Perovskite Materials for Energy and Environmental Applications. Группа авторов

on the DC voltage of the cell. The device conductance G(ω)/capacitance C(ω) at the specified frequency ω may be determined from the real/imaginary parts of the AC cell current:

      When conducted under open-circuit situations, the impedance of a ZnO/a-Si:H(n)/c-Si(p)/Al heterojunction silicon solar cell is quite susceptible to the interface state density D_it. For larger D_it, 10¹⁰ cm⁻²≤ D_it ≤ 10¹² cm⁻², the resonant frequency (maximum of the phase shift) shifts toward higher frequencies. On the other hand, if operated under dark or short-circuit conditions, the D_it sensitivity is low: for instance, the conductance depending on the temperature in the dark changes only for D_it≥10¹² cm⁻² which is related to a change of the band bending at the equilibrium.

Because of a random shift in external parameters, the system’s time response may be determined by using a transient. “.ttd” file detailing the variations, for example, it may simulate the increase and decline of the SPV signal or the spectral integrated PL signal because of a short monochromatic laser pulse.

      If ZnO/a-Si:H(n)/c-Si(p)/Al silicon heterojunction solar cells are excited by a monochromatic laser pulse at a wavelength of 900 nm, the generation of extra carriers occurs only in the c-Si(p) wafer. Hence, the recombination of the a-Si:H/c-Si interface may be efficiently tested. The SPV signal essentially tracks the shift in band bending at the surface because of excessive generation/recombination of carrier, whereas the PL signal is straight linked to excess generation/recombination of carrier. When laser pulse of 10 ns is used, all signals during the pulse do not enter conditions of steady state. Nevertheless, the two signals’ degeneration behavior occur in different time domains. It is inside the range of ms for decay of SPV and within the range of 100 ns for decay of PL. The density of states at a-Si:H/c-Si interface, D_it critically relies not only on the early values immediately after the pulse but also on the transient decay.

      Until now, the AFORS-HET software has been primarily used to (1) determine total achievable amorphous/crystalline solar cells efficiencies, (2) create criteria for designing for such solar cells, (3) establish calculation approaches for controlling a-Si:H/c-Si recombination interface.

1.6 Solar Cell Capacitance Simulator (SCAPS)

      SCAPS is a one-dimensional program for simulation of solar cells established at the University of Ghent’s Department of Electronics and Information Systems (ELIS). It analyzes the behavior and characteristics of solar cell structures numerically. Various measurements of the output parameters of solar cells could be performed by SCAPS. It can calculate the open circuit voltage (V_oc), the short circuit current (J_sc), the output characteristic J-V, the fill factor (FF), the quantum efficiency (QE), the output efficiency of the cell, the generation and recombination profiles, and so on [27–30].

Similar to any other numerical simulation program, SCAPS solves the elementary equations for semiconductors: the Poisson equation, which relate the charge and the electrostatic potential ψ, as well as the electron and hole continuity equations. The maximum length of the cell L is split into N intervals in one direction, and the magnitude of ψi and the concentrations of electron and hole ni and pi in each interval are the unknown variables of the problem. These may be identified by solution of nonlinear 3N equations numerically, i.e., the elementary equations at each of the i intervals. Additionally, instead of (ψi, ni, pi), one can choose ψ_i, E_Fni, and E_Fpi as independent variables. Here, for electrons and holes, E_Fn and E_Fp are the quasi-Fermi energy levels. As the continuity equations include a nonlinear term for recombination in n and p, the basic equations become nonlinear [31–35].

      The electrical characteristics can be determined according to the defined physical structure and conditions of bias. This can be done by the assumption that the device’s function can be approximated into a grid in one dimension composed of a set of grid points also called as nodes. The transport of carriers through the system can be modeled by adding the set of differential equations (Poisson’s equation and continuity equations) on this grid (or the discretization of the equation). The grid with finite element can be used to depict the domain of the simulation.

      SCAPS is a program for Windows, and some of its chief characteristics are classified here as follows:

       It is possible to include up to seven layers of semiconductor to the solar cell device.

       When required, gradation of nearly all physical parameters can be done in a new window.

       Capacity to approximate band diagram in steady state, profile for recombination, and transport of carrier.

       Variable bias of voltage, different temperature, and various illumination options

       Can be used to measure concentrations and currents, J-V, C-f, C-V, and Q-V, AC characteristics and spectral response.

       Calculation of single and batch values.

       Final, as well as intermediate values, graphs, and other significant data, can be obtained as output and can be exported

SCAPS are arranged in several panels where the parameters can be defined by the user. The key panel is the “action panel” (Figure 1.3), in which one can set an operation point (temperature, voltage, frequency, and illumination) and the action list for calculation to be performed (J-V, C-f, C-V, Q(λ)). The running parameter (V, f, or λ) vary in the definite range in each calculation, whereas the values quantified in the operation point are for all other parameters. The user can also view the results earlier calculated directly, i.e., J-V, C-f, C-V, Q(λ), likewise, band diagrams, electric field, densities of the carrier, partial currents of recombination [36–45].

We click on the “set problem” button for the problem definition, i.e., the geometry, materials, and different characteristics of the solar cell. As depicted in Figure 1.4, the Solar Cell Definition Panel opens. In this panel, we can define up to 9 layers of structures. The back contact is the first layer; the last one is the front contact. The properties of intermediate semiconductor layers can be specified by the user. Each layer contains the following semiconductor properties, except for front and back contact:

       thickness (μm),

       electron affinity (eV),

       band gap (eV),

       dielectric permittivity (relative),

       valance band effective density of states (1/cm3),

       conduction band effective density of states (1/cm3),

       hole thermal velocity (cm/s),

       electron thermal velocity (cm/s),

       acceptor shallow density (ND),

       donor shallow density (ND).

Figure 1.3 Action panel of SCAPS.

Figure 1.4 Solar cell definition panel of SCAPS.

      The optical absorption of the semiconductor layers can be taken from a user file. Examples of such user files are distributed with the program: Si.abs,