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.This work has been partially supported by the Australian Research Council

Carrier population control and surface passivation in solar cells

Controlling the concentration of charge carriers near the surface is essential for solar cells. It permits to form regions with selective conductivity for either electrons or holes and it also helps to reduce the rate at which they recombine. Chemical passivation of the surfaces is equally important, and it can be combined with population control to implement carrier-selective, passivating contacts for solar cells. This paper discusses different approaches to suppress surface recombination and to manipulate the concentration of carriers by means of doping, work function and charge. It also describes some of the many surface-passivating contacts that are being developed for silicon solar cells, restricted to experiments performed by the authors.Funding from the Australian Government via ARENA (project RND003), ACAP (project on "Passivated contacts") and the ARC (DP150104331) is gratefully acknowledged

A magnesium/amorphous silicon passivating contact for n-type crystalline silicon solar cells

Among the metals, magnesium has one of the lowest work functions, with a value of 3.7 eV. This makes it very suitable to form an electron-conductive cathode contact for silicon solar cells. We present here the experimental demonstration of an amorphous silicon/magnesium/aluminium (a-Si:H/Mg/Al) passivating contact for silicon solar cells. The conduction properties of a thermally evaporated Mg/Al contact structure on n-type crystalline silicon (c-Si) are investigated, achieving a low resistivity Ohmic contact to moderately doped n-type c-Si (∼5 × 1015 cm−3) of ∼0.31 Ω cm2 and ∼0.22 Ω cm2 for samples with and without an amorphous silicon passivating interlayer, respectively. Application of the passivating cathode to the whole rear surface of n-type front junction c-Si solar cells leads to a power conversion efficiency of 19% in a proof-of-concept device. The low thermal budget of the cathode formation, its dopant-less nature, and the simplicity of the device structure enabled by the Mg/Al contact open up possibilities in designing and fabricating low-cost silicon solar cells.This work was supported by the Australian Government through the Australian Research Council and the Australian Renewable Energy Agency (ARENA). Some facilities at the Australian National Fabrication Facility at the ANU were used

Methods to Improve Bulk Lifetime in n-Type Czochralski-Grown Upgraded Metallurgical-Grade Silicon Wafers

This paper investigates the potential of three different methods-tabula rasa (TR), phosphorus diffusion gettering (PDG), and hydrogenation, for improving the carrier lifetime in n-type Czochralski-grown upgraded metallurgical-grade (UMG) silicon samples. Our results show that the lifetimes in the UMG wafers used in this study were affected by both mobile metallic impurities and as-grown oxygen precipitate nuclei. Thus, the dissolution of grown-in oxygen precipitate nuclei via TR and the removal of mobile impurities via PDG step were found to significantly improve the electronic quality of the UMG wafers. Finally, we report bulk lifetimes and 1-sun implied open-circuit voltages of the UMG wafers after boron and phosphorus diffusions, as typically applied in n-type cell fabrication.This work has been supported by the Australian Renewable Energy Agency (ARENA) through research grant RND009.

In situ recombination junction between p-Si and TiO2 enables high-efficiency monolithic perovskite/Si tandem cells

Increasing the power conversion efficiency of silicon (Si) photovoltaics is a key enabler for continued reductions in the cost of solar electricity. Here, we describe a two-terminal perovskite/Si tandem design that increases the Si cell’s output in the simplest possible manner: by placing a perovskite cell directly on top of the Si bottom cell. The advantageous omission of a conventional interlayer eliminates both optical losses and processing steps and is enabled by the low contact resistivity attainable between n-type TiO2 and Si, established here using atomic layer deposition. We fabricated proof-of-concept perovskite/Si tandems on both homojunction and passivating contact heterojunction Si cells to demonstrate the broad applicability of the interlayer-free concept. Stabilized efficiencies of 22.9 and 24.1% were obtained for the homojunction and passivating contact heterojunction tandems, respectively, which could be readily improved by reducing optical losses elsewhere in the device. This work highlights the potential of emerging perovskite photovoltaics to enable low-cost, high-efficiency tandem devices through straightforward integration with commercially relevant Si solar cells

Tantalum oxide electron-selective heterocontacts for silicon photovoltaics and photoelectrochemical water reduction

Crystalline silicon (c-Si) solar cells have been dominating the photovoltaic (PV) market for decades, and c-Si based photoelectrochemical (PEC) cells are regarded as one of the most promising routes for water splitting and renewable production of hydrogen. In this work, we demonstrate a nanoscale tantalum oxide (TaOx, ∼6 nm) as an electron-selective heterocontact, simultaneously providing high-quality passivation to the silicon surface and effective transport of electrons to either an external circuit or a water-splitting catalyst. The PV application of TaOx is demonstrated by a proof-of-concept device having a conversion efficiency of 19.1%. In addition, the PEC application is demonstrated by a photon-to-current efficiency (with additional applied bias) of 7.7%. These results represent a 2% and 3.8% absolute enhancement over control devices without a TaOx interlayer, respectively. The methods presented in this Letter are not limited to c-Si based devices and can be viewed as a more general approach to the interface engineering of optoelectronic and photoelectrochemical applications

Phosphorus-diffused polysilicon contacts for solar cells

This paper describes the optimization of a technique to make polysilicon/SiO x contacts for silicon solar cells based on doping PECVD intrinsic polysilicon by means of a thermal POCl3 diffusion process. Test structures are used to measure the recombination current density Joc and contact resistivity ρ c of the metal/n+ polysilicon/SiO x /silicon structures. The phosphorus diffusion temperature and time are optimized for a range of thicknesses of the SiO x and polysilicon layers. The oxide thickness is found to be critical to obtain a low contact resistivity ρ c , with an optimum of about 1.2nm for a thermal oxide and ~1.4nm for a chemical oxide. A low J oc ≤5fA/cm2 has been obtained for polysilicon thicknesses in the range of 32nm-60nm, while ρ c increases from 0.016Ωcm2 to 0.070Ω-cm2 due to the bulk resistivity of polysilicon. These polysilicon/SiO x contacts have been applied to the rear of n-type silicon solar cells having a front boron diffusion, achieving Voc =674.6mV, FF=80.4% and efficiency=20.8%, which demonstrate the effectiveness of the techniques developed here to produce high performance solar cells

N- and p-type silicon Solar Cells with Molybdenum Oxide Hole Contacts

AbstractThis paper provides an experimental proof-of-concept for simple solar cell designs on n- and p-type crystalline silicon (c-Si) substrates which utilise sub-stoichiometric MoOx (x < 3) films to collect holes. The n-type cell design (referred to as ‘moly-poly’) features a planar rear SiOx / poly-Si(n+) stack with a planar front SiOx / MoOx / ITO stack. We demonstrate an un-optimised conversion efficiency of ∼16.7±1% for a 3 x 3cm cell using a simple 10-step fabrication procedure. The p-type cell design (referred to as ‘moly-BSR’) is comprised of a simple SiNx passivated, textured, front phosphorus diffusion with a rear MoOx / Ag hole contact. A conversion efficiency of ∼16.4±1% is achieved for 2 x 2cm using an 11-step fabrication procedure. Beyond the proof-of-concept results achieved, a number of future improvements are also outlined