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- 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.
- 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.
- Exploiting the Optical Limits of Thin‐Film Solar Cells: A Review on Light Management Strategies in Cu(In,Ga)Se 2Publication . Oliveira, António J. N.; Teixeira, Jennifer P.; Ramos, Duarte; Fernandes, P. A.; Salomé, Pedro M. P.Light management strategies are of utmost importance to allow Cu(In,Ga)Se2 (CIGS) technology market expansion, as it would enable a conversion efficiency boost as well as thinner absorber layers, increasing sustainability and reducing production costs. However, fabrication and architecture constraints hamper the direct transfer of light management architectures from other photovoltaic technologies. The demand for light management in thin and ultrathin CIGS cells is analyzed by a critical description of the optical loss mechanisms in these devices. Three main pathways to tackle the optical losses are identified: front light management architectures that assist for an omnidirectional low reflection; rear architectures that enable an enhanced optical path length; and unconventional spectral conversion strategies for full spectral harvesting. An outlook over the challenges and developments of light management architectures is performed, establishing a research roadmap for future works in light management for CIGS technology. Following the extensive review, it is expected that combining antireflection, light trapping, and conversion mechanisms, a 27% CIGS solar cell can be achieved.
- Cu(In,Ga)Se2 based ultrathin solar cells: the pathway from lab rigid to large scale flexible technologyPublication . Lopes, Tomás; Teixeira, Jennifer; Curado, Marco; Ferreira, Bernado; Oliveira, Antonio; Cunha, José; Monteiro, Margarida; Violas, André; Barbosa, João; Sousa, Patricia; Çaha, Ihsan; Borme, Jérôme; Oliveira, Kevin; Ring, Johan; Chen, Wei; Zhou, Ye; Takei, Klara; Niemi, Esko; Francis, Leonard; Edoff, Marika; Brammertz, Guy; Fernandes, Paulo; Vermang, Bart; Salomé, PedroFor the first time, the incorporation of interface passivation structures in ultrathin Cu(In,Ga)Se2 (CIGS) based solar cells is shown in a flexible lightweight stainless-steel substrate. The fabrication was based on an industry scalable lithography technique - nanoimprint lithography (NIL) - for a 15x15 cm2 dielectric layer patterning, needed to reduce optoelectronic losses at the rear interface. The nanopatterning schemes are usually developed by lithographic techniques or by processes with limited scalability and reproducibility (nanoparticle lift-off, spin-coating, etc). However, in this work the dielectric layer is patterned using NIL, a low cost, large area, high resolution, and high throughput technique. To assess the NIL performance, devices with a NIL nanopatterned dielectric layer are benchmarked against electron-beam lithography (EBL) patterning, using rigid substrates. Up to now, EBL is considered the most reliable technique for patterning laboratory samples. The device patterned by NIL shows similar light to power conversion efficiency average values compared to the EBL patterned device - 12.6 % vs 12.3 %, respectively - highlighting the NIL potential for application in the solar cell sector. Moreover, the impact of the lithographic processes, such as different etch by-products, in the rigid solar cells’ figures of merit were evaluated from an elemental point of view via X-ray Photoelectron Spectroscopy and electrically through a Solar Cell Capacitance Simulator (SCAPS) fitting procedure. After an optimised NIL process, the device on stainless-steel achieved an average power conversion efficiency value of 11.7 % - a slightly lower value than the one obtained for the rigid approach, due to additional challenges raised by processing and handling steel substrates, even though scanning transmission electron microscopy did not show any clear evidence of impurity diffusion towards the absorber. Notwithstanding, time-resolved photoluminescence results strongly suggested the presence of additional non-radiative recombination mechanisms in the stainless-steel absorber, which were not detected in the rigid solar cells, and are compatible with elemental diffusion from the substrate. Nevertheless, bending tests on the stainless-steel device demonstrated the mechanical stability of the CIGS-based device up to 500 bending cycles.