Antonio J. N. Oliveira

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.

The impact of high temperature annealing (HTA) treatments on the performance of Sb-doped CdSeTe solar cells containing a CdSe0.25Te0.75/CdTe front stack and Sb-doped CdTe main absorber was studied. The HTA treatment of the thermally evaporated CdSe0.25Te0.75/CdTe front stack converts the mixed-phase film into a single zinc blende structure through recrystallization of the photo-inactive wurtzite phase. Subsequently, HTA treatment of the full device stack after vapor transport deposition of Sb-doped CdTe (CdTe:Sb) absorber promotes Se-Te intermixing and enhances CdTe:Sb grain growth. Comprehensive characterization using x-ray diffraction, scanning electron microscopy, cathodoluminescence spectrum imaging, and time-resolved terahertz spectroscopy revealed that the combined HTA treatments significantly improved film crystallinity, reduced defect densities, and enhanced carrier dynamics. With HTA treatments, the open-circuit voltage (VOC) of CdSeTe:Sb devices increased from ∼ 400 mV to ≥ 600 mV, and when combined with optimized CdCl2 treatment, the device’s VOC reached 849 mV. The findings demonstrate the potential of HTA treatments to improve the structural and electrical properties of CdSexTe1-x solar cells.

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.

In this study, we developed a new method for in situ Sb doping of CdTe thin films combining vapor transport deposition with a Group V pyrolyzer to address Sb doping concentration and doping efficiency. The Sb doped CdSeTe (CdSeTe:Sb) films were deposited in solar cell structures under variations of Sb dopant source heater, vapor pyrolyzer temperature, and Cd vapor excess. Results indicate that although these parameters do not affect the CdTe morphology or crystal structure, they critically influence doping efficiency and trap concentration. Capacitance–voltage measurements show that a higher dopant heater (T_D) or pyrolyzer (T_P) temperature leads to higher net carrier concentration, achieving a net carrier concentration of 10¹⁶ cm⁻³ and 20% doping efficiency with a T_D/T_P combination of 600 °C/1100 °C. By tuning the Cd/Sb flux ratio during CdSeTe:Sb deposition, the lowest defect concentration is achieved at Cd/Sb of 1.4:1, which produced the best V_OC CdSeTe:Sb cell. This demonstrates a path to produce high net carrier concentration polycrystalline CdTe thin film with a low concentration of dopant-induced defects.

The conversion efficiency of CdTe solar cells may be improved by bandgap engineering, i.e., changing the bandgap value through the addition of Se in the absorber. The Se alloying enables a short-circuit current density improvement, as it leads to a bandgap energy value decrease. Furthermore, it has been associated with increased minority carrier lifetimes, assuring high open-circuit voltage values. An Se gradient profile control can further optimize the solar cell performance. Thus, an optical model baseline of the CdSe_xTe_1−x (CST) compound was developed. Spectroscopic ellipsometry measurements were conducted to accurately extract the optical constants of ten CST layers deposited through coevaporation with x varying from 0 to 1. Using the measured dielectric function spectra from the discrete CST layers with varying x, and considering the composition-induced shift in the critical point energies, an energy-shift model was employed to develop the accurate optical library for the CST compound for any x value to provide data for future modeling and optimization. The library accuracy was validated through optical simulations of the quantum efficiency of a graded CST solar cell using the finite-difference time-domain method by replicating the Se profile in the absorber layer measured through secondary ion mass spectrometry.

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.