- Email: [email protected]
p-InP
Optical Materials 86 (2018) 576–581
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Heterojunction solar cell based on n-MoS2/p-InP a
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Sachin A. Pawar , Donghwan Kim , Ansoon Kim , Joo Hyung Park , Jae Cheol Shin , TaeWan Kime,∗, Hyo Jin Kimd,∗∗ a
Department of Physics, Yeungnam University, Gyeongsan, Gyeongbuk, 38541, South Korea Industrial Metrology, Korea Research Institute of Standards and Science, Nano Science, University of Science and Technology, Daejeon, 34113, South Korea Photovoltaics Laboratory, Korea Institute of Energy Research, Daejeon, 34129, South Korea d Korea Photonics Technology Institute, Gwangju, 61007, South Korea e Department of Electrical Engineering and Smart Grid Research Center, Chonbuk National University, Jeonju, 54896, South Korea b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Heterojunction n-MoS2 p-InP Solar cell MOCVD
In recent days few-layered or ultrathin molybdenum disulfide (MoS2) one of the transition metal dichalcogenides (TMDs) is gaining serious attention for photovoltaics. TMD materials are able to absorb up to 5–10% of incident sunlight in less than 1 nm thickness to achieve an order of magnitude higher solar light absorption. Metal organic chemical vapor deposition (MOCVD) was employed to obtain atomic layered MoS2 on SiO2/Si substrate which subsequently transferred on InP substrate using a simple transfer protocol. We realized photovoltaic operation in atomically few-layered MoS2 by forming a type II heterojunction with low dopped p-InP, instead of widely used p-Si. The built-in electric field arised due to interface between n-MoS2 and p-InP heterojunction is beneficial for the photogenerated carrier generation and separation. The solar cell device could achieve a power conversion efficiency of 0.11% with 1.87 mA/cm2 current density, first attempt of TMD based solar cell on InP substrate, demonstrating a promising solar cell performance. The obtained results pave the way for the integration of TMD materials with the InP substrate to obtain efficient solar cells.
1. Introduction Presently the global warming effect is causing us to seek for the renewable energy alternatives for our everyday life. The huge population around us is stressing to search for the long lasting, clean and affordable energy resources. Solar energy is clean, renewable, abundant in availability and is affordable to the mankind. Now, it is the need of hour to focus our attention towards the solar cells as an energy generators to fulfill the modern tech savvy lifestyle. It is the most promising technologies that directly convert solar light into electricity. In order to be able to apply solar cells to practical applications that should meet the three factors of efficiency, stability and low cost of production. The stability of thin film solar cells especially silicon solar cells are stable, with long lifetime around 25 years and high power conversion efficiency [1]. Two dimensional (2D) materials are finding greater attention very recently owing to their versatile abilities of uses in upcoming electronics devices. Transistors, photodetectors, light emitting diodes (LEDs), photocatalysis and solar cells are few such fields where 2D materials are exploited [2–6]. Various transition metal dichalcogenides (TMDs) such
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as MoS2, MoSe2, WS2 and WSe2 are extensively researched due to their resembling properties to graphene [7–10]. Molybdenum disulfide (MoS2) is an indirect band gap semiconductor with an energy gap of ∼1.2 eV in the bulk form. Interestingly, the band gap of MoS2 is crystal thickness dependent due to quantum confinement. It's been found that to the increase in its size, the nature of its band gap changes from indirect to direct for single monolayer. Monolayer MoS2 has a carrier mobility of around 200 cm2/V s [11]. Recently, single layer or mono layer thick TMDs like MoS2 have attracted serious attention due to their novel electronic properties. Similarly, few-layered MoS2 have been used in excellent photodetectors with exhibiting a photogain greater than 108 demonstrating the emerging applications of 2D materials for high efficiency photonic devices [12]. It should be noted that 2D TMD materials are able to absorb up to 5–10% of incident sunlight in less than 1 nm thickness to achieve an order of magnitude higher solar light absorption than other solar absorbers like GaAs and Si [13]. MoS2 is readily synthesized with the versatile techniques such as exfoliation, liquid-phase exfoliation, chemical vapor deposition and molecular beam epitaxy. Exfoliation of MoS2 is widely used synthesis route while metal organic chemical vapor deposition (MOCVD) of MoS2
Corresponding author. Corresponding author. E-mail addresses: [email protected] (T. Kim), [email protected] (H.J. Kim).
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https://doi.org/10.1016/j.optmat.2018.10.052 Received 8 October 2018; Received in revised form 24 October 2018; Accepted 26 October 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. (a) AFM image showing the smooth surface of MoS2 and corresponding height profile. (b) Raman spectra of MoS2 on InP and SiO2 substrates (c) Photoluminescence study of MoS2 on InP and SiO2 substrates.
pressure was lowered to the growth pressures 10 Torr with 100 sccm H2S gas flow rate and 400 °C growth temperature at Mo(CO)6 sublimation temperatures of 28 °C. The substrates were unloaded to the load-lock chamber after growth and cooled down for 1 h with 400 sccm H2 and 30sccm H2S flow.
is also gaining recent attraction due to its added advantage of highly reproducibility and controlled growth [14]. It is essential to have a scalable and controlled deposition technique for practical application and MOCVD is once such kind of technique to rely upon. In this work, we report on the synthesis of few-layered MoS2 sheets on silicon/silicon dioxide (Si/SiO2) for characteristics, transferred on p-Si and indium phosphide (InP) substrates for the physical properties measurements and the heterojunction solar cell, respectively. The synthesized six atomic layer MoS2 have been characterized for their structural, morphological and optical properties. The fabricated MoS2/InP heterostructure solar cell has been investigated.
2.2. Characteristics The surface morphology of the films was examined by optical microscope (OM, Olympus, BX-51) and atomic force microscope (AFM, XE-150, Park System). Raman spectroscopy and steady-state PL spectra were done with an excitation wavelength of 488 nm and a laser power of 10 mW power using a Renishaw Raman spectroscope integrated. To determine the surface chemical composition and electronic structures (valence band edge and work function), X-ray and ultra-violet photoelectron spectroscopy (XPS, UPS) measurements were carried out with a PHI5000 Versa Probe II (Ulvac-PHI) using a monochromatic Al Kα source. The base pressure was below 3 × 10−10 Torr. The instrument work function was calibrated based on the ISO-15472 by using Au, Cu, and Ag metals. The thickness of MoS2 film employed in the solar cell was evaluated using as AFM (XE-150, Park System).
2. Experimental details 2.1. Growth procedure MoS2 layers were grown by showerhead-type MOCVD reactor using molybdenum hexacarbonyl (Mo(CO)6) as a precursor. This was grown on a highly doped(< 0.005Ωcm−1) p-type Si substrate with a 300 nmthick SiO2 layer. In an attempt to grow the multilayer MoS2 sheets, the SiO2/Si substrate was pre-cleaned and loaded into the MOCVD reactor without any delay to prevent any contamination in the ambient environment. A high-purity Hydrogen sulfide (H2S) was used as the sulfur reaction gas. The molar flow of H2S and Mo(CO)6, were precisely controlled using a mass flow controller and a precursor canister covered with a chiller heater tape, respectively. The heating block in the MOCVD reactor was pre-heated to 400 °C under ambient H2S, Mo(CO)6 was added. For the growth of the few-layer MoS2 film, the reactor
2.3. Device fabrication Solar cell was fabricated on p-type InP substrate was low dopped (∼1016 cm3). The InP substrate was first cleaned with Acetone, IPA and DI water and dried with N2 gas. Then the native oxide was removed by using buffered oxide etch solution. PMMA (950 PMMA A6, Micro chem) 577
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232.55 and 229.35 eV could be assigned to Mo 3d3/2 and Mo 3d5/2 of Mo4+ in MoS2. Moreover, the lower energy peak located at 226.70 eV could be assigned to S 2s core level. These peaks correspond to the MoS2 reported in previous reports [25,26]. There are no peaks related to MoO3 or MoxSy resulting in neither oxidation nor reduction of MoS2. Therefore, the transferred MoS2 sample is stoichiometric [19]. Similarly, the peaks at 162.23 and 163.33 eV can be assigned to S 2p3/2 and S 2p1/2 of divalent sulfide ions (S2−), respectively, as shown in Fig. 2 (b). XPS data of the MoS2 sample reveal S-to-Mo ratio is 1.82, which indicates good agreement of stoichiometry of the MoS2 film.
was used to transfer MoS2 to InP substrate. The PMMA was spin coated at 1500 rpm on the MoS2 film side then baked using a hot plate. The MoS2 layer was then floated on the surface of Di water, and transferred to the InP substrate by scooping with the substrate. The PMMA was eliminated using a Methylene Chloride (Dichloromethane, J. T. Baker) and a Dichloromethane with IPA for before and after cleaning samples, respectively. The anode was fabricated by using electron-beam evaporation to deposit Ti/Pt/Au (25/25/150 nm) on the bottom of the InP substrate. The cathode was deposited through a shadow mask by electron-beam evaporation for Ti/Au (5/50 nm) onto MoS2 layer. The total area of the solar cell is 1 cm2 and the aperture area, excluding bus lines, is 0.7 cm2. The electrical measurement of the device was performed at room temperature, using an in-house four-probe station with a semiconductor parameter analyzer (4156A, Hewlett-Packard). The photovoltaic current-density-voltage (J-V) characteristics of the fabricated cells were measured under an AM 1.5 (100 mW/cm2) solar simulator at 25 °C.
3.1. UPS and energy band diagram In order to get an insight into the electronic structures of MoS2/p-Si heterojunction, the ultraviolet photoemission spectroscopy (UPS) study was carried out (Fig. 3 (a) and (b)). It is well established that the valence band maximum calculation can be performed by considering the cutoff of the lowest binding energy [27]. And, the calculation of work function can be performed by considering the difference between the photon energy of excited radiation and spectra width which can be measured from the valence band and secondary edges. The work function of multilayer MoS2 is 4.01 eV, showing the increase of work function compared with bare Si surface (3.73 eV), as shown in Fig. 3 (a). On the other hand the energy difference between the Fermi level and valence band maximum is 1.32 eV, which suggests the n-type behavior of MoS2. Based on the above results, the energy band diagrams are constructed as shown in Fig. 4. This energy band diagrams show the band bending behavior at the few-layered MoS2 and p-Si interface. This junction (Fig. 4) is a type II heterojunction with a built-in potential which is highly beneficial for the extraordinary solar cell performance. Fig. 5 (a) shows the photograph of actual MoS2-transferred solar cell device with 1 × 1 cm2 device area. The corresponding OM image in Fig. 5 (b) shows the distinguishing MoS2 layer on InP substrate. To form the front contacts on solar cell device e-beam evaporation is carried out by depositing Ti/Au contacts (Fig. 5 (b)). The electrical patterns on MoS2 can be clearly visualized in OM image Fig. 5 (b) with Au contacts on a smooth MoS2 layer. After device fabrication, the MoS2/p-InP solar cell is further characterized for the photovoltaic measurements. Fig. 5 (c) shows the J-V characteristics of MoS2/p-InP heterojunction solar cell. The solar cell parameters of the device are tabulated in Table 1. The J-V performance shows significant enhancement after IPA cleaning compared to before IPA cleaning MoS2 samples. The best performance with short circuit current density (Jsc) = 1.87 mA/cm2, open circuit voltage (Voc) = 0.28 V, fill factor (FF) = 22% and 0.11 efficiency was observed for the sample after cleaning. While the MoS2 sample before cleaning shows Jsc = 0.695 mA/cm2, Voc = 0.23 V with highest FF value of 32.32% but the overall efficiency was as low as 0.05%. The enhancement in the Jsc is attributed to the increased absorption of the few-layered MoS2 layers and the formation of the depletion region at the MoS2/p-InP interface. This depletion region causes separation in photoexcited electron hole pairs leading to increased current density in heterojunction solar cells [28]. It is noted that the FF value of after cleaning sample reduced dramatically due to lower Voc. The reduction in the Voc in our devices may also attributed to the “rollover effect” observed due to distortion of current-voltage (J-V) characteristics. Rollover effect is a distortion of J-V characteristics under illumination at forward bias with saturating current at higher voltage [29]. Rollover effect is dominant due to existence of a barrier which prevents carrier transport in the solar cell devices. In most of the thin film solar cells like CIGS, CdS/CdTe solar cells, the rollover effect arises due to barrier origin caused by back contact. If the contact is non-Ohmic and instead if it is Schottky diode, rollover effect arises. Existence of such phenomenon in our solar cell device can't be ruled out as there is a distortion in J-V curves of both the MoS2 solar cell devices is observed. Fig. 5 (c), stems from formation of a Schottky contact between electrodes and MoS2 (top contact). Further study in this direction is underway in our
3. Results and discussion The AFM was used to analyze the surface topography of MoS2 layers which shows the smooth surface as shown in Fig. 1 (a). The corresponding height profile is also shown, where the thickness of fewlayered MoS2 evaluated by AFM was around 4 nm, indicating the six layer structure of MoS2 film. The Raman spectra of MoS2 on InP and Si substrate is as shown in Fig. 1 (b). The in-plane vibrations at ∼383.08 and 384.14 cm−1 and out-of-plane vibrations at ∼408.05 and 403.97 cm−1 with (E21g ) and ( A1g ) modes on InP and SiO2/Si substrate, respectively, are significant peaks due to MoS2 [15]. Vibration of the (E21g ) mode involves the in-plane opposing motions of sulfur (S) and molybdenum (Mo) atoms and that of ( A1g ) mode is the out-of-plane relative motions of S atoms. The peak at 340.22 cm−1 is due to InP substrate. The separation distance between (E21g ) and ( A1g ) Raman modes is Δ = 24.96 and 24.99 on InP and SiO2/Si substrates, respectively. If the separation distance between the Raman peaks is greater than 23 cm−1 in case of MoS2, it confirms the multilayer formation, whereas if Δ falls below 20 cm−1 monolayer formation is affirmed [16]. In order to determine the optical properties, defect states and the band gap of MoS2 steady state photoluminescence (PL) study is carried out. Fig. 1 (c) shows PL spectra of MoS2 on SiO2/Si and InP substrates. It is noted that both the PL shows prominent luminescence emissions at 689 and 629 nm corresponding to the A1 and B1 direct excitonic transitions at, 1.79 and 1.97 eV, respectively for MoS2 on SiO2/Si. Theses PL peaks are due to radiative recombination of direct spin-split excitonic transitions [17]. Similarly, the PL for MoS2 on InP shows weak signal at 670 nm (1.85 eV) corresponds to A1 direct excitonic transitions with the energy split from valence band spin-orbital coupling. There is an unexpected stronger and broader low energy peak at ∼716 nm (1.73 eV) was observed for MoS2 on InP. This PL peak and the red-shifted of both E21g and A1g Raman modes (Fig. 1 (b)) may be arised due to the recombination of defect-bound excitons. This defect may have caused due to strain formation during heterogeneous nucleation [18–20]. It is interesting that the PL is absent in the bulk MoS2 rather it's presence is observed in monolayered MoS2 since bulk MoS2 is indirect band gap semiconductor [21]. Similar behavior is observed by Eda et al. where they observed that the thinnest samples exhibited the strongest photoluminescence while the PL emission intensity decreased upon increased thickness of MoS2 [22]. It is noted that the PL is significantly quenched in the MoS2/InP heterostructure region. This reduced PL intensity is attributed to the interlayer exciton dissociation which confirms the type II heterojunction formation in MoS2/InP heterostructure [23,24]. XPS analysis was conducted to get an insight of chemical states and relative chemical composition of Mo and S. Fig. 2 (a) and (b) shows high resolution XPS spectra of Mo 3d and S 2p regions for multilayer MoS2 transferred onto p-Si(100)substrate. The binding energy peaks at 578
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Fig. 2. Chemical composition of MoS2 transferred onto Si(100) substrate. (a) Mo 3d and (b) S 2p XPS spectra of the MoS2 sample.
Fig. 3. UPS study to determine the (a) work function and (b) valence band edges of Si(100) (black) and MoS2/Si(100) (red) surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4. Conclusions
laboratory and the efforts are being made to create ideal Ohmic contact between electrode metal and TMDs, which can lead to eliminate the rollover effect, so that Voc and FF of solar cell devices can be further improved. Eventually, the MoS2/p-InP device exhibited an efficiency of 0.11%, first of its kind for TMD few-layered solar cell device with the use of InP substrate.
We fabricated a solar cell device based on n-MoS2/p-InP junction and the photovoltaic measurements are studied therein. Realization of n-MoS2/p-InP heterojunction solar cell is achieved through few-layered MoS2 by versatile MOCVD technique coupled with a simple transfer
Fig. 4. Energy band diagram after p-Si/n-MoS2 heterojunction formation. 579
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Fig. 5. (a) MoS2 Solar cell on InP substrate photograph of actual 1 × 1 cm2 solar cell device (b) the optical microscopic image of gold patterns on the solar cell device by e-beam (c) Photovoltaic (J–V) characteristics of MoS2 solar cell device on InP substrate. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Solar cell parameters for MoS2 solar cells before and after IPA cleaning. Sample Name
Voc (V)
Jsc (mA/cm2)
Eff (%)
FF (%)
MoS2-before IPA cleaning MoS2-after IPA cleaning
0.238 0.289
0.695 1.879
0.053 0.119
32.32 21.99
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protocol. UPS measurement reveal the work function of 4.01 eV and energy difference between the Fermi level and valence band maximum of 1.32 eV, which arises from the n-type behavior of MoS2 few-atomiclayer. The solar cell device of MoS2/p-InP after IPA cleaning shows better performance with 0.11% efficiency and 1.87 mA/cm2 current density, first of its kind TMD based solar cell on InP substrate. The builtin potential near the interface between n-MoS2 and p-InP is responsible for the effective separation of photogenerated electron hole pair. MOCVD technique is guaranteed, highly reproducible and is capable of producing large scale solar cells of above kind and can be further used for other TMDs based solar cells. This work demonstrates layered structures of TMDs can be fully integrated into the III-V substrates and holds promise for realizing other TMDs in different electronic and optoelectronic devices.
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Acknowledgment
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This work was supported by the National Research Foundation of Korea (NRF-2017M1A2A2048904). This research was supported by “Research Base Construction Fund Support Program” funded by Chonbuk National University in 2018.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Heterojunction solar cell based on n-MoS2/p-InP a
a
b
T c
a
Sachin A. Pawar , Donghwan Kim , Ansoon Kim , Joo Hyung Park , Jae Cheol Shin , TaeWan Kime,∗, Hyo Jin Kimd,∗∗ a
Department of Physics, Yeungnam University, Gyeongsan, Gyeongbuk, 38541, South Korea Industrial Metrology, Korea Research Institute of Standards and Science, Nano Science, University of Science and Technology, Daejeon, 34113, South Korea Photovoltaics Laboratory, Korea Institute of Energy Research, Daejeon, 34129, South Korea d Korea Photonics Technology Institute, Gwangju, 61007, South Korea e Department of Electrical Engineering and Smart Grid Research Center, Chonbuk National University, Jeonju, 54896, South Korea b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Heterojunction n-MoS2 p-InP Solar cell MOCVD
In recent days few-layered or ultrathin molybdenum disulfide (MoS2) one of the transition metal dichalcogenides (TMDs) is gaining serious attention for photovoltaics. TMD materials are able to absorb up to 5–10% of incident sunlight in less than 1 nm thickness to achieve an order of magnitude higher solar light absorption. Metal organic chemical vapor deposition (MOCVD) was employed to obtain atomic layered MoS2 on SiO2/Si substrate which subsequently transferred on InP substrate using a simple transfer protocol. We realized photovoltaic operation in atomically few-layered MoS2 by forming a type II heterojunction with low dopped p-InP, instead of widely used p-Si. The built-in electric field arised due to interface between n-MoS2 and p-InP heterojunction is beneficial for the photogenerated carrier generation and separation. The solar cell device could achieve a power conversion efficiency of 0.11% with 1.87 mA/cm2 current density, first attempt of TMD based solar cell on InP substrate, demonstrating a promising solar cell performance. The obtained results pave the way for the integration of TMD materials with the InP substrate to obtain efficient solar cells.
1. Introduction Presently the global warming effect is causing us to seek for the renewable energy alternatives for our everyday life. The huge population around us is stressing to search for the long lasting, clean and affordable energy resources. Solar energy is clean, renewable, abundant in availability and is affordable to the mankind. Now, it is the need of hour to focus our attention towards the solar cells as an energy generators to fulfill the modern tech savvy lifestyle. It is the most promising technologies that directly convert solar light into electricity. In order to be able to apply solar cells to practical applications that should meet the three factors of efficiency, stability and low cost of production. The stability of thin film solar cells especially silicon solar cells are stable, with long lifetime around 25 years and high power conversion efficiency [1]. Two dimensional (2D) materials are finding greater attention very recently owing to their versatile abilities of uses in upcoming electronics devices. Transistors, photodetectors, light emitting diodes (LEDs), photocatalysis and solar cells are few such fields where 2D materials are exploited [2–6]. Various transition metal dichalcogenides (TMDs) such
∗
as MoS2, MoSe2, WS2 and WSe2 are extensively researched due to their resembling properties to graphene [7–10]. Molybdenum disulfide (MoS2) is an indirect band gap semiconductor with an energy gap of ∼1.2 eV in the bulk form. Interestingly, the band gap of MoS2 is crystal thickness dependent due to quantum confinement. It's been found that to the increase in its size, the nature of its band gap changes from indirect to direct for single monolayer. Monolayer MoS2 has a carrier mobility of around 200 cm2/V s [11]. Recently, single layer or mono layer thick TMDs like MoS2 have attracted serious attention due to their novel electronic properties. Similarly, few-layered MoS2 have been used in excellent photodetectors with exhibiting a photogain greater than 108 demonstrating the emerging applications of 2D materials for high efficiency photonic devices [12]. It should be noted that 2D TMD materials are able to absorb up to 5–10% of incident sunlight in less than 1 nm thickness to achieve an order of magnitude higher solar light absorption than other solar absorbers like GaAs and Si [13]. MoS2 is readily synthesized with the versatile techniques such as exfoliation, liquid-phase exfoliation, chemical vapor deposition and molecular beam epitaxy. Exfoliation of MoS2 is widely used synthesis route while metal organic chemical vapor deposition (MOCVD) of MoS2
Corresponding author. Corresponding author. E-mail addresses: [email protected] (T. Kim), [email protected] (H.J. Kim).
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https://doi.org/10.1016/j.optmat.2018.10.052 Received 8 October 2018; Received in revised form 24 October 2018; Accepted 26 October 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. (a) AFM image showing the smooth surface of MoS2 and corresponding height profile. (b) Raman spectra of MoS2 on InP and SiO2 substrates (c) Photoluminescence study of MoS2 on InP and SiO2 substrates.
pressure was lowered to the growth pressures 10 Torr with 100 sccm H2S gas flow rate and 400 °C growth temperature at Mo(CO)6 sublimation temperatures of 28 °C. The substrates were unloaded to the load-lock chamber after growth and cooled down for 1 h with 400 sccm H2 and 30sccm H2S flow.
is also gaining recent attraction due to its added advantage of highly reproducibility and controlled growth [14]. It is essential to have a scalable and controlled deposition technique for practical application and MOCVD is once such kind of technique to rely upon. In this work, we report on the synthesis of few-layered MoS2 sheets on silicon/silicon dioxide (Si/SiO2) for characteristics, transferred on p-Si and indium phosphide (InP) substrates for the physical properties measurements and the heterojunction solar cell, respectively. The synthesized six atomic layer MoS2 have been characterized for their structural, morphological and optical properties. The fabricated MoS2/InP heterostructure solar cell has been investigated.
2.2. Characteristics The surface morphology of the films was examined by optical microscope (OM, Olympus, BX-51) and atomic force microscope (AFM, XE-150, Park System). Raman spectroscopy and steady-state PL spectra were done with an excitation wavelength of 488 nm and a laser power of 10 mW power using a Renishaw Raman spectroscope integrated. To determine the surface chemical composition and electronic structures (valence band edge and work function), X-ray and ultra-violet photoelectron spectroscopy (XPS, UPS) measurements were carried out with a PHI5000 Versa Probe II (Ulvac-PHI) using a monochromatic Al Kα source. The base pressure was below 3 × 10−10 Torr. The instrument work function was calibrated based on the ISO-15472 by using Au, Cu, and Ag metals. The thickness of MoS2 film employed in the solar cell was evaluated using as AFM (XE-150, Park System).
2. Experimental details 2.1. Growth procedure MoS2 layers were grown by showerhead-type MOCVD reactor using molybdenum hexacarbonyl (Mo(CO)6) as a precursor. This was grown on a highly doped(< 0.005Ωcm−1) p-type Si substrate with a 300 nmthick SiO2 layer. In an attempt to grow the multilayer MoS2 sheets, the SiO2/Si substrate was pre-cleaned and loaded into the MOCVD reactor without any delay to prevent any contamination in the ambient environment. A high-purity Hydrogen sulfide (H2S) was used as the sulfur reaction gas. The molar flow of H2S and Mo(CO)6, were precisely controlled using a mass flow controller and a precursor canister covered with a chiller heater tape, respectively. The heating block in the MOCVD reactor was pre-heated to 400 °C under ambient H2S, Mo(CO)6 was added. For the growth of the few-layer MoS2 film, the reactor
2.3. Device fabrication Solar cell was fabricated on p-type InP substrate was low dopped (∼1016 cm3). The InP substrate was first cleaned with Acetone, IPA and DI water and dried with N2 gas. Then the native oxide was removed by using buffered oxide etch solution. PMMA (950 PMMA A6, Micro chem) 577
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232.55 and 229.35 eV could be assigned to Mo 3d3/2 and Mo 3d5/2 of Mo4+ in MoS2. Moreover, the lower energy peak located at 226.70 eV could be assigned to S 2s core level. These peaks correspond to the MoS2 reported in previous reports [25,26]. There are no peaks related to MoO3 or MoxSy resulting in neither oxidation nor reduction of MoS2. Therefore, the transferred MoS2 sample is stoichiometric [19]. Similarly, the peaks at 162.23 and 163.33 eV can be assigned to S 2p3/2 and S 2p1/2 of divalent sulfide ions (S2−), respectively, as shown in Fig. 2 (b). XPS data of the MoS2 sample reveal S-to-Mo ratio is 1.82, which indicates good agreement of stoichiometry of the MoS2 film.
was used to transfer MoS2 to InP substrate. The PMMA was spin coated at 1500 rpm on the MoS2 film side then baked using a hot plate. The MoS2 layer was then floated on the surface of Di water, and transferred to the InP substrate by scooping with the substrate. The PMMA was eliminated using a Methylene Chloride (Dichloromethane, J. T. Baker) and a Dichloromethane with IPA for before and after cleaning samples, respectively. The anode was fabricated by using electron-beam evaporation to deposit Ti/Pt/Au (25/25/150 nm) on the bottom of the InP substrate. The cathode was deposited through a shadow mask by electron-beam evaporation for Ti/Au (5/50 nm) onto MoS2 layer. The total area of the solar cell is 1 cm2 and the aperture area, excluding bus lines, is 0.7 cm2. The electrical measurement of the device was performed at room temperature, using an in-house four-probe station with a semiconductor parameter analyzer (4156A, Hewlett-Packard). The photovoltaic current-density-voltage (J-V) characteristics of the fabricated cells were measured under an AM 1.5 (100 mW/cm2) solar simulator at 25 °C.
3.1. UPS and energy band diagram In order to get an insight into the electronic structures of MoS2/p-Si heterojunction, the ultraviolet photoemission spectroscopy (UPS) study was carried out (Fig. 3 (a) and (b)). It is well established that the valence band maximum calculation can be performed by considering the cutoff of the lowest binding energy [27]. And, the calculation of work function can be performed by considering the difference between the photon energy of excited radiation and spectra width which can be measured from the valence band and secondary edges. The work function of multilayer MoS2 is 4.01 eV, showing the increase of work function compared with bare Si surface (3.73 eV), as shown in Fig. 3 (a). On the other hand the energy difference between the Fermi level and valence band maximum is 1.32 eV, which suggests the n-type behavior of MoS2. Based on the above results, the energy band diagrams are constructed as shown in Fig. 4. This energy band diagrams show the band bending behavior at the few-layered MoS2 and p-Si interface. This junction (Fig. 4) is a type II heterojunction with a built-in potential which is highly beneficial for the extraordinary solar cell performance. Fig. 5 (a) shows the photograph of actual MoS2-transferred solar cell device with 1 × 1 cm2 device area. The corresponding OM image in Fig. 5 (b) shows the distinguishing MoS2 layer on InP substrate. To form the front contacts on solar cell device e-beam evaporation is carried out by depositing Ti/Au contacts (Fig. 5 (b)). The electrical patterns on MoS2 can be clearly visualized in OM image Fig. 5 (b) with Au contacts on a smooth MoS2 layer. After device fabrication, the MoS2/p-InP solar cell is further characterized for the photovoltaic measurements. Fig. 5 (c) shows the J-V characteristics of MoS2/p-InP heterojunction solar cell. The solar cell parameters of the device are tabulated in Table 1. The J-V performance shows significant enhancement after IPA cleaning compared to before IPA cleaning MoS2 samples. The best performance with short circuit current density (Jsc) = 1.87 mA/cm2, open circuit voltage (Voc) = 0.28 V, fill factor (FF) = 22% and 0.11 efficiency was observed for the sample after cleaning. While the MoS2 sample before cleaning shows Jsc = 0.695 mA/cm2, Voc = 0.23 V with highest FF value of 32.32% but the overall efficiency was as low as 0.05%. The enhancement in the Jsc is attributed to the increased absorption of the few-layered MoS2 layers and the formation of the depletion region at the MoS2/p-InP interface. This depletion region causes separation in photoexcited electron hole pairs leading to increased current density in heterojunction solar cells [28]. It is noted that the FF value of after cleaning sample reduced dramatically due to lower Voc. The reduction in the Voc in our devices may also attributed to the “rollover effect” observed due to distortion of current-voltage (J-V) characteristics. Rollover effect is a distortion of J-V characteristics under illumination at forward bias with saturating current at higher voltage [29]. Rollover effect is dominant due to existence of a barrier which prevents carrier transport in the solar cell devices. In most of the thin film solar cells like CIGS, CdS/CdTe solar cells, the rollover effect arises due to barrier origin caused by back contact. If the contact is non-Ohmic and instead if it is Schottky diode, rollover effect arises. Existence of such phenomenon in our solar cell device can't be ruled out as there is a distortion in J-V curves of both the MoS2 solar cell devices is observed. Fig. 5 (c), stems from formation of a Schottky contact between electrodes and MoS2 (top contact). Further study in this direction is underway in our
3. Results and discussion The AFM was used to analyze the surface topography of MoS2 layers which shows the smooth surface as shown in Fig. 1 (a). The corresponding height profile is also shown, where the thickness of fewlayered MoS2 evaluated by AFM was around 4 nm, indicating the six layer structure of MoS2 film. The Raman spectra of MoS2 on InP and Si substrate is as shown in Fig. 1 (b). The in-plane vibrations at ∼383.08 and 384.14 cm−1 and out-of-plane vibrations at ∼408.05 and 403.97 cm−1 with (E21g ) and ( A1g ) modes on InP and SiO2/Si substrate, respectively, are significant peaks due to MoS2 [15]. Vibration of the (E21g ) mode involves the in-plane opposing motions of sulfur (S) and molybdenum (Mo) atoms and that of ( A1g ) mode is the out-of-plane relative motions of S atoms. The peak at 340.22 cm−1 is due to InP substrate. The separation distance between (E21g ) and ( A1g ) Raman modes is Δ = 24.96 and 24.99 on InP and SiO2/Si substrates, respectively. If the separation distance between the Raman peaks is greater than 23 cm−1 in case of MoS2, it confirms the multilayer formation, whereas if Δ falls below 20 cm−1 monolayer formation is affirmed [16]. In order to determine the optical properties, defect states and the band gap of MoS2 steady state photoluminescence (PL) study is carried out. Fig. 1 (c) shows PL spectra of MoS2 on SiO2/Si and InP substrates. It is noted that both the PL shows prominent luminescence emissions at 689 and 629 nm corresponding to the A1 and B1 direct excitonic transitions at, 1.79 and 1.97 eV, respectively for MoS2 on SiO2/Si. Theses PL peaks are due to radiative recombination of direct spin-split excitonic transitions [17]. Similarly, the PL for MoS2 on InP shows weak signal at 670 nm (1.85 eV) corresponds to A1 direct excitonic transitions with the energy split from valence band spin-orbital coupling. There is an unexpected stronger and broader low energy peak at ∼716 nm (1.73 eV) was observed for MoS2 on InP. This PL peak and the red-shifted of both E21g and A1g Raman modes (Fig. 1 (b)) may be arised due to the recombination of defect-bound excitons. This defect may have caused due to strain formation during heterogeneous nucleation [18–20]. It is interesting that the PL is absent in the bulk MoS2 rather it's presence is observed in monolayered MoS2 since bulk MoS2 is indirect band gap semiconductor [21]. Similar behavior is observed by Eda et al. where they observed that the thinnest samples exhibited the strongest photoluminescence while the PL emission intensity decreased upon increased thickness of MoS2 [22]. It is noted that the PL is significantly quenched in the MoS2/InP heterostructure region. This reduced PL intensity is attributed to the interlayer exciton dissociation which confirms the type II heterojunction formation in MoS2/InP heterostructure [23,24]. XPS analysis was conducted to get an insight of chemical states and relative chemical composition of Mo and S. Fig. 2 (a) and (b) shows high resolution XPS spectra of Mo 3d and S 2p regions for multilayer MoS2 transferred onto p-Si(100)substrate. The binding energy peaks at 578
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Fig. 2. Chemical composition of MoS2 transferred onto Si(100) substrate. (a) Mo 3d and (b) S 2p XPS spectra of the MoS2 sample.
Fig. 3. UPS study to determine the (a) work function and (b) valence band edges of Si(100) (black) and MoS2/Si(100) (red) surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
4. Conclusions
laboratory and the efforts are being made to create ideal Ohmic contact between electrode metal and TMDs, which can lead to eliminate the rollover effect, so that Voc and FF of solar cell devices can be further improved. Eventually, the MoS2/p-InP device exhibited an efficiency of 0.11%, first of its kind for TMD few-layered solar cell device with the use of InP substrate.
We fabricated a solar cell device based on n-MoS2/p-InP junction and the photovoltaic measurements are studied therein. Realization of n-MoS2/p-InP heterojunction solar cell is achieved through few-layered MoS2 by versatile MOCVD technique coupled with a simple transfer
Fig. 4. Energy band diagram after p-Si/n-MoS2 heterojunction formation. 579
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Fig. 5. (a) MoS2 Solar cell on InP substrate photograph of actual 1 × 1 cm2 solar cell device (b) the optical microscopic image of gold patterns on the solar cell device by e-beam (c) Photovoltaic (J–V) characteristics of MoS2 solar cell device on InP substrate. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Solar cell parameters for MoS2 solar cells before and after IPA cleaning. Sample Name
Voc (V)
Jsc (mA/cm2)
Eff (%)
FF (%)
MoS2-before IPA cleaning MoS2-after IPA cleaning
0.238 0.289
0.695 1.879
0.053 0.119
32.32 21.99
[2] [3] [4]
protocol. UPS measurement reveal the work function of 4.01 eV and energy difference between the Fermi level and valence band maximum of 1.32 eV, which arises from the n-type behavior of MoS2 few-atomiclayer. The solar cell device of MoS2/p-InP after IPA cleaning shows better performance with 0.11% efficiency and 1.87 mA/cm2 current density, first of its kind TMD based solar cell on InP substrate. The builtin potential near the interface between n-MoS2 and p-InP is responsible for the effective separation of photogenerated electron hole pair. MOCVD technique is guaranteed, highly reproducible and is capable of producing large scale solar cells of above kind and can be further used for other TMDs based solar cells. This work demonstrates layered structures of TMDs can be fully integrated into the III-V substrates and holds promise for realizing other TMDs in different electronic and optoelectronic devices.
[5]
[6]
[7]
[8]
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Acknowledgment
[11]
This work was supported by the National Research Foundation of Korea (NRF-2017M1A2A2048904). This research was supported by “Research Base Construction Fund Support Program” funded by Chonbuk National University in 2018.
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