Haopeng Dong

Study on the stability of CH3NH3PbI3 films and the effect of post-modification by aluminum oxide in all-solid-state hybrid solar cells

Degradation of perovskite has been a big problem in all-solid-state perovskite solar cells, although many researchers mainly focus on the high efficiency of these solar cells. This paper studies the stability of CH3NH3PbI3 films and finds that CH3NH3PbI3 is sensitive to moisture. The degradation reaction is proposed according to UV-Vis spectra and XRD results. In order to improve the degradation of CH3NH3PbI3, we introduce aluminum oxide as a post-modification material into all-solid-state perovskite solar cells for the first time. UV-Vis spectra show that Al2O3 modification could maintain the absorption of CH3NH3PbI3 after degradation. XRD results reveal that Al2O3 could protect perovskite from degradation. Moreover, the device post-modified by Al2O3 has shown more brilliant stability than that without modification when exposed to moisture. EIS results and dark current illustrate that the modification increased interface resistance in the dark, indicating the restrained electron recombination process.

Montmorillonite as bifunctional buffer layer material for hybrid perovskite solar cells with protection from corrosion and retarding recombination

4-tert-Butylpyridine (TBP) has been an important component in hole transport layer for hybrid perovskite solar cells. However, our study shows that TBP can corrode the perovskite absorption layer (CH3NH3PbI3) and interfere with the stability of the solar cells. To address this problem, montmorillonite (MMT) was used to form a buffer layer on top of the hole transport layer. XRD results revealed that TBP was intercalated in the MMT structure and UV-vis spectroscopy analysis revealed that this structure could prevent the corrosion of the CH3NH3PbI3 layer. Moreover, the MMT buffer layer could limit charge recombination in the solar cells. A delayed corrosion led to an increased current density owing to enhanced absorption, while a reduced charge recombination led to an increased fill factor and open voltage circuit values. Consequently, the corresponding efficiency largely increased from 9.0% to 11.9%, with an improvement of 32.2%. Therefore, the application of MMT as a bifunctional buffer coating layer on the hole transport layer is a useful strategy for preparing highly efficient hybrid perovskite solar cells with anti-corrosion and delayed charge recombination properties.

Graphene oxide as dual functional interface modifier for improving wettability and retarding recombination in hybrid perovskite solar cells

The interface between perovskite and the hole transport layer (HTL) is sensitive to photoelectric conversion properties. However, this study shows that the interface wettability of the HTL solution on a perovskite surface could be improved. To address this problem, graphene oxide (GO) with amphiphilic function was used to form a buffer layer between the perovskite and the HTL. After the GO modification, the contact angles of the HTL solution on the perovskite film decreased to zero degrees. X-ray photoelectron spectroscopy revealed that the GO interacts with the perovskite by forming Pb–O bonds, and Raman spectroscopy analysis revealed that the two-dimensional carbon–carbon bonds absorbed the hole transport material, 2,29,7,79-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,99-spirobifluorene (Spiro-MeOTAD) via π–π interactions. The GO layer improved the contact between the perovskite and HTL, resulting in an enhancement of the short circuit current (JSC). Moreover, using GO as an insulating buffer layer can retard charge recombination in solar cells, as revealed by EIS measured in dark conditions, leading to a significant increase in the open-circuit voltage (VOC) and the fill factor (FF). Consequently, the corresponding average efficiency greatly increased by 45.5%, from 10.0% to 14.5%. Therefore, application of GO as a dual-functional buffer layer on the perovskite layer is a useful strategy for preparing highly efficient hybrid perovskite solar cells.

Multifunctional perovskite capping layers in hybrid solar cells

In this study, the crucial role of perovskites capping layers in the TiO2/CH3NH3PbI3 hybrid solar cells is investigated. The capping layers are realized by controlling the concentration of PbI2 solutions in the sequential deposition process. The morphologies of the active layers are studied by high-resolution scanning electron microscopy (HR-SEM). The amount of perovskites in capping layers increases with the concentration of PbI2 solution, and the coverage of perovskite capping layers on TiO2 films is better developed. Except for the correlation between photocurrents and coverages of perovskite proposed by Snaith, we revealed a more detailed relationship between the photovoltaic performances and perovskite capping layers. It is noteworthy that UV-vis absorption increased with perovskites in capping layers. Moreover, according to the diffuse reflection spectra, light scattering, which is beneficial for the conversion efficiency of photons to electrons by directly preventing most of the incident light from transmitting out, is also enhanced due to both the emergence of larger-size particles in the capping layers and the higher effective dielectric coefficient. All of the aforementioned aspects result in high photocurrents up to 20.6 mA cm−2. Efficiency as high as 10.3% is ultimately achieved by a simple control of PbI2 concentration in the sequential deposition process.

Multifunctional MgO Layer in Perovskite Solar Cells.

A multifunctional magnesium oxide (MgO) layer was successfully introduced into perovskite solar cells (PSCs) to enhance their performance. MgO was coated onto the surface of mesoporous TiO(2) by the decomposition of magnesium acetate and, therefore, could block contact between the perovskite and TiO(2). X-ray photoelectron spectroscopy and infrared spectroscopy showed that the amount of H(2)O/hydroxyl absorbed on the TiO(2) decreased after MgO modification. The UV/Vis absorption spectra of the perovskite with MgO modification revealed an enhanced photoelectric performance compared with that of unmodified perovskite after UV illumination. In addition to the photocurrent, the photovoltage and fill factor also showed an enhancement after modification, which resulted in an increase in the overall efficiency of the cell from 9.6 to 13.9 %. Electrochemical impedance spectroscopy (EIS) confirmed that MgO acts as an insulating layer to reduce charge recombination.

Constructing nanorod–nanoparticles hierarchical structure at low temperature as photoanodes for dye-sensitized solar cells: combining relatively fast electron transport and high dye-loading together

ZnO nanorod–nanoparticles (NR–NPs) hierarchical structure was prepared via a two-step hydrothermal process at 70 °C. In the hierarchical structure, ZnO nanorods prepared at step one served as the backbone for direct electron transport while ZnO nanoparticles synthesized at step two offered large surface area for dye-loading. Both reaction temperature and reaction time at step two had a significant influence on the morphology of the product. At a higher temperature, microspheres appeared above the nanorod film instead of nanoparticles surrounding the nanorods. Prolonging the reaction time to 24 h, the NR–NPs structure would transform to nanorod–nanoplants. Intensity-modulated photocurrent spectroscopy results showed that the photoanode composed of the NR–NPs hierarchical structure had an electron diffusion coefficient (Dn) much higher than that of the nanoparticles. The dye desorption results showed that the dye adsorption amount for the NR–NPs structure was as much as 250% of that for the nanorods. Compared with dye-sensitized solar cells (DSCs) based on nanorods, the incident photon-to-electron conversion efficiency of the DSCs based on NR–NPs hierarchical structure improved remarkably. Under AM 1.5G illumination (100 mW cm−2), the power conversion efficiency of DSCs based on photoanodes composed of NR–NPs hierarchical structure exhibited a significant improvement (more than 120%) compared with that of ZnO nanorods.

Interface engineering of perovskite solar cells with PEO for improved performance

Interface engineering is an important and efficient way to further improve the conversion efficiency of perovskite solar cells. In this study, we report the modification of the electron transport layer (ETL) using a thin layer of PEO. Characterizations showed that PEO was uniformly coated on top of the original TiOx ETL, without resulting in an evident change of the surface morphology, hydrophilic ability or transparency. With the interface dipole formed at the interface, the work function of the ETL greatly decreased. Compared with devices with TiOx only, devices based on the modified ETL gave a nearly 15% enhancement to the overall conversion efficiency, with both Voc and Jsc improved. Further studies showed that the improved performance could mainly be attributed to the better retardation of back recombination and the enhanced electron collection efficiency by means of the PEO thin layer modification.

Morphology-controlled CH3NH3PbI3 films by hexane-assisted one-step solution deposition for hybrid perovskite mesoscopic solar cells with high reproductivity

The morphology and crystal structure of perovskite films are critical for achieving high-performance perovskite solar cells; however in most cases, the conventional one-step solution deposition method hardly yields a homogeneous perovskite film over a large area, especially for CH3NH3PbI3. Here, we propose a facile and environmentally friendly hexane-assisted one-step solution approach for dense and uniform CH3NH3PbI3 thin films. According to the phase diagram of immiscible liquids, n-hexane was chosen as the assistant solvent to speed-up the evaporation of the main solvent N,N-dimethylformamide (DMF) during the solution deposition process, thus significantly promoting the nucleation and crystallization process of CH3NH3PbI3 perovskite. The as-prepared CH3NH3PbI3 films demonstrated a uniform and dense morphology, and enhanced light absorption in a long-wavelength range. In particular, such n-hexane treatment could help eliminate the residual DMF and greatly improve the thermal stability of the obtained perovskite films. The full solution-processed CH3NH3PbI3 solar cells using n-hexane treatment exhibited a maximum power conversion efficiency of 11.7% and average efficiencies of 11.3 ± 0.4% under standard AM 1.5 conditions in comparison with 7.0 ± 0.4% of the conventional cells. The recombination resistance of the cells increased nearly 5 times by using n-hexane. These results suggest that this hexane-assisted one-step solution approach is promising for controlling the crystallization process of perovskites to achieve high-performance perovskite solar cells.

Inorganic iodide ligands in ex situ PbS quantum dot sensitized solar cells with I−/I3− electrolytes

In this paper, inorganic iodide ligands were used in PbS quantum dot sensitized solar cells (QDSCs) with iodide/triiodide electrolytes. Inorganic ligands are employed to replace organic ligands in QDSCs for the first time. They combined effects of passivating surface states and decreasing the interface resistance between QDs and sensitized TiO2, QDs and electrolyte. Then the corrosion of PbS QDs by triiodide in iodide/triiodide electrolytes was suppressed and electron injection and hole transfer was much easier. Stability test verified iodide ligands could prevent PbS from corrosion of iodide/triiodide electrolytes. Electrochemical impedance spectroscopy (EIS) showed that iodide ligands effectively decreased the interface resistance and improved the electron transfer. Finally, the performance with iodide ligands was significantly improved and achieved 3.7 times that of the untreated cells in efficiency.

Cesium carbonate as a surface modification material for organic–inorganic hybrid perovskite solar cells with enhanced performance

Cs2CO3 has been employed as a new surface modification material for inorganic–organic hybrid perovskite solar cells. With the optimized modifying process, 14.2% power conversion efficiency (PCE) was obtained, enhanced by nearly 20% compared with the control devices. Further studies showed that the PCE improvement mainly came from the retarded back recombination.