Decreased emitter sheet resistivity loss in high-eficiency silicon solar cells

State-of-the-art two-dimensional (2D) numerical semiconductor device simulation tools are applied to bifacially contacted silicon solar cells of practical dimensions in order to investigate the 2D effects arising from ohmic voltage drops in cell emitters due to finite front metal grid line spacings. the 2D simulations show that for typical front finger spacings of high-efficiency silicon solar cells the minority carrier flow in the base deviates strongly from the purely linear flow assumed by one-dimensional (1D) theory. Compared to conventional 1D theory, this 2D effect results in reduced emitter sheet resistivity losses, an increased optimum front finger spacing and a reduced impact of finger spacing on cell efficiency. the 2D effects are of particular importance for concentrator solar cells. The 2D simulations presented in this work considerably improve the general understanding of internal device physics of high-efficiency silicon solar cells and reveal the limits of 1D models for the simulation of these devices.     

Experimental and simulated analysis of front versus all‐back‐contact silicon heterojunction solar cells: effect of interface and doped a‐Si:H layer defects

Front silicon heterojunction and interdigitated all‐back‐contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high efficiency and low cost because of their good surface passivation, heterojunction contacts, and low temperature fabrication processes. The performance of both heterojunction device structures depends on the interface between the crystalline silicon (c‐Si) and intrinsic amorphous silicon [(i)a‐Si:H] layer, and the defects in doped a‐Si:H emitter or base contact layers. In this paper, effective minority carrier lifetimes of c‐Si using symmetric passivation structures were measured and analyzed using an extended Shockley–Read–Hall formalism to determine the input interface parameters needed for a successful 2D simulation of fabricated baseline solar cells. Subsequently, the performance of front silicon heterojunction and IBC‐SHJ devices was simulated to determine the influence of defects at the (i)a‐Si:H/c‐Si interface and in the doped a‐Si:H layers. For the baseline device parameters, the difference between the two device configurations is caused by the emitter/base contact gap recombination and the back surface geometry of IBC‐SHJ solar cell. This work provides a guide to the optimization of both types of SHJ device performance, predicting an IBC‐SHJ solar cell efficiency of 25% for realistic material parameters. Copyright © 2013 John Wiley & Sons, Ltd.     

DC‐sputtered ZnO:Al as transparent conductive oxide for silicon heterojunction solar cells with µc‐Si:H emitter

Silicon heterojunction (SHJ) solar cells are highly interesting, because of their high efficiency and low cost fabrication. So far, the most applied transparent conductive oxide (TCO) is indium tin oxide (ITO). The replacement of ITO with cheaper, more abundant and environmental friendly material with texturing capability is a promising way to reduce the production cost of the future SHJ solar cells. Here, we report on the fabrication of the SHJ solar cells with direct current‐sputtered aluminum‐doped zinc oxide (ZnO:Al) as an alternative TCO. Furthermore, we address several important differences between ITO and the ZnO:Al layers including a high Schottky barrier at the emitter/ZnO:Al interface and a high intrinsic resistivity of the ZnO:Al layers. To overcome the high Schottky barrier, we suggest employing micro‐crystalline silicon (µc‐Si:H) emitter, which also improves temperature threshold and passivation of the solar cell precursor. In addition, we report on the extensive studies of the effect of the ZnO:Al deposition parameters including layer thickness, oxygen flow, power density and temperature on the electrical properties of the fabricated SHJ solar cells. Finally, the results of our study indicate that the ZnO:Al deposition parameters significantly affect the electrical properties of the obtained solar cell. By understanding and fine‐tuning all these parameters, a high conversion efficiency of 19.2% on flat wafer (small area (5 × 5 mm²) and without any front metal grid) is achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

Co‐optimisation of the emitter region and the metal grid of silicon solar cells

The procedure for the simultaneous optimisation of the dopant density profile and the front metal grid of silicon solar cells is illustrated with representative cases of laboratory and commercial devices. Contour plots for the saturation current density and the photo‐generated current density of phosphorus doped emitter regions of silicon solar cells are calculated as a function of emitter thickness and dopant concentration, for the particular case of Gaussian dopant profiles. To expose the competing factors of surface and bulk emitter recombination, grid shading and series resistance, the other regions of the device are assumed ideal, that is, loss‐less. The calculated contour plots of the conversion efficiency indicate that relatively thick emitters are optimum if surface passivation is available, whereas thin, heavily doped emitters are preferable in the absence of surface passivation. Copyright © 2000 John Wiley & Sons, Ltd.     

Low-cost back contact silicon solar cells

Back-contacted solar cells offer multiple advantages in regard of reducing module assembling costs and avoiding grid shadowing losses. The investigated emitter-wrap-through (EWT) device design has an electrical connection of the front emitter and the rear emitter grid in form of small holes drilled into the crystalline silicon wafer. The obtained cell structure is especially suitable for low-cost base material with small minority carrier diffusion lengths. Different industrially applicable solar cell manufacturing processes for EWT devices are described and compared. The latest experimental results are presented and interpreted; the photocurrent is found to be distinctly increased. The relation between open circuit voltage and rear side passivation is discussed based on two-dimensional (2-D) computer simulations     

A permeable-base switching device using stacked layers of silicon, silicon dioxide, and silicon-rich silicon dioxide

A novel permeable-base device that is a type of solid-state analog to the vacuum tube has been-constructed using stacked layers of silicon, silicon dioxide, and silicon-rich silicon dioxide. The base or grid, which is formed from thin patterned polycrystalline silicon with degenerate doping, is separated from a single-crystal silicon substrate that acts as the collector or anode by a layer of silicon dioxide. The emitter or cathode is formed on top of the base using a stack of silicon dioxide, silicon-rich silicon dioxide, and degenerately doped polcrystalline silicon (known as an electron injector structure). Current flow from the emitter to the base and collector is modulated by the electric fields created in the silicon dioxide layers by the voltages applied to the various terminals. This unipolar device, which has only electrons carrying the current, is shown to be capable of operation over a wide range of voltages and gains depending on design.     

Study of base series resistance losses in single and double emitter silicon solar cells through simulations and experiments

This work focuses on base series resistance influence on the performance of single and double emitter rear point contact silicon solar cells. This study is performed through measurements on experimental devices with different rear contact sizes and spacings, which were designed and fabricated using standard silicon integrated circuit technology, while the results were compared with simulation data based on a 3D model developed at our institute. Simulation and experimental results show that the series resistance of the double junction structure is significantly lower compared to the single junction equivalent. In addition, it was demonstrated that the operation of both junctions under slightly different voltages improves device efficiency. Copyright © 2008 John Wiley & Sons, Ltd.     

High efficiency polycrystalline silicon solar cells using lowtemperature PECVD process

Conventionally directionally solidified (DS) and silicon film (SF) polycrystalline silicon solar cells are fabricated using gettering and low temperature plasma enhanced chemical vapor deposition (PECVD) passivation. Thin layer (~10 nm) of PECVD SiO₂ is used to passivate the emitter of the solar cell, while direct hydrogen rf plasma and PECVD silicon nitride (Si₃N₄) are implemented to provide emitter and bulk passivation. It is found in this work that hydrogen rf plasma can significantly improve the solar cell blue and long wavelength responses when it is performed through a thin layer of PECVD Si₃N₄. High efficiency DS and SF polycrystalline silicon solar cells have been achieved using a simple solar cell process with uniform emitter, Al/POCl₃ gettering, hydrogen rf plasma/PECVD Si₃N₄ and PECVD SiO₂ passivation. On the other hand, a comprehensive experimental study of the characteristics of the PECVD Si₃N₄ layer and its role in improving the efficiency of polycrystalline silicon solar cells is carried out in this paper. For the polycrystalline silicon used in this investigation, it is found that the PECVD Si₃N₄ layer doesn't provide a sufficient cap for the out diffusion of hydrogen at temperatures higher than 500°C. Low temperature (⩽400°C) annealing of the PECVD Si₃N ₄ provides efficient hydrogen bulk passivation, while higher temperature annealing relaxes the deposition induced stress and improves mainly the short wavelength (blue) response of the solar cells     

Modelling implications of recent silicon bandgap narrowing expressions

Bandgap narrowing expressions used in the recent silicon solar cell literature allow simple insight into their implications for device modelling. This paper addresses issues such as minority carrier concentrations, effective doping densities, built-in electric fields and optimum surface doping concentrations, according to these expressions. © 1997 John Wiley & Sons, Ltd.     

Simulation of losses in thin‐film silicon modules for different configurations and front contacts

A simulation tool for the quantification of electrical losses in thin‐film modules using a one‐ and two‐dimensional electrical PSpice model is presented. Two main sources of electrical losses are examined: monolithic contacts (MC) and front contacts made of a transparent conductive oxide (TCO) layer with or without a metal finger grid. Our study was focussed on amorphous and micromorph silicon modules in substrate or superstrate configuration. Results show that front contact losses (TCO losses and finger losses) prevail. While, under assumption that their subcell performances are the same, performance of amorphous silicon (a‐Si) modules do not depend on the configuration, the superstrate micromorph silicon module has a relatively slight (below 2%) advantage over the substrate counterpart due to lower electrical losses in the MC. Losses of the front contact made of a thick TCO layer or of thin TCO layer and metal finger grid on top were studied for both modules in substrate configuration and optimisation results are presented. Use of thin TCO layer and optimised finger grid and solar cell geometry is competitive and these modules can even outperform the optimised amorphous or micromorph silicon module with thick TCO front contact. In all optimised cases under standard test conditions, total relative losses can be minimised to around 10%. Copyright © 2008 John Wiley & Sons, Ltd.     

Optical properties of thin‐film silicon solar cells with grating couplers

The effect of grating couplers on the optical properties of silicon thin‐film solar cells was studied by a comparison of experimental results with numerical simulations. The thin‐film solar cells studied are based on microcrystalline silicon (μc‐Si:H) absorber layers of thickness in the micrometer range. To investigate the light propagation in these cells, especially in the red wavelength region, three‐dimensional power loss profiles are simulated. The influence of different grating parametres—such as period size, groove height, and shape of the grating—was studied to gain more insight into the light propagation within thin‐film silicon solar cells and to determine an optimized light trapping scheme. The effect of the TCO front and TCO back side layer thickness was investigated. The calculated quantum efficiencies and short‐circuit current densities are in good agreement with the experimental data. The simulations predict further optimization criteria. Copyright © 2006 John Wiley & Sons, Ltd.     

High efficiency UMG silicon solar cells: impact of compensation on cell parameters

High efficiency solar cells have been fabricated with wafers from an n‐type Czochralski grown (Cz) ingot using 100% Upgraded Metallurgical‐Grade (UMG) silicon feedstock. The UMG cells fabricated with a passivated emitter and rear totally diffused (PERT) structure have an independently confirmed cell efficiency of 19.8%. This is the highest efficiency reported for a cell based on 100% UMG silicon at the time of publication. The current and power losses are analysed as a function of measured material parameters, including carrier mobility, lifetime and the presence of the boron–oxygen defect. Dopant compensation is shown to reduce both the minority carrier lifetime and mobility, which significantly affects both the current and voltage of the device. Copyright © 2015 John Wiley & Sons, Ltd.     

Design,fabrication, and analysis of transparent silicon solar cells for multi‐junction assemblies

Transparent silicon solar cells can lead to an increased efficiency of silicon‐based multi‐junction assemblies by transmitting near and below band gap energy light for conversion in a low band gap solar cell. This analysis shows that the maximum efficiency gain for a low band gap solar cell beneath silicon at a concentration of 50 suns is 5.8%, based on ideal absorption and conversion of the photons. This work analyzes the trade‐offs between increased near band edge absorption in the silicon and silicon solar cell transparency. Application of these results to real cases including a germanium bottom solar cell is analyzed, leading to a range of cases with increased system efficiency. Non‐ideal surfaces and real silicon and germanium solar cell device performance are presented. The range of practical system gains may be as low as 2.2 – 1% absolute when compared with the efficiency of a light‐trapped silicon solar cell for 1‐sun operation, based on this work. Copyright © 2012 John Wiley & Sons, Ltd.     

Application of junction capacitance measurements to the characterization of solar cells

The quasi-static capacitance-voltage ( C-V) technique measures the dependence of junction capacitance on the bias voltage by applying a slow, reverse-bias voltage ramp to the solar cell in the dark, using simple circuitry. The resulting C-V curves contain information on the junction area and base dopant concentration, as well as their built-in potential. However, in the case of solar cells made on low to medium resistivity substrates and having thick emitters, the emitter dopant profile has to be taken into account. A simple method can then be used to model the complete C-V curves, which, if the base doping is known, permits one to estimate the emitter doping profile. To illustrate the method experimentally, several silicon solar cells with different base resistivities have been measured. They comprise a wide range of areas, surface faceting conditions and emitter doping profiles. The analysis of the quasi-static capacitance characteristics of the flat surface cells resulted in good agreement with independent data for the wafer resistivity and the emitter doping profile. The capacitance in the case of textured surfaces is a function of the effective junction area, which is otherwise difficult to measure, and is essential to understand the emitter and space charge region recombination currents. The results indicate that the effective area of the junction is not as large as the area of the textured surface.     

Analytical models for the series resistance of selective emitters in silicon solar cells including the effect of busbars

A theoretical analysis of the power loss and series resistance of the front side emitter in silicon solar cells is presented. Existing 1D models (infinitely long finger) and 2D models (including the effect of busbars) of emitter series resistance contribution are extended to the case of selective emitters. The general case of different current densities for both emitters in the selective emitter scheme is considered in these extensions. The resulting models depend on the individual sheet resistances and current densities in both emitters and the device's overall grid geometry. The models are corroborated by finite element simulation of the potential in the emitter. An excellent agreement is found between the analytical models, and the simulations for a wide range of sheet resistances typically encountered in silicon solar cells. Grid simulations using the 2D model are applied to solar cells with selective emitters, where the width of the low‐resistive emitter was varied. The simulations demonstrate that the 2D model can explain the absolute change in fill factor observed in these cells. Copyright © 2013 John Wiley & Sons, Ltd.     

A high-low junction emitter structure for improving silicon solar cell efficiency

A new device structure which is designed to substantially increase the open-circuit voltage and the power-conversion efficiency of p-n junction silicon solar cells is described. The structure differs from the conventional cell structure in that it contains a high-low (H-L) junction in the emitter. Based on numerical solutions of the fundamental Shockley differential equations, efficiency improvements of about 15 percent at AM1 and about 40 percent at 50 suns can be epxected. The improvement at 50 suns results in an efficiency of about 20 percent at 27°C.     

Use of porous silicon antireflection coating in multicrystallinesilicon solar cell processing

The latest results on the use of porous silicon (PS) as an antireflection coating (ARC) in simplified processing for multicrystalline silicon solar cells are presented. The optimization of a PS selective emitter formation results in a 14.1% efficiency multicrystalline (5×5 cm²) Si cell with evaporated contacts processed without texturization, surface passivation, or additional ARC deposition. Specific attention is given to the implementation of a PS ARC into an industrially compatible screen-printed solar cell process. Both the chemical and electrochemical PS ARC formation method are used in different solar cell processes, as well as on different multicrystalline silicon materials. Efficiencies between 12.1 and 13.2% are achieved on large-area (up to 164 cm² ) commercial Si solar cells     

Alternative contact designs for thin epitaxial silicon solar cells

Two major opportunities for increasing the performance of crystalline silicon solar cells involve reducing their thickness and reducing the losses associated with their front metallic grid contacts. Front grid contacted thin epitaxial silicon solar cells based on the growth of crystalline silicon films on a substrate or superstrate have been reported for many years, as have wafer‐based solar cells with alternative contact approaches. Integrating these two concepts into a single device presents an opportunity for simultaneously reducing two major loss mechanisms associated with crystalline silicon solar cells. The opportunities that exist and challenges that must be overcome in order to realize such a device are described in this paper. The design space is defined and explored by considering a wide range of possible approaches. A specific approach was chosen and used to design, grow, and fabricate a proof‐of‐concept thin epitaxial silicon solar cell with an embedded semiconductor grid as an alternative to a conventional front metallic grid. The work presented here has resulted in a thin epitaxial silicon solar cell with a 7·8% designated area conversion efficiency, well isolated contacts, negligible series resistive power loss, and less than 1% shading of the designated area. Copyright © 1999 John Wiley & Sons, Ltd.     

Performance of bifacial HIT solar cells on n-type silicon substrates

The performance of amorphous silicon(a-Si:H) /crystalline silicon(c-Si) heterojunction is studied,and the effects of the emitter layer thickness,doping concentration,intrinsic layer thickness,back heavily-doped n layer,interface state and band offset on the optical and electrical performance of bifacial heterojunction with intrinsic thin-layer(HIT) solar cells on ntype silicon substrates are discussed.It is found that the HIT solar cells on n-type substrates can obtain a higher conversion efficiency than th...