Improved performance of colloidal quantum dot solar cells using high-electric-dipole self-assembled layers

High performance colloidal quantum dot (CQD) solar cells were developed by modifying ZnO electron accepting layers (EALs) using self-assembled monolayers (SAMs) of highly polar molecules. A high molecular dipole moment of −10.07D was achieved by conjugating a strong electron donor, julolidine, to an electron acceptor, a cyanoacetic acid unit, through a thiophene moiety. The energetic properties of ZnO EALs were manipulated with respect to the dipole moment of the modifying molecules. The built-in potential (V_bi) and internal electric field (E_int) of CQD solar cells could thereby be tuned. The power conversion efficiency (PCE) of the SAM modified devices was improved from 3.7% to 12.9% relative to the unmodified devices as a function of molecular dipole moments (from −5.13D to −10.07D). All figures-of-merit of solar cells were improved simultaneously by SAM modification due to enhanced V_bi, E_int, and charge collection efficiency. The PCE of the highly polar molecule modified devices reached 10.89% with a V_OC of 0.689 V, whereas that of the unmodified devices was 9.65% with a V_OC of 0.659 V. Notably, the remarkably low energy loss of 0.433 eV is achieved in the SAM modified devices.