Photovoltaics (solar panels)

 

Benefits offered by Oxford Instruments Plasma Technology

Our tools handle substrates of various shapes and sizes. They can cluster and merge PVD, CVD, Etch and ALD technologies on the same platform, allowing multiple process steps to be carried out without a vacuum break. The cluster tool has the option of integrating a Kelvin Probe for in-situ measurement capability of material work functions and photo voltage, without vacuum break.

Other Key Benefits:

• High vacuum options available for handler and CVD, resulting in reduced background contamination levels
• Very high frequency and remote plasma CVD options for deposition onto damage sensitive surfaces
• Flexibility of power ramping within the software, minimising potential damage at the start of process
• High temperature substrate table options up to 800ºC
• Refractive index tuning for single or multiple layer anti reflection coatings, enable improved cell efficiency
• Low pinhole density to avoid electrical shorts, particularly for larger area solar cells
• Substrate handling up to 210 mm2 available

 

Example photovoltaic applications include:

• Anti-reflective coating

• Transparent conducting oxide
• Base material
• Photonic crystals
• Etched lenses
• Device isolation
• Edge isolation etch
• Passivation of interface defects

ALD Surface Passivation

Surface passivation is a major issue for crystalline silicon solar technology. The high level of built-in negative charges in an ultra-thin aluminium oxide layer deposited by plasma ALD can almost entirely eliminate electronic losses at the solar cell surfaces.

ALD Surface Passivation

FlexAL ALD tool

Infrared Mapping of 100mm Diameter p-type

ALD Transparent Conductive Oxide (TCO)

ZnO is a transparent conducting oxide that can be deposited in
a highly controlled and conformal manner by ALD.
ZnO:Al – ALD is ideally suited for controlled doping of
small amounts of aluminium to reduce the conductivity.

Oxford Instruments ALD Capabilities

• Proven process capability in solar cell technology
• FlexAL® (load lock) and OpAL™ (open load) tools customised for PV applications
• Proven low damage remote plasma source whilst still producing high plasma densities for excellent film quality
• Scale up route with cassette to cassette handling
  

The use of alumina deposited by ALD as front passivation over n-type crystalline silicon solar cells has shown significant improvements in conversion efficiency.1 An Al2O3 and SiNx or SiOx stack deposited by PECVD has shown to be an effective combination for passivation.

High solar cell efficiencies using a FlexAL tool

High solar cell efficiencies using a FlexAL tool
A record efficiency has been demonstrated for PERL solar cells based on n-type silicon by the application of an ultra-thin Al2O3 layer at the front of the solar cell.
Research carried out between the Eindhoven University of Technology (TUe). All data is courtesy of TUe.

Good lifetime uniformity

Lifetime mapping of a 100 mm diameter p-type c-Si wafer,passivated with 30 nm Al2O3 deposited by plasma ALD.

PVD of Transparent Conductive Oxide (TCO)

Indium Tin Oxide Deposition by PVD

Sputter deposited ITO with resistivity down to 3x10-4 Ohm cm and optical transmission of up to 92%. Sputter deposition is the favoured method for depositing thin layers of indium tin oxide used in many photovoltaic devices.
Oxford Instruments Plasma Technology has specifically developed a pulsed DC magnetron process to deposit ITO films using a typical commercially available ceramic oxide target.
Key parameters and trends such as pressure and oxygen/argon gas flow ratios have been identified. The process can

Image courtesy of Adam Williamson at X-FAB Semiconductor Foundries

be adjusted to provide films with optimum film properties.
Low resistivity down to 3.0x10-4 Ω cm is achieved while maintaining high optical
transmission in the visible and near infrared spectrum at up to >90%.

Surface texturing by ICP etch

Black silicon surface texturing produced in the PlasmalabSystem100 ICP380 may be used to reduce light reflection losses below 2%.
Surface analysis (AES/AFM/EDX/TEM) has confirmed that the Si is still crystalline and of device quality – the needles are essentially structured crystalline Substrate.

Silicon Nitride Anti-Reflection Layer and Passivation Layers by PECVD

Up to 30% sunlight can be reflected without the use of an anti-reflection coating. Processes can be tuned to give a range of refractive
indices and hydrogen content.
Refractive index tuning for single or multiple layer anti reflection coatings result in improved cell efficiency.

Amorphous Silicon Deposition by High Frequency PECVD

Gas chemistry or kinetics are important factors in photovoltaic applications of amorphous silicon. Oxford Instruments Plasma Technology offers a wide variation in possible process schemes that can influence not only bulk film properties but also impingement of plasma
species at the surface or interface.
Various gas chemistries can be supplied and source options include RF, very high frequency RF and remote
or downstream plasma.

Remote Plasma PECVD

• Various plasma source options available
• High density plasma at 5 x 1011 / cm2
• Reduced surface ion damage
• Ion flux control through transmission plates (patent filed)
• High quality films deposited at reduced temperature

Hot table option

• High temperature table option is available, with radiation shielding
• Incorporates a dual heater assembly to optimise temperature uniformity
• Fast ramping from ambient to 700°C in less than 40 minutes

Process Tool Software

Oxford Instruments PC2000TM Software is renowned for its clarity and ease of use, making it quick to train process operators while retaining full functionality for fab managers and service staff. The front end visual interface which
controls and monitors the process tool is configured exactly for the customer’s system.

Process Tool Software Benefits:

Ability to control a tool cluster from a single interface

• Process recipes are written, stored and recalled through the same software, building a library
• Password controlled user login allows different levels of user access and tasks, from ‘one button’ run operation to full system functions
• Continuous system data logging ensures traceability of each wafer and                       process run
• Fully GEM/SECS compatible