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 C-60 and a fluorinated zinc phthalocyanine derivative (F4-ZnPc) that leads to a 0.1-0.15 V higher open circuit voltage V-proportional to than the commonly used ZnPc. The second heterojunction incorporates C-60 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 j(sc) due to optical interference effects, whereas V-proportional to and fill factor are hardly affected. The maximum efficiency eta 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 eta 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: j(sc) and eta closely follow a cosine behavior between 0 degrees and 70 degrees. Investigations of the temperature behavior of the tandem device show an increase in eta of 0.016 percentage points per Kelvin between -20 degrees C and 25 degrees C followed by a plateau up to 50 C. Finally, further optimization of the tandem stack results in a certified. of (6.07 +/- 0.24)% on (1.9893 +/- 0.0060) cm(2) (Fraunhofer ISE), i.e., areas large enough to be of relevance for modules.