Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 28062−28068
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Phototriggered Depolymerization of Flexible Poly(phthalaldehyde) Substrates by Integrated Organic Light-Emitting Diodes Kyung Min Lee,† Oluwadamilola Phillips,‡ Anthony Engler,‡ Paul A. Kohl,‡ and Barry P. Rand*,†,§ †
Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, United States Department of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, United States § Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey 08544, United States Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on January 24, 2019 at 16:37:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: We demonstrate phototriggered depolymerization of a low ceiling temperature (Tc) polymer, poly(phthalaldehyde) (PPHA), via internal light emission from integrated organic light-emitting diodes (OLEDs) fabricated directly on flexible PPHA substrates with silver nanowire electrodes. The depolymerization of the PPHA substrates is triggered by absorption of the OLED emission by a sensitizer that activates a photoacid generator via energetically favorable electron transfer. We confirm with Fourier-transform infrared spectroscopy that the photon doses delivered by the integrated OLED are sufficient to depolymerize the PPHA substrates. We determine this critical dosage by measuring the operating lifetimes of the OLEDs whose failure is believed to be due to significant mechanical softening during the liquefaction of decomposed phthalaldehyde monomers. KEYWORDS: transient electronics, transient polymer, low ceiling temperature polymer, organic light-emitting devices
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INTRODUCTION Transient electronics with triggerable and controlled degradation are an emerging class of devices with the unique property that allows them to become untraceable after a period of stable operation. Many transient systems have been demonstrated in imaging,1 template materials,2,3 and biocompatible devices4−9 in which their disappearance is accomplished by vaporization, liquefaction, or dissolution in solvents. A particularly effective class of transient materials well-suited for dry applications are low ceiling temperature (Tc) polymers; such low Tc polymers can be rendered kinetically stable in ambient conditions by end-capping or cyclization of the chain and can be very efficiently depolymerized by backbone bond cleavage when triggered by external stimuli such as heat or acid.10,11 Poly(phthalaldehyde) (PPHA) is a low Tc (−43 °C) polymer that has received much attention for their ease of synthesis, promising mechanical properties, and fast depolymerization rates.12,13 Copolymers of phthalaldehyde and other aldehydes which have different ceiling temperatures and vaporization rates have also been demonstrated.14 As such, PPHA has been used as substrates for transient devices and successfully depolymerized by applying heat15 or acid.16 In the latter case, the acid trigger is generated by incorporating a UVabsorbing photoacid generator (PAG) in the PPHA and using UV light to activate the PAGs. We also note that poly(phthalaldehyde) is an amorphous polymer that thermally degrades before reaching a glass transition. Neat PPHA films © 2018 American Chemical Society
are brittle, with modulus values in the range of 1−3 GPa at higher molecular weight. The mechanical properties of PPHA films loaded with PAG and photosensitizer vary minimally compared to the neat polymer. The addition of liquid plasticizers greatly improves the flexibility of films at the cost of lowering the modulus. Examples of how PPHA properties respond to different plasticizers have been reported elsewhere.17,18 A particularly useful development in extending the wavelength selectivity of PAGs from ultraviolet to visible is the use of visible light sensitizers that absorb lower-energy photons and activate the PAGs via electron transfer.19,20 In this work, we employ visible light sensitive PPHA films using a photoacid generator Rhodorsil-Faba (4-isopropyl-4′-methyldiphenyliodonium tetrakis(pentafluorophenyl)borate) and BPET (5,12bis(phenylethynyl)tetracene) as a photosensitizer. Absorption of wavelengths between 450 and 600 nm transitions BPET to an excited singlet state where an exothermic electron transfer reaction occurs upon collision with a PAG molecule. Products of the photoinduced electron transfer undergo rapid, radical decomposition which results in the release of a strong Brønsted acid. A simplified mechanism is shown in Scheme 1a. Received: May 17, 2018 Accepted: July 24, 2018 Published: July 24, 2018 28062
DOI: 10.1021/acsami.8b08181 ACS Appl. Mater. Interfaces 2018, 10, 28062−28068
Research Article
ACS Applied Materials & Interfaces
Scheme 1. (a) Generation of Acid via Electron Transfer from the Excited Singlet State of BPET to PAG; (b) Depolymerization of PPHA to Monomers by Protonation of Acetal Linkages; and (c) Proposed Mechanism of Phototriggering the Depolymerization of s-PPHA with an Integrated OLED and the Resulting System That Is Mechanically Softeneda
a
The phototriggered substrate (PHA) contains reacted PAG and BPET that are chemically altered.
Figure 1. (a) Device structure of the OLEDs. (b) Current density (J) and forward luminance (L) vs voltage for green phosphorescent OLEDs on a PPHA/AgNW substrate. (c) External quantum efficiency (EQE) vs J and a photograph of two neighboring working devices (inset). (d) Normalized electroluminescence (EL) spectra for various viewing angles.
spectral power, as opposed to broadband or high energy sources, such as sunlight or UV lamps. We show that integrated flexible OLEDs can successfully depolymerize s-PPHA substrates before the end of their lifetimes (defined as the time before failure to emit light). This finite OLED lifetime is attributed to mechanical failure of one or more of the thin organic layers within the OLED as significant softening16 of the substrate occurs as depolymerized monomers liquefy before solidifying again by removal of heat by convection or vaporization.14 We confirm by Fouriertransform infrared spectroscopy (FTIR) that the phototriggered substrates predominantly contain depolymerized monomeric species. Furthermore, as the depolymerization mecha-
We trigger this depolymerization mechanism (Scheme 1a and b) with a green-emitting organic light-emitting diode (OLED) fabricated directly on a flexible sensitized PPHA (sPPHA) substrate with embedded silver nanowire (AgNW) electrodes. Scheme 1c illustrates this concept in which operation of the OLED renders the substrate depolymerized and mechanically softened, causing the OLED to fail. Such integrated devices enable a compact and flexible packaging and footprint, and to our knowledge this is the first report of a transient polymer substrate whereby the triggering mechanism is fully integrated. Moreover, the use of light-emitting device emission coupled to a sensitizer with an appropriate absorption spectrum results in a far more efficient utilization of their 28063
DOI: 10.1021/acsami.8b08181 ACS Appl. Mater. Interfaces 2018, 10, 28062−28068
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Absorbance and normalized photoluminescence (PL) spectra of s-PPHA film with 1% PAG and 0.5% BPET. Normalized EL spectrum of the Ir(ppy)2(acac) OLED. (b) Device structures of the OLEDs on glass/ITO (left) and flexible AgNW substrate (right). (c) Measured spectra from the glass/ITO OLED with a s-PPHA (10% PAG) film integrated on the substrate backside. The structure is given in panel b (left). Continuous voltage bias is applied for 10 min after which the device is stored in the dark for 7 days. The inset shows a photograph of a substrate backside where the two pixels located in dashed boxes have been depolymerized. The pixel on the bottom right is photographed after obtaining the spectra in panel c (10 min irradiation). (d) Measured spectra from a flexible OLED on a s-PPHA (1% PAG) substrate with the structure given in panel b (right). Continuous voltage bias is applied for 90 s. The inset shows a photograph of the OLEDs showing phototriggered pixels in a dashed box.
nism is catalytic12 (i.e., one acid molecule will depolymerize an entire polymer chain), the bulk of the phototriggered substrate will continue to depolymerize, and their vaporization is limited by the low vapor pressure of phthalaldehyde monomers.14
show virtually no dependence on viewing angle. A green phosphorescent emitter, Ir(ppy)2(acac), with an electroluminescence (EL) peak at approximately 520 nm, is selected for its large spectral overlap with the sensitizer absorption (Figure 2a) and its preferential horizontal orientation that allows for efficient light outcoupling into the substrate,25−27 necessary to ensure a sufficient photon dose is delivered to the substrate. Additionally, PPHA exhibits a relatively high refractive index (n > 1.6, Figure S1b) for a plastic material which further traps light in the substrate at the substrate/air interface. We expect a reflection loss of about 60% at this substrate/air interface and estimate the total power delivered to PPHA substrates (both neat and sensitized) from the measured outcoupled power from the OLEDs on a neat PPHA substrate. Identical OLEDs are fabricated on s-PPHA substrates (structure shown in Figure 2b, right), fabricated under red LED lighting at wavelengths beyond the sensitizer absorption spectrum to minimize acid generation during processing. The s-PPHA substrates used in this work are prepared by adding equimolar blends of PAG and BPET, or 2:1 weight ratio, to neat PPHA, which is optimized for one-to-one electron transfer. We compare two concentrations: 10 wt % PAG (5 wt % BPET) and 1 wt % PAG (0.5 wt % BPET). To gain insight about the response of s-PPHA exposed to OLED irradiation, we fabricate OLEDs on a rigid glass/ITO substrate and deposit a film of s-PPHA (10% PAG) on the backside of the glass. This configuration, shown in Figure 2b
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RESULTS AND DISCUSSION To properly characterize the performance of phosphorescent OLEDs on PPHA substrates, we start with devices on neat PPHA substrates lacking the sensitizer and PAG as OLEDs on s-PPHA substrates possess phototriggered degradation. A schematic of the structure of OLEDs fabricated on AgNW electrodes embedded in PPHA films is shown in Figure 1a, and the current density−luminance−voltage characteristics of the OLEDs are given in Figure 1b. We obtain a peak external quantum efficiency (EQE) of 8.5% (Figure 1c), a value significantly lower than the optimized OLEDs21 using cathodes such as Al, due to lower reflectivity and increased absorption losses in the ytterbium (Yb) cathode (Figure S1a). However, the use of a low work function metal, such as Yb, that evaporates at lower temperatures22 is necessary to prevent the PPHA substrate from thermally decomposing during fabrication as PPHA has a decomposition temperature of ∼130 °C. In calculating the EQE of the flexible OLED in Figure 1, we assume a Lambertian emission pattern which is an appropriate approximation for substrates that are strongly scattering, in our case due to a dense AgNW network.23,24 This assumption is further supported by the emission spectra in Figure 1d, which 28064
DOI: 10.1021/acsami.8b08181 ACS Appl. Mater. Interfaces 2018, 10, 28062−28068
Research Article
ACS Applied Materials & Interfaces
Figure 3. FTIR spectra of s-PPHA on the substrate backside of phototriggered vs nonphototriggered (fresh) pixels fabricated on glass/ITO and AgNW electrodes showing: (a) decreased polymer acetal backbone peaks over 900−1100 cm−1 and emergence of C−O peak over 1150−1200 cm−1 and (b) increased monomer carbonyl peak across 1650−1700 cm−1.
(left), in which the s-PPHA film is external to the OLEDs allows us to study transient behavior of the s-PPHA films without causing the OLEDs to fail. As we continuously bias the OLED at 7 V for 10 min, we observe output emission spectra that change with time, as shown in Figure 2c. Initially (0 s), the spectrum reflects the emission of Ir(ppy)2(acac) filtered through the s-PPHA layer containing the absorbing BPET molecule that cuts off the higher energy emission of the OLED. Additionally, the device features down converted photoluminescence (PL) from BPET, as shown in Figure 2a. At slightly later time (1 min), we capture the emission from the OLED but now with considerable photobleaching of BPET where it can be seen that the OLED emission spectrum features dips associated with absorption peaks of BPET at wavelengths of 522 and 561 nm. Ultimately (10 min and later), the emission of the OLED reflects that of Ir(ppy)2(acac) because of complete photobleaching of BPET, indicating successful electron transfer events from BPET to PAG, which irreversibly render BPET a chemically different and nonabsorbing species.19 These electron transfer events initiate acid generation which depolymerizes PPHA even in the absence of continuous irradiation. We show that in Figure S2a, subsequent PL and excitation scans of a s-PPHA (10% PAG) film lead to decreased absorption and PL intensity, due to photobleaching of BPET. Therefore, after 10 min of irradiation, the spectrum almost completely returns to that of Ir(ppy)2(acac) predominantly due to photobleaching, and to a far lesser degree, thinning of the s-PPHA film due to vaporization of depolymerized phthalaldehyde monomers. Phthalaldehyde monomers typically take days to vaporize due to their low vapor pressure, and hence a significant fraction of the 60 μm-thick s-PPHA film likely remains after 10 min irradiation. The photograph of the phototriggered substrate backside (inset, Figure 2c) shows a visibly discolored s-PPHA film because of depolymerization during the 10 min irradiation. After 48 h in the dark, nearly complete vaporization of the sPPHA film can be seen visibly (Figure S3a) as the acids generated in the initial 10 min irradiation diffuse and depolymerize the unreacted polymer chains throughout the bulk of the film. After 7 days in the dark, we recover the Ir(ppy)2(acac) spectrum indicating further vaporization of the monomers. In contrast, the OLEDs integrated on s-PPHA (10% PAG) substrates do not behave the same way as the rigid OLEDs
with s-PPHA on the backside. Instead of photobleaching while still functioning, the flexible OLEDs entirely absorb the Ir(ppy)2(acac) emission and emit exclusively above 600 nm before they fail (Figure S2b), which on average takes about 20 s at 7 V. We note that the OLEDs on ITO with Al top contacts emit at least 3× more strongly than those on PPHA substrates with Yb contacts, and hence it is likely that the flexible OLEDs fail before acquiring enough dose (we estimate that at least 3 min of exposure is needed) to photobleach. We find that the conductive AgNW/TiO2 network embedded in a flexible sPPHA (10% PAG) substrate remains robust during the depolymerization process as it maintains reasonable sheet resistance well into 200 mJ/cm2 (1000 mJ/cm2 for 1% PAG) of monochromatic irradiation peaked at 520 nm (Figure S1c). However, we believe the rest of the OLEDs comprised of nmthick organic layers is less likely to survive the depolymerization process in which liquefaction of the depolymerized monomers leads to significant softening of the substrate. When we reduce the PAG loading to 1%, we begin to observe changes in outcoupled spectra from the flexible sPPHA substrate with a continued voltage bias at 7 V (Figure 2d). As this s-PPHA (1% PAG) is less absorptive, the OLED is partially transparent to the Ir(ppy)2(acac) emission at 0 s. Photobleaching of BPET evidenced by the decreased long wavelength emission becomes nearly complete by 90 s, and the OLED fails shortly thereafter. The observation of sensitizer photobleaching is made possible by the increased OLED lifetime (20 s in 10% PAG to 106 s in 1% PAG) and hence the increased irradiation dose delivered before the device failure. This increased lifetime is due to slower photolysis at a lower PAG and BPET loading. The photograph of a phototriggered s-PPHA (1% PAG) substrate clearly shows discoloration of the substrate backside similar to that of the phototriggered pixels on the glass/ITO substrate. We monitor a phototriggered sPPHA (1% PAG) substrate over the following 3 days in the dark and find that slow but significant monomer vaporization takes place (Figure S4). This is evidenced by the exposed Yb electrodes seen from the s-PPHA substrate backside. Unfortunately, the proximity between the phototriggered and the fresh pixels as well as acid diffusion makes it difficult to prevent the untriggered parts of the substrate from depolymerizing. Additionally, we believe that the prolonged (∼4 h) exposure to a high-vacuum environment (base pressure ∼1 × 10−7 Torr) during the evaporation of the OLED 28065
DOI: 10.1021/acsami.8b08181 ACS Appl. Mater. Interfaces 2018, 10, 28062−28068
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Measured lifetimes of the OLEDs on s-PPHA/AgNW substrates at various operating voltages; each data set is fit to an accumulated photon intensity in the s-PPHA substrate estimated from the power outcoupled from the PPHA/AgNW device. (b) Photograph of OLEDs on sPPHA (10% PAG) showing 5 phototriggered pixels (lit until failure at various voltages) in dashed boxes with decomposed substrate backside. The scale bar in the upper left corner is 1 cm. (c) Photograph of OLEDs on s-PPHA (1% PAG) showing 4 phototriggered pixels (lit until failure) in dashed boxes. The scale bar is 1 cm.
expect them to be about 10× longer-lived if we assume 10% PAG is 10× more absorptive. The best fit to the 1% PAG data set is 600 mJ cm−2, which is about 7.5× that of 10% PAG. Possible sources of error include uncertainties in nonuniform sPPHA thickness and AgNW density. Despite the short OLED lifetimes, the phototriggered substrate backsides exhibiting visible discoloration (Figure 4b and c), as well as the FTIR signals (Figure 3), confirm the presence of significant monomeric species. Additionally, we qualitatively observe that the phototriggered pixels are extremely fragile to the touch and even ordinary handling practices such as putting down the sensitized substrate with a phototriggered pixel on a surface and picking it back up carefully leaves the pixel torn apart from the rest of the substrate (Figure S2b). The mechanical properties of sensitized PPHA films (10% PAG) are further quantified by holding a film at a constant force of 1.39 N in a tensilometer and measuring the elastic modulus before and after photoexposure (730 mJ cm−2). The elastic modulus (Figure S5) gradually decays after photoexposure, resulting in a near-linear loss in mechanical properties with time. It can be seen that the elastic modulus nearly halves approximately 20 min after the photoexposure. The resulting film is a sticky liquid made of depolymerized phthalaldehyde monomers and various additives including sensitizer and plasticizer. We thus far present both qualitative and quantitative assessment of successful depolymerization of PPHA films by integrated OLED irradiation. Our findings suggest that the OLED failure rate is significantly faster than the vaporization rate of phthalaldehyde monomers which are depolymerized both via direct acid generation from phototriggering and acid diffusion in the absence of irradiation. We believe that OLED lifetime and monomer vaporization can be engineered to comparable rates by accelerating the monomer vaporization by copolymerization of PPHA with more volatile monomers14 and reducing the substrate thickness. Bilayer systems in which a neat PPHA film is placed between an OLED and a sensitized PPHA film may also mitigate OLED failure modes and increase the OLED lifetimes.
components accelerates the depolymerization of nearby untriggered pixels. Fourier-transform infrared spectroscopy (FTIR) is a useful tool to assess chemical changes upon depolymerization without exposing the samples to visible or UV light, and therefore we use FTIR on the substrate backsides of phototriggered OLEDs to quantitatively look for byproducts of the depolymerization reaction. We first perform FTIR on phototriggered PPHA on the rigid glass/ITO OLED after 10 min irradiation at 7 V and compare the spectrum to that of a fresh OLED on the same substrate. In Figure 3a, the strong ether signal in the range of 900−1100 cm−1 from the fresh pixel nearly disappears in the phototriggered pixel, and the C− O peak around 1200 cm−1 appears for the phototriggered pixel which is believed to be the aldehyde functional in phthalaldehyde monomers. A strong peak across 1650−1700 cm−1 in the phototriggered pixel (Figure 3b) is also assigned to the CO bond of phthalaldehyde monomers. The FTIR spectra strongly suggest successful depolymerization of sPPHA via OLED irradiation. Similarly, we repeat the experiment for the phototriggered and fresh pixels on flexible s-PPHA OLED (1% and 10% PAG) substrates after operating the OLEDs to failure at 7 V. The slightly decreased ether signal from the phototriggered pixels on the flexible substrates (especially for 10% PAG) compared to that on the rigid substrate points to an early onset of depolymerization despite our best efforts to perform the fabrication without exposure to stray light. Nonetheless, the signals from the phototriggered pixels on the flexible substrates closely match those of the control signal on the glass/ITO substrate, confirming that depolymerization is also successful in the flexible OLEDs. We estimate the approximate doses to bring the integrated OLEDs to failure by fitting the device lifetimes (i.e., time to failure) to the OLED output power predicted by the outcoupled emission from the OLED on a neat PPHA substrate. Since the OLED output power increases exponentially with voltage, we see a fast decay in the OLED lifetime with increasing voltage bias. Assuming that the dose is the main factor contributing to the OLED failure, a linear leastsquares fit minimizing percent error from the projected OLED lifetime yields 80 mJ cm−2 for the 10% PAG data set. The experimentally obtained OLED lifetimes and the corresponding fits are shown in Figure 4a. The lifetimes of the 1% PAG devices are much longer due to slower photolysis, and we
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CONCLUSION
We have demonstrated successful integration of OLEDs with transient PPHA substrates sensitized with photoacid generator and sensitizer. The polymer substrates are rendered transient by phototriggering the electron transfer between PAG and 28066
DOI: 10.1021/acsami.8b08181 ACS Appl. Mater. Interfaces 2018, 10, 28062−28068
Research Article
ACS Applied Materials & Interfaces
lamp irradiation with a peak wavelength of 520 nm through monochromator gratings. The monochromatic irradiation (spot size ∼18 mm2) was focused on the AgNW side of the conductive s-PPHA film, and its power was measured using a calibrated silicon photodiode. The same monochromator system coupled to an integrating sphere (LabSphere) and a calibrated silicon photodiode to obtain reflection and absorption spectra of Al and Yb films. The films were thermally evaporated on glass substrates, and the measurements were taken after encapsulating the devices to prevent oxidization.
BPET via the integrated OLED irradiation, generating acids that depolymerize the low-Tc PPHA backbone. We show that our OLEDs can provide sufficient doses to initiate and cause significant depolymerization of the PPHA substrates. We confirm this depolymerization by FTIR and by qualitatively studying visible and mechanical changes to the phototriggered substrates.
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EXPERIMENTAL METHODS
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Synthesis of Poly(phthalaldehyde). The synthesis of poly(phthalaldehyde) (PPHA) is provided in more detail in ref 13. A brief description is provided here. Phthalaldehyde is dissolved into anhydrous dichloromethane (DCM) at a concentration of 0.75 M under a dry, nitrogen atmosphere. Boron trifluoride diethyl etherate (BF3−OEt2) is added to the reaction vessel at an initiator to monomer molar ratio of 1:500. The reaction mixture is cooled to −80 °C for 3 h. The reaction is quenched using pyridine. The material is precipitated into methanol, vacuum filtered, dried, and then redissolved in tetrahydrofuran (THF) for a second precipitation into methanol. Two different batches of PPHA were used in this work: (Mw = 306 kDa, Đ = 1.98), and (Mw = 285 kDa, Đ = 1.44). Synthesis of 5,12-Bis(Phenylethynyl)tetracene (BPET). The synthesis of BPET is detailed in ref 19. Formulation of PPHA Precursor and Films. The PPHA was dissolved in dioxane (Sigma-Aldrich) and 30 wt % diethylene glycol dibenzoate (TCI America) was added as a plasticizer to improve flexibility of the films. The optical constants (n, k) were measured by ellipsometry (Woollam M-2000 ellipsometer). For sensitized films, precursors of PAG and BPET were first separately dissolved in dioxane and added to the PPHA formulation in 2:1 weight ratios. The optical characterization of sensitized PPHA (1% PAG) films were performed using UV−vis Cary 5000 spectrometer and Edinburgh Instruments FLS980 PL spectrometer. The resulting formulations (pristine or sensitized) were approximately 40 mg/mL (PPHA to dioxane) and were dropcast onto 3 × 3 cm2 substrates prepatterned with AgNW/TiO2 and allowed to dry overnight in the dark. Freestanding PPHA films of approximately 60 μm thickness were obtained by carefully delaminating them from the supporting substrates in a deionized water bath. Characterization of Elastic Modulus of PPHA Film. The elastic modulus was measured by holding a sensitized PPHA (10% PAG) film at a constant force of 1.39 N in a tensilometer. The sample dimensions were (l × w × t) = (16 × 8 × 0.18 mm). Electrode Preparation and Fabrication of OLEDs. Silver nanowires (ACS Materials, diluted to 2.5% in ethanol) were spin coated 8× on 3 × 3 cm2 glass or silicon substrates before spin coating sol−gel TiO2. Detailed preparation closely follows ref 21. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, Heraeus) was diluted in deionized water in 1:1 ratio and modified with 0.05%(v/v) Triton X-100 surfactant (Sigma-Aldrich) for improved wetting on the PPHA films. The modified PEDOT:PSS solution was dip-coated 5 times on the PPHA substrates and were dried for 20 min in air. The substrates were then transferred to a vacuum thermal evaporator (EvoVac, Angstrom, base pressure of ∼1 × 10−7 Torr) for organic and metal film growth. We evaporated 5 nm MoOx (Alfa Aesar), 20 nm CBP (4,4′-bis(N-carbazolyl)-1,1′-biphenyl), 15 nm CBP doped with 8% Ir(ppy)2(acac) (bis[2-(2-pyridinyl-N)phenylC]), 30 nm TPBi (2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1Hbenzimidazole), and 200 nm Yb (Kurt J. Lesker). All materials were purchased from Nichem unless mentioned otherwise. Characterization of OLEDs, AgNW Resistance, and Top-Contact Electrodes. The fabricated OLEDs were tested in a nitrogen glovebox with a Keithley 2400 source-measure unit, a calibrated silicon photodiode (Thorlabs), and a fiber optic spectrophotometer (StellarNet Inc.). The FTIR spectra of the degraded and fresh OLEDs were collected in the ATR (attenuated total reflectance) mode on Nicolet iN 10 MX with a Ge crystal. The sheet resistance of freestanding s-PPHA/AgNW (10% and 1% PAG) films were measured with a Keithley 2400 source-measure unit under xenon
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b08181. Reflection and absorption spectra, refractive index and extinction coefficient, normalized sheet resistance, photographs of phototriggered s-PPHA, flexible OLED on s-PPHA, and substrate backside of flexible OLEDs on s-PPHA, and measured storage modulus of a sensitized PPHA film (PDF) Video of a working OLED on flexible s-PPHA with 10% PAG (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Barry P. Rand: 0000-0003-4409-8751 Author Contributions
O.P. and A.E. contributed equally. O.P., A.E., and P.A.K. synthesized the PPHA starting material and BPET sensitizer. K.M.L. prepared the conductive PPHA substrates, fabricated the OLEDs, and characterized the films and the devices. All authors discussed the results and contributed to the manuscript. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge Jisu Jiang of the Georgia Institute of Technology for measuring the storage modulus of the poly(phthalaldehyde) film and Dr. Jared Schwartz of the Georgia Institute of Technology for early discussions on PPHA. We acknowledge funding for this work by a DARPA Young Faculty Award (B.P.R. and K.M.L. Award D15AP00093).
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REFERENCES
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DOI: 10.1021/acsami.8b08181 ACS Appl. Mater. Interfaces 2018, 10, 28062−28068
Research Article
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DOI: 10.1021/acsami.8b08181 ACS Appl. Mater. Interfaces 2018, 10, 28062−28068
