NOVELTY - The tandem photovoltaic device for small molecule heterojunction organic solar cells, comprises a transparent substrate, a transparent first electrode on the substrate, a first cell configured to receive incident electromagnetic radiation in a light propagation path from the substrate, a second cell configured to receive electromagnetic radiation passing out of the first cell in a light propagation path, a first titanium oxide layer interposed between the first and second cell, and a second titanium oxide layer adjacent to the second cell.    USE - The tandem photovoltaic device is useful for small molecule heterojunction organic solar cells and for hybrid organic solar cells.    ADVANTAGE - The photovoltaic device has improved performance, high efficiency and high open circuit voltage, prevents interlayer mixing during the spin-casting of the second cell using a thermally evaporated metal layer, and is fabricated in large areas using low cost printing and coating technologies.    DETAILED DESCRIPTION - The tandem photovoltaic device for small molecule heterojunction organic solar cells, comprises a transparent substrate, a transparent first electrode on the substrate, a first cell configured to receive incident electromagnetic radiation in a light propagation path from the substrate, a second cell configured to receive electromagnetic radiation passing out of the first cell in a light propagation path, a first titanium oxide layer interposed between the first and second cell, and a second titanium oxide layer adjacent to the second cell. The first and second cell has first and second charge separating layers having first and second semiconducting polymers adapted to create electric charge carriers generated by electromagnetic radiations respectively. The first and the second titanium oxide layers are amorphous, and have a general formula (TiOx), where x is 1-1.96. The semiconducting polymers have band gaps, of which the first band gap is lower than second one. The first charge separating layer and/or the second charge-separating layer comprises a heterojunction layer having a semiconducting conjugated polymer and a fullerene derivative. An INDEPENDENT CLAIM is included for a method of preparing a tandem photovoltaic device.    DESCRIPTION OF DRAWING(S) - The figure shows a tandem photovoltaic device.