Jiangjian Shi

Hole-conductor-free perovskite organic lead iodide heterojunction thin-film solar cells: High efficiency and junction property

Efficient hole-conductor-free organic lead iodide thin film solar cells have been fabricated with a sequential deposition method, and a highest efficiency of 10.49% has been achieved. Meanwhile, the ideal current-voltage model for a single heterojunction solar cell is applied to clarify the junction property of the cell. The model confirms that the TiO2/CH3NH3PbI3/Au cell is a typical heterojunction cell and the intrinsic parameters of the cell are comparable to that of the high-efficiency thin-film solar cells.

Interfaces in Perovskite Solar Cells

The interfacial atomic and electronic structures, charge transfer processes, and interface engineering in perovskite solar cells are discussed in this review. An effective heterojunction is found to exist at the window/perovskite absorber interface, contributing to the relatively fast extraction of free electrons. Moreover, the high photovoltage in this cell can be attributed to slow interfacial charge recombination due to the outstanding material and interfacial electronic properties. However, some fundamental questions including the interfacial atomic and electronic structures and the interface stability need to be further clarified. Designing and engineering the interfaces are also important for the next-stage development of this cell.

Modified Two-Step Deposition Method for High-Efficiency TiO2/CH3NH3PbI3 Heterojunction Solar Cells

Hybrid organic-inorganic perovskites (e.g., CH3NH3PbI3) are promising light absorbers for the third-generation photovoltaics. Herein we demonstrate a modified two-step deposition method to fabricate a uniform CH3NH3PbI3 capping layer with high-coverage and thickness of 300 nm on top of the mesoporous TiO2. The CH3NH3PbI3 layer shows high light-harvesting efficiency and long carrier lifetime over 50 ns. On the basis of the as-prepared film, TiO2/CH3NH3PbI3 heterojunction solar cells achieve a power conversion efficiency of 10.47% with a high open-circuit voltage of 948 mV, the highest recorded to date for hole-transport-material-free (HTM-free) perovskite-based heterojunction cells. The efficiency exceeding 10% shows promising prospects for the HTM-free solar cells based on organic lead halides.

Highly efficient planar perovskite solar cells with a TiO2/ZnO electron transport bilayer

A TiO2/ZnO electron transport bilayer, which combines the advantages of high electron extraction and low interfacial recombination together, has been designed for planar perovskite solar cells. With this bilayer, the front surface recombination in the cell is significantly suppressed, and a high efficiency exceeding 17% has been achieved. These results suggest a promising and simple approach to further design photovoltaic devices from the aspect of charge transport and recombination.

Efficient hybrid mesoscopic solar cells with morphology-controlled CH3NH3PbI3-xClx derived from two-step spin coating method.

A morphology-controlled CH3NH3PbI3-xClx film is synthesized via two-step solution deposition by spin-coating a mixture solution of CH3NH3Cl and CH3NH3I onto the TiO2/PbI2 film for the first time. It is revealed that the existence of CH3NH3Cl is supposed to result in a preferential growth along the [110] direction of perovskite, which can improve both the crystallinity and surface coverage of perovskite and reduce the pinholes. Furthermore, the formation process of CH3NH3PbI3-xClx perovskite is explored, in which intermediates containing chlorine are suggested to exist. 13.12% of power conversion efficiency has been achieved for the mesoscopic cell, higher than 12.08% of power conversion efficiency of the devices fabricated without CH3NH3Cl via the same process. The improvement mainly lies in the increasing open-circuit photovoltage which is ascribed to the reduction of reverse saturation current density.

Mg-doped TiO2 boosts the efficiency of planar perovskite solar cells to exceed 19%

Mg as an effective ion dopant has been introduced into compact TiO2 ETL for planar perovskite solar cells. It is found that Mg doping can shift the Fermi energy level of the ETL upward and decrease the resistivity of the TiO2 film. Carrier transport velocity is thus enhanced and high charge collection efficiency has been achieved at high bias voltages, leading to a higher fill factor (FF) and better cell efficiency. More than 19% power conversion efficiency has been obtained.

Efficient CH3NH3PbI3 perovskite solar cells with 2TPA-n-DP hole-transporting layers

CH3NH3PbI3 perovskite solar cells with 2TPA-n-DP (TPA = 4,4′-((1E, 1′E,3E,3′E)-[1,1′-biphenyl]-4,4′-diylbis(buta-1,3-diene-4,1-diyl)); DP = bis(N,N-di-p-tolylaniline); n = 1, 2, 3, 4) as hole-transporting materials (HTMs) have been fabricated. After optimization of the mesoporous TiO2 film thickness, devices based on 2TPA-2-DP with power conversion efficiencies (PCEs) of up to 12.96% have been achieved, comparable to those of devices with (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) (spiro-OMeTAD) as HTM under similar conditions. Further time-resolved photoluminescence (PL) measurements showed a fast charge transfer process at the perovskite/2TPA-2-DP interface. With the aid of electrochemical impedance spectra, a study of the electron blocking ability of 2TPA-2-DP in the device reveals that the presence of 2TPA-2-DP can greatly increase charge transfer resistance at the HTM/Au interface in the device, thus reducing the recombination. Furthermore, the perovskite solar cells based on these four HTMs exhibit good stability after testing for one month.

Perovskite thin-film solar cell: excitation in photovoltaic science

As a new member of thin-film solar cells, the perovskite solar cells have inspired a new research hot in new photoelectric materials and devices, and have given a new energy to the photovoltaic science. Currently, various device structures, including mesoporous and planar, with and without hole transport material have been developed. In this review, much focus has been addressed to the deposition of high-quality perovskite films, structural optimization, and interface engineering as well as the understanding of the charge generation, transport, and recombination mechanisms of the devices. Furthermore, cost, stability, and environment issues of the cell are also discussed for commercial application.

Pressure-assisted CH3NH3PbI3 morphology reconstruction to improve the high performance of perovskite solar cells

The pressure is introduced as a parameter in the post-treat process for perovskite solar cells. Via a hot-pressing method, the rough surface of perovskite film becomes smooth, and the pin-holes can be cured. This modified perovskite morphology can help to improve charge transporting and eliminate recombination in the perovskite solar cells. Moreover, significantly enhanced photovoltaic performances with high PCEs of 10.84% and 16.07% are thus achieved in HTM-free type and spiro-OMeTAD based cells, respectively.

Control of Charge Transport in the Perovskite CH3NH3PbI3 Thin Film

Carrier density and transport properties in the CH3 NH3 PbI3 thin film have been investigated. It is found that the carrier density, the depletion field, and the charge collection and transport properties in the CH3 NH3 PbI3 absorber film can be controlled effectively by different concentrations of reactants. That is, the carrier properties and the self-doping characteristics in CH3 NH3 PbI3 films are strongly influenced by the reaction thermodynamic and kinetic processes. Furthermore, by employing mixed solvents with ethanol and isopropanol to deposit the CH3 NH3 PbI3 film, the charge collection and transport efficiencies are improved significantly, thereby yielding an overall enhanced cell performance.