We note that both exciton and charge transport in organic materials usually require hopping from molecule to molecule. Thus, close packing of the molecules is assumed to decrease the width of the intermolecular barriers and a flat molecular structure should generally lead to better transport properties than bulky 3 dimensional molecules. We note that dense packing also favours a higher absorption coefficient. In order to meet these specific requirements for efficient photon to charge conversion different device architectures have been developed in the past. We give below a brief summary of the strengths and weak points of the three different architectures known today.