Outdoor performance of organic photovoltaics: Diurnal analysis, dependence on temperature, irradiance, and degradation Cite as: J. Renewable Sustainable Energy 7, 013111 (2015); https://doi.org/10.1063/1.4906915 Submitted: 06 October 2014 . Accepted: 17 January 2015 . Published Online: 29 January 2015 N. Bristow, and J. Kettle ARTICLES YOU MAY BE INTERESTED IN Effects of concentrated sunlight on organic photovoltaics Applied Physics Letters 96, 073501 (2010); https://doi.org/10.1063/1.3298742 Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions Journal of Applied Physics 90, 5343 (2001); https://doi.org/10.1063/1.1412270 Two-layer organic photovoltaic cell Applied Physics Letters 48, 183 (1986); https://doi.org/10.1063/1.96937 J. Renewable Sustainable Energy 7, 013111 (2015); https://doi.org/10.1063/1.4906915 7, 013111 © 2015 AIP Publishing LLC. JOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 7, 013111 (2015) Outdoor performance of organic photovoltaics: Diurnal analysis, dependence on temperature, irradiance, and degradation N. Bristow and J. Kettlea) School of Electronic Engineering, Bangor University, Dean St., Bangor, Gwynedd, Wales, United Kingdom (Received 6 October 2014; accepted 17 January 2015; published online 29 January 2015) The outdoor dependence of temperature and diurnal irradiance on inverted organic photovoltaic (OPV) module performance has been analysed and benchmarked against monocrystalline-silicon (c-Si) photovoltaic technology. This is first such report and it is observed that OPVs exhibit poorer performance under low light conditions, such as overcast days, as a result of inflexion behaviour in the current- voltage curves, which limits the open-circuit voltage (VOC) and fill factor. These characteristics can be removed by photo-annealing at higher irradiance levels, which occur diurnally as irradiance increases after sunrise. We also report the first temperature coefficients for OPVs from outdoor data; the OPV modules showed a positive temperature coefficient, which compared to a negative coefficient from the c-Si modules. Overall, the cell degradation outdoors appears very severe for these modules and highlights the need for improved barrier. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4906915] I. INTRODUCTION Remarkable improvements in organic photovoltaic (OPV) performance have been achieved by the use of bulk heterojunction OPV devices, in which conjugated polymer donors are blended with electron acceptors such as fullerene derivatives in solution. Currently, record power conver- sion efficiencies (PCEs) of over 10% has been demonstrated in laboratory conditions by a number of companies1 and 11% has been reported in the literature.2 Whilst such efficiencies are impres- sive, understanding how such devices operate in the real world under a variety of atmospheric conditions is imperative in order to fully commercialise the technology. To support the development of the technology, ISOS consensus standards have been devel- oped to standardise testing protocols between international laboratories.3 Included in these ISOS standards are three levels of outdoor performance measuring protocols. These tests require addi- tional test facilities not commonly available in laboratories, including suitable measurement equip- ment and complex data analysis software packages. Due to the complexity of this, few groups have developed test facilities for conducting outdoor tests and therefore limited data exists on the outdoor performance of OPVs. However, these are some of the most useful tests as they allow for performance of materials and systems to be understood in an environment where the modules are likely be deployed and to understand how the behaviour of OPVs during outdoor testing is affected by the climatic conditions of the dynamic testing environment.4,5 Despite these challenges, there have been some comprehensive studies. Katz et al. published a study of outdoor degradation based on OPVs, encapsulated with glass and aluminium, with different semiconductors in the active layer6 and showed that the main losses occur due to falls in short circuit current density (JSC) and fill factor (FF), which were equated to degradation of the back electrode. Overall, the cells were relatively unstable and showed after 90 h, the best cell dropped to 50% of the original a)Email: [email protected]. Tel.: 01248 382471. 1941-7012/2015/7(1)/013111/12/$30.00 7, 013111-1 VC 2015 AIP Publishing LLC 013111-2 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015) efficiency, despite the glass encapsulation. Another approach was adopted by Hauch et al., who used an additional barrier film to encapsulate flexible Poly(3-hexylthiophene-2,5-diyl):Phenyl-C61- butyric acid methyl ester (P3HT:PCBM) modules on Polyethylene terephthalate (PET)7 and showed over 1 year, the performance dropped by only 20% of the original efficiency. Other work includes that of Krebs who showed how edge sealing plays a significant role in preserving the OPV from environmental degradation by using glass-fibre reinforced thermosetting epoxy (in this case, prepreg8) as an edge sealant, which demonstrated a reduction in efficiency of only 65% over ayear.9 In addition there have been a number of inter-laboratory stability tests; Gevorgyan et al. conducted an inter-laboratory monitoring programme of flexible modules of P3HT:PCBM on PET, which were encapsulated by a barrier film from Amcor Flexibles and studied at different outdoor locations.10 On average, the performance dropped by 40% of the original efficiency after approxi- mately 1000 h (42 day) of outdoor exposure. Few of these reports present data showing how an OPV performs relative to other modules positioned at the same site or have presented measure- ments to explain how climatic conditions affect the OPVs performance. In this work, data are presented for two separate outdoor monitoring campaigns performed on OPV modules made by the Technical University of Denmark (DTU), supplied as part of their freeOPV programme.11–13 The focus of this work is benchmarking the performance of OPV modules relative to c-Si modules and identifying how climatic conditions affect solar cell performance. For the first time in literature, the temperature coefficient of OPV modules is reported based on outdoor performance data. Results are also shown for how module degrada- tion is affected by seasonal variation during the summer and winter. II. EXPERIMENTAL A. Organic photovoltaic modules Roll-to-roll coated OPV modules produced at the DTU were used for the tests. The fabrication of these modules is reported by Krebs et al.12 and were encapsulated using flexible Amcor packag- ing barrier foil and epoxy adhesive. The devices had an ITO free structure of Ag-grid/Poly(3,4- ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)/ZnOX/P3HT:PCBM/PEDOT:PSS/ Ag-grid/PET-substrate. The device terminals were connected to easily accessible electric plugs, which were connected to a switch matrix and Source Measurement Unit (SMU). The modules have a PET/SiOX barrier layer with a refractive indexwhichissimilartothatofglass(1.57). Prior to monitoring, the modules were fixed on a platform so that air could circulate behind the panels. B. Outdoor monitoring setup The OPV modules were measured in at the School of Electronics, Bangor, Gwynedd, North Wales, which has latitude and longitude of 53.2280 N, 4.1280 W and is located at low altitude (20 m above sea level) and 250 m from the Menai Straits (Irish Sea). Long term cli- matic average temperatures for the winter are 4.7 C and for the summer are 14 C. However, the first of these tests was conducted during the summer of 2013, which corresponded to a pe- riod of abnormally hot and humid conditions, with an average mean temperature of 16.5 C and an average maximum temperature of 20.2 C. Data are also supplied for a winter measurement campaign during which climatic conditions were milder than usual with an average mean tem- perature of 8.8 C, an average minimum of 3.1 C, and an average maximum temperature of 11.7 C. The humidity levels for both summer and winter were very similar: with an average mean of 79%, an average maximum of 90%, and the average minimum being 61% in summer and 65% in winter. UV indices were very different with an average of 1.07 in the summer (av- erage maximum 5.45) and 0.43 in the winter (average maximum 2.38). The outdoor monitoring system at Bangor University is on the roof of the School of Electronic Engineering. Two 185 Wp silicon modules (manufactured by Pure Wafer Solar Ltd., Swansea) which are monitored using an Egnitec PVMS250 PV measurement system (manufac- turer Egnitec Ltd., Caernarfon, Gwynedd, UK). The modules are kept at maximum power point in between periodic current-voltage (IV) sweeps (once every minute) and each has a PT100 013111-3 N. Bristow and J. Kettle J. Renewable Sustainable Energy 7, 013111 (2015) temperature sensor fixed to its backplane. Current and voltage at the maximum power point (IMPP,VMPP) and PT100 measurements are taken every 15 s. The OPV measurement system comprises an 8-channel measurement unit with switch matrix and two separately adjustable mounting boards on a rack (Figure 1). Each board can have up to nine OPV modules mounted on it and an IMT GmbH solar silicon reference cell for irradiance measurements. There are two PT100 temperature measurement channels: one was mounted on a horizontal module and the other on an inclined module. For the purpose of this experiment, one silicon module and one OPV mounting board were mounted horizontally and the other silicon module and OPV mounting board were inclined at 35, which is the optimum inclination for maximising solar power over a year in Bangor, Gwynedd. The outdoor measurement setup conforms to the ISOS-O-1 outdoor measuring protocol.3 The data were analysed using a combination of MySQL, MS Access, and MS Excel.