Alexander To

Improved understanding of the recombination rate at inverted p+ silicon surfaces

The effect of positive fixed charge on the recombination rate at SiN x -passivated p+ surfaces is studied in this work. It is shown that a high positive fixed charge on a low defect density, passivated doped surface can result in a near injection level independent lifetime in a certain injection level range. This behaviour is modelled with advanced computer simulations using Sentaurus TCAD, which replicates the measurements conditions during a photoconductance based effective minority carrier lifetime measurement. The resulting simulations show that the shape of the injection level dependent lifetime is a result of the surface recombination rate, which is non-linear due to the surfaces moving into inversion with increasing injection level. As a result, the surface recombination rate switches from being limited by electrons to holes. Equations describing the surface saturation current density, J 0s, during this regime are also derived in this work.

The Effect of Bifacial AlOx Deposition on PERC Solar Cell Performance

Single-sided atomic layer deposition (ALD) is challenging as often some degree of wrap-around deposition onto the nontarget side occurs. This is particularly challenging for aluminum oxide (AlO<italic>_x</italic>) deposition on silicon solar cells, as is required on the rear of passivated emitter rear cell (PERC) solar cells. The effects of unintended ALD-deposited AlO<italic>_x</italic> on the front side of PERC solar cells are explored in this work, with a specific focus on the impact on contact resistance. The contact resistance is determined from transmission line measurement structures on samples with up to 10 nm of AlO<italic>_x</italic> on top of silicon nitride (SiN<italic>_x</italic>). These values are used as input parameters for computer simulations to simulate the effect of varying degrees of wrap-around on <italic>p</italic>-PERC cells. Finally, <italic>p</italic>-type aluminum back surface field solar cells with 3, 5, 7, and 10 nm AlO<italic>_x</italic> layers on top of the SiN<italic>_x</italic> are fabricated using various firing recipes and screen printing pastes to assess the effect on fabricated devices. It was found that a 3 nm AlO<italic>_x</italic> layer can improve the cell performance relative to the uncapped case, whereas 5, 7, and 10 nm resulted in a lower solar cell efficiency due to higher series resistance.

Empirical and Quokka simulated evidence for enhanced V OC due to limited junction area for high efficiency silicon solar cells

A new design for high efficiency silicon solar cells with the use of limited p-n junction area and point Al rear contact is proposed. In this work, planar test devices with different emitter area and spacing are fabricated to demonstrate the limited junction area (LIA) design can improve VOC for a well-designed cell structure. Interim lifetime and PL measurements during fabrication confirm lower dark saturation current (I0) and higher implied VOC (iVOC) in the LIA samples with a robust surface passivation scheme. Higher VOC and efficiency are also simulated with Quokka on some LIA devices compared to the full area emitter control sample.

Extracting dielectric fixed charge density on highly doped crystalline-silicon surfaces using photoconductance measurements

A novel method for the extraction of fixed interface charge, Qf, and the surface recombination parameters, Sn0 and Sp0, from the injection-level dependent effective minority carrier lifetime measurements is presented. Unlike conventional capacitance-voltage measurements, this technique can be applied to highly doped surfaces provided the surface carrier concentration transitions into strong depletion or inversion with increased carrier injection. By simulating the injection level dependent Auger-corrected inverse lifetime curve of symmetrically passivated and diffused samples after sequential annealing and corona charging, it was revealed that Qf, Sn0, and Sp0 have unique signatures. Therefore, these important electronic parameters, in some instances, can independently be resolved. Furthermore, it was shown that this non-linear lifetime behaviour is exhibited on both p-type and n-type diffused inverted surfaces, by demonstrating the approach with phosphorous diffused n+pn+ structures and boron diffused p+...

Inkjet patterned ALD aluminium oxide for rear PERC metal contacts

A method of patterning atomic layer deposition (ALD) deposited aluminium oxide dielectrics (Al2O3) using an inkjet printer is outlined. This method has applications in creating PERC cell type rear contacts. It is simpler, and uses fewer chemicals than immersive etching techniques such as those involved in photolithography, and the patterning does not cause damage to the silicon evident in laser ablation techniques. Holes with varying diameter between 61-159 μm were etched onto a 50 nm thick ALD Al2O3 passivating dielectric at numerous print settings. The relationship between the printing parameters and the feature size of the holes is discussed.

Modeling of silicon solar cells voltage increase on localized emitter area approach

A limited area p-n junction silicon solar cell is designed and the effect of limited area emitter on the output open circuit voltage (VOC) and efficiency is studied by a 3-D simulator Quokka. The recombination property of emitter, surface and bulk are all extracted from the test structures and this paper reports potential voltage that can be achieved on the limited area junction solar cells by the modelling. The result indicates that this approach will help improve VOC when the emitter area is reduced and the voltage will be more than 700mV. The result also shows that there is an optimum emitter width for the highest efficiency on the limited area junction solar cell.

Silicon solar cell voltage increase based on limited area junction

A study of limited area p-n junction silicon solar cells using photoluminescence (PL) is used to demonstrate the pathways to an increase in open circuit voltage (VOC). A high voltage baseline structure is used to ensure that the junction dominates the recombination. Both quasi-steady-state photo-conductance (QSSPC) and PL measurements indicate higher implied voltage on the limited p-n junction area solar cells compared to full area junction cells. This work aims at achieving more than 740 mV VOC on the limited area junction structure, through optimising the passivation layers and comprehensive analysis of voltage losses throughout the structure. Various choices of passivation layers and a new structure design for separating and analysing J0 from each component on the limited area junction solar cell will be presented in this paper.