| Recently, a new type of solar cell, e.g. perovskite solar cells (PSCs), applying organolead halide perovskite (e.g., CH3NH3PbX3 X=I, Br, Cl) materials as light absorbing layers have experienced the fastest increase in efficiencies. New progress has pushed the power conversion efficiency (PCE) up to 20%. Many researches are currently focusing on the preparation technology, device instability and large-scale products for commercial propose. However, consideration over the toxicity of lead in the lead halide perovskite solar cells which is the inherent defects of this material may impede the promotion for such cells. And thus it stimulates the search for an altanetive and lead-free light absorbing materials for PSCs.In this thesis, the growth and physical properties of inorganic and lead-free Cs-Sn-I perovskite thin films and their utilization as light absorbers in perovskite solar cells were selected as the main research subjects. We prepared B-γ-CsSnI3 and Cs2SnI6 films with high quality through vacuum thermal evaporation deposition and solution process. We systematically investigated the stability of the B-y-CsSnI3 and Cs2SnI6 films and their optical and electrical properties, and their application as light absorbing materials (LAM) in the conversional N-i-P planar and mesoscopic PSCs. The main research contents and results are as follows:1. Inorganic B-γ-CsSnI3 thin film was prepared by thermal evaporation and its optical and electrical properties were studied. Using CsI and SnI2 with high purity as raw materials, we inveatigated the influence of the growth conditions on the films, such as evaporation rate, reaction temperature and deposition order. By optimizing the growth parameters, lead-free B-y-CsSnI3 films with high absorption coefficient (-104 cm-1), lower carrier concentration (-6.6×108 cm-3) but higher mobility (～120 cm2 V-1 s-1) were obtained, which indicates their further application as LAM in PSCs.2. The stability of B-γ-CsSnI3 perovskite thin film in air was explored and the mechanism of phase change was discussed. With increased exposure time, the experimental results showed that light absorption of the B-y-CsSnI3 film changed significantly and its colour varied from black (Black-y phase) to yellow (Yellow phase), finally black. The XRD data indicated that the spontaneous phase change of the B-γ-CsSnI3 in air was irreversible and the formation of CS2SnI6 with a direct optical band gap of-1.48 eV, which was because of the Sn4+ oxidized from Sn2+ easily.3. The B-γ-CsSnI3 film and Cs2SnI6 film acting as LAMs in conventional N-i-P planar perovskite solar cells was demonstrated. We initially explored device assembly, and fabricated PSCs with the planar architecture of FTO/TiO2 compact layer/inorganic perovskite film/hole transporting materials (HTM)/metal electrode. The results indicated that the more stable Cs2SnI6 film exhibited better device characteristics with a typial PCE of～1%.4. Mesoscopic perovskite solar cells based on ZnO nanorods using typical Cs2SnI6 film and CH3NH3PbI3 film as LAMs were compared. (1) We prepared ZnO seed layer through palsed laser deposition method and spin-coating method. The morphology of the ZnO seed layer and precursor concentrations both influced the ZnO nanorods grwoth, and consequently affected the device performace. PLD-ZnO seed layer with optimized surface morphology was demonstrated to be more efficiently suppress charge recombination, and resulted in higher PCE; (2) CH3NH3PbI3 films deposited directly on the ZnO surface can be decomposed into PbI2 under annealing process. Using CdS as passive layer to form a core-shell structure could significantly suppress the interface degradation between CH3NH3PbI3 and ZnO. Annealing at 80℃ for 30 min, we obtained CH3NH3PbI3 films with larger grain size, decreased grain boundaries and lower interface electron transfer resistance, and thus increased the open circuit voltage and PCE; (3) Comparing the performance of CH3NH3PbI3 and Cs2SnI6 mesoscopic PSCs, we found that the former got a PCE of～3.7%, while the later was only～0.86%。 High-quality CH3NH3PbI3 film can be grown now, however, Cs2SnI6 film is newly found, and is still in the primary exploratory stage. So the lower efficiency is caused by its unknown physical property. But, there is no doubt that such attempt opens up a new way for lead-free perovskite solar cells. |
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