Improvement Of Surface Passivation Research Articles

Tunnel silicon oxynitride phase transformation for n-type polysilicon passivating contacts in crystalline silicon solar cells

The study investigates the fabrication and characterization of tunnel oxynitride (TON) layers produced using Plasma-Enhanced Chemical Vapor Deposition (PECVD) at different pressure and power variations. The aim is to explore TON layers as potential substitutes for conventional SiO2 in Tunnel Oxide Passivated Contact (TOPCon) solar cells. Foe this purpose, an effective oxide thickness (EOT) of 1.1–1.2 nm and 1.5–1.6 nm has been achieved by treating the TON samples with NH3 and N2 gas plasma, respectively. Fourier Transform Infrared Spectroscopy (FTIR) analysis revealed critical variations in the chemical bonding within TON layers, which exhibit different properties based on the deposition conditions. The presence of nitrogen-nitrogen and nitrogen–hydrogen bonds is prominently influenced by changes in the PECVD parameters, directly affecting the layer’s optical and electronic properties. Notably, the presence of hydroxyl (O–H) and amino (NH2) groups within the TON structure suggests an improvement in surface passivation, essential for optimizing solar cell performance and durability. A lifetime of 737 and 823 μs has been recorded for N2 and NH3 samples, compared to the NAOS 710 μs reference sample. NH3 plasma-treated samples show an improved iVoc of 736 mV, indicating better phase transformation and passivation quality than N2 plasma treatment and the NAOS reference sample.

Modifications of Textured Silicon Surface Morphology and Its Effect on Poly-Si/SiO x Contact Passivation for Silicon Solar Cells

High-efficiency crystalline silicon ( c -Si) solar cells require textured surfaces for efficient light trapping. However, passivation of a textured pyramidal surface to reduce carrier recombination is challenging due to the presence of sharp tips, edges, and valleys. Using electrical and surface microscopies combined with lifetime measurements, we report on the effect of HF:HNO3 etching on the pyramidal textured Si surface morphology, and on poly -Si/SiO x contact performance. Preferential rounding of either the valleys between pyramids, or the pyramid tips is obtained depending on the HF:HNO3 solution temperature. Both these morphologies make the pyramid shape irregular, with the pyramid faces no longer being predominantly a Si(111) surface. Our atomic force microscopy measurements further show that the nanoscale roughness over the pyramid face also reduces after HF:HNO3 etching. Thus, etching affects the microscopic pyramidal shape, the dominant crystallographic orientation, and the nanoscale roughness of the surface. We speculate that these three surface effects result in the improvement of surface passivation via poly -Si/SiO x contacts. However, this improved passivation is accompanied by increased reflectance of the HF:HNO3-etched textured surface. Finally, our electron-beam-induced current measurements reveal thickness nonuniformities in the thermally grown SiO x layer on the HF:HNO3-etched textured Si surface: SiO x is thicker near the pyramid tips, edges, and faces as compared with near the valleys between pyramids. This nonuniformity in the SiO x layer may explain the poorer passivation obtained on a KOH-textured and HF:HNO3-etched textured surface as compared with a planar surface.

Thiol and Halometallate, Mutually Passivated Quantum Dot Ink for Photovoltaic Application.

Tunable-band-gap colloidal QDs are a potential building block to harvest the wide-energy solar spectrum. The solution-phase surface passivation with lead halide-based halometallate ligands has remarkably simplified the processing of quantum dots (QDs) and enabled the proficient use of materials for the development of solar cells. It is, however, shown that the hallometalate ligand passivated QD ink allows the formation of thick crystalline shell layer, which limits the carrier transport of the QD solids. Organic thiols have long been used to develop QD solar cells using the solid-state ligand exchange approach. However, their use is limited in solution-phase passivation due to poor dispersity of thiol-treated QDs in common solvents. In this report, a joint passivation strategy using thiol and halometallate ligand is developed to prepare the QD ink. The mutually passivated QDs show a 50% reduction in shell thickness, reduced trap density, and improved monodispersity in their solid films. These improvements lead to a 4 times increase in carrier mobility and doubling of the diffusion length, which enable the carrier extraction from a much thicker absorbing layer. The photovoltaic devices show a high efficiency of 10.3% and reduced hysteresis effect. The improvement in surface passivation leads to reduced oxygen doping and improved ambient stability of the solar cells.

Surface passivation of n-type doped black silicon by atomic-layer-deposited SiO2/Al2O3 stacks

Black silicon (b-Si) nanotextures can significantly enhance the light absorption of crystalline silicon solar cells. Nevertheless, for a successful application of b-Si textures in industrially relevant solar cell architectures, it is imperative that charge-carrier recombination at particularly highly n-type doped black Si surfaces is further suppressed. In this work, this issue is addressed through systematically studying lowly and highly doped b-Si surfaces, which are passivated by atomic-layer-deposited Al2O3 films or SiO2/Al2O3 stacks. In lowly doped b-Si textures, a very low surface recombination prefactor of 16 fA/cm2 was found after surface passivation by Al2O3. The excellent passivation was achieved after a dedicated wet-chemical treatment prior to surface passivation, which removed structural defects which resided below the b-Si surface. On highly n-type doped b-Si, the SiO2/Al2O3 stacks result in a considerable improvement in surface passivation compared to the Al2O3 single layers. The atomic-layer-deposited SiO2/Al2O3 stacks therefore provide a low-temperature, industrially viable passivation method, enabling the application of highly n- type doped b-Si nanotextures in industrial silicon solar cells.

Hydrogen passivation effect on the conversion efficiency of Si solar cells by low-energy proton implantation

We investigated the hydrogen passivation effect on the conversion efficiency of Si solar cells by including low-energy hydrogen ion implantation in the cell fabrication process. The solar cells were implanted with various proton energies and doses just before the co-firing process. The quantum efficiency specifically in the blue wavelengths of the solar cells increased with a low implantation dose and energy indicating improvement of surface passivation. However, quantum efficiency decreased when the implantation dose and energy were increased, indicating a high defect-assisted surface recombination of minority carriers. The cells implanted with a low dose and energy in the presence of SiNx showed interfacial passivation with enhanced photocurrent, minority carrier lifetime and short-circuit current density for improved conversion efficiency.

Impact of the Deposition and Annealing Temperature on the Silicon Surface Passivation of ALD Al2O3 Films

The effect of the deposition and annealing temperature on the surface passivation of atomic layer deposited Al2O3 films was investigated on n-type Cz silicon wafers. The deposition temperature was varied between 200 and 500̊C and the annealing temperature between 300 and 450̊C, respectively. Films prepared at 200 and 300̊C showed an improvement of surface passivation with increasing anneal temperature. The Al2O3 films grown at 400 and 500̊C did not improve by annealing. By corona charging experiments it was revealed that the improvement in surface passivation with increasing anneal temperature of films grown at 300̊C can be attributed to a significant increase in chemical passivation with a minor increase in field-effect passivation. For Cz and FZ wafers an identical surface passivation was achieved with the chemical passivation being lower for Cz wafers due to the surface morphology and the field-effect passivation being quite similar. Consequently the field-effect passivation was found to be the more important passivation mechanism.

Role of hydrogen bonding environment in a-Si:H films for c-Si surface passivation

The search for an ideal surface passivation layer of crystalline silicon (c-Si) to be employed in a silicon heterojunction photovoltaic device has garnered much attention. The leading candidate is a few nanometers of intrinsic amorphous silicon ((i)a-Si:H) film. Reported dependencies of film surface passivation quality on substrate preparation, orientation, and deposition temperature have been extended in this work to include H2 to SiH4 dilution ratio and postdeposition annealing. Simple avoidance of the deposition regimes that lead to epitaxial growth of Si on the c-Si substrate produces decent lifetimes on the order of 500μs. Subsequent low temperature annealings cause an important restructuring of Si–H bonding at the a-Si:H∕c-Si interface increasing the amount of monohydride at the c-Si surface. This restructuring would reduce the c-Si surface defect density and cause an improvement of surface passivation as confirmed by effective lifetime measurements. Final effective carrier lifetimes up to 2550μs are achieved postannealing. Initial results indicate the improvement depends on surplus SiH2 from the interface region.

Hydrogen Reintroduction by Forming Gas Annealing to LPCVD Silicon Nitride Coated Structures

The effect of high-temperature anneals on low-pressure chemical-vapor deposited (LPCVD) /thermally grown stacks on silicon wafers has been studied. Annealing in a nitrogen atmosphere at 900 or for causes a substantial reduction in both and bond concentration in the nitride film and a significant degradation of surface passivation. Subsequent anneals in forming gas (95% Ar/5% ) at temperatures in the range lead to a gradual reformation of and bonds and an improvement in surface passivation. It is hypothesized that the chief mechanism responsible for the improvement in surface passivation is the passivation of defects at the interface with atomic hydrogen, supplied by the breaking of and bonds in the nitride film. Samples with stacks display excellent surface passivation following high-temperature forming gas anneals, indicating that such treatments can be used for photovoltaic applications, where surface passivation as well as good optical properties are important.

Characterization and application of a-SiC x:H films for the passivation of the c-Si surface

In this work we investigate the effects of typical thermal treatments on c-Si surface passivation by intrinsic a-SiC x :H films. First, a forming gas anneal (FGA), (400 °C, 30 min in H 2/N 2) is applied resulting in an improvement in surface passivation. The best result is a surface recombination velocity lower than 23 cm s −1. A second thermal step (730 °C, 30 s) is studied to try and simulate the firing step for screen-printed contacts. This annealing has no effect in surface passivation indicating that a-SiC x :H films are promising candidates for c-Si surface passivation in the photovoltaic industry. In order to analyze the improvement of surface passivation after FGA, a physical model of surface recombination at the a-SiC x :H/c-Si interface is developed. This improvement is directly connected to a reduction in fundamental recombination velocities of electrons and holes at the interface. A possible explanation could be the reduction in the interface state density due to hydrogen saturation of dangling bonds.