Christian Samundsett

Millisecond minority carrier lifetimes in n-type multicrystalline silicon

Exceptionally high minority carrier lifetimes have been measured in n-type multicrystalline silicon (mc-Si) grown by directional solidification and subjected to phosphorus gettering. The highest effective lifetimes, up to 1.6 ms averaged over several grains and 2.8 ms within some of them, were measured for relatively lowly doped, 2–3 Ωcm, wafers. The lifetime was found to decrease for lower resistivities, still reaching 500 μs for 0.9 Ωcm and 100 μs for 0.36 Ωcm. Several important findings are reported here: (i) achievement of carrier lifetimes in the millisecond range for mc-Si, (ii) effectiveness of phosphorus gettering in n-type mc-Si, and (iii) demonstration of good stability under illumination for n-type mc-Si.

Recombination and trapping in multicrystalline silicon

Minority carrier recombination and trapping frequently coexist in multicrystalline silicon (mc-Si), with the latter effect obscuring both transient and steady-state measurements of the photoconductance. In this paper, the injection dependence of the measured lifetime is studied to gain insight into these physical mechanisms. A theoretical model for minority carrier trapping is shown to explain the anomalous dependence of the apparent lifetime with injection level and allow the evaluation of the density of trapping centers. The main causes for volume recombination in mc-Si, impurities and crystallographic defects, are separately investigated by means of cross-contamination and gettering experiments. Metallic impurities produce a dependence of the bulk minority carrier lifetime with injection level that follows the Shockley-Read-Hall recombination theory. Modeling of this dependence gives information on the fundamental electron and hole lifetimes, with the former typically being considerably smaller than the latter, in p-type silicon, Phosphorus gettering is used to remove most of the impurities and reveal the crystallographic limits on the lifetime, which can reach 600 /spl mu/s for 1.5 /spl Omega/cm mc-Si. Measurements of the lifetime at very high injection levels show evidence of the Auger recombination mechanism in mc-Si. Finally, the surface recombination velocity of the interface between mc-Si and thermally grown SiO/sub 2/ is measured and found to be as low as 70 cm/s for 1.5 /spl Omega/cm material after a forming gas anneal and 40 cm/s after an anneal. These high bulk lifetimes and excellent surface passivation prove that mc-Si can have an electronic quality similar to that of single-crystalline silicon.

Proof-of-concept p-type silicon solar cells with molybdenum oxide partial rear contacts

This paper explores the application of transparent MoOx (x<;3) films as hole-collecting contacts on the rear-side of crystalline silicon solar cells. 2D simulations, which consider experimental contact recombination J0c and resistivity ρc values, demonstrate that the benefits of the MoOx based contacts are best exploited by reducing the rear contact fraction. This concept is demonstrated experimentally using simple p-type cells featuring a ~5% rear fraction MoOx contact. These cells attain a conversion efficiency of 20.4%, a promising result, given the early stage of development for this technology.

Upgraded metallurgical-grade silicon solar cells with efficiency above 20%

We present solar cells fabricated with n-type Czochralski–silicon wafers grown with strongly compensated 100% upgraded metallurgical-grade feedstock, with efficiencies above 20%. The cells have a passivated boron-diffused front surface, and a rear locally phosphorus-diffused structure fabricated using an etch-back process. The local heavy phosphorus diffusion on the rear helps to maintain a high bulk lifetime in the substrates via phosphorus gettering, whilst also reducing recombination under the rear-side metal contacts. The independently measured results yield a peak efficiency of 20.9% for the best upgraded metallurgical-grade silicon cell and 21.9% for a control device made with electronic-grade float-zone silicon. The presence of boron-oxygen related defects in the cells is also investigated, and we confirm that these defects can be partially deactivated permanently by annealing under illumination.

Evidence of impurity gettering by industrial phosphorus diffusion

The possible benefits of phosphorus gettering as applied to production multicrystalline silicon wafers have been evaluated. After optimization of an open tube POCl/sub 3/ process, relatively low temperatures and short times have been found to significantly improve the minority carrier lifetime of most wafers. The possible gettering action stemming from the industrial process of phosphorus diffusion has also been investigated and found to be similarly effective. Average lifetimes of 45 /spl mu/s (diffusion length of 360 /spl mu/m) were obtained, with some wafers reaching maximum values up to 130 /spl mu/s. Lifetime monitoring of a commercial cell fabrication line has also enabled characterization of the voltage limits imposed by the standard emitter and aluminum back-surface-field. The results indicate that the bulk, as improved by emitter gettering, is generally not the limiting factor on cell performance.

Demonstration of c-Si Solar Cells With Gallium Oxide Surface Passivation and Laser-Doped Gallium p + Regions

Gallium oxide (Ga₂O₃) deposited by plasma enhanced atomic layer deposition (PEALD) is shown to passivate crystalline silicon surfaces via a combination of a high negative charge and a reduction in the density of surface defects to below 1×10¹¹ cm^-2 eV^-1 at midgap. The passivation, as determined by the injection dependent excess carrier lifetime, is demonstrated to be commensurate to that of PEALD aluminium oxide (Al₂O₃). In addition, Ga₂O₃ is used as a gallium source in a laser doping process, resulting in an efficiency of 19.2% and an open circuit voltage of 658 mV in a partial rear contact p-type cell design. As such, we demonstrate that Ga₂O₃ is comparable to Al₂O₃ in terms of performance and utility, with potential material advantages over Al₂O₃.

Towards industrial advanced front-junction n-type silicon solar cells

Recent progress in the development of advanced front-junction n-type monocrystalline solar cells for potential industrial fabrication is presented. The textured, boron-diffused front surface is passivated with a stack of Atmospheric Pressure Chemical Vapor Deposited (APCVD) Al2O3 and Plasma-Enhanced Chemical Vapor Deposited (PECVD) SiNx. A champion cell with an in-house measured efficiency of 21.6% is obtained for small-area cells (i.e., 2×2 cm2) fabricated at the ANU. The high open-circuit voltage of 664 mV demonstrates the excellent passivation of both the front and rear surfaces. The cell design and process have demonstrated a good tolerance to substrate resistivity variations, with an average cell efficiencies close to 21% for resistivity varying between 3 and 10 Ω·cm. Moreover, with an adaption of the process developed at the ANU, large-area cells (i.e., 12.5×12.5 cm2) are fabricated at Trina Solar on n-type Cz substrates with a resistivity of 2.5 Ω·cm. A champion cell with an in-house measured efficiency of 20.5% is obtained, demonstrating a high potential in commercializing the advanced cells developed in this work. Finally, simulations reveal that further improvements in cell efficiency are to be mainly achieved through further optimisations of the rear side contact geometry and rear surface passivation.

Materials and manufacturing processes for high-efficiency flexible photovoltaic modules

An overview of the materials, processing techniques, and characterisation procedures for flexible solar modules is presented. Flexible modules are lightweight, roll-able, and/or foldable for storage and transport. The design approach selected by the Australian National University incorporates very thin, high-efficiency crystalline silicon solar cells embedded between flexible coversheets, and supported by silicone encapsulant and flexible electrical contacts. The modules can be fabricated using a number of approaches including constructing the circuitry separately to the packaging, or using the packaging as both a protective layer and a base for circuitry.

Passivation of highly boron doped silicon surfaces by sputtered AlO x and PECVD SiN, a comparison

We show that boron-diffused emitters can be passivated with AlO<inf>x</inf> deposited using RF sputtering of an Al target. The surface passivation achieved so far is inferior to that obtained using an optimised PECDV SiN process that includes a chemically grown SiO<inf>2</inf> interfacial layer. Nevertheless, the levels of passivation obtained, expressed by emitter recombination current densities of J<inf>oE</inf>=228–349 fA/cm² for sheet resistances of 88–210 Ω/□, are already consistent with solar cell with efficiencies in the 20% range.

Microscopic Distributions of Defect Luminescence From Subgrain Boundaries in Multicrystalline Silicon Wafers

We investigate the microscopic distributions of sub-band-gap luminescence emission (the so-called D-lines D1/D2/D3/D4) and the band-to-band luminescence intensity, near recombination-active subgrain boundaries in multicrystalline silicon wafers for solar cells. We find that the sub-band-gap luminescence from decorating defects/impurities (D1/D2) and from intrinsic dislocations (D3/D4) has distinctly different spatial distributions, and is asymmetric across the subgrain boundaries. The presence of D1/D2 is correlated with a strong reduction in the band-to-band luminescence, indicating a higher recombination activity. In contrast, D3/D4 emissions are not strongly correlated with the band-to-band intensity. Based on spatially resolved, synchrotron-based micro-X-ray fluorescence measurements of metal impurities, we confirm that high densities of metal impurities are present at locations with strong D1/D2 emission but low D3/D4 emission. Finally, we show that the observed asymmetry of the sub-band-gap luminescence across the subgrain boundaries is due to its inclination below the wafer surface. Based on the luminescence asymmetries, the subgrain boundaries are shown to share a common inclination locally, rather than being orientated randomly.