Engineering and investigation of Zinc oxide cathode interfacial layer in inverted organic solar cells Implications for charge transport and collection

Abstract

The aim of this thesis is to investigate and engineer Zinc oxide (ZnO) cathode buffer layer which bridges the transparent electrode and the active layer in an inverted OSC. We have used current density-voltage (J-V), space charge limited current (SCLC), capacitance-voltage (C-V) newlineviii newlineand impedance spectroscopy (IS) techniques to study charge transport, charge carrier recombination and charge collection efficiency in devices. A quantitative and qualitative analysis of the structural and surface aspects of sol-gel processed ZnO was done using X-ray reflectivity (XRR) and atomic force microscopy (AFM) techniques. On the basis of our understanding of the structural-property correlation in the sol-gel processed ZnO based inverted OSC devices, we developed an approach to engineer the ZnO layer by using an alternate precursor. Further, with the same precursor, we developed a PC61BM doped ZnO-organic hybrid cathode buffer layer and implemented in OSC devices. A self-assembled approach to realize an ideal p-i-n structure in conjunction with ZnO buffer layers is employed and investigated in inverted OSCs. Here, we have used a specifically designed a ternary active layer involving one donor (PTB7-Th) and two structurally different acceptors, ellipsoidal PC71BM and a spherical PC61BM. The inverted OSC devices having the ternary active layer with ZnO cathode buffer layer(s) are investigated to understand the impact of these improvisations on the electron transport and their collection. Further, the improved device performances and their electrical characteristics are interpreted in the context of structural and chemical changes occurred in ZnO buffer layer and at their interfaces.