Interfacial layers (IL) are important components of high efficient organic solar cells as they provide efficient charge carrier extraction towards the electrodes avoiding hereby losses such as non-Ohmic contact, charge carrier recombination and exciton quenching at the interfaces.1 In the past, solution processed materials such as metal oxides have been successfully introduced as hole and electron blocking layers for organic solar cells. Especially ZnO based extraction layers are nowadays one of the most promissing IL systems leading to efficiencies of 10.3% in single junction devices.2 In order to further improve such metal oxide ILs, deeper understanding of the physics taking place at the interfaces with ZnO nanoparticles and polymer blends is necessary. Furthermore ZnO ILs are usually included as very thin layers to keep the fill factor of the solar cells high due to their poor conduction properties. Therefore new materials for thick layer and thus robust solution processing are another key parameter for printed OPV. For this purpose, we studied ZnO nanoparticle ILs in combination with P3HT as standard and PTB7 as well as PTB7-Th as actual world record donor materials that allow efficiencies over 9%3 and 10%2, respectively. By using P3HT/ZnO bilayer solar cells, we demonstrate that ZnO nanoparticles in direct contact with the donor can modify strongly LUMO and HOMO level of the donor polymer making specific interfacial engineering necessary to avoid charge carrier recombination.4 Furthermore solution processed ZnO nanoparticles ILs using specific ligands were developed for PTB7:PC70BMC and PTB7-Th:PC70BMC solar cells using normal device structures. Optimization of optical, morphological, and electronic effects of the Ils lead to efficiencies of 7.6% for PTB7 5 and 9.2 % for PTB7-Th devices (Figure 1). We further applied Al doping to increase conductivity of the ZnO Ils towards robust thick layer processing of Ils. The corresponding results revealed that Al doped ZnO (AZO) nanoparticles allow processing of highly efficient ILs of 80 nm layer thickness with fill factors up to 64 %. In combination with polymer blends with optimal layer thickness, AZO layers generate optical spacer effects leading to strongly improved photocurrent generation by 20% compared to devices using thin IL layers and more importantly, new opportunity for color design in OPV. [1] XU, C. Z., HONG, Z., YANG, Y., J. Mater, Chem. 2010, 20, 2575. [2] LIAO, S.-H., JHUO, H.-I., YEH, P.-N., CHENG, Y. S., LI, Y.-L., LEE, Y.-H., SHARMA, S., CHEN. S.-A. Scientific Report, DOI: 10.1038/srep06813 [3] HE, Z., ZHONG, C., SU, S., XU, M., WU, H., CAO Y., Nat. Photonics 2012, 6, 591 [4] MATTIOLI, G., BEN DKHIL, S., SABA, M. I., MALLOCI, G. MELIS, C., ALIPPI, P., FILIPPONE, F. GIANNOZZI, P. THAKUR, A., GACEUR, M., MARGEAT, O. DIALLO, A. K., VIDELOT-ACKERMANN, C., ACKERMANN, J., BONAPASTA, A. A., MATTONI, A. Ad. Energy Mater. 2014 DOI: 10.1002/aenm.201301694. [5] BEN DKHIL, S. DUCHE, D. GACEUR M., THAKUR, A., ABOURA, F. B., ESCOUBAS, L., SIMON, J.-J., GUERRERO, A., BISQUERT J., GARCIA_BELMONTE, G., BAO, Q., FAHLMAN, M., VIDEOT-ACKERMANN, C., ACKERMANN, J. 2014. Ad. Energy Mater 2014. DOI: 10.1002/aenm.201400805.