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- Cu(In,Ga)Se$$_2$$-based solar cells for space applications: Proton irradiation and annealing recoveryPublication . Candeias, Maria B.; Fernandes, Tiago V.; Falcão, Bruno P.; Cunha, António F.; Cunha, José M. V.; Barbosa, João; Teixeira, Jennifer P.; Fernandes, Paulo A.; Peres, Marco; Lorenz, Katharina; Salomé, Pedro M. P.; Leitão, Joaquim P.In this work, we present an experimental study of a Cu(In,Ga)Se2 (CIGS)-based solar cell (SC), irradiated with protons of energy 80 and 180 keV and with fuences of 1012 , 1013 , and 1014 cm−2 , as well as a strategy to recover the induced damage. The possible modifcations of the structural, electrical, and optical properties, induced by the proton irradiation, were investigated. Although the irradiation did not promote any major modifcation in the crystalline structure, it did induce the creation of defects responsible for changes in the electronic structure which caused a partial PL quenching and signifcant changes in the PL spectral shape, as well as a reduction of the power conversion efciency and open-circuit voltage of up to 30% as revealed by J–V measurements. The photoluminescence results showed a broadening, redshift and decrease in the signal-to-noise ratio. The recovery of damage induced by irradiation in several SCs was tested through annealing steps performed at diferent temperatures and time intervals. It was found that the best recovery strategy for the investigated irradiation parameters was carrying out several isothermal annealing at 200°C for 30 min. This strategy is compatible with the intermitent variation of the temperature in space and allowed to recover a power conversion efciency comparable to that of the as grown cell. In particular, it must be highlighted that keeping the SC at room temperature in ambient atmosphere and in the dark, did not promote signifcant recovery in contradiction with some previous reports. This recovery methodology was applied in parallel for non-irradiated SCs and no increase in power conver sion efciency was found, but rather a slight decrease. The dominant radiative recombination channel was, apparently, unchanged with the irradiation and the subsequent recovery process. Nonetheless, changes in the concentration of defects of diferent types cannot be excluded, which is in line with a signifcant infuence of fuctuating potentials in both as grown and after recovery stages of the solar cell. This work constitutes a frst systematic study that simultaneously encompasses the infuence of proton irradiation on the optical and electrical properties of CIGS SCs and a damage recovery methodology with a high potential to be explored in space applications. Additionally, it contributes to reinforcing the high potential of CIGS technology in the context of creating constellations of small satellites that are being developed by diferent entities, particularly private ones.
- Coupling of plasmonic nanoparticles on a semiconductor substrate via a modified discrete dipole approximation methodPublication . Carvalho, Diogo F.; Martins, Manuel A.; Fernandes, P. A.; Correia, M. Rosário P.Understanding the plasmonic coupling between a set of metallic nanoparticles (NPs) in a 2D array, and how a substrate affects such coupling, is fundamental for the development of optimized optoelectronic structures. Here, a simple semi-analytical procedure based on discrete dipole approximation (DDA) is reported to simulate the far-field and near-field properties of arrays of NPs, considering the coupling between particles, and the effect of the presence of a semiconductor substrate based on the image dipole approach. The method is validated for Ag NP dimers and single Ag NPs on a gallium nitride (GaN) substrate, a semiconductor widely used in optical devices, by comparison with the results obtained by the finite element method (FEM), indicating a good agreement in the weak coupling regime. Next, the method is applied to square and random arrays of Ag NPs on a GaN substrate. The increase in the surface density of NPs on a GaN substrate mainly results in a redshift of the dipolar resonance frequency and an increase in the near-field enhancement. This model, based on a single dipole approach, grants very low computational times, representing an advantage to predict the optical properties of large NP arrays on a semiconductor substrate for different applications.
- Electronic Conduction Mechanisms and Defects in Polycrystalline Antimony SelenidePublication . Cifuentes, N.; Ghosh, Santunu; Shongolova, A.; Correia, M. R.; Salomé, P. M. P.; Fernandes, P. A.; Ranjbar, S.; Garud, S.; Vermang, B.; Ribeiro, G. M.; González, J. C.A study of the electronic conduction mechanisms and electrically active defects in polycrystalline Sb2Se3 is presented. It is shown that for temperatures above 200 K, the electrical transport is dominated by thermal emission of free holes, ionized from shallow acceptors, over the intergrain potential barriers. In this temperature range, the temperature dependence of the mobility of holes, limited by the intergrain potential barriers, is the main contributor to the observed thermal activation energy of the conductivity of 485 meV. However, at lower temperatures, nearest-neighbor and Mott variable range hopping transport in the bulk of the grains turn into the dominant conduction mechanisms. Important parameters of the electronic structure of the Sb2Se3 thin film such as the average intergrain potential barrier height ϕ = 391 meV, the intergrain trap density Nt = 3.4 × 1011 cm−2, the shallow acceptor ionization energy EA0 = 124 meV, the acceptor density NA = 1 × 1017 cm−3, the net donor density ND = 8.3 × 1016 cm−3, and the compensation ratio k = 0, 79 were determined from the analysis of these measurements.
- Over 100 mV VOC improvement for rear passivated ACIGS ultra‐thin solar cellsPublication . Oliveira, Antonio; Rocha Curado, Marco; Teixeira, J. P.; Tomé, Daniela; Çaha, Ihsan; Oliveira, Kevin; Lopes, Tomás; Monteiro, Margarida; Violas, André; Correira, Maria; Fernandes, Paulo; Deepak, Francis; Edoff, Marika; Salomé, PedroA decentralized energy system requires photovoltaic solutions to meet new aesthetic paradigms, such as lightness, flexibility, and new form factors. Notwithstanding, the materials shortage in the Green Transition is a concern gaining momentum due to their foreseen continuous demand. A fruitful strategy to shrink the absorber thickness, meeting aesthetic and shortage materials consumption targets, arises from interface passivation. However, a deep understanding of passivated systems is required to close the efficiency gap between ultra-thin and thin film devices, and to mono-Si. Herein, a (Ag,Cu)(In,Ga)Se2 ultra-thin solar cell, with 92% passivated rear interface area, is compared with a conventional nonpassivated counterpart. A thin MoSe2 layer, for a quasi-ohmic contact, is present in the two architectures at the contacts, despite the passivated device narrow line scheme. The devices present striking differences in charge carrier dynamics. Electrical and optoelectronic analysis combined with SCAPS modelling suggest a lower recombination rate for the passivated device, through a reduction on the rear surface recombination velocity and overall defects, comparing with the reference solar cell. The new architecture allows for a 2% efficiency improvement on a 640 nm ultra-thin device, from 11% to 13%, stemming from an open circuit voltage increase of 108 mV.
- Design of experiments optimization of fluorine-doped tin oxide films prepared by spray pyrolysis for photovoltaic applicationsPublication . Pinheiro, X.L.; Vilanova, A.; Mesquita, D.; Monteiro, M.; Eriksson, J.A.M.; Barbosa, J.R.S.; Matos, C.; Oliveira, A.J.N.; Oliveira, K.; Capitão, J.; Loureiro, E.; Fernandes, Paulo A.; Mendes, A.; Salomé, P.M.P.Transparent conducting oxides (TCOs) have witnessed an ever-expanding use in our lives through many optoelectronic applications, namely photovoltaic (PV) devices. Indium-tin oxide (ITO) is the most used and studied TCO, but it lacks thermal and chemical stability and indium is a scarce and toxic element. Fluorine-doped tin oxide (FTO) emerged as the most promising alternative to ITO, presenting better thermal and chemical stability. Among the numerous techniques for depositing FTO thin films over glass substrates, spray pyrolysis is the simplest and most economical, with great potential for upscaling. However, the relative importance of the experimental variables that influence the optoelectronic properties remains barely addressed. Following this premise, the present work aimed at optimizing the deposition of FTO films on soda lime glass (SLG) substrates by spray pyrolysis following a Design of Experiments (DoE) methodology. The optoelectronic properties of FTO-SLG substrates was evaluated based on their optical transmittance and sheet resistance, both combined in a figure of merit (FoM) tailored for PV applications. It was concluded that the volume of sprayed solution and the fluorine/tin ratio in the precursor have the greatest influence in the FoM, being the optimal deposition conditions a sprayed volume of 60.8 ml and a [F]/[Sn] ratio of 0.45. FTO-SLG substrates prepared with these conditions achieved a FoM of 0.680 Ω□−1/10, corresponding to a sheet resistance of 3.40 Ω□ and a transmittance equivalent to 77% of the maximum current generated in the considered spectrum. The improved FoM was validated in dye-sensitized and perovskite solar cells (DSSCs and PSCs, respectively), assembled with in-house optimized and commercial substrates. The efficiency of DSSCs was improved by 8.9% (relative), whereas PSCs achieved a light-to-power efficiency of 17% (absolute), corresponding to an improvement of 4.7% (relative).
- Decoupling of Optical and Electrical Properties of Rear Contact CIGS Solar CellsPublication . Cunha, Jose M. V.; Fernandes, P. A.; Salome, Pedro M. P.; Lopes, Tomas S.; Bose, Sourav; Hultqvist, Adam; Chen, Wei-Chao; Donzel-Gargand, Olivier; Ribeiro, Rodrigo M.; Oliveira, Antonio J. N.; Edoff, MarikaA novel architecture that comprises rear interface passivation and increased rear optical reflection is presented with the following advantages: i) enhanced optical reflection is achieved by the deposition of a metallic layer over the Mo rear contact; ii) improved interface quality with CIGS by adding a sputtered Al 2 O 3 layer over the metallic layer; and, iii) optimal ohmic electrical contact ensured by rear-openings refilling with a second layer of Mo as generally observed from the growth of CIGS on Mo. Hence, a decoupling between the electrical function and the optical purpose of the rear substrate is achieved. We present in detail the manufacturing procedure of such type of architecture together with its benefits and caveats. A preliminary analysis showing an architecture proof-of-concept is presented and discussed.
- Rear Optical Reflection and Passivation Using a Nanopatterned Metal/Dielectric Structure in Thin-Film Solar CellsPublication . Lopes, Tomas S.; Cunha, Jose M. V.; Bose, Sourav; Barbosa, Joao R. S.; Borme, Jerome; Donzel-Gargand, Olivier; Rocha, Celia; Silva, Ricardo; Hultqvist, Adam; Chen, Wei-Chao; Silva, Ana G.; Edoff, Marika; Fernandes, P. A.; Salome, Pedro M. P.Currently, one of the main limitations in ultrathin Cu(In,Ga)Se 2 (CIGS) solar cells are the optical losses, since the absorber layer is thinner than the light optical path. Hence,light management, including rear optical reflection, and light trapping is needed. In this paper, we focus on increasing the rear optical reflection. For this, a novel structure based on having a metal interlayer in between the Mo rear contact and the rear passivation layer is presented. In total, eight different metallic interlayers are compared. For the whole series, the passivation layer is aluminum oxide (Al 2 O 3 ). The interlayers are used to enhance the reflectivity of the rear contact and thereby increasing the amount of light reflected back into the absorber. In order to understand the effects of the interlayer in the solar cell performance both from optical and/or electrical point of view, optical simulations were performed together with fabrication and electrical measurements. Optical simulations results are compared with current density-voltage (J-V) behavior and external quantum efficiency measurements. A detailed comparison between all the interlayers is done, in order to identify the material with the greatest potential to be used as a rear reflective layer for ultrathin CIGS solar cells and to establish fabrication challenges. The Ti-W alloy is a promising a rear reflective layer since it provides solar cells with light to power conversion efficiency values of 9.9%, which is 2.2% (abs) higher than the passivated ultrathin sample and 3.7% (abs) higher than the unpassivated ultrathin reference sample.
- Cu(In,Ga)Se2 based ultrathin solar cells the pathway from lab rigid to large scale flexible technologyPublication . Lopes, T. S.; Teixeira, J. P.; Curado, M. A.; Ferreira, B. R.; Oliveira, A. J. N.; Cunha, J. M. V.; Monteiro, M.; Violas, A.; Barbosa, J. R. S.; Sousa, P. C.; Çaha, I.; Borme, J.; Oliveira, K.; Ring, J.; Chen, W. C.; Zhou, Y.; Takei, K.; Niemi, E.; Deepak, F. L.; Edoff, M.; Brammertz, G.; Fernandes, P. A.; Vermang, B.; Salomé, P. M. P.The incorporation of interface passivation structures in ultrathin Cu(In,Ga)Se2 based solar cells is shown. The fabrication used an industry scalable lithography technique—nanoimprint lithography (NIL)—for a 15 × 15 cm2 dielectric layer patterning. Devices with a NIL nanopatterned dielectric layer are benchmarked against electron-beam lithography (EBL) patterning, using rigid substrates. The NIL patterned device shows similar performance to the EBL patterned device.The impact of the lithographic processes in the rigid solar cells’ performance were evaluated via X-ray Photoelectron Spectroscopy and through a Solar Cell Capacitance Simulator. The device on stainless-steel showed a slightly lower performance than the rigid approach, due to additional challenges of processing steel substrates, even though scanning transmission electron microscopy did not show clear evidence of impurity diffusion. Notwithstanding, time-resolved photoluminescence results strongly suggested elemental diffusion from the flexible substrate. Nevertheless, bending tests on the stainless-steel device demonstrated the mechanical stability of the CIGS-based device.
- SiOx Patterned Based Substrates Implemented in Cu(In,Ga)Se2 Ultrathin Solar Cells: Optimum ThicknessPublication . Oliveira, Kevin; Teixeira, Jennifer P.; Chen, Wei-Chao; Lontchi Jioleo, Jackson; Oliveira, Antonio J. N.; Caha, Ihsan; Francis, Leonard Deepak; Flandre, Denis; Edoff, Marika; Fernandes, Paulo A.; Salome, Pedro M. P.Interface recombination in sub-µm optoelectronics has a major detrimental impact on devices’ performance, showing the need for tailored passivation strategies to reach a technological boost. In this work, SiOx passivation based substrates were developed and integrated into ultrathin Cu(In,Ga)Se2 (CIGS) solar cells. This study aims to understand the impact of a passivation strategy, which uses several SiOx layer thicknesses (3, 8, and 25 nm) integrated into high performance substrates (HPS). The experimental study is complemented with 3D Lumerical finite-difference time-domain (FDTD) and 2D Silvaco ATLAS optical and electrical simulations, respectively, to perform a decoupling of optical and electronic gains, allowing for a deep discussion on the impact of the SiOx layer thickness in the CIGS solar cell performance. This study shows that as the passivation layer thickness increases, a rise in parasitic losses is observed. Hence, a balance between beneficial passivation and optical effects with harmful architectural constraints defines a threshold thickness to attain the best solar cell performance. Analyzing their electrical parameters, the 8 nm novel SiOx based substrate achieved a light to power conversion efficiency value of 13.2 %, a 1.3 % absolute improvement over the conventional Mo substrate (without SiOx).
- Will ultrathin CIGS solar cells overtake the champion thin-film cells? Updated SCAPS baseline models reveal main differences between ultrathin and standard CIGSPublication . Violas, André F.; Oliveira, António J.N.; Teixeira, Jennifer P.; Lopes, Tomás S.; Barbosa, João R.S.; Fernandes, Paulo A.; Salomé, Pedro M.P.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.