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. Perovskite LEDs with emission in the IR through to blue have been developed. New device architectures that allow close to 100% internal quantum efficiency for near IR and green have been demonstrated.

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.

With the organic semiconductors, new materials and LED architectures have been developed. A new class of organo-metallic complexes, carbene-metal-amides, was found to support very efficient LEDs, made possible because the metal (typically gold) allows rotational flexibility between electron donor and acceptor group and allows strong spin-orbit coupling that enables efficient use of both the spin singlet and spin triplet excitons produced by electron-hole capture during LED operation.

A big advance was made in using spin-radical organic semiconductors; contrary to expectations, it was possible to achieve electron injection into the half-filled spin-bearing SOMO level and also achieve hole injection into the lower-lying HOMO level, allowing efficient luminescence. Operation within the spin doublet manifold made it possible to achieve high efficiency (competing non-emissive excitations such as triplet excitons are avoided).
Exploitation Route optimisation of future solar cell design and performance
Sectors Electronics,Energy

 
Description The development within the project of perovskite semiconductors with very high luminescence efficiency is important for the efficient operation of both solar cells and also luminescent devices such as LEDs. Patents have been filed to cover aspects of this work. LEDs with close to 100% internal quantum efficiency, in the IR and also green, have been developed. A new company, Helio Display Materials (first registered as Heliochrome) has been founded to exploit light emission from perovskite semiconductors.
First Year Of Impact 2016
Sector Electronics,Energy
Impact Types Economic

 
Title Perovskite/Colloidal Quantum Dot Tandem Solar Cells: Theoretical Modeling and Monolithic Structure 
Description © 2018 American Chemical Society. Metal-halide perovskite-based tandem solar cells show great promise for overcoming the Shockley-Queisser single-junction efficiency limit via low-cost tandem structures, but so far, they employ conventional bottom-cell materials that require stringent processing conditions. Meanwhile, difficulty in achieving low-bandgap (<1.1 eV) perovskites limits all-perovskite tandem cell development. Here we propose a tandem cell design based on a halide perovskite top cell and a chalcogenide colloidal quantum dot (CQD) bottom cell, where both materials provide bandgap tunability and solution processability. A theoretical efficiency of 43% is calculated for tandem-cell bandgap combinations of 1.55 (perovskite) and 1.0 eV (CQDs) under 1-sun illumination. We highlight that intersubcell radiative coupling contributes significantly (>11% absolute gain) to the ultimate efficiency via photon recycling. We report an initial experimental demonstration of a solution-processed monolithic perovskite/CQD tandem solar cell, showing evidence for subcell voltage addition. We model that a power conversion efficiency of 29.7% is possible by combining state-of-the-art perovskite and CQD solar cells. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Research data supporting "Dedoping of Lead Halide Perovskites Incorporating Monovalent Cations" 
Description Raw data files pertaining to the materials characterization, DFT calculations and solar cell measurements. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Research data supporting "Efficient singlet exciton fission in pentacene prepared from a soluble precursor" 
Description Underlying datasets for all figures in the Manuscript and the SI. Data comprises results from absorption-, photoluminescence-, ellipsometry-, transient spectroscopy-, FTIR-, X-ray and atomic force microscopy measurements. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Title Research data supporting "Efficient singlet exciton fission in pentacene prepared from a soluble precursor" 
Description Underlying datasets for all figures in the Manuscript and the SI. Data comprises results from absorption-, photoluminescence-, ellipsometry-, transient spectroscopy-, FTIR-, X-ray and atomic force microscopy measurements. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Title Research data supporting "High Circular Polarization of Electroluminescence Achieved via Self-Assembly of a Light-Emitting Chiral Conjugated Polymer into Multidomain Cholesteric Films" 
Description Left and right-handed circularly polarized components of Electro-Luminescence and corresponding dissymmetry factors. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Research data supporting "High-efficiency perovskite-polymer bulk heterostructure light-emitting diodes" 
Description Datasets for figures in "High-efficiency perovskite-polymer bulk heterostructure light-emitting diodes" 
Type Of Material Database/Collection of data 
Provided To Others? Yes  
 
Title Research data supporting "High-performance light-emitting diodes based on carbene-metal-amides" 
Description This dataset includes the experimental results of optical spectroscopy (cryogenic transient photoluminescence, transient absorption, ultrafast transient photoluminescence, transient electroluminescence, steady-state emission/absorption and Raman), OLED device characterization, electrochemistry, as well as data associated with quantum chemical (DFT) calculations. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Research data supporting "Improving the photoluminescence quantum yields of quantum dot films through a donor/acceptor system for near-IR LEDs" 
Description Data underlying each Figure in the paper, namely absorption spectra, emission spectra, transient absorption data and LED characteristics for mixed quantum dot films. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? Yes  
 
Title Research data supporting Lead-Free Perovskite Semiconductors Based on Germanium-Tin Solid Solutions: Structural and Optoelectronic Properties 
Description The data has been collected from multiple instruments including XRD, PL, PDS and simulations. The details of which are mentioned in the linked manuscript. 
Type Of Material Database/Collection of data 
Provided To Others? Yes  
 
Title Research data supporting: "Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy" 
Description Research data supportingthe findings of the manuscript "Ultrafast carrier thermalization in lead iodide perovskite probed with two-dimensional electronic spectroscopy". All data is provided in form of embedded Origin graphs in a PPT file. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Company Name HELIOCHROME LIMITED 
Description Heliochrome is developing the use of lead halide perovskite semiconductors as light-emitting materials, for use a colour-converting phosphors for lighting and displays, and for use as the active semiconductor in light-emitting diodes, LEDs, for displays and other applications. 
Year Established 2016 
Impact Heliochrome is developing new materials and LEDs based on published and patented discoveries at the Universities of Oxford and Cambridge