First, AMPS measure the simple band diagram, built-in potential, electric field, trapped carrier populations, and free carrier populations found in a device if there is no bias (voltage or light) of any kind. These solutions obtained at thermodynamic equilibrium permit to “see what the device will look like.”
AMPS will then use such solutions obtained at thermodynamic equilibrium as starting presumptions for the iterative scheme that will contribute to the full characterization of a device structure under voltage, illumination, or both voltage and illumination bias. AMPS produce output, such as band diagram (which include quasi-Fermi levels), carrier populations, recombination profiles, currents, current-voltage (I-V) characteristics, and spectral response for device structures with different voltage, illumination or voltage and illumination bias [14–16].
Given the range of voltage bias in the window for specifying the conditions for voltage bias, this range of voltages applies to both the dark I-V and light I-V. If the user wishes to get a biased band diagram, he/she can open the selected biased window to give AMPS the value that must (1) lie in the voltage detailed in the previous window and (2) be constant with the previously selected voltage step.
The only distinction at the user input window between light and dark I-V is in the lighting process, the user has to check “light on.” AMPS received AM1.5 by default but the user may specify the photon flux and spectrum. A box called “light-x,” is provided as a neutral filter/concentrator. The absorption coefficient must be manually entered by the user. Yet AMPS provide the user with an alternative in the “Eopt” box to shift the absorption coefficient linearly. Because absorption coefficient information is not easy to obtain most of the time, this linear shift will help the user check the band gap adjustment. You can see details on the user interface.
If the user wishes to see the current produced at each wavelength, the “spectral response” box must be checked. With and without light bias, AMPS will always produce spectral response. The spectrum and the flux in the window of the spectrum determine the light bias. The user-defined spectrum, therefore, defines the range of the QE graph. Users can also adjust the frequency of the probe laser. At the defined voltage bias, AMPS will produce QE.
1.5 Automat for Simulation of Heterostructures (AFORS-HET)
A lot of different experimental approaches are used to analyze (thin film) heterojunction solar cells, varying from traditional techniques for the characterization of solar cell, such as current-voltage characteristics or quantum efficiency to more sophisticated ones, such as surface photovoltage, photo- and electroluminescence, impedance, capacitance/conductance, decay in photoconductance intensity modulated photocurrent spectroscopy or electrically detected magnetic resonance [17–20].
AFORS-HET allows these measures to be interpreted AFORS-HET simulates solar cells with heterojunction, as well as the observables of the related techniques of measurement. A visual framework enables all simulation information to be displayed, stored, and compared. It is possible to perform arbitrary variations of parameters, multi-dimensional parameter fitting and optimization.
AFORS-HET is used specifically to model heterojunction solar cells based on amorphous/crystalline Si with the TCO/a-Si:H(n, p)/c-Si(p, n)/ a-Si:H(p, n)/Al form where ultra-thin (5 nm) a-Si:H sheets of hydrogenated amorphous silicon are mounted on top of a thick (300 μm) crystalline wafer of Silicon. Up to 19.8% of experimental efficiencies have been found.
It is possible to model an arbitrary sequence of semiconducting layers, laying down the properties of corresponding layer and interface, i.e., the defect state distribution (DOS). Utilizing recombination statistics by Shockley-Read-Hall, semiconductor equations are solved in one dimension (1) for thermodynamic equilibrium, (2) for stable conditions under externally applied voltage or current and/or illumination, (3) for small extra sinusoidal externally applied voltage/illumination modulations, (4) for temporary conditions due to general external variations. It is, therefore, possible to determine the internal characteristics of the cell-like band diagrams, local rates of generation and recombination, local currents of the cell, current densities, and phase shifts. In addition, it is possible to simulate a range of characterization methods, i.e., I-V, transient, pulselength, wavelength, intensity, and voltage-dependent photovoltaic surface (TR-SPV, PD-SPV, WD-SPV, ID-SPV, VD-SPV), transient luminescence (TR-PEL, intTR-PEL), internal and external quantum efficiency (QE), impedance/admittance spectrum, spectral-resolved photo, and electroluminescence (PEL) [21–23].
External users can implement new characterization approaches and new numerical segments (open-source on request). To date, these numerical modules have been established: (a) modules for front contact: metal/ semiconductor Schottky- or Schottky-Bardeen- or metal/insulator/semiconductor contact; (b) modules for interface: no interface or drift diffusion or thermionic emission semiconductor/semiconductor heterojunction interface; (c) modules for bulk layer: arbitrary 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