Control of spin and coherence in electronic excitations in organic and hybrid organic/inorganic semiconductor structures

Lead Research Organisation: University of Cambridge
Department Name: Physics

Abstract

The field of organic electronics has continued to make great technological and scientific progress over the last 5 years and has given rise to a significant industry. The worldwide market for organic/printable electronics reached $12 billion in 2012, about half of which were organic light-emitting diode (OLED) displays. The efficiency of phosphorescent red and green as well as fluorescent blue OLEDs is already close to their theoretical maximum; to achieve this requires complex multilayer architectures. For future OLED applications, such as lighting, there is an important need for simpler device architectures and cheaper materials to meet demanding cost targets. Also organic field-effect transistors (OFETs) are now being used in commercial applications, including flexible active-matrix electronic paper displays. There continues to be an important need for organic semiconductors with higher carrier mobilities (>10-50 cm2/Vs) and electrical stability to enable a wider range of applications. Also organic solar cells based on distributed donor-acceptor heterojunctions have achieved steady improvements in performance with power conversion efficiencies of 10-11% now being reported for the best single-junction cells. However, in spite of intense research efforts the performance/efficiency and resulting cost of electricity of organic photovoltaics (OPV) is still not competitive with crystalline silicon solar cells. Two very significant breakthroughs made in the last two years have the potential to change this: (i) Our group in Cambridge has demonstrated 200% quantum efficiency in solar cells through the use of singlet fission, which opens up completely new architectures for solar energy harvesting. (ii) Hybrid organic-inorganic heterojunctions solar cells based on mixed halide perovskites have shown unexpected performance with efficiencies up to 16-17%, achieved in part through long exciton/charge diffusion lengths and low energetic disorder in the perovskite materials. This discovery may provide a solar cell technology that could realistically be competitive with silicon in a few years time.
Within this steadily advancing field of science and technology we identify three spectacular and unanticipated discoveries that create the opportunity for discontinuous advances. These are the focus of our programme: (i) Wavefunction delocalisation / coherence - We have been surprised that the degree of energetic disorder in conjugated polymers can now be reduced to levels at which it is no longer dominating the transport physics. It is very unexpected that this can be found in low-temperature processed non-crystalline materials. The associated coherence and delocalisation of excited state wavefunctions enables long-range electron transfer in non-covalent materials and heterojunctions; (ii) Organic-inorganic heterojunctions - The Oxford work on lead halide perovskites reveal low-temperature processed inorganic semiconductors with unexpectedly clean properties both in the bulk properties and also at interfaces with organic semiconductors. Understanding why it is possible to avoid electronic defect/trap states at these interfaces will form a major part of the programme. (iii) Spin - The unique spin physics of organic materials offers novel routes for controlling electronic processes that are not available in conventional, inorganic semiconductors. In particular, the process of singlet exciton fission to a pair of triplet excitons offers the potential of overcoming the Shockley-Queisser (SQ) efficiency limit in solar cells. The exploitation of these phenomena requires hybrid systems comprising both organic and inorganic semiconductors. Our programme grant builds on recent breakthroughs and is centered around the engineering of wavefunction delocalisation in organic and perovskite semiconductors. It will bring about a paradigm shift in the field of organic and inorganic large-area electronics and achieve step-changes in device performance.

Planned Impact

The programme is focussed on upstream, fundamental research that has the potential to form the scientific basis for the device performance of organic and hybrid inorganic-organic structures to rival and exceed that of conventional inorganic semiconductors. On a 5-20 year time scale organic and hybrid solar cells with current maximum power conversion efficiencies of 11% and 16%, respectively, have the potential to lower the cost of photovoltaic electricity to what is achievable by burning natural gas and to bring significant societal benefits. The level of control of the electronic processes at the critical heterojunction that we are aiming for in this programme could make efficiencies match or exceed those of silicon-based solar cells. Exploitation of the process of singlet fission in solar cells could break the fundamental Shockley-Queisser efficiency limit. A key feature of our proposal is that it also opens up new areas of application for solution-processed semiconductors, notably in quantum information. Applications in quantum information processing may appear ambitious at present, but we note that the solid-state quantum information community is engaging in a wider exploration of materials as limitations of widely investigated systems, such as nitrogen vacancy (NV) centres in diamond, are becoming apparent (see October 2013 focus issue of the MRS Bulletin on "Materials Issues in Quantum Computation").

All Investigators have extensive experience with the commercialisation of fundamental research, not only through founding successful start-up companies (Cambridge Display Technology (Friend), Plastic Logic (Sirringhaus, Friend), Eight19 (Friend, Greenham, Sirringhaus), Oxford Photovoltaics (Snaith), and Flexink (McCulloch)), but also through engaging with large, multinational technology and end user companies. The practical arrangements for protection and exploitation of project IP will follow the well-established technology transfer processes at the three Universities involved, using the services provided by their respective technology transfer offices (TTOs) Cambridge Enterprise, Isis Innovation, and Imperial Innovations. When, as expected, joint inventions arise, our collaboration agreement will encourage assignment to the TTO best placed to ensure successful exploitation, with appropriate sharing of future revenues.

Our impact strategy recognises that some of our results will not fit within this simple linear technology transfer model. Given the long-term nature of our research goals, we anticipate that some of our developments will be outside the immediate fields of interest of our industrial partners; they may be at low technology readiness level, require significant further research and development resources to establish commercial viability and/or their main application may be uncertain. In such situations we will explore alternative exploitation paths, including proof-of-concept studies supported by our TTOs, collaborations with the High Value Manufacturing Technology Innovation Centre and the EPSRC Centre for Innovative Manufacturing in Large-Area Electronics to establish feasibility or open-innovation collaborations with industrial partners (such as Hitachi) who have appropriately long-term vision.

The programme will provide an ideal interdisciplinary training environment for the researchers and associated PhD students, who will move on to become highly skilled researchers in academia and industry. Several of the senior post-doctoral researchers on our current programme grant have successfully attracted lectureships or personal EPSRC or Royal Society research fellowships. In their ability to bridge between fundamental science and industrial applications all five investigators have become role models to students and aspiring researchers and they will continue to be inspiring advocates for Science and Engineering in the UK.

Publications

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Description Research on the optoelectronic properties of lead halide perovskite semiconductors has revealed a very close connection between high solar cell efficiency and high luminescence efficiency. In particular, in efficient solar cells made with these materials, photo generated charges can recombine to regenerate a photon that is then re-absorbed and this re-generates electronic charges. This process of re-cycling of photons helps raise the open circuit voltage of the solar cell. Further advances have been made in the performance of lead halide perovskite LEDs. New transient optical spectroscopy techniques have been developed to probe early time electron-hole separation in organic solar cell devices. Measurements of the fission of singlet excitons to pairs of triplet excitons have been extended to a range of new molecular semiconductors.
Exploitation Route optimisation of future solar cell design and performance
Sectors Electronics,Energy