What links metrology with solar energy?

Solar growth

Power generation from solar cells - also known as photovoltaic (PV) cells - has grown significantly in popularity over the last decade and shows no sign of fading. The application of solar technology is vital for smart grids, distributed power generation and the full participation of consumers in the energy market. In the near-term, large-scale solar power generation is becoming increasingly popular and now competes on a price-per-kilowatt basis with traditional energy sources such as nuclear and gas.

Growth in the solar power market has shown a significant upturn in recent years with a 50 % growth in installed capacity between 2015 and 2016. China is the largest market and was responsible for nearly half of this new capacity. The Asia-Pacific (APAC) area has now become the largest solar-powered region in the world with 147.2 GW of total installed capacity (2016), exceeding that of Europe and the US.

PV manufacturing

Motion control is required at every step and all handling steps in-between. One of the most challenging processes is the precise deposition of the metal contact layers. Silver and aluminium paste is screen printed on the front and back surfaces of each wafer. The sunward side of the PV cell has a printed series of fine contact fingers approximately 100 microns wide, with a pitch of 2 mm, overlaid with two or three perpendicular bus bars. The rear side has a corresponding set of bus bars over a metalized area. The main function of the front and rear bus bars is to collect the current and to enable mechanical contact with conducting electrodes.

A screen printing system for PV applications may utilise the following steps:

• Wafer inspection - The system delivers the wafer to an inspection zone and images at least two fiducials (alignment marks) on the upper surface of the wafer.
  
• Image processing - Precise position data for each fiducial is then determined by image processing software and periodically updated in memory to improve accuracy over many printed layers.
• Wafer alignment - The wafer stage is finely adjusted in X, Y and θ (see Figure 3) to correct for offsets and then transferred beneath a mesh printing screen (stencil). These adjustments overlap the fiducials on the wafer with the stored reference fiducials. Both linear and angular encoders provide the requisite position feedback to drive each axis to accurately align the wafer with the screen.
  
• Printing - After alignment, the wafer is clamped in place before direct application of silver and aluminium paste by conventional screen printing.

Motion control requirements

Current monocrystalline PV cell efficiencies approach 20 %: the maximum theoretical limit is about 29 % for a silicon single-junction cell. Improvements to conversion efficiency drive down costs per KWh and reduce the physical footprint of solar power installations, so manufacturers are constantly improving their production processes to achieve efficiency gains.

A typical silicon solar cell production process requires multiple screen printing runs, and there are at least two separate printing steps for the front and back sides of the cell. For improved efficiency, contact lines on the front side must be printed as finely as possible without reducing conductivity, which requires the printing of layers that are overlaid with high accuracy and repeatability.

By making the lines thinner and taller, more of the cell can participate in solar energy conversion (Figure 4). For instance, moving from contact fingers with 120 µm line width to 70 µm line width, with twice the thickness, gives a potential conversion efficiency gain of 0.5 %. Another technique for improving performance is the use of selective emitters - the n-doped front side of the solar cell. By strongly doping the area directly below the metal contacts and only weakly doping the other areas, cell efficiency at shorter wavelengths can be improved (Figure 5).

There are several techniques available for manufacturing selective emitters and most involve the highly-accurate alignment and deposition of layers. Screen alignment accuracy is the most critical metric for multi-layer printed contacts because subsequent print layers must be precisely placed over the first. State-of-the-art alignment systems with high-resolution cameras are now capable of ±10 µm alignment accuracy. High-precision position encoders, such as Renishaw's RESOLUTE absolute encoder system, are key to improving print screen overlay accuracy and control. RESOLUTE encoders are capable of high translational speeds to 100 m/s, high resolutions to 1 nm, and low cyclic error of ±40 nm.

Summary

Solar energy is likely to become a major source of electrical power over the coming decades with applications in both centralised and distributed generation e.g. rooftop solar (Figure 6). There are many commercial opportunities for successful supply chain participants. Motion control is involved at every stage of the manufacturing process of PV cells and is particularly essential for high-precision screen printing. Renishaw's motion control expertise and extensive encoder product ranges provide OEMs and end users with the latest metrology solutions for their motion control needs.