Supplemental Material
Rhenium oxide as an efficient p-dopant to overcome S-shaped current density-voltage curves in organic photovoltaics with a deep highest occupied molecular orbital level donor layer
Dae-Ho Kim, Tae-Min Kim, Won-Ik Jeonga) and Jang-Joo Kimb)
Department of Materials Science and Engineering, WCU Hybrid Materials Program and Center for Organic Light Emitting Diode, Seoul National University, 151-742, Seoul, South Korea
a)Present address: IT Material R&D, LG Chem Research Park. 104-1, Moonji-dong, Yuseong-gu, Daejeon, 305-380, Korea
b)Electronic mail:
In this supplemental material, we report the effect of thin interfacial layer on the shape of the current density-voltage (J-V) curve of organic photovoltaic (OPV) cells with a deep highest occupied molecular orbital (HOMO) level donor layer. Molybdenum oxide (MoO3) with high work function (WF) of 6.6 eV is used as an interfacial layer with thickness of 3 nm.
Devices without and with thin interfacial layer have the following simple structure: 150 nm-thick indium tin oxide (ITO)/ (3 nm-thick MoO3)/7 nm-thick DCV5T/35 nm-thick C60/8 nm-thick BCP/100 nm-thick Al. The 150 nm-thick ITO coated glass substrate was successively cleaned with acetone and isopropyl alcohol. The substrate was exposed to UV-O3 for 10 min before use. MoO3 with high WF of 6.6 eV is used as an interfacial layer with thickness of 3 nm. α,α’-bis(2,2-dicyanovinyl)-quinquethiophene (DCV5T) and fullerene (C60) were used as the donor and acceptor molecules, respectively. All the organic layers were successively deposited using thermal evaporation at the base pressure of ~10-7 Torr without breaking the vacuum. The evaporation rate was 1 Å/s for all the organic materials except for MoO3 (0.2 Å/s) and DCV5T (0.5 Å/s). The 100 nm thick Al was deposited at a rate of 4 Å/s as the cathode, using a shadow mask. The active area was 4 mm2, defined using a patterned insulator on the ITO anode. After fabrication, the devices were encapsulated using glass cans in an N2 environment. The device characteristics were measured using an AM 1.5G solar simulator (Oriel 69911) and a source measurement unit (Keithley 237). The measurement setup was calibrated with a National Renewable Energy Laboratory-certified reference Si solar cell covered with a KG-5 filter before every measurement. In the IPCE set up, a 1000 W Xe lamp was used with a monochromator and its intensity was calibrated with a Si photodiode. The UV-vis absorption spectra of the films were recorded with a VARIAN Cary 5000 UV-vis spectrometer.
Fig. S1. (a) shows the J-V curves of OPV cells without and with the thin interfacial layer. When the thin interfacial layer or p-doped hole transport layer (p-HTL) are not used, the J-V curve shows a strong S-shape because of the contact resistance between ITO anode and DCV5T. On the other hand, when the interfacial layer of MoO3 is used, the S-shape of the J-V curve is removed but the short circuit current density (JSC) significantly decreases from 4.3 mA/cm2 to 3.4 mA/cm2. The photovoltaic parameters of the OPV cells are summarized in Table S1.
Fig. S1. (b) shows the incident photon to current conversion efficiency (IPCE) curves of the OPV cells and the absorption spectra of the active materials. DCV5T has the absorption peaks at 390 nm and 570 nm. C60 has the absorption peaks at 350 nm and 450 nm. When the interfacial layer is used, the IPCE peak at 570 nm decreased by 34%, indicating that the excitons generated in the DCV5T layer are quenched by MoO3.
TABLE SⅠ1. The photovoltaic parameters of OPV cells without and with interfacial layer.
VOC(V) / FF / JSC(mA/cm2) / PCE(%)w/o interfacial layer / 0.91 / 0.49 / 4.3 / 2.0
w/ interfacial layer / 1.00 / 0.57 / 3.4 / 2.0
Figure captions
Fig. S1. (a). J-V curves of OPV cells without (square) and with (circle) thin interfacial layer. (b) IPCE curves of OPV cells without (square) and with (circle) thin interfacial layer. Absorption spectrum of C60 (50nm, upper triangle) and DCV5T (10nm, down triangle) films on glass substrate.
Figure. S1.