The dark conductivity of hydrogenated amorphous silicon (a-Si:H) films deposited from dichlorosilane (SiCl2H2) and silane (SiH4), and doped with diborane (B2H6), increases by as much as a factor of 100 over the usual a-Si:H,B films deposited without SiCl2H2. The effect is observed at gas phase concentrations of diborane ranging from 0.006 to 0.5 vol %, and for both direct current (dc) and 13.56 MHz radio frequency plasma depositions, although it is more noticeable for the dc discharge. An increase in dark conductivity is also observed in boron doped hydrogenated amorphous silicon carbon alloys (a-SiC:H) deposited with dichlorosilane, albeit coupled with a change in the Tauc gap. Chlorine reduces the conductivity of undoped and phosphorus doped a-Si:H films. A B–Cl cluster acceptor that is not passivated by H is proposed as a possible mechanism for chlorine enhanced conductivity. This increase in p-layer conductivity translates into an increase of solar cell efficiency, but surprisingly by an increase in photocurrent rather than open circuit voltage. © 2000 American Institute of Physics.