ITO surface modification for inverted organic photovoltaics

Abstract

The work function (WF) of indium-tin-oxide (ITO) substrates plays an important role on the inverted organic photovoltaic device performance. And electrode engineering has been a useful method to facilitate carrier extraction or charge collection to enhance organic photovoltaic (OPV) performance. By using self-assembly technique, we have deposited poly(dimethyl diallylammonium chloride) (PDDA) layers onto ITO coated glass substrates. The results indicate that the surface WF of ITO is reduced by about 0.3 eV after PDDA modification, which is attributed to the modulation in electron affinity. In addition, the surface roughness of ITO substrate became smaller after PDDA modification. These modified ITO substrates can be applied to fabricate inverted OPVs, in which ITO works as the cathode to collect electrons. As a result, the photovoltaic performance of inverted OPV is substantially improved, mainly reflecting on the increase of short circuit current density.

Keywords

organic photovoltaic (OPV) / indium tin oxide (ITO) / inverted structure / surface modification / work function (WF)

References

[1] Hoppe H, Sariciftci N S. Organic solar cells: an overview. Journal of Materials Research, 2004, 19(7): 1924–1945

[2] Cnops K, Rand B P, Cheyns D, Verreet B, Empl M A, Heremans P. 8.4% efficient fullerene-free organic solar cells exploiting long-range exciton energy transfer. Nature Communications, 2014, 5: 3406

[3] Cao W, Xue J. Recent progress in organic photovoltaics: device architecture and optical design. Energy & Environmental Science, 2014, 7(7): 2123–2144

[4] Jørgensen M, Norrman K, Krebs F. Stability/degradation of polymer solar cells. Solar Energy Materials and Solar Cells, 2008, 92(7): 686–714

[5] 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. Nature Photonics, 2012, 6(9): 591–595

[6] Zhou Y, Fuentes-Hernandez C, Shim J, Meyer J, Giordano A J, Li H, Winget P, Papadopoulos T, Cheun H, Kim J, Fenoll M, Dindar A, Haske W, Najafabadi E, Khan T M, Sojoudi H, Barlow S, Graham S, Brédas J L, Marder S R, Kahn A, Kippelen B. A universal method to produce low-work function electrodes for organic electronics. Science, 2012, 336(6079): 327–332

[7] Min X, Jiang F, Qin F, Li Z, Tong J, Xiong S, Meng W, Zhou Y. Polyethylenimine aqueous solution: a low-cost and environmentally friendly formulation to produce low-work-function electrodes for efficient easy-to-fabricate organic solar cells. ACS Applied Materials & Interfaces, 2014, 6(24): 22628–22633

[8] Guo Z, Shen Y, Wang M, Zhao F, Dong S. electrochemistry and electrogenerated chemiluminescence of SiO₂ nanoparticles/Tris (2,2-bipyridyl)ruthenium(II) multilayer films on Indium Tin oxide electrodes. Analytical Chemistry, 2004, 76(1): 184–191

[9] Li L S, Li A D Q, Jia Q X. Effects of self-assembled multilayers on the evolution of surface physical properties of indium-tin-oxide. Applied Surface Science, 2003, 219(3-4): 199–202

[10] Ma W, Yang C, Gong X, Lee K, Heeger A J. Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology. Advanced Functional Materials, 2005, 15(10): 1617–1622

[11] Park Y, Choong V, Gao Y, Hsieh B R, Tang C W. Work function of indium tin oxide transparent conductor measured by photoelectron spectroscopy. Applied Physics Letters, 1996, 68(19): 2699–2701

[12] Manor A, Katz E A. Open-circuit voltage of organic photovoltaics: Implications of the generalized Einstein relation for disordered semiconductors. Solar Energy Materials and Solar Cells, 2012, 97: 132–138

[13] Zhang C, You H, Lin Z, Hao Y. Inverted organic photovoltaic cells with solution-processed zinc oxide as electron collecting layer. Japanese Journal of Applied Physics, 2011, 50(8R): 082302

[14] Zhao D W, Sun X W, Jiang C Y, Kyaw A K K, Lo G Q, Kwong D L. Efficient tandem organic solar cells with an Al/MoO₃ intermediate layer. Applied Physics Letters, 2008, 93(8): 083305

[15] Tao C, Ruan S, Zhang X, Xie G, Shen L, Kong X, Dong W, Liu C, Chen W. Performance improvement of inverted polymer solar cells with different top electrodes by introducing a MoO₃ buffer layer. Applied Physics Letters, 2008, 93(19): 193307

Acknowledgement

The financial support from the Director Fund of the Wuhan National Laboratory for Optoelectronics, the Major State Basic Research Development Program of China (Nos. 2014CB643506 and 2013CB922104) is acknowledged.

RIGHTS & PERMISSIONS

2014 Higher Education Press and Springer-Verlag Berlin Heidelberg