Generation and recombination of electrons and holes in organic solar cells occurs via charge transfer states located at the donor/acceptor interface. The energy of these charge transfer states is a crucial factor for the attainable open-circuit voltage and its correct determination is thus of utmost importance for a detailed understanding of such devices. This work reports on drastic changes of electroluminescence spectra of bulk heterojunction organic solar cells upon variation of the absorber layer thickness. It is shown that optical thin-film effects have a large impact on optical out-coupling of luminescence radiation for devices made from different photoactive materials, in configurations with and without indium tin oxide. A scattering matrix approach is presented which accurately reproduces the observed effects and thus delivers the radiative recombination spectra corrected for the wavelength-dependent out-coupling. This approach is proven to enable the correct determination of charge transfer state energies.

However, in comparison to established inorganic-based photovoltaic technologies organic solar cells possess still a much lower efficiency. [2,[9][10][11][12] The efficiency η of solar cells is determined bywhere J SC is the short-circuit current density, V OC is the open-circuit voltage, FF is the fill factor, and P in is the incident power density. The relatively poor transport properties (low mobilities) of the disordered organic materials [13,14] applied in OSC lead to a strong accumulation of charge carriers under current flow and thus to an increase in nongeminate recombination. [10] Consequently, the FF is reduced and for cases of extremely low charge carrier mobilities also J SC , which results in a low overall power conversion efficiency. [10,11,15] Further, due to the relatively low permittivity of the organic materials a significant fraction of the absorbed photon energy is required as driving force for the exciton dissociation of the coulombic bound electron-hole pair (offset between the lowest unoccupied molecular orbitals of donor and acceptor) which lowers the achievable power conversion efficiency significantly. [9] In addition to the latter, a key loss mechanism originates from additional nonradiative recombination pathways. Besides the nongeminate recombination via trap states as a first order recombination process, the recombination at the electrodes, i.e., surface recombination, is an additional loss channel. Surface recombination in OSC was investigated intensively in different perspectives which indicates the importance of a thorough understanding of the underlying mechanisms. [16][17][18][19][20] Kirchartz et al. derived the ideality factor differentially by the light intensity dependence of the open-circuit voltage and demonstrated that the presence of surface recombination leads to the occurrence of an ideality factor n id < 1. [18] But also the charge carrier mobility µ dependent efficiency of OSC has been investigated in terms of surface recombination. It seems to be counterintuitive to assume that the device efficiency is maximum at a finite optimum mobility. Yet, this case is in detail discussed by Deibel et al. and Kirchartz et al., who showed that an optimum mobility exists in the presence of not ideally selective contacts and that faster kinetics by mobilities higher than the optimum will result in a reduction of the overall efficiency. [21,22] TressThe selectivity of electrodes of solar cells is a critical factor that can limit the overall efficiency. If the selectivity of an electrode is not sufficient both electrons and holes recombine at its surface. In materials with poor transport properties such as in organic solar cells, these surface recombination currents are accompanied by large gradients of the quasi-Fermi energies as the driving force. Experimental results from current-voltage characteristics, advanced photo-and electroluminescence as well as charge extraction of three different photoactive materials are shown and compared to drift-diffusion simulations. It can be conc...

Simple organic molecules with permanent dipole moments -amino acids and heterocycleshave been successfully employed in bulk-heterojunction organic solar cells as interlayer between photoactive material and electron contact. A large increase of open-circuit voltage and fill factor could be observed for 4 different polymers as donor material in the photoactive layer. A combination of current-voltage curves, Scanning Kelvin-Probe AFM, ultraviolet photoelectron spectroscopy and electroluminescence measurements as well as numerical simulations were carried out to clarify in detail the underlying mechanisms. All results fully confirm the hypothesis that the main effect is an accumulation of electrons and a depletion of

In organic photovoltaics, high power conversion efficiencies (PCE) are mostly achieved on device areas well below 0.1 cm2. Herein, organic solar cells based on a D18:Y6 absorber layer on an active area of ≥ 1 cm2 with a certified PCE of 15.24% are reported. The impacts of the sheet resistance of the transparent electrode and the cell design are quantified by means of full optical device simulations and an analytical electrical model. Three imaging methods (light beam‐induced current, dark lock‐in thermography, and electroluminescence [EL]) are applied and reveal a strong homogeneity of the record cell. Nevertheless, it is found that there is substantial room for improvement mostly in current but also in fill factor and that a PCE of 18.6% on ≥1 cm2 is feasible with this absorber material. Further, photoluminescence (PL) and EL spectroscopy reveal that both emissions occur at the same wavelength(s) and are very similar to the PL spectrum of a pure Y6 acceptor film. The latter points strongly toward electronic coupling between the S1 states of the acceptor and the charge transfer states at the donor/acceptor interface.