Applied Science and Convergence Technology 2019; 28(6): 217-220
Published online November 30, 2019
https://doi.org/10.5757/ASCT.2019.28.6.217
Copyright © The Korean Vacuum Society.
Seungsun Choi , Wonsik Kim , Woojin Shin , Sohyun Park , and Hyunbok Lee*
Department of Physics, Kangwon National University, Gangwon-do 24341, Republic of Korea
Correspondence to:hyunbok@kangwon.ac.kr
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Interface engineering plays a critical role in the device performance of organic photovoltaics (OPVs). In inverted OPVs, the top anode must have a high work function to match the highest occupied molecular orbital level of a p-type organic semiconductor. Therefore, a functional interlayer with a high work function is inserted between the light-absorbing layer and anode. OPVs with a dual-anode buffer layer have been reported to exhibit a superior performance than those with a single-anode buffer layer. Herein, the device performance of inverted OPVs with a poly(3-hexylthiophene-2,5-diyl):[
Keywords: Inverted organic photovoltaics, Anode buffer layer, MoO3, HAT-CN, Interface engineering
Organic photovoltaics (OPVs) have attracted significant attention owing to their low-cost fabrication, low weight, and mechanical flexibility [1–3]. Generally, OPVs have a sandwich structure with an organic light-absorbing layer between the anode and cathode. In this multilayer structure, interface engineering between the organic semiconductor and metal electrodes plays an important role in the device performance [4–6]. To attain high device performance, the basic requirement of the anode is a high work function. To increase the work function, a functional interlayer is inserted between the anode and p-type organic semiconductor. Additionally, an inverted structure of the OPVs, where the top electrode is used as an anode, is known to have a long device lifetime [7]. However, Al and Ag, which are commonly used as the top electrode materials, have low work functions that do not match the highest occupied molecular orbital (HOMO) level of a conventional p-type organic semiconductor. Therefore, a suitable anode buffer layer, such as MoO3, should be used to form an efficient energy-level alignment [8]. In addition, MoO3 enables charge generation owing to the deep conduction band minimum [9]. Hence, MoO3 is frequently inserted between the organic light-absorbing layer and top anode in inverted OPVs. However, it has been recently reported that MoO3 cannot achieve a perfect Ohmic contact, and therefore, an additional organic layer should be inserted between MoO3 and p-type organic semiconductors for further enhancement of the hole transport [10]. Some functional materials have been incorporated into MoO3, and the dual-anode buffer layer has shown a more enhanced device performance. For example, OPVs with a MoO3 and CuI dual-anode buffer layer exhibit a better power conversion efficiency (PCE) than a single MoO3 or CuI buffer layer [11]. As a similar strategy, in an inverted top-emission organic light-emitting diode, a 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN) and MoO3 dual-anode buffer layer has been reported to exhibit superior performance than a single MoO3 anode buffer layer [12]. However, this HAT-CN and MoO3 dual-anode buffer layer has not been employed in inverted OPVs.
In this study, a dual-anode buffer layer consisting of MoO3 and HAT-CN is investigated in inverted OPVs. The MoO3 and HAT-CN layers are deposited onto the poly(3-hexylthiophene-2,5-diyl) (P3HT): [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk hetero-junction layer by varying the deposition sequence. Thereafter, the device performances of the OPVs and hole-only devices with the MoO3/Al, HAT-CN/MoO3/Al, and MoO3/HAT-CN/Al anode systems are compared. The possible cause of the differences in the device performance is discussed based on the capacitance-voltage measurements.
The inverted OPVs were fabricated with an Al/anode buffer layer/P3HT:PCBM/polyethylenimine ethoxylated (PEIE)/indium tin oxide (ITO) structure. An ITO-patterned glass substrate was cleaned by ultrasonication in deionized water, detergent, acetone, and methanol bath. It was then dried with N2 gas flow and treated with ultraviolet ozone at 100 °C for 15 min. At the cathode buffer layer, PEIE (Sigma-Aldrich) was deposited by spin coating onto ITO at a spin rate of 5000 rpm for 60 s [13]. Subsequently, the sample was annealed at 100 °C for 10 min. P3HT (Mw: > 45,000, regioregularity: > 93 %) and PCBM (purity: > 99.5 %) were purchased from Luminescence Technology. P3HT and PCBM were dissolved in chlorobenzene (purity: > 99.9 %, Sigma-Aldrich) at a concentration of 80 mg mL−1 (1:1 wt %) and stirred overnight before use. The P3HT:PCBM light-absorbing layer was deposited by spin coating onto PEIE/ITO from the mixed solution at a spin rate of 2000 rpm for 60 s. Then, the film was annealed at 150 °C for 15 min. All solution processes were accomplished at ambient conditions, and the annealing was performed using a hot plate. The sample was thereafter transferred into a vacuum chamber. MoO3 (purity: > 99.97 %) was purchased from Sigma-Aldrich and HAT-CN (purity: > 99.9 %) from EM INDEX. MoO3 (15 nm) and HAT-CN (5 nm) were deposited via thermal evaporation at a deposition rate of 0.01 nm s−1. Finally, the 100 nm-thick Al layer was deposited via thermal evaporation with deposition rates of 0.01 nm s−1 for the initial 10 nm thickness and 0.05 – 0.1 nm s−1 for a thickness of 90 nm. The hole-only devices were fabricated with the Al/anode buffer layer/P3HT/poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS)/ITO structure. PEDOT:PSS (Clevios™ P VP AI 4083) was purchased from Heraeus. The PEDOT:PSS layer was deposited onto ITO by spin coating at a rate of 500 rpm for the initial 5 s, followed by 3000 rpm for the next 30 s, and then annealed at 150 °C for 10 min. Subsequently, the P3HT layer was deposited onto PEDOT:PSS/ITO by spin coating from a chlorobenzene solution at a concentration of 40 mg mL−1, and then annealed at 150 °C for 15 min. The anode buffer layer and Al were deposited in the same manner as the OPVs. The device area of the OPVs and hole-only devices was 0.04 cm2.
Current density-voltage (
Figure 1 shows (a) the chemical structures of the organic materials (P3HT, PCBM, and HAT-CN), and (b) the energy-level alignment of the OPVs with the MoO3 and HAT-CN dual-anode buffer layer used in this study. The energy levels are obtained from the charge transport levels measured by ultraviolet and inverse photoelectron spectroscopy [9,14,15] under the assumption of vacuum-level alignment. P3HT and PCBM were used as the donor and acceptor, respectively. Owing to the low work function of PEIE, the cathode interface was efficiently contacted. HAT-CN has a deep HOMO level and high work function, similar to MoO3 [15,16]. However, the reported transport gap of HAT-CN is much larger than that of MoO3.
Figure 2(a) shows the
Figure 2(b) shows a semi-log plot of the
Figure 3 shows the
where
Figure 4 shows the
In this study, the effect of deposition sequence in a MoO3 and HAT-CN dual-anode buffer layer on inverted OPVs is investigated. When HAT-CN is deposited prior to MoO3, the device performance of the OPV significantly deteriorates compared to that with a single MoO3 anode buffer layer. From the Mott-Schottky plots, it is concluded that the low PCE of the OPVs with HAT-CN/MoO3/Al is attributed to the low
This study was supported by the National Research Foundation of Korea (NRF-2018R1D1A1B07051050 and 2018R1A6A1A03025582) and Supporting Business for College Innovation from Kangwon National University.
Photovoltaic parameters of the inverted OPVs using different anode systems. Statistical analysis was performed for 32 devices.
Anode system | FF (%) | PCE (%) | ||
---|---|---|---|---|
MoO3/Al | 10.60 ± 0.34 | 0.63 ± 0.004 | 56.6 ± 2.12 | 3.74 ± 0.20 |
HAT-CN/MoO3/Al | 10.28 ± 0.34 | 0.58 ± 0.008 | 53.1 ± 1.38 | 3.19 ± 0.12 |
MoO3/HAT-CN/Al | 11.40 ± 0.32 | 0.63 ± 0.005 | 55.6 ± 1.38 | 3.97 ± 0.12 |