Solution-Processed Organic Optical Upconversion Device | ACS

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23428−23435

Solution-Processed Organic Optical Upconversion Device Karen Strassel,†,‡,∥ Santhanu Panikar Ramanandan,†,§,∥ Sina Abdolhosseinzadeh,†,§ Matthias Diethelm,†,§ Frank Nüesch,†,§ and Roland Hany*,†

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†

Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Functional Polymers, CH-8600 Dübendorf, Switzerland ‡ Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, EPFL, Station 6, CH-1015 Lausanne, Switzerland § Institute of Materials Science and Engineering, Ecole Polytechnique Fédérale de Lausanne, EPFL, Station 12, CH-1015 Lausanne, Switzerland S Supporting Information *

ABSTRACT: Imaging in the near-infrared (NIR) is getting increasingly important for applications such as machine vision or medical imaging. NIR-to-visible optical upconverters consist of a monolithic stack of a NIR photodetector and a visible lightemitting unit. Such devices convert NIR light directly to visible light and allow capturing a NIR image with an ordinary camera. Here, five-layer organic solution-processed upconverters (OUCs) are reported which consist of a squaraine dye NIR photodetector and a fluorescent poly(para-phenylene vinylene) copolymer (super yellow)-based organic light-emitting diode (OLED) or light-emitting electrochemical cell (LEC), respectively. Both OLED−OUCs and LEC−OUCs convert NIR light at 980 nm to yellow light at around 575 nm with comparable device metrics of performance, such as a turn-on voltage of 2.7−2.9 V and a NIR-to-visible photon conversion efficiency of around 1.6%. Because of the presence of a salt in the emitting layer, the LEC−OUC is a temporally dynamic device. The LEC−OUC turn-on and relaxation behavior is characterized in detail. It is demonstrated that a particular ionic distribution and thereby the LEC−OUC status can be frozen by storing the device in the presence of a small voltage applied. This provides a test chart for quantitative measurements. KEYWORDS: upconverter, photodetector, near-infrared, squaraine, light-emitting electrochemical cell

1. INTRODUCTION Sensing and imaging in the near-infrared (NIR) region is increasingly used in a number of applications such as process control in agriculture and pharmacy, noninvasive subsurface vision or medical imaging.1−3 NIR-to-visible optical upconverters consist of a series-connected NIR photodetector with a visible light-emitting unit. These devices convert a NIR image pixel-free directly to a visible image, avoiding intermediate electronics and an external display for image visualization. Upconverters have emerged as an attractive low-cost addition to the current technology based on inorganic semiconductor photodiode arrays interconnected with read-out-integrated circuitry. Great progress is being made with upconverters fabricated entirely from organic and hybrid materials, which include metal-halide perovskites and colloidal quantum dots.4−16 In particular, the fraction of incident NIR photons converted to © 2019 American Chemical Society

visible photons, the photon-to-photon conversion efficiency (P2PCE), is an important performance metrics. By using different material systems and device architectures, the P2PCE has rapidly increased from 2.7% in 20109 to 29.6%14 and over 100%8 in 2018. Other major achievements during the last decade include the upconversion of NIR light at 1300 nm,11 which is beyond the silicon band edge at around 1100 nm, upconverters that are visibly transparent12 or the direct imaging of subcutaneous blood vessels.4 Recent advances with optical upconverters made from all-organic and hybrid materials have been reviewed in ref 17. So far, organic and hybrid upconverters have been fabricated entirely via thermal evaporation or by a combination of Received: April 17, 2019 Accepted: June 10, 2019 Published: June 10, 2019 23428

DOI: 10.1021/acsami.9b06732 ACS Appl. Mater. Interfaces 2019, 11, 23428−23435

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ACS Applied Materials & Interfaces

Figure 1. (a) Chemical structures of the compounds used in this study. SQ-880 is an inner salt but the net charge of the entire molecule is zero; no (mobile) charge-compensating counter ion is present. (b) OUC stacks with a LEC or an OLED as visible light-emitting component. (c) Visual device operation for a LEC−OUC in the presence of NIR light and an applied voltage above the band gap of SY.

The further realization of solely solution-processed OUCs is challenging because the evaporated charge-injecting/electrode layers must be replaced by solution-processable alternative materials. For photodetectors and OLEDs made in the inverted architecture, a coated top Pedot:PSS layer provides a solution, optionally completed by a metal.19−21 In our case, however, the OUC was built in the regular device geometry and electron injection occurs through the top electrode. A survey of solution-processable electron-injection materials has been presented.22,23 Among these, thin layers of polyethyleneimine (PEI) or PEI ethoxylated stand out and substantially reduce the electrode work function and electron-injection barrier.24,25 Rather than relying on these established routes for the fabrication of solution-processed electron-injection layers, we attempted here to develop an OUC where such a layer is not needed at all. This was realized by adding an electrolyte to the SY emitter, thereby transforming the OLED into a lightemitting electrochemical cell (LEC).26−30 LECs possess advantages over the more developed OLEDs such as faulttolerant processability from solution or a low driving voltage. The LEC performance also depends less on the work function of the metal cathode compared to an OLED, which is explainable by the formation of electric double layers by ions at the electrodes that facilitate the injection of electrons and holes.31 A further LEC characteristic is the formation of p- and n-doped regions at the anode and cathode and an intrinsic (i) region in between where electron−hole recombination and light-emission takes place. We combined the NIR photodetector with a SY LEC and fabricated LEC−OUCs without the Ca electron-injection layer, thereby reducing the required number of thermally

solution-processing and thermal evaporation. It can be argued that to harness the true low-cost potential of upconverters it is required that large-area devices on flexible substrates can be realized entirely by printing and coating processes. Organic and hybrid materials are predestinated for that purpose because thin and uniform films can be deposited from solution. In this respect, Ding and Zhu et al.18 reported recently an upconverter that consisted of a solution-processed multilayer stack of zinc oxide/NIR photodetector/Pedot:PSS/CsPbBr3, where the NIR photodetector was a blend of a low-band gap polymer and PC70BM or a nonfullerene acceptor, Pedot:PSS denotes poly(3,4-ethylenedioxythiophene)−poly(styrene sulfonate), and CsPbBr3 was used as the visible light-emitting material. This layer stack was sandwiched between a glass/ITO substrate and a thermally evaporated triple layer consisting of an electron-transporting layer, electron-injection layer, and a top metal contact. Here, we report on five-layer solution-processed all-organic upconversion devices (OUCs). The NIR organic photodetector was based on a NIR squaraine dye (SQ-880): PCBM ([6,6]-phenyl C61 butyric acid methyl ester) blend, which we developed recently,12 with a peak sensitivity at 980 nm and an internal photon-to-current conversion efficiency under reverse bias of almost 100%. In a first step, we combined this photodetector with an organic light-emitting diode (OLED) comprising the fluorescent poly(para-phenylene vinylene) copolymer called super yellow (SY) and demonstrated OLED−OUCs with a low turn-on voltage (2.7 V) and a P2PCE (1.6%), close to the expected maximum. In the OLED−OUC, the top Ca/Al (calcium/aluminium) electrode was deposited by thermal evaporation. 23429


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Figure 2. Performance characteristics of OLED−OUCs with different architectures: complete stack (with SY coated from toluene), and devices without Ca or QUPD. (a) Luminance vs voltage trends with and without NIR light, (b) current density vs voltage trends, (c) current efficacy, and (d) luminance on/off ratio and NIR to visible photon to photon conversion efficiency (P2PCE).

UV−ozone treatment step. The cross-linkable QUPD layer was spin-coated from toluene. After spin-coating the QUPD films were exposed to UV-light and annealed at 120° to promote cross-linking of the oxetane groups at the side chains by cationic ring-opening polymerization using OPPI (4octyloxydiphenyliodonium hexafluoroantimonate) as the initiator.32,33 As the cross-linked QUPD layer is insoluble in common solvents, there is no need for an orthogonal solvent for the subsequent coating of the SY emitter, which was toluene for OLED−OUCs and tetrahydrofuran (THF) for LEC−OUCs. Process optimization of the Pedot:PSS and QUPD layers and an evaluation against thermally evaporated hole-transporting and electron-blocking layers are detailed in the Supporting Information S1. Luminance−voltage characteristics for OLED−OUCs are shown in Figure 2a. Devices with the full-layer stack performed best with a luminance turn-on voltage at 2.7 V and a luminance level that reached 760 cd m−2 at 7.5 V. The reproducibility of device fabrication and a linear dependence of the luminance on the NIR light intensity over two orders of magnitude are demonstrated in Figures S4 and S5. As expected, omitting the Ca-injection layer in a OLED stack was detrimental to the device performance and the turn-on voltage for the luminance increased to 5.8 V. Likewise, removal of the QUPD layer resulted in a significant increase of the turn-on voltage (to 3.6 V) and decrease of the luminance (70 cd m−2 at 7.5 V). Dark currents in the absence of NIR light for OLED−OUCs with and without the QUPD layer were rather similar (Figure 2b). Omitting the QUPD layer introduced a hole-injection barrier and in the presence of NIR light the current density was reduced. Also, the current efficacy was considerably lower when QUPD was left out (Figure 2c), which indicates that probably an imbalance between the number of electrons and

evaporated layers by one compared to the OLED−OUC. However, because of the presence and mechanistic role of the mobile ions, LEC−OUC is a temporally dynamic device. We characterize the dynamic response of LEC−OUCs in detail and present test charts for quantitative measurements. We find that an appropriate preconditioning voltage step results in a similar device performance as measured for the OLED−OUC.

2. RESULTS AND DISCUSSION The OLED−OUC and LEC−OUC device architectures and the used materials are shown in Figure 1a,b. Both in the off(without NIR light) and in the on- (with NIR light) state a voltage bias is applied. Figure 1c shows a schematic of the LEC−OUC device operation in the on-state. NIR light is absorbed by the squaraine dye (SQ-880) and charge generation takes place in the SQ-880:PCBM blend. Electrons are extracted via TiO2 at the ITO anode while holes drift via the Pedot:PSS and QUPD (N4,N4′-bis(4-(6-((3-ethyloxetan3-yl)methoxy)hexyloxy)phenyl)-N4,N4′-bis(4methoxyphenyl)biphenyl-4,4′-diamine) layers into the LEC/ OLED where they recombine in the SY layer with electrons injected from the cathode under the emission of visible light. The function of the TiO2 layer is to reduce hole injection at the anode, whereas QUPD is used as a hole-transporting layer and blocking layer for electrons flowing through the device. This suppresses the dark current, results in a high on/off ratio of the emitted light and consequently in a high image contrast. Methods for the deposition of the TiO2 and SQ-880:PCBM layers are detailed in ref 12. The Pedot:PSS layer was inserted as a hole-injection layer into the OLED/LEC and a protective layer against the subsequent solvents which dissolve the photodetector blend film. Best results for a continuous and pinhole-free Pedot:PSS film were obtained with a preceding 23430


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ACS Applied Materials & Interfaces holes developed. For the full-layer stack and at 1000 cd m−2, the highest current efficacy was 7.5 cd A−1 (EQE = 2.56%, calculated with the electroluminescence spectrum shown in Figure S6), in good agreement with reported values for the corresponding SY OLED (7.68 ± 0.21 cd A−1, EQE = 2.89%).34 Figure 2d displays the luminance on/off ratios as a function of the voltage applied. For the optimized OLED−OUC, the on/off ratio was 800 at 4 V and continuously decreased with increasing voltage because of an increase of the dark luminance. Figure 2d also shows the P2PCE versus voltage trends, calculated from the measured luminance and the incident NIR light intensity. For the optimized device, P2PCE was 0.75% at 7.5 V and increased to 1.6% at 12 V. The P2PCE can also be approximated from the individual external quantum efficiencies (EQEs) of the photodetector and the OLED. The EQE of the photodetector at reverse bias was estimated from an ITO/TiO2/SQ-880:PCBM/MoO3/Ag stack.12 The EQE was ∼30% at an electric field of 7.5 V/active-layer thickness. Therefore, the value for the approximated P2PCE is EQE(photodetector) × EQE(OLED) ≈ 0.3 × 2.89 = 0.87% at 7.5 V, in good agreement with the experimental result. To simplify the OUC architecture further and to effectively eliminate the evaporated Ca layer used for OLED−OUCs, we turned our attention to LEC−OUC devices. Therefore, we added the electrolyte TMPE/Li+CF3SO3− to the SY solution before film coating and used a recently optimized blend ratio of SY/TMPE/Li+CF3SO3− = 1:0.075:0.025 and an emitter film thickness of 70 nm.29 Because the salt is not soluble in toluene, the solvent for LEC fabrication was changed to THF. The reproducibility of LEC−OUC device fabrication is demonstrated in Figure S4. As explained above, the Li+ cations and CF3SO3− anions in the SY film are mobile when an electric field is applied and their function is to form Ohmic contacts at the electrodes and to assist with the electrochemical doping of the SY layer. Because it takes a certain time for the ions to rearrange, the LEC turn-on shows a delay when a voltage is applied, followed by a luminance increase and saturation when the optimum p− i−n situation is established. Depending on the ionic conductivity, the active-layer thickness, and the applied voltage, this process in LECs can take seconds to hours.35 We performed a series of experiments to demonstrate the dynamic LEC−OUC turn-on behavior. Figure 3a shows luminance versus voltage trends for a slow and fast voltage step increase. Below 5.5 V, the two luminance curves overlap but with increasing voltage the luminance for the slow voltage scan is higher. This is because during a slow voltage ramp, the ions have more time to rearrange already during the measurement. This behavior makes quantitative measurements more challenging because the luminance level changes continuously when upconverted NIR light is measured over a longer period. Also, when the same LEC−OUC device is used for consecutive identical image acquisitions, the luminance level will not be constant. One solution to this problem is to apply a voltage preconditioning step in the off-state. Figure 3b shows dark luminance versus time trends for different constant voltages applied. For a bias below 7 V, the luminance does not change over a measurement time of 0.5 h because for a low voltage the ionic movement is slow. However, at 7 V the luminance slowly increased with time which implies that the ions now accumulate at the electrodes and because of the

Figure 3. Dynamic behavior of LEC−OUCs. (a) Luminance vs voltage trends for a slow and fast voltage scan. (b) Dark luminance vs time trends for varying constant voltages applied. (c) Luminance vs voltage trends after prebiasing at 7 V for different times in the dark.

facilitated electronic charge injection, a small dark current (Figure S7) and luminance develops. Figure 3c shows luminance versus voltage trends using a preconditioning 7 V bias for different periods. Over a conditioning time of 90 min, both the dark and NIR lightinduced luminance level increased continuously. Between 4 and 7 V, the luminance on/off ratios were in the range of 30− 40, independent of the biasing time. We note, however, that the device is not yet in a steady-state after a biasing time of 90 min and that the luminance increases further over a time of several hours before saturation (see below, Figure 6). 23431


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out a number of experiments for OLED-based devices using THF as solvent for coating of the emitter layer. Figure 5 shows

When the voltage is turned off and the LEC−OUC device is idled, the p- and n-regions de-dope and ion relaxation occurs. These processes, however, are slow and take place over a period of several days, in contrast to the turn-on dynamics that occurs over a period of several hours. The behavior is demonstrated in Figure 4a where luminance versus voltage

Figure 5. Performance comparison of OUC devices with SY coated from THF.

luminance versus voltage trends for an OLED−OUC and a LEC−OUC where in both cases THF was used as solvent for SY. Interestingly, the OLED−OUC luminance was now well below the LEC−OUC luminance, in clear contrast to the achievable luminance level of toluene-based OLED−OUCs (760 cd m−2 at 7.5 V, Figure 2a). In addition, the luminance turn-on voltage increased from 2.8 V for the LEC−OUC to 3.6 V for the THF-based OLED−OUC. Also, for single ITO/ Pedot:PSS/SY(toluene or THF)/Ca/Ag OLEDs the influence of the SY solvent was evident and over the range from 4 to 7 V the luminance for toluene-based OLEDs was at least 8 times higher (Figure S8). Interestingly, however, the current efficacy (cd A−1) was higher by around 5% for THF-based OLEDs. These observations are in line with results of an extensive solvent effect study using the structurally related polymer MEH−PPV, poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene).36 The authors studied the degree of polymer interchain interactions (aggregation) in MEH−PPV films coated from the solvents chlorobenzene (CB) or THF. Aggregation was promoted in CB where the polymer chains have an open and straight conformation, and was restricted when using THF where chains tend to form a tight coil. Results from ITO/Pedot/MEH−PPV(CB or THF)/Ca/Al OLEDs showed that devices fabricated from the THF-cast films performed poorly compared to devices based on CB-cast films. This difference in the device performance directly reflected the degree of interchain interactions in the films. Aggregation (from CB) facilitated charge transport between chains, enhanced the mobility of charge carriers, and allowed high current to flow. On the other hand, the tightly coiled chains in THF solutions formed a film with weak aggregation. Consequently, there were few paths over which carriers could be transported through the film, and it was difficult both to inject and transport charge carriers through the THFcoated film. In line with our findings, it was also observed that despite poor performance, the current efficacy of devices fabricated from THF-cast film was actually higher compared to CB-based devices.36 This is because once an exciton is formed, the

Figure 4. Temporal relaxation behavior of LEC−OUCs (a) at open circuit and short circuit, and (b) when a constant voltage is applied during storage. All luminance vs voltage trends were measured in the presence of NIR light.

characteristics are displayed for an untreated device, for an upconverter after a prebiasing step at 7 V for 30 min, as well as for devices after storage at open circuit and at short circuit. Even after extensive storage, the luminance has not returned to the level of the pristine device. For quantitative NIR imaging, it is also important to note that the ions can be frozen in a prepared state. Figure 4b shows luminance versus voltage trends for a pristine device, after prebiasing at 7 V for 30 min and after a storage time of 2 days during which a constant voltage of 2 V was applied. The luminance curves after prebiasing and storage overlap, which means that a small voltage present during storage is already enough to suppress device relaxation completely. Such a test chart is suitable for quantitative LEC−OUC imaging because once the device is driven into equilibrium, this state can be frozen and comparable luminance levels can be obtained during multiple measurements. To study whether potential differences in the performance of OLED−OUCs and LEC−OUCs devices can be attributed to the different solvents used for coating of the SY film, we carried 23432


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ACS Applied Materials & Interfaces luminescence efficiency is higher for a film with minimal interchain interactions. Initial device stability tests are promising. Figure 6a shows that OLED−OUCs are rather stable during storage under inert

addition, the conductivity of the n- and p-doped SY regions is strongly increased. Therefore, despite reduced interchain interactions in a SY film coated from THF, charges in a LEC can be easily injected into the SY film and a large current can flow through the doped regions to the intrinsic region where emission occurs. An interesting observation is that in the LEC−OUC the current stabilized before the luminance, in contrast to conventional LECs where the opposite is true. For a constant voltage-driven LEC, the current rises after electric double layer formation and continues to grow because of electrochemical doping. The luminance shows a similar temporal behavior but in many cases the luminance efficiency peaks before the current that continuously increases. This is attributed to a reduction of the intrinsic region, resulting in increased exciton quenching by polarons involved in the doping of the nearby pand n-doped regions.37,38 In the on-state, the current in the LEC−OUC is limited by the number of generated charge carriers in the photodetector that are injected into the SY emitter layer, which explains that the current saturates (Figure 6b). We also note that in the offstate, the dark current after 30 min and at 6 V has increased only to 0.45 mA cm−2 (Figure S7), compared to 4.8 mA cm−2 in the presence of NIR light. One possible explanation for the continuous luminance increase after current stabilization relates to a dynamic light-emission zone, viz. to a nonstabilized recombination zone that moves through the emitter layer during the period of ∼0.5−5 h. The emitting layer is sandwiched between a strongly reflecting Al metal and a weakly reflecting (glass/ITO/TiO 2 /photodetector/Pedot:PSS/QUPD) multilayer stack, resulting in optical interference effects (weak microcavity) as function of the emission zone. Recently,29,39 it has been demonstrated that these interference effects can indeed be pronounced, and data analysis29 suggest that in our LEC−OUC device, the direction of the emission zone shift is from Al toward QUPD.

Figure 6. Device stability. (a) Luminance vs voltage trends of an OLED−OUC when stored in the glovebox. (b) Luminance and current vs time trend of a LEC−OUC, continuously operated at 6 V and irradiated with NIR light at full intensity (49 mW cm−2).

3. CONCLUSIONS We have demonstrated solution-processed optical upconverters by combining an organic NIR photodetector with an OLED or a LEC. Pedot:PSS served as a protection layer for the photodetector from subsequently used solvents and QUPD was used as a hole-injection and electron-blocking layer. This material combination is generic and provides a platform to combine other photodetectors with a light-emitting unit via solution-processing. It is anticipated that optical upconverters can provide an interesting alternative to the existing NIR imaging technology that is less practical for consumer or lowend applications. In this respect, the prospect of low-cost, large-area, and simple solution-processed upconverters is appealing. The performance of a LEC is largely independent of the precise active-layer thickness and therefore LEC−OUCs possess a processing advantage compared to OLED−OUCs. In addition to this, the use of a LEC eliminates the need of an electron-injection layer that in most cases is still applied via thermal evaporation. It must be admitted, however, that the full low-cost potential of LEC−OUCs can only be tapped when also the evaporated top metal electrode can be deposited by a simple and effective process from solution.

conditions and device degradation became apparent only after a period of 3 months. Figure 6b displays the long-term luminance and current versus time trends for a LEC−OUC. The current stabilized after approximately 0.5 h, the luminance after 5 h and no degradation can be observed over a period of 15 h. The LEC−OUC stabilized at a current of 4.8 mA cm−2 and a luminance of 210 cd m−2 (4.4 cd A−1, Figure 6b). This can be compared with the correspondingly reduced data (2 mA cm−2, 38.7 cd m−2, 1.9 cd A−1) of a THF-based OLED−OUC at 6 V (Figures 5 and S9). With the assumption that the angular emission pattern is Lambertian, the luminous efficacy of the THF-based OLED−OUC is only 1 lm/W. For the LEC−OUC, the efficacy is 2.3 lm/W, in close agreement with the luminous efficacy of toluene-based OLED−OUCs that was 2.8 lm/W at 6 V (5.6 mA cm−2, 295 cd m−2, 5.3 cd A−1, average from ten devices, Figure S4). These results suggest that the weakly aggregated SY film coated from THF that is detrimental to the OLED device efficiency has almost no negative effect on the performance of a LEC−OUC. First, we attribute this to the ions in a LEC that redistribute in the SY layer in the presence of an electric field and effectively reduce the injection barriers for electrons at the SY/Al interface and for holes at the QUPD/SY interface. In

4. EXPERIMENTAL METHODS Details on the cleaning of the ITO substrates (GEOMATEC, ≈11 Ω square−1), preparation of the TiO2 layer, and synthesis of SQ-880 are 23433


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ACS Applied Materials & Interfaces Notes

reported in ref 12. SQ-880 (2.5 mg) and PCBM (7.5 mg, Solenne BV, 99%) were dissolved in chloroform (1 mL) and spin-coated at 4000 rpm for 60 s to give (72 ± 5) nm thick films. N,N′-Bis(3methylphenyl)-N,N′-diphenylbenzidine (TPD, 99%), molybdenum(VI) oxide (MoO3, 99.97%), and tris-(8-hydroxyquinoline)aluminium (Alq3, 99.995%) were purchased from Merck and were evaporated at 2 × 10−6 mbar. Pedot:PSS HTL Solar (Ossila) was spin-coated on the UV-ozone (30 s)-treated SQ-880:PCBM blend film at 400 rpm for 30 s to yield a ∼75 nm thick film and was then annealed at 120 °C for 10 min. A solution of QUPD (5 mg mL−1, Lumtec, >99%) and OPPI (1.5 wt %, abcr, 95%) was spin-coated from toluene onto Pedot:PSS at 4000 rpm for 60 s. To promote cross-linking, the film was exposed to UV-illumination (365 nm, 20 s) and annealed at 120 °C for 2 min before rinsing with THF to remove excess initiator. For the OLED−OUC, SY (5 mg mL−1, Merck, average Mn > 400 000, dried for 24 h, 0.1 mbar, 40 °C) was dissolved in toluene or THF. The solution was stirred over night at room temperature (toluene) or at 60 °C (THF) before spin coating at 1500 rpm for 30 s. Films with a thickness of ∼90 nm were dried at 50 °C for 20 min. For the LEC−OUC, Li+CF3SO3− (Sigma-Aldrich, dried for 24 h, 0.1 mbar, 160 °C), trimethylolpropane ethoxylate (TMPE, SigmaAldrich, average Mn 450, dried for 24 h, 0.1 mbar, room temperature) and SY were separately dissolved in anhydrous THF (Sigma-Aldrich) in concentrations of 10 mg mL−1 (Li+CF3SO3−, TMPE) and 5 mg mL−1 (SY). The SY solution was stirred for 7 h at 60 °C inside a glovebox. The precursor solutions were then mixed in mass ratios of 1:0.075:0.025 (SY/TMPE:Li+CF3SO3−) and the blend was stirred for at least 17 h at 60 °C. Solutions were let to cool down for 20 min before spin coating at 2000 rpm for 60 s. The films with a thickness of ∼70 nm were annealed at 60 °C for 1 h. The top electrode, consisting of either Ca (10 nm)/Al (100 nm) or Al (100 nm) was evaporated at 2 × 10−6 mbar through a shadow mask defining the device area to 3.1 or 7.1 mm2. Film thicknesses were measured using an Ambios XP1 profilometer. For characterization devices were placed in a N2-filled airtight holder. Luminance−current−voltage characteristics were recorded using a Keithley 2400 and a Konica Minolta LS-110 luminance meter with a close-up lens 110. As the NIR-light source, a 980 nm laser (49 mW cm−2) was used. In ref 12 the experimental setup for device characterization is shown and the evaluation of the P2PCE is explained. UV−vis spectra were recorded using a Varian Cary 50 Scan spectrophotometer. Electroluminescence spectra were measured using an integrating sphere connected to an Ocean Optics QE Pro spectrometer.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Swiss National Science Foundation (grant number IZRJZ2_164179/1) is acknowledged. The authors gratefully acknowledge use of the facilities of the Coating Competence Center at Empa. Financial support from the project FOXIP in the framework of the Strategic Focus Area (SFA) Advanced Manufacturing of the ETH Board is acknowledged.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06732.

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REFERENCES

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Comparison of device results against optimized, partially evaporated upconverters; reproducibility of device fabrication and linearity of response; LEC−OUC dark current; EL spectra; and solvent effects on the device performance (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +41 58 765 4084. ORCID

Karen Strassel: 0000-0002-4783-7129 Matthias Diethelm: 0000-0002-8899-1755 Frank Nüesch: 0000-0003-0145-7611 Roland Hany: 0000-0003-0569-119X Author Contributions ∥

K.S. and S.P.R. contributed equally. 23434


Research Article

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Solution-Processed Organic Optical Upconversion Device | ACS

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