OPV technology companies need to provide reliable data to demonstrate reliability of their offering
By Charu Malik
Pittsburgh, Pennsylvania-based Plextronics has deployed a set of its test solar modules at the National Renewable Energy Laboratory (NREL) for outdoor monitoring and performance.
The data over the next few months will be used to improve outdoor stability of the company’s OPV technology and pave the way for testing more modules and refinement of the method, according to the company.
NREL is equipped with field-testing capabilities that allow the lab to evaluate, analyse, and test PV systems and modules under both normal and accelerated field conditions. The Measurements and Characterisation (M&C) division at the NREL and the National Center for Photovoltaics provides characterisation support, collaborative research, and the development of new measurement techniques for the advancement of the photovoltaic generation of energy. The focus is on solving problems in all phases of material and device development.
In order to know more about the measurement procedure, standard reporting and other relevant issues, OPV Today spoke to Keith Emery, device performance, research staff, NREL. Excerpts:
Although the efficiencies of organic solar cells are still low at 5-7%, they have improved rapidly during this decade and efficiencies over 10% are expected. Recently, Plextronics says it has become the first company to deploy organic solar modules at NREL’s Outdoor Test Facility. How do you assess this development from NREL’s perspective?
Keith Emery: It is a testament to the rapid progress in organic PV that a manufacturer is even considering deploying a prototype. To be a viable competitor to other PV technologies for kW and above type applications, the reliability must be improved to the point where organic PV modules can pass the IEC 61646 thin-film module qualification sequence.
Passing the 85 C, 85% humidity for 1000 hours and the 10 cycles from 85 C 85% humidity to -40 C with the power after stress testing greater than 90% of the manufactures assigned nameplate rating is major challenge.
A set of 10 Plextronics test solar modules were installed on the roof of the facility. Can you elaborate on measurement procedure specifically for OPV?
Keith Emery: The modules were mounted on the Photovoltaic Energy Ratings Test bed (PERT). The modules are mounted on a latitude tilted test bed with open frame racks and measured with a Science wares custom MOSFET load that sweeps in about 10 to 30 seconds to minimise bias rate artifacts.
The samples are actively maintained at their maximum power point when IV curves are not being taken. The PERT monitors the back of module temperature and the spectra is also measured along with the broadband plane of array irradiance along with the IV data. All relevant resource data is monitored at the test bed and at a central site. The modules are characterised indoors under standard conditions prior to deployment to assist in interpreting the data and monitoring long-term performance.
What are the major considerations for measurement related to OPV cells? Is there any major difference in its measurement in comparison with others from the PV community?
Keith Emery: The major consideration for testing OPV cells is to follow procedures that have been established for testing PV cells over the last 30 years. This involves using reference detectors that are closely matched in relative spectral responsivity to cells being tested (typically a filtered Si reference cell), and being aware that current collection from outside the cell area is possible and to take measures such as using an aperture to prove that this artifact is not present. From a calibration lab’s perspective, the major difference is that the stability of current generation organic cells prevents the lab’s normal procedure of measuring the QE first from being followed. Another issue is that some organic devices have slow response times which must be considered or shown not to be relevant. For our lab, we had worked out all these issues with testing amorphous Si cells in the 1980’s and dye sensitised cells in the 1990’s.
NREL measures the performance of PV cells and modules with respect to standard reporting conditions-defined as a reference temperature (25°C), total irradiance (1000 W/m2), and spectral irradiance distribution (IEC standard 60904-3). How do you assess the procedure for the OPV sector at this stage and which are the areas which this sector needs to focus on?
Keith Emery: The procedure we follow conforms to international standards.
The reference spectrum IEC standard 60904-3 was recently changed from edition 1 to edition 2. The new spectra is the same as ASTM G173. NREL made this switch in September 2008. Fraunhofer ISE also switched around the same time. The effect for Si technologies is an approximate 0.8% increase in the power rating. For organic PV, the increase is 0.5 to 1.5% depends on the responsivity. The main issue for procedures will be for devices that have slow response times meaning that the time it takes Isc to respond to light is longer than 0.1 seconds. This will prevent the modules from being tested on pulsed solar simulators used by nearly all PV module qualification group in the world. The guidelines, established in August last year, will greatly improve the credibility in reported efficiencies that were not independently confirmed.
Can you provide an overview of key findings across testing operations at CENER/NREL and provide an assessment of where OPV operators are most likely to find big technological gains?
Keith Emery: Not really. It is not clear to me where OPV operators are most likely to find big gains. The stability against moisture must be significantly improved. Understanding why OPV degrades is critical to addressing the problem. Reliance on packaging to solve stability issues related to moisture sensitivity has historically not worked very well for thin-film technologies because of trade-off in increased cost for a package with lower moisture ingress offsets the benefits of a low-cost deposition process. Because of increasing balance of system costs with decreasing efficiencies, it is critical to improve module efficiencies to 10% in the lab and above around 8% in the field. Sheet resistance seems to be an issue with modules that is minimised in small area cells. Increasing the conductivity of the transparent conductors is another area for significant improvement.
What do you recommend when it comes to preparing for durability assessment?
Keith Emery: Demonstrate that the sample can service with less than 10% degradation in power after 85 C, 85% RH for 1000 hours. After accomplishing this, it may be worth the time and money to proceed with more comprehensive stress testing. Also, one can learn from thin film manufactures who have historically faced difficulties passing the damp heat and wet insulation resistance tests.
How do you think OPV sector has progressed this year and where do you foresee it in the next year or so?
Keith Emery: The field is rapidly advancing in terms of increasing cell and module efficiencies and in terms of stability. This is seen from multiple groups including Plextronics, Konarka, and Solarmer. I see this trend continuing. At some point without a major technological breakthrough it is not clear that OPV will reach efficiency and stability levels comparable to other commercial thin-film technologies. It should be remembered that the commercial thin-film technologies have been around for a lot longer.
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