Thin Films Characterization and Study of N749-Black Dye for Photovoltaic Applications

Abstract

This paper reports on the fabrication and photovoltaic characteristics of a heterojunction solar cell based on an organic small molecular semiconductor, N-749 black dye (N749-BD). To investigate the photovoltaic characteristics of N749-BD, an ITO/PEDOT:PSS/N749-BD/Ag device is prepared by spin casting a 100 ± 5-nm thin film of N749-BD on the poly(3,4, ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) film, which acts as buffer/hole transport layer (HTL) and indium tin oxide (ITO) is employed as a transparent conducting substrate. Under standard testing conditions (STC), i.e., 25 °C, 1.5 AM global and 100 mW/cm2 irradiation, the photovoltaic parameters of the device, such as fill factor (FF) and power conversion efficiency (PCE), are found to be 0.65 and 3.8% ± 0.5%, respectively. Current-voltage (I–V) characteristics of the device are also studied in dark conditions to measure reverse saturation current (I0), series resistance at the interface, rectification ratio (RR), barrier height (ϕb) and ideality factor (n). Optical bandgaps (Eg) of N749-BD thin film are found by applying Tauc’s plot on its ultraviolet-visible (UV-Vis) spectrum, which are measured to be 1.68, 2.67, 3.52 and 4.16 eV. External quantum efficiency (EQE) measurements of the fabricated device are studied, which demonstrate large value of EQE ≈ 12.89%, with peak intensity at 626 nm. Bond dynamics and compositional analysis of N749-BD is carried out via Fourier transformed infrared (FTIR) spectroscopy. Morphology of the thin film of N749-BD on quartz glass are investigated via scanning electron microscopy (SEM) with in-situ energy dispersive X-ray (EDX) spectroscopy which exhibits random distribution of N749-BD grains across the surface with nearly uniform grain sizes and shapes. The larger values of FF, PCE and EQE of ITO/PEDOT:PSS/N749-BD/Ag device suggests the potential of N749-BD to be utilized in low cost, simple manufacturing process and high performance of solar cells.