Ultra-thin intrinsic amorphous silicon (a-Si) hybrid structure with inorganic/organic materials and its applications
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The conventional amorphous silicon (a-Si) photovoltaic (PV) has been designed with intrinsic a-Si interfaced with p- and n- doped layers that are summed around 40-50 nm. Since the dopants, as intentional impurities, annihilate photogenerated carriers, the doped material should be as thin as possible while creating an internal electric field. Also, the doped layer thickness limits the possible minimum thickness of a-Si photoactive layer. Thus, in this dissertation, we present studies suggesting removing dopants in a-Si PV for high internal quantum efficiency as well as previously undiscovered decorative applications.
We propose an intrinsic a-Si hybrid structure with inorganic/organic materials for PVs without any doping. The average power conversion efficiency of the a-Si hybrid PV is 6.7 % by 180 nm-thick undoped a-Si layer. The PV cell shows short circuit current (Jsc) of 13.6 mA/cm2, open circuit voltage (Voc) of 0.77 V, and fill factor (FF) of 64 %. We also characterize the hybrid cells by capacitance-voltage measurement to identify a built-in potential, consequently determining Voc of the hybrid cells. The proposed a-Si hybrid device is further investigated by capacitance-frequency characteristics in order to verify Voc change (0.6 V – 0.8 V) in ultra-thin a-Si regime. We also analyze transient photocurrent and photovoltage responses of the hybrid cells for quantifying ambipolar diffusion length (~ 80 nm), along with higher solar concentration test (1 – 7 Sun), to explain FF change (62 % – 68 %) with regard to varied a-Si thickness.
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. The electric power-generating American flag and University of Michigan logo panels provide the potential examples of decorative PVs. We also suggest applying the ultra-thin a-Si hybrid structure for large area high-speed photo-detectors and transparent interactive screens.