Self-Doped Conjugated Polyelectrolyte With Tuneable Work Function For Effective Hole Transport In Polymer Solar Cells

A water-soluble conjugated polyelectrolyte (CPE), PEDOT-S (poly(4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl-methoxy)-1-butanesulfonic acid)), is demonstrated to be an excellent hole transport material in several polymer solar cells with different donor's HOMO (highest occupied molecular orbital). With a P3TI:PC71BM (poly[6,6'-bis(5'-bromo-3,4'-dioctyl-[2,2'-bithiophen]-5-yl)-1,1'-bis(2-hexyldecyl)-[3,3'-biindolinylidene]-2,2'-dione]:[6,6]-phenyl C-71 butyric acid methyl ester) active layer, the device using PEDOT-S as a hole transport layer (HTL) outperforms the PEDOT:PSS-based devices due to an increased FF (fill factor). The devices' current density-voltage characteristics (J-V) show that a PEDOT-S layer can operate well with a wide range of thicknesses as well, helped by its high conductivity and decent transparency. With UV-ozone treatment, the work function of the PEDOT-S can increase from 4.9 eV to 5.2 eV. In TQ1:PC71BM (poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl]:PC71BM) devices, which have a deeper donor HOMO than P3TI, V-oc is improved from 0.81 V to 0.92 V by 7 min UV-ozone treatment, along with a suppressed reverse injection current and increased J(sc) (short-circuit current density) and FF. Topography study shows the excellent coating ability of PEDOT-S. Conductive atomic force microscopy (C-AFM) shows the out-of-plane current in PEDOT-S film is one thousand times higher than that in PEDOT: PSS PH 4083 film under the same electric field and has much more uniformly distributed current pathways.