| Improving conversion efficiency and reducing cost are the focus of the research on the solar cell. The light trapping layers and structure can minimize the light reflection and enhance the transmittance, therefore, improve the solar collection efficiency and the overall solar-to-electricity efficiency. However, the light trapping structure, which has been developed in the solar cell, works only at a certain spectral range and at the normal incidence. The omni-directional broadband antireflection characteristic is important, especially in thin film solar cell and third generation solar cell.In this present study, the TiO2single layer, double layers and gradient-index antireflection coatings were investigated. The preparation parameters, deposition model, structure, optical properties of the TiO2thin films and their correlationships have been investigated; The mechanism for the transition of anatase to rutile has been found; The novel methods, which are developed to adjust the refractive index of the TiO2films, have been used to prepare the nanostructured antireflection coatings; The effect of light trapping structure on the impurity photovoltaics effect has been studied. These researches have provided theoretical and experimental support for the development of the high efficient solar cells.TiO2thin films were deposited by direct current reactive magnetron sputtering. The relationship between the deposition model, structure, optical properties and the preparation parameters were investigated. It is demonstrated that the light absorption of the dangling bond, and the scattering of the grain boundaries and surface reduce the transmission of the films. The deposition model of the TiO2thin films is determined by the intensity of the Ti+, TiO+and TiO2+present in the plasma. When the oxygen flow rate is less than3mL/min and the sputtering power is greater than400W, the deposition is dominated by Ti+, this deposition is in the metal model. When the oxygen flow rate increases from3to5mL/min, the deposition is dominated by both Ti+and TiO+, the films are golden yellow. When the oxygen flow rate is bigger than7mL/min, the deposition is dominated by TiO2+and the films are transparent.The films with mixtures of anatase and rutile phases were deposited for the first time. The research on the transition of anatase to rutile demonstrates that the columnar structure restricts the transition, and the crystallite size is doubled by the transition. It is because that the transition of anatase to rutile was carried out in three stages:Firstly, the anatase particles grow large enough to meat each other during the annealing. Secondly, the rutile starts to locally nucleate at the boundary of two anatase particles. Thirdly, the rutile further migrates towards the exterior of the new larger particle, and two anatase particles combine together to form one rutile particle.The oblique angle deposition method was developed to adjust the nano structure and the refractive index of the TiO2thin films. Results confirm that the column particles start to separate from each other at the deposition angle of30°, some separated column particles appeared in the films at deposited angle60°, many holes penetrated through the films at deposition angel of80°. The column structure angle (3has a relationship of β=tan-1(0.347tanα) with the deposition angle a, while the relationship between the films refractive index n and deposition angle is n=2.50619-1.1×10-4α2. The oblique angle deposition methods can prepare the film with a refractive index of1.81～2.53, this controllable refractive index allows the realization of grade-index profile. With this method, the decrease of the grain size increases the optical transmittance in the visible-light region, while the decrease of the nanorod diameter increases the optical transmittance in the ultraviolet region.Properties of the solar cell with the single layer, double layers and gradient-index TiO2based antireflection coating were simulated. The results show that, the solar cell with antireflection coating has a main improvement in the short-circuit current density and the efficiency, and this improvement is more remarkable in double layers antireflection coating. The least weighted average reflection can be acquired is7.8%in single layer TiO2antireflection coating, and an increase of66and65%can be gotten for short-circuit current density and the efficiency respectively. A weighted average reflection of1.5%and4.6%can be got in the SiO2/TiO2and TiO2/TiO2double layers antireflection coating, respectively. When used in the encapsulated solar cell, the gradient-index antireflection coating constructed all by TiO2has a very small weighted average reflection of1.5%. The short-circuit current density and the efficiency have an increase of76%and75%, respectively; This gradient-index antireflection coating has a cubic-index profile, and can work well if the light incidence angle is lower than70°. Used in the non-encapsulated solar cell, its reflection increases due to lake of low index material. After using of SiO2thin films, the antireflection property was improved.The solar cell with dopping of the deep level impurity can absorb more near-infrared photons. The short-circuit current density and the efficiency increase first then decrease as the increase of T1impurity concentration. The highest efficiency was acquired at Nt=ND=1017cm-1. In the impurity photo voltage effect, there are two competing absorption mechanisms for the photons with energy between mid gap and the full band gap. One photon is needed to excite an electron from the valence band to the defect level, and another photon is needed to excite the electron from the defect level to the conduction band. The impurity photo voltage effect of thallium extends the spectral sensitivity in the sub-band gap range from1000nm to about1400nm. If higher values of the light trapping were possible, the IPV effect becomes appreciable (ΔJsc≈9mA/cm2and Δη≈2%). |
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