Effect of dimethyl sulfoxide (DMSO) on the electrical properties of PEDOT:PSS/n-Si heterojunction diodes has been studied. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) was deposited on n-type Si wafer using facile process of spin coating. The DMSO content was varied from 0 to 8 vol%. Electrical characterization of these heterojunction diodes as performed using both current-voltage (I-V) and capacitance-voltage (C-V) measurements. All diodes showed rectifying behavior. AFM measurement revealed that the surface became more rough after the DMSO treatment of PEDOT:PSS films. The RMS values were found in the range of 4-6 nm. The resistivity of the PEDOT: PSS films decreased with increase in temperature. The addition of DMSO into PEDOT:PSS solution results in a decrease in resistivity of films by approximately two orders of magnitude. PEDOT:PSS films showed high transmission more than 85% in the entire visible region. Raman spectroscopy indicated effect of the DMSO treatment on the chemical structure of PEDOT chains, suggesting a conformational change of PEDOT chain in the film. An optimal value of DMSO was obtained with 5 vol% content, and it showed the best PEDOT: PSS films properties and good quality heterojunction diodes characteristics with ideality factor of 2.4 and barrier height 0.80 eV. (C) 2015 Elsevier B.V. All rights reserved.