A vertically integrated solar-powered electrochromic window for energy efficient buildings.

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A Vertically Integrated Solar-Powered Electrochromic Window for Energy Efficient Buildings Aubrey L. Dyer, Rayford H. Bulloch, Yinhua Zhou, Bernard Kippelen, John R. Reynolds,* and Fengling Zhang* It is well recognized that the most abundant source for renewable energy is solar radiation, and with continued improvements in research and development, efforts in adopting photovoltaic (PV) technologies are at an all-time high. However, while solar radiation provides renewable sources for electricity, it is also one of the main causes for energy consumption in residential and commercial buildings through increased cooling demands in warm months and increased interior lighting needs to counteract added shading used to cut daytime glare.[1] One report estimates that 41% of the energy consumed in the U.S. in 2009 was in the buildings sector (commercial and residential) which is an increase of 48% from 1980.[1] Of this, 19% can be attributed to space cooling and lighting combined (37% is attributed to space heating and the remainder to water heating and consumer electronics/appliances).[1] A technology that holds much promise for decreasing both lighting and cooling energy use through modulation of transmitted light and solar heat is variable transmission electrochromic windows (ECWs).[2,3] The ECWs offer tunable shading, allowing lighting energy use savings of 48-67%, while decreasing annual peak cooling loads up to 19–26% when compared to efficient low-e

Dr. A. L. Dyer, R. H. Bulloch, Prof. J. R. Reynolds School of Chemistry and Biochemistry Center for Organic Photonics and Electronics Georgia Institute of Technology Atlanta, GA 30332, USA E-mail: [email protected] Prof. J. R. Reynolds School of Materials Science and Engineering Center for Organic Photonics and Electronics Georgia Institute of Technology Atlanta, GA 30332, USA Dr. Y. H. Zhou, Prof. B. Kippelen School of Electrical and Computer Engineering Center for Organic Photonics and Electronics Georgia Institute of Technology Atlanta, GA 30332, USA Dr. Y. H. Zhou Wuhan National Laboratory for Optoelectronics Huazhong University of Science and Technology Wuhan 430074, China Prof. F. Zhang Biomolecular and Organic Electronics Department of Physics Chemistry and Biology Linköping University 58183, Linköping, Sweden E-mail: [email protected]

DOI: 10.1002/adma.201401400

Adv. Mater. 2014, DOI: 10.1002/adma.201401400

windows.[4] However, while ECWs can offer the ability to lower cooling needs, these windows still require an external power source, offsetting some of the energy savings. To address this last point, there have been efforts towards integration of electrochromics with photovoltaic devices, resulting in so-called self-powered, or photoelectrochromic, devices.[5–11] Many device types incorporate the PV component externally, or at the periphery of the ECW,[6,7,11] while others incorporate it vertically, as part of the device stack.[8–10] This latter device type requires the PV to be highly transparent and is typically based on low-gap a-Si or dye-sensitized solar cells as the power-generating component and metal oxides (e.g., WO3) as the active electrochrome.[8–10] While these are early proof-ofprinciple demonstrations of the vertical self-powered window concept, inherent drawbacks remain as these examples require sputter-deposited layers, and/or an external power source to switch the device back to the bleached state. Here, we demonstrate a self-powered, solution processable and vertically integrated, polymeric variable transmission electrochromic/photovoltaic (EC/PV) device, where the EC and PV components share common transparent conducting polymer electrodes. Solution processability offers the advantage of processing under ambient conditions and the promise of high throughput roll-to-roll processing of fully flexible devices for retrofit integration. This EC/PV concept is shown in Figure 1 with device schematics and photographs. The device consists of two inverted PV cells in tandem with an ECD, which shares two transparent electrodes with the PV cells, as shown in Figure 1a. The PV cells (PV1 and PV2 in the schematic) are based on an inverted geometry with the photoactive layer comprised of a blend of the low band gap polymer, poly(diketopyrrolopyrroleterthiophene (PDPP3T), and phenyl-C61-butyric acid methyl ester (PC61BM) as previously reported.[12,13] To produce these cells, ITO surfaces were first converted to electron-collecting contacts via modification with solution-processed ZnO and poly(ethylenimine ethoxylate) (PEIE) (Electrodes 1 and 4).[14] The use of PEIE on the ZnO allows further reduction of the work function of the electron-collecting layer and improves the electron collection efficiency, which has previously been confirmed by Kyaw et al.[15] Next, the PDPP3T:PC61BM layer was deposited through spin casting (PV layers). This is followed by a layer of high-conductivity poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) spin cast on top to serve simultaneously as transparent hole-collecting electrodes for the PV cells and as transparent electrodes for the ECD portion of the device (labeled as electrodes 2 and 3). Fabrication of the EC portion was performed by airbrush spraying of two electrochromic layers onto exposed PEDOT:PSS

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Figure 1. Concept and structure of solar-powered electrochromic device. a) Schematic cross section structure of the EC/PV laminated vertical stack. The device consists of two inverted PV cells (PV1 and PV2) and an EC cell. The electrodes comprising the device are (Electrode 1) the electron-collecting electrode of the bottom PV cell, (Electrode 2) the hole-collecting electrode of the same PV cell/working electrode of the ECD, (Electrode 3) the hole-collecting electrode of the top PV cell/counter electrode of the ECD, and (Electrode 4) the electron-collecting electrode of the top PV cell. b) Schematic of device layer components (numbering corresponds to electrodes in a). c) Photograph and schematic of EC/PV self-powering concept showing connections required to cause bleaching (top) or coloring (bottom) of ECD under illumination.

PV electrodes (EC layers). The EC polymers are PProDOT(CH2OEtHx)2, referred to as ECP-Magenta, and PProDOPN-C18H37, referred to as MCCP. ECP-Magenta is the actively 2


coloring electrochrome—switching from highly absorptive magenta in the neutral state to highly transmissive colorless in the oxidized state, while MCCP exhibits minimal color in either redox state—providing only charge balance within the ECD.[16,17] The repeat unit structures for each of the polymeric materials used in the PV and EC portions of the device are provided in Figure S1. An ionically conducting gel electrolyte was then deposited between the two separate EC layers and the device laminated to complete the fully integrated EC/PV device as shown in Figure 1b. Device operation between colored and bleached states (Figure 1c and Video 1 in SI) is achieved through connection of the appropriate PV electrodes to the EC electrodes. The bleached state is induced (top schematic) by connecting electrode 1 (PV1) to electrode 3 (MCCP-coated electrode). This causes reduction of the MCCP layer and, to maintain charge balance, oxidation of the ECP-Magenta layer to result in the device bleached state as shown in the photograph to the right. In order to switch the device to the colored state, electrode 4 (PV2) is connected to electrode 2 (ECP-Magenta-coated electrode) to cause ECP-Magenta to reduce and color, while the MCCP layer oxidizes as shown in the bottom schematic and the photograph to the left of Figure 1c. Uniquely, this device provides vertical integration of solution-processed polymeric materials with common electrodes for different devices; a single PV cell providing sufficient voltage to drive the EC layers, and a high degree of visible transparency from the low bandgap photovoltaic polymer:PCBM layer. As can be seen in Figure 2a, a single cell has a transmittance at 550 nm (the peak wavelength of photopic sensitivity) of 76%, while two stacked cells maintain a high transmittance of 58%. This can also be seen in the inset photograph of a single cell. This high level of transparency is attributed both to the low bandgap and relatively narrow absorption profile of the photoactive polymer (PDPP3T), which absorbs in the NIR with a peak at 850 nm (with an optical bandgap of 1.37 eV), and the highly transparent electrode layers. As provided in Figure 2b (with values reported in Table 1) a PV cell, with an active area of 1 cm2, generates 0.70 V open-circuit voltage (Voc), ∼3 mA short-circuit current (Isc), and 0.8 mW at the Maximum Power Point (MPP) before application of an electrochromic layer by spray-casting. The initial modest performance of these cells is not unexpected as the active layer is thin (∼40 nm) for high transparency and relatively large area (1 cm2) and use of the less conductive PEDOT:PSS top electrode (as compared to metal electrodes). After application of the electrochromic layers (exemplified by ECP-Magenta onto PV cell 1), the cell performance drops, possibly due to sprayprocessing of the EC layers from an organic solvent. It should be noted that a decrease in current and voltage for the PV2 cell coated with MCCP was more than that for PV1 (Table 1). We attribute this to the low oxidation potential of MCCP resulting in electron exchange at the interface to oxidize the MCCP and reduce a thin layer of the PEDOT:PSS, decreasing the conductivity of the conducting polymer electrode and resulting in a drop in PV cell efficiency. It is evident that the desire for transparent PV cells (giving away photon absorption to allow for transparent windows) and orthogonal processing of multiple layers are at the expense of




Table 1. Semi-transparent photovoltaic device characteristics. PV cells

Voc [V]

Jsc [mA cm−2]

FF

Power [mW]

PV1 (before spray)a)

0.70

3.2

0.38

0.84

PV2 (before spray)a)

0.68

2.8

0.41

0.79

b)

PV1 (after spray)

0.66

3.2

0.28

0.61

PV2 (after spray)b)

0.42

1.8

0.26

0.20

PV1/PV2 tandem-Parallelc)

0.51

3.3

0.30

0.50

PV1/PV2 tandem-Seriesd)

1.04

1.4

0.38

0.54

a)PV device performance prior to addition of EC layer; b)PV device performance after spray-casting of EC layer; c)tandem PV devices in EC/PV stack connected in parallel as shown schematically in Figure S2a; d)tandem PV devices in EC/PV stack connected in series as shown schematically in Figure S2b.




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Figure 2. Current-Voltage Profile and Transmittance Spectra for Transparent Tandem Solar Cells used in EC/PV Stack. a) Spectral transmittance across the visible and near infrared for two PV cells (blue line) and for a single PV cell (red line) illustrating high level of transmittance of cells with inset photograph of a single PV cell. b) Current-Voltage curves for a single PV cell after coating with ECP-Magenta (triangles) and current-voltage curves for PV cells in complete EC/PV stack when connected in parallel (solid circles) and in series (open circles).

device efficiency.[18,19] This is exemplified in Figure S2 where the PV device characteristics while not powering the EC device are illustrated. On the other hand, while the efficiencies are not targeted to high values, they are sufficient to operate the EC device in a net positive power mode. When comparing the current, voltage, and power needs of the electrochromic cell to that delivered by the photovoltaic cells, it is apparent that the energy demands for electrochromic operation are far less. As is shown in Figure S3, the initial current demands of the electrochromic layers peaks at 2.3–2.9 mA (corresponding to a power of 3-4 mW) and quickly (within 2 seconds) decays to a background current of 0.2-0.3 µA (corresponding to a power of 20-33 µW). This current profile is typical of electrochemical devices of this type where a majority of the current demands are instantaneous and compromise of both redox and doublelayer charging currents. As the double-layer is established and redox process complete, a minimal background current can be applied to maintain the redox (i.e., color) state of the device. However, in thin layer electrochromic devices, such as the one employed here, there is sufficient electrochromic memory (the color of the device is maintained even with no voltage applied) that the device can be operated for longer periods of time with even lower power requirements. This is demonstrated in Figure S4, where the percent reflectance maintained after removal of the solar cell connections is monitored over a period of up to 75 s. The percent drift (the amount by which the reflectance decreases at the end of 75 s relative to when the photovoltaic connection is removed indicated by the dotted lines) is less than 5%, after allowing enough time (≥3 s) for the device to switch completely. With the low power demands of the EC cell and the electrochromic memory discussed, power needs for switching the electrochromic layers from fully colored to fully bleached (or reverse) and maintaining those colored states, for even a short amount of time, can be estimated. For example, to afford a switch from colored to bleached would require as little as 0.4 mW for a 1 cm2 device over a 5 s period of time (or energy of 2 mJ). This 5 s hold is more than enough time to allow for the EC device to fully bleach, as is shown in Figure 3a where the transmittance contrast saturates at 2 s of full illumination (inset). Figure S5a shows the time of illumination required to reach saturation as 3 s for the coloring process. When considering the electrochromic memory, a further 70 s of time can pass (or longer) with less than a 5% decrease in transmittance before a refresh voltage pulse is required from the solar cell. During this additional 70 s where the electrochromic cell is not powered, the energy generated by the PV devices can be utilized to power other functions. Further, given that there are two PV cells in tandem with the EC layers, these cells can be connected in either parallel or series to generate additional current or voltage as shown in Figure 2b and in Table 1. For example, when not powering the EC portion of the device, PV1 and PV2 can be connected in series to generate additional power to operate peripheral devices (e.g., lighting, fans, consumer electronics, etc.), or to charge a battery that would allow for operation of the windows when no lighting is present. This, in effect, allows the EC/PV windows to operate as net energy positive devices as the power generated by the PV cells is in excess of what is required for EC operation. It should be noted that in the

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55

25

50

20

45

Δ% T

Percent transmittance (% T)

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a)

40 35

15 10 5

30

0

1

2

3

4

5

6

Time (s)

25 20 15 10 5 0

400

500

600

700

Wavelength (nm)

b)

55 20

45

Δ% T

Percent transmittance (% T)

50 40 35 30

18 16 14

25

25

50

75

100

% Sun

20 15 10 5 0

400

500

600

700

Wavelength (nm) Figure 3. Transmittance spectra across the visible region for a fully assembled EC/PV device during bleaching. a) Full device spectra when illuminated for increasing time periods under AM 1.5 (1 sun) illumination. Filled square = fully colored state at start, open square = 1 s illumination, circle = 2 s, up-triangle = 3 s, down-triangle = 4 s, and diamond = 5 s. Inset shows delta percent transmittance (as measured at 550 nm) for various illumination times illustrating fully contrast achieved for 2 s of illumination under 1 sun. b) Full device spectra when illuminated for 3 s under varying intensities of light. Filled square = fully colored state at start, open square = 25% sun (i.e., 25 mW/cm2), circle = 50% sun (i.e., 50 mW/cm2), up-triangle = 75% sun (i.e., 75 mW/cm2), and downtriangle = 1 sun (i.e., 100 mW/cm2). Inset shows delta percent transmittance (as measured at 550 nm) for various illumination intensities.

tandem configuration, there are limitations in obtained power in both series and parallel connections. This can be attributed to the drop in OPV performance after spraying of the organic EC layers as the parallel/series measurements provided were performed on these altered cells. In a series connection, the combined current density is limited by the subcell with the smaller current (PV2 in this case) while in the parallel configuration, the overall voltage of the tandem configuration is limited by the subcell with the lower voltage (PV2 again). The result is a drop in combined overall power for both configurations. It is expected that as improvements are gained in preparing these layered devices, this loss in overall power will be minimized.

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Of course, it is understood that operation of PV windows occurs under conditions markedly different than those in typical laboratory settings. In fact, the angle at which the solar radiation hits a building window is not at normal incidence at midday, while typical laboratory devices are characterized at normal incidence. Add to this the fact that there are very few days that the illumination intensity is consistently 1 sun, which raises the question as to whether the EC/PV windows would operate with full efficiency under these non-ideal conditions. With that in mind, the EC/PV devices were characterized for contrast and switching speeds under illuminations that are less than 1 sun for 3 s of illumination (i.e., 75%, 50%, and 25% of 1 sun, correlating to 75, 50, and 25 mW/cm2 intensity). As is shown in Figures 3b and S5b, there is a difference in contrast achieved under reduced illumination when switching from the colored to bleached (Figure 3b) and bleached to colored (Figure S5b) states. The maximum contrast (20%) is achieved with intensities greater than 75 mW/cm2 when switching to the bleached state and full intensity (100 mW/cm2) is needed to achieve the maximum contrast when switching to the colored state (Figure S5b inset). Yet there is only a 4% difference in contrast when lower intensities (50 or 75 mW/cm2) are utilized. To gain a practical perspective on these intensities, we measured the solar irradiance under non-ideal conditions (i.e., different times of day in Atlanta during the month of December, from different angles) to allow correlation with how these devices would be expected to perform in real applications. For example, the measured intensity of solar radiation varied from 0.09 sun (facing East), to 0.20 sun (facing North), 0.40 sun (facing West), and 0.88 sun (facing South).[20] Additionally, we measured the solar irradiation in Southern Sweden (Latitude: +58.41) at 2 pm in early December with a measured value of 0.19 sun. To further characterize device performance under these conditions, we monitored the contrast after different illumination times under 25 mW/cm2 illumination shown in Figure S6. Even under this reduced illumination, full device contrast is achieved, albeit with longer illumination times required (6 s for bleaching and 3 s for coloring). This is not unexpected as the PV voltage (what drives EC operation) is less sensitive to illumination intensity and the PV current (what drives EC operation speed) is linearly dependent on illumination intensity, hence the longer illumination times required.[21–23] These results show that this device concept is promising for practical solarpowered, net energy positive windows for lighting and solar heat control for energy efficient buildings. However, it should be noted that while this is the first proof of concept demonstration of a polymeric, solution processed net energy positive EC/ PV device, further work is needed to improve device operation, optimize device components, increase device area, and incorporate fully solution processed and flexible device layers while understanding limitations on device lifetime and durability. Future work will tackle these improvements and challenges. In conclusion, we demonstrate the first solution processed polymer EC/PV window type device by vertically integrating a polymer ECD with two inverted transparent organic PV cells using EC and PV active materials that are all solution processable. Selectively connecting two of four electrodes of the EC/PV device, a self-powered ECD can be reversibly switched between transparent and colored states driven by two integrated PV cells.




Experimental Section Preparation of transparent PV cells: ITO-coated glass substrates (15 Ω sq−1, Colorado Concept Coatings LLC) were cleaned in sequential ultrasonic baths of detergent, distilled water, acetone, and isopropanol. The ZnO precursor was prepared by dissolving zinc acetate dihydrate (Sigma-Aldrich, 0.5 g) and ethanolamine (Aldrich, 0.14 g) in 5 mL 2-methoxyethanol (Sigma-Aldrich) and stirred overnight in air. Then the solution was spin coated on the plasma-treated ITO substrates at a speed of 3000 rpm and annealed at 150 °C in air for 1 h. The ZnO film was 30 nm thick. A polymer surface modification layer of polyethylenimine ethoxylate (Sigma-Aldrich, PEIE) was spin coated on top of the ITO/ ZnO samples at a speed of 5000 rpm and annealed at 100 °C in air for 10 min. The film thickness is nominally 10 nm derived from ellipsometry as previously demonstrated.[14] Then the samples were transferred into a N2-filled glove box to coat the photoactive layer PDPP3T (Solarmer Materials Inc.): phenyl-C61-butyric acid methyl ester (PCBM, Nano-C Inc.). The photoactive layer was prepared by spin coating from the overnight stirred solution of PDPP3T:PCBM (1:2, weight ratio) in a mixture of dichlorobenzene:chloroform,1,8-diiodooctane (79:16:5, v/v/v) solution with a total concentration of 18 mg ml−1 at a speed of 1000 rpm for 1 min. The photoactive layer was pumped for 10 min and annealed for 5 min at 75 °C to remove residual solvent. 5% dimethyl sulfoxide was added into PH1000 solution and stirred overnight at 300 rpm at room temperature to increase its conductivity. PH1000 solution was spin coated on the active layer at 1000 rpm for 15 s and annealed on a hot plate at 80 °C for 5 min in the glove box. Prior to the spin coating of PH1000, the surface of PDPP3T:PCBM was exposed for a short time (1 s) to plasma treatment to render the surface hydrophilic. Current-voltage (I-V) characteristics were measured inside a N2-filled glove box by using a source meter (Keithley Instruments 2400) controlled by a LabVIEW program. To test the solar cell properties under illumination, an Oriel lamp with an intensity of 100 mW/cm2 was used as the light source. Fabrication of electrochromic devices (ECDs): Synthesis and purification of the electrochromic polymers used in device fabrication, referred to as ECP-Magenta (26 kDa, PDI – 1.7) and MCCP (63 kDa, PDI – 1.7), has been described elsewhere.[8,11] Polymers were dissolved into toluene at a concentration of 2 mg mL−1, and passed through 0.45 µm PTFE syringe filters prior to spray casting. Solutions were then spray cast, using an Iwata-Eclipse HP-BC airbrush with nitrogen at a pressure of 20 psi, onto the outermost PEDOT:PSS layer of the transparent PV cells fabricated as described. Spray casting was performed under ambient conditions with the airbrush at an approximate distance of 10 cm from the substrate. The film thicknesses of both ECP-Magenta and MCCP were respectively


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Considering that the EC/PV operates effectively even under low intensity illumination of 0.25 sun with a switching time of 3–6 s demonstrates the feasibility of the EC/PV in window applications under oblique illumination of the Sun. Furthermore, the two PV cells can be connected in parallel or series to function as a tandem PV cell generating power for other applications. Therefore, the EC/PV cell is a net energy positive device, which generates electricity in addition to modulating incoming light and heat through windows. This work may have broad impact as such EC/PV device concepts can lead to solar powered EC windows which will decrease the energy demand for space cooling and significantly cut the energy consumption of buildings, especially for skyscrapers and passive houses. Moreover, the EC/PV fully processed from organic solutions is compatible with printing or roll-to-roll techniques for large-scale production to lower ultimate cost. The EC/PV windows may also have a large impact on the building and glass industries by adding two new functions to conventional windows to offset electricity consumption of buildings and relieve our dependence on fossil fuels.

180 nm (+/− 20 nm) and 250 nm (+/− 30 nm) as measured by stylus profilometry (Bruker Dektak XT). Strips of VHB (3M) foam acrylic tape were then used to define the device area. A gel electrolyte (0.5 M LiBTI in propylene carbonate, 8 wt% PMMA) was pipetted into this area, and sealed in with the second transparent PV cell onto the surface of which the second ECP has been spray cast. Contact to the completed device was facilitated by the addition of ½ inch copper tape (series 1181, 3M) at the substrate edges. Characterization of ECDs: Photography of films and devices was performed using a Nikon D90 DLSR camera, with a Nikon 18-105 mm VR lens. UV-Vis-NIR transmittance spectra (Figure 2a and Figure S7a) and device switching kinetic data (Figure S8b) were recorded using a Cary 5000 spectrophotometer. Colorimetric data (Figure S7b) was generated using this data. Potential control for device switching in Figure S7 and Figure S8b was facilitated by a Princeton Applied Research potentiostat/galvanostat (model PAR273A), under CorrWare control. Visible region transmittance spectra for device switching (Figure 3, Figure S5, Figure S6) was recorded using an Ocean Optics USB2000+ fiber optic spectrometer. Reflectance data (Figure S4 and Figure S8a) was gathered using a multichannel spectroradiometer (Optronix, model OL770-DMS) with a CCD imaging telescope attachment (Optronix, model OL610). Samples were placed on a PTFE diffuse reflectance standard (Optronic Laboratories, model OL25RS) in order to reflect light to the CCD imaging telescope for recording. Illumination for device switching and reflectance spectroscopy was provided by a Newport lamp (model 94021A).

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements F. Z. greatly appreciates financial support from Swedish Governmental Agency for Innovation Systems (VINNOVA) through program VINNMER (2008-03422). J.R.R thanks BASF and the Office of Naval Research (N00014-11-1-0245) for funding, while Y.Z. and B.K. acknowledge funding from the Office of Naval Research (N00014-14-1-0126). Received: March 28, 2014 Revised: April 23, 2014 Published online:

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