Solar perks of laser micromachining
Over the past decade, laser micromachining has been applied in multiple ways to increase the conversion efficiency of solar cells. Thermal laser separation is a prime example; it won the Intersolar Technology Award in 2015 and was a SNEC conference Top10 Technology Highlight in 2016. Laser-Fired Contact (LFC) processing has also been recognised recently, winning a Fraunhofer award in May.
Thermal laser separation is a dicing method that reduces electrical losses in the cell caused by resistances. Slicing the cell into various energy-producing units can result in a two to three per cent increase in power output across a solar panel, around five extra watts.
One of the big advantages of dicing the cell using thermal laser separation is the technique’s throughput, according to Frederick Bamberg, product manager at German laser machine manufacturer 3D-Micromac. He said the process can be two to three times faster than the current industrial standard solution of laser ablation and mechanical cleaving.
‘We [3D-Micromac] use a process that was originally developed for chip dicing in the semiconductor industry, but we found that it was beneficial for solar cell manufacturers as well,’ said Bamberg. The technique uses a combination of a laser and deionised water to heat up and then rapidly cool down the substrate. This introduces a field of both compressive and tensile stress into the silicon wafers.
‘By having a starting point defined by laser ablation, you can now guide a crack at high speed through the wafer with the heating laser and the spraying nozzle,’ continued Bamberg.
Standard laser ablation is a comparably slow process and uses a lot of equipment. This is because laser ablation requires overlapping passes and an ablation depth of 90µm on standard 180µm thick solar wafers, as well as requiring an additional cleaving step.
Thermal laser separation speeds up this process. It also improves solar cell efficiencies by reducing the number of defects that occur during production. ‘We now offer a tool that has at least double the throughput of conventional tools,’ said Bamberg. ‘It works at higher speeds and the efficiency of the manufactured solar cells is also better because it produces less heat impact.’
Excessive heat is known to reduce the electrical output of a solar cell during operation; however, it can also affect solar cells during their production. A higher heat impact can lead to melting and recrystallisation effects of silicon in the cell, which causes electrical defects.
Laser micromachining is based on using short, high-energy pulses of light to ablate material without putting too much heat into the surrounding substrate. For its micromachining processes, 3D-Micromac uses laser pulses in the nanosecond range. ‘Nanosecond lasers are better known in the industry,’ commented Bamberg. ‘They are more advanced, more stable and much cheaper.’
In terms of wavelength, 532nm and 1,064nm are commonly used; 3D-Micromac bases its solutions on the latter. ‘Infrared laser sources allow higher pulse repetition rates, which meets our goal to offer tools with higher throughput,’ continued Bamberg. ‘We also saw that the lifetime of these laser sources is higher compared to the green wavelengths and at least equal cell efficiencies are obtained.’
Increasing efficiency from the rear
Minimising the amount of heat put into the substrate is especially important for next-generation solar cells, such as passivated emitter and rear cell (PERC) and hetero junction cells (HJC). PERC technology has been under development for the past 20 years. The technology increases the conversion efficiency of solar cells by adding a dielectric passivation layer on the rear side of the cell. ‘It’s an insulator that deactivates some crystal defects that are on the rear surface of the solar cell,’ explained Bamberg. This insulator removes traps for the electrons at the rear of the cell. According to Bamberg, this is the key to PERC cell efficiency.
‘You want to make use of all the electrons and electron holes on the rear side,’ continued Bamberg. ‘Because it’s an insulator you still need some local contact with the metal layer on the rear side.’ This is achieved through a laser process that has been industrialised over the last few years.
In order for the aluminium on the rear side to form a contact with the silicon, a certain area of approximately three to six per cent of the total area on the rear side of the cell must be opened. Laser micromachining tools can achieve this via a process called laser contact opening (LCO). ‘Our lasers form these openings,’ said Bamberg. During micromachining, lasers can be directed using a range of pulse durations and patterns to remove different amounts of material to suit the process. ‘There are dot patterns, dash patterns, line patterns – it depends on the conductive paste, the firing conditions and the quality of the crystals being used in the cell,’ commented Bamberg.
In total, this process increases the absolute efficiency of a PERC solar cell by at least one per cent compared to previous processes. This can result in a 20W increase in module power delivered by a sum of interconnected PERC cells.
With the improvements in efficiency from micromachining, PERC solar cells are being considered increasingly over their conventional aluminium back surface field (Al-BSF) counterparts. ‘The market share of this [PERC] is about 20-30 per cent right now, and it is expected to be 60 per cent in the next five to seven years,’ remarked Bamberg.
During Fraunhofer’s 2016 awards ceremony held in Essen, Germany in May, one of the three ‘Joseph von Fraunhofer’ awards was presented to Dr Jan Nekarda, head of the Laser Process Technologies group at the Fraunhofer Institute for Solar Energy Systems, and his colleague Dr Ralf Preu for their development of Laser-Fired Contact (LFC) technology. This technique improves the efficiency and reduces the manufacturing costs of PERC solar cells.
The purpose of LFC technology is to connect the rear side electrode of a PERC solar cell to the cell’s semiconductor. ‘By means of single laser pulses the aluminium electrode is locally melted through the passivation layer and alloyed with the underlying silicon,’ explained Nekarda. ‘Such a contact is about 30µm small and up to 150,000 contacts are used per solar cell.’ To achieve this, pulsed lasers – in combination with fast galvo scanning heads – are used to produce a cycle time of less than two seconds, which is required to carry out the process.
Pulsed infrared lasers are used for LFC technology thanks to their reliability, low cost and their ability to provide the several hundred micro-joules of pulse energy, required to form the local contacts used in PERC solar cells. As in all laser micromachining processes, the pulse duration used in LFC technology is a critical parameter. During manufacture, pulse duration needs to be adapted to the layout of the electrode in the solar cell.
‘If the duration is too short, the aluminium does not penetrate through the dielectric layer and no contact is formed,’ Nekarda said. ‘If the pulse duration is too long, either too much aluminium is molten in the region around the contact… or the aluminium penetrates too deep into the silicon.’ Pulse durations in the region of microseconds are therefore used in LFC technology.
LFC reduces the cost of mass-producing PERC solar cells by providing a simple way to create the additional local contacts required in PERC cells. ‘The LFC process – for the first time – enabled economical production of high efficiency PERC solar cells,’ explained Nekarda. Despite the higher production costs required to incorporate the extra process steps of LFC technology, the process is still economically viable thanks to the resulting increase in solar cell efficiency. ‘The cost per watt peak output power is what counts for a cell manufacturer,’ Nekarda continued. ‘Additionally solar cells with higher efficiency could be sold as a premium product.’
Lasers can also be used to aid the doping process to create the junction separating positive and negative charges. According to Bamberg, laser processing is the most efficient way to do this.
During the manufacture of standard solar cells, the front side of a boron-doped silicon wafer undergoes phosphorus doping. This creates a junction that separates the positive and negative charges generated in the semiconductor material on exposure to sunlight. If this area is doped too heavily, the lifetime and mobility of the electrons will drop, reducing the energy conversion efficiency of the cell. However, heavier doping can also provide advantages.
Heavy doping means the metal electrodes create a better ohmic contact to a doped region, noted Bamberg. Therefore, it is beneficial to increase the doping in certain parts of the cell where good ohmic contact is needed. The structures which use this alternate doping are known as ‘selective emitters’ and increase solar cell efficiency by optimising the ohmic contact between the metal electrodes and the silicon wafer of the cell. Selective emitters feature heavier phosphorus doping at the areas of metal electrodes and reduced doping between them.
A laser can be used to heat up the dopant-rich surface of a cell in a process called laser-assisted diffusion, which enables direct formation of strongly phosphorous doped regions. With it the selective emitters can be formed and the efficiency of the solar cell increased.
Selective emitters can increase the efficiency of a standard silicon-based solar cell by up to two per cent, making this process worthwhile if a large number of cells are produced.
Choose a stable laser
In general, because of the delicacy of micromachining processes, choosing a laser with a stable power source is essential. The micrometre and nanometre dimensions used in photovoltaics results in a very low tolerance level of variation in thickness along the substrate. An unstable power source could lead to fluctuations in the amount of material removed during micromachining, which in turn introduces damage in the silicon. Because of this, two elements must be closely monitored throughout: ‘One is the layer thickness stability and the other is the power stability,’ said Bamberg. ‘Whether it’s too low or too high power, you will suffer additional losses to the solar cell efficiency.’
In addition, incorporating a new manufacturing process into a production line can often be quite costly. On a small production scale – producing solar cells for residential housing for example – introducing a new micromachining process would not be profitable. However, it is advantageous for the larger companies to invest in this new machinery.
Looking to the future, Bamberg believes that shortening the pulse length of micromachining processes is a key factor to further improve solar cell efficiencies. ‘Having a shorter pulse length will result in less heat impact, therefore ablation could take place without any melting and this will improve solar cell efficiency.’ Bamberg went on to say that solar cell efficiencies will continue to improve as a result of finer cell structures and higher repetition rates of passes achieved during micromachining.
Similar to Bamberg, Nekarda emphasised that the key to making more efficient solar cells in the future lies in reducing the damage that laser micromachining processes induce in solar cell materials. According to Nekarda, this can be done by tuning laser parameters. ‘The spot size for some applications has to decrease and the positioning accuracy must improve,’ he remarked. Nekarda believes that the relevance of laser processing will likely increase in photovoltaics in the future. ‘The challenge is to build reliable 24/7 working machines that process several thousand solar cells per hour,’ he concluded.