- Email: [email protected]
Highly sensitive non-contact shunt detection of organic photovoltaic modules
Solar Energy Materials & Solar Cells 101 (2012) 176–179
Contents lists available at SciVerse ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Highly sensitive non-contact shunt detection of organic photovoltaic modules Jonas Bachmann a,n, Claudia Buerhop-Lutz a, Roland Steim b, Pavel Schilinsky b, Jens A. Hauch b, Eitan Zeira b, Brabec Christoh J. a,c a
The Bavarian Center for Applied Energy Research (ZAE Bayern), Am Weichselgarten 7, 91058 Erlangen, Germany Konarka Technologies GmbH, Landgrabenstr. 94, 90443 N¨ urnberg, Germany c Chair of Materialscience VI, Institute of Materials for Electronics and Energy Technologies (I-MEET), Friedrich-Alexander University, Martensstraße 7, 91058 Erlangen, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2011 Received in revised form 19 January 2012 Accepted 20 January 2012 Available online 21 February 2012
Organic photovoltaics (OPV) have been shown to have excellent indoor efficiency at light intensities as low as one 10 W/m2. To fully explore their low light efficiency potential, the density as well as severeness of shunts must be kept as low as possible. In this communication we discuss a novel thermography method allowing to image the shunt distribution in organic solar modules with no electrical contact to the module. This thermography method is based on light excitation (ILIT—illuminated lock in thermography) instead of dark current excitation (DLIT—dark lock-in thermography). Comparison of ILIT with DLIT images reveals the equivalence of these two modes for shunt detection, with ILIT having the advantage of being performed in non-contact mode. & 2012 Elsevier B.V. All rights reserved.
Keywords: Lock-in thermography Shunt Organic solar cell
1. Introduction Recent studies identified hot spots as sites with a locally increased dark current flow under reverse injection [1]. Since this local currents are leading to a local heating of the cell at the position of the shunt, thermographic imaging methods are very powerful to detect and visualize the hot spot distribution in organic solar cells [1–3]. The detection limit and also the spatial resolution of thermographic imaging with IR cameras was considerably improved by a lock-in algorithm using several thermographic images taken within defined time intervals [4,5]. Here, a pulsed bias is used to modulate the dissipated power of the electrical active layers of the cell (module) which results in a modulated surface temperature of the device at a selected lock-in frequency, with the lock-in algorithm being exclusively sensitive to temperature variations modulated at this frequency. This method applied to solar cells is typically referred to as dark lock-in thermography (DLIT) or also voltage modulated LIT (VOMO-LIT) [6] and is correlated with the dark I–V characteristics of the sample. But excitation of solar-cells is not limited to voltage modulation. Using a modulated light source to drive the measurement, this method is typically referred to as illuminated LIT (ILIT) [5] or light modulated LIT (LIMO-LIT) [6], has the advantage of not needing an electrical contact to the sample.
n
Corresponding author. Tel.: þ49 913193698223. E-mail address: [email protected] (J. Bachmann).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2012.01.022
In terms of quality control, this could also hold true for semi-finished cells, i.e. cells and modules without the top electrode.
2. Experimental All thermographic measurements were done in an opaque measurement station. The setup is shown in Fig. 1. The thermal radiation of the samples was detected by an infrared camera (Taurus 110k SM pro, Ircam GmbH) equipped with a cooled focal plane array of MCT-detectors (mercury, cadmium, tellurium), sensitive to IR radiation between 2 mm and 5 mm. The details of the measurement principle, lock-in algorithm and image calculation, are best reviewed by Breitenstein [4]. Briefly, the major benefits of the lock-in thermography imaging method can be summarized as follows: first, all non-modulated, unwanted signals (such as reflections of ambient sources) are completely suppressed by the lock-in filter, and only temperature differences correlated to the modulation are observed. Second, statistical noise can be drastically reduced by increasing the total number of measured lock-in periods. A power amplified function generator (Agilent 3120) was used to generate the square wave voltage modulation required for the DLIT measurement. For the ILIT measurements, an array of water cooled LEDs (Ediline II, COB, 5 W, cold white, Edison) was used as excitation source, optical output power approximately 40 W/m2, while signal detection was realized with the DLIT setup. Parasitic IR radiation from the LEDs was filtered by an 3 cm thick PMMA
J. Bachmann et al. / Solar Energy Materials & Solar Cells 101 (2012) 176–179
trigger signal
177
data
3,000 Hz function generator
square wave signal
personal computer for lock-in calculation, camera control
power amplifier
sample (organic photovoltaic module)
infrared filter (visually transparent)
25°C
cryostat LED
LED
LED
heat sink Fig. 1. Schematic of the measurement station used for presented LIT measurements. DLIT measurement setup makes no use of led light sources, instead the power is delivered directly to the module integrated contacts. ILIT measurements use LEDs for optical excitation of the sample. The samples are put up side down, so the camera detects the infrared radiation emitted by the backside of the modules.
encapsulant (back)
of 5.5 V. Prior to measurements all modules were light soaked for 15 min by an AM 1.0 100 W/m2 halogen bulb lamp array, operating at approximately 2500 K.
opaque metal layer 3. Results and discussion
electron blocking layer bulk heterojunction-layer hole blocking layer transparent conductive oxide layer encapsulant (front) Fig. 2. Cross-section schematic of the modules.
plate. The investigated organic modules were provided by Konarka Technologies Incorporated (KTI). Details on the Konarka technology can be found in various publications [7,1,8]. A typical I–V characteristic measured with a So1A Class ABB Solar Simulator (Oriel) is presented in Fig. 3 (image F). A schematic cross section of the modules is presented in Fig. 2. The module contains 10 cells in series connection and has a total active area of 168 cm2. The open circuit voltage of the module is in the order
Several organic modules were investigated with ILIT and DLIT. Three different voltage regimes were identified to give the most relevant information for the DLIT measurements: regime 1 was chosen at low reverse bias between 4 V and 0 V, where shunts determine the jV characteristics. The second regime was chosen between 0 V and þ4 V, and contains information on shunts as well as on the cell’s injection properties. Finally, the third regime was placed at a bias higher than the threshold voltage of the diode. Thus, the regime between 0 V and þ8 V dominantly gives information about the injection properties. By switching between forward and reverse bias, series resistance losses (high forward bias) can be clearly differentiated from leakage losses (low reverse bias). ILIT measurements were run under low light excitation, either at open circuit voltage or at short circuit current conditions, with the module contacts left either open (Voc) or short circuited (Isc). A full set of LIT images of one module is presented in Fig. 3. Measurement times of the images presented in Fig. 3 are adjusted in a way to guarantee not to miss any thermal anomaly present at the given parameters. The hot spots visualized in Fig. 3A,B are of a very weak nature, this can be recognized by a still noisy background. Measurement time for all images was 900 s, lock-in frequency was set to 1 Hz. The ILIT results (Fig. 3D,E) are dominated by the local none-radiative recombination of excited charge carriers, resulting in an offset signal of the active area. This local offset covers
178
J. Bachmann et al. / Solar Energy Materials & Solar Cells 101 (2012) 176–179
1
2
3
4
5
6
7
8
9 10
module current [mA]
300
200
dark illuminated
100
0
5
-100
voltage [V] Fig. 3. Amplitude LIT images of an organic solar module, measurement time 900 s, lock-in frequency 1 Hz. (A) DLIT—low reverse bias (0 V to 4 V); (B) DLIT—low forward bias (0 V to þ 4 V); (C) DLIT—high forward bias (0 V to þ 8 V); (D) ILIT—Isc (E) ILIT—Voc; (F) I–V characteristic. The rectangle marks an erroneous detector area. The triangle marks hot spots, which are just detectable at positive biases. The circles mark hot spots which are difficult to visualize under forward bias and ILIT, albeit they are present in all images. The temperature resolution of each image is identical, but for visualization purposes the color scaling has been optimized individually. The spatial resolution is 350 mm=pixel. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
some of the very weakest hot spots (marked with circles) as measured by low bias DLIT because the results presented here are optimized for visualization purposes. This is always a compromise, because it is impossible to optimize for both weak and strong signals in one and the same image at the same time. Classic Shunts, i.e. ohmic leakage paths bypassing the diode, are detectable at low reverse bias and low forward bias. It was
found recently that DLIT imaging is less sensitive to shunts under high forward injection conditions [3]. At high current injection other parasitic losses like the series resistance of the semiconductor and the electrodes, the contact resistance at the interconnect regime as well as the sheet resistance of the electrodes become current limiting. As a consequence, heat dissipation by series resistance losses are in competition with heat dissipation at
J. Bachmann et al. / Solar Energy Materials & Solar Cells 101 (2012) 176–179
shunts. Overall, this results in an inhomogeneous current allocation across the module, dominated by the sheet resistances of the electrodes and the module’s interconnect resistance. Only major shunts shortening the two electrodes remain visible in DLIT at the injection regime. In the following we will analyze the DLIT pictures. Fig. 3A (reverse bias) shows a series of hot spots, which are found with decreasing intensity at increasing forward bias (Fig. 3B,C), as expected by theory. With moderate forward bias two hot spots are found in the top left and bottom right corner, which are not observed under reverse bias. Such features are not fully understood, but are typically interpreted as regions where the series resistance of the semiconductor is lowered. However, the origin of hot spots as well as the nature of the shunts in organic solar cells will be discussed in a separate manuscript. In summary, we define a classic shunt as a region in the device which allows currents to bypass the diode. Such shunts can be uniquely identified by comparing the DLIT images under low forward bias with the one under low reverse bias. Hot spots observed in both imaging modes are classified as a classic shunt. Next, we analyze the ILIT images shown in Fig. 3 (images D, E). The most striking observation is that both ILIT images show a kind of photographic image of the OPV module. Details like the electrodes, the interconnection regimes as well as the bus bars are clearly recognized. A more detailed analysis suggests that both electrodes, the transparent ITO as well as the opaque Ag electrode get considerably heated by the white light illumination from the LEDs. As such, the ILIT images under both conditions are dominated by the heat emission of the electrodes according to their specific heat conductivity and emissivity of the electrodes. A significant difference between the Isc and Voc is notified when analyzing the hot spots. Image D (Isc condition) does not (or only very weakly) show hot spots. In contrast to that, Voc measurements (image E) visualizes a series of hot spots. A detailed comparison with the DLIT images confirms that all hot spots found in the ILIT Voc mode are also existing in the DLIT reverse bias or DLIT low forward bias mode and vice versa. We have carried out this series of thermographic measurements at five different measurement conditions for several modules, and always found the same trend regarding shunt imaging. In all cases the ILIT Voc image was representative for the shunt distribution in the organic photovoltaic modules. In summary, we conclude that the ILIT Voc mode is a highly sensitive measurement mode for detecting shunts. Moreover, ILIT Voc was found to be identical effective for shunt hunting as the DLIT reverse bias or DLIT low forward bias mode, but having the advantage of being a non-contact measurement mode. In the following, we will comment on the difference between the ILIT Isc and ILIT Voc images. At first glance it seems surprising that ILIT Isc is so insensitive to shunts. The main difference between Isc and Voc conditions are the place where the photoinduced carriers recombine. In the case of Isc, the dominant part of
179
the carriers is extracted by the electrodes and can recombine in the outer circuitry. These carriers are lost for thermographic imaging. Voc conditions differ significantly to that, since no potential is available to extract the charge carriers. In that case, all photo-induced carriers have to recombine within the semiconductor layer or at the interface between the electrodes and the semiconductor. All photo-generated charge carriers recombine within the active area of the cell, and the heat dissipated by their recombination can be detected in the thermographic image. While this argumentation explains the difference between the ILIT Isc and Voc images, it does not explain the detailed mechanism how recombination of photo-induced charge carriers can lead to an increased heat dissipation in shunts.
4. Conclusions In summary, we have introduced ILIT Voc as a new thermographic imaging mode to visualize shunts in organic solar modules. ILIT Voc was proven to be equivalent effective in shunt hunting as the well established DLIT modes, but with having the advantage of allowing non-contact measurements. This establishes ILIT Voc as a powerful imaging mode for quality control of organic photovoltaic module manufacturing. Moreover, ILIT Voc is compatible to sheet based as well as roll to roll based production technologies. The expansion of this method to semi-finished cells (i.e. before deposition of the top electrode) will be investigated in the near future as a highly interesting extension of this imaging mode.
References [1] R. Steim, S.A. Choulis, P. Schilinsky, U. Lemmer, C. Brabec, Formation and impact of hot spots on the performance of organic photovoltaic cells, Applied Physics Letters 94 (2009) 43303–43304. [2] H. Hoppe, J. Bachmann, B. Muhsin, K.H. Drue, I. Riedel, G. Gobsch, C. BuerhopLutz, C.J. Brabec, V. Dyakonov, Quality control of polymer solar modules by lock-in thermography, Journal of Applied Physics 107 (2010) 14504–14505. [3] J. Bachmann, C. Buerhop-Lutz, C. Deibel, I. Riedel, H. Hoppe, C.J. Brabec, V. Dyakonov, Organic solar cells characterized by dark lock-in thermography, Solar Energy Materials and Solar Cells 94 (2010) 642–647. [4] O. Breitenstein, M. Langenkamp, Lock-in Thermography, Springer-Verlag, Berlin, Heidelberg, New York, 2003. 3-540-00405-X. [5] J. Isenberg, W. Warta, Realistic evaluation of power losses in solar cells by using thermographic methods, Journal of Applied Physics 95 (2004) 5200–5209. [6] M. Kaes, S. Seren, T. Pernau, G. Hahn, Light-modulated lock-in thermography for photosensitive pn-structures and solar cells, Progress in Photovoltaics: Research and Applications 12 (2004) 355–363. [7] R. Steim, S.A. Choulis, P. Schilinsky, C.J. Brabec, Interface modification for highly efficient organic photovoltaics, Applied Physics Letters 92 (2008) 093303, doi:10.1063/1.2885724. [8] C. Waldauf, M. Morana, P. Denk, P. Schilinsky, K. Coakley, S.A. Choulis, C.J. Brabec, Highly efficient inverted organic photovoltaics using solution based titanium oxide as electron selective contact, Applied Physics Letters 89 (2006) 233513–233517.
Contents lists available at SciVerse ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Highly sensitive non-contact shunt detection of organic photovoltaic modules Jonas Bachmann a,n, Claudia Buerhop-Lutz a, Roland Steim b, Pavel Schilinsky b, Jens A. Hauch b, Eitan Zeira b, Brabec Christoh J. a,c a
The Bavarian Center for Applied Energy Research (ZAE Bayern), Am Weichselgarten 7, 91058 Erlangen, Germany Konarka Technologies GmbH, Landgrabenstr. 94, 90443 N¨ urnberg, Germany c Chair of Materialscience VI, Institute of Materials for Electronics and Energy Technologies (I-MEET), Friedrich-Alexander University, Martensstraße 7, 91058 Erlangen, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2011 Received in revised form 19 January 2012 Accepted 20 January 2012 Available online 21 February 2012
Organic photovoltaics (OPV) have been shown to have excellent indoor efficiency at light intensities as low as one 10 W/m2. To fully explore their low light efficiency potential, the density as well as severeness of shunts must be kept as low as possible. In this communication we discuss a novel thermography method allowing to image the shunt distribution in organic solar modules with no electrical contact to the module. This thermography method is based on light excitation (ILIT—illuminated lock in thermography) instead of dark current excitation (DLIT—dark lock-in thermography). Comparison of ILIT with DLIT images reveals the equivalence of these two modes for shunt detection, with ILIT having the advantage of being performed in non-contact mode. & 2012 Elsevier B.V. All rights reserved.
Keywords: Lock-in thermography Shunt Organic solar cell
1. Introduction Recent studies identified hot spots as sites with a locally increased dark current flow under reverse injection [1]. Since this local currents are leading to a local heating of the cell at the position of the shunt, thermographic imaging methods are very powerful to detect and visualize the hot spot distribution in organic solar cells [1–3]. The detection limit and also the spatial resolution of thermographic imaging with IR cameras was considerably improved by a lock-in algorithm using several thermographic images taken within defined time intervals [4,5]. Here, a pulsed bias is used to modulate the dissipated power of the electrical active layers of the cell (module) which results in a modulated surface temperature of the device at a selected lock-in frequency, with the lock-in algorithm being exclusively sensitive to temperature variations modulated at this frequency. This method applied to solar cells is typically referred to as dark lock-in thermography (DLIT) or also voltage modulated LIT (VOMO-LIT) [6] and is correlated with the dark I–V characteristics of the sample. But excitation of solar-cells is not limited to voltage modulation. Using a modulated light source to drive the measurement, this method is typically referred to as illuminated LIT (ILIT) [5] or light modulated LIT (LIMO-LIT) [6], has the advantage of not needing an electrical contact to the sample.
n
Corresponding author. Tel.: þ49 913193698223. E-mail address: [email protected] (J. Bachmann).
0927-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2012.01.022
In terms of quality control, this could also hold true for semi-finished cells, i.e. cells and modules without the top electrode.
2. Experimental All thermographic measurements were done in an opaque measurement station. The setup is shown in Fig. 1. The thermal radiation of the samples was detected by an infrared camera (Taurus 110k SM pro, Ircam GmbH) equipped with a cooled focal plane array of MCT-detectors (mercury, cadmium, tellurium), sensitive to IR radiation between 2 mm and 5 mm. The details of the measurement principle, lock-in algorithm and image calculation, are best reviewed by Breitenstein [4]. Briefly, the major benefits of the lock-in thermography imaging method can be summarized as follows: first, all non-modulated, unwanted signals (such as reflections of ambient sources) are completely suppressed by the lock-in filter, and only temperature differences correlated to the modulation are observed. Second, statistical noise can be drastically reduced by increasing the total number of measured lock-in periods. A power amplified function generator (Agilent 3120) was used to generate the square wave voltage modulation required for the DLIT measurement. For the ILIT measurements, an array of water cooled LEDs (Ediline II, COB, 5 W, cold white, Edison) was used as excitation source, optical output power approximately 40 W/m2, while signal detection was realized with the DLIT setup. Parasitic IR radiation from the LEDs was filtered by an 3 cm thick PMMA
J. Bachmann et al. / Solar Energy Materials & Solar Cells 101 (2012) 176–179
trigger signal
177
data
3,000 Hz function generator
square wave signal
personal computer for lock-in calculation, camera control
power amplifier
sample (organic photovoltaic module)
infrared filter (visually transparent)
25°C
cryostat LED
LED
LED
heat sink Fig. 1. Schematic of the measurement station used for presented LIT measurements. DLIT measurement setup makes no use of led light sources, instead the power is delivered directly to the module integrated contacts. ILIT measurements use LEDs for optical excitation of the sample. The samples are put up side down, so the camera detects the infrared radiation emitted by the backside of the modules.
encapsulant (back)
of 5.5 V. Prior to measurements all modules were light soaked for 15 min by an AM 1.0 100 W/m2 halogen bulb lamp array, operating at approximately 2500 K.
opaque metal layer 3. Results and discussion
electron blocking layer bulk heterojunction-layer hole blocking layer transparent conductive oxide layer encapsulant (front) Fig. 2. Cross-section schematic of the modules.
plate. The investigated organic modules were provided by Konarka Technologies Incorporated (KTI). Details on the Konarka technology can be found in various publications [7,1,8]. A typical I–V characteristic measured with a So1A Class ABB Solar Simulator (Oriel) is presented in Fig. 3 (image F). A schematic cross section of the modules is presented in Fig. 2. The module contains 10 cells in series connection and has a total active area of 168 cm2. The open circuit voltage of the module is in the order
Several organic modules were investigated with ILIT and DLIT. Three different voltage regimes were identified to give the most relevant information for the DLIT measurements: regime 1 was chosen at low reverse bias between 4 V and 0 V, where shunts determine the jV characteristics. The second regime was chosen between 0 V and þ4 V, and contains information on shunts as well as on the cell’s injection properties. Finally, the third regime was placed at a bias higher than the threshold voltage of the diode. Thus, the regime between 0 V and þ8 V dominantly gives information about the injection properties. By switching between forward and reverse bias, series resistance losses (high forward bias) can be clearly differentiated from leakage losses (low reverse bias). ILIT measurements were run under low light excitation, either at open circuit voltage or at short circuit current conditions, with the module contacts left either open (Voc) or short circuited (Isc). A full set of LIT images of one module is presented in Fig. 3. Measurement times of the images presented in Fig. 3 are adjusted in a way to guarantee not to miss any thermal anomaly present at the given parameters. The hot spots visualized in Fig. 3A,B are of a very weak nature, this can be recognized by a still noisy background. Measurement time for all images was 900 s, lock-in frequency was set to 1 Hz. The ILIT results (Fig. 3D,E) are dominated by the local none-radiative recombination of excited charge carriers, resulting in an offset signal of the active area. This local offset covers
178
J. Bachmann et al. / Solar Energy Materials & Solar Cells 101 (2012) 176–179
1
2
3
4
5
6
7
8
9 10
module current [mA]
300
200
dark illuminated
100
0
5
-100
voltage [V] Fig. 3. Amplitude LIT images of an organic solar module, measurement time 900 s, lock-in frequency 1 Hz. (A) DLIT—low reverse bias (0 V to 4 V); (B) DLIT—low forward bias (0 V to þ 4 V); (C) DLIT—high forward bias (0 V to þ 8 V); (D) ILIT—Isc (E) ILIT—Voc; (F) I–V characteristic. The rectangle marks an erroneous detector area. The triangle marks hot spots, which are just detectable at positive biases. The circles mark hot spots which are difficult to visualize under forward bias and ILIT, albeit they are present in all images. The temperature resolution of each image is identical, but for visualization purposes the color scaling has been optimized individually. The spatial resolution is 350 mm=pixel. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
some of the very weakest hot spots (marked with circles) as measured by low bias DLIT because the results presented here are optimized for visualization purposes. This is always a compromise, because it is impossible to optimize for both weak and strong signals in one and the same image at the same time. Classic Shunts, i.e. ohmic leakage paths bypassing the diode, are detectable at low reverse bias and low forward bias. It was
found recently that DLIT imaging is less sensitive to shunts under high forward injection conditions [3]. At high current injection other parasitic losses like the series resistance of the semiconductor and the electrodes, the contact resistance at the interconnect regime as well as the sheet resistance of the electrodes become current limiting. As a consequence, heat dissipation by series resistance losses are in competition with heat dissipation at
J. Bachmann et al. / Solar Energy Materials & Solar Cells 101 (2012) 176–179
shunts. Overall, this results in an inhomogeneous current allocation across the module, dominated by the sheet resistances of the electrodes and the module’s interconnect resistance. Only major shunts shortening the two electrodes remain visible in DLIT at the injection regime. In the following we will analyze the DLIT pictures. Fig. 3A (reverse bias) shows a series of hot spots, which are found with decreasing intensity at increasing forward bias (Fig. 3B,C), as expected by theory. With moderate forward bias two hot spots are found in the top left and bottom right corner, which are not observed under reverse bias. Such features are not fully understood, but are typically interpreted as regions where the series resistance of the semiconductor is lowered. However, the origin of hot spots as well as the nature of the shunts in organic solar cells will be discussed in a separate manuscript. In summary, we define a classic shunt as a region in the device which allows currents to bypass the diode. Such shunts can be uniquely identified by comparing the DLIT images under low forward bias with the one under low reverse bias. Hot spots observed in both imaging modes are classified as a classic shunt. Next, we analyze the ILIT images shown in Fig. 3 (images D, E). The most striking observation is that both ILIT images show a kind of photographic image of the OPV module. Details like the electrodes, the interconnection regimes as well as the bus bars are clearly recognized. A more detailed analysis suggests that both electrodes, the transparent ITO as well as the opaque Ag electrode get considerably heated by the white light illumination from the LEDs. As such, the ILIT images under both conditions are dominated by the heat emission of the electrodes according to their specific heat conductivity and emissivity of the electrodes. A significant difference between the Isc and Voc is notified when analyzing the hot spots. Image D (Isc condition) does not (or only very weakly) show hot spots. In contrast to that, Voc measurements (image E) visualizes a series of hot spots. A detailed comparison with the DLIT images confirms that all hot spots found in the ILIT Voc mode are also existing in the DLIT reverse bias or DLIT low forward bias mode and vice versa. We have carried out this series of thermographic measurements at five different measurement conditions for several modules, and always found the same trend regarding shunt imaging. In all cases the ILIT Voc image was representative for the shunt distribution in the organic photovoltaic modules. In summary, we conclude that the ILIT Voc mode is a highly sensitive measurement mode for detecting shunts. Moreover, ILIT Voc was found to be identical effective for shunt hunting as the DLIT reverse bias or DLIT low forward bias mode, but having the advantage of being a non-contact measurement mode. In the following, we will comment on the difference between the ILIT Isc and ILIT Voc images. At first glance it seems surprising that ILIT Isc is so insensitive to shunts. The main difference between Isc and Voc conditions are the place where the photoinduced carriers recombine. In the case of Isc, the dominant part of
179
the carriers is extracted by the electrodes and can recombine in the outer circuitry. These carriers are lost for thermographic imaging. Voc conditions differ significantly to that, since no potential is available to extract the charge carriers. In that case, all photo-induced carriers have to recombine within the semiconductor layer or at the interface between the electrodes and the semiconductor. All photo-generated charge carriers recombine within the active area of the cell, and the heat dissipated by their recombination can be detected in the thermographic image. While this argumentation explains the difference between the ILIT Isc and Voc images, it does not explain the detailed mechanism how recombination of photo-induced charge carriers can lead to an increased heat dissipation in shunts.
4. Conclusions In summary, we have introduced ILIT Voc as a new thermographic imaging mode to visualize shunts in organic solar modules. ILIT Voc was proven to be equivalent effective in shunt hunting as the well established DLIT modes, but with having the advantage of allowing non-contact measurements. This establishes ILIT Voc as a powerful imaging mode for quality control of organic photovoltaic module manufacturing. Moreover, ILIT Voc is compatible to sheet based as well as roll to roll based production technologies. The expansion of this method to semi-finished cells (i.e. before deposition of the top electrode) will be investigated in the near future as a highly interesting extension of this imaging mode.
References [1] R. Steim, S.A. Choulis, P. Schilinsky, U. Lemmer, C. Brabec, Formation and impact of hot spots on the performance of organic photovoltaic cells, Applied Physics Letters 94 (2009) 43303–43304. [2] H. Hoppe, J. Bachmann, B. Muhsin, K.H. Drue, I. Riedel, G. Gobsch, C. BuerhopLutz, C.J. Brabec, V. Dyakonov, Quality control of polymer solar modules by lock-in thermography, Journal of Applied Physics 107 (2010) 14504–14505. [3] J. Bachmann, C. Buerhop-Lutz, C. Deibel, I. Riedel, H. Hoppe, C.J. Brabec, V. Dyakonov, Organic solar cells characterized by dark lock-in thermography, Solar Energy Materials and Solar Cells 94 (2010) 642–647. [4] O. Breitenstein, M. Langenkamp, Lock-in Thermography, Springer-Verlag, Berlin, Heidelberg, New York, 2003. 3-540-00405-X. [5] J. Isenberg, W. Warta, Realistic evaluation of power losses in solar cells by using thermographic methods, Journal of Applied Physics 95 (2004) 5200–5209. [6] M. Kaes, S. Seren, T. Pernau, G. Hahn, Light-modulated lock-in thermography for photosensitive pn-structures and solar cells, Progress in Photovoltaics: Research and Applications 12 (2004) 355–363. [7] R. Steim, S.A. Choulis, P. Schilinsky, C.J. Brabec, Interface modification for highly efficient organic photovoltaics, Applied Physics Letters 92 (2008) 093303, doi:10.1063/1.2885724. [8] C. Waldauf, M. Morana, P. Denk, P. Schilinsky, K. Coakley, S.A. Choulis, C.J. Brabec, Highly efficient inverted organic photovoltaics using solution based titanium oxide as electron selective contact, Applied Physics Letters 89 (2006) 233513–233517.