Tandem organic solar cells with an inverted structure

Abstract

In this dissertation, systematic study on organic solar cells (OSCs) with inverted and tandem structures and a new polymer based solar cell is presented. An inverted device has been demonstrated using low work function (LWF) metal Ca modified ITO as cathode and MoO3 modified Ag as anode. In tandem devices, an efficient Al/MoO3 intermediate layer achieving optical and electrical sub-cell connection, has been developed and employed in various types of tandem devices, including polymer/small molecule and polymer/polymer double- and triple-tandem devices with improved or comparable efficiencies. Subsequently, inverted tandem OSCs with MoO3/Ag/Al/Ca intermediate layer have been realized for the first time, exhibiting good performance. Optical and structural properties of these intermediate layers have been studied in details, along with their applications in corresponding devices. The intermediate layer is rather crucial for realization of conventional and inverted tandem devices. Furthermore, new polymer F6T2 based solar cells have been studied in terms of the effect of the weight ratio of F6T2 to PCBM on the photo-physics, morphology, and electrical characteristics.

Conventional OSCs suffer interface instability due to the usage of PE-DOT:PSS at anode side and LWF metals as cathode. Inverted structure could solve these problems by reversing cathode and anode via interfacial modifica-tion. Interfacial layers used for modifying ITO as cathode have significantly affected the realization and performance of inverted devices. An ultrathin Ca as a simple interfacial layer is found to effectively modify ITO as cathode; mean-while, MoO3 as a buffer layer modifies Ag anode. The functions of Ca and MoO3 have been discussed correspondingly.