Effect of titanium chelate as a function of thickness on the electron mobility and electron transport and collection efficiency

Electron extraction layers (EELs) with shallow lowest unoccupied molecular orbitals (LUMO) featuring good energy level alignment with LUMO of acceptor materials are desirable for high-performance polymer solar cells (PSCs). Here, the use of alcohol-soluble titanium chelate, titanium (IV) oxide bis(2, 4-pentanedionate) (TOPD) as EEL layer and to tune the work function (WF) of EEL is proposed. The enhanced photovoltaic performance can be experimentally explained by the decreased barrier potential between photoactive layer and TOPD/ITO composite electrode from the ultraviolet photoelectron spectroscopy (UPS) curve, and the enhancement of electron mobility and electron transport and collection efficiency from space-charge-limited current (SCLC), time of flight (TOF) curve and the photocurrent density versus effective voltage (Jph-Veff) curve. Simultaneously, the weaker bimolecular recombination between photoactive layer and the TOPD layer were confirmed by the short-circuit current (JSC) value as a function of light intensity. Consequently, the PCE of TOPD-based PSCs arrives to 9.16% with 15 nm TOPD layer as EEL, which is higher than the PCE of 8.83% for ZnO-based PSCs.