Optimum Design Of Nanocrystalline Silicon Solar Cells And Study Of I-V Explicit Formulation

The solar cells which is made by nc-Si:H thin film are the most important points forthe third generation solar cells. In this paper, we shall discuss the effects of the band gap,the thickness of intrinsic layer and the back surface field on performance of nc-Si/c-Si HIT(Heterojunction with an Intrinsic Thin layer) solar cell. The results showed that with theincreasing of doping density in p layer, the electric field in most region of this layer becomestronger. Meanwhile, the electric field helps to improve the solar cell efficiency because theinner electric field in p layer, the short circuit current and open circuit voltage of solar cellsare all increased. The thickness of intrinsic layer has significant influence on solar cells.The battery efficiency (η) and fill factor (FF) decrease sharply when the thickness is morethan30nm. Based on above analysis, the optimal thickness of intrinsic layer is10nm.Finally we have analyzed the effect of a-Si back surface field on efficiency of solar cells.The battery efficiency improved from21.677%to24.163%after adding a-Si back surfacefield, the thickness of which is10nm.The principle of AMPS simulation software is calculation of the Poisson equation,electron continuity equation and hole continuity equation with the finite element methodand the Newton-Raphson algorithm. To calculate the self-consistent solution according tothe properties of materials and boundary co nditions of devices. The simulated I-Vcharacteristic curves are all based on the ideal case. The I-V equation of solar cells, whichwas derived from solid state physics theory, has became the research foundation of solarcell output performance. But this I-V equation belongs to the implicit equation. The opencircuit voltage and short circuit current cann’t be explicitly expressed by the elementaryfunctions. So we use some approximate methods to avoid the tedious calculation processand derivation when we use the implicit equation to study solar cells. In order to get theexplicit formula, we add the LambertW function in the I-V equations. According to the dataof four types of solar cells, we verificate the accuracy of the explicit formula by MATLABsoftware, we have drawn some I-V curve of cells. The results prove that the explicitformula can accurately express the I-V character of solar cells.In the fifth chapter, we have described how to derive the optimum load for solar cellsby the explicit formula which is calculated above. The derivation process is particularly complicated and requires very high math skills when we use implicit equations. Becausethe equation contains the short circuit current and open circuit voltage on the conditions ofmaximum power, it’s very difficult to calculate the optimal value of load directly. However,through the explicit formula of voltage and current, we could get the optimum loadexpression formula conveniently according to the optimum load definition. We can alsocalculate the optimum load value by these model parameters. Finally, we have analyzed theinfluences of series resistance, parallel resistance, ideal factor, light current and reversesaturation current on the optimum load.