Improved polymer solar cell performance by engineering of cathode interface

¹ Department of Physics, Concordia University, Montréal, Québec, Canada
² NanoQAM, Département d’informatique, Université du Québec à Montréal (UQAM), Montréal, Québec, Canada
³ Optical-Bio Microsystems Laboratory, Mechanical & Industrial Engineering Department, Concordia University, Montréal, Québec, Canada

^a e-mail: jbar@alcor.concordia.ca
^b e-mail: izquierdo.ricardo@uqam.ca

Received: 3 February 2011
Accepted: 30 May 2011
Published online: 18 August 2011

Abstract

By engineering the interface between the intermediate photoactive layer and the cathode aluminum (Al) electrode, through the introduction of ultra-thin layers of various materials, in a standard bulk heterojunction (BHJ) polymer solar cell (PSC) fabricated of regioregular poly(3-hexylthiophene) (rr-P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM), the power conversion efficiency (PCE) has been effectively improved. The devices fabricated using individual single interlayer of bathocuproine (BCP), lithium fluoride (LiF) and Buckminster fullerene C₆₀ have shown optimal efficiencies of ~2.40%, ~3.21% and ~1.92% respectively. Further improvement of the photovoltaic efficiency was achieved by introducing a composite bilayer made of LiF in combination with BCP as well as with C₆₀ at the BHJ/cathode interface. The best results were obtained by interposing a 9 nm of C₆₀ interlayer in combination with a 0.9 nm thick LiF layer, with the PCE of the PV cells being effectively increased up to 3.94% which represents an improvement of 23% as compared to the standard device with LiF interlayer alone. The photocurrent density (J_sc) versus voltage (V) characteristic curves shows that the increase of the efficiency is essentially due to an increase in J_sc. Moreover, all the sets of devices fabricated using various interlayers over a certain range of thickness exhibit an optimum PCE that is inversely proportional to the series resistance of the PV cells. We presume that interposing a C₆₀/LiF layer at the interface could repair the poor contact at the electron acceptor/cathode interface and improve the charge career extraction from the BHJ.