Fabrication of Ultra-Thin Photo-Multiplication Photodiode by Using a Non Fullerene-Based 2D Planar Small Molecular Semiconductor as an Efficient Optical Sensitizer

Abstract

In this work, we explore the possibility of using nonfullerene and a planar n-type small molecular semiconductor, 2,2′‐((2Z,2′Z)‐((4,4,9,9‐tetrahexyl‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(methanylylidene))bis(3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (IDIC) as an optical sensitizer to improve the EQE and to reduce the thickness of the photoactive layer to 70 nm. A key idea of this work is utilizing the unique photophysical properties of IDIC with an anisotropic electron transport. As is well known, contrary to spherical PCBMs (PC61BM and PC71BM) with an isotropic charge transport property, the 2D planar IDIC with an inherently anisotropic packing structure tends to hinder the formation of the effective electron percolation pathways. This is a very important requirement for the optical sensitizer of PM-OPDs because it leads to more efficient charge trapping. In addition, IDIC possesses a relatively higher absorption coefficient in the visible range compared to PC71BM, which can contribute to a higher photocurrent. Together with a deeper lowest unoccupied molecular orbital (LUMO) level of IDIC compared to PC71BM, all the mentioned photophysical properties of IDIC can be much more beneficial as optical sensitizers of the PM-OPDs. Layer-by-layer deposition of P3HT as a photoactive layer and IDIC as an optical sensitizer enables more effective PM operation, yielding high EQE exceeding 130,000% and specific detectivity over 1012 Jones at 150-nm-thick active layer. Furthermore, due to more facile spatial confinement of the charge carriers, the photoactive layer thickness was further decreased down to 70 nm while maintaining reasonably high EQE of 60,000% as well as specific detectivity over 1012 Jones. Physical origins of such synergetic effects of using IDIC as an optical sensitizer are fully discussed with various photophysical analyses in the forthcoming sections.