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
a Linköping University, Sweden, SE-581 83, Linköping, Sweden
b Lund University, Department of Chemical Physics, Getingevägen 60, Lund, 22241, Sweden
c Chalmers 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.3 However, 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 (Jsc) 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 (Voc) 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 Jscand 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 Voc of 0.83V, a Jscof 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 Voc of 0.86V, a Jscof 3.84 mA/cm2, a FF of 0.51 and a PCE of 1.68%.6

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