Graphitic carbon nitride hybrid thin films for energy conversion: A mini-review on defect activation with different materials

1 Introduction

With the increasing global population, the demand for energy is also increasing worldwide. Fossil fuels will not be able to fulfill this energy demand. Moreover, the production of fossil fuels is a strong contributor to global warming. Solar energy is one of the viable candidates that can produce clean and high-output energy to supply this demand [1]. Solar cells use solar energy efficiently and convert it into electrical energy via the photovoltaic effect. Many technologies and materials are available commercially and are the subject of current research, such as Si [2], CIGS [3], CZTSSe [4], perovskite solar cells (PSC) [5], perovskite-tandem solar cells [6], and dye-sensitized and organic solar cells [7]. Graphitic carbon nitride (g-C₃N₄) is an important material that has gained much attention from scientists worldwide and has many applications, such as the degradation of pollutants, sensing, heavy metal ions reduction, NO _x and CO₂ reduction, oxygen activation site differentiation, biomedical, nitrogen fixation, light-emitting diodes, photocatalysis, and hydrogen reduction reaction [8]. Modification with other organic/inorganic materials helps improve their performance. Electrochemical properties and their layered structure allow for energy storage applications. Two of the main devices used in this subject are batteries and supercapacitors. Alkali metal ion batteries consist of a cathode, an anode, a physical separator, and an electrolyte. The working mechanism of this device consists of the insertion/extraction of the metal ion, such as Li or Na, between the electrodes during the charge/discharge cycles [9]. Cyclic voltammetry measurements of a polypyrrole/carbon nitride/Nb₂O₅ yielded a specific capacitance of 1,177 F g⁻¹ at a current density of 5 A g⁻¹, which allows for its application as a supercapacitor. The supercapacitive behavior is due to the linked agglomerates of Nb₂O₅ on the carbon nitride [10]. By employing Ta₂O₅ nanocomposites of carbon nitride and polypyrrole, Xavier and Vinodhini developed an electrode for supercapacitor applications, in which an in situ oxidation polymerization and adsorption were employed to obtain the Ta₂O₅ nanoparticles on the Py/g-C₃N₄ composite. They proved a highly stable electrode by showing a 2% decrease in the initial capacitance of 1,190 F g⁻¹ at a current density of 5 A g⁻¹ after 10,000 cycles [11]. The same strategy can be used to develop a polyindole/carbon nitride/MnO₂ nanocomposite electrode for supercapacitor applications [12]. Li and co-workers developed a smart nanohybrid Zn₂GeO₄/g-C₃N₄ ultra-thin layer for Li-ion batteries. In their work, the Zn₂GeO₄ acted as spacers between the carbon nitride to prevent the ultra-thin layers from restacking, and they also contributed to the high charge capacity via exposed vacancies [13]. A few layers of g-C₃N₄ are synthesized by annealing the carbon nitride with a zinc catalyst at 800°C, allowing for a good Na ion battery electrode. The layers have an interplanar distance of 0.51 nm, and the N-rich carbon nitride tunes the interlayer distance and the electrical conductivity. Their theoretical calculations support their experimental results [14]. Modifications to improve the photocatalytic properties can also be performed. Gujjulla et al. proved the synthesis of an efficient photoelectrocatalytic material Ag@g-C₃N₄/r-GO to produce hydrogen using a simple sonochemical method. Their results indicate that the photoelectrolytic activity is enhanced when adding the Ag nanoparticles on the surface of carbon nitride, thus providing active sites for the hydrogen evolution reaction [15]. Bi et al. developed a Z-scheme ZnO/g-C₃N₄ in which ZnO helped to improve the surface area of carbon nitride, which, in turn, led to more active sites for the photocatalytic degradation process [16]. Naveed’s group fabricated a TiO₂/g-C₃N₄ composite that allowed for efficient photocatalytic degradation of Congo red dye due to the high surface area, pore volume, and band gap of the materials. In addition, this composite was used for the esterification process to fabricate biofuels [17]. Due to its optoelectronic properties, this material finds applications in photovoltaic devices. On dye-sensitized solar cells, g-C₃N₄ can function as a blocking layer in TiO₂ solar cells [18]. On PSC, it can act either as an electron transport layer [19], as an additive [20], or as a passivator of surface defects of the perovskite absorbing layer [21]. Hybrid films can efficiently passivate the surface of the perovskite absorbing layer, as the authors have previously demonstrated [22]. Hybrid materials are usually a blend of inorganic and organic materials or nanostructures. When used in conjunction with electron and hole transport layers in solar cells, they usually increase the efficiency. Typically, materials employed to form hybrid films are Si, chalcogenides, and metal oxides [23]. These materials are used to improve the optical and electrical properties of the absorbing layers, such as electron mobility, conductivity, injection, and transport of charge carriers, and to improve the stability of the layers. In this sense, g-C₃N₄ hybrid thin films can be efficiently employed to improve the performance of photovoltaic devices due to the improvement of the properties of the layers by defect engineering and surface passivation effects. Also, g-C₃N₄ can be obtained by using various solution processing methods, compatible with other solution techniques typically employed in solar cell fabrication [24]. Theoretical studies have demonstrated that defects can enhance photocatalytic activity. Density functional theory (DFT) allows for understanding the material’s behavior with different additives and dopants from an atomistic point of view. Carbon nitride can be modified by doping or co-doping with metal atoms, surface decoration with metallic atoms, and g-C₃N₄ chalcogenide or oxide-based composites. Each kind of modification yields different interactions with the neighboring atoms, depending on the geometry and location of the impurity atom or material. Different functionals estimate the band gap of the resulting composite; the HSE06 functional described the band gap of carbon nitride, while the GGA functional underestimated it [25]. The simulations help predict the properties and the application of the materials used to optimize experiments. For example, Lin et al. studied the anatase TiO₂ (101) structure interface with g-C₃N₄ using spin-polarized DFT + U. They showed that the heterojunction leads to an improved rate of oxidation and redox reactions due to the migration from carbon nitride into anatase TiO₂ due to the type II band alignment. The type of bonding formed in this hybrid material is a van der Waals structure, as evidenced by the interfacial binding energy of 24 meV/Å [26]. The theoretical calculations carried out by Qian et al., using DFT with generalized gradient approximation, helped to corroborate the improved HER activity by proving that there is a transfer of charge from carbon nitride to MoS₂ due to the localized activated nitrogen sites, which indicates an N defect activation upon the acidification process carried out to exfoliate the g-C₃N₄ [27]. Koutsouroubi’s group performed DFT calculations to study the effect of Ni substitutional doping and S vacancies on MoS₂ and its subsequent effect on g-C₃N₄. The effect of the Ni substitutional doping is to decrease the formation energy of S vacancies when using a small concentration of Ni, and this leads to forming defect states in the gap toward the conduction band edge of MoS₂, yielding enhanced n-type conductivity [28]. Yan’s group produced a self-assembled ternary graphene/MoS₂/g-C₃N₄ aerogel and studied their band structures and effective sites for HER applications by DFT. They proved that upon adding the graphene layer to g-C₃N₄/MoS₂, the band structure of the resulting material changed to be a near-zero gap semiconductor at the K-point, indicating an increase in the material’s conductivity. Also, the adsorption sites of H atoms are studied on their free energy. Their theoretical calculations showed that the H atom adsorbs at the S site on MoS₂ and N sites on g-C₃N₄ [29]. Coupling with an MoS₂ monolayer, the valence band of the carbon nitride moves downward significantly, thus improving its ability for water splitting. Indeed, the conduction band minimum and the valence band maximum for pristine carbon nitride are −2.15 and −5.40 eV, respectively, located at Γ point, with a calculated direct band gap of 3.18 eV in agreement with the graphitic structure. After adding the MoS₂ layers to add S impurities, active sites are generated by substitution of the N edge and corner atoms, as well as C atoms with S; this leads to the formation of gap states that reduce the band gap, but they do not lower the valence band enough to allow efficient water splitting. Indeed, the valence band maximum is −4.8 eV. The same trend is observed by considering vacancies of the same sites, in which N and C vacancies lower the valence band to −5.75 and −5.74 eV, respectively. When considering the interfacial coupling between MoS₂ and g-C₃N₄ layers. The heterostructures are a monolayer of MoS₂ coupled with pristine g-C₃N₄, S-doped g-C₃N₄, and N and C vacancies of g-C₃N₄, respectively. The band gap was enlarged for the pristine case, and the conduction band minimum and valence band maximum moved to −2.6 and −6.4 eV, respectively. In the case of S-doped heterostructure, the band gap was increased to 3.9 eV, but gap states at −4 eV compensate for this. The same trend is observed for the N and C vacant heterostructures [30]. Figure 1 shows the calculated valence band and conduction band positions and the gap states of the described materials.

Figure 1

Calculated valence band and conduction band positions for S-doped g-C₃N₄ and MoS₂ supported on g-C₃N₄ considering the same S-doped sites on carbon nitride. The oxidation and reduction potentials for water splitting are shown in the figure (reprinted (adapted) with permission from the study of Qian et al. [27], Copyright (2022) American Chemical Society).

The previous lines imply that vacancy engineering is vital in applying this material in energy conversion. This brief review is centered on hybrid g-C₃N₄ thin films, specifically in relation to their application in photovoltaic systems, while also considering other potential uses. We first briefly introduce the properties and processing techniques of g-C₃N₄. Next, we give a small but comprehensive review of the available literature on hybrid carbon nitride thin films with chalcogenide materials, layered inorganic materials, inorganic halogen materials, and perovskite materials. We deal with the subject of how these materials improve the properties of the absorbing layers.

2 Properties of g-C₃N₄

Carbon nitride is a polymeric 2-D π conjugated organic semiconductor. It has a band gap of around 2.7 eV, which can be tuned by various means to enhance its optical absorption edge to the visible region [31]. The valence band comprises a lone pair N density of states, and the π-bonding stabilizes this lone pair state. Thus, nitrogen content modulates the band gap by modifying the electronic structure [32]. This material is a n-type semiconductor with a low conductivity value of around 10⁻⁹ (Ω cm)⁻¹. Various means can increase this value, such as making a hybrid nanocomposite with sulfur [33]. The material is photoconductive, which makes it attractive to photocatalytic and photovoltaic applications. There are different forms of carbon nitride, each one having different properties. It can be found as α-C₃N₄, β-C₃N₄, pseudocubic and cubic C₃N₄, and g-C₃N₄, the most stable form of carbon nitride [34]. Generally, it is accepted that tri-s-triazine is the building unit of the material. The structure is a graphitic plane formed by the sp² hybridization of carbon and nitrogen. This gives remarkable chemical and thermal stability to the material [35]. The defect engineering over g-C₃N₄ controls the electronic structure, increasing the light-harvesting ability through the meliorate separation of electron–holes. For this, many different synthesis routes can be employed. For example, the material can be obtained by chemical vapor deposition, solvothermal method, sputtering, thermal decomposition of other precursors, pulsed laser ablation, etc. [36]. The synthetic methodologies utilized to obtain g-C₃N₄ enable the production of hybrid materials with chalcogenides, layered chalcogenides, and metal halides. Indeed, all these synthesis methods can be utilized to obtain g-C₃N₄ to be used in different types of applications, for example, hydrogen production through water splitting, water purification and treatment, air purification, and pollutant removal, solar-driven carbon dioxide reduction, and solar fuel cells [37]. Thus, we focus on hybrid g-C₃N₄ with these materials and how their properties are modified. Photovoltaics and energy storage subjects are studied but are not limited to just these applications. Figure 2 shows a schematic representation of the most used synthesis routes to obtain this material.

Figure 2

Schematic representation of the most used synthesis routes to obtain graphitic carbon nitride: (a) chemical vapor deposition (reprinted (adapted) with permission from the study of Chubenko et al. [38], Copyright (2022) American Chemical Society), (b) solvothermal method (reprinted (adapted) with permission from the study of Ye et al. [39], Copyright (2016) American Chemical Society), (c) pulsed laser processing (reprinted (adapted) with permission from the study of Kim et al. [40], Copyright (2022) American Chemical Society), and (d) thermal decomposition of selected precursors (reprinted (adapted) with permission from the study of Majdoub et al. [41], Copyright (2020) American Chemical Society). Also, applications range from photocatalysis and solar cells (reprinted (adapted) with permission from the study of Mamba and Mishra [34], Copyright (2023) Royal Chemical Society).

3 g-C₃N₄/chalcogenide hybrids

In general, chalcogenide materials have several applications in energy conversion. They can be obtained by solution processing methods, which makes them attractive in this area of research. Moreover, their shape and morphology determine their capability for energy conversion. As such, by varying the processing method, the morphology of the materials can be easily modified [42]. Hybrid g-C₃N₄ can be obtained by employing several chalcogenide materials. Bashir et al. obtained a core–shell Fe₃S₄/g-C₃N₄ hybrid structure using a solvothermal method for electrochemical sensing of Pb²⁺ and UO²⁺ ions and mineralization of methylene blue dye. The authors report a significant increase in dye degradation, higher than 98% upon adding the hybrid material, and a detection limit for the lead and uranium ions of 0.71 and 0.22 μM, respectively. An efficient charge separation process explained the higher mineralization rate due to the built-in heterojunction of the hybrid material. As such, there is an efficient charge transfer of the heavy metal ions, as confirmed by XPS analysis [43]. Zahid et al. demonstrated how ZnS and In₂S₃ quantum dots can be deposited on top of g-C₃N₄ to enhance the photocatalytic activity of the carbon nitride thin film. The incorporation of the chalcogenide material was done using a wet chemical route. The addition of the metal chalcogenide to g-C₃N₄ allowed for a surface passivation effect of the layer so that the electron–hole pair remains active for a sufficiently long time to allow the photocatalytic process. Their study achieved a double passivation effect that was achieved by employing g-C₃N₄/ZnS/In₂S₃ that allowed a 93% of RhB dye degradation, compared to the control sample (38%) [44]. Li et al. achieved an in situ g-C₃N₄/MoS₂ hybrid. In short, they mixed melamine and (NH₄)₂MoS₄ and performed a thermal decomposition of the mixture to obtain the hybrid nanoplates. Molten salts of LiCl–NaCl–KCl at a melting temperature of 550°C served as a medium for forming the 2D material. The authors showed a remarkable interaction in the hybrid material that increased the photocatalytic activity compared to pure g-C₃N₄, primarily due to the molten salts forming a material with a high surface area [45]. Fu et al. developed a g-C₃N₄/Ag composite decorated with MoS₂. The material was obtained with a two-step method, in which a microemulsion-assisted reduction was employed, followed by a wetness impregnation method. The resulting composite had an enhanced absorption near the visible region with an improved photocatalytic activity due to the efficient separation of photoexcited charge carriers, resulting in excellent oxidation and reduction processes [46]. He et al. developed a g-C₃N₄/NiS nanocomposite by calcinating urea, thiourea, and nickel acetate. In this case, nickel sulfide acts as a co-catalyst, trapping the photoexcited charge and carrying more negative conduction band levels, leading to an efficient charge carrier separation and improved H₂ evolution activity. The increase in photocatalytic activity is due to the stronger NiS light absorption, the red-shift on the absorption edge, and the proper charge carrier separation at the interfaces [47]. Thin films of MoS₂/g-C₃N₄ were obtained by the thermal decomposition of melamine and thiourea and hydrothermal decomposition’s subsequent deposition of the MoS₂ film. In short, the substrate was placed on top of the respective crucible or autoclave to obtain the hybrid thin film. The addition of MoS₂ extends the absorption of the film. The authors explain the charge carrier mechanism: electrons are transferred to ITO from the hybrid composite material, which implies that the composite material acts as a photoanode [39]. Liu et al. obtained a composite using a hydrothermal method. The composite exhibited a red-shifted absorption feature due to the addition of the composite on carbon nitride in the form of microspheres, which promoted light absorption. The heterojunction of g-C₃N₄–Sb₂S₃ with Sb₄O₅Cl₂ reduces the recombination rate of the photogenerated charge carriers and increases the photocatalytic activity [48].

4 g-C₃N₄/layered chalcogenide hybrids

Layered chalcogenide materials are compatible with carbon nitride owing to their structure. They can form a bulk heterojunction, and several nanocomposites have been reported in the literature. For example, Figure 3 shows the Z-scheme diagram of a MoS₂/g-C₃N₄ bulk heterojunction. The process is carried out by photoexciting the electrons to the conduction band of one semiconductor from the valence band of the other through a donor/acceptor pair via redox reactions.

Figure 3

Z-scheme of the MoS₂/g-C₃N₄ bulk heterojunction (reprinted (adapted) with permission from the study of Lu et al. [49], Copyright (2017) American Chemical Society).

Tran Huu et al. developed a MoS₂/g-C₃N₄ composite using a simple one-pot synthesis, in which the respective precursors are calcinated to obtain the materials. Thiourea, the precursor of carbon nitride, functions as a buffer to exfoliate MoS₂, which also accelerates the decomposition of carbon nitride to obtain N vacancies and enhance charge transport. MoS₂ also enhances absorption near the visible region, which, combined with the enhanced charge transport properties, boosts the photocatalytic activity [50]. Zhang et al. proposed applying these composites to reduce heavy metals, such as U(VI). They produced a MoS₂/g-C₃N₄ composite by calcinating the appropriate precursors under different conditions. Next, both materials were dispersed in ethanol at different molar ratios for 5 h to obtain the desired composite material. The morphology of the nanocomposite is a nanoflower, in which MoS₂ is intercalated in g-C₃N₄, offering a fluent electron injection from the CB of the carbon nitride to MoS₂ [51]. Wang et al. developed a co-catalyst by growing CoS _x and MoS₂ on the surface of g-C₃N₄ to obtain a double heterojunction that yielded higher hydrogen production activity (6.5 times higher than CN, 46 times higher than MoS₂, and 98% higher than CoS _x ). The double heterojunction yields a higher active surface area and more active sites for hydrogen production. Also, the heterojunction’s built-in electric field inhibits the recombination rate of electron–hole pairs. Sulfur vacancies on the layered materials also provide the electron transfer rate from carbon nitride to the layered semiconductors [52]. Cao et al. developed a planar p–n junction nanocomposite with MoS₂–g-C₃N₄ via a hydrothermal method. The addition of MoS₂ improved light absorption, created a high surface area, and improved the visible part of the spectrum. Also, the bulk heterojunction produces active species and less charge recombination of the charge carriers [53]. Yuan et al. developed a MoS₂/g-C₃N₄ photocatalyst by thermal decomposition of ammonium tetrathiomolybdate and thiourea. Then, the authors employed thermal quenching in liquid nitrogen to prepare the catalyst. Their results show that the synergistic effect of the precursors, and especially the quenching in liquid nitrogen, yields a laminated structure of MoS₂ with only a few layers in thickness, which offers an efficient injection of the electrons from the CB of carbon nitride into MoS₂, which enhances the photocatalytic activity [54]. Kumar et al. showed the construction of MoS_2−x /g-C₃N₄/Ca–α-Fe₂O₃ heterojunction by controlling the molar ratios of the reagents employed in a multi-step hydrothermal method. S vacancies tune the band gap of the composite and allow efficient trapping of the photogenerated electrons, enhancing the photocatalytic activity. The authors showed that employing a lattice-compatible component, such as MoS₂ and g-C₃N₄, reduces the lattice mismatch, and by generating S vacancies, they can achieve more active sites and higher surface area. Oxygen vacancies can be generated by doping the structure with Ca and Fe₂O₃, enhancing the photocatalytic activity. The generated built-in electric field from the heterojunction improved charge carrier dynamics [55]. Liu et al. developed a MoS₂/g-C₃N₄ hybrid material for bisphenol A (BPA) degradation. The authors report a dual defect Z-scheme composite, in which urea was calcinated in a furnace to prepare the defective carbon nitride, previously treated with HCl. MoS₂ was obtained hydrothermally with hexammonium heptamolybdate tetrahydrate and thiourea. Both powders were ultrasonicated in anhydrous ethanol and annealed in nitrogen. The authors demonstrated that the simultaneous dispersion of the materials is a self-assembled layer of the materials, which allows for the stability of the dispersion and avoids the disordered formation of MoS₂ layers. TEM analysis shows many dislocations, distortions, and crystal fringes, which are evidence of the rich defects of the samples. The electron diffraction rings confirm this observation, with the appearance of six diffraction arcs. XPS analysis reveals that the atomic ratio of C/N is 0.85, compared to the theoretical value of 0.75, indicating a nitrogen deficiency. The N 1s peaks move to lower binding energies with the addition of MoS₂, which indicates a strong interaction between both materials. The addition of thiourea in the precursor solution of MoS₂ reduces Mo⁶⁺ to Mo⁴⁺, which is evidenced by the Mo core level, and thiourea induces S vacancies due to the shift to higher binding energies of the S 2p core levels compared to pristine MoS₂. These results indicate a defect-rich material and the formation of the MoS₂/g-C₃N₄ hybrid materials. With the addition of chalcogen, the absorption edge red-shifts to the visible region, enhancing photon absorption in this region. The bulk heterojunction of the hybrid material allows for an efficient charge carrier transfer by inhibiting recombination. The radiation lifetime of the charge carriers is increased with the addition of chalcogen, allowing electrons to transfer to the surface of the hybrid material and participate in the catalytic reaction due to the defect-rich nature of the composite and the band match of the photoexcited electrons [56]. Shi et al. developed a g-C₃N₄/MoS₂ hybrid, in which MoS₂ was in the form of quantum dots instead of a layered material, by an in situ one-step photodeposition technique. The quantum dot junction offers a higher electron concentration, as shown by XPS. The N 1s core level peaks move toward lower binding energy, indicating a strong interaction between carbon nitride and the quantum dots. This interaction comes from unsaturated S atoms lying on the surface of MoS₂, as revealed by the S 2p core level spectra. The photogenerated electrons from carbon nitride are efficiently transferred into the quantum dots, and the recombination is retarded when the quantum dots are employed, compared to when using a MoS₂ monolayer. More active sites are available when using quantum dots, so less recombination of the charge carriers is expected [57]. Active photocatalytic sites can be obtained not just by adding MoS₂ to g-C₃N₄, but also chemically activated sites can be achieved by appropriate treatment, as demonstrated by Shi et al., in which they prepared carbon nitride by protonating it in concentrated HCl. Afterward, the g-C₃N₄/MoS₂ composites were fabricated by an electrostatic self-assembly process. XPS analysis shows that HCl treatment promotes more positively charged C-NH₂ groups. Using this method, MoS₂ is partially oxidized, as a small signal of Mo⁶⁺ appears in XPS. The exfoliation process and the chemical protonation using HCl leads to a hybrid layered structure with a porous network, which allows for efficient photodegradation of organic pollutants: methyl orange and phenol [58]. Zhu et al. developed a 1T-2H MoS₂/g-C₃N₄ composite using a simple two-step calcination and solvothermal method. A built-in homojunction exists between the 1T and 2H MoS₂ since the 1T phase is metallic and the 2H phase is semiconducting. This homojunction enhances electron injection and transport into carbon nitride. The composite consists of MoS₂ intercalated in the carbon nitride as small nanoflakes. These two materials formed a stable heterojunction and did not destroy the matrix material [59]. The hybrids formed with a metallic phase of MoS₂ 1-T of a few layers (5–8) facilitate charge transfer due to the heterojunction between carbon nitride and the metallic phase. MoS₂ offers active sites to improve photocatalytic activity [60]. Liang et al. developed a ternary MoS₂/black-P/g-C₃N₄ nanocomposite in which the heterojunction and the existence of N vacancies enhance photocatalytic activity. The addition of black-P to carbon nitride allows for a porous structure. By introducing black-P and MoS₂ during the polymerization process of carbon nitride, N vacancies can be formed by replacing the atom with these two materials, as evidenced by the C/N ratio of 0.82 for carbon nitride and 1.01 for the composite [61].

In short, defects can be modulated easily by adding layered chalcogenides, improving the properties of the base material, such as higher surface area, efficient charge transfer dynamics, and enhanced photocatalytic activity. The next section of this review aims to study the defect modulation of carbon nitride with perovskite materials.

5 g-C₃N₄/perovskite hybrids

Metal halide perovskite materials are applied in different areas of energy conversion, such as photovoltaic devices, photocatalysis, and hydrogen evolution reactions [62,63]. Their remarkable optoelectronic properties are suitable for energy conversion and photoelectrochemical applications. Also, they can be easily tuned due to the very simple synthesis methods by which they can be obtained [64]. Figure 4 shows examples of some of the most common applications of perovskite/g-C₃N₄ hybrids.

Figure 4

Different applications of perovskite/g-C₃N₄ hybrids. The most common are as follows: catalysis (reprinted (adapted) with permission from the study of Cheng et al. [65], Copyright (2020) American Chemical Society), hydrogen evolution reaction (reprinted (adapted) with permission from the study of Huang et al. [66], Copyright (2020) American Chemical Society), photovoltaic and supercapacitor applications (reprinted (adapted) with permission from the study of Tuc Altaf et al. [67], Copyright (2023) American Chemical Society) and photodetectors (reprinted (adapted) with permission from the study of Liu et al. [68], Copyright (2019) American Chemical Society). In the figure, we observe the perovskite crystal structure (reprinted (adapted) with permission from the study of Akkerman and Manna [69], Copyright (2020) American Chemical Society) and the carbon nitride structure (reprinted (adapted) with permission from the study of Wang et al. [70], Copyright (2012) American Chemical Society).

Hybrid materials of carbon nitride and halide perwsovskites boost these properties. Liu and Ma developed a novel CsPbI₃/g-C₃N₄ composite by mixing the exfoliated carbon nitride on the perovskite bulk dispersed in hexane for photocatalytic degradation of RhB dye. The perovskite crystal was decorated with carbon nitride, allowing active photocatalytic sites. It is important to note that the g-C₃N₄ morphology was also porous on the perovskites, which is beneficial for the degradation process. Upon illumination with visible light, there is a strong increase in the photocurrent density of CsPbI₃/g-C₃N₄, compared to pure materials, which allows for an efficient charge transfer process. Impedance spectroscopy measurements confirm these observations, in which the composite has the least electron transfer resistance. The authors attribute the enhancement of the photocatalytic activity to the porous structure of carbon nitride anchored to the perovskite and to the relative band positions vs SHE of the materials. The valence band positions of perovskite and carbon nitride are 0.54 and 1.76 eV, respectively; meanwhile, the conduction band positions are −1.67 and −0.56 eV, respectively, in which XPS valence band spectra were used to obtain the valence band positions. The effect between both materials can improve the charge separation under illumination and, thus, enhance photocatalytic activity [71]. Sheng et al. developed a g-C₃N₃/CsPb(Br _x I_1−x )₃ hybrid. TEM studies proved that the cubic perovskite nanocrystals are supported on top of the carbon nitride monolayers. PL studies show an efficient charge transfer between g-C₃N₄ and the perovskite by observing quenching in the PL spectra. The hybrid forms a type-II heterojunction, allowing photoexcited electrons from perovskite into carbon nitride. Also, carbon nitride helps to passivate the surface defects from perovskite, thus allowing a longer carrier lifetime. The resulting hybrid is an efficient photocatalytic material in the presence of air since the photogenerated charge carriers produce reactive O 2 − species that act as active sites for photodegradation. This mechanism is explained by the relative band positions of both materials upon illumination; the valence band positions of carbon nitride and perovskite vs vacuum are −6.36 and −5.45 eV, respectively; meanwhile, the conduction band positions are −3.88 and −3.51 eV. These observations confirm the type-II heterojunction scheme and the relative band positions allow the photogenerated charge carriers to be injected from perovskite to carbon nitride by the conduction band offset [72]. Bai et al. developed g-C₃N₄/Cs₃Bi₂Br₉ composite by an electrostatic self-assembly process. The carbon nitride composite allowed for efficient photocatalytic effects that allowed the degradation of benzaldehyde. Carbon nitride acts as a scaffold that supports the Cs₃Bi₂I₉ crystals. The conduction band offsets were estimated using Mott–Schottky analysis. The conduction band levels for carbon nitride and perovskite were estimated as −0.94 and −0.56 eV vs NHE, respectively. Together with the band gap of both materials, valence band levels of 1.83 and 2.04 eV, respectively, were obtained, confirming a type II heterojunction between both materials, with a low charge transfer resistance and a suppressed charge recombination [73]. Bresolin et al. obtained a g-C₃N₄/Cs₃Bi₂I₉ composite. The composite, obtained by mixing both materials in appropriate solvents and mass ratios of the precursors, was anchored to carbon nitride in the form of particles. The addition of perovskite to carbon nitride forms a type II heterojunction and allows for the separation of the photogenerated charge carriers in both semiconductors. Also, the electrons from the conduction band of carbon nitride combine with the holes from the valence band of the perovskite, thus acting like a scavenger. These matrix and composite material action mechanisms enhance photocatalytic activity [74]. Wang et al. obtained a core–shell Cs₂AgBiBr₆@g-C₃N₄ Z-scheme photocatalyst. The heterojunction of the composite material red-shifted the absorption spectra, compared to bulk g-C₃N₄ and Cs₂AgBiBr₆, thus enhancing the photocatalytic activity in the visible region. The authors report that when varying the concentration of carbon nitride and with different aging conditions to prepare the composite materials, one can also obtain a type II heterojunction as a function of surface coverage of carbon nitride. Depending on the type of heterojunction, a different type of charge transfer mechanism is proposed: in the type II scheme, the electrons in the conduction band of carbon nitride are transferred to the conduction band of the perovskite, whereas in Z-scheme, the electrons in the perovskite recombine with holes from carbon nitride, thus the remaining electrons are on the conduction band of carbon nitride and the remaining holes are on the valence band of the perovskite. Reduction of CH₄ to CO was performed on the surface of the material, i.e., on carbon nitride. By using different solvents, toluene, and CH₂Cl₂, different types of composites can be obtained. On toluene, perovskite does not completely cover carbon nitride. Meanwhile, with CH₂Cl₂, the composite coverage is almost complete [75]. Quantum dots of CsPbBr₃ anchored to g-C₃N₄ via a self-assembly process were obtained by Ou et al. The porous nature of carbon nitride after exfoliation yielded NH _x sites, which are bonded with the quantum dots by an N–Br interaction. The N–Br bonding enables charge separation of the photogenerated charge carriers from carbon nitride to the perovskite quantum dots. Thus, the C in C–NH _x becomes electron-rich, and the bromine becomes electron-deficient. These defects and the interaction between carbon nitride and the quantum dots enhance photocatalytic CO₂ reduction [76]. Romani et al. obtained a photocatalytic active nanocomposite of Cs₃Bi₂Br₉/g-C₃N₄. When loading small amounts of the composite, i.e., the perovskite, onto carbon nitride, favorable band alignment and efficient charge transport properties are observed, as evidenced by the enhanced photocatalytic degradation of methylene blue compared to bulk materials alone. However, when increasing the wt% load of the perovskite beyond 15% yields a lower hydrogen evolution rate. This lower efficiency is due to self-trapped excitons in the composites. On the other hand, better photocatalytic efficiency comes from localized in-gap states at the perovskite/carbon nitride interface [77].

The above discussion is not just limited to photoelectrochemical applications. Also, these composites can be employed for photovoltaic applications since the working mechanism of the composites is similar to that of a photovoltaic device. A fine example of this is the work by Liu et al., in which a methanolic dispersion of g-C₃N₄ is deposited on the interface between the electron transport layer (ETL) and the perovskite CH₃NH₃PbI₃ (MAPI) and the perovskite and the hole transport layer (HTL). This layer helps passivate the device’s interfacial defects and enhance its efficiency. The increase in the charge transfer of the device upon illumination is due to enhanced light absorption from carbon nitride and the increase in shunt resistance. Also, the device performance was improved due to the decrease in the series resistance. These parameters are affected by the film quality, the heterojunction quality, and the material’s surface defects. Carbon nitride is employed as a buffer layer for perovskite and enhances the transport of the charge carriers at the interfaces. As such, carbon nitride and MAPI perovskite form a Lewis adduct that passivates the surface trap states of the absorber layer. The authors report the best device with V _oc of 1.14 V, J _sc of 21.45 mA/cm², FF of 0.807, and efficiency of 19.67% [78]. Liu et al. employed a CsPbBr₃/g-C₃N₄ nanocomposite by adding carbon nitride nanosheets into the PbBr₂ solution. The resulting perovskite yielded large grains (from 350 to 1,300 nm) owing to surface passivation of the grain boundaries due to carbon nitride. The interaction of the Pb²⁺ dangling bonds with the lone pair electrons from carbon nitride offers an electrostatic interaction that increases the bond strength of the C═N and C–N bonds. Grain boundaries of the perovskite are passivated by the polymeric nature of carbon nitride, and due to the formation of an intermediate g-C₃N₄@PbBr₂ complex that when in contact with CsBr, delays the crystallization process and yields large crystals that reduce non-radiative recombination in the perovskite. Carbon nitride concentrates on the grain boundaries of the perovskite; since the defects on the perovskite are unsaturated Pb²⁺ atoms, i.e., Br vacancies, the interaction between the lone pairs of the N atom in carbon nitride helps to passivate the grain boundaries and slows the fast crystallization process. For a better understanding of the atomistic origin of the passivation at the grain boundary, the authors present a scheme of the energy diagrams of the material and the device. The valence band positions of the perovskite and carbon nitride found by UPS are −5.60 and −5.37. The perovskite and carbon nitride’s conduction band positions are −3.30 and −2.62 eV. Some photoexcited electrons in the conduction band can be trapped in the trap states formed by the dangling Pb²⁺ bonds. After passivation, these trap states are reduced. The difference in the conduction band offsets helps act as an energy barrier to avoid the recombination of the electrons and holes at the CsPbBr₃/g-C₃N₄ boundaries. As a result, the authors report the best device with V _oc of 1.278 V, J _sc of 7.8 mA/cm², FF of 0.8, and efficiency of 8% [79]. Jiang et al. studied the addition of a g-C₃N₄ solution to the perovskite precursor. They found that upon adding the solution of carbon nitride, the conductivity of the perovskite increases with an increase in electron mobility. Also, there is a surface passivation defect by an interaction of carbon nitride on the grain boundaries of the perovskite. Larger grains of the absorbing layer are obtained by retarding the crystallization process with carbon nitride, thus, overall, increasing device efficiency and stability. The authors report the champion device with V _oc of 1.07 V, J _sc of 24.31 mA/cm², FF of 0.74, and efficiency of 19.49% [80]. Cao et al. studied passivation employing I-doped g-C₃N₄ on a mixed triple cation perovskite. Their results show that carbon nitride coordinates with unsaturated Pb bonds on the perovskite grain boundaries, passivating the trap state density, leading to higher crystallinity of the material and yielding a more efficient charge carrier and transport to the electrical contacts, and thus, higher efficiency. The authors reported the champion device with V _oc of 1.07 V, J _sc of 22.97 mA/cm², FF of 0.74, and efficiency of 18.28% [81]. Functionalized ( SO 3 − , NH 3 − , OH⁻, and SO 3 − ) g-C₃N₄ was added to perovskite as a surface passivator by Li et al. In their work, the functionalization of carbon nitride yielded higher crystallite sizes than the control sample (increase from 60 to 68 nm), indicating an increase in the film quality. Film quality is important for the efficient charge transport properties of the photogenerated charge carriers. The functional groups of carbon nitride coordinate with Pb²⁺ atoms. In general, the higher electronegative N atom compared to the S atom has a higher coordination with Pb²⁺, and thus, NO₃–g-C₃N₄ has a higher electron lifetime compared to the other functional groups since this heals the defect states of the grain boundaries of the perovskite. This modification yielded a champion device with V _oc of 1.11 V, J _sc of 22.84 mA/cm², FF of 0.792, and efficiency of 20.08% [82]. Wang et al. developed a novel carbon nitride modified with a 4-carboxyl-3-fluorophenylboronic acid additive for a triple cation perovskite. Their results indicate that the additive forms strong bonds with the perovskite that help to passivate the trap states. Also, the efficient charge carrier is proven by the additional charge transfer channels provided by this composite on the grain boundaries to yield a champion device with V _oc of 1.19 V, J _sc of 24.79 mA/cm², FF of 0.8, and efficiency of 23.71% [83]. Hemasiri’s group added g-C₃N₄ to NiO _x as an interfacial modification between the HTL and the perovskite. This interfacial engineering method avoids direct contact between NiO _x and the perovskite, which, in turn, passivates this interface by eliminating charge build-up due to defects and avoiding surface charge recombination. Also, it can act as a stabilizer by avoiding I oxidation and deprotonation of cationic amines, which results in a champion device with a V _oc of 0.85 V, a J _sc of 22.48 mA/cm², an FF of 0.78, and an efficiency of 19.08% [84]. Cao et al. developed an ultrathin Co(iii)-grafted carbon nitride incorporated into the spiro-OMeTAD HTM by a bottom-up strategy followed by a self-assembly process to yield the Co(iii)-CN nanosheets with a lateral size of micrometers. These materials possess the Co(iii)-CN/Co(ii)-CN redox couple that controls the oxidation process of the HTM and reduces the I² released from the perovskite to form I⁻ and enhance its electrical properties. Moreover, the material passivates the surface trap states on the perovskite/HTM interface. The Co(ii)-CN forms a d–π conjugated structure that helps in the healing of the I₂ defects. Incorporating this material yields a champion device with a V _oc of 1.138 V, a J _sc of 25.43 mA/cm², FF of 0.795, and efficiency of 23.01% [85]. Xie’s group incorporated g-C₃N₄ doped with I into a ZnTiO₃ perovskite ETL. The incorporation of the I-doped CN improved the interfacial contact with the perovskite to improve the crystallinity of the absorbing layer and the charge separation due to the favorable energy levels between the ETL and the perovskite, which is not possible when considering only the ZnTiO₃ perovskite as ETL. I-doped carbon nitride helps the cohesion between the ETL and perovskite, improving the crystal quality and avoiding charge recombination. With this modification, the authors report a champion device with a V _oc of 1.012 V, a J _sc of 26.32 mA/cm², an FF of 0.72, and an efficiency of 19.07% [86]. Table 1 summarizes the results of photovoltaic devices modified with carbon nitride materials.

Based on the previous survey of the literature about carbon nitride applied on photovoltaic devices, we can address some of the following observations: first, carbon nitride can be employed as either an additive to the perovskite absorber layer or as a surface modification between the electron transport layer or the hole transport layer of the device. Second, carbon nitride boosts the photovoltaic performance of the devices by enhancing the open-circuit voltage and the stability of the devices by healing defects and suppressing trap states on the perovskite. Usually, carbon nitride bonds with Pb²⁺ dangling bonds suppress bulk defect states, improve film crystallinity and quality, and enhance charge carrier separation and migration. When employed as a surface modifier, carbon nitride helps to passivate surface defects between each layer, with additional effects depending on the layer. In the case of the HTL, carbon nitride allows for more channels for the charge transfer of the photogenerated electrons in the perovskite, which allows for the healing of halogen defects. In the case of the ETL, the carbon nitride bonds with perovskite to enhance the film quality and suppress bulk defects on the interfacial contact. An interesting trend is observed in the literature review: very few studies on HTL-free PSC are available with carbon nitride modifications due to the less efficient devices produced with this architecture. However, when eliminating the HTL, costs are reduced by avoiding costly chemicals that give the spiro OMeTAD its necessary properties. Also, we can avoid using a glovebox by employing perovskites such as CsPbBr₃ or its mixed halides resistant to ambient conditions. On the other hand, we can also modify different types of HTL layers with carbon nitride for photovoltaic devices, such as CuI, to improve the charge separation and injection into the contacts and the healing of the interfacial defects between the HTL and perovskite.

Figure 5 shows a schematic diagram showing the flat energy levels of carbon nitride’s conduction and valence bands with some of the above perovskites. This helps us understand the solar energy harvesting these hybrid materials provide and how they can be used as photocatalytic materials or efficiency boosters on photovoltaic devices.

Figure 5

Conduction band and valence band positions of some perovskite/g-C₃N₄ hybrids: (a) CsPbBr₃ hybrid for photocatalytic applications (reprinted (adapted) with permission from the study of Bai et al. [73], Copyright (2018) Wiley), (b) CsPbBr₃ hybrid for photovoltaic applications (reprinted (adapted) with permission from the study of Ou et al. [76], Copyright (2021) Elsevier), (c) CsPb(Br _x I_1−x )₃ for photocatalytic applications (reprinted (adapted) with permission from the study of Liu et al. [68], Copyright (2020) Wiley), and (d) Cs₂AgBiBr₆ for photocatalytic applications (reprinted (adapted) with permission from the study of Sheng et al. [72], Copyright (2021) Elsevier).

6 Conclusions

In conclusion, g-C₃N₄ hybrids have three significant roles in the defect states of semiconducting materials for energy conversion:

• They can generate active sites via doping or forming controlled defects. For example, doping with S on an N vacancy can improve photocatalytic performance by generating active sites on carbon nitride.

• By generating bulk defects on the structure using quantum dots or pores to enhance surface area and generating photocatalytic active sites, such as having greater surface area and void volume by incorporating them on layered semiconductors and other materials like MoS₂ and metal halide perovskites, also, more active sites for photocatalysis can be achieved by decorating carbon nitride with such materials that can activate defect states such as N vacancies by the materials.

• By passivating grain boundaries on perovskite materials, the efficiency of the devices was enhanced. For example, it heals bulk defects on the MAPI perovskite by coordinating with the Pb²⁺ dangling bonds and suppresses the bulk defects on the interface between the ETL and HTL.

DFT theoretical calculations corroborate these results, as the predicted formation energies of the favorable defects on carbon nitride agree with what is observed experimentally. Calculation predicts more photocatalytic sites upon modification with different materials, such as layered semiconductors or oxides, by enabling favorable vacancies and defects and lowering their formation energies. Also, recent trends in photovoltaic studies with carbon nitride prove the boosting efficiency and stability of the absorbing perovskite materials. Very few studies on HTL-free PSC modified with carbon nitride were found. As such, this presents an area of opportunity for researchers to explore. Modifications on the HTL also yield better efficiencies, and thus, this encourages the modification of different types of hole transport materials, such as CuI, to obtain more efficient devices.