"Influence of the hole-transport layer on the initial behavior and lifetime of inverted organic photovoltaics"

Matthew T. Lloyd: Craig H. Peters, Andres Garcia, Isaac V. Kauvar, Joseph J. Berry, Matthew O. Reese, Michael D. McGehee, David S. Ginley, Dana C. Olson; Solar Energy Materials and Solar Cells, 05/01/11.

Additional Authors: Craig H. Peters, Andres Garcia, Isaac V. Kauvar, Joseph J. Berry, Matthew O. Reese, Michael D. McGehee, David S. Ginley, Dana C. Olson

Abstract:

The inverted organic photovoltaic (OPV) device architecture represents an important advancement due to the relative environmental stability of the electron transport layer (ETL) and hole-collecting contact. We investigated the initial and long-term behavior of inverted devices to identify changes taking place at the Ag hole-collecting contact. We show that efficient hole collection can be obtained after modifying the Ag contact by thermal annealing, long-term exposure to ambient atmosphere, or employing a high work function organic hole-transport layer (HTL). We find that whether or not the device employs an organic HTL, degradation of the photocurrent initially follows a simple exponential decay. After prolonged illumination (>500 h), devices with an organic HTL fail catastrophically due to a precipitous drop in photocurrent. Based on evidence for pinhole-induced degradation observed in photocurrent maps, we propose a nucleation and island growth mechanism and a model for the photocurrent behavior employing a modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation. Devices that do not contain an HTL appear to degrade by a mechanism other than pinhole ingress resulting in a more uniform degradation of the photocurrent across the active area.