Realistic Multidimensional Optoelectrical Modeling Guide for Copper Indium Gallium Diselenide Solar Cells

Abstract

Herein, an optoelectrical model is presented for copper indium gallium diselenide (CIGSe) solar cells in COMSOL Multiphysics, capable of multidimensional simulations, and it is applied to ultrathin (500 nm absorber thickness) solar cells. First, the modeling approach is shown. Special attention is paid to back contact materials, interface states, and defect application and their impact on the current–voltage (J–V) characteristics. To address whether the back contact is Schottky or Ohmic, the influence of the Schottky barrier height, recombination velocity, and interface states is shown. Then, the additional application of an acceptor defect gradient at the absorber back and a donor defect density distribution at the p–n junction is investigated. The results of these parameter adjustments are discussed, and the trends are shown to enable fast fitting of experimental J–V curves. Finally, the results are compared to the experimental J–V curves for indium tin oxide and Mo back contact, and challenges encountered are discussed while fitting. The optoelectrical model for CIGSe solar cells, established in two dimensions here, paves the way for comprehensively describing 2D and 3D solar cell structures, e.g., nanotextured or microsolar cells, as well as for considering different absorber thicknesses.

Influence of Bulk Defect Density on the Efficiency of CIGS Based Photovoltaic Cell/Solar Cell

“…In the four-contact point configuration, two points inject current while the other two points measure potential difference using the LCR meter [16]. By knowing both the injected currents and voltages at specific film thickness, resistivity or impedance can be computed using the Van der Pauw technique [3]. Its complex impedance, Zx is then computed using:…”

“…It determines how photon absorption and charge carrier generation takes place [19]. When gallium (Ga) is introduced into CIS absorber to form CIGS by gallium levels between x = 0 and x =1, its band gap widens from 1.021 eV to about 1.672 eV [3,9]. It is a clear indicator that the introduced bulk defects influence band gap.…”

“…forming a CuIn1-xGaxSe2 composite. Parameters such as absorption coefficient [3], electron affinity [7] and bulk defect density [15] are adjusted by some correction factor that can also depend on bulk defect density. The density of bulk defects is increasing becoming a key factor to consider when analysing the limits by which CIGS photocells can perform even at higher gallium concentration levels that may be above the theoretical values.…”

“…The spacing between tip points was each varied at fixed values of 1mm to 5 mm and resistivity determined as a function of frequency, thickness and type of configuration. Five points were tested for frequency ranges from 5×10 3 Hz to 4×10 4 Hz. Two different film with thicknesses 3.3 μm and 5.2 μm were chosen and were tested both in aligned and in square configurations.…”

“…It is the most important layer [1] of the cell and it is usually covered by an n-type buffer layer to provide a P-N junction interface [2]. Most of these heterojunction devices use a thin layer of CdS on top of the buffer layer with a transparent conductive oxide of a higher resistivity forming a third thin layer [3]. This complete assembly is then deposited on a recommended substrate to offer mechanical support.…”