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Oxide Based Solar Cell: Impact of Layer Thicknesses on the Device Performance Shrabani Panigrahi, Daniela Nunes, Tomás Calmeiro, Kasra Kardarian, Rodrigo Martins, and Elvira Fortunato ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.6b00154 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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ACS Combinatorial Science

Oxide Based Solar Cell: Impact of Layer Thicknesses on the Device Performance Shrabani Panigrahi*, Daniela Nunes, Tomás Calmeiro, Kasra Kardarian, Rodrigo Martins, and Elvira Fortunato* i3N/CENIMAT, Department of Materials Science, Faculty of Sciences and Technology, Universidade NOVA de Lisboa, Campus de Caparica, 2829-516 Caparica

KEYWORDS: Spray pyrolysis, ZnO/Cu2O heterojunction, Combinatorial, Layer thickness, Interfacial phenomena

ABSTRACT: ZnO/Cu2O based combinatorial hetero-junction device library was successfully fabricated by a simple spray pyrolysis technique using ITO coated glass as substrate. The combinatorial approach has been introduced for analyzing the impact of the ZnO and Cu2O layer thicknesses on the performances of the cells. The thickness of the ZnO layer varied from ~ 50320 nm and Cu2O layer was deposited orthogonal to the ZnO thickness gradient. In case of Cu2O, the thickness varied from ~ 200-800 nm. The photovoltaic performances of the cell are strongly dependent on the absorber layer thickness for a particular window layer thickness and reaching a maximum short circuit current density of 3.9 mA/cm2 when the absorber layer thickness just cross ~700 nm. Reducing the thickness of the active layers leads to a sharp decrease in the device performances. It is shown that the entire built-in bias of the hetero-

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junction is created in the absorber layer due to the low carrier density. Poor performances of the devices having lower thicknesses are attributed due to different interfacial phenomena such as optical losses due to thin Cu2O layer, back contact recombination of the carriers due to thin layer thickness because minimum heterojunction thickness is required for the formation of the full build-in bias that slows down the recombination of the carriers with other factors.

Introduction One of the most abundant energy resources on Earth is solar energy and it is the most promising future energy resources without harmful environmental consequences.1-3 The conversion from solar energy to electric current by a solar cell or photovoltaic cell using the photovoltaic effect is one of the most important solar active technologies.3-4 Crystalline siliconbased solar cells provide high efficiency advantages but oxide materials based solar energy converters have potential applications due to non-toxicity characteristics of the materials and can also be prepared by cost-effective and low-temperature synthesis techniques.

5-6

Metal oxides

exhibit a broad range of functional properties depending on their crystal structure and bonding between the metal cation and oxygen.7 Nowadays, these materials are effectively used in different commercial applications such as for making solar cells as a transparent conducting front electrodes, transistors, UV sensors, gas sensors etc.8-11 Recently, Cu2O, TiO2, ZnO, SnO2, Al:ZnO (AZO) and other oxide semiconducting materials were developed for large-scale applications using several techniques.6,

12-18

In particular, there has been a renewal interest in

copper (I) oxide (Cu2O) based solar cells since this semiconducting material shows large absorption coefficient in the visible region (~ 104 cm-1) and a long minority carrier diffusion length (1 ~ 4 µm) with a direct energy gap of 2.1 eV.19 It is used in several hetero-junction solar cells such as CdO/Cu2O,20 Ga2O3/Cu2O, 21 ZnO/Cu2O, 22 TiO2/Cu2O23 where CdO, Ga2O3, ZnO,


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TiO2 are as the most successful n-type counterpart. Among these cells, ZnO/Cu2O heterojunction is one of the systems exhibiting the most favorable conduction band edge alignment for solar cell applications.24-25 ZnO is usually an intrinsic n - type semiconductor with wide band gap (3.3 eV), which can be used as the transparent conducting window layer in solar cells. There are many reports on ZnO/Cu2O based solar cells where most of the reports highlighted the efficiency of the cells by inserting buffer layer or observed the interface quality and solar cell performance by changing the atmospheric conditions.21-22,

26-29, 30-31

The influence of the active layer

thicknesses on the performance of the solar cell is rarely reported. Recently, Musselman et al.32 observed the performances of the bilayer and nanowire based Cu2O-ZnO hetero-junction solar cells with respect to the thickness of the absorber layer, length of ZnO nanowires and seed layer. A thick depletion layer was observed in the Cu2O layer of bilayer devices due to the low carrier density of electrodeposited Cu2O. It was confirmed from their observation that the lower performances of the devices is due to the inappropriateness of the length scales where larger length is necessary for the formation of the full built-in bias to slow down the recombination and short length scale is necessary for efficient charge collection. In the present work, the nature of the performances of ZnO/Cu2O based combinatorial heterojunction device library is systematically observed as a function of the active layers thicknesses. It provides a complete data rather than one conventional single device measurement having a fixed thickness. Spray pyrolysis technique was used for the deposition of both metal oxide layers. A series of spray cycles with a progressive larger scan area were used to produce the linear ZnO and Cu2O thickness gradients. Cu2O layer was deposited orthogonal to the ZnO thickness gradient. The structural, optical and electrical characterizations of this device library had been carried out after the rapid thermal annealing treatment. Due to low carrier concentration of the


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Cu2O layer deposited by spray pyrolysis technique, the entire built-in bias of the hetero-junction is created in this layer. When the thickness of the absorber layer is lower than the thickness of depletion layer, the reduced performances of the device library have been observed. This work also shows different ways to improve the performances of the low cost solution processed oxide based solar cells.

Experimental Section Materials ITO glass substrate, zinc acetate dihydrate [Zn(CH3COO)2·2H2O], methanol (solvent), acetic acid (catalyst), copper (II) acetate monohydrate (Cu(CH3COO)2·H2O), glucose (reducing agent), isopropanol (solvent) were used for this study. All chemicals (reagent grade) were purchased from Sigma-Aldrich and used without purification. Deionised water from Millipore equipment was used. Deposition of ZnO by spray pyrolysis technique ZnO layers on the ITO coated glass substrate were deposited by spray pyrolysis technique using a commercial spraying system (SONOTEK Exacta Coat). The substrates were thoroughly washed by soap solution and then rinsed by de-ionized water, acetone and iso-propanol respectively and finally these were dried in a dry air stream. The substrate was kept at 340 ◦C and the precursor solution of zinc acetate dehydrate (0.27 M), methanol and acetic acid (mixing ratio 49:1) was sprayed on the substrate with a pneumatic spray nozzle (Spraying Systems Co.) at a flow rate of 1 mL/min. Acetic acid was used as a catalyst in this method. The spray system was controlled by a syringe pump (Razel Scientific Instruments) where the airflow pressure was 5.4 bars. The nozzle was mounted onto a commercial x–y scanner (EAS GmbH) using x–y scan


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velocity of 30 mm/s and a nozzle - substrate distance was 18 cm. The thickness of the film was produced using a series of spray cycles. By using a selective program in the system the gradient structures for both layers had been produced. Deposition of Cu2O by spray pyrolysis technique After the deposition of ZnO, the Cu2O layer was deposited onto the ZnO layer using the same system. A precursor solution of copper (II) acetate monohydrate (0.02 M) and glucose (0.03 M) dissolved in water and isopropanol (mixing ratio 4:1) was sprayed by the ultrasonic nozzle onto the substrate at temperature of 275 oC using a hot plate. The solution flow rate and the airflow pressure were the same as the time of ZnO deposition. Structural Characterizations The structural characterization of the ZnO/Cu2O based combinatorial device was performed by X-ray diffraction (XRD) patterns using the computer-controlled Panalytical XpertPRO system (Cu Kα radiation; λ=1.5405 Å) and X´Celerator 1D detector. The morphologies were analyzed by Scanning electron microscopy (SEM) measurements using a Carl Zeiss AURIGA CrossBeam workstation, equipped for Energy dispersive X-Ray spectroscopy (EDS) measurements. For focused ion beam (FIB) experiments, Ga+ ions were accelerated to 30 kV at 10 pA and the etching depth was kept around 800 nm. When we observed the cross-sectional SEM images of the ITO/ZnO/Cu2O based combinatorial heterojunction, we put a sacrificial layer of AZO ~200 nm on the top of Cu2O layer to avoid the charging effects as Cu2O reacts with Ga from FIB. AZO layer was deposited by sputtering technique in Ar atmosphere. EDS maps were carried out after FIB experiments with an accelerating voltage of 15 kV and at a tilt angle of 54 º properly indicated on INCA software from Oxford instruments. Thicknesses of the different layers of the cell were measured by the profilometer instrument (AMBIOS TECHNOLOGY, XP-200).


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Moreover, in order to confirm, after FIB measurements, the dimensions of each individual layer have been determined from SEM micrographs using the Image-J software.33 The surface topographies were observed with an Asylum Research MFP-3D Stand Alone AFM. Image acquisition was done in alternate contact mode with a resolution of 512 X 512 points, using commercial silicon probes (Olympus AC240TS, k = 2 N/m, freq = 70 kHz). Using the Hall measurement system (BiO-RAD), the carrier concentration of the different layers had been measured. Optical Characterizations For absorption measurement, UV-Vis-NIR spectrometer (Perkin Elmer; Lambda 950) was used.

Solar cell Characterizations To show the current-voltage (I-V) characteristics of the hetero-junction, Au back contacts were deposited by thermal evaporation (Vinci Technologies) technique onto the Cu2O layer using a shadow mask where each contact patch had a diameter of 1.75 mm, corresponding to an area of 0.0237 cm2. For gradient hetero-junction 8 × 8 back contacts were deposited. I–V characteristics of all 64 devices in the dark and under illumination were measured by a Keithley 2400 source meter. Illumination was provided through AM 1.5 illumination conditions with a Sciencetech SS1.6kW-A-2-Q system consisting of a Xe lamp with a light intensity of 100 mW cm-2. Solar cells parameters from I-V curves were derived using a Sciencetech SOLAR photo I-V measurement system software. All the measurements have been done in air.


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Results and discussion The schematic diagram of the ZnO/Cu2O based combinatorial hetero-junction device library is shown in Figure 1. At first ZnO layer was deposited on the ITO coated glass substrate with a linear thickness gradient parallel to the y-axis (Figure 1a). Thickness of ZnO varied from ~50320 nm as shown in color map. Then, the Cu2O absorber layer was deposited with a linear gradient parallel to the x-axis (Figure 1b) that means orthogonal to the ZnO thickness gradient leading to a total hetero-junction thickness profile with a diagonal thickness gradient (Figure 1c). The Cu2O thickness varied from ~200-800 nm leading to the total thickness variation from ~2501150 nm. Figure 2a shows the schematic three dimensional view of the combinatorial device library, where the gradient nature of the thicknesses for both layers is clearly observed and they are orthogonal to each other. Figure 2b shows the cross-sectional SEM image of the one portion of the device library where the width of the ZnO and Cu2O layers are 145 ± 27 nm and 377 ± 53 nm, respectively. For elemental analysis, EDS maps have been performed after FIB milling experiments indicating a uniform distribution of Au, Cu, Zn and In along the sample. FIB experiments were carried out in several positions along the device for demonstrating the presence of different layer thicknesses and the cross-sectional SEM images are shown in supporting information (Figure S2). The surface elemental analysis of the hetero-junction has also been performed with EDS measurements (Figure 3) to observe that all the elements are homogeneously distributed along the film. To observe the device performances at different active layers thicknesses, a grid of 8 × 8 round Au back contacts were deposited onto the Cu2O layer. Figure 4a shows the photograph of the device library with the typical yellow/orange color of Cu2O. The top left corner is darker due to a higher thickness of Cu2O and ZnO. The color becomes lighter in the right side since the


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thickness of Cu2O decreases gradually from the left to right. Figure 4b shows the example of current density-voltage (J-V) scan for three different points marked by circles (Figure 4a) with three different Cu2O thicknesses at a constant window layer thickness. It confirms that a higher short circuit current density (JSC) and open circuit voltage (VOC) were observed for the cell with thicker Cu2O layer. The nature of the dependence of JSC and VOC values of the combinatorial device library as a function of the absorber layer thicknesses are presented in the Figure 4c and d, respectively. At any particular Cu2O thickness, there are many values corresponding to different ZnO thicknesses. For lower thicknesses of Cu2O (at different thicknesses of ZnO), the devices show the diminished performances. When the thickness of the absorber layer increases, cells performances enhance sharply. The strong dependence of the JSC and VOC values on the thickness of the absorber layer may be related to the deposition process. At the time of deposition of gradient structure, thicker layers stayed for longer time on the hot plate compared to thinner layers which improved the interface quality between the two layers of ZnO and Cu2O and reduced the defect concentration. That means, for higher thickness devices, the interfaces are much better than the lower thickness devices for more time annealing effect at the time of layer deposition. Heat treatment generally influences to increase the grain size and agglomerations, which decreases the surface roughness of the film (shown in supporting information). Kumar et al.34 also observed better photovoltaic performances from the perovskite solar cell after the heat treatment of the device because of the improvement of the interface between perovskite layer and back contact. It is reported that the better interface stoichiometry improved the device characteristics of ZnO/Cu2O hetero-junction solar cells.30 Surface quality improvement of the layers of the devices happened due to proper annealing effect at the time of deposition, which influence the performances quality of the devices.35 Color maps of the solar cell parameters JSC,


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VOC, maximum power density (Pmax) and fill factor (FF) which are derived from the J–V curve under illumination are shown in Figure 5. The highest JSC is attained when the absorber layer thickness just cross ~700 nm (Figure 5a). It is clearly shown from the Figure 4(c) and 5 (a) that JSC increased with the increase of the thickness of the absorber layer and again decreased for the devices having the highest thickness of the absorber layer. It is probably due to the increase of recombination rate of the charge carriers at the interface. At higher thickness of the devices, the pathway for charge collection is longer and the drift of charge carriers becomes slower under short circuit condition so the probability of recombination increases for separated charges.36 In case of VOC, the higher values have been observed on the top left corner of the library (Figure 5b), where the thicknesses of ZnO as well as Cu2O are higher. This result is in agreement with the previously reported results where low VOC observed for Cu2O-ZnO hetero-junctions using thin Cu2O layers. 37-38 A similar pattern is observed for the color map of Pmax (Figure 5c) and FF (Figure 5d) respectively. The higher Pmax and FF are observed for the higher thickness of the devices. For the lower thicknesses of both layers that means at the right corner of the library where the values of Pmax and FF both are minor than the other part of the library. Figure 6 shows the optical absorption spectra of this device library at different positions with different Cu2O thicknesses (at a particular ZnO thickness). It shows high absorption in the range of 400–600 nm, which are due to Cu2O crystals. The absorbance difference among higher and lower thicknesses of Cu2O based devices would be related to the (111) orientations of Cu2O grains (X-ray diffraction pattern is available in supporting information). As the intensity of (111) orientations of Cu2O layers were increased for the higher thickness of Cu2O, the absorptions also increased due to the light confinement by the arrangement of the Cu2O grain with favored orientations.39 The degradation of the device performances in case of thinner absorber layer may be due to


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lower absorption and consequently low short circuit current densities of the cells. Furthermore, it is already reported that for increasing the series resistance or decreasing the shunt resistance at the thinner absorber layer of the devices, photovoltaic performances also decreased.15,

40

Additionally, for thinner absorber layer, the electrons will be generated closer to the back contact which creates higher probability for back contact recombination and the number of micro short circuits can be increased due to the roughness of the layers. Figure 7a shows the energy level diagram of the heterojunction solar cell. According to the view point of the device physics, at the interface of the two layers, a favorable band alignment is developed for the electron transport from the conduction band of Cu2O to the conduction band of ZnO, as shown in Figure 7b. It is well known that, the built-in bias is an important factor for the heterojunction solar cells as it can resist the flow of undesirable dark current and provide a VOC results from the diffusion of electrons in the n-ZnO to the p-Cu2O. At the interface, depletion layers are developed and the built-in bias Vbi (n) and Vbi (p) are produced in the ZnO and Cu2O layers, respectively. By applying Poisson's equation, the ratio of the two components of the build-in bias is given by: () ()

=

…………………….(1)

Where, NA, ND are the acceptor and donor densities and εCu2O, εZnO are the absolute permittivity of Cu2O and ZnO, respectively. Detailed derivation is presented in supporting information. From the Hall measurement data sheet (shown in Table 1), it is clearly shown that after annealing, the carrier densities of Cu2O and ZnO are typically in the order of 1015 /cm3 and 1018 /cm3, respectively. Therefore, it follows from equation (1) that almost the entire built-in bias of the hetero-junction is created in the Cu2O layer ( εCu2O = 6.2, εZnO = 8.0 41).


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Thus, total depletion region width ( ) is given by =

( + )

…………………….(2)

where, q is the charge, εs is the permittivity of the semiconductor and Vbi is the built-in voltage across the p-n junction. The values of the depletion layer thicknesses in the ZnO and Cu2O layers (i.e., xn and xp) can be determined by using equation (2). Detailed derivation is in supporting information. For Cu2O, the depletion layer thickness xp is given by

=

ε .

…………………………………….(3)

That means,

…………………………………….(4)

Musselman et al.32 reported that for an expected built-in bias in the range of 0.4 to 0.7 V, the depletion layer thickness in the Cu2O (xp) is approximately 2.3 to 3.0 µm where carrier density approximately 6x1013/cm3. Experimentally, they observed that if the thickness of the Cu2O layer is lower than the calculated value of the depletion layer thickness then the values of VOC decreased sharply. That means the calculated value of depletion layer thickness matches well with the practical observation. In our case, the carrier density of Cu2O (NA) is approximately 4.17×1015/cm3 which is much higher than the reported value of carrier density by Musselman et al. By using the same equation, the calculated value of depletion layer thickness is ~250-300 nm for the same expected built-in bias in the range of 0.4 to 0.7 V. In the present case, the calculated value of depletion layer thickness (~250-300 nm) is much less than the value of the depletion width (2.3 to 3.0 µm) of the electrodeposited Cu2O-ZnO heterojunctions reported by Musselman


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et al. group.32 When the Cu2O thickness is lower than the thickness of depletion layer, we also observed the poor performances of the devices. This implies that the drop in VOC is due to the inhibition of the full built-in bias. The loss in the VOC may be due to the recombination in the back contact region for the thinnest devices.42 The large interface state density can also influence the built-in bias of a heterojunction and depletion layer thickness. Therefore, the importance of the depletion layer thickness in a particular heterojunction will depend on the material and interfacial conditions. On the other hand, the smaller depletion layer thickness in the ZnO means by UV absorption carriers are generated in the ZnO and collected by a diffusive process. The degradation of the solar cell efficiency for thinner absorber layer have also been observed in case of CIGS and Cu(In,Ga)Se2 based solar cell.43-45 They reported that the reduction of Jsc, Voc, and FF values for thinner absorber layer were attributed due to optical losses, increase in recombination due to the degradation of the material, back contact recombination and shunting problems. For future investigation, if we introduce an electron reflector at the back contact to avoid recombination as well as use a buffer layer to control the current flow, then the performance of the cell may be improved. This study has important implications for the design of heterojunctions for photovoltaics and photodetectors applications. Moreover, the higher quality of the ZnO and Cu2O films is reflected at the performances of the solar cell, which revealed to be stable for 2-3 months. Regarding to the production route, a low cost spray deposition technique has been used in this work for making the combinatorial device library which has the potential for fast development in industry and it is quite difficult to develop this type of device library using other techniques like electrodeposition, sputtering or copper oxidation at high temperatures.


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Conclusions The systematic study on the performances of the device library as a function of the individual layer thicknesses provides a more complete data rather than one conventional single device measurement with a particular layer thickness. It gives the clear idea about the basic device physics including the formation of the built-in bias inside the cell. It is shown that the cell performances of the device library are strongly dependent on the thickness of the absorber layer. The device shows poor performances when the thickness of the absorber layer is lower than that of depletion layer. Diminished performances of the cells having lower thicknesses are attributed due to less absorption, low carrier density and mobility in Cu2O, back contact recombination of the carriers, inhibition of the full built-in bias etc. The back contact could be superior by increasing the total optical reflection and diffusing reflectance by texturing. The incorporation of an electron reflector at the back contact to avoid recombination was also recommended. The lack of other light absorbing p-type oxides creates doped Cu2O or other semiconducting low band gap oxide materials with high carrier concentration and enhanced mobility for future investigation. The improvement of the electrical and morphological properties of the nanostructured oxide materials are also required to achieve the improved performances of the devices.


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Thickness

y (mm)

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60

0.25

x

Figure 1. Schematic representation (left) and measured thickness data (right) of the spray pyrolysis deposited combinatorial device library on ITO substrates to which (a) ZnO layer is deposited with a linear gradient in y-direction and (b) a linear Cu2O gradient in x-direction. (c) Joint thickness profile of both layers.


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Figure 2. (a) Schematic view of the combinatorial heterojunction. (b) Cross-sectional SEM image of the heterojunction, SEM image after FIB milling experiment showing in false colors of the corresponding X-ray maps of Au, Cu, Zn and In. The dashed lines indicate the limits of each individual layer.


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(a)

(b)

Figure 3. (a) Surface elemental analysis of ZnO/Cu2O heterojunctions as well as after FIB milling showing the corresponding X-ray maps of In, Cu, Zn, O. (b) The EDS spectrum of the heterojunction.


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Figure 4. (a) Photograph of the combinatorial photovoltaic device library, (b) J-V characteristics of the three different points which is marked by circles in the photograph of the device library. Variation of (c) JSC and (d) VOC values for the ZnO/Cu2O device library as a function of Cu2O thicknesses. For any particular Cu2O thickness, there are many points corresponding to different ZnO thicknesses.


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Cu2O thickness

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Figure 5. Color maps of the (a) JSC, (b) VOC, (c) maximum power output (Pmax) and (d) fill factor derived from the J–V curve under illumination. The black points indicate for dead cells.


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250 nm 400 nm 550 nm 750 nm

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400

500

600 700 Wavelength (nm)

800

Figure 6. UV-Vis absorption spectra of ZnO/Cu2O based combinatorial hetero-junction device at different Cu2O thicknesses. ZnO thickness is fixed (200 nm).


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Figure 7. (a) Energy level diagram of ZnO/Cu2O-based solar cells on ITO. (b) Schematic energy band diagram of the hetero-junction solar cell.

Table 1. Resistivity, Hall mobility and carrier concentration data sheet of ZnO and Cu2O film.

Sample ZnO (n-type) Cu2O (p-type)

Resistivity

Hall mobility

Carrier concentration

Sheet 5.54 x103 Ohm/sq

Co eff. -4.39 m2/c

Sheet -1.42x1014/cm2

Bulk 0.0941 Ohm-cm

Mobility 7.93 cm2/v-s

Bulk -8.37x1018/cm3

Sheet 1.91x107 Ohm/sq

Co eff. 2.49x103 m2/c

Sheet 2.50x1011 /cm2

Bulk 1.15x103 Ohm-cm Mobility 1.3 cm2/v-s

Bulk 4.17x1015/cm3


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ASSOCIATED CONTENT Supporting Information. Details XRD pattern of the film, cross-sectional SEM images of the ZnO/Cu2O based combinatorial heterojunction at two different positions, optical density curve at different Cu2O thicknesses, SEM and AFM images of Cu2O layer after different annealing time, and theoretical calculation of the built-in-bias and depletion width are in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Shrabani Panigrahi - [email protected] and *Elvira Fortunato - [email protected] (EF) ACKNOWLEDGMENTS This study was funded by the European Commission under the FP7 All Oxide PV project “Novel Composite Oxides by Combinatorial Material Synthesis for Next Generation All-OxidePhotovoltaics” number 309018 and the FP7 ERC AdG project “Transparent Electro-nics” number 228144. This work was partially supprted by FEDER funds through the COMPETE 2020 Programme and National Funds throught FCT - Portuguese Foundation for Science and Technology under the project UID/CTM/50025/2013. I want to acknowledge Ms. Sofia Ferreira for her help to deposit AZO layer by using sputtering technique on the upper layer of heterojunction (ITO/ZnO/Cu2O).


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