The analysis of light trapping and internal quantum efficiency of a solar cell with DBR back reflector

A theoretical analysis of the total internal quantum efficiency (IQE) of a flat-band p-n homo-junction silicon solar cell with back reflector using distributed Bragg reflectors to improve the light trapping is presented and contributions of different regions of the structure to IQEs are simulated. An optical model for the determination of generation profile of the cell is adopted and multiple light passes are considered and compared to previous single light pass approach. It is found that the spatial widths of the cell, the surface recombination velocities, the front surface transmittance and the back reflector have significant impacts on the IQEs. With two light passes and normal incident light, the simulation result shows the IQEs can be increased over the one pass value by 6.34% and with a 60° light reflection angle, the IQEs can be further increased by 9.01% while assuming the reflectance at back structure closed to 100%. The effect on IQEs by back reflectance is more significant than that by front transmittance. Under multiple light passes simulation, up to 51 light trapping passes have been considered at wavelength range 900-1100 nm, the cell IQEs can be enhanced by about 26.98%.     

Internal quantum efficiency analysis of solar cell by genetic algorithm

To investigate factors limiting the performance of a GaAs solar cell, genetic algorithm is employed to fit the experimentally measured internal quantum efficiency (IQE) in the full spectra range. The device parameters such as diffusion lengths and surface recombination velocities are extracted. Electron beam induced current (EBIC) is performed in the base region of the cell with obtained diffusion length agreeing with the fit result. The advantage of genetic algorithm is illustrated.     

Analysis of thin film surface barrier solar cells with a microrelief interface

The influence of a microrelief interface between a thin conductive film (emitter) and a semiconductor substrate (absorber) on the optical and recombination losses in surface barrier solar cells is analyzed.Equations for the calculation of monochromatic light transmission through a thin absorbing film with one or two rough surfaces, on an absorbing substrate, are presented. For a weakly texturized interface and a normal direction of the incident light, the influence of the microroughness is taken into account by the formulae obtained. A model of patches with microsurfaces parallel to the structure surface and ones inclined to it at a certain angle (the most probable statistically) was used to describe the experimental results. Au/GaAs surface barrier structures with interface microrelief of a quasigrating or a dendritic type, obtained by chemical anisotropic etching of GaAs, were investigated. The geometric and statistical parameters of the microrelief were determined by atomic force microscopy techniques. The recombination parameters of the textured interfaces were determined from the experimental spectra of the external quantum efficiency, taking into consideration the calculated spectra of light transmittance. A comparison of the internal quantum efficiency spectra with the calculated ones allows a determination of the electronic parameters of the interface. Both the photoconversion parameters under AM0 simulated illumination and the tolerance for ⁶⁰Co γ-irradiation, obtained for structures with various interface microrelief morphologies, allowed identification of microrelief of the quasigrating type as more suitable for solar cell applications.     

Efficiency improved by acid texturization for multi-crystalline silicon solar cells

In this paper, we will show that efficiency of multi-crystalline silicon (mc-Si) solar cells may be improved by acid texturization. In order to enhance overall efficiency of mc-Si for solar-cell applications, the surface treatment of texturization with wet etching using appropriate solutions can improve incident light into the cell. Alkali etchant cannot produce uniformly textured surface to generate enough open circuit voltage (V_OC) and high efficiency of the mc-Si due to the unavoidable grain randomly oriented with higher steps formed during etching process. Optimized acid etching conditions can be obtained by decreasing the reflectance (R) for mc-Si substrate below levels generated by alkali etching. Short-circuit current (I_SC) measurements on acid textured cells reveal that current gain can be significantly enhanced by reducing reflection. The optimal acid etching ratio HF:HNO₃:H₂O = 15:1:2.5 with etching time of 60 s and lowering 42.7% of the R value can improve 112.4% of the conversion efficiency (η) of the developed solar cell. In order to obtain more detailed information of different defect region, high-resolution light beam induced current (LBIC) is applied to measure the internal quantum efficiency (IQE) and the lifetime of minority carriers. Thus, the acid texturing approach is instrumental to achieve high efficiency in mass production using relatively low-cost mc-Si as starting material with proper optimization of the fabrication steps.     

The use of the self-calibration method for accurate determination of silicon solar cell internal quantum efficiency

The application of the self-calibration method as a means of increasing the accuracy of spectral response, SR, and internal quantum efficiency, Q(λ), measurements is discussed. The principle of the method is the precise calculation of Q(λ_m) of a test cell for light at λ_m≈0.8 μm, where the response is weakly dependent on the emitter and base parameters. The ratio of the calculated and measured Q(λ_m) values gives the corresponding factor for shifting the experimental spectral response curve. The sequence of calculations is described, and an algorithm of the necessary operations for a computer is developed. Several examples of the use of the self-calibration method for correction of SR measurements of solar cells with low shunt resistance demonstrate its very high effectiveness. The corrected Q(λ) values follow the respective actual data with very high accuracy even when the measured SR is decreased by factor 2–3 due to low shunt resistance of the solar cell.     

Self-calibration as a tool for high-accuracy determination of silicon solar cell internal quantum efficiency

Light nonuniformity, uncertainty in the illuminated photoactive area, and relative, but not absolute radiometric data for the reference detector, can be the reasons for the inaccuracy or impossibility of solar cell spectral response and quantum efficiency determination. The use of a self-calibration principle permits minimization of the errors caused by the above factors. This principle consists of quite precise calculation of the internal quantum efficiency Q(λ_m) of the test cell at λ_m≈0.8 μm, where the cell response is weakly dependent on emitter and base parameters. Experimentally determined short- and long-wavelength internal quantum efficiencies, Q(0.4) and Q(0.95), respectively, based on relative radiometric data for a reference detector, are used as starting data for the Q(λ_m) calculation. The ratio of the calculated to measured Q(λ_m) values gives the correction factor for shifting the experimental quantum efficiency curve. Computer modeling supports the assumption that uniform deviation of measured Q(λ) can be precisely corrected by calculation. Analysis of the accuracy of the self-calibration method demonstrates very small uncertainties in the corrections of quantum efficiency measurements, attainable for many practical situations. Confirmation of correctness of the proposed method is shown by analysis of the results of spectral response measurements of several solar cells.     

A contactless photoconductance technique to evaluate the quantum efficiency of solar cell emitters

A new technique, the spectral response of the steady-state photoconductance, is proposed for solar cell characterisation in research and development. The emphasis of this paper is on the evaluation of the carrier collection efficiency of the emitter region based on a simple, two-wavelength approach. We show that in addition to the well-established determination of the wafer recombination properties that results from a long-wavelength photoconductance measurement, detailed emitter quantum-efficiency information can be obtained by performing a second measurement with short-wavelength light. The method is experimentally demonstrated with silicon solar cell precursors having emitters with markedly different levels of surface and bulk recombination losses. The main advantages of the spectral photoconductance technique are that it is fast, contactless, and can be used immediately after junction formation before metallisation; these properties make it very appropriate for routine monitoring of the emitter region, including in-line process control.     

The analysis of light trapping and internal quantum efficiency of a solar cell with grating structure

A multiple light paths analysis of the internal quantum efficiency (IQE) of a silicon solar cell with back reflector using grating structure to improve the light trapping is presented and the contributions of diffusion length of base regions to IQEs are simulated. An optical model for the determination of generation profiles of the cell is adopted and for a refractive index n material up to 4n² light paths are considered and compared with no light trapping structure. It is found that the spatial widths of the cell, the increase of diffusion length, the diffraction angle distribution, number of light trapping paths and transmitted angle can significantly affect the IQE_b for lower absorption wavelength (i.e., 1000-1100 nm). With light trapping paths, the simulation results show that the best IQEs (≈IQE_b) with transmitted angle can reach up to 73% which is 14% more than that with normal incident, the best achievable IQE_b with grating structure is 49% at cell thickness w_b = 50 μm, and the IQE_b with diffraction angle θ_m = 60° is 9% and 39% larger than that with transmitted angle θ_l = 0° and without light trapping, respectively. For the case of normal transmitted angle, the IQE_b with diffusion length 1000 μm is about 81% and is 37% higher than that with diffusion length 25 μm. The obtained results can provide essential information for designing a high-efficiency solar cell.     

A detailed study of p–n junction solar cells by means of collection efficiency

The operation of a crystalline silicon solar cell was studied by a methodology based on collection efficiency. The collection efficiencies of the base, emitter, and depletion layers were determined separately using numerical solutions. The quantum efficiency was then determined by the reciprocity theorem. It is shown that the model can provide useful new insights and can be used to extract device parameters by fitting the modelled results to experimental data.     

Minority carrier lifetime and efficiency improvement of multicrystalline silicon solar cells by two-step process

Impurities and defects are of significant interest in multicrystalline silicon, due to the detrimental effect they can have on carrier lifetimes and electrical properties. In view of that, it is important to incorporate certain processing steps to decrease the recombination activities. In this study, a novel experiment was applied as a beneficial approach to improve the electronic quality of low-resistivity mc-Si substrates via a two-step process. Initially, the first step involves gettering multicrystalline substrates using sacrificial porous silicon layer on both sides, which was introduced as a simple sequence for efficient extrinsic gettering schemes. The gettering experiment was performed at 600–900 °C, and optimum results were obtained at 900 °C. Then, the second step involves coating the front surface of gettered mc-Si at 900 °C with vanadium oxide that serves as an excellent antireflection layer and leads to improve furthermore the electrical properties. Significant improvements were obtained after the deposition of vanadium oxide antireflection coating, in view of the fact that gettered mc-Si substrate at 900 °C provides the highest minority carrier lifetime and the lowest effective surface recombination velocity. An overall increase of the electrical properties was obtained after the described two-step process. The conversion efficiency increases from 6% (reference) and reached 13.7%.     

Evaluation of external quantum efficiency of a 12.35% tandem solar cell comprising dye-sensitized and CIGS solar cells

A tandem solar cell is constructed by series connection of a semi-transparent dye-sensitized solar cell (DSSC) as a top cell and a Cu(In, Ga)Se₂ (CIGS) solar cell as a bottom cell, where the isolated DSSC and CIGS cells show the conversion efficiency of 8.27% and 11.71%, respectively. The DSSC/CIGS tandem cell exhibits the improved conversion efficiency of 12.35% with photocurrent density of 14.1 mA/cm², open-circuit voltage of 1.435 V and fill factor of 0.61. External quantum efficiency (EQE) of the tandem cell is investigated under DC and AC modes. EQE of the isolated DSSC and CIGS cell can be measured by either DC mode or AC mode, whereas EQE for the tandem cell is detected only under AC mode with bias light. Bias light intensity is found to play the crucial role in determining the precise EQE of the tandem cell. At the given chopping frequency as low as 10 Hz, the measured EQE at bias light corresponding to 1 sun intensity is consistent with the simulated EQE data.     

Voltage-dependent quantum efficiency measurements of amorphous silicon multi-junction mini-modules

Multi-junction solar cells have the potential to provide higher efficiencies than single junction devices and to reduce the impact of Staebler-Wronski degradation on amorphous silicon (a-Si) devices. They could, therefore, reduce the cost of solar electricity. However, their characterization presents additional challenges over that of single junction devices. Achieving acceptable accuracy of any current-voltage calibration requires correction of the current-voltage data with external quantum efficiency measurements and spectral mismatch calculations. This paper presents voltage-dependent EQE curves for both single junction and double junction a-Si solar cells, along with dispersion curves extracted from these data. In the case of single junction a-Si devices the mismatch factor is known to be voltage-dependent and a similar trend is shown to apply to multi-junction devices as well. However, the error introduced into current-voltage calibrations due to this bias dependence is found to be <1% for spectral mismatch calculations.     

Short-circuit current distribution in solar cells

This paper describes a method to determine the contribution of each region of a solar cell to the short-circuit current, using spectral response measurements and dynamic inner collection efficiency (DICE) analysis. The photon flux is calculated along the perpendicular axis of the cell surface at a given illumination condition, normally AM0 or AM1. This enables to calculate the current at each surface-parallel plane for which the DICE parameter is known. The method is applied to silicon solar cells to exemplify its capability.     

Modelling the quantum efficiency of cadmium telluride solar cells

A simple analytical model is presented describing the quantum efficiency of cadmium telluride (CdTe) solar cells. This model is based on a consistent set of parameters that were extracted from electrical and optical measurements. These measurements also reveal the CdTe solar cells to mainly rely on carrier generation as well as carrier collection within the space-charge region. Recombination in this part of the cell is hence taken into account. As a result, quantum efficiency spectra can be closely fitted by an expression that includes the lifetime of the minority carriers and the width of the space-charge region as free variables. The comparison of the calculated quantum efficiency curves with the experimental ones gives fundamental insight into the specific operation of CdTe solar cells.     

Internal quantum efficiency improvement of polysilicon solar cells with porous silicon emitter

The present study aimed to develop a simple analytical model to investigate the potential use and implications of porous silicon (PS) as an antireflective coating in thin polysilicon solar cells. It analytically solved the complete set of equations necessary to determine the contribution that this material has on the internal quantum efficiency (IQE) of the cell when acting as an antireflective coating agent. The increase in the IQE, the contribution of the different regions of the cell, and the effects of the physical parameters of each region were derived and investigated in comparison with conventional solar cells.The findings revealed that the internal quantum efficiency of the solar cell with PS emitter is higher than that of the conventional one particularly for short-wavelengths (λ < 0.6 μm). Furthermore, for photons with higher energy, the emitter contribution in the IQE is more significant than the base and depletion regions. For photons with smaller energy, on the other hand, the absorption coefficients are also smaller, which leads to a higher generation rate in the base region and, hence, to a more pronounced contribution from this region to IQE. Last but not least, the improvement of IQE is observed to increase with decreased PS thickness and with heavily doped PS emitter (N_d⁺⁺ = 10²⁰ cm⁻³).     

Colloidal quantum dot solar cells

In recent years colloidal quantum dots solar cells have been the subject of extensive research. A promising alternative to existing silicon solar cells, quantum dot solar cells are among the candidates for next generation photovoltaic devices. Colloidal quantum dots are attractive in photovoltaics research due to their solution processability which is useful for their integration into various solar cells. Here, we review the recent progresses in various quantum dot solar cells which are prepared from colloidal quantum dots. We discuss the preparation methods, working concepts, advantages and disadvantages of different device architectures. Major topics discussed in this review include integration of colloidal quantum dots in: Schottky solar cells, depleted heterojunction solar cells, extremely thin absorber solar cells, hybrid organic-inorganic solar cells, bulk heterojunction solar cells and quantum dot sensitized solar cells. The review is organized according to the working principle and the architecture of photovoltaic devices.     

Investigation of texturization for crystalline silicon solar cells with sodium carbonate solutions

We investigate a new texturization technique for crystalline silicon solar cells with sodium carbonate (Na₂CO₃) solutions. We show the dependence of the hemispherical surface reflectance on solution temperature, the etching time and the Na₂CO₃ concentration. Furthermore, we investigate what element in Na₂CO₃ solution influences the texturing for reducing the texturing time. As a result of experiments, we find it possible to get low reflectance in a shorter texturing time by the addition of NaHCO₃. The size of texture becomes smaller by the addition of NaHCO₃ but the etching rate does not change. We conclude carbonic ion and/or its compound seems to play an important role as the initiator of pyramidal structure. This texturing method is cost effective because there is no need of expensive IPA, and the surface reflectance is reduced sufficiently in a short time. This method is promising for a large-scale production of crystalline silicon solar cells.     

Reactive ion etching (RIE) technique for application in crystalline silicon solar cells

Saw damage removal (SDR) and texturing by conventional wet chemical processes with alkali solution etch about 20 micron of silicon wafer on both sides, resulting in thin wafers with which solar cell processing is difficult. Reactive ion etching (RIE) for silicon surface texturing is very effective in reducing surface reflectance of thin crystalline silicon wafers by trapping the light of longer wavelength. High efficiency solar cells were fabricated during this study using optimized RIE. Saw damage removal (SDR) with acidic mixture followed by RIE-texturing showed the decrease in silicon loss by ∼67% and ∼70% compared to conventional SDR and texturing by alkaline solution. Also, the crystalline silicon solar cells fabricated by using RIE-texturing showed conversion efficiency as high as 16.7% and 16.1% compared with 16.2%, which was obtained in the case of the cell fabricated with SDR and texturing with NaOH solution.     

Latest results on semitransparent POWER silicon solar cells

The paper presents the latest results of the polycrystalline wafer engineering result (POWER) silicon solar cell research (G. Willeke, P. Fath, The POWER silicon solar cell, Proceedings of the 12th EPVSEC, Amsterdam, 1994, pp. 766–768). Mono – as well as bifacially active semitransparent silicon solar cells have been created by forming perpendicularly overlapping grooves on the front and the rear side of a silicon wafer resulting in a regular pattern of holes. The developed very simple manufacturing process is fully compatible with an industrial production and uses POCl₃-tube diffusion, PECVD silicon nitride as single ARC and screen-printing metallization. Maximum efficiencies of η=11.2% for monofacial POWER cells on 0.4 Ω cm Cz material with a transparency of 18.2% and η=12.9% for bifacial cells on 1 Ω cm Cz material with a transparency of 16% have been obtained. Results for multicrystalline (mc) semitransparent mono- and bifacially active silicon solar cells are also presented.     

Phosphorus-doped silicon quantum dots for all-silicon quantum dot tandem solar cells

Doping of Si quantum dots is important in the field of Si quantum dots-based solar cells. Structural, optical and electrical properties of Si QDs formed as multilayers in a SiO₂ matrix with various phosphorus (P) concentrations introduced during the sputtering process were investigated for its potential application in all-silicon quantum dot tandem solar cells. The formation of Si quantum dots was confirmed by transmission electron microscopy. The addition of phosphorus was observed to modify Si crystallization, though the phosphorus concentration was found to have little effect on quantum dot size. Secondary ion mass spectroscopy results indicate minimal phosphorus diffusion from Si QDs layers to adjacent SiO₂ layers during high-temperature annealing. Resistivity is significantly decreased by phosphorus doping. Resistivity of slightly phosphorus-doped (0.1 at% P) films is seven orders of magnitude lower than that of intrinsic films. Dark resistivity and activation energy measurements indicate the existence of an optimal phosphorus concentration. The photoluminescence intensity increases with the phosphorus concentration, indicating a tendency towards radiative recombination in the doped films. These results can provide optimal condition for future Si quantum dots-based solar cells.