Field Effect Surface Passivation Research Articles

Graphene oxide films for field effect surface passivation of silicon for solar cells

In recent years it has been shown that graphene oxide (GO) can be used to passivate silicon surfaces resulting in increased photocurrents in metal-insulator-semiconductor (MIS) tunneling diodes, and in improved efficiencies in Schottky-barrier solar cells with either metal or graphene barriers, however, the source of this passivation is still unclear. The suggested mechanisms responsible for the enhanced device performance include the dangling bond saturation at the surface by the diverse functional groups decorating the GO sheets which reduce the recombination sites, or field effect passivation produced by intrinsic negative surface charge of GO. In this work through a series of measurements of minority carrier lifetime with the microwave photo-conductance decay (µPCD) technique, infrared absorption spectra, and surface potential with Kelvin probe force microscopy (KPFM) we show that there is no evidence of significant chemical passivation coming from the GO films but rather negative field effect passivation. We also discuss the stability of GO's passivation and the flexibility of this material for its application as temporary passivation layer for bulk lifetime measurements, or as a potential cheap alternative to current passivation materials used in solar cell fabrication.

Corona Field Effect Surface Passivation of n-type IBC Cells

Passivation of silicon surfaces is an important requirement in achieving high energy conversion efficiencies in interdigitated back contact cells. Surface passivation, commonly achieved by dielectric coatings, can be greatly improved by extrinsic addition of chemical and field effect components. In particular, cell performance is strongly dependent on front surface passivation. In this work device modelling is used to show that 200% relatively better performance can be achieved using charge extrinsically added to a passivation coating without the need for a front surface field. Research scale cells have been tested as a function of front surface corona charge. On average an absolute improvement in efficiency of 0.3% ±0.3 and 0.13% ±0.12 has been observed on cells without and with a front surface field. These results show that extrinsic field effect passivation can produce significant improvements in cell efficiency.

Investigation of parasitic absorption and charge carrier recombination losses in plasmonic silicon solar cells using quantum efficiency and impedance spectroscopy

Quantum efficiency and impedance spectroscopy tools are employed for understanding the influence of parasitic absorption losses and partial field effect surface passivation by the silver nanoparticles (Ag NPs) on electrical properties of textured silicon solar cells without and with Si3N4 spacer layer. The parasitic absorption losses from Ag NPs reduced the internal quantum efficiency near the surface plasmon resonance region. The passive components like; series and parallel resistances, chemical capacitance of solar cells without and with Ag NPs are estimated after fitting impedance semicircles, which are further used for estimating effective carrier lifetime (τeff) values. Under AM1.5G illumination, cells with Si3N4 spacer layer showed a large decrease in the τeff due to the strong parasitic absorption losses from the Ag NPs. But, the cells without Si3N4 spacer layer showed a small decrease in the τeff due to the reduced surface recombination after partial field effect passivation from near-fields of Ag NPs’ surface plasmon resonances on the emitter surface.

Very low surface recombination velocity in n-type c-Si using extrinsic field effect passivation

In this article, field-effect surface passivation is characterised as either intrinsic or extrinsic, depending on the origin of the charges present in passivation dielectric layers. The surface recombination velocity of float zone, 1 Ω cm, n-type silicon was reduced to 0.15 cm/s, the lowest ever observed for a passivating double layer consisting of thermally grown silicon dioxide and plasma enhanced chemical vapour deposited silicon nitride. This result was obtained by enhancing the intrinsic chemical and field-effect passivation of the dielectric layers with uniform, extrinsic field-effect passivation induced by corona discharge. The position and stability of charges, both intrinsic and extrinsic, were characterised and their passivation effect was seen stable for two months with surface recombination velocity <2 cm/s. Finally, the intrinsic and extrinsic components of passivation were analysed independently. Hydrogenation occurring during nitride deposition was seen to reduce the density of interfacial defect states from ∼5 × 1010 cm−2 eV−1 to ∼5 × 109 cm−2 eV−1, providing a decrease in surface recombination velocity by a factor of 2.5. The intrinsic charge in the dielectric double layer provided a decrease by a factor of 4, while the corona discharge extrinsic field-effect passivation provided a further decrease by a factor of 3.

A technique for field effect surface passivation for silicon solar cells

The recombination of electric charge carriers at the surface of semiconductors is a major limiting factor in the efficiency of optoelectronic devices, in particular, solar cells. The reduction of such recombination, commonly referred to as surface passivation, is achieved by the combined effect of a reduction in the trap states present at the surface via a chemical component, and the reduction in the charge carriers available for a recombination process, via a field effect component. Here, we propose a technique to field effect passivate silicon surfaces using the electric field effect provided by alkali ions present in a capping oxide. This technique is shown to reduce surface recombination in a controlled manner, and to be highly stable. Surface recombination velocities in the range of 6–15 cm/s are demonstrated for 1 Ω cm n-type float zone silicon using this technique, and they are observed to be constant for over 300 days, without the use of any additional surface chemical treatment. A model of trapping-mediated ionic injection is used to describe the system, and activation energies of 1.8–2 eV are deduced for de-trapping of sodium and potassium alkali ionic species.

Electric Field Effect Surface Passivation for Silicon Solar Cells

Effective reduction of front surface carrier recombination is essential for high efficiency silicon solar cells. Dielectric films are normally used to achieve such reduction. They provide a) an efficient passivation of surface recombination and b) an effective anti-reflection layer. The conditions that produce an effective anti-reflection coating are not necessarily the same for efficient passivation, hence both functions are difficult to achieve simultaneously and expensive processing steps are normally required. This can be overcome by enhancing the passivation properties of an anti-reflective film using the electric field effect. Here, we demonstrate that thermally grown silicon dioxide is an efficient passivation layer when chemically treated and electrically charged, and it is stable over a period of ten months. Double layers of SiO2 and SiN also provided stable and efficient passivation for a period of a year when the sample is submitted to a post-charge anneal. Surface recombination velocity upper limits of 9 cm/s and 19 cm/s were inferred for single and double layers respectively on n-type, 5 Ωcm, Cz-Si.

Stable Field Effect Surface Passivation of n-type Cz Silicon

Surface recombination of carriers in solar cells can cause a significant reduction in their efficiency and is most commonly minimized by the deposition of surface dielectric layers which simultaneously perform two functions; efficient passivation of surface recombination and the provision of an effective anti-reflection layer. This can be difficult to achieve in practice since the conditions that produce an optimum anti-reflection coating are not necessarily the same as those required for effective passivation. In this work we describe the use of external electrical charging of dielectric layers which serves to improve their passivation properties without affecting their reflection properties. This provides a method by which, to some extent, the electrical and optical properties of the films can be decoupled so allowing better overall performance to be achieved. It is demonstrated that SiO2 and SiO2/SiN stacks deposited on a silicon surface can provide a stable reduction of surface recombination when chemically treated, electrically charged using a corona discharge and then annealed at low temperature. Surface recombination velocity upper limits of 19cm/s and 16cm/s were inferred for single and double layers respectively on n-type, 5Ωcm, Cz-Si.

Modelling and Simulation of Field-effect Surface Passivation of Crystalline Silicon-based Solar Cells

An effective surface passivation plays a vital role in the performance of crystalline silicon (c-Si) solar cells. Experimental research shows that fixed charge-induced field-effect passivates the c-Si efficiently. In this work, n-type and p-type c-Si wafers symmetrically passivated by the negative-charge dielectric Al2O3 were numerically modelled with SENTAURUS TCAD. Surface recombination is traditionally modelled by employing the extended Shockley-Read-Hall (SRH) model with single energy level of interface traps. However, experiments show interface traps distribute across the silicon bandgap. Thus, we implemented the extended SRH model with the ability to describe arbitrary energy distribution of interface traps within the bandgap. The extended SRH model predicts a constant effective surface recombination velocity at low injection levels for lightly doped n-type and p-type c-Si passivated by dielectrics with either a high negative or positive charge density. However, this prediction contradicts experimental results which show a significant reduction of the effective lifetime at low injection levels if the polarity of the fixed charge is attracting the minority bulk charge carriers. One explanation is assuming the presence of a thin defect-rich layer close to the c-Si surface in which the lifetimes are degraded. However, the choice of the lifetime and depth of such damaged surface region seem rather arbitrary and its physical origin is still unclear. We show that unequal electron and hole bulk lifetime parameters can also account for the phenomenon. This proposition is physically plausible and the simulation results also show a good agreement with the experimental data from both n-type and p-type c-Si wafers passivated by Al2O3. Our modelling results predict a simple but unambiguous experiment to distinguish between these two explanations.

Field effect surface passivation of SiO 2/Si interfaces by heat treatment with high-pressure H 2O vapor

We investigated a simple field effect passivation of the silicon surfaces using the high-pressure H 2O vapor heating. Heat treatment with 2.1×10 6 Pa H 2O vapor at 260°C for 3 h reduced the surface recombination velocity from 405 cm/s (before the heat treatment) to 38 cm/s for the thermally evaporated SiO x film/Si. Additional deposition of 140 nm-SiO x films ( x<2) with a high density of fixed positive charges on the SiO 2/Si samples further decreased the surface recombination velocity to 22 cm/s. We also demonstrated the field effect passivation for n-type silicon wafer coated with thermally grown SiO 2. Additional deposition of 210 nm SiO x films on both the front and rear surfaces increased the effective lifetime from 1.4 to 4.6 ms. Combination of thermal evaporation of SiO x film and the heat treatment with high-pressure H 2O vapor is effective for low-temperature passivation of the silicon surface.

Numerical analysis of field-effect surface passivation for solar cells

In order to investigate the influence of surface potential on the electric characteristics of solar cells, the characteristics of conventional cells and back-contact type high-efficiency silicon cells were analyzed using 2-dimensional numerical simulation, varying the surface electrical potential. The locations where surface electrical potential is controlled are the rear side in conventional cells and the front side in back-contact cells. As a result of the calculations, it was found that field-effect surface passivation yields cell characteristics equivalent to those of a cell with effective surface recombination velocity of 0 cm/s, even if the cell has a poor Si SiO 2 interface (i.e., D it > 1.0 × 10 11cm −2eV −1). It was also found that both the use of a higher resistivity wafer and — especially in p-type substrates — the formation of inversion layers causes the field-effect surface passivation to work the fullest effect. In addition, a computer simulation based on physical-parameter measurements taken from actual materials forecasts that a back-contact cell would realistically be able to exceed 25% efficiency under AM1.5 global, one-sun illumination.