Stratified Oxygen Vacancies Enhance the Performance of Mesoporous TiO2 Electron Transport Layer in Printable Perovskite Solar Cells

The low electrical conductivity and the high surface defect density of the TiO₂ electron transport layer (ETL) limit the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Here, the conductivity and defect modulation of the mesoporous TiO₂ (mp-TiO₂) ETL via oxygen vacancy (OV) management by the reduction and oxidation treatment are reported. Reduction treatment via reducing agent introduces abundant OVs into the TiO₂ nanocrystalline particles on the surface and at the subsurface. The following oxidation treatment via hydrogen peroxide removes the surface OVs while remains the subsurface OVs, resulting in stratified OVs. The stratified OVs improve the conductivity of TiO₂ ETL by increasing carrier donors and decrease nonradiative centers by reducing surface defects. Such synergy ensures the capability of mp-TiO₂ as the well-performed ETL with improved energy level alignment, suppressed interface recombination, enhanced carrier extraction, and transport. As a result, printable hole-conductor-free carbon-based mesoscopic PSCs based on the modulated mp-TiO₂ ETL demonstrate a highest reported PCE of 18.96%.     

Red-Carbon-Quantum-Dot-Doped SnO2 Composite with Enhanced Electron Mobility for Efficient and Stable Perovskite Solar Cells

An efficient electron transport layer (ETL) plays a key role in promoting carrier separation and electron extraction in planar perovskite solar cells (PSCs). An effective composite ETL is fabricated using carboxylic-acid- and hydroxyl-rich red-carbon quantum dots (RCQs) to dope low-temperature solution-processed SnO₂, which dramatically increases its electron mobility by ≈20 times from 9.32 × 10⁻⁴ to 1.73 × 10⁻² cm² V⁻¹ s⁻¹. The mobility achieved is one of the highest reported electron mobilities for modified SnO₂. Fabricated planar PSCs based on this novel SnO₂ ETL demonstrate an outstanding improvement in efficiency from 19.15% for PSCs without RCQs up to 22.77% and have enhanced long-term stability against humidity, preserving over 95% of the initial efficiency after 1000 h under 40–60% humidity at 25 °C. These significant achievements are solely attributed to the excellent electron mobility of the novel ETL, which is also proven to help the passivation of traps/defects at the ETL/perovskite interface and to promote the formation of highly crystallized perovskite, with an enhanced phase purity and uniformity over a large area. These results demonstrate that inexpensive RCQs are simple but excellent additives for producing efficient ETLs in stable high-performance PSCs as well as other perovskite-based optoelectronics.     

SnO2: A Wonderful Electron Transport Layer for Perovskite Solar Cells

The highest power conversion efficiency of perovskite solar cells is beyond 22%. Charge transport layers are found to be critical for device performance and stability. A traditional electron transport layer (ETL), such as TiO₂, is not very efficient for charge extraction at the interface, especially in planar structure. In addition, the devices using TiO₂ suffer from serious degradation under ultraviolet illumination. SnO₂ owns a better band alignment with the perovskite absorption layer and high electron mobility, which is helpful for electron extraction. In this Review, recent progresses in efficient and stable perovskite solar cells using SnO₂ as ETL are summarized.     

Improving the Electron Transport Performance of TiO2 Film by Regulating TiCl4 Post-Treatment for High-Efficiency Carbon-Based Perovskite Solar Cells

Titanium oxide (TiO₂) has been widely used as an electron transport layer (ETL) in perovskite solar cells (PSCs). Typically, TiCl₄ post-treatment is indispensable for modifying the surfaces of TiO₂ ETL to improve the electron transport performance. However, it is challenging to produce the preferred anatase phase-dominated TiO₂ by the TiCl₄ post-treatment due to the higher thermodynamic stability of the rutile phase. In this work, a mild continuous pH control strategy for effectively regulating the hydrolysis process of TiCl₄ post-treatment is proposed. As the weak organic base, urea has been demonstrated can maintain a moderate pH decrease during the hydrolysis process of TiCl₄ while keeping the hydrolysis process relatively mild due to the ultra-weak alkalinity. The improved pH environment is beneficial for the formation of anatase TiO₂. Consequently, a uniform anatase-dominated TiO₂ surface layer is formed on the mesoporous TiO₂, resulting in reduced defect density and superior band energy level. The interfacial charge recombination is effectively suppressed, and the charge extraction efficiency is improved simultaneously in the fabricated solar cells. The efficiency of the fabricated carbon electrode-based PSCs (C-PSCs) is improved from 16.63% to 18.08%, which is the highest for C-PSCs based on wide-bandgap perovskites.     

Novel Bilayer SnO2 Electron Transport Layers with Atomic Layer Deposition for High-Performance α-FAPbI3 Perovskite Solar Cells

Perovskite solar cells (PSCs) based on the SnO₂ electron transport layer (ETL) have achieved remarkable photovoltaic efficiency. However, the commercial SnO₂ ETLs show various shortcomings. The SnO₂ precursor is prone to agglomeration, resulting in poor morphology with numerous interface defects. Additionally, the open circuit voltage (V_oc) would be constrained by the energy level mismatch between the SnO₂ and the perovskite. And, few studies designed SnO₂-based ETLs to promote crystal growth of PbI₂, a crucial prerequisite for obtaining high-quality perovskite films via the two-step method. Herein, we proposed a novel bilayer SnO₂ structure that combined the atomic layer deposition (ALD) and sol-gel solution to well address the aforementioned issues. Due to the unique conformal effect of ALD-SnO₂, it can effectively modulate the roughness of FTO substrate, enhance the quality of ETL, and induce the growth of PbI₂ crystal phase to develop the crystallinity of perovskite layer. Furthermore, a created built-in field of the bilayer SnO₂ can help to overcome the electron accumulation at the ETL/perovskite interface, leading to a higher V_oc and fill factor. Consequently, the efficiency of PSCs with ionic liquid solvent increases from 22.09% to 23.86%, maintaining 85% initial efficiency in a 20% humidity N2 environment for 1300 h.     

Nanoporous p-type NiOx electrode for p-i-n inverted perovskite solar cell toward air stability

Designing air-stable perovskite solar cells (PSCs) is a recent trend in low-cost photovoltaic technology. Metal oxide-based electron transporting layers (ETLs) and hole transporting layers (HTLs) have attracted tremendous attention in PSCs, because of their excellent air stability, high electron mobility, and optical transparency. Herein, we report a co-precipitation method for the synthesis of p-type nanoporous nickel oxide (np-NiOx) thin films as the HTL for inverted (p-i-n) PSCs. The best-performing p-i-n PSC having np-NiOx HTL, (FAPbI₃)_0.85(MAPbBr₃)_0.15 (herein FAPbI₃ stands for formamidinium lead iodide and MAPbBr₃ stands for methylammonium lead bromide) perovskite and phenyl-C₆₁-butyric acid methyl ester (PCBM)/ZnO ETL exhibited a 19.10% (±1%) power conversion efficiency (PCE) with a current density (J_SC) of 22.76?mA?cm^?2, open circuit voltage (V_OC) of 1.076?V and fill factor (FF) of 0.78 under 1?sun (100?mW?cm^?2). Interestingly, the developed p-i-n PSCs based on p-type NiOx and n-type ZnO could retain >80% efficiency after 160?days, which is much higher than conventional PEDOT:PSS HTL-based PSCs. Our findings provide air-stable perovskite solar cells with high efficiency.     

20.67%-Efficiency Inorganic CsPbI3 Solar Cells Enabled by Zwitterion Ion Interface Treatment

All-inorganic CsPbI₃ perovskite solar cells (PSCs) have been extensively studied due to their high thermal stability and unprecedented rise in power conversion efficiency (PCE). Recently, the champion PCE of CsPbI₃ PSCs has reached up to 21%; however, it is still much lower than that of organic–inorganic hybrid PSCs. Interface modification to passivate surface defects and minimize charge recombination and trapping is important to further improve the efficiency of CsPbI₃ PSCs. Herein, a new zwitterion ion is deposited at the interface between electron transporting layer (ETL) and perovskite layer to passivate the defects therein. The zwitterion ions can not only passivate oxygen vacancy (V_O) and iodine vacancy (V_I) defects, but also improve the band alignment at the ETL-perovskite interface. After the interface treatment, the PCE of CsPbI₃ device reaches up to 20.67%, which is among the highest values of CsPbI₃ PSCs so far. Due to the defect passivation and hydrophobicity improvement, the PCE of optimized device remains 94% of its original value after 800 h storing under ambient condition. These results provide an efficient way to improve the quality of ETL-perovskite interface by zwitterion ions for achieving high performance inorganic CsPbI₃ PSCs.     

Room-temperature synthesis of ZrSnO_4 nanoparticles for electron transport layer in efficient planar hetrojunction perovskite solar cells

The interface engineering plays a key role in controlled optoelectronic properties of perovskite photovoltaic devices,and thus the electron transport layer(ETL) material with tailored optoelectronic properties remains a challenge for achieving high photovoltaic performance of planar perovskite solar cells(PSCs).Here,the fine and crystalline zirconium stanate(ZrSnO_4) nanoparticles(NPs) was synthesized at low temperature,and its optoelectronic properties are systematically investigated.Benefiting from the favorable electronic structure of ZrSnO_4 NPs for applications in ETL,efficient electron transport and extraction with suppre s sed charge recombination are achieved at the interface of perovskite layer.As a result,the optimized ZrSnO_4 NPs synthesized at room-temperature deliver the optimized power conversion efficiency up to 16.76% with acceptable stability.This work opens up a new class of ternary metal oxide for the use in ETL of the planar PSCs and should pave the way toward designing new interfacial materials for practical optoelectronic devices.     

Surface modified NiOx as an efficient hole transport layer in inverted perovskite solar cells

Hole transporting materials play a vital role in improving the performance of perovskite solar cells (PSCs). In this work, different concentrations of poly[bis(4-phenyl)(2, 4, 6-trimethylphenyl)amine] (PTAA) are used to modify the surface of sputtered nickel oxide (NiO_x) as hole transport layer (HTL) in inverted PSCs. After the introduction of PTAA, the roughness of the sputtered NiO_x films decreases, while the crystallinity of the perovskite layer increases. The carrier transport across the interface between the sputtered NiO_x film and the perovskite layer is significantly improved. A power conversion efficiency of 18.5% is achieved based on PTAA-modified sputtered NiO_x, exhibiting a 15.2% improvement compared to its pristine counterpart.

     

Gradient Energy Alignment Engineering for Planar Perovskite Solar Cells with Efficiency Over 23%

An electron-transport layer (ETL) with appropriate energy alignment and enhanced charge transfer is critical for perovskite solar cells (PSCs). However, interfacial energy level mismatch limits the electrical performance of PSCs, particularly the open-circuit voltage (V_OC). Herein, a simple low-temperature-processed In₂O₃/SnO₂ bilayer ETL is developed and used for fabricating a new PSC device. The presence of In₂O₃ results in uniform, compact, and low-trap-density perovskite films. Moreover, the conduction band of In₂O₃ is shallower than that of Sn-doped In₂O₃ (ITO), enhancing the charge transfer from perovskite to ETL, thus minimizing V_OC loss at the perovskite and ETL interface. A planar PSC with a power conversion efficiency of 23.24% (certified efficiency of 22.54%) is obtained. A high V_OC of 1.17 V is achieved with the potential loss at only 0.36 V. In contrast, devices based on single SnO₂ layers achieve 21.42% efficiency with a V_OC of 1.13 V. In addition, the new device maintains 97.5% initial efficiency after 80 d in N₂ without encapsulation and retains 91% of its initial efficiency after 180 h under 1 sun continuous illumination. The results demonstrate and pave the way for the development of efficient photovoltaic devices.     

Bifacial Contact Junction Engineering for High‐Performance Perovskite Solar Cells with Efficiency Exceeding 21%

Ordered 1D metal oxide structure is desirable in thin film solar cells owing to its excellent charge collection capability. However, the electron transfer in 1D electron transporting layer (ETL)‐based devices is still limited to a submicrometer‐long pathway that is vertical to the substrate. Here, an innovative closely packed rutile TiO₂ nanowire (CRTNW) network parallel to the facet of fluorine‐doped tin oxide (FTO) substrate is reported, which can serve as a 1D nanoscale electron transport pathway for efficient perovskite solar cells (PSCs). The PSC constructed using newly prepared CRTNW ETL achieves an impressive power conversion efficiency of 21.10%, which can be attributed to the facilitated electron extraction induced by the favorable junctions formed at FTO/ETL and ETL/perovskite interfaces and also the suppressed charge recombination originating from improved perovskite morphology with large grains, flat surface, and good surface coverage. The bifacial contact junctions engineering also enables large‐area device fabrication. The PSC with 1 cm² aperture yields an efficiency of 19.50% under one sun illumination. This work highlights the significance of controlling the orientation and packing density of the ordered 1D oxide nanostructured thin films for highly efficient optoelectronic devices in a large‐scale manner.     

Selectively patterned TiO2 nanorods as electron transport pathway for high performance perovskite solar cells

Organic-inorganic hybrid perovskite solar cells (PSCs) are attracting tremendous attention for new-generation photovoltaic devices because of their excellent power conversion efficiency and simple fabrication process. One of the various approaches to increase the efficiency of PSCs is to change the material or structure of the carrier transport layer. Here, optically long and electrically short structural concept is proposed to enhance the characteristics of a PSC by employing selectively grown single crystalline TiO₂ nanorods. The approach has the merit of increasing the electron-hole separation effectively and enables a thicker active layer to be coated without electrical loss by using TiO₂ nanorods as an electron pathway. Moreover, selectively grown TiO₂ nanorods increase the optical path of the incident light via scattering effects and enable a smooth coating of the active layer. Nanoimprint lithography and hydrothermal growth were employed to fabricate selectively grown TiO₂ nanorod substrates. The fabricated solar cell exhibits an efficiency of 19.86% with a current density, open-circuit voltage, and fill factor of 23.13 mA/cm², 1.120 V, and 76.69%, respectively. Time-resolved photoluminescence, ultraviolet-visible (UV–Vis) spectroscopy, and the incident photon to current efficiency (IPCE) analysis were conducted to understand the factors responsible for the improvement in characteristics of the fabricated PSCs.

     

Low‐Temperature In Situ Amino Functionalization of TiO2 Nanoparticles Sharpens Electron Management Achieving over 21% Efficient Planar Perovskite Solar Cells

Titanium oxide (TiO₂) has been commonly used as an electron transport layer (ETL) of regular‐structure perovskite solar cells (PSCs), and so far the reported PSC devices with power conversion efficiencies (PCEs) over 21% are mostly based on mesoporous structures containing an indispensable mesoporous TiO₂ layer. However, a high temperature annealing (over 450 °C) treatment is mandatory, which is incompatible with low‐cost fabrication and flexible devices. Herein, a facile one‐step, low‐temperature, nonhydrolytic approach to in situ synthesizing amino‐functionalized TiO₂ nanoparticles (abbreviated as NH₂‐TiO₂ NPs) is developed by chemical bonding of amino (‐NH₂) groups, via Ti? N bonds, onto the surface of TiO₂ NPs. NH₂‐TiO₂ NPs are then incorporated as an efficient ETL in n‐i‐p planar heterojunction (PHJ) PSCs, affording PCE over 21%. Cs_0.05FA_0.83MA_0.12PbI_2.55Br_0.45 (abbreviated as CsFAMA) PHJ PSC devices based on NH₂‐TiO₂ ETL exhibit the best PCE of 21.33%, which is significantly higher than that of the devices based on the pristine TiO₂ ETL (19.82%) and is close to the record PCE for devices with similar structures and fabrication procedures. Besides, due to the passivation of the surface trap states of perovskite film, the hysteresis of current–voltage response is significantly suppressed, and the ambient stability of devices is improved upon amino functionalization.     

Multifunctional Polymer-Regulated SnO2 Nanocrystals Enhance Interface Contact for Efficient and Stable Planar Perovskite Solar Cells

Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar-structured PSCs can be fabricated with low-temperature (≤150 °C) solution-based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO₂ electron-transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO₂ nanocrystals, and induce vertically aligned crystal growth of perovskites on top. Correspondingly, SnO₂–HP-based devices can demonstrate an average efficiency of 23.03% on rigid substrates with enhanced open-circuit voltage (V_OC) of 1.162 V and high reproducibility. Attributed to the strengthened interface binding, the devices obtain high operational stability, retaining 97% of their initial performance (power conversion efficiency, PCE > 22%) after 1000 h operation at their maximum power point under 1 sun illumination. Besides, the HP-modified SnO₂ ETL exhibits promising potential for application in flexible and large-area devices.     

<Emphasis Type="Italic">In situ</Emphasis> fabrication of 2D SnS<Subscript>2</Subscript> nanosheets as a new electron transport layer for perovskite solar cells

To promote commercialization of perovskite solar cells (PSCs), low-temperature processed electron transport layer (ETL) with high carrier mobility still needs to be further developed. Here, we reported two-dimensional (2D) tin disulfide (SnS₂) nanosheets as ETL in PSCs for the first time. The morphologies of the 2D SnS₂ material can be easy controlled by the in situ synthesized method on the conductive fluorine-doped tin oxide (FTO) substrate. We achieved a champion power conversion efficiency (PCE) of 13.63%, with the short-circuit current density (J_SC) of 23.70 mA/cm², open-circuit voltage (V_OC) of 0.95 V, and fill factor (FF) of 0.61. The high J_SC of PSCs results from effective electron collection of the 2D SnS₂ nanosheets from perovskite layer and fast electron transport to the FTO. The low V_OC and FF are the results of the lower conduction band of 2D SnS₂ (4.23 eV) than that of TiO₂ (4.0 eV). These results demonstrate that 2D material is a promising candidate for ETL in PSCs.

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High-Efficiency Carbon-based CsPbI2Br Perovskite Solar Cells from Dual Direction Thermal Diffusion Treatment with Cadmium Halides

In recent years, carbon-based CsPbI₂Br perovskite solar cells (PSCs) have attracted more attention due to their low cost and good stability. However, the power conversion efficiency (PCE) of carbon-based CsPbI₂Br PSCs is still no more than 16%, because of the defects in CsPbI₂Br or at the interface with the electron transport layer (ETL), as well as the energy level mismatch, which lead to the loss of energy, thus limiting PCE values. Herein, a series of cadmium halides are introduced, including CdCl₂, CdBr₂ and CdI₂ for dual direction thermal diffusion treatment. Some Cd²⁺ ions thermally diffuse downward to passivate the defects inside or on the surface of SnO₂ ETL. Meanwhile, the energy level structure of SnO₂ ETL is adjusted, which is in favor of the transfer of electron carriers and blocking holes. On the other hand, part of Cd²⁺ and Cl⁻ ions thermally diffuse upward into the CsPbI₂Br lattice to passivate crystal defects. Through dual direction thermal diffusion treatment by CdCl₂, CdI₂ and CdBr₂, the performance of devices has been significantly improved, and their PCE has been increased from 13.01% of the original device to 14.47%, 14.31%, and 13.46%, respectively. According to existing reports, 14.47% is one of the highest PCE of carbon-based CsPbI₂Br PSCs with SnO₂ ETLs.     

Significant efficiency enhancement of carbon-based CsPbI2Br perovskite solar cells enabled by optimizing tin oxide electron transport layer

Carbon-based perovskite solar cells (C-PSCs) have been popular for achieving low-cost and stable photovoltaics. To overcome an obstacle of high-temperature annealing process for producing titanium dioxide (TiO₂), CsPbI₂Br C-PSCs based on a device structure of FTO/tin oxide (SnO₂)/CsPbI₂Br/carbon electrode can be fabricated at the low-temperature annealing process of 280 °C for 180 s, where SnO₂ is used as the electron transporting layer (ETL). Experimental results showed that the suitable concentration of SnO₂ ETL could yield smooth surface CsPbI₂Br films with free-pinhole and larger grain-sized crystallization. In combination with prolonging annealing time, a champion power conversion efficiency of 9.68% with a larger open-circuit voltage (V_oc) of 1.14 V was obtained for CsPbI₂Br C-PSC based on SnO₂ ETL. Here, a simple low-temperature fabricating process of SnO₂ ETL can be adapted to flexible substrates for C-PSCs and furtherly reduce the manufacturing cost.

     

Planar perovskite solar cells: eco-friendly synthesized cone-like ZnO nanostructure for efficient interfacial electron transport layer

The process of interfacial engineering in planar perovskite solar cells (PPSCs) plays an important role not only in transferring charges from active perovskite layer but also in enhancing the device performance. Considering this, the present study aims to synthesize an eco-friendly solution processed ZnO nanostructure interfacial electron transport layer (ETL) in PPSCs. The optical, structural, morphological and chemical properties of the prepared ZnO nanostructured material are analysed using ultraviolet–visible spectroscopy (UV–Vis), X-ray diffraction analysis (XRD), field emission-scanning electron microscopy (FE-SEM), energy-dispersive X-ray analysis (EDX), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis, respectively. Under ambient conditions, the device performance in terms of efficiency, stability and device degradation are investigated for both single layer (c-TiO₂ and cone-like ZnO nanostructure) and bilayer (c-TiO₂/cone-like ZnO nanostructure) ETL. Furthermore, the effective way of constructing cone-like nanostructured ZnO ETL on top of c-TiO₂ surface, found to be improved in faster charge transfer at the ETL/perovskite interfaces and reduced recombination losses. As a result, it exhibits maximum power conversion efficiency (PCE), short-circuit current density, fill factor and open-circuit voltage as 8.02%, 15.33 mA cm^?2, 0.52% and 0.99 V, respectively. Besides, the stability of PPSCs fabricated with bilayer exhibits better air stability of?~?87.40% with retained rate of 250 h from its initial value.

     

Efficient Bifacial Passivation with Crosslinked Thioctic Acid for High-Performance Methylammonium Lead Iodide Perovskite Solar Cells

Defects, inevitably produced within bulk and at perovskite-transport layer interfaces (PTLIs), are detrimental to power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). It is demonstrated that a crosslinkable organic small molecule thioctic acid (TA), which can simultaneously be chemically anchored to the surface of TiO₂ and methylammonium lead iodide (MAPbI₃) through coordination effects and then in situ crosslinked to form a robust continuous polymer (Poly(TA)) network after thermal treatment, can be introduced into PSCs as a new bifacial passivation agent for greatly passivating the defects. It is also discovered that Poly(TA) can additionally enhance the charge extraction efficiency and the water-resisting and light-resisting abilities of perovskite film. These newly discovered features of Poly(TA) make PSCs herein achieve among the best PCE of 20.4% ever reported for MAPbI₃ with negligible hysteresis, along with much enhanced ultraviolet, air, and operational stabilities. Density functional theory calculations reveal that the passivation of MAPbI₃ bulk and PTLIs by Poly(TA) occurs through the interaction of functional groups ( COOH,  C S) in Poly(TA) with under-coordinated Pb²⁺ in MAPbI₃ and Ti⁴⁺ in TiO₂, which is supported by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy.     

Versatility of Carbon Enables All Carbon Based Perovskite Solar Cells to Achieve High Efficiency and High Stability

Carbon‐based perovskite solar cells (PVSCs) without hole transport materials are promising for their high stability and low cost, but the electron transporting layer (ETL) of TiO₂ is notorious for inflicting hysteresis and instability. In view of its electron accepting ability, C₆₀ is used to replace TiO₂ for the ETL, forming a so‐called all carbon based PVSC. With a device structure of fluorine‐doped tin oxide (FTO)/C₆₀/methylammonium lead iodide (MAPbI₃)/carbon, a power conversion efficiency (PCE) is attained up to 15.38% without hysteresis, much higher than that of the TiO₂ ones (12.06% with obvious hysteresis). The C₆₀ ETL is found to effectively improve electron extraction, suppress charge recombination, and reduce the sub‐bandgap states at the interface with MAPbI₃. Moreover, the all carbon based PVSCs are shown to resist moisture and ion migration, leading to a much higher operational stability under ambient, humid, and light‐soaking conditions. To make it an even more genuine all carbon based PVSC, it is further attempted to use graphene as the transparent conductive electrode, reaping a PCE of 13.93%. The high performance of all carbon based PVSCs stems from the bonding flexibility and electronic versatility of carbon, promising commercial developments on account of their favorable balance of cost, efficiency, and stability.