Small-Molecule Organic Photovoltaic Devices: Applications and Reliability.

Abstract

The development of organic optoelectronic devices has moved forward at an incredible pace over the past three decades. Prototype organic solar cell panels have emerged in the personal electronic market, and displays using organic light-emitting diodes have become an essential part of consumer electronics. This thesis focuses on the application and reliability of organic photovoltaic cells (OPVs), as they are promising candidates as low-cost, flexible solar energy conversion sources. Top-illuminated organic photovoltaic cells present opportunity to widen OPV applications, with potential use as power generating coatings on flexible and low-cost substrates. In the first part of the thesis, we explore inverted small-molecule organic photovoltaic cells on reflective metal substrates. We investigate the design of inverted OPVs. We demonstrate methods to overcome challenges in device performance and achieve inverted OPV on metal substrates with comparable efficiency to conventional devices. In the second part of the thesis, we study the reliability of small-molecule OPVs. We start with an extensive overview of characterization and reporting of OPV operational lifetime, device packaging and current research progress. We present a fully-automated, compact experimental setup for long-term reliability testing. We identify exciton-induced product as a dominant degradation mechanism in organic solar cells, and describe the physical theory that accurately predicts the burn-in period of device degradation. Device reliability can be greatly improved by reducing exciton lifetime, such as employing a mixed donor-acceptor active layer. The degradation mechanism also applies to photodegradation of neat OPV materials, and the stability is substantially improved in the mixed donor-acceptor film. In long-term lifetime study, we show that oxygen diffusion into the active layer is the primary cause of degradation, leading to increased recombination current.