Name: improved heterojunction quality in cu2o-based solar cells through the optimization of atmospheric pressure spatial atomic layer deposited zn1-xmgxo
Uploaded: 2016-07-31T13:00:00.000+00:00
Duration: 8 min 14 s
Description: Watch this Scientific Journal Video about Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO at JoVE.co

The authors acknowledge the support of the Cambridge Overseas and Commonwealth Trust, the Rutherford Foundation of New Zealand, Girton College Cambridge. This work has been funded by ERC Advanced Investigator Grant, Novox, ERC-2009-adG247276 and by the EPSRC (under RGS3717).

1. Preparation of Cuprous Oxide Substrates
<ol>
	<li>Oxidation of copper foil
	<ol>
		<li>Cut 0.127 mm thick Cu foil into 13 mm x 13 mm squares and clean by sonicating in acetone.</li>
		<li>Heat up copper foil to 1,000 &#176;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 &#176;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.</li>
		<li>Cool down the furnace to 500 &#176;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.</li>
	</ol>
	</li>
	<li>Etching of Cu2O
	<ol>
		<li>Etch one side of the substrates by repeatedly applying a drop of dilute nitric acid (1:1 mixture of H2O and 70% HNO3) to remove any cupric oxide from the surface. Continue etching until no grey film is visible on the Cu2O surface. CAUTION: This procedure is performed in a fume hood.</li>
		<li>Immediately after etching, rinse each substrate in deionized water and sonicate in isopropanol. Dry with an air gun.</li>
		<li>Deposit 80 nm of gold onto the etched side of the Cu2O substrates by evaporating a 1 g gold pellet placed in a tungsten boat inside a resistance evaporator. Use base pressure 8 x 10-6 mbar and current of 4 A to reach the evaporation rate of 0.8 &#197;/sec.</li>
		<li>Etch the other side of the substrates in dilute nitric acid by applying a drop of acid onto the surface. Make sure the acid does not etch the golden film on the other side. Rinse and sonicate as described in section 1.2.2.</li>
		<li>Cover the substrates with a black insulating paint (use high temperature engine enamel) by using a paint brush, leaving an unmasked area of approximately 0.1 cm2 as the active area of the solar cell. Cover the golden electrode on the back side with a marker pen completely.</li>
	</ol>
	</li>
</ol>
2. Depositing Zn1-xMgxO Using AP-SALD Reactor
Note: Deposit Zn1-xMgxO films on the unmasked side of Cu2O 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.
<ol>
	<li>Set-up the AP-SALD system as follows:
	<ol>
		<li>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.</li>
		<li>For zinc oxide deposition, adjust the bubbling rate of nitrogen gas through the diethylzinc to 25 ml/min, which is contained at RT (20 &#176;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 &#176;C) to control to Zn to Mg ratio in the Zn1-xMgxO.</li>
		<li>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.</li>
		<li>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.</li>
	</ol>
	</li>
	<li>Keep the gas manifold at a temperature of 40 &#176;C via circulating water. Heat up the stage (moving platen) to the desired temperature (50 - 150 &#176;C).</li>
	<li>Set the desired sample-to-head distance, sample size, platen speed (50 mm/sec) and number of oscillations (cycles) with the software controlling the platen. The ZnO deposition rate is 1.1 nm/sec (or per cycle) and the Zn1-xMgxO deposition rate is approximately 0.54 nm/sec at 150 &#176;C. A typical number of deposition cycles is 200.</li>
	<li>Deposit the desired oxide on a glass slide for 400 oscillations or until a clear thick homogeneous film can be seen.</li>
	<li>Place the substrate on a glass mask if required, then place it under the gas manifold. Adjust the head (gas manifold) height to 50 &#181;m above the substrate.</li>
	<li>Deposit the Zn1-xMgxO films by first opening the valves for the Mg precursor bubbler, then the Zn precursor bubbler, then start moving the platen under the gas manifold by clicking &#34;start deposition&#34; in the software. Open the H2O bubbler only after scanning the substrate with 5 oscillations of metal precursors in order to avoid Cu2O surface exposure to the oxidant while heated.</li>
	<li>When the deposition is finished, remove the Cu2O 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. Start the next deposition cycle as described in 2.6.</li>
	<li>When finished, purge the system for 30 min before closing the nitrogen valves.</li>
</ol>
3. Sputtering of ITO
<ol>
	<li>Sputter 175 nm of indium tin oxide (ITO) by direct current magnetron sputtering20 at the following conditions: power 20 W, base pressure &#60;10-9 mbar, Ar pressure 2.5 Pa. At a sputtering rate of 35 nm/min, sputter ITO for 5 min for a 175 nm thick ITO film. The resulting ITO/ZnO/Cu2O heterojunction is shown in Figure 3.</li>
</ol>
4. Finishing of the Devices
<ol>
	<li>Clean the marker pen from the gold electrode with acetone to expose the golden electrode.</li>
	<li>Apply electrical contacts by sticking 2 thin wires with Ag paste onto the ITO and Au electrodes.</li>
</ol>

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The authors declare that they have no competing financial interests.

Critical steps within the protocol are stipulated by the Cu2O 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 Zn1-xMgxO deposition and finally, deposition of Zn1-xMgxO on Cu2O 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 Cu2O 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 Cu2O exposure to air and moisture, a six-fold increase in the conversion efficiency of ZnO/Cu2O devices made using AP-SALD was achieved. The improvement came from the understanding that Cu2O 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 Cu2O is avoided in vacuum based techniques3,18, for large-scale manufacturing, it is important that this problem can be minimized in atmospheric fabrication processes. In AP-SALD, the substrate surface can be exposed to reducing agents prior to the formation of the heterointerface, and by balancing the oxidation of Cu2O with the reduction of CuO by using forming gas during the deposition of the n-type oxide.25 The reducing agent used in AP-SALD could be a mixture of an inert gas with a reducing gas (e.g., N2 + 5% H225), or a number of cycles with a reducing precursor prior to the deposition, i.e., water-free cycles, in order to reduce CuO back to Cu2O just before the ZnO oxide starts to grow on its surface.
In this work, a standard protocol has been developed that minimizes CuO formation optimizing fabrication steps from Cu2O processing and etching to p-n junction formation by AP-SALD in open-air. The success of this work demonstrates the potential of AP-SALD as a promising method for application in cheap and scalable photovoltaic devices. The technique can be used for fast deposition of a variety of n- and p-type semiconducting metal oxides as well as blocking, buffer and barrier layers in solar cells on heat-sensitive substrates including plastics.

Cuprous oxide (Cu2O) is an earth-abundant non-toxic p-type semiconductor material1. 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, Cu2O is known to be paired with a variety n-type large band gap semiconductors such as ZnO2 and its doped variations3,4, Ga2O35,6 and TiO27 (For a more detailed overview on Cu2O photovoltaics see Ref.8). The development of Cu2O 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 Cu2O-based solar cells. The advancement of AP-SALD over conventional ALD is that in the former, precursors are separated in space rather than in time10. 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 (&#60;150 &#176;C) on a variety of substrates including plastics, which enables AP-SALD films to be applied to low-cost functional devices such as solar cells13, light emitting diodes14 and thin-film transistors15.
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 &#181;m 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 (Zn1-xMgxO16,17). Incorporating Mg into ZnO allows the conduction band to be tuned, which is important for reducing losses due to band-tail thermalization13 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 Cu2O based solar cells: recombination at the heterojunction interface due to an excessive formation of cupric oxide (CuO) on the Cu2O surface.

<table><tbody><tr><td>Copper foil</td><td>Avocado Research Chemicals LTD T/A Alfa Aesar</td><td>13380</td><td>0.127 mm thick, 99.9% (metals basis), annealed</td></tr><tr><td>Rapidox Oxygen analyzer</td><td>Rapidox</td><td>Model 2100</td><td></td></tr><tr><td>Alumina boat</td><td>Almath Crucibles LTD</td><td>6121203</td><td>Dimensions 20 mm x 50&amp;nbsp;mm x 5&amp;nbsp;mm</td></tr><tr><td>Gold pellets</td><td>KJLC</td><td>EVMAUXX40G</td><td>99.99% pure, 1/8&quot; x 1/8&quot;, sold by the gram</td></tr><tr><td>Diethylzinc</td><td>Aldrich</td><td>256781</td><td>&amp;ge;52 wt. % Zn basis</td></tr><tr><td>Bis(ethylcyclopentadienyl)magnesium</td><td>Strem Chemicals UK</td><td>12-0510</td><td>5 g</td></tr><tr><td>ITO target</td><td>GoodFellow Cambridge Limited</td><td>LS 427438</td><td>Indium Oxide/Tin Oxide target (In&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; 90 / SnO&lt;sub&gt;2&lt;/sub&gt; 10). Condition: Hot-pressed. Thickness: 2.0&amp;nbsp;mm &amp;#177; 0.5&amp;nbsp;mm. Size: 35.5&amp;nbsp;mm x 55.5&amp;nbsp;mm &amp;#177; 0.5&amp;nbsp;mm</td></tr><tr><td>VHT engine enamel paint</td><td>Halfords</td><td>325019</td><td>very high temperature engine enamel black paint</td></tr><tr><td>Nitric acid HNO&lt;sub&gt;3, &lt;/sub&gt;ACS reagent 70%&amp;nbsp;</td><td>Sigma-Aldrich Co Ltd</td><td>438073-2.5L</td><td>Harmful, irritant</td></tr><tr><td>2% Oxygen/Argon 200 bar</td><td>BOC Limited</td><td>225757-L</td><td>gas mixture for Cu foil oxidation, to be diluted with Ar</td></tr></tbody></table>

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.

Thermodynamically, CuO is the only stable phase of copper oxide in air at RT, as the Cu-O phase stability diagram reveals21-23. To verify the presence of CuO on the surface of Cu2O, absorption spectra of the etched and unetched thermally oxidized Cu2O substrates were taken with photothermal deflection spectroscopy (PDS) &#8212; a highly sensitive technique which allows for sub-band gap absorption measurement24 (Figure 4). Both spectra showed absorption above 1.4 eV, which coincides with the band gap of CuO, before saturating at 2 eV (Cu2O band gap). The unetched substrate had a higher absorption below 2 eV, suggesting a thicker layer of CuO on the surface of unetched Cu2O than on the etched substrate. The inset in Figure 4 shows a grey layer of CuO on the as-oxidized (unetched) Cu2O 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 Cu2O substrates was also confirmed with x-ray photoelectron spectroscopy (XPS)19,25. Cupric oxide present on the Cu2O surface introduces deep level trap states (Cu2+)18 at the heterojunction interface that can act as recombination centers and, therefore, CuO presence at the p-n junction is undesirable.
Heating Cu2O substrates in the presence of oxidants (e.g., air and moisture) facilitates the oxidation of Cu2O to CuO. In order to obtain polycrystalline ZnO by AP-SALD, the substrates are heated to 150 &#176;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 Cu2O surface. Figure 5 shows scanning electron microscopy (SEM) images of an etched Cu2O substrate before and after spending 3 min on the AP-SALD platen at 150 &#176;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 &#176;C for 400 sec on top of the etched thermally oxidized Cu2O 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 Cu2O exposure to air and oxidants. Table 1 and Figure 7 (&#39;ZnO/Cu2O standard&#39; curve) demonstrate the relatively poor performance of this device.
In order to avoid CuO formation on the Cu2O surface, the conditions for depositing ZnO by AP-SALD on the etched thermally oxidized Cu2O 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 Cu2O substrates in air just before the start of deposition. The optimal parameters of ZnO deposition on Cu2O were found to be 100 &#176;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&#8211;voltage (J-V) characteristic of the optimized ZnO/Cu2O device is compared with the standard device in Figure 7. The photovoltaic performance of both standard and optimized ZnO/Cu2O 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 &#176;C and 100 &#176;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/Cu2O 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 further15. Zn1-xMgxO/Cu2O solar cells were made with the optimized Zn0.8Mg0.2O films, resulting in 2.2% device PCE &#8212; the highest to date for Cu2O-based solar cells with open-air fabricated heterojunctions (see the device performance in Figure 7 and Table 1).
<img alt="Figure 1" src="/files/ftp_upload/53501/53501fig1.jpg" /> 
Figure 1. Cu2O-based solar cell efficiency by year of publication (This figure has been modified from Ref.8). Markers indicate whether the interface was formed in vacuum or in atmosphere (non-vacuum) and labels indicate the method of heterojunction formation. MSP - magnetron sputtering, IBS - ion beam sputtering, VAPE - vacuum arc plasma evaporation. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53501/53501fig2.jpg" /> 
Figure 2. Schematic of AP-SALD deposition process (compared with conventional ALD) and set-up for producing multicomponent metal oxides. (A) Sequential exposure of each precursor and purge step in conventional ALD (delta-doping) (This figure has been reproduced from Ref. 11). 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 &#39;purge&#39; 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 Zn1-xMgxO deposition (This figure has been reproduced from Ref. 13). <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53501/53501fig3.jpg" /> 
Figure 3. Cross-sectional SEM image of ITO/ZnO/Cu2O heterojunction (This figure has been reproduced from Ref.8). Conformal coating of Cu2O substrate with ZnO and ITO films can be observed. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53501/53501fig4.jpg" /> 
Figure 4. PDS spectra of etched and unetched (as-oxidized) Cu2O substrates (This figure has been modified from Ref.8). The insets show photographs of the etched and unetched cuprous oxide substrates. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/53501/53501fig5.jpg" /> 
Figure 5. SEM images of the surface of a Cu2O substrate when (A) freshly etched and (B) after annealing at 150 &#176;C in air for 3 min&#160;(This figure has been reproduced from Ref.8). Insets show surface composition acquired with EDX. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/53501/53501fig6.jpg" /> 
Figure 6. SEM images of the surface of ZnO/Cu2O solar cells made using (A) standard conditions and (B) optimized conditions of AP-SALD ZnO (This figure has been reproduced from Ref.8). Various outgrowths can be seen in the standard device. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/53501/53501fig7.jpg" /> 
Figure 7. Light J-V characteristics for Zn1-xMgxO/Cu2O solar cells fabricated at standard and optimized AP-SALD conditions (This figure has been modified from Ref.8). The J-V curves demonstrate solar cell performance improvement when the composition and AP-SALD conditions of the Zn1-xMgxO films are optimized. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" src="/files/ftp_upload/53501/53501fig8.jpg" /> 
Figure 8. The effect of AP-SALD parameters on the performance of ZnO/Cu2O solar cells. (A) and (B) The effect of AP-SALD ZnO deposition time and temperature on the open-circuit voltage (Voc) of the devices (This figure has been reproduced from Ref.8), (C) correlation of water-free cycles with the Voc of the devices. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 9" src="/files/ftp_upload/53501/53501fig9.jpg" /> 
Figure 9. EQE spectra of ZnO/Cu2O solar cells with ZnO deposited at 100 &#176;C and 150 &#176;C. (This figure has been reproduced from Ref.8). Open-circuit voltage of the devices is indicated in the legend. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53501/53501fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table fo:keep-together.within-page="1">
	<tbody>
		<tr>
			<td>Solar cell</td>
			<td>Deposition temperature, &#176;C</td>
			<td>Deposition time, sec</td>
			<td>Jsc, mA/cm2</td>
			<td>Voc, V</td>
			<td>FF, %</td>
			<td>PCE, %</td>
		</tr>
		<tr>
			<td>ZnO/Cu2O Standard</td>
			<td>150</td>
			<td>400</td>
			<td>3.7</td>
			<td>0.18</td>
			<td>35</td>
			<td>0.23</td>
		</tr>
		<tr>
			<td>ZnO/Cu2O Optimized</td>
			<td>100</td>
			<td>100</td>
			<td>7.5</td>
			<td>0.49</td>
			<td>40</td>
			<td>1.46</td>
		</tr>
		<tr>
			<td>Zn0.8Mg0.2/Cu2O Optimized</td>
			<td>150</td>
			<td>100</td>
			<td>6.9</td>
			<td>0.65</td>
			<td>49</td>
			<td>2.20</td>
		</tr>
	</tbody>
</table>
Table 1. Standard and optimized AP-SALD Zn1-xMgxO deposition parameters and performance of the best corresponding ITO/Zn1-xMgxO/Cu2O solar cells (This table has been modified from Ref.8). JSC- short circuit current density, FF - fill factor.

Watch this Scientific Journal Video about Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO at JoVE.co

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

Here we present a protocol for synthesizing Zn1-xMgxO/Cu2O heterojunctions in open-air at low temperature via atmospheric pressure spatial atomic layer deposition (AP-SALD) of Zn1-xMgxO 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.

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 ...

improved-heterojunction-quality-cu2o-based-solar-cells-through

Nanyang Technological University

Research

JoVE Journal

Chemistry

12.1K Views.  University of Cambridge.  Hier presenteren we een protocol voor het synthetiseren Zn 1-x Mg x O / Cu 2 O heterojunctions in de open lucht bij lage temperatuur via luchtdruk ruimtelijke atomic layer deposition (AP-SALD) van Zn 1-x Mg x O op koperoxide. Dergelijke hoge kwaliteit conforme metaaloxiden kan worden geteeld op een verscheidenheid van substraten, waaronder kunststoffen door deze goedkope en schaalbaar methode. 

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

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Engineering

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Synthesis and Characterization of High c-axis ZnO Thin Film by Plasma Enhanced Chemical Vapor Deposition System and its UV Photodetector Application

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto silicon (Si) substrates via different synthesized temperatures by using plasma enhanced chemical vapor deposition (PECVD) system. The effects of different synthesized temperatures on the crystal structure, surface morphologies and optical properties have been investigated. The X-ray diffraction (XRD) patterns indicated that the intensity of (0002) diffraction peak became stronger with increasing synthesized temperature until 400 oC. The diffraction intensity of (0002) peak gradually became weaker accompanying with appearance of (10-10) diffraction peak as the synthesized temperature up to excess of 400 oC. The RT photoluminescence (PL) spectra exhibited a strong near-band-edge (NBE) emission observed at around 375 nm and a negligible deep-level (DL) emission located at around 575 nm under high c-axis ZnO thin films. Field emission scanning electron microscopy (FE-SEM) images revealed the homogeneous surface and with small grain size distribution. The ZnO thin films have also been synthesized onto glass substrates under the same parameters for measuring the transmittance.
For the purpose of ultraviolet (UV) photodetector application, the interdigitated platinum (Pt) thin film (thickness ~100 nm) fabricated via conventional optical lithography process and radio frequency (RF) magnetron sputtering. In order to reach Ohmic contact, the device was annealed in argon circumstances at 450 oC by rapid thermal annealing (RTA) system for 10 min. After the systematic measurements, the current-voltage (I-V) curve of photo and dark current and time-dependent photocurrent response results exhibited a good responsivity and reliability, indicating that the high c-axis ZnO thin film is a suitable sensing layer for UV photodetector application.

We offered a method to directly synthesize high c-axis (0002) ZnO thin film by plasma enhanced chemical vapor deposition. The as-synthesized ZnO thin film combined with Pt interdigitated electrode was used as sensing layer for ultraviolet photodetector, showing a high performance through a combination of its good responsivity and reliability.

In this study, zinc oxide (ZnO) thin films with high c-axis (0002) preferential orientation have been successfully and effectively synthesized onto ...

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, we present a protocol for the fabrication of this material via the low-pressure vapor assisted solution process (LP-VASP) method, which yields ~19% power conversion efficiency in planar heterojunction perovskite solar cells. First, we report the synthesis of methylammonium iodide (CH3NH3I) and methylammonium bromide (CH3NH3Br) from methylamine and the corresponding halide acid (HI or HBr). Then, we describe the fabrication of pinhole-free, continuous methylammonium-lead halide perovskite (CH3NH3PbX3 with X = I, Br, Cl and their mixture) films with the LP-VASP. This process is based on two steps: i) spin-coating of a homogenous layer of lead halide precursor onto a substrate, and ii) conversion of this layer to CH3NH3PbI3-xBrx by exposing the substrate to vapors of a mixture of CH3NH3I and CH3NH3Br at reduced pressure and 120 &#176;C. Through slow diffusion of the methylammonium halide vapor into the lead halide precursor, we achieve slow and controlled growth of a continuous, pinhole-free perovskite film. The LP-VASP allows synthetic access to the full halide composition space in CH3NH3PbI3-xBrx with 0&#160;&#8804; x&#160;&#8804; 3. Depending on the composition of the vapor phase, the bandgap can be tuned between 1.6 eV&#160;&#8804; Eg&#160;&#8804; 2.3 eV. In addition, by varying the composition of the halide precursor and of the vapor phase, we can also obtain CH3NH3PbI3-xClx. Films obtained from the LP-VASP are reproducible, phase pure as confirmed by X-ray diffraction measurements, and show high photoluminescence quantum yield. The process does not require the use of a glovebox.

Here, we present a protocol for the synthesis of CH3NH3I and CH3NH3Br precursors and the subsequent formation of pinhole-free, continuous CH3NH3PbI3-xBrx thin films for the application in high efficiency solar cells and other optoelectronic devices.

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule ...

Fabrication of Schottky Diodes on Zn-polar BeMgZnO/ZnO Heterostructure Grown by Plasma-assisted Molecular Beam Epitaxy

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device applications. Zinc oxide (ZnO), a semiconductor with a wide bandgap (3.4 eV) and high electron saturation velocity has gained a great deal of attention as an attractive material for high speed devices. Efficient gate modulation, however, requires high-quality Schottky contacts on the barrier layer. In this article, we present our Schottky diode fabrication procedure on Zn-polar BeMgZnO/ZnO heterostructure with high density 2DEG which is achieved through strain modulation and incorporation of a few percent Be into the MgZnO-based barrier during growth by molecular beam epitaxy (MBE). To achieve high crystalline quality, nearly lattice-matched high-resistivity GaN templates grown by metal-organic chemical vapor deposition (MOCVD) are used as the substrate for the subsequent MBE growth of the oxide layers. To obtain the requisite Zn-polarity, careful surface treatment of GaN templates and control over the VI/II ratio during the growth of low temperature ZnO nucleation layer are utilized. Ti/Au electrodes serve as Ohmic contacts, and Ag electrodes deposited on the O2 plasma pretreated BeMgZnO surface are used for Schottky contacts.

Attainment of high-quality Schottky contacts is imperative for achieving efficient gate modulation in heterostructure field effect transistors (HFETs). We present the fabrication methodology and characteristics of Schottky diodes on Zn-polar BeMgZnO/ZnO heterostructures with high-density two dimensional electron gas (2DEG), grown by plasma-assisted molecular beam epitaxy on GaN templates.

Heterostructure field effect transistors (HFETs) utilizing a two dimensional electron gas (2DEG) channel have a great potential for high speed device ...

Developing High Performance GaP/Si Heterojunction Solar Cells

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar cells. In this work, we present high performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime and high crystal quality of epitaxial GaP layers. It is shown that by applying phosphorus (P)-diffusion layers into the Si substrate and a SiNx layer, the Si minority-carrier lifetime can be well-maintained during the GaP growth in the molecular beam epitaxy (MBE). By controlling the growth conditions, the high crystal quality of GaP was grown on the P-rich Si surface. The film quality is characterized by atomic force microscopy and high-resolution x-ray diffraction. In addition, MoOx was implemented as a hole-selective contact that led to a significant increase in the short-circuit current density. The achieved high device performance of the GaP/Si heterojunction solar cells establishes a path for further enhancement of the performance of Si-based photovoltaic devices.

Here, we present a protocol to develop high-performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime.

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar ...

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se2 Thin-film Solar Cells

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells

In this protocol, we describe the detailed experimental procedure for the fabrication of a robust nanoscale contact between a silver nanowire network and CdS buffer layer in a CIGS thin-film solar cell.

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes ...

Close-Space Sublimation-Deposited Ultra-Thin CdSeTe/CdTe Solar Cells for Enhanced Short-Circuit Current Density and Photoluminescence

Developments in photovoltaic device architectures are necessary to make solar energy a cost-effective and reliable source of renewable energy amidst growing global energy demands and climate change. Thin film CdTe technology has demonstrated cost-competitiveness and increasing efficiencies due partially to rapid fabrication times, minimal material usage, and introduction of a CdSeTe alloy into a ~3 &#956;m absorber layer. This work presents the close-space sublimation fabrication of thin, 1.5 &#181;m CdSeTe/CdTe bilayer devices using an automated in-line vacuum deposition system. The thin bilayer structure and fabrication technique minimize deposition time, increase device efficiency, and facilitate future thin absorber-based device architecture development. Three fabrication parameters appear to be the most impactful for optimizing thin CdSeTe/CdTe absorber devices: substrate preheat temperature, CdSeTe:CdTe thickness ratio, and CdCl2 passivation. For proper sublimation of the CdSeTe, the substrate temperature prior to deposition must be ~540 &#176;C (higher than that for CdTe) as controlled by dwell time in a preheat source. Variation in the CdSeTe:CdTe thickness ratio reveals a strong dependence of device performance on this ratio. The optimal absorber thicknesses are 0.5 &#956;m CdSeTe/1.0 &#956;m CdTe, and non-optimized thickness ratios reduce efficiency through back-barrier effects. Thin absorbers are sensitive to CdCl2 passivation variation; a much less aggressive CdCl2 treatment (compared to thicker absorbers) regarding both temperature and time yields optimal device performance. With optimized fabrication conditions, CdSeTe/CdTe increases device short-circuit current density and photoluminescence intensity compared to single-absorber CdTe. Additionally, an in-line close-space sublimation vacuum deposition system offers material and time reduction, scalability, and attainability of future ultra-thin absorber architectures.

This work describes the complete fabrication process of thin absorber cadmium selenium telluride/cadmium telluride photovoltaic devices for enhanced efficiency. The process utilizes an automated in-line vacuum system for close-space sublimation deposition that is scalable, from fabrication of small area research devices as well as large-scale modules.

Developments in photovoltaic device architectures are necessary to make solar energy a cost-effective and reliable source of renewable energy amidst ...

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

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