Tom Hopper is a Postdoctoral Scholar in Photon Science at SLAC National Accelerator Lab. After completing a Masters degree in Chemistry at Newcastle University, he became the first graduate student of Artem Bakulin at Imperial College London, which began with building a brand new femtosecond laser suite for the Chemistry department, and culminated in numerous awards for his thesis on ultrafast photophysics in organic, hybrid and nanoscale materials and devices. Following an EPSRC Doctoral Prize Fellowship in the same lab, he joined the group of Aaron Lindenberg at Stanford University in 2021 as a TomKat Postdoctoral Fellow in Sustainable Energy, continuing research into energy transport in emerging photovoltaics. Tom currently works with Adi Natan and Kelly Gaffney at SLAC, where he is developing pulse sequences to exert coherent control of photochemical structural and solvation dynamics, and image these processes with the world’s brightest X-rays at the LCLS free electron laser.

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Honors & Awards

  • Semiconductor Physics PhD Thesis Prize, The Institute of Physics (Sep 2021)
  • Postdoctoral Fellowship in Sustainable Energy, TomKat Center, Stanford University (Jul 2021 - Present)
  • Best Thesis Prize, Department of Chemistry, Imperial College London (Mar 2021)
  • Energy Sector PhD Thesis Award (3rd prize), Royal Society of Chemistry (Mar 2021)
  • Doctoral Prize Fellowship, EPSRC (Oct 2020 - Jun 2021)
  • The Edward Steers Award, The Association of British Spectroscopists (Feb 2020)
  • Best Speaker Prize (across all departments), Faculty of Natural Sciences, Imperial College London (Sep 2019)
  • Best Talk Prize, Department of Chemistry, Imperial College London (Jul 2019)
  • Sustainable Energy & Fuels Talk Prize for Best Communication, Centre for Processable Electronics, Imperial College London (Jun 2019)
  • Early Career Travel Grant, Royal Society of Chemistry (Jun 2019)
  • Research Excellence Award, MKS Instruments & SPIE (May 2019)
  • Conference Bursary, Wilkinson Charitable Trust for Inorganic Chemistry (May 2019)
  • C. R. Barber Trust Conference Bursary, The Institute of Physics (Apr 2019)
  • Research Student Conference Bursary, The Institute of Physics (Mar 2019)
  • International Conference Bursary, SUPERGEN SuperSolar (Feb 2019)
  • Best Science Poster, Centre for Processable Electronics (Jun 2018)
  • Doctoral Training Scholarship, EPSRC (Sep 2016 - Dec 2019)
  • Roger Griffin Prize, Newcastle University (Jul 2016)
  • Chemistry Excellence Prize, Newcastle University (Oct 2015)
  • Undergraduate Research Bursary, Royal Society of Chemistry (Jun 2015 - Jul 2015)
  • Society of Chemical Industry Prize, Newcastle University (Sep 2014)
  • Vacation Student Scholarship, Newcastle University (Jul 2014 - Aug 2014)
  • Society of Chemical Industry Prize, Newcastle University (Sep 2013)
  • Access Scholarship, Newcastle University (Sep 2012 - Jul 2016)

Boards, Advisory Committees, Professional Organizations

  • Chartered Chemist (CChem), Royal Society of Chemistry (2021 - Present)
  • Member of The Institute of Physics (MInstP), The Institute of Physics (2019 - Present)
  • Member of Royal Society of Chemistry (MRSC), Royal Society of Chemistry (2019 - Present)
  • Registered Scientist (RSci), The Science Council (2018 - Present)

Professional Education

  • Doctor of Philosophy, Imperial College of London (2020)
  • Master of Science, Newcastle University (2016)
  • PhD, Imperial College London, Physical Chemistry (2020)
  • MChem, Newcastle University, Chemistry (2016)

Stanford Advisors

2022-23 Courses

All Publications

  • Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials. ACS nano Carwithen, B. P., Hopper, T. R., Ge, Z., Mondal, N., Wang, T., Mazlumian, R., Zheng, X., Krieg, F., Montanarella, F., Nedelcu, G., Kroll, M., Siguan, M. A., Frost, J. M., Leo, K., Vaynzof, Y., Bodnarchuk, M. I., Kovalenko, M. V., Bakulin, A. A. 2023


    The relaxation of the above-gap ("hot") carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier-carrier and carrier-phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump-push-probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials. In cuboidal nanocrystals, we observe only a weak size effect on the cooling dynamics. In contrast, two-dimensional systems show suppression of the hot phonon bottleneck effect common in bulk perovskites. The proposed kinetic model describes the intrinsic and density-dependent cooling times accurately in all studied perovskite systems using only carrier-carrier, carrier-phonon, and excitonic coupling constants. This highlights the impact of exciton formation on carrier cooling and promotes dimensional confinement as a tool for engineering carrier-phonon and carrier-carrier interactions in LHP optoelectronic materials.

    View details for DOI 10.1021/acsnano.2c12373

    View details for PubMedID 36939330

  • Hot Carrier Cooling and Trapping in Atomically Thin WS2 Probed by Three-Pulse Femtosecond Spectroscopy. ACS nano Wang, T., Hopper, T. R., Mondal, N., Liu, S., Yao, C., Zheng, X., Torrisi, F., Bakulin, A. A. 2023


    Transition metal dichalcogenides (TMDs) have shown outstanding semiconducting properties which make them promising materials for next-generation optoelectronic and electronic devices. These properties are imparted by fundamental carrier-carrier and carrier-phonon interactions that are foundational to hot carrier cooling. Recent transient absorption studies have reported ultrafast time scales for carrier cooling in TMDs that can be slowed at high excitation densities via a hot-phonon bottleneck (HPB) and discussed these findings in the light of optoelectronic applications. However, quantitative descriptions of the HPB in TMDs, including details of the electron-lattice coupling and how cooling is affected by the redistribution of energy between carriers, are still lacking. Here, we use femtosecond pump-push-probe spectroscopy as a single approach to systematically characterize the scattering of hot carriers with optical phonons, cold carriers, and defects in a benchmark TMD monolayer of polycrystalline WS2. By controlling the interband pump and intraband push excitations, we observe, in real-time (i) an extremely rapid "intrinsic" cooling rate of ∼18 ± 2.7 eV/ps, which can be slowed with increasing hot carrier density, (ii) the deprecation of this HPB at elevated cold carrier densities, exposing a previously undisclosed role of the carrier-carrier interactions in mediating cooling, and (iii) the interception of high energy hot carriers on the subpicosecond time scale by lattice defects, which may account for the lower photoluminescence yield of TMDs when excited above band gap.

    View details for DOI 10.1021/acsnano.2c10479

    View details for PubMedID 36939760

  • Multipulse Terahertz Spectroscopy Unveils Hot Polaron Photoconductivity Dynamics in Metal-Halide Perovskites. The journal of physical chemistry letters Zheng, X., Hopper, T. R., Gorodetsky, A., Maimaris, M., Xu, W., Martin, B. A., Frost, J. M., Bakulin, A. A. 2021: 8732-8739


    Hot carriers in metal-halide perovskites (MHPs) present a foundation for understanding carrier-phonon coupling in the materials as well as the prospective development of high-performance hot carrier photovoltaics. While the carrier population dynamics during cooling have been scrutinized, the evolution of the hot carrier properties, namely mobility, remains largely unexplored. Here we introduce novel ultrafast visible pump-infrared push-terahertz probe spectroscopy to monitor the real-time conductivity dynamics of cooling carriers in methylammonium lead iodide. We find a decrease in mobility upon optically re-exciting the carriers, as expected for band transport. Surprisingly, the conductivity recovery is incommensurate with the hot carrier population dynamics measured by infrared probe and exhibits a negligible dependence on the hot carrier density. Our results reveal the importance of localized lattice heating toward the hot carrier mobility. This collective polaron-lattice phenomenon may contribute to the unusual photophysics of MHPs and should be accounted for in hot carrier devices.

    View details for DOI 10.1021/acs.jpclett.1c02102

    View details for PubMedID 34478291

  • Materials, Photophysics and Device Engineering of Perovskite Light-Emitting Diodes. Reports on progress in physics. Physical Society (Great Britain) Chen, Z., Li, Z., Hopper, T., Bakulin, A. A., Yip, H. 2021


    Here we provide a comprehensive review of a newly developed lighting technology based on metal halide perovskites (i.e. perovskite light-emitting diodes) encompassing the research endeavours into materials, photophysics and device engineering. At the outset we survey the basic perovskite structures and their various dimensions (namely three-, two- and zero-dimensional perovskites), and demonstrate how the compositional engineering of these structures affects the perovskite light-emitting properties. Next, we turn to the physics underpinning photo- and electroluminescence in these materials through their connection to the fundamental excited states, energy/charge transport processes and radiative and non-radiative decay mechanisms. In the remainder of the review, we focus on the engineering of perovskite light-emitting diodes, including the history of their development as well as an extensive analysis of contemporary strategies for boosting device performance. Key concepts include balancing the electron/hole injection, suppression of parasitic carrier losses, improvement of the photoluminescence quantum yield and enhancement of the light extraction. Overall, this review reflects the current paradigm for perovskite lighting, and is intended to serve as a foundation to materials and device scientists newly working in this field.

    View details for DOI 10.1088/1361-6633/abefba

    View details for PubMedID 33730709

  • Kinetic modelling of intraband carrier relaxation in bulk and nanocrystalline lead-halide perovskites PHYSICAL CHEMISTRY CHEMICAL PHYSICS Hopper, T. R., Jeong, A., Gorodetsky, A. A., Krieg, F., Bodnarchuk, M., Huang, X., Lovrincic, R., Kovalenko, M., Bakulin, A. A. 2020; 22 (31): 17605–11


    The relaxation of high-energy "hot" carriers in semiconductors is known to involve the redistribution of energy between hot and cold carriers, as well as the transfer of energy from hot carriers to phonons. Over the past few years, these two processes have been identified in lead-halide perovskites (LHPs) using ultrafast pump-probe experiments, but their interplay is not fully understood. Here we present a practical and intuitive kinetic model that accounts for the effects of both hot and cold carriers on carrier relaxation in LHPs. We apply this model to describe the dynamics of hot carriers in bulk and nanocrystalline CsPbBr3 as observed by multi-pulse "pump-push-probe" spectroscopy. The model captures the slowing of the relaxation dynamics in the materials as the number of hot carriers increases, which has previously been explained by a "hot-phonon bottleneck" mechanism. The model also correctly predicts an acceleration of the relaxation kinetics as the number of cold carriers in the samples is increased. Using a series of natural approximations, we reduce our model to a simple form containing terms for the carrier-carrier and carrier-phonon interactions. The model can be instrumental for evaluating the details of carrier relaxation and carrier-phonon couplings in LHPs and other soft optoelectronic materials.

    View details for DOI 10.1039/d0cp01599g

    View details for Web of Science ID 000560847500014

    View details for PubMedID 32808944

  • Hot Carrier Dynamics in Perovskite Nanocrystal Solids: Role of the Cold Carriers, Nanoconfinement, and the Surface NANO LETTERS Hopper, T. R., Gorodetsky, A., Jeong, A., Krieg, F., Bodnarchuk, M., Maimaris, M., Chaplain, M., Macdonald, T. J., Huang, X., Lovrincic, R., Kovalenko, M., Bakulin, A. A. 2020; 20 (4): 2271–78


    Carrier cooling is of widespread interest in the field of semiconductor science. It is linked to carrier-carrier and carrier-phonon coupling and has profound implications for the photovoltaic performance of materials. Recent transient optical studies have shown that a high carrier density in lead-halide perovskites (LHPs) can reduce the cooling rate through a "phonon bottleneck". However, the role of carrier-carrier interactions, and the material properties that control cooling in LHPs, is still disputed. To address these factors, we utilize ultrafast "pump-push-probe" spectroscopy on LHP nanocrystal (NC) films. We find that the addition of cold carriers to LHP NCs increases the cooling rate, competing with the phonon bottleneck. By comparing different NCs and bulk samples, we deduce that the cooling behavior is intrinsic to the LHP composition and independent of the NC size or surface. This can be contrasted with other colloidal nanomaterials, where confinement and trapping considerably influence the cooling dynamics.

    View details for DOI 10.1021/acs.nanolett.9b04491

    View details for Web of Science ID 000526413400008

    View details for PubMedID 32142303

  • Control of Donor-Acceptor Photophysics through Structural Modification of a "Twisting" Push-Pull Molecule CHEMISTRY OF MATERIALS Hopper, T. R., Qian, D., Yang, L., Wang, X., Zhou, K., Kumar, R., Ma, W., He, C., Hou, J., Gao, F., Bakulin, A. A. 2019; 31 (17): 6860–69
  • Impact of Marginal Exciton-Charge-Transfer State Offset on Charge Generation and Recombination in Polymer:Fullerene Solar Cells ACS ENERGY LETTERS Vezie, M. S., Azzouzi, M., Telford, A. M., Hopper, T. R., Sieval, A. B., Hummelen, J. C., Fallon, K., Bronstein, H., Kirchartz, T., Bakulin, A. A., Clarke, T. M., Nelson, J. 2019; 4 (9): 2096–2103
  • Block Junction-Functionalized All-Conjugated Donor-Acceptor Block Copolymers ACS APPLIED MATERIALS & INTERFACES Nuebling, F., Hopper, T. R., Kuei, B., Komber, H., Untilova, V., Schmidt, S. B., Brinkmann, M., Gomez, E. D., Bakulin, A. A., Sommer, M. 2019; 11 (1): 1143–55


    Junction-functionalized donor-acceptor (D-A) block copolymers (BCPs) enable spatial and electronic control over interfacial charge dynamics in excitonic devices such as solar cells. Here, we present the design, synthesis, morphology, and electronic characterization of block junction-functionalized, all-conjugated, all-crystalline D-A BCPs. Poly(3-hexylthiophene) (P3HT), a single thienylated diketopyrrolopyrrole (Th xDPPTh x, x = 1 or 2) unit, and poly{[ N, N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]- alt-5,5'-(2,2'-bithiophene)} (PNDIT2) are used as donor, interfacial unit, and acceptor, respectively. Almost all C-C coupling steps are accomplished by virtue of C-H activation. Synthesis of the macroreagent H-P3HT-Th xDPPTh x, with x determining its C-H reactivity, is key to the synthesis of various BCPs of type H-P3HT-Th xDPPTh x- block-PNDIT2. Morphology is determined from a combination of calorimetry, transmission electron microscopy (TEM), and thin-film scattering. Block copolymer crystallinity of P3HT and PNDIT2 is reduced, indicating frustrated crystallization. A long period lp is invisible from TEM, but shows up in resonant soft X-ray scattering experiments at a length scale of lp ∼ 60 nm. Photoluminescence of H-P3HT-Th xDPPTh x indicates efficient transfer of the excitation energy to the DPP chain end, but is quenched in BCP films. Transient absorption and pump-push photocurrent spectroscopies reveal geminate recombination (GR) as the main loss channel in as-prepared BCP films independent of junction functionalization. Melt annealing increases GR as a result of the low degree of crystallinity and poorly defined interfaces and additionally changes backbone orientation of PNDIT2 from face-on to edge-on. These morphological effects dominate solar cell performance and cause an insensitivity to the presence of the block junction.

    View details for DOI 10.1021/acsami.8b18608

    View details for Web of Science ID 000455561200124

    View details for PubMedID 30523687

  • Efficient non-fullerene organic solar cells employing sequentially deposited donor-acceptor layers JOURNAL OF MATERIALS CHEMISTRY A Zhang, J., Kan, B., Pearson, A. J., Parnell, A. J., Cooper, J. K., Liu, X., Conaghan, P. J., Hopper, T. R., Wu, Y., Wan, X., Gao, F., Greenham, N. C., Bakulin, A. A., Chen, Y., Friend, R. H. 2018; 6 (37): 18225–33

    View details for DOI 10.1039/c8ta06860g

    View details for Web of Science ID 000448340100039

  • Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites ACS ENERGY LETTERS Hopper, T. R., Gorodetsky, A., Frost, J. M., Mueller, C., Lovrincic, R., Bakulin, A. A. 2018; 3 (9): 2199–2205


    The rapid relaxation of above-band-gap "hot" carriers (HCs) imposes the key efficiency limit in lead-halide perovskite (LHP) solar cells. Recent studies have indicated that HC cooling in these systems may be sensitive to materials composition, as well as the energy and density of excited states. However, the key parameters underpinning the cooling mechanism are currently under debate. Here we use a sequence of ultrafast optical pulses (visible pump-infrared push-infrared probe) to directly compare the intraband cooling dynamics in five common LHPs: FAPbI3, FAPbBr3, MAPbI3, MAPbBr3, and CsPbBr3. We observe ∼100-900 fs cooling times, with slower cooling at higher HC densities. This effect is strongest in the all-inorganic Cs-based system, compared to the hybrid analogues with organic cations. These observations, together with band structure calculations, allow us to quantify the origin of the "hot-phonon bottleneck" in LHPs and assert the thermodynamic contribution of a symmetry-breaking organic cation toward rapid HC cooling.

    View details for DOI 10.1021/acsenergylett.8b01227

    View details for Web of Science ID 000445052900025

    View details for PubMedID 30450410

    View details for PubMedCentralID PMC6231231

  • Design rules for minimizing voltage losses in high-efficiency organic solar cells NATURE MATERIALS Qian, D., Zheng, Z., Yao, H., Tress, W., Hopper, T. R., Chen, S., Li, S., Liu, J., Chen, S., Zhang, J., Liu, X., Gao, B., Ouyang, L., Jin, Y., Pozina, G., Buyanova, I. A., Chen, W. M., Inganas, O., Coropceanu, V., Bredas, J., Yan, H., Hou, J., Zhang, F., Bakulin, A. A., Gao, F. 2018; 17 (8): 703–9


    The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

    View details for DOI 10.1038/s41563-018-0128-z

    View details for Web of Science ID 000439573400015

    View details for PubMedID 30013057

  • Field-Assisted Exciton Dissociation in Highly Efficient PffBT4T-2OD:Fullerene Organic Solar Cells CHEMISTRY OF MATERIALS Weu, A., Hopper, T. R., Lami, V., Kress, J. A., Bakulin, A. A., Vaynzof, Y. 2018; 30 (8): 2660–67