Original Title: Huang Jinsong Nature Subjournal Latest Review: Physical Properties of Hybrid Perovskites Applied in Photovoltaic Devices [Introduction] Over the past few decades, researchers have been working to find new photovoltaic materials that are self-cleaning, renewable and low-cost. Organic-inorganic hybrid perovskites (OIHPs) have attracted much attention in the field of photovoltaic devices due to their abundant raw materials, low cost, low-temperature batch synthesis and high conversion efficiency. However, the disadvantages of OIHPs, such as heavy metal Pb and poor hydrothermal stability, greatly hinder its commercial application. Although scientists have made a lot of efforts on these issues, the gains are very small. In addition, there are still no clear design rules in the design of materials. Therefore, a deep understanding of hybrid perovskite materials is essential to improve the performance of perovskite solar cells and the development of next-generation photovoltaic materials. Recently, Professor Huang Jinsong of the University of Nebraska at Lincoln (corresponding author) and others published an article entitled "Understanding the physical properties of hybrid perovskites" in the Nature Reviews Materials. For photo voltaicapplications "for your article. In this review, we summarize the latest research progress on the physical properties of three-dimensional and low-dimensional OIHPs with small open-circuit voltage deficit and high efficiency, including physical defects, ferroelectricity, exciton separation process, carrier recombination lifetime and photon cycle. In addition, the article critically evaluates the impact of ion migration on the efficiency and stability of solar cells, and discusses some challenges for the commercial application of OIHPs. Overview map 1. Charge recombination in perovskite 1.1 Optical absorption and charge transfer Photovoltaic materials are required to have a large light absorption coefficient. The active layer of the material with large absorption coefficient can absorb almost all photons above the band gap in a very thin case, reduce the saturation dark current caused by carrier recombination, and thus has higher light harvesting efficiency and larger open circuit voltage (VOC). The relationship between film thickness and VOC can be expressed by the Shockley – Queisser model: Expand the full text Where K is the Boltzmann constant, T is the temperature, Q is the elemental charge, JSC is the short-circuit current density, J0 is the onset current density, ND is the doping density, τeff is the carrier recombination lifetime, Ni is the intrinsic carrier concentration, and d is the thickness of the absorber layer. OIHPs are almost the most effective absorber among photovoltaic materials. For example, the absorption coefficient of MAPbI3 in the visible region is larger than 3.0 × 104 cm − 1, which is an order of magnitude larger than that of Si. The thickness of the thin film in the high-efficiency OIHPs solar cell reported at the present stage is 0.3-0.6 μm, while the thickness of the thin film in the silicon solar cell is 300 μm. The VOC deficit of the photovoltaic materials synthesized by the solution method mainly comes from the energy separating the photogenerated Frenkel excitons, and the exciton binding energy (EB) is 0.2-1.0 eV. Therefore, an acceptor/donor heterojunction interface with a large energy offset (> 0.3 eV) is required to provide the driving force to separate the excitons. For photovoltaic devices, the smaller EB can reduce the energy loss. For example, the excitons of Si (EB ~ 15 meV) and GaAs (EB ~ 4 mV) with small band gaps can be separated at room temperature. The EB of the hybrid perovskites is 2-75 meV, indicating that OIHPs in photovoltaic applications can be classified as non-excitonic materials. The non-excitonic character allows perovskite solar cells to be fabricated as planar homojunction or heterojunction thin film structures. Figure 1 VOC deficit of OIHPs solar cells EB is sensitive to microstructure. In perovskite films with many small grains, disordered and different potentials will reduce EB and inhibit the generation of excitons. Therefore, the stronger exciton peak of MAPbI3 can only appear in larger single crystals.
Recent studies have shown that excitons are more likely to be generated at high crystallinity, and free charges appear at the center of grains with more small grains. Figure 2 Photogenerated excitons or photogenerated free carriers of OIHPs (A) Actual (ε ′) and theoretical (ε ′ ′) dielectric constants of MAPbI3; (B) The proportion of free carriers to total carriers in thermodynamic equilibrium; (C) measured EQE of non-excitonic MAPbBr3 and excitonic organic PCBM in the same device; (D) Transient absorption spectra of small crystals (< 200 nm) and large crystals (~ 1 μm); (E) Normalized EQEs of MAPbBr3 polycrystalline thin film with thickness of 3 mm, polycrystalline thin film with thickness of 200 mm, and single crystal; (F) Absorption spectrum of MAPbBr3. In addition to the chemical composition and microstructure,nickel titanium wire, the dimensionality of the crystal also affects the EB. 2D materials have a large EB due to the spatial quantum confinement effect. With the increase of the number of layers, EB tends to be a 3D material. 1.2 Charge recombination rate A longer carrier recombination lifetime requires a larger VOC to provide a higher carrier concentration. The carrier recombination lifetime in a semiconductor material can be expressed by the following formula: Where G is the carrier generation rate, K1 is the unimolecular carrier recombination constant, K2 is the bimolecular carrier recombination constant, and K3 is the Auger carrier recombination constant. In solar cells, the recombination probability of Auger carriers is much smaller than that of the other two carriers. A series of K1 and K2 values were obtained by time-resolved fluorescence spectroscopy (TRPL), transient spectroscopy, and time-resolved microwave conductance (TRMC) (Table 1). It can be seen from the table that the single-molecule carrier recombination dominates the carrier recombination in the perovskite film with lower exciton concentration. Table 1 Values of K1 and K2 for different materials 1.3 Charge screening Almost all high-efficiency solar cells, such as GaAs and organic bulk-phase heterojunctions, are non-Langevin-type bimolecular complexes. The Langevin-type bimolecular recombination coefficient is e (μe + μh)/ (εε0), where e, μe, μh and εε0 are the electron charge, electron mobility, hole mobility and dielectric constant,titanium round bar, respectively. The recombination coefficient of Langevin-type bimolecular recombination system is related to the carrier mobility. Non-Langevin behavior is important for organic molecules with large carrier mobilities. In general, the charge screening mechanism needs to reduce secondary charge recombination, and OIHPs with large dielectric constants can screen electrons and holes. There are some other mechanisms to explain why the photogenerated electron-hole recombination of OIHPs is slow. Some researchers have hypothesized that perovskites may be ferroelectric, creating a polarizing electric field that separates electrons from holes. Domain structures have been observed in perovskite structures by piezoelectric force microscopy (PFM). However, after excluding the effect of charge leakage, no stable magnetization was detected in MAPbI3 by the classical Sawyer – Tower circuit method (Figure 3A). Ferroelastic properties in MAPbI3 polycrystalline thin films and single crystals were confirmed by micro-nano technology (fig. 3B). Obvious domain walls at 70 ° and 109 ° were observed by polarized light microscopy and PFM, which may be due to the presence of twinned domains with different orientations in the tetragonal MAPbI3 (Figure 3C). Some studies observed ferroelastic domain motion in single crystal and polycrystalline films by polarized light microscopy and PFM (Figure 3D). Researchers have also studied the role of organic cations in carrier recombination. It is shown that although the type of organic cation has no obvious effect on the band edge recombination, the organic cation plays a large charge screening role in the thermalization of carriers (Fig. 3 e, f).
Figure 3 Electrochemical performance of OIHPs (A) Polarization properties of single-crystalline MAPbI3 for solar cells tested by the Sawyer – Tower circuit method; (B) inversion of domain group observed by reflection polarize microscopy; (C) Schematic diagram of winding structure with two crystal orientations in MAPbI3 single crystal and twin crystal; (D) AFM and PFM of polycrystalline MAPbI3; (E) PL decay dynamics at 2.3 eV and 2.6 eV; (F) Crystal structure of MAPbI3. 1.4 Doping of perovskite semiconductor Doping plays an important role in the electrical properties of semiconductors, such as conductivity and carrier mobility. In solar cells, doping can directly change the carrier recombination rate, diffusion length, contact resistance and VOC. The lower doping concentration reduces the scattering and recombination of carriers, so that the material has a longer carrier diffusion length. Proper doping concentration can reduce the internal resistivity of the solar cell, resulting in a larger VOC. However, it is difficult to achieve extrinsic doping because the structure of OIHPs has low fault tolerance and low activation energy of ion migration. In OIHPs, self-doping can be achieved because the formation energies of donors and acceptors are similar. The self-doping of OIHPs is affected by the composition and synthesis parameters of the precursor, for example, in the precursor of MAPbI3 thin film, more PbI2 makes the thin film become n-type doped material; with the change of MAI content, thermal annealing can convert p-type doped material into n-type doped material. 1.5 Photon Cycle The experiment proves that the photon circulation effect of the GaAs solar cell (the reabsorption and reemission of the photoactive layer itself) makes the carriers of the photoactive layer gather, thus enhancing the splitting of the quasi-Fermi level, which makes it have a higher VOC, and the PCE increases from 25% to 29%. However, it is not known whether there is a photon cycle effect in OIHPs. In fact, there are many similarities between perovskite and GaAs, for example, they are both direct band gap materials and have high band edge absorption coefficient, which makes the carriers of the materials have high self-separation rate. These properties provide preconditions for photon circulation. Recently, titanium filler rod ,titanium exhaust tubing, a study has demonstrated for the first time the photon recycling effect in MAPbI3 polycrystalline films. The results show that there are photogenerated charges in the light absorption region of more than 50 μm in the polycrystalline thin film (Fig. 4a-c). At the same time, the photon recycling efficiency of single crystal OIHPs was determined based on the way their polarization differences distinguish between emitted and reabsorbed photons. The results show that the photoluminescence signal of the crystal is dominant, and the reabsorption and reemission are weak (Figure 4d-f). In these systems, the photon recycling efficiency is less than 0.5%, which is negligible for solar cells. The lower photon recycling efficiency may be caused by the lower internal PLQY of the perovskite film. Fig. 4 Photon circulation effect of OIHPs (A) Schematic of the microscope setup for measuring photon circulation; (B) luminescence plot for different separation distances between excitation and collection; (C) Comparison of the experimentally measured (solid line) and theoretically calculated (dashed line) attenuation of the material at 765 and 800 nm; (D) Schematic diagram of photon circulation in perovskite single crystal; (E) Surface PL (blue line) and filtered recovered PL (PLF + PLR; red line) spectra of MAPbBr3 single crystals of 1.3 mm thickness; (F) Filtered recovered PL (PLF + PLR; red line) and recovered PL (PLR, blue line) spectra. 1.6 High electronic dimension Recently, the concept of electronic dimension has been used to explain why MAPbI3 has better photovoltaic performance than other metal halide perovskites. Generally speaking, the structural dimension is an important index to describe the photovoltaic characteristics and device performance of the absorber. Electron dimension refers to the connectivity of electron orbitals, which can lower the conduction band and raise the valence band. Compared with the structural dimension, the electronic dimension can better describe the physical properties of materials, such as band gap, carrier mobility and defect energy levels.
Although some perovskite materials have three-dimensional structure and electronic dimensions, some perovskites with three-dimensional structure have low electronic dimensions. The high electronic dimension provides important guidance for understanding the performance of solar cells. 2. Defects in OIHPs 2.1 Band tail of perovskite The calculated VOCSQ is calculated assuming a perfect break or step absorption coefficient for the light absorber, which results in a calculated VOC maximum that is greater than the actual one. In real solar cells, the absorption spectrum is not abrupt, and there is always an absorption band tail in the absorption spectrum due to defects and other reasons. Therefore, the definition of the maximum achievable VOC (the radiative recombination limit VOCrad) is the balance of the radiative charge recombination current and the photocurrent. For solar cells, VOCrad (providing a path for VOC losses due to sub-bandgap reorganization) is much smaller than VOCSQ. OIHPs photovoltaic devices have a smaller VOC deficit due to their smaller subband gap absorption. The absorption spectra of OIHPs have a steep band edge, which is characterized by the Urbach energy (the exponential part of the absorption curve). The Urbach energy of polycrystalline MAPbI3 is ~ 15 meV at room temperature, which is close to that of single crystal Si (~ 11 meV) and GaAs (~ 7.5 meV), indicating that the polycrystalline MAPbI3 thin films synthesized by solution method have orderly microstructure and low deep level defect density. The experimental results show that the improved crystallinity and phase purity of Cs + -doped perovskite with a wide band gap reduce the Urbach energy, thus improving the VOC and efficiency of the device. 2.2 Point Defect Tolerance of OIHPs It is generally believed that in OIHPs solar cells, the lower density of charge traps makes them have a larger VOC. However, at this stage, the understanding of charge traps is still in the preliminary stage. Although most of the OIHP devices are synthesized by the low-temperature solution method, the defect concentration of the OIHP devices is smaller than that of many polycrystalline inorganic solar cells. Figs. 5 (B) and (C) show the calculated possible point defects of MAPbI3. All the point defects that can form deep levels have large defect formation energies and will not affect the device performance, while most of the point defects that exist in the bulk MAPbI3 will form shallow levels. In addition, the study shows that the energy level of MAPbI3 does not change even if 20% of the I ions in the film are lost (fig. 5d). Fig. 5 Defect tolerance of OIHPs (A) Defect state density of MAPbI3 polycrystalline thin film and single crystal; (B) Formation energy of intrinsic point defects in MAPbI3; (C) Energy levels of intrinsic donors (left) and intrinsic acceptors (right) in MAPbI3; (D) Defect tolerance of MAPbI3. Polycrystalline OIHPs have a lower defect density, another reason is that perovskite materials are easy to crystallize. It is found that the crystallization barrier of perovskite is 56.6 – 97.3 kJ/mol, while that of polysilicon is ~ 471 kJ/mol. 2.3 Defects on surface and grain boundary In addition to point defects, extended defects at grain boundaries and interfaces also increase the defect concentration in OIHPs. Atoms at grain boundaries and surfaces are not strictly stoichiometric, which results in a higher defect concentration here. For example, after annealing, the volatilization of organic components on the surface of the crystal will result in a larger defect density on the surface. Even the surface of OIHPs single crystal has many defects, and the recombination of surface photogenerated carriers will terminate the photoresponse of short-wavelength light. Photoresponse occurs only when the energy of the photogenerated charge is close to the energy band. This surface recombination can also explain the phenomenon that the fluorescence lifetime of perovskite single crystals changes with different penetration depths of light (Figure 6a). The surface charge recombination mechanism of perovskite materials is complex. The defect density of each crystal plane of the perovskite surface is different. There are two crystal planes (100) and (112) on the surface of MAPbI3 single crystal, and there are many orientations (110), (202), (310) and (314) on the surface of MAPbI3 polycrystalline film.
The results show that only (100) is uncharged and the others are charged, so there are other ions to balance the surface charge. In addition, the binding energy of the organic components of different crystal planes is different, so that the surface defect density shows crystal plane dependence. Fig. 6 B shows that the floating of VOC of different crystal planes on the same crystal grain reaches 0.6 V. In addition, the surface recombination rate is sensitive to the composition and the ambient humidity. The ozone treated MAPbBr3 single crystals showed significant fluorescence enhancement and growth of radiative recombination lifetime at the crystal surface (Fig. 6C). Fig. 6 Surface charge recombination of OIHPs (A) Schematic of charge recombination and collection under the influence of surface defects and its effect on fluorescence lifetime (bottom left) and narrow-band light detection (bottom right); (B) AFM of the polycrystalline thin film; (C) PL of MAPbBr3 single crystal after vacuum and UV – O3 treatment (left), fluorescence lifetime of MAPbBr3 single crystal after vacuum and air treatment. 2.4 Characterization of Defect Density Accurate characterization of the charge defect density of OIHPs materials is crucial for understanding the relevant optoelectronic properties. There is still a lot of defect data unknown at this stage. An early method to measure the defect density is thermal admittance spectroscopy, which can give the density of defect States at different depth levels. The results show that the physical contact of MAPbI3 polycrystalline film with PCBM and C60 can reduce the shallow and deep energy levels by 1 to 2 orders of magnitude, indicating that the defects are mainly on the surface of the film (Fig. 7A). In order to understand the internal defect density of the single crystal, the defect density was evaluated by the trap filling space charge limited current method (fig. 7 B). Understanding the defect density of electrons and holes can provide better conditions for designing new perovskite materials with better performance. However, the current method of measuring defect density can only get the total defect density. To separate electrons from holes, fig. 7c shows a photoconductive element with a carrier acceptor layer of only one type of carrier. Fig. 7 Charge defect density and surface passivation of OIHPs (A) reducing the defect density of States of deep and shallow level defects by surface passivation of C60 and PCBM; (B) Current-voltage diagram of MAPbI3 single crystal device; (C) Schematic diagram of the symmetrical lateral contact device; (D) Defect density of States (NT) within the band gap. 2.5 Role of ion migration Ion migration is the unique property that distinguishes OIHPs from other photovoltaic materials. In this paper, we focus on the effect of ion migration on device efficiency and stability. Ion migration and aggregation can form a chemical doping effect, which is the result of ohmic contacts of high-efficiency solar cells with p – I – n or n – I – p structures (Figure 8 a). A recent study shows that solar cells with n – I – p structure have abnormal photovoltaic effect (Fig. 8B). This effect was observed in solar cells with MAPbI3, MAPbBr3, and CsPbBr3 as active layers. Some studies have shown that ion migration can improve the short-term stability of materials, but weaken the long-term stability of perovskite solar cells. Figure 8C shows that the performance of the device degrades under light and can be partially or fully restored after a period of time in the dark. This self-healing effect is most likely related to ion migration (Figure 8 d). Because that time it take for the ions to migrate in the film (a few second or minutes) is much shorter than the aging time of the film (day or months), the crystalline phase transition between MAPbI3 and PbI2 causes reversible ion migration, so that ion migration has the effect of "repairing" local lattice distortions. Self-healing due to ion migration requires sealing of the sample to prevent volatilization of the substance and chemical reaction of the surface with airborne substances. In some studies, the grain boundaries of MAPbI3 films were filled with polyvinyl alcohol to prevent the volatilization of MA + cations, thus playing a self-healing effect to enhance the stability of the device (Fig. 8e).
Figure 8 Ion migration in OIHPs (A) Schematic diagram of p-type, n-type in-situ doping and ohmic contact interface in MAPbI3 solar cell device; (B) Scattered tunnel junctions due to local ion aggregation in electrically poled polycrystalline perovskite films, resulting in band shifts and anomalous photovoltaic effects; (C) Self-healing phenomena of two types of perovskite solar cells; (D) in the MAPbI3 film, the defect concentration is reduced due to the redistribution of photogenerated carriers; (E) Illustration of self-healing achieved by filling MAPbI3 grain boundaries with polyvinyl alcohol. 3. Conclusion and prospect In the past few years, great progress has been made in understanding the unique physical properties of OIHPs. In this paper, the intrinsic optoelectronic properties of OIHPs are discussed, and these properties are combined with the device performance. The properties of OIHPs prepared by solution method, such as optical absorption, carrier defect depth, exciton binding and charge diffusion length, are much better than those of covalent-bonded photovoltaic materials (such as Si, GaAs and CdTe) prepared by high temperature and vacuum method. However, the influence of intrinsic optical and electrical properties, such as intrinsic defect concentration, charge screening and ion migration, on the performance of materials is still not well understood. Although great progress has been made in the study of mechanism, there are still many important problems to be further explored. The ferroelectric properties of MAPbI3 still need to be further studied, and the substitution of FA +, Rb +, Cs + or Co2 + for MA + cations will provide a way to understand the role of ferroelectric dipoles. Recent studies have improved the device efficiency by introducing external ions and adjusting the composition. The role of external ions needs to be further understood, which is conducive to further understanding the shortcomings of MAPbI3 materials with single component. Another direction is to understand the internal correlation between the intrinsic characteristics of materials (electrical, mechanical, optical and spin properties) and device properties. Although the larger lattice constant of OIHPs makes the material have ion migration effect, the reason why these properties make the material have larger dielectric constant and better structural stability is still unclear. The understanding of the unique optoelectronic properties of OIHPs materials will provide conditions for the discovery of perovskite light absorbers that are non-toxic and stable in air for a long time. Literature Link: Understanding the physical properties of hybrid perovskites for photovoltaicapplications (Nat. Rev. Mater., 2017, DOI: 10.1038/natrevmats.2017.42） This article is contributed by Zhu Xiaoxiu, a material person, and edited by Material Niu. Material Cattle Network focuses on tracking the progress of science and technology and industry in the field of materials. It brings together students from universities, first-line researchers and industry practitioners. If you are interested in tracking the progress of science and technology in the field of materials, interpreting high-level articles or commenting on the industry, Contribution and content cooperation can be added to the editor's WeChat: xiaofire-18, Wu Mei, we will invite teachers to join the expert group.
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