Optical and electrical properties of p-type doped hydrogenated amorphous silicon (p-a-Si:H), used as window emitter layer in silicon based hetero junction (a-Si:H/c-Si HJ) solar cells, are investigated. These properties were investigated by comparing diborane (B2H6) and trimethylboron (B(CH3)3, TMB) as doping gas analyzing the deposition temperature dependence. A wider optical band gap (E04>1.9 eV) and lower refractive index (n) is observed for TMB-doped layers, which we ascribe to carbon incorporation. a-Si:H/c-Si HJ solar cells with p-doped emitter layers using TMB and B2H6 were fabricated. For cells with TMB-doped layers an increased photocurrent of up to 1.5 mA/cm2 is found, which is due to a reduction in parasitic absorption. However, cells with TMB-doped emitter also show a higher series resistance, which we attribute to increased electrical losses at the TCO/p-contact.