Ultra-thin Intrinsic Amorphous Silicon (a-Si) Hybrid Structure with Inorganic/Organic Materials and Its Applications.

Abstract

The conventional a-Si photovoltaic (PV) has been designed with intrinsic a-Si interfaced with p- and n- type doped layers, total around 40-50 nm. Here, the intrinsic layer needs to be much thicker than the doped layers in order to maximize the conversion of the photogenerated carriers to electric current. Since the dopants, as intentional impurities, annihilate photogenerated carriers, the doped material should be as thin as possible unless it reduces an internal electric field. Eventually the unwanted carrier recombination with dopants causes light-induced degradation of a-Si PV. In this dissertation, we present studies suggesting removing dopants in a-Si PV for higher internal quantum efficiency as well as previously undiscovered novel PV applications. We propose intrinsic a-Si hybrid structure with inorganic/organic materials for PVs without any doping. The highest average power conversion efficiency of the a-Si hybrid PV is 6.7% by 180 nm-thick undoped a-Si layer. Its current density-voltage (J-V) curve shows the average short circuit current (Jsc) of 13.6 mA/cm2, open circuit voltage (Voc) of 0.77, and fill factor (FF) of 64 %. We also characterize the hybrid cells by capacitance-voltage (C-V) measurement to identify a built-in potential, from 0.7 V to 0.85 V, according to metal oxide Schottky contact at anode. The built-in potential consequently determines Voc of the hybrid cells. We exploit the proposed a-Si hybrid device further by running capacitance-frequency (C-f) measurement in order to quantify the interfacial amount of charge annihilation with regard to varied a-Si thicknesses, thereby affecting Voc ranging from 0.6 V to 0.8 V. The Voc change is also evaluated under concentrated sun condition (1 Sun ~ 7 Sun). Since the a-Si hybrid cell shows FF decrease (68 % to 62 %) with increased a-Si thickness (50 nm to 180 nm), we exploit the ambipolar diffusion length (~ 80 nm) by transient photocurrent and photovoltage responses. Notably, we demonstrate the use of the a-Si hybrid cells for decorative colored PV applications by designing a-Si thickness below ~30 nm, even a few nanometers for semi-transparent PVs. We also suggest applying the ultra-thin a-Si hybrid structure for various photo-sensing applications including large area high-speed photo-detectors.