Monograin layer (MGL) solar cell combines the features of a monocrystalline solar cell and a thin film solar cell. The photoactive layer is formed from the kesterite-stannite semiconductor Cu2SnZn(S,Se)4 (CZTSSe) material with the single-crystalline grains embedded into the epoxy resin (Altosaar et al., 2003). With the graphite back contact, cadmium sulphide (CdS) buffer layer, and zinc oxide (ZnO:Al/i-ZnO) window layer, the remainder of the structure resembles a thin film CIS solar cell in the superstrate configuration (Fig. 1).

Cu2SnZn(Se,S)4 monograin layer solar cells are based on a two-dimensional matrix of monograin crystals embedded into the epoxy resin. The rest of the solar cell resembles a thin film CIGS solar cell in the superstrate configuration. Laboratory AM1.5 conversion efficiency of these solar cells reaches up to 5.3% along with the open-circuit voltages up to 660 mV and fill-factors of up to 65%. One way to improve the efficiency is to increase the packing density of the monograins. An alternative is to enhance the properties of the solar cell structure. By using the numerical modelling approach we have investigated the latter possibility. The results indicate the presence of the surface defect layer between the buffer and the CZTSSe monograin absorber layers with the high concentration of the mid-gap defects. The open-circuit voltage seems to be limited by the non-thermodynamic effect. Besides the shallow acceptor doping in the CZTSSe layer, also wider distribution of deep compensating donor traps occurs, which might be responsible for the low fill-factor and for the specific temperature trend of the short-circuit current. A relatively narrow space charge region and a large difference between the absorbed flux and the collected current density points to good prospects for the short-circuit current improvement.

A study of the effects of grain shape, size, and recombination activity in the thin film polycrystalline Cu(In,Ga)Se2 absorber layer on a solar cell’s performance by means of computer-aided simulations in two dimensions is presented. This paper addresses the influence of the grain boundary area with its spatial position and electronic activity on the output performance. Simulation results indicate the highest conversion efficiency of a solar cell when grains are large enough to extend beyond the entire absorber thickness and the surface recombination velocity at the grain boundaries is below 103 cm s−1. The influence of the grain boundaries was found to be most detrimental when placed in the space charge region close to the CdS/Cu(In,Ga)Se2 heterointerface. Simulations indicate that this also holds true for differently shaped grains. The output performance depends primarily on the amount and electronic activity of grain boundaries within the space charge region regardless of the shape of grains.

The purpose of this work was to build a numerical model of thermally activated tunneling transport by upgrading the drift-diffusion transport model in one-dimensional semiconductor simulator ASPIN, and to identify the current limiting mechanisms of the CIS monograin layer solar cells. The superposition of the classical drift-diffusion transport and the semi-classical thermionic-field transport is applied across the whole solar cell structure. The implemented model correctly predicts the shapes of temperature dependent current density–voltage characteristics and the temperature dependence of the photogenerated current density at the short-circuit condition in the range from 320K down to 160K.

We have investigated, by means of numerical simulation, the possible nature of the recombination heterointerface in the monolithic CGS (top)/CIGS (bottom) tandem solar cell. The results show that current transport by tunnelling might be of less importance. On the other hand, a good recombination junction can be achieved by a formation of a semi-metal at the back contact. It is proposed that a thin interlayer of MoSe2, which is of semi-metallic nature, is introduced at the CGS back contact in order to have a good ohmic contact between the two cells in the tandem. Numerical simulation of the tandem structure reveals that there exists an optimal thickness of the top absorber. The most probable limitations of the efficiency of this tandem structure are a low open circuit voltage and a low fill factor.

In this work numerical modeling of Cu(In,Ga)Se 2 (CIGS) thin-film solar cells is presented. The main features of the optical simulator SunShine and the electrical simulator Aspin that are used for solar cell analysis are demonstrated. Modeling of light scattering at rough interfaces in the solar cell structures is described. The two descriptive scattering parameters that are used in optical simulations are the haze parameter and the angular distribution function of scattered light. The results of optical and electrical simulations of a thin CIGS solar cell with an absorber layer thickness of 360 nm are presented. The calculated external parameters of the solar cell are verified with the experimental data.

Band-gap grading in a CIGS absorber and a conduction band offset at n/p hetero-interface are two important parameters of band-gap engineering aiming at high efficient CIGS solar cells. To obtain optimal CIGS absorber's band-gap grading profile an automatic optimization loop based on Nelder–Mead simplex optimization algorithm has been implemented. The optimization problem is described with an objective function, which—by varying the input parameters—is minimized or maximized. In our study two types of objective functions are used; optical and electrical. As the most optimal profile a parabolic, double graded band-gap profile with a positive or nearly zero conduction band offset at n/p hetero-interface is calculated. Structures with different CIGS absorber thicknesses and bulk and/or hetero-interface recombination lifetimes are examined and their optimized parameters are discussed in the light of experimental achievements.

Optical modeling and results of numerical simulation of thin-film Cu(In,Ga)Se2 solar cells are presented. Main features of the developed optical simulator SunShine are explained. An approach to model light scattering at rough interfaces in the solar cell is described. Two types of scattering parameters are used: haze and angular distribution function of scattered light. Simulation results - total reflectance, absorptance in CIGS absorber and carrier generation rate profile - are shown and verified for the solar cell with thin (d=360 nm) CIGS absorber

We developed a numerical procedure, based on Nelder-Mead simplex optimization algorithm, which, by optimizing an objective function, shapes the solar cell absorber's band-gap profile. The minimization of the thermalization power loss and the heat power loss is investigated in order to obtain solar cell with the overall maximum open-circuit voltage and/or maximum output power.

We have investigated, by means of numerical simulation, the possible nature of the recombination heterointerface in the monolithic CGS (top)/CIGS (bottom) tandem solar cell. The results show that current transport by tunnelling might be of less importance. On the other hand, a good recombination junction can be achieved by a formation of a semi–metal at the back contact. It is proposed that a thin interlayer of MoSe2, which is of semi–metallic nature, is introduced at the CGS back contact in order to have a good ohmic contact between the two cells in the tandem. Numerical simulation of the tandem structure reveals that there exists an optimal thickness of the top absorber. The most probable limitations of the efficiency of this tandem structure are a low open circuit voltage and a low fill factor.