Joaquim Pratas Leitao

In this study, we investigated the effect of annealing time at a fixed temperature (120 °C or 180 °C) on the recovery of Cu(In,Ga)Se2 (CIGS) thin solar cells (SCs) irradiated with protons. The energy range, 80–180 keV, allowed the proton stopping region to encompass the depletion region. The photovoltaic performance and optical properties were investigated using photoluminescence (PL), J – V measurements, and external quantum efficiency measurements along the irradiation and recovery stages. The irradiation caused a reduction in power conversion efficiency of ∼92% and a quite strong modification of the radiative recombination channels, namely involving deep energy levels in the band gap. The increase in defect density resulted in an increase in Urbach energy, from 33 meV to ∼38 meV, and a reduction in the estimated band gap energy from ∼1.159 eV to ∼1.146 eV. The annealing temperature used in the recovery of the SCs has a decisive influence on the recovery. At 120 °C, the efficiency and the radiative recombination channels recover more rapidly during the first 18 h, but maximum recovery is only reached after 58 h. In contrast, at 180 °C, these aspects recover predominantly within the first 30 min, although maximum recovery takes 8 h. Analysis of the PLs temperature dependence after recovery highlighted the involvement of the V _Cu defect in radiative recombination channels, independently of the annealing temperature. The results provide evidence that some defects formed during irradiation persist even after the recovery process, leading to an increased total defect density relative to the pre-irradiation condition. The relationship between annealing time, temperature, and the self-healing capacity of the CIGS is quite important for accurately predicting the behavior of these SCs to in-orbit temperature cycles. Our results support the potential of this technology for long-term missions in hostile space environments.

This work presents an optimisation study of silicon surface preparation for silicon heterojunction solar cells (SHJ) fabricated from surface-damaged wafers, aiming to ensure minimal surface defects and improved passivation at the crystalline silicon-hydrogenated amorphous silicon (c-Si/a-Si:H) interface. KOH etching conditions were optimized to remove diamond‑wire‑saw (DWS) damage on c‑Si while tuning nanoscale faceting for improved interface passivation. Namely, KOH 45% was used to smooth the surface and remove saw damage, while KOH 5% generated approximately 20 nm pyramidal‑like features in a single‑step, IPA‑free process. For the fabrication of SHJ solar cells with an Aluminium Back Surface Field (Al-BSF), intrinsic and n-type doped a-Si:H layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) on the front side of the devices. Two different (i)a-Si:H films, one dense, deposited with 50% hydrogen dilution, (id)a-Si:H, and the other, under-dense, using pure silane gas, (iud)a-Si:H, were used in different solar cells. A relative improvement of 18% in implied PCE (Suns‑Voc) was observed for KOH 5% with dense (id)a‑Si:H versus KOH 45%, which was attributed to the effect of nanoscale faceting created on the c‑Si surface by the 5% KOH treatment.

Laboratory irradiation tests are crucial to evaluate the viability of photovoltaic technologies for space applications. This work investigates the effect of the proton flux on the radiation hardness of Cu(In,Ga)Se ₂ ‐based solar cells. Two solar cells were irradiated with 1 MeV protons at equal fluence (∼10 ¹³ H ⁺ cm ^{− 2} ) under distinct fluxes. The optical and electrical characterization revealed that the proton irradiation induces a higher density of defects, some deep in the bandgap, leading to the emergence of new recombination channels and the degradation of the device's performance. Besides the common degradation observed for both cells, the extent of damage shows a dependency on the irradiation flux. The cell irradiated with a lower proton flux exhibited greater degradation of its open‐circuit voltage, fill factor, power conversion efficiency, and a higher density of deep defects in the bandgap. On the other hand, the higher flux caused stronger losses both in external quantum efficiency and short‐circuit current, suggesting a different nature of the induced damage. Altogether, these findings point to a flux‐dependent balance between defect generation and dynamic annealing processes that occur during irradiation, highlighting the need to consider the irradiation flux as a critical parameter in radiation hardness studies of space‐aimed photovoltaic technologies.

Scandium nitride (ScN) is a cubic NaCl-structured, degenerate, narrow-bandgap, n-type semiconductor that exhibits remarkable semiconducting, thermoelectric and plasmonic properties. However, its properties are sensitive to several types of defects, such as crystal defects, morphology, intentional or unintentional doping. For the purpose of reducing the deposition temperature of ScN, a series of films were deposited in the temperature range of 250–850 °C using a high-power impulse magnetron sputtering technique. While the stoichiometry and crystal structure remained unaffected in the sample series, the optical and electrical properties were affected when the temperature was decreased. Using in-depth XRD, optical and electrical characterizations, the effect of strain and dislocations on the semiconductor properties of ScN was evaluated. A reduction in the deposition temperature from 850 °C to 450 °C yielded a slow change in the electrical and optical properties, while a drastic change occurred for the films deposited below 450 °C. The main cause of the deterioration of the electrical transport properties (σ/10 000; n/100, and µ/100) was attributed to a high dislocation density (10¹¹ cm⁻²) along with a rhombohedral distortion of the ScN cell (α: 90° → 88.6°), which was the main cause of the variation in the electrical transport. The presence of dislocations/crystal defects in the film generated defect states near the edges of the valence and conduction bands, softening the edges and impacting the electron density and mobility. The best thermoelectric properties of ScN were obtained when it was grown at 850 °C and were further modified and altered by strain engineering.

Luminescence study of the c-Si surface for treatment with 5% KOH and deposition of a-Si:H in a SiH ₄ and 50% H ₂ environment.

Zn₂GeO₄ (ZGeO) phosphor exhibits a wide bandgap energy, making it highly suitable for luminescence-based applications spanning the entire electromagnetic spectrum, from ultraviolet (UV) to near-infrared (NIR) wavelengths.

The resilience of chalcogenide-based solar cells (SCs) in the space environment is crucial for their application in satellite power systems. In this experimental study, we investigate the effects of 1 MeV proton irradiation at fluences ranging from 3 × 10 10 to 10¹⁴ H + cm⁻² on the optoelectronic properties of Cu(In,Ga)Se₂ (CIGS) based SCs. In situ and ex situ electrical characterization revealed that irradiation causes degradation of the diode parameters and the electrical performance of the device, in particular the open-circuit voltage ( V OC ), fill factor and power conversion efficiency (η). Fluences above 10¹³ H + cm⁻² caused significant degradation of the SCs. Keeping the SCs at room temperature, ambient atmosphere and in the dark did not contribute significantly to the SC recovery. The photoluminescence (PL) analysis revealed an increase in the defect density leading to the formation of deeper radiative recombination channels, as well as non-radiative ones, and an increase on the magnitude of the fluctuating potentials. In order to revert this degradation, we explored a heat-light soaking (HLS) treatment that consisted in exposing the SCs to light incidence at a temperature slightly above 90 ° C. This treatment significantly restored the device’s electrical performance and the luminescence properties, demonstrating partial defect annihilation and restoration of the CIGS electronic structure. Interestingly, a non-irradiated SC subjected to the same HLS treatment showed no significant beneficial effect on the performance of the SC, which confirms the link between the observed recovery of the irradiated SCs and the annihilation of irradiation-induced defects. The findings of this work suggest that HLS is an effective, space-compatible self-healing mechanism that mitigates radiation damage in CIGS SCs, potentially enabling satellite SCs to be designed without cover glass.