Color-neutral, semitransparent organic photovoltaicsfor power window applicationsYongxi Lia, Xia Guob, Zhengxing Pengc,d, Boning Que, Hongping Yanf, Harald Adec,d, Maojie Zhangb,1

,and Stephen R. Forresta,e,g,1

aDepartment of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109; bCollege of Chemistry, Chemical Engineeringand Materials Science, Soochow University, 215123 Suzhou, Jiangsu, People’s Republic of China; cDepartment of Physics, North Carolina State University,Raleigh, NC 27695; dOrganic and Carbon Electronics Laboratories (ORaCEL), North Carolina State University, Raleigh, NC 27695; eDepartment of MaterialsScience and Engineering, University of Michigan, Ann Arbor, MI 48109; fStanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory,Menlo Park, CA 94025 and gDepartment of Physics, University of Michigan, Ann Arbor, MI 48109

Contributed by Stephen R. Forrest, July 1, 2020 (sent for review May 13, 2020; reviewed by Yonggang Huang and Jiangeng Xue)

Semitransparent organic photovoltaic cells (ST-OPVs) are emerg-ing as a solution for solar energy harvesting on building facades,rooftops, and windows. However, the trade-off between power-conversion efficiency (PCE) and the average photopic transmission(APT) in color-neutral devices limits their utility as attractive,power-generating windows. A color-neutral ST-OPV is demon-strated by using a transparent indium tin oxide (ITO) anode alongwith a narrow energy gap nonfullerene acceptor near-infrared(NIR) absorbing cell and outcoupling (OC) coatings on the exit sur-face. The device exhibits PCE = 8.1 ± 0.3% and APT = 43.3 ± 1.2%that combine to achieve a light-utilization efficiency of LUE = 3.5 ±0.1%. Commission Internationale d’eclairage chromaticity coordi-nates of (0.38, 0.39), a color-rendering index of 86, and a correlatedcolor temperature of 4,143 K are obtained for simulated AM1.5illumination transmitted through the cell. Using an ultrathin metalanode in place of ITO, we demonstrate a slightly green-tinted ST-OPV with PCE = 10.8 ± 0.5% and APT = 45.7 ± 2.1% yielding LUE =5.0 ± 0.3% These results indicate that ST-OPVs can combine bothefficiency and color neutrality in a single device.

organic solar cell | efficiency | nonfullerene acceptor |building-integrated photovoltaic | semitransparent

Transparent solar cells are attractive energy-conversion de-vices for integration onto window panes, skylights, and

building facades, providing an opportunity for increasing solarenergy harvesting on building surfaces (1–12). Compared to in-organic semiconductors, the narrow excitonic absorption bandsof organic semiconductors offer opportunities for organic pho-tovoltaics (OPVs) as power-generating windows since many or-ganics selectively absorb outside of the visible wavelengths.Semitransparent OPV (ST-OPV) efficiencies have hoveredaround 7% for the last few years, which is deemed too low forsubstantial market penetration. Additionally, only a few ST-OPVs have achieved visible transparency ∼50%, which is criti-cal for many power-window applications. The performance ofST-OPVs is ultimately limited by the trade-off between power-conversion efficiency (PCE) and average photopic transmission(APT; see Methods), which is the perceived transparency of theappliance. As a consequence, to our knowledge their light-utilization efficiencies, LUE = PCE × APT, have recentlyreached 4.3% (SI Appendix) (13–17). Another challenge faced byan ST-OPV is that the most efficient devices exhibit an unwantedtint (14, 18–22). Except for relatively few instances, it is impor-tant to develop power-generating windows with aestheticallyacceptable neutral colors that are easily applied in the broadestpossible applications.To date, demonstrations of efficient, color-neutral ST-OPVs

have primarily focused on designing materials with strong near-infrared (NIR) absorption (6, 14, 15, 18, 20, 23–28), incorpo-rating multijunction device structures to minimize thermalizationlosses (14, 28–31), and employing antireflection coatings (ARC)or aperiodic dielectric reflectors (ADR) to enhance absorption

(3, 19, 32, 33). For example, Zhang et al. developed a color-neutral ST-OPV with an LUE = 1.95% by using ternary blendsof two polymer donors with an NIR nonfullerene acceptor(NFA) to balance absorption in the visible region (34). Recently,our group demonstrated a nearly color-neutral ST-OPV with aPCE = 5.8%, APT = 45%, which combine to LUE = 2.56%under 1 sun, AM 1.5G spectral illumination by employing a bi-layer outcoupling (OC) coating on the exit surface (13). Thisdevice combined a two-component NFA blend absorbing be-tween wavelengths of 600 and 900 nm with an ultrathin metalanode. The absorption of the active layer covers wavelengths λ <900 nm, leaving substantial solar energy at longer wavelengthsunused. Furthermore, the thin metal films are rough, and theirtransmittance is spectrally dependent across the visible, oftenlending a tinted appearance.Here, we demonstrate an ST-OPV that achieves PCE = 10.8 ±

0.6% and APT = 45.7 ± 2.1%, leading to LUE = 5.0 ± 0.3. Thedevice employs an NFA molecule with strong NIR absorption,that requires only a few steps in its synthesis. Contrary to ex-pectations, strong intermolecular π–π interactions and closemolecular packing are observed in these simple NFAs that

Significance

We demonstrate a semitransparent organic photovoltaic cellthat achieves a power conversion efficiency of 10.8% and vis-ible transparency of ∼50% using a nonfullerene acceptor (NFA)featuring strong near-infrared (NIR) absorption and simplesynthesis. Contrary to expectations, stronger NIR absorptionand closer molecular packing are obtained by employing anadditive in these partially, instead of fully fused, rigid NFAs. Bycombining NIR-absorbing material sets with an optical out-coupling structure as well as transparent electrode, we over-come the trade-offs between efficiency, transparency, anddevice appearance. These results surpass other semitranspar-ent solar cell technologies based on organic and other thin-filmmaterials systems, showing a promising future for ST-OPVs aspower-generating windows and other solar energy harvestingapplications.

Author contributions: Y.L. and S.R.F. designed research; Y.L., X.G., Z.P., B.Q., and H.Y.performed research; Y.L., X.G., Z.P., B.Q., H.Y., H.A., M.Z., and S.R.F. analyzed data; andY.L., X.G., Z.P., B.Q., H.Y., H.A., M.Z., and S.R.F. wrote the paper.

Reviewers: Y.H., Northwestern University; and J.X., University of Florida.

Competing interest statement: S.R.F. has an ownership interest in one of the sponsors ofthis research, Universal Display Corp. This conflict is under management by the Universityof Michigan Office of Research.

Published under the PNAS license.1To whom correspondence may be addressed. Email: mjzhang@suda.edu.cn or stevefor@umich.edu.

This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2007799117/-/DCSupplemental.

First published August 17, 2020.

www.pnas.org/cgi/doi/10.1073/pnas.2007799117 PNAS | September 1, 2020 | vol. 117 | no. 35 | 21147–21154

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feature partially covalently fused ring backbones rather thanrigid, fully fused rings (Fig. 1). By combining the elements ofNIR-absorbing material sets with an OC structure on the exitsurface and transparent electrodes, we overcome the trade-offsbetween efficiency, transparency, and device appearance. Acolor-neutral ST-OPV using a transparent indium tin oxide(ITO) anode exhibits PCE = 8.1 ± 0.3%, APT = 43.3 ± 1.5%,and LUE = 3.5 ± 0.1%. Commission Internationale d’eclairage(CIE) chromaticity coordinates of (0.38, 0.39), a color-renderingindex of CRI = 86, and a correlated color temperature of CCT =4,143 K are obtained for simulated AM1.5 illumination trans-mitted through the cell. These results suggest that there is apromising future for ST-OPVs employed building integratedapplications.

ResultsThe molecular structural formulae of the three NFAs studiedare shown in Fig. 1A. One NFA, SBT-FIC, features a fully fusedmolecular backbone. The other two NFAs [A078 (22) andA134] with partially fused cores are isomers of SBT-FIC com-prising four thiophenes, two cyclopentadienes, and one ben-zene ring. The details of molecular design and the syntheticroutes to these molecules are provided in SI Appendix, SchemesS1–S3. In contrast to SBT-FIC that requires 10 synthetic stepsstarting from 4,8-dihydrobenzo[1,2b:4,5-b′]dithiophene-4,8-dione, the costs of A078 and A134 are potentially lower sincethey entail only 4–6 synthetic steps starting from 2,5-dibromo-terephthalic acid diethyl ester. In addition, the low synthesiscomplexity, high yield, less toxic precursors, and inexpensivestarting materials enable A078 and A134 to be economicallyproduced at large scale.The ultraviolet-visible (UV-vis) absorption spectra of the

NFAs are shown in Fig. 1 B and C. Surprisingly, thin films ofA078 and A134 exhibit significant bathochromic shifts of∼135 nm compared to SBT-FIC with an absorption peak atλmax = 900 nm. Contrary to expectations, closer molecularpacking leading to the spectral shifts is obtained when a rotationis imposed by the single bond between indaceno[1,2-b:5,6-b′]dithieno[3,2-b]thiophene (IDT) and the flanking thiophene in

A078 and A134. Density-functional theory (DFT) calculations atthe generalized gradient approximation/triple zeta polarized setlevel show that the S···S distance between 2-ethylhexanethiolgroup on the thiophene and the IDT (3.18 Å) unit is far shorterthan the sum of the van der Waals radii of the two sulfur atoms(3.68 Å) (35). This reveals that noncovalent S···S interactionsrigidify the conjugated structure (36), resulting in a reduction ofthe torsion angle between the thiophene and IDT from 20° to2.5°(SI Appendix, Fig. S1).Cyclic voltammetry in SI Appendix, Fig. S2 gives the highest

occupied (HOMO) and lowest unoccupied (LUMO) molecularorbital energies of EH = −5.81 (±0.02) and EL = −4.15 (±0.03)eV for SBT-FIC, −5.58 (±0.02) and −4.06 (±0.03) eV for A078,and −5.54 (±0.02) and −4.05 (±0.03) eV for A134, respectively.Both A078 and A134 show a lower HOMO–LUMO energy gap(∼1.40 eV) than SBT-FIC (∼1.65 eV), which is consistent withoptical measurements. Moreover, A078 and A134 exhibit shal-lower LUMO energies compared with SBT-FIC, which leads toan improvement of VOC in OPVs.The NFAs blended with PCE-10 were employed in OPVs with

the structure ITO/ZnO (30 nm)/active layer (∼100 nm)/MoO3(20 nm)/Ag (100 nm) (see fabrication details in Methods). Theircurrent density–voltage (J−V) characteristics are plotted inFig. 2A, with a summary of performance measured under 1 sunintensity, simulated AM1.5G solar spectral illumination in Ta-ble 1. Here, PCE = 13.0 ± 0.4% is achieved in the A078-baseddevice, with VOC = 0.75 ± 0.01 V, JSC = 24.8 ± 0.7 mA cm−2, andFF = 0.70 ± 0.04. In contrast, the A134-based OPV exhibitedPCE =7.6 ± 0.2% with VOC = 0.75 ± 0.01 V, JSC = 16.7 ± 0.5mA cm−2, and FF = 0.61 ± 0.03. For the PCE-10:SBT-FIC de-vice, PCE = 7.8 ± 0.3% with VOC = 0.70 ± 0.01 V, JSC = 17.2 ±0.7 mA cm−2, and FF = 0.65 ± 0.02. Interestingly, the1-phenylnathalene (PN) additive results in dramatic improve-ments in efficiency of the A078 and A134 devices comparedwith SBT-FIC, which is due to the improved molecular packingof the A078 and A134 as well as more favorable molecularorientations in the blends (see below, this section). Note thatthe PCE-10:A134 device shows a lower PCE compared to the

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Fig. 1. (A) Molecular structural formulae of the SBT-FIC, A078, and A134. (B) UV-vis absorption spectra of SBT-FIC, A078, and A134 in toluene solution, and(C) in thin films.

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PCE-10:A078 OPV due to the crystallinity of A134, leading toits lower solubility.Fig. 2B shows the external quantum efficiency (EQE) spectra

of the several devices. The significant improvement in JSC for theA078 compared to the SBT-FIC OPV is attributed to its ∼200-nm absorption redshift that provides solar spectral coveragefurther into the NIR. The long-wavelength cutoff of A078 andA134 at λ = 1,000 nm is compared to ∼800 nm for SBT-FIC.The EQE of the A078 OPV reaches 80%, between λ = 700 and900 nm, while leaving a transparency window between the vis-ible wavelengths of 400 and 650 nm. The integrated JSC areshown in Table 1, which are consistent with the solar simulationmeasurements.To better understand the performance of these devices, the

absorption profiles of the NFA neat films and PCE-10:NFAsblends with and without the PN additive are shown in SI Ap-pendix, Fig. S3 and Fig. 3 A–C. The PCE-10:SBT-FIC film ab-sorption shows little change by employing PN. In contrast, a new,pronounced aggregation peak around 900 nm is found in bothPCE-10:A078 and PCE-10:A134 blends, indicating that the ad-ditive enhances intermolecular π–π interactions on partiallyfused acceptors rather than on the polymer donor.The morphological properties are investigated using grazing-

incidence wide-angle X-ray scattering (GIWAXS), with a sum-mary of parameters obtained in Table 2 and SI Appendix, Fig. S2.A078 shows a broad (100) diffraction peak at 0.31 Å−1 with alamellar coherence length of Lc = 7.5 nm, while a narrower andsharper diffraction peak (100) at 0.36 Å−1 is observed in A134with increased Lc = 15 nm (SI Appendix, Fig. S4). This suggestsA134 has increased ordering compared to A078, afforded byreplacing the bulk p-hexylphenyl sidechain with the compactlinear alkyl chains (37). On the other hand, SBT-FIC shows adiffraction peak at 0.34 Å−1 with the smallest lamellar coherencelength of Lc = 3.7 nm due to its amorphous nature. The (010)diffraction peaks of PCE-10:A078 and PCE-10:A134 in Fig. 3Eat 1.79 and 1.82 Å−1 (due to NFAs) are shifted and show anincreased coherence length (24 vs. 52 Å, for A078) and (30 vs.63 Å, for A134) when employing the PN additive. In contrast, Lc

of PCE-10 is unchanged, which further confirms that morpho-logical differences arise primarily from the NFAs rather than thedonor. There is no obvious diffraction peak and coherencelength variation for both the donor and acceptor in PCE-10:SBT-FIC blends by employing PN. These results explain the signifi-cant improvement in PCE for the A078 and A134 OPV by usingthe additive, leading to improved stacking of large and orderedaggregates compared to SBT-FIC. Additionally, a dependenceon molecular orientation from edge-on to face-on is found inusing PN. For PCE-10:A078, the face-on/edge-on ratio extractedfrom the pole figures of (100) peaks increases from 2.37 to 3.64(Fig. 3D). Since the face-on orientation is favorable for inter-molecular charge transport, it helps to describe why the highestefficiency is achieved in the A078 device.The A078 OPV is further exploited by ST-OPVs with the

structure ITO/ZnO (30 nm)/PCE-10: A078 (95 nm)/MoO3 (20nm)/Ag (16 nm). The J-V, optical transmission, and EQE spec-tral characteristics are shown in Fig. 4, with results summarizedin Table 3. The ST-OPV showed the LUE = 2.8 ± 0.1%, withPCE = 11.0 ± 0.7% and APT = 25.0 ±1.3%. Although PCE >10% is obtained, the low APT limits its applications for archi-tectural glass that requires APT ∼50% (1). To solve this prob-lem, a structure was designed to control the device opticalproperties to achieve maximum transmission in the visible whilereflection is maximized in the NIR. An optical OC coatingconsisting of four layers: CBP (35 nm, index of refraction, nCBP =1.90)/MgF2 (100 nm, nMgF2 = 1.38)/CBP (70 nm)/MgF2 (45 nm)was deposited onto the Ag anode surface, and an ARC con-sisting of a bilayer of 120-nm-thick MgF2 and 130 nm of lowrefractive index SiO2 (nSiO2 = 1.12) (38) was deposited onto thedistal surface of the glass substrate. The ST-OPV with an OCand ARC shows APT increases from 25.0 ± 1.3% to 45.7 ± 2.1%,representing a nearly 80% improvement than that of a cellwithout these layers. As shown in Fig. 4C, the PCE of the ST-OPV is almost as same as its initial value, with only a slightdecrease in JSC (20.4 ± 0.8 vs. 20.9 ± 1.2 mA cm−2). Hence, theLUE = 5.0 ± 0.3% is achieved with the OC and ARC coatings,which to our knowledge, is the highest reported efficiency forST-OPVs.The device appearance was examined using AM1.5G simu-

lated solar illumination. The transmitted light of the OC andARC-coated device has 1931 CIE chromaticity coordinates of(0.33, 0.39) with CCT = 5,585 K. Note that the high reflectivity ofthe ultrathin Ag cathode at λ > 600 nm gives the device a greentint shown in SI Appendix, Fig. S5, which is apparent from itscolor coordinates. In contrast to Ag, ITO has a higher trans-parency with a flat in transmission spectrum across the visible.Using an ITO cathode and anode results in a more neutral hue.The ITO-based ST-OPVs with the following structure: MgF2(120 nm)/ITO glass/ZnO (30 nm)/PCE-10: A078 (105 nm)/MoO3 (20 nm)/sputtered ITO (140 nm)/MgF2 (145 nm)/MoO3(60 nm)/MgF2 (190 nm)/MoO3 (105 nm) exhibit J-V, transmis-sion, and EQE spectral characteristics in Fig. 5 and SI Appendix,

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Fig. 2. (A) Current density (J)–voltage (V) characteristics, and (B) EQEspectra of organic photovoltaic cells based on PCE-10: SBT-FIC (1:2, wt/wt),PCE-10:A078 (1:2, wt/wt), and PCE-10:A134 (1:2, wt/wt).

Table 1. Operating characteristics of OPVs under simulated of AM 1.5G, 100 mW cm−2,illumination

Device* JSC†, mA/cm2 VOC, V FF PCE, % Eloss, eV

PCE-10:SBT-FIC (wo/PN) 18.1 ± 0.6 (17.2) 0.70 ± 0.01 0.62 ± 0.03 7.9 ± 0.3 0.79PCE-10:SBT-FIC (w/PN) 17.2 ± 0.7 (16.5) 0.70 ± 0.01 0.65 ± 0.02 7.8 ± 0.3 0.79PCE-10:A078 (wo/PN) 22.2 ± 1.0 (21.3) 0.76 ± 0.01 0.56 ± 0.02 9.5 ± 0.3 0.55PCE-10:A078 (w/PN) 24.8 ± 0.7 (24.3) 0.75 ± 0.01 0.70 ± 0.04 13.0 ± 0.4 0.55PCE-10:A134 (wo/PN) 18.5 ± 0.5 (17.4) 0.75 ± 0.01 0.45 ± 0.03 6.2 ± 0.3 0.54PCE-10:A134 (w/PN) 16.7 ± 0.5 (16.1) 0.75 ± 0.01 0.61 ± 0.03 7.6 ± 0.2 0.54

*The values in parentheses are calculated from the integral of the EQE spectra.†The average value is based on measurement of 16 devices with donor: acceptor blending ratio of 1:2.

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Fig. S6, with results summarized in Table 3. Compared to theAg-based ST-OPV, the ITO-based device shows differences inFF and VOC due to its higher work function and sheet resistance(∼50 Ω/sq). The largest differences are found in JSC and PCE. Asthe device becomes increasingly transparent, the reflection fromthe ITO anode into the thin active region is decreased, elimi-nating the double pass of photons. To minimize the loss of low-energy photons, the OC coating is designed with a transmissionmaximum in the visible while being more reflective at longerwavelengths. Therefore, the device with the OC has a 15%higher JSC and PCE compared with the uncoated ITO device,although the visible transparency is nearly unchanged. The OC-coated ITO device exhibits an LUE = 3.5 ± 0.1% with PCE =8.1 ± 0.3% and APT = 43.3 ± 1.5%, and it has a near-neutralappearance. Fig. 5C shows the MacAdam ellipses drawn alongthe Planckian locus in the 1931 CIE color space. The chroma-ticity coordinates of (0.38, 0.39) are achieved in the OC-coated

ITO device, shown by the blue boxes used for binning white light-emitting diode illumination sources. Moreover, this ST-OPVachieves a color-rendering index of CRI = 86 and a CCT = 4143K. This high CRI indicates that illumination through the OPVwindow accurately renders the color of an object (Fig. 5D). Thetransmission spectra are dependent on light incidenceangles >60° from normal, which can be adjusted for each latitudeof window application by appropriate design of the opticalcoatings in SI Appendix, Fig. S7.

DiscussionThe foregoing results point to a trade-off between transparencyand efficiency that can be minimized by the appropriate choiceof absorbing materials, transparent contacts, and optical coat-ings. From these results, we can estimate the performanceachievable for ST-OPVs based on thermodynamic limitations forideal OPVs calculated by Giebink et al. (39). To arrive at a

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Fig. 3. (A) UV-vis absorption spectra of SBT-FIC and PCE-10:SBT-FIC (1:2, wt/wt) blend films. (B) A078 and PCE-10:A078 (1:2, wt/wt) blend films. (C) A134 andPCE-10:A134 (1:2, wt/wt) blend films with and without the PN additive. (D) Corrected polar (100) diffraction peaks and (E) in-plane (black line) andout-of-plane (red line) X-ray scattering patterns extracted from GIWAXS data of PCE-10:SBT-FIC, PCE-10:A078, and PCE-10:A134 blends with and without thePN additive.

Table 2. Morphological parameters extracted from GIWAXS measurements

Device Peak*, Å−1π–π stackingdistance, Å FWHM†, Å−1

Coherencelength, Å Face-on/edge-on ratio

Donor NFA Donor NFA Donor NFA Donor NFA

PCE-10:SBT-FIC (wo/PN) 1.70 1.61 3.69 3.90 0.26 0.56 24 11 0.65PCE-10:SBT-FIC (w/PN) 1.70 1.59 3.69 3.95 0.27 0.56 23 11 0.71PCE-10:A078 (wo/PN) 1.67 1.79 3.76 3.51 0.53 0.26 12 24 2.37PCE-10:A078 (w/PN) 1.70 1.83 3.69 3.43 0.50 0.12 13 52 3.64PCE-10:A134 (wo/PN) 1.69 1.82 3.72 3.45 0.50 0.21 13 30 2.34PCE-10:A134 (w/PN) 1.70 1.87 3.69 3.36 0.47 0.10 13 63 2.57

*The value extracted from the (010) peak.†Full width at half-maximum.

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practical estimate, we assume that EQE = 90% is constant acrossthe visible spectrum up to the optical energy gap of the absorbinglayer, and FF = 0.75––values that are close to the state-of-the-artOPV performance. The Eloss is defined as the difference betweenthe lowest absorbed photon energy and VOC (40). Fig. 6A plotsthe calculated efficiency as a function of the optical energy gapwith different Eloss (41). The visible transparency of T = 50%(T50) provides a neutral density appearance that is typical ofmany windows used in residences and commercial buildings, andT = 100% (T100) corresponds to a completely transparent fixture.Interestingly, compared to the T50 device, the efficiency of T100 isnot significantly reduced. For example, for Eloss = 0.5 eV and anenergy gap of Eg = 1.4 eV, we obtain PCE = 15.5% for T = 50%,and 10.5% for T = 100%. This is indicative of the dispropor-tionate amount of solar flux available in the NIR, and the im-portance of employing cells that are designed to preferentiallyharvest long-wavelength radiation. Indeed, the ease of tailoringcell transparency is a particular advantage of OPVs compared toinorganic thin-film solar cells. Therefore, OPVs with appropriateoptical designs can achieve a combination of both high transparencyand efficiency.Another finding is the remarkable bathochromic shift of the

absorption spectra of partially fused compared to fully fusedmolecules achieved by employing a solvent additive. As previ-ously, molecules with rigid and planar conformations allowparallel p-orbital interactions to extend their effective conjuga-tion and facilitate π-electron delocalization between molecules(42, 43). This, in turn, leads to a decrease in the bond-length

alternation and reduction of the energy gap, Eg (44). However, alower Eg is obtained in molecules (A078 and A134) with ap-parent rotational disorder rather than rigid molecules with fullyfused rings (SBT-FIC). DFT calculations show that intermolec-ular S···S interactions in A078 and A134, result in a larger torsionangle than otherwise would exist between the central fused-ringcore and the flanking thiophenes. This allows for substantialoverlap between neighboring molecules in the solid state, givingrise to more ordered molecular packing and a reduced Eg. Totest the importance of these noncovalent interactions, we studiedthree additional materials systems in SI Appendix, Fig. S8 andTable S3 (23, 44–48). Similar to the S···S interaction, the O···Sinteraction in IEICO-4F (23) also shows a bathochromic shift ofabsorption compared to the fully fused molecule BT-FIC (46). Incontrast, IEIC (48) with a 2-ethylhexanethiol group leads to ahigher rotational disorder and larger Eg compared to the cova-lent rigid molecule CBT-IC (47). These results are consistentwith the conclusion that noncovalent conformational locks pro-vide a driving force to planarize and rigidify π-conjugatedbackbones that lead to a reduction in band gap (49–51).With rapid development of new NFAs, ST-OPVs exhibit

considerable potential for achieving high performance and apleasing appearance. However, their relatively low stability re-mains a barrier for their commercialization. For example, themost stable NFA devices reported degrade within only a fewyears (extrapolated to 80% of their initial PCE) (52). This iscompared to the extrapolated intrinsic lifetimes of fullerene-based cells of thousands of years, as recently reported by our

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Fig. 4. (A) Schematic of the semitransparent device showing optimized layer thicknesses and compositions. (Right) Detailed layer structures of the OC and ARlayers. (B) Current density vs. voltage characteristics, (C) EQE spectra, and (D) measured optical transmission and reflection of the optimized semitransparentcells with and without the OC and ARC layers.

Table 3. Operating characteristics of semitransparent, neutral-colored OPVs

Device* JSC, mA/cm2 VOC, V† FF PCE, % APT, % LUE, % CIE CCT

Ag wo/OC and ARC 20.9 ± 1.2 0.75 0.70 ± 0.03 11.0 ± 0.7 25.0 ± 1.3 2.8 ± 0.1 (0.27, 0.34) 9,021Ag w/OC and ARC 20.4 ± 0.8 0.75 0.70 ± 0.03 10.8 ± 0.5 45.7 ± 2.1 5.0 ± 0.1 (0.33, 0.39) 5,585ITO wo/OC and ARC 14.3 ± 0.5 0.73 0.68 ± 0.04 7.1 ± 0.4 46.7 ± 1.0 3.3 ± 0.1 (0.34, 0.40) 5,266ITO w/OC and ARC 16.3 ± 0.4 0.73 0.68 ± 0.04 8.1 ± 0.3 43.3 ± 1.5 3.5 ± 0.1 (0.38, 0.39) 4,143

*The average value is based on measurement of eight devices.†Error is ±0.01 V.

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group (53). Nevertheless, stretchable, foldable, lightweight andmechanically resilient form factors enabled by OPVs will en-courage continued developments in reliability as these devicesopen new applications for attachment to irregular surfaces ontextiles and robotic machinery (54, 55).

ConclusionsWe demonstrated an ST-OPV with PCE = 10.8 ± 0.6%, APT =45.7 ± 2.1%, and LUE = 5.0 ± 0.3% by utilizing a partly fusedNFA-based, NIR absorbing donor–acceptor in a bulk hetero-junction. The S···S interaction between 2-ethylhexanethiol groupand central IDT unit planarizes and rigidifies the π-conjugatedmolecules. Consequently, the partly fused NFAs aggregate,leading to a reduced Eg with absorption peak at λ > 900 nm.Interestingly, the intermolecular π–π interactions of the partlyfused NFAs are enhanced by employing a PN additive thatcontributes to an increased JSC and FF. Furthermore, with theapplication of a transparent ITO anode, we demonstrated an ST-OPV with both neutral color and high efficiency. The optimizeddevice exhibits PCE = 8.1 ± 0.3%, APT = 43.3 ± 1.5%, and

LUE = 3.5 ± 0.1%, with CIE coordinates of (0.38, 0.39), CCT =4,143 K, and CRI = 86.

MethodsMaterials. All devices were grown on patterned ITO substrates with sheetresistances of 15 Ω/sq. The acceptors (4,4,10,10-tetrakis(4-hexylphenyl)-5,11-(2-ethylhexylsulfanyl)-4,10-dihydro-dithienyl[1,2-b:4,5b′]benzodi-thiophene-2,8-diyl)bis(2-(3-oxo-2,3-dihydroinden-5,6-difluro-1-ylidene)malononitrile)SBT-FIC; 2-((E)2-((5-(7-(5-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2ylidene)methyl)-3-((2-ethylhexyl)thio)-thiophen-2-yl)-4,4,9,9tetrakis(4-hexylphenyl)4,9-dihydrosindaceno[1,2b:5,6b′]dithiophen-2-yl)-4-((2ethylhexyl)thio)thiophen-2yl)methylene)-5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile A078; 2-((E)2-((5-(7-(5-(((Z)-1-(dicya-nomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2Hinden-2ylidene)methyl)-3-((2-ethylhexyl)thio)thio-phen-2-yl)-4,4,9,9-tetrakis-hexyl4,9-dihydrosindaceno[1,2b:5,6b′]dithiophen-2-yl)-4-((2-ethylhexyl)thio)thiophen-2yl)methylene)-5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1ylidene)malononitrile A134were synthesizedin our laboratories. Other materials were purchased from commercial suppliers:MoO3 (Acros Organics); 4,4′-Bis(N-carbazolyl)-1,1′- biphenyl (CBP, LuminescenceTechnology Corp.); MgF2 (Kurt J. Lesker Corp.); Ag (Alfa Aesar); and Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′] dithiophene-2,6-diyl -alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2–6-diyl)](PCE-10, 1-Material).

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Fig. 5. (A) Current density vs. voltage characteristics, (B) optical transmission and reflection of ITO-based semitransparent cells with and without the OC andARC layers, (C) MacAdam “ellipses” along the Planckian locus. The black boxes are the American National Standards Institute, ANSI C78.377 standard forvariations of an acceptable lighting source at a particular CCT) and the blue boxes show bins used to group illuminants whose CCT and CRI fit within ap-proximately a three-step ellipse. Chromaticity coordinates of the transmission spectra of the ITO cathode device with an OC and ARC (blue cross), ITO devicewithout OC and ARC (red circle), 16-nm Ag device with OC and ARC (orange triangle) using an AM1.5G solar reference input spectrum. (D) Photograph of theoutdoor image through the (Left) ultrathin Ag and (Right) ITO semitransparent device.

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Fig. 6. Calculated PCE vs. optical energy gap with energy loss as parameters for a (A) cell transparency of T = 50% and (B) T = 100% at wavelengths <650 nm.

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Device Fabrication and Characterization. Prepatterned ITO-coated glass sub-strates with sheet resistances of 15 Ω/sq were purchased from Lumtec. Thesubstrates were cleaned using a detergent (tergitol solution) and solventsfollowed by CO2 snow cleaning and exposed to UV ozone for 15 min beforefilm growth. The ZnO layer precursor solution was spin cast and then ther-mally annealed at 150 °C for 45 min in air. The active layer, PCE-10:NFA (1:2wt/wt), was spin-coated from toluene solution (total concentration 14 mgmL−1), followed by the 1-phenylnathalene additive (0.5%). The vacuum-deposited MoO3 and Ag were grown at ∼0.6 Å/s in a vacuum chamberwith a base pressure of 2 × 10−7 Torr. The areas of the OPVs are defined bythe patterned ITO cathode and the Ag anode deposited through a 50-μm-thick shadow mask, resulting in a device area of 0.04 cm2. ST-OPVs used thesame fabrication procedures as the opaque cells. The ITO anode was de-posited at a rate of 1 Å/s from a In2O3:Sn target by direct current magnetronsputtering in a chamber with a base pressure of 1 × 10−6 Torr. The dopingdensity was varied by adjusting the flow of oxygen into the chamber whilethe substrate holder was rotated at 10 rpm. The OC coating was grown byvacuum thermal evaporation (VTE) in a chamber with a base pressure of 1 ×10−7 Torr at 1 Å/s for MgF2, 0.6 Å/s for CBP and 0.6 Å/s for MoO3. The ARCwas grown onto the glass substrate after the devices were completed. MgF2was deposited by VTE at a rate of 1 Å/s, and the SiO2 was grown by electron-beam deposition on the substrate held at an angle of 85° to the source.Glancing incidence deposition results in a porous film with a refractive indexof 1.1.

The current density–voltage (J-V) characteristics and EQEs of the cells weremeasured in a ultrapure N2 atmosphere. The EQE measurements were per-formed using a 200-Hz chopped, monochromated, and focused beam from aXe lamp. The beam is focused to underfill the device area. The current fromthe devices and from a National Institute of Science and Technology-traceable Si reference detector were recorded using a lock-in amplifier.Light from a Xe lamp filtered to achieve a simulated AM 1.5G spectrum(American Society for Testing and Materials, ASTM G173-03) was used as thesource for J-Vmeasurements. The spectral mismatch factors are calculated tobe from 1.006 to 1.009. The lamp intensity is varied using neutral densityfilters and calibrated by a National Renewable Energy Laboratory certified Sireference cell. The cells were measured using a 3.24-mm2 metal mask atintensities from 0.001 to 1 sun (100 mW/cm2). ST-OPVs were measured fromthe ITO side with no object behind the cells. Errors account for measurementvariations from three or more cells. There is also a systematic error of 5% forJSC and PCE.

Optical and Electrochemical Characterization. The reflection spectra of thedevices were determined using an F20, Filmetrics thin-film measurementinstrument integrated with a spectrometer and light source. The layerthicknesses and refractive indexes were measured using spectroscopicellipsometry (WVASE32, J. A. Woollam). The absorption and transmissionspectra were measured using UV-vis spectrometer (Perkin-Elmer 1050). Op-tical simulations of the single junction based on the transfer matrix methodused MATLAB along with the measured J-V characteristics of each cell. Thefour-point probe method (FPP-5000, Miller Design & Equipment) was usedfor sheet-resistance measurements. Cyclic voltammetry employed acetonitrile

with 0.1 M of tetrabutylammonium hexafluorophosphate at a scan rate of100 mV s−1. ITO, Ag/AgCl, and Pt mesh were used as the working, reference,and counter electrode, respectively. All measurements were performed at ascan rate of 100 mVs−1.

GIWAXS. GIWAXS patterns of the thin films were performed at the StanfordSynchrotron Radiation Light Source beamline 11–3 (56) in a He-filledchamber with an X-ray energy of 12.7 KeV (at the critical angle of 0.13° ofthe films) and LaB6 was used for geometry calibration. Samples for GIWAXSwere prepared on top of Si (100) substrates. The raw two-dimensional X-raydata were processed with a modified version of NIKA into one-dimensionalscattering profiles I(q).

Average Photopic Transparency. The APT is calculated using

APT = ∫ T (λ)P(λ)S(λ)d(λ)∫ P(λ)S(λ)d(λ) ,

where λ is the wavelength, T is the transmission, P is the normalized phot-opic spectral response of the eye, and S is the solar irradiance.

Data Availability. All study data are included in the article and SI Appendix,including cyclic voltammetry measurements, genetic algorithm calculations,and synthesis details.

ACKNOWLEDGMENTS. This work is supported by the US Department ofEnergy’s Office of Energy Efficiency and Renewable Energy under Solar En-ergy Technologies Office Agreement DE-EE0008561. This report was pre-pared as an account of work sponsored by an agency of the USgovernment. Neither the US government nor any agency thereof, nor anyof their employees, makes any warranty, express or implied, or assumes anylegal liability or responsibility for the accuracy, completeness, or usefulnessof any information, apparatus, product, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference herein toany specific commercial product, process, or service by trade name, trade-mark, manufacturer, or otherwise does not necessarily constitute or imply itsendorsement, recommendation, or favoring by the US government or anyagency thereof. The views and opinions of authors expressed herein do notnecessarily state or reflect those of the US g or any agency thereof. Also, theauthors received support from the US Department of the Navy, Office ofNaval Research under Award N00014-17-1-221, and Universal Display Corp.X.G. and M.Z. acknowledge financial support from National Natural ScienceFoundation of China (Grants 51773142 and 51973146), the Jiangsu ProvincialNatural Science Foundation (Grant BK20190099), Collaborative InnovationCenter of Suzhou Nano Science & Technology. Z.P. and H.A. gratefully ac-knowledge the support by ONR Grant N000141712204. X-ray data were ac-quired at both the Advanced Light Source, Lawrence Berkeley NationalLaboratory, and the Stanford Synchrotron Radiation Lightsource, SLAC Na-tional Accelerator Laboratory. Use of the Advanced Light Source was sup-ported by the Director, Office of Science, Office of Basic Energy Sciences, ofthe US Department of Energy under Contract DE-AC02-05CH11231.

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