The second International Summit on OPV Stability on April 21-22, 2009 in Amsterdam, The Netherlands was organized in a collaborative effort between OrgaPVnet and OPERA, (European coordinators) together with Konarka, NREL and Plextronics.
OPV stability and lifetime measurement standardization was the main focus of this summit. Talks were contributed from some companies that worked together in the breakout sessions in order to put together proposals for measurement standardization and the performance of round robin testing.
Highlights are presented below:
Andreas Riedl spoke about the correlation between accelerated tests and real life ageing. Weathering instruments and systems are Atlas' specialty along with testing services and consultancy. Primary weather factors -such as heat, solar radiation, cold, snow, rain, oxygen, ozone- together with secondary ones - sand/dust/ hail abrasion, acid rain, salt deposits, even bird droppings - are the kind of parameters that influence the overall acceleration factor for tests.
An important issue that was raised was that some samples might pass, for example, separate testing of UV radiation and then wet cycles but in real life, environmental effects might be happening simultaneously and synergistic effects might lead to failure.
Andreas also pointed out the following three observations:
- There is NO universal correlation factor. Acceleration factor can be estimated using an Arrhenius like equation and known activation energies but it's just an estimate. But it's always dependent on many different parameters, not least of all, the geographic location in which an outdoor test is taking place and the weather effects there.
- Trueness vs. Precision: High trueness, low precision, and vice versa.
- Correlation doesn't necessarily mean causality.
Frederik Krebs described how at the Riso National Research Lab in Denmark, roll 2 roll processing of organic solar cells at 2m2/hr was achieved in 2008, while this year they can reach up to 6m2/hr. Out of their costs, ~25% is the cost of ITO. The encapsulation used is the one that's commercially available such as the one from Alcan Packaging. Much longer life achieved using the lower cost barrier material from Alcan than with just PET.
Best performing cells were described at an efficiency of ~2.3%.
David Ginely made a very important comment on the consideration of both molecular reasons for lifetime limitations on OPV as well as macro-issues such as a sudden hail storm that ruins a solar cell array.
In terms of standards that should be followed by OPV cells IEC 61215 and 61646 (same as other thin film PV technologies) would be the ones that will most likely have to be followed. It will be hard for OPVs to look into utilizing a different one to other TFPVs.
David also suggested the set up of a round robin test:
Make a calibrating Si-based device (this first step to be started by the end of this year), properly filtered to act as close to an OPV one as possible and then make standard OPV devices that will be distributed and sent to different testing labs around the globe. The Si-KG filtered solar panel will be made by NREL, Konarka or Plextronics will be providing the OPVs. The testing would be made with standardized packaging, spectra, temperature ranges etc.
In terms of a roadmap for efficiencies and lifetimes, these are the targets set by the previous ISOS conference.
David also commented that perceptions seem to have changed: At last year's ISOS, people involved in OPV research thought that OPV lifetimes can reach up to 1000 hrs or less; this year though, the survey returned opinions for lifetimes of up to 5000 hrs.
David Laird gave a long list of important factors contributing to OPV stability:
Charge carrier build up, pretreatment, relative humidity, cell/module packaging, initial efficiency, cycling of temp, light %RH, electrodes, light source & spectrum, load conditions, hole/electron transport layer, light intensity and variation, test method and parameters, photoactive layer...
Plextronics has so far had solar cells certified by the NREL at 5.9% efficiency.
Limiters: Light, water, oxygen, voltage, temperature, time - intrinsic factors such as surface chemical reactions and material instability.
Darrin also pointed out that the definition for what is 1 sun for indoor testing depends on what lamp is used, xenon and sulfur lamps come quite close on actual outdoor irradiation. Behaviour under these two lamps is different though, maybe sensitivity to specific wavelengths is important, and should be investigated.
Jens Hauch of Konarka stated that the theoretical capacity of 1GW in the New Bedford manufacturing facility will take quite a while until it is reached.
The certified maximum efficiency of Konarka's best performing cells is 6.4% on a 0.8cm2 area.
Jens pointed out how it is important not to just rely on packaging alone for stability but it is equally important to make the device intrinsically more stable. Showing some lifetime test results from a konarka sample with a barrier layer with a WVTR of 0.05g/m2/day which has an estimated lifetime of ~3 years, brought up the hypothesis that maybe 10-6 g/m2/day is not necessary after all.