Energy level alignments and photocurrents in crystalline Si/organic semiconductor heterojunction diodes

We investigate electronic energy level alignment and photocurrent in crystalline silicon/organic/semitransparent metal heterojunction diodes. Optically thin films of poly[2-methoxy,5-(2(')-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV), poly(9,9-dioctylfluorene) [PFO], pentacene (Pc), and C-60 were deposited on n and p type Si wafers and diode structures were formed by depositing either a Au anode or Al cathode onto the organic film. The energy level alignment was assessed using built-in potential and capacitance-voltage measurements. In all cases, the results are consistent with near ideal vacuum energy level alignment between the organic and inorganic semiconductor. The diode current-voltage (I-V) characteristics are consistent with the electronic structure of the heterojunction interface. For n-Si/MEH-PPV/Au, the I-V curves are quantitatively described by an organic device model. For photocurrent measurements the diodes were illuminated through the semitransparent metal contact with optical wavelengths from 350-1100 nm. The photocurrent in the diode structure can be due to absorption either in the organic layer or Si substrate. For n-Si diodes, the 0 bias photocurrent is small with external quantum efficiencies (EQEs) less than 5x10(-3) in all cases. The photocurrent is dominated by absorption in the organic layer for MEH-PPV, PFO, and C-60 and by absorption in Si for Pc. For p-Si diodes, the 0 bias photocurrent is large with EQEs of similar to 0.2 and is dominated by absorption in silicon for all organic layers. Both MEH-PPV and PFO form type I heterostructures with Si and photocurrent due to organic exciton dissociation is less efficient than in commonly used type II organic/organic heterostructures. Silicon/Pc and C-60 heterojunctions are most likely type II with small valence (Pc) or conduction (C-60) energy level differences. Surprisingly, no photocurrent was observed due to optical absorption in Pc most likely due to a chemical reaction between Pc and Si that prevented exciton dissociation at the heterojunction interface. In n-type/C-60 structures photoconductive gain with EQE >3 was obtained and in p-type structures, the EQE was >0.15 and the spectral response of the photocurrent could be changed significantly from broad band absorption in Si to relatively narrow band absorption in C-60.