In organic tandem solar cells, the morphology of the photo-active layers is not usually stable when their fabrication processes are made with several thermal treatments. Here, we report pentacene-assisted planar tandem organic photovoltaic device based on the active layers of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61-butyric acid methyl ester (PCBM), fabricated in air-ambient. An additive of pentacene was introduced into the active layers of the bottom sub-cells for planarization on their surfaces. The surface morphology of pentacene-based active layer was maintained to be flat from 80 to 160 degrees C, and allowed for fabricating the planar active layers and interconnecting layer of tandem devices against thermal treatment. State-of-the-art homo-tandem organic solar cells were achieved with an average power conversion efficiency (PCE) of 3.5%, while their best single junction solar cells of P3HT:PCBM achieved a PCE of 3.2%. We showed the first application of high performance tandem organic photovoltaics with three times of high temperature annealing processes at 160 degrees C. Our work demonstrates a practical way to design highly efficient tandem organic solar cells with other powerful and well-chosen bandgap energy active layers during thermal treatments.