André Filipe Violas

This study investigates the interface between (Ag,Cu)(In,Ga)Se ₂ (ACIGS) and In ₂ O ₃ :Sn (ITO) in bifacial solar cells with X-ray Photoelectron Spectroscopy (XPS), Scanning Transmission Electron Microscopy (STEM) and Energy-dispersive X-ray Spectroscopy (STEM-EDS). The aim is to characterize the GaO _x layer forming at the rear interface in solar cell samples with varying NaF precursor layer, maximum deposition temperature, deposition time, and Ga-concentration. The GaO _x layer is present in all samples, even when no NaF is added. Temperature and excess NaF appear to increase the thickness or roughness of the GaO _x layer, yet a continuous layer of < 3 nm is found at the ACIGS/ITO interface in all samples. Good performance could be reached, with FF∽72-76% even with a GaO _x layer present. Considering the limited thickness of the GaO _x layer measured in this study, it does not block the current flow and could have a degree of passivation on the rear interface of the solar cells, leading to the high FF.

Building-integrated photovoltaics (BIPV) represents a promising approach for energy generation in urban environments, although balancing aesthetics and photovoltaic performance remains challenging. This work presents and validates a novel tunable coloring strategy for flexible Cu(In,Ga)Se2 (CIGS) solar cells that allows to balance optical induced losses with gains while reducing color angle dependence. Inspired by the light-emitting diode display technology, the strategy combines micropatterned Red, Green, and Blue Bragg reflector (BR) coatings in adjustable proportions to achieve tailored colors. Two line-based microarchitectures were fabricated on soda lime glass and flexible CIGS solar cells. White coloration was selected due to its compatibility with common building facades, with patterns designed to appear visually uniform at distances higher than a 1 m viewing distance. The developed process demonstrated reproducible defect-free microfabrication and successful color perception. Compared to conventional uniform BRs, the RGB pixel coloration strategy significantly reduced color angle dependence and improved optical performance, limiting short-circuit current density losses to below 1.5 mA·cm–2. Moreover, individual color BR led to a short-circuit current loss of 1% to 4% or an enhancement of nearly 10% depending on the final color. The proposed strategy demonstrates strong potential for efficient, aesthetically integrated colored BIPV applications.

Thin‐film Cu(In,Ga)Se ₂ solar cells are a key photovoltaic technology for addressing global energy and environmental challenges. Reducing absorber thickness lowers manufacturing cost and critical raw material consumption but introduces optical and electrical losses due to incomplete light absorption and increased rear interface recombination. Rear recombination can be mitigated through Ga grading and/or rear contact passivation, for example using tunnel oxide as in c‐Si TOPCon technology. In this work, a full coverage plasma enhanced chemically vapor deposited 8 nm porous SiO _x passivation layer was implemented in ultra‐thin (Ag,Cu)(In,Ga)Se ₂ (ACIGS) solar cells and compared with patterned passivation layers (∼70% coverage) and an unpassivated reference. Standard current‐voltage and external quantum efficiency characterization methods show that the complete passivation yielded the highest performance, achieving 16.4% efficiency, a 2% absolute improvement over the reference. Patterned samples showed enhanced performance to a lower extent. Elemental and structural analysis showed that the SiO _x layers remained intact after 550°C deposition of the 700 nm ACIGS absorber. Notably, a MoSe ₂ interfacial layer formed between the full SiO _x layer and Mo contact, likely due to Na and/or Se diffusion through SiO _x pores. Combining near‐complete passivation and the ohmic MoSe ₂ contact may explain the improved performance.

Reducing the absorber thickness in thin‐film photovoltaics has a direct impact on low raw material consumption and high manufacturing throughput. However, it leads to low optical performance due to incomplete light absorption. In (Ag,Cu)(In,Ga)Se ₂ (ACIGS)‐based technology, this limitation has been manifested in a photogenerated current density gap between standard and ultrathin devices, hindering their competitiveness and scalability. In this study, it is demonstrated that the optical performance gap of ultrathin ACIGS solar cells to standard devices can be closed by implementing a scalable light management structure at the rear contact. The core of such advancement is a nanoscale‐structured SiO _x periodic array, designed on a transparent rear contact through optical simulations to promote light scattering and increase the optical path length inside the absorber, and developed by a suitable nanoimprint lithography process. This strategy enables 500–600 nm ACIGS solar cells to reach a short‐circuit current density of 36.2 mA cm ⁻² , surpassing 80% of the Shockley–Queisser limit, which has not been reported for such ultrathin devices. The proposed architecture provides gains in bifacial configuration. This way, a fundamental performance limitation of ultrathin solar cells is overcome, enabling material‐efficient and integrated photovoltaic applications.

Rear interface passivation on submicrometer Cu(In,Ga)Se₂ (CIGS) solar cells through nanopatterned dielectric layers has enabled performance gains. The combined optimization of the rear contact interface and the absorber properties enables high-efficiency ultrathin CIGS solar cells. However, the absorber quality of a standard thin 2000 nm CIGS has not been suitably maintained in absorbers with thickness values around 500 nm. Establishing an optimized Na doping, absorber crystallinity, and bandgap profile on such submicrometer CIGS solar cells has been challenging. Moreover, the Na doping may also be influenced by the rear contact architectures implemented in passivated solar cells, e.g., affecting the diffusion of Na from the soda-lime glass. Nonetheless, a proper Na supply into CIGS is critical for an optimized absorber quality. Therefore, this study evaluates the impact of the amount of Na supply controlled through a NaF precursor layer on a rear passivated ultrathin CIGS solar cell. The NaF layer thickness leading to the best performance solar cells in this study, attributed to high absorber crystallinity and low ordered vacancy compound content, is lower than that used in standard thin 2000 nm cells. It suggests that the transition to ultrathin solar cells is not straightforward, and the cell architecture influences the conditions to reach the optimum absorber properties.

Ultrathin (Ag,Cu)(In,Ga)Se₂ (ACIGS) solar cells enable material savings, high manufacturing throughput, and application versatility. Moreover, the reduced absorber thickness relieves European concerns about critical raw material shortages. However, ultrathin device performance does not yet compete with their thicker counterparts due to increased rear contact recombination and incomplete light absorption. This work proposes a rear contact architecture that encapsulates a 25 nm Au patterned layer with 20 nm of Al₂O₃, increasing the rear contact reflectance while passivating the rear interface defects. However, one of several challenges with nanofabrication is aligning two nanopatterns. Therefore, the nanofabrication of the rear architecture is optimized to encompass one‐step nanoimprint lithography without requiring alignment. Two ACIGS absorber growth temperature values of 550 and 450 °C are evaluated, with attention to their effect on rear architecture integrity. The rear contact remains intact when the absorber growth temperature is 450 °C. In such conditions, the proposed architecture increases the solar cell conversion efficiency by 1.5% abs. compared to a reference cell with Mo, resulting from a short‐circuit current density gain of 3.7 mA cm⁻². Therefore, this rear contact architecture may greatly benefit the performance of ultrathin solar cells deposited at low temperatures.

In response to climate and resource challenges, the transition to a renewable and decentralized energy system is imperative. Ultrathin Cu(In,Ga)Se₂ (CIGS)‐based solar cells are compatible with such transition due to their low material usage and improved production throughput. Despite the benchmark efficiency of CIGS technology, ultrathin configurations face efficiency drops arising from increased rear interface recombination and incomplete light absorption. Dielectric passivation schemes address rear interface recombination, but achieving simultaneous electrical and optical gains is crucial for thinning down the absorber. Plasmonic nanoparticles emerge as a solution, enhancing light interaction through resonant scattering. In the proposed architecture, the nanoparticles are encapsulated within a dielectric rear passivation layer, combining effective passivation and light trapping. A controlled deposition and encapsulation of individualized nanoparticles is achieved by an optimized process flow using microfluidic devices and nanoimprint lithography. With the developed plasmonic and passivated architecture, a 3.7 mA cm⁻² short‐circuit current density and a 23 mV open‐circuit voltage improvements are obtained, leading to an almost 2% increase in light‐to‐power conversion efficiency compared to a reference device. This work showcases the developed architecture potential to tackle the electrical and optical downfalls arising from the absorber thickness reduction, contributing to the dissemination of ultrathin technology.