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.

The nanoscale morphology of polymer blends is a key parameter to reach high efficiency in bulk heterojunction solar cells. Thereby, research typically focuses on optimal blend morphologies while studying non-optimized blends may give insight into blend design that can be more robust against morphology defects. Here we focus on the direct correlation of morphology and device performance of PTB7:PC71BM bulk heterojunction (BHJ) blends processed without additive in different donor:acceptor weight ratios. We show that while blends of a 1:1.5 ratio are composed of large donor enriched and fullerene domains beyond exciton diffusion length, reducing the ratio below 1:0.5 leads to blends composed purely of polymer enriched domains. Importantly photocurrent density in such blends can reach values between 45 to 60% of those reached of fully optimized blend using additives. We provide here a direct visual evidence that fullerenes in the donor enriched domains are not distributed homogeneously but fluctuate locally. To this end, we performed compositional nanoscale morphology analysis of the blend using spectroscopic imaging of low energy-loss electrons in the transmission electron microscope. Charge transport measurement in combination with molecular dynamics simulations show that the fullerene sub-structures inside the polymer phase generate efficient electron transport in the polymer enriched phase. Furthermore, we show that the formation of densely 3 packed regions of fullerene inside the polymer phase is driven by the PTB7:PC71BM enthalpy of mixing. The occurrence of such a nanoscale network of fullerene clusters leads to a reduction of electron trap states and thus efficient extraction of photocurrent inside the polymer domain. Suitable tuning of the polymer acceptor interaction can thus introduce acceptor sub-networks in polymer enriched phases improving the tolerance for high efficiency BHJ towards morphological defects such as donor enriched domains exceeding exciton diffusion length.