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We report on the photovoltaic properties of air-stable inverted organic solar cells in which zinc oxide (ZnO) of varying thicknesses is formed as the electron-transport layer by an atomic layer deposition (ALD) method. The device performance was found to be dependent on the ZnO thickness. Air-stable inverted solar cells with an optimized ZnO thickness reached a power conversion efficiency of 2.91%. This efficiency was found to be comparable to those of conventional organic solar cells. The use of the ZnO electron-transport layer led to improved air stability: the power conversion efficiencies of unencapsulated organic solar cells remained above 80% of their original values even after storage in air for thirty days.Organic solar cells (OSCs) have attracted much attention recently because of their advantages in solution processes, their low cost, and their compatibility with flexible substrates. [1][2][3][4][5] In general, OSCs with conventional structures are composed of a photoactive layer sandwiched between an indium tin oxide (ITO) electrode and a top metal electrode with a low work function. The performance of such devices is degraded quickly because of the hygroscopic nature of the hole-transport layer of poly(3,4-ethylene dioxythiophene doped with polystyrene sulfonate) (PEDOT:PSS), 6,7 and the oxidation of the top metal electrode during exposure in ambient conditions; the resulting oxide layer has been shown to produce an insulating barrier that reduces the conductivity of the electrode, effectively increasing the serial resistance of the device. 8,9 For these reasons, an OSC device with an inverted geometry has been proposed. 10,11 The inverted OSC showed better long-term air stability under ambient conditions through the use of inorganic materials functioning as the electron-transport layer interposed between the ITO cathode and the photoactive layer of zinc oxide (ZnO), 12,13 titanium oxide (TiO x ), 14,15 or aluminum oxide (Al 2 O 3 ). 16 Inorganic materials are used as electron transport layers on top of the ITO cathode because of their large bandgaps and good electron-extraction properties.Here, we employed an air-stable inverted OSC structure using ZnO prepared by atomic layer deposition (ALD) as the electrontransport layer. The use of the ALD deposition method is appropriate for controlling the thickness because it uses a self-limiting process that allows layer-by-layer growth, and therefore, the growth of layers with very precise thicknesses can be achieved on the nanometer scale. In addition, ALD is expected to produce defect-free, uniform, and highly conformal films. [17][18][19] We studied the structural, electrical, and optical properties of ZnO layers with different thicknesses in inverted OSCs. The photovoltaic performance was found to be dependent on the thickness of the ZnO layer. Air-stable inverted solar cells with an optimized ZnO thickness reached a power conversion efficiency of 2.91%, and the device performance remained at approximately 80% of the original value even after storage in ...

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