Arivazhagan Valluvar Oli

Wide‐gap Cu(In,Ga)S ₂ solar cells with In ₂ O ₃ :Sn (ITO) as transparent back contact are evaluated for the application as top cells in tandem devices. The effect of Na on the solar cell performance is investigated by supplying additional Na by NaF co‐evaporation or exclusively by Na diffusion from glass. An efficiency of 12.7% is achieved for a semitransparent solar cell with a band gap of 1.6 eV, with sufficient Na diffusion from glass only, allowed by a thin ITO layer. Absorber grown with additional NaF co‐evaporation during Cu(In,Ga)S ₂ growth on thicker ITO show a comparable efficiency of 12%. High‐temperature growth at = 630°C enhances overall absorber quality and results in wide‐gap absorbers, with photoluminescence quantum yield improved to 1.5 × 10 ⁻⁵ , two orders of magnitude higher than absorber grown at low temperature. NaF co‐evaporation is effective in suppressing deep defects, thereby reducing non‐radiative recombination and enhancing photoluminescence quantum yield further. A GaO _x interfacial layer is formed at the rear contact, likely contributing to the passivation of the back contact. With the presence of thick GaO _x layer, current blocking effects are visible in the current–voltage curves. On the contrary, a thinner ITO tends to result in thinner GaO _x layer and no current blocking is observed.

This study underscores the pivotal role of sodium (Na) supply in optimizing the optoelectronic properties of wide bandgap (~1.6 eV) Cu(In,Ga)S ₂ (CIGS) thin‐film absorbers for high‐efficiency solar cells. Our findings demonstrate that the synergistic use of Na from the glass substrate, in conjunction with in situ sodium fluoride (NaF) co‐evaporation, significantly enhances the structural and optoelectronic properties of the CIGS. CIGS grown under either Na‐deficient or excess conditions exhibit inferior microstructural and optoelectronic properties, whereas an optimal Na supply leads to enhanced photovoltaic performance. Optimal Na incorporation minimizes vertical gallium fluctuations and improves the grain size and crystallinity. An absolute 1 sun calibrated photoluminescence (PL) measurement reveals a substantial suppression of bulk defects and a reduction in non‐radiative losses, resulting in a high quasi‐Fermi level splitting (Δ E _F ) of 1.07 eV, 93 meV higher than in Na‐deficient CIGS with the same bandgap. Optimal Na supply further increases excited carrier decay time, as revealed from time‐resolved PL, and hole doping density. Cross‐sectional hyperspectral cathodoluminescence mapping reveals that optimal Na supply significantly reduces defect density near the surface, thereby effectively translating Δ E _F to open‐circuit voltage ( V _OC ). As a result, a champion wide bandgap CIGS solar cell with a cadmium‐free ZnSnO _x buffer layer achieved an impressive V _OC of 971 mV and an active area power conversion efficiency of 15.7%, highlighting its potential for advancing tandem photovoltaic technologies with a stable inorganic top cell.

Ultrathin ZnO interlayer boosts TiO ₂ –based Sb ₂ S ₃ solar cells to record efficiencies 7.5% AM 1.5G and 18% indoor. Reduced nonradiative losses, passivated front interface, and larger Sb ₂ S ₃ grains enable stable power for IoT applications.

In thin film photovoltaic devices, the control of grain structure and local crystallography are fundamental for high power conversion efficiency and reliable long‐term operation. Structural defects, grain boundaries, and unwanted phases can stem from compositional inhomogeneities or from specific synthesis parameters, and they need to be thoroughly understood and carefully engineered. However, comprehensive studies of the crystallographic properties of complex systems, including different phases and/or a large number of grains, are often prohibitively challenging. Here, the use of 4D Scanning Transmission Electron Microscopy (4D‐STEM) is demonstrated on cross‐sections to unravel the nanoscale properties of three different materials for photovoltaics: Cu(In,Ga)S ₂ , halide perovskite, and Sb ₂ Se ₃ . These materials are chosen because of the variety of challenges they present: the presence of multiple phases and complex stoichiometry, electron beam sensitivity, and very high density of grains. 4D‐STEM provides comprehensive insights into crystallinity and microstructure, but navigating its large datasets and extracting actionable, statistically sound information requires advanced algorithms. How unsupervised machine learning, including dimensionality reduction and hierarchical clustering, can extract key information from 4D‐STEM datasets is demonstrated. The analytical framework follows FAIR principles, employing open‐source software and enabling data sharing.

Cu(In, Ga)S ₂ demonstrates potential as a top cell material for tandem solar cells. However, achieving high efficiencies has been impeded by open‐circuit voltage (V _OC ) deficits arising from In‐rich and Ga‐rich composition segregation in the absorber layer. This study presents a significant improvement in the optoelectronic quality of Cu(In, Ga)S ₂ films through the mitigation of composition segregation in three‐stage co‐evaporated films. By elevating the substrate temperature during the first stage, the intermixing of In and Ga is promoted, leading to reduced Cu(In, Ga)S ₂ composition segregation. Furthermore, the optimization of Cu‐excess during the second stage minimizes non‐radiative voltage loss. These combined strategies yield quasi‐Fermi level splitting exceeding 1 eV and a record V _OC of 981 mV in Cu(In, Ga)S ₂ devices. Consequently, a champion device achieves an in‐house power conversion efficiency (PCE) of 16.1% (active area) and a certified PCE of 14.8%, highlighting the potential of Cu(In, Ga)S ₂ as a stable and efficient top‐cell device for tandem photovoltaics.

Perovskite solar cells (PSCs) are receiving renewed interest since they have reached high power conversion efficiency (PCE) and show potential for application not only on rigid and flexible substrates but also on mechanically deformable substrates for integration on nonplanar curvilinear surfaces. Here we demonstrate PSCs fabricated on transparent conducting oxide-free ultrathin polyethylene terephthalate substrates capable of efficiently harvesting indoor light even under compressive strain. Interface engineering with poly(bis(4-phenyl)(2,4,6-trimethylphenyl)amine) improved the shunt resistance and band alignment at the perovskite-hole transport layer interface, which resulted in enhanced charge extraction, leading to 114% improvement in PCE from 5.57 to 11.91% under 500 lx indoor white LED (4000 K) illumination. The champion device exhibited a PCE of 18.37% under 250 lx cool white LED (4000 K) light. The maximum power output (P _max) of the devices varied from 13.78 to 25.38 μW/cm² by changing the indoor light illumination from 250 to 1000 lx, respectively. Moreover, the devices showed impressive performance even after mechanical deformation and retained 83 and 76% for 1 sun and indoor light, respectively, under 30% compressive strain. Our approach paves the way for fabrication of efficient indoor light harvesting PSCs on mechanically deformable substrates for integration on nonplanar surfaces prone to compressive strain.

Indoor light-energy-harvesting solar cells have long-standing history with perovskite solar cells (PSCs) recently emerging as potential candidates with high power conversion efficiencies (PCEs). However, almost all of the reported studies on indoor light-harvesting solar cells utilize white light in the visible wavelength. Low wavelength near-ultraviolet (UV) lights used under indoor environments are not given attention despite their high photon energy. In this study, perovskite solar cells have been investigated for the first time for harvesting energy from a commercially available near-UV (UV-A) indoor LED light (395-400 nm). Also called black lights, these near-UV lights are commonly used for decoration (e.g., in bars, pubs, aquariums, parties, clubs, body art studios, neon lights, and Christmas and Halloween decorations). The optimized perovskite solar cells with the n-i-p architecture using the CH3NH3PbI3 absorber were fabricated and characterized under different illumination intensities of near-UV indoor LEDs. The champion devices delivered a PCE and power output of 20.63% and 775.86 mu W/cm(2), respectively, when measured under UV illumination of 3.76 mW/cm(2). The devices retained 84.10% of their initial PCE when aged under near-UV light for 24 h. The effects of UV exposure on the device performance have been comprehensively characterized. Furthermore, UV-stable solar cells fabricated with a modified electron transport layer retained 95.53% of its initial PCE after 24 h UV exposure. The champion devices delivered enhanced PCE and power output of 26.19% and 991.21 mu W/cm(2), respectively, when measured under UV illumination of 3.76 mW/cm(2). This work opens up a novel direction for energy harvesting from near-UV indoor light sources for applications in microwatt-powered electronics such as internet of things sensors.

Bismuth (Bi) based solar cells have recently attracted significant attention in line with lead-free perovskite solar cells development. In this study, 2-aminothiazolium iodide (ATI) is used to prepare highly crystalline and environmentally stable lead free ATBiI₄ photoactive thin films, which were comprehensively characterized for integration into planar solar cells with the n–i–p device architecture. The ATBiI₄ solar cells delivered a power conversion efficiency of 0.53% with open-circuit voltage of 0.83 V under 1 sun, a 12.8% increase in the power conversion efficiency arising mainly due to 124% increase in the open circuit voltage as compared to the triple mesoscopic devices explored previously. Furthermore, in this study the solar cells have been explored for harvesting energy from indoor white light emitting diodes (LED) for the first time revealing a power output of 81.75 μW cm⁻² at 500 lux. As ATBiI₄ has its absorption maximum in the ultra-violet wavelength region, the solar cells have been further tested under 9 W UV LED (395–400 nm), which yielded a power output of 8.07 μW cm⁻² at 150 lux. This work demonstrates that 2-aminothiazolium bismuth iodide solar cells are promising microwatt energy sources which can find potential application for powering smart electronics.

Hole transporting layers (HTLs) play a crucial role in the realization of efficient and stable perovskite solar cells (PSCs). Copper phthalocyanine (CuPc) is a promising HTL owing to its thermal stability and favorable band alignment with the perovskite absorber. However, the power conversion efficiency (PCE) of PSCs with a CuPc HTL is still lagging behind highly efficient solar cells. Herein, a p-type tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) is employed as an interlayer between the perovskite and CuPc HTL in all-vacuum deposited PSCs. The F4-TCNQ interlayer improves the conductivity of both MAPbI ₃ and CuPc, reduces the shunt pathway and facilitates an efficient photoexcited holes transfer from the valance band of the MAPbI ₃ to the LUMO of the F4-TCNQ. Consequently, the best solar cell device with an F4-TCNQ interlayer achieved a PCE of 13.03% with a remarkable improvement in fill factor. Moreover, the device showed superior stability against thermal stress at 85 °C over 250 h and retained ∼95% of its initial efficiency. This work demonstrates a significant step towards all-vacuum deposited perovskite solar cells with high thermal stability.

Fullerene (C ₆₀ ) has been demonstrated, using vapor deposition, to be a good electron transport layer (ETL) and different thicknesses have been utilized in n-i-p configuration perovskite solar cells. However, an underlying reason for the variation in thicknesses, which hinders the reproducibility of perovskite solar cells employing a C ₆₀ ETL, has not been well-examined. This study reveals that the surface roughness of the conducting glass, such as fluorine doped tin oxide (FTO), affects the photovoltaic performance while optimizing the thickness of the vacuum deposited compact C ₆₀ ETL. A low-thickness C ₆₀ ETL retains a surface roughness and Ohmic behavior similar to bare FTO due to physical defects at the C ₆₀ /FTO interface. Increasing the thickness further reduces the defects at the C ₆₀ /FTO interface and facilitates an enhanced electron extraction from a vacuum co-deposited methyl-ammonium lead iodide perovskite light absorber. As a result, a perovskite solar cell with a homogenously covered C ₆₀ ETL on an FTO substrate delivers a power conversion efficiency of 14.63%. These comprehensive characterizations support the finding that suppression of defects at the C ₆₀ /FTO interface results in an improved photovoltaic performance. This work demonstrates that the surface roughness of FTO needs to be considered for a decisive compact ETL for enhanced photovoltaic performance and reproducibility.