Cu2O heterojunction solar cells

Cu2O heterojunction solar cells

Available online at www.sciencedirect.com Electrochimica Acta 53 (2008) 2226–2231 Electrodeposited ZnO/Cu2O heterojunction solar cells S.S. Jeong, A...

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Available online at www.sciencedirect.com

Electrochimica Acta 53 (2008) 2226–2231

Electrodeposited ZnO/Cu2O heterojunction solar cells S.S. Jeong, A. Mittiga ∗ , E. Salza, A. Masci, S. Passerini ∗ Agency for the New Technologies, Energy and the Environment (ENEA), Casaccia Research Center, Via Anguillarese 301, 00123 Rome, Italy Received 19 June 2007; received in revised form 12 September 2007; accepted 18 September 2007 Available online 22 September 2007

Abstract In this paper the fabrication and the characterization of heterojunction solar cells based on electrodeposited ZnO and Cu2 O is described. The effect of the electrodeposition conditions (pH and temperature) on the cell performance has been investigated. The cells made with a Cu2 O layer deposited at high pH (12) and moderate temperature (50 ◦ C) have shown conversion efficiency as high as 0.41%. © 2007 Elsevier Ltd. All rights reserved. Keywords: Zinc oxide; Copper oxide; ZnO/Cu2 O; Electrodeposition; Photovoltaic cells

1. Introduction In view of the severe future ecological impacts of energy production by combustion of fossil fuels, solar energy is seriously considered as an alternative. However, the development of new solar energy converters with improved performance and lower cost requires new approaches focused on the use of cheap and non-toxic materials prepared via low energy intensity processes such as electrodeposition. The possibility of using electrodeposition to realize solar cells with energy conversion efficiencies larger that 10% is witnessed by the work regarding CdS/CdTe heterojunctions [1]. However, the well-known toxicity of cadmium represents a serious obstacle to the development of industrial processes for the production of electrodeposited solar cells. On the other hand, cuprous oxide (Cu2 O) is a non-toxic direct energy gap semiconductor that can be easily produced with a minority carrier diffusion length suited for the use as solar cell absorber layer. Despite that, however, the highest energy conversion efficiency of a Cu2 O solar cell obtained up to now is much lower than the Shockley–Queisser theoretical limit which is about 20%. The main hindrance to the optimization of Cu2 O solar cells is the difficulty in the doping process. Cu2 O is spontaneously a p-type semiconductor and all



Corresponding author. E-mail addresses: [email protected] (A. Mittiga), [email protected] (S. Passerini). 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.09.030

the efforts to form homojunctions by n-doping of Cu2 O have, so far, failed. An exception is a very recent report [2] in which, however, no photovoltaic action is claimed. The best approach is therefore to use a heterojunction between Cu2 O and a n-type oxide. Among all possible combinations, ZnO/Cu2 O heterojunctions have recently attracted a renewed interest of several researchers [3–7] because of the favorable alignment of the conduction band edges. As a result of the new research efforts, cells with photovoltaic conversion efficiencies as high as 2% [3] have been recently made in our laboratories. The best performing cells [3,4] were fabricated on Cu2 O substrates obtained by oxidation of copper sheets at high temperature. Their fill factor, however, was still limited by the low electric conductivity of the thick Cu2 O substrates. The use of a thin film structure could solve this problem and, also, avoid the use of large amounts of high purity copper. Thin film solar cells based on Cu2 O were obtained by depositing cuprous oxide and zinc oxide by reactive rf magnetron sputtering [5]. In that work it was demonstrated that the two deposition sequences, ZnO deposited on Cu2 O and Cu2 O deposited on ZnO, were not equivalent with the latter sequence giving significantly better current–voltage characteristics. The difference was ascribed to a better crystallographic matching due to the spontaneous high orientation of the ZnO crystals in the (0 0 0 1) orientation that induced the growth of the Cu2 O crystals with the (1 1 1) preferential orientation. Since ZnO is hexagonal (wurtzite) while Cu2 O is cubic (cuprite) this matching gives a similar hexagonal atomic arrangement at the interface with

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a lattice mismatch of only 7.1%. Under AM1.5 standard illumination these cells reached Voc = 0.26 V, Jsc = 2.8 mA cm−2 , FF = 0.55 giving a conversion efficiency equal to 0.4%. Thin film ZnO/Cu2 O heterojunctions have been obtained using the cheaper and low-energy intensive electrodeposition technique [6,7] also. This process involves the cathodic electrodeposition of ZnO on a TCO coated glass substrate followed by the cathodic electrodeposition of Cu2 O to form the heterojunction. It must be noted that good quality Cu2 O can be grown by electrodeposition: epitaxial growth of Cu2 O on InP and on Si [8,9] has been reported. However, till a few months ago, the efficiency of these cells remained quite low (about 0.1%) [6]. Only very recently a large improvement in the efficiency has been reported (Jsc = 3.8 mA cm−2 , Voc = 0.59 V, FF = 0.58 and conversion efficiency of 1.28%) [7], but a clear understanding of the factors limiting the cell performance is not yet available. 2. Experimental ZnO/Cu2 O heterojunction solar cells were made by consecutive cathodic electrodeposition of ZnO and Cu2 O on glass plates covered with a SnO2 :F transparent conductive oxide layer (Asahi glass). Three-electrode cells equipped with a Ag/AgCl reference electrode and a metal foil counter electrode (zinc or copper depending on the layer to be deposited) were used for the electrodeposition of the two layers. ZnO layers were galvanostatically electrodeposited from unstirred 0.05 M aqueous solution of Zn(NO3 )2 on SnO2 conductive glass substrates as described in reference [10]. The sample thickness ranged between 1 and 2 ␮m depending on the deposition time and current. After deposition the sample was rinsed with water and transferred in the Cu2 O electrodeposition bath (a 0.25 M aqueous solution of CuSO4 stabilized with lactic acid as chelating agent [11,12]). The pH of the copper sulfate solution was adjusted in the range extending from 9 to 12 using NaOH. The Cu2 O electrodeposition was galvanostatically driven immediately after the immersion of the sample in the CuSO4 bath to prevent ZnO corrosion. The sample thickness ranged between 2 and 4 ␮m depending on the deposition time and current. Finally, the substrates were rinsed with water, dried with air and quickly transferred into a thermal evaporator for the deposition of the gold back contact with an area of 0.5 cm2 .

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ZnO/TiO2 /Cu2 O cells were also made. The intermediate TiO2 film was deposited by spinning a 10−2 M solution of tetrabutyltitanate in isopropanol on the ZnO film. After solvent evaporation the samples were annealed in air at 450 ◦ C for 30 min. For comparison purposes, single layers of ZnO and Cu2 O were electrodeposited on conductive glass substrates by performing only one of the electrodeposition processes described above. The films were characterized using an XRD apparatus (Rigaku MiniFlex) and a Philips-ESEM-XL30 electron microscope. The conductivity versus temperature measurements were performed using a Keithley 236 electrometer. The temperature was controlled by hosting the samples inside a cryostat cooled by liquid nitrogen (Janis VPF-100). The temperature controller was a Lakeshore 330. The current–voltage curve under standard AM1.5 illumination were made using a Wacom WXS-140SSuper Solar simulator. All chemicals used (reagent grade) were purchased from Aldrich. Deionized water from Millipore equipment was used. 3. Results and discussion In Fig. 1 panel A are illustrated the potential–time curves recorded during the electrodeposition of ZnO on SnO2 conductive glass at 70 ◦ C and different current densities. The ZnO layer is formed through a multistep process initiated by the development of OH− resulting from the reduction of nitrate to nitrite anions (NO3 − + H2 O + 2e− → NO2 − + 2OH− ) [10]. The generation of OH- causes the precipitation onto the electrode surface of Zn(OH)2 that spontaneously transforms in ZnO. The bath temperature was fixed at 70 ◦ C to favour the latter step (higher temperatures would result in a high rate evaporation of water from the aqueous electrolyte). The stationary solution (unstirred) was required to favour the precipitation of ZnO onto the electrode and to avoid the formation of particles in the electrolyte. Regarding the electrodeposition current, the best results in terms of homogeneity of the ZnO layers were obtained with the intermediate current density of 1 mA cm−2 (see SEM picture in Fig. 2). Films electrodeposited at higher or lower current densities showed macroscopic inhomogeneities in terms of thickness. Based on this observation, only ZnO layers electrodeposited at

Fig. 1. Panel A: potential–time curves during the electrodeposition of ZnO on conductive glass at different current densities (a: 0.5 mA cm−2 , b: 1 mA cm−2 , c: 1.5 mA cm−2 ). Panel B: optical transmittance curve of a ZnO layer deposited at 70 ◦ C for 60 min at 1 mA cm−2 corresponding to a film thickness of about 2 ␮m (solid line). The transmittance of the substrate is also reported (dashed line).

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Fig. 2. Panel A: potential–time curves during the electrodeposition of Cu2 O (on ZnO) at different temperatures (a: 30 ◦ C, b: 40 ◦ C, c: 50 ◦ C, d: 60 ◦ C, e: 70 ◦ C) and pH 12. Panel B: SEM image showing the columnar structure of a ZnO (2 ␮m)/Cu2 O (2.8 ␮m) heterojunction (Cu2 O was deposited at 70 ◦ C, pH 12 and I = 0.75 mA cm−2 ). Panel C: XRD data of three heterojunctions differing for the pH of the solution used to deposit Cu2 O.

1 mA cm−2 and 70 ◦ C from unstirred solutions were prepared and used for further testing. Layers of various thickness were obtained simply changing the deposition time. In Fig. 1 panel B is reported the optical transmittance of an electrodeposited ZnO (deposition time of 60 min corresponding to about 2 ␮m). At wavelengths higher than 400 nm the optical transmittance of

the glass/SnO2 /ZnO was about 80%, i.e., nearly identical to that of glass/SnO2 , that confirms the good optical quality of the ZnO layer. The absorption edge of the sample with the ZnO layer is located at larger wavelengths due to the lower band gap of this material compared to that of SnO2 . An accurate determination of the ZnO gap value is difficult due to the light scattering from the surface roughness and the free-carrier absorption in the SnO2 layer. A rough estimate gives Eg of about 3.4 eV that is in agreement with the value expected for a non-degenerate material which has a Fermi level below or at the conduction band edge. To fabricate the heterojunctions, Cu2 O was electrodeposited on ZnO layers at different temperatures, solution pH and current densities. The morphology and the crystalline structure of the electrodeposited cells were characterized with SEM and XRD. The SEM picture of a cell (Fig. 2B) shows the columnar structure of both layers. It has been reported [12] that at pH 9 the orientation of the crystals is [1 0 0] while at pH 12 the orientation changes to [1 1 1]. Recently, a narrower pH range (from 9.4 to 9.9) has been found [13] in which the preferred orientation is [1 1 0]. XRD measurements taken on full cells confirm this dependence of the Cu2 O orientation on the pH value. As seen in Fig. 2C the orientation of the Cu2 O film was [1 0 0] at pH lower than 9 and [1 1 1] at pH of 12. The layer deposited at intermediate pH (10.35) is mostly [1 1 1] oriented but with smaller crystals. Also important to note is that the low pH orientation of Cu2 O ([1 0 0]) is maintained even if the layer is grown on the [0 0 0 1] oriented ZnO film that would offer a good matching to the [1 1 1] Cu2 O structure. Beside the effect on the crystal orientation, the pH of the Cu2 O electrodeposition bath was also found to be a critical parameter in determining the cell efficiency. The best performing heterojunction solar cells were obtained by depositing the Cu2 O layer at pH 12 (see Table 1). The deposition of Cu2 O at pH 10.35 gave devices with lower photocurrents and larger series resistances This behaviour could be explained supposing that oxygen-rich Cu2 O with higher conductivity (due to the holes generated by the negatively charged copper vacancies) is formed at high pH (see the Pourbaix diagram of the Cu2 O–H2 O system). To test this hypothesis, the dark conductivity of electrodeposited Cu2 O films was measured at temperatures below the RT on SnO2 /Cu2 O samples. To perform the measurements, a gold dot (1 mm diameter) was evaporated on the Cu2 O surface. The conductivity was therefore measured in the direction normal to the film surface using the SnO2 as the other contact. The resistance of the conductive SnO2 was first checked and found to be nearly independent on the temperature and neg-

Table 1 Comparison of the photovoltaic parameters of various ZnO/Cu2 O cells Cell #

Conditions

Voc (mV)

Jsc (mA cm−2 )

FF (%)

Eff (%)

52 93 104 80 80

Cu2 O at pH 10.35

200 283 372 316 217

1.42 2.64 1.38 2.69 2.5

30.2 36.8 41.7 48 40.7

0.086 0.276 0.215 0.41 0.22

With TiO2 Just made Stabilized

If not indicated, the Cu2 O layer was galvanostatically electrodeposited at 0.75 mA cm−2 , pH 12, Tsol = 50 ◦ C. The ZnO layer was always galvanostatically electrodeposited at 1.0 mA cm−2 and Tsol = 70 ◦ C.

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Fig. 3. Panel A: dark conductivity vs. 1/T of Cu2 O films (2 ␮m thick) electrodeposited on SnO2 conductive glass substrates at different temperatures and pH (see legend). Panel B: photoconductivity of a Cu2 O film electrodeposited on SnO2 conductive glass substrate at 50 ◦ C and pH 12.

ligible with respect to that of the Cu2 O. The results reported in Fig. 3A indicate that indeed the room temperature dark conductivity of films deposited at 70 ◦ C increases of 3 orders of magnitude (from 1.95 × 10−8 to 1.73 × 10−5 −1 cm−1 ) going from pH 9 to 12. Their conductivity shows an activated behaviour σ(T) = σ 0 exp(−EA /kT) with activation energies EA which decrease from 0.6 to 0.37 eV (see label in the figure). These trends are in good agreement with those measured by Hall effect on electrodeposited Cu2 O layers mechanically transferred to nonconductive epoxy glue [11]: a pH increase from 9.5 to 12.5 resulted in a conductivity increase from 3 × 10−7 to 3.7 × 10−5 −1 cm−1 due to the increase of both the hole concentration (from 1012 to 1014 cm−3 ) and mobility μh (from 0.4 to 1.8 cm2 V−1 s−1 ). It has to be noted that these hole mobility values obtained for the electrodeposited films are much lower than those found in bulk Cu2 O made by oxidation [2] (μh = 100 cm2 V−1 s−1 ) and in sputtered Cu2 O films [4] (μh = 50 cm2 V−1 s−1 ). In Fig. 3A the dark conductivity of a film electrodeposited at 50 ◦ C and pH 12 is also reported. Unluckily, samples deposited at pH 9 and 10.3 and 50 ◦ C were not uniform in morphology and their conductivity measurements were not reliable. The conductivity of the sample grown at 50 ◦ C is much lower than that of the corresponding film grown at 70 ◦ C probably as a results of a lower grain size. However, the large series resistance, which could inhibit the fabrication of working solar cells, is mitigated by the film photoconductivity. In panel B of Fig. 3 is reported the conductivity of the film grown at pH 12 and 50 ◦ C in response to a 5 min period of AM1.5 standard illumination. The film photoconductivity reaches a value of 4 × 10−6 −1 cm−1 (corresponding to a photogenerated carriers lifetime of some ns) in about 300 s, which is adequately high for the realization of thin film solar cells. From Fig. 3B it is also seen that the rise and the fall of the photoconductivity show a very slow dynamics unexplainable by the photocarriers lifetime and reminding the Persistent PhotoConductivity (PPC) observed in other semiconductors [14]. The temperature of the Cu2 O electrodeposition bath also plays a key-role on the cell performance. It was found that temperatures exceeding 50 ◦ C quite often gave shunted diodes with very low Voc under illumination. This problem was most likely due to the solubility of ZnO in high pH solutions [15] leading

to the formation of pinholes through the ZnO layer. These pinholes were then filled by Cu2 O during the electrodeposition to form ohmic contacts (i.e., short circuits) with the conductive layer (SnO2 :F). As a matter of fact, we have observed this phenomenon to take place even at 50 ◦ C and lower temperatures when the ZnO-covered substrates where left immersed in the Cu2 O deposition bath in rest conditions. In order to avoid the ZnO corrosion by the CuSO4 solution, a thin layer of TiO2 was coated on the ZnO layer by spin-coating followed by annealing in air. Although these cells showed higher Voc values, their photocurrents were lower with a net result of an efficiency decrease (see Table 1). Overall, the best performing devices were obtained electrodepositing the Cu2 O at pH 12 and Tsol = 50 ◦ C without the TiO2 inter-layer. The photovoltaic parameters of two cells made in such conditions, measured under AM1.5 illumination, are also reported in Table 1. Occasionally, cells with better performance were obtained (see cell 80 in Table 1), however, these cells degraded to the average values reported above within a few hours. Nevertheless, this indicates that there is still large room for improvements as it is also indicated by the comparison with the performance of ZnO/Cu2 O solar cells produced using large grain polycrystalline Cu2 O substrates prepared by copper oxidation [16]. The most critical point in the electrodeposited cells reported in this work is the low value of the photocurrent. An increase of this value would result in the increase of all the other photovoltaic parameters (Voc , FF and Eff ). In order to understand the reason of the low photocurrent detected, the quantum yield (Q.Y.) energy dependence of the cell 93 was measured (see Fig. 4). The first consideration to be done is that the Q.Y. is entirely due to the photocarriers generated inside the Cu2 O layer since, as shown before, the electrodeposited ZnO films have a very low absorbance in the range extending from 380 to 800 nm with the higher limit well above the Cu2 O absorption range. Nevertheless, above 500 nm the Q.Y. of the cell is very low. This behavior can be explained supposing that in the region with the high Q.Y. (between 380 and 460 nm) the photons are absorbed in the depletion layer region of the heterojunction where the high electric field causes a high collection probability. For larger wavelengths, however, the carriers are photogenerated deeper inside the Cu2 O layer. This results in longer diffusion paths for them to reach the

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The absorption coefficient energy dependence was described by a curve that fits data from the literature [18–20] (ref. 20 for λ < 490 nm only). The absorption coefficient drops abruptly below 105 cm−1 at 490 nm. The solid line in Fig. 4 shows the result of the simulation using this set of parameters. The good agreement between the experimental data and the numerical simulation confirms the validity of the assumption adopted then allowing to state that the low Q.Y. (i.e., the low photocurrent) is mostly due to the high defect concentration at the ZnO/Cu2 O interface and in the Cu2 O bulk. Wang et al. [13] reported similar values of defect density in electrodeposited Cu2 O obtained from capacitance versus voltage measurements in Cu2 O/liquid electrolyte cells. 4. Conclusions Fig. 4. Quantum yield energy dependence of a typical ZnO/Cu2 O heterojunction (squares). The solid line shows the result of a numerical simulation obtained using the AFORS-HET package. The parameters used in the simulation are discussed in the text.

depletion region edge. For these wavelengths the collection efficiency drops dramatically indicating that the Cu2 O quality (and consequently its electron diffusion length) is very low. To quantitatively assess this interpretation, numerical simulations of the ZnO/Cu2 O devices were performed. AFORS-HET (version 2.2) [17], a numerical computer program for simulation of heterojunction solar cells, was used. To simplify the simulation, ZnO was not included in the numerical calculation since it gives a negligible contribution to the photocurrent. However, the effect of the glass/SnO2 /ZnO stack on the optical absorption of the Cu2 O layer was included in the AFORS-HET simulations using the experimental data (Fig. 1B). The main effect of the ZnO is the generation of a band bending (the so-called Built-in Potential Vbi ) in the Cu2 O layer that was directly set in AFORS-HET. The value of Vbi is given, in a first approximation, by the difference between the work function of the ZnO and Cu2 O layers. From the conductivity measurement it can be roughly estimated that the Fermi level in the Cu2 O film of cell 93 is located about 0.49 eV above the valence band edge. Using an electron affinity of 3.2 eV and an energy gap of 1.95 eV we obtain a work function of 4.66 eV. A similar evaluation could not be done for the ZnO layer, thus the Fermi level for this material was estimated to be very near to its conduction band edge and therefore the work function was assumed to be equal to the electron affinity (4.1 eV). Using these values a Vbi of 0.46 eV was estimated. The recombination processes taking place near the defective ZnO/Cu2 O interface were simulated, as usual, using an “interface recombination velocity” for which the worst possible value, namely the thermal velocity of carriers (about 107 cm s−1 ), was selected. The generally accepted picture of Cu2 O as a compensated semiconductor has been used [16]. The acceptor levels are located very near the Fermi level while the compensating donor levels are located 0.7 eV above the valence band edge. In order to account for the very low collection efficiency at λ > 500 nm, a large (5 × 1018 cm−3 ) acceptor concentration (mainly due to copper vacancies) in the Cu2 O was used resulting in a space charge layer thickness of 25 nm only.

Heterojunction solar cells based on electrodeposited ZnO and Cu2 O were fabricated. The effect of the Cu2 O electrodeposition conditions (pH and temperature) on the cell performance has been investigated. The cells made with a Cu2 O layer deposited at high pH (12) and moderate temperature (50 ◦ C) have shown conversion efficiency as high as 0.41%. This achievement is most likely due to the highest conductivity of the Cu2 O layer electrodeposited at pH 12. Nevertheless, the cell performance is still low for practical applications. However, there is a lot of room for improvements through, for example, a careful control of the electrodeposition conditions in order to reduce the defect density in the Cu2 O layer. Further work in this direction is on going in our group. Acknowledgment S.S.J. gratefully acknowledges the postdoctoral fellowship program of the Italian Foreign Ministry. References [1] S.K. Das, G.C. Morris, Sol. Energy Mater. Sol. Cells 30 (1993) 107. [2] L. Wang, M. Tao, Electrochem. Solid-State Lett. 10 (2007) H248. [3] A. Mittiga, E. Salza, F. Sarto, M. Tucci, R. Vasanthi, Appl. Phys. Lett. 88 (2006) 163502. [4] T. Minami, T. Miyata, K. Ihara, Y. Minamino, S. Tsukada, Thin Solid Films 494 (2006) 47. [5] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, G.K. Paul, T. Sakurai, Solar Energy 80 (2006) 715. [6] J. Katayama, K. Ito, M. Matsuoka, J. Tamaki, J. Appl. Electrochem. 34 (2004) 687. [7] M. Izaki, T. Shinagawa, K. Mizuno, Y. Ida, M. Inaba, A. Tasaka, J. Phys. D: Appl. Phys. 40 (2007) 3326. [8] R. Liu, E.W. Bohnnan, J.A. Switzer, F. Oba, F. Ernst, Appl. Phys. Lett. 83 (2003) 1944. [9] F. Oba, F. Ernst, Y. Yu, R. Liu, H.M. Kothari, J.A. Switzer, J. Am. Ceram. Soc. 88 (2005) 253. [10] M. Izaki, T. Omi, J. Electrochem. Soc. 143 (1996) L53. [11] K. Mizuno, M. Izaki, K. Murase, T. Shinagawa, M. Chigane, M. Inaba, A. Tasaka, Y. Awakura, J. Electrochem. Soc. 152 (2005) C179. [12] T.D. Golden, M.G. Shumsky, Y. Zhou, R.A. VanderWerf, R.A. Van Leeuwen, J.A. Switzer, Chem. Mater. 8 (1996) 2499. [13] L.C. Wang, N.R. de Tacconi, C.R. Chenthamarakshan, K. Rajeshwar, M. Tao, Thin Solid Films 515 (2007) 3090.

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