Enhanced performance of CsPbBr3 perovskite solar cells by reducing the conduction band offsets via a Sr-modified TiO2 layer

Enhancing photovoltaic performance of carbon-based perovskite solar cells by introducing plasmonic Au NPs

In recent years, perovskite solar cells have obtained a rapid development due to their large light absorption coefficient, low-cost and high power conversion efficiency (PCE). In this study, the one-dimensional ordered ZnO nanorod arrays were prepared on FTO glasses by chemical bath deposition at low temperature. TiO2 nanoparticles from different kinds of solution were further spin-coated onto the ZnO nano arrays to form ZnO/TiO2 composite nano arrays, as the electron transfer layer in perovskite solar cells. The microstructure of different ZnO/TiO2 composite nano arrays and their corresponding photovoltaic performance of the solar cells were investigated. It was found that the cells based on ZnO nano arrays treated by TiO2 nanoparticle paste exhibit the highest PCE. The influence of TiO2 paste concentration on the photovoltaic performance of cells was further investigated. It indicated that the cell achieves best photovoltaic performance at TiO2 paste concentration of 0.1 mol/L: open circuit voltage ( V oc) of 0.93 V, short circuit current ( J sc) of 15.30 mA cm−2, filling factor ( FF ) of 43% and PCE of 6.07%. The treatment of TiO2 paste on ZnO nano arrays results in perovskite nanoparticles can effectively fill into the cracks between ZnO nanorods and a flat and compact perovskite layer can also form on the top of ZnO nano arrays. These effectively enhance the loading of perovskite and suppress the recombination between carriers in cells, resulting in an improved photovoltaic performance. A further treatment of ZnO/TiO2 paste arrays with TiCl4 aqueous solution can significantly improve the photovoltaic performance of the perovskite solar cells: V oc=0.99 V, J sc=19.09 mA cm−2, FF =58%, and PCE of 11%. The TiCl4 treatment of ZnO/TiO2 composite arrays introduces small TiO2 nanoparticles (~3 nm) into nano arrays. The small nanoparticles can fully fill the cracks between the nanorods and create better contact between perovskite (both in top layer and the arrays) and nano arrays. The photo induced carrers can rapidly transfer via ZnO nanorods to the conductive substrates. Furthermore, the introduce of small TiO2 nanoparticles also increases the surface area of electrode to absorb more perovskite, and hence improve the adsorption of light and result in an improved photovoltaic performance of the cells.

Lead-free tin perovskite solar cells

In recent years, organic-inorganic hybrid perovskite solar cells have aroused the interest of a large number of researchers due to the advantages of large optical absorption coefficient, tunable bandgap and easy fabrication. Recently, the power conversion efficiency of organic-inorganic hybrid perovskite solar cells has been enhanced to more than 23% in laboratory. In solution processed perovskite solar cells, perovskite and charge transport layer are stacked together, due to the different crystallization rates leading to lattice mismatch near the surface region of perovskite film, resulting in a lot of interface defects, especially at the interface between perovskite and charge transport layer. What is more, the photo-induced free carriers must transfer across the interfaces to be collected. But the defects near the interface can trap photogeneration electrons, thus reducing the carrier lifetime and causing the charges to be recombined, which greatly influence the performance and stability of perovskite solar cells. Therefore, reducing and passivating these defects is critical for obtaining the high performance perovskite solar cells. Now, there have been made tremendous efforts devoting to advancing passivation techniques, such as doping and surface modification, for high efficiency perovskite solar cell with improved stability and reduced hysteresis. These approaches also contribute to improving the energy band alignment between carrier transport layers and perovskite absorber improving device performance, or resistance moisture to enhance device stability. In this review we mainly introduce the formation and the effect of defects on perovskite solar cells, analyze the mechanism for passivating the interfacial defects between charge transport layer and perovskite photo absorption layer for different materials, compare the effects of different passivation materials on the photovoltaic performance of perovskite solar cells, and summarize the role of these materials in passivating the defects. Finally we discuss the research trend and development direction of passivation defects in perovskite solar cells.

Introduction The NiOx layer is actively used as hole extraction layer for inverted structure perovskite solar cells. However, clear guidelines on NiOx chemical composition, manufacturing methods, energy levels, and the hole-transport process remain unclear. In this conference, high-density NiOx layers with various manufacturing temperatures are produced with Nickel acetylacetonate using the SPD method in order to improve the performances of inverted perovskite solar cells. The chemical composition of the NiOx is investigated using XPS, and GIXRD. Furthermore, the influence of Na+ ions in the valence band of the NiOx layer is investigated by vacuum ultraviolet photoelectron spectroscopy using synchrotron radiation. The results showed that the conversion efficiency of the perovskite solar cell is high as the crystallite size of NiOx is large. The highest perovskite solar cell conversion efficiency was obtained at a NiOx layer production temperature of 550 °C. Inverted perovskite solar cells were fabricated with over 16.1% conversion efficiency. Conversion efficiency of solar cell The conversion efficiencies of the perovskite solar cells using NiOx films with various deposition temperatures are summarized in Table 1. The perovskite solar cells conversion efficiency increased along with the NiOx layer production temperature to 550 and 600 °C; however, it decreased slightly when the temperature increased further to 600 °C. As a result, the perovskite solar cell conversion efficiency reached its peak when the NiOx layer production temperature was 500 °C. To highlight the performance change, we herein investigated the composition, structure and the band state of the NiOx layers. Influence of valence band and valence of Ni ion of the NiOx layers To investigate the states of the regions near the edges of the valence bands of the NiOx layers, PES measurements were carried out with a BL-07B at the NewSUBARU synchrotron radiation facility, and the results are shown in Figure 1. The regions near the edges of the valence bands of the NiOx correspond to Ni 3d (label A), O 2p (label B), and the charge transfer transition from an O 2p level to Ni 3d photoionization (label C). From the PES measurement results, the region (around 4-5 eV) corresponding to O 2p was stronger at 300 and 600 °C than at 500 °C. This is because the influence of the O 2p orbital increased because the proportion of oxygen in NiOx increased owing to the loss of Ni ions. Although the ratio of Ni2+ to Ni3+ ions affects the energy levels in the vicinity of the HOMO, a change in these energy levels does not contribute to hole extraction in perovskite solar cells. From these results, it was found that the valence of the Ni ions has little influence on the performance of the solar cells. Crystal analysis of the NiOx layers Both the structures and crystallite sizes of the thin NiOx films formed by SPD were investigated via GIXRD, as shown in Figure 2. The five peaks that appeared at 2θ values of approximately 37.6° 43.6°, 63.3°, 75.6°, and 79.6° were assigned to the 111, 200, 220, 311, and 222 planes, respectively, and then attributed to the randomly oriented NiOx crystal. At 250 °C, no diffraction peaks were observed because the NiOx film did not crystallise. This implies that crystallised NiOx films formed with annealing temperatures above 300 °C. The NiOx crystallite sizes were calculated from the intensities of the 200 peaks using the Scherrer equation, and the results are shown in Figure 1, which indicates that their sizes increased with increasing SPD temperatures. The size of a NiOx crystallite and the tendency of conversion efficiency were consistent, except at 550 and 600 °C, where the FTO layer broke beyond the glass transition point. In other words, as a guideline for manufacturing high-performance inverted perovskite solar cells, it is important to enlarge the crystallites in the NiOx film. Conclusions We demonstrated that it is related to the NiOx crystallite size and the conversion efficiency of the perovskite solar cells, which can be used as a guideline to achieve high performance. The highest perovskite solar cell conversion efficiency was obtained at a NiOx layer production temperature of 500 °C. In the inverse perovskite solar cell, the conversion efficiency exceeded 16.1%, and hysteresis was not observed. Moreover, the measurements of the valence band using synchrotron radiation indicated that the ratio of Ni2+ to Ni+3 had little effect on the solar cell performance. This is an important guideline for improving the performance of perovskite solar cells. Acknowledgement This work was supported by New energy and industrial Technology Development Organization (NEDO), Japan.

Organometal hybrid perovskite material has emerged as an attractive competitor in the field of photovoltaics due to its promising potential of low-cost and high-efficiency photovoltaic applications. Although organometal halide perovskite solar cell shows great potential to meet future energy needs, the degradation raises serious questions about its commercialization viability. At present, the stability of perovskite solar cells has been studied in various environmental conditions. Nonetheless, an understanding of the degradation and its performance of CH3NH3PbI3-xClx perovskite solar cell is limited. Herein, we report the mechanical and structural degradation of CH3NH3PbI3-xClx perovskite films at room temperature as a function of time and thermal instability of perovskite solar cells during the heating and cooling processes. For mechanical degradation measurement, we used nanoindentation for CH3NH3PbI3-xClx perovskite films fabricated on FTO/PEDOT:PSS substrate. The hardness and elastic modulus of perovskite films were measured as a function of time. In addition, the mechanical degradation of perovskite thin films was correlated with X-ray diffraction, steady-state and time-resolved photoluminescence (PL). We also investigated the thermal instability of perovskite thin films and the irreversible performance of perovskite solar cells. Particularly, the irreversible performance of CH3NH3PbI3-xClx was analyzed by measuring the development of crystallinity, charge trapping/detrapping, trap depth, and PbI- phase while varying the temperature of perovskite films and solar cells between room temperature and 82 °C. Surprisingly, we found that the degradation of both perovskite films and solar cells occurred at ~70°C. Remarkably, even after the perovskite solar cell temperature cooled down to room temperature, the performance of solar cells continuously degraded. The underlying mechanism of irreversibly degraded performance of perovskite films and solar cells were explained in terms of the development of phase separation, increased trapping rates and deep trap depth of defect states of perovskite films.

Reducing hysteresis and enhancing performance of perovskite solar cells using acetylacetonate modified TiO2 nanoparticles as electron transport layers

An analysis comparing the performance of lead and tin halides organic Perovskite Solar Cells and numerical simulation with SCAPS

Two-dimensional Bi2OS2 doping improves the performance and stability of perovskite solar cells

Effect of doping engineering in TiO2 electron transport layer on photovoltaic performance of perovskite solar cells

Power conversion efficiencies of the perovskite solar cells (PSCs) are drastically increased in this 5 years. PSCs without a mesoporous TiO2 layer, planar-type cells, commonly obtained poorer cell performance as compared to cells with a porous TiO2 layer, owing to inefficient electron transfer from the perovskite layer to the compact TiO2 layer in the former case. The matching of the conduction band edge (CBE) levels of perovskite and the compact TiO2 layer is thus essential for enhancing solar cell performance. In this work, we discussed about the interface between perovskite and electron transport layer (ETL) to improve the performance of the PSCs with adjustment of the CBE levels for reducing the energy loss at the material interfaces in the PSCs. The CBE levels were evaluated with the electrochemical methods. The chemical bath treatment with TiCl4 (TiCl4 treatment) for the compact TiO2 layer to increase the performance of PSC has been reported. The effect of the TiCl4 treatment was considered as the cause of the wettability increases of the precursor solution of the perovskite on TiO2 compact layer and the efficient charge transport from perovskite layer to TiO2 layer. However, TiCl4 treatment may change the CBE potentials. The adjustment of the CBE potentials will be important issue to obtain the low barrier potentials for electron transfer from perovskite to TiO2 compact layers. To study the effect of the TiCl4 treatment on the CBE potentials, we prepared the electrochemical cells with the iodide and tri-iodide redox electrolyte and treated substrate with TiCl4 aqueous solution and the relation between open circuit voltages ( V oc) and the electron density in TiO2 layer of the cells were measured with the charge extraction methods. The V oc as the quasi CBE potential was negatively shifted with the TiCl4 treatment but positively shifted with heating after TiCl4 treatment. We compared the PSCs performance as the different heating temperature at 150 °C, 300 °C, and 450 °C of the TiCl4 treated compact TiO2 layers. The Voc was decreased and the short circuit current was increased with increasing the temperature. The highest PCE was obtained with the heating at 300 °C.

Organic-inorganic hybrid perovskite solar cells have attracted much attention due to their high power conversion efficiency (>25%) and low-cost fabrication. Yet, improvements are still needed for more stable and higher-performing solar cells. In this work, a series of TiO2 nanocolumn photonic structures have been intentionally fabricated on half of the compact TiO2-coated fluorine-doped tin oxide substrate by glancing angle deposition with magnetron sputtering, a method particularly suitable for industrial applications due to its high reliability and reduced cost when coating large areas. These vertically aligned nanocolumn arrays were then applied as the electron transport layer into triple-cation lead halide perovskite solar cells based on Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3. By comparison to solar cells built onto the same substrate without nanocolumns, the use of TiO2 nanocolumns can significantly enhance the power conversion efficiency of the perovskite solar cells by 7% and prolong their shelf life. Here, detailed characterizations on the morphology and the spectroscopic aspects of the nanocolumns, their near-field and far-field optical properties, solar cells characteristics, as well as the charge transport properties provide mechanistic insights on how one-dimensional TiO2 nanocolumns affect the performance of perovskite halide solar cells in terms of charge transport, light harvesting, and stability, knowledge necessary for the future design of higher-performing and more stable perovskite solar cells.

Organic-inorganic lead halide perovskites are promising materials for realization of low-cost and high-efficiency solar cells. Because of the toxicity of lead, Sn-based perovskite materials have been developed as alternatives to enable fabrication of Pb-free perovskite solar cells. However, the solar cell performance of Sn-based perovskite solar cells (Sn-PSCs) remains poor because of their large open-circuit voltage (VOC) loss. Sn-based perovskite materials have lower electron affinities than Pb-based perovskite materials, which result in larger conduction band offset (CBO) values at the interface between the Sn-based perovskite and a conventional electron transport layer (ETL) material such as TiO2. Herein, the relationship between the VOC and the CBO in these devices was studied to improve the solar cell performances of Sn-PSCs. It was found that the band offset at the ETL/perovskite layer interface affects the VOC of the Sn-PSCs significantly but does not affect that of the Pb-PSCs because the Sn-based perovskite material is a p-type semiconductor, unlike the Pb-based perovskite. It was also found that Nb2O5 has the CBO that is closest to zero for Sn-based perovskite materials, and the VOC values of Sn-PSCs that use Nb2O5 as their ETL are higher than those of Sn-PSCs using TiO2 or SnO2 ETLs. This study indicates that control of the energy alignment at the ETL/perovskite layer interface is an important factor in improving the VOC values of Sn-PSCs.

The dynamic defect tolerance under light soaking is a crucial aspect of halide perovskites. However, the underlying physics of light soaking remains elusive and is subject to debate, exhibiting both positive and negative effects. In this investigation, we demonstrated that surface defects in perovskite films significantly impact the performance and stability of perovskite solar cells, closely correlated with light soaking behaviors. Removing the top surface layer through adhesive tape, the surface defect density noticeably decreases, leading to enhanced photoluminescence (PL) efficiency, prolonged carrier lifetime, and higher conductivity. Consequently, the power conversion efficiency (PCE) of solar cells improves from 17.70% to 20.5%. Furthermore, we confirmed a positive correlation between surface defects and the light soaking effect. Perovskite films with low surface defects surprisingly exhibit a 3-fold increase in PL intensity and an 85% increase in carrier lifetime under 500 s of continuous illumination at an intensity of 100 mW/cm2. Beyond the conventional strategy of suppressing defect trapping, we propose increasing the capability of dynamic defect tolerance as an effective strategy to enhance the optoelectronic properties and performance of perovskite solar cells.

Maximizing photovoltaic performance of all-inorganic perovskite CsSnI3-xBrx solar cells through bandgap grading and material design