Early-Access Review (Pt 2): Epishine Light Energy Harvesting Module Evaluation Kit

RE: Early-Access Review (Pt 2): Epishine Light Energy Harvesting Module Evaluation Kit

Gough Lui — Thu, 06 Aug 2020 09:59:34 GMT

Readers may have noticed that I didn't make a comparison in the above comment - that was actually rather deliberate, as I had to wait for the sun to go down to eliminate its influence on the results. Now evening in Sydney, I can test using the ambient overhead lighting provided by a neutral white LED panel, providing about 200 lux to my workbench surface.

First, we can see the results for the LEH module (apologies - I realised my previous graph had LIH instead, a typo):

[View:/resized-image/__size/620x447/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-22684c77-8b62-471b-9b8c-3fe4a1786992/0363.contentimage_5F00_192046.png:620:447]

This time around, the power delivered is a little less, but that's expected as the light is a little less. The fill factor maintains a very similar value, although it must be remembered that this is from the tortured and partially compromised cell - it cannot be discounted that a fresh cell may indeed perform better.

The big question for me was, how would the ordinary 17-year old reference hobby cell fare under the same conditions. You might think that if something works at high intensities, it would certainly work better at low intensities ...

[View:/resized-image/__size/620x448/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-22684c77-8b62-471b-9b8c-3fe4a1786992/8867.contentimage_5F00_192047.png:620:448]

... but that is not the case in this instance. While the cells are a very old product and perhaps newer monocrystalline cells may not exhibit such poor low light behaviour, this reversal of fortunes is pretty similar in appearance to what the LEH was showing at high intensities. The reason is very likely due to the cell's internal shunt resistance - while easily overcome with plenty of photocurrent, at low photocurrents, these parasitic losses result in a severe loss of Voc and the fill factor drops massively as a result. The delivered power is about 8% of that from the LEH OPV cell - and that's even before I've compensated for the fact that the silicon monocrystalline module has about three times the area.

As a result, it's a clear case of "horses for courses".

- Gough

RE: Early-Access Review (Pt 2): Epishine Light Energy Harvesting Module Evaluation Kit

Gough Lui — Wed, 05 Aug 2020 23:45:45 GMT

Thanks for all your inputs –

With hindsight, perhaps it’s easy to say that I’ve put too much light into the cell @DAB, but this was the product of a number of considerations, many to do with testing technicalities and conventions.

For example, standard photovoltaic cells are usually characterised at Standard Test Conditions which involves 1000W/m² AM1.5 spectra. While I know the LEH is targeting indoor usage, the reference cell I am comparing to would almost certainly have been tested at STC, thus a “halfway” point is necessary to try to get a comparable output from both that could be related to cell efficiency in some way. Another consideration is the precision of current measurement is a problem, especially as currents get smaller and smaller. The maximum current was ~12mA in the OPV case which is very small – thankfully I am using an SMU, but even then, I did have concerns as my setup does not use any shielded/guarded cables, thus the resulting current values are likely to be even noisier as the light intensity is reduced. Add to this that my bench does receive stray light – be it natural light from the windows that face north, or the ambient room lights. To reduce error, I determined that the test light source should (as much as possible) shadow the other sources of light and provide a signal that “overwhelms” the background – think of this like maintaining a signal-to-noise ratio. Additionally, in the previous tests, using an LED lamp in that situation did produce the maximum harvested power, which didn’t suggest to me that it was a major issue for the cell.

As a result, that’s how I ended up with the testing methodology shown in this post. Perhaps I should have documented my thought processes more.

As a result, the above tests while perhaps being a little unfair to the LEH’s intended use more closely mirror what might be expected from conventional solar panel tests. It’s now clear from the test that the LEH is indeed engineered with thinner organic electrodes which results in series resistances when too much light impinges on the cell – that’s definitely what I’d call useful information to know given we have no official documentation on the cell – as alluded to by [mention:d214a0a0f5594ee19515b2a3782e7070:e9ed411860ed4f2ba0265705b8793d05]. One thing I did not mention was the open circuit voltage, which alludes to the sort of organic PV compounds that might be in use.

With the input from [mention:b7e9caa5f1b648b3a0700b362e380293:e9ed411860ed4f2ba0265705b8793d05] , I have decided to be ambitious and give it another test – this time under 220 lux of neutral white LED light. While my cell does have partial delamination which would affect its output somewhat, at least we should be able to demonstrate the fill factor improvement from a reduced photocurrent –

[View:/resized-image/__size/620x403/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-22684c77-8b62-471b-9b8c-3fe4a1786992/0363.contentimage_5F00_192045.png:620:403]

This was a challenging measurement to make given the small quantities at play, but I hope this adds some balance to the results above. My first attempt looked absolutely nothing like that – it looked more like my local mountain range as stray light from the rising sun outside competed with my indoor lighting through the cracks in my curtains. However, the second time (increasing the scan speed) allowed me to capture a decent looking curve with a fill factor of 0.736, right in the range as noted by Jonas.

Unfortunately, as I stated in the beginning, I don’t have access to a proper solar PV characterisation setup, I would say sending me another LEH module is perhaps unnecessary (as it would probably cost a lot more in postage, and I wouldn’t know what to do with it next). However, it also shows that these OPV cells are perhaps not suited to the standard characterisation procedures – which is why I’ve seen some characterised under 220 lux of neon lamps etc. Indeed, the intensity measured by my smartphone's onboard sensors exceeded its maximum 32,768 lux value, agreeing with your estimates.

After all, learning is why I do what I do – your valuable inputs are much appreciated!

- Gough

RE: Early-Access Review (Pt 2): Epishine Light Energy Harvesting Module Evaluation Kit

DAB — Wed, 05 Aug 2020 18:37:25 GMT

Very good addition to your initial testing.

Given the manufacturers intended use, I think you put way too much energy on the organic cell.

Putting the light about a meter away would have probably been a more realistic test of the cell and its intended use.

DAB

RE: Early-Access Review (Pt 2): Epishine Light Energy Harvesting Module Evaluation Kit

genebren — Wed, 05 Aug 2020 14:53:04 GMT

Interesting data. The I-V and power curves on the Epishine solar cell are quite unusual (in my limited experience). It is very difficult to test modules like this, without complete documentation. You end up doing a lot of guessing and trial and error testing, which leaves any data as suspect or at best incomplete.

Nice job!

RE: Early-Access Review (Pt 2): Epishine Light Energy Harvesting Module Evaluation Kit

jonas_epishine — Wed, 05 Aug 2020 14:45:07 GMT

Great work to desolder the module! And great to see your extensive tests!

A comment on the performance and fill factor. From your short circuit current densities I would estimate the light intensity for both LED and incandescent illumination tests to be over 50 000 lux (one sun is equivalent to 109 000 lux). Our energy harvesting modules are optimized for indoor illumination in the range from zero up to a few thousand lux. Typical usage are at 50-1000 lux where our fill factor is 0.7-0.75.

The low fill factor in your test is due to a huge voltage drop over the electrodes. To reduce the environmental footprint and cost we use organic electrodes instead of typical transparent conductors such as indium tin oxide (ITO). As our modules are designed for low light applications the thickness of the electrodes is low and thereby the sheet resistance is high. This way we do minimize the absorption losses in the electrodes. At low light intensities the photo current is significantly lower and thereby the voltage drop is negligible, but when light intensity increases above 1000 lux the fill factor starts to drop and at 50 000 lux the voltage drop significantly impacts the fill factor as you have seen in your tests.

We would be happy to provide a new free standing module as I see that your module was slightly damaged after the desoldering.

Best,

Jonas CTO at Epishine