Compact modeling of physical mechanisms in organic solar cells

Abstract

Organic solar cells (OSCs) are promising devices in the field of solar energy. Their many advantages are intrinsic of the organic/polymeric technology, such as light weight, flexibility and low manufacturing costs. However, the degradation of OSCs hinders a predictable and a stable performance. In order to improve the device performance, accurate physics-based models, including boundary conditions, are needed. In this thesis, a model that relates the charge carrier density at the metal-organic boundaries with the current density in OSCs is considered for simulation and modeling purposes. The model is proposed after initial studies on single-carrier and bipolar organic diodes in darkness. The work begins with the proposal of a model for the current-voltage characteristics of organic and polymeric single-carrier diodes. The model unifies two different mechanisms in the structure, the injection and transport of charge, and includes a proper boundary condition for the free charge density at the metal-organic interface and a temperature and electric-field dependent mobility model. The results of the model highlight the importance of the boundary condition at the interface, which is used to explain different trends and their transitions in experimental current–voltage characteristics: linear, quadratic and a higher than quadratic trend at high electric fields. Una de las regiones más sensibles de una célula solar orgánica es la zona de contacto entre el electrodo metálico y el material orgánico. Por un lado, los contactos controlan el flujo de la corriente. Por otro lado, la región del contacto es altamente sensible a la degradación. La formación de una capa aislante cerca a la interfaz metal-orgánico o el decrecimiento de la velocidad de recombinación en el contacto son efectos desfavorables que reducen la eficiencia de las células solares [3, 4]. Para optimizar el rendimiento de estos dispositivos es requisito indispensable una detallada descripción de los mecanismos físicos que tienen lugar en la estructura metal-orgánico. El modelado y la simulación de estas estructuras son herramientas muy adecuadas para conseguir este objetivo.