Trap-Assisted Non-radiative Recombination in Organic-Inorganic Perovskite Solar Cells (original) (raw)
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Journal of Luminescence, 2018
The degradation of methyl-ammonium lead iodide (MAPbI 3) upon exposure to air is a potentially limiting effect for large scale MAPbI 3 photovoltaic production. Here, we report a systematic study on effects of air-exposure on the structural and optical properties of MAPbI 3 thin films. The X-ray diffraction studies indicate a shrinking volume of MAPbI 3 upon air-exposure as the material decomposes back to its precursors, methyl amine (CH 3 NH 2) and lead iodide (PbI 2). However, the photoluminescence (PL) yield and carrier lifetime measured with time-resolved photoluminescence, show an increasing trend upon air-exposure. These phenomena can be explained by self-passivation of MAPbI 3 grains by PbI 2 layer that reduces the number of nonradiative recombination centres at the grain boundaries. However, this process is not self-limiting and it eventually leads to a film that has completely reverted back to its precursor state. It is shown that this conversion of MAPbI 3 film back to its precursors is also accelerated by exposure to laser illumination. Furthermore, we report unusual variation of PL intensity on a shorter time scale of a few seconds in all the films used for the experiment. The variations are found to follow different trends in the encapsulated samples as compared to the un-encapsulated samples. We propose that the decomposition followed by the ionic diffusion through film is responsible for such unusual behaviours.
Advanced Science, 2015
and iodide fi lms (CH 3 NH 3 PbX 3 , X = I, Br) by monitoring the spatially resolved PL. These two materials were chosen due to their use in photovoltaic cells (CH 3 NH 3 PbI 3 ) and light-emitting diodes (CH 3 NH 3 PbBr 3 ). We employ scanning near-fi eld optical microscopy (SNOM), which has been previously used to study local PL variations in nanostructured systems like polymer thin fi lms, inorganic quantum structures, and polymer blends. Recently, high spatial resolution techniques have been used to study charge extraction in hybrid lead halide perovskite cells and recombination in perovskite nanocrystals. Spectrally resolved PL emission maps of thin fi lms on glass substrates were measured simultaneously with topography maps using a setup that combines SNOM optical excitation and atomic force microscopy (AFM). Local variations in emission intensity of several orders of magnitude are observed and localized hot-spots are detected, which appear to act as radiative recombination centers in the fi lms. The origin of these recombination centers is studied with time-resolved PL and spectral analysis, which attributes the increased emission from these centers to the formation of longlived excited states in regions of high order and fi lm quality.
Impact of Residual Lead Iodide on Photophysical Properties of Lead Triiodide Perovskite Solar Cells
Energy Technology
The role of residual lead iodide on the photophysical properties of methylammonium lead iodide is still unclear and contradictory views exists about its impact. Herein, we report that there is a critical amount of residual lead iodide, which is beneficial for the solar cell performance. We employed transient absorption spectroscopy to investigate the charge carrier recombination dynamics in perovskite solar cells and to address the role of different amounts of residual lead iodide. The amount of lead iodide is varied through the perovskite thin film preparation protocol upon a modified two-step fabrication process. We observe slower carrier dynamics at the perovskite/titanium dioxide interface in the presence of residual lead iodide when exciting the perovskite at the perovskite/titanium interface, which correlates with improved solar cell device performance. Excitation from the perovskite side indicates that the effect of residual lead iodide is primarily at the titanium dioxide interface. Increasing the lead iodide content further did not alter the carrier recombination; on the contrary, it resulted in lower device performance. Our study confirms that the presence of lead iodide can have a beneficial effect, as it reduces charge carrier recombination at the perovskite/titanium dioxide interface.
Advanced Energy Materials, 2014
postulate stating that a solar cell functions as a light-emitting diode (LED) when charges are injected. This law is a more generalized expression of the equilibrium between absorption and emission, which holds generally and determines the maximum energy-conversion effi ciency of a solar cell. This is known as the Shockley-Queisser limit describing the case of a single band gap semiconductor where recombination of electron-hole pairs is only radiative. The more generalized form based on the quantum effi ciencies allows for an internal quantum effi ciency smaller than unity and includes non-radiative recombination. According to this reciprocity relation, only two quantities of a solar cell have to be measured to predict V oc : the photovoltaic quantum effi ciency spectrum (EQE PV , also called IPCE) and the external quantum effi ciency of the electroluminescence (EQE EL ) at V oc .
The Journal of Physical Chemistry Letters, 2015
Spectrally resolved photoluminescence is used to measure the bandto-band absorption coefficient α BB (ℏω) of organic−inorganic hybrid perovskite methylammonium lead iodide (CH 3 NH 3 PbI 3 ) films from 675 to 1400 nm. Unlike other methods used to extract the absorption coefficient, photoluminescence is only affected by band-to-band absorption and is capable of detecting absorption events at very low energy levels. Absorption coefficients as low as 10 −14 cm −1 are detected at room temperature for long wavelengths, which is 14 orders of magnitude lower than reported values at shorter wavelengths. The temperature dependence of α BB (ℏω) is calculated from the photoluminescence spectra of CH 3 NH 3 PbI 3 in the temperature range 80−360 K. Based on the temperature-dependent α BB (ℏω), the product of the radiative recombination coefficient and square of the intrinsic carrier density, B(T) × n i 2 , is also obtained.
Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites
ACS Energy Letters
Fundmental comprehension of light-induced processes of perovskites are still scarce. One active debate surrounds the influence of excess lead iodide (PbI 2) on device performance, as well as optoelectronic properties, where both beneficial and detrimental traits have been reported. Here, we study its impact on the charge-carrier recombination kinetics by simultaneously acquiring photoluminescence quantum yield and time-resolved photoluminescence as a function of excitation wavelength (450 nm-780 nm). The presence of PbI 2 in the perovskite film is identified via a unique spectroscopic signature in the PLQY spectrum. Probing the recombination in the presence and absence of this signature, we detect a radiative bimolecular recombination mechanism induced by PbI 2. Spatially resolving the photoluminescence, we determine that this radiative process occurs in a small volume at the PbI 2 /perovskite interface, which is only active when charge carriers are generated in PbI 2 , and therefore provide deeper insight into how excess PbI 2 may improve the properties of perovskite based devices.
Recombination Study of Combined Halides (Cl, Br, I) Perovskite Solar Cells
The Journal of Physical Chemistry Letters, 2014
We report on the preparation of a series of solution-processed perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbX 3 , which show tunable optical properties depending on the nature and ratio of the halides employed (X = Cl, Br, and I). Devices have been prepared with different cell architecture, thin film, and mesoporous scaffold (TiO 2 and Al 2 O 3). We have analyzed different sample sets focusing on the characterization of the charge recombination by means of impedance spectroscopy (IS). On the one hand, our study discloses that the insertion of both Cl and Br in the perovskite lattice reduces the charge recombination rates in the light absorber film, thus determining the open circuit voltage (V oc) of the device. The samples prepared on a mesoporous Al 2 O 3 electrode present lower charge recombination rates than those devices prepared on mesoporous TiO 2. Furthermore, the addition of Br in the perovskite structure was demonstrated to improve slightly the lifetime of the devices; in fact, the efficiencies of all devices tested remained at least at the 80% of the initial value 1 month after their preparation. These results highlight the crucial role of the charge-recombination processes on the performance of the perovskite solar cells and pave the way for further progress on this field.
Radiative efficiency of lead iodide based perovskite solar cells
Scientific Reports, 2014
The maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. We herein determine the important figure of merit of radiative efficiency for Methylammonium Lead Iodide perovskite solar cells and, to put in context, relate it to an organic photovoltaic (OPV) model device. We evaluate the reciprocity relation between electroluminescence and photovoltaic quantum efficiency and conclude that the emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage ,0.14 V closer to its radiative limit than the OPV cell. Additionally, and in contrast to OPVs, we show that the photoluminescence of the perovskite solar cell is substantially quenched under short circuit conditions in accordance with how an ideal photovoltaic cell should operate.
Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics
Chemical Science, 2015
We report on reversible, light-induced transformations in (CH 3 NH 3)Pb(Br x I 1Àx) 3. Photoluminescence (PL) spectra of these perovskites develop a new, red-shifted peak at 1.68 eV that grows in intensity under constant, 1-sun illumination in less than a minute. This is accompanied by an increase in sub-bandgap absorption at $1.7 eV, indicating the formation of luminescent trap states. Light soaking causes a splitting of X-ray diffraction (XRD) peaks, suggesting segregation into two crystalline phases. Surprisingly, these photo-induced changes are fully reversible; the XRD patterns and the PL and absorption spectra revert to their initial states after the materials are left for a few minutes in the dark. We speculate that photoexcitation may cause halide segregation into iodide-rich minority and bromide-enriched majority domains, the former acting as a recombination center trap. This instability may limit achievable voltages from some mixed-halide perovskite solar cells and could have implications for the photostability of halide perovskites used in optoelectronics.