Interfacial Materials And Perovskite Film Morphology In Perovskite Solar Cells

As an emerging thin film solar cell technology, Organic-inorganic hybrid perovskite solar cells (PSCs) employing CH3NH3PbX3 (X=I, Br, Cl) as light absorbers have been attracting more and more attention due to its advantages in terms of large absorption coefficients, high carrier mobility, especially long charge carrier diffusion lengths, solution processibility and low-cost. Up to now, the power conversion efficient (PCE) of PSCs have reached 22.1%. There are two key issues in solar cells, to improve the PCE and to reduce the manufacturing cost. Hence, at present much effort of PSCs have been focused on replacing the expensive materials, developing novel perovskite materials with high performance/stability and interfacial materials, optimizing morphology and crystallinity of perovskite layer. In this dissertation, we focused on improving the performance of PSCs via developing novel interfacial materials and controlling perovskite layer morphology, and carried out the following works:(1) Kesterite-structured quaternary semiconductor Cu2ZnSnS4 (CZTS) has been commonly used as light absorber in inorganic semicondutor thin film solar cells on the basis of its optimal bandgap of 1.5 eV, high absorption coefficient, and earth-abundant elemental constituents. We applied CZTS nanoparticles as a novel inorganic hole transporting material (HTM) for organo-lead halide PSCs for the first time, achieving a power conversion efficiency (PCE) of 12.75% and quite comparable to that obtained for PSCs based on commonly used organic HTM such as 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (spiro-MeOTAD). The size of CZTS nanoparticles and its incorporation condition as HTM were optimized, and the effects of CZTS HTM on the optical absorption, crystallinity, morphology of the perovskite film and the interface between the perovskite layer and the Au electrode were investigated and compared with the case of spiro-MeOTAD HTM, revealing the role of CZTS in efficient hole transporting from the perovskite layer to the top Au electrode as confirmed by the prohibited charge recombination at the perovskite/Au electrode interface. On the basis of the effectiveness of CZTS as a low-cost HTM competitive to spiro-MeOTAD in PSCs, we demonstrate the new role of CZTS in photovoltaics as a hole conductor beyond the traditional light absorber.(2) Ionic liquid (IL) was incorporated for the first time as a novel electron transport material in PSCs, resulting in efficiency enhancement. Fistly, IL layer was spin-coated on TiO2 electron transport layer (ETL), and then perovskite layer was deposited on IL layer. Compared with the reference device (14.2±0.43%), the device with TiO2/IL ETL can exhibit an average power conversion efficiency (PCE) of 18.42±0.65%, which is significantly increased by 29.1%. Also, we used IL layer to directly substitute TiO2 ETL without high temperature preparation and the device with IL layer as ETL shows an average power conversion efficiency (PCE) of 13.23±0.55%. The IL ETL based device shows quite comparable to that obtained for device with TiO2 ETL which means that IL shows good potential as an ETL in PSCs. On the basis of a series of characterizations including Ultraviolet Photoelectron Spectroscopy (UPS), Atomic Force Microscope (AFM) and Electrochemical Impedance Spectroscopy (EIS), etc. and a possible mechanism of efficiency enhancement upon IL incorporation was proposed:IL incorporation may induce a dipole layer on the FTO substrate of T1O2 layer, which may enhance the work function of FTO or TiO2 and then lower the energy barrier, consequently facilitating electron transport from perovskite layer to FTO.(3) Two-step method has been popularly adopted to fabricate perovskite film of planar heterojunction organo-lead halide (PSCs). However, this method often generates uncontrollable film morphology with poor coverage. We developed a facile method to improve perovskite film morphology by incorporating a small amount of acetate (CH3COO-, Ac-) salts (NH4AC, NaAc) as non-halogen additives in CH3NH3I solution used for immersing PbI2 film, resulting in improved CH3NF3PbI3 film morphology. Under the optimized NH4Ac additive concentration of 10 wt%, the best power conversion efficiency (PCE) reaches 17.02%, which is enhanced by ～23.2% relative to the pristine device without additive, whereas NaAc additive does not lead to efficiency enhancement despite of the improvement on CH3NH3PbI3 film morphology. SEM study reveals that NH4Ac additive can both effectively improve perovskite film morphology by increasing the surface coverage via diminishing pinholes. The improvement on CH3NH3PbI3 film morphology is beneficial for increasing the optical absorption of perovskite film and improving the interfacial contact at the perovskite/Spiro-OMeTAD interface, leading to the increase of short-circuit current and consequently efficiency enhancement of the PSC device.