layer (standard) or layer of crystalline silicon; (d) modules for optical layer: Lambert-Beer absorption or coherent/incoherent multiple reflection.
A suitable series of semiconducting layers and interfaces must be specified before measurement. For example, the properties of the semiconductor, viz. the thin film emitter a-Si:H(n) and the silicon wafer c-Si(p), should be specified. Therefore, the defect state distribution (DOS) must be defined for each layer and interfaces if appropriate. In particular, the boundary contacts must be specified: the TCO surface has been demonstrated as an optical layer for the chosen example (requiring the reflectivity and absorption measured). The contact with TCO/a-Si:H is supposed to be a depleted Schottky contact, whereas the contact’s calculated barrier height represents an input parameter. The metallic c-Si(p)/Al back contact is supposed to be a flat band with a recombination speed of 107 cm/s for simplicity.
Once the external cell parameters (temperature, illumination, and cell voltage or cell current) have been specified, the internal cell results, such as local rates of recombination, densities of carrier, currents, band energies, and so on, can be computed both under steady-state conditions (DC) or by arbitrarily changing external cell parameters using small additional sinusoidal perturbations, and the ratio of amplitude of all these quantities can be observed.
Variation in input parameter and output parameter to monitor can be specified.
In addition, characterization procedures (measurements) can be simulated by changing external variables, as in a specific experimentation and by doing certain postprocessing analysis of the data with the results of internal cells. The following methods of measurement have been used so far: methods of DC-mode characterization: I-V, SPV, QE, PEL, ASSPC, C-V, C-f, C-T; methods of TR-mode characterization: TR-PEL, TR-SPV, intTR-PEL, TR-PC. External users can insert or implement other characterization methods (open-source on request) [24–26].
The resulting total cell current (I-V) can be calculated by variation in the voltage of the external cell at a specified value of illumination. The number of photons emitted because of band-to-band radiative recombination can be computed using the generalized Planck equation from the quasi-Fermi energy splitting within the cell.
ZnO/a-Si:H(n)/c-Si(p)/Al heterojunction silicon solar cells’ solar cell performance is critically dependent on the Dit interface state density a-Si:H/c-Si. A Dit = 1012 cm−2 state density at interface lowers the solar cell’s open-circuit voltage by more than 100 mV. Open-circuit photoluminescence is a quick and nondestructive characterization method susceptible to Dit and needs no contact. The photoluminescence signal is quenched by a growing recombination at interface because of a higher Dit.
Using complex numbers, small values of additional sinusoidal disturbances of the external parameters of the cell are treated. The shift in phase and the ratio of the amplitude between the AC cell voltage and the AC cell current can be computed as dependent on the frequency of perturbation if a small additional sinusoidal (AC) external signal voltage is superimposed 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 Dit. For larger Dit, 1010 cm−2≤ Dit ≤ 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 Dit sensitivity is low: for instance, the conductance depending on the temperature in the dark changes only for Dit≥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, Dit 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 (Voc), the short circuit current (Jsc), 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 ψi, EFni, and EFpi as independent variables. Here, for electrons and holes, EFn and EFp 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
