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

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(3-hexylthiophene) with [6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PC₆₀BM), and a state-of-the-art narrow-band gap non-fullerene-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)3-fluorothieno[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-1H-indene-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.