Browsing by Author "Edoff, Marika"
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- Characterization of the Interfacial Defect Layer in Chalcopyrite Solar Cells by Depth‐Resolved Muon Spin SpectroscopyPublication . Alberto, Helena V.; Vilão, Rui C.; Ribeiro, Eduardo F. M.; Gil, João M.; Curado, Marco A.; Teixeira, Jennifer P.; Fernandes, Paulo A.; Cunha, José M. V.; Salomé, Pedro M. P.; Edoff, Marika; Martins, Maria I.; Prokscha, Thomas; Salman, Zaher; Weidinger, AloisAs devices become smaller and more complex, the interfaces between adjacent materials become increasingly important and are often critical to device performance. An important research goal is to improve the interface between the absorber and the window layer by inserting buffer layers to adjust the transition. Depth-resolved studies are key for a fundamental understanding of the interface. In the present experiment, the interface between the chalcopyrite Cu(In,Ga)Se2 absorber and various buffer layers are investigated using low-energy muon spin rotation (μSR) spectroscopy. Depth resolution in the nm range is achieved by implanting the muons with different energies so that they stop at different depths in the sample. Near the interface, a region about 50 nm wide is detected where the lattice is more distorted than further inside the absorber. The distortion is attributed to the long-range strain field caused by defects. These measurements allow a quantification of the corresponding passivation effect of the buffer layer. Bath-deposited cadmium sulfide provides the best defect passivation in the near interface region, in contrast to the dry-deposited oxides, which have a much smaller effect. The experiment demonstrates the great potential of low energy μSR spectroscopy for microscopic interfacial studies of multilayer systems.
- A comparison between thin film solar cells made from co-evaporated CuIn1-xGaxSe2using a one-stage process versus a three-stage processPublication . Salomé, Pedro M.P.; Fjällström, Viktor; Szaniawski, Piotr; Leitão, Joaquim P.; Hultqvist, Adam; Fernandes, Paulo A.; Teixeira, Jennifer P.; Falcão, Bruno P.; Zimmermann, Uwe; Cunha, António F. da; Edoff, MarikaUntil this day, the most efficient Cu(In,Ga)Se2 thin film solar cells have been prepared using a rather complex growth process often referred to as three-stage or multistage. This family of processes is mainly characterized by a first step deposited with only In, Ga and Se flux to form a first layer. Cu is added in a second step until the film becomes slightly Cu-rich, where-after the film is converted to its final Cu-poor composition by a third stage, again with no or very little addition of Cu. In this paper, a comparison between solar cells prepared with the three-stage process and a one-stage/in-line process with the same composition, thickness, and solar cell stack is made. The one-stage process is easier to be used in an industrial scale and do not have Cu-rich transitions. The samples were analyzed using glow discharge optical emission spectroscopy, scanning electron microscopy, X-ray diffraction, current–voltage-temperature, capacitance-voltage, external quantum efficiency, transmission/reflection, and photoluminescence. It was concluded that in spite of differences in the texturing, morphology and Ga gradient, the electrical performance of the two types of samples is quite similar as demonstrated by the similar J–V behavior, quantum spectral response, and the estimated recombination losses.
- 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.
- 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.
- 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.
- 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.
- 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).