NOVELTY - A thio derivative is new.    USE - A thio derivative for forming a poly(thiophene-co-diphenylthienopyrazine) for use as a hole-conducting polymer in a photovoltaic device for forming a conducting polymer based photovoltaic device (claimed).    ADVANTAGE - The derivative is soluble in organic solvent(s) before thermal cleavage, and insoluble in organic solvents and water after thermal cleavage thus permitting a robust, insoluble bulk heterojunction layer.    DETAILED DESCRIPTION - A thio derivative of formula (I) or (II) is new.    n=not defined.    INDEPENDENT CLAIMS are included for:    (1) a method of forming a conducting polymer based photovoltaic device comprising providing a transparent first electrode; providing the transparent first electrode with a layer of metal oxide A nanoparticles where metal oxide A is titanium oxide or zinc oxide; providing a layer of metal oxide B nanoparticles with a bulk heterojunction layer comprising metal oxide nanoparticles and a hole conducting polymer containing thermocleavable groups where the metal oxide B is titanium oxide, cerium oxide, niobium oxide or zinc oxide; heating the bulk heterojunction layer to cleave the thermally cleavable groups to produce an insoluble hole containing polymer; providing the bulk heterojunction layer with a hole transporting layer; and providing the hole transporting layer with a second electrode;    (2) a conducting polymer based photovoltaic device comprising a transparent first electrode; a metal oxide A nanoparticle layer; a bulk heterojunction layer comprising metal oxide B nanoparticles and a hole conducting polymer which has been thermally treated to decrease its solubility; a hole transporting layer; UV filter; and a second electrode;    (3) a device comprising no layers or coatings that exclude oxygen from contact with the bulk heterojunction layer while the device is in use; and    (4) a method of forming a poly(thiophene-co-diphenylthienopyrazine) by subjecting (I) or (II) to thermal cleavage at 310 degrees C.    DESCRIPTION OF DRAWING(S) - The drawing is a schematic view of a construction of a cell.