Oxygen-Terminated Nanocrystalline Diamond Film as an Efficient Anode in Photovoltaics By Candy Haley Yi Xuan Lim, Yu Lin Zhong, Stoffel Janssens, Milos Nesladek, and Kian Ping Loh* 1. Introduction Organic photovoltaics are light-to-electricity conversion devices that utilize mainly organic materials as the active layer. These include small organic molecules, polymers, and hybrid materials with inorganic nanoparticles. [1] The advantages of organic solar cells arise from their ease of processing and low production costs, as well as upward scalability on flexible, large-area substrates. [2] However, there are two major limitations of organic solar cells: poorer stability and lower efficiency compared to their inorganic counterparts. The former can be addressed by improving the encapsulation methods to prevent oxidative degradation. However, the poor interface stability between the organic active layer and inorganic electrodes remain the limiting factors for the widespread com- mercial development of organic solar cells, [3] and therefore the search for good anode material remains highly relevant. Although indium tin oxide (ITO) is widely used as the transparent and con- ducting electrode for optoelectronic devices, there are rising concerns over the limited supply of indium and its chemical stability. [4] Alternative candidate electrodes such as transparent conducting oxides (TCOs), [5] conducting polymer films, [6] and graphene films have been considered, [7] but the issue of chemical stability and low efficiency remains. Another point is that ITO is essentially a degenerate n-type semiconductor and the hole injection barrier is considerably high for many organic dyes. One way to reduce the hole injection barrier and increase the open-circuit voltage (V oc ) is to use p-type electrodes. In this work, we propose that boron-doped nanocrystalline diamond films grown on glass are potential candidates for a new type of optically transparent electrode (OTE) for solar cells. Although the growth parameters have not been fully optimized in this study, our work here shows that this material, because of its robustness and unique surface properties, shows immense potential. There has been tremendous progress in the plasma-enhanced chemical vapor deposition (PECVD) of nanocrystalline diamond (NCD) films prepared on large-area glass, quartz, and even polymer substrates at low temperatures (<400 8C), [8] which enable many engineering applications. In principle, the precursors for the synthesis of these diamond films are inexpensive, dilute methane- in-hydrogen mixtures gas feeds, thus large-area coating on glass substrates is commercially viable. Introducing boron (for example from a trimethylboron source) into the gas feeds creates acceptor states in the diamond (0.36 eV from the valence band maximum, or metallic states for boron concentration >5  10 20 cm 3 ) film for hole transport in the otherwise insulating material. In ITO, both substitutional tin and oxygen vacancies contribute to its n-type conductivity. Whilst ITO has been traditionally used as the anode material in solar cells, it can be envisaged that p-type diamond with high conductivity and transparency can perhaps act better as an anode due to its intrinsic hole-acceptor characteristics. One unique aspect of diamond is that its surface energy levels can be readily tuned by controlling its surface termination, [9] which affords the FULL PAPER www.MaterialsViews.com www.afm-journal.de [*] Prof. K. P. Loh, C. H. Y. X. Lim, Y. L. Zhong Department of Chemistry National University of Singapore Science Drive 3, 117543 (Singapore) E-mail: chmlohkp@nus.edu.sg C. H. Y. X. Lim NUS Graduate School for Integrative Sciences and Engineering Centre for Life Sciences #05-01 28 Medical Drive, 117456 (Singapore) Dr. M. Nesladek, S. Janssens IMOMEC, Hasselt University Wetenschapspark, B 3590 Diepenbeek (Belgium) DOI: 10.1002/adfm.200902204 The potential of using p-doped nanocrystalline diamond as the anode for organic solar cells, because of its outstanding photostability and well- matched energetics with organic dyes, is demonstrated. The interface dipole and open-circuit potential can be tuned by varying the surface termination on diamond. Oxygenated nanocrystalline diamond (O-NCD) exhibits the best photocurrent conversion among all the surface-treated electrodes studied in this work because of its large open-circuit potential. The good energy alignment of the valence band of O-NCD with the HOMO of poly(3- hexylthiophene), as well as its p-doped characteristics, suggest that O-NCD can replace the hole transport layer, such as PEDOT:PSS, needed for efficient performance on indium tin oxide (ITO) electrodes. If the sheet resistance and optical transparency on NCD can be further optimized, chemical-vapor- deposited diamond electrodes may offer a viable alternative to ITO and fluorinated tin oxide (FTO). Adv. Funct. Mater. 2010, 20, 1313–1318 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1313