To reach higher performances in organic solar cells, each layer has to be optimised with respect to its purpose. In the case of a p-i-n structured solar cell, the layers are the absorber system, the doped electron and hole transport layers, and the bottom and top contacts. This work focuses oil the investigation and characterisation of the transparent hole transport materials PV-TPD, PV-TPDoM, Di-NPB, and MeO-Spiro-TPD, as used in organic p-i-n solar cells. The motivation is to replace the hole transport, material MeO-TPD, which has been used so far despite its morphological instability at elevated temperatures, with an energetically and morphologically more suitable material. The hole, transport materials were investigated for dopability, hole mobility,. absorption, reflection, cyclic voltammetry, and glass transition temperature. Further specific material properties were determined with simplified structures, e.g. m-i-p diodes, and the standard solar cells, consisting of the fullerene C-60 as acceptor and ZnPc as the donor material. The Di-NPB has turned out to be the best choice with respect to its intrinsic properties and device parameters. The deep lying HOMO, the high hole mobility of mu = 1.9 . 10(-4) cm(2)/Vs, the morphological stability of T-g = 158 degrees C, and the excellent results of the C-60:ZnPc bulk heterojunction solar cell inakes the Di-NPB highly suitable for replacement of the MeO-TPD in organic solar cells.