In this study, we investigated inverted hybrid bulk-heterojunction solar cells with the following configuration: fluorine-doped tin oxide (FTO) jTiO2/RGOjP3HT:PC61BMjV2O5 or PEDOT:PSS|Ag. The TiO2/GO dispersions were prepared by sol-gel method, employing titanium isopropoxide and graphene oxide (GO) as starting materials. The GO concentration was varied from 0.1 to 4.0 wt%. The corresponding dispersions were spin-coated onto FTO substrates and a thermal treatment was performed to remove organic materials and to reduce GO to reduced graphene oxide (RGO). The TiO2/RGO films were characterized by X-ray diffraction, Raman spectroscopy, and microscopy techniques. Atomic force microscopy (AFM) images showed that the addition of RGO significantly changes the morphology of the TiO2 films, with loss of uniformity and increase in surface roughness. Independent of the use of V2O5 or PEDOT: PSS films as the hole transport layer, the incorporation of 2.0 wt% of RGO into TiO2 films was the optimal concentration for the best organic photovoltaic performance. The solar cells based on TiO2/RGO (2.0 wt%) electrode exhibited a ~22.3% and ~28.9% short circuit current density (Jsc) and a power conversion efficiency enhancement, respectively, if compared with the devices based on pure TiO2 films. Kelvin probe force microscopy images suggest that the incorporation of RGO into TiO2 films can promote the appearance of regions with different charge dissipation capacities.51 Hau, S.K., Yip, H.L., Jen, A.K.Y., A review on the development of the inverted polymer solar cell architecture (2010) Polym. Rev., 50 (4), pp. 474-510Liang, Y.Y., For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4% (2010) Adv. Mater., 22 (20), pp. E135-E138Liu, S.J., High-efficiency polymer solar cells via the incorporation of an amino-functionalized conjugated metallopolymer as a cathode interlayer (2013) J. Am. Chem. Soc., 135 (41), pp. 15326-15329Cabanetos, C., Linear side chains in benzo[1, 2-b:4, 5-b]dithiophene-thieno[3, 4-c]pyrrole-4, 6-dione polymers direct self-assembly and solar cell performance (2013) J. Am. Chem. Soc., 135 (12), pp. 4656-4659Yeo, K.S., Application of sputter-deposited amorphous and anatase TiO2 as electroncollecting layers in inverted organic photovoltaics (2013) Org. Electron., 14 (7), pp. 1715-1719Sasajima, I., Flexible inverted polymer solar cells containing an amorphous titanium oxide electron collection electrode (2011) Org. Electron., 12 (1), pp. 113-118Bekci, D.R., Erten-Ela, S., Effect of nanostructured ZnO cathode layer on the photovoltaic performance of inverted bulk heterojunction solar cells (2012) Renewable Energy, 43, pp. 378-382Jagadamma, L.K., Efficient inverted bulk-heterojunction solar cells from low-temperature processing of amorphous ZnO buffer layers (2014) J. Mater. Chem. A, 2 (33), pp. 13321-13331Sun, H.Y., UV light protection through TiO2 blocking layers for inverted organic solar cells (2011) Sol. Energy Mater. Sol. C, 95 (12), pp. 3450-3454Zhang, H., Enhanced performance of inverted organic photovoltaic cells using CNTs-TiOx nanocomposites as electron injection layer (2013) Nanotechnology, 24 (35), p. 355401Zhang, Y., Enhanced electron collection in inverted organic solar cells using titanium oxide/reduced graphene oxide composite films as electron collecting layers (2014) Electrochim. Acta, 117, pp. 438-442Zhang, Y., Yuan, S., Liu, W., Inverted organic solar cells employing RGO/TiOx composite films as electron transport layers (2014) Electrochim. Acta, 143, pp. 18-22Lee, H.W., Highly efficient inverted polymer solar cells with reduced grapheneoxide-zinc-oxide nanocomposites buffer layer (2013) Appl. Phys. Lett., 102 (19), p. 193903Xiang, Q.J., Yu, J.G., Jaroniec, M., Graphene-based semiconductor photocatalysts (2012) Chem. Soc. Rev., 41 (2), pp. 782-796Li, X.S., Graphene films with large domain size by a two-step chemical vapor deposition process (2010) Nano Lett., 10 (11), pp. 4328-4334Hass, J., De Heer, W.A., Conrad, E.H., The growth and morphology of epitaxial multilayer graphene (2008) J. Phys. Condens. Matter, 20 (32), p. 323202Chen, F., Electrochemical gate-controlled charge transport in graphene in ionic liquid and aqueous solution (2009) J. Am. Chem. Soc., 131 (29), pp. 9908-9909Li, D., Processable aqueous dispersions of graphene nanosheets (2008) Nat. Nanotechnol., 3 (2), pp. 101-105Pei, S.F., Cheng, H.M., The reduction of graphene oxide (2012) Carbon, 50 (9), pp. 3210-3228Stankovich, S., Stable aqueous dispersions of graphitic nanoplatelets via the reduction of exfoliated graphite oxide in the presence of poly(sodium 4-styrenesulfonate) (2006) J. Mater. Chem., 16 (2), pp. 155-158Jasuja, K., Microwave-reduced uncapped metal nanoparticles on graphene: Tuning catalytic, electrical, and Raman properties (2010) J. Phys. Chem. Lett., 1 (12), pp. 1853-1860Aksay, I.A., Single sheet functionalized graphene by oxidation and thermal expansion of graphite (2007) Chem. Mater., 19 (18), pp. 4396-4404Ding, J.N., The influence of temperature, time and concentration on the dispersion of reduced graphene oxide prepared by hydrothermal reduction Diam Relat. Mater., 21 (11-15), p. 2012Chen, L., Enhanced photovoltaic performance of a dye-sensitized solar cell using graphene-TiO2 photoanode prepared by a novel in situ simultaneous reduction-hydrolysis technique (2013) Nanoscale, 5 (8), pp. 3481-3485Shu, W., Synthesis and photovoltaic performance of reduced graphene oxide-TiO2 nanoparticles composites by solvothermal method (2013) J. Alloy Compd., 563, pp. 229-233Pei, F.Y., TiO2 nanocomposite with reduced graphene oxide through facile blending and its photocatalytic behavior for hydrogen evolution (2013) Mater. Res. Bull., 48 (8), pp. 2824-2831Wang, D.T., Enhanced photoelectrocatalytic activity of reduced graphene oxide/TiO2 composite films for dye degradation (2012) Chem. Eng. J., 198, pp. 547-554Zhen, M.M., Well-distributed TiO2 nanocrystals on reduced graphene oxides as highperformance anode materials for lithium ion batteries (2013) RSC Adv., 3 (33), pp. 13696-13701Yeh, M.H., Dye-sensitized solar cells with reduced graphene oxide as the counter electrode prepared by a green photothermal reduction process (2014) Chem. Phys. Chem., 15 (6), pp. 1175-1181Wan, L., Room-temperature fabrication of graphene films on variable substrates and its use as counter electrodes for dye-sensitized solar cells (2011) Solid State Sci., 13 (2), pp. 468-475Zhang, H., Effects of TiO2 film thickness on photovoltaic properties of dye-sensitized solar cell and its enhanced performance by graphene combination (2014) Mater. Res. Bull., 49, pp. 126-131Cheng, G., Novel preparation of anatase TiO2 at reduced graphene oxide hybrids for high-performance dye-sensitized solar cells (2013) ACS Appl. Mater. Inter., 5 (14), pp. 6635-6642Tang, B., Hu, G.X., Two kinds of graphene-based composites for photoanode applying in dye-sensitized solar cell (2012) J. Power Sources, 220, pp. 95-102Kovtyukhova, N.I., Layer-by-layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations (1999) Chem. Mater., 11 (3), pp. 771-778Teran-Escobar, G., Low-temperature, solution-processed, layered V2O5 hydrate as the hole-transport layer for stable organic solar cells (2013) Energy Environ. Sci., 6 (10), pp. 3088-3098Wang, H., Hu, Y.H., Effect of oxygen content on structures of graphite oxides (2011) Ind Eng. Chem. Res., 50 (10), pp. 6132-6137Shen, J.F., One step hydrothermal synthesis of TiO2-reduced graphene oxide sheets (2011) J. Mater. Chem., 21 (10), pp. 3415-3421Kaniyoor, A., Ramaprabhu, S., A Raman spectroscopic investigation of graphite oxide derived graphene (2012) AIP Adv., 2 (3), p. 032183Some, S., Can commonly used hydrazine produce n-type graphene (2012) Chem. Eur. J., 18 (25), pp. 7665-7670Stankovich, S., Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide (2007) Carbon, 45 (7), pp. 1558-1565Shen, J.F., Ionic liquid-assisted one-step hydrothermal synthesis of TiO2-reduced graphene oxide composites (2011) Nano Res., 4 (8), pp. 795-806Jeong, H.K., Thermal stability of graphite oxide (2009) Chem. Phys. Lett., 470 (4-6), pp. 255-258Filik, J., XPS and laser Raman analysis of hydrogenated amorphous carbon films (2003) Diamond Relat. Mater., 12 (3-7), pp. 974-978Zu, Y.H., Graphite oxide-supported CaO catalysts for transesterification of soybean oil with methanol (2011) Bioresour. Technol., 102 (19), pp. 8939-8944Han, H.S., Synthesis of graphene oxide grafted poly(lactic acid) with palladium nanoparticles and its application to serotonin sensing (2013) Appl. Surf. Sci., 284, pp. 438-445Mattevi, C., Evolution of electrical, chemical, and structural properties of transparent and conducting chemically derived graphene thin films (2009) Adv. Funct. Mater., 19 (16), pp. 2577-2583Bagri, A., Structural evolution during the reduction of chemically derived graphene oxide (2010) Nat. Chem., 2 (7), pp. 581-587Yang, D., Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and micro-Raman spectroscopy (2009) Carbon, 47 (1), pp. 145-152Tsai, T.H., Chiou, S.C., Chen, S.M., Enhancement of dye-sensitized solar cells by using graphene-TiO2 composites as photoelectrochemical working electrode (2011) Int. J. Electrochem. Sci., 6 (8), pp. 3333-3343Wu, M.C., Correlation between nanoscale surface potential and power conversion efficiency of P3HTTiO2 nanorod bulk heterojunction photovoltaic devices (2010) Nanoscale, 2 (8), pp. 1448-1454De Morais, A., Enhancing in the performance of dye-sensitized solar cells by the incorporation of functionalized multi-walled carbon nanotubes into TiO2 films: The role of MWCNT addition (2013) J. Photochem. Photobiol. A, 251, pp. 78-84Liscio, A., Photovoltaic charge generation visualized at the nanoscale: A proof of principle (2008) J. Am. Chem. Soc., 130 (3), p. 780Xiang, Q.J., Microwave-hydrothermal preparation and visible-light photoactivity of plasmonic photocatalyst Ag-TiO2 nanocomposite hollow spheres (2010) Chem. Asian J., 5 (6), pp. 1466-1474Huang, J.Z., Incorporation of graphene oxide in quantum dot sensitized photocatalyst based on ZnO nanorods (2014) J. Nanosci. Nanotechnol., 14 (4), pp. 3001-3005Kyaw, A.K.K., Top-illuminated dye-sensitized solar cells with a room-temperatureprocessed ZnO photoanode on metal substrates and a Pt-coated Ga-doped ZnO counter electrode (2011) J. Phys. D Appl. Phys., 44 (4), p. 045102Lira-Cantu, M., Oxide/polymer interfaces for hybrid and organic solar cells: Anatase vs. Rutile TiO2 (2011) Sol. Energy Mater. Sol. C, 95 (5), pp. 1362-1374