Applied Science and Convergence Technology 2019; 28(6): 217-220
Published online November 30, 2019
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
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 . 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 . In addition, MoO3 enables charge generation owing to the deep conduction band minimum . 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 . 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 . 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 . 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 . 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
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|