D. Sirbu, F. H. Balogun, R. L. Milot and P. Docampo
Advanced Energy Materials (May 2021)
Hot carriers in mixed Pb-Sn halide perovskite semiconductors cool slowly while retaining their electrical mobility
Metal composition influences optoelectronic quality in mixed-metal lead-tin triiodide perovskite solar absorbers
M. T. Klug, R. L. Milot, J.B. Patel, T. Green, H. C. Sansom, M. D. Farrar, A. J. Ramadan, S. Martani, Z. Wang, B. Wenger, J. M. Ball, L. Langshaw, A. Petrozza, M. B. Johnston, L. M. Herz and H. J. Snaith
Energy & Environmental Science (May 2020) [ pdf ] [ ref ]
Current designs for all-perovskite multi-junction solar cells require mixed-metal Pb–Sn compositions to achieve narrower band gaps than are possible with their neat Pb counterparts. The lower band gap range achievable with mixed-metal Pb–Sn perovskites also encompasses the 1.3 to 1.4 eV range that is theoretically ideal for maximising the efficiency of single-junction devices. Here we examine the optoelectronic quality and photovoltaic performance of the ((HC(NH2)2)0.83Cs0.17)(Pb1−ySny)I3 family of perovskite materials across the full range of achievable band gaps by substituting between 0.001% and 70% of the Pb content with Sn. We reveal that a compositional range of “defectiveness” exists when Sn comprises between 0.5% and 20% of the metal content, but that the optoelectronic quality is restored for Sn content between 30–50%. When only 1% of Pb content is replaced by Sn, we find that photoconductivity, photoluminescence lifetime, and photoluminescence quantum efficiency are reduced by at least an order of magnitude, which reveals that a small concentration of Sn incorporation produces trap sites that promote non-radiative recombination in the material and limit photovoltaic performance. While these observations suggest that band gaps between 1.35 and 1.5 eV are unlikely to be useful for optoelectronic applications without countermeasures to improve material quality, highly efficient narrower band gap absorber materials are possible at or below 1.33 eV. Through optimising single-junction photovoltaic devices with Sn compositions of 30% and 50%, we respectively demonstrate a 17.6% efficient solar cell with an ideal single-junction band gap of 1.33 eV and an 18.1% efficient low band gap device suitable for the bottom absorber in all-perovskite multi-junction cells.
Ultrafast Optoelectronic Processes in 1D Radial van der Waals Heterostructures: Carbon, Boron Nitride, and MoS2 Nanotubes with Coexisting Excitons and Highly Mobile Charges
M.G. Burdanova, R.J. Kashtiban, Y. Zheng, R. Xiang, S. Chiashi, J.M. Woolley, M. Staniforth, E. Sakamoto-Rablah, X. Xie, M. Broome, J. Sloan, A. Anisimov, E.I. Kauppinen, S. Maruyama and J. Lloyd-Hughes
Nano Lett. 20 5, 3560 (Apr 2020) [ free e-print ] [ preprint pdf ] [ ref ]
Heterostructures built from 2D, atomically thin crystals are bound by the van der Waals force and exhibit unique optoelectronic properties. Here, we report the structure, composition and optoelectronic properties of 1D van der Waals heterostructures comprising carbon nanotubes wrapped by atomically thin nanotubes of boron nitride and molybdenum disulfide (MoS2). The high quality of the composite was directly made evident on the atomic scale by transmission electron microscopy, and on the macroscopic scale by a study of the heterostructure’s equilibrium and ultrafast optoelectronics. Ultrafast pump–probe spectroscopy across the visible and terahertz frequency ranges identified that, in the MoS2 nanotubes, excitons coexisted with a prominent population of free charges. The electron mobility was comparable to that found in high-quality atomically thin crystals. The high mobility of the MoS2 nanotubes highlights the potential of 1D van der Waals heterostructures for nanoscale optoelectronic devices.
K.D.G.I. Jayawardena, R.M.I. Bandara, M. Monti, E. Butler-Caddle, T. Pichler, H. Shiozawa, Z. Wang, S. Jenatsch, S.J. Hinder, M.G. Masteghin, M. Patel, H.M. Thirimanne, W. Zhang, R.A. Sporea, J. Lloyd-Hughes and S. R. P. Silva
J. Mater. Chem. A 8 693 (Jan 2020) [ pdf ] [ ref ]
The performance of all solar cells is dictated by charge recombination. A closer to ideal recombination dynamics results in improved performances, with fill factors approaching the limits based on Shockley-Queisser analysis. It is well known that for emerging solar materials such as perovskites, there are several challenges that need to be overcome to achieve high fill factors, particularly for large area lead-tin mixed perovskite solar cells. Here we demonstrate a strategy towards achieving fill factors above 80% through post-treatment of a lead-tin mixed perovskite absorber with guanidinium bromide for devices with an active area of 0.43 cm2. This bromide post-treatment results in a more favourable band alignment at the anode and cathode interfaces, enabling better bipolar extraction. The resulting devices demonstrate an exceptional fill factor of 83%, approaching the Shockley–Queisser limit, resulting in a power conversion efficiency of 14.4% for large area devices.
The Effects of Doping Density and Temperature on the Optoelectronic Properties of Formamidinium Tin Triiodide Thin Films
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.
Efficient Intraband Hot Carrier Relaxation in the Perovskite Semiconductor Cs1-xRbxSnI3 Mediated by Strong Electron-Phonon Coupling
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 Cs1-xRbxSnI3, 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 Cs1-xRbxSnI3 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.
We show that films of the 3-dimensional perovskite Cs1−xRbxSnI3 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 Cs0.8Rb0.2SnI3 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.