Improved Heterojunction Quality in Cu₂O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited
Zn_1-xMg_xO

Here we present a protocol for synthesizing Zn_1-xMg_xO/Cu₂O heterojunctions in open-air at low temperature via atmospheric pressure spatial atomic layer deposition (AP-SALD) of Zn_1-xMg_xO on cuprous oxide. Such high quality conformal metal oxides can be grown on a variety of substrates including plastics by this cheap and scalable method.

The overall goal of this procedure is to obtain a high quality interface in zinc oxide cuprous oxide heterojunctions synthesized outside a vacuum. This is achieved by atmospheric pressure spatial atomic layer deposition or AP-SALD of zinc oxide films on thermally oxidized cuprous oxide. Atmospheric spatial ALD is an oxide printing technique that allows for deposition of conformal oxide films with precise thickness control in their own dual compatible process at atmospheric pressure and low temperature.

Our atmospheric pressure spatial ALD system is ideal for the rapid synthesis of high quality, uniform, crystalline, multi-component metal oxides for electronics, as demonstrated for zinc magnesium oxide in this work. First, cut a 0.127 millimeter thick copper foil into 13 by 13 millimeter squares and clean by sonicating in acetone. Dry the copper foil squares with an air gun to remove the residual acetone.

Then, place the dried substrates into an aluminum crucible and place the crucible in a furnace. Heat the copper foil squares to 1, 000 degrees Celsius with a continuous flow of argon. Monitor the gas ambient in the furnace with the gas analyzer throughout the oxidation.

When a temperature of 1, 000 degrees Celsius is reached, introduce oxygen to the furnace at a flow rate to obtain 10, 000 parts per million oxygen partial pressure and maintain for at least two hours. After two hours, turn off the oxygen flow. With the argon gas flowing, cool the furnace to 500 degrees Celsius.

Crunch the oxidized substrates by rapid withdrawal of the crucibles from the furnace. Then, dip them into deionized water to cool. Next, etch one side of the substrates by repeatedly applying a drop of dilute nitric acid to remove any cupric oxide from the surface.

Continue etching until no gray film is visible on the cupric oxide surface. Immediately after etching, rinse each substrate in deionized water and sonicate in isopropanol, then dry the substrates with an air gun. After depositing gold onto the etched side of the substrates, etch the other side of the substrates in dilute nitric acid by applying a drop of acid onto the surface, making sure not to etch the gold electrode on the other side.

After rinsing and drying the substrates, cover them with a black insulating paint using a paintbrush, leaving an unmasked area of approximately 0.1 centimeters squared as the active area of the solar cell. Cover the gold electrode on the back side with a marker. After setting up the AP-SALD reactor, adjust the bubbling rate through the diethyl zinc precursor to 6 milliliters per minute and 200 milliliters per minute through the magnesium precursor to deposit zinc magnesium oxide.

Next, set the flow rate of the nitrogen carrier gas for the metal precursor mixture to 100 milliliters per minute and bubble nitrogen through deionized water, which serves as the oxidant, diluted with nitrogen gas flowing at 200 milliliters per minute. Now, flow nitrogen at 500 milliliters per minute to the gas manifold. Keep the gas manifold at a temperature of 40 degrees Celsius via circulating water.

Then, heat the stage or moving platen to the desired temperature. Set the sample size, platen speed, and number of oscillations with the software controlling the platen. Deposit the desired oxide on a glass slide for 400 oscillations or until a clear, thick, homogeneous film can be seen.

Following deposition, place the subtrate on a glass mask and place it under the gas manifold. Adjust the head, or gas manifold height, to 50 micrometers above the substrate. Deposit the zinc magnesium oxide films by first opening the valve for the magnesium precursor bubbler, followed by the valve for the zinc precursor bubbler.

Then, start moving the platen under the gas manifold by clicking Start Deposition in the software. Open the water bubbler only after scanning the substrate with five oscillations of metal precursors in order to avoid cuprous oxide surface exposure to the oxidant while heated. When the deposition is finished, remove the substrates from the heated platen as quickly as possible and close the bubbler valves of the metal precursors.

Clean the gas channels in the manifold with a blade to remove any deposited oxide powder. It is important to minimize the time that the etched cuprous oxide substrates spend in open air on the heated platen, as the growth of cuprous oxide on the surface is accelerated with temperature. After sputtering indium tin oxide on the substrates, remove the marker from the gold electrode with acetone to expose the electrode.

Finally, apply electrical contacts by sticking two thin wires with silver paste onto the indium tin oxide and gold electrodes. The photothermal deflection spectra of etched and unetched cuprous oxide substrates show absorption above 1.4 electron volts before saturating at two electron volts, which can be attributed to the presence of cuprous oxide on the substrate surface. The unetched substrate has a higher absorption below two electron volts, suggesting a thicker cupric oxide layer on the surface.

The presence of cupric oxide outgrowths on cuprous oxide substrates was verified by energy dispersive X-ray spectroscopy. The SEM image of a standard photovoltaic cell surface shows cupric oxide outgrowths formed due to cuprous oxide exposure to air and oxidants. In contrast, the surface of the optimized device is free of cupric oxide outgrowths.

A six-fold increase in device efficiency was achieved through optimization of the zinc oxide deposition conditions. Devices with the optimized zinc magnesium oxide films yielded even higher efficiency of 2.2 percent. The external quantum efficiency spectra of the two devices differ below 475 nanometers, which is the range of wavelengths absorbed close to the interface.

The external quantum efficiency of the heterojunction made at higher temperature is less than half that of the lower temperature heterojunction, suggesting a lower quality interface due to more cupric oxide. Optimizing the conditions for zinc oxide growth by atmospheric ALD for thermally oxidized cuprous oxide allowed for an improvement in heterojunction interface quality and solar cell performance. The same optimization strategy can be applied to electrochemically deposited cuprous oxide solar cells.

We've atmospherically deposited crystalline zinc magnesium oxide on copper oxide to increase the open circuit voltage in heterojunction solar cells. This work reported the highest to date efficiency of 2.2 percent for cuprous oxide heterojunctions obtained outside a vacuum.