An analytical model for high-low-emitter (HLE) solar cells in concentrated sunlight

A current-voltage characteristic is derived for the high-low emitter (HLE) solar cell in concentrated sunlight. For high-level injection, the ambipolar approach is used to yield the complete information of the low emitter concentration region, including the ohmic drop, the Dember voltage, the minority carrier current density, the minority-carrier distribution and the electric field distribution. High doping effects including Auger recombination and bandgap narrowing are considered. The dependences of short-circuit current, open-circuit voltage, fill factor and conversion efficiency on the variations of the geometrical dimensions and material parameters are discussed in detail for silicon single crystal materials. It is shown that the maximum conversion efficiency of 22% at 100 suns AMO can be obtained for silicon high-low emitter solar cell.     

Evaluation of advanced p‐PERL and n‐PERT large area silicon solar cells with 20.5% energy conversion efficiencies

In this paper, we evaluate p‐type passivated emitter and rear locally diffused (p‐PERL) and n‐type passivated emitter and rear totally diffused (n‐PERT) large area silicon solar cells featuring nickel/copper/silver (Ni/Cu/Ag) plated front side contacts. By using front emitter p‐PERL and rear emitter n‐PERT, both cell structures can be produced with only a few adaptations in the entire process sequence because both feature the same front side design: homogeneous n⁺ diffused region with low surface concentration, SiO₂/SiN_x:H passivation, Ni/Cu/Ag plated contacts. Energy conversion efficiencies up to 20.5% (externally confirmed at FhG‐ISE Callab) are presented for both cell structures on large area cells together with power‐loss analysis and potential efficiency improvements based on PC1D simulations. We demonstrate that the use of a rear emitter n‐PERT cell design with Ni/Cu/Ag plated front side contacts enables to reach open‐circuit voltage values up to 676 mV on 1–2 Ω cm n‐type CZ Si. We show that rear emitter n‐PERT cells present the potential for energy conversion efficiencies above 21.5% together with a strong tolerance to wafer thickness and bulk resistivity. Copyright © 2014 John Wiley & Sons, Ltd.     

Opto‐electrical modelling and optimization study of a novel IBC c‐Si solar cell

Interdigitated back contact (IBC) crystalline silicon (c‐Si) solar cells are attracting a lot of attention because of their capability to reach world record conversion efficiency. Because of the relatively complex contact pattern, their design and optimization typically require advanced numerical simulation tools. In this work, a TCAD‐based simulation platform has been developed to account accurately and in detail the optical and passivation mechanisms of front texturization. Its validation has been carried out with respect to a novel homo‐junction IBC c‐Si solar cell based on ion implantation and epitaxial growth, comparing measured and simulated reflectance, transmittance, internal quantum efficiency, external quantum efficiency spectra, and current density–voltage characteristics. As a result of the calibration process, the opto‐electrical losses of the investigated device have been identified quantitatively and qualitatively. Then, an optimization study about the optimal front surface field (FSF) doping, front‐side texturing morphology, and rear side geometry has been performed. The proposed simulation platform can be potentially deployed to model other solar cell architectures than homo‐junction IBC devices (e.g., passivated emitter rear cell, passivated emitter rear locally diffused cell, hetero‐IBC cell). Simulation results show that a not‐smoothed pyramid‐textured front interface and an optimal FSF doping are mandatory to minimize both the optical and the recombination losses in the considered IBC cell and, consequently, to maximize the conversion efficiency. Similarly, it has been showed that recombination losses are affected more by the doping profile rather than the surface smoothing. Moreover, the performed investigation reveals that the optimal FSF doping is almost independent from the front texturing morphology and FSF passivation quality. According to this result, it has been demonstrated that an IBC cell featuring an optimal FSF doping does not exhibit a significant efficiency improvement when the FSF passivation quality strongly improves, proving that IBC cell designs based on low‐doped FSF require a very outstanding passivation quality to be competitive. Deploying an optimization algorithm, the adoption of an optimized rear side geometry can potentially lead to an efficiency improvement of about 1%_abs as compared with the reference IBC solar cell. Further, by improving both emitter and c‐Si bulk quality, a 22.84% efficient solar cell for 280‐μm thick c‐Si bulk was simulated. Copyright © 2017 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.     

17.6% efficient tricrystalline silicon solar cells with spatially uniform texture

Up to now solar cells fabricated on tricrystalline Czochralski‐grown silicon (tri‐Si) have shown relatively low short‐circuit current densities of about 31–33 mA/cm² because the three {110}‐oriented grains cannot effectively be textured by commonly used anisotropic etching solutions. In this work, we have optimised a novel chemical texturing step for tri‐Si and integrated it successfully into our solar cell process. Metal/insulator/semiconductor‐contacted phosphorus‐diffused n⁺p junction silicon solar cells with a silicon‐dioxide‐passivated rear surface and evaporated aluminium contacts were manufactured, featuring a spatially uniform surface texture over all three grains on both cell sides. Despite the simple processing sequence and cell structure, an independently confirmed record efficiency of 17.6% has been achieved. This excellent efficiency is mainly due to an increased short‐circuit current density of 37 mA/cm² obtained by substantially reduced reflection and enhanced light trapping. Copyright © 2003 John Wiley & Sons, Ltd.     

18.45%‐Efficient Multi‐Crystalline Silicon Solar Cells with Novel Nanoscale Pseudo‐Pyramid Texture

Silicon‐based cells could convert more solar energy to electrical energy if the cells could absorb more light. However, the nanostructured cells have demonstrated relatively low power conversion efficiency even when its reflection is very low; thus, they are still far from becoming real products of the photovoltaic industry. Here, nanoscale pseudo‐pyramid textured multi‐crystalline silicon (Pmc‐Si) solar cells, with the best efficiency of ≈18.45%, are fabricated by using a metal‐catalyzed chemical etching plus a post alkaline etching on an industrial production line. Such Pmc‐Si solar cells have showed similar light trapping ability as single crystalline silicon solar cells of micrometer pyramid texture, and the improved efficiency is mainly ascribed to its enhanced light absorption while the nanostructured surface still keeps acceptable passivation quality, that is, the short‐circuit current density has an increase of ≈300 mA cell^–1, while the open‐circuit voltage has only a slight decrease of ≈1 mV. Further elevations of the efficiency are expected by optimizing both micrometer‐ and nanotextures, and exploring more effective passivation technique. More excitingly, the technique presented here has been verified in the production line for several batches as a real technique of low cost and high efficiency.     

PERC背钝化结构提升多晶硅太阳电池的光电转换性能研究

在工业产线上制备了PERC结构的多晶硅太阳电池,并研究了在电池背表面引入PERC背钝化结构对其光电转换性能的影响。结果表明:PERC背钝化结构能够提升电池的短路电流和开路电压,光电转换效率超过了20%。结合光学仿真及分析电池的关键光电参数知,其光电转换性能改善的原因可归结为PERC背钝化结构降低了长波太阳光子在背铝电极的寄生吸収损失和光生载流子的背表面复合损失。PERC背钝化结构能够提升多晶硅太阳电池的光电转换效率,并且其制备工艺与传统产线兼容,是一种优选的产业电池结构。     

Progress with epitaxy wrap‐through crystalline silicon thin‐film solar cells

Wafer‐Equivalents are thin‐film solar cells that use a low‐cost silicon substrate to epitaxially grow a high‐quality crystalline silicon active layer. The epitaxy wrap‐through (EpiWT) cell is a back‐contact version of the Wafer‐Equivalent that aims to increase currents and gain other benefits of back contacts. The EpiWT cell can be made in a symmetrically interdigitated configuration with 50% back emitter coverage, or using an isolation layer to lower the back emitter coverage to ∼10%, which will theoretically increase voltages. The epitaxial deposition through via holes in the substrate depends on many factors, including the sealing of the deposition chamber, and produces various thicknesses and geometrical forms of the layers in the holes. An extended process has been developed to incorporate a passivated selective emitter and the first batch has been fabricated. The best result was an efficiency of 13.2% with ∼22 µm base layer thickness. The results are limited most by the fill factors at this stage, e.g. 75% for this cell, which is due to a processing difficulty encountered with screen‐printing in via holes. A new isolation layer was tested and successfully implemented for the low back‐emitter configuration. Comparable voltages and currents were achieved but the fill factors were lower than for the 50% back emitter cells, resulting in a best efficiency of 11.2%. Copyright © 2011 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.     

InGaP//GaAs//c‐Si 3‐junction solar cells employing spectrum‐splitting system

In this work, we practically demonstrated spectrum‐splitting approach for advances in efficiency of photovoltaic cells. Firstly, a‐Si:H//c‐Si 2‐junction configuration was designed, which exhibited 24.4% efficiency with the spectrum splitting at 620 nm. Then, we improved the top cell property by employing InGaP cells instead of the a‐Si:H, resulting in an achievement of efficiency about 28.8%. In addition, we constructed 3‐junction spectrum‐splitting system with two optical splitters, and GaAs solar cells as middle cell. This InGaP//GaAs//c‐Si architecture was found to deliver 30.9% conversion efficiency. Our splitting system includes convex lenses for light concentration about 10 suns, which provided concentrated efficiency exceeding 33.0%. These results suggest that our demonstration of 3‐junction spectrum‐splitting approach can be a promising candidate for highly efficient photovoltaic technologies. Copyright © 2016 John Wiley & Sons, Ltd.     

Comparison of the open circuit voltage of simplified PERC cells passivated with PECVD silicon nitride and thermal silicon oxide

Plasma enhanced chemical vapor deposited silicon nitride films have been used to passivate both the front and rear surface of simplified PERC silicon solar cells (planar surface, single‐step emitter). An independently confirmed open circuit voltage (V_oc) of 667 mV was measured, proving the outstanding surface passivation provided by the silicon nitride films. The achieved V_oc represents a significant improvement for all‐SiN passivated silicon solar cells. A conversion efficiency of 17˙8% was obtained. For comparison, similar cells with different passivation schemes, including high quality, thermally grown TCA oxides and thin SiO₂/SiN double layers, were also investigated. Open circuit voltages up to 673 mV and conversion efficiencies up to 18˙3% were achieved. Copyright © 2000 John Wiley & Sons, Ltd.     

Performance enhancement of multicrystalline silicon solar cells and modules using double‐layered SiNx:H antireflection coatings

Silicon nitride coating deposited by the plasma‐enhanced chemical vapor deposition method is the most widely used antireflection coating for crystalline silicon solar cells. In this work, we employed double‐layered silicon nitride coating consisting of a top layer with a lower refractive index and a bottom layer (contacting the silicon wafer) with a higher refractive index for multicrystalline silicon solar cells. An optimization procedure was presented for maximizing the photovoltaic performance of the encapsulated solar cells or modules. The dependence of their photovoltaic properties on the thickness of silicon nitride coatings was carefully analyzed. Desirable thicknesses of the individual silicon nitride layers for the double‐layered coatings were calculated. In order to get statistical conclusions, we fabricated a large number of multicrystalline silicon solar cells using the standard production line for both the double‐layered and single‐layered antireflection coating types. On the cell level, the double‐layered silicon nitride antireflection coating resulted in an increase of 0.21%, absolute for the average conversion efficiency, and 1.8 mV and 0.11 mA/cm² for the average open‐circuit voltage and short‐circuit current density, respectively. On the module level, the cell to module power transfer factor was analyzed, and it was demonstrated that the double‐layered silicon nitride antireflection coating provided a consistent enhancement in the photovoltaic performance for multicrystalline silicon solar cell modules than the single‐layered silicon nitride coating. Copyright © 2015 John Wiley & Sons, Ltd.     

Screen‐printed Emitter‐Wrap‐Through solar cell with single step side selective emitter with 18.8% efficiency

A fabrication process for Emitter‐Wrap‐Through solar cells on monocrystalline material with high quality gap passivation by wet thermal silicon dioxide is investigated. Masking and structuring steps are performed by screen‐printing technology. Via‐holes are created by an industrially applicable high‐speed laser drilling process. The cell structure features a selective emitter structure fabricated in a single high temperature step: a highly doped emitter at the via‐holes and the rear side, allowing for a low via‐hole resistivity as well as a low resistivity contact to screen‐printed pastes, and a moderately doped front side emitter exhibiting high quantum efficiency in the low wavelength range. Therefore a novel approach is applied depositing either doped or undoped PECVD silicon dioxide layers on the front side. It is shown that doping profiles advantageous for the EWT‐cell structure can be achieved. The screen‐printed aluminum paste is found to penetrate the underlying thermal dioxide layer at appropriate contact firing conditions leading to a zone of high recombination in the overlap region of aluminum and silicon dioxide. It is shown that conventional PECVD‐anti‐reflection silicon nitride acts as effective protection layer reducing the recombination in this region. Designated area conversion efficiencies up to 18.8% on FZ material are obtained applying the single step side selective emitter fabrication technique. Copyright © 2010 John Wiley & Sons, Ltd.     

Front and Back‐Junction Carbon Nanotube‐Silicon Solar Cells with an Industrial Architecture

In the past, the application of carbon nanotube‐silicon solar cell technology to industry has been limited by the use of a metallic frame to define an active area in the middle of a silicon wafer. Here, industry standard device geometries are fabricated with a front and back‐junction design which allow for the entire wafer to be used as the active area. These are enabled by the use of an intermixed Nafion layer which simultaneously acts as a passivation, antireflective, and physical blocking layer as well as a nanotube dopant. This leads to the formation of a hybrid nanotube/Nafion passivated charge selective contact, and solar cells with active areas of 1–16 cm² are fabricated. Record maximum power conversion efficiencies of 15.2% and 18.9% are reported for front and back‐junction devices for 1 and 3 cm² active areas, respectively. By placing the nanotube film on the rear of the device in a back‐junction architecture, many of the design‐related challenges for carbon nanotube silicon solar cells are addressed and their future applications to industrialized processes are discussed.     

Annual degradation rates of recent crystalline silicon photovoltaic modules

Long‐term reliability and durability of recently installed photovoltaic (PV) systems are currently unclear because they have so far only been operated for short periods. Here, we investigated the quality of six types of recent crystalline silicon PV modules to study the viability of PV systems as dispersed power generation systems under operating conditions connected to an electric power grid. Three indicators were used to estimate the annual degradation rates of the various crystalline silicon PV modules: energy yield, performance ratio, and indoor power. Module performance was assessed both with indoor and outdoor measurements using electric measurements taken over a 3‐year period. The trends in the results of the three indicators were almost consistent with each other. Although the performance of the newly installed PV modules decreased by over 2% owing to initial light‐induced degradation immediately after installation, little to no degradation was observed in all the PV modules composed of p‐type solar cells over a 3‐year operation period. However, the PV modules composed of n‐type solar cells clearly displayed performance degradation originating from the reduction of open‐circuit voltage or potential‐induced degradation. The results indicate that a more continuous and detailed outdoor actual investigation is important to study the quality of new, high‐efficiency solar cells, such as heterojunction, interdigitated back contact solar cells, and passivated emitter rear cells, which are set to dominate the PV markets in the future. © 2017 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons Ltd.     

Open‐circuit voltage recovery in type II GaSb/GaAs quantum ring solar cells under high concentration

We report on the open‐circuit voltage recovery in GaSb quantum ring (QR) solar cells under high solar concentration up to 2500 suns. The detailed behaviour of type II GaSb/GaAs QR solar cells under solar concentration, using different temperatures and light illumination conditions, is analysed through optical and electrical measurements. Although enhancement of the short‐circuit current was observed because of sub‐bandgap photon absorption in the QR, the thermionic emission rate of holes was found to be insufficient for ideal operation. The direct excitation of electron–hole pairs into QRs has revealed that the accumulation of holes is one of the causes of the open‐circuit voltage (V_OC) degradation. However, using concentrated light up to 2500 suns, the GaSb QR cell showed much quicker V_OC recovery rate than a GaAs control cell. Copyright © 2015 John Wiley & Sons, Ltd.     

24·5% Efficiency silicon PERT cells on MCZ substrates and 24·7% efficiency PERL cells on FZ substrates

This paper reports the recent improvements in the energy conversion efficiencies of solar cells on magnetically-confined Czochralski grown (MCZ) and float zone (FZ) silicon substrates at the University of New South Wales. A PERT (passivated emitter, rear totally-diffused) cell structure has been used to reduce the cell series resistance from higher resistivity substrates. The total rear boron diffusion in this PERT structure appears to improve the surface passivation quality of MCZ and some FZ substrates. Hence, higher open-circuit voltages were observed for some PERT cells. One of these cells on MCZ substrates demonstrated 24·5% energy conversion efficiency at Sandia National Laboratories under the standard global spectrum (100 mW/cm²) at 25°C. This is the highest efficiency ever reported for a MCZ silicon solar cell. The cells made on MCZ substrates also showed stable cell performance rather than the usually reported unstable performance for boron-doped CZ substrates. Also reported is a PERL (passivated emitter, rear locally-diffused) cell on a FZ substrate of 24·7% efficiency, which is the highest efficiency ever reported for any silicon solar cell. Copyright © 1999 John Wiley & Sons, Ltd.     

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.     

Economic feasibility of bifacial silicon solar cells

Bifacial solar cells and modules are a promising approach to increase the energy output of photovoltaic systems, and therefore decrease levelized cost of electricity (LCOE). This work discusses the bifacial silicon solar cell concepts PERT (passivated emitter, rear totally diffused) and BOSCO (both sides collecting and contacted) in terms of expected module cost and LCOE based on in‐depth numerical device simulation and advanced cost modelling. As references, Al‐BSF (aluminium back‐surface field) and PERC (passivated emitter and rear) cells with local rear‐side contacts are considered. In order to exploit their bifacial potential, PERT structures (representing cells with single‐sided emitter) are shown to require bulk diffusion lengths of more than three times the cell thickness. For the BOSCO concept (representing cells with double‐sided emitter), diffusion lengths of half the cell thickness are sufficient to leverage its bifacial potential. In terms of nominal LCOE, BOSCO cells are shown to be cost‐competitive under monofacial operation compared with an 18% efficient (≙ p_MPP = 18 mW/cm²) multicrystalline silicon (mc‐Si) Al‐BSF cell and a 19% mc‐Si PERC cell for maximum output power densities of p_MPP ≥ 17.3 mW/cm² and p_MPP ≥ 18.1 mW/cm², respectively. These values assume the use of $10/kg silicon feedstock for the BOSCO and $20/kg for the Al‐BSF and PERC cells. For the PERT cell, corresponding values are p_MPP ≥ 21.7 mW/cm² and p_MPP ≥ 22.7 mW/cm², respectively, assuming the current price offset (≈50%, at the time of October 2014) of n‐type Czochralski‐grown silicon (Cz‐Si) compared with mc‐Si wafers. The material price offset of n‐type to p‐type Cz‐Si wafers (≈15%, October 2014) currently accounts for approximately 1 mW/cm², which correlates to a conversion efficiency difference of 1%_abs for monofacial illumination with 1 sun. From p‐type mc‐Si to p‐type Cz‐Si (≈30% wafer price offset, October 2014), this offset is approximately 2.5 mW/cm² for a PERT cell. When utilizing bifacial operation, these required maximum output power densities can be transformed into required minimum rear‐side illumination intensities for arbitrary front‐side efficiencies η_front by means of the performed numerical simulations. For a BOSCO cell with η_front = 18%, minimum rear‐side illumination intensities of ≤ 0.02 suns are required to match a 19% PERC cell in terms of nominal LCOE. For an n‐type Cz‐Si PERT cell with η_front = 21%, corresponding values are ≤ 0.11 suns with 0.05 suns being the n‐type to p‐type material price offset. This work strongly motivates the use of bifacial concepts to generate lowest LCOE. Copyright © 2016 John Wiley & Sons, Ltd.     

InGaAsP/InGaAs双结太阳电池研究

本文采用MOCVD设备生长了与InP晶格匹配的InGaAs(P)光伏器件。分析了InGaAsP/InGaAs (1.07/0.74 eV)双结太阳电池的QE与I-V特性。在AM1.5D光谱下,InGaAsP/InGaAs双结太阳电池的开路电压,短路电流,填充因子及转换效率分别为0.977 V, 10.2 mA/cm,80.8%,8.94%。对于InGaAsP/InGaAs双结太阳电池,在聚光条件下,其最大转换效率在280个聚光倍数下达到了13%。这一结果预示了GaInP/GaAs/InGaAsP/InGaAs四结太阳电池的潜在应用前景。