Effect of Junction Morphology on the Performance of Polycrystalline

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Effect of Junction Morphology on the Performance of Polycrystalline Cu2O Homojunction Solar Cells Colleen M. McShane, Withana P. Siripala,‡ and Kyoung-Shin Choi* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907

ABSTRACT Cu2O p-n homojunction solar cells were fabricated by the consecutive electrochemical deposition of p-Cu2O layer, followed by n-Cu2O layer. The surface morphology of p-type Cu2O, which determines the p-n junction interface, was modified to investigate its effect on the performance of the homojunction solar cell. The results showed that the junction quality and the cell efficiency varied significantly depending on the crystals faces exposed at the p-n junction, although the resistivity of the p- and n-layers remained comparable. The best performance of the homojunction cell fabricated in this study was VOC = 0.423 V, ISC = 2.5 mA/ cm2, fill factor (ff) = 27%, and η = 0.29%. The main limiting factor for the cell efficiency was the high resistivity of both p- and n-layers. Doping studies and finetuning of the junction morphology will be necessary to improve the performance of the Cu2O homojunction solar cells further. SECTION Energy Conversion and Storage

uprous oxide (Cu2O) is an environmentally benign, abundant, low-cost, direct band gap semiconductor (Eg = 1.9 to 2.2 eV) that has been investigated for use in photoelectrochemical and photovoltaic devices.1-7 As Cu2O is typically produced p-type because of the presence of copper vacancies in the lattice,1 previous Cu2O photovoltaic devices were constructed using p-Cu2O/metal Schottky junctions or p-n heterojunctions formed between the p-Cu2O layer and an n-type semiconductor layer (e.g., n-CdO and n-ZnO).2,6-12 However, the combination of p- and n-type Cu2O layers to form a Cu2O homojunction is thought to be a likely way to achieve its theoretical efficiency of ca. 12%.3 Despite this prediction, construction of p-n Cu2O homojunction solar cells has been very limited to date because of the lack of methods producing n-type Cu2O. There have been only two Cu2O homojunctions reported to date.13,14 The most recent reports an efficiency (η) of 0.103%, a short-circuit photocurrent (ISC) of 1.228 mA/cm2, an open-circuit voltage (VOC) of 0.321 V, and a fill factor (ff) of 35.33%.14 Recently, we have reported a new electrochemical deposition condition to produce n-type Cu2O electrodes using slightly acidic aqueous media buffered with acetate (pH 4.9).15,16 In this study, we explored the possibility of electrochemically fabricating a p-n Cu2O homojunction solar cell by electrodepositing n-Cu2O layers using the new condition on top of a p-type Cu2O layer deposited from a conventional alkaline copper lactate solution.17-20 In this case, because n-type Cu2O is deposited on p-type Cu2O, the surface morphology of p-type Cu2O determines the p-n junction morphology. Therefore, we modified the composition and pH of the copper lactate solution to alter the morphology of the p-Cu2O layer and investigated its effect on the efficiency of the homojunction solar cell.

C

r 2010 American Chemical Society

The p-type layer that resulted in the highest efficiency of the homojunction solar cell was cathodically deposited on an indium-doped tin oxide substrate (ITO) using a solution containing 0.02 M CuSO4 and 0.4 M lactic acid that was adjusted to pH 11 (Solution I). The resulting p-Cu2O electrode shows a good coverage of the substrate with Cu2O crystals that uniformly have cubic surface morphology exposing only {100} planes at the interface (Figure 1a). (All apexes observed in the SEM image possess a three-fold rotational axis created by three symmetrically equivalent {100} planes, which is indicative of a cubic morphology.) When a more concentrated Cu2þ solution (0.4 M CuSO4 and 3 M lactic acid, pH 11: Solution II), which is more commonly used for p-Cu2O deposition,17-20 is used, the surface morphology of the Cu2O crystals was changed to octahedral or cuboctahedral, and both {100} and {111} planes were observed at the interface (Figure 1b). (The apexes shown in the SEM image possess either a four-fold rotational axis created by four {111} planes from an octahedral morphology or a two-fold rotational axis created by two {100} planes and two {111} planes from a cuboctahedral morphology.) This is because the increase in the CuSO4 concentration resulted in more sulfate ions available in the plating solution, which are known to selectively stabilized the {111} planes of Cu2O.21 Depositions using Solutions I and II under more alkaline conditions (pH g 12) resulted in lower nucleation density and therefore the growth of larger crystals (Figure 1c,d). In this case, complete coverage of the ITO substrate with Cu2O crystals became very difficult, and the resulting p-Cu2O electrodes could not be used to form a good p-n junction. Received Date: July 20, 2010 Accepted Date: August 20, 2010 Published on Web Date: August 26, 2010

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DOI: 10.1021/jz100991e |J. Phys. Chem. Lett. 2010, 1, 2666–2670

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Figure 2. SEM images showing the top view of n-Cu2O layers deposited (a) on p-Cu2O(I) layer and (b) on p-Cu2O(II) layer. (c) SEM image and (d) I-V curve of n-Cu2O layer in the dark. An ITO substrate was used as back contact, and sputter-coated AuPd dots were used as front contacts.

Figure 1. SEM images of p-Cu2O electrodes deposited from (a) Solution I and (b) Solution II (pH 11). (c,d) p-Cu2O crystals deposited from Solutions I and II, respectively, when the pH was elevated to 13. I-V curves of (e) p-Cu2O(I) layer and (f) p-Cu2O(II) layer in the dark. For both layers, ITO substrates were used as back contacts, and sputter-coated AuPd dots were used as front contacts.

Dark I-V measurements for p-Cu2O layers deposited from Solutions I and II at pH 11 (hereafter referred to as p-Cu2O(I) and p-Cu2O(II), respectively) are shown in Figure 1e,f. Because the I-V curves do not show ideal Ohmic behavior, accurate resistivity values could not be obtained. However, using the slope of the linear portion, these layers were estimated to have comparable resistivities on the order of 105 to 106 Ω 3 cm. On top of p-Cu2O(I) and p-Cu2O(II) layers, n-Cu2O layers were deposited at 0.02 V versus Ag/AgCl using a solution containing 0.02 M copper acetate and 0.08 M acetic acid with pH adjusted to 4.9 (Figure 2 a,b). Deposition of n-Cu2O on an ITO substrate using this condition is known to cause mass transport-limited growth, resulting in the formation of dendritically branched Cu2O crystals, as shown in Figure 2c.16 However, when deposited on top of the p-Cu2O layer using the same condition, n-Cu2O was grown as well-faceted polyhedral crystals (Figure 2 a,b). This is because the deposition on a highly resistive p-type Cu2O layer significantly lowers the deposition current; therefore, the growth of n-type Cu2O was not limited by mass transport. The dark I-V measurement of the n-Cu2O layer deposited on an ITO substrate is shown in Figure 2d, and its resistivity was also estimated to be in the range of 105 to


Figure 3. Side view SEM images of (a) p-Cu2O(I) layer and (b) p-Cu2O(I)/n-Cu2O homojunction. I-V curves of (c) p-Cu2O(I)/ n-Cu2O and (d) p-Cu2O(II)/n-Cu2O homojunction solar cells under 1 sun, AM 1.5 illumination. The inset shows a schematic assembly of a p-n Cu2O homojunction solar cell used in this study. The sideview SEM image of p-Cu2O(II)/n-Cu2O homojunction looks comparable to that of p-Cu2O(I)/n-Cu2O homojunction and can be found in the Supporting Information.

106 Ω 3 cm. (The I-V curves of the p-Cu2O(I), p-Cu2O(II), and n-Cu2O layers obtained under illumination can be found in Supporting Information.) The typical side view SEM image of the Cu2O homojunction assembled in this study is shown in Figure 3a,b, where the p-Cu2O layer is ∼700 nm thick, and the n-Cu2O

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layer is ∼300 nm thick. These thicknesses were found to allow for sufficient photon absorption by both the p-Cu2O layer and the n-Cu2O layer. (With the 700 nm thick p-Cu2O layer, ca. 40% of the insident light reached the n-Cu2O layer.) The I-V curve of the p-n junction created on the p-Cu2O(I) layer, which achieves a higher efficiency than was previously reported, is shown in Figure 3c. The values of VOC, ISC, ff, and η are found to be 0.423 V, 2.5 mA/cm2, 27%, and 0.29%, respectively, under 1 sun, AM 1.5 illumination. Except for the fill factor, all other values obtained in this study show an improvement from the previously reported values for the p-n Cu2O homojunction solar cell (VOC = 0.321 V, ISC = 1.228, ff = of 35.33%, and η = 0.103%).14 The low ISC and extremely poor fill factor suggest that the high resistivity of Cu2O layers is the main factor that limits the cell performance. Therefore, increasing carrier densities for both the p- and n-layers through proper doping will be critical to improve the efficiency of the cell further. When the p-n junction was fabricated with the p-Cu2O(II) layer, the performance of the cell was significantly lowered with a VOC of 0.268 V, an ISC of 1.1 mA/cm2, a ff of 27%, and an η of 0.08% (Figure 3d). This considerable change in performance was not expected because Cu2O(I) and p-Cu2O(II) layers have comparable charge transport properties. The conductivities of n-layers deposited on p-Cu2O(I) and p-Cu2O(II) layers should also be similar because both n-layers were deposited from the same solution using identical deposition conditions. The only obvious difference between the two homojunction cells that can lead to the observed performance difference appears to be the crystal face of the p-Cu2O layer exposed at the p-n junction (i.e., different surface terminations). Because each crystal face contains a different atomic arrangement, the termination of atoms and the deviation of surface coordination from ideal bulk coordination are expected to be different depending on which plane is exposed at the surface. This difference in turn can affect the energy level, density, and distribution of interface states at the junction and, therefore, the recombination losses at the interface. Therefore, if the {100} terminated p-Cu2O(I) layer generates fewer surface states in the interband region, then the homojunction built on the p-Cu2O(I) layer can achieve a higher efficiency than the cell built on the p-Cu2O(II) layer when all other factors are comparable. We obtained experimental evidence correlating exposed crystal faces and the quality of the junction by comparing XRD patterns of p- and n-Cu2O layers. Figure 4a shows that p-Cu2O(I) and p-Cu2O(II) layers have the identical crystal orientations judging from the same relative intensity ratios of (110), (111), and (200) peaks (I110/I111/I200 1:22:1). However, their difference in surface terminations resulted in a difference in crystal orientations in the subsequently deposited n-Cu2O layers. The XRD pattern of the n-Cu2O layer deposited on p-Cu2O(I) layer shows an almost identical intensity ratio for the (110), (111), and (200) peaks (I110/I111/I200 1:23:1), indicating that the same crystal orientation is maintained in the n-layer (Figure 4b). This means that the n-Cu2O crystals grow on p-Cu2O crystals in a seamless manner, maintaining the continuity of the atomic level crystal lattice. In this case,


Figure 4. Comparison of XRD patterns: (a) p-Cu2O(I) layer (black) versus p-Cu2O(II) layer (gray), (b) p-Cu2O(I) layer (black) versus n-Cu2O layer deposited on p-Cu2O(I) layer (gray), and (c) p-Cu2O(II) layer (black) versus n-Cu2O layer deposited on p-Cu2O(II) layer (gray). * denotes peaks generated by an ITO substrate.

abrupt structural discontinuities or severe interface states are not expected to exist at the p-n junction. The XRD pattern of the n-Cu2O layer deposited on p-Cu2O(II) layer shows a considerable change in intensity ratio (I110/I111/I200 1:30:1), indicating a corresponding change of crystal orientation in the n-layer (Figure 4c). This suggests that the coordination of atoms on {111}-terminated p-Cu2O crystal surface may deviate significantly from the ideal coordination of atoms on {111} planes in the bulk crystal structure, preventing well-aligned epitaxial growth of n-Cu2O crystals on p-Cu2O crystals. The resulting structural discontinuity at the junction can generate many interface states, which lowers the quality of the homojunction and the overall cell efficiency. These preliminary results on constructing Cu2O homojunction solar cells show that the shape of individual crystals in the polycrystalline electrodes, which determines the atomic arrangement at the junction, plays an important role in affecting the overall performance of the solar cells. Therefore, whereas decreasing the series resistance through the doping of both p- and n-Cu2O is the most critical task to improve cell characteristics further, a continued effort in modifying the junction morphologies will be necessary to better understand their effect and optimize the cell performance.

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EXPERIMENTAL METHODS

ACKNOWLEDGMENT This work was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through grant DE-FG02-05ER15752 and made use of the Life Science Microscopy Facility at Purdue University.

Electrodeposition. The formation of the p-Cu2O/n-Cu2O junction took place as a two-step electrochemical deposition process, electrodeposition of p-Cu2O, followed by electrodeposition of n-Cu2O. A layer of p-Cu2O was cathodically deposited (2 Cu2þ þ 2 OH- þ 2e- f Cu2O þ H2O) on an ITO substrate (sheet resistance of 8-12 Ω) from aqueous media containing either 0.02 M CuSO4 (anhydrous, 98% purity, Alfa Aesar) and 0.4 M L-(þ)-lactic acid (85-90% aqueous solution, Alfa Aesar) or 0.4 M CuSO4 and 3 M L-(þ)-lactic acid. A glass slide coated with 1000 Å of platinum deposited on 300 Å of titanium served as a counter electrode, and the reference electrode was a Ag/AgCl (4 M KCl) electrode. The pH was adjusted to 11 or higher prior to deposition using NaOH and H2SO4. Depositions were carried out at 60 °C and -0.4 V versus Ag/AgCl for a set amount of charge (Q = 0.68 C/cm2) so that the resulting Cu2O electrodes would contain comparable amounts of Cu2O. A layer of n-Cu2O was electrodeposited on top of a previously deposited p-layer. The deposition media for n-Cu2O consists of 0.02 M Cu(CH3COO)2 3 H2O (98-102%, Alfa Aesar) and 0.08 M CH3COOH (80þ%, Mallinckrodt). The pH was adjusted to 4.9 prior to deposition. We carried out 30 min depositions at 70 °C and þ0.02 V versus Ag/AgCl (Q = 0.45 C/cm2). After deposition, AuPd top contacts were sputtered using a Technics Hummer I sputter coater operated at 100 mTorr and 10 DC mA with an AuPd target. The sputtered spot size was 2.0 mm2 Characterization. The purity and orientation of each Cu2O layer were examined by X-ray diffraction using a Scintag X2 diffractometer with a Cu-KR radiation source. Scanning electron microscope (SEM) images were taken on a FEI NOVA nanoSEM 200 operated at 3 kV. The films were coated with Pt by sputtering before imaging to minimize charging problems. I-V measurements were performed using a Keithly 2400 source meter. Blunt tip, spring-loaded Au probes were used to make contacts to the AuPd top contact spots and copper tape attached to the back ITO contact. Samples were back-illuminated through the ITO back contact to avoid shadowing of the solar cell by the top contact and the measurement probe. An optical fiber leading from a 300 W xenon arc lamp with a series of neutral density filters, an IR filter, and an AM 1.5G filter provided solar simulating light (set to 100 mW/cm2).

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SUPPORTING INFORMATION AVAILABLE Side view SEM

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image of p-Cu2O(II)/n-Cu2O homojunction and the I-V curves of p-Cu2O(I), p-Cu2O(II), and n-Cu2O layers obtained under illumination. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

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Corresponding Author: *To whom correspondence should be addressed. E-mail: kchoi1@ purdue.edu. Tel: 1-765-494-0049. Fax: 1-765-494-0239.

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Notes (18)

‡

On sabbatical leave from Department of Physics, University of Kelaniya, Kelaniya, Sri Lanka.


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Effect of Junction Morphology on the Performance of Polycrystalline

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