Effect of two-dimensional-Langevin and trap-assisted recombination on the device performance of organic solar cells

Using drift-diffusion simulations, we have clarified the effect of two-dimensional lamellar ordering on the device performance and, in particular, the open circuit voltage in donor­acceptor type organic solar cells. The simulations are performed both in systems where direct (band-to-band) recombination dominates and in systems where trap-assisted recombination dominates. Results show that lamellar ordering reduces both the amount of direct and trap-assisted recombination, which is beneficial for device performance. The effect is particularly prominent for small lamellar thicknesses (1 nm). It is furthermore shown that in the case of s-shaped current­voltage characteristics due to electrostatic injection barriers the s-shape becomes less prominent for thinner lamellar thicknesses. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.JPE.6.014501] Keywords: drift-diffusion simulation; organic solar cells; open circuit voltage; recombination. Paper 15078P received Dec. 8, 2015; accepted for publication Feb. 2, 2016; published online Mar. 2, 2016. 1 Introduction In order to further advance the power conversion efficiencies (PCE) of solar cells based on conjugated polymers, a profound understanding of the underlying mechanisms limiting device performance is essential. The PCE is defined as PCE ¼ JSC V OC FF × 100%; Plight (1) EQ-TARGET;temp:intralink-;e001;116;354 where J SC ¼ the short circuit current, V OC ¼ the open circuit voltage, FF ¼ the fill factor, and Plight is the power of the light source. Limitations in performance can thus be directly related to the J SC , V OC , or FF. One of the most critical loss mechanisms in bulk heterojunction solar cells is the (bimolecular) recombination of photo generated charges in the active layer. The decay of photo generated charges due to bimolecular recombination is determined by dp ¼ G - R ¼ G - np; dt (2)...