TECHNOLOGY FOCUS - ELECTRONICS - Preferred Device: The photovoltaic cell is a tandem photovoltaic cell.    TECHNOLOGY FOCUS - POLYMERS - Preferred Compound: The cyclopenta dithiophene moiety is substituted at 4-position, and is of formula (I). The comonomer repeating unit (II) comprises 3,4-benzo-l,2,5-thiadiazole moiety, and 20 claimed moieties such as benzothiadiazole moiety of formula (II), thiophene dioxide moiety of formula (VIII) and thiophene moiety of formula (XX), preferably thiophene moiety.The substituent is (OCH2CH2)2OCH3, or OCH2CF2OCF2CF2OCF3.Preferred Polymer: The polymer is at least one compound of formulae (1,2). The polymer is an electron donor material or an electron acceptor material.R1-R6= H, 1-20C alkyl, 1-20C alkoxy, aryl, heteroaryl, 3-20C cycloalkyl, 3-20C hetero cycloalkyl, halogen atom, CN, NO2, or SO2R, preferably R1 and R2 is hexyl, 2-ethylhexyl, 3,7-dimethyloctyl, or 1-20C alkoxy optionally substituted by 1-20C alkoxy or halogen atom, and R5 and R6 is H;R=H, 1-20C alkyl, 1-20C alkoxy, aryl, heteroaryl, 3-20C cycloalkyl, or 3-20C hetero cycloalkyl; andn=integer greater than 1.The polymer further comprises a comonomer repeating unit (III) consisting of a thiophene moiety or a fluorene moiety. The thiophene or fluorene moiety is substituted with 1-20C alkyl, 1-20C alkoxy, aryl, heteroaryl, 3-20C cycloalkyl, 3-20C hetero cycloalkyl, halogen atom, CN, NO2, or SO2R, where R is same as defined above. Preferred Material: The photoactive material further comprises an electron acceptor material containing a fullerene or 61C-phenyl-butyric acid methyl ester. Preferred Property: The lowest energy unoccupied molecular orbital (LUMO) energy level of the polymer is 0.2 eV less than the LUMO energy level of the electron acceptor material. EXAMPLE - Bis(trimethyl stannyl)-4,4-di(2-ethylhexyl)-cyclopenta(2,1-b:3,4-b') dithiophene (in g) (0.686) and 4,7-dibromo-2,1,3-benzothiadiazole (0.269) were dissolved in toluene (20 ml) and reacted in nitrogen atmosphere. Then, tris(dibenzylidene acetone) dipalladium (0) (25.1 mg) and triphenyl phosphine (57.6 mg) were added and reaction was continued for 10 minutes. Further, the reaction mixture was heated to 120degreesC for 24 hours in nitrogen atmosphere. The solvent was removed under vacuum and residue was dissolved in chloroform. The resulting mixture was poured into methanol, blue precipitate obtained was separated, washed with methanol and dried. The precipitate was dissolved in chloroform, filtered and purified to obtain high purity 4,4-bis(2-ethyl-hexyl)-2,6-bis-(trimethylstannanyl)-4H-cyclopenta(2,1-b:3,4-b') dithiophene (120 mg) having number average molecular weight of 35000.