Performance improvement of organic solar cell via incorporation of donor type self-assembled interfacial monolayer

Highlights: • Hybrid TiO₂ is employed in electron transport layer. • The effective work function of TiO₂ is changed by monolayer. • The solar cell performance is improved by TiO₂ modification. • Exciton association is enhanced by monolayer on the TiO₂ surface. -- Abstract: Surface engineering of solar cell subsrate plays an important role to achieve high performance organic solar cell (OSC). The surface modification of the electron selective electrodes on which active organic layers form is a main research issue to improve interface between the semiconductor and the electron selective electrode. Here an enhancement of performance of the inverted type organic solar cells (i-OSCs) was provided by the use of a facile method of titanium dioxide (TiO₂) surface modification with triphenylamine based self-assembled monolayer (SAM) molecules. The surface potential as well the effective work function of SAMs treated TiO₂ layer was better aligned to lowest unoccupied molecular orbital (LUMO) level of acceptor organic semiconductor for efficient electron charge extraction. As a result, the best maximum power conversion efficiency (PCE) of i-OSCs with and without SAMs modified TiO₂ electrodes increased from 2.65% to 2.95%, respectively. The overall electrical parameters (short circuit current density, open circuit voltage and fill factor) of i-OSCs on the SAMS treated TiO₂ electrode exhibited better results compared to unmodified TiO₂ electrode. Series resistance (R_s) and shunt resistance (R_sh) of SAM treated devices was also evaluated to discuss an interface effects on the i-OSCs performance. These significant performance improvements reveal the potential application of SAMs for all solar cell technologies using TiO₂ electrode.