Optimization of PCDTBT Metal-Insulator-Metal Hole-Only Photodiodes

Abstract

While there have been significant strides in improvements to organic solar cell (OSC) power conversion efficiencies using non-fullerene acceptors and novel benzodithophene and benzodithiophene-dione co-polymers, many of these new polymers still suffer from intrinsic degradation via photooxidation or light-induced cross-linking [1]. PCDTBT, a co-polymer of carbazole and dithienyl-benzothiadiazole, has remained among one of the most stable donor polymers to-date [2,3]. PCDTBT has recently been employed as a donor component in ternary blend OSCs [4], and as a hole transport layer in perovskite solar cells, resulting in improvements to device efficiency and stability [5]. Thus, understanding properties of neat PCDTBT thin-films is of great importance for determining how to best employ this stable co-polymer in highly efficient and stable OSC devices.

In this work, we fabricated PCDTBT unipolar hole-only devices in a metal-insulator-metal Schottky photodiode geometry. Neat PCDTBT thin-films are challenging to prepare due to their poor solubility in typical organic solvents and ease of aggregation. We optimized PCDTBT thin-films by varying the solvent, molecular weight, heating times and temperatures, and filtering conditions. To evaluate the quality of PCDTBT thin-films with low degree of aggregation, we used a relatively unexplored technique of polarized light microscopy (PLM) [6]. PLM is a non-destructive, wide-field technique that allows elucidation of birefringent materials, such as conjugated polymers, giving enhanced contrast compared to standard bright-field light microscopy (BFM). We used PLM to rapidly evaluate the quality of PCDTBT thin-films in order to find optimal conditions for uniform thin-films with low aggregation.

By using optical transfer matrix methods (TMM) simulations and experimental measurements of transmission and sheet resistivity, we further optimized conditions for transparent Au electrodes. TMM simulations revealed the optimal thickness of each layer to maximize photocurrent generation. Finally, using optimized conditions, we fabricated PCDTBT photodiodes in a Au/MoOx/PCDTBT/Ag geometry for thick (200 nm) and thin (80 nm) PCDTBT layers. We extracted the Schottky barrier height and hole mobility of PCDTBT from current-voltage measurements and drift-diffusion simulations, respectively.