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Letter
Unraveling Charge Carriers Generation, Diffusion and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film Piotr Piatkowski, Boiko Cohen, Carlito S. Ponseca, Manuel Salado, Samrana Kazim, Shahzada Ahmad, Villy Sundstrom, and Abderrazzak Douhal J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02648 • Publication Date (Web): 24 Dec 2015 Downloaded from http://pubs.acs.org on December 26, 2015
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The Journal of Physical Chemistry Letters
Unraveling Charge Carriers Generation, Diffusion and Recombination in Formamidinium Lead Triiodide Perovskite Polycrystalline Thin Film
Piotr Piatkowski1, Boiko Cohen1, Carlito S. Ponseca Jr2, Manuel Salado3, Samrana Kazim3, Shahzada Ahmad3, Villy Sundström2 and Abderrazzak Douhal*,1
1
Departamento de Química Física, Facultad de Ciencias Ambientales y Bioquímica, and
INAMOL, Universidad de Castilla-La Mancha, Avda. Carlos III, S.N., 45071 Toledo, Spain. 2
Division of Chemical Physics, Lund University, Box 124, 22100 Lund, Sweden.
3
Abengoa Research, Abengoa, Campus Palmas Altas, C/Energia Solar, 41014
Sevilla, Spain.
Corresponding author:
[email protected] 1
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Abstract We report on studies of the formamidinium lead triiodide (FAPbI3) perovskite film, using time-resolved THz spectroscopy (TRTS) and flash photolysis to explore charge carriers generation, migration and recombination. The TRTS results show that upon
femtosecond
excitation
above
the
absorption
edge,
the
initial
high
photoconductivity (~ 75 cm2V-1s-1) remains constant at least up to 8 ns, and corresponds to a diffusion length of 25 m. Pumping below the absorption edge results in a mobility of 40 cm2V-1s-1, which suggests lower mobility of charge carriers located at the bottom of the conduction band or shallow sub-band-gap states. Furthermore, the analysis of the THz kinetics reveals rising components of < 1 ps and 20 ps, reflecting dissociation of excitons having different binding energies. Flash photolysis experiments indicate that trapped charge carriers persist for milliseconds.
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Over the past 3 years, great advances have been made to improve the solar light to electricity conversion using the now well-known perovskites-based solar cells.1-10 Methylammonium lead triiodide (MAPbI3) and the methylammonium iodide/chloride mixed halides (MAPbI3-xClx) are the most used light absorbers in this family.1-6, 8, 10-26 The improvements in the solar cell architecture and film quality, as well as compositional engineering development, resulted in a solar-to-electricity power conversion efficiency (PCE) of 19.3% for devices based on MAPbI3-xClx.27 Recently, a solar cell based on formamidinium lead triiodide (FAPbI3) film reached a PCE over 20%.1 It is believed that the superior performance of this material is due to its broader absorption of the solar spectrum and higher mobility of the excited charge carriers.1, 28, 29 The high efficiency of perovskite solar cells can be traced as a combination of high light absorption, slow charge recombination, and relatively high charge carrier mobility caused by the presence of free charges formed upon exciton dissociation in a few picoseconds.5, 10, 13-15, 30 The polycrystalline film morphology also plays a crucial role in the photovoltaic performance due to electron and hole trapping on the surface of the crystals.6, 8, 24, 25 Other reports have shown a strong dependence of the photoluminescence lifetime on the size of the crystallites and concentration of traps, which may depend on details of the preparation procedure.8, 12, 23, 31 Light illumination of a MAPbI3 thin layer in presence of oxygen resulted in enhanced photoluminescence intensity and longer lifetimes reflecting a reduced trap concentration and decreased photoluminescence quenching. 29, 31, 32 Moreover, the optical properties of perovskites strongly depend on the organic components and on the post-synthetic treatment of the freshly prepared film.7, 3134
Very recently, picosecond timescale (0.3 ps and 3 ps) rotational motion of the methyl
group within the MAPbI3 lattice was observed.33, 35 Several studies have demonstrated the importance of the organic cation in tuning the optical properties of the perovskite based
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devices. The increase of the effective cation radius when switching from methylammonium (217 pm) to formamidinium (253 pm) results in a decrease of the optical bandgap energy from 1.54 to 1.47 eV.36 A change in the organic part leads to a change in the lattice symmetry - trigonal for FAPbI3 and tetragonal for MAPbI3 at room temperature7, 37, 38 As a result, the diffusion lengths of the charge carriers in FAPbI3 have been reported to be longer than in MAPbI3.3, 26, 28 This difference has been proposed to be a result of a change in the perovskite band structure giving rise to unbalanced hole and electron effective masses.28 Ultrafast processes in perovskites, such as exciton generation and dissociation into free charges, their mobilities and trapping, are key factors influencing the overall efficiency of a solar cell.5, 8, 14, 30 Time-resolved optical spectroscopy studies have been instrumental in unravelling many of these processes in MAPbI3, and have demonstrated high charge carrier mobilities and their slow recombination. Furthermore, the role of trap states and dark carriers in MAPbI3 has been reported. On the other hand, there are only a limited number of studies of these processes in FAPbI3. Taking into account the considerable improvement in the overall efficiency of the FAPbI3 and mixed FA/MAPbI3 based solar cells, and the significant role of the ultrafast processes in the overall photophysical properties of these systems, there is an urgent need for a time-resolved optical spectroscopy study of the dynamical behavior of FAPbI3. Herein we present and discuss our time-resolved terahertz (THz) spectroscopy (TRTS) and nanosecond to millisecond flash photolysis results of thin (210 ± 5 nm) film of this material (Figure S1).29 We found that the charge mobility is 75 cm2V-1s-1, considerably higher than that in a similarly prepared MAPbI3 film. Moreover, we present experimental evidence that states, located at the bottom of the conduction band (CB) and shallow sub-band-gap states contribute to the charge dynamics. The THz signal kinetics exhibit an initial ultrafast rise
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comparable to the time resolution of the setup (
