Institute of Nanostructures, Nanomodelling and Nanofabrication

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Cu(In,Ga)Se2 based ultrathin solar cells the pathway from lab rigid to large scale flexible technology

Publication . 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.

2023Journal article

Open access

Cu(In,Ga)Se$$_2$$-based solar cells for space applications: Proton irradiation and annealing recovery

Publication . 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.

2023Journal article

Restricted access

Multidefect detection tool for large-scale PV plants: Segmentation and classification

Publication . Rocha, Daniel; Alves, Joao; Lopes, Vitor; Teixeira, Jennifer P.; Fernandes, Paulo A.; Costa, Mauro; Morais, Modesto; Salome, Pedro M. P.

Unmanned aerial vehicles (UAVs) with highresolution optical and infrared (IR) imaging have been introduced in recent years to perform inexpensive and fast inspections in operation and maintenance activities of solar power plants, reducing the labor needed, while lowering the on-site inspection time. Even though UAVs can acquire images extremely quickly, the analysis of those images is still a time-consuming procedure that should be performed by a trained professional. Therefore, a computer vision approach may be used to accelerate image analysis. In this work, a dataset of IR images was created from a 10-MW solar power plant and a comparative analysis between mask R- convolutional neural network (CNN) and U-Net was performed for two experiments. Concerning the defective module segmentation, the mask R-CNN algorithm achieved a mean average precision at intersection over union (IoU) = 0.50 of 0.96, using augmentation data. Regarding the segmentation and classification of failure type, the algorithm reached a value of 0.88 considering the same evaluation metric and data augmentation.When compared to the U-Net in terms of IoU, the mask R-CNN outperformed it with 0.87 and 0.83 for the first and second experiments, respectively.

2023Journal article

Restricted access

Over 100 mV VOC improvement for rear passivated ACIGS ultra‐thin solar cells

Publication . 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, Pedro

A 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.

2023Journal article

Restricted access

Cu(In,Ga)Se2 based ultrathin solar cells: the pathway from lab rigid to large scale flexible technology

Publication . 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, Pedro

For 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.

2022Journal article

Open access