Integration of Aluminum Plasmonic Nanostructures for Manipulating Organic Ultraviolet Photodetectors

Incorporating surface plasmonics into optoelectronic devices can manipulate the performance of devices such as solar cells and photodetectors. This effect has been well-established for metals such as gold (Au), but has not been studied as extensively for metals with UV-range transitions such as aluminum (Al). UV-specific photodetectors have a wide variety of applications including environmental monitoring, scientific research, imaging, and flame and missile detection. Many of the devices currently in use are based on inorganic wide-bandgap materials such as GaN and ZnO, but organic materials offer several advantages over inorganic materials such as flexibility, tunability, and low material cost. In this work, a wide-bandgap (2.4 eV) polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)] (F8T2) was blended with a fullerene derivative [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) for device active layers. We integrated Al plasmonic nanostructures into the organic UV photodetectors with the photodiode structures either as a transparent electrode or at the top metal electrode. For the devices with the Al plasmonic nanostructures as the transparent conducting electrodes, we replaced the indium tin oxide (ITO) with Al plasmonic nanohole arrays (Al-NHAs) with the device structure of glass/Al-NHA/PEDOT:PSS/F8T2:PC₇₁BM/LiF/Al, where PEDOT:PSS and LiF were used as hole and electron transport layers, respectively. We applied 3-dimensional finite-difference time-domain (3D-FDTD) electromagnetic simulations to design the Al-NHA to produce strong UV absorption in the active layer and enhanced internal electric field intensity. We successfully fabricated the Al-NHA electrodes using nanosphere lithography and incorporated into photodetectors, which produced two narrow photoresponse peaks with specific detectivity (D*) values of 4.0 x 10⁹ and 4.6 x 10⁹ jones under 340 and 515 nm illumination, respectively, and −2 V bias and one broad photoresponse peak with a peak D* of 8.8 x 10⁹ jones under 450 nm illumination and 2 V bias. For the devices with the Al plasmonic nanostructures at the top metal electrodes, we imprinted nanohemisphere arrays (NHSAs) into the top surface of the active layer (F8T2:PC₇₁BM) in conventional organic UV photodetectors with the structure of glass/ITO/PEDOT:PSS/F8T2:PC₇₁BM/LiF/Al. NHSAs were transferred to the top Al cathode in the following thermal deposition of LiF and Al to generate plasmonic Al nanostructures. 3D-FDTD simulations revealed that the devices with plasmonic NHSA top electrodes exhibit stronger UV absorption in the active layer and enhanced electric fields at the interface of the top of the active layer and the Al electrode, especially for devices with thinner active layers. The inclusion of a NHSA was found to improve the photoresponse strength, sensitivity and speed through increased UV absorption and enhanced electric fields resulting from angular reflection of light in the active layer and the plasmonic effects of the Al NHSA electrode. The novel bias dependent response switching improves the applicability of UV photodetectors through the cost-effective, flexible, and performance-enhancing plasmonic Al-NHA transparent conducting electrodes. Our work also demonstrates that imprinting an NHSA into the top of the active layer, thus structuring the top metal electrode, is an effective approach for incorporating performance-enhancing plasmonic nanostructures into organic UV photodetectors.