REAL-WORLD DEGRADATION OF ORGANIC PHOTOVOLTAIC DEVICES IN THE SHEFFIELD SOLAR FARM

Transcription

1 REAL-WORLD DEGRADATION OF ORGANIC PHOTOVOLTAIC DEVICES IN THE SHEFFIELD SOLAR FARM Dawn Scott E-Futures DTC miniproject Supervisor Dr Alastair Buckley 19 th May 2011

2 OUTLINE What are organic photovoltaic devices? How do they work? Current challenges Current research Solar Farm research Further work

3 WHAT ARE ORGANIC PHOTOVOLTAIC DEVICES? Third generation photovoltaics Organic ~7% Dye-sensitized ~11% Hybrid ~10% Multijunction

4 HOW DO THEY WORK? p-type ev oc n-type Inorganic device ε Fe ε Fh 1. Creation of electronhole pair by photon absorption 2. Thermalisation of the electron in the conduction band 3. Minority carriers diffuse to the junction, are swept away and accumulate on the other side

5 HOW DO THEY WORK? External current flow Organic device 1. An exciton is generated by photon absorption 2. The exciton diffuses to the polymerfullerene interface where it is quenched by electron transfer from the polymer to the fullerene 3. If the exciton is fully dissociated, the free charge carriers are transported to the device electrodes

6 CURRENT CHALLENGES Efficiencies are around 7% Commercial silicon technologies have efficiencies of 15-20% Materials are relatively unstable While silicon devices can last for over 25 years, the best reported lifetimes for organic devices is slightly over 5 years Degradation mechanisms are complex and still not well understood Reaction of the active components with oxygen and water Direct photodegradation of the active components Thermal degradation of active components Corrosion at electrode surfaces Interlayer diffusion leading to active component reaction Particle formation There has only been a handful of outdoor studies performed The range of operating conditions experienced by a module over its lifetime cannot be fully simulated in the lab

7 CURRENT RESEARCH In 2001, Katz et al. investigated the temperature and irradiance dependence of polymer-fullerene solar cell parameters Measurements were performed under a solar simulator and under the sun in Sede Boqer in Israel on cloudless days around midday The cell temperature was controlled by a thermoelectric cooling plate during measurements Cells were kept refrigerated in the dark between measurements In 2006 Katz et al. performed a long term study of device performance under outdoor conditions Measurements were performed on cloudless days in Sede Boqer, Israel over a period of 32 days The devices were mounted on a solar tracker At night the cells were kept in a dark glove box with a nitrogen atmosphere

8 CURRENT RESEARCH In 2007 Krebs et al. reported the results of a year-long outdoor study of large area polymer solar cells A module 40 cm 25 cm was made by connecting together 91 cells The module was tested outdoors in Roskilde, Denmark between Jan 2004 and Feb 2005 In 2008 Hauch et al. reported testing polymer-fullerene devices for over a year outdoors Devices were tested under load condition over a period of 14 months at the Konarka rooftop testing setup in Lowell, MA (USA)

9 SOLAR FARM RESEARCH Aim to investigate novel devices fabricated in the University outdoors in the Sheffield Solar Farm Four devices with a photoactive layer of PCDTBT:PC 70 BM(1:4) or P3HT:PCBM(1:0.6), 4.5 mm 2, were investigated Each was mounted on the roof and monitored until failure (in this case most likely due to moisture infiltration) An IV curve was taken every 10 seconds from which the short circuit current, open circuit voltage and maximum power point were extracted Incident irradiation was recorded before and after each IV curve

10 SOLAR FARM RESEARCH Device 2 results Adjusted current (a.u.) /03/ /03/ /03/ /03/2011 General decrease in device performance observable Voltage (V) Sample IV curves on successive days (Current values have been adjusted according to the incident solar radiation)

11 SOLAR FARM RESEARCH Device 2 results Adjusted short circuit current (a.u.) Adjusted Isc Voc Open circuit voltage (V) A decrease in both open circuit voltage and short circuit current is evident over 4 days. Changing irradiation levels are responsible for the scattering of data points Time in daylight (hours) Principle PV parameters as a function of sunlight exposure (Current values have been adjusted according to the incident solar radiation)

12 SOLAR FARM RESEARCH Device 2 results Efficiency decreases from ~4% to ~3% over 4 days. The device shows a rise and fall in efficiency during the 2 nd and 3 rd days. The fill factor remains relatively constant at ~55%. Efficiency Efficiency FF Time in daylight (hours) Fill factor Principle PV parameters as a function of sunlight exposure (Current values have been adjusted according to the incident solar radiation)

13 FURTHER WORK Test further devices over a longer period of time Test devices in parallel to allow comparison Measure humidity and device temperature Both have been shown to significantly affect organic device performance Improve the method used for IV curve fitting for more accurate parameter extraction Parameterisation techniques provide a route to understanding degradation mechanisms

14 ACKNOWLEDGEMENTS Dr Alastair Buckley, EPMM group, University of Sheffield Darren Watters, PhD student, EPMM group, University of Sheffield Dr Giuseppe Colantuono, visiting researcher, University of Sheffield

15 REFERENCES F. C. Krebs, Ed., Polymer Photovoltaics A Practical Approach. SPIE, 2008 C. J. Brabec, et al., "Polymer-fullerene bulk-heterojunction solar cells," Advanced Materials, vol. 22, pp , 2010 T. M. Clarke and J. R. Durrant, "Charge photogeneration in organic solar cells," Chemical Reviews, vol. 110, pp , 2010 E. A. Katz, et al., "Out-door testing and long-term stability of plastic solar cells," EPJ Applied Physics, vol. 36, pp , 2006 E. A. Katz, et al., "Temperature and irradiance effect on the photovoltaic parameters of a fullerene/conjugated-polymer solar cell," Proceedings of SPIE, 4108 pp , 2001 E. A. Katz, et al., "Temperature dependence for the photovoltaic device parameters of polymer-fullerene solar cells under operating conditions," Journal of Applied Physics, vol. 90, pp , 2001 J. A. Hauch, et al., "Flexible organic P3HT:PCBM bulk-heterojunction modules with more than 1 year outdoor lifetime," Solar Energy Materials and Solar Cells, vol. 92, pp , 2008