Improved optical absorption and photocurrent for polythiophene-fullerene bulk heterojunction photovoltaic devices is demonstrated using a unique self-assembled monolayer of Ag nanoparticles formed from a colloidal Solution. With the presence of suitable nanoparticle organic capping groups that inhibit its propensity to agglomerate, the particle-to-particle spacing can be tailored. Transmission electron microscopy reveals the self-assembled Ag nanospheres are highly uniform with an average diameter of similar to 4 nm and controllable particle-to-particle spacing. The localized surface plasmon resonance peak is similar to 465 nm with a narrow full width at half maximum (95 nm). In the spectral range of 350-650 nm, where the organic bulk heterojunction photoactive film absorbs, an enhanced optical absorption is observed due to the increased electric field in the photoactive layer by excited localized surface plasmons within the Ag nanospheres. Under the short-circuit condition, the induced photocurrent efficiency (IPCE) measurement demonstrates that the maximum IPCE increased to similar to 51.6% at 500 nm for the experimental devices with the self-assembled layer of Ag nanoparticles, while the IPCE of the reference devices without the plasmon-active Ag nanoparticles is similar to 45.7% at 480 nm. For the experimental devices under air mass 1.5 global filtered illuminations with incident intensity of 100 mW/cm(2), the increased short-circuit current density is observed due to the enhancement of the photogeneration of excitons near the plasmon resonance of the Ag nanoparticles. (C) 2009 Elsevier B.V. All rights reserved.