TECHNOLOGY FOCUS - ELECTRONICS - Preferred System: When the system comprises the recombination layer, then the third layer is between the first photoactive layer and the fifth layer. When the system comprises the recombination layer, then the system further comprises a hole carrier layer (420) between the first photoactive layer and the first electrode; and a hole blocking layer (480) between the second photoactive layer and the second electrode. In the system, at least one of the first and second photoactive layers comprises an electron donor material and an electron acceptor material. The fifth layer comprises a semiconductive material or an electrically conductive material. When the system comprises the recombination layer, then the first photoactive layer has a first band gap and the second photoactive layer has a second band gap different from the first band gap. In the system, at least one of the first and second electrodes comprises a mesh electrode.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: When the system comprises the recombination layer, then the fifth layer comprises a metal oxide (preferably titanium oxides, indium tin oxides, tin oxides, and zinc oxides); or metallic particles. The p-type semiconductor material comprises metal oxide (preferably copper oxide, strontium copper oxide and/or strontium titanium oxide); or p-doped metal oxide (preferably p-doped zinc oxide or p-doped titanium oxide). The n-type semiconductor material comprises a metal oxide (preferably titanium oxide, zinc oxide, tungsten oxide and/or molybdenum oxide). The hole blocking layer comprises a material selected from LiF and/or metal oxides (preferably titanium oxide or zinc oxide). The electron acceptor materials and the n-type semiconductor materials comprise a material selected from inorganic nanoparticles and inorganic nanorods.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Componnets: The electron acceptor materials and the n-type semiconductor materials comprise a material selected from fullerenes, oxadiazoles, discotic liquid crystals and carbon nanorods (preferably substituted fullerene, especially 61C-phenyl-butyric acid methyl ester (PCBM)).    TECHNOLOGY FOCUS - POLYMERS - Preferred Components: In the system, the third layer comprises an n-type semiconductor material and the fourth layer comprises a p-type semiconductor material. The p-type semiconductor material comprises a polymer selected from polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide)s, polythiadiazoloquinoxalines, polybenzoisothiazoles, polybenzothiazoles, polythienothiophenes, poly(thienothiophene oxide), polydithienothiophenes, poly(dithienothiophene oxide), polytetrahydroisoindoles, and their copolymers (preferably poly(3,4-ethylene dioxythiophene)). The hole carrier layer comprises a polymer selected from polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, and their copolymers (preferably poly(3,4-ethylene dioxythiophene)). The electron donor material comprises a polymer selected from polythiophenes, polyanilines, polyvinylcarbazoles, polyphenylenes, polyphenylvinylenes, polysilanes, polythienylenevinylenes, polyisothianaphthanenes, polycyclopentadithiophenes, polysilacyclopentadithiophenes, polycyclopentadithiazoles, polythiazolothiazoles, polythiazoles, polybenzothiadiazoles, poly(thiophene oxide)s, poly(cyclopentadithiophene oxide), polythiadiazoloquinoxaline, polybenzoisothiazole, polybenzothiazole, polythienothiophene, poly(thienothiophene oxide), polydithienothiophene, poly(dithienothiophene oxide), polytetrahydroisoindoles, and their copolymers (preferably polythiophenes, polycyclopentadithiophenes, and their copolymers, especially poly(3-hexylthiophene) or poly(cyclopentadithiophene-co-benzothiadiazole)). The electron acceptor materials and the n-type semiconductor materials comprise a material selected from polymers containing CN group and/or polymers containing CF 3 groups. EXAMPLE - A substrate with indium tin oxide (ITO) was cleaned sequentially with acetone and isopropanol for 10 minutes in an ultrasonic bath at room temperature. Tetra-n-butyl-titanate diluted in anhydrous isopropanol (1:199) was applied onto the ITO via doctor-blading and hydrolyzed by distilled water. The coating thus obtained was dried for 10 minutes to give a titanium oxide layer having a thickness of 10 plus minus 5 nm. A solution of poly-(3-hexylthiophen) (P3HT):61C-phenyl-butyric acid methyl ester (PCBM) in ortho-xylene (1.5 mg: 1.2 mg:100 plus minus l) was then applied onto the titanium oxide layer via doctor-blading to give a P3HT:PCBM layer having a thickness of 100 plus minus 10 nm. A solution of poly(3,4-ethylene dioxythiophene) (PEDOT) in isopropanol (1 ml:5 ml) was subsequently coated on the P3HT:PCBM layer via doctor-blading to give in a PEDOT layer of 30 plus minus 10 nm. After the device thus obtained was baked for 10 minutes at 140 degrees C in nitrogen atmosphere, tetra-n-butyl-titanate diluted in anhydrous isopropanol (1:199) was applied onto the PEDOT layer via doctor-blading. The coating was hydrolyzed and dried for 10 minutes to give a second titanium oxide layer of 10 plus minus 5 nm. The PEDOT layer and the s obtained second titanium oxide layer constituted as the recombination layer in the final tandem photovoltaic cell. A solution of P3HT:PCBM in ortho-xylene (1.5 mg:1.2 mg:100 plus minus l) was then applied onto the second titanium oxide layer via doctor-blading to give a second P3HT:PCBM layer having a thickness of 300 plus minus 30 nm. Subsequently, a solution of PEDOT in isopropanol (1 ml:5 ml) was applied onto the second P3HT:PCBM layer via doctor-blading to give a second PEDOT layer having a thickness of 30 plus minus 10 nm. After the device thus obtained was baked for 20 minutes at 140 degrees C in nitrogen atmosphere, a 100 nm layer of silver was applied onto the second PEDOT layer via thermal evaporation to give a tandem photovoltaic cell having the structure of ITO/TiO 2/P3HT:PCBM/PEDOT/TiO 2/P3HT:PCBM/PEDOT/Ag.