Dual mode OPV-OLED device with photovoltaic and light-emitting functionalities

The rapid development of organic optoelectronic devices such as organic photovoltaics (OPVs) and organic light-emitting devices (OLEDs) is largely attributable to their advantageous properties of their large area, ultrathin thickness, flexiblility, transparency, and solution processability. Herein, we fabricate and characterize a dual mode OPV-OLED device with three-terminal structure comprising a polymer-based bulk-heterojunction inverted OPV unit and a top-emission white phosphorescent OLED unit back-to-back connected via intermediate metal alloy electrode. Sputter-deposited indium tin oxide was used as a transparent cathode of the inverted OPV unit, whereas Ag-doped Al served as a common OPV/OLED anode, allowing the decoupling of electricity generation and light mission functions. Notably, the doping of Al by Ag facilitated the reduction of surface roughness, allowing the above electrode to be used as a common anode and dramatically reducing the leakage current. Finally, the top-emission OLED unit featured an ultrathin layer of Ag-doped Mg as a semitransparent cathode. Thus, successful integration of the OPV-OLED elements results in the decoupling of electricity generation and light emission functionalities, achieving a power conversion efficiency of 3.4% and an external quantum efficiency of 9.9%.

unit was less than 70% at 530 nm, which implies that OLED-emitted light unit was able to pass through the OPV unit with a small absorption loss. However, in this integrated device, light incident into the OPV was required to be oriented in the same direction as that emitted by the OLED. Therefore, the development of a dual mode OPV-OLED device is required to decouple the above directions without absorption loss.
In addition, the dual mode OPV-OLED device enables simultaneously photovoltaic and light-emitting characteristics in one device. These novel features of dual mode device can be applied to the "smart window blinds". In general, window blinds block out the sunlight, whereas smart window blinds by dual mode OPV-OLED device harness the potential sunlight as a solar power during the day. Moreover, dual mode OPV-OLED device can be used for the lighting application at night.
Herein, to decoupled electricity generation and light emission function, we fabricated a dual mode OPV-OLED device with three terminal structure (active are = 1 cm 2 ) featuring a polymer-based bulk-heterojunction inverted OPV unit and a top-emission white phosphorescent OLED unit back-to-back connected via an intermediate metal alloy electrode Sputter-deposited indium tin oxide (ITO) with relatively high transmittance (up to 90%) was used as the transparent cathode of the inverted OPV unit. The intermediate connecting electrode comprising Ag-doped Al and played a key role in realizing the individual operation of OPV and OLED units, with its smooth surface 38,39 resulting in an acceptably small device leakage current. Ultrathin Ag-doped Mg was used as a semitransparent cathode for the top-emission OLED unit, and the fabricated dual mode device exhibited a PCE of 3.4% and an EQE of 9.9%.

Results and Discussions
To realize the abovementioned dual mode device, we employed a three-terminal structure, namely ITO transparent cathode/inverted OPV/Ag-doped Al intermediate connecting anode/top-emission OLED/Ag-doped Mg semitransparent cathode, as shown in Fig. 1. The active area of fabricated device equaled 1 cm 2 , being fairly large compared to the conventional laboratory-scale value of 0.04 cm 2 . Thus, the OPV unit was irradiated by solar light from the transparent ITO-coated glass side, whereas OLED-emitted light passed through the semitransparent ultrathin Ag-doped Mg cathode. We prepared the various deposition masks with active area of 1 cm 2 such as sputter-deposited ITO mask, intermediate electrode mask, OLED mask, and transparent top electrode mask, as shown in Fig. S1.
The intermediate electrode was utilized for charge collection from the OPV unit and charge injection into the OLED unit, with its smooth surface being of high importance. Ag films are commonly used as inverted OPV anodes due to featuring appropriate energy level alignment. However, the Ag film employed herein showed a high surface roughness (R a = 1.84 nm, and R max = 18.3 nm) (Fig. 2a)), which resulted in increased leakage current when OLED unit was directly fabricated onto high roughness Ag film. To reduce surface roughness, we fabricated an Ag-doped Al alloy electrode by co-evaporation of Ag and Al. Figure 2b-d show atomic force microscopy (AFM) images of Ag-doped (Ag contents of 0, 30, and 60 wt%, respectively) Al films onto Ag film. The surface roughness of the Ag-doped Al film with a thickness of 70 nm decreased from 2.71 to 1.16 nm (R a ) and from 36.4 to 10.7 nm (R max ) as the Ag content increased from 0 to 60 wt%, reflecting the fact that co-evaporation prevented the aggregation of homogeneous metal clusters and hence reduced the device leakage current and optical loss of the device 38 . In addition, relatively small R a and R max values (2.13 and 21.2 nm, respectively) were observed even when Ag-doped Al was deposited over the whole inverted OPV unit (Fig. S6). Figure 3a shows the current density-voltage curves for the OPV in the dual mode device and for a single inverted OPV under typical air mass 1.5 global irradiation (AM1.5 G, 100 mW cm −2 ). The dual mode OPV-OLED featured PCE = 3.31%, short-circuit current (Jsc) = 13.2 mA cm −2 , open-circuit voltage (Voc) = 0.73 V, and fill factor (FF) = 33%, with the observed efficiency thus being almost identical to that of the single inverted OPV (PCE = 3.36%, FF = 35%). This result suggested that the OPV unit of dual mode device could be effectively operated as an electricity generation source similarly to a conventional single OPV. Figure 3b shows the external quantum efficiency (EQE) curves the above OPVs, revealing their close similarity. Notably, a low EQE was observed at 300-450 nm, which was attributed to the low transmittance of sputter-deposited ITO film in this short-wavelength range. The characteristics of the utilized OPVs are summarized in Table 1.
Thus, based on the obtained results, the dual mode OPV-OLED device could be effectively operated as both a top-emission white OLED and a single-OLED.

Conclusion
In summary, we have successfully fabricated and characterized a dual mode three-terminal OPV-OLED device comprising a polymer-based bulk-heterojunction inverted-OPV unit and a top-emission white phosphorescent OLED unit back-to-back connected by an intermediate metal alloy electrode. Sputter-deposited transparent ITO was used as the cathode of the inverted OPV unit, whereas the abovementioned metal alloy (Ag-doped Al) electrode was used to enable the decoupling of electricity generation and light emitting functions. The doping of Al by Ag played a significant role in the reduction of surface roughness, resulting in the almost complete absence of leakage current. Ultrathin Ag-doped Mg was used as a semitransparent cathode for the top-emission white OLED unit of dual mode device, which exhibited a PCE of 3.4% and an EQE of 9.9%.

Methods
Materials. PTB7 and PC 71 BM were purchased from Solarmer Energy. HATCN6, TAPC, TCTA, and Liq were purchased from eRay. 26DCzPPy and FIrpic were purchased from Chemipro Kasei, and PQ 2 Ir(dpm) was purchased from Lumtec. ZnO and B3PyPB were synthesized according to the kown literature procedure.

ITO film fabrication.
Glass substrates were cleaned with ultra-purified water and neutral detergent, being subsequently, dry-cleaned by 10-min exposure to an UV-ozone ambient. The cleaned substrates were coated with ITO films at room temperature by the radio frequency (RF) sputtering (NRF-technologies NR05NP-03) using an ITO target (90% In 2 O 3 -10%SnO 2 , 99.99%) supplied by Kojundo Chemical. The target-substrate distance equaled 200 mm, and the sputtering chamber was evacuated to less than 5 × 10 −5 Pa prior to deposition. High-purity Ar (99.999%) and O 2 (99.999%) were introduced at rates of 40 and 0.4 sccm, respectively. Before deposition, the target was decontaminated by 5-min pre-sputtering in Ar-O 2 . The working pressure equaled 0.3 Pa, and the RF power was set to 160 W, resulting in the deposition of a 130-nm-thick ITO film with an active area of 1 cm 2 .

Device fabrication.
To fabricate an inverted OPV with a polymer-based bulk-heterojunction, a dispersion of ZnO nanoparticles (10 mg mL −1 ) in 2-ethoxyethanol was spin-coated onto a cleaned ITO substrate and annealed at 100 °C for 10 min to produce a 30-nm-thick layer. A blend of PTB7:PC 71 BM (2:3 w/w) in chlorobenzene:1,8-diiodooctane (97:3 v/v) with a concentration of 25 mg mL −1 was spin coated onto ZnO to afford a 100-nm-thick layer. All spin coating and annealing procedures were performed in a nitrogen-filled glove box (<0.1ppm O 2 and H 2 O). Other layers (MoO 3 , Ag, and Ag-doped Al) were deposited by thermal evaporation in vacuum (~10 −5 Pa). To fabricate the top-emission white phosphorescent OLED unit, functional organic layers were deposited onto the Ag-doped Al anode by thermal evaporation in vacuum (~10 −5 Pa) using an organic patterned shadow mask. The top cathode (Ag-doped Mg) was deposited by co-evaporation (Ag:Mg 1:9), and was patterned using a shadow mask with an array of 1-cm 2 openings without breaking the vacuum (~10 −5 Pa). Immediately after preparation, the obtained device was encapsulated under a nitrogen atmosphere using epoxy glue and transparent glass lids.
Characterization. Film thickness was measured using Dektek8 profile meter, and the sheet resistivity of ITO film was measured utilizing a conventional four-probe technique. Surface roughness was analyzed using a Bruker Dimension Icon atomic force microscope. The optical transmittance of ITO film was measured using a Shimadzu UV-3150UV-vis-NIR spectrophotometer. To characterize the bulk-heterojunction inverted OPV, current density-voltage curves were recorded using a Keithley 2400 source measure unit. Light intensity was determined by a monosilicon detector (with a KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory to reduce spectral mismatch. For the characterization of the top-emission white phosphorescent OLED unit, EL spectra were acquired using an optical multichannel analyzer (Hamamatsu Photonics PMA-11). Current density-voltage and luminance-voltage curves were recorded using a Keithley source measure unit 2400 and a KonicaMinolta CS200 luminance meter, respectively. External quantum efficiencies were calculated from front luminances, current densities and EL spectra.  Table 2. OLED characteristics of composite and single devices.