Inkjet Printed Transition Metal Dichalcogenides and Organohalide Perovskites for Photodetectors and Solar Cells
Authors:   , Hossain Ridwan Fayaz 
Published: February 2025

This dissertation is devoted to the development of novel devices for optoelectronic and photovoltaic applications using the promise of inkjet printing with two-dimensional (2D) materials. A systematic approach toward the characterization of the liquid exfoliated 2D inks comprising of graphene, molybdenum disulfide (MoS2), tungsten diselenide (WSe2), and 2D perovskites is discussed at depth. In the first study, the biocompatibility of 2D materials -- graphene and MoS2 -- that were drop cast onto flexible PET and polyimide substrates using mouse embryonic fibroblast (STO) and human esophageal fibroblast (HEF) cell lines, was explored. The polyimide samples for both STO and HEF showed high biocompatibility with a cell survival rate of up to ~ 98% and a confluence rate of 70-98%. An inkjet printed, biocompatible, heterostructure photodetector was constructed using inks of photo-active MoS2 and electrically conducting graphene, which facilitated charge collection of the photocarriers. The importance of such devices stems from their potential utility in age-related-macular degeneration (AMD), which is a condition where the photosensitive retinal tissue degrades with aging, eventually compromising vision. The biocompatible inkjet printed 2D heterojunction devices were photoresponsive to broadband incoming radiation in the visible regime, and the photocurrent scaled proportionally with the incident light intensity, exhibiting a photoresponsivity R ~ 0.30 A/W. Strain-dependent measurements were also conducted with bending, that showed Iph ~ 1.16 µA with strain levels for curvature up to ~ 0.262 cm-1, indicating the feasibility of such devices for large format arrays printed on flexible substrates. Alongside the optoelectronic measurements, temperature-dependent (~ 80 K to 573 K) frequency shifts of the Raman-active E12g and A1g modes of multilayer MoS2 exhibited a red-shift with increasing temperature, where the temperature coefficients for the E12g and A1g modes were determined to be ~ - 0.016 cm-1/K and ~ - 0.014 cm-1/K, respectively. The phonon lifetime τ was determined to be in the picosecond range for the E12g and A1g modes, respectively, for the liquid exfoliated multilayer MoS2. Secondly, an all inkjet printed WSe2-graphene hetero-structure photodetector on flexible polyimide substrates is also studied, where the device performance was found to be superior compared to the MoS2-graphene photodetector. The printed photodetector was photo responsive to broadband incoming radiation in the visible regime, where the photo responsivity R ~ 0.7 A/W and conductivity σ ~ 2.3 × 10-1 S/m were achieved at room temperature. Thirdly, the synthesis of solution-processed 2D layered organo-halide (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 2, 3, and 4) perovskites is presented here, where inkjet printing was used to fabricate heterostructure flexible photodetector devices on polyimide substrates. The ON/OFF ratio was determined to be high, ~ 2.3 × 103 while the photoresponse time on the rising and falling edges was measured to be rise ~ 24 ms and fall ~ 65 ms, respectively. The strain-dependent measurements, conducted here for the first time for inkjet printed perovskite photodetectors, revealed the Ip decreased by only ~ 27% with bending (radius of curvature of ~ 0.262 cm-1). This work demonstrates the tremendous potential of the inkjet printed, composition tunable, organo-halide 2D perovskite heterostructures for high-performance photodetectors, where the techniques are readily translatable toward flexible solar cell platforms as well. Fourthly, metal contacts and carrier transport in 2D (CH3(CH2)3NH3)2(CH3NH3)n-1PbnI3n+1 (n = 4) perovskites is a critical topic, where the use of silver (Ag) and graphene (Gr) inks as metallic contacts to 2D perovskites was investigated. The all inkjet printed Gr-perovskite and Ag-perovskite photodetectors were found to be photo-responsive to broadband incoming radiation where measurements were conducted from λ ~ 400 nm to 2300 nm. The photoresponsivity R and detectivity D were compared between the Gr-perovskite and Ag-perovskite photodetectors, which revealed the higher performance for the Ag-perovskite photodetector. The superior performance of the Ag-perovskite photodetector was also justified with the Schottky barrier analysis using the thermionic emission model through temperature-dependent transport measurements. Finally, this dissertation ends with the description of the first steps for using solution-processed, inkjet printed perovskites for solar cells. The preliminary investigations include the discussion of the chemical formulations for the carrier separation layers, dispersion route, and the variation of solar cell figures of merit with processing.