Variation of the layer thickness to study the electrical property of PECVD Al2O3 / c-Si interface

AbstractThis paper focusses in particular on the influence of the layer thickness on the passivation quality, the charge density and the interface defects of PECVD Al2O3 passivation layers on c-Si surfaces. The surface recombination velocity and the interface defect density are observed to increase by decreasing the layer thickness. However, the density of negative charges remains almost constant with values around 3 1012cm-2. An optimal passivation quality is obtained for thicknesses of 15nm and higher. A linear relation between surface recombination velocity and Dit was established, allowing the estimation of the electron capture cross section (σn ∼ 10-13cm-2).Additionally, we measured the capture cross section of holes and electrons using DLTS measurement. The results are found to be very similar to reported values for silicon dioxide. This supports the idea that the chemical passivation of crystalline silicon by Al2O3 is performed by the interstitial SiO2 layer

Numerical modeling of highly doped Si:P emitters based on Fermi–Dirac statistics and self-consistent material parameters

We have established a simulation model for phosphorus-doped silicon emitters using Fermi–Dirac statistics. Our model is based on a set of independently measured material parameters and on quantum mechanical calculations. In contrast to commonly applied models, which use Boltzmann statistics and apparent band-gap narrowing data, we use Fermi–Dirac statistics and theoretically derived band shifts, and therefore we account for the degeneracy effects on a physically sounder basis. This leads to unprecedented consistency and precision even at very high dopant densities. We also derive the hole surface recombination velocity parameter Spo by applying our model to a broad range of measurements of the emitter saturation current density. Despite small differences in oxide quality among various laboratories, Spo generally increases for all of them in a very similar manner at high surfacedoping densities Nsurf. Pyramidal texturing generally increases Spo by a factor of five. The frequently used forming gas anneal lowers Spo mainly in low-doped emitters, while an aluminumanneal(Al deposit followed by a heat cycle) lowers Spo at all Nsurf.P.P.A. is on a Postdoctoral Fellowship from the Australian Research Council ~ARC!. The Center for Photovoltaic Engineering is supported by ARC’s Special Research Centres Scheme. A.C. and M.K. also acknowledge funding by the ARC

Silicon solar cell–integrated stress and temperature sensors for photovoltaic modules

We propose silicon solar cell–integrated stress and temperature sensors as a new approach for the stress and temperature measurement in photovoltaic (PV) modules. The solar cell–integrated sensors enable a direct and continuous in situ measurement of mechanical stress and temperature of solar cells within PV modules. In this work, we present a proof of concept for stress and temperature sensors on a silicon solar cell wafer. Both sensors were tested in a conventional PV module setup. For the stress sensor, a sensitivity of (−47.41 ± 0.14)%/GPa has been reached, and for the temperature sensor, a sensitivity of (3.557 ± 0.008) × 10$^{-3}$ K$^{-1}$ has been reached. These sensors can already be used in research for increased measurement accuracy of the temperature and the mechanical stress in PV modules because of the implementation at the precise location of the solar cells within a laminate stack, for process evaluation, in‐situ measurements in reliability tests, and the correlation with real exposure to climates

Improved Silicon Surface Passivation by ALD Al2O3/SiO2 Multilayers with In‐Situ Plasma Treatments

Abstract Al2O3 is one of the most effective dielectric surface passivation layers for silicon solar cells, but recent studies indicate that there is still room for improvement. Instead of a single layer, multilayers of only a few nanometers thickness offer the possibility to tailor material properties on a nanometer scale. In this study, the effect of various plasma treatments performed at different stages during the ALD deposition of Al2O3/SiO2 multilayers on the silicon surface passivation quality is evaluated. Significant improvements in surface passivation quality for some plasma treatments are observed, particularly for single Al2O3/SiO2 bilayers treated with a H2 plasma after SiO2 deposition. This treatment resulted in a surface recombination parameter J0 as low as 0.35 fA cm−2 on (100) surfaces of 10 Ω cm n‐type silicon, more than a factor of 5 lower than that of Al2O3 single layers without plasma treatment. Capacitance‐voltage measurements indicate that the improved surface passivation of the plasma‐treated samples results from an enhanced chemical interface passivation rather than an improved field effect. In addition, a superior temperature stability of the surface passivation quality is found for various plasma‐treated multilayers

Realization of TOPCon using industrial scale PECVD equipment

This paper discusses the successful realization of tunnel oxide passivated contacts (TOPCon) using industry-relevant PECVD equipment. It will be shown that batch-type direct plasma PECVD allows for a damage-free deposition of doped a-Si onto an ultra-thin oxide layer. Using symmetric test structures the impact of thermally or wet-chemically grown ultra-thin interfacial SiOx layer, as well as the influence of the poly-Si doping level on the surface passivation quality will be discussed in detail. Maximum values of 736 mV iVoc and 87.4 % iFF were achieved. Additionally, asymmetric lifetime samples featuring an n-type TOPCon at the rear and a p-type TOPCon at the front demonstrated 720 mV Voc

Stable reverse bias or integrated bypass diode in HIP‑MWT+ solar cells

The Metal Wrap Through+ (HIP-MWT+) solar cell is based on the PERC concept but features two additional electrical contacts, namely the Schottky contact between p-type Si bulk and Ag n-contact and the metal-insulator-semiconductor (MIS) contact on the rear side of the cell below the n-contact pads. To prevent thermal hotspots under reverse bias, both contacts shall either restrict current flow or allow a homogenous current flow at low voltage. In this work we present both options. First the stable reverse bias characteristics up to −15 V with a MIS contact using industrially manufactured SiON passivation and second, an integrated by-pass diode using AlOX as insulator in the passivation stack allowing current flows at approximately Vrev = –3.5 V depending on the chosen screen-print paste. The examined Schottky contacts break down at around Vrev = –2.5 V. Reverse bias testing of the cells reveals a solid performance of the cells under reverse bias and an average conversion efficiency of η = 21.2% (AlOX) and η = 20.7% (SiON), respectively

Numerical simulation of silicon heterojunction solar cells featuring metal oxides as carrier-selective contacts

The applicability of different high (low) work function contact materials for the formation of alternative passivating and hole (electron) selective contacts is currently re-explored for silicon solar cells. To assist the engineering of those contacts, which is still in its infancy, numerical device simulations are used to improve knowledge regarding relevant heterojunction and thin film properties with the focus on metal oxide based hole contacts. The importance of 1) a high metal oxide work function for the induced c-Si pn-junction is shown. It is elucidated that for an efficient hole transport from this induced c-Si junction into the external electrode, via the buffer and the metal oxide, 2) the metal oxide's conduction band must be below the valence band of the buffer (or c-Si absorber) for direct band-to-band tunneling, or 3) bulk traps near the valence band edge of the buffer (or c-Si absorber) are needed for trap-assisted tunneling

Long Term Stability Analysis of Copper Front Side Metallization for Silicon Solar Cells

AbstractIn this work the development and evaluation of a copper solar cell front side metallization based on a screen printed silver seed layer, a plated nickel diffusion barrier, a plated copper conductive layer and a plated silver capping is presented. Due to a slight adjustment of the standard screen printing process, and subsequent plating of Ni, Cu and Ag, silver consumption was reduced to < 16 mg per cell front side on standard BSF 15.6 cm x 15.6cm mono crystalline wafers, produced only with inline capable techniques. Contact adhesion forces of 1.5 N/mm and, efficiencies of 18.4 % were achieved. For the long term stability analysis on cell and module level, cells with different nickel diffusion barrier masses (10-40mg/cell) were treated on hotplates at 200, 225 and 250°C while detecting degradation due to copper diffusion by measuring the pFF. Sufficient cell life times are predicted for 10 mg Ni by the resulting Arrhenius plot. After 750h damp heat test of one cell modules no visible degradation was determined. This means that accelerated degradation on hotplates show the degradation due to copper diffusion more strongly than 750hours damp heat test. In an additional SEM analysis the plated nickel diffusion barrier showed a closed layer on the fingers and an incomplete layer on the busbars, indicating busbars as possible weak points for copper diffusion due to inhomogeneous Ni platin