Eur. Phys. J. Appl. Phys. 38, 111-114 (2007)
https://doi.org/10.1051/epjap:2007062

Empirical study of the characteristics of current-state organic bulk heterojunction solar cells^*

University of Gent, Dept. ELIS, Pietersnieuwstraat 41, 9000 Gent, Belgium

Corresponding author: Ben.Minnaert@elis.UGent.be

Received: 25 July 2006
Revised: 3 January 2007
Accepted: 20 February 2007
Published online: 21 March 2007

Abstract

We studied and compared the reported characteristics of 22 different bulk heterojunction organic solar cells fabricated and characterized by different research institutes. We only considered bulk heterojunction solar cells where both the acceptor (the n-type) and the donor (the p-type) are organic. All cells were characterized under illumination with the standard A.M. 1.5 spectrum and an intensity of 100 mW/cm². The material properties (the highest occupied (HOMO) and lowest unoccupied molecular orbital (LUMO) of donor and acceptor) and the device characteristics (the open circuit voltage V_oc, the short circuit current density J_sc, the fill factor FF and the efficiency) are compared and related to each other. One finding is that not the V_oc, but the J_sc is the limiting factor for obtaining a high efficiency with the current state of technology. Also an empirical threshold of 0.2 eV is found between the LUMO's of the donor and acceptor, necessary for exciton dissociation. There has long been a debate about the origin of the V_oc. In recent literature, it is proposed that the V_oc is not related with the work function difference of the contacts, but with the energy difference between the LUMO of the acceptor, and the HOMO of the donor (called `the interface bandgap'). A relation between the V_oc and the energy levels of donor and acceptor is searched from our empirical study.