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).     

A simple general analytical solution for the quantum efficiency of front-surface-field solar cells

The optimisation of a FSF solar cell or a BSF thin film solar cell necessitates an understanding of the characteristics of an illuminated high-low junction. However, except for minority carrier reflection, the photocurrent collection property of the light-generated carriers in the high region by a high-low junction has rarely been treated previously. It is the purpose of this paper to present a simple solution for the contribution of the light-generated current from the high region. The model shows that a high-low junction may be very efficient in light-generated current collection, and this property is primarily responsible for the increase of the short wavelength quantum efficiency using a FSF. The effects of the FSF layer doping concentration and its thickness on the quantum efficiency are discussed by using the computed results; the optimisation of the FSF is then made. The calculations take into account the high doping effects of degeneracy and bandgap narrowing.     

Improving the efficiency of crystalline silicon solar cells by an intersected selective laser doping

The selective emitter technology is the most effective method to improve the silicon solar cell performance. However, the strict alignment requirements between the Ag gridlines and the selective emitters should be addressed. In the present study, a novel front metal contact patterning scheme for crystalline silicon (c-Si) solar cells is developed. An intersected selective laser doping process is applied. The control experiment indicates that the best efficiency increases for ∼23.8% from ∼14.39% to ∼17.74% after receiving the laser doping process. Moreover, our laser doping process does not affect the open-circuit voltage and short-circuit current significantly and the emitter properties have not been degraded in terms of the emitter saturation current. The improved front metal contact results in the improved energy conversion efficiency. The developed process offers an elegant and reliable approach for the low-cost, high throughput industrial-scale production process.     

A new model of very high efficiency buried emitter silicon solar cell

In this work, we report a new model of symmetrical silicon solar structure where the emitter is buried, thus, creating many depletion regions in series inside the cell. The photocurrent in this model is computed for AM0 solar spectrum and is compared to the classical p-n junction. The first results of the calculation show that the buried emitter solar cell (BESC) has 10–35% more short-circuit current than the classical cell depending on the surface recombination velocity S. The biggest difference is obtained for S = 10⁵cm/s. The ratio of the photocurrent in the BESC to the classical photocurrent as a function of the absorption coefficient a goes from 1.9 for small α to 6.5 times for α = 10⁷cm⁻¹ with a minimum of 1.1 for α around 1800 cm⁻¹ for S = 10⁵cm/s.     

Study for improvement of solar cell efficiency by impurity photovoltaic effect

This paper is to study for efficiency improvement of solar cells by utilizing impurity traps introduced in the band gap of semiconductor, that is, impurity photovoltaic (IPV) effect. It is revealed theoretically that there is a certain energy range where impurity-traps act as stepping stones in two-step excitation of electrons from the valence band to the conduction band under suppression of carrier recombination through such traps. Indium is selected as one of proper impurities that satisfy this condition in crystalline silicon, and theoretical prediction is experimentally verified. A good agreement between theory and experiment is obtained concerned with photoconductive properties. It is concluded that the IPV effect is useful to improve the cell efficiency.     

Advances in monocrystalline Si thin film solar cells by layer transfer

The transfer of monocrystalline Si films enables the fabrication of efficient thin film solar cells on glass or plastic foils. Chemical vapor deposition serves to epitaxially deposit Si on quasi-monocrystalline Si films obtained from thermal crystallization of a double-layer porous Si film on a Si wafer. A separation layer that forms during this crystallization process allows one to separate the epitaxial layer on top of the quasi-monocrystalline film from the starting Si wafer after solar cell processing. Independently confirmed thin film solar cell efficiencies are 15.4% and 16.6% for thin film solar cells transferred to a glass superstrate with a total Si film thickness of 24.5 and 46.5 μm, respectively, and a cell area of 4 cm². Device simulations indicate an efficiency potential above 20%.     

Transient analytical solution of solar energy devices

This note presents a transient analytical solution of fluid temperature variation in solar energy devices, viz. suspended plate solar air heater [Fitzgibbons and Kline (1978), Fig. 1], parallel flat plate collector (Fig. 2), solar stills [Tiwari and Malik (1982), Fig. 3] and pebble bed storage [Duffie and Beckman, (1980)] etc. as a function of time and space coordinate. Till now no such analytical solutions are available for such types of devices, except periodic and numerical analysis.     

Thin film solar cells on glass based on the transfer of monocrystalline Si films

Thin film solar cells based on monocrystalline Si films are transferred onto a glass superstrate. Chemical vapor deposition serves to epitaxially deposit Si on quasi-monocrystalline Si films obtained from thermal crystallization of a double-layer porous Si film on a Si wafer. A separation layer that forms during this crystallization process allows one to separate the epitaxial layer on top of the quasi-monocrystalline film from the starting Si wafer. At present, we achieve an independently confirmed efficiency of 10.6% with a thin film solar cell of an area of 1.92 cm² that consists of a 24.5 μm thick Si film transferred to glass. Device simulation indicates an efficiency potential of around 17%.     

An analytical expression for efficiency of solar still

In this Technical Note, an analytical expression for the efficiency of a solar still has been derived as a function of solar still and climatical parameters. The analysis is based on the basic energy balance of the solar still. It is observed that an expression for the efficiency of a solar still is of the same form as obtained for a flat-plate collector.     

High efficiency ultra-thin sputtered CdTe solar cells

Large scale manufacturing of CdTe PV modules at the GW/yr level may be constrained due to the limited availability of the relatively rare (Te) element and the volume of potentially hazardous (Cd) material being used in the typically 3–8 μm thick CdTe absorber layer. However, we find that it is possible to reduce the CdTe layer thickness without much compromise in efficiency. The CdS/CdTe solar cells were fabricated using magnetron sputtering with ultra-thin CdTe layers in the range of 0.5–1.28 μm. The ultra-thin films and cells were characterized using X-ray diffraction (XRD), optical transmission, scanning electron microscopy (SEM), current–voltage and quantum efficiency measurements. These results were compared with those of standard 2.3 μm thick CdTe sputtered cells. Different post-deposition processing parameters were required for cells with ultra-thin and standard CdTe thicknesses to achieve high efficiency. Ultra-thin CdTe cells showed crystallographic texture and CdTe_1−xS_x alloy formation after CdCl₂ treatment very similar to standard CdTe cells. Optimization of the post-deposition CdCl₂ treatment and back-contact processing yielded cells of 11.2% efficiency with 0.7 μm CdTe compared to 13.0% obtained with standard 2.3 μm CdTe cells.     

Lateral current effects on the voltage distribution in the emitter of solar cells under concentrated sunlight

The design of the grid contact in silicon solar cells is one of the most important steps for the optimization and fabrication of these energy conversion devices. The voltage drop due to the lateral flow of current towards the grid fingers can be a limiting factor causing the reduction of conversion efficiency. For low current levels this voltage drop can be made small, for typical values of sheet resistance in the emitter, but for solar cells made to operate at high sun concentrations this efficiency loss can be important, unless there is a clear vision of the current and voltage distribution so that the emitter and grid design can be improved. Hence, it is important to establish and solve the current and voltage distribution equations for solar cells with a grid contact. In this work, first these equations are established and then they are solved in order to show the effects that the lateral current flow in the emitter cause on the voltage distribution, particularly at high illumination levels. In addition, it will be shown that the open circuit voltage is significantly reduced due to the lateral current flow as compared to the value predicted from a simple equivalent circuit with a lumped resistance model.     

The efficiency of solar cells immersed in liquid dielectrics

The effect of an increase in the efficiency of solar cells (SCs) (in particular, common silicon SCs) by their immersion in an isotropic liquid dielectric is described. The presence of a dielectric thin film results in an increase in the SCs efficiency by 40–60% from the reference value. The current–voltage characteristic, fill factor and other characteristics of SCs are analyzed. The mechanisms of the increase in the efficiency of SCs are discussed.     

The limiting efficiency of band gap graded solar cells

Two fundamental mechanisms limit the maximum attainable efficiency of solar cells, namely the radiative recombination and Auger recombination. We show in this paper that proper band gap grading of the solar cell localizes the Auger recombination around the metallurgical junction. Two beneficial effects result from this Auger recombination localization; first the cell is less sensitive to the surface conditions, and second, the previous estimates for the limiting efficiency of solar cells by Shockley, Tiedje, and Green are revised upwardly. We calculate the optimum bandgap grading profile for several real material systems, including GaInAsP lattice matched to InP, and a-SiGe on a-Si substrate.     

2D-numerical analysis and optimum design of thin film silicon solar cells

Device modeling for p–i–n junction μc-Si basis thin film polycrystalline Si solar cells has been examined with a simple model of columnar grain structure and its boundary condition utilizing two-dimensional device simulator. As the simulation results of solar cell characteristics show, open-circuit voltage (V_oc) and curve fill factor (FF) considerably depend on those structural parameters, while short-circuit current density (J_sc) is comparatively stable by courtesy of homogeneous built-in electric field in the i layer. It has also been found that conversion efficiency over 12% could be expected with 1 μm grain size and well-passivated condition with 3 μm thick i-layer.     

A new technique for boosting efficiency of silicon solar cells

A new type of photovoltaic system with higher generation power density has been studied in detail. The feature of the system is a V-shaped module (VSM) with two tilted monocrystalline solar cells. Compared to solar cells in a flat orientation, the VSM enhances external quantum efficiency and leads to an increase of 31% in power conversion efficiency. Due to the VSM technique, short-circuit current density was raised from 24.94 to 33.7 mA/cm², but both fill factor and open-circuit voltage were approximately unchanged. For the VSM similar results (about 30% increase) were obtained for solar cells fabricated by using mono-crystalline silicon wafers with only conventional background impurities.     

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.     

Transient and analytical solution of suspended flat-plate solar air heater

Based on a simple transient analysis, an explicit expression for the temperature of air, flowing through the channel of a suspended flat-plate solar air heater, has been developed as a function of time and space co-ordinates. Investigations regarding the effect of various parameters, such as air velocity, air channel depth and inlet air temperature, on the performance of the system have been carried out under two modes of operation; (1) the time is kept constant while the space co-ordinate along the flow direction is varied, and (2), the latter is kept constant while the former is varied.     

Energy conversion efficiency of solar cells coated with fluorescent coloring agent

A coating of fluorescent coloring agent (FCA) on the solar cells gives 30% increase in the energy conversion efficiency of the solar cell. This increase is attributable to the reduction of the reflection of incident light. The reflectances show low values at the excitation wavelengths, where the incident light is absorbed to excite the FCA. The fluorescence quantum yield for a dried FCA was much larger than that for FCAs dissolved in paint thinner.     

Understanding defect-related issues limiting efficiency of CIGS solar cells

Many electrical characteristics of Cu(In,Ga)Se₂-based solar cells, including current-voltage characteristics, are affected by specific properties of negative-U defects in the absorber. We present these characteristics and discuss them in the framework of a V_Se-V_Cu defect model proposed by Lany and Zunger. We show how these defects influence photocarrier transport and the dominant recombination mechanism, and hence also the photovoltaic parameters of the cells. Numerical simulations validating our approach will also be presented.     

Analytical modelling and minority current measurements for the determination of the emitter surface recombination velocity in silicon solar cells

A new analytical model is used to describe the emitter of silicon solar cells in order to gain information on the surface recombination velocity S. The procedure takes advantage of the combined use of experimental measurements, done to determine the emitter saturation current J_oe, and analytical modelling to relate J_oe to S. Several experiments have been carried out on silicon solar cells having different emitter profiles subjected to various surface treatments. The influence of the surface on significant cell parameters has been analysed.