ИСТИНА

Fundamental limits for efficiency of organic solar cells (OSC) are intensively discussed in comparison with the Shockley-Queisser limit (SQL) calculated for silicon p-n junction solar cells [1]. The cell efficiency depends on the form of its current-voltage (J-V) characteristics characterized by short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF). Green calculated a theoretical limit for FF, which depends on open-circuit voltage [2], as shown in Fig. 1b. In this work, we show that organic solar cells can have FF exceeding the SQL and discuss the possible reasons of high FF in OSC. Using a numerical drift-diffusion model, we recently found that planar heterojunction (bilayer) OSC can have very high FF for the doped active layers [5]. Figure 1 shows a calculated J-V characteristic of a bilayer OSC (blue solid line) with input parameters for polymer/fullerene (P3HT/PCBM) blend [5] and a doping level of 1024 m-3, its FF is 92% The red dashed line in Fig. 1a shows the J-V curve for ideal p-n junction inorganic solar cell (ISC) with JSC and VOC the same as for the calculated above bilayer OSC. The FF is 86%, and this is a limit for p-n junction ISC at given VOC. The doping is not necessary for FF exceeding SQL, Fig. 1b shows other modeled OSC with FF higher than SQL (colored circles). To find the possible reasons of high FF, we extended our drift-diffusion model for OSC to p-n junction ISC, such model gives the maximal FF according to the SQL. It was found that the key reason for FF exceeding SQL is field dependence of interface recombination rate of free charges at the donor-acceptor interface in bilayer OSC. It was substantiated by following numerical experiment: first, recombination rate made field-independent, constant in OSC model – as a result FF became within SQL (unfilled circle); second, the field dependent interfacial recombination was introduced into ISC model, which led to slight increase of FF over SQL (unfilled square). We discuss how interface charge generation/recombination and energy barriers in heterojunction could contribute to high FF in OSC. This work was supported by RFBR (project № 14-02-31823). 1. W. Shockley and H.J. Queisser, J. Appl. Phys. 1961, 32, 510. 2. M.A. Green, Solid-State Electronics 1977, 20, 265. 3. X. Guo, et al., Nat. Photon. 2013 7, 825 4. J. Wagner, et al., Adv. Func. Mater., 2010 20, 4295. 5. V.A. Trukhanov, V.V. Bruevich and D.Yu. Paraschuk, Phys. Rev. B 2011 84, 205318.