The bi-direction tuning of work function by authigenic buffer layer in all polymer solar cells

Abstract

A better energy level alignment between active layer and electrode is crucial in the manufacture of a high-performance all polymer solar cells (all-PSCs). Here, the surface work function of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophe-ne-alt-3-flu-orothieno[3,4-b]thiophene-2-carboxy-late]:poly[[N,N-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)] (PTB7-Th:N2200) active layer could be bi-direction tuned via poor-solvent methanol soaking for different time. Kelvin probe force microscope (KPFM) shows that the methanol treatment made the active layers work function changed from 4.68 eV to 4.52 eV and then turned to 4.81 eV when the soaking time were 0, 40 and 140 s, which lead to the reversal variation of potential barrier between active layers and electrodes. According to the principle of polarity, the stronger polarized polymers N2200 have a stronger solvation effect with polar solvents methanol than PTB7-Th. The results of X-ray photoelectron spectroscopy (XPS) indicate that the surface work function reversal mainly come from the change of surface component. Under the driving force of methanol, more N2200 migrated from the bulk phase to surface at first, then left surface and lead to more PTB7-Th appearing on surface. The authigenic buffer layers for different composition were formed in this progress which was responsible to the bi-direction tuning of surface work function. The power conversion efficiency (PCE) of inverted all polymer solar cells experienced a fluctuated progress of decreased first and then increased, and a opposite progress subjected to conventional solar cells that was increased first and then decreased of the PCE. Depending on different treatment time methanol soaking benefits to both inverted and conventional all-PSCs and both PCE of them have over 10% improvement than control device.