Nickel oxide (NiO) is a prominent electron-blocking material in organic and perovskite photovoltaic (PV) devices and is of interest for future silicon-based PV designs. In these devices, the NiO is generally sputtered or solution-deposited, and sometimes it is doped in order to increase its conductivity or change its band alignment. Solution deposition may be less than optimal because of the desire for thickness control on the order of nm and the high annealing temperatures (400°C) of many solution deposition processes. Because of its ability to conformally deposit films with precisely-tuned thicknesses at relatively low temperatures, atomic layer deposition (ALD) presents potentially useful advantages.

Here, we report on the first copper-doped nickel oxide films deposited by ALD and the application of ALD-NiO as an electron-blocker in perovskite solar cells. First, a nickel amidinate precursor (Bis(N,N'-di-t-butylacetamidinato)nickel(II)) was tested with various oxidizers at temperatures from 150 to 200°C. Only ozone was shown to produce near-stoichiometric NiO, with hydroxide content decreasing as process temperature increases. Observed growth rates varied, but were in the range of 0.4 – 1.0 Å/cycle. A copper oxide (CuO) ALD process, based on Cu(dmap)₂ (Bis(dimethylamino-2-propoxy)copper(II)) and ozone, was also developed, and co-ALD of the two precursors was used to deposit Cu_xNi_1-xO. UV-ozone-treated ALD-NiO films were also fabricated. 20-30 nm films were analyzed in detail by X-ray photoelectron spectroscopy and X-ray diffraction. At low concentrations (on the order of 5-10%), the copper is incorporated into the polycrystalline NiO lattice as Cu²⁺, as shown in Figure 1a, while at high concentrations (30%) the CuO precipitates out, as shown in Figure 1b. Temperature-dependent electrical measurements (Figure 1c) showed that copper doping decreases the lateral resistivity, from 5.2⋅10⁴ Ohm-cm at room temperature in undoped NiO to 5.4⋅10³ Ohm-cm for Cu_0.1Ni_0.9O. Activation energy differences between doped (0.34 eV) and undoped (0.28 - 0.29 eV) samples imply that either the conduction mechanisms are different or that copper inclusion significantly reduces dipole screening in the ALD films (see Austin & Mott (2001), “Polarons in crystalline and non-crystalline materials,” Advances in Physics). Ultraviolet photoelectron spectroscopy (UPS) and optical spectroscopy revealed that copper doping increased the NiO:Cu work function by 170 meV while decreasing the optical bandgap and leaving the ionization energy unchanged. Ozone treatment of the NiO film reduced the resistivity to 2.4⋅10³ Ohm-cm. It also deepened the work function by 700 meV and the ionization energy by approximately 130 meV.

ALD-NiO deposited at 200°C with ozone as oxidizer was also tested as an electron-blocking layer in ITO/NiO/perovskite/PCBM/BCP solar cells. Compared to a similar device made with solution-deposited NiO, the ALD-NiO device showed an approximately 20 meV V_OC improvement, suggesting reduced recombination and/or better electron blocking. The fill-factor and short-circuit-current were both slightly smaller, as shown in Figure 1d, most likely in part because of the high vertical resistivity of the ALD-NiO film or because of surface layers inhibiting charge transfer or current flow. Future work will focus on incorporating ALD-NiO:Cu into PV designs, optimizing film processing conditions, and investigating activation and conduction mechanism in the ALD-NiO:Cu films themselves.

Fig. 1 Caption: a) Cu2p XPS Spectrum for a 25%-Cu-by-cycle ALD-NiO:Cu film, and reference CuO spectrum. The close fit indicates the copper is being incorporated as Cu²⁺. b) GIXRD data of NiO and NiO:Cu films. The NiO has a polycrystalline structure, with no CuO peaks visible at lower concentrations. At 30%-Cu NiO:Cu, the CuO precipitates out and CuO peaks are visible. c) Temperature-dependent conductivities for NiO, NiO:Cu, and UV-ozone-treated NiO, along with Arrhenius fits. Both UV-ozone treatment and copper doping increase the conductivity, but the differing activation energies point to some difference in transport mechanism or film dielectric environment. d) Experimental JV curves for ITO/NiO/CH₃NH₃PbI₃/PCBM/BCP/Ag solar cells, using both ALD-NiO and solution-deposited NiO.