Modeling of 2D PV Devices/Light Generation and Harvesting in a van der Waals Heterostructure

Note[edit | edit source]

This is a literature review page to take optical and electrical parameters from existing 2D PV devices at the lab scale and scale them up to make a theoretical module. Then do simulation on them e.g. AOI, antireflection coating etc. in something like pvlib to provide system designers with the new module. It would be our pleasure if you share your experience in this area with us. (Discussion tab is top left of this page)

Light Generation and Harvesting in a van der Waals Heterostructure[1][1][1][edit | edit source]

Abstract: Two-dimensional (2D) materials are a new type of materials under intense study because of their interesting physical properties and wide range of potential applications from nanoelectronics to sensing and photonics. Monolayers of semiconducting transition metal dichalcogenides MoS2 or WSe2 have been proposed as promising channel materials for field-effect transistors. Their high mechanical flexibility, stability, and quality coupled with potentially inexpensive production methods offer potential advantages compared to organic and crystalline bulk semiconductors. Due to quantum mechanical confinement, the band gap in monolayer MoS2 is direct in nature, leading to a strong interaction with light that can be exploited for building phototransistors and ultrasensitive photodetectors. Here, we report on the realization of light-emitting diodes based on vertical heterojunctions composed of n-type monolayer MoS2 and p-type silicon. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of direct excitons related to the optical transitions between the conduction and valence bands. Our pn diodes can also operate as solar cells, with typical external quantum efficiency exceeding 4%. Our work opens up the way to more sophisticated optoelectronic devices such as lasers and heterostructure solar cells based on hybrids of 2D semiconductors and silicon.

• Not that much relevant to our topic
• focus mostly on LED and photodetector

Flexible Graphene Electrode-Based Organic Photovoltaics with Record-High Efficiency[2][2][2][edit | edit source]

Abstract Advancements in the field of flexible high-efficiency solar cells and other optoelectronic devices will strongly depend on the development of electrode materials with good conductivity and flexibility. To address chemical and mechanical instability of currently used indium tin oxide (ITO), graphene has been suggested as a promising flexible transparent electrode, but challenges remain in achieving high efficiency of grahene-based polymer solar cells (PSCs) compared to their ITO-based counterparts. Here we demonstrate graphene anode- and cathode based flexible PSCs with record-high power conversion efficiencies of 6.1% and 7.1%, respectively. The high efficiencies were achieved via thermal treatment of MoO3 electron blocking layer and direct deposition of ZnO electron transporting layer on graphene. We also demonstrate graphene-based flexible PSCs on polyethylene naphthalate substrates and show the device stability under different bending conditions. Our work paves a way to fully graphene electrode-based flexible solar cells using a simple and reproducible process.

• graphene Cathode based and Anode Based PSCs tried, MoO3 was used in both structure as electron blocking to prevent recombination
• 3 monolayers graphene is used
• Same structure tried by ITO, very close PCE tested
• Both structure fabricated on polyethylene naphthalate (PEN) and tested PCE=6.1% for Anode & PCE=7.1% for cathode
• performance remain the same up to 100 tensile flexing cycles

Stability of graphene–silicon heterostructure solar cells[3][3][3][edit | edit source]

Abstract: The stability of undoped graphene–silicon heterostructure solar cells was investigated. Single-layer graphene was grown by chemical vapor deposition on copper foil. Prior to the transfer of graphene to the silicon wafer, the flat Si(111) surface was passivated with hydrogen or methyl groups (CH3). The conversion efficiency, h, of the H terminated Si device was negligible small (0.1%), whereas that of the CH3 passivated Si was 2 and 4.2% at 100mW (AM 1.5) and 20mW of light intensity, respectively. After 28 days in ambient atmosphere h decreased only slightly to 1.5 and 3.7%. This small change of h is due to the high stability of the CH3 passivated graphene– Si(111) interface. The methylated Si surface shows a high degree of chemical stability especially during the graphene transfer process.

• graphene cannot placed directly on Si, dangling bonds will destroy monolayer graphene properties. This is due to a reduced interface defect-density.
• Graphene and Si interface is investigated by comparing two different methods for si passivation; H-termination and methyl (CH3) while methylated showed much better result

Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics[4][4][4][edit | edit source]

Abstract: Graphene and transition metal dichalcogenides (TMDCs) are the two major types of layered materials under intensive investigation. However, the zero-bandgap nature of graphene and the relatively low mobility in TMDCs limit their applications. Here we reintroduce black phosphorus (BP), the most stable allotrope of phosphorus with strong intrinsic in-plane anisotropy, to the layered-material family. For 15-nm-thick BP, we measure a Hall mobility of 1,000 and 600cm2V-1 s-1 for holes along the light (x) and heavy (y) effective mass directions at 120 K. BP thin films also exhibit large and anisotropic in-plane optical conductivity from 2 to 5 mm. Field-effect transistors using 5 nm BP along x direction exhibit an on–off current ratio exceeding 105, a field-effect mobility of 205cm2V-1 s- 1, and good current saturation characteristics all at room temperature. BP shows great potential for thin-film electronics, infrared optoelectronics and novel devices in which anisotropic properties are desirable

• This paper look physical, electrical, and optical properties of BP
• It has angle-dependent optical conductivity

Role of Interfacial Oxide in High-Efficiency Graphene–Silicon Schottky Barrier Solar Cells[5][5][5][edit | edit source]

Abstract: The advent of chemical vapor deposition (CVD) grown graphene has allowed researchers to investigate large area graphene/n-silicon Schottky barrier solar cells. Using chemically doped graphene, efficiencies of nearly 10% can be achieved for devices without antireflective coatings. However, many devices reported in past literature often exhibit a distinctive s-shaped kink in the measured I/V curves under illumination resulting in poor fill factor. This behavior is especially prevalent for devices with pristine (not chemically doped) graphene but can be seen in some cases for doped graphene as well. In this work, we show that the native oxide on the silicon presents a transport barrier for photogenerated holes and causes recombination current, which is responsible for causing the kink. We experimentally verify our hypothesis and propose a simple semiconductor physics model that qualitatively captures the effect. Furthermore, we offer an additional optimization to graphene/n-silicon devices: by choosing the optimal oxide thickness, we can increase the efficiency of our devices to 12.4% after chemical doping and to a new record of 15.6% after applying an antireflective coating.

• Si native oxide layer thicker than 15 A make recombination issue cannot solved by graphene doping.
• Native SiO2 thickness=10A 1 hour after HF dip and 20A after 1 week

Interface designed MoS2/GaAs heterostructure solar cell with sandwich stacked hexagonal boron nitride[6][6][6][edit | edit source]

Abstract: MoS2 is a layered two-dimensional semiconductor with a direct band gap of 1.8 eV. The MoS2/ bulk semiconductor system offers a new platform for solar cell device design. Different from the conventional bulk p-n junctions, in the MoS2/bulk semiconductor heterostructure, static charge transfer shifts the Fermi level of MoS2 toward that of bulk semiconductor, lowering the barrier height of the formed junction. Herein, we introduce hexagonal boron nitride (h-BN) into MoS2/GaAs heterostructure to suppress the static charge transfer, and the obtained MoS2/h-BN/GaAs solar cell exhibits an improved power conversion efficiency of 5.42%. More importantly, the sandwiched h-BN makes the Fermi level tuning of MoS2 more effective. By employing chemical doping and electrical gating into the solar cell device, PCE of 9.03% is achieved, which is the highest among all the reported monolayer transition metal dichalcogenide based solar cells.

The photovoltaic performance of the heterojunction is greatly influenced by the junction barrier height, which means suppressing the static charge transfer between 2D materials and semiconductor substrate are highly desirable. Herein, we introduce 2D hexagonal boron nitride (h-BN) into the MoS2/GaAs heterostructure to suppress the static charge transfer. More importantly, the inserted h-BN layer makes the tuning of Fermi level of MoS2 more effective, which greatly improves the performance of solar cells.

• GaAs passivated by NH3 plasma 5 min
• Both structures tested, with and without hBN layer
• In this study, AuCl3 solution in nitromethane (1 mM) is used to doping 2D MoS2 to increase the PCE of the MoS2/h-BN/GaAs solar cell. this chemical doping enhanced PCE from 5.38% to 7.15%
• electrical gating even improved from 6.87% to 9.03% with Vg= -1V

18.5% efficient graphene/GaAs van der Waals heterostructure solar cell[7][7][7][edit | edit source]

Abstract: High efficient solar cell is highly demanded for sustainable development of human society, leading to the cutting-edge research on various types of solar cells. The physical picture of graphene/semiconductor van der Waals Schottky diode is unique as Fermi level of graphene can be tuned by gate structure relatively independent of semiconductor substrate. However, the reported gated graphene/semiconductor heterostructure has power conversion efficiency (PCE) normally less than 10%. Herein, utilizing a designed graphene-dielectric-graphene gating structure for graphene/GaAs heterojunction, we have achieved solar cell with PCE of 18.5% and open circuit voltage of 0.96 V. Drift-diffusion simulation results agree well with the experimental data and predict this device structure can work with a PCE above 23.8%. This research opens a door of high efficient solar cell utilizing the graphene/semiconductor heterostructure.

• a graphene-dielectric-graphene structure, where the top graphene layer functions as the gating electrode, This structure combines the advantages of anti-reflection property of the dielectric layer, high transparency and highly tunable Fermi level of graphene.
• graphene transferred on top of GaAs using PMMA, 68 nm of Al2O3 was deposited as ARC layer, extra graphene layer on top of active area as electrode structure and Ag paste in last as electrode
• active graphene layer was doped by TFSA (bis(trifluoromethanesulfonyl)amide)
• 1-9 layer Graphene investigated, 3 layer showed max PCE
• very bad written paper, hard to follow

Functionalized graphene and other two-dimensional materials for photovoltaic devices: device design and processing[8][8][8][edit | edit source]

Abstract: This is a review paper that focus on application of Graphene, TMDs and black phophorous on various photovoltaic devices such as; organic solar cells, Shottky junction solar cells, dye-sensitized solar cells, quantum dot-sensitized solar cells, some other inorganic solar cells and perovskite solar cells. This paper first introduce mostly used preparation methods such as; Exfoliation methods and CVD methods and then focus on some properties;

Properties[edit | edit source]

• mobility : Generally 2D materials are famous for their very high in plane mobility, This number for graphene is ~ 200 000 cm2V-1 S-1
• Absorbance: theoretical equation for absorbance of single layer Graphene is presented, absorbance for single layer is ~2.3%, TMDs also showing absorbance 5-10% which is one order of magnitude higher than Si.
• Conductivity: equation provided. Four layers Graphene has 30 ohm squared cm conductivity but still 90% transmittance which can be a perfect candidate for transparent electrode. TMDs don't have high conductivity because of their semiconducting behavior but because of high mobility and absorbance they can be used as active layer or charge transport layer.
• Specific surface area: surface area of graphene measured as high as 2150 m2g-1 make graphene as promising electrode material for super-capacitors, solar cells, and sensors

Application in OPVs Graphene is being used in OPVs as transparent electrodes (anode and cathode), electron transport layers (ETLs), hole transport layers (HTLs), n-type acceptors and packaging layers (there is a table of previous work on OPVs) Other PV devices Application of Graphene in other PV devices such as shottky junction solar cell, dye-sensitized solar cells (DSSCs), quantum dot-sensitized solar cells (QDSSCs) and Perovskite solar cells are summarized in a table

Enhanced photovoltaic performances of graphene/Si solar cells by insertion of a MoS2 thin film[9][9][9][edit | edit source]

Abstract: This paper represent of adding a MoS2 large area CVD grown film to a graphene Si Schottky junction solar cell to improve the efficiency. This extra layer will work as an effective electron-blocking/hole-transporting layer. It is also demonstrated the solar cell efficiency will improve by increasing graphene layer thickness and decreasing MoS2 thickness. Highest efficiency achieved by this structure was 11.1% ; trilayer-graphene/MoS2/n-Si. single layer/bilayer and trilayer graphene/MoS2/ n-Si are compared in performance. The thickness of MoS2 layer kept constant as 17 nm (is not single layer). Trilayer graphene showed better performance compared with others.

Enhanced photon absorption in spiral nanostructured solar cells using layered 2D materials[10][10][10][edit | edit source]

Abstract: MoS2 can absorb the light but in solar cell, because of its sub-wavelength thickness, only a few percentage of incident light will be absorbed. This paper introduce a spiral structure for MoS2/Graphene/hBN stack to increase optical absorption up to 90%. The thickness of the stack is about 1 um. They investigate Spiral with and without metal contacts to be able to monitor MoS2 influence into the structure more accurately. The thickness of hBN layer has critical role to enhance optical absorption (up to 90%) or the absorption relative to the amount of photoactive material used (up to 762% enhancement)

Epitaxial growth of a monolayer WSe2-MoS2 lateral p-n junction with an atomically sharp interface[11][11][11][edit | edit source]

Abstract: Two-dimensional transition metal dichalcogenides (TMDCs) such as molybdenum sulfide MoS2 and tungsten sulfide WSe2 have potential applications in electronics because they exhibit high on-off current ratios and distinctive electro-optical properties. Spatially connected TMDC lateral heterojunctions are key components for constructing monolayer p-n rectifying diodes, light-emitting diodes, photovoltaic devices, and bipolar junction transistors. However, such structures are not readily prepared via the layer-stacking techniques, and direct growth favors the thermodynamically preferred TMDC alloys. We report the two-step epitaxial growth of lateral WSe2-MoS2 heterojunction, where the edge of WSe2 induces the epitaxial MoS2 growth despite a large lattice mismatch. The epitaxial growth process offers a controllable method to obtain lateral heterojunction with an atomically sharp interface.

• lateral heterojunctions is challenging because TMDC alloys are thermodynamically preferred and perform alloy in the interface
• First grow single crystalline triangular WSe2 at 950 and then growth MoS2 in a separate tube at 750 C Mo and S vapor pressure should be in certain range, otherwise will growth vertically or WS2 (page 625 middle column)
• Calculated PCE=0.2%
• This paper mostly focused on the fab technique and physics of the device than PV behavior

A Van Der Waals Homojunction: Ideal p–n Diode Behavior in MoSe 2[12][12][12][edit | edit source]

Abstract: A MoSe2 p–n diode with a van der Waals homojunction is demonstrated by stacking undoped (n‐type) and Nb‐doped (p‐type) semiconducting MoSe2 synthesized by chemical vapor transport for Nb substitutional doping. The p–n diode reveals an ideality factor of ≈1.0 and a high external quantum efficiency (≈52%), which increases in response to light intensity due to the negligible recombination rate at the clean homojunction interface.

• MoS2, WS2 and MoSe2 are n-type while WSe2 and MoTe2 are P-type as intrinsic
• in this study they doped MoSe2 with Nb to make it P-type and make vdW homojunction p-n diode

Combined effect of double antireflection coating and reversible molecular doping on performance of few-layer graphene/n-silicon Schottky barrier solar cells[13][13][13][edit | edit source]

Abstract: Few layer graphenes were deposited by CVD onto unpolished Cu foils and then transferred to n-type silicon wafer to make graphene/n-silicon Schottky barrier solar cells. Graphene was doped with nitric acid vapor and an antireflection layer was added on top. The total power conversion efficiency reached to 8.5%. It is shown Double layer Anti-reflective Coating (DARC), MgF2/ZnS gives the best results. Graphene/n-Si and DARC/Graphene/n-Si are compared by simulation and demonstration indicating DARC/G/n-Si has the highest efficiency.

Diffractive nanostructures for enhanced light-harvesting in organic photovoltaic devices[14][14][14][edit | edit source]

Abstract: An in-coupling gratings introduced in this paper to improve performance of thin film organic solar cells. It is claimed this gratings will improve the light absorption of 14.8% and it is independent from active layer. This was applied on blade-coated devices and a 12% improvement in efficiency was measured. 2 dimensional grating yields the best result compared to 1D. This will be compatible with roll-to-roll production for OPVs. Different geometry structures for 1D and 2D examined to reach to optimum design.

Broadband Absorption Enhancement in Solar Cells with an Atomically Thin Active Layer[15][15][15][edit | edit source]

Abstract: In this paper they are trying to use of resonances made by a photonic crystal slab for broadband enhancement of above bandgap absorption of a single layer MoS2. In this structure design, a single layer MoS2 is placed on top a photonic crystal layer that it sit on top of a mirror. By doing multiple resonances absorption can be improved over the entire frequency range above the bandgap of MoS2. By doing this average absorption reached over 51% for visible spectrum while it is around 10% for single-pass absorption for single layer MoS2. A photonic crystal structure design study was done to find the optimum Lattice Constant, Hole Radius and the Slab thickness to maximize the resonance and light absorption. They comes up with some rules, First; grating period need to be subwavelength to most of the frequency region under interest to eliminate those waves travelling away from active layer and cannot be absorbed. The large holes works better than small holes, Finally thin layer works better than a thick layer. This geometry design is a general design and can be used for other TMDs materials as well.

Angle-selective perfect absorption with two-dimensional materials[16][16][16][edit | edit source]

Abstract: In this paper is tried to use a simple structure, a mirror, a dielectric spacer and a 2D material on top to enhance the light absorption. In this design, and in 2D material wavelength range which the material has some loss, always there is an angle of incidence that absorption reaches to unity in that angle. They demonstrated this structure can exceed 77% absorption for mid-infrared range (13 um). This was shown for Graphene but it is claimed it works with other 2D materials with different doping levels. It is claimed because this structure is simple and no nano-patterning required, so it can be scaled to large area and high yield production.

Lateral black phosphorene P–N junctions formed via chemical doping for high performance near-infrared photodetector[17][17][17][edit | edit source]

Abstract: Black phosphorene (BP), a newly discovered elemental two-dimensional material, is attractive for optoelectronic and photonic applications because of its unique in-plane anisotropy, thickness-dependent direct bandgap and high carrier mobility. Since its discovery, black phosphorene has become an appealing candidate well-suited for polarization-resolved near- and mid-infrared optoelectronics due to its relative narrow bandgap and asymmetric structure. Here, we employ benzyl viologen (BV) as an effective electron dopant to part of the area of a (p-type) few-layer BP flake and achieve an ambient stable, inplane P–N junction. Chemical doping with BV molecules modulates the electron density and allows acquiring a large built-in potential in this in-plane BP P–N junction, which is crucial for achieving high responsivity photodetectors and high quantum efficiency solar cells. As a demonstrative example, by illuminating it with a near-infrared laser at 1.47 mm, we observe a high responsivity up to �180 mA/W with a rise time of 15 ms, and an external quantum efficiency of 0.75%. Our strategy for creating environmentally stable BP P–N junction paves the way to implementing high performance BP phototransistors and solar cells, which is also applicable to other 2D materials.

• the most stable allotrope of element phosphorus with strong anisotropy and a direct bandgap ranging from 0.3 eV to 2 eV depending on # of layers
• photoresponse in the visible and near infrared ranges
• they claim PCE=0.75% for power density of 10 W/cm2

Mixed-dimensional van der Waals heterostructures[18][18][18][edit | edit source]

Abstract: This is a review paper about VdW structure devices using 2D materials (Graphene, hBN or TMDs). VdW structures are 2D-nD (n=0,1 or 3).

• this paper looks the physics of 2D and heterostructure devices
• different type of devices such as FET, LED and photodetectors covered in this paper
• They also looked PV devices.
• must read PV part in page 176 of journal, a good track provided

Sub-nanometer planar solar absorber[19][19][19][edit | edit source]

Abstract: This paper focus on a method to optimize solar absorption for 2D material or any sub-namo thickness films using simple planar structures. Two structures offered, sub-nanometer film placed (1) onto a transparent layer on a metallic film or (2) between the transparent layer and the substrate for thr oblique sunlight illumination condition These methodology will give two parameter freedom to enhance the absorption film thickness and incident angle while previous structure had to deal with film thickness only, So can reach to almost perfect absorption.

• they show in theory and experiment that always there is a pair of transparent layer thickness and incident angle that absorption reach to 100%
• first they choose a thickness for target wavelength and then find the incident angle to reach 100% absorption
• they reached 92% absorption my MoS2 at 660 nm
• the PCE for broadband and wide angle was measured to be 4.4%

Graphene coupled with Pt cubic nanoparticles for high performance, air-stable graphene-silicon solar cells[20][20][20][edit | edit source]

Abstract:

• Graphene itself has low carrier concentration and therefore low work function so not good for PV
• chemical doping is not permanent solution, it degrade fast
• they add some Pt nano particle onto Gr to enhance solar absorption be Plasmonic effect
• adding Pt nano particles improved also carrier concentration and work function ( works as physical doping)
• PCE reached to 7%
• Adding TiO2 antireflective film enhance PCE to 10%
• this can be called as 0, 2 and 3 dimensional structure