  An approach is presented to modify the WF of solution-processed sol-gel
derived ZnOover an exceptionally wide range of more than 2.3 eV. This approach
relies on the formation of dense and homogeneous self-assembled monolayers
based on phosphonic acids with different dipole moments. This allows us to
apply ZnO as charge selective bottom electrodes in either regular or inverted
solar cell structures, using P3HT:PCBM as the active layer. These devices
compete with or even exceed the performance of the reference cell on
ITO/PEDOT:PSS. Our finding challenges the current view that bottom electrodes
in inverted solar cells need to be electron-blocking for good device
performance.
