Effect of interface modification in polymer solar cells: An in-depth investigation of the structural variation of organic dye for interlayer material

摘要

In order to investigate the effect of the structural variation of interlayer materials on the photovoltaic properties of polymer solar cells (PSCs) in depth, we designed and synthesized three types of small-molecule dyes with a structure of 1,1'-bis(1-alkyl)-4,4'-bipyridine-1,1'-diium benzenesulfonate (V-alkyl-OTs). Here, the alkyl groups were butyl, hexyl, and dodecyl, which denoted as C4, C6, and C12, respectively. The magnitudes dipole moments will be in the order of V-C4-OTs < V-C6-OTs < V-C12-OTs due to the increased the alkyl chain length from C4 to C12. The work function of the ZnO layer with V-alkyl-OTs is exhibited to depend on the alkyl chain length, indicating that a Schottky barrier can be tuned by the size of cation part. Thus, The power conversion efficiencies (PCEs) of the PSCs based on the blend of PTB7 and PC71BM as the photoactive layer with V-alkyl-OTs were improved over for the device with pristine ZnO (without V-Alkyl-OTs) from 7.6% (short circuit current (J(SC)) = 16.0 mA/cm(2), open circuit voltage (V-oc) = 0.72 V, fill factor (FF) = 65.6%) to 8.1% (V-C4-OTs, J(SC) = 16.8 mA/cm(2), V-OC = 0.73 V, FF = 65.9%), 8.3% (V-C6-OTs, J(SC) = 17.2 mA/cm(2), V-OC = 0.72 V, FF = 67.3%), and 8.6% (V-C12-OTs, J(SC) = 18.0 mA/cm(2), V-OC = 0.72 V, FF = 66.4%). The enhancement of the PCE is strongly related to the alkyl chain length, and the major contribution was by the improvement of the J(SC) due to the reduction in the energy offset at the cathode interface.