Enhancing performance of Organic solar cells with ZnO as either electron transport layer or electron acceptor

Zaifei Ma^a, Fengling Zhang^a, Shuyan Shao^a, Zheng Tang^a, Tõnu Pullerits^b, Kaibo Zheng^b, Ergang Wang^c

^aLinköping University, Sweden, SE-581 83, Linköping, Sweden

^bLund University, Department of Chemical Physics, Getingevägen 60, Lund, 22241, Sweden

^cChalmers University of Technology, Sweden, Fysikgränd, 3, Gothenburg, Sweden

International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)

Ecublens, Switzerland, 2014 May 11th - 14th

Organizers: Michael Graetzel and Mohammad Nazeeruddin

Poster, Fengling Zhang, 139
Publication date: 1st March 2014

ZnO nanoparticles are widely used in organic solar cells (OSCs) as electron transport layers (ETL) or electron acceptors due to their low cost and environmental-friendly property. ZnO nanoparticles as the ETL in inverted OSCs have been extensively investigated not only because ZnO can considerably improve the stability of the OSCs but also their roll-to-toll processing compatibility.^1,2 Furthermore, the power conversion efficiencies (PCEs) obtained from inverted OSCs contained ZnO nanoparticles as ETL can be as big as those of conventional OSCs.³However, the correlation between surface roughness of the ZnO ETL and the performance of the inverted OSCs is seldom studied. Here we present our investigation on connection between the roughness of the ZnO ETL and the performance of the inverted OSCs.⁴ The results show that smooth ZnO interlayer favours big short-circuit current density (J_sc) due to a larger donor/acceptor (D/A) interfacial area in the active layer facilitating exciton dissociation. The improvement in both fill factor (FF) and open-circuit voltage (V_oc) is ascribed to a reduced trap concentration at the interface between the ZnO interlayer and the active layer because of reduced ZnO surface roughness.

In addition, it is found that modified ZnO nanoparticles with poly(ethylene oxide) (PEO) can effectively passivate the surface traps of ZnO, suppress the recombination loss of carriers, reduce the series resistance, and improve the electrical coupling of ZnO/active layer. Consequently, both J_scand FF of the inverted OSCs composed of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-dithienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):PC71BM blends are considerably improved. The PCE of the device based on the PEO-modified ZnO layer is increased to 6.59% from 5.39% of the reference device based on the ZnO layer.⁵

The performance of hybrid solar cells composed of polymer and ZnO is mainly hindered by the defects of ZnO. Here, we extend application of PEO modified ZnO nanoparticles to hybrid solar cells based on poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and ZnO nanoparticles as electron acceptors. The reference device using ZnO nanoparticles as electron acceptors shows a V_oc of 0.83V, a J_scof 3.00 mA/cm2, a FF of 0.46 and a PCE of 1.15%. After modified with small amount of PEO (0.05%), the device typically shows a V_oc of 0.86V, a J_scof 3.84 mA/cm2, a FF of 0.51 and a PCE of 1.68%.⁶

References:

[1] Hau, S. K.; Yip, H.-L.; Baek, N. S.; Zou, J.; O’Malley, K.; Jen, A. K. Y. High performance ambient processed inverted polymer solar cells through interfacial modification with a fullerene self-assembled monolayer. Appl. Phys. Lett. 2008, 92, 253301.

[2] Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. A roll-to-roll process to flexible polymer solar cells: model studies, manufacture and operational stability studies. J. Mater. Chem.2009, 19, 5442-5451.

[3] Small, C.; Chen, S.; Subbiah, J,; Amb, C. M.; Tsang, S. W.; Lai, T.-H.; Reynolds, J. R.; and So, F. High-Efficiency Inverted Dithienogermole-thienopyrrolodione-based Polymer Solar Cells. Nature Photonics 2012, 6, 115-120.

[4] Ma, Z. F.; Tang, Z.; Wang, E. G.; Andersson, M. R.; Inganas, O.; Zhang, F.L. Influences of Surface Roughness of ZnO Electron Transport Layer on the Photovoltaic Performance of Organic Inverted Solar Cells. J. Phys. Chem. C 2012,116(46),24462-4468.

[5] Shao, S. Y.; Zheng, K. B.; Pullerits, T.; Zhang, F. L. Enhanced Performance of Inverted Polymer Solar Cells by Using Poly(ethylene oxide)-Modified ZnO as an Electron Transport Layer. ACS. Applied Materials & Interfaces, 2013, 5(2), 380-383.

[6] Shao, S. Y.; Zheng, K. B.; Zidek, K.; Chabera, P.; Pullerits, T.; Zhang, F. L. Optimizing ZnO nanoparticle surface for bulk heterojunction hybrid solar cells. Solar Energy Mater. and Solar Cells, 2013, 118, 43-47.

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