PEDOT:PSS is one of the commonly used HTM.

Start with "Fundamental of Photonics," second edition, 2007, by Saleh and Teich, before reading the heavier stuff. 

Using PEDOT:PSS often leads to instability of the device due to its acidic nature. So, in my opinion MoOx is the best HTL (although it cannot be solution processed).

Unfortunately, there are not a unique material, the HTL best depends on the HOMO-LUMO levels of the materials in the active layer in order to optimize the photocurrent.