Unusual photocapacitance properties of a mono-crystalline silicon solar cell for optoelectronic applications

The current-voltage characteristics of mono-crystalline solar cell device under dark and illumination of 100 mW/m² (AM1.5) were measured. The efficiency of the studied device under AM1.5 was found to be 14.22%±0.2 compared with the company standards. The capacitance properties of mono-crystalline silicon solar cell device were investigated under dark and illumination conditions. The studied mono-crystalline silicon solar cell exhibits an unusual photocapacitance ranging from 50.4 to 4585 nF under dark and 100 mW/m² (AM1.5) of white light, respectively. The drastic increase in the capacitance of the solar cell is due to the space charge polarization induced by the increasing number of photogenerated carriers. The photocapacitance mechanism of the solar cell was interpreted by modified Goswami and Goswami (MGG) model. The relative capacitance C_ph/C_d (the ratio between the capacitance under illumination to the capacitance under dark) and the relative resistance R_ph/R_d (the ratio between the resistance under illumination to the resistance under dark) as a function of the applied frequency at different illuminations were interpreted. The values of the interface state density N_ss and interface capacitance C_ss are increased with the increasing illumination intensities. The prepared mono-silicon solar cell device is a good candidate for photocapacitive and photoresistive sensors in modern electronic and optoelectronic devices.     

Measurement of silicon and GaAs/Ge solar cells ac parameters

The ac parameters (cell capacitance and cell resistance) of Silicon (Si) and Gallium Arsenide (GaAs/Ge) solar cells are measured at different temperatures using time domain technique. The cell capacitance is calculated from the Open circuit voltage decay (OCVD) and the cell resistance from solar cell I–V characteristics measured under dark condition. It is observed that the solar cell capacitance increases whereas the cell resistance decreases with increase in temperature.     

Silicon (BSFR) solar cell AC parameters at different temperatures

The AC parameters of back surface field refiected (BSFR) silicon solar cell are measured at different cell temperatures (198–348 K) both in forward and reverse bias under dark condition using impedance spectroscopy technique. It is found that cell capacitance increases with temperature whereas cell resistance decreases, in forward bias voltage. Beyond maximum power point voltage, the cell inductance (0.28 μH) is measured, as the inductive reactance is comparable with cell series resistance. The measured cell parameters (cell capacitance, dynamic resistance, etc) are used to calculate the mean carrier lifetime and diode factor at different cell temperatures.     

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.     

GaAs/Ge solar cell AC parameters at different temperatures

The AC parameters of Gallium Arsenide (GaAs/Ge) solar cell were measured at different cell temperatures (198–348 K) by varying the cell bias voltage (forward and reverse) under dark condition using impedance spectroscopy technique. It was found that the cell capacitance increases with the cell temperature where as the cell resistance decreases, at any bias voltage. The measured cell parameters were used to calculate the intrinsic concentration of electron–hole pair, cell material relative permittivity and its band gap energy. The diode factor and the cell dynamic resistance at the corresponding maximum power point decrease with the cell temperature.     

Measurement of CuInSe2 solar cell AC parameters

The AC parameters (cell capacitance and cell resistance) of Copper Indium Diselenide (CuInSe₂) solar cell are measured using time-domain technique. The cell capacitance is calculated from the open circuit voltage decay (OCVD) and cell resistance with solar cell I–V characteristics measured in dark. The solar cell exhibits high parallel resistance and low parallel capacitance. The doping concentration and built in voltage are derived from the 1/C_P² versus bias voltage graph. The built-in voltage of the solar cell shows good agreement with measurements published in the literature.     

GaAs/Ge and silicon solar cell capacitance measurement using triangular wave method

The capacitance of GaAs/Ge and silicon (BSFR) solar cells are measured at different temperature ranging from 288 to 338 K under dark condition using triangular wave method. It is a frequency domain technique. In the proposed method, the solar cells are biased externally using DC voltage at the desired operating voltage and the AC triangle wave small signal of desired amplitude with variable frequencies are applied. The resultant AC current of the device is measured and the cell capacitance is calculated. GaAs/Ge solar cell has shown only transition capacitance throughout its operating voltage while silicon (BSFR) solar cell exhibited both transition and diffusion capacitances. It is a direct and simple measurement technique in comparison to impedance spectroscopy and other bridge methods.     

Study of silicon solar cell at different intensities of illumination and wavelengths using impedance spectroscopy

The electrical properties of an n⁺–p–p⁺ structure-based single-crystalline silicon solar cell were studied by impedance spectroscopy, I–V and spectral response. The impedance spectrum is measured in dark, under different intensities (14, 43, 57, 71, 86, 100 mW/cm²) of illumination and wavelengths (400–1050 nm) of light. Under dark and at low intensities of illumination (<50 mW/cm²) the impedance spectra show perfect semicircles but at high intensities the semicircles are distorted at low frequencies. It is found that illumination provides an additional virtual R₁C₁ network parallel to the initial bulk R_pC_p network observed under dark conditions. The value of virtual resistance R₁ depends on the illumination wavelength and shows an inverse relationship with the spectral response of the device.     

Depletion layer characteristics of Al/microcrystalline chlorophyll a/Ag sandwich cells

The capacitance measurements of Al/microcrystalline chlorophyll a/Ag sandwich cells have been carried out at various frequencies in the dark and under illumination. The voltage-dependent capacitance, C, and the linear Schottky plots obtained at only low frequencies, 0.1 Hz, and under illumination suggest that the depletion layer mainly consists of trapped charges that are not able to follow the variations in applied voltage at high frequencies 100 Hz. From the linear Schottky plots, the depletion parameters, e.g., built-in potential, width at zero bias, and space charge density have been determined in the dark and under illumination (11 μW cm⁻²), which are, respectively, 620 mV, 300 Å and 2 × 10²³ m⁻³, and 700 mV, 205 Å and 4.78 × 10²³ m⁻³. The ageing of the cells in ambient air has been found to have dramatic effect on the capacitance characteristics of the cells. The diffusion of oxygen and presence of water vapour are most probably responsible for an inhomogeneous space charge developing within the depletion layer and the decrease of space charge density with time, respectively.     

A review and comparison of different methods to determine the series resistance of solar cells

This work presents a review of five different methods to determine the lumped series resistance R_S of solar cells and an experimental investigation of these to find the most reliable and robust method(s) for cell characterization under operating conditions. The methods under consideration are: fitting of the two-diode equation function to a dark IV-curve, comparison of a one-sun with a dark IV-curve, comparison of a Suns-V_OC with a one-sun IV-curve, comparison of two or more IV-curves measured at different illumination intensities, and computation of the area under a one-sun IV-curve. Firstly, for a quantitative evaluation, all series resistance values were plotted against the fill factor FF of the corresponding cell. The accuracy of the methods is quantified using a wide range of solar cells. Secondly, the robustness of the methods in the presence of other FF-limitations such as shunts is also explored. The results and the interpretation of a first analysis of small 2×2 cm² solar cells of the integration method led to a successful improvement of this method, which was proven by a second measurement. All the conducted investigations led us to the conclusion that of these five methods under consideration, the illumination intensity variation, the comparison of a Suns-V_OC with a one-sun IV-curve, and the modified comparison of a one-sun IV-curve with a dark IV-curve method are the most reliable and robust ways to determine the series resistance under operating conditions.     

Blue-photon modification of nonstandard diode barrier in CuInSe2 solar cells

The current–voltage curves of many ZnO/CdS/CuInSe₂ solar cells display significant distortion when only red light illumination is employed. This distortion generally disappears or partially disappears for a period of time following illumination with blue light. Similarly, the dark diode curve shortly after illumination containing blue light is shifted significantly from the equilibrium dark curve. This effect is more common than generally realized and indicates a mechanism that is potentially detrimental to photovoltaic efficiency. A model is presented that is based on a low free-electron concentration and a high concentration of deep levels in the CdS window layer. This model is consistent with observed variations in current–voltage, capacitance, and laser scan data with illumination wavelength and history.     

Photodiodes based on graphene oxide–silicon junctions

Schottky barrier diode based on graphene oxide (GO) with the structure of Al/GO/n-Si/Al was fabricated. The current–voltage characteristics of the diode were investigated under dark and various light intensity. It was observed that generated photocurrent of the diode depends on light intensity. Various junction parameters were presented using I–V characteristics. The transient photocurrent measurement indicated that the Al/GO/n-Si/Al diode was very sensitive to illumination. The photocurrent of the diode increases with increase in illumination intensity. The capacitance–voltage–frequency (C–V–f) measurements indicated that the capacitance of the diode depends on voltage and frequency. The capacitance decreases with increasing frequency due to a continuous distribution of the interface states. These results suggest that the Al/GO/n-Si/Al diode can be utilized as a photosensor.     

Capacitance measurements of photovoltaic cells based on mixed monolayers of chlorophyll a and sulfoquinovosyldiacylglycerol

The capacitance measurements of Al/Chlorophyll a/Ag and Al/Chlorophyll a-Sulfoquinovosyldiacylglycerol/Ag sandwich cells have been carried out at different frequencies in dark and under illumination. The results show that while a voltage-dependent capacitance and a linear 1/C² versus V_a plot is obtained for chlorophyll a (Chl a) at low frequencies, the addition of sulfoquinovosyldiacylglycerol (SQDG) in proportions 0.25 results in totally voltage-invariant capacitance. The capacitance characteristic of Chi a-SQDG (0.25) cells resemble more those for an insulator than those for a Schottky barrier. These results are explained in terms of negatively charged SQDG playing a role of traps which immobilize the holes. Alternatively, the interaction of negative polar head group of SQDG with Mg²⁺ of Chl a can also be invoked as a possible reason for the observed results. The bound Chl a- SQDG species possibly does not form a Schottky barrier or forms a weak barrier with either Al or Ag electrodes, resulting in insulator-like capacitance characteristics of Al/Chl a-SQDG(0.25)/Ag cells.     

Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy

The photocurrent-voltage characteristic of a photoelectrochemical cell for solar hydrogen production via water splitting, using undoped-hematite as photoanode, was obtained. Photoelectrochemical characteristics of the cell were also investigated by electrochemical impedance spectroscopy. Both techniques were carried out in the dark and under illumination. The analysis of the frequency spectra for the real and imaginary parts of the complex impedance allowed obtaining equivalent electrical analogs for the PEC cell operating in the dark and under 1 sun simulated illumination. Additionally, different electrode configurations were used (two and three-electrode arrangements). The two-electrode configuration allowed the study of the overall charge transfer phenomena occurring at the semiconductor, within the electrolyte and at the counter-electrode side of the cell, whereas the three-electrode configuration gave more detailed information concerning the double charged layer at the semiconductor/electrolyte interface.     

Spatially distributed model for the analysis of laser beam induced current (LBIC) measurements of thin film silicon solar modules

A 3D distributed model is developed and implemented based on circuit analysis software for the investigation of spatial variation in performance due to the distributed nature and non-uniformity of solar cell properties. This is applied to LBIC measurements where it is used for sensitivity analysis of the measurements with respect to certain parameters in series connected thin film PV modules.The model is used to explain the differences in dark and illuminated measurements, which clearly shows the illuminated LBIC signal is largely dependent on the homogeneity of the background illumination. The dark LBIC is largely affected by the shunt resistance of the neighbouring cells rather than by the signal strength of the cell under test. It is required to bring the cell into limiting conditions, which then gives a signal one order of magnitude stronger than that in the non-limiting case. The simulations are validated against measurements taken in these regimes.     

Photoconductive Schottky diode based on Al/p-Si/SnS2/Ag for optical sensor applications

The Schottky barrier junctions of tin disulfide (SnS₂) on p-silicon were fabricated using sol–gel spin technique. The photoresponse and junction properties of the diode were investigated. The ideality factor and barrier height of the Al/p-Si/SnS₂/Ag diode were obtained to be 1.54 and 0.53 eV, respectively. The photocurrent properties of the device under various illuminations were also explored. The photocurrent in the reverse bias voltage is increased by increasing photo-illumination intensity. The transient photocurrent results indicate that photocurrent under illumination is higher that the dark current. The capacitance–voltage characteristics of diode were also investigated at different frequencies. The capacitance decreases with increasing frequency due to a continuous distribution of the interface states. These results suggest that the fabricated diode can be used for optical sensor applications.     

Measurement and comparison of AC parameters of silicon (BSR and BSFR) and gallium arsenide (GaAs/Ge) solar cells used in space applications

The AC parameters of silicon (BSR and BSFR) solar cells and GaAs/Ge solar cell have been measured using impedance spectroscopy. Each cell capacitance, dynamic resistance and series resistance were measured and compared. GaAs/Ge solar cell has shown only the transition capacitance throughout its operating range while silicon (BSR and BSFR) solar cells exhibited both transition and diffusion capacitance. The theoretical and experimental values of dynamic resistance were compared and found in good agreement while the diode factor in silicon solar cells varies from 2 to 1, where as in GaAs/Ge solar cell it varies from 4 to 2 to 1.     

Dielectric and conduction studies on hot-wall deposited CdSe films

CdSe films are deposited using hot-wall deposition technique on glass and ITO substrates. From the XRD analysis, the structural parameters like crystallite size, dislocation density and strain were calculated. Films had preferential orientation along (0 0 2) and the structure of the film corresponded to wurtzite nature. From the EDAX analysis a slight increase in the cadmium content is observed as thickness increases. The dielectric study has been carried out on the stoichiometric films at different frequencies and temperatures to study their effect on capacitance, dielectric constant and dielectric loss. To explore the effect of illumination on these fundamental dielectric parameters, measurements are taken in dark as well as under an illumination of 1000 lx. and observed slight variation in these parameters. The temperature coefficient of capacitance, relative permittivity and linear expansion coefficient are evaluated. From the AC conduction studies the conduction was found to be due to hopping. Variation of conductivity with temperature reveals the presence of two activation energies. The Mott–Schottky plot for the films yields the value for carrier concentration in the range 10¹⁷–10¹⁸ cm⁻³ and the conduction was found to be n-type.     

Fill factor losses in ZnO/CdS/CuGaSe2 single-crystal solar cells

Current–voltage characteristics of ZnO/CdS/CuGaSe₂ single-crystal solar cells with solar conversion efficiency values of η=3.5%, 6.0%, 6.7% and 9.7% were analyzed using the single diode equation. The effect of each of the achieved parameters on the fill factor was calculated. The calculations revealed that the fill factor reduction due to the series resistance remained below Δff=4.4% under illumination, while this effect would have been much higher if the illumination had not reduced the series resistance markedly. The calculation furthermore revealed that the fill factor reduction due to the shunt resistance remained below Δff=3.6% under illumination. This effect would have been negligible if the illumination had not also reduced the shunt resistance in all studied cells. The increase of the saturation current density under illumination has brought about considerably high fill factor losses (at least Δff=8.3%) in all studied cells. Already the dark saturation current density and the diode ideality factor in such cells have been found to be much higher than the ones in the cells based on CuInSe₂. This seems to be the most substantial restriction to the fill factor, and thus to the performance, of solar cells based on CuGaSe₂. An explanation for this different behavior seems to lie in the different band structures of these cells.     

Measurement of solar cell parameters using impedance spectroscopy

Measurement of solar cell parameters is important for the design of satellite power systems. These parameters can be measured using impedance spectroscopy and an equivalent circuit model developed. In this study parameters of a Back Surface Reflector Field solar cell (BSFR) have been measured using impedance spectroscopy. The results show high diffusion capacitance of BSFR cells and their exponential relation to the operating voltage. Cell dynamic resistance, diode factor, transition capacitance, and series resistance could also be measured. The minority carrier life time also has been calculated.