Mitigating Plasmonic Absorption Losses at Rear Electrodes in High‐Efficiency Silicon Solar Cells Using Dopant‐Free Contact Stacks

Although charge‐carrier selectivity in conventional crystalline silicon (c‐Si) solar cells is usually realized by doping Si, the presence of dopants imposes inherent performance limitations due to parasitic absorption and carrier recombination. The development of alternative carrier‐selective contacts, using non‐Si electron and hole transport layers, has the potential to overcome such drawbacks and simultaneously reduce the cost and/or simplify the fabrication process of c‐Si solar cells. Nevertheless, devices relying on such non‐Si contacts with power conversion efficiencies (PCEs) that rival their classical counterparts are yet to be demonstrated. In this study, one key element is brought forward toward this demonstration by incorporating low‐pressure chemical vapor deposited ZnO as the electron transport layer in c‐Si solar cells. Placed at the rear of the device, it is found that rather thick (75 nm) ZnO film capped with LiF_x/Al simultaneously enables efficient electron selectivity and suppression of parasitic infrared absorption. Next, these electron‐selective contacts are integrated in c‐Si solar cells with MoO_x‐based hole‐collecting contacts at the device front to realize full‐area dopant‐free‐contact solar cells. In the proof‐of‐concept device, a PCE as high as 21.4% is demonstrated, which is a record for this novel device class and is at the level of conventional industrial solar cells.     

Study of Back Contacts for CdTe Solar Cells

ZnTe/ZnTe:Cu layer is used as a complex back contact.The parmeters of CdTe solar cells with and without the complex back contacts are compared.The effects of un-doped layer thickness,doped concentration and post-deposition annealing temperature of the complex layer on solar cells preformance are investigated.The results show that ZnTe/ZnTe:Cu layer can improve back contacts and largely increase the conversion efficiency of CdTe solar cells.Un-doped layer and post-deposition annealing of high temperature can increase open voltage.Using the complex back contact,a small CdTe cell with fill factor of 73.14% and conversion efficiency of 12.93% is obtained.     

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.     

Solar cell contact resistance—A review

An overview of ohmic contacts on solar cells is presented. The fundamentals of metal-semiconductor contacts are reviewed, including the Schottky approach, Fermi level pinning by surface states, and the mechanisms of thermionic emission, thermionic/field emission, and tunneling for current transport. The concept of contact resistance is developed and contact resistance data for several different contact materials on both silicon and gallium arsenide over a range of doping densities are summarized. Finally, the requirements imposed by solar cells on contact resistance are detailed.     

Solar based large scale power plants: what is the best option?

There are very few published data comparing performance and cost of thermal and photovoltaic (PV) based solar power generations. With recent intense technology and business developments there is a need to establish a comparison between these two solar energy options. We have developed a simple model to compare electricity cost using these two options without any additional fuel source of hybridization. Capital along with operation and maintenance (O&M) costs and other parameters from existing large scale solar farms are used to reflect actual project costs. To compete with traditional sources of power generation, solar technologies need to provide dispatchable electric power to respond to demand during peak hours. Different solutions for energy storage are available. In spite of their high capital cost, adding energy storage is considered a better long term solution than hybrid solar systems for large scale power plants. For this reason, a comparison between the two solar options is also provided that include energy storage. Although electricity storage is more expensive than thermal storage, PV power remains a competitive option. Expenses related to O&M in solar thermal plant are about ten times higher than PV, an important factor resulting in higher energy cost. Based on data from proven commercial technologies, this study showed that PV holds a slight advantage even when energy storage is included. Copyright © 2010 Crown in the right of Canada. Published by John Wiley & Sons, Ltd.     

The truncated-pyramid MIS inversion-layer solar cell: a comprehensive analysis

Metal–insulator–semiconductor inversion-layer (MISIL) solar cells are of significant interest because of their simple fabrication process. In this work a comprehensive analysis of the improved front surface design of truncated-pyramid MISIL silicon solar cells is presented. This analysis reveals the two most important effects that have led to an increase in the open-circuit voltage of more than 50 mV. Firstly, passivation of the non-grid area at the front surface is optimized to meet the special requirements of MISIL solar cells. Secondly, the MIS contact is investigated very thoroughly. This includes the recombination properties of the contact and the current transport via the MIS contact. The results of this investigation show that the contact area can be reduced without an increase in the series resistance of the MISIL solar cells and therefore the recombination losses at the metal contacts are reduced. As a result of these improvements, independently confirmed 1-sun efficiencies above 17% are achieved with truncated-pyramid MISIL solar cells. © 1998 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.     

Analysis of buried contact solar cells and some rules for their optimum design

One of the most promising structures for solar cells is the Buried Contact Solar Cell (BCSC), because it offers the possibility of production at low cost, at the same time that it allows the combination of several high efficiency concepts without any design compromise as occurs in conventional cells. Therefore, there is a growing interest for such kind of solar cells. Unfortunately, this structure poses new problems for its design since the buried contact will cause two-dimensional carrier flow effects under both light and dark current conditions. However, as we describe in this paper, it is possible to make a simple analysis of the conversion efficiency as a function of the buried contact depth in the BCSC cell. In this article we will show that the efficiency is minimum for depths in the range between 50 and 70 μm when the assumed parameters are typical for silicon solar cells. In addition, we will show that in order to have good performance for these cells, the buried contact depth should be very small (below 5 μm), or very large (bigger than 150 μm) instead, but not in-between these values, in order to avoid any efficiency degradation caused by the buried contacts. Finally, we shall show that the efficiency can be better than in conventional cells only when the buried contacts have a large depth, whenever they are designed properly such that they allow additional collection of charge carriers generated by sunlight.     

Diffusion‐based model of local Al back surface field formation for industrial passivated emitter and rear cell solar cells

In this work, the back surface field (BSF) formation of locally alloyed Al‐paste contacts employed in recent industrial passivated emitter and rear cell solar cell designs is discussed. A predictive model for resulting local BSF thickness and doping profile is proposed that is based on the time‐dependent Si distribution in the molten Al paste during the firing step. Diffusion of Si in liquid Al away from the contact points is identified as the main differentiator to a full‐area Al‐BSF; therefore, a diffusion‐based solution to the involved differential equation is pursued. Data on the Si distribution in the Al and the resulting BSF structures are experimentally obtained by firing samples with different metal contact geometries, peak temperature times and pastes as well as by investigating them by means of scanning electron microscopy and energy dispersive X‐ray spectroscopy. The Si diffusivity in the Al paste is then calculated from these results. It is found that the diffusivity is strongly dependent on the paste composition. Furthermore, the local BSF doping profiles and thicknesses resulting from different contact geometries and paste parameters are calculated from the Si concentration at the contact sites, the diffusivity and solubility data. These profiles are then used in a finite element device simulator to evaluate their performance on solar cell level. With this approach, a beneficial paste composition for any given rear contact geometry can be determined. Two line widths are investigated, and the effects of the different paste properties are discussed in the light of the solar cell results obtained by simulation. Copyright © 2013 John Wiley & Sons, Ltd.     

Substrates,contacts and monolithic integration

Substrates and contacts play a critical role in thin-film solar cell device and module performance. They influence light trapping, film growth, impurity levels, doping, stability, yield and laser scribing for monolithic integration. The substrate is also a major cost factor, often accounting for the largest component of the module cost. The interaction between the substrates or contacts with the semiconductor layers can also limit the range of the subsequent semiconductor layer processing parameters. The panel and audience discussed these factors in relation to fabrication, performance and characterization of today's thin-film solar cells and modules. © 1997 John Wiley & Sons, Ltd.     

太阳能电池的基本特性

能源危机与环境污染是人类正面临的重大挑战,开发新能源和可再生清洁能源是21世纪最具决定影响的技术领域之一。太阳能是一种取之不尽、用之不竭的可再生清洁能源,对太阳能电池的研究与开发也变得日益重要。从太阳能电池的结构、工作原理出发,系统地论述了表征太阳能电池特性的短路电流、开路电压和填充因子等参数以及外界条件对他们的影响。     

A life cycle assessment of perovskite/silicon tandem solar cells

Given the rapid progress in perovskite solar cells in recent years, perovskite/silicon (Si) tandem structure has been proposed to be a potentially cost‐effective improvement on Si solar cells because of its higher efficiency at a minimal additional cost. As part of the evaluation, it is important to conduct a life cycle assessment on such technology in order to guide research efforts towards cell designs with minimum environmental impacts. Here, we carry out a life cycle assessment to assess global warming, human toxicity, freshwater eutrophication and ecotoxicity and abiotic depletion potential impacts and energy payback time associated with three perovskite/Si tandem cell structures using silver (Ag), gold (Au) and aluminium (Al) as top electrodes compared with p–n junction and hetero‐junction with intrinsic inverted layer Si solar cells. It was found that the replacement of the metal electrode with indium tin oxide/metal grid in the tandem cell reduces the environmental impacts significantly compared with the perovskite cell. For all the impacts assessed, we conclude that the perovskite/Si tandem using Al as top electrode has better environmental outcomes, including energy payback time, when compared with the other tandem structures studied. Use of Al in preference to noble metals for contacts, Si p–n junction in preference to intrinsic inverted layer and the avoidance of 2,20,7,70‐tetrakis(N ,N‐di‐p‐methoxyphenylamine)9,90‐spirobifluorene (Spiro‐OMeTAD) are environmentally beneficial. The key result found of this work is that the most important factor for the better environmental impacts of these tandem solar cells is the transparency and electrical conductivity of the perovskite layer after it fails. Copyright © 2017 John Wiley & Sons, Ltd.     

Transition resistances of ohmic contacts to p-type cdte and their time-dependent variation

Ohmic contacts to p-type CdTe are important for the development of solar cells based on this semiconductor, as for instance CdS/CdTe or ITO/ CdTe solar cells. Ohmic contacts to CdTe Bridgman crystals, doped with phosphorus, have been examined with respect to their resistivity dependence and their variation as a function of time. The ‘specific’ contact resistance r shows a linear dependence on the bulk resistivity; in addition, it is affected by the oxygen content of the CdTe. The lowest r obtained was 0.07Ω cm. With one exception, ali the contacts with nickel, gold and platinum deposited on different crystals show a more or less pronounced increase of r as a function of time.     

Intermediate band solar cells: Comparison with shockley-read-hall recombination

Intermediate band solar cells are characterized by the existence of a collection of energy levels in the middle of the otherwise conventional semiconductor band gap. According to the standard Shockley-Read-Hall recombination theory, the states corresponding to these energy levels behave as nonradiative recombination centers and, therefore, are detrimental to solar cell performance. Nevertheless, the theory of the intermediate band solar cells predicts an enhancement of the solar cell efficiency well above the limiting efficiency of single gap solar cells (63.2% vs. 40.7%) when these levels exist. This paper clarifies the reasons.     

Silicon solar cells based on all‐laser‐transferred contacts

Crystalline silicon solar cells based on all‐laser‐transferred contacts (ALTC) have been fabricated with both front and rear metallization achieved through laser induced forward transferring. Both the front and rear contacts were laser‐transferred from a glass slide coated with a metal layer to the silicon substrate already processed with emitter formation, surface passivation, and antireflection coating. Ohmic contacts were achieved after this laser transferring. The ALTC solar cells were fabricated on chemically textured p‐type Cz silicon wafers. An initial conversion efficiency of over 15% was achieved on a simple cell structure with full‐area emitter. Further improvements are expected with optimized laser transferring conditions, front grid pattern design, and surface passivation. The ALTC process demonstrates the advantage of laser processing in simplifying the solar cell fabrication by a one‐step metal transferring and firing process. Copyright © 2013 John Wiley & Sons, Ltd.     

Influence of Hole‐Transport Layers and Donor Materials on Open‐Circuit Voltage and Shape of I–V Curves of Organic Solar Cells

The effect of injection and extraction barriers on flat heterojunction (FHJ) and bulk heterojunction (BHJ) organic solar cells is analyzed. The barriers are realized by a combination of p‐type materials with HOMOs varying between –5.0 and –5.6 eV as hole‐transport layer (HTL) and as donor in vacuum‐evaporated multilayer p‐i‐metal small‐molecule solar cells. The HTL/donor interface can be seen as a model for the influence of contacts in organic solar cells in general. Using drift‐diffusion simulations we are well able to reproduce and explain the experimental I–V curves qualitatively. In FHJ solar cells the open‐circuit voltage (V_oc) is determined by the donor and is independent of the HTL. In BHJ solar cells, however, V_oc decreases if injection barriers are present. This different behavior is caused by a blocking of the charge carriers at a spatially localized donor/acceptor heterojunction, which is only present in the FHJ solar cells. The forward current is dominated by the choice of HTL. An energy mismatch in the HOMOs leads to kinks in the I–V curves in the cases for which V_oc is independent of the HTL.     

Electrons and holes in solar cells with partial rear contacts

When the metal contact of a silicon solar cell is restricted to a fraction of the rear surface, the flow of electrons and holes towards that contact is constricted, which is beneficial for minority charge carriers but detrimental for majority carriers. It is possible to describe their 2D/3D transport and determine their concentration in the vertical and transversal dimensions of the solar cell by separately studying the central region near the contact and the peripheral region surrounding it. A virtue of such geometric approach is that it establishes a link between analytical models and computer simulations, providing both physical insight and sufficient accuracy to optimise partial rear contact devices. In this paper, we extend a previous version of the geometric model to solar cells having a full‐area, locally contacted dopant diffusion on the rear surface. The case for n‐type versus p‐type wafers is considered, point contacts are compared with line contacts, including the impact of the metal/semiconductor resistance and bulk recombination is evaluated. Copyright © 2013 John Wiley & Sons, Ltd.     

Optimizing annealing steps for crystalline silicon solar cells with screen printed front side metallization and an oxide‐passivated rear surface with local contacts

Silicon solar cells that feature screen printed front contacts and a passivated rear surface with local contacts allow higher efficiencies compared to present industrial solar cells that exhibit a full area rear side metallization. If thermal oxidation is used for the rear surface passivation, the final annealing step in the processing sequence is crucial. On the one hand, this post‐metallization annealing (PMA) step is required for decreasing the surface recombination velocity (SRV) at the aluminum‐coated oxide‐passivated rear surface. On the other hand, PMA can negatively affect the screen printed front side metallization leading to a lower fill factor. This work separately analyzes the impact of PMA on both, the screen printed front metallization and the oxide‐passivated rear surface. Measuring dark and illuminated IV‐curves of standard industrial aluminum back surface field (Al‐BSF) silicon solar cells reveals the impact of PMA on the front metallization, while measuring the effective minority carrier lifetime of symmetric lifetime samples provides information about the rear side SRV. One‐dimensional simulations are used for predicting the cell performance according to the contributions from both, the front metallization and the rear oxide‐passivation for different PMA temperatures and durations. The simulation also includes recombination at the local rear contacts. An optimized PMA process is presented according to the simulations and is experimentally verified. The optimized process is applied to silicon solar cells with a screen printed front side metallization and an oxide‐passivated rear surface. Efficiencies up to 18.1% are achieved on 148.8 cm² Czochralski (Cz) silicon wafers. Copyright © 2009 John Wiley & Sons, Ltd.     

New experimental evidence for minority-carrier reflection at negative-barrier MIS contacts

Recently, measurements of the open-circuit voltage of solar cells with negative-barrier metal-insulator-semiconductor (MIS) back contacts have been used to demonstrate that such contacts can function as the electrical analogues of metallurgical high-low junctions. In this brief, further experimental evidence for the minority-carrier reflecting properties of the negative-barrier MIS junction is presented. First, it is shown that a negative-barrier Mg-SiOx-nSi back contact can be used to enhance the long-wavelength photoresponse of p⁺-n solar cells in the same manner as a diffused n⁺back-surface field. Secondly, measurements of the effective surface-recombination velocity for an Mg-SiOx-nSi contact and for a diffused n-n⁺high-low junction formed on an identical substrate are reported. Both junctions gave very low values of recombination velocity, on the order of 50 cm/s.     

A review on solar forecasting and power management approaches for energy‐harvesting wireless sensor networks

For rechargeable wireless sensor nodes, effective power management is of prime importance because of the stochastic behaviour of the environmental resources. A key issue in integrating solar resources with wireless sensor networks (WSNs) is the need of precise irradiance measurements and power to resource modelling. WSNs are employed in an adhoc manner comprises of numerous sensing nodes and organised as a network for the sake of checking and balancing the environmental factors. Each node has sensing, computation, communication, and locomotion capabilities but operates with limited battery life. Energy harvesting is a way of powering these WSNs by harvesting energy from the environment. By considering harvested energy as an energy source, certain considerations are different from that of battery‐operated networks. Nondeterministic energy availability with respect to time is the reason behind these differences, which put a limit on the maximum rate at which energy can be used. Thus, reliable knowledge of solar radiation is essential for informed design, deployment planning, and optimal management of energy in rechargeable WSNs. Further, power management is essential in self‐powerssed networks to efficiently utilize the available energy. In this paper, a detailed survey on different solar forecasting techniques has been presented for precise energy estimates. A detailed study on energy efficient power management techniques is also proposed to address the feasibility of energy‐harvesting approach in WSNs.