R. L. Milot, M. T. Klug, C. L. Davies, Z. Wang, H. Kraus, H. J. Snaith, M. B. Johnston, and L. M. Herz
Advanced Materials (Sept 2018) [ pdf ] [ ref ]

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI₃) thin films, whose background hole doping density is varied through SnF₂ addition during film fabrication. Monomolecular charge‐carrier recombination exhibits both a dopant‐mediated part that grows linearly with hole doping density and remnant contributions that remain under tin‐enriched processing conditions. At hole densities near 10²⁰ cm⁻³, a strong Burstein–Moss effect increases absorption onset energies by ≈300 meV beyond the bandgap energy of undoped FASnI₃ (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (10²⁰ cm⁻³), temperature‐dependent measurements indicate that the effective charge‐carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 10¹⁹ cm⁻³ and below, the charge‐carrier mobility increases with decreasing temperature according to ≈T^−1.2, suggesting that it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2 × 10¹⁸ cm⁻³, charge‐carrier mobilities reach a value of 67 cm² V⁻¹ s⁻¹ at room temperature and 470 cm² V⁻¹ s⁻¹ at 50 K. Intraexcitonic transitions observed in the THz‐frequency photoconductivity spectra at 5 K reveal an exciton binding energy of only 3.1 meV for FASnI₃, in agreement with the low bandgap energy exhibited by this perovskite.

M. Monti, S. Tao, M. Staniforth, A. Crocker, E. Griffin, A. Wijesekara, R.A. Hatton, and J. Lloyd-Hughes
J. Phys. Chem. C (Aug 2018) [ pdf ] [ ref ]

The dynamic increase in THz photoconductivity resulting from energetic intraband relaxation was used to track the formation of highly mobile charges in thin films of the tin iodide perovskite Cs_1-xRb_xSnI₃, with x=0 and x=0.1. Energy relaxation times were found to be around 500fs, comparable to those in the prototypical inorganic semiconductor GaAs. At low excess energies the efficient intraband energy relaxation in the lowest conduction and valence bands of Cs_1-xRb_xSnI₃ can be understood within the context of the Fröhlich electron-phonon interaction, with a strong coupling strength. For higher excess energies the photoconductivity rise time lengthens in accordance with carrier injection into multiple bands, identified by quantitative first-principles bandstructure calculations and photoluminescence spectroscopy. The findings contribute to the development of design rules for photovoltaic devices capable of extracting hot carriers from perovskite semiconductors.

Q. Sun, X. Liu, J. Cao, R.I. Stantchev, Y. Zhou, X. Chen, E.P.J. Parrott, J. Lloyd-Hughes, N. Zhao, and E. Pickwell-MacPherson
J. Phys. Chem. C 122 17552 (June 2018) [ pdf ] [ ref ]

Terahertz (THz) thin-film total internal reflection (TF-TIR) spectroscopy is shown to have an enhanced sensitivity to the vibrational properties of thin films in comparison with standard THz transmission spectroscopy. This increased sensitivity was used to track photoinduced modifications to the structure of thin films of methylammonium (MA) lead halide, MAPbI_3–xBr_x (x = 0, 0.5, 1, and 3). Initially, illumination strengthened the phonon modes around 2 THz, associated with Pb–I stretch modes coupled to the MA ions, whereas the 1 THz twist modes of the inorganic octahedra did not alter in strength. Under longer term illumination, the 1 THz phonon modes of encapsulated films slowly reduced in strength, whereas in films exposed to moisture and oxygen, these phonons weaken more rapidly and blue-shift in frequency. The rapid monitoring of environmentally induced changes to the vibrational modes afforded by TF-TIR spectroscopy offers applications in the characterization and quality control of the perovskite thin-film solar cells and other thin-film semiconductors.

K.P. Marshall, S. Tao, M. Walker, D.S. Cook, J. Lloyd-Hughes, S. Varagnolo, A. Wijesekara, D. Walker, R.I. Walton and R.A. Hatton
Materials Chemistry Frontiers 2:1515 (June 2018) [ pdf ] [ ref ]

We show that films of the 3-dimensional perovskite Cs_1−xRb_xSnI₃ can be prepared from room temperature N,N-dimethylformamide solutions of RbI, CsI and SnCl₂ for x ≤ 0.5, and that for x ≤ 0.2 film stability is sufficient for utility as the light harvesting layer in inverted photovoltaic (PV) devices. Electronic absorption and photoluminescence spectroscopy measurements supported by computational simulation, show that increasing x increases the band gap, due to distortion of the lattice of SnI₆ octahedra that occurs when Cs is substituted with Rb, although it also reduces the stability towards decomposition. When Cs_0.8Rb_0.2SnI₃ perovskite is incorporated into the model inverted PV device structure; ITO|perovskite|C₆₀|bathocuproine|Al, an ∼120 mV increase in open-circuit is achieved which is shown to correlate with an increase in perovskite ionisation potential. However, for this low Rb loading the increase in band gap is very small (∼30 meV) and so a significant increase in open circuit-voltage is achieved without reducing the range of wavelengths over which the perovskite can harvest light. The experimental findings presented are shown to agree well with the predictions of density functional theory (DFT) simulations of the stability and electronic structure, also performed as part of this study.