Structure-Property-Performance Relationships for Organic Bulk Heterojunction Solar Cells

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

Organic semiconductors combine the semiconductor properties traditionally associated with inorganic materials with the more desirable properties of plastics such as low cost, flexibility and ease of processing and patterning. Moreover, the organic syntheses of these materials allow for great flexibility in the tuning of their electronic and optical properties. By combining these properties, organic semiconductors such as conjugated polymers and small molecules have been demonstrated as the active layer in a wide range of optical and electronic devices including photovoltaic solar cells. The leading design of organic solar cells is based on the bulk heterojunction , in which organic blends comprising an electron donating component (usually a conjugated polymer) and an electron accepting component (such as a fullerene derivative or a conjugated polymer) that are dissolved in the same solvent and then spin-coated from the solution to form a thin film, sandwiched between two different electrodes. Recent developments in materials and device fabrication processes are leading to rapid improvements in performance of these devices. For example, solar conversion efficiencies up to 5-6 % were reported for solution-processed organic solar cells. Despite their significantly improved device performance, a number of scientific challenges remain to more fully understand, quantify, and predict the behaviour of the organic bulk heterojunction solar cells. In particular, the dependence of device performance on the thin film morphology of organic blends and the factors affecting the development of specific thin film structures from blend solutions are still poorly understood. To understand the interplay between the organic blend thin films and devices, it is therefore important to identify the structure-property relationship of the organic materials and its effects on device performance.Our work at Imperial College London seeks to provide key fundamental and technological insights into this issue. We aim to clarify the structure-property-performance relationships of organic bulk heterojunction solar cells. We will achieve this aim through a combined application of scanning probe microscopic techniques and optical and electrical techniques to identify the nature of organic nanostructures and interfaces in terms of their chemical, physical, optical, and electrical properties, and thus to clarify the role of these organic nanostructures and interfaces on device performance. It is our vision that only the fully complementary information obtained by measurements of these properties will be able to provide the necessary powerful tool to study organic nanostructures and interfaces and thus to further develop organic bulk heterojunction solar cells.

Publications

10 25 50
 
Description Organic semiconductors combine the semiconductor properties traditionally associated with inorganic materials with the more desirable properties of plastics such as low cost, flexibility and ease of processing and patterning. Recent developments in materials and device fabrication processes are leading to rapid improvements in performance of organic devices (e.g. solar conversion efficiencies up to 9-10 % were reported for solution-processed organic solar cells). Despite their significantly improved device performance, a number of scientific challenges remain to more fully understand, quantify, and predict the behaviour of the organic bulk heterojunction solar cells.

The main aim of the project was to clarify the structure-property-performance relationships of organic bulk heterojunction solar cells. We have achieved this aim through a combined application of scanning probe microscopic techniques and optical and electrical techniques to identify the nature of organic nanostructures and interfaces in terms of their chemical, physical, optical, and electrical properties, and thus to clarify the role of these organic nanostructures and interfaces on OPV device performance.

First, we demonstrated the use of AR-XPS (also UPS) and PC-AFM techniques to probe thin film structures of OPV blend films and devices. Second, we developed resonant and polarised Raman spectroscopy techniques as a simple and powerful technique to probe the degree of local molecular order and orientation of organic semiconductors in OPV films. Third, we built and demonstrated PC-AFM technique to image the local optoelectronic properties of organic blends in working OPV devices with high spatial and depth resolution. Fourth, we demonstrated simultaneous Raman-AFM measurement for simultaneous determination of physical, chemical and structural properties of organic semiconductors using light emitting polymer nanowires.

Based on this combined application of advanced techniques, we clarified the crucial structure-property-performance relationships in the OPV blend films and devices. They include important findings that an increase in the regularity of the side-chain of P3HT polymer enhances its degree of molecular order and substituting the sulphur atoms in P3HT with selenium atoms leads to better crystal quality but less fraction of crystalline-phase. An amorphous phase of polymer can mix better with PCBM small molecules suppressing the formation of micrometer sized PCBM aggregates, which strongly influence photocurrent generation and thus ultimate device performance. In addition, self-assembly of P3HT polymers into nanowires leads to higher degree of molecular order enhancing charge transport properties and thus increasing OPV device efficiency.

The key advances in organic semiconductor nanostructures and interfaces achieved throughout this project provide important insight into a design rule of organic semiconductor and devices, not only for future development in organic solar cells but also for other novel molecular electronics. Based on this project, we have published 10 refereed papers in high impact journals including J. Am. Chem. Soc., Adv. Funct. Mater. and Energy Environ. Sci. and 1 invited book chapter in "Optoelectronic Devices and Properties". Two more papers have been recently submitted. I believe that this project has been very successful and our research has been greatly benefited from this EPSRC-NPL post-doctoral research partnerships grant.
Exploitation Route This project was a basic research programme that leaded to advances in the science and technology of organic semiconductors and their devices, with a particular focus on organic bulk heterojunction solar cells. Its impact based on fundamental understanding of organic semiconductors and their nanostructures is, however, expected towards much broader areas of organic electronics. The experimental techniques including (resonant)Raman and PC-AFM developed under this work will be used for others to carry out analysis of other advanced functional materials. On the other hand, our scientific findings will be critical to establish the working principles of organic solar cells.

Furthermore, this project was multidisciplinary, providing us a unique and invaluable opportunity to establish a strong working relationship with NPL (UK), which led to publication of high impact scientific papers and additional new research grants for further collaboration. It also provided us an opportunity to work closely with other national and international academic and industrial organisations with world-leading experts in organic semiconductors and devices. Several industrial organisations such as Cambridge Display Technology Ltd (UK)/ Sumitomo Chemical Company (Japan), Cytec Engineered Materials (UK) and Samsung Mobile Display (South Korea) have shown strong interest in our research outputs, in particular scanning probe microscopic techniques developed in this project, which has already led to an industrial support for CASE studentship (one awarded from CDT/Sumitomo Chemical Company). Two additional studentships (from Cytec Engineered Materials and CDT/Sumitomo Chemical Company) as well as research support from Samsung Mobile Display are under negotiation. Awarding these grants will enable me to continue research towards future potential industrial exploitation of this project.

The key results obtained in this project have been presented at various international/ national conferences/ workshops and industrial organisations as invited lectures/ seminars and published in scientific journals of high visibility. Further research focusing on Organic Semiconductors for Plastic Electronics will be carried out based on further research support that has arisen as a result of the work supported under this grant. I have been awarded 7 research grants in this research area since 2009 from various research funding sources including EPSRC, Technology Strategy Board (TSB), Industrial Organisations, British Embassy and Imperial College. By securing these research grants, I have been able to set up new nanoanalysis facilities including scanning probe microscope techniques, which are essential for my research and also have been able to establish my research group with 7 PhD students and 3 Post-doctoral researchers so far. At the appropriate stage, main research results achieved in this project will be continuously presented at scientific meetings and published in high quality scientific journals as before.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy

URL http://www3.imperial.ac.uk/people/ji-seon.kim
 
Description 1. Energy Environ. Sci., 8, (2015), 3222-3232, DOI: 10.1039/C5EE01974E: Recent advances in the performance of organic photovoltaics have largely arisen through the development of novel molecular structures using a donor-acceptor copolymer motif. Copolymers incorporating diketopyrrolopyrrole (DPP) units have attracted strong, widespread interest, but the main role of this unit in the optoelectronic properties and device performance was not clear. This work clearly identifies the natures of the main optical absorption transitions of DPP-based copolymers and challenges the assumption that the DPP-unit behaves as a traditional acceptor unit. This insight leads to a clearer understanding of the excitation energy dependent photodegradation mechanism of the materials, providing ways to improve the operational stability of organic solar cells. I am the lead scientist who directed this work. This work was introduced as a cover article of the journal as important showcasing research from the collaborative team led by Dr Ji-Seon Kim at the Imperial College London, UK. This work also leads to two new industrial CASE PhD projects entitled "Printed Large Area Organic Solar Cells" sponsored by CSEM Brasil (1/10/2016-30/9/2020) and "Towards Full Device Stability for Organic and Hybrid Solar Cells" sponsored by Cambridge Display Technology/ Sumitomo Chemical Company (UK/Japan) (1/10/2015-30/9/2019). 2. Adv. Energy Mater. (2014), 1400527, DOI: 10.1002/aenm.201400527: Revealed a critical relationship between the chemical structure of polymers and photovoltaic properties of bulk heterojunction organic solar cells by elucidating detailed understanding of the impact of heteroatom on the polymer backbone on the molecular conformation, thin-film morphology, and hence device performance. This work offers a new outlook for the design of new electron donor polymers using the powerful characterisation tool of Raman spectroscopy, which allow us to draw precise conclusions about how subtle chemical structural differences are related to the long-range molecular packing and macroscopic optoelectronic properties. I am the lead scientist who directed this work. This work has further provided an important basis to establish resonant Raman spectroscopy as a direct structure probe for important organic semiconductors [Adv. Energy Mater. 2014, 1400527, J. Am. Chem. Soc. 2015, 137, 6866 and Nature Communications 2015, 6, 5977]. 3. J. Chem. Phys. 139(6) (2013) 064901. - placed fourth on the Journal of Chemical Physics' list of the 25 most accessed articles of 2014. 4. The outcome of research performed under this grant has produced 14 refereed papers in high impact journals and numerous invited talks in international workshops/ conferences.
First Year Of Impact 2012
Sector Electronics,Energy,Environment
Impact Types Economic

 
Description Korea-UK Joint Government Committee Meeting on Science and Technology
Geographic Reach Asia 
Policy Influence Type Gave evidence to a government review
 
Description UK delegate for the Korea-UK Joint Government Committee Meetings on Science and Technology held by the Korean Ministry of Science, ICT and Future Planning (MSIP) & the UK Foreign Office, Department for Business, Energy and Industrial Strategy (BEIS, 2017)
Geographic Reach Asia 
Policy Influence Type Gave evidence to a government review
Impact UK-Korea S&T collaboration was strongly encouraged and potential funding schemes were introduced by Korean NRF. One of these funding schemes, GRL grant (> 2billion KRW, 2017-2023) fully funded by Korean NRF, was awarded for research collaboration between UK and Korea, generating potential economic impact in both countries through scientific research.
 
Description 2015 Samsung Global Research Outreach (GRO) Programme Grant
Amount $100,000 (USD)
Organisation Samsung 
Department Samsung Advanced Institute of Technology
Sector Private
Country Korea, Republic of
Start 10/2015 
End 09/2016
 
Description Development of Flexible and Printed Perovskite/Organic Integrated Photovoltaic Modules with Ultrahigh Efficiency, Long-Term Stability and Large-Area Reproducibility
Amount ₩2,147,483,647 (KRW)
Funding ID GRL 
Organisation National Research Foundation of Korea 
Sector Academic/University
Country Korea, Republic of
Start 06/2017 
End 02/2023
 
Description Global Partnerships Fund (GPF)
Amount £43,000 (GBP)
Organisation Department for Business, Energy & Industrial Strategy 
Sector Public
Country United Kingdom
Start 04/2013 
End 03/2016
 
Description Industrial CASE top-up fund for a PhD studentship
Amount £25,000 (GBP)
Organisation CSEM Brasil 
Sector Charity/Non Profit
Country Brazil
Start 10/2016 
End 09/2020
 
Description Industrial CASE top-up fund for a PhD studentship
Amount £45,000 (GBP)
Organisation Cambridge Display Technology 
Sector Private
Country United Kingdom
Start 10/2015 
End 09/2019
 
Title Raman spectroscopy 
Description Raman spectroscopy is an advanced structural nanoprobe for conjugated molecular semiconductors. Utilising selective resonant and polarisation dependent excitations, together with in situ control of temperature, pressure, electrical, and electrochemical potential, we have demonstrated its unique capability to elucidate the properties of molecular semiconductors, including: chemical structure, molecular conformation, order, orientation, and fundamental photo- and electro-chemical processes and stability - all of which are critically important to the performance of a wide range of optical and electronic organic semiconductor devices. 
Type Of Material Improvements to research infrastructure 
Year Produced 2015 
Provided To Others? Yes  
Impact The fast growing field of Nanotechnology strongly needs novel experimental techniques capable of characterising the structure and composition of matter at the nanoscale molecular level. However, not many nanoscale imaging techniques have been developed and established, that have provided the required information with high chemical/ structural sensitivity and high spatial resolution. My team has developed and established an advanced structural nanoimaging technique based on Raman spectroscopy, integrating it with other functionalities such as selective resonant and polarisation dependent excitations, together with in situ control of temperature, pressure, electrical, and electrochemical potential. This method provides a very valuable nanometrology for functional materials and devices. As a result of its unique capabilities, the use of Raman is expected to grow rapidly not only for molecular materials in organic electronics, but also much more broadly into bio- and nano-material related research areas. 
URL https://www.imperial.ac.uk/nanoanalysis-group/research/developing-nanometrology/
 
Description Industrial collaboration with CSEM Brasil 
Organisation CSEM Brasil
Country Brazil 
Sector Charity/Non Profit 
PI Contribution Wrote a research proposal entitled - Printed Large Area Organic Solar Cells Recruited a PhD student Performing / directing the project
Collaborator Contribution Financial support of the project (25000 GBP) Participating in recruitment of a PhD student Contributing to the project including hosting a student in CSEM
Impact Industrial engagement in multi-disciplinary research (Chemistry, Materials, Physics and Engineering)
Start Year 2016
 
Description Industrial collaboration with Cambridge Display Technology 
Organisation Cambridge Display Technology
Country United Kingdom 
Sector Private 
PI Contribution Wrote a research proposal entitled - Towards Full Device Stability for Organic and Hybrid Solar Cells Recruited a PhD student Performing / directing the project
Collaborator Contribution Financial support of the project Contributing to the project via regular project review meetings Hosting a student in the company for a short period
Impact Industrial engagement in multi-disciplinary research (Chemistry, Materials, Physics and Engineering)
Start Year 2015
 
Description Industrial collaboration with Samsung Electronics 
Organisation Samsung
Country Global 
Sector Private 
PI Contribution Wrote a research proposal - High-Performance Organic Near-IR Photodetectors via Advanced Molecular Structure Control and Analysis Performing / directing the project
Collaborator Contribution Financial support of the project Contributing to the project via regular project review meetings
Impact Industrial engagement in multi-disciplinary research (Chemistry, Materials, Physics and Engineering)
Start Year 2015
 
Description Annual Plastic Electronics CDT Symposium 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact The Plastic Electronics CDT holds an annual 1 day symposium in September every year. PhD students give talks and present posters to an audience of other postgraduate and undergraduate students, postdocs, academics, external invited academics, industrial sponsors, and the PE-CDT external advisory board. The symposium also features invited talks from high profile academic and industrial researchers, and from PE-CDT alumni.
Year(s) Of Engagement Activity 2013,2014,2015,2016,2017
 
Description Annual Plastic Electronics Winter School 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact The Plastic Electronics week-long Winter School is held in January every year. Plastic Electronics CDT students attend, as well as "aligned" students and postdocs, PE-CDT/Centre for Plastic Electronics academics, and invited external academics from UK and European institutions/companies. The programme includes talks from PhD students on their research, invited talks from PE-CDT/CPE academics, and invited talks/tutorials from external academics/industry researchers. The students and postdocs also undertake a week-long group challenge project.
Year(s) Of Engagement Activity 2013,2014,2015,2016,2017
 
Description Annual symposium on Plastic Electronics 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact The annual UK-Korea Symposium on Plastic Electronics is held alternately between the UK (usually Imperial College London) and South Korea. Academics from several different institutions take part, including Imperial College London, Oxford University, Gwangju Institute of Science & Technology, and POSTECH. There are often also visits to companies, e.g. Flexenable and Cambridge Display Technologies in the UK, and Samsung and LG Displays in Korea.
Year(s) Of Engagement Activity 2013,2014,2015,2016,2017
 
Description Korea-UK Joint Government Committee Meetings 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Policymakers/politicians
Results and Impact UK delegate for the 13th and 12th Korea-UK Joint Government Committee Meetings on Science and Technology held by the Korean Ministry of Science, ICT and Future Planning (MSIP) & the UK Foreign Office, Department for Business, Energy and Industrial Strategy (BEIS, 2017) and Department of Business, Invocation & Skills (BIS, 2015). The UK-Korea scientific collaboration was encouraged and potential funding schemes were introduced. One of Korea-UK international funding schemes, the Global Research Laboratory (GRL) research grant was applied and awarded (>2billion KRW, 2017-2023).
Year(s) Of Engagement Activity 2015,2017
 
Description Public lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact "Plastic Electronics - Next Generation Technology", Public Lecture at Institution of Engineering and Technology (IET), London (15 Sept 2015).
Year(s) Of Engagement Activity 2015
URL https://www.imperial.ac.uk/nanoanalysis-group/group-news/
 
Description UK-Korea Workshops on Plastic Electronics 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Organised the UK-Korea workshops on Plastic Electronics (PE). Based on the strong interactions with Korean HEIs and Industry, the UK-Korea PE Consortium was formed (Dec 2013) involving more than 30 UK and Korea academic & industrial partners.
Year(s) Of Engagement Activity 2012,2013,2014,2015,2016
URL http://www3.imperial.ac.uk/newsandeventspggrp/imperialcollege/centres/plasticelectronics/newssummary...