Multilayer tandem polymer solar cells were prepared by solution processing using thermocleavable polymer materials that allow for conversion to an insoluble state through a short thermal treatment. The problems associated with solubility during application of subsequent layers in the stack were efficiently solved. Devices comprised a transparent front cathode based on solution processed zinc oxide nanoparticles, a large band gap active layer based on a bulk heterojunction between zinc oxide and poly(3-carboxydithiophene) (P3CT) followed by a layer of PEDOT:PSS processed from water. The second cell in the stack employed a zinc oxide front cathode processed on top of the PEDOT:PSS layer from an organic solvent, a low band gap active layer based on a bulk heterojunction between zinc oxide and the novel poly(carboxyterthiophene-co-diphenylthienopyrazine) (P3CTTP) followed by a layer of PEDOT:PSS again processed from water and finally a printed silver electrode. The devices were prepared without the use of fullerenes and vacuum steps and employ only thermal treatments and orthogonal solvents. The devices exhibited operational stability in air without any form of encapsulation. (C) 2008 Elsevier B.V. All rights reserved.