Optical, structural and morphology study of Cu2O/Cu and GO/Cu2O/Cu films prepared by pulsed electrodeposition and electrophoresis.

In this work, copper (I) oxide films were prepared by pulsed electrodeposition onto copper substrates. Graphene oxide was deposited on the Cu2O/Cu films by cathodic electrophoresis. The films were studied by X-ray diffraction, Raman spectroscopy, optical reflectance and atomic force microscopy. The bandgap of the Cu2O/Cu films is close to 1.8 eV due to the presence of defects and decreases to close to 1.1 eV with GO deposition due to the oxidation of Cu2O to CuO on the surface. When GO was deposited, a reduction in the mean height was observed, indicating coverage of the entire surface. A topographic transformation of the surface was also observed, consisting of an increase in grain size and homogenization of the grain shape after GO deposition, possibly due to phase transformation. This work is the first step to prepare fully wet deposited thin film ZnO/GO/Cu2O/Cu solar cells.


Introduction
Three generations are known in photovoltaic technology, the first is based on crystalline silicon wafers, the second uses thin films deposited on low-cost substrates like glass or polymers, and the third generation includes organic cells, dye sensitized solar cells and emergent solar cells like those based on quantum dots or perovskites [3].Although thin film solar cells based on CdTe or Cu(In,Ga)Se2 have efficiencies comparable with those of c-Si, these cells still present problems such as high-cost production and the use of scarce, strategic or toxic raw materials [4], thus, abundant, non-toxic and avaliable materials are required for thin film solar cells with competitive efficiencies [5].As an example, the Cu 2 O/ZnO heterojunction thin film solar cells have been widely studied [6] because of low-cost, abundance and low toxicity, although, due to interdiffusion at the ZnO-Cu2O interface results in low voltages and reduced lifespan.Therefore, a graphene oxide (GO) layer is proposed in this work as a separator between n-type (ZnO) and p-type (Cu2O) semiconductors to avoid ion migration and to favor device stability.On the other hand, it has been demonstrated that pulsed electrodeposition of semiconductors films produces changes in nanostructuration, crystallinity, grain size and optical properties [10].In this work, the GO deposition effect onto Cu2O films deposited in different frequencies on its structure and optical properties is studied, as a first step for a full device development.

Experimental
Copper (I) oxide films were prepared by pulsed electrodeposition on copper substrates using an electrolytic solution bath (copper (II) sulphate 0.4 M) and lactic acid as complexing agent, varying pulse frequency (10, 50 and 100 Hz) applied with a function generator using a square wave and a 50% duty cycle.Graphene oxide was deposited onto Cu2O/Cu films by cathodic electrophoresis using a 20A DC source, using a ladder-shaped run, by increasing voltage from 2 to 15 V, increasing 1 V each step, keeping the voltage during ten seconds.Films were characterized using a Bruker D8 Advance diffractometer using Cu K (= 0.15406n nm) radiation, equipped with Lynx Eye detector in Bragg-Brentano configuration from 20°-80° in 2 with 0.02° step size each second and sample rotation.Raman spectra were collected using a Micro Raman i-Plus in a spectral range of 100-3000 cm -1 using a laser line of 532 nm with a fluence of 6 mW/cm 2 .Optical reflectance spectroscopy was performed from 350 to 1000 nm using a Stellarnet system composed of a tungsten-halogen source, a bifurcated 600 m optical fiber and a Blue Wave spectrometer.Atomic Force Microscopy (AFM) was performed in the contact mode in a TT-AFM Workshop instrument, using Sb-doped Si, diamond-like carbon coated tip.

Results and discussion
X ray diffractograms of Cu2O/Cu films are presented in Figure 1, where cuprite (Cu2O) presence is confirmed by PDF card #050667, peaks at 2 29.5°, 36.4°,42.3°, 61.3° and 73.5° corresponding to the (110), ( 111), ( 200), ( 220) and (311) planes with a preferential growth in (111) plane.The calculated lattice parameter was a= 0.42 nm.The intensity of the peaks suggest that it is directly proportional to the deposition frequency.Raman spectra of the GO/Cu2O/Cu structure are presented in Figure 2. Graphene oxide D and G bands about 1348 cm -1 and 1590 cm -1 respectively were observed.Also, a signal at 211 cm -1 is observed for Cu2O which increases with the deposition frequency.It can be observed that as the Cu2O deposition frequency increases, D and G bands are depicted with greater intensity, suggesting a higher GO content probably due to higher surface area in the Cu2O layer, which may have increased with the deposition frequency.Additionally, non-assigned Raman band was observed at 819 cm -1 , but its identity shall be elucidated in a future work.In Figure 3a) it can be observed that reflectance values are between 3.6% and 4.4%, decreasing as the deposition frequency increments.After GO (Figure 3b)) deposition reflectance increases to 6%-10.3%.Kubelka-Munk spectra obtained from the reflectance measurements of the Cu2O/Cu and GO/Cu2O/Cu structures are shown in Figure 3(c,d).The figure shows Cu2O band gap values obtained from the Kubelka-Munk formalism.In Figure 3c) is noticed that band gap values increase with deposition frequency from 1.83 to 1.88 eV.The difference with characteristic Cu 2 O band gap value (2.2 eV) could be due to the presence of defects [10].
Lower band gap values are observed in Figure 3d) with respect to the films without GO, i.e. 1.12-1.16eV.This value is consistent with the cupric oxide (CuO) band gap value, which is near to 1.2 eV, suggesting that the GO electrophoretic deposition process produces Cu2O oxidation in the surface.
Figure 4 shows 20 x 20 µm 2 AFM images of Cu2O/Cu (Figure 4 a-c) and GO/Cu2O/Cu (Figure 4 df) films.In the images of the Cu2O/Cu films, an increase in the grain size is appreciated with deposition frequency.After graphene oxide deposition (Figure 4 d-f), an increase in the grain size in all films is observed, which would be consistent with the phase change at surface from Cu2O to CuO suggested in the optical spectra.5 presentsthe height distribution from the AFM images.In the Cu2O/Cu films it can be noticed a narrow height distribution and a mean height of 0.5 m for the film grown at 10 Hz; meanwhile the films grown at 50 and 100 Hz an enlargement in the height distribution with grains up to 2.6 m, related to multiple nucleation and growth events is observed.However, after GO deposition changes in the film mean heights are observed, which are related to phase transformation of material.

Conclusions
Cu2O films were prepared by pulsed electrodeposition on copper substrates in different frequencies.GO was deposited on these films by cathodic electrophoresis.The Cu2O/Cu band gap is about 1.8 eV due to the presence of defects, but it decreases to about 1.16 eV after GO deposition, possibly due to Cu2O oxidation to CuO in the surface, because of the action of graphene oxide as a strong oxidant.It was also observed a topographic transformation of surface consisting in a grain size increase and shape grain homogenization after GO deposition, possibly because of phase transformation.Future studies will compare the GO/ZnO deposition to determine the best configuration for the device.

Figure 3 .Figure 4 .
Figure 3. Reflectance spectra of Cu2O/Cu (a) and GO/ Cu2O/Cu (b); and reflectance spectra modified as a result of the application of the Kubelka-Munk function to determine the Cu2O/Cu band gap (b) and the effect of graphene deposition on the band gap (c)

Figure
Figure5presentsthe height distribution from the AFM images.In the Cu2O/Cu films it can be noticed a narrow height distribution and a mean height of 0.5 m for the film grown at 10 Hz; meanwhile the films grown at 50 and 100 Hz an enlargement in the height distribution with grains up to 2.6 m, related to multiple nucleation and growth events is observed.However, after GO deposition changes in the film mean heights are observed, which are related to phase transformation of material.

Figure 5 .
Figure 5. Mean height distribution obtained from AFM images.a) Cu2O/Cu deposition at different frequencies, b) deposition with GO.