Optical Expediency of Back Electrode Materials for Organic Near-Infrared Photodiodes

Abstract

Organic semiconductor devices, including organic photodetectors (OPDs) and organic photovoltaics (OPVs), have undergone vast improvements, thanks to the development of non-fullerene acceptors. The absorption range of such NFA-based systems is typically shifted toward the near-infrared (near-IR) region compared to early-generation fullerene-based systems, rendering organic semiconductor devices suitable for near-IR sensing applications. While most efforts are concentrated on the photoactive materials, less attention is paid to the impact of the back electrodes on the device performance. Therefore, this work focuses on the optical expediency of gold (Au), silver (Ag), aluminum (Al), and graphite as back electrode materials in organic optoelectronics. This work shows that the "one size fits all" methodology is not a valid approach for choosing the back electrode material. Instead, considering the active layer absorption, the active layer thickness, and the intended application is necessary. A traditional polymer/fullerene-based system, poly(3hexylthiophene) with [6,6]-phenyl C 61 butyric acid methyl ester (P3HT:PC 60 BM), and a state-of-the-art narrow-band gap nonfullerene-based system, poly [4,8bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b; 4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethy-lhexyl)3fluorothieno[3,4-b]thiophene-)-2-carboxylate-(2-6-diyl)] and 2,2′-((2Z,2′Z)-((5,5′-(4,4-bis(2-ethylhexyl)4H-cyclopenta[1,2-b:5,4-b′]dithiophene-2,6-diyl)bis(4-(( 2ethylhexyl)oxy)thiophene-5,2-diyl))bis(methanylylidene)) bis(5,6-difluoro3-oxo-2,3-dihydro-1Hindene-2,1-diylidene))dimalononitrile (PCE10:COTIC-4F), are investigated by combining optical transfer matrix modeling simulations with experimentally determined recombination and extraction losses. We find that the narrow-band gap system shows performance gains when employing Au as the back electrode. Furthermore, we show that these performance gains are dependent on active layer thickness, yielding the most significance for thin active layers (<100 nm). Such thin, ultra-narrow-band gap devices are the focus of near-IR sensing applications, highlighting the importance of methodically choosing the back electrode. Lastly, the impact of the back electrode on the OPV device performance is outlined.

Unraveling Device Physics of Dilute‐Donor Narrow‐Bandgap Organic Solar Cells with Highly Transparent Active Layers

“…To obtain the charge‐carrier generation rates depending on the spatial position G ( x ) in the active layer, we obtained the optical properties of the blends with variable angle spectroscopic ellipsometry (VASE) and combined them with the already known optical properties of the other materials used in the device stack to serve as the input for optical transfer matrix simulations (TMM). [ 40,52 ] The resulting G ( x ) curves are shown in Figure 3c. Here, too, a clear trend can be seen with G ( x ) being reduced noticeably as the donor concentration is reduced from 40% to 30% and 20%.…”

A Review on the Materials Science and Device Physics of Semitransparent Organic Photovoltaics

“…Schopp et al have demonstrated, that the ideal electrode choice depends on the absorber band gap and the thickness of the active layer. Many opaque systems rely on Ag electrodes, whereas a change to Au electrodes for thin narrow-band gap active layers can boost the performance [125].…”

The role of structural parameters on efficiency and transparency of semi-transparent non-fullerene organic solar cell

“…In addition, both electrodes must be transparent. Although in OSC, the amount of interfacial recombination is affected significantly by the electrode choice, a broader range of materials and deposition techniques can be considered for transparent electrodes because the surface recombination at the active layer interfaces is not a dominant recombination mechanism in these systems, therefore, the limitations due to high series resistance decrease 11 . There are several ways to fabricate transparent electrodes.…”