Fabrication And The Performance Of Hybrid Perovskite Solar Cells

In recent five years, hybrid perovskite solar cells have attracted considerable interests due to their potential of high efficiency, low cost, and simple fabrication process. Different organic-inorganic hybrid perovskite materials and the structure of cells have been developed to improve the power conversion efficiency(PCE) of solar cells. Up to date, the best PCE has exceeded more than 20%.Generally, the basic structure of the perovskite cells is: FTO/dense Ti O2 layer/ mesoporous TiO2 layer/perovskite layer / Spiro-OMeTAD / Au. Spiro-OMeTAD is often used as a hole transporting material, but it has some disadvantages, such as low solubility, weak stability in the air, which may affect the performance of the solar cells. To improve the stability and performance of hybrid perovskite solar cells, we try to select different hole-transporting materials, and optimize the structures. Thus, the stability of the solar cells is greatly improved. In this work, hybrid lead halide perovskites(CH3NH3PbI3 and CH3NH3PbIXCl3-X) were used as light absorber layers. Meanwhile, poly(3-hexylthiophene-2,5-diyl)(P3HT) was used as the hole transporting material in hybrid perovskite solar cells. At the same time, impedance spectroscopy analysis revealed carrier transport and collection of physical mechanisms of perovskite solar cells.In the first section, a review was made of the latest research on solar cells. The significance of this work was stated.In the second section, the fabrication and properties of TiO2/CH3NH3PbI3 heterojunction solar cells were investigated. The device structure is: FTO/compact TiO2 layer/ TiO2 layer/perovskite layer/Au. TiO2/CH3NH3PbI3 solar cells were fabricated by spin-coating and characterized by current–voltage measurements, impedance spectroscopy and capacitance–voltage measurements. It was demonstrated that the TiO2/CH3NH3PbI3 layers form an ideal p–n heterojunction suitable for the photovoltaic applications. The built-in potential of 0.67 eV in the TiO2/CH3NH3PbI3 heterojunction were derived by the Mott–Schottky relationship. The degradation of TiO2/CH3NH3PbI3 solar cells showed that the efficiency remained 35.5% after storage under ambient laboratory conditions for 15 days. These results indicated that the oriented TiO2 layers provide a possible route to fabricate stable perovskite-based photovoltaic devices without hole transporting materials.In the third section, the fabrication and properties of CH3NH3PbI3-xClx-based perovskite solar cells employing P3 HT as the hole transporting material layer were investigated. Two structures are fabricated respectively, one is FTO/compact TiO2 layer/ oriented TiO2 layer/perovskite layer/Au, and another is FTO/compact TiO2 layer/ oriented TiO2 layer/perovskite layer/ P3HT/Au. The compact and oriented TiO2 films are prepared by a solvothermal method, and used as electron transporting layers in perovskite CH3NH3PbI3-xClx based solar cells incorporating P3 HT as the hole transporting material layer. The devices with P3 HT exhibit a remarkably increase in power conversion efficiency, open circuit voltage, and fill factor, compared with the reference device without P3 HT. Impedance spectroscopy measurements demonstrate that the present P3 HT layer decreases the internal resistance in solar cells and allows the interface between oriented TiO2 and CH3NH3PbI3-xClx to form more perfect in electronics. It is also found that the electron lifetime in the devices with P3 HT is much longer than that of the device without P3 HT. Thus, the charge collection efficiency of the device with P3 HT is markedly enhanced, compared with the devices without P3 HT.Analysis of the energy levels of the involved materials indicates that the P3 HT film between the CH3NH3PbI3-xClx layer and the Au electrode provides a better energy level matching for efficient transporting holes to the anode. Meanwhile, the stability of such P3 HT solar cells is enhanced because of the compact and oriented TiO2 film preventing the possible interaction between TiO2 and perovskite as time went on.In the fourth section, the process and properties of CH3NH3PbIXCl3-X-based perovskite solar cells were optimized. The entire process of the fabrication of CH3NH3PbIXCl3-X-based perovskite solar cells, including the thickness of the compact layer, pH value effect in TiO2 solution, the crystallization of perovskite and the P3HT-doping, was optimized to improve the performance of perovskite solar cells. A slow drying method was developed to increase the coverage rate of perovskite on the TiO2. Meanwhile, the P3 HT solution mixed with optimized rate of Li salt and pyridine, which can remarkably enhance open-circuit voltage of perovskite solar cells, compared with those without Li salt and pyridine. Under illumination, the optimum solar cell exhibits a short circuit current density of 20.8 mA /cm2, an open circuit voltage of 0.93 V, a fill factor of 58.9% and a PCE of 11.6%. From the top-view SEM images of the perovskite coated on compact and the oriented rod-type TiO2 films, it can be seen that the coverage of the perovskite is increased significantly, which leads to the photocurrent of solar cells enlarge. The result is agreed with the obtained by J-V.In the fifth section, the conclusions and perspective of this work on hybrid perovskite solar cells were made.