Heavily doped transparent-emitter regions in junction solar cells, diodes, and transistors

An analytical treatment of heavily doped transparentemitter devices is presented that includes the effects of bandgap narrowing, Fermi-Dirac statistics, a doping concentration gradient, and a finite surface recombination velocity S at the emitter surface. Transparency of the emitter to minority carrier is defined by the condition that the transit time τtis much smaller than the minority carrier lifetime in the emitter τp,tau_{t} ll tau_{p}. As part of the analytical treatment, a self-consistency test is formulated that checks the validity of the assumption of emitter transparency for any given device. The transparent-emitter model is applied to calculate the dependence of the open-circuit voltage VOCof n⁺-p junction silicon solar cells made on low-resistivity substrates. The calculated VOCagrees with experimental values for highS_{P}( geq5 \× 10^{4}cm/s) provided the effects of bandgap narrowing (modified by Fermi-Dirac statistics) are included in the transparent-emitter model.     

Design, fabrication, and analysis of 17-18-percent efficient surface-passivated silicon solar cells

A simple analytical model has been developed which provides useful guidelines for fabricating high-efficiency silicon solar cells. Consistent with the model calculations, both surfaces of n⁺-p-p⁺solar cells were passivated by a thin layer of thermally grown SiO2. Oxide passivation resulted in 17.2-percent efficient solar cells on 4 Ω . cm base material. Passivated cells show about 3 mA/cm²increases in JSC, ∼20 mV improvement in VOC, and ∼2-percent increase in absolute cell efficiency compared to the counterpart 15.2-percent efficient unpassivated cells. The majority of improvement in VOCcame from the emitter surface passivation, while both front- and back-surface passivation contributed to the increase in JSC. The emitter region should not be regarded as a "dead layer" because emitter surface passivation can increase the quantum efficiency at short wavelengths from 40 percent to greater than 75 percent.     

Visualization of Photoexcited Carrier Responses in a Solar Cell Using Optical Pump—Terahertz Emission Probe Technique

We observed photoexcited carrier responses in solar cells excited by femtosecond laser pulses with spatial and temporal resolution using an optical pump-terahertz emission probe technique. We visualized the ultrafast local variation of the intensity of terahertz emission from a polycrystalline silicon solar cell using this technique and clearly observed the change in signals between a grain boundary and the inside of a grain in the solar cell. Further, the time evolution of the pump–probe signals of the polycrystalline and monocrystalline silicon solar cells was observed, and the relaxation times of photoexcited carriers in the emitter layers of crystalline silicon solar cells were estimated using this technique. The estimated relaxation time was consistent with the lifetime of the Auger recombination process that was dominant in heavily doped silicon used as an emitter layer for the silicon solar cells, which is difficult to obtain with photoluminescence method commonly used for the evaluation of solar cells.     

Analysis of high-efficiency silicon solar cells

Performance data on high-efficiency concentrator (>18 percent at 25 suns) silicon solar cells are compared to results from an exact numerical model in which all parameters for the calculation are taken from the existing literature on bulk silicon. The numerical solution of the transport equations includes the effects of Fermi-Dirac statistics, bandgap narrowing, and Auger recombination. Cell performances as a function of sunlight concentration are predicted with reasonable accuracy using this model. Evidence for the existence of bandgap-narrowing effects is found by comparing experimental data to calculated values of spectral quantum efficiencies and open-circuit voltages under a variety of lifetime assumptions. The validity of using superposition with simple diode equations to approximate the behavior of silicon solar cells also is examined.     

Effect of trap location and trap-assisted Auger recombination onsilicon solar cell performance

Model calculations were performed to investigate and quantify the effect of trap location and trap-assisted Auger recombination on silicon solar cell performance. Trap location has a significant influence on the lifetime behavior as a function of doping and injected carrier concentration in silicon. It Is shown in this paper that for a high quality silicon (τ=10 ms at 200 ohm-cm, no intentional doping), high resistivity (⩾200 ohm-cm) is optimum for high efficiency one sun solar cells if the lifetime limiting trap is located near midgap. However, if the trap is shallow (E_t-E_v⩽0.2 eV), the optimum resistivity shifts to about 0.2 ohm-cm. For a low quality silicon material or technology (10 μs at 200 ohm-cm, prior to intentional doping) the optimum base resistivity for one sun solar cells is found to be ~0.2 ohm-cm, regardless of the trap location. It is shown that the presence of a shallow trap can significantly degrade the performance of a concentrator cell fabricated on high-resistivity high-lifetime silicon material because of an undesirable injection level dependence in the carrier lifetime. The effect of trap assisted Auger recombination on the cell performance has also been modelled in this paper. It is found that the trap-assisted Auger recombination does not influence the one sun cell performance appreciably, but can degrade the concentrator cell performance if the trap-assisted Auger recombination coefficient value exceeds 2×10^-14 cm³/s. Therefore, it is necessary to know the starting lifetime as well as trap location in order to specify base resistivity in order to predict or achieve the best cell performance for a given one sun or concentrator cell design     

Recombination in highly injected silicon

Recent advances in solar cells designed to operate under high-level injection conditions have produced devices that are approaching some of the limits imposed by the fundamental band-to-band Auger recombination in Silicon. A device has been optimized to study this recombination by using the fabrication technology developed for point-contact solar cells. Using both steady-state and transient measurements, the recombination rates in high-resistivity Si in the injected carrier density range of 10¹⁵to 2 × 10¹⁷carriers / cm³were investigated. The coefficient of the recombination, which depends on the carrier density cubed, is found to be 1.66 × 10^-30cm⁶/s ± 15 percent. This result is four times higher than the ambipolar Auger coefficient commonly used in the modeling of devices that operate in this injected carrier density range and lowers the expected limit efficiencies for silicon solar cells.     

Collection efficiency in amorphous silicon solar cells

A simple relation is presented for the field assisted collection efficiency for amorphous silicon solar cells, assuming zero geminate recombination. The ratio of the collection efficiency under forward biasing condition to short circuited condition is computed. It is found to agree reasonably well with the experimentally observed values.     

Unified model of fundamental limitations on the performance of silicon solar cells

Previous attempts to explain the substantial discrepancy between observed and predicted efficiencies in silicon solar cells are shown to have treated inadequately two important features of typical devices: 1) In the diffused region the electric-field distribution is much wider than generally believed and the field values away from the junction are generally higher; 2) Auger processes in heavily doped regions have a more pervasive impact than has been recognized. By incorporating a suitable modification of the junction model and a consistent treatment of Auger effects into the analysis, a unified model is developed for the principal limitations on the performance of Si solar cells. This model accounts for limits to Iscand Vocarising in either the front or base region. The present analysis reinterprets the "violet-cell" observations of the effect of the diffusion profile and presents an alternative to the bandgap-narrowing model of the heavy-doping effect on Voc. A new method is developed for evaluating the junction saturation current in heavily doped regions of such solar cells and transistor emitters.     

Separation of local bulk and surface recombination in crystalline silicon from luminescence reabsorption

Spontaneous photoemission of crystalline silicon provides information on excess charge carrier density and thereby on electronic properties such as charge carrier recombination lifetime and series resistance. This paper is dedicated to separating bulk recombination from surface recombination in silicon solar cells and wafers by exploiting reabsorption of spontaneously emitted photons. The approach is based on a comparison between luminescence images acquired with different optical short pass filters and a comprehensive mathematical model. An algorithm to separate both front and back surface recombination velocities and minority carrier diffusion length from photoluminescence (PL) images on silicon wafers is introduced. This algorithm can likewise be used to simultaneously determine back surface recombination velocity and minority carrier diffusion length in the base of a standard crystalline silicon solar cell from electroluminescence (EL) images. The proposed method is successfully tested experimentally. Copyright © 2009 John Wiley & Sons, Ltd.     

Analytical model for the diode saturation current of point‐contacted solar cells

Point‐contacted solar cells exhibit three‐dimensional transport effects due to a spatially inhomogeneous surface recombination. Complex multi‐dimensional finite element simulations are commonly applied to model such devices. This paper presents an empirical analytic equation for the diode saturation current of a point‐contacted base of a solar cell that accounts for three‐dimensional transport. The input parameters of the model that characterize the back surface are: recombination velocity at the contacts; recombination velocity between the contacts; fraction of surface area covered by the contacts; and the contact spacing. We test this model experimentally by conducting spatially resolved minority‐carrier lifetime measurements on silicon wafers with point contacts of various sizes and spacings. The diode saturation currents derived from the lifetime measurements agree with the values predicted by the analytic model. Copyright © 2005 John Wiley & Sons, Ltd.     

PC1D方法对铝背场钝化技术的分析

降低单晶硅原材料成本,采用更薄的硅片作为太阳电池的原料是晶体硅太阳电池产业发展的趋势之一。对薄片化的太阳电池,铝背场的背表面钝化工艺显得愈加重要。采用PC1D太阳电池软件模拟的方法,对以商业用p型硅为衬底的单晶硅125×125太阳电池的铝背场的背表面钝化技术进行了模拟,分析得出,对一定厚度的电池片来说,尤其是当少数载流...     

High-eficiency silicon solar cells: Si/SiO2, interface parameters and their impact on device performance

This article presents the first measurements of the parameters of the Si/SiO₂, interfaces employed on the record-efficiency silicon solar cells made at the University of New South Wales (UNSW). the UNSW oxides are characterized by very low values of the surface state density (∼4 × 10⁻⁹ cm⁻⁻² eV-⁻⁻¹), low values for the positive fixed oxide charge density (∼7 × 10⁻¹⁰ cm⁻⁻²) and a pronounced asymmetry in the capture cross-sections of electrons and holes. Using the microwave-detected photoconductance decay method, we verify experimentally that these oxide parameters produce a strong dependence of the effective surface recombination velocity at oxidized p-type silicon surfaces on the excess electron concentration within the wafer. In high-performance silicon solar cells, this phenomenon produces a long-wavelength spectral response that strongly depends on the bias light intensity, a characteristic ‘hump’ in the dark current-voltage characteristics and fill factors that are lower than would be expected from the high open-circuit voltages of 700 mV.     

Spatially resolved analysis and minimization of resistive losses in high-efficiency Si solar cells

This paper presents an improved method for measuring the total lumped series resistance (R_s) of high-efficiency solar cells. Since this method greatly minimizes the influence of non-linear recombination processes on the measured R_s values, it is possible to determine R_s as a function of external current density over a wide range of illumination levels with a significantly improved level of accuracy. This paper furthermore explains how resistive losses in the emitter, the base, the metal/silicon contacts and the front metal grid can be separately determined by combining measurements and multidimensional numerical simulations. A novel combination of device simulation and circuit simulation is introduced in order to simulate complete 2 × 2 cm² PERL (‘passivated emitter and rear locally-diffused’) silicon solar cells. These computer simulations provide improved insight into the dynamics of resistive losses, and thus allow new strategies for the optimization of resistive losses to be developed. The predictions have been experimentally verified with PERL cells, whose resistive losses were reduced to approximately half of their previous values, contributing to a new efficiency world record (24.0%) for silicon solar cells under terrestrial illumination. The measurement techniques and optimization strategies presented here can be applied to most other types of solar cells, and to materials other than silicon.     

Influence of the junction area to edge area ratio on the open-circuit voltage of silicon solar cells

This work shows experimentally the decrease in open-circuit voltage produced by edge recombination in silicon solar cells. The effect is related to the edge area to junction area ratio.     

GaAs MIS solar cells with evaporated tin oxide interfacial layers

GaAs metal-insulator-semiconductor (MIS) solar cells with vacuum-deposited tin oxide interfacial layers have been investigated. Open-circuit voltages of 0.765 V were observed, 62 percent higher than those of cells made without the tin oxide layer. The dependence of open-circuit voltage on interfacial layer thickness indicated a peak at 20-Å width. The results suggest that VOCcould be further improved by reduction of pinhole area in the interfacial layer.     

Reduced fill factors in multicrystalline silicon solar cells due to injection‐level dependent bulk recombination lifetimes

Recombination lifetimes of multicrystalline silicon solar cell precursors have been measured experimentally as a function of injection‐level, and modeled using Shockley–Read–Hall statistics. The expressions for the variable lifetimes are then used to predict the final cell open‐circuit voltages and fill factors using a simple analytic method. When accurate recombination lifetimes measurements are possible, the predicted parameters match well with the measured values on finished cells. The cells are shown to be limited by the presence of bulk recombination, which not only limits the open‐circuit voltage through lower lifetimes, but also reduces the fill factor due to a strong injection‐level dependence around one‐sum maximum‐power conditions. It is shown that such non‐ideal behaviour cannot be adequately explained by junction recombination. The specific effect of interstitial iron, an important impurity in silicon, on voltages and fill factors is modeled numerically and discussed. Copyright © 2000 John Wiley & Sons, Ltd.     

Limiting efficiency of crystalline silicon solar cells due to Coulomb‐enhanced Auger recombination

Excitonic effects are known to enhance the rate of intrinsic recombination processes in crystalline silicon. New calculations for the limiting efficiency of silicon solar cells are presented here, based on a recent parameterization for the Coulomb‐enhanced Auger recombination rate, which accounts for its dopant type and dopant density dependence at an arbitrary injection level. Radiative recombination has been included along with photon recycling effects modeled by three‐dimensional ray tracing. A maximum cell efficiency of 29.05% has been calculated for a 90‐μm‐thick cell made from high resistivity silicon at 25°C. For 1 Ω cm p‐type silicon, the maximum efficiency reduces from 28.6% for a 55‐μm‐thick cell in the absence of surface recombination, down to 27.0% for a thickness in the range 300–500 μm when surface recombination limits the open‐circuit voltage to 720 mV. Copyright © 2002 John Wiley & Sons, Ltd.     

Quantum efficiency analysis of thin-layer silicon solar cells withback surface fields and optical confinement

Thin-layer silicon solar cells utilize surface textures to increase light absorption and back surface fields to prevent recombination at the silicon-substrate interface. We present an analytical model for the internal quantum efficiency that accounts for light trapping and also considers carrier generation and recombination in back surface fields or substrates. We introduce a graphical representation of experimental data, the so-called Parameter-Confidence-Plot, which allows one to draw maximum information on diffusion lengths and surface recombination velocities from quantum efficiency measurements. The analysis is exemplified for state of the art thin-layer silicon solar cells with and without back surface fields     

The limiting efficiency of silicon solar cells under concentrated sunlight

The intrinsic limits on the energy conversion efficiency of silicon solar cells when used under concentrated sunlight are calculated. It is shown that Auger recombination processes are even more important under concentrated sunlight than nonconcentrated sunlight. However, light trapping can be far more effective under concentrated light due to the better defined direction of incident light. As a result of these effects, the limiting efficiency lies in tile 36-37-percent range regardless of concentration ratio compared to the limiting value of 29.8 percent for a nonconcentrating cell with isotropic response.