Fundamental studies of the electronic structure of lead halide perovskites using synchrotron-based resonant electron and X-ray spectroscopies

Synopsis

Lead halide perovskites (HaPs) of the form APbX₃ have attracted substantial renewed research interest for over a decade, motivated by initially dramatic gains in HaP solar cell power conversion efficiencies and now other optoelectronic applications.  At present, in spite of the substantial growth of many new subclasses of HaP-related materials and their applications, many fundamental questions related to the prototypical HaPs remain unanswered (D.A. Egger et al., Adv. Mater. 30, 1800691 (2018)), despite a history of basic research dating back to as early as the 1970’s (D. Weber, Z. Naturforsch. 33b, 1443 (1978)). 

Here we focus on two questions: (i) what is the optoelectronic role of the A-cation and (ii) what is the mechanism of slow hot carrier cooling in HaPs?  Since 2015/16, multiple A-cation HaP films have been used in devices (Saliba et al., Energy Environ. Sci. 9, 1989 (2016)), primarily to enhance their ambient stabilities, and the optoelectronic role of the A-cation, a well-debated issue, remains controversial.  We have reported the first application of resonant Auger electron spectroscopy (RAES) to HaPs and found evidence for electronic coupling in the unoccupied states via femtosecond-timescale charge transfer from the organic A-cation to the iodide-lead sublattice (G.J. Man et al., PRB 104, L041302 (2021)).  Through the use of High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS), we uncover a previously hidden feature in the conduction band states, the σ-π splitting, and find that its magnitude is strongly influenced by the strength of hydrogen-bonding between the A-cation and the bromide-lead sublattice (G.J. Man et al., arXiv:2109.08587 [cond-mat] (2021)).  Our work provides conclusive evidence of A-cation effects on the conduction band states.  Additionally, the σ-π splitting explains numerous optical spectroscopy-based experimental observations reported in the literature and provides an alternative mechanism to the commonly discussed polaronic screening and hot phonon bottleneck carrier cooling mechanisms.

Slow hot carrier cooling in HaPs has been reported nearly a decade ago (Xing et al., Science 342, 344 (2013)) and HaPs show hot carrier decay constants orders of magnitude higher than all competing materials used in solar cells.  The mechanism of slow hot carrier cooling in HaPs remains controversial.  Hot carrier solar cells, which require the existence of slow hot carrier cooling in the solar absorber, have potential to surpass the thermodynamic power conversion efficiency limit of ~33% (Shockley-Queisser limit) and were proposed 40 years ago (Ross, Nozik, J. Appl. Phys. 53, 3813 (1982)).  To-date, no actual devices exist (Ferry et al., J. Appl. Phys. 128, 220903 (2020)).  Our work sheds new light on the mechanism of slow hot carrier cooling in HaPs, which are the most promising class of materials for hot carrier photovoltaics to-date.  Halide perovskite solar cells are widely considered a game changer due to their potential for low-cost.  The development of low-cost, hot-carrier HaP solar cells could be a double game changer.

Bio

Gabriel Man received his B.A.Sc. (2007) and M.A.Sc. (2010) degrees in Electrical Engineering from UBC (Vancouver, Canada).  Following a year of materials science and solar cell research at the Weizmann Institute of Science (Rehovot, Israel), supervised by Professor David Cahen, he started a PhD program at Princeton University (New Jersey, USA).  He received his Ph.D. degree in Electrical Engineering and Materials Science in 2017, under the guidance of Professor Antoine Kahn.  His dissertation work, partially supported by an NSERC fellowship, focused on design principles for carrier-selective contacts in solar cells and the use of direct and inverse photoelectron spectroscopy (UPS, XPS, IPES) to elucidate the energy level alignment of interfaces involving silicon and lead halide perovskite (HaP) solar absorbers.  Gabriel spent roughly a year working as a HaP solar cell device engineer and materials scientist at Hunt Perovskite Technologies (Dallas, USA, now CubicPV) before returning to academic research.  He started working as a postdoctoral researcher at Uppsala University (Sweden) in 2018 and is now a staff researcher.  He has led and is leading projects that involve the use of synchrotron-based resonant and non-resonant electron and X-ray spectroscopy to uncover features in the electronic structure of HaPs that are relevant for optoelectronic functionality.