Electrical Junction Behavior of Poly(3,4-ethylenedioxythiophene) (PEDOT) Contacts to H-Terminated and CH3-Terminated p-, n-, and n+-Si(111) Surfaces

The electronic and photovoltaic properties of junctions between the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) and Si(111) surfaces have been investigated for a range of doping types, doping levels, and surface functionalization of the Si. PEDOT–poly(styrenesulfonate) (PSS) formed ohmic, low resistance contacts to H-terminated and CH3-terminated p-type Si(111) surfaces. In contrast, PEDOT formed high barrier height (0.8–1.0 V) contacts to n-Si(111) surfaces, with CH3-terminated n-Si(111)/PEDOT contacts showing slightly higher barrier heights (1.01 eV) than H-terminated n-Si(111)/PEDOT contacts (0.89 V). PEDOT contacts to CH3-terminated and H-terminated n-Si(111) surfaces both produced photovoltages under illumination in accord with the Shockley diode limit based on bulk/recombination diffusion in the semiconductor. Such devices produced solar energy-conversion efficiencies of 5.7% under 100 mW cm–2of simulated air mass 1.5 illumination. The electrical properties of PEDOT contacts to CH3-terminated Si surfaces were significantly more stable in an air ambient than the electrical properties of PEDOT contacts to H-terminated Si surfaces. PEDOT films produced a low resistance, tunnel-barrier type of ohmic contact to n+-Si(111) surfaces. Hence, through various combinations of doping type, doping level, and surface functionalization, the PEDOT/Si contact system offers a wide range of opportunities for integration into monolithic photovoltaic and/or artificial photosynthetic systems.