High-voltage solar cell with high short wavelength sensitivity on the base of GaAs–AlGaAs heterosystem

In this contribution two structures of high-voltage solar cells are discussed and analyzed. The first one is a high-voltage solar cell with only vertical p-n junctions. Therefore, the photo-current value of this structure and — consequently — the efficiency are low because of the small active p-n junction area. In order to improve the main parameters of the above mentioned structure — particularly the photo-current and the efficiency — a new structure of high-voltage solar cells, combining both horizontal and vertical p-n junctions is suggested. This structure has been elaborated by combining liquid phase epitaxy with gas-phase zinc diffusion technologies, and was grown on semi-insulating GaAs substrates with ρ=10¹²Ω cm. This new structure has shown better parameters than the parameters of the high-voltage solar cell with only vertical p-n junctions. It exhibits appreciable values of photo-current and output voltage. Furthermore, this structure provides both high short wavelength sensitivity, and independent efficiencies of spectral composition of solar radiation. Therefore, it can be successfully utilized in many aspects of modern science and technology, particularly as power supplies in high-located areas, and as photoacceptors of ultra-violet radiation.     

A numerical model of p–n junctions bordering on surfaces

Many solar cell structures contain regions where the emitter p–n junction borders on the surface. If the surface is not well passivated, a large amount of recombination occurs in such regions. This type of recombination is influenced by the electrostatics of both the p–n junction and the surface, and hence it is different from the commonly described recombination phenomena occurring in the p–n junction within the bulk. We developed a two-dimensional model for the recombination mechanisms occurring in emitter p–n junctions bordering on surfaces. The model is validated by reproducing the experimental I–V curves of specially designed silicon solar cells. It is shown under which circumstances a poor surface passivation, near where the p–n junction borders on the surface, reduces the fill factor and the open-circuit voltage. The model can be applied to many other types of solar cells.     

Design issues in the fabrication of CdS–CdTe solar cells on molybdenum foil substrates

Investigations on CdTe–CdS solar cells on molybdenum foil substrates revealed that the depletion layer spans the entire CdS and CdTe film thickness and the cell should be conceived as a single junction device instead of the three separate junctions (Mo–CdTe, CdTe–CdS, CdS–TCO). Higher open circuit voltages were achieved when two CdS layers (separated by an air anneal) were used instead of a single CdS layer. The high series resistance of this solar cell continues to be the limiting factor in cell performance. Modeling and design issues for improving cell performance are presented.     

Improved photovoltaic method for measurement of minority carrier diffusion length applied to silicon solar cells

The photovoltage spectrum measured on back illuminated silicon solar cells of the (passivated emitor solar cell) (PESC) type without original bottom ohmic electrode is evaluated with the aim to find the diffusion length of minority carriers in bulk of the absorber (L). Two junctions, namely pn⁺ junction of the cell and that spontaneously created on the free surface generally exist in such samples. They give rise to two signals of opposite signs with one point of exact compensation. Six parameters (including L) are needed to characterize the spectrum. Special simple arrangement removes influence of spontaneously created junction on the free surface, which, in this way, reduces the number of parameters needed for fitting to three and enhances reliability of the measurement.     

Effect of edge junction isolation on the performance of laser doped selective emitter solar cells

The effect of laser and chemical edge junction isolation on electrical performance of industrially manufactured laser doped selective emitter solar cells with light induced plated n-type contacts is investigated in this work. Directly after the formation of the aluminium back surface field, photoluminescence images indicates that laser edge junction isolation causes substantial damage around the perimeter of the cell, extending several millimeters from the laser edge isolation groove. On finished devices, regions of high series resistance are evident around the perimeter, caused by parasitic plating nucleating in the damaged laser grooved region which induce shunting and inhibits further plating taking place in the surrounding regions. The use of chemical edge junction isolation eliminates both of these issues and can result in efficiency gains of more than 2% absolute compared to that fabricated using laser edge isolation, suggesting a far superior method of edge junction isolation for the industrial manufacture of laser doped selective emitter solar cells with light induced plated contacts.     

Performance study of solar cell arrays based on a Trough Concentrating Photovoltaic/Thermal system

The performances of solar cell arrays based on a Trough Concentrating Photovoltaic/Thermal (TCPV/T) system have been studied via both experiment and theoretical calculation. The I–V characteristics of the solar cell arrays and the output performances of the TCPV/T system demonstrated that among the investigated four types of solar cell arrays, the triple junction GaAs cells possessed good performance characteristics and the polysilicon cells exhibited poor performance characteristics under concentrating conditions. The optimum concentration ratios for the single crystalline silicon cell, the Super cells and the GaAs cells were also studied by experiments. The optimum concentration ratios for the single crystalline silicon cells and Super cells were 4.23 and 8.46 respectively, and the triple junction GaAs cells could work well at higher concentration ratio. Besides, some theoretical calculations and experiments were performed to explore the influences of the series resistances and the working temperature. When the series resistances R_s changed from 0 Ω to 1 Ω, the maximum power P_m of the single crystalline silicon, the polycrystalline silicon, the Super cell and the GaAs cell arrays decreased by 67.78%, 74.93%, 77.30% and 58.07% respectively. When the cell temperature increased by 1 K, the short circuit current of the four types of solar cell arrays decreased by 0.11818 A, 0.05364 A, 0.01387 A and 0.00215 A respectively. The research results demonstrated that the output performance of the solar cell arrays with lower series resistance was better and the working temperature had a negative impact on the current under concentration. In addition, solar irradiation intensity had certain effects on the solar cell’s performance. For the crystalline silicon solar cell arrays, when the solar direct radiation exceeded a certain value, the I–V curves almost became a straight line and the output performances decreased due to the high series resistance leading to the high power loss. For the triple junction GaAs solar cell array, its performance was always excellent.     

Evaluation of InGaP/InGaAs/Ge triple-junction solar cell and optimization of solar cell's structure focusing on series resistance for high-efficiency concentrator photovoltaic systems

The series resistance of an InGaP/InGaAs/Ge triple-junction solar cell was evaluated in detail. Series resistance components such as electrode resistance, tunnel junction resistance and lateral resistance between electrodes were estimated separately. The characteristics of the triple-junction solar cell under concentrated light were evaluated by equivalent circuit calculation with a simulation program with integrated circuit emphasis (SPICE). By equivalent circuit calculation, the optimization of cell designs was performed, focusing on series resistance and cell current in order to realize high-efficiency concentrator cells.     

Cu(In,Ga)Se2 solar cells with superstrate structure using lift-off process

Cu(In,Ga)Se₂ (CIGS) solar cells with a superstrate structure were fabricated using a lift-off process. To widen the variety of substrate choices for CIGS solar cells, a lift-off process was developed without an intentional sacrificial layer between the CIGS and Mo back-contact layers. The CIGS solar cells fabricated on Mo/soda-lime glass (SLG) were transferred to an alternative SLG substrate. The conversion efficiency of the CIGS solar cells with the superstrate structure was 5.1%, which was almost half that of the CIGS solar cells with a substrate structure prior to the lift-off process. The low conversion efficiency was caused by the high series resistance and low shunt resistance, which would be due to the junction resistance between the CIGS/back contact and cracks introduced during the lift-off process, respectively.     

A simple analytical treatment of edge-illuminated VMJ silicon solar cells

The series connected silicon vertical multi-junctions (VMJs) solar cells have been suggested as means for ensuring high voltage high efficiency solar cells. This study includes a review of some previously published work concerning the edge-illuminated VMJs solar cells. We introduce a simple one-dimensional analysis to study the high voltage series connected silicon VMJs solar cells. The cell, under study, consists of 40 VMJs. Each junction (unit cell) has dimensions of 250 μm × 0.78 cm × 500 μm. We calculate the short circuit current, the open circuit voltage and the efficiency for an ideal cell, having perfect carrier collection at short circuit conditions, and for real cells. An optimization with respect to the base doping, the emitter doping, the surface recombination velocity and the number of junctions is done for the real cell. A conversion efficiency of 20% has been calculated under AM1.5 light spectrum for real cells having a base doping of 10¹⁶ cm⁻³ and an emitter doping of 10¹⁷ cm⁻³.     

High efficiency AlxGa1−xAs/GaAs solar cells: Fabrication, irradiation and annealing effect

High efficiency Al_xGa_1−xAs/GaAs heteroface solar cells have been fabricated by an improved multi-wafer squeezing graphite boat liquid phase epitaxy (LPE) technique, which enables simultaneous growth of twenty 2.3 × 2.3cm² epilayers in one run. A total area conversion efficiency of 17.33% is exhibited (1sun, AMO, 2.0 × 2.0cm²). The shallow junction cell shows more resistance to 1 MeV electron radiation than the deep one. After isochronal or isothermal annealing the density and the number of deep level traps induced by irradiation are reduced effectively for the solar cells with deep junction and bombardment under high electron fluences.     

ANALYTICAL AND GRAPHICAL METHODS OR DETERMINATION OF SOLAR CELL PARAMETERS AND INVESTIGATIONS OF SHADOWING EFFECT

In actual solar cells, the main power loss is due to the effect of the internal series resistance and the shunt resistance of the solar cell. Two methods; mathematical and graphical, were used to determine these two resistances for an Iraqi monocrystalline solar cell (type AI-Mansour). The results show that both of the series resistance (0·09 Ω) and the shunt resistance (210 Ω) can usually be neglected in an array performance evaluation for systems which don't use concentration arrangements

In addition to the series and shunt resistances computations, the analysis of the mismatching among solar cells as well as the power dissipation by shadowed or faulty cells for different module configurations are discussed in detail in this paper. As a result it was found that the maximum number of cells that can be safely series, parallel connected are 50 and 6 cells respectively.     

Development of both-side junction silicon space solar cells

This paper reports the recent results of improving the radiation hardness of silicon solar cells, which is SHARP and NASDA's project since 1998 (Tonomura et al., Second World Conference on Photovoltaic Solar Energy, 1998, pp. 3511–3514). Newly developed 2×2 cm² Si solar cells with ultrathin substrates and both-side junction (BJ) structure showed 72.0 mW (13.3% efficiency) maximum output power at AM0, 28°C after 1 MeV electron irradiation up to 1×10¹⁵ e/cm² and the best cell showed 72.5 mW (13.4%) maximum output power. These solar cells have p–n junctions at both front and rear surfaces and showed less radiation degradation and better remaining factor than previous solar cells.     

Effect of temperature on dark current characteristics of silicon solar cells and diodes

The influence of temperature on the dark forward current–voltage characteristics of a single crystalline silicon solar cell and a small silicon diode within the range from 295–373 K has been analysed. It was shown that the forward voltage of the solar cell degrades 2 mV and in the case of diode 1 mV per 1 K temperature increases at constant forward current of 100 mA. Thermal resistance and heat transfer from the solar cell by using a thick copper plate as a heat sink have also been discussed. For the series resistance determination the current–voltage I–U characteristics of single crystalline silicon solar cells in different temperatures were measured in the dark. It was proved that series resistance of the silicon solar cells and diodes is temperature dependent and increases with temperature increase 0.65% K^?1. Therefore, protection of silicon solar cells as well as silicon diodes against overheating is essential during their exploitation. Copyright © 2005 John Wiley & Sons, Ltd.     

Extended description of tunnel junctions for distributed modeling of concentrator multi-junction solar cells

One of the key components of highly efficient multi-junction concentrator solar cells is the tunnel junction interconnection. In this paper, an improved 3D distributed model is presented that considers real operation regimes in a tunnel junction. This advanced model is able to accurately simulate the operation of the solar cell at high concentrations at which the photogenerated current surpasses the peak current of the tunnel junction. Simulations of dual-junction solar cells were carried out with the improved model to illustrate its capabilities and the results have been correlated with experimental data reported in the literature. These simulations show that, under certain circumstances, the solar cell's short circuit current may be slightly higher than the tunnel junction peak current without showing the characteristic dip in the J-V curve. This behavior is caused by the lateral current spreading toward dark regions, which occurs through the anode/p-barrier of the tunnel junction.     

Series resistance in n-MoSe2 (TMDC)-based PEC solar cells

Transition metal dichalcogenides (TMDCs) have been noticed as potentials for the PEC solar cells because they are inherently stable against the electrolytic environment. Since MoSe₂—a member of group VI TMDCs—possesses an optically matching band gap of around 1.4 eV, it holds relatively more promise as a better material for such devices. In this article, the authors report their investigations on PEC solar cells fabricated using n-MoSe₂ crystals grown by a direct vapour transport technique. The photoconversion characteristics of n-MoSe₂/I₂/I⁻/pt PEC solar cells were investigated under polychromatic illumination from an incandescent lamp at various intensities. Since the series resistance of TMDC-based PEC solar cells is expected to be high, it may be one of the major parameters blocking the available power on photoconversion from such devices. Efforts have been made here to estimate its value. In addition, the effect of thermal treatment of the photoelectrode on the series resistance was also investigated. It has been found that the series resistance decreases from 4.01 K ohms to 1.93 K ohmson controlled thermal treatment of the photoelectrode. This is accompanied by a marked increase in the photoconversion efficiency (from 3% to around 12%). Thus, it can be concluded that the contribution of the series resistance in TMDC-based PEC solar cells is quite significant and can be reduced by giving controlled thermal treatment to the photoelectrodes.     

ZnSe太阳电池

通过掺少量Te提高ZnSe受主杂质浓度，从而制成ZnSe p—n结太阳电池。单结ZnSe光电池在AM1条件下光谱响应半宽度范围365-450nm，波长为400nm时量子效率最大。ZnSe是一种好的叠层多结光电池的顶层材料，可以提供较高的开路电压，并且是底层电池的窗口；在单结ZnSe光电池的基区指数掺杂，95％的区域内能产生大于10^4V／cm的附加电场，在400—475nm波长范围内，该电场可以使其外量子效率增加1～1．5倍。     

Microcrystalline silicon solar cells fabricated by VHF plasma CVD method

A series of systematic investigations on microcrystalline silicon (μc-Si:H) solar cells at high deposition rates has been studied. The effect of high deposition pressure and narrow cathode-substrate (CS) distance on the deposition rate and quality of microcrystalline silicon is discussed. The microcrystalline silicon solar cell is adopted as middle cell and bottom cell in a three-stacked junction solar cell. The characteristics of large area three-stacked junction solar cells, whose area is 801.6 cm² including grid electrode areas, are studied in various deposition rates from 1 to 3 nm/s of microcrystalline silicon. An initial efficiency of 13.1% is demonstrated in the three-stacked junction solar cell with microcrystalline silicon deposited at 3 nm/s.     

Theoretical possibilities of InxGa1−xN tandem PV structures

We designed a model of In_xGa_1−xN tandem structure made of N successive p–n junctions going from two junctions for the less sophisticated structure to six junctions for the most sophisticated. We simulated the photocurrent density and the open-circuit voltage of each structure under AM 1.5 illumination in goal to optimize the number of successive junctions forming one structure.For each value of N, we assumed that each junction absorbs the photons that are not absorbed by the preceding one. From the repartition of photons in the solar spectrum and starting from the energy gap of GaN, we fixed the gap of each junction that gives the same amount of photocurrent density in the structure. Then we calculated the current density accurately and optimized the thicknesses of p and n layers of each junction to make it give the same output current density. The evaluation of n_i: the intrinsic concentration permitted to calculate the saturation current density and the open-circuit voltage of each junction. Assuming an overall fill factor of 80%, we divided the output peak power by the incident solar power and obtained the efficiency of each structure.The numerical values for In_xGa_1−xN were taken from the relevant literature. The calculated efficiency goes from 27.49% for the two-junction tandem structure to 40.35% for a six-junction structure. The six-junction In_xGa_1−xN tandem structure has an open-circuit voltage of about 5.34 V and a short circuit current density of 9.1 mA/cm².     

Third generation multi-layer tandem solar cells for achieving high conversion efficiencies

Different ways of connecting solar cell structures to form multi-layer tandem solar cells have been considered by re-visiting relevant device designs. It is found that the present use of a series connection or tunnel junction approach is detrimental to charge-carrier collection in the tandem cells. Each tunnel junction introduced to the solar cell structure decelerates the charge carriers and allows them to recombine at the vicinity of the tunnel junction. The adoption of parallel connections has several advantages over series connections and there is high potential for achieving enhanced efficiencies in third generation tandem solar cells. In these devices, charge carriers are continuously accelerated across the whole device and collected in the external circuit. Multi charge-carrier production and impurity photo-voltaic mechanisms are also built into this system to enhance its performance by increasing the short-circuit current density.     

A new method of determination of series and shunt resistances of silicon solar cells

A new method of measurement of series resistance R_s and shunt resistance R_sh of a silicon solar cell is presented. The method is based on the single exponential model and utilizes the steady state illuminated I–V characteristics in third and fourth quadrants and the V_oc–I_sc characteristics of the cell. It enables determination of values of R_sh and R_s with the intensity of illumination. For determination of R_s it does not require R_sh to be assumed infinite and realistic values of R_sh can be used. The method is very convenient to use and in the present study it has been applied to silicon solar cells having finite values of R_sh. We have found that R_sh is independent of intensity but the R_s decreases with both the intensity of illumination and the junction voltage.