Monolithic CIGS-Perovskite Tandem Cell for an Optimal Light Harvesting Without Current Matching

We present a novel monolithic architecture for optimal light harvesting in multijunction thin film solar cells. In the configuration we consider, formed by a perovskite (PVK) cell overlying a CIGS cell, the current extracted from the two different junctions is decoupled by the insertion of a dielectric nonperiodic photonic multilayer structure. This photonic multilayer is designed by an inverse integration approach to confine the incident sunlight above the PVK band gap in the PVK absorber layer, while increasing the transparency for sunlight below the PVK band gap for an efficient coupling into the CIGS bottom cell. To match the maximum power point voltages in a parallel connection of the PVK and CIGS cells, the latter is divided into two subcells by means of a standard three-laser scribing connection. Using realistic parameters for all the layers in the multijunction architecture we predict power conversion efficiencies of 28%. This represents an improvement of 24% and 26% over the best CIGS and PVK single-junction cells, respectively, while at the same time outperforms the corresponding current-matched standard tandem configuration by more than two percentage points.Peer ReviewedPostprint (author's final draft

Will ultrathin CIGS solar cells overtake the champion thin-film cells? Updated SCAPS baseline models reveal main differences between ultrathin and standard CIGS

Cu(In,Ga)Se2 (CIGS) solar cells are amongst the best performing thin-film technologies, with the latest performance gains being mainly due to recent years improvements obtained with post-deposition treatments (PDT). Moreover, thinning of the absorber layer down to sub-micrometre values (ultrathin absorbers) is of extreme importance for CIGS to be even more cost-effective and sustainable. However, electrical and optical limitations, such as rear interface recombination and insufficient light absorption, prevent the widespread implementation of ultrathin CIGS devices. The recent electrical CIGS simulation baseline models have failed to keep up with the experimental developments. Here an updated and experimentally based baseline model for electrical simulations in the Solar Cell Capacitor Simulator (SCAPS) software is presented and discussed with the incorporation of the PDT effects and increased optical accuracy with the support from Finite-Difference Time-Domain (FDTD) simulation results. Furthermore, a champion solar cell with an equivalent architecture validates the developed thin-film model. The baseline model is also applied to ultrathin CIGS solar cell devices, validated with the ultrathin champion cell. Ultimately, these ultrathin models pave the way for an ultrathin baseline model. Simulations results reveal that addressing these absorbers' inherent limitations makes it possible to achieve an ultrathin solar cell with at least 21.0% power conversion efficiency, with open-circuit voltage values even higher than the recent thin-film champion cells.This work was supported by the Fundação para a Ciência e Tecno-logia (FCT) grant numbers DFA/BD/7073/2020, DFA/BD/4564/2020, SFRH/BD/146776/2019, IF/00133/2015, UIDB/50025/2020, UIDP/50025/2020, UIDB/04730/2020, and UIDP/04730/2020. The authors want to acknowledge the funding from the project NovaCell (PTDC/ CTM-CTM/28075/2017). The authors also acknowledge the financial support of the project Baterias 2030, with the reference POCI-01-0247-FEDER-046109, co-funded by Operational Programme for Competitiveness and Internationalization (COMPETE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDFinfo:eu-repo/semantics/submittedVersio

High Efficiency Ultrathin CIGS Solar Cells

The global demand for renewable energy is growing rapidly. Increasing the global share of alternative sources of energy would not only bring environmental benefits, but also enhance overall energy security by diversifying energy supply. Many technology options exist nowadays to harvest the power of the sun, a sustainable energy source, and generate electricity directly from this source via the photovoltaic effect. Among them, Cu(In,Ga)Se2 (CIGS) has gained significant momentum as a possible high efficiency and low cost thin film solar cell material. The capacity to scale up any photovoltaic technology is one of the criteria that will determine its long term viability. In the case of CIGS, many manufacturers are showing the way for GW-scale production capacity. However, as CIGS technology continues to increase its share of the market, the scarcity and high price of indium will potentially affect its ability to compete with other technologies. One way to avoid this bottleneck is to reduce the importance of indium in the fabrication of the cell simply by reducing its thickness without significant efficiency loss. Reducing the thickness of CIGS thin film will not only save the material but will also lower the production time and the power needed to produce the cell. The material properties of Cu(In,Ga)Se2 thin films are different with deposition process. Many different methods to deposit Cu(In,Ga)Se2 thin film have been tried until now but Cu(In,Ga)Se2 thin films prepared by co-evaporation of elemental sources are the most successful due to the control over the sequence of evaporation of individual material. However, the co-evaporation process is a complex process and, depending on the individual sources and substrate temperature, the thin films are grown with different characteristics. The characteristics of these thin films changes with the change in the atomic percentage (at.%) of Cu, In, Ga and Se, which depends on the evaporation conditions. Among different co-evaporation techniques to grow Cu(In,Ga)Se2, 1-stage, 2-stage and 3-stage co-evaporation processes are the most successful processes. Co-evaporation process is the best technique for highly efficient CIGS solar cell but this process needs a precise control of the elemental composition in the vapor flux in order to achieve high quality material, which is not easy to obtain due to the low sticking coefficient of Selenium. One of the major concerns in Cu(In,Ga)Se2 thin film solar cell fabrication that affect significantly the cell performance during deposition is stoichiometry. CIGS with low Se samples exhibited very low Cu content, additional chalcopyrite phases, very small grain size, and poor solar cell performance. So it is very important to find the minimal selenium flux to obtain high quality Cu(In,Ga)Se2 thin film. During the deposition of ultrathin Cu(In,Ga)Se2 film with 1-stage, 2-stage and 3-stage co-evaporation processes, real time spectroscopic ellipsometry (RTSE) is implemented to study the material properties as well as to monitor the process. The deposition processed for individual layer of CIGS device are optimized to enhance the efficiency of the CIGS solar cell

Optimization Of Process Parameters For Faster Deposition Of Cuin1-xgaxs2 And Cuin1-xgaxse2-ysy Thin Film Solar Cells

Thin film solar cells have the potential to be an important contributor to the world energy demand in the 21st century. Among all the thin film technologies, CuInGaSe2 (CIGS) thin film solar cells have achieved the highest efficiency. However, the high price of photovoltaic (PV) modules has been a major factor impeding their growth for terrestrial applications. Reduction in cost of PV modules can be realized by several ways including choosing scalable processes amenable to large area deposition, reduction in the materials consumption of active layers, and attaining faster deposition rates suitable for in-line processing. Selenization-sulfurization of sputtered metallic Cu-In-Ga precursors is known to be more amenable to large area deposition. Sputter-deposited molybdenum thin film is commonly employed as a back contact layer for CIGS solar cells. However, there are several difficulties in fabricating an optimum back contact layer. It is known that molybdenum thin films deposited at higher sputtering power and lower gas pressure exhibit better electrical conductivity. However, such films exhibit poor adhesion to the soda-lime glass substrate. On the other hand, films deposited at lower discharge power and higher pressure although exhibit excellent adhesion show lower electrical conductivity. Therefore, a multilayer structure is normally used so as to get best from the two deposition regimes. A multi-pass processing is not desirable in high volume production because it prolongs total production time and correspondingly increases the manufacturing cost. In order to make manufacturing compliant with an in-line deposition, it is justifiable having fewer deposition sequences. Thorough analysis of pressure and power relationship of film properties deposited at various parameters has been carried out. It has been shown that it is possible to achieve a molybdenum back contact of desired properties in a single deposition pass by choosing iv the optimum deposition parameters. It is also shown that the film deposited in a single pass is actually a composite structure. CIGS solar cells have successfully been completed on the developed single layer back contact with National Renewable Energy Laboratory (NREL) certified device efficiencies \u3e 11%. The optimization of parameters has been carried out in such a way that the deposition of back contact and metallic precursors can be carried out in identical pressure conditions which is essential for in-line deposition without a need for load-lock. It is know that the presence of sodium plays a very critical role during the growth of CIGS absorber layer and is beneficial for the optimum device performance. The effect of sodium location during the growth of the absorber layer has been studied so as to optimize its quantity and location in order to get devices with improved performance. NREL certified devices with efficiencies \u3e 12% have been successfully completed

Optimization of Top Contact for Cu(In,Ga)Se2 Solar Cells

As world energy demands continue to increase, the need to generate electricity from a broader variety of sources, including renewables, is more critical than ever. With costs still 30% higher than those of natural gas, solar energy is a viable contender, but more progress is needed to level the playing field with other forms of energy generation. The overall energy security can be enhanced by diversifying the energy supply. Among them, Cu(In,Ga)Se2 (CIGS) has gained significant momentum as a possible high efficiency and low cost thin film solar cell material. The capacity to scale up any photovoltaic technology is one of the criteria that will determine its long term viability. In the case of CIGS, many manufacturers are showing the way for GW-scale production capacity. However, as CIGS technology continues to increase its share of the market, the scarcity and high price of indium will potentially affect its ability to compete with other technologies. One way to avoid this bottleneck is to reduce the importance of indium in the fabrication of the cell simply by reducing its thickness without significant efficiency loss. Reducing the thickness of CIGS thin film will not only save the material but will also lower the production time and the power needed to produce the cell. As the properties of the absorber and buffer layers are modified with each enhancement, it is also important to continue developing a better and effective light trapping mechanism. The overall reflection losses can be minimized to a great extent by applying an efficient anti-reflective (AR) coating, thus increasing the power conversion efficiency of the device. We describe a method using in-situ real time spectroscopic ellipsometry and optical modeling allowing for the optimization of the thickness of the anti-reflective (AR) coating for Cu(In1-xGax)Se2 (CIGS) solar cells. The model is based on a transfer matrix theory as well as accurate measurement of the dielectric function and thickness of each layer in the stack by spectroscopic ellipsometry. The AR coating thickness is then optimized in real time to optically enhance the performance of the device for various device configurations by varying the thickness and properties of different layers. In ultra-thin CIGS solar cells, multi-layered anti-reflective coatings are essential since a single layer AR coating is not capable of suppressing the reflectance as it increases. Thus it is very important to obtain an enhanced light trap in the red and near infra-red region. Multi-layer AR coatings are used to obtain at least five passes in the internal reflection from the bottom surface of the cell

Physical routes for the synthesis of kesterite

This paper provides an overview of the physical vapor technologies used to synthesize Cu2ZnSn(S,Se)4 thin films as absorber layers for photovoltaic applications. Through the years, CZT(S,Se) thin films have been fabricated using sequential stacking or co-sputtering of precursors as well as using sequential or co-evaporation of elemental sources, leading to high-efficient solar cells. In addition, pulsed laser deposition of composite targets and monograin growth by the molten salt method were developed as alternative methods for kesterite layers deposition. This review presents the growing increase of the kesterite-based solar cell efficiencies achieved over the recent years. A historical description of the main issues limiting this efficiency and of the experimental pathways designed to prevent or limit these issues is provided and discussed as well. Afinal section is dedicated to the description of promising process steps aiming at further improvements of solar cell efficiency, such as alkali doping and bandgap grading1. R Caballero and M León acknowledge financial support via the Spanish Ministry of Science, Innovation and Universities project (WINCOST, ENE2016-80788-C5-2-R) and thank H2020 EU Programme under the project INFINITE-CELL (H2020-MSCA-RISE-2017-777968). 2. S Canulescu and J Schou acknowledge the support from Innovation Fund Denmark. 3. D-H Kim acknowledges financial support via the DGIST R&D Program of the Ministry of Science and ICT, KOREA (18-BD-05). 4.C. Malerba acknowledges the support from the Italian Ministry of Economic development in the framework of the Operating Agreement with ENEA for the Research on the Electric System. 5.A Redinger acknowledges financial support via the FNR Attract program, Project : SUNSPOT, Nr.11244141. 6. E Saucedo thanks H2020 EU Programme under the projects STARCELL (H2020-NMBP-03-2016-720907) and INFINITE-CELL (H2020-MSCA-RISE-2017-777968), the Spanish Ministry of Science, Innovation and Universities for the IGNITE project (ENE2017-87671-C3-1-R), and the European Regional Development Funds (ERDF, FEDER Programa Competitivitat de Catalunya 2007–2013). IREC belong to the SEMS (Solar Energy Materials and Systems) Consolidated Research Group of the ‘Generalitat de Catalunya’ (Ref. 2017 SGR 862). 7. Taltech acknowledges financial support via the Estonian Ministry of Education and Research funding project IUT19-28 and the European Union Regional Development Fund, Project TK141. 8. B Vermang has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (Grant Agreement No 715027

On the importance of joint mitigation strategies for front, bulk, and rear recombination in ultrathin Cu(In,Ga)Se2 solar cells

Several optoelectronic issues, such as poor optical absorption and recombination limit the power conversion efficiency of ultrathin Cu(In,Ga)Se2 (CIGS) solar cells. To mitigate recombination losses, two combined strategies were implemented: a Potassium Fluoride (KF) Post-Deposition Treatment (PDT) and a rear interface passivation strategy based on an Aluminium Oxide (Al2O3) point contact structure. The simultaneous implementation of both strategies is reported for the first time on ultrathin CIGS devices. Electrical measurements and 1-D simulations demonstrate that, in specific conditions, devices with only KF-PDT may outperform rear interface passivated based devices. By combining KF-PDT and rear interface passivation, an enhancement in open-circuit voltage of 178 mV is reached over devices that have a rear passivation only and of 85 mV over devices with only a KF-PDT process. Time-Resolved Photoluminescence measurements showed the beneficial effects of combining KF-PDT and the rear interface passivation at decreasing recombination losses in the studied devices, enhancing charge carrier lifetime. X-ray photoelectron spectroscopy measurements indicate the presence of a In and Se rich layer that we linked to be a KInSe2 layer. Our results suggest that when bulk and front interface recombination values are very high, they dominate and individual passivation strategies work poorly. Hence, this work shows that for ultrathin devices, passivation mitigation strategies need to be implemented in tandem.publishe

Review on Light Management by Nanostructures in Chalcopyrite Solar Cells

Light management has gained wide interest for various types of solar cells. This paper reviews the application of nanostructures for light management to chalcopyrite (CIGSe) type solar cells. Firstly, the relevance of light management for CIGSe solar cells will be introduced and applicable concepts of nanostructures for absorption enhancement discussed. The development of ultra- thin CIGSe solar cells and examples for nanoparticle fabrication techniques together with their chances and challenges for application to CIGSe will be presented. Particular attention will be paid to nanostructures that have been applied to CIGSe solar cells, revealing many theoretical and some experimental results. Metallic and dielectric nanostructures as well as intrinsic nanotextures will be covered. For the future, combined considerations of optical and electrical properties will gain in importance

On the importance of joint mitigation strategies for front, bulk, and rear recombination in ultrathin Cu(In,Ga)Se2 solar cells

Several optoelectronic issues, such as poor optical absorption and recombination limit the power conversion efficiency of ultrathin Cu(In,Ga)Se2 (CIGS) solar cells. To mitigate recombination losses, two combined strategies were implemented: a Potassium Fluoride (KF) Post-Deposition Treatment (PDT) and a rear interface passivation strategy based on an Aluminium Oxide (Al2O3) point contact structure. The simultaneous implementation of both strategies is reported for the first time on ultrathin CIGS devices. Electrical measurements and 1-D simulations demonstrate that, in specific conditions, devices with only KF-PDT may outperform rear interface passivated based devices. By combining KF-PDT and rear interface passivation, an enhancement in open-circuit voltage of 178 mV is reached over devices that have a rear passivation only and of 85 mV over devices with only a KF-PDT process. Time-Resolved Photoluminescence measurements showed the beneficial effects of combining KF-PDT and the rear interface passivation at decreasing recombination losses in the studied devices, enhancing charge carrier lifetime. X-ray photoelectron spectroscopy measurements indicate the presence of a In and Se rich layer that we linked to be a KInSe2 layer. Our results suggest that when bulk and front interface recombination values are very high, they dominate and individual passivation strategies work poorly. Hence, this work shows that for ultrathin devices, passivation mitigation strategies need to be implemented in tandem.publishe

Impact of Sb and Na Doping on the Surface Electronic Landscape of Cu2ZnSnS4 Thin Films

Open-circuit voltage deficiency is the key limiting factor in Cu2ZnSnS4 (CZTS) thin-film solar cells, which is commonly associated with band tails and deep gap states arising from elemental disorder. The introduction of dopants such as Na and Sb has led to improvement in device performance, yet their effects on the optoelectronic properties of CZTS are yet to be fully elucidated. In this Letter, we unraveled the effect of Sb and Na:Sb co-doping on the surface energy landscape of solution-processed CZTS films employing energy-filtered photoelectron emission microscopy. In the absence of the additives, 150 nm resolution photoemission maps reveal oscillations in the local effective work function as well as areas of low photoemission energy threshold. The introduction of dopants substantially reshapes the photoemission maps, which we rationalize in terms of Cu:Zn and Sn disorder. Finally, we establish unprecedented correlations between the photoemission landscape of thin films and the performance of over 200 devices