Читать книгу Perovskite Materials for Energy and Environmental Applications - Группа авторов - Страница 30

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 D_it interface state density a-Si:H/c-Si. A D_it = 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 D_it and needs no contact. The photoluminescence signal is quenched by a growing recombination at interface because of a higher D_it.

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 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.