Near infrared (NIR) emission was studied in rubrene: fullerene planar and bulk heterojunction bifunctional devices. The degree of mixing was controlled by changing the substrate temperature for planar heterojunctions and changing the ratio of the two materials for bulk heterojunctions. We found that there was no simple correlation between the presence of the NIR emission and the photovoltaic efficiency and the turn-on voltage of the devices. The origin of the NIR emission and the relationship between EL spectra, photovoltaic efficiency, and the degree of mixing of the donor and acceptor materials is discussed. (C) 2009 Elsevier B. V. All rights reserved.