| The solar cells using a new type of structure can reduce the cost and improve the conversion efficiency,has become a hot research, especially the heterojunction thin film solar cells. Comparing TiO2 with CuInSe2(CuInS2), heterojunction thin film solar cells devices of low cost and high conversion efficiency is built.CuInSe2 is a multi-compound p-type semiconductor. Its photoelectric conversion efficiency is as high as19.5% and optical absorption coefficient is up to 105 orders of magnitude, which is in favor of photon absorption and the minority carrier collection in the solar cells base area. TiO2 is a kind of non-toxic and wide band gap n-type indirect semiconductor materials, and its chemical properties is excellent,which has a strong oxidizing ability and a resistance of strong acid and alkali. The compound of TiO2 and CuInSe2 can form a PN heterojunction, which can promote the separation of photo-generated electron-hole pairs, and reduce the average diffusion length of minority carrier, thereby reducing the recombination and lastly increasing the charge collection efficiency. Moreover, the compound is able to reduce the purity requirements for CuInSe2 material. Currently CIS/TiO2 composite thin film solar cells, reaches the energy conversion efficiency of 7%, and has broad application prospects in the all-solid-state thin film solar cells.In the CIS/TiO2 composite thin film solar cells, the PCE increases from 0.2% to 2.8% by improving the charge transmission in the TiO2 thin film. On the basis, when In2S3 is added as a buffer layer between TiO2 and CIS, the PCE is up to 7%. As can be seen, interfacial properties might be able to determine the device performance. Therefore, it is necessary to make a study of photo-charge separation, transport and recombination process in the interface.Most studies of TiO2/CIS solar cells are based on TiO2 nanoparticles, but the one-dimensional structure is more conducive to transmission. Here, on the basis of FTO, we prepared one-dimensional Ti O2 nanowire arrays by the method of hydrothermal synthesis. The CIS was deposited on the nw-TiO2 array films using the one step electrochemical method, thus forming the heterojunction structure. In order to study the influence of the interfacial properties on the charge separation and transport, we conducted the following research work.(1) The alcohol sol of TiO2 was coated on the FTO using a spin coating method and the TiO2suspension(P-25) was deposited onto FTO substrates at room temperature by the doctor blade techniqueafter annealing. By field emission scanning electron microscope(FSEM), steady-state surface photovoltage(SPV) and phase spectrum, electrochemical impedance spectroscopy(EIS), the effect of TiO2 compact layer on the charge transmission in the interface between TiO2 nanoparticle films and FTO substrate.Experimental results show that TiO2 compact layer improved the interface charge transmission from TiO2 nanoparticle films to FTO substrate, and reduced the recombination.(2) The anatase and the rutile nw-TiO2 arrays were grown on transparent FTO substrates with compact layer by a hydrothermal method respectively. By field emission scanning electron microscope(FSEM),steady-state surface photovoltage(SPV) and phase spectrum, Kelvin probe(Kelvin) and X-ray diffraction(XRD), the crystal structure, morphology and optical properties of the TiO2-nw composite film were characterized and analyzed. Morever, the interface band structure of CIS/TiO2 composite film and the charge transmission process were studied. Experimental results show that: different growth environment of PH value can influence the surface work function of the composite film, thus affecting the interface/surface photo-generated charge separation and transmission.(3) Using one-dimensional charge diffusion model and under certain assumptions, the relationship derivation between excess charge carrier concentration and time, location was made. Combined with the Poisson equation of electric field, the transient response was obtained. Since the process is complex, we can only get help from mathematical software(MATLAB) for numerical analysis. Under the simplistic conditions, the influence of different factor on the transient response was analyzed and the effects of TiCl4 solution on photo-generated charge transmission in the TiO2 film was studied. |
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