(Invited) ALD Enabling High-Efficiency Solar Cells

Polydimethylsiloxane (PDMS) is an important polymer material widely used for microfluidic device fabrication, microcontact lithography, and surface morphology molding. However, the hydrophobic surface limits its functionality. Low temperature atomic layer deposition (ALD) has recently been used to functionalize a wide range of polymer surfaces. In previous research, the authors were able to produce a uniform hydrophilic alumina film coating on PDMS using trimethyl aluminum/water ALD. However, the surface recovered its hydrophobicity after 24–48 h in ambient air or under inert gas storage, which was ascribed to organic species outdiffusion through the ALD layer. This paper reports a stable hydrophilic ALD surface modification on the PDMS. The PDMS substrate was first sequentially exposed to trimethylaluminum and water vapor, allowing the vapors to infiltrate and react to create a mechanical and diffusion buffer layer in the PDMS surface region. This buffer layer helps to nucleate a cohesive hydrophilic ALD or molecular layer deposition (MLD) coating and also helps prevent organic outdiffusion that typically leads to PDMS hydrophobic recovery. The results provide valuable insight into reliable surface energy and mechanical modification of PDMS using vapor-phase precursor/polymer reactions. In addition, molecular layer deposition of “alucone” coatings was also investigated to modify the PDMS surface. The experiment result showed the vapor infiltration and MLD coatings produce a viable and stable hydrophilic surface on PDMS.

Light and elevated temperature induced degradation (LeTID) varies significantly in multicrystalline PERC silicon solar cells, depending mainly on the solar cell processes. We show that despite high firing temperatures, LeTID can manifest itself in two different ways: 1) strong LeTID in good grains (jsc and Voc losses >7%rel.), or 2) low LeTID in good grains with stronger LeTID at dislocation clusters. Such LeTID at dislocation clusters is not only caused by a bulk recombination increase but also by an increased back-surface recombination velocity.

Crystalline silicon (c-Si) photovoltaics dominate the solar industry, yet further advancements hinge on passivating and carrier-selective contacts to overcome efficiency limitations. This review explores the pivotal role of atomic layer deposition (ALD) in enabling metal oxide films for high-performance c-Si solar cells, bridging material innovation with industrial scalability. Historically, ALD-grown Al2O3 enabled the effective passivation of p-type Si surfaces via its high negative fixed charge, which made localized rear contacts viable and facilitated the transition from aluminum back surface field to passivated emitter rear contact architectures, ultimately lowering J0 and boosting efficiency. However, emerging carrier-selective contacts demand materials that simultaneously minimize recombination and resistive losses while avoiding parasitic absorption. Metal oxides, leveraging tunable optoelectronic properties and ALD's atomic-scale precision, offer a promising alternative to conventional silicon-based films (e.g., a-Si:H and poly-Si). We analyzed 373 studies to map trends in ALD metal oxide applications, highlighting the dominance of Al2O3 and TiO2, alongside growing interest in multi-metal oxides. The review underscores ALD's unique ability to tailor chemical and field-effect passivation mechanisms while addressing challenges in stoichiometric control and interfacial engineering. Targeting both ALD specialists and PV engineers, we propose standardized metrics for evaluating passivating contacts, aiming to accelerate cross-disciplinary innovation. Finally, we outline future opportunities for ALD-derived metal oxide in next-generation photovoltaics, including tandem and thin-film technologies, advocating for systematic research to unlock their full potential.

Analysis of variation in recombination characteristics due to light and heat in industrial silicon solar cells

Crystalline silicon solar cells are highly efficient, but their long-term stability is compromised by Light and Elevated Temperature Induced Degradation (LeTID), which occurs under moderate illumination and elevated temperatures. While much research has focused on LeTID in multi-crystalline and monocrystalline silicon cells, with mitigation techniques like thermal treatments and optimized passivation, the use of alternating current (AC) biasing waveforms for LeTID mitigation and performance recovery has not been thoroughly explored. This study investigates the regeneration of a heterojunction with intrinsic thin-layer (HIT) solar cells (4 × 4 cm2), subjected to LeTID conditions (1 sun illumination at 75 °C for 11 h), using sinusoidal, square, and triangular AC waveforms at frequencies of 100 kHz, 500 kHz, and 1 MHz. LeTID exposure significantly reduced efficiency, open-circuit voltage (Voc), short-circuit current (Jsc), and fill factor (FF). After applying AC waveforms, substantial recovery was observed across all performance metrics, with square waveforms at 1 MHz providing the most significant recovery, restoring Voc, Jsc, FF, and efficiency to 99.99% of their initial values. Sinusoidal waveforms showed effective recovery, reaching 99%–99.5%, while triangular waveforms exhibited somewhat lower recovery, in the range of 97%–99%. These findings highlight the importance of waveform shape and frequency in facilitating hydrogen redistribution and defect passivation. The results suggest that high-frequency AC biasing techniques could be integrated into industrial processes to enhance the long-term stability and reliability of HIT solar cells, supporting the broader deployment of high-efficiency photovoltaic systems.

System performance loss due to LeTID

Silicon passivated emitter and rear contact (PERC) solar cells with V-groove texture were fabricated using maskless alkaline solution etching with in-house developed additive. Compared with the traditional pyramid texture, the V-groove texture possesses superior effective minority carrier lifetime, enhanced p–n junction quality and better applied filling factor (FF). In addition, a V-groove texture can greatly reduce the shading area and edge damage of front Ag electrodes when the V-groove direction is parallel to the gridline electrodes. Due to these factors, the V-groove solar cells have a higher efficiency (21.78%) than pyramid solar cells (21.62%). Interestingly, external quantum efficiency (EQE) and reflectance of the V-groove solar cells exhibit a slight decrease when the incident light angle (θ) is increased from 0° to 75°, which confirms the excellent quasi omnidirectionality of the V-groove solar cells. The proposed V-groove solar cell design shows a 2.84% relative enhancement of energy output over traditional pyramid solar cells.

TCAD based numerical exploration of industrially feasible tunnel oxide passivated contact on p-type silicon

Corrosion is one of the main PV module failure mechanisms, as it can cause severe electrical performance degradation in PV modules exposed to hot and humid environments. Moisture penetrating a photovoltaic (PV) module may react with the metallic components causing corrosion. In addition, acetic acid which is produced by hydrolysis of ethylene vinyl acetate (EVA), the most common encapsulant, may further degrade metallic components. Corrosion is one of the main PV module failure mechanisms, as it can cause severe electrical performance degradation in PV modules exposed to hot and humid environments. The specific chemical reactions involved in the corrosion mechanisms for the different components are well understood. However, which of these causes the most serious degradation in the field, and therefore, most severe power loss is unknown. Moreover, the severity of corrosion in the absence of acetic acid is not yet well researched. This work distinguished between the front and rear side corrosion mechanisms and identified the different electrical signatures observed due to them. The experiment included damp‐heat (DH) conditioning of single‐cell mini‐modules, containing passivated emitter and rear contact (PERC) solar cells, laminated with a polyethylene terephthalate (PET) based backsheet. Furthermore, half‐encapsulated PERC PV cells were tested, with either the front or the rear side exposed. Electrical and material characterisation were conducted for the investigation of the sample degradation, and the performance decrease, related to the degradation of the rear surface passivation, was examined.

Herein, an approach for integrating gate insulator deposition and graphene transfer steps in the fabrication of graphene field‐effect devices is reported. A thin layer of Al2O3 is deposited by atomic layer deposition (ALD) onto as‐grown graphene on copper, where the improved surface wettability of graphene on copper aids in obtaining a uniform deposition of the ALD layer. The ALD Al2O3/graphene stack is then mechanically delaminated from the copper surface and transferred onto the desired target substrate. An ALD layer thickness between 20 and 30 nm is optimal for facilitating such transfer. The ALD layer protects graphene from process contamination during subsequent transfer and device fabrication, resulting in reduced doping in measured field‐effect devices.

Atomic layer deposition (ALD) is exceptional deposition technique because it allows for growth of high quality films on large are substrates at low deposition temperatures. In our contribution ALD was employed for preparation of transparent conducting electrodes and for encapsulation of fabricated solar cells. Al-doped ZnO films were used for transparent conducting electrodes. Deposition was carried out at temperatures between 150 and 250 °C on Si and quartz substrates. Diethyl zinc and trimethyl aluminum were used as precursors and water vapors as reactant. Al-doping was performed by inserting Al2O3 cycles in ZnO growth. Deposition of 1 Al2O3 layer per 7 deposited ZnO layers gave the best results. Resistivity of the Al-doped ZnO films depended on the deposition temperature and film thickness. Transition electron microscopy revealed typical columnar growth of the films with fine grained polycrystalline region close to the substrate. The films with the thickness above 100 nm prepared at 250 °C showed resistivity of 1 mΩcm. This corresponds to the sheet resistance of 40 Ω/square. Hall measurement revealed electron concentration of 4*1020 cm-3 and mobility of 15 cm2Vs-1 for the films with resistivity of 1 mΩcm. Work function of 4.4 eV was measured by Kelvin probe. Optical transmittance of the films was higher than 80% in the wavelength range between 400 and 900 nm. Al-doped ZnO transparent electrodes were used for fabrication of perovskite-type conventional planar solar cell. CH3NH3 PbI3 perovskite was used as absorbing layer, Spiro-OMeTAD as hole transport layer and Au/Ag as top contact layer. Properties of the solar cell with Al-doped ZnO electrode are compared with inverted planar solar cell using indium tin oxide as a transparent electrode. Inorganic-organic hybrid perovskite solar cells suffer from poor stability in ambient atmosphere. Oxygen and moisture are believed to be reason for the solar cell degradation. To protect the cells against ambient atmosphere influence ALD prepared Al2O3 films were employed for encapsulation. Trimethyl aluminum was used as precursor while either water vapors or ozone were applied as reactants. Deposition of the Al2O3 films took place at 50°C. Power conversion efficiency of the solar cell encapsulated by 24 nm Al2O3 thin film deposited using water vapors as reactant decreased to 60% while unprotected solar cell degraded down to 10% of its initial value after 20 days in ambient atmosphere. In our contribution we compare properties of perovskite solar cell encapsulated by ALD Al2O3 films prepared using water vapors or ozone as reactants.

Encapsulation methods have shown to be effective in imparting improved stability to metal‐halide perovskite nanocrystals (NCs). Atomic layer deposition (ALD) of metal oxides is one of the promising approaches for such encapsulation, yet better control on the process parameters are required to achieve viable lifetimes for several optoelectronic and photocatalytic applications. Herein, we optimize the ALD process of amorphous aluminum oxide (AlO x ) as an encapsulating layer for CsPbBr 3 NC thin films by using oxygen (O 2 ) as a molecular diffusion probe to assess the uniformity of the deposited AlO x layer. When O 2 reaches the NC surface, it extracts the photogenerated electrons, thus quenching the PL of the CsPbBr 3 NCs. As the quality of the ALD layer improves, less quenching is expected. We compare three different ALD deposition modes. We find that the low temperature/high temperature and the exposure modes improve the quality of the alumina as a gas barrier when compared with the low temperature mode. We attribute this result to a better diffusion of the ALD precursor throughout the NC film. We propose the low temperature/high temperature as the most suitable mode for future implementation of multilayered coatings.

Keywords: atomic layer deposition; batteries; energy conversion; electrochemistry; electrolysis; fuel cells; photovoltaics; solar cells; thin films

Industrail Technology of Passivated Emitter and Rear Cells with Silicon Oxynitride and Silicon Nitride as Rear Passivation for High Efficiency BIPV Modules

The contact formation on high efficiency solar cells using a high temperature process is the subject of this research. The chemical reactions between ink components and solar cell during the contact firing process are studied in detail by thermal gravimetric - differential thermo analysis. The mechanism behind the etching process and the opening of the dielectric layer are explained and the impact of the glass frit is investigated. Based on these studies, a seed layer ink was developed, optimized and tested on silicon solar cells. The developed ink was applied on high efficiency solar cells with printed front and rear contacts. At the rear side, we used a firing stable passivation layer consisting of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> , which additionally resists the reactive compounds of the screen printed Al-paste. After applying laser fired contacts at the rear and light induced silver plating at the front, cell efficiencies of η = 21% and fill factors of 81% could be measured.