Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.


Introduction
Cuprous oxide (Cu 2 O) is an earth-abundant non-toxic p-type semiconductor material 1 . With a band gap of 2 eV, cuprous oxide can fulfill the role of light absorber in heterojunction or tandem solar cells. In heterojunction solar cells, Cu 2 O is known to be paired with a variety n-type large band gap semiconductors such as ZnO 2 and its doped variations 3,4 , Ga 2 O 3 5,6 and TiO 2 7 (For a more detailed overview on Cu 2 O photovoltaics see Ref. 8 ). The development of Cu 2 O based heterojunction solar cells is presented in Figure 1, where the method of synthesizing the heterojunction is indicated next to each data point. One can note that vacuum based methods such as pulsed laser deposition (PLD) or atomic layer deposition (ALD) allowed for higher power conversion efficiencies to be achieved (up to 6.1% 9 ). In contrast, the efficiencies for non-vacuum synthesis methods such as electrochemical deposition (ECD) have remained low. However, for low-cost photovoltaics it is better to synthesize the heterojunction outside a vacuum. While a vacuum-free, scalable technique of heterojunction formation is a more suitable alternative, it remains challenging to produce an interface of high quality by such methods. In this work we utilize an open-air, scalable thin film deposition process called atmospheric pressure spatial atomic layer deposition (AP-SALD) to grow n-type oxides for Cu 2 O-based solar cells. The advancement of AP-SALD over conventional ALD is that in the former, precursors are separated in space rather than in time 10 . During the deposition process, a substrate oscillates back and forth on a heated platen under a gas manifold which contains precursor gas channels separated by inert gas channels, as shown in Figure 2. The nitrogen gas carrying the precursors flows vertically through the gas manifold down towards the laterally moving platen. Due to the oscillations of the platen, each point on the substrate is sequentially exposed to the oxidant and metal precursors, as illustrated in Figure 2. This allows the metal oxide film to grow layer by layer. A detailed description of AP-SALD reactor design and operation can be found elsewhere. 11,12 This approach allows the deposition to occur one to two orders of magnitude faster than conventional ALD and outside vacuum, which is compatible with roll-to-roll processing. High quality conformal oxide films produced by AP-SALD can be deposited at low temperatures (<150 °C) on a variety of substrates including plastics, which enables AP-SALD films to be applied to low-cost functional devices such as solar cells 13 , light emitting diodes 14 and thin-film transistors 15 .
The custom made AP-SALD gas manifold used in this work was mechanically maintained over the substrate placed on the platen. This allowed control of the substrate-manifold spacing independent of the gas flow rates. A large spacing of 50 µm was used, which resulted in intermixing between the metal precursor and oxidant in the gas phase. Therefore, the AP-SALD reactor was operated in chemical vapor deposition (CVD) mode. This was found to be advantageous over operating in ALD mode because the films were grown at a higher rate, but still with high thickness uniformity and were crystalline when deposited at the same temperatures as ALD films. 12 Herein, we still refer to the reactor as an AP-SALD reactor because it has the same fundamental design principles as other AP-SALD reactors. 11 We used our reactor to deposit the n-type layer for our solar cells, in particular zinc oxide and zinc magnesium oxide (Zn 1-x Mg x O 16,17 ). Incorporating Mg into ZnO allows the conduction band to be tuned, which is important for reducing losses due to band-tail thermalization 13 and interfacial recombination. 18,19 Here we show how tuning the conditions for depositing zinc oxide and zinc magnesium oxide films on thermally oxidized cuprous oxide substrates allowed for improved interface quality and hence better solar cell performance to be obtained. This improvement was made possible through the identification of the major limiting factor in Cu 2 O based solar cells: recombination at the heterojunction interface due to an excessive formation of cupric oxide (CuO) on the Cu 2 O surface.

Preparation of Cuprous Oxide Substrates
1. Oxidation of copper foil 1. Cut 0.127 mm thick Cu foil into 13 mm x 13 mm squares and clean by sonicating in acetone. 2. Heat up copper foil to 1,000 °C while continuously flowing Ar gas through the furnace. Monitor the gas ambient in the furnace with a gas analyzer throughout the oxidation. When the temperature of 1,000 °C is reached, introduce oxygen to the furnace at a flow rate to obtain 10,000 ppm oxygen partial pressure and keep that for at least 2 hr. After 2 hr, turn off the oxygen but keep the Ar gas flowing. 3. Cool down the furnace to 500 °C (keep the Ar gas flowing). Quench the oxidized samples by rapid withdrawal of the crucibles from the furnace. Dip the substrates into deionized water to cool them faster.

Depositing Zn 1-x Mg x O Using AP-SALD Reactor
Note: Deposit Zn 1-x Mg x O films on the unmasked side of Cu 2 O substrates. 13 In this work, a custom-made AP-SALD reactor was used, adapted from the original design developed by Kodak. 11,12 Details of the reactor customization are given in Ref. 12.
1. Set-up the AP-SALD system as follows: 1. Use diethylzinc (DEZ) as the Zn precursor and bis(ethylcyclopentadienyl)magnesium as the Mg precursor. These are liquid precursors each contained in their separate glass bubblers. The precursors are pyrophoric and should never come in contact with air or water. The deposition system is gas-tight. 2. For zinc oxide deposition, adjust the bubbling rate of nitrogen gas through the diethylzinc to 25 ml/min, which is contained at RT (20°C ). For zinc magnesium oxide deposition, adjust the gas fraction of each precursor by setting the bubbling rate through the diethylzinc to 6 ml/min and 200 ml/min through the bis(ethylcyclopentadienyl)magnesium (which is heated to 55 °C) to control to Zn to Mg ratio in the Zn 1-x Mg x O. 3. Set the flow rate of the nitrogen carrier gas for the metal precursor mixture to 100 ml/min. Bubble nitrogen gas at 100 ml/min through deionized water, which is employed as the oxidant. This vapor is diluted with nitrogen carrier gas flowing at 200 ml/min. 4. Flow nitrogen gas at 500 ml/min to the gas manifold. In the AP-SALD gas manifold, this nitrogen gas is split to four separate channels.
Each channel serves to spatially separate the two oxidant channels from the metal precursor mix channel between them.

Representative Results
Thermodynamically, CuO is the only stable phase of copper oxide in air at RT, as the Cu-O phase stability diagram reveals [21][22][23] . To verify the presence of CuO on the surface of Cu 2 O, absorption spectra of the etched and unetched thermally oxidized Cu 2 O substrates were taken with photothermal deflection spectroscopy (PDS) -a highly sensitive technique which allows for sub-band gap absorption measurement 24 ( Figure   4). Both spectra showed absorption above 1.4 eV, which coincides with the band gap of CuO, before saturating at 2 eV (Cu 2 O band gap). The unetched substrate had a higher absorption below 2 eV, suggesting a thicker layer of CuO on the surface of unetched Cu 2 O than on the etched substrate. The inset in Figure 4 shows a grey layer of CuO on the as-oxidized (unetched) Cu 2 O substrate. While no grey film could be detected visually on the etched substrate, some CuO was still present on its surface, as the PDS measurements suggests. The presence of a very thin CuO film on the surface of Cu 2 O substrates was also confirmed with x-ray photoelectron spectroscopy (XPS) 19,25 . Cupric oxide present on the Cu 2 O surface introduces deep level trap states (Cu 2+ ) 18 at the heterojunction interface that can act as recombination centers and, therefore, CuO presence at the p-n junction is undesirable.
Heating Cu 2 O substrates in the presence of oxidants (e.g., air and moisture) facilitates the oxidation of Cu 2 O to CuO. In order to obtain polycrystalline ZnO by AP-SALD, the substrates are heated to 150 °C. As the substrate is held at elevated temperature in open-air or under the oxidant gas during the deposition, CuO quickly forms on the Cu 2 O surface. Figure 5 shows scanning electron microscopy (SEM) images of an etched Cu 2 O substrate before and after spending 3 min on the AP-SALD platen at 150 °C under the flow of nitrogen. Multiple CuO outgrowths can be seen on the annealed substrate, with their composition being close to that of CuO as verified by energy-dispersive X-ray spectroscopy (EDX).
Photovoltaic devices were made with ZnO deposited by AP-SALD at 150 °C for 400 sec on top of the etched thermally oxidized Cu 2 O substrates. Figure 6A shows the surface of this standard device. One can notice numerous rod-and flower-like outgrowths present in the device. As confirmed earlier with EDX and PDS, these outgrowths are cupric oxide and occur due to Cu 2 O exposure to air and oxidants. Table 1 and Figure  7 ('ZnO/Cu 2 O standard' curve) demonstrate the relatively poor performance of this device.
In order to avoid CuO formation on the Cu 2 O surface, the conditions for depositing ZnO by AP-SALD on the etched thermally oxidized Cu 2 O substrates were optimized. The following measures were taken in order to minimize CuO growth: reduction of deposition temperature ( Figure  8A); reduction of deposition time ( Figure 8B); scanning the substrate surface for a few oscillations without exposure to the oxidant gas, i.e., with only metal precursors and inert channels open ( Figure 8C); and finally, avoidance of unnecessary heating of naked Cu 2 O substrates in air just before the start of deposition. The optimal parameters of ZnO deposition on Cu 2 O were found to be 100 °C, 100 sec and 5 water-free cycles. The surface of the optimized device was free of CuO outgrowths, as is demonstrated in Figure 6B. The current density-voltage (J-V) characteristic of the optimized ZnO/Cu 2 O device is compared with the standard device in Figure 7. The photovoltaic performance of both standard and optimized ZnO/Cu 2 O devices is presented in Table 1. It can be seen that by following the four above-mentioned measures, a six-fold increase in power conversion efficiency of the devices was achieved.
To further elucidate the effect of optimization of AP-SALD conditions on the reduction of CuO and the heterojunction quality, external quantum efficiency (EQE) measurements were performed on devices with ZnO deposited at 150 °C and 100 °C (Figure 9). The EQE spectra of the two devices, while similar at wavelengths above 475 nm, differed significantly at wavelengths below 475 nm, which is the range of wavelengths absorbed close to the interface. For the shorter wavelength radiation, the EQE of the device with ZnO made at higher temperature was less than half that of the device with ZnO made at lower temperature. This suggests that more cupric oxide was present at the ZnO/Cu 2 O interface made at higher temperature, which reduced charge collection from the region close to the heterointerface due to increased recombination.
Mg was incorporated into AP-SALD ZnO films in order to raise the conduction band of ZnO and to reduce recombination further 15 .  ). In the context of this manuscript, M1 is diethylzinc vapor, M2 bis(ethylcyclopentadienyl)magnesium vapor, and O1 and O2 water vapor. (B) Sequential exposure of metal precursor mixture (co-injection), inert gas channels (equivalent to 'purge' step) and oxidant in AP-SALD (This figure has been reproduced from Ref. 11 ). (C) Schematic of a general AP-SALD reactor, showing the precursors spatially separated by inert gas channels, with the substrate oscillated beneath the different channels (This figure has been reproduced from Ref. 11 , which is a modification from one in Ref. 26 ). (D) Overview schematic of the important components of an AP-SALD system with atomic force microscopy (AFM) images showing the morphology of the substrate before and after Zn 1-x Mg x O deposition (This figure has been reproduced from Ref. 13 ). Please click here to view a larger version of this figure.       Open-circuit voltage of the devices is indicated in the legend. Please click here to view a larger version of this figure. ). J SC -short circuit current density, FF -fill factor.

Discussion
Critical steps within the protocol are stipulated by the Cu 2 O to CuO substrate surface oxidation. These include etching of the substrates in dilute nitric acid to remove any CuO after oxidation as well as after the evaporation of the golden electrode, minimizing the time substrates spend in open air before the Zn 1-x Mg x O deposition and finally, deposition of Zn 1-x Mg x O on Cu 2 O substrates by AP-SALD.
The advantage of AP-SALD compared to conventional ALD is that films can be grown outside a vacuum with a growth rate that is one to two orders of magnitude higher. However, this implies that the Cu 2 O substrates have to be exposed to oxidants in air at elevated temperature at least just before the deposition, which is sufficient for a thin CuO layer to form on the surface. This seemingly limits the application of the AP-SALD method to some oxidation-sensitive materials. However, by optimizing AP-SALD conditions such as temperature and time, as well as minimizing Cu 2 O exposure to air and moisture, a six-fold increase in the conversion efficiency of ZnO/Cu 2 O devices made using AP-SALD was achieved. The improvement came from the understanding that Cu 2 O to CuO oxidation is the major limiting factor of copper oxide as a material in heterojunction solar cells and modifying the fabrication protocol accordingly.
In order to completely avoid oxidation of cuprous oxide, the substrates have to be kept in an inert atmosphere or in vacuum all the time, which can be challenging when employing an open-air deposition technique such as AP-SALD. While the oxidation of Cu 2 O is avoided in vacuum based techniques 3,18