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In this paper, two vacuum processed single heterojunction organic solar cells with complementary absorption are described and the construction and optimization of tandem solar cells based on the combination of these heterojunctions demonstrated. The red‐absorbing heterojunction consists of C60 and a fluorinated zinc phthalocyanine derivative (F4‐ZnPc) that leads to a 0.1–0.15 V higher open circuit voltage Voc than the commonly used ZnPc. The second heterojunction incorporates C60 and a dicyanovinyl‐capped sexithiophene derivative (DCV6T) that mainly absorbs in the green. The combination of both heterojunctions into one tandem solar cell leads to an absorption over the whole visible range of the sun spectrum. Thickness variations of the transparent p‐doped optical spacer between both subcells in the tandem solar cell is shown to lead to a significant change in short circuit current density jsc due to optical interference effects, whereas Voc and fill factor are hardly affected. The maximum efficiency η of about 5.6% is found for a spacer thickness of 150‐165 nm. Based on the optimized 165nm thick spacer, effects of intensity and angle of illumination, and temperature on a tandem device are investigated. Variations in illumination intensity lead to a linear change in jsc over three orders of magnitude and a nearly constant η in the range of 30 to 310 mW cm−2. Despite the stacked heterojunctions, the performance of the tandem device is robust against different illumination angles: jsc and η closely follow a cosine behavior between 0° and 70°. Investigations of the temperature behavior of the tandem device show an increase in η of 0.016 percentage points per Kelvin between −20 °C and 25 °C followed by a plateau up to 50 °C. Finally, further optimization of the tandem stack results in a certified η of (6.07 ± 0.24)% on (1.9893 ± 0.0060)cm2 (Fraunhofer ISE), i.e., areas large enough to be of relevance for modules.

A series of p- and n-GaAs-S-C(n)H(2n+1) || Hg junctions are prepared, and the electronic transport through them is measured. From current-voltage measurements, we find that, for n-GaAs, transport occurs by both thermionic emission and tunneling, with the former dominating at low forward bias and the latter dominating at higher forward bias. For p-GaAs, tunneling dominates at all bias voltages. By combining the analysis of the transport data with results from direct and inverse photoemission spectroscopy, we deduce an energy band diagram of the system, including the tunnel barrier and, with this barrier and within the Simmons tunneling model, extract an effective mass value of 1.5-1.6m(e) for the electronic carriers that cross the junctions. We find that transport is well-described by lowest unoccupied and highest occupied states at 1.3-1.4 eV above and 2.0-2.2 eV below the Fermi level. At the same time, the photoemission data indicate that there are continua of states from the conduction band minimum and the valence band maximum, the density of which varies with energy. On the basis of our results, it appears likely that, for both types of junctions, electrons are the main carrier type, although holes may contribute significantly to the transport in the p-GaAs system.

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In this study the charge dissociation at the donor/acceptor heterointerface of thermally evaporated planar heterojunction merocyanine/C 60 organic solar cells is investigated. Deposition of the donor material on a heated substrate as well as post-annealing of the complete devices at temperatures above the glass transition temperature of the donor material results in a twofold increase of the fill factor. An analytical model employing an electric-fielddependent exciton dissociation mechanism reveals that geminate recombination is limiting the performance of as-deposited cells. Fourier-transform infrared ellipsometry shows that, at temperatures above the glass transition temperature of the donor material, the orientation of the dye molecules in the donor films undergoes changes upon annealing. Based on this finding, the influence of the dye molecules' orientations on the charge-transfer state energies is calculated by quantum mechanical/molecular mechanics methods. The results of these detailed studies provide new insight into the exciton dissociation process in organic photovoltaic devices, and thus valuable guidelines for designing new donor materials.

The occupied and unoccupied states of poly(9,9‘-dioctylfluorene) (F8) and poly(9,9‘-dioctylfluorene-co-bis-N,N‘-(4-butylphenyl)diphenylamine) (TFB) are investigated using ultraviolet photoelectron and inverse photoemission spectroscopies, cyclic voltammetry, and density functional theory calculations. Hole injection barriers are determined for interfaces between substrates with work function ranging from 4.3 to 5.1 eV and these two polymers as well as poly(9,9‘-dioctylfluorene-co-bis-N,N‘-(4-butylphenyl)-bis-N,N‘-phenyl-1,4-phenylenediamine) (PFB). Vacuum level alignment with flat bands away from the interface is found when the interface hole barrier is 0.6 eV or larger. Band bending away from the Fermi level occurs when the hole barrier is smaller than 0.4 eV. This is due to the accumulation of excess interface charges on the polymer when the barrier is small. The resulting field shifts the polymer levels to limit charge penetration in the bulk of the film.