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Nature Photonics |
Journal name:
Nature Photonics
Year published:
doi:10.1038/nphoton.
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Organohalide-perovskite solar cells have emerged as a leading next-generation photovoltaic technology. However, despite surging efficiencies, many questions remain unanswered regarding the mechanisms of operation. Here we report a detailed study of the electro-optics of efficient CH3NH3PbI3-perovskite-only planar devices. We report the dielectric constants over a large frequency range. Importantly, we found the real part of the static dielectric constant to be ~70, from which we estimate the exciton-binding energy to be of order 2 meV, which strongly indicates a non-excitonic mechanism. Also, Jonscher's Law behaviour was consistent with the perovskite having ionic character. Accurate knowledge of the cell's optical constants allowed improved modelling and design, and using this information we fabricated an optimized device with an efficiency of 16.5%. The optimized devices have ~100% spectrally flat internal quantum efficiencies and minimal bimolecular recombination. These findings establish systematic design rules to achieve silicon-like efficiencies in simple perovskite solar cells.
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Figure 1: Materials and their properties used in the CH3NH3PbI3 non-oxide perovskite solar cells.
a, Chemical structures of the polymer interlayers: PCDTBT, PCPDTBT, P3HT, DPP-DTT and PC60BM. b, Schematic illustration of the energy levels of CH3NH3PbI3 perovskite (conduction and valence bands), PC60BM ‘electron affinity’ and p-type polymer interlayer ionization potential from the photoelectron spectroscopy in air. c, XRD spectra of CH3NH3PbI3-perovskite films grown on different polymer interlayers via thermal evaporation. The inset is the schematic crystal structure of CH3NH3PbI3 perovskite.
Figure 2: Optical and dielectric properties of the CH3NH3PbI3 perovskite.
a, Optical constants of the CH3NH3PbI3: refractive index (n) and extinction coefficient (k), as determined by employing spectroscopic ellipsometry, near-normal incidence reflectance and total transmittance. Three different spectral regions occur: λ > 800 nm with minimal absorption, 500 nm < λ < 800 nm with moderate absorption (comparable with the absorption of typical organic semiconductors) and λ < 500 nm with the very strong absorption characteristic of PbI2. b, Dielectric constants of CH3NH3PbI3, real and imaginary parts, in the optical (high) frequency regime as determined from n, k and low-frequency and static values from impedance analysis and CELIV, respectively. A high static dielectric constant of ~70 is notable. The 1/f behaviour of the imaginary part of the dielectric constant is indicative of the ionic nature of this perovskite.
Figure 3: Electro-optical modelling of perovskite solar cells.
a, Optical-field distribution in a CH3NH3PbI3-perovskite device for four wavelengths: for λ < 500 nm the optical-field distribution follows the Beer-Lambert law and no optical field reaches to the back electrode as a result of the high absorption coefficient. In such cases the absorption is saturated and no optical interference occurs. For λ > 500 nm the optical field is governed by low-finesse cavity interference. b, EQE spectra of devices with different CH3NH3PbI3-layer thicknesses. For λ < 500 nm there is minimal influence of the film thickness, but for λ > 500 nm the EQE is strongly thickness dependent because of the optical interference. c, Comparison of experimental short-circuit current density Jsc and modelled Jsc (assuming IQE is 100%). It was predicted that by increasing the thickness, the photocurrent density would reach a maximum of ~21 mA cm-2 at an active layer thickness of 350 nm. This was confirmed experimentally. As a result of morphological effects, the maximum photocurrent density falls off at thicker active layers.
Figure 4: Solar-cell performance of the ‘hero’ CH3NH3PbI3 perovskite.
a, Dark and light J-V curves (scan from -1 V to 1.2 V with a scan rate of 0.1 V s-1) of the best-performing CH3NH3PbI3-perovskite solar cell optimized based on . b, EQE and IQE spectra of a corresponding device. The IQE is spectrally flat and ~100% to within the error of measurement (±5%). The inset shows the ratio of EQE measured without bias and with a -1 V reverse bias and shows that carrier extraction is essentially lossless.
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Contributions
Q.L. synthesized the MAI, prepared perovskite films and performed the basic characterization. Q.L. fabricated the solar-cell devices and Q.L. and A.A. tested them. Q.L., A.A., P.M. and P.L.B. designed the devices and experiments. R.C.R.N. and P.M. performed the spectroscopic ellipsometry and reflectometry and R.C.R.N., P.M. and A.A. fitted the data. A.A. and P.M. carried out the dielectric constant/binding energy analysis. All the authors contributed in writing the manuscript.
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The authors declare no competing financial interests.
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Qianqian LinArdalan ArminRavi Chandra Raju NagiriPaul L. BurnPaul Meredith
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