Building mechanism for a high open-circuit voltage in an all-solution-processed tandem polymer solar cell

Additional post-processing techniques, such as post-thermal annealing and UV illumination, were found to be required to obtain desirable values of the cell parameters in a tandem polymer solar cell incorporated with solution-processed basic n-type titanium sub-oxide (TiOx)/acidic p-type poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) interlayers. Subsequent to the fabrication of the tandem polymer solar cells, the open-circuit voltage (V-OC) of the cells exhibited half of the expected value. Only after the application of the post-treatments, the VOC of a tandem cell increased from the initial half-cell value (similar to 0.6 V) to its full-cell value (similar to 1.2V). The selective light-biased incident photon-to-current efficiency (IPCE) measurements indicated that the initial V-OC originated from the back subcell and that the application of the post-processing treatments revived the front subcell, such that the net photocurrent of the tandem cell was finally governed by a recombination process of holes from the back subcell and electrons from the front subcell. Based on our experimental results, we suggest that a V-OC enhancement could be ascribed to two types of subsequent junction formations at the interface between the TiOx and PEDOT: PSS interlayers: an 'ion-mediated dipole junction', resulting from the electro-kinetic migration of cationic ions in the interlayers during post-thermal annealing in the presence of a low-work-function metal cathode, and a 'photoinduced Schottky junction', formed by increasing the charge carrier density in the n-type TiOx interlayer during UV illumination process. The two junctions separately contributed to the formation of a recombination junction through which the electrons in TiOx and the holes in PEDOT:PSS were able to recombine without substantial voltage drops.