www.afm-journal.de FULL PAPER © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2993 www.MaterialsViews.com wileyonlinelibrary.com Jesse R. Manders, Sai-Wing Tsang, Michael J. Hartel, Tzung-Han Lai, Song Chen, Chad M. Amb,* John R. Reynolds,* and Franky So* Solution-Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells 1. Introduction In a society concerned with environ- mental, economic, and geopolitical con- sequences of energy consumption, new alternatives to traditional energy sources are needed. With rapid progress being made in organic photovoltaics, they are becoming a viable source of renewable energy, as power conversion efficiencies (PCEs) exceeding 8% have been demon- strated. [1,2] Traditional bulk heterojunction polymer solar cells consist of a transparent indium tin oxide (ITO) anode, a hole trans- port layer, a photoactive layer, and a top cathode. Hole transport layers must have high optical transparency, good chemical stability, a large ionization potential, and good electron blocking capability. In a typical polymer solar cell, poly(3,4- ethylenedioxythiophene):poly(styrenes ulfonate) (PEDOT:PSS) is used as the hole-transporting layer (HTL) and has a work function ( Φ) of 5.2 eV. However, its acidity, tendency to absorb water, and inability to block electrons effectively are factors which contribute to device per- formance problems and degradation. [3] Nickel oxide is emerging as an alternative HTL for polymer solar cells. [4–14] Pure, stoichiometric NiO is an excellent insulator, with room temperature conductivity on the order of 10 −13 S cm −1 , [15] while non-stoichiometric NiO is a wide bandgap p-type semiconductor. [16–21] The p-type conductivity of NiO originates from two positively charged holes which accom- pany each Ni 2 + vacancy in the lattice for charge neutrality. [16,22,23] These holes are quasi-localized on Ni 2 + ions near the vacancy in the lattice, generating two Ni 3 + ions for each Ni 2 + vacancy. [16,24] The valence band edge of NiO is well-aligned to the highest occupied molecular orbital (HOMO) levels of many p-type con- jugated polymers for photovoltaics. [12,21] Irwin et al. first demonstrated an enhancement in polymer solar cell performance with a NiO electron blocking layer depos- ited via pulsed laser deposition. [12,25] Recently, solution-proc- essed NiO was also reported for polymer photovoltaics. [13,14,21] A nickel ink made from nickel formate and ethylenediamine was used as the precursor in those reports. Here, we chose nickel acetate tetrahydrate and monoethanolamine precursors in an ethanolic solution, as this presents a set of materials not yet The detailed characterization of solution-derived nickel (II) oxide (NiO) hole- transporting layer (HTL) films and their application in high efficiency organic photovoltaic (OPV) cells is reported. The NiO precursor solution is examined in situ to determine the chemical species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decompose to form non-stoichiometric NiO. Specifically, the [Ni(MEA) 2 (OAc)] + ion is found to be the most prevalent species in the precursor solution. The defect-induced Ni 3 + ion, which is present in non-stoichiometric NiO and signifies the p-type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using X-ray photoelectron spectroscopy. Bulk hetero- junction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly-dithienogermole-thienopyrrolodione (pDTG-TPD) and [6,6]-phenyl-C71-butyric acid methyl ester (PC 71 BM) are fabricated using these solution-processed NiO films. The resulting devices show an average power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homoge- neous donor/acceptor morphology in the active layer at the NiO/BHJ inter- face. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS. J. R. Manders, Dr. S.-W. Tsang, M. J. Hartel, T.-H. Lai, S. Chen, Prof. F. So Department of Materials Science and Engineering University of Florida Gainesville, FL 32611 USA E-mail: fso@mse.ufl.edu Dr. C. M. Amb The George and Josephine Butler Polymer Research Laboratory Department of Chemistry Center for Macromolecular Science and Engineering University of Florida Gainesville, FL 32611 USA E-mail: chad.amb@hotmail.com Prof. J. R. Reynolds School of Chemistry and Biochemistry School of Materials Science and Engineering Center for Organic Photonics and Electronics Georgia Institute of Technology Atlanta, GA 30332-0400 USA E-mail:r eynolds@chemistry.gatech.edu DOI:10. 1002/adfm.201202269 Adv. Funct. Mater. 2013, 23, 2993–3001