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Department of Physics

The Cavendish Laboratory
 

Biography

Richard Friend’s research explores the electronic properties of novel semiconductors, currently carbon-based organic semiconductors and metal halide perovskites. We use these as the active component in range of semiconductor devices, including photovoltaic diodes, FETs, and LEDs, both to study their fundamental electronic structure and also to explore applications in display technologies and solar cells.

 

Research

We have substantial facilities for the fabrication of thin film structures and active semiconductor devices, both using vacuum and solution-based processing methods.  Besides our wide range of standard structural and optoelectronic characterisation techniques, we have a substantial investment in ultra-fast optical spectroscopy, with time resolution down to 10 femtoseconds, that we use to track the time evolution of photoexcitations.

Molecular semiconductors. 

The electronic properties of organic molecular semiconductors are controlled by the relatively strong Coulomb interactions present, causing photoexcitations to form as strongly bound excited states, excitons, that show a large spin-exchange energy between higher energy spin singlet and lower energy spin triplet states.  Triplet excitons are formed by electron-hole capture in both LEDs and in solar cells, and generally lead to non-radiative losses that limit device performance.  Much of our current research is concerned with spin management, to explore new physics and to develop better engineered LED and solar cell device architectures.

For LEDs we are exploring methods to control the exchange energy using molecular structures that give charge-separated excited states with low exchange energies [1].  For organic solar cells, we are developing new systems where we can control the spin-dependent kinetics for formation and dissociation of intermolecular electron-hole bound states [2]. We have shown how this can prevent losses through triplet exciton formation and this may allow higher organic solar cell efficiencies, which though now beyond 18% are still limited by non-radiative decay channels.

A new approach to spin management that we have found is to use semiconductor molecules that have an unpaired spin in the ground state.  These ‘spin radical’ materials can be designed to show very high luminescence efficiencies and we have found that we can operate efficient LEDs entirely within the spin doublet manifold [3-5].   In doing so, we have used materials designed without lower energy non-emissive states (such as spin quartets) .  This work opens up a new range of molecular designs and possible novel spintronic control of optoelectronic behaviour that we are exploring in a new (October 2021) ERC-funded project, SCORS.

Some molecular semiconductors can be engineered so that the spin singlet exciton is close to twice the energy of the spin triplet exciton.  When this condition is met there can be very fast interconversion between the singlet exciton and a spin-zero pair of triplet excitons.  The fusion of two triplet excitons to form a singlet exciton is widely used to optimise efficiency in current OLED displays. The reverse process, of singlet exciton fission to a pair of triplet excitons can also be fast and efficient.  This process provides a route to improved solar cell efficiencies, where higher energy photons can be down-converted to pairs of lower energy states that are then both used to generate charge in a standard single junction solar cell [6]. We work within a large programme in Cambridge that is exploring materials [7] and device architectures to develop this photon conversion technology.

We are exploring a new class of well-ordered molecular nanofibres that we find show unprecedented long range diffusion of photogenerated excitons, beyond 300 nm [8].  This distance is large enough to use these structures as light-harvesting antenna structures for solar cell and photodetectors a we are exploring materials and structures to demonstrate this.  We want to know why this long range energy transfer is possible and our current work reveals this may be due to transient thermal excitation to delocalised excited states [9].

LEDs and lasers based on lead halide perovskites.

Lead halide perovskites have recently been found to provide unexpectedly efficient thin-film solar cells. We have looked instead at their light-emitting properties and find that they can show remarkably efficient luminescence which we have explored in novel LED structures [10] and in optically-pumped lasers.  Our current research, now supported by a new EPSRC research grant (Oct 2021), explores new quantum well structures that we assemble with perovskites with controlled bandgaps, using two-dimensional perovskites as larger energy gap components that we use to confine electronic excitations within lower gap three-dimensional perovskite regions [11, 12].

Publications

Key publications: 

1               "Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states", L. S. Cui, A. J. Gillett, S. F. Zhang, H. Ye, Y. Liu, X. K. Chen, Z. S. Lin, E. W. Evans, W. K. Myers, T. K. Ronson, H. Nakanotani, S. Reineke, J. L. Bredas, C. Adachi, and R. H. Friend, Nature Photonics (2020) http://dx.doi.org/10.1038/s41566-020-0668-z

 

2               "The role of charge recombination to triplet excitons in organic solar cells", A. J. Gillett, A. Privitera, R. Dilmurat, A. Karki, D. P. Qian, A. Pershin, G. Londi, W. K. Myers, J. Lee, J. Yuan, S. J. Ko, M. K. Riede, F. Gao, G. C. Bazan, A. Rao, T. Q. Nguyen, D. Beljonne, and R. H. Friend, Nature 597, 666-+ (2021) http://dx.doi.org/10.1038/s41586-021-03840-5

 

3               "Efficient radical-based light-emitting diodes with doublet emission", X. Ai, E. W. Evans, S. Dong, A. J. Gillett, H. Guo, Y. Chen, T. J. H. Hele, R. H. Friend, and F. Li, Nature 563, 540 (2018) http://dx.doi.org/10.1038/s41586-018-0695-9

 

4               "High stability and luminescence efficiency in donor–acceptor neutral radicals not following the Aufbau principle", H. Guo, Q. Peng, X.-K. Chen, Q. Gu, S. Dong, E. W. Evans, A. J. Gillett, X. Ai, M. Zhang, D. Credgington, V. Coropceanu, R. H. Friend, J.-L. Brédas, and F. Li, Nature Materials (2019) http://dx.doi.org/10.1038/s41563-019-0433-1

 

5               "Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes", A. Abdurahman, T. J. H. Hele, Q. Y. Gu, J. B. Zhang, Q. M. Peng, M. Zhang, R. H. Friend, F. Li, and E. W. Evans, Nature Materials (2020) http://dx.doi.org/10.1038/s41563-020-0705-9

 

6               "Harnessing singlet exciton fission to break the Shockley-Queisser limit", A. Rao and R. H. Friend, Nature Reviews Materials 2 (2017) http://dx.doi.org/10.1038/natrevmats.2017.63

 

7               "Singlet exciton fission in a modified acene with improved stability and high photoluminescence yield", P. J. Budden, L. R. Weiss, M. Muller, N. A. Panjwani, S. Dowland, J. R. Allardice, M. Ganschow, J. Freudenberg, J. Behrends, U. H. F. Bunz, and R. H. Friend, Nature Communications 12 (2021) http://dx.doi.org/10.1038/s41467-021-21719-x

 

8               "Long-range exciton transport in conjugated polymer nanofibers prepared by seeded growth", X.-H. Jin, M. B. Price, J. R. Finnegan, C. E. Boott, J. M. Richter, A. Rao, S. M. Menke, R. H. Friend, G. R. Whittell, and I. Manners, Science 360, 897 (2018) http://dx.doi.org/10.1126/science.aar8104

 

9               "Efficient energy transport in an organic semiconductor mediated by transient exciton delocalization", A. J. Sneyd, T. Fukui, D. Palecek, S. Prodhan, I. Wagner, Y. F. Zhang, J. Sung, S. M. Collins, T. J. A. Slater, Z. Andaji-Garmaroudi, L. R. MacFarlane, J. D. Garcia-Hernandez, L. J. Wang, G. R. Whittell, J. M. Hodgkiss, K. Chen, D. Beljonne, I. Manners, R. H. Friend, and A. Rao, Science Advances 7 (2021) http://dx.doi.org/10.1126/sciadv.abh4232

 

10            "Ligand-engineered bandgap stability in mixed-halide perovskite LEDs", Y. Hassan, J. H. Park, M. L. Crawford, A. Sadhanala, J. Lee, J. C. Sadighian, E. Mosconi, R. Shivanna, E. Radicchi, M. Jeong, C. Yang, H. Choi, S. H. Park, M. H. Song, F. De Angelis, C. Y. Wong, R. H. Friend, B. R. Lee, and H. J. Snaith, Nature 591, 72-+ (2021) http://dx.doi.org/10.1038/s41586-021-03217-8

 

11            "Efficient light-emitting diodes from mixed-dimensional perovskites on a fluoride interface", B. D. Zhao, Y. X. Lian, L. S. Cui, G. Divitini, G. Kusch, E. Ruggeri, F. Auras, W. W. Li, D. X. Yang, B. N. Zhu, R. A. Oliver, J. L. MacManus-Driscoll, S. D. Stranks, D. W. Di, and R. H. Friend, Nature Electronics 3, 704-+ (2020) http://dx.doi.org/10.1038/s41928-020-00487-4

 

 

12            "High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes", B. Zhao, S. Bai, V. Kim, R. Lamboll, R. Shivanna, F. Auras, J. M. Richter, L. Yang, L. Dai, M. Alsari, X.-J. She, L. Liang, J. Zhang, S. Lilliu, P. Gao, H. J. Snaith, J. Wang, N. C. Greenham, R. H. Friend, and D. Di, Nature Photonics (2018) http://dx.doi.org/10.1038/s41566-018-0283-4 .

 

Teaching and Supervisions

Research supervision: 

I am currently looking to fill PhD and post-doctoral positions on current ERC and EPSRC grants

Fellow of St. John's College

Contact Details

Maxwell 2.90
Cavendish Laboratory
JJ Thomson Avenue
Cambridge
CB3 0HE
+44 (0)1223 337218