This review discusses various parameters that influence and control the organo-metal halide perovskite crystallization process. The effect of the perovskite morphology on the photovoltaic performance is a critical factor. Moreover, it has a dramatic effect on the stability of the perovskite, which has significant importance for later use of the organo-metal perovskite in assorted applications. In this review, we brought together several research investigations that describe the main parameters that significantly influence perovskite crystallization, for example, the annealing process, the precursor solvent, anti-solvent treatment, and additives to the iteite solutions.mechanisms. Control over morphology is a key parameter to understand this attractive material; morphology control will be an additional step throughout its commercialization process. The review is divided to four parts, presenting various parameters influencing perovskite crystallization and morphology. Section 2 discusses the effect of the annealing process; Section 3 describes the precursor solvent, presenting the possible solvents being used in the deposition process; Section 4 presents the anti-solvent treatment and its effect on the cell properties; and Section 5 discusses additives that can be added to the perovskite solution before deposition.
Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. We report the in-plane spatial distribution and degree of chlorine incorporation in organic−inorganic lead halide perovskite absorbers by means of nondestructive synchrotronbased nanoprobe X-ray fluorescence. The presence of chlorine is positively identified in CH3NH3PbI3 films synthesized with Clcontaining precursors and as an impurity in some films synthesized with nominally Cl-free precursors. The impurity may be introduced from precursors or as contaminants during film synthesis. The films formed from Cl-containing precursors contain roughly an order of magnitude higher amount of chlorine, with Cl:I values greater than 0.02 found whether Cl is present in either the organic or the inorganic precursor for both one- and two-step fabrication processes. A spatial variation in the Cl incorporation is observed within single particles and as well as between particles within a given film, and the standard deviation of the Cl:I ratio across the films is up to 30% of the average value. Understanding and controlling the heterogeneous distribution of chlorine in hybrid perovskite layers may offer a path to improve their photovoltaic performance.
We report a hybrid mesoporous–planar architecture of methylammonium lead iodide perovskite based solar cells, to combine the benefits of both the mesoporous and planar architectures in a single device. A mesoporous-TiO2 grid was fabricated on a compact TiO2 layer, through a self-assembly process based on directional wetting, providing regions with and without mesoporous-TiO2, followed by perovskite deposition and back contact evaporation (hybrid cells). The hybrid cells showed up to 10.7% power conversion efficiency (PCE) as compared to 13.5% and 6.3% for their mesoporous and planar counterparts, respectively. Interestingly, the hybrid cells are found to show a short circuit current density (Jsc) as high as the Jsc of the mesoporous TiO2 based cells and proved to conserve the current density even in the absence of mesoporous-TiO2 from planar parts of the hybrid cells. The cells showed the best fill factor as compared to their mesoporous and planar counterparts. The areal variation in the meso to planar ratio has also been realized by changing the grid size to demonstrate the effect of the architecture on the cell performance. Charge extraction measurements have been used to obtain insight into the recombination inside different solar cells architectures. The hybrid cell structure emerged as a novel promising design for perovskite solar cells.
Organo-metal halide perovskite is an efficient light harvester in photovoltaic solar cells. Organometal halide perovskite is used mainly in its “bulk” form in the solar cell. Confined perovskite nanostructures could be a promising candidate for efficient optoelectronic devices, taking advantage of the superior bulk properties of organo-metal halideperovskite, as well as the nanoscale properties. In this paper, we present facile low-temperature synthesis of two-dimensional (2D) lead halide perovskite nanorods (NRs). These NRs show a shift to higher energies in the absorbance and in the photoluminescence compared to the bulk material, which supports their 2D structure. X-ray diffraction (XRD) analysis of the NRs demonstrates their 2D nature combined with the tetragonal 3D perovskite structure. In addition, by alternating the halide composition, we were able to tune the optical properties of the NRs. Fast Fourier transform, and electron diffraction show the tetragonal structure of these NRs. By varying the ligands ratio (e.g., octylammonium to oleic acid) in the synthesis, we were able to provide the formation mechanism of these novel 2D perovskite NRs. The 2D perovskite NRs are promising candidates for a variety of optoelectronic applications, such as light-emitting diodes, lasing, solar cells, and sensors.
High time resolution broadband pump-probe experiments on CH3NH3PbI3 and CH3NH3PbBr3 films are described. The improved time resolution delineates instantaneous and delayed relaxation related effects on sample absorption and assists in clarifying controversial assignment of the underlying mechanisms. Analysis of the data in terms of finite difference spectra and spectral band integrals reveals that photoexcitation is high in the inter-band continuum leading to partial bleaching and red-shifts of the exciton band just below the absorption-edge instantaneously. Increased pump intensity saturates the exciton bleach and progressively reduces inter-band absorption in a broad range extending from the band edge to higher photon energies. Both effects are attributed to reduced Coulomb enhancement due to hot carrier screening. The spectral extent of the inter-band absorption attenuation provides estimated binding energies in the range of 20–30 meV in both materials. Sub-picosecond carrier cooling reverses the initial exciton transition red-shift and induces transmission near the band edge due to state filling and stimulated emission. Finally, 1–100 ps signals are dominated by reverse state filling due to non-geminate recombination. These results demonstrate that both inter-band and exciton absorptions are essential for unraveling photo-induced dynamics in these materials, and that insights obtained from many-body theoretical analysis of dynamic screening are essential for correctly assigning the recorded spectral evolution.
In this work we study the kinetics of cesium lead halide perovskite nanoparticle (NP) growth; the focusing and de-focusing of the NP size distribution. Cesium lead halide perovskite NPs are considered to be attractive materials for optoelectronic applications. Understanding the kinetics of the formation of these all-inorganic perovskite NPs is critical for reproducibly and reliably generating large amounts of uniformly sized NPs. Here we investigate different growth durations for CsPbI3 and CsPbBr3 NPs, tracking their growth by high-resolution transmission electron microscopy and size distribution analysis. As a result, we are able to provide a detailed model for the kinetics of their growth. It was observed that the CsPbI3 NPs exhibit focusing of the size distribution in the first 20 seconds of growth, followed by de-focusing over longer growth durations, while the CsPbBr3 NPs show de-focusing of the size distribution starting from the beginning of the growth. The monomer concentration is depleted faster in the case of CsPbBr3 than in the case of CsPbI3, due to faster diffusion of the monomers, which increases the critical radius and results in de-focusing of the population. Accordingly, focusing is not observed within 40 seconds of growth in the case of CsPbBr3. This study provides important knowledge on how to achieve a narrow size distribution of cesium lead halide perovskite NPs when generating large amounts of these promising, highly luminescent NPs.
This work demonstrates antisolvent treatment of organo-metal halide perovskite film in hole conductor-free perovskite-based solar cell, achieving impressive power conversion efficiency of 11.2% for hole-conductor-free cells with gold contact. We found that antisolvent (toluene) surface treatment affects the morphology of the perovskite layer, and importantly, it also affects the electronic properties of the perovskite. Conductive atomic force microscopy (cAFM) and surface photovoltage show that the perovskite film becomes more conductive after antisolvent treatment. Moreover, the antisolvent treatment suppresses the hysteresis commonly obtained for perovskite-based solar cells. When the perovskite alone is characterized, a I−V plot of a single perovskite grain measured by cAFM shows that hysteresis vanishes after toluene treatment. During toluene treatment, excess halide and methylammonium ions are removed from the perovskite surface, leading to a net positive charge on the Pb atoms, resulting in a more conductive perovskite surface, which is beneficial for the hole-conductor-free solar cell structure. The reliability of the surface treatment was proved by calculating the statistical parameters Z score and p value, which were 2.5 and 0.012, respectively. According to these values, it can be concluded with 95% confidence that the average efficiency of cells fabricated via surface treatment is greater than the average efficiency of cells without surface treatment. The statistical data support the impact of surface treatment on the photovoltaic performance of perovskite solar cells.
In this work we demonstrate the planar configuration on hole conductor (HTM) free perovskite based solar cells. The CH3NH3PbI3 perovskite was deposited using the spray technique to achieve micrometer size perovskite crystals. The number of spray passes changes the CH3NH3PbI3 film thickness; for example, 10 spray passes achieved a film thickness of 3.4 μm of perovskite. Surprisingly, power conversion efficiency of 6.9% was demonstrated for this novel, simple solar cell structure with thick perovskite film that has no HTM. Capacitance−voltage measurements reveal charge accumulation at the CH3NH3PbI3/Au interface while the compact TiO2/CH3NH3PbI3 junction showed a space charge region, which inhibits the recombination. Studying these interfaces is key to understanding the operation mechanism of this unique solar cell structure. This simple planar HTM free perovskite solar cell demonstrates the potential to make large-scale solar cells while maintaining a simple, low-cost architecture.
Recent discoveries have revealed a breakthrough in the photovoltaics (PVs) ! eld using organometallic perovskites as light harvesters in the solar cell. The organometal perovskite arrangement is self-assembled as alternate layers via a simple low-cost procedure. These organometal perovskites promise several bene! ts not provided by the separate constituents. This overview concentrates on implementing perovskites in PV cells such that the perovskite layers are used as the light harvester as well as the hole-conducting component. Eliminating hole-transport material (HTM) in this solar-cell structure avoids oxidation, reduces costs, and provides better stability and consistent results. Aspects of HTM-free perovskite solar cells discussed in this article include (1) depletion regions, (2) high voltages, (3) panchromatic responses, (4) chemical modi! cations, and (5) contacts in HTM-free perovskite solar cells. Elimination of HTM could expand possibilities to explore new interfaces in these solar cells, while over the long term, these uniquely structured HTM-free solar cells could offer valuable bene! ts for future PV and optoelectronics applications.
This work reports on the preparation of semitransparent perovskite solar cells. The cells transparency is achieved through a unique wet deposition technique that creates perovskite grids with various dimensions. The perovskite grid is deposited on a mesoporous TiO 2 layer, followed by hole transport material deposition and evaporation of a semitransparent gold fi lm. Control of the transparency of the solar cells is achieved by changing the perovskite solution concentration and the mesh openings. The semitransparent cells demonstrate 20–70% transparency with a power conversion effi ciency of 5% at 20% transparency. This is the fi rst demonstration of the possibility to create a controlled perovskite pattern using a direct mesh-assisted assembly deposition method for fabrication of a semitransparent perovskite-based solar cell.
This paper presents for the first time Sb2S3-based solar cells operating on scaffold film. The scaffolds studied are Al2O3 and ZrO2, for which no electron injection from the Sb2S3 to the Al2O3 or ZrO2 is possible. As a result, one of the highest open circuit voltages (Voc) of 0.712 V was observed for this solar cell configuration. Electron dispersive spectroscopy (EDS) was performed, revealing complete pore filling of the Sb2S3 into the metal oxide pores (e.g., Al2O3 or ZrO2); the complete pore filling of the Sb2S3 is responsible for the photovoltaic performance (PV) of this unique solar cell structure. In addition, intensity modulated photovoltage and photocurrent spectroscopy (IMVS and IMPS) were performed to extract the electron diffusion length. Electron diffusion length in the range of 900 nm to 290 nm (depending on the light intensity) was observed, which further supports the operation of metal oxide/Sb2S3 solar cell configuration. Moreover, the Al2O3-based cells have longer electron diffusion length than the TiO2-based cells, supporting the higher open circuit voltage of the noninjected metal oxide-based cells. This work demonstrates the potential of Sb2S3 to gain high voltage and to perform on a scaffold substrate without requiring electron injection.
Cross-sections of a hole-conductor-free CH3NH3PbI3 perovskite solar cell were characterized with Kelvin probe force microscopy. A depletion region width of about 45 nm was determined from the measured potential profiles at the interface between CH3NH3PbI3 and nanocrystalline TiO2, whereas a negligible depletion was measured at the CH3NH3PbI3/Al2O3 interface. A complete solar cell can be realized with the CH3NH3PbI3 that functions both as light harvester and hole conductor in combination with a metal oxide. The band diagrams were estimated from the measured potential profile at the interfaces, and are critical findings for a better understanding and further improvement of perovskite based solar cells.
We present a density functional theory (DFT) study aimed at understanding the injection and recombination processes that occur at the interface between PbS QDs and TiO2 oxide nanoparticles with different morphologies. The calculated injection rates fall in the picosecond timescale in good agreement with the experiments. In addition, our simulations show that the (101) facet of TiO2 more favourably accommodates the QD, resulting in stronger electronic couplings and faster electron injections than the (001) surfaces. Despite this, the (101) slab is also more prone to faster electron recombination with the valence band of the QD, which can lead to overall lower injection efficiencies than the (001) surface.
We report on accelerated degradation testing of MAPbX3 films (X = I or Br) by exposure to concentrated sunlight of 100 suns and show that the evolution of light absorption and the corresponding structural modifications are dependent on the type of halide ion and the exposure temperature. One hour of such exposure provides a photon dose equivalent to that of one sun exposure for 100 hours. The degradation in absorption of MAPbI3 films after exposure to 100 suns for 60 min at elevated sample temperature (∼45−55 °C), due to decomposition of the hybrid perovskite material, is documented. No degradation was observed after exposure to the same sunlight concentration but at a lower sample temperature (∼25 °C). No photobleaching or decomposition of MAPbBr3 films was observed after exposure to similar stress conditions (light intensity, dose, and temperatures). Our results indicate that the degradation is highly dependent on the hybrid perovskite composition and can be light- and thermally enhanced.
In the recent years, the heterojunction solar cells based on quantum dots (QDs) have attracted attention due to strong light absorbing characteristics and the size effect on the bandgap tuning. This paper reports on the kinetics of interfacial charge separation of PbS QDs/(001) TiO 2 nanosheets heterojunction solar cells. PbS QDs are deposited using a bifunctional linker molecule on two different TiO 2 fi lms, i.e., TiO 2 nanosheets (with 001 dominant exposed facet) and TiO 2 nanoparticles (with 101 dominant exposed facet). Upon bandgap excitation, electrons are transferred from the PbS QDs conduction band to the lower lying conduction band of TiO 2 . Based on the ultrafast pump-probe laser spectroscopy technique, the kinetics of charge separation is scrutinized at the PbS/TiO 2 interface. The interfacial charge separation at PbS/TiO 2 nanosheets films made of (001) dominant exposed facets is fi ve times faster than that on (101) dominant exposed facets TiO 2 nanoparticles. The quantum yields for charge injection are higher for the (001) TiO 2 nanosheets than the (101) TiO 2 nanoparticles due to enhanced interfacial interaction with (001) surface compared to the (101) nanoparticles. The superior interfacial charge separation at PbS/(001) nanosheets respect to PbS/(101) nanoparticles is consistent with the higher photocurrent and enhanced power conversion effi ciency in the PbS QDs/(001) TiO 2 heterojunction solar cell. The use of (001) TiO 2 nanosheets can be a better alternative to conventional mesoporous TiO 2 fi lms in QD heterojunction solar cells and perovskites-based heterojunction solar cells.