pairingofnear-ultravioletsolarcellswith ... · of active layers comprising d/a1 (red), d/a2 (blue),...

ARTICLESPUBLISHED: 30 JUNE 2017 | VOLUME: 2 | ARTICLE NUMBER: 17104

Pairing of near-ultraviolet solar cells withelectrochromic windows for smart management ofthe solar spectrumNicholas C. Davy1, Melda Sezen-Edmonds1, Jia Gao1, Xin Lin2, Amy Liu1, Nan Yao3, Antoine Kahn2

and Yueh-Lin Loo1,4*

Current smart window technologies o�er dynamic control of the optical transmission of the visible and near-infrared portionsof the solar spectrum to reduce lighting, heating and cooling needs in buildings and to improve occupant comfort. Solarcells harvesting near-ultraviolet photons could satisfy the unmet need of powering such smart windows over the samespatial footprint without competing for visible or infrared photons, and without the same aesthetic and design constraints.Here, we report organic single-junction solar cells that selectively harvest near-ultraviolet photons, produce open-circuitvoltages eclipsing 1.6V and exhibit scalability in power generation, with active layers (10 cm2) substantially larger than thosetypical of demonstration organic solar cells (0.04–0.2 cm2). Integration of these solar cells with a low-cost, polymer-basedelectrochromic window enables intelligent management of the solar spectrum, with near-ultraviolet photons powering theregulation of visible and near-infrared photons for natural lighting and heating purposes.

Heating, cooling, and lighting needs in buildings compriseroughly 40% of the energy demand in the United States1,with heating accounting for nearly twice the energy required

for cooling2,3. Smart windows can regulate the transmission ofvisible and near-infrared light, thereby offsetting building energyneeds by up to 40% (refs 4–6). Among smart window technologies,electrochromic windows have garnered the most attention becausethey offer dynamic control7,8. Together with temperature and lightsensors, smart windows promise to work in concert with buildingclimate control and lighting systems to afford significant energy sav-ings over passive or static alternatives, such as photochromic coat-ings4. Electrochromic windows require intermittent or continuouspower for operation, necessitating external wiring and complicatinginstallation2,4,9–11. Low-cost, visibly transparent photovoltaic cells(TPVs) can provide local, renewable power to such electrochromicwindows2 without altering building aesthetics or imposing fur-ther design constraints. Existing TPV technologies12–14, however, donot currently meet this application need because they target thesame near-infrared (near-IR) wavelengths for power generation thatsmart windows aim to regulate dynamically. Selectively harvestingnear-ultraviolet (near-UV) light for powering smart window tech-nologies avoids competition for the same spectral range.

Organic semiconductors, with inherently narrow and tunableabsorption, offer the opportunity for wavelength-specific lightharvesting12,15. Here, we report solar cells based on contortedhexabenzocoronene (cHBC) derivatives that selectively harvestnear-UV light, and demonstrate their pairing with polymer-basedelectrochromic windows (ECWs) to modulate the transmission ofvisible and near-IR light. These single-junction cHBC-based solarcells exhibit photovoltages exceeding 1.6V; the high photovoltagesof the near-UV cells (relative to typical solar cells) enhances their

utility for switching the electrochemical states of ECWs. Thesecells produce power that increases linearly with active area, andcan drive similarly sized or larger ECWs over the same spatialfootprint with switching times on the order of seconds. This workimplicates near-UV solar cells as promising power sources foremerging dual-band smart window technologies to independentlyregulate visible and near-IR/infrared wavelengths over the samespatial footprint (Fig. 1)4,7. Beyond powering smart windows for

Near-UV Visible Near-IR

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Figure 1 | Proposed management of the solar spectrum. ASTM G173spectral irradiance at global tilt conditions (grey filled). The colour legend(top panel) is scaled to the wavelength on the x-axis. We propose thepairing of organic solar cells that absorb near-ultraviolet (near-UV) light topower ECWs that modulate the transmission of visible and near-infrared(near-IR) photons for natural lighting and heating purposes, respectively.

1Department of Chemical and Biological Engineering, Princeton University, Princeton, New Jersey 08544, USA. 2Department of Electrical Engineering,Princeton University, Princeton, New Jersey 08544, USA. 3Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University,Princeton, New Jersey 08544, USA. 4Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, USA.*e-mail: [email protected]

NATURE ENERGY 2, 17104 (2017) | DOI: 10.1038/nenergy.2017.104 | www.nature.com/natureenergy 1

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

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http://dx.doi.org/10.1038/nenergy.2017.104

www.nature.com/natureenergy

ARTICLES NATURE ENERGY

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Figure 2 | Chemical structures and thin-film optoelectronic properties.a, Chemical structures of contorted hexabenzocoronene derivatives used aselectron donor (D) and electron acceptor (A1 or A2). b, Energy levels of thehighest occupied and lowest unoccupied molecular orbitals of theconstituents determined by ultraviolet photoelectron and inversephotoemission spectroscopies, respectively. The energy levels of D werereported previously20 and those of C60 were obtained from the literature25.c, Absorption coe�cients of thin films of D (green), A1 (red), A2 (blue),and C60 (black).

buildings and automobiles, solar cells that selectively harvest near-UV light can benefit a broad range of other applications, such astransparent displays, wearables, and sensors for internet-of-things.They can also serve as power sources for vertically integratedelectrochemical devices, and as top cells for multi-junctionphotovoltaic systems15–18.

Optoelectronic characterization of cHBC derivativesHarvesting near-UV light selectively using organic solar cellsrequires a pair of wide-bandgap organic semiconductors withcomplementary frontier orbital energy levels19. Our near-UV solarcells comprise organic semiconductors from a class of polyaromatichydrocarbons based on cHBC20. Thin films of cHBC exhibit peakabsorption at 386 nm, with an absorption coefficient exceeding200,000 cm−1, and an electronic bandgap of 3.0 eV (ref. 20). cHBC

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is thus an excellent starting point for subsequent functionalizationto create derivatives with favourable properties for harvesting near-UV light in solar cells. The synthesis of these cHBC derivativesis facile, uses widely available precursors, and has produced alibrary of chemically stable molecules with varied optoelectronicproperties20–22. The chemical structures of the electron donor(D) and the two halogenated cHBC electron acceptors21(A1 = 8Cl-cHBC, A2 = 12Cl-cHBC) used in our solar cellsare shown in Fig. 2a. These molecules can be handled in ambientconditions and deposited as thin films using vacuum thermalevaporation23,24, a deposition technique already demonstrated toproduce organic electronic devices commercially18, due to theirthermal stability (Supplementary Fig. 1).We evaluated the thin-filmenergetics of A1 and A2 using ultraviolet photoelectron (UPS)and inverse photoemission (IPES) spectroscopies (SupplementaryFig. 2). The frontier orbital energy levels of A1 and A2 are shownin Fig. 2b along with those of D and C60 obtained from theliterature20,25. Compared to the lowest unoccupied molecular orbital(LUMO) energy level of C60, those of A1 and A2 are raised towardsthe vacuum level by 0.9 eV and 0.6 eV, respectively. The energy-levelalignment of D/A1 and D/A2 pairs should thus allow for solarcells that harvest near-UV photons at energies substantially higher

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NATURE ENERGY 2, 17104 (2017) | DOI: 10.1038/nenergy.2017.104 | www.nature.com/natureenergy



NATURE ENERGY ARTICLES

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Figure 4 | Solar cell characterization. a, J–V characteristics of D/A1 (red), D/A2 (blue), and D/C60 (black) planar-heterojunction solar cells, each havingan active area of 0.18 cm2. Cells were measured under 100 mW cm−2 AM1.5G simulated solar illumination. b, Open-circuit voltage as a function of theLUMO energy level of the electron acceptor used in each cell type. c, Histogram of Voc across 57 D/A1 solar cells. d, A high-resolution TEM image of thecross-section of a D/A2 cell on silicon with interfaces marked in red. e,f, Power–voltage dependence (e) and spectral power response (f) of the solar cells.The spectral power response is the product of spectral responsivity and operating voltage.

than those accessed by D/C60 cells. The optical responses of thinfilms of the absorbers are shown in Fig. 2c. Thin films of D,A1, and A2 exhibit maximum absorption coefficients exceeding200,000 cm−1 at 372, 403, and 409 nm, respectively. Thus, A1 andA2 films both complement the optical response of films of D at theshort-wavelength edge of the visible spectrum. With an absorptioncoefficient of 131,000 cm−1, the maximum near-UV absorption byC60 thin films is not only lower, it occurs at 345 nm where the solarflux is almost three times weaker than at 400 nm. The absorptionof C60 is also broader and extends into the visible, with an averageabsorption coefficient of 45,500 cm−1 at 400–550 nm, and mildabsorption to 700 nm. A1 and A2 are thus stronger absorbers, andeach exhibits more selective absorption of near-UV light than C60.

400–420 nm light is strongly absorbed by A1 and A2 films.These wavelengths are barely detectable by the human eye butrepresent 36%of the solar flux that is available between 300–420 nm.

Figure 3a shows the transmittance of each active layer. The D/A1and D/A2 active layers absorb 52 and 56% of light between 300and 420 nm, respectively, while transmitting 83% and 81% of visiblelight. Figure 3b illustrates the transparency of a 25 cm2 D/A1 activelayer on glass.

Performance characterization of near-UV solar cellsPlanar-heterojunction solar cells comprising active layers of D withA1, A2, or C60 were optimized and tested under 1 sun AM1.5Gillumination. The J–V characteristics of the cells are shown inFig. 4a and tabulated in Supplementary Table 1. The open-circuitvoltages (Voc) of D/A1, D/A2, and D/C60 cells are 1.63, 1.46, and0.82V, respectively. D/A1 and D/A2 cells both exhibit Vocs that arehigher than any previously reported single-junction solar cell, withtheVoc of D/A1 cells holding the record for any single-junction thin-film PV technology26,27. The fact that the Voc of the D/A1 cell is



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(black), 0.48 cm2 (blue), 0.96 cm2 (red), and 2.22 cm2 (green) D/A1planar-heterojunction cells fabricated on ITO-coated glass. The dashed lineis a fit of the data with the intercept fixed at the origin, with its linearitydemonstrating quantitative scaling of power generation with active area.

double that of the D/C60 cell is a manifestation of the raised LUMOenergy level of A1 relative to that of C60. Figure 4b reveals a linearcorrelation between Voc and the LUMO energy level of the acceptor.The 0.81 and 0.64 V difference in Voc between D/A1 and D/A2 cellsrelative toD/C60 cells, respectively, is accordingly consistent with the0.9 and 0.6 eV difference in the LUMO energy levels of A1 and A2relative to that of C60. Importantly, the fill factors of the D/A1 andD/A2 cells are 58 and 60%, respectively. With a few exceptions28,29,

previous solar cells with Vocs exceeding 1.3V exhibit fill factors of30–45%, indicating recombination as a bottleneck to operation insuch devices26,30,31.

Figure 4c details the Voc values recorded for all D/A1 cellstested in this study. The judicious selection of conformationallynon-planar cHBC derivatives favours the formation of amorphouspinhole-free films20, and enables 100% yield of the 57 cells fabri-cated. The high-resolution cross-sectional micrograph in Fig. 4dconfirms the amorphous nature of the donor and acceptor layers,and reveals the preservation of a pristine donor–acceptor interface.

Reduced visible light absorption by A1 and A2 relative to C60increases active-layer transparency at the expense of photocurrent;the short-circuit current density (Jsc) of these cells are almost halfthat of the D/C60 cell (Fig. 4a). At 1.3–1.5mWcm−2, however, thepower densities of D/A1 and D/A2 cells (Fig. 4e) are comparableto that of the D/C60 cell because of their high Voc and fill factor.With photovoltages at the maximum-power points of 1.15V and1.28V, D/A1 andD/A2 cells produce power densities comparable tothe most transparent near-IR-absorbing organic solar cells12,32, butour cells deliver this power at almost twice the photovoltage whileabsorbing significantly less solar flux.

At 100mWcm−2 AM1.5G illumination, the metrics in Fig. 4atranslate to power-conversion efficiencies (PCEs) of 1.3–1.5%.By targeting the near-UV, our cells neglect approximately 93%of spectral irradiation; the standard metric of PCE is thus notinformative in quantifying their targeted performance. We insteaddefine a wavelength-specific efficiency as the product of the spectralresponsivity (Supplementary Fig. 3A) and operating voltage (Fig. 4e)for each type of cell, which we refer to as the spectral powerresponsivity (SPR) in Fig. 4f. Conversion of highly energetic near-UV photons to free charge carriers is known to be very inefficientdue to thermalization losses. For reference, the SPR at 400 nm ofthe most efficient crystalline silicon solar cells is less than 19%(ref. 33). The SPR at 400 nm of cells comprising D/A1 or D/A2 is12.4 and 14.0%, respectively. With an external quantum efficiency(Supplementary Fig. 3B) approximately a third that of a championsilicon solar cell33, the SPR of D/A1 and D/A2 cells at 400 nmremains competitive because their photovoltages are approximatelya factor of two higher than that of the silicon cell. D/C60 cells havean SPR of 5.9% at 400 nm. Thus, D/A1 and D/A2 cells are twice asefficient at harvesting near-UV photons as D/C60 cells.

Scalability and stability of near-UV solar cellsFigure 5a,b shows the J–V characteristics and power-density pro-files of D/A1 planar-heterojunction cells with increasing active areasof 0.24 cm2, 0.48 cm2, 0.96 cm2 and 2.22 cm2. The performance ofthe cells is invariant despite an active-area increase by an orderof magnitude. The cells produce a photocurrent that scales witharea. At constant photovoltage and fill factor, power productionthus increases linearly with active area for these cells (Fig. 5c).Comparatively, near-IR-absorbing organic solar cells exhibit sig-nificant reductions in performance with increasing areas34. In thecase of a high-efficiency low-bandgap polymer bulk-heterojunctionsolar cell, increasing the active area from 0.27 cm2 to 2 cm2 resultedin a 20% reduction in fill factor and a 24% loss in power den-sity, even after significant device optimization34. We attribute thescalability of our cells to two factors. First, high-voltage powerhas lower resistive losses compared to high-current power becausethe total resistive power loss per unit area is proportional to thesquare of the current density at the maximum-power point35. Ourchoice to target near-UV light, as opposed to near-IR light, resultsin cells that output high-voltage/low-current power. Second, theactive layers in our cells are amorphous and inherently free ofpinholes20. Contrary to cells that use polycrystalline active layers36,this feature has enabled us to scale up the active area with the samesingle-junction architecture while mitigating the shorting of cells.

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Figure 6 | Large-area solar cell with an alternative electrode. a–c, Photograph (a) and optical micrographs (b,c) of D/A1 active layer prepared on a 10 cm2

ITO/Ag-grid/PEDOT:PSS bottom electrode. d, J–V characteristic of 1 cm2 (black) and 10 cm2 (red) D/A1 cells fabricated on an ITO/PEDOT:PSS transportlayer and an ITO/Ag-grid/PEDOT:PSS transport layer, respectively. Cells were measured under 100 mW cm−2 AM1.5G simulated solar illumination.

Although lower charge-carrier mobilities in amorphous thin filmscan be a concern23, charge transport appears not to be the bottleneckin these planar-heterojunction cells given their very thin (10–20 nm)active layers37.

Leveraging the ability to scale D/A1 films, we fabricated a 10 cm2

D/A1 cell on an indium tin oxide (ITO)-coated glass substrate. A30-nm-thick honeycomb-inspired Ag-grid38, shown in Fig. 6a–c,that is 97% transparent between 300–800 nm (Supplementary Fig. 4)was added to decrease the series resistance associated with ITOcarrying current over extended distances. The J–V characteristicsof this 10 cm2 D/A1 cell are shown in Fig. 6d along with a similarlyconstructed 1 cm2 reference cell. We observe comparable perfor-mance when the cell area is increased from 1 cm2 to 10 cm2. Thefact that power production is scalable to 10 cm2 with an alternativeelectrode is further testament that D/A1 active layers are pinhole-free and amenable to large-area power production without sacri-ficing performance metrics. The vast majority of publications onorganic solar cells report performance based on active areas that are0.04–0.2 cm2 (ref. 39). However, the building blocks of organic solarmodules are solar cells having active areas of 5–10 cm2 (refs 40–42).Developing organic solar cells that minimize power loss when theactive area is scaled up to sizes relevant for integration into mod-ules has been investigated in numerous papers and texts40–42. Thismanuscript represents the first study that quantifies performancescalability of near-UV solar cells, and the first report of organic solarcells having properties inherently amenable to such scale up.

We subjected encapsulated D/A1 cells to >150 h of continuousillumination to probe their stability under direct sunlight(Supplementary Fig. 5). Importantly, theVoc is invariant throughoutthis experiment. Because the frontier orbital energy levels of cHBCacceptors are determined by the extent of halogenation21, the Vocinvariance in D/A1 cells negates the possibility of photo-inducedchlorine loss from A1, especially at the D/A1 interface. The Jscand fill factor suffer an initial decrease, resulting in a power-density reduction of 27% over the initial 30 h, beyond which thedevice characteristics are stable. This burn-in behaviour is typical;comparable reductions in performance metrics have been reportedin low-bandgap polymer solar cells, even when the UV portion ofthe solar spectrum was filtered43–45. As another example, similarlyconstructed planar-heterojunction solar cells comprising boronsubphthalocyanine chloride (SubPc) and C60, an archetypicaldonor/acceptor pair, exhibit a burn-in power loss of 28% after 10 hof illumination at 1 sun AM1.5G (ref. 46). Thus, despite harvestinghigh-energy near-UV photons, D/A1 cells have comparable stabilityto other promising organic solar cell technologies15,47. Extendingthe lifetime of organic solar cells is an ongoing area of research

that is commonly embraced in the organic electronics community;efforts span the design of new active materials, electrodes andbuffer layers, optimization of device architecture, and introductionof new encapsulation technologies47.

Self-powered electrochromic windowsNear-UV solar cells offer the opportunity to integrate power gen-eration with ECWs that modulate sunlight over the same foot-print without competing for the same spectral range. As a proof ofconcept, we demonstrate the side-by-side48 operation of a 2.25 cm2

ECW using the high-voltage power produced by a 1.38 cm2 D/A1solar cell (Supplementary Video 1, set-up shown in SupplementaryFig. 6). Characterization of the operation of an ECW powered bya D/A1 cell is presented in Fig. 7. Figure 8 offers the optical trans-mittance of a vertically aligned ECW and PV stack. Compared toECWs previously powered with near-IR-absorbing solar cells2,11,49,we demonstrate power generation with near-UV light, leaving near-IR—in addition to the visible wavelengths—to be regulated by theECW for heating and lighting purposes, respectively1,7.

Figure 7a shows a scheme of the ECW comprising a primaryelectrochromic layer of polyaniline that is doped with poly(2-acrylamido-2-methyl-1-propane-sulfonic acid) (PANI-PAAMPSA)(refs 50–54), a poly(3,4–ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) counter electrode, and a gel electrolytecomposed of an aprotic ionic liquid, 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)imide (EMIM TFSI) with 5wt%fumed silica55. PEDOT:PSS and PANI-PAAMPSA are bleachedwhen the former is oxidized and the latter is reduced(Supplementary Fig. 7A)50,56. Similarly, both polymers are colouredwhen PEDOT:PSS is reduced and PANI-PAAMPSA is oxidized(Supplementary Fig. 7B). Thus, the complementarity of PEDOT:PSSand PANI-PAAMPSA enhances the optical contrast of the resultingECW56. Applying a negative or positive bias from a single-junction D/A1 cell between the PANI-PAAMPSA and PEDOT:PSSelectrodes drives the ECW toward its bleached or colouredtransmittance states, respectively, as illustrated by Fig. 7b. The useof solution-processable electrochromic materials allows for tuningof transmittance by varying the thickness of each polymer film at theoutset (Supplementary Fig. 8), with thinner electrochromic layersresulting in higher bleached-state transmittance. For any givenPANI-PAAMPSA and PEDOT:PSS electrode thickness, the opticaltransmittance contrast is further determined by the applied voltages(Supplementary Fig. 9). In Fig. 7c,d we compare the transmittancespectra in the bleached and coloured states achievable in an ECWhaving PANI-PAAMPSA and PEDOT:PSS of thicknesses 370and 355 nm, respectively, when powered by operating voltages



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characteristic of our solar cells (±1.2V) and when powered byoperating voltages characteristic of near-IR solar cells (±0.6V)2,12.Having 1.2V instead of 0.6V to drive ECW switching leads to asubstantial increase in the transmittance contrast at 650 nm (from17% to 42%). Our near-UV solar cells are thus more effective atpowering ECWs that regulate visible light than near-IR solar cells.While one could connect multiple near-IR solar cells in series or usea tandem cell to access higher operating voltages17, our near-UVsolar cells obviate the need for such fabrication complexity, allowingfor vertically integrated designs that directly connect single-junction solar cell and ECW components. Additionally, for ECWsthat regulate near-IR light, near-UV cells are the only appropriatepower source to avoid competition for the same spectral range.

Figure 7e shows the transmission response of the ECW duringrepeated potential cycling. The optical transmittance switches stablyand reversibly when the applied bias between electrodes is pulsedat either positive or negative 1.2 V. When the applied potential isremoved (see Fig. 7e; between 5,000 and 6,000 s), a slow decayin transmittance from 58% to 50% occurs over 10min. TheECW quickly recovers its stable maximum transmittance with asubsequent pulse of −1.2V. The kinetics of a single colouring andbleaching cycle are shown in Fig. 7f, along with the instantaneouscurrent draw.Within 40 s of the applied potential, the transmittanceof the ECW reaches 95% of the saturated values corresponding toits bleached and coloured states. Faster ECW switching speeds, ifnecessary, can be achieved by replacing the electrochromic polymers

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Figure 8 | Combined ECW and PV stack optical transmittance.a, Combined stack geometry for ECW and D/A1 cell, with the positive (+)and negative (−) circuit connections for powering the coloured andbleached states. b, Optical transmittance of the bleached (red) andcoloured (blue) states of the combined stack using air as the background,along with transmittance of the glass/ITO/electrolyte stack as background(black). c, Optical transmittance of the bleached (red) and coloured (blue)states of the combined stack after subtraction of the background spectrumprovided in b.

employed here with those requiring less charge for switching and/orthe electrolyte to reduce mass transfer limitations2,57.

When deployed, this technology should be implemented withan energy storage component, trickle-charged by our solar cell,to enable uninterrupted operation of the ECW regardless of solarintermittency. Our D/A1 solar cell produces 4.7 J cm2 of energy perhour of 1 sun illumination. The total charge and energy needs for acolouring and bleaching cycle (Fig. 7f) are given in SupplementaryTable 2. At an applied voltage of ±1.2V, complete colouring andbleaching of the ECW requires 4.7 and 5.0mJ cm−2, respectively,and maintenance pulses every 10min each require approximately1.2mJ cm−2. Assuming a need to cycle the ECW between its

coloured and bleached states every hour, as well as the need tomaintain the transmittance while the ECW is in these respectivestates, the total energy needed by the ECW is approximately406mJ cm−2 per 24 h. A D/A1 solar cell thus produces an order ofmagnitudemore energy in one hour thanwhat is required to operatethe ECW for 24 h. With an energy storage component, our solar-powered ECW should be able to operate without external powerdespite solar intermittency and in a wide variety of weather andinsolation conditions.

ChallengesLonger-term testing of PV and ECW components under differentillumination and temperature conditions is needed to inform thedesign of and extend the lifetime of next-generation devices.Like other electrochromic technologies11, the ECW reported hereachieves its coloured state using absorbing layers, necessitatingfuture study of heat management and optimization for combineddevice thermal stability. In this regard, smart windows that reflectlight may improve thermal stability and aid heat management of theintegrated stack.

Replacement of ITO electrodes with inexpensive alternativescomprising earth-abundant materials will also be needed for low-cost and sustainable access to smart window technologies. A furtherchallenge stems from our thin active layers, which necessitatesalternative electrodematerials to be ultra-smooth.We speculate thatelectrode conductivity may be less vital for sufficient near-UV solarcell function, given the low currents produced by such cells, and theconservative energy needs of our ECW. In comparison, electrodetransmittance in the visible and colour neutrality are probably stifferconstraints for such smart window applications.

ConclusionWe demonstrate the complementary pairing of near-UV solarcells with a low-cost, single-band ECW option based on water-dispersible conducting polymers. The realization of solar cells thatselectively harvest near-UV light necessitates the use of organicsemiconductor pairs in which each has a wide bandgap. Leveragingour initial success and new machine-learning techniques58, effortsare ongoing to predict, and design, synthesize and characterizeaccordingly promising organic semiconductors for solar cellswith improved wavelength selectivity and enhanced spectralresponsivity. This exciting technological advance also raises newscientific questions about the fundamental operational limits ofdevices that incorporate wide-bandgap organic semiconductorpairs. Simultaneously, future progress is being enabled by time-resolved spectroscopic studies that elucidate exciton dynamicsof these materials. Dual-band ECWs are on the horizon1,7,11,50,59;vertical integration with optimized near-UV solar cells can enablelocal, self-contained power generation and expedite deployment ofthese energy-saving technologies.

MethodsMolecular semiconductors D, A1, A2. D (or cTBFDBC; contortedtetrabenzofuranyldibenzocoronene; dibenzo(3,4:9,10)benzo(4′ ,5′ )furan(3′ ,2′ :5,6)-benzo(4′ ,5′ )furan(2′ ,3′ :7,8)benzo(4′ ,5′ )furan-(3′ ,2′ :11,12)coroneno(1,2-b)benzofuran) was synthesized according to previously reported methods20.Carbon tetrabromide (99%), triisopropyl phosphite (95%),tris(dibenzylideneacetone)dipalladium(0) (97%),2-Dicyclohexylphosphino2′ ,6′ -dimethoxybiphenyl (SPhos, %), potassiumphosphate tribasic (≥98%), 2-benzofuranylboronic acid MIDA ester (97%),anhydrous dichloromethane (99.9%), dioxane (99.5%), iodine (≥99.8%), andpropylene oxide (≥99%) were all purchased from Sigma-Aldrich and usedas-received. Dichloromethane and toluene were purchased from FisherScientific and used as-received. A1 (or 8Cl-cHBC; 1,3,6,8,13,15,18,20-octachlorotrinaphtho(1,2,3,4-fgh:1′ ,2′ ,3′ ,4′ -pqr:1′ ′ ,2′ ′ ,3′ ′ ,4′ ′ -za1b1)trinaphthylene)and A2 (or 12Cl-cHBC; 1,3,6,7,8,13,14,15,18,19,20-dodecachlorotrinaptho(1,2,3,4-fgh:1′ ,2′ ,3′ ,4′ -pqr:1′ ′ ,2′ ′ ,3′ ′ ,4′ ′ -za1b1)trinapthylene) were synthesizedaccording to previously reported methods21. Bis(triphenylphosphine)



7




palladium(II) dichloride (≥99.99%), potassium carbonate (99+%), and3,5-dichlorophenylboronic acid (98%) were purchased from Sigma-Aldrich andused as-received. 3,4,5-trichlorophenylboronic acid (AstaTech, 98%) was usedas-received.

Thin-film preparation and characterization. C60 (99.5%, NanoC) and D werepurified once using horizontal physical-vapour transport with Ar (99.999%,Airgas), as the inert carrier gas. A1 and A2 were purified via recrystallization intoluene. Thin films of C60, D, A1 and A2 were thermally evaporated at a rate of1Å s−1 in a chamber with a pressure < 2 × 10−6 torr. Absorption spectra of thinfilms of C60, D, A1, and A2 on glass slides were collected on an Agilent Cary 5000UV–Vis–NIR spectrophotometer. Film thicknesses for Figs 2c and 3a wereidentical to the thicknesses used in devices: 23 nm (D), 17 nm (A1, A2) and 40 nm(C60). Ultraviolet photoelectron spectroscopy (UPS) and inverse photoelectronspectroscopy (IPES) measurements were performed at room temperature, in adedicated ultrahigh vacuum chamber with a base pressure of <2 × 10−10 torr.10-nm-thick films of A1 or A2 were evaporated in a thermal evaporator locatedin a nitrogen-filled glovebox and transported to the measurement vacuum systemin nitrogen. UPS was performed using a Specs gas discharge lamp operating withhelium, yielding helium I photons at 21.22 eV. The energy resolution in UPS was0.15 eV. IPES was performed in isochromat mode using a home-built set-up60,with an energy resolution of 0.45 eV. A Helios G3 DualBeam Focused Ion Beamwas used to prepare a cross-section of a D/A2 stack on silicon for imaging with aTitan Themis 80–300 Cubed Double Cs-corrected Scanning/TransmissionElectron Microscope. We chose A2 for this imaging experiment instead of A1because it has a higher electron density contrast relative to the donor layer (D).

Solar cell fabrication and testing. MoO3 (99.97%, Sigma-Aldrich)and bathocuproine (99.99%, Sigma-Aldrich) were used as-received. Glasssubstrates (23mm × 27mm) pre-patterned with an ITO strip 7.5mm wide and23mm long (20 �/square, TFD) were cleaned by sonication in deionized water,acetone, and isopropyl alcohol, and then dried with nitrogen. The substrateswere then immediately transferred to a nitrogen glovebox for sequentialevaporation of device layers. MoO3 and bathocuproine (BCP) layers were each5 nm thick, deposited at 1Å s−1. After deposition of BCP, 60 nm of Al (99.999%,Kurt J. Lesker) was evaporated through patterned masks at a rate of 2Å s−1to define the active area. The devices were placed under AM1.5G 100mWcm−2illumination in a nitrogen-filled glovebox and the current density–voltage (J–V)characteristics were acquired with a Keithley 2400 (Fig. 4 and SupplementaryFig. 5) or a Keithley 2602B source measurement unit (Figs 5 and 6), respectively.J–V and external quantum efficiency (EQE) measurements were performed usinga 300 W xenon arc lamp (Newport Oriel), with filtered monochromatic lightfrom a Cornerstone 260 1/4 M double grating monochromator (Newport 74125)for the EQE measurements. A Newport calibrated photodetector (model 71582)served as the reference cell. The spectral mismatch factor, M , was calculatedto be 0.97, 0.96, and 1.11 for the D/A1, D/A2, and D/C60 cells, respectively.

We fabricated and tested 57, 28, and 24 devices comprising A1, A2, and C60

as the acceptor, respectively. The active area of each device for these experimentswas 0.18 cm2. The yield on functional devices is 100% with all three acceptors; theaverage and standard deviation values for relevant device characteristics aresummarized in Supplementary Table 1. The solar cells exhibit identical J–Vcharacteristics in forward and reverse directions (Supplementary Fig. 10).

D/A1 cells (active area 0.18 cm2) were encapsulated in a nitrogen-filledglovebox using a glass-on-glass architecture and DELO-KATIOBOND LP686solvent-free epoxy. The cells remained in the glovebox overnight to ensure fullcuring of the epoxy before they were brought out to the ambient. A Keithley 2400source measurement unit was used to continuously monitor the maximum-powerpoint and periodically probe the current density–voltage (J–V ) characteristics ofthe cells during continuous exposure to AM1.5G 100mWcm−2 illuminationprovided by a 300 W xenon arc lamp (Newport Oriel). The cell stage wasmaintained at 21 ◦C throughout the ageing measurements.

To fabricate cells with areas larger than our standard test cell, ITO-coatedglass substrates (20�/square, Colorado Concept Coatings) having dimensions23mm × 27mm were patterned with an ITO strip having dimensions12mm × 27mm. Photomasks with varying sizes were used during aluminium topelectrode deposition to define device active areas of 0.24, 0.48, 0.96, and 2.22 cm2.

The substrates used to prepare the 10 cm2 devices reported in Fig. 6 wereobtained by a multi-step patterning process. ITO-coated glass sheets(20�/square, Colorado Concept Coatings) were cut into 5 cm × 5 cm squaresand cleaned by sonication in soap and deionized water, followed by deionizedwater, acetone, isopropanol, and then dried with nitrogen. Photolithography wasthen performed to define the bottom electrode (Fig. 6b, light brown) and the30-nm-thick Ag-grid current collector (Fig. 6b,c, white). These patternedsubstrates were inspected for defects using an optical microscope before a finalcleaning via sonication in 2-propanol for 3min followed by drying with nitrogen.To create a smooth surface for device layers, the Ag-grid substrates were

augmented with a PEDOT:PSS layer per literature38. PEDOT:PSS (Heraeus,Clevios pH 1000) was mixed with 5 vol% dimethyl sulfoxide and stirred overnightin the dark before the dispersion was spin-coated atop the patterned substrates at4,000 r.p.m. (1,000 r.p.m. ramp) for 40 s. The substrates were then annealed at140 ◦C for 30min in a glovebox before they were transferred to the evaporatorchamber for deposition of device layers as outlined above.

PANI-PAAMPSA synthesis. Aniline (Sigma-Aldrich, 99.5%), ammoniumperoxydisulfate (98.9%, Fisher Scientific), and PAAMPSA (Scientific PolymerProducts, 10.36wt% in water, reported MW 800 kgmol−1) were used in thesynthesis of PANI-PAAMPSA as-purchased. PANI-PAAMPSA was synthesized bytemplate polymerization of aniline along PAAMPSA at a 1:1 monomer-to-acidmolar ratio in deionized water, as described by Yoo and colleagues54. Ammoniumperoxydisulfate was added to the mixture dropwise at a 1:0.9monomer-to-oxidizing agent molar ratio. The reaction mixture was stirred in anice bath for 24 h. Acetone was slowly added to precipitate the PANI-PAAMPSAcomplex, which was dried under vacuum for 24 h.

Electrochromic window (ECW) fabrication and characterization. A monolayerof 12-(phosphonododecyl)phosphonic acid (Sigma-Aldrich, 97%) was covalentlybound to pre-cleaned ITO/glass by the tethering-by-aggregation-and-growthmethod developed by Schwartz and colleagues61. A 5wt% PANI-PAAMPSAdispersion in water that were stirred for at least 10 days—or as-purchasedPEDOT:PSS (Heraeus, Clevios pH 1000)—were spin-coated onto phosphonicacid-modified ITO/glass substrates. We varied the spin-coating rates from700 r.p.m. to 1,000 r.p.m., and deposited 1 to 2 layers of PANI-PAAMPSA and 1to 4 layers of PEDOT:PSS films to vary the thicknesses of the electrochromicpolymers. These polymer films were dried for 1min at 100 ◦C between eachdeposition. These films were then vigorously shaken in dichloroacetic acid (DCA,Sigma-Aldrich, 99%) for 3min as described by Yoo and colleagues51. DCAtreatment is performed in order to eliminate any hysteresis in electrochromicswitching due to ion-transport limitation50. Excess DCA was removed by bakingPANI-PAAMPSA films at 170 ◦C and PEDOT:PSS films at 140 ◦C. The films werekept under vacuum for at least 3 h prior to testing to remove any residual DCA.Film thicknesses were measured using a Dektak 3.21 profilometer.

To bring PANI-PAAMPSA and PEDOT:PSS to opposite oxidation states,PEDOT:PSS was oxidized for 200 s at 0.9 V (versus Ag/AgCl) andPANI-PAAMPSA was reduced for 200 s at −0.6V (versus Ag/AgCl) in pH 8,100mM phosphate buffer solution using CH-Instruments CHI-660electrochemical workstation. 1-Ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide (EMIM TFSI, Sigma-Aldrich, >97%) mixedwith 5wt% fumed silica (Sigma-Aldrich, 0.007 µm) was used as the gelelectrolyte55. UV–Vis–NIR spectra of the ECWs were collected using an Agilent8453 UV–Vis–NIR spectrometer while simultaneously performingchronoamperometry experiments using a CH-Instruments CHI-660electrochemical workstation to switch between the bleached and the colouredstates of our ECW. Spectra in Fig. 7 were background subtracted, with twoITO/glass substrates and EMIM TFSI/silica ionogel used as the background.

ECW video methods. A 2.25 cm2 ECW was prepared as described above. A1.38 cm2 solar cell with D/A1 as the active layers was used to drive the switchingof this ECW between its coloured and bleached states. The cathode and anode ofthe solar cell were connected to the PANI-PAAMPSA and PEDOT:PSSelectrodes, respectively, to achieve the coloured state of the ECW. Theconnections were reversed to access the bleached state.

Film thicknesses for various ECWs. ECW video: PANI-PAAMPSA: 350 nm,PEDOT:PSS: 285 nm. Potential map (Fig. 7c,d and Supplementary Fig. 8):PANI-PAAMPSA: 260 nm, PEDOT:PSS: 320 nm. Stability/cyclability andcharge/energy calculations (Fig. 7e,f and Supplementary Table 2):PANI-PAAMPSA: 370 nm, PEDOT:PSS: 355 nm. Combined device (Fig. 8):PANI-PAAMPSA: 245 nm, PEDOT:PSS 200 nm.

Data availability. The data that support the plots within this paper and otherfindings of this study are available from the corresponding author uponreasonable request.

Received 10 February 2017; accepted 30 May 2017;published 30 June 2017; corrected 18 September 2017

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AcknowledgementsN.C.D. acknowledges financial support from the National Science Foundation GraduateResearch Fellowship Program under Grant No. DGE 1148900. We (N.C.D., M.S.-E. andY.-L.L.) acknowledge support from NSF MRSEC funding through Princeton Center forComplex Materials (DMR-1420541) and the Wilke Family Fund administered by the



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http://dx.doi.org/10.1002/aenm.201602209

http://www.heliatek.com/en/press/press-releases/details/heliatek-sets-new-organic-photovoltaic-world-record-efficiency-of-13-2

http://www.heliatek.com/en/press/press-releases/details/heliatek-sets-new-organic-photovoltaic-world-record-efficiency-of-13-2




School of Engineering and Applied Science at Princeton University. A.K. acknowledgessupport from the National Science Foundation under Grant No. DMR-1506097. Wethank M. Panzer and H. Qin of Tufts University for valuable discussions regardingionogels, and G. Man of Princeton University for assistance with UPS/IPES analysis.

Author contributionsN.C.D., M.S.-E. and Y.-L.L. wrote the manuscript; N.C.D. synthesized molecularsemiconductors; X.L. and A.K. provided UPS/IPES measurements and analysis; N.C.D.and J.G. fabricated and characterized PV cells; M.S.-E. synthesized PANI-PAAMPSA;M.S.-E. and A.L. fabricated and characterized ECWs; M.S.-E. and N.D. integrated ECWand PV components for solar-powered ECW demonstration. N.Y. conducted TEMsample preparation and imaging. All authors discussed the results and reviewedthe manuscript.

Additional informationSupplementary information is available for this paper.Reprints and permissions information is available at www.nature.com/reprints.Correspondence and requests for materials should be addressed to Y.-L.L.How to cite this article: Davy, N. C. et al. Pairing of near-ultraviolet solar cells withelectrochromic windows for smart management of the solar spectrum. Nat. Energy 2,17104 (2017).Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.

Competing interestsThe authors declare no competing financial interests.

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Author CorreCtionDOI: 10.1038/s41560-017-0017-8

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

Author Correction: Pairing of near-ultraviolet solar cells with electrochromic windows for smart management of the solar spectrumNicholas C. Davy1, Melda Sezen-Edmonds1, Jia Gao1, Xin Lin2, Amy Liu1, Nan Yao3, Antoine Kahn2 and Yueh-Lin Loo1,4*

Nature Energy 2, 17104 (2017); published online 30 June 2017; corrected online 18 September 2017.

In the version of this Article originally published, Fig. 4f was incorrect. This error has now been corrected (see new figure below).

D/A1

D/C60

D/A2

D/A1

1.0a b

c d

e f

0.5

0.0

−0.5

−0.5 0.0 0.5 1.0Voltage (V)

1.5

D/A1D/A2D/C60

D/A1D/A2D/C60

D/A1D/A2D/C60

2.0 −4.2 −4.0 −3.8 −3.6LUMO (V)

−3.4 −3.2 −3.0

Cur

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den

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(mA

cm

−2)

Cou

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V oc

(V)

Voc (V)

Voltage (V)

−1.0

−1.5

−2.0

−2.5

−3.0

−3.5

1.8

1.6

1.4

1.2

1.0

0.8

40

30

20

10

01.60 1.61 1.62 1.63 1.64 1.65

Pow

er d

ensi

ty (m

W c

m−2

)

2.0

1.5

1.0

0.5

0.00.0 0.5 1.0 1.5

Wavelength (nm)

300 400 500 600 700

30 nm

MoO3

D

A2

BCP

Si

Al

0

4

8

12

16

Spec

tral

pow

er re

spon

se (%

)

Nature eNergy | www.nature.com/natureenergy

http://www.nature.com/natureenergy

pairingofnear-ultravioletsolarcellswith ... · of active layers comprising d/a1 (red), d/a2 (blue),...

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