Mini-project report Real world degradation of organic photovoltaic devices in the Sheffield Solar Farm Dawn Scott dtp10dms@sheffield.ac.uk 20/05/2011 1
Abstract The aim of this project was to investigate the behaviour and degradation of organic photovoltaic devices operating outdoors under real world conditions. Four organic devices were monitored until failure on the Sheffield Solar Farm rooftop testing setup over a period of two months. The devices had a photoactive layer of PCDTBT:PC 70 BM(1:4) or P3HT:PCBM(1:0.6) and front and back electrodes composed of ITO/MoO 3 or ITO/PEDOT and Al. They were encapsulated between glass slides sealed with epoxy resin and had a pixel size of 4.5 mm 2. The short circuit current, open circuit voltage and maximum power point were recorded every 10 seconds. The cell efficiency, open circuit voltage and short circuit current were all observed to decrease appreciably over four days while the fill factor remained relatively constant. 1. Introduction Organic photovoltaic devices have emerged over the last 25 years with the advance of the science and engineering of organic semiconducting materials. One of the most significant early examples of such devices was that fabricated by Tang in 1986 which showed a greatly improved power conversion efficiency of about 1% under AM2 radiation over previous organic cells with similar architectures [1]. They belong to the group known as third generation photovoltaics along with dye-sensitized, multijunction and hybrid solar cells [2]. A good overview of the development of third generation solar cell materials is given in reference [3]. At the moment, despite the high costs associated with their fabrication and the materials themselves, silicon solar panels (first generation photovoltaics) still dominate the PV market [4]. This is due in part to the existence of an established process technology for silicon coming from the semiconductor industry, and also to the difficulties involved in the controlled manufacture of thin-film photovoltaics (second generation) [5]. Effort is going into developing organic photovoltaic devices to overcome the disadvantages of first and second generation photovoltaics. Organic materials do not require the high temperatures necessary for the production of inorganic cells, reducing energy consumption and, correspondingly, production cost. This also increases the range of possible substrates which can be used, including flexible plastics [6]. The fabrication of polymer devices processed from solution lends itself to a screen printing technique which can easily be scaled up for commercial production. characteristic of the device. This is achieved by adapting the equivalent circuit model method previously applied to inorganic solar cells [7-11]. Such parameterisation techniques are important as they provide a theory to understand and then to predict device behaviour. 2.2 OPV device operation Figure 1 depicts a widely used cell architecture for organic devices. It consists of a bulk heterojuction formed from a polymer-fullerene mix to maximise the interface region between the electron donor (polymer) and acceptor (fullerene) materials for charge separation. This is sandwiched between two electrodes along with thin layers of PEDOT:PSS or other materials to improve conduction and contact with the electrodes. Charge photogeneration in an organic device differs slightly from that occurring in inorganic devices. A photon of sufficient energy incident on an organic device will create a bound electron-hole pair, or exciton, which then diffuses towards the polymer:fullerene interface. Here the electron can transfer from the donor to the acceptor and possibly cause dissociation of the exciton to generate free charge carriers. The binding energy of an exciton can be 100 times greater or more than that of an electron-hole pair in an inorganic material, and twenty times greater or more than the thermal energy at room temperature [12]. Consequently, exciton generation by photon absorption does not guarantee the creation of free charge carriers. A more thorough treatment of charge photogeneration can be found in reviews of organic solar cells [6, 12]. 2. Background 2.1 Parameter extraction There exist many attempts to develop models which accurately describe the physical phenomena underlying the behaviour of organic solar cells. One of the main areas of interest is the extraction of parameters such as the short circuit current (I sc ), open circuit voltage (V oc ), photogenerated current (I ph ), and series and shunt resistances (R s and R p respectively) from fitting the IV 2
monitoring of devices with lifetimes of several years. These make use of the accelerating effect of increased illumination or temperature on device aging. However, it has been reported that enhanced illumination introduces effects which are not observed under illumination of 1 sun, and that accelerated degradation of organic solar cells by concentrated sunlight must therefore be carefully conducted [16]. Furthermore, laboratory conditions, being controlled by their very nature, will never fully simulate the constantly fluctuating real world conditions OPVs are subjected to during their operational lifetime. Figure 1. Device architecture and charge photogeneration in an organic photovoltaic device [12]. 2.3 Degradation mechanisms in OPVs Unlike more mature inorganic technologies which can operate reliably for over 25 years, OPVs exhibit relatively short device lifetimes due to the unstable nature of the materials used to make them. Currently reported lifetimes range from 100 hours to over 5 years [3, 13-15]. These figures are generally derived from controlled laboratory tests, sometimes using accelerated lifetime testing. The reasons for device failure are numerous and degradation mechanisms are often interrelated. They can be divided into externally applied conditions and internal processes. The most common degradation mechanisms arising from external conditions are reaction of the active components with oxygen and water penetrating the device, direct photodegradation of the active components, and thermal degradation of the active components. Internally, the device can be subject to corrosion at electrode surfaces, interlayer diffusion leading to reaction of the active components, and particle formation among other processes. The interplay of different degradation mechanisms makes the analysis of such processes very complex. For example, illumination both directly causes photodegradation and raises the temperature of the device which encourages thermal degradation. There are numerous physical and chemical characterization techniques for studying these mechanisms, ranging from time-of-flight secondary ion mass spectrometry (TOF-SIMS) to accelerated lifetime measurements [2]. While such tests are essential to identify and understand individual degradation mechanisms, they do not necessarily accurately predict device lifetimes. In some cases they might highlight degradation paths, such as from continuous light exposure, which would not be encountered during the normal operation of the cell. Accelerated lifetime measurements are used to overcome laboratory time constraints which prevent ongoing 2.4 Outdoor testing of OPVs There are relatively few reports of the outdoor testing of OPV devices. Katz et al. have recorded current-voltage measurements of devices mounted on a solar tracker in Sede Boqer in Israel over a period of a month [17]. During the night the devices were stored in a dark glove box with a nitrogen atmosphere. A previous study by Katz controlled the device temperature, storing it in the dark in a refrigerator in between 200 second periods of exposure to the sun on cloudless days [18] while another investigated the temperature dependence of the device parameters [19]. More recently in 2007 Krebs et al. produced a large organic module measuring 40 cm 25 cm from 91 cells which operated outdoors continuously for over a year in Roskilde, Denmark [20]. Hauch et al. also recorded the performance of smaller devices mounted on the Konarka rooftop testing setup in Lowell, MA (USA), operating continuously over a period of 14 months in 2008 [21]. 3. Experimental technique 3.1 Device testing In total, four organic devices were mounted on the Sheffield Solar Farm rooftop testing setup over a period of two months. The devices, fabricated by the EPMM group in the University of Sheffield, had a photoactive layer of PCDTBT:PC 70 BM(1:4) or P3HT:PCBM(1:0.6) and front and back electrodes composed of ITO/MoO 3 or ITO/PEDOT and Al. The pixel size of the devices was 4.5 mm 2. The devices were encapsulated between glass slides sealed with epoxy resin which provided a poor barrier to air and moisture infiltration. As a result they degraded beyond recovery when it rained. Examples of devices before and after destruction are shown in Figure 2. 3
A positive correlation between organic device efficiency and temperature has been reported by Katz et al. [19]. The fill factor remains relatively constant over the four days at around 55%. This is because the maximum power point is decreasing in step with the short circuit current and open circuit voltage. Figure 2. Test devices before (left) and after (right) rainfall. Each device was mounted and monitored until it failed. Devices 1 and 2 were mounted separately while devices 3 and 4 were mounted simultaneously to allow comparison of their performance under the same environmental conditions. IV curves were recorded every 10 seconds using a voltage sweep from -1 to +1 V. The short circuit current and open circuit voltage and maximum power point were extracted from each IV characteristic. The total and diffuse solar radiation incident on the device was recorded at the beginning and end of each voltage sweep. 3.2 Parameter extraction The APTIV method described by Haouari-Merbah et al. [22] was chosen to fit each IV curve to extract parameters such as the photogenerated current and series resistance as well as the short circuit current and open circuit voltage, based on the two-diode model. Although this method is intended to arrive at a good fit without the need for accurate initial guesses at the parameters, it was found that the system of equations generated would not converge. In the end the short circuit current and open circuit voltage were extracted by linearly fitting the relevant regions of the IV curve. 4. Results and discussion Only data for device 2 (P3HT:PCBM) is presented here since problems with the recording equipment meant that the data for the other devices was corrupted. Figure 3 and Figure 4 show the evolution of the principle parameters for device 2 over four days. Although only four days data is available, there is still an appreciable decrease visible in the open circuit voltage from around 0.82 V to 0.78 V. The short circuit current can also be seen to decline. Likewise, the device efficiency falls from around 4% to 3% over the four days. There also appears to be a daily rise and fall in the efficiency on the second and third days. This might also occur on the fourth day, but the data does not span the entire day. This could be a positive response to temperature since the device will heat up, particularly over the noontime period, with prolonged exposure to the sun. Adjusted short circuit current (a.u.) 0.0000010 0.0000008 0.0000006 0.0000004 0.0000002 0.0000000 0.64 0 5 10 15 20 25 30 35 40 Figure 3. Short circuit current and open circuit voltage as a function of sunlight exposure for device 2 over four days from 22/03/11 to 25/03/11. The short circuit current has been adjusted by dividing by the level of incident solar radiation. Efficiency 0.10 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 Time in daylight (hours) Efficiency FF Adjusted Isc Voc Figure 4. Power conversion efficiency and fill factor of device 2 as a function of sunlight exposure over four days from 22/03/11 to 25/03/11. 5. Conclusions and further work Small organic photovoltaic devices were monitored continuously outdoors until failure. It was possible to track the device performance through the principle photovoltaic parameters: short circuit current, open circuit voltage, efficiency and fill factor. However, four days worth of data is not enough to draw firm conclusions from. There is much scope for further work. Monitoring further devices simultaneously would allow comparison between them, helping to determine which degradation mechanisms are taking place. This could be backed up with chemical analysis of the degraded cells. 0.82 0.80 0.78 0.76 0.74 0.72 0.70 0.68 0.66 0.0 0 5 10 15 20 25 30 35 40 Time in daylight (hours) 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Open circuit voltage (V) Fill factor 4
Also, recording humidity and the temperature of the device would provide further information from which degradation mechanisms could be determined. With a longer study, trends could be more easily picked out. Parameterisation techniques also provide a route to understanding degradation mechanisms. Developing a fitting algorithm to extract device parameters from the IV characteristic rather than using a direct approach which depends heavily on the accuracy of the points chosen for the calculation would improve the results [23]. Acknowledgements I would like to acknowledge the help and guidance provided during this project by my supervisor Dr Alastair Buckley, and also Dr Giuseppe Colantuono and Darren Watters, all of the University of Sheffield. References [1] C. W. Tang, "Two-layer organic photovoltaic cell," Applied Physics Letters, vol. 48, pp. 183-185, 1986. [2] F. C. Krebs, Ed., Polymer Photovoltaics A Practical Approach. SPIE, 2008. [3] C. J. Brabec, et al., "Polymer-fullerene bulkheterojunction solar cells," Advanced Materials, vol. 22, pp. 3839-3856, 2010. [4] S. Pizzini, et al., "From electronic grade to solar grade silicon: Chances and challenges in photovoltaics," Physica Status Solidi (A) Applications and Materials, vol. 202, pp. 2928-2942, 2005. [5] B. Sinha, "Trends in global solar photovoltaic research: Silicon versus non-silicon materials," Current Science, vol. 100, pp. 654-660, 2011. [6] B. Kippelen and J. L. Brédas, "Organic photovoltaics," Energy and Environmental Science, vol. 2, pp. 251-261, 2009. [7] P. Junsangsri and F. Lombardi, "Modelling and extracting parameters of organic solar cells," Electronics Letters, vol. 46, pp. 1462-1464, 2010. [8] J. Chuan, et al., "Research on the characteristics of organic solar cells," Journal of Physics: Conference Series, vol. 276, 2011. [9] A. Jain and A. Kapoor, "A new approach to study organic solar cell using Lambert W-function," Solar Energy Materials and Solar Cells, vol. 86, pp. 197-205, 2005. [10] B. Mazhari, "An improved solar cell circuit model for organic solar cells," Solar Energy Materials and Solar Cells, vol. 90, pp. 1021-1033, 2006. [11] S. Z. Wei, et al., "Modeling of the J-V characteristics for ITO/CuPc/C60/Al heterostructure solar cells," Journal of the Chinese Chemical Society, vol. 57, pp. 1185-1190, 2010. [12] T. M. Clarke and J. R. Durrant, "Charge photogeneration in organic solar cells," Chemical Reviews, vol. 110, pp. 6736-6767, 2010. [13] E. Voroshazi, et al., "Long-term operational lifetime and degradation analysis of P3HT:PCBM photovoltaic cells," Solar Energy Materials and Solar Cells, vol. 95, pp. 1303-1307, 2011. [14] G. Dennler and R. Gaudiana, "Future challenges for the industry of organic photovoltaics," 2010. [15] T. Riedl, et al., "Reliability aspects of organic light emitting diodes," 2010, pp. 327-333. [16] T. Tromholt, et al., "Reversible degradation of inverted organic solar cells by concentrated sunlight," Nanotechnology, vol. 22, 2011. [17] E. A. Katz, et al., "Out-door testing and long-term stability of plastic solar cells," EPJ Applied Physics, vol. 36, pp. 307-311, 2006. [18] E. A. Katz, et al., "Temperature and irradiance effect on the photovoltaic parameters of a fullerene/conjugated-polymer solar cell," 2001, pp. 117-124. [19] E. A. Katz, et al., "Temperature dependence for the photovoltaic device parameters of polymerfullerene solar cells under operating conditions," Journal of Applied Physics, vol. 90, pp. 5343-5350, 2001. [20] F. C. Krebs, et al., "Large area plastic solar cell modules," Materials Science and Engineering B: Solid-State Materials for Advanced Technology, vol. 138, pp. 106-111, 2007. [21] 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. 727-731, 2008. [22] M. Haouari-Merbah, et al., "Extraction and analysis of solar cell parameters from the illuminated current-voltage curve," Solar Energy Materials and Solar Cells, vol. 87, pp. 225-233, 2005. [23] R. Gottschalg, et al., "The influence of the measurement environment on the accuracy of the extraction of the physical parameters of solar cells," Measurement Science and Technology, vol. 10, pp. 796-804, 1999. 5