Device Simulation On Cu（InGa）Se₂Thin Film Solar Cells

As the excellent device performance and the promising manufacture prospect,CuInGaSe₂（CIGS） thin film solar cell is the highest efficiency thin film solar cell,and has become a hot research topic in the Photovoltaic academy field and the most-concerned solar cell by the industry for its high-performance and low cost. Since theabsorber material of CIGS is a complicated compound composed of four elements,and the heterojunction structure is related to the complex interface state and tunnelingmechanism, the inner device principle of CIGS solar cell is difficult to understand,and there are still lots of subjects that need further investigation. Device simulation isa very useful theoretical analysis method, which is one important assistant approachin the high-level solar cell research.For various limitaions, the previous solar cell simulation softwares are notadequate to satify the requirements of the developing CIGS theoretical research. Inorder to better implement the simulation studies on some of the CIGS researchsubjects, this work studies the device physics of CIGS solar cell and updates onepopular simulation code AMPS, and carries out modeling researches on some specifictopics of CIGS solar cell. The improved code is named wxAMPS and has beenpublished for the free download of the international PV community.Lots of theoretical knowledge is involved in the improvements of the simulationsoftware, including semiconductor device physics and numerical modelingtechnology. And only the device mechanisms, such as carriers’ transportationmechanism, defects behaviors, are well understood, the complex mechanisms ofCIGS solar cell can be studied thoroughfully through using simulation toolsefficiently, and the simulation results can be explained correctly. Therefore, thisthesis elucidates the device physics theory and numerical simulation method at firstchapters.In Chapter1, the thesis introduces the frontier progress of CIGS solar celltheoretical research, summarizes a variety of popular modeling software whosefeatures are compared, and explains why AMPS is chosen to be revised and updated. The general solar cell device physics is described in Chapter2. According to thequasi-neutral region assumption which is applied to the PN junction analysis in thesemiconductor physics, the current-voltage characteristic under dark and lightsituations are deduced by the analytical method. The effects and the fitting method ofthe series resistance and shunt resistance are also discussed. In Chapter3, theanalytical approach is applied to study the CIGS solar cell characteristics, analyzesthe dependence of open-circuit voltage to the temperature, of the quality factor to thetemperature, the relationship between the activation energy of saturation current andthe band gap of CIGS material, the Fermi-level pinning phenomena at heterojunctioninterface, and the effects of the trap-assisted tunneling and the Ga back grading to thedevice performance.Assumptions and simplifications are required in the analytical approach toobtain the closed-form solution of the models and fulfill the analysis. But theconditions of these assumptions may not be satisfied in complicated devicemechanisms. Hence, numerical methods are needed for more accurate modeling.Chapter4describes the theory fundamentals of implementing numerical simulationfor the solar cell modeling in detail, and elucidates the defect mechanisms and thenumerical solution for varieties of defect types in thin film solar cell. The tunnelingmodels and optical models used in the solar cell simulation are also explained andcompared.Through studying the basic modeling theory of thin film solar cells andconsidering the specific requirement of CIGS solar cell simulation, wxAMPS makesup for the shortcomings of AMPS which does not consider the tunneling effects, addstwo tunneling models to enhance the code capability to better analyze the physicalmechanism in thin film solar cells. Moreover, wxAMPS improves the solvingalgorithm of the model by combining the Newton iteration method and the Gummeliteration method, and ameliorate the stability and the convergence property of thecode. The wxAMPS softward is also capable of simulating the effects of arbitraygrading of material parameters to the device performance.By using wxAMPS, Chapter5applies the numerical modeling techniche to themechanism studies of CIGS solar cell and compares the experimental resutls, in order to reveal the micro mechanisms behind the macro experimental observations. Thiswork simulates and analyzes the effects of carriers density, thickness of CIGS thinfilm material, studies the device response to the low temperature and low irradiance,and explaines the trend in simulation results by analytical modeling. The simulationresearch found that, it is the effect of CdS/CIGS hetero-interface barrier that causesthe series resistance of CIGS solar cell to increase non-linearly when temperaturedecreases below300K. This thesis proposes a new opinion and theoreticalexplanation for this issue that is still in dabate in the academic field. This chapter alsostudies a new modeling approach that can analyze the effects of arbitray Ga gradingto the cell performance. Based on the measurement Ga gradient data and generating adevice-modeling file that is compatible to wxAMPS, the effects of the experimentalmeasured Ga grading can be analyzed theoretically, and more inner information ofthe device details can be revealed by comparing the experimental data and modelingresults.At last, the main work and the achievements in the Ph.D. project are concluded,and future developments of wxAMPS and the device modeling of CIGS solar cell areforesighted as well. This thesis explicates the latest theoretical understanding on theCIGS solar cell, and the contribution of the work in this dissertation to this field.