Thin-Film Solar: Interpreting Efficiencies

Reporting the efficiency of a solar cell or module depends on a number of assumptions, and unrealistic assumptions are sometimes made in order to report the highest possible efficiency.

There are some cases where it is difficult to make fair and consistent assumptions. For example, when comparing different technologies (thin film, crystalline silicon, and CPV) which have different values for temperature coefficient, spectral dependence, land coverage, and other properties, it can be difficult to come up with a completely standard, comparable, and realistic method of rating power and efficiency.

However, within a particular type of technology (thin-film solar, for example), there is really no excuse for making non-standard assumptions and then omitting this critical information when you report your results. For this post I’m going to focus on particular misleading reporting practices I have seen used multiple times in the thin-film industry (that is, solar technologies such as CIS, CIGS, CdTe, and thin-film silicon, which includes amorphous silicon, and “micromorph” tandem devices).

I do understand that this poses a challenge to each individual company: If your competitors are reporting their efficiency in a misleading way, it is difficult to report your own results in the “correct” way if it gives the appearance that you have a lower-efficiency product. This is a form of arms race or “Prisoner’s Dilemma”, but this is also not my problem: my point here is to make sure people who read results know the right questions to ask to make sure companies are giving them realistic numbers with full disclosure on the most critical measurement assumptions.

Assumption: Aperture Area

One detail often glossed over is the notion of aperture area compared to full area. An aperture area efficiency for a module divides the output power by only the area covered with the active absorber, not including the border around the module where the edge seal is or the frame. In addition, in some cases (especially during R&D), companies mask or scribe away regions of the absorber where there is significant thickness variation due to edge effects in the deposition chamber, and don't include those edge regions in the measurement. A 25mm border on a Gen 5 glass sheet (1.1 x 1.4m) corresponds to about an 8% relative difference in area, or a roughly 1% difference in reported absolute efficiency for a 12% efficient module.

Note that in some cases aperture area is a reasonable way to report efficiency, especially during R&D when you want to report the overall potential of a technology and ignore some process issues, or if the deposited materials are a large fraction of your cost and your substrate is a low-cost plastic (in which case extra area of this plastic may not add as much cost as active module area). However, it’s important to clearly state that an aperture efficiency is what’s being reported.

Assumption: Light-Induced Degradation

Another reporting issue most relevant to thin-film silicon is the reporting of initial efficiency compared to stabilized efficiency. Stabilized efficiency is the relevant number for real-world applications, and refers to performance after the degradation that amorphous silicon undergoes when exposed to light for the first time (the Staebler-Wronski effect, also referred to as LID for Light Induced Degradation). However, some companies report the higher initial/unstabilized efficiencies, and only mention this is what they’re doing if you ask them directly “is that a stabilized efficiency?” This is important because a typical Staebler-Wronski degradation can be 10% relative, or even more. At the recent EUPVSEC, a number of companies mentioned that their LID at the cell level was 10%, but then were vague about their LID at the module level, or said something along the lines of “there are some challenges with LID on large area [Gen 8.5 glass = 2.2m x 2.6m!] modules,” effectively telling the audience that LID was significantly worse than 10% in their modules.

In other cases, thin-film manufacturers would report the power output and area of modules, but not the efficiency, or even the area, initial power, initial efficiency, and stabilized power, but not the stabilized efficiency. This is a bit silly, as anyone in the room can calculate the stabilized efficiency from these numbers.

So, thin-film manufacturers: If you’re going to show the performance of a new cell or module in a conference presentation, I suggest you include a summary table like the below, rather than making your audience guess or ask you about your assumptions. It only takes one slide, and saves everyone time; and if you don’t do this, savvy people in the audience realize you’re trying to pull the wool over their eyes when you are vague about your measurement conditions.

			Area	Power [W]	Efficiency [%]	LID [% relative]
Cell	Initial	Aperture Area	1 cm2		11.1%
Cell	Stabilized	Aperture Area	1 cm2		10.0%	-10%
Module	Initial	Aperture Area	1.5m2	143	9.3%
Module	Stabilized	Aperture Area	1.5m2	123	8.0%	-14%
Module	Initial	Total Area	1.5m2	136	9.1%
Module	Stabilized	Total Area	1.5m2	116	7.8%	-14%

(the numbers above are arbitrary, just to provide an example)

Or if you really don’t want to use a table, a least attach the most relevant test details to your reported values, for example:

• 10.4% (1cm² Cell, Aperture, Stabilized)
• 9.3% (Gen 5: 1.1x1.4m, Module, Aperture, Initial)
• 8.7% (Gen 8.5: 2.2x2.6m, Module, Aperture, Initial)
• 7.8% (Gen 8.5: 2.2x2.6m, Module, Aperture, Stabilized)

Assumption: Deposition Rate

Another question to ask when new efficiency results are reported is: “What was the deposition rate used to achieve the reported efficiency, and is that a realistic throughput according to your cost models?”

Typically, film quality and electrical efficiency go down as the deposition rate goes up. So a company may be able to get a higher efficiency “champion” module by using a very low deposition rate (one that is too low a throughput for volume manufacturing) when making that particular module. This result does have some value: it shows that a certain module efficiency is technically possible on large areas, and perhaps in the future with improved tools and processes, that efficiency might be achievable in manufacturing.

However, in production, companies can run cost models and pick some deposition rate that is a balance of throughput and module efficiency that leads to the best overall cost per watt and cost per area for the module. It’s not a trivial calculation because as efficiency goes down, module area at a given power goes up, so balance of system costs such as installation and land also go up slightly. But it’s also not economically sensible to use a process with dramatically lower throughput just to get an extra 0.1% of efficiency.

Assumption: Deposition Process

Another issue to consider is whether the fabrication processes used are industry standard ones, or less-used processes. If the latter, is there a path to scale-up of the process? It is certainly acceptable to explore new deposition processes in pursuit of better performance, lower cost, or even proof-of-concept devices. However, if a company is presenting results in an established industry where there is a common deposition tool and method (for example, a-si deposited by 13.56MHz PECVD), the company should identify if their devices were made with a different process (VHF plasma, HWCVD, or so on).

Conclusions

There are a number of common ways in which efficiencies reported by thin-film solar companies can be difficult to interpret and compare, based on variation in how the numbers are calculated. Three of the most critical are whether an aperture-area measurement is made, whether light-induced degradation of thin-film silicon is included, and whether the deposition rate was unrealistically low. It is the responsibility of companies to clearly disclose their measurement assumptions, and also of people receiving results to demand this disclosure. I have included a very simple table for reporting results which would avoid the need for perhaps 40% of the questions I have heard asked of thin-film manufacturers at conference presentations. Now let’s all get back to engineering and solving problems!