This journal is©The Royal Society of Chemistry 2017 J. Mater. Chem. C

Cite this:DOI: 10.1039/c7tc00275k

A transparent solar cell based on a mechanicallyexfoliated GaTe and InGaZnO p–n heterojunction†

Ah-Jin Cho,ab Kyung Park,b Solah Park,ab Min-Kyu Song,ab Kwun-Bum Chungc andJang-Yeon Kwon *ab

Two-dimensional (2D) materials are known for their unique properties and potential for application in

various electronic and optoelectronic devices. Since 2D semiconductors have weak bonding between

the layers, they can be easily separated into several nanometer-thick layers which still maintain their

characteristics. GaTe is a p-type 2D semiconductor having a direct bandgap. By combining multi-layer

GaTe and thin-film IGZO, we have fabricated a p–n heterojunction, a fundamental unit of optoelectronic

devices. In this paper, we propose the first fully transparent solar cell using a 2D material, based on a

GaTe/IGZO heterostructure. The device shows a high transparency of B90% and an efficiency of 0.73%

with a fill factor of 37%. It exhibits instantaneous generation of photo-carriers under periodic light

pulses. Further analysis of the operating mechanism was conducted by studying its band alignments.

The transparency of our GaTe/IGZO solar cell can overcome its relatively low efficiency, as it can be

installed in a much larger scale and the total amount of generated power will surpass that of the

conventional solar cell. Furthermore, advances in the large-scale growth of GaTe will enhance the

power conversion efficiency, and finally enable the adoption of 2D active layer based highly transparent,

thin-film solar cells for building integrated photovoltaic systems.

Introduction

Solar energy is considered to be one of the most powerful andreliable renewable energy sources. Consequently, solar cells, thedevices that enable the direct conversion of solar energy intoelectrical energy, are widely studied in order to achieve highenergy conversion efficiency at a low cost. Conventional crystallinesilicon based solar cells have reached a power conversion effi-ciency (PCE) of B25%, and multi-junction III–V semiconductorbased cells show an even higher efficiency of B40%.1–3 Never-theless, the widespread application of solar energy harvestingsystems is often hampered by the limitation of installation space,which requires a large area, resulting in a high installation cost.In order to overcome this hurdle, solar cells with a transparent

and light-weight form, which can be integrated into buildings, arereceiving attention.4–6 Transparent thin-film solar cells possessa great advantage in that they can utilize the existing exterior ofa building, car or mobile device for energy harvesting, ratherthan occupying additional installation sites.

The term ‘transparent solar cell’ somewhat contains contra-dictory concepts in that transparency indicates no interactionwith incident light, while a solar cell is a device that absorbslight and converts it into electrical energy. In order to satisfythese two requirements at once, the cell has to transmit most ofthe visible light to achieve transparency but also utilize part ofthe visible light and rays of a different spectrum for energyharvesting. This results in an inherent limitation of efficiency forthe transparent solar cell. However, its potential to be installedin a much larger area without consideration of aesthetic andspatial aspects can easily offset the low PCE value, because theamount of the total generated power could be even higher.

Two-dimensional (2D) materials are known for their peculiarelectrical, mechanical and optical properties.7–10 The crystal struc-ture of layered materials is composed of vertically stacked layers,which are weakly held together by van der Waals interactions.11,12

As a result, they are easily separated into atomic layers showinghigh transparency due to their extreme thinness of around 1 nm.Unlike most bulk materials, they can maintain their own electricalproperties even with such a thin layer.13 In particular, 2D semi-conductors have a tunable bandgap with thickness variation.14

a School of Integrated Technology, Yonsei University, Incheon, 21983, Korea.

E-mail: jangyeon@yonsei.ac.krb Yonsei Institute of Convergence Technology, Incheon, 21983, Koreac Division of Physics and Semiconductor Science, Dongguk University, Seoul, 04620,

Korea

† Electronic supplementary information (ESI) available: Fig. S1: electrical andoptical properties of the 150 nm-thick rf sputtered ITO film. Fig. S2: semi-logarithmic I–V plot of the GaTe/IGZO p–n heterojunction in dark state withleakage current level. Fig. S3: spectral intensity of the halogen light source, whichwas used for the photovoltaic performance measurement. Fig. S4: photovoltaicperformance of other samples. Fig. S5: rise time plot of the GaTe/IGZO solar cellwith respect to the data acquisition speed during the measurement. Fig. S6: bandstructure analysis on IGZO film. See DOI: 10.1039/c7tc00275k

Received 17th January 2017,Accepted 30th March 2017

DOI: 10.1039/c7tc00275k

rsc.li/materials-c

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MoS2, one of the most well-known 2D semiconductors, exhibitsan indirect-to-direct bandgap transition when thinned downto a monolayer.15,16 In addition, 2D semiconductors tend toshow extraordinarily high light absorption compared to 3D semi-conductors with the same thickness. Compared to 1 nm-thickSi and GaAs, two of the most widely used absorber materials forsolar cells, 1 nm-thick MoS2 is known to absorb 39 and 13 timesmore sunlight, respectively.17 Due to such characteristics, the2D semiconductor can be a good candidate as an absorptionlayer for transparent solar cells.

The p–n junction is one of the most fundamental elements torealize optoelectronic devices. There have been several studiesthat fabricated p–n junctions by combining 2D semiconductorsand bulk materials with the opposite electrical properties. Lopez-Sanchez et al. and Tsai et al. reported the photovoltaic propertiesof a heterostructure of an n-type MoS2 monolayer and p-Si.18,19

Hao et al. utilized bulk-like MoS2 deposited by magnetronsputtering with p-Si to form a p–n junction.20 Lin et al. fabricateda MoS2/GaAs heterostructure solar cell with interface tuning byinserting h-BN sheets.21 In another study, they utilized InP as ap-type semiconductor and monolayer MoS2 as an n-type layerto form p–n junctions.22 Although these devices are not trans-parent, they show sufficient feasibility of photovoltaic cells using2D-bulk hybrid p–n junctions.

We fabricated transparent solar cells with 2D-bulk hybridp–n heterojunctions consisting of GaTe and IGZO as p-type andn-type semiconductors individually. GaTe is a typical III–VIlayered semiconductor with p-type transfer characteristics.23

Unlike MoS2, multi-layer GaTe shows a direct bandgap, whichmakes it highly advantageous for optoelectronic applications.It has been reported that GaTe photodetectors show a very highphotoresponsivity of B104 A W�1, which is the highest valueamong the 2D semiconductor-based photodetectors.11 Becauseof its thinness, the multi-layer GaTe transmits a lot of theincident visible light. IGZO is an n-type oxide semiconductorthat is transparent due to its wide bandgap of B3.4 eV.24,25

Unlike crystalline silicon or other III–IV semiconductors,IGZO can be easily deposited by sputtering and can operatein a thin-film form.26 It is also a direct bandgap semiconductor.27

Therefore, by forming a transparent electrode with ITO on theGaTe/IGZO heterostructure and utilizing glass substrates, wefabricated a transparent solar cell. The transparency of our cellenables wide applications such as a building-integrated photo-voltaic system (BIPV).

Results and discussion

Fig. 1a shows the schematic of our 2D-bulk hybrid heterostruc-ture based transparent solar cell. The multiple layers of GaTe,which belongs to a family of layered semiconductors, and thinfilms of rf sputtered IGZO (50 nm) are partly overlapped,forming a p–n heterojunction. This heterostructure is connectedto the transparent ITO electrodes so that the photo-generatedcarriers from the active region can be collected. IGZO, an n-typesemiconductor layer, has a wide bandgap of 3.4 eV, causing the

Fig. 1 Fundamental information on the device and material. (a) A schematic image of the structure of the GaTe/IGZO transparent solar cell underillumination. (b) The crystal structure of GaTe, a layered semiconductor. (c) The Raman spectra of the mechanically exfoliated GaTe multi-layertransferred onto the glass substrate. (d) The transmittance spectrum of the entire device after finishing fabrication. The inset shows a photograph and anoptical microscope image of the device.

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visible light rays to pass through it rather than be absorbed.ITO is one of the most famous transparent conducting oxide(TCO) materials. It has low resistivity and high transparency atthe same time, enough to be utilized as a transparent electrode(Fig. S1, ESI†). As a substrate, alkali-free glass was chosen. As few-layer GaTe, the only intrinsically opaque material used in ourdevice, transmits most of the incident light due to its extremethinness, the entire device is highly transparent.

GaTe is a typical III–VI layered semiconductor which has highpotential for optoelectronic applications due to its optimum sizeand the type of bandgap. It has a direct bandgap of 1.7 eV whichmakes it advantageous for photon–carrier transition.11 Fig. 1bindicates the monoclinic crystal structure of GaTe, which isdifferent from the hexagonal structure of other III–VI layeredmaterials such as GaS or GaSe.11,12,28 GaTe has a much morecomplicated and less symmetric structure, which results in largelydifferent properties from other III–IV compounds, such as havinga direct bandgap. Overall, strongly covalent bonded Ga and Teatoms form each layer and the weak van der Waals force betweenthe layers maintains the layered structure. Hence, it can be easilyseparated into mono- or multi-layers from the bulk crystal. Inparticular, the GaTe mono-layer is composed of two types of Ga–Gabonding: two-thirds are perpendicular to the layer and one-third isoriented nearly parallel to the layer.23,28,29

The mechanically exfoliated GaTe multi-layer (B70 nm) wasused for the fabrication of our transparent GaTe/IGZO solarcell. The Raman spectrum of the GaTe flake exfoliated from thebulk crystal is shown in Fig. 1c. It shows the strongest Ag peak

at 126.2 cm�1 and another Ag peak at 114.5 cm�1. A weak Bg peakat 173.7 cm�1 is also observed, with other peaks at 109 cm�1,141.1 cm�1 and 160.6 cm�1, which are well-matched with theprevious reports on the Raman spectra and the calculatedphonon dispersion curves of GaTe.28,29 The results confirm thehigh quality of the exfoliated GaTe utilized in our experiment.

Before discussing the electrical and photoresponse behaviorof the GaTe/IGZO heterostructure based photovoltaic cell, weexamined its transparency. Transmittance from 380 nm to780 nm, throughout the visible light spectrum, was measuredusing UV-Vis-NIR spectrophotometer (Fig. 1d). As the photo-graph of the device in the inset of the transmittance plot shows,our GaTe/IGZO solar cell is highly transparent in the visiblelight region, with an average transmittance of 90.86%. Inaddition, the device is almost colorless due to its relativelyuniform absorbance throughout the visible light spectrum.

After characterizing the physical properties of the GaTe/IGZOsolar cell, its electrical performance was measured. Fig. 2apresents the I–V characteristics of our heterostructure in thedark, operating clearly as a p–n diode. It shows current rectifyingbehavior with a peak forward-to-reverse bias current ratio (recti-fication coefficient) of around 560 at a voltage bias of �3.7 V.A leakage current of B10 pA was observed under a reverse bias of�4 V (Fig. S2, ESI†). In addition, the highly linear currenttransport shown in the I–V plot of ITO–IGZO and the ITO–GaTestructure (inset of Fig. 2a) confirms that the ITO electrode formsohmic contacts with both IGZO and GaTe flakes, which enablesoptoelectronic measurements without the harsh effect of the

Fig. 2 Electrical characteristics of the device. (a) I–V characteristics of the GaTe/IGZO p–n heterojunction diode with the rectification ratio. The insetexhibits the formation of ohmic contact between ITO–IGZO and ITO–GaTe. (b) The J–V plot of the device showing the photovoltaic effect, undervarious illumination intensities. (c) Current and generated power from the device as a function of bias voltage and extracted power conversion efficiency(measured under a light power of 4.25 mW cm�2). (d) The experimentally measured value of short-circuit current density Jsc (blue symbols), open-circuitvoltage Voc (orange symbols), and fit of the power law (blue dashed line) with respect to the incident power density of light. (e) The power conversionefficiency (blue symbols) and fill factor (orange symbols) extracted from the experimental data, with respect to the incident power density of light.

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Schottky barrier height. As a result, it is confirmed that theGaTe/IGZO heterostructure has successfully formed an ohmiccontact p–n diode.

The photovoltaic performance of our device under illumina-tion was then tested. A halogen lamp was used as the irradia-tion source and was connected to a vacuum chamber by anoptic fiber. (The spectral intensity of the halogen light sourceis shown in Fig. S3, ESI.†) All of the photovoltaic response wasmeasured under vacuum conditions with light shining verti-cally onto the active region of the device. Fig. 2b demonstratesthe existence of an open-circuit voltage (Voc) and short-circuitcurrent density (Jsc) under light, which is clear evidence that theGaTe/IGZO heterostructure operates as a photovoltaic cell. Asthe intensity of incident light increases, the J–V curve shiftstoward the lower right quadrant. When the heterojunction isunder illumination with a light power of 4.42 mW cm�2, arectifying J–V curve with Voc of 0.14 V and Jsc of 0.63 mA cm�2 isobserved. We fabricated B10 samples, and most of themexhibited a photovoltaic effect with similarly shaped J–V curves(Fig. S4, ESI†). This result indicates that the band alignment ofthe two semiconductors formed a p–n heterojunction as wedesired and successfully shows a photovoltaic effect. Furtherstudy of the energy-band diagram of our device will be describedin the last part of this paper. Fig. 2c is a current and power plot ata fixed light power of 4.25 mW cm�2. The device reached amaximum power point of 5.68 pW at V = 0.08 V and I = 71.01 pA.At this point, the PCE of the GaTe/IGZO heterojunction basedsolar cell was calculated as 0.73% with a fill factor of 0.37. Thearea of the active region was measured as an overlapped regionof two semiconductor materials, yielding 18.41 mm2. Previouslyreported solar cells using a heterojunction of 2D semiconductorsand opaque bulk semiconductors such as exfoliated MoS2/p-Si(0.12%),18 CVD MoS2/p-Si (5.23%),19 sputtered MoS2/p-Si (1.3%)20

and CVD MoS2/InP (7.1%)22 tend to show about tens of timeshigher efficiency. However, when compared with studies utilizing2D–2D heterojunctions (MoS2/WSe2 (0.2%)30 and ReSe2/MoS2

(0.072%)31), which have the potential to be applied as an activelayer for a transparent solar cell, our GaTe/IGZO solar cell exhibitshigher power conversion efficiency. Furthermore, when the large-scale CVD growth of GaTe is achieved and the configuration of theGaTe/IGZO photovoltaic cell is optimized, the PCE will be furtherenhanced to a great extent.

In Fig. 2d and e, we present the main parameters of thephotovoltaic cell: Jsc, Voc, PCE and FF, as a function of incidentpower density (extracted from Fig. 2b). Jsc varies from 0.25 to0.63 mA cm�2 as the irradiation power increases. In Fig. 2d, theblue-colored symbols are the experimentally obtained value ofJsc and the dashed line shows the fit of the power law. It isknown that the short-circuit current of a solar cell with respectto the light intensity (I) usually follows a power law, Jsc p Ia,where a varies with the recombination type.32 When extractingfrom a linear fitted line of a log-scale I–Jsc plot, a = 1.09 (nearunity) which indicates that most of the carrier loss occurredby monomolecular recombination via trap states.32,33 Voc scaleswith ln(I), as expected from the conventional p–n junctiontheory.30 When the correlation between open-circuit voltage

and light intensity was calculated, the Pearson product-momentcorrelation coefficient (Pearson’s r) turned out to be 0.99, whichindicates a clear positive correlation between these two factors.Fig. 2e indicates the efficiency and fill factor of our solar cell withrespect to the incident light power density. The power conversionefficiency is defined as PCE = Pmax/Pinc = (Voc�Jsc�FF/Pinc) � 100%,where Pmax is the output at the maximum power point and Pinc isthe power density of incident light. The fill factor is calculatedusing the equation FF = Pmax/Voc�Jsc. Orange-colored symbols inFig. 2e indicate the FF values, which do not show large variationsthroughout the measured range. The Pearson r value between thePCE (blue-colored symbol) and light intensity was calculated as0.97, which indicates high positive correlation. Such a propor-tional relationship can be explained from the equation definingthe efficiency of a solar cell. As Voc and Jsc increase with theenhancement of light power and FF remains stable without largevariation, PCE, which is proportional to the product of the threeparameters, must scale with light intensity.

After testing the general photovoltaic performance of theGaTe/IGZO transparent solar cell, we also measured its responseunder monochromatic light with a timescale. For the measure-ment, light generated from a Hg–Xe lamp and filtered by amonochromator was used. By controlling the electronic shutterwith a digital timer, we generated a light pulse that alternatesbetween on and off. The GaTe/IGZO p–n heterojunction wasexposed to this light pulse inside a vacuum chamber and theelectrical response was simultaneously measured in real time.Considering the bandgap size of GaTe (1.7 eV), the mainabsorbing layer, the wavelengths of incident light were set at400 and 550 nm. Before testing the device, we measured thepower density of light at each wavelength using a power meter.Visible light of 400 and 550 nm was found to have a powerdensity of 0.79 mW cm�2 and 1.31 mW cm�2 individually.

Fig. 3a shows the short-circuit current variation of the GaTe/IGZO photovoltaic cell with respect to time, when it is exposedto a monochromatic light pulse repeating 0.5 seconds on and0.5 seconds off. As the short-circuit current is defined as acurrent flowing through the solar cell without any applied bias,the bias voltage was set as zero throughout the measurementand its current change between two electrodes was observed.When the light was off, the ‘‘low’’ current state of around 6 pAwas maintained, but it changed into a ‘‘high’’ current state ofaround �20 pA (400 nm light) or �15 pA (550 nm light) rightafter the light was turned on. As the photocurrent changeoccurs very quickly, the Isc plot along the time axis followsthe original pulse-form of incident light with an interval of0.5 seconds. According to the wavelength of irradiation, the‘‘high’’ current level, which includes the photocurrent, changes.Upon comparing the responses under different wavelengths ofmonochromatic illumination, 400 nm light resulted in higherphotocurrent even though its power density was less than thatof 550 nm light. This result might have originated from thedifference in the absorption rate of the GaTe/IGZO heterostruc-ture based on the wavelength.

Fig. 3b shows the time dependence of the open-circuit voltagefor the device under periodic light pulses with 3 seconds on and

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3 seconds off. As the open-circuit voltage is defined as a voltageacross the electrodes when the net current flowing through thesolar cell is zero, the change in voltage between two electrodeswas observed while maintaining zero current. During the light-on period, the open-circuit voltage increased and saturated at a‘‘high’’ level. On the other hand, when the light was off, thevoltage decayed to a ‘‘low’’ level. Unlike the rapid variationshown in the Isc plot (Fig. 3a), the Voc curve tends to changemuch more slowly in response to the change in illumination.In particular, the recovery took much longer compared to therelatively sharp increase and saturation of Voc upon incidentlight. This indicates a relatively long carrier lifetime within theGaTe/IGZO heterojunction. Trap-state mediated electron–holerecombination may be responsible for this slow photovoltagerecovery by reducing the recombination rate for photogeneratedcarriers.34 Otherwise, the type II band alignment (further expla-nation in Fig. 4) of GaTe and IGZO, which leads to the accumu-lation of electrons at IGZO and holes at GaTe, might reduce therecombination possibility and increase the carrier lifetime.35

The dynamic performance of the GaTe/IGZO solar cell wasfurther studied by extracting the response time from the Isc

time-scale plot. Fig. 3c and d show the rising and falling edgesof the Isc plot in the second period, respectively. The rise time isdefined as the time taken by a signal to change from 10% to90% of its step amplitude while the fall time is determined bythe time spent for the signal to change from 90% to 10% of itsheight. The rise time and fall time extracted from the Isc curve

under the same wavelength of light showed similar values ofB130 ms, which verified the highly symmetric and instanta-neous response for light on and light off. It also implied therapid transfer of the photogenerated electrons and holes toIGZO and GaTe. As the data acquisition speed of the measuringequipment is still limiting the response speed, the responsetime will be even lower as the measuring speed is increased.We have experimentally determined that one sample, whichshowed a rise of 133 ms under the data acquisition interval of166 ms, tends to exhibit a linear decrease of the response timealong with an increase in measuring speed (Fig. S5, ESI†). Therise time reached 34 ms under the maximum measuring speedof 42 ms�1, which is the limitation of our machine’s technicalspecification.

The responsivity and external quantum efficiency (EQE)values were calculated from Fig. 3a as parameters indicatingthe solar cell performance. Responsivity at a certain wavelength(Rl) can be calculated by the following equation, Rl = Iph/Pl�S,where Iph is the current generated by photons, Pl is the powerdensity of incident light and S refers to the active area. Bysubstituting the amount of Isc change with Iph, the responsivityof the GaTe/IGZO solar cell was calculated to be 182.91 and87.63 mA W�1 for 400 and 550 nm light individually. In the caseof EQE, it is defined as EQE = h�c�Rl/q�l, where h is the Planckconstant, c refers to the speed of light and q is the elementarycharge. The resulting EQE values were found to be 56.76%(400 nm) and 19.78% (550 nm).

Fig. 3 Device response to dynamic light. Time domain of (a) short-circuit current and (b) open-circuit voltage plots obtained from our GaTe/IGZO solarcell, under periodic monochromatic light at wavelengths of 400 and 500 nm. (c) Rise edge with extracted rise time and (d) falling edge with extracted falltime, from the second period of the short-circuit current plot.

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In order to understand the operating mechanism, weattempted to determine the band alignment for the multi-layerGaTe/IGZO p–n heterojunction photovoltaic cell. The bandgap sizeand work function difference between two semiconductors are themain parameters that determine the type of heterojunction. Thebandgap was determined using spectroscopic ellipsometry (SE)data and previously reported values (Fig. S6, ESI†). In order todetermine the work function difference, Kelvin probe force micro-scopy (KPFM) was utilized. KPFM measures the contact potentialdifference (CPD) between a conducting AFM tip and a sample.The CPD between the tip and the sample is defined as: VCPD =(jtip � jsample)/(�e), where jtip and jsample are the work functionsof the tip and the sample individually, and e is the electroniccharge.36 Fig. 4a and b show the line profile of CPD across SiO2–GaTe and SiO2–IGZO, and their CPD mapping image (inset).Considering the CPD of the SiO2 substrate as a reference, GaTeand IGZO exhibit 0.31 eV and 0.66 eV higher potential than thereference. From the KPFM measurement data, the work functiondifference between GaTe and IGZO can be extracted as 0.35 eV. Asthis value is extracted from the CPD line profile fitted with a stepfunction, it has an error range of B7.1%, but the variance is smallenough to leave the overall band alignment unchanged. Synthesiz-ing the entire information on the band structure of the twosemiconductors, the band alignment of our p–n junction can bevisualized as in Fig. 4c. The potential difference between the fermilevel and the valence band of GaTe is not clear yet. Nevertheless,from the information we have collected, we can make a reasonableinference that both the conduction band and valence bandof IGZO are positioned at a lower level than those of GaTe. There-fore, it can be concluded that GaTe and IGZO form a type II

staggered-gap heterojunction, which is advantageous for opto-electronic applications by enabling spontaneous electron and holeseparation. Fig. 4d shows the equilibrium state energy-band dia-gram when the GaTe/IGZO heterojunction is formed, and Fig. 4eexplains how the photovoltaic effect occurs under illumination.When the incident photon falls onto the active area of the solarcell, it generates electron–hole pairs. The photo-excited electronsand holes move to the n- and p-type semiconductor, respectively.Finally, the electron–hole pair generation and the separationprocess cause a potential difference, which is referred to as aphotovoltage. Theoretically, the maximum photovoltage is deter-mined by the difference between the conduction band minimumof IGZO and the valence band maximum of GaTe. In our case,as the work function difference is 0.35 eV, the theoreticallycalculated maximum open-circuit voltage is above 0.35 eV. Theexperimentally measured Voc of our GaTe/IGZO transparentsolar cell was 0.14 V, which is far below the maximum value.Increasing the incident light intensity or decreasing the seriesresistance of the device by changing the configuration willresult in the improvement of the photovoltaic performance.37

It has also been confirmed that the photovoltaic effect in theGaTe/IGZO solar cell is fully originated by a p–n junction, notthe Schottky barrier at the interface between active layers andthe ITO electrode (Fig. S7, ESI†).

Conclusions

We have demonstrated the first fully transparent 2D/bulk hybridstructure based solar cell, by utilizing a mechanically exfoliated

Fig. 4 Band structure and operating mechanism of the device. The KPFM mapping image (inset) and the contact potential difference profile (a) betweenthe SiO2 substrate and the GaTe flake, and (b) between the SiO2 substrate and IGZO thin film. (c) The energy–band diagram of GaTe and a-IGZO beforejoining. The band alignment of the GaTe/IGZO p–n heterojunction at (d) equilibrium, and (e) under illumination.

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GaTe/IGZO p–n heterojunction. For p-type semiconductorlayers, a newly explored layered material, GaTe, was chosenbecause of its excellent optoelectronic property originating fromthe direct bandgap structure. Our photovoltaic device achievedan average transmittance of 91% in the visible light region and aPCE of 0.73%. Compared with the theoretical limit, there isstill plenty of room to enhance the photovoltage of our GaTe/IGZOphotovoltaic cell. In addition, when the wafer-scale GaTe synthesisis realized, the GaTe/IGZO solar cell with a vertical configurationcan be fabricated, which will enable further improvement of PCE,compared with that of the current lateral structure.30,38 The GaTe/IGZO transparent cell also showed instantaneous response tolight with various wavelengths. Its working mechanism wasverified with a study of the band structure of the heterojunction.This work demonstrates the possibility of a 2D material basedsolar cell, showing high transparency, by utilizing only part of thevisible light for electricity generation. It will lead to the realizationof a BIPV system by enabling the substitution of glass walls withtransparent, thin-film solar cells.

Methods

Commercially available crystals of GaTe (2D semiconductors)were exfoliated using the micromechanical cleavage method. Inorder to minimize the tape residue, blue Nitto tape was utilizedfor exfoliating and transferring GaTe multi-layer flakes to the topof glass substrates. Alkali-free glass, Eagle2000 (Corning), wasused as a transparent substrate for our solar cell. 50 � 50 mm2

sized IGZO in an array of 30 by 30 was patterned on top of thesubstrate with randomly located GaTe flakes through a conven-tional lithography process followed by rf sputtering of IGZOand lift-off. GaTe–IGZO heterostructures were identified byoptical microscopy and their positions were recorded in an x–ycoordinate system. Once identified, the anode and cathode, eachconnected to a 150 � 150 mm2 sized bonding pad, were formedthrough a second lithography step. As an electrode, one ofthe most widely used transparent conducting oxides, ITO wasdeposited in a 100 nm-thick layer by rf sputtering. After thefabrication of the solar cell structure, the samples were annealedunder 200 1C in a vacuum atmosphere for 1 hour.

GaTe was characterized by Raman spectroscopy (Horiba)with a 532 nm laser and its thickness was identified by atomicforce microscopy (Park Systems). The transmittance of ourGaTe–IGZO heterostructure based solar cell was measuredusing a UV-Vis-NIR spectrophotometer (Agilent). The electricalcharacterization of the device was conducted under vacuumconditions using a Keithley 4200 parameter analyzer. Thephotovoltaic response under various intensities of light wasmeasured with white light from a halogen lamp. The real-timeresponse of the solar cell under repetitive pulses of 400 and550 nm light was measured using a monochromatic lightsource system (Oriel Instruments) equipped with a Hg–Xelamp, a monochromator, an electronic shutter and a digitaltimer. The light power density was calibrated with a powermeter in advance.

The band alignment of GaTe–IGZO was predicted basedon the work function data measured with Kelvin probe forcemicroscopy (Bruker).

Acknowledgements

This research was supported by the MSIP (Ministry of Science,ICT and Future Planning), Korea, under the ‘‘ICT ConsilienceCreative Program’’ (IITP-R0346-16-1008) supervised by the IITP(Institute for Information & Communications TechnologyPromotion).

References

1 M. A. Green, K. Emery, Y. Hishikawa, W. Warta andE. D. Dunlop, Prog. Photovoltaics, 2016, 24, 905–913.

2 M. Taguchi, A. Yano, S. Tohoda, K. Matsuyama, Y. Nakamura,T. Nishiwaki, K. Fujita and E. Maruyama, IEEE J. Photovoltaics,2014, 4, 96–99.

3 F. Dimroth, M. Grave, P. Beutel, U. Fiedeler, C. Karcher,T. N. D. Tibbits, E. Oliva, G. Siefer, M. Schachtner, A. Wekkeli,A. W. Bett, R. Krause, M. Piccin, N. Blanc, C. Drazek, E. Guiot,B. Ghyselen, T. Salvetat, A. Tauzin, T. Signamarcheix, A. Dobrich,T. Hannappel and K. Schwarzburg, Prog. Photovoltaics, 2014, 22,277–282.

4 R. R. Lunt and V. Bulovic, Appl. Phys. Lett., 2011, 98, 113305.5 Y. Zhao and R. R. Lunt, Adv. Energy Mater., 2013, 3,

1143–1148.6 J. L. H. Chau, R. T. Chen, G. L. Hwang, P. Y. Tsai and

C. C. Lin, Sol. Energy Mater. Sol. Cells, 2010, 94, 588–591.7 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang,

Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov,Science, 2004, 306, 666–669.

8 S. Yang, S. Tongay, Y. Li, Q. Yue, J. B. Xia, S. S. Li, J. Li andS. H. Wei, Nanoscale, 2014, 6, 7226–7231.

9 C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 2008, 321,385–388.

10 L. Falkovsky, Optical properties of graphene, Dubna, Moscowregion, Russia, 2008.

11 F. Liu, H. Shimotani, H. Shang, T. Kanagasekaran,V. Zolyomi, N. Drummond, V. I. Fal’ko and K. Tanigaki,ACS Nano, 2014, 8, 752–760.

12 D. J. Late, B. Liu, J. Luo, A. Yan, H. S. Matte, M. Grayson,C. N. Rao and V. P. Dravid, Adv. Mater., 2012, 24, 3549–3554.

13 G. H. Lee, Y. J. Yu, X. Cui, N. Petrone, C. H. Lee, M. S. Choi,D. Y. Lee, C. Lee, W. J. Yoo, K. Watanabe, T. Taniguchi,C. Nuckolls, P. Kim and J. Hone, ACS Nano, 2013, 7, 7931–7936.

14 C. Gong, H. J. Zhang, W. H. Wang, L. Colombo, R. M. Wallaceand K. J. Cho, Appl. Phys. Lett., 2013, 103, 053513.

15 K. F. Mak, C. Lee, J. Hone, J. Shan and T. F. Heinz, Phys. Rev.Lett., 2010, 105, 136805.

16 B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti andA. Kis, Nat. Nanotechnol., 2011, 6, 147–150.

17 M. Bernardi, M. Palummo and J. C. Grossman, Nano Lett.,2013, 13, 3664–3670.

Journal of Materials Chemistry C Paper

Publ

ishe

d on

31

Mar

ch 2

017.

Dow

nloa

ded

by Y

onse

i Uni

vers

ity o

n 28

/04/

2017

05:

41:3

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J. Mater. Chem. C This journal is©The Royal Society of Chemistry 2017

18 O. Lopez-Sanchez, E. Alarcon Llado, V. Koman, A. Fontcuberta iMorral, A. Radenovic and A. Kis, ACS Nano, 2014, 8, 3042–3048.

19 M. L. Tsai, S. H. Su, J. K. Chang, D. S. Tsai, C. H. Chen,C. I. Wu, L. J. Li, L. J. Chen and J. H. He, ACS Nano, 2014, 8,8317–8322.

20 L. Z. Hao, Y. J. Liu, W. Gao, Z. D. Han, Q. Z. Xue, H. Z. Zeng,Z. P. Wu, J. Zhu and W. L. Zhang, J. Appl. Phys., 2015,117, 114502.

21 S. Lin, X. Li, P. Wang, Z. Xu, S. Zhang, H. Zhong, Z. Wu,W. Xu and H. Chen, Sci. Rep., 2015, 5, 15103.

22 S. S. Lin, P. Wang, X. Q. Li, Z. Q. Wu, Z. J. Xu, S. J. Zhang andW. L. Xu, Appl. Phys. Lett., 2015, 107, 153904.

23 Z. Wang, K. Xu, Y. Li, X. Zhan, M. Safdar, Q. Wang, F. Wangand J. He, ACS Nano, 2014, 8, 4859–4865.

24 A. Takagi, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya,M. Hirano and H. Hosono, Thin Solid Films, 2005, 486,38–41.

25 J. Yang, H. Kwak, Y. Lee, Y. S. Kang, M. H. Cho, J. H. Cho,Y. H. Kim, S. J. Jeong, S. Park, H. J. Lee and H. Kim, ACSAppl. Mater. Interfaces, 2016, 8, 8576–8582.

26 H. Yabuta, M. Sano, K. Abe, T. Aiba, T. Den, H. Kumomi,K. Nomura, T. Kamiya and H. Hosono, Appl. Phys. Lett.,2006, 89, 112123.

27 A. Thakur, H. Yoo, S. J. Kang, J. Y. Baik, I. J. Lee, H. K. Lee,K. Kim, B. Kim, S. Jung, J. Park and H. J. Shin, ECS J. SolidState Sci. Technol., 2012, 1, Q11–Q15.

28 U. S. Shenoy, U. Gupta, D. S. Narang, D. J. Late, U. V. Waghmareand C. N. R. Rao, Chem. Phys. Lett., 2016, 651, 148–154.

29 Q. Zhao, T. Wang, Y. Miao, F. Ma, Y. Xie, X. Ma, Y. Gu, J. Li,J. He, B. Chen, S. Xi, L. Xu, H. Zhen, Z. Yin, J. Li, J. Ren andW. Jie, Phys. Chem. Chem. Phys., 2016, 18, 18719–18726.

30 M. M. Furchi, A. Pospischil, F. Libisch, J. Burgdorfer andT. Mueller, Nano Lett., 2014, 14, 4785–4791.

31 X. T. Wang, L. Huang, Y. T. Peng, N. J. Huo, K. D. Wu, C. X. Xia,Z. M. Wei, S. Tongay and J. B. Li, Nano Res., 2016, 9, 507–516.

32 L. J. A. Koster, V. D. Mihailetchi, H. Xie and P. W. M. Blom,Appl. Phys. Lett., 2005, 87, 203502.

33 A. Pospischil, M. M. Furchi and T. Mueller, Nat. Nanotechnol.,2014, 9, 257–261.

34 T. Majumder, J. J. L. Hmar, K. Debnath, N. Gogurla, J. N. Roy,S. K. Ray and S. P. Mondal, J. Appl. Phys., 2014, 116, 034311.

35 Z. M. Zhang, X. D. Li, C. T. Gao, F. Teng, Y. Q. Wang,L. L. Chen, W. H. Han, Z. X. Zhang and E. Q. Xie, J. Mater.Chem. A, 2015, 3, 12769–12776.

36 W. Melitz, J. Shen, A. C. Kummel and S. Lee, Surf. Sci. Rep.,2011, 66, 1–27.

37 A. Pezeshki, S. H. Shokouh, T. Nazari, K. Oh and S. Im, Adv.Mater., 2016, 28, 3216–3222.

38 S. Memaran, H. R. Pradhan, Z. Lu, D. Rhodes, J. Ludwig,Q. Zhou, O. Ogunsolu, P. M. Ajayan, D. Smirnov, A. I. Fernandez-Dominguez, F. J. Garcia-Vidal and L. Balicas, Nano Lett., 2015, 15,7532–7538.

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