Advances in solar energy harvesting integrated by van der Waals graphene heterojunctions

Abstract

Graphene has garnered increasing attention for solar energy harvesting owing to its unique features. However, limitations hinder its widespread adoption in solar energy harvesting, comprising the band gapless in the molecular orbital of graphene lattice, its vulnerability to oxidation in oxidative environments, and specific toxic properties that require careful consideration during development. Beyond current challenges, researchers have explored doping graphene with ionic liquids to raise the lifespan of solar cells (SCs). Additionally, they have paid attention to optimizing graphene/Si Schottky junction or Schottky barrier SCs by enhancing the conductivity and work function of graphene, improving silicon's reflectivity, and addressing passivation issues at the surface/interface of graphene/Si, resulting in significant advancements in their power conversion efficiency. Increasing the functional area of graphene-based SCs and designing efficient grid electrodes are also crucial for enhancing carrier collection efficiency. Flaws and contaminants present at the interface between graphene and silicon pose significant challenges. Despite the progress of graphene/Si-based photovoltaic cells still needs to catch up to the efficiency achieved by commercially available Si p-n junction SCs. The low Schottky barrier height, design-related challenges associated with transfer techniques, and high lateral resistivity of graphene contribute to this performance gap. To maximize the effectiveness and robustness of graphene/Si-based photovoltaic cells, appropriate interlayers have been utilized to tune the interface and modulate graphene's functionality. This mini-review will address ongoing research and development endeavors using van der Waals graphene heterojunctions, aiming to overcome the existing limitations and unlock graphene's full potential in solar energy harvesting and smart storage systems.

Fig. 1. The publication numbers of “solar energy harvesting” in terms of 2010–2023 (WOS Source).

Fig. 2. (a) Design of G/n-Si SC, a layer of graphene was applied onto SiO₂/Si substrate, forming a junction. (a) has been reproduced from ref. with permission from American Chemical Society. (b) Energy band diagram in G/n-Si SB-SCs and the displacement of electron–hole pairs. (b) has been reproduced from ref. with permission from Wiley-VCH. (c) A visual representation depicting the uncomplicated creation of a G/Si SB-SCs through a heterojunction structure is shown. This process involves three key factors: interfacial layer, chemical doping, and ARC coating. These factors collectively contribute to improving efficiency and prolonging the durability of G/Si SCs. (c) reproduced from ref. with permission from Royal Society of Chemistry.

Fig. 3. Voltage–current characteristics and FF of a theoretical G/Si SCs with various (a) series resistance, (b) sheet resistance. (a) and (b) have been reproduced from ref. with permission from Wiley-VCH. Experimental G/Si SCs with (c) series resistance and sheet resistance, which is estimated from the reverse bias I–V sweep. (c) has been reproduced from ref. with permission from Wiley-VCH.

Fig. 4. (a) The diagram of band structure and ϕ_wf of the MLG/Si interface prior to and following the doping process. (b) Raman spectroscopy results showing the spectra of MLG and FLG with and without the presence of Au NPs. (c) Dark J–V curves of the FLG/Si SCs with and without Au NPs. The inset presents the graphs of dV/d(ln J) versus I, enabling the determination of the series resistance R_S of cells. (b)–(d) have been reproduced from ref. with permission from American Institute of Physics.

Fig. 5. Tilted cross-sectional SEM images visuals portraying (a) arrays of nanowires measuring 2 μm and (b) arrays of nanowires measuring 5 μm. Scale bars are 10 μm for both images. In addition, (c) transmission spectra of thin Si structures are provided for the comparison: prior to etching (red), subsequent to etching into 2 μm (green), and following etching into 5 μm (black) nanowires. (a)–(c) have been reproduced from ref. with permission from American Chemical Society.

Fig. 6. SEM image of Si pillar without (a) and with coated graphene (b), (c) the reflectance spectra of Si pillars with different pillar height and planar Si substrate. Inset presents the principle of antireflective effect of Si pillar substrate. (d) Schematic presentation of a G/Si pillar SB-SCs. (e) Photograph of a G/Si pillar SC with junction area of 0.09 cm². (a)–(e) have been reproduced from 72 with permission from American Institute of Physics.

Fig. 7. (a) Schematics diagrams of G/planar Si, (b) G/Si NW junctions. (a) and (b) have been reproduced from ref. with permission from American Chemical Society. (c) Schematic presentation of the G/Si HA SB-SCs, (d) top-view SEM image of the G/Si HA device. The area included by graphene films shows darker contrast in the SEM image. (e) Top-view and (f) cross-sectional view SEM images of the as-prepared SiHA. Insets present the enlarged SEM images, (g) PV characteristics, and (h) EQE spectra of the G/Si HA SB-SCs with various hole depths. (c)–(h) have been reproduced from ref. with permission from Royal Society of Chemistry.

Fig. 8. (a) Patterning of the Si substrate, (b) relationship between refractive index and wavelength for certain ARC materials. (a) and (b) have been reproduced from ref. with permission from Royal Society of Chemistry. (c) Light reflection spectra of a g/Si SCs prior to (black) and following (red) the application of a TiO₂ colloid coating, illustrating antireflection effect, (d) J–V characteristics of an as-deposited G/Si SCs, after HNO₃ vapor doping, and after TiO₂ coating (together with HNO₃ doping), respectively. (c) and (d) Have been reproduced from ref. with permission from American Chemical Society. (e) Reflectance spectra of PMMA-extracted and PMMA-grown graphene samples on quartz slides, (f) J–V curves of PMMA-removed and PMMA-coated (2000 rpm) G/Si SCs before and after HNO₃ doping. (e) and (f) Have been reproduced from ref. with permission from Royal Society of Chemistry.

Fig. 9. G/Si SB-SCs with (a) schematic presentation of the color device, (b) photographs of device area: 1 cm² with various colors, and (c) coordinates of seven colors in CIE chromaticity. (a) and (b) Has been reproduced from ref. with permission Elsevier.

Fig. 10. (a) Modeled (dash lines) and observed (solid lines) reflection spectra of DL-MgF₂/ZnS-coated G/Si heterojunction SCs (b) EQE spectra of the uncoated and G/Si SCs with various structural colors, (c) J–V spectra of the devices with various structural colors. J–V spectra of the devices without coating and with enhanced anti-reflection coating were also indicated for comparison, (d) contrast of the J–V and EQE (inset) among SL-ZnS-coated and DL-MgF₂/ZnS-coated G/Si devices. (a)–(d) Have been reproduced from ref. with permission from Elsevier.

Fig. 11. Energy band diagrams of the G/Si SB-SCs (a) without and (b) with an h-BN electron blocking layer. (a) and (b) have been reproduced from ref. with permission from Elsevier Presentations of band diagrams for the SCs. The PV procedures (c) in the graphene/n-Si and (d) in the graphene/MoS₂/n-Si SCs are indicated. (c) and d have been reproduced from ref. with permission from Royal Society of Chemistry.

Fig. 12. Band diagram schematic of hybrid SCs. Figure has been reproduced from ref. with permission from American Chemical Society.

Fig. 13. The promising prospects of van der Waals graphene heterojunctions integrated into the solar energy harvesting.