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Strong Photocurrent Enhancements in Highly Efficient Flexible Organic Solar Cells by Adopting a Microcavity Configuration Kung-Shih Chen, Hin-Lap Yip, José-Francisco Salinas, Yun-Xiang Xu, Chu-Chen Chueh, and Alex K.-Y. Jen* Organic photovoltaics (OPV) based technology has potential to provide cost effective solar energy conversion from solutionprocessed light-weight, flexible, and large-area devices.[1–3] Limited by the low charge carrier mobility of organic semiconductors, the thickness of the photoactive layer in most OPV devices is often ∼100 nm in order to avoid serious charge recombination loss. However, this may cause insufficient light absorption and inhomogeneous electromagnetic field distribution due to coherent interference between the transmitted and reflected waves. Both effects can compromise the collection of the incident sunlight, especially at wavelengths with lower absorption coefficients such as those near the band edge of the semiconductors. To address this deficiency in absorption, various light-trapping approaches have been proposed. For example, a common approach is to introduce an optical spacer[4,5] to modulate the phases of the standing electromagnetic waves to eliminate absorption dead zones from the photoactive layers. Other strategies such as multi-junction stack,[6–8] plasmonics,[9–13] diffraction gratings,[14,15] photonic crystal,[16] microlens,[17] and wrinkled photonic structures[18] have also been proposed to alleviate this problem. In addition to the aforementioned strategies, the application of a microcavity architecture represents a promising alternative for OPV devices.[19–25] The term microcavity refers to an optical resonator structure that has a spacer layer sandwiched by two reflecting faces. Light bounded inside a microcavity travels back and forth between the reflectors until getting absorbed or escaping from the device. Due to coherent interference, radiations having the resonant frequencies will be reinforced and the other out-of-phase waves will be depressed. Such improved optical confinement through multi-reflection can extend the length of absorption path and potentially reduce collection loss without using thick photoactive layers. Although the stratified structures of OPV devices resembles a microcavity geometry, the commonly used highly transparent conducting electrode such as indium tin oxide (ITO) in OPV devices will allow the unabsorbed photons to escape from the
K.-S. Chen, Dr. H.-L. Yip, J.-F. Salinas, Dr. Y.-X. Xu, Dr. C.-C. Chueh, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington Seattle, Washington 98195-2120, USA E-mail: [email protected]
DOI: 10.1002/adma.201306323
Adv. Mater. 2014, DOI: 10.1002/adma.201306323
devices easily. This makes the ordinary OPV devices function as a very weak and inefficient microcavity. A stronger microcavity can be realized by replacing the ITO electrode with a more reflective transparent metal thin film[26,27] to enhance the optical confinement and eliminate problems associated with ITO electrode, such as poor flexibility, limited lateral conductance, high temperature processing, and low natural reserve of indium which strongly limit the development of OPV devices.[28,29] However, unlike the electrically pumped optoelectronic devices such as organic light-emitting diodes (OLED),[30–34] using highly reflective mirrors for optically pumped devices like OPV cells on both ends of the cavity can significantly hinder light entrance and degrade the device performance. Furthermore, strong resonance effects can lead to narrow spectral distribution, which is disadvantageous for harvesting broadband radiation from the sun. Therefore, careful control of the optical field distribution and resonance effects is very crucial for achieving balance between the entry and the confinement of the incident radiations. Recently, the performance of OPV devices adopting the ultrathin metal films (UTMF) microcavity structures has been progressively improved to reach comparable power conversion efficiency (PCE) with those of regular ITO-based devices using the same photoactive organic semiconductors.[19,20,23,24,27,35] These encouraging results have affirmed the feasibility of using UTMF electrodes to replace ITO in OPV and many other optoelectronic devices. However, the optical resonant cavity approach hardly demonstrates significant improvements in enhancing light collection for high efficiency OPV devices like other aforementioned light-trapping approaches. In this Communication, we report OPV devices with an ITOfree microcavity structure that reach high PCE of 8.50% on both glass and flexible plastic substrates, which corresponds to ∼20% improvement in PCE when compared to the ITO-based devices. The significantly enhanced performance is ascribed to the substantially improved photon collection by resonant microcavity structure, which contributes to improved photocurrent compared with devices built on ITO-coated substrates. Optical models based on the transfer matrix method (TMM) have been employed to assist understanding and designing the OPV devices. The results from strongly enhanced absorption and the possibility to use ITO-free structure show the promise of applying the microcavity architecture on OPV devices. Throughout this work, bulk-heterojunction (BHJ) blends of the semiconducting polymer PIDTT-DFBT[36] and [6,6]-phenylC71-butyric acid methyl-ester (PC71BM) are employed to
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Figure 1. Device configurations studied in this work: (a) Normal ITObased OPV device configuration (b) Microcavity configuration using semitransparent Ag film and top-capping light incoupling layer as the optical incident electrode.
demonstrate the microcavity effect. Previously, this BHJ system with a normal ITO-based device configuration (Figure 1) has been shown to reach a relatively high PCE of 7.03%. Although these devices have shown high open-circuit voltage (Voc ∼0.96 V), balanced charge transport, and complementary absorption of the two photoactive materials, there is still room to further improve their performance.[36] The relatively thin photoactive layer (∼80 nm) and low loading of the semiconducting polymer PIDTT-DFBT (25 wt%) in the photoactive blend may not be sufficient to harvest all the incident light. To investigate how incident light is collected in the aforementioned PIDTT-DFBT:PC71BM device (Figure 1), an optical model based on the TMM is established. TMM has been demonstrated as a useful tool for understanding the optical field distribution within the layer structures of OPV devices to estimate the potential achievable photocurrent. The detailed device configuration for simulation is the following: glass (2 mm)/ITO (120 nm)/ PEDOT:PSS (50 nm)/PIDTT-DFBT:PC71BM (0 – 1 µm)/Bis-C60 salt (8 nm)/Ag (120 nm). PEDOT:PSS represents the conducting
Figure 2. Calculated lossless photocurrent (Jph_100%IQE) and the measured short-circuit current (Jsc) of the PIDTT-DFBT:PC71BM OPV devices in the normal ITO-based device configuration plotted against the thickness of the BHJ layer (tBHJ).
2
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polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) for hole transport, and Bis-C60 salt is a dual-function fullerene surfactant which simultaneously modifies the work-function of the metal electrode and transports electron.[37,38] Each composing layer is represented by its complex refractive indices (ñ(λ) = n(λ) + ik(λ)) acquired by the spectral ellipsometric technique, which are shown in Figure S1 in the supporting information (SI). The light source is assumed to be the AM 1.5 global solar spectrum of one sun intensity (1000 W m–2). By assuming a unity photon-to-electron conversion, i.e., internal quantum efficiency (IQE) = 100% at all wavelengths, we can calculate the theoretical lossless photocurrent (Jph-100%IQE) of each simulated condition. The thickness of the photoactive layer (tBHJ) is varied from 0 to 1 µm and the dependence of Jph-100%IQE on tBHJ are plotted in Figure 2. The Jph-100%IQE curve in Figure 2 shows common oscillating characteristics due to coherent interference.[39,40] The best condition for thin absorber layer appears to be at tBHJ = 70 nm with Jph-100%IQE reaching 13.97 mA cm–2. On the other hand, the Jph-100%IQE of device with thick absorber layer (tBHJ = 1 µm) can reach > 19 mA cm–2. The large photocurrent variation between different thicknesses of absorber layers in devices clearly reveals the substantial absorption loss when the tBHJ is
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Strong Photocurrent Enhancements in Highly Efficient Flexible Organic Solar Cells by Adopting a Microcavity Configuration Kung-Shih Chen, Hin-Lap Yip, José-Francisco Salinas, Yun-Xiang Xu, Chu-Chen Chueh, and Alex K.-Y. Jen* Organic photovoltaics (OPV) based technology has potential to provide cost effective solar energy conversion from solutionprocessed light-weight, flexible, and large-area devices.[1–3] Limited by the low charge carrier mobility of organic semiconductors, the thickness of the photoactive layer in most OPV devices is often ∼100 nm in order to avoid serious charge recombination loss. However, this may cause insufficient light absorption and inhomogeneous electromagnetic field distribution due to coherent interference between the transmitted and reflected waves. Both effects can compromise the collection of the incident sunlight, especially at wavelengths with lower absorption coefficients such as those near the band edge of the semiconductors. To address this deficiency in absorption, various light-trapping approaches have been proposed. For example, a common approach is to introduce an optical spacer[4,5] to modulate the phases of the standing electromagnetic waves to eliminate absorption dead zones from the photoactive layers. Other strategies such as multi-junction stack,[6–8] plasmonics,[9–13] diffraction gratings,[14,15] photonic crystal,[16] microlens,[17] and wrinkled photonic structures[18] have also been proposed to alleviate this problem. In addition to the aforementioned strategies, the application of a microcavity architecture represents a promising alternative for OPV devices.[19–25] The term microcavity refers to an optical resonator structure that has a spacer layer sandwiched by two reflecting faces. Light bounded inside a microcavity travels back and forth between the reflectors until getting absorbed or escaping from the device. Due to coherent interference, radiations having the resonant frequencies will be reinforced and the other out-of-phase waves will be depressed. Such improved optical confinement through multi-reflection can extend the length of absorption path and potentially reduce collection loss without using thick photoactive layers. Although the stratified structures of OPV devices resembles a microcavity geometry, the commonly used highly transparent conducting electrode such as indium tin oxide (ITO) in OPV devices will allow the unabsorbed photons to escape from the
K.-S. Chen, Dr. H.-L. Yip, J.-F. Salinas, Dr. Y.-X. Xu, Dr. C.-C. Chueh, Prof. A. K.-Y. Jen Department of Materials Science and Engineering University of Washington Seattle, Washington 98195-2120, USA E-mail: [email protected]
DOI: 10.1002/adma.201306323
Adv. Mater. 2014, DOI: 10.1002/adma.201306323
devices easily. This makes the ordinary OPV devices function as a very weak and inefficient microcavity. A stronger microcavity can be realized by replacing the ITO electrode with a more reflective transparent metal thin film[26,27] to enhance the optical confinement and eliminate problems associated with ITO electrode, such as poor flexibility, limited lateral conductance, high temperature processing, and low natural reserve of indium which strongly limit the development of OPV devices.[28,29] However, unlike the electrically pumped optoelectronic devices such as organic light-emitting diodes (OLED),[30–34] using highly reflective mirrors for optically pumped devices like OPV cells on both ends of the cavity can significantly hinder light entrance and degrade the device performance. Furthermore, strong resonance effects can lead to narrow spectral distribution, which is disadvantageous for harvesting broadband radiation from the sun. Therefore, careful control of the optical field distribution and resonance effects is very crucial for achieving balance between the entry and the confinement of the incident radiations. Recently, the performance of OPV devices adopting the ultrathin metal films (UTMF) microcavity structures has been progressively improved to reach comparable power conversion efficiency (PCE) with those of regular ITO-based devices using the same photoactive organic semiconductors.[19,20,23,24,27,35] These encouraging results have affirmed the feasibility of using UTMF electrodes to replace ITO in OPV and many other optoelectronic devices. However, the optical resonant cavity approach hardly demonstrates significant improvements in enhancing light collection for high efficiency OPV devices like other aforementioned light-trapping approaches. In this Communication, we report OPV devices with an ITOfree microcavity structure that reach high PCE of 8.50% on both glass and flexible plastic substrates, which corresponds to ∼20% improvement in PCE when compared to the ITO-based devices. The significantly enhanced performance is ascribed to the substantially improved photon collection by resonant microcavity structure, which contributes to improved photocurrent compared with devices built on ITO-coated substrates. Optical models based on the transfer matrix method (TMM) have been employed to assist understanding and designing the OPV devices. The results from strongly enhanced absorption and the possibility to use ITO-free structure show the promise of applying the microcavity architecture on OPV devices. Throughout this work, bulk-heterojunction (BHJ) blends of the semiconducting polymer PIDTT-DFBT[36] and [6,6]-phenylC71-butyric acid methyl-ester (PC71BM) are employed to
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. Device configurations studied in this work: (a) Normal ITObased OPV device configuration (b) Microcavity configuration using semitransparent Ag film and top-capping light incoupling layer as the optical incident electrode.
demonstrate the microcavity effect. Previously, this BHJ system with a normal ITO-based device configuration (Figure 1) has been shown to reach a relatively high PCE of 7.03%. Although these devices have shown high open-circuit voltage (Voc ∼0.96 V), balanced charge transport, and complementary absorption of the two photoactive materials, there is still room to further improve their performance.[36] The relatively thin photoactive layer (∼80 nm) and low loading of the semiconducting polymer PIDTT-DFBT (25 wt%) in the photoactive blend may not be sufficient to harvest all the incident light. To investigate how incident light is collected in the aforementioned PIDTT-DFBT:PC71BM device (Figure 1), an optical model based on the TMM is established. TMM has been demonstrated as a useful tool for understanding the optical field distribution within the layer structures of OPV devices to estimate the potential achievable photocurrent. The detailed device configuration for simulation is the following: glass (2 mm)/ITO (120 nm)/ PEDOT:PSS (50 nm)/PIDTT-DFBT:PC71BM (0 – 1 µm)/Bis-C60 salt (8 nm)/Ag (120 nm). PEDOT:PSS represents the conducting
Figure 2. Calculated lossless photocurrent (Jph_100%IQE) and the measured short-circuit current (Jsc) of the PIDTT-DFBT:PC71BM OPV devices in the normal ITO-based device configuration plotted against the thickness of the BHJ layer (tBHJ).
2
wileyonlinelibrary.com
polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) for hole transport, and Bis-C60 salt is a dual-function fullerene surfactant which simultaneously modifies the work-function of the metal electrode and transports electron.[37,38] Each composing layer is represented by its complex refractive indices (ñ(λ) = n(λ) + ik(λ)) acquired by the spectral ellipsometric technique, which are shown in Figure S1 in the supporting information (SI). The light source is assumed to be the AM 1.5 global solar spectrum of one sun intensity (1000 W m–2). By assuming a unity photon-to-electron conversion, i.e., internal quantum efficiency (IQE) = 100% at all wavelengths, we can calculate the theoretical lossless photocurrent (Jph-100%IQE) of each simulated condition. The thickness of the photoactive layer (tBHJ) is varied from 0 to 1 µm and the dependence of Jph-100%IQE on tBHJ are plotted in Figure 2. The Jph-100%IQE curve in Figure 2 shows common oscillating characteristics due to coherent interference.[39,40] The best condition for thin absorber layer appears to be at tBHJ = 70 nm with Jph-100%IQE reaching 13.97 mA cm–2. On the other hand, the Jph-100%IQE of device with thick absorber layer (tBHJ = 1 µm) can reach > 19 mA cm–2. The large photocurrent variation between different thicknesses of absorber layers in devices clearly reveals the substantial absorption loss when the tBHJ is
