Type-Ⅱ Heterojunction Nanostructured Solar Cells

With the rapid development of economy and the continuous growth of population,the energy demand in human society is increasing.The huge consumption of fossil energy resources causes not only the global energy crisis but also the great damage to environment.Solar energy attracts widely attentions owing to its rich reserves and universal availability,and the exploitation of solar energy will certainly lead to a revolution in the energy industry and energy structure all over the world.Since the appearance of the Si-based solar cell in middle of last century,Si-based solar cells have made substantial progress.Currently,Si-based solar cells hold the dominant position in the photovoltaic market,however,their high costs restrict the large-scale commercial practical applications.In order to produce cheap and high-efficiency solar cells,third generation solar cells based on type-II heterojunction nanostructures emerge.Comparing to conventional planar structures,the coaxial nanowire is in favor of high-efficient solar cells,due to its larger surface-to-volume-rate,longer light absorption length and short carrier separation distance.In addition,organic-inorganic hybrid metal halide perovskite materials show great potential for the applications in solar cells,and recently receive enormous interest because of their high light absorption coefficients and super long carrier diffusion lengths.Herein,this work put emphasis on the two research hotspots,using both theoretical and experimental analysis,we intensely investigated the photoelectric properties and the applications in solar cells based on three kinds of type-Ⅱheterojunction nanostructures,namely ZnO/ZnSe coaxial nanowire,ZnO/perovskite coaxial nanowire and TiO2/perovskite planar film.(1)Realization of nearly full spectral response in ZnO/ZnSe coaxial nanowires.We proposed a method to study the large strains and the band structures under nanometer scale by combining the mechanical analytic method and first-principles simulation.Both ZnO and ZnSe have good radiation stabilities and photocatalytic abilities,but their wide band gaps(3.37 eV and 2.7 eV,respectively)are disadvantageous to light absorption.When directly applying into solar cells,they usually show low photoelectric conversion efficiencies(PCE).In this work,we found that the effective band gap(EBG)of ZnO/ZnSe coaxial nanowires could be modified by the strain field,as a result,the photo-response threshold could be widely extended.Firstly,the strained structures of ZnO/ZnSe coaxial nano wires were investigated based on the elastic equilibrium theory,it was found that a pseudomophic layer of metastable wurtzite ZnSe is preferentially grown on the interface,and the tunable range of the axial strain exceeds 20%.With the increase of the ZnO diameter,the critical thickness of the ZnSe pseudomophic layer first increases and then decreases to a constant value of 2nm;the thickest ZnSe pseudomophic layer is about 6.5 nm when the diameter of ZnO core is 9.0 nm.By using first-principles calculations(VASP software),we calculated the EBGs of ZnO/ZnSe coaxial nanowires.Results showed that the threshold could range from 0.4 eV to 1.8 eV.Experimentally,ZnO/ZnSe coaxial nanowires were synthesized by a two-step CVD method.A totally coherent ZnSe pseudomorphic shell layer around a ultra thin ZnO core was observed.Moreover,the ultra thin ZnO cores exhibited large lattice distortions and their interplanar spacings were very close to that of ZnSe.The lowest photo-response energy tested by transmission spectra was found to be 0.82eV,which covered up to 94%of the solar power.The photo-response threshold identified by EQE spectrum extended to infrared region of 0.91 eV,which almost realized the nearly full spectral responds.This work provided a strategy and method to design and develop a stable and efficient photovoltaic devices based on wide-bandgap semiconductors.(2)Design of highly efficient type-Ⅱ ZnO/CH3NH3PbI3 coaxial nano wire solar cells.Coaxial structures exhibit great potential for the application of high-efficiency solar cells due to the novel mechanism of radial charge separation.It’s the first time to discuss how the carrier separation mechanism in type-Ⅱ heterojunction effects the performances of coaxial nanowire solar cells.We deeply investigated the nonuniformity of the carrier separation efficiency(CSE),the light absorption inside ZnO/CH3NH3Pbl3 nano wires,and their influence on PCEs of nano wire solar cells by combining the semiconductor diffusion theory and FDTD simulations.Results show that the CSE rapidly decreases along the radial direction in the shell,and the value at the outer side becomes extremely low for the thick shell.Meanwhile,the absorbed energy does not uniformly distribute in the shell layer,and the peak position gradually moves towards the outer side with the increase of the shell thickness.The nonuniform CSE and light absorption in type-II nanowires will result in a decrease of PCE,and the decreasing becomes more evident for the nanowire with the thicker shell.Therefore,there is an optimal shell thickness in ZnO/CH3NH3PbI3 nanowire structure.In our case,,the maximal PCE is 19.5%for the nanowire solar cell with the shell thickness of 50 nm.It is believed that the PCE definitely can be improved by optimizing other parameters,such as nanowire length and core radius.This work provides guidance on the design of high-efficiency solar cells,especially the type-II coaxial nanowire solar cells.(3)Research and fabrication of highly efficient type-Ⅱ planar heterojunction perovskite solar cells.Vapor-assisted solution process method and hot-casting technique were adopted to fabricate the type-Ⅱ heterojunction planar perovskite solar cells at ambient condition.By optimizing the parameters,such as spin coating rate and reacting time,we obtained the best solar cell with the PCE of 11.26%,open-circuit voltage of 0.92 V,short-circuit current density of 20.3 mA/cm2 and filling factor of 0.603.Storing the cell in a dry environment for 2 weeks,the PCE still maintained 76%of its original level.Besides,it was found that a very thin PbI2 on the cl-TiO2 could enhance the cell’s performance;long-term stability test suggested that carefully eliminating the probable moisture could relieve the instability of the perovskite solar cells when exposed to humid air.This work provides information for making high-efficiency stable type-II heterojunction planar perovskite solar cells.