Graphical Abstract

Highly efficient and stable polymer solar cells (PSCs) have been fabricated by adopting solution-derived hybrid poly(ethylene glycol)-titanium oxide (PEG-TiOx) nanocomposite films as a novel and universal cathode buffer layer (CBL), which can greatly improve device performance by reducing interface energy barriers and enhancing charge extraction/collection. The performance of inverted PSCs with varied bulk-heterojunctions (BHJs) based on this hybrid nanocomposite CBL was found to be much better than those of control devices with a pure TiOx CBL or without a CBL. An excellent power conversion efficiency up to 9.05% under AM 1.5G irradiation (100 mW·cm-2) was demonstrated, which represents a record high value for inverted PSCs with TiOx-based interface materials.
Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated polymer-based organic solar cells. Chem. Rev. 2007, 107, 1324-1338.
Servaites, J. D.; Ratner, M. A.; Marks, T. J. Organic solar cells: A new look at traditional models. Energy Environ. Sci. 2011, 4, 4410-4422.
Li, G.; Zhu, R.; Yang, Y. Polymer solar cells. Nat. Photonics 2012, 6, 153-161.
Li, K.; Li, Z.; Feng, K.; Xu, X.; Wang, L.; Peng, Q. Development of large band-gap conjugated copolymers for efficient regular single and tandem organic solar cells. J. Am. Chem. Soc. 2013, 135, 13549-13557.
Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Stability of polymer solar cells. Adv. Mater. 2012, 24, 580-612.
Liao, S. -H.; Jhuo, H. -J.; Cheng, Y. -S.; Chen, S. -A. Fullerene derivative-doped zinc oxide nanofilm as the cathode of inverted polymer solar cells with low-bandgap polymer (PTB7-Th) for high performance. Adv. Mater. 2013, 25, 4766-4771.
Ma, H.; Yip, H. L.; Huang, F.; Jen, A. K. Y. Interface engineering for organic electronics. Adv. Funct. Mater. 2010, 20, 1371-1388.
He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat. Photonics 2012, 6, 591-595.
You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C. -C.; Gao, J.; Li, G.; Yang, Y. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2013, 4, 1446.
Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. -T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%. Adv. Mater. 2010, 22, E135-E138.
Yin, Z.; Zhu, J.; He, Q.; Cao, X.; Tan, C.; Chen, H.; Yan, Q.; Zhang, H. Graphene-based materials for solar cell applications. Adv. Energy Mater. 2014, 4, 1300574.
Zhang, M.; Gu, Y.; Guo, X.; Liu, F.; Zhang, S.; Huo, L.; Russell, T. P.; Hou, J. Efficient polymer solar cells based on benzothiadiazole and alkylphenyl substituted benzodithiophene with a power conversion efficiency over 8%. Adv. Mater. 2013, 25, 4944-4949.
Xiao, Z.; Ye, G.; Liu, Y.; Chen, S.; Peng, Q.; Zuo, Q.; Ding, L. Pushing fullerene absorption into the near-IR region by conjugately fusing oligothiophenes. Angew. Chem. Int. Ed. 2012, 51, 9038-9041.
Li, Y. Molecular design of photovoltaic materials for polymer solar cells: Toward suitable electronic energy levels and broad absorption. Acc. Chem. Res. 2012, 45, 723-733.
Wong, W. -Y.; Wang, X. -Z.; He, Z.; Djurišić, A. B.; Yip, C. -T.; Cheung, K. -Y.; Wang, H.; Mak, C. S. K.; Chan, W. -K. Metallated conjugated polymers as a new avenue towards high-efficiency polymer solar cells. Nat. Mater. 2007, 6, 521-527.
Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin, P. G.; Kim, Y.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley, D. D. C.; Nelson, J. Morphology evolution via self- organization and lateral and vertical diffusion in polymer: fullerene solar cell blends. Nat. Mater. 2008, 7, 158-164.
Yin, Z.; Zheng, Q. Controlled synthesis and energy applications of one-dimensional conducting polymer nanostructures: An overview. Adv. Energy Mater. 2012, 2, 179-218.
Lee, B. R.; Jung, E. D.; Nam, Y. S.; Jung, M.; Park, J. S.; Lee, S.; Choi, H.; Ko, S. -J.; Shin, N. R.; Kim, Y. -K.; Kim, S. O.; Kim, J. Y.; Shin, H. -J.; Cho, S.; Song, M. H. Amine- based polar solvent treatment for highly efficient inverted polymer solar cells. Adv. Mater. 2014, 26, 494-500.
Chen, S.; Manders, J. R.; Tsang, S. W.; So, F. Metal oxides for interface engineering in polymer solar cells. J. Mater. Chem. 2012, 22, 24202-24212.
Song, M.; Park, J. H.; Kim, C. S.; Kim, D. -H.; Kang, Y. -C.; Jin, S. -H.; Jin, W. -Y.; Kang, J. -W. Highly flexible and transparent conducting silver nanowires/ZnO composite film for organic solar cells. Nano Res. 2014, 7, 1370-1379.
Li, W.; Furlan, A.; Hendriks, K. H.; Wienk, M. M.; Janssen, R. A. J. Efficient tandem and triple-junction polymer solar cells. J. Am. Chem. Soc. 2013, 135, 5529-5532.
Zhang, F.; Xu, X.; Tang, W.; Zhang, J.; Zhuo, Z.; Wang, J.; Xu, Z.; Wang, Y. Recent development of the inverted configuration organic solar cells. Sol. Energy Mater. Sol. Cells 2011, 95, 1785-1799.
Williams, G.; Wang, Q.; Aziz, H. The photo-stability of polymer solar cells: Contact photo-degradation and the benefits of interfacial layers. Adv. Funct. Mater. 2013, 23, 2239-2247.
Xu, Z.; Chen, L.; Yang, G.; Huang, C. H.; Hou, J.; Wu, Y.; Li, G.; Hsu, C. S.; Yang, Y. Vertical phase separation in poly(3-hexylthiophene): Fullerene derivative blends and its advantage for inverted structure solar cells. Adv. Funct. Mater. 2009, 19, 1227-1234.
Motiei, L.; Yao, Y.; Choudhury, J.; Yan, H.; Marks, T. J.; van der Boom, M. E.; Facchetti, A. Self-propagating molecular assemblies as interlayers for efficient inverted bulk- heterojunction solar cells. J. Am. Chem. Soc. 2010, 132, 12528-12530.
Yin, Z.; Zheng, Q.; Chen, S.; Cai, D. Interface control of semiconducting metal oxide layers for efficient and stable inverted polymer solar cells with open-circuit voltages over 1.0 Volt. ACS Appl. Mater. Interfaces 2013, 5, 9015-9025.
Yin, Z.; Zheng, Q.; Chen, S.; Cai, D.; Zhou, L.; Zhang, J. Bandgap tunable Zn1-xMgxO thin films as highly transparent cathode buffer layers for high-performance inverted polymer solar cells. Adv. Energy Mater. 2014, 4, 1301404.
Wang, D. H.; Kim, J. K.; Seo, J. H.; Park, I.; Hong, B. H.; Park, J. H.; Heeger, A. J. Transferable graphene oxide by stamping nanotechnology: Electron-transport layer for efficient bulk-heterojunction solar cells. Angew. Chem. Int. Ed. 2013, 52, 2874-2880.
You, J.; Chen, C. C.; Dou, L.; Murase, S.; Duan, H. S.; Hawks, S. A.; Xu, T.; Son, H. J.; Yu, L.; Li, G.; Yang, Y. Metal oxide nanoparticles as an electron-transport layer in high-performance and stable inverted polymer solar cells. Adv. Mater. 2012, 24, 5267-5272.
Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted polymer solar cells integrated with a low- temperature-annealed sol-gel-derived ZnO film as an electron transport layer. Adv. Mater. 2011, 23, 1679-1683.
Huang, J.; Yin, Z.; Zheng, Q. Applications of ZnO in organic and hybrid solar cells. Energy Environ. Sci. 2011, 4, 3861- 3877.
Waldauf, C.; Morana, M.; Denk, P.; Schilinsky, P.; Coakley, K.; Choulis, S. A.; Brabec, C. J. Highly efficient inverted organic photovoltaics using solution based titanium oxide as electron selective contact. Appl. Phys. Lett. 2006, 89, 233517.
Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.; Heeger, A. J. Efficient tandem polymer solar cells fabricated by all-solution processing. Science 2007, 317, 222-225.
Zhang, D.; Choy, W. C. H.; Xie, F.; Sha, W. E. I.; Li, X.; Ding, B.; Zhang, K.; Huang, F.; Cao, Y. Plasmonic electrically functionalized TiO2 for high-performance organic solar cells. Adv. Funct. Mater. 2013, 23, 4255-4261.
Huang, J.; Li, G.; Yang, Y. A semi-transparent plastic solar cell fabricated by a lamination process. Adv. Mater. 2008, 20, 415-419.
Jiang, C.; Sun, X.; Zhao, D.; Kyaw, A. K. K.; Li, Y. Low work function metal modified ITO as cathode for inverted polymer solar cells. Sol. Energy Mater. Sol. Cells 2010, 94, 1618-1621.
Liu, J.; Shao, S.; Fang, G.; Meng, B.; Xie, Z.; Wang, L. High-efficiency inverted polymer solar cells with transparent and work-function tunable MoO3-Al composite film as cathode buffer layer. Adv. Mater. 2012, 24, 2774-2779.
Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S. W.; Lai, T. H.; Reynolds, J. R.; So, F. High-efficiency inverted dithienogermole-thienopyrrolodione-based polymer solar cells. Nat. Photonics 2012, 6, 115-120.
Sun, C.; Wu, Y.; Zhang, W.; Jiang, N.; Jiu, T.; Fang, J. Improving efficiency by hybrid TiO2 nanorods with 1, 10- phenanthroline as a cathode buffer layer for inverted organic solar cells. ACS Appl. Mater. Interfaces 2014, 6, 739-744.
Kang, H.; Hong, S.; Lee, J.; Lee, K. Electrostatically self-assembled nonconjugated polyelectrolytes as an ideal interfacial layer for inverted polymer solar cells. Adv. Mater. 2012, 24, 3005-3009.
Choi, H.; Park, J. S.; Jeong, E.; Kim, G. H.; Lee, B. R.; Kim, S. O.; Song, M. H.; Woo, H. Y.; Kim, J. Y. Combination of titanium oxide and a conjugated polyelectrolyte for high- performance inverted-type organic optoelectronic devices. Adv. Mater. 2011, 23, 2759-2763.
Jo, S. B.; Lee, J. H.; Sim, M.; Kim, M.; Park, J. H.; Choi, Y. S.; Kim, Y.; Ihn, S. G.; Cho, K. High performance organic photovoltaic cells using polymer-hybridized ZnO nanocrystals as a cathode interlayer. Adv. Energy Mater. 2011, 1, 690-698.
Sun, B.; Sirringhaus, H. Solution-processed zinc oxide field-effect transistors based on self-assembly of colloidal nanorods. Nano Lett. 2005, 5, 2408-2413.
Lee, H. S.; Kim, D. H.; Cho, J. H.; Hwang, M.; Jang, Y.; Cho, K. Effect of the phase states of self-assembled monolayers on pentacene growth and thin-film transistor characteristics. J. Am. Chem. Soc. 2008, 130, 10556-10564.
Hu, T.; Li, F.; Yuan, K.; Chen, Y. Efficiency and air- stability improvement of flexible inverted polymer solar cells using ZnO/poly(ethylene glycol) hybrids as cathode buffer layers. ACS Appl. Mater. Interfaces 2013, 5, 5763-5770.
Wong, K. H.; Mason, C. W.; Devaraj, S.; Ouyang, J.; Balaya, P. Low temperature aqueous electrodeposited TiOx thin films as electron extraction layer for efficient inverted organic solar cells. ACS Appl. Mater. Interfaces 2014, 6, 2679-2685.
Yu, J.; Zhao, X.; Zhao, Q. Effect of surface structure on photocatalytic activity of TiO2 thin films prepared by sol-gel method. Thin Solid Films 2000, 379, 7-14.
Kumar, P. M.; Badrinarayanan, S.; Sastry, M. Nanocrystalline TiO2 studied by optical, FTIR and X-ray photoelectron spectroscopy: Correlation to presence of surface states. Thin Solid Films 2000, 358, 122-130.
Tan, Z.; Zhang, W.; Zhang, Z.; Qian, D.; Huang, Y.; Hou, J.; Li, Y. High-performance inverted polymer solar cells with solution-processed titanium chelate as electron-collecting layer on ITO electrode. Adv. Mater. 2012, 24, 1476-1481.
Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk heterojunction solar cells with internal quantum efficiency approaching 100%. Nat. Photonics 2009, 3, 297-302.
Shao, S.; Zheng, K.; Pullerits, T.; Zhang, F. Enhanced performance of inverted polymer solar cells by using poly(ethylene oxide)-modified ZnO as an electron transport layer. ACS Appl. Mater. Interfaces 2013, 5, 380-385.
Shi, J.; Liu, Y.; Peng, Q.; Li, Y. ZnO hierarchical aggregates: solvothermal synthesis and application in dye-sensitized solar cells. Nano Res. 2013, 6, 441-448.
Thapa, A.; Zai, J.; Elbohy, H.; Poudel, P.; Adhikari, N.; Qian, X.; Qiao, Q. TiO2 coated urchin-like SnO2 microspheres for efficient dye-sensitized solar cells. Nano Res. 2014, 7, 1154-1163.
Li, H.; Wang, J.; Liu, M.; Wang, H.; Su, P.; Wu, J.; Li, J. A nanoporous oxide interlayer makes a better Pt catalyst on a metallic substrate: Nanoflowers on a nanotube bed. Nano Res. 2014, 7, 1007-1017.
Guan, B.; Wang, T.; Zeng, S.; Wang, X.; An, D.; Wang, D.; Cao, Y.; Ma, D.; Liu, Y.; Huo, Q. A versatile cooperative template-directed coating method to synthesize hollow and yolk-shell mesoporous zirconium titanium oxide nanospheres as catalytic reactors. Nano Res. 2014, 7, 246-262.
Song, H.; Jo, K.; Jung, B.Y.; Jung, G.Y. Fabrication of periodically aligned vertical single-crystalline anatase TiO2 nanotubes with perfect hexagonal open-ends using chemical capping materials. Nano Res. 2014, 7, 104-109.
Park, S.; Kim, D.; Lee, C.W.; Seo, S. -D.; Kim, H.J.; Han, H.S.; Hong, K.S.; Kim, D. -W. Surface-area-tuned, quantum- dot-sensitized heterostructured nanoarchitectures for highly efficient photoelectrodes. Nano Res. 2014, 7, 144-153.