Detailed investigation of dependencies of photovoltaic performances of P3HT:PC61BM based solar cells on anodic work function modified by surface treatment of indium-tin-oxide electrode with benzenesulfonyl chloride derivatives

Abstract

The photovoltaic (PV) characteristics of bulk-heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) were improved using indium-tin-oxide (ITO) anode electrodes modified chemically with CH3O-, H-, Cl-, CF3-, and NO2-terminated benzenesulfonyl chlorides as a self-assembled monolayer (SAM). The ITO electrode surfaces were easily treated through the chemical modification of the reactive –SO2Cl binding group, and the work function (WF) of the modified ITO was effectively changed depending on the permanent dipole moments introduced in the para-position of benzenesulfonyl chloride. We examined the correlation between the ITO WFs corrected by the change in the contact potential difference and the calculated dipole moments of the SAM models. Moreover, we examined the PV characteristics of the P3HT:PC61BM based BHJ organic PV cells using the SAMs or poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-treated ITOs with different WFs lying within ±0.2eV from the highest occupied molecular orbital (HOMO) level of P3HT. We found that the enhancement effect of the SAMs on the power-conversion efficiency (ηP) reached a maximum with Cl (ηP=3.72%), and became larger than that of PEDOT:PSS (ηP=3.62%). Two distinct Jsc dependencies, increasing and decreasing with the increasing WF of the anode ITO, were observed at higher and lower WFs than the HOMO level of the donor, respectively. Almost constant Voc values (around 0.6V) were observed with different SAM-modified ITOs, which suggested that Fermi level pinning was achieved by aligning the anode Fermi level and positive polaronic level of the donor polymer.