Silevo VP of Business Development and Marketing Chris Beitel on n-type substrates, tunneling junction cells, efficiency, and production costs

Christopher Beitel
Christopher Beitel

Christopher Beitel is vice president of business development and marketing at Silevo, and is responsible for sales and marketing, market strategy, and a key partner network to enable industry leading solar system installation cost and performance with the company. Prior to Silevo, Beitel brings his extensive experience in business commercialization to Silevo, having grown multiple products at Applied Materials, Inc. into billion dollar product lines.

Solar Server: Can you give our readers an overview of how Triex cells achieve higher efficiencies and lower temperature co-efficients, including how the c-Si n-type substrates, thin-film passivation layer and oxide layers work together?

Chris Beitel: Triex is the brand name for the product that we are commercializing now. We have developed a new cell device concept that leverages some learnings from various devices that have been studied in the past, as well as leverages the benefits of certain materials that can be incorporated into one single device concept.

The Triex cell leverages the n-type mono-silicon substrate as its first lever. N-type substrates have inherent advantages versus the traditional crystalline technologies, which are based on p-type substrates. N-type allows for very high carrier lifetimes, so you can drive better efficiency with an n-type substrate.

The other improvement with n-type, is that there's a concept called light-induced degradation (LID). Typically panels will decay in the first hours of exposure in the range of 1% to 5% from their initial performance levels. N-type substrates do not have this degradation, this LID effect, and so you will get more kWh per kW peak with a concept that employs that kind of material.

Then after we utilize an n-type wafer, we then deposit an emitter on top of that, and that's done through an oxide layer and a thin amorphous silicon layer. The oxide step is a proprietary step that Silevo has developed. Oxide layers have been used in the semiconductor microelectronics industry for decades. They basically drive excellent passivation characteristics, so that you can harvest the electricity better. So when we couple the oxide passivation layer with a doped amorphous silicon thin-film layer, we can drive an excellent junction, and we can drive a device that has high voltage.

High voltage is one of the three aspects of creating efficiency: You have voltage, you have current, and then you have fill factor, which is the ratio of the two together. And with our types of materials, we drive excellent high voltage properties, in the range of 725 mV, which is probably 60-70 mV better than any other traditional-type crystalline silicon substrates that are in production today.

The amorphous silicon layer is directly ported over from thin-film silicon, which has been studied by thin-film players. That's a material that we understand how to deposit, there is a very robust supply chain in terms of the equipment that is available. And so we are able to scale that in a commercial environment in a straightforward manner.

Then the last advantageous aspect of our Triex cell is that we've incorporated a copper-based metallization scheme. Traditional cells use silver nano-paste, and silver is a precious metal that in the last few years has seen a significant increase in terms of spot price.

So what we do is we eliminate the use of silver completely and replace it with copper-based metals that have been used in other industries. In this case, myself as well as the other founders of the company helped the semiconductor industry transition to copper-based interconnects in the late 90's.

We can leverage those techniques to bring a new material into the solar space, to drive down costs, but also help improve performance. Copper is something that as you scale the size of the substrate, you can maintain a low resistivity, and low stresses, so that you can continue to garner more power out per given device.

Those are the three key things. The n-type substrates, the passivation layer that we create through an oxide as well as the amorphous silicon thin-film deposition, and then the copper-based metalization scheme for the electrodes.

 

Solar Server: Can you tell us more about tunneling junction cells, how they work, and why Silevo chose this approach?

Chris Beitel: Tunneling junction is a concept that has been studied for many decades. The first work on tunnel oxide films was done in the late 70's on a solar cell device called an MIS - metal insulated semiconductor solar cell. And one of the most prestigious professors that did characterization was Dr. Martin Green of the University of New South Wales. He published several papers and did a lot of characterization on MIS cells in the late 70's.

And the advantages of that is that the oxide gave great passivation properties. And so they were able to demonstrate a cell that could get 17.6% efficiency. And the voltage in those days was around 655 mV. 655 mV was probably about 40-50 mV better than anything that had been demonstrated in that time period.

MIS cells had a lot of promise, in that they could show high efficiency, but they also were done in a low-temperature manufacturing environment, or at least research environment, where potentially you could preserve the carrier life of the silicon substrates by not overheating them. Modeling showed that you could get as high as 23% efficiency with a cell that employed this tunneling oxide.

The problem that they had was scaling from a research environment to a production environment. So the MIS cells used some other materials to basically induce a P-N junction. MIS cells typically used an inversion layer that was cesium doped. Cesium is an element that is very similar to sodium or salt, so it is very unstable and can be corrosive.

And the challenge that they had was they were never able to demonstrate that they could scale from, let's say, a 2 x 2 cm cell to a substrate that could be viable from a production-worthiness point of view. Typically you want to have a greater than 140 cm-squared cell size to be able to get enough power to be able to package a cost-effective module. So that was their primary challenge, was figuring out a way to get away from cesium, but still leverage the benefits of this passivation from the oxide layer.

So what we did was we replaced cesium with an amorphous layer, but we retained the properties of that oxide layer. And really, what the benefit is, that there is something called an interface trap density (DiT), and it’s basically, in layman's terms, it's the purity or quality of the film that you deposit.

And so you can drive lower and lower interface trap densities, by leveraging different passivation techniques, and especially improving the oxide formation that it can create, along with the amorphous and the emitter. What our scientists have been able to do is to continue to package these two materials together, oxide and amorphous, to drive down the DiT level, the interface trap density, and raise that voltage of that device. As well as give excellent junction properties to perform very low temperature co-efficients.

So when you look at the research industry, people have shown that it is possible to get down to a 5e7 DiT, and what we believe, those are done with very elaborate, expensive passivation steps, usually dry plasma steps. We believe that we can use wet-process steps to get about halfway down from where we are at today to where they are at, in a research environment, and in that case we would be at a 24% production efficiency based on those metrics.

 

Solar Server: I understand that Silevo has a roadmap to 24% efficiency, and USD 0.65 production costs. How is work progressing on that?

Chris Beitel: We are in the process right now of scaling our first high-volume manufacturing facility, this will be coming on line here in the next few months, and we will be shipping product to customers that have already qualified our product through the pilot line that we have, in California.

Basically at a single line-scale, meaning a 32 MW line, producing cells between 20% and 21% conversion efficiency, we will be at around USD 0.98 per watt in terms of your all-in production costs. So that would be the wafers, the cell conversion as well as the module packaging.

Then, especially in the place of China, entities have been able to leverage economies of scale to significantly drive down production costs. And so, as we move to a 200 MW scale, which is our plan in 2013, we will be able to reduce that cost by 15%-20%.

And then as we continue to scale, as well to improve the efficiency performance, from 21% that we have demonstrated today to the 24% that we believe that we can get over the next few years, then the cost model shows that we can achieve as low as USD 0.61 per watt in a production environment.

So it's really two things, it is take a process that we have developed that is fairly straightforward, with about nine process steps. You look at some of the other high-efficiency players, the one that everyone knows is SunPower. The belief is that they have a very complex device, with their integrated back-contact approach, and they have an elaborate set of tools and process steps probably in the range of 20-23 steps to build their device.

So the simplicity of our approach, with only nine steps, helps us on capital costs, and therefore the cell conversion costs. The baseline process is cheaper, but then as we build bigger factories, as we can leverage larger purchasing power, because we are consuming more material, we can cluster the equipment into having potentially three tools that serve the right throughput that typically it would take five to do so if it were in separate lines, then we can really drive the capital costs down, as well as the labor and factory costs.

It's a story of low costs to start with, based on our fundamental design, and then leverage the traditional economies of scale benefit as we mature.

 

Solar Server: Great. So when does Silevo plan to open its 30 MW manufacturing facility in Hangzhou?

Chris Beitel: The Hangzhou facility is completed in terms of the building, the equipment is all moved in. We are in the process right now of hooking all of equipment together with the automation, starting up all of the tools, and our hope is that in the next 1-2 months we will be in commercial production.

 

Solar Server: Is there anything that we haven't talked about regarding Silevo and your process that you feel our readers should know?

Chris Beitel:
I think one point that I talked briefly about, that I think it is important to comment on more is energy harvest: Triex is about excellence in three areas: Efficiency, cost and harvest. So the harvest is basically the energy yield that our installation will get in real-world environments. And our cell technology, the tunneling-junction cell, has what we believe is the industry's best-based crystalline-based temperature coefficient.

So typically crystalline panels that are produced with your standard diffusion junction technology have a temperature coefficient of -.45% to -.5% per degree Celsius. And so what that means is that if you had a 200 watt panel, as it heats up in a typical installation - and there is a lot of data that supports that panel temperatures can get as high as 70 degrees C - if you have a 200 watt panel measured at what they call standard test conditions, when it heats up in a real-world environment, it actually will only output around 160 watts. So it is a significant loss, that happens in the installation, and ultimately effects the performance ratio and the kWh per kW peak that an installation will generate.

In the case of Silevo's cells, the cell temperature coefficient is -.22% per degree Celsius, so less than half, so that means that we have a 180 watt panel, when others have a 160, assuming that they were both 200 watts to start.

So it’s the third aspect of the Triex in terms of efficiency at the right cost, but then with the right energy harvest, to really provide the best value and best performance to cost ratio modules that are on the market today.

Interview conducted by Solar Server International Correspondent Christian Roselund on April 18th, 2012