Research interests
Perovskite-based photovoltaic materials, devices and specroscopy
Publications
- Charge carrier recombination channels in the low-temperature phase of organic-inorganic lead halide perovskite thin films,
C Wehrenfennig, M. Liu, HJ Snaith, MB Johnston, LM Herz
APL Mater., 2:081513 (2014)
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pdf | doi:10.1063/1.4891595 ]
The optoelectronic properties of the mixed hybrid lead halide perovskite CH3NH3PbI3−xClx have been subject to numerous recent studies related to its extraordinary capabilities as an absorber material in thin film solar cells. While the greatest part of the current research concentrates on the behavior of the perovskite at room temperature, the observed influence of phonon-coupling and excitonic effects on charge carrier dynamics suggests that low-temperature phenomena can give valuable additional insights into the underlying physics. Here, we present a temperature-dependent study of optical absorption and photoluminescence (PL) emission of vapor-deposited CH3NH3PbI3−xClx exploring the nature of recombination channels in the room- and the low-temperature phase of the material. On cooling, we identify an up-shift of the absorption onset by about 0.1 eV at about 100 K, which is likely to correspond to the known tetragonal-to-orthorhombic transition of the pure halide CH3NH3PbI3. With further decreasing temperature, a second PL emission peak emerges in addition to the peak from the room-temperature phase. The transition on heating is found to occur at about 140 K, i.e., revealing significant hysteresis in the system. While PL decay lifetimes are found to be independent of temperature above the transition, significantly accelerated recombination is observed in the low-temperature phase. Our data suggest that small inclusions of domains adopting the room-temperature phase are responsible for this behavior rather than a spontaneous increase in the intrinsic rate constants. These observations show that even sparse lower-energy sites can have a strong impact on material performance, acting as charge recombination centres that may detrimentally affect photovoltaic performance but that may also prove useful for optoelectronic applications such as lasing by enhancing population inversion. - Charge-carrier dynamics in vapour-deposited films of the organolead halide perovskite {CH$_3$NH$_3$PbI$_{3-x}$Cl$_x$},
C Wehrenfennig, M. Liu, HJ Snaith, MB Johnston, LM Herz
Energy Environ. Sci., 7:2269--2275 (2014)
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pdf | doi:10.1039/C4EE01358A ]
We determine high charge-carrier mobilities [greater-than-or-equal] 33 cm2 V-1 s-1 and bi-molecular recombination rates about five orders of magnitude below the prediction of Langevin{'}s model for vapour-deposited CH3NH3PbI3-xClx using ultrafast THz spectroscopy. At charge-carrier densities below [similar]1017 cm-3 intrinsic diffusion lengths are shown to approach 3 microns{,} limited by slow mono-molecular decay processes.} - Homogeneous Emission Line Broadening in the Organo Lead Halide Perovskite $CH_3NH_3PbI_{3-x}Cl_x$,
C Wehrenfennig, M. Liu, HJ Snaith, MB Johnston, LM Herz
J. Phys. Chem. Lett., 5:1300-1306 (2014)
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pdf | doi:10.1021/jz500434p ]
The organic−inorganic hybrid perovskites methylammonium lead iodide (CH3NH3PbI3) and the partially chlorine-substituted mixed halide CH3NH3PbI3−xClx emit strong and broad photoluminescence (PL) around their band gap energy of ∼1.6 eV. However, the nature of the radiative decay channels behind the observed emission and, in particular, the spectral broadening mechanisms are still unclear. Here we investigate these processes for high-quality vapor-deposited films of CH3NH3PbI3−xClx using time- and excitation-energy dependent photoluminescence spectroscopy. We show that the PL spectrum is homogenously broadened with a line width of 103 meV most likely as a consequence of phonon coupling effects. Further analysis reveals that defects or trap states play a minor role in radiative decay channels. In terms of possible lasing applications, the emission spectrum of the perovskite is sufficiently broad to have potential for amplification of light pulses below 100 fs pulse duration. - Efficient planar heterojunction perovskite solar cells by vapour deposition,
M. Liu, MB Johnston, HJ Snaith
Nature, 501:395--398 (2013)
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pdf | doi:10.1038/nature12509 ]
Many different photovoltaic technologies are being developed for large-scale solar energy conversion1, 2, 3, 4. The wafer-based first-generation photovoltaic devices1 have been followed by thin-film solid semiconductor absorber layers sandwiched between two charge-selective contacts3 and nanostructured (or mesostructured) solar cells that rely on a distributed heterojunction to generate charge and to transport positive and negative charges in spatially separated phases4, 5, 6. Although many materials have been used in nanostructured devices, the goal of attaining high-efficiency thin-film solar cells in such a way has yet to be achieved7. Organometal halide perovskites have recently emerged as a promising material for high-efficiency nanostructured devices8, 9, 10, 11. Here we show that nanostructuring is not necessary to achieve high efficiencies with this material: a simple planar heterojunction solar cell incorporating vapour-deposited perovskite as the absorbing layer can have solar-to-electrical power conversion efficiencies of over 15 per cent (as measured under simulated full sunlight). This demonstrates that perovskite absorbers can function at the highest efficiencies in simplified device architectures, without the need for complex nanostructures.
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