- Email: [email protected]
Large-area organic photovoltaic module—Fabrication and performance
ARTICLE IN PRESS Solar Energy Materials & Solar Cells 93 (2009) 442–446
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Large-area organic photovoltaic module—Fabrication and performance Ritesh Tipnis , Jan Bernkopf, Shijun Jia, John Krieg, Sergey Li, Mark Storch, Darin Laird Plextronics, Inc., 2180 William Pitt Way, Pittsburgh, PA 15238, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 September 2008 Received in revised form 13 November 2008 Accepted 20 November 2008 Available online 8 January 2009
Here we describe the fabrication of the largest (233 cm2 total area) organic photovoltaic (OPV) module (polymer:fullerene) to be certified by the National Renewable Energy Laboratory (NREL). OPV solar cells were fabricated at Plextronics by spin coating a blend of poly 3-hexylthiophene-2,5 diyl (P3HT) and [6,6] phenyl C61 butyric acid methyl ester (PCBM) on top of our hole transport layer (HTL), Plexcores OC. In laboratory-scale devices (0.09 cm2), this system routinely exhibits power conversion efficiencies exceeding 3.7%. This P3HT:PCBM active layer and HTL ink system was used to scale up to the larger area module (15.2 cm 15.2 cm module size, i.e. 233 cm2 total area; 108 cm2 active area), which was certified by NREL as having 1.1% total area efficiency (3.4% active area efficiency). & 2008 Elsevier B.V. All rights reserved.
Keywords: Organic photovoltaics P3HT:PCBM Plexcore Module fabrication Lifetime Solar cells
1. Introduction Recently, power generation has seen rising demand for renewable energy through solar, wind, and biofuel options. However, the cost of many of these renewable energy sources is not competitive with fossil fuels, resulting in only a fraction of the United States’ energy needs being derived from these sources. According to the Energy Information Administration (EIA), the world’s energy consumption has been forecasted to increase by 50% between 2005 and 2030 [1]. This will drive the continuing expansion of the renewable energy market. Of the renewable sources, only solar energy based on photovoltaic technology is capable of serving a significant fraction of this demand [2]. However, silicon-based solar energy, the dominant technology currently available in the marketplace, has not proven to be grid competitive in the absence of heavy subsidies [3]. This has drastically limited the widespread implementation of this technology. The world’s demand for less expensive and cleaner energy has created a surge of investment in alternative solar energy technologies, such as those based on cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). These technologies, though less expensive than silicon, have not yet achieved widescale commercialization, mostly due to the manufacturing hurdles associated with the complex chemistry needed for CIGS [4] and the potential environmental concerns related to cadmium in CdTe [5]. Amorphous silicon and related technologies have been
Corresponding author. Tel.: +1 412 423 2030x129.
E-mail address: [email protected] (R. Tipnis). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.11.018
demonstrated and are currently being installed but are yet to prove commercial viability. Due to low-cost manufacturing methods, and the potential for thin dimensions and light-weight composition, polymer-based organic photovoltaic (OPV) cells offer the potential for dramatic cost savings over current solar technologies, significantly expanding the solar energy markets. The primary commercialization challenge for OPV technology is achieving the required combination of operating efficiency and device lifetime, which are key inputs into cost models. We address these challenges through the development of new materials and inks as well as the development of process technology to enable low-cost commercial solar cells for both off-grid and on-grid applications. Though others are currently working on OPV modules [6,7], we currently hold the record for the largest National Renewable Energy Laboratory (NREL)-certified OPV modules as well as the highest single-layer OPV efficiency for a laboratory-scale device. The OPV cell is an excitonic solar cell [8] based on an active layer that is a composite of a p-type light-harvesting polymer and n-type intrinsic semiconductors [9]. Fig. 1 shows the basic device architecture for an OPV cell comprising a photoactive layer fabricated using a solution of the p- and n-type components, referred to here as PV ink. The active layer is deposited on top of an organic hole transport layer (HTL) that planarizes the transparent anode and facilitates the collection of positive charge carriers (holes) from the light-harvesting layer. The two organic layers are sandwiched between anode and cathode electrodes and sealed with a cavity glass and getter encapsulation system. For the purposes of this study, the active layer is based on a blend of poly 3-hexylthiophene-2,5 diyl (P3HT) and [6,6] phenyl C61 butyric
ARTICLE IN PRESS R. Tipnis et al. / Solar Energy Materials & Solar Cells 93 (2009) 442–446
acid methyl ester (PCBM), which has been identified by many researchers as one of the efficient systems for an OPV system [10–13], due to the phase separation that occurs between the p- and n-components of the composite, which generates discrete manifolds beneficial for charge transport and exciton dissociation.
2. Module design OPV modules were fabricated on a 15.2 cm 15.2 cm ITOcoated glass substrate, which had been fortified with a metal frame for improved collection of holes. The modules have been designed with a specific configuration of cells to allow maximum utilization of area, while delivering the optimum operating voltage for a particular application. Fig. 2a shows the design of a Plextronics module that consists of 54 identical cells, each having an active area of 1 cm 2 cm. The cells are organized in six parallel columns, each containing nine cells that are connected in series by internal wiring (see Fig. 2b). The columns are electrically isolated, which provides an opportunity to independently measure each column’s performance. External wiring is used to configure the module in several ways. In this paper, we will refer to configuration 54S (all cells connected in series) and 9S,6P (six columns connected in parallel). This design was adopted for development purpose, with the commercial modules expected to be redesigned to increase the active area.
Fig. 1. OPV device stack.
443
3. Module fabrication The pre-patterned ITO-coated glass substrates (10 O/square) were purchased from a vendor and used in processing steps as outlined below:
1. Substrate preparation: The substrates were cleaned in a fourstage ultrasonic bath process; first, the substrates were washed with DI water with detergent, followed by DI water wash, followed by solvent wash in acetone, and rinsed with isopropyl alcohol. The substrates were dried using a nitrogen gun and UV-ozone treated to improve their wettability. 2. Deposition of HTL: The HTL was applied immediately after the UV-ozone treatment, by spin coating the ink under ambient conditions. Following this step, the substrates were annealed on a hot plate at 110–175 1C for 15–30 min in an inert atmosphere. 3. Deposition of active layer: The regioregular poly(3-hexylthiophene) used in this study was synthesized by the Grignard metathesis (GRIM) method according to the previously described procedure [14–16]. Typical molecular weights for the P3HT used herein were 45,000–65,000 with a PDI (Mw/Mn) 1.5–1.7 (Plextronics’ Plexcores OS2100). The optical properties of the polymers studied revealed lmax in the range of 555–559 nm for solid spin cast films from o-dichlorobenzene (ODCB) with a band gap in the range of 1.9–2.0 eV. The PV ink was applied by spin coating in an inert atmosphere. The lid of the spinner was closed during the process to slow down the drying of the film and thus promote film uniformity. Following this, the substrates were annealed under similar conditions as with the HTL. 4. Laser ablation: Plextronics uses a Resonetics 120 laser system operating at 248 nm wavelength and using an ATLEX-300-SI excimer laser (300 Hz repetition rate, 6 W average power, 20 mJ maximum pulse energy) to remove organic layers from the contact and seal areas of the module. In this ablation process, the laser beam breaks the material’s molecular bonds, resulting in high-velocity ejection of the ‘‘vaporized’’ materials into the collection path of a debris nozzle, which helps keep the active area of the device clean. This material-removing process, done in a closed-cycle nitrogen environment, enables
Fig. 2. (a) Design of Plextronics OPV module on 15.2 cm 15.2 cm glass substrate. (b) Module schematics of 54 identical cells organized in six parallel columns of nine cells each.
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Fig. 3. Flow chart of OPV module processing steps (top) and pictures (bottom).
Fig. 4. OPV module measurement configurations—54S (left) and 9S,6P (right).
Fig. 3 summarizes the OPV module-fabrication process from the input of pre-patterned ITO glass substrate to a finished OPV module, including pictures of equipment utilized in some of the steps.
Device ID: 2752 MAr 18, 2008 10:49 Spectrum: AM1.5-G (IEC 60904)
Device Temperature: 25.0 ± 1.0°C Device Area: 232.800 cm2 Irradiance: 1000.0 W/m2
X25 IV System CONFIDENTIAL PV Performance Characterization Team 18 16 14 12 10 Current (mA)
better encapsulation of the devices and facilitates series connections. The process does not damage the other components of the device. 5. Cathode deposition: A shadow mask was used to pattern the Ca/ Al cathode during the metal evaporation, carried out under high vacuum (10 6 mbar). 6. Encapsulation: The modules were transferred under an inert atmosphere from the evaporator to the encapsulation station. A robotic dispensing arm was used to deposit a bead of UV-curable sealant around the module periphery. Cavity glass with an attached getter was mated with the substrate and pressed together to produce a thin sealant layer which was UV cured. A special fixture was used to maintain good alignment between substrate and cavity glass.
8 6 4 2
4. Module testing 4.1. Power conversion efficiency Compared to laboratory cells, the power conversion efficiency of OPV modules is impacted by several factors, including a small aperture ratio, effects of defective cells, higher series resistance, and difficulties in scaling up the fabrication processes [17]. Through our module design process, we are optimizing the various fabrication steps and establishing accurate measurement
0 -2
-10 -5 0 5 Voc = 29.3466 V Isc = 16.906 mA Jsc = 0.072618 mA/cm2 Fill Factor = 50.98 %
10
15
20 25 30 Imax = 13.210 mA Vmax = 19.1466 V Pmax = 0.2529 W Efficiency = 1.09 %
Fig. 5. NREL certification of Plextronics’ OPV module made with P3HT:PCBM active layer.
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445
4.2. Lifetime testing facility
techniques, in order to maximize the potential of the active layer ink system. The module testing was conducted under a 1 Sun solar simulator, i.e. 100 mW/cm2, with KG-5 Si reference and spectral mismatch was used to correct the current density. The six-module columns were measured independently and then combined into 54S and 9S,6P configurations (see Fig. 4) and measured. High-performance modules were shipped to National Renewable Energy Laboratory for certification. Fig. 5 indicates the typical performance of our OPV module that was certified at NREL. This module, which was made using P3HT:PCBM blend as active layer, demonstrated 1.1% efficiency over its total area of 233 cm2. It is widely accepted that reliable encapsulation is essential for consistent efficiency measurements, and ultimately successful commercialization of OPVs [18]. We are currently evaluating various sealants and encapsulation techniques to improve the hermeticity of the seal. Penetration of water/oxygen, due to any by-products of the curing of glue can lead to module degradation. Additional problems associated with intra-module uniformity were revealed by IR imagery, namely presence of shorted cells and/or cells with higher forward current that negatively affect achievable fill factor of the module. With better processing and encapsulation, we expect the module performance to improve due to higher uniformity of the columns (within the module) and longer lifetime due to superior encapsulation.
0.1
Plextronics has established world-class OPV testing facilities. A large-area, high-intensity xenon lamp (Atlas Specialty Lighting, PE240E-13FM) has been installed, which enables better spectral match to the solar spectrum during device testing (see Fig. 6). Six modules can be tested simultaneously under one lamp with illumination tracking on a temperature-controlled stage. An adjustable DC load maintains the devices at their maximum power point. Various parameters such as power output, open-circuit voltage (VOC), and short-circuit current (JSC) are intermittently monitored. A PolyScience 6000 Chiller is used to maintain the temperature of the modules and an unfiltered Si photodiode is used to track the lamp illumination. This state-of-the-art set-up has allowed Plextronics to accurately monitor the lifetime of its modules. Plextronics currently defines OPV lifetime as the duration over which an OPV device or module diminishes to 80% of its ‘stabilized’ power output (or power conversion efficiency), normalized by the illumination intensity (lamp variation or decay) under 1 Sun Xe-arc lamp with (or converted to) 50% duty cycle. The actual data are collected at 100% duty cycle; the 50% duty cycle conversion may translate to performance under a day–night cycle; however, the temperature swings of such a cycle are not factored into this conversion. Shelf life of these modules appears to be quite stable, i.e. 43000 h to above T95. Fig. 7a shows the normalized output power of Plextronics module (3070) after being normalized by the input power decay of Xe-lamp irradiation. The increased scattering of normalized data is due to stochastic variations in the lamp intensity, while the red arrow indicates the inflection of the power output due to the module position change after J–V measurement. Application of a linear fit to the data gives a decay rate of 9.82% per 1000 h (the decay rate is 5% per 1000 h for 50% duty cycle). This decay rate would correspond to a lifetime of 5000 h. Fig. 7b shows the change of J–V characteristics (AM1.5G solar simulation) of Plextronics’ module (3070) after being illuminated under 1 Sun Xe-lamp for 648 and 1320 h. The module performance is essentially unchanged, typically 73%, within the variation of test error, and indicates that the module is very stable.
0.0
5. Conclusion
0.6 0.5
Lamp spectrum Metal Halide AM 1.5 Xe High intensity Xe
0.4 0.3 0.2
300
400
500 600 Wavelength (nm)
700
With optimized materials synthesis, photoactive layer ink formulation, and module design and fabrication, we have demonstrated the largest OPV module to be certified at NREL. Using a blend of P3HT and PCBM on top of a proprietary HTL, the
800
Fig. 6. Improved lamp spectrum with Xe lamp.
0.10
1.0 0.8 0.6
The inflection of the power output is due to the module position change after JV measurement
0.4 0.2
Current Density (mA/cm2)
NP (on Lamp In) (a.u.)
Module 3070 under 100% Duty Cycle
Module 3070 0.05 Fresh 600 H 1320 H
0.00 -0.05 -0.10 -0.15
0.0 0
200 400 600 800 1000 1200 Life Time (hours)
0
5
10 15 20 Voltage (V)
25
30
Fig. 7. (a) Normalized power output of module 3070 under Xe lamp. This decay rate translates to 45000 h of lifetime to T80 at 50% duty cycle with 1 Sun illumination. Data were smoothed (reduction of data points and removing of outliers) by interpolation in Origin software. (b) AM1.5G solar simulation J–V characteristics of module 3070 operated for 648 and 1320 h under 1 Sun Xe lamp.
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module exhibited 1.1% efficiency over its total area of 233 cm2. A state-of-the-art lifetime measurement set-up was used to evaluate the module and the decay rate was observed to correspond to a lifetime of 5000 h.
Acknowledgements Plextronics would like to thank the PV Performance Characterization Team at NREL, headed by Dr. Keith Emory for the cell and module measurements shown in Fig. 5.
References [1] Energy Information Administration, International Energy Outlook 2008, p. 7. [2] K. Zweibel, The terawatt challenge for thin-film PV, NREL Technical Report, NREL/TP-520-38350, 2005, pp. 1–49. [3] /http://findarticles.com/p/articles/mi_m0EIN/is_2004_Jan_27/ ai_112591535S, Government subsidies vital in making solar power technology price-competitive, Business Wire, January 27, 2004 (date retrieved: November 12, 2008). [4] F. Kessler, D. Rudmann, Technological aspects of flexible CIGS solar cells and modules, Sol. Energy Mater. Sol. Cells 77 (2004) 685–695. [5] V. Fthenakis, K. Zweibel, CdTe PV: real and perceived EHS risks, NCPV and Solar Program Review Meeting, NREL/CD-520-33586, 2003, pp. 413–415. [6] S. Yoo, W.J. Potscavage, B. Domercq, J. Kim, J. Holt, B. Kippelen, Integrated organic photovoltaic modules with a scalable voltage output, Appl. Phys. Lett. 89 (2006) 233516-1–233516-3.
[7] F.C. Krebs, H. Spanggard, T. Kjær, M. Biancardo, J. Alstrup, Large area plastic solar cell modules, Mater. Sci. Eng. 138 (2007) 106–111. [8] B.A. Gregg, Excitonic solar cells, J. Phys. Chem. B 107 (2003) 4688–4698. [9] N.S. Sariciftci, D. Braun, C. Zhang, V.I. Srdanov, A.J. Heeger, G. Stucky, F. Wudl, Semiconducting polymer–buckminsterfullerene heterojunctions: diodes, photodiodes, and photovoltaic cells, Appl. Phys. Lett. 62 (1993) 585–587. [10] G. Li, V. Shotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang, High-efficiency solution processable polymer photovoltaic cells by selforganization of polymer blends, Nat. Mater. 4 (2005) 864–868. [11] W. Ma, C. Yang, X. Gong, K. Lee, A.J. Heeger, Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology, Adv. Funct. Mater. 15 (2005) 1617–1622. [12] L.L. Kazmerski, Solar photovoltaics R&D at the tipping point: a 2005 technology overview, J. Electron Spectrosc. Relat. Phenom. 150 (2006) 105–135. [13] A.J. Moule´, K. Meerholz, Intensity-dependent photocurrent generation at the anode in bulk-heterojunction solar cells, Appl. Phys. B 92 (2008) 209–218. [14] R.S. Lowe, S.M. Khersonsky, R.D. McCullough, A simple method to prepare head-to-tail coupled, regioregular poly(3-alkylthiophenes) using Grignard metathesis, Adv. Mater. 11 (1999) 250–253. [15] E.E. Sheina, J. Liu, M.C. Iovu, D.W. Laird, R.D. McCullough, Chain growth mechanism for regioregular nickel-initiated cross-coupling polymerizations, Macromolecules 37 (2004) 3526–3528. [16] M.C. Ivou, E.E. Sheina, R.R. Gil, R.D. McCullough, Experimental evidence for the quasi-‘‘living’’ nature of the Grignard metathesis method for the synthesis of regioregular poly(3-alkylthiophenes), Macromolecules 38 (2005) 8649–8656. [17] B. von Roedern, Thin-film module review, Renewable Energy Focus July/ August (2006) 34–39. [18] M. Jørgensen, K. Norman, F.C. Krebs, Stability/degradation of polymer solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 686–714.
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Large-area organic photovoltaic module—Fabrication and performance Ritesh Tipnis , Jan Bernkopf, Shijun Jia, John Krieg, Sergey Li, Mark Storch, Darin Laird Plextronics, Inc., 2180 William Pitt Way, Pittsburgh, PA 15238, USA
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 September 2008 Received in revised form 13 November 2008 Accepted 20 November 2008 Available online 8 January 2009
Here we describe the fabrication of the largest (233 cm2 total area) organic photovoltaic (OPV) module (polymer:fullerene) to be certified by the National Renewable Energy Laboratory (NREL). OPV solar cells were fabricated at Plextronics by spin coating a blend of poly 3-hexylthiophene-2,5 diyl (P3HT) and [6,6] phenyl C61 butyric acid methyl ester (PCBM) on top of our hole transport layer (HTL), Plexcores OC. In laboratory-scale devices (0.09 cm2), this system routinely exhibits power conversion efficiencies exceeding 3.7%. This P3HT:PCBM active layer and HTL ink system was used to scale up to the larger area module (15.2 cm 15.2 cm module size, i.e. 233 cm2 total area; 108 cm2 active area), which was certified by NREL as having 1.1% total area efficiency (3.4% active area efficiency). & 2008 Elsevier B.V. All rights reserved.
Keywords: Organic photovoltaics P3HT:PCBM Plexcore Module fabrication Lifetime Solar cells
1. Introduction Recently, power generation has seen rising demand for renewable energy through solar, wind, and biofuel options. However, the cost of many of these renewable energy sources is not competitive with fossil fuels, resulting in only a fraction of the United States’ energy needs being derived from these sources. According to the Energy Information Administration (EIA), the world’s energy consumption has been forecasted to increase by 50% between 2005 and 2030 [1]. This will drive the continuing expansion of the renewable energy market. Of the renewable sources, only solar energy based on photovoltaic technology is capable of serving a significant fraction of this demand [2]. However, silicon-based solar energy, the dominant technology currently available in the marketplace, has not proven to be grid competitive in the absence of heavy subsidies [3]. This has drastically limited the widespread implementation of this technology. The world’s demand for less expensive and cleaner energy has created a surge of investment in alternative solar energy technologies, such as those based on cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). These technologies, though less expensive than silicon, have not yet achieved widescale commercialization, mostly due to the manufacturing hurdles associated with the complex chemistry needed for CIGS [4] and the potential environmental concerns related to cadmium in CdTe [5]. Amorphous silicon and related technologies have been
Corresponding author. Tel.: +1 412 423 2030x129.
E-mail address: [email protected] (R. Tipnis). 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.11.018
demonstrated and are currently being installed but are yet to prove commercial viability. Due to low-cost manufacturing methods, and the potential for thin dimensions and light-weight composition, polymer-based organic photovoltaic (OPV) cells offer the potential for dramatic cost savings over current solar technologies, significantly expanding the solar energy markets. The primary commercialization challenge for OPV technology is achieving the required combination of operating efficiency and device lifetime, which are key inputs into cost models. We address these challenges through the development of new materials and inks as well as the development of process technology to enable low-cost commercial solar cells for both off-grid and on-grid applications. Though others are currently working on OPV modules [6,7], we currently hold the record for the largest National Renewable Energy Laboratory (NREL)-certified OPV modules as well as the highest single-layer OPV efficiency for a laboratory-scale device. The OPV cell is an excitonic solar cell [8] based on an active layer that is a composite of a p-type light-harvesting polymer and n-type intrinsic semiconductors [9]. Fig. 1 shows the basic device architecture for an OPV cell comprising a photoactive layer fabricated using a solution of the p- and n-type components, referred to here as PV ink. The active layer is deposited on top of an organic hole transport layer (HTL) that planarizes the transparent anode and facilitates the collection of positive charge carriers (holes) from the light-harvesting layer. The two organic layers are sandwiched between anode and cathode electrodes and sealed with a cavity glass and getter encapsulation system. For the purposes of this study, the active layer is based on a blend of poly 3-hexylthiophene-2,5 diyl (P3HT) and [6,6] phenyl C61 butyric
ARTICLE IN PRESS R. Tipnis et al. / Solar Energy Materials & Solar Cells 93 (2009) 442–446
acid methyl ester (PCBM), which has been identified by many researchers as one of the efficient systems for an OPV system [10–13], due to the phase separation that occurs between the p- and n-components of the composite, which generates discrete manifolds beneficial for charge transport and exciton dissociation.
2. Module design OPV modules were fabricated on a 15.2 cm 15.2 cm ITOcoated glass substrate, which had been fortified with a metal frame for improved collection of holes. The modules have been designed with a specific configuration of cells to allow maximum utilization of area, while delivering the optimum operating voltage for a particular application. Fig. 2a shows the design of a Plextronics module that consists of 54 identical cells, each having an active area of 1 cm 2 cm. The cells are organized in six parallel columns, each containing nine cells that are connected in series by internal wiring (see Fig. 2b). The columns are electrically isolated, which provides an opportunity to independently measure each column’s performance. External wiring is used to configure the module in several ways. In this paper, we will refer to configuration 54S (all cells connected in series) and 9S,6P (six columns connected in parallel). This design was adopted for development purpose, with the commercial modules expected to be redesigned to increase the active area.
Fig. 1. OPV device stack.
443
3. Module fabrication The pre-patterned ITO-coated glass substrates (10 O/square) were purchased from a vendor and used in processing steps as outlined below:
1. Substrate preparation: The substrates were cleaned in a fourstage ultrasonic bath process; first, the substrates were washed with DI water with detergent, followed by DI water wash, followed by solvent wash in acetone, and rinsed with isopropyl alcohol. The substrates were dried using a nitrogen gun and UV-ozone treated to improve their wettability. 2. Deposition of HTL: The HTL was applied immediately after the UV-ozone treatment, by spin coating the ink under ambient conditions. Following this step, the substrates were annealed on a hot plate at 110–175 1C for 15–30 min in an inert atmosphere. 3. Deposition of active layer: The regioregular poly(3-hexylthiophene) used in this study was synthesized by the Grignard metathesis (GRIM) method according to the previously described procedure [14–16]. Typical molecular weights for the P3HT used herein were 45,000–65,000 with a PDI (Mw/Mn) 1.5–1.7 (Plextronics’ Plexcores OS2100). The optical properties of the polymers studied revealed lmax in the range of 555–559 nm for solid spin cast films from o-dichlorobenzene (ODCB) with a band gap in the range of 1.9–2.0 eV. The PV ink was applied by spin coating in an inert atmosphere. The lid of the spinner was closed during the process to slow down the drying of the film and thus promote film uniformity. Following this, the substrates were annealed under similar conditions as with the HTL. 4. Laser ablation: Plextronics uses a Resonetics 120 laser system operating at 248 nm wavelength and using an ATLEX-300-SI excimer laser (300 Hz repetition rate, 6 W average power, 20 mJ maximum pulse energy) to remove organic layers from the contact and seal areas of the module. In this ablation process, the laser beam breaks the material’s molecular bonds, resulting in high-velocity ejection of the ‘‘vaporized’’ materials into the collection path of a debris nozzle, which helps keep the active area of the device clean. This material-removing process, done in a closed-cycle nitrogen environment, enables
Fig. 2. (a) Design of Plextronics OPV module on 15.2 cm 15.2 cm glass substrate. (b) Module schematics of 54 identical cells organized in six parallel columns of nine cells each.
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R. Tipnis et al. / Solar Energy Materials & Solar Cells 93 (2009) 442–446
Fig. 3. Flow chart of OPV module processing steps (top) and pictures (bottom).
Fig. 4. OPV module measurement configurations—54S (left) and 9S,6P (right).
Fig. 3 summarizes the OPV module-fabrication process from the input of pre-patterned ITO glass substrate to a finished OPV module, including pictures of equipment utilized in some of the steps.
Device ID: 2752 MAr 18, 2008 10:49 Spectrum: AM1.5-G (IEC 60904)
Device Temperature: 25.0 ± 1.0°C Device Area: 232.800 cm2 Irradiance: 1000.0 W/m2
X25 IV System CONFIDENTIAL PV Performance Characterization Team 18 16 14 12 10 Current (mA)
better encapsulation of the devices and facilitates series connections. The process does not damage the other components of the device. 5. Cathode deposition: A shadow mask was used to pattern the Ca/ Al cathode during the metal evaporation, carried out under high vacuum (10 6 mbar). 6. Encapsulation: The modules were transferred under an inert atmosphere from the evaporator to the encapsulation station. A robotic dispensing arm was used to deposit a bead of UV-curable sealant around the module periphery. Cavity glass with an attached getter was mated with the substrate and pressed together to produce a thin sealant layer which was UV cured. A special fixture was used to maintain good alignment between substrate and cavity glass.
8 6 4 2
4. Module testing 4.1. Power conversion efficiency Compared to laboratory cells, the power conversion efficiency of OPV modules is impacted by several factors, including a small aperture ratio, effects of defective cells, higher series resistance, and difficulties in scaling up the fabrication processes [17]. Through our module design process, we are optimizing the various fabrication steps and establishing accurate measurement
0 -2
-10 -5 0 5 Voc = 29.3466 V Isc = 16.906 mA Jsc = 0.072618 mA/cm2 Fill Factor = 50.98 %
10
15
20 25 30 Imax = 13.210 mA Vmax = 19.1466 V Pmax = 0.2529 W Efficiency = 1.09 %
Fig. 5. NREL certification of Plextronics’ OPV module made with P3HT:PCBM active layer.
ARTICLE IN PRESS R. Tipnis et al. / Solar Energy Materials & Solar Cells 93 (2009) 442–446
445
4.2. Lifetime testing facility
techniques, in order to maximize the potential of the active layer ink system. The module testing was conducted under a 1 Sun solar simulator, i.e. 100 mW/cm2, with KG-5 Si reference and spectral mismatch was used to correct the current density. The six-module columns were measured independently and then combined into 54S and 9S,6P configurations (see Fig. 4) and measured. High-performance modules were shipped to National Renewable Energy Laboratory for certification. Fig. 5 indicates the typical performance of our OPV module that was certified at NREL. This module, which was made using P3HT:PCBM blend as active layer, demonstrated 1.1% efficiency over its total area of 233 cm2. It is widely accepted that reliable encapsulation is essential for consistent efficiency measurements, and ultimately successful commercialization of OPVs [18]. We are currently evaluating various sealants and encapsulation techniques to improve the hermeticity of the seal. Penetration of water/oxygen, due to any by-products of the curing of glue can lead to module degradation. Additional problems associated with intra-module uniformity were revealed by IR imagery, namely presence of shorted cells and/or cells with higher forward current that negatively affect achievable fill factor of the module. With better processing and encapsulation, we expect the module performance to improve due to higher uniformity of the columns (within the module) and longer lifetime due to superior encapsulation.
0.1
Plextronics has established world-class OPV testing facilities. A large-area, high-intensity xenon lamp (Atlas Specialty Lighting, PE240E-13FM) has been installed, which enables better spectral match to the solar spectrum during device testing (see Fig. 6). Six modules can be tested simultaneously under one lamp with illumination tracking on a temperature-controlled stage. An adjustable DC load maintains the devices at their maximum power point. Various parameters such as power output, open-circuit voltage (VOC), and short-circuit current (JSC) are intermittently monitored. A PolyScience 6000 Chiller is used to maintain the temperature of the modules and an unfiltered Si photodiode is used to track the lamp illumination. This state-of-the-art set-up has allowed Plextronics to accurately monitor the lifetime of its modules. Plextronics currently defines OPV lifetime as the duration over which an OPV device or module diminishes to 80% of its ‘stabilized’ power output (or power conversion efficiency), normalized by the illumination intensity (lamp variation or decay) under 1 Sun Xe-arc lamp with (or converted to) 50% duty cycle. The actual data are collected at 100% duty cycle; the 50% duty cycle conversion may translate to performance under a day–night cycle; however, the temperature swings of such a cycle are not factored into this conversion. Shelf life of these modules appears to be quite stable, i.e. 43000 h to above T95. Fig. 7a shows the normalized output power of Plextronics module (3070) after being normalized by the input power decay of Xe-lamp irradiation. The increased scattering of normalized data is due to stochastic variations in the lamp intensity, while the red arrow indicates the inflection of the power output due to the module position change after J–V measurement. Application of a linear fit to the data gives a decay rate of 9.82% per 1000 h (the decay rate is 5% per 1000 h for 50% duty cycle). This decay rate would correspond to a lifetime of 5000 h. Fig. 7b shows the change of J–V characteristics (AM1.5G solar simulation) of Plextronics’ module (3070) after being illuminated under 1 Sun Xe-lamp for 648 and 1320 h. The module performance is essentially unchanged, typically 73%, within the variation of test error, and indicates that the module is very stable.
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5. Conclusion
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With optimized materials synthesis, photoactive layer ink formulation, and module design and fabrication, we have demonstrated the largest OPV module to be certified at NREL. Using a blend of P3HT and PCBM on top of a proprietary HTL, the
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Fig. 6. Improved lamp spectrum with Xe lamp.
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Fig. 7. (a) Normalized power output of module 3070 under Xe lamp. This decay rate translates to 45000 h of lifetime to T80 at 50% duty cycle with 1 Sun illumination. Data were smoothed (reduction of data points and removing of outliers) by interpolation in Origin software. (b) AM1.5G solar simulation J–V characteristics of module 3070 operated for 648 and 1320 h under 1 Sun Xe lamp.
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module exhibited 1.1% efficiency over its total area of 233 cm2. A state-of-the-art lifetime measurement set-up was used to evaluate the module and the decay rate was observed to correspond to a lifetime of 5000 h.
Acknowledgements Plextronics would like to thank the PV Performance Characterization Team at NREL, headed by Dr. Keith Emory for the cell and module measurements shown in Fig. 5.
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