Monomolecular and Bimolecular Recombination of Electron–Hole Pairs at the Interface of a Bilayer Organic Solar Cell

• Published in: Advanced functional materials Vol. 27; no. 1; pp. np - n/a
• Main Authors: Hahn, Tobias, Tscheuschner, Steffen, Kahle, Frank‐Julian, Reichenberger, Markus, Athanasopoulos, Stavros, Saller, Christina, Bazan, Guillermo C., Nguyen, Thuc‐Quyen, Strohriegl, Peter, Bässler, Heinz, Köhler, Anna
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 01.01.2017
• Subjects: Buckminsterfullerene; charge carrier extraction; Computer simulation; Diffusion; Electric potential; fill factor; Fullerenes; Hole mobility; Luminous intensity; Materials science; Monte Carlo simulation; organic semiconductors; Photovoltaic cells; planar heterojunctions; Solar cells; Thickness
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201604906

While it has been argued that field‐dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT‐type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p‐DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV‐characteristics as a function of intensity and charge‐extraction‐by‐linearly‐increasing‐voltage‐type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p‐DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s‐shape of the JV‐curves and the concomitant additional decrease of the FF with increasing layer thickness. Back diffusion of holes is identified to control the fill factor and thus the efficiency of bilayer organic solar cells. The competition between recombination at the donor–acceptor interface and extraction at the electrodes is studied by varying the donor layer thickness and light intensity such as to differentiate between monomolecular, i.e., geminate recombination and bimolecular, nongeminate recombination.