Study of Si Nanowires Produced by Metal-Assisted Chemical Etching as a Light-Trapping Material in n-type c-Si Solar Cells (original) (raw)

A simple passivation technique for the edge area of silicon solar cells improves the efficiency

Solar Energy Materials and Solar Cells, 2002

The efficiency of silicon solar cells (SC) can strongly be degraded by localized defects especially at the edge of SC (e.g. scratches) which are introduced during the production of the SC and may cause local shunts. A new optimized chemical etching procedure has been developed which allows a very effective passivation of shunts at the SC edges without reducing the surface area, i.e. without a reduction of the I sc current. In contrast to other techniques like plasma etching ("coin staking") or cutting off the edges, this procedure could be implemented cheaply in a large scale production. The newly developed passivation method always leads to an improvement in the efficiency η of slightly or severely degraded SCs which is typically around 10%-30%, but can be as large as 100%, while good SCs are totally uneffected with respect to η while still showing an improvement of the leakage current.

PROGRESS IN THE SURFACE PASSIVATION OF SILICON SOLAR CELLS

In order to increase the efficiency of silicon-wafer-based solar cells in production well above 20%, it is indispensable to improve the currently applied level of surface passivation at the front as well as at the rear of the cells. This paper focuses on two main challenges: (i) the low-temperature passivation of lowly doped p-type silicon surfaces at the cell rear and (ii) the passivation of highly boron-doped p + emitter surfaces as used at the front of solar cells on high-lifetime n-type silicon wafers. In the past, low surface recombination velocities (< 20 cm/s) have been achieved on low-resistivity (~1 Ωcm) p-type silicon using plasma-enhanced chemical-vapour-deposited (PECVD) silicon nitride (SiN x ) as well as amorphous silicon (a-Si). However, the high density of fixed positive charges within the PECVD-SiN x layer induces an inversion layer at the rear of p-type Si cells, producing a detrimental parasitic shunting, which reduces the short-circuit current density by up to 3 mA/cm 2 . The passivation quality of a-Si on the other hand is very temperature sensitive. More recently it has been shown that atomic-layer-deposited (ALD) aluminium oxide (Al 2 O 3 ) provides an outstanding level of surface passivation, which can be attributed to its extremely high negative fixed charge density in combination with the very gentle deposition technique ALD, leading to low interface state densities. The application of these ALD-Al 2 O 3 layers to the rear of p-type solar cells shows that this new passivation scheme is indeed suitable for high efficiencies and that due to the large negative fixed charge density no parasitic shunting occurs. We also demonstrate that ALD-Al 2 O 3 seems to be the ideal passivation layer for borondoped p + emitter surfaces. In a direct comparison with other passivation schemes, it is found that Al 2 O 3 even outperforms optimized thermally grown SiO 2 and opens the possibility of achieving very large open-circuit voltages up to V oc = 740 mV.

Advances in surface passivation of c-Si solar cells

Materials for Renewable and Sustainable Energy, 2012

In order to avoid an unacceptably large efficiency loss when moving towards thinner silicon materials, the near-term challenge in the c-Si PV industry is to implement an effective passivation method for both cell surfaces. This paper discussed several suitable passivation schemes available. While the efficiency potential of industrially produced thin film poly-Si cells on foreign substrates cannot yet reliably be predicted, it is clear that wafer-based c-Si solar cells will allow to maintain (or even improve) today's efficiency levels while at the same time reducing the consumption of (expensive) crystalline silicon by up to 50 %. Given the trend towards these Si materials, the most promising surface passivation methods are identified to date. The key issues to be considered are cost-effectiveness, added complexity, additional benefits, reliability, and efficiency potential. The efficiency increase for best cells is around 0.5-0.6 %abs and the current efficiency potential already demonstrated for all technologies is around 19.0 %. Average efficiencies in industrial mass production for selected technologies are 18.5-18.6 % for Cz and 17.1 % for mc-Si.

Passivation of all-angle black surfaces for silicon solar cells

Solar Energy Materials and Solar Cells

Optical losses at the front surface of a silicon solar cell have a significant impact on efficiency, and as such, efforts to reduce reflection are necessary. In this work, a method to fabricate and passivate nanowire-pyramid hybrid structures formed on a silicon surface via wet chemical processing is presented. These high surface area structures can be utilised on the front surface of back contact silicon solar cells to maximise light absorption therein. Hemispherical reflectivity under varying incident angles is measured to study the optical enhancement conferred by these structures. The significant reduction in reflectivity (<2%) under low incident angles is maintained at high angles by the hybrid textured surface compared to surfaces textured with nanowires or pyramids alone. Finite Difference Time Domain simulations of these dual micro-nanoscale surfaces under varying angles supports the experimental results. In order to translate the optical benefit of these high surface area structures into improvements in device efficiency, they must also be well passivated. To this end, atomic layer deposition of alumina is used to reduce surface recombination velocities of these ultra-black silicon surfaces to below 30 cm/s. A decomposition of the passivation components is performed using capacitance-voltage and Kelvin Probe measurements. Finally, device simulations show power conversion efficiencies exceeding 21% are possible when using these ultra-black Si surfaces for the front surface of back contact silicon solar cells.

Advances in the Surface Passivation of Silicon Solar Cells

Energy Procedia, 2012

The surface passivation properties of aluminium oxide (Al 2 O 3) on crystalline Si are compared with the traditional passivation system of silicon nitride (SiN x). It is shown that Al 2 O 3 has fundamental advantages over SiN x when applied to the rear of p-type silicon solar cells as well as to the p + emitter of n-type silicon solar cells. Special emphasis is paid to the transfer of Al 2 O 3 into industrial solar cell production. We compare different Al 2 O 3 deposition techniques suitable for mass production such as ultrafast spatial atomic layer deposition, inline plasma-enhanced chemical vapour deposition and reactive sputtering. Finally, we review the most recent cell results with Al 2 O 3 passivation and give a brief outlook on the future prospects of Al 2 O 3 in silicon solar cell production.

Passivation of thin film silicon solar cells

2016

Passivation of thin film polycrystalline silicon solar cells in hydrogen plasma and in water vapour at various processing conditions to achieve maximum passivation effect. 2) In-situ analysis of the water vapour passivation process. 3) Analysis of the passivated solar cells by available methods to explain and describe mechanisms of the water vapour passivation process. 4) Comparison of the passivation in hydrogen plasma and in water vapour. Chapter 4 describes some of measuring methods used in this thesis Suns-VOC method, a sun simulator, EQE. Chapter 5 aims to present the state-of-the-art of solid phase crystallized (SPC) and liquid phase crystallized (LPC) thin film silicon solar cells, their structure, achieved results and perspectives. Chapter 6 researches optimum processing conditions of the water vapour passivation for SPC Si. Chapter 7 investigates a passivation effect of hydrogen plasma on SPC and LPC poly-Si solar cells. Chapter 8 reports on a possible synergetic passivation effect of water vapour and hydrogen plasma. Chapter 9 compares plasma hydrogenation and water vapour passivation. Chapter 10 presents optical pump transient terahertz probe spectroscopy as a useful tool for a contactless investigation of ultrafast processes in SPC Si solar cells. Chapter 11 brings a conclusion with the most important results and observations. Chapter 12 summarizes contributions of the thesis in the fields of silicon passivation and characterization. Chapter 13 presents further possible research steps in the field of silicon passivation processes and material characterization.

Performance limitation of Si nanowire solar cells: Effects of nanowire length and surface defects

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE ON ADVANCED MATERIALS: ICAM 2019

In Si nanowire (SiNW) solar cells enhanced light confinement property in addition to decoupling of charge carrier collection and light absorption directions plays a significant role to resolve the draw backs of bulk Si solar cells. In this report we have studied the dependence of the phovoltaic properties of Si NW array solar cells on the SiNW length and enhanced surface defect states as a result of enhanced surface area of the NWs. The SiNW arrays have been fabricated using metal catalyzed electroless etching (MCEE) technique. p-n junction has been produced by spin-ondopant technique followed by thermal diffusion process. Front and rear electrodes have been deposited by e-beam evaporation techniques. SiNW lengths have been controlled from ~ 320 nm to 6.4 m by controlling the parameters of MCEE technique. Photovoltaic properties of the solar cells have been characterized by measuring quantum efficiency and photocurrent density vs. voltage characteristics. Morphological studies have been carried out by using scanning electron microscopy. Reduction in light trapping capability comes at the benefit of reduced surface defects. The reduction of surface defects has been proved to be more advantageous in comparison to the decrement of light trapping capability. The major contribution to the changes in cell efficiency comes from the enhancement of short circuit current density with a very weak dependence on open circuit voltage. This work is beneficial for the production of commercial Si solar cells where SiNW arrays could be used as an antireflection coating instead of using separate antireflection layers. Thus could reduced the production cost.

Rear surface passivation for high efficiency silicon solar cells

2013

Symbol Description Unit E d Defect energy level eV ∆ Phase difference between the parallel and orthogonal electric field component Deg ∆n Excess electron concentration cm-3 ∆p Excess hole concentration cm-3 µ n Electron mobility cm 2 .V-1 s-1 µ p Hole mobility cm 2 .V-1 s-1 AM 1.5 Solar irradiance function versus wavelength for one sun intensity through the atmosphere with an incidence angle of 48.2° W.cm-2 B Radiative constant cm-3 .s-1

Surface passivation of silicon solar cells using industrially relevant Al2O3 deposition techniques

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