The electronic structure of a large sample set of CH3 NH3 PbI3 -based perovskites is studied. Combined investigations by UV/X-ray photoelectron spectroscopy and X-ray diffraction reveal that interstitials present in the film lead to changes in the occupied density of states close to the valence band, which in turn influences the performance of solar cells. Changes in elemental composition tune the ionization energy of the perovskite film by almost 1 eV without introducing significant amounts of gap states.

A series of new symmetrical donor-acceptor-donor (D−A−D) dyes based on s -indacene-1,3,5,7(2 H ,6 H )-tetraone as an acceptor unit containing varying electron donating moieties and analogous donor-acceptor (D−A) chromophores with indane-1,3-dione as an acceptor are synthesized. By employing these two sets of dyes, the infl uence of a scaffold change from unsymmetric push-pull (D−A) to symmetrical (D−A−D) systems on optical, electrochemical, and photovoltaic properties are explored. Detailed comparative studies reveal favorable optical characteristics and considerably decreased bandgaps for the D−A−D dyes compared to those of the reference D−A chromophores. Accordingly, the evaluation of the present dyes as donor materials in bulk heterojunction (BHJ) solar cells in combination with fullerene derivatives PC 61 BM or PC 71 BM as acceptors afforded signifi cantly improved performance for devices based on D−A−D blends (up to a factor of 4 compared to the respective D-A reference) with power conversion effi ciencies of up to 2.8%. In less polar solvents such as toluene, some of the novel D−A−D chromophores exhibit unexpectedly high fl uorescence quantum yields Φ em of up to unity, in striking contrast to their weakly fl uorescent D-A counterparts.

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH3NH3PbI3 perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

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