistryworld.com/feature/the-next-generation/1010134.article
 
Description COST Vilnius Ivan 10/15 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Ivan's COST STSM presentation "Stability of dilute organic blend solar cells" at the StableNextSol Meeting in Vilnius, Lithuania 10/16
Year(s) Of Engagement Activity 2015
 
Description Conference for Undergraduate Women in Physics 03/17 
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 Undergraduate students
Results and Impact Labtour to the participants of the Conference for Undergraduate Women in Physics
Year(s) Of Engagement Activity 2017
URL https://twitter.com/AFMDGroup/status/845663190503604232
 
Description GRC 2016 Ivan 06/16 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Ivan's poster "Photo-degradation in dilute heterojunction organic solar cells" at the GRC conference, Italy, 06/16
Year(s) Of Engagement Activity 2016
 
Description Invited Presentation CPE Symposium, London, 06/17 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Invited presentation at the Centre for Plastic Electronics Symposium 2017, London
Year(s) Of Engagement Activity 2017
 
Description Invited Seminar Moritz Riede, AMOLF, NL, 03/2018 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Invited Seminar at AMOLF in Amsterdam, The Netherlands, and meeting with academics there
Year(s) Of Engagement Activity 2018
 
Description Oxford Physics Industry Day Sameer 09/16 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Presenting Poster at the Oxford Physics Industry Day
Year(s) Of Engagement Activity 2016
URL https://twitter.com/AFMDGroup/status/779428417334808576
 
Description Presentation Ivan Ramirez F-Pi13, Hongkong, 06/17 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Contributed Presentation at the 13th international Symposium on Pi-conjugated systems in Hongkong
Year(s) Of Engagement Activity 2017
 
Description Presentation Moritz Riede, F-Pi 13, Hongkong, 06/17 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Contributed presentation at the 13th International Symposium on Pi-conjugated systems
Year(s) Of Engagement Activity 2017
 
Description RSC London 01/16 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact invited talk "Diluted Heterojunction Organic Solar Cells" at "RSC Next Generation Materials for Solar Photovoltaics" conference
Year(s) Of Engagement Activity 2016
 
Description Regional School Physicist of the Year Event Sameer 10/16 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact Engagement with students during the Regional School Physicist of the Year Event hosted by the Physics Department in Oxford, judging of the posters presented by the students.
Year(s) Of Engagement Activity 2016
URL https://twitter.com/AFMDGroup/status/792047256631246848
 
Description UBC ECE Presentation 11/16 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact invited seminar "Organic Solar Cells: a disruptive Technology?" in the department "Electrical and Computer Engineering" at UBC
Year(s) Of Engagement Activity 2016
 
Description Uni Glasgow 03/16 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact invited seminar "Vacuum-processed Organic Solar Cells" at Uni Glasgow
Year(s) Of Engagement Activity 2016
 
Description Visit MP Blackwood Ivan 12/16 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact Presenting a Poster during Symposium on 2nd December for Ms Nicola Blackwood MP
Year(s) Of Engagement Activity 2016
URL https://twitter.com/nicolablackwood/status/804781026765578241
 
Description Wolfson Careers Day, 03/2016 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Postgraduate students
Results and Impact podium discussion (three invited speakers: academia, policy, industry) for career advice
Year(s) Of Engagement Activity 2016