Preparation And Characterization Of Novel Semiconductor Absorber Materials For Thin Film Solar Cell Applications

Faced with the increasingly serious environmental pollution and energy crisis, it becomes a hot topic in new materials and new energy fields to develop a kind of novel solar cell with low-cost and high-efficiency by exploiting inexhaustible solar energy.On the one hand, Cu2ZnSnS4(CZTS) and Cu2ZnSnSe4(CZTSe) are an ideal candidate of absorber materials for solar cells due to its high absorption coefficient (>104 cm-1), proper direct band gap, and earth-abundant compositions. However, owing to the high equipment cost and processing complexity, the conventional vacuum method is not suitable for mass production. The hydrazine-based solution process brings about safety issue because of a highly toxic hydrazine solvent. Therefore, it is critical to develop an economical and efficient Cu2ZnSn(SxSe1-x)4 (CZTSSe) absorbing layer preparation technique for cheap and efficient thin film solar cells. On the other hand, novel CH3NH3bI3 perovskite materials have attracted considerable attention since strong light-absorption characteristic, high carrier transport property, and large defect tolerance with the photoelectric conversion efficiency (PCE) as high as 20% in thin-film solar cells. However, its stability and repeatability need further improving.This thesis focuses on the preparation and optical properties of new semiconductor light-absorber materials in thin film solar cells. A novel and simple two-step route to synthesize CZTSSe nanoparticles has been developed. The effect of sulfurization time on the structural, compositional, and optical properties of CZTSSe nanoparticles has been characterized systematically. The fabrication and performance of CZTSSe thin film solar cells was explored preliminarily. Meanwhile the CH3NH3PbI3-xClx perovskite thin film solar cells have also been explored. And a two-step/one-step deposition procedure has been investigated. The impact of the Cl dopant and the introduction of hole transporting material (HTM) of P3HT on the structure, composition, morphology and device performance of CH3NH3PbI3 have also been characterized in depth.The main achievements are listed below:1. We have developed a facile and low-cost route to synthesize Cu2ZnSn(SxSe1-x)4 (CZTSSe) nanocrystals with tunable composition and optical band gap by a two-step process. It involves a solvothermal reaction of Cu2ZnSnSe4 (CZTSe) and a sulfurization post-annealing. The effects of sulfurization time, ranging from 0 min to 90 min, on the structural, compositional, and optical properties of CZTSSe have been investigated deeply. X-ray diffraction, Raman scattering, field-emission scanning electron microscopy, and transmission electron microscopy confirm that the Se2- is gradually replaced by the S2- to form the CZTSSe solid solutions with a tetragonal structure and uniform composition distribution when increasing the sulfurization time from 30 min to 90 min. The A1 Raman modes of CZTSSe display a typical two-mode behavior, revealing that S composition plays an important role on tuning the vibrating modes. The absorption spectra show that the optical band gap becomes large gradually from 0.91 to 1.30 eV with increasing the S content from 0 to 0.76. CZTSSe nanocrystals derived from this method can be used as low-cost absorber layer for photovoltaic applications.2. The CZTSSe film with tetragonal structure was prepared by nanoparticle ink method and a sulfurization post-annealing. The solar cell devices with stacking structure of Mo/CZTSSe/CdS/i-ZnO/ZnO:Al was produced. And the photovoltaic properties was also tested with open circuit voltage (Voc) of 76.8 mV, short-circuit current (Jsc) of 0.348 mA/cm2, and fill factor (FF) of 26.57%. The preparation process of CZTSSe film solar cells still needs further optimization.3. The pure CH3NH3PbI3 film with tetragonal structure was prepared by a two-step deposition method. Meanwhile CH3NH3PbI3-xClx was prepared by a one-step deposition method. When Cl element is doped, the films become denser with smaller grain size and larger light absorption ability. After adding the P3HT as the hole-transporting material, the films exhibit strong absorption peak in the wavelength range of 450-550 nm, which enhances the sunlight utilization of the device.4. The perovskite thin film solar cells with the structure of FTO/TiO2/CH3NH3PbI3-xClx/Au have been obtained. Compared to CH3NH3PbI3 device, when Cl elements is doped, the Voc, Jsc, FF and the PCE of HTM-free solar cells increase to 402 mV,0.101 mA/cm2,58.73% and 0.02%, respectively. It shows that the introduction of Cl elements can improve the carrier transport property, increase the diffusion length of the electron/hole pairs, and enhance the PCE of the devices. After adding the P3HT as the HTM on the top of CH3NH3Pbl3-xClx layer, both the open circuit voltage and short circuit current of CH3NH3PbX3 perovskite thin film solar cells have been greatly improved with a PCE of 0.05% (VOC=558 mV, Jsc=0.251 mA/cm2, FF=35.94%). It means that the P3HT materials can promote the separation of electron/hole pairs at interface and reduce the charge recombination, leading to the improved device performance.