Sustaining Sub-Bandgap Photons via Upconversion for Solar Splitting Cells

Abstract

Conservation of the entire spectrum of the sun is crucial to raising the efficiency of solar splitting cells or any photochemical conversion. With the aid of upconversion nanomaterials, it could potentially be achievable. In general, solar splitting technologies are associated with numerous losses. Remarkably, inadequate utilization of the light spectrum is the primary cause of losses in photophysical processes. This is usually caused by a particular band gap in semiconductor materials, where higher-energy photons dissipate as energy and lower-energy photons, or sub-bandgap photons, are unable to be absorbed. The process of absorbing two or more photons and then emitting one photon with more energy than the sum of the individual energies of the previously absorbed ones is known as upconversion. Introducing an appropriate upconverter can significantly improve the photoconversion process’s efficiency. Efforts have been made in the past few years to enhance the efficiency, broad-range sensitivity, and activity of semiconductors by integrating upconversion systems. This chapter provides a detailed discussion of the upconversion strategies that have been used thus far to increase the efficiency of solar splitting cells. It will undoubtedly assist the researchers in advancing in this area.

1. Introduction

1.1 Sub-bandgap photons

When photons strike the surface of a semiconductor, they can be absorbed, reflected, or transmitted through the material. The energy of a photon is a key factor in determining whether it is transmitted or absorbed during photonic interaction. Photon interaction with a semiconductor depends entirely on the energy of the photon in relation to the semiconductor’s band gap. There are two pathways: one when the photon energy (E_photon) is less than the band gap energy (E_bg), resulting in weak interactions with the semiconductor, that photon is known as sub-bandgap photons, and the other, when E_photon is greater than or equal to E_bg, leading to efficient absorption and the formation of an electron-hole pair. Since photons with energy below the bandgap are not absorbed, cells often lose a large portion of the energy that is incident upon them from the sun. Sub-bandgap light photon upconversion (UC) offers a practical solution to this core problem while preserving the advantages of semiconductor devices.

1.2 Photon upconversion

In a process known as photon upconversion (UC), light at a wavelength lower than the excitation wavelength is released because of the consecutive absorption of two or more photons. Multiple mechanisms can lead to upconversion in both organic and inorganic materials. Figure 1 highlights the distinctions from the traditional photoluminescence process and illustrates the general idea of the upconversion luminescence method. In upconversion luminescence, an excitation photon can be used by a luminous center in the ground state 1 to absorb energy and move toward the excited state 2. Following that, the luminous center will be promoted to the excited state by an additional excitation photon 3. A higher-energy photon emission is the outcome of a radiative transition from this excited state 3 back to the ground state or another lower-energy state [1]. In actuality, the metastable intermediate excited state is responsible for the upconversion luminescence process. Two primary strategies have been established for achieving upconversion luminescence emission with low-energy stimulation. These are triplet−triplet annihilation (TTA)-based upconversion [2] and the upconversion luminescence emission of lanthanide ions [3]. Both upconversion luminescence methods rely on intermediate excitation states [2, 3].

Figure 1.

Figure representing (a) conventional photoluminescence and (b) the upconversion luminescence process.

1.3 Upconversion material

Upconversion can occur in both organic and inorganic materials via various methods.

Polycyclic aromatic hydrocarbons (PAHs) are capable of achieving photon upconversion by triplet-triplet annihilation [4, 5, 6, 7, 8, 9, 10]. Triplet-triplet annihilation (TTA) through photon upconversion (UC) often produces high-energy (specifically, UV) light from lower-energy (visible) light. The TTA-based upconversion system relies on a combination of a sensitizer and an acceptor (Figure 2).

Figure 2.

Presentation of photochemical events linked to the triplet-triplet annihilation (TTA) through photon upconversion (UC) approach; ISC = intersystem crossing; TTEnt = triplet-triplet energy transfer; and TTA = triplet-triplet annihilation.

The fundamental properties of these two components play a crucial role in determining TTA-UC properties. The sensitizer must possess two primary properties. (i) The capacity to capture light in the visible to near-infrared range of the electromagnetic spectrum, facilitating low-energy stimulation. (ii) The triplet excited state can last several microseconds or more, allowing for efficient and progressive energy dissipation. Later, this quenching process occurs when the energy of the acceptor’s triplet state is smaller than that of the sensitizer. A higher energy differential between the triplet sensitizer and triplet acceptor leads to a stronger driving force for the TTA, resulting in a more favorable triplet energy transfer mechanism. The conditions for observing upconverted fluorescence from the sample are adequate if the energy criteria are satisfied and the combined triplet energy from two acceptor molecules is larger than or equal to the acceptor singlet state energy. Table 1 summarizes the available annihilators and sensitizers for the TTA-based upconversion process.

Sensitizer with core properties	(2,3,7,8,12,13,17,18-octaethylporphyrinato) Pt(II), exhibits a substantial PL quantum yield (QY) of 50% [4].
	Palladium(II) octaethylporphyrin employed as an upconversion sensitizer, similar to the above one [5].
	Phthalo cyanine (Pc) has a strong ability to absorb light in the red to near-infrared (NIR) ranges [6].
	Ruthenium(II) polyimine complexes demonstrate highly effective intersystem crossover [7].
	Rhenium(I) diimine tricarbonyl complexes are typically constrained in their ability to function as triplet sensitizers due to their low absorption in the visible spectrum [8].
	Cyclometalated iridium(III) complexes have durable excited states and absorption bands extend into the visible spectrum [9].
Activators	Anthracene, pyrene, tetracene, perylene, fluorene, and its derivatives are used [10]. 9,10-diphenylanthracene, 9,10-bis(phenylethynyl)anthracene are most extensively utilized.

Table 1.

Lists the common available annihilators and sensitizers for organic-based UC material.

Currently, the most widely recognized upconversion sensitizer is (2, 3, 7, 8, 12, 13, 17, 18-octaethylporphyrinato) Pt(II), which exhibits a substantial phosphorescence quantum yield (QY) of 50%. Phthalo cyanine has a strong ability to absorb light in the red to near-infrared (NIR) ranges. It also has a low level of energy in its triplet state, which makes it excellent for enhancing long wavelength emissions in upconversion systems based on TTA (triplet-triplet annihilation). The introduction of a heavy-metal atom, such as Pd, into phthalocyanine significantly increases its capacity for intersystem crossing. Ruthenium(II) polyimine complexes demonstrate highly effective intersystem crossover. The singlet to triplet transition in Ru(II) complexes is highly efficient, approaching unity. This, together with the extended duration of the triplet excited state, makes these complexes advantageous as sensitizers. Platinum(II) polyimine acetylide complexes are appealing as triplet sensitizers due to their ability to adjust their photophysical properties by employing various acetylide ligands. Rhenium(I) diimine tricarbonyl complexes are typically constrained in their ability to function as triplet sensitizers due to their low absorption in the visible spectrum. Cyclometalated iridium(III) complexes have durable excited states and exhibit absorption bands that extend into the visible spectrum. This enables these complexes to function as sensitizers for TTA-based upconversion.

Regarding acceptor or annihilators combination, polycyclic aromatic hydrocarbons, such as anthracene, tetracene, pyrene, perylene, fluorene, and their derivatives have been widely used as the primary annihilators in TTA-based upconversion systems documented in scientific literature. Despite being colorless, anthracene emits a blue fluorescence of approximately 420 nm with an 18% quantum yield.

Inorganic materials that contain ions of d- or f-block elements have the ability to upconvert photons. Inorganic-based upconverting particles are usually made of a host material doped with optically active activator and sensitizer ions [11, 12]. Among all common host materials, including oxides (ZrO₂, Y₂O₃), vanadates (YVO₄, GdVO₄), and phosphates (LuPO₄, YPO₄), it has been demonstrated that the NaREF₄ family of host matrices is one of the most successful due to its higher chemical stability and lower phonon energy [1, 3]. The purpose of the sensitizers in the UC materials is to increase the sensitivity of the activators. The properties of an ideal sensitizer include a significant absorption cross section at the targeted excitation wavelength, a sufficiently long lifespan of the excited state, and resonant energy levels that are compatible with the activators’ appropriate excited energy states. For the reasons described above, Yb³⁺ and Nd³⁺ have been extensively studied as sensitizers. All things considered, Yb³⁺ is the best candidate for a sensitizer because its energy level diagram only has one excited state (²F_5/2) that roughly matches the f-f transitions of many RE activators, including Er³⁺, Ho³⁺, and Tm³⁺ [13]. Ions that are employed as activators have a to some degree rich energy level structure along with a comparatively extended duration of UCL states. Following the stimulation of ground-state electrons to the metastable state, activators are likely to absorb energy from nearby sensitizers to become more excited at higher levels. Luminescent Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Tm³⁺ are popular activator ions [1, 3]. The six primaries proposed upconversion mechanisms in lanthanide include cross relaxation (CR), photon avalanche (PA), cooperative upconversion (CU), cooperative luminescence (CL), ground state and excited state absorption (GSA/ESA), energy transfer upconversion (ETU), and cooperative upconversion (CU) [14].

1.4 Solar splitting cells

Solar splitting cells include the utilization of semiconductor material where chemical reactions occur upon the interaction of a light source with the material surface. In this process, it is important for a minimum of two processes to take place simultaneously: the oxidation reaction, involving holes generated by light and the reduction reaction, involving electrons generated by light. The primary obstacle in the development of efficient photocatalytic cells for water splitting, degradation, or conversions is the search for cost-effective materials that meet the criteria of an ideal photoelectrode. The criteria for finding an ideal photoelectrode in this context include [15] (i) the capacity to capture all types of solar radiation, (ii) strong chemical stability in water-based electrolyte solutions in both dark and illuminated conditions, (iii) appropriate alignment of energy levels for specific oxidation and reduction reactions, (iv) minimal excess energy required for these reactions to occur, and finally (v) the ability to selectively transfer charges at the interface between the semiconductor and electrolyte for the purpose of splitting or converting substances. The book chapter offers a thorough examination of an upconversion integrated semiconductor nanosystem (Section 2) that possesses the capability to efficiently capture sub-bandgap photons for photo reactions.

2. Literature outlook of UC integrated semiconductors systems

2.1 Organic upconverter integrated semiconductor system (TTA-based upconversion systems)

Barawi et al. [16] presented a theoretical model of a photoelectrochemical (PEC) device utilizing photon upconversion, particularly in the visible-to-ultraviolet spectrum. The system used titanium dioxide (TiO₂) as the photoanode to generate hydrogen gas (H2). Photoelectrochemical tests were carried out using a standard three-electrode PEC cell that included the TTA-UC system (Figure 3). The photoanode employed was a semiconductor made of TiO₂ with a bandgap energy (E_g) of 3.2 eV. A laser pointer with a wavelength (λ_exc) of 445 ± 10 nm, power of 2 W, and intensity of 1.64 mW cm⁻² was used as the irradiation source. The group selected 2,3-butanedione (biacetyl, BA) and 2,5-diphenyloxazole (PPO) as an appropriate TTA-UC pair system. BA absorbed visible light with an excitation wavelength of 430-450 nm. The UV band at 370 nm appeared by the delayed emission of PPO (1 PPO*), which has an excited singlet energy (ES) of 3.6 eV in dimethylformamide.

Figure 3.

Diagram depicting the 2,3-butanedione (biacetyl, BA) and 2,5-diphenyloxazole (PPO) (TTA-UC) powered PEC cell for hydrogen generation via sulfite to sulfate conversion.

The TiO₂ photoanode displayed a photocurrent density of approximately 4.5 μA cm⁻² and demonstrated good stability throughout the experiment. When exposed to visible light, the upconversion (TTA-UC) system emits UV photons that activate the TiO₂ photoanode, resulted in the formation of charge separation or hole and electron pairs. Sulfite ions trap the empty spaces in the valence band, facilitated the formation of sulfate. Concurrently, the electrons in the conduction band had moved to the platinum counter electrode. At the counter electrode, they participated in water reduction, leading to the production of hydrogen gas. The conversion of visible light into UV radiation was accomplished by the interaction of two basic organic chromophores using the triplet-triplet annihilation mechanism. Felix N. Castellano and his colleagues [17] introduced a system that utilizes upconversion to enhance photoelectrochemical processes. The system involved the combination of palladium(II) octaethylporphyrin (PdOEP) and 9,10-diphenylanthracene (DPA) in toluene. The aforementioned combination, consisting of noncoherent green photons, was discovered to efficiently enhance the sensitivity of nanostructured WO₃ photoanodes with an energy gap of 2.7 eV. The experiment represented in Figure 4 utilized thin WO₃ colloidal photoanodes, with a thickness of 10 mm, that were coated onto FTO glass. These photoanodes of active area 1 cm² were employed as the working electrode in the experiment, using a 1.0 M H₂SO₄ electrolyte. The electrode was set to a potential of +0.9 V compared to the Ag/AgCl reference electrode. The upconversion pumping commenced using a noncoherent lamp that emitted light with wavelengths over 500 nm.

Figure 4.

Schematic representation of PdOEP and DPA (TTA-UC) powered PEC cells for hydrogen generation via acidic water spitting.

The blue photons (with a maximum wavelength of 432 nm) produced by the upconversion process, manifesting as singlet DPA fluorescence, were haphazardly absorbed by the WO₃ anode, leading to detectable photocurrents under relatively moderate noncoherent excitation conditions. The photo responses obtained in these proof-of-concept studies were low because of the imperfect capture of photons at the WO₃ photoanode. The experiment showed that the WO₃ electrode could not sense anything below the bandgap level because there was no TTA-UC in the electrolyte. Nevertheless, the authors believe that there is considerable scope for enhancement if there is a change in geometric configuration, light condition, and photon retrieval. D. Choi et al. [18] showed that incorporating a luminescent back reflector capable of upconverting photons (UC LBR) improves the efficiency of water splitting in BiVO₄. The LBR was created by dispersing a mixture of meso-tetraphenyltetrabenzoporphine palladium (PdTPBP) and perylene organic fluorophore. PdTPBP absorbs light between 600 and 650 nm and emits light at 470 nm with higher photon energy via Dexter energy transfer and triplet-triplet annihilation (TTA) (Figure 5). Perylene was discovered to absorb light at 350-450 nm and emit light at 470 nm by photoluminescence. The BiVO₄ photoelectrode absorbs the emissions, improving the water splitting process. Using the light bending reflector (LBR) increased the photocurrent for water splitting in BiVO₄ electrodes by 17%, achieving photocurrent densities of up to 5.25 mA/cm² at 1.23 V vs. RHE. Figure 5 displays the attachment of the LBR film to the rear of the Mo:BiVO₄ photoelectrode. The Mo:BiVO₄ film was found very transparent for wavelengths above 500 nm but possessed transmittance of around 10% for wavelengths below 500 nm due to photon absorption.

Figure 5.

Luminescent rear reflector with the ability to upconvert photons fabricated using combination of meso-tetraphenyltetrabenzoporphine palladium (PdTPBP) and perylene organic fluorophore (TTA-UC system) powered PEC cells for water splitting application.

The J-V characteristics of Mo:BiVO₄ with PdTPBP LBR films were changed by changing the concentrations to 0.10, 0.15, and 0.19 mM and the film thicknesses to 1, 2, and 3 mm. A direct association was found between the thickness of the LBR coating and the increase in emission intensity. A maximum water splitting photocurrent of 5.34 mA/cm² was achieved when a Mo:BiVO₄ photoelectrode was paired with a 3 mm LBR film containing 0.15 mM PdTPBP, primarily due to the TTA-UC effect. The value was approximately 16% more than the photocurrent detected using only the Mo:BiVO₄ photoelectrode, under the same conditions.

2.2 Inorganic upconverter integrated semiconductor system (lanthanide-based upconversion systems)

Yun-Mo Sung et al. [19] described infrared (IR)-powered photoelectrochemical (PEC) cells with upconversion glass-ceramics as bases. UC glass-ceramics as substrates for photocatalysts can address the chemical instability of fluoride nanoparticles, prevent incident light blocking, and enhance the exposure of photocatalysts to liquid electrolytes. Oxyfluoride glass-ceramics containing nanocrystals doped with (Yb,Er)-doped YF₃ and (Yb,Tm)-doped YF₃ have generated green and ultraviolet/blue emissions, respectively, when exposed to 980 nm light. High-density ZnO nanorods were grown on upconversion glass-ceramic substrates using the hydrothermal technique. They were then covered with CdSe nanocrystals to create CdSe/ZnO heterostructures using chemical bath deposition. CdSe nanoparticles were stimulated by the UC UV emission from Tm and the visible emission from Er and Tm, whereas ZnO nanorods were mostly stimulated by the UC UV emission from Tm. The photocurrent densities of CdSe/ZnO-Er and CdSe/ZnO-Tm were around 0.65 μA cm⁻² and 1.4 μA cm⁻², respectively. The PEC performances of CdSe/ZnO-Er and CdSe/ZnO-Tm were enhanced compared to ZnO-Er (0.03 μA cm⁻²) and ZnO-Tm (0.2 μA cm⁻²). Neither ZnO nor CdSe/ZnO on the ITO substrate exhibited any photocurrent densities when exposed to 980 nm irradiation. The photocurrent density of CdSe/ZnO-Tm is higher than that of CdSe/ZnO-Er. The CdSe/ZnO material created on the glass-ceramics with (Yb,Tm)-doped YF₃ had higher photocurrent density than that created on the glass-ceramics with (Yb,Er)-doped YF₃ attributed to charge separation induced by the type-II cascade structure. Yoongu Lim et al. [20] developed a Core@shell@shell UCNPs (NaYF₄:Yb,Nd,Tm [19/1/0.5 mol%] @NaYF₄:Yb,Nd [10/20 mol%]@NaYF₄) -coated ZnFe₂O₄/TiO₂ photocatalyst (UCNPs-ZFO/TiO₂) by layering ZFO nanoparticles and UCNPs onto TiO₂ nanopillars on a transparent fluorine-doped tin oxide (FTO) substrate. The UCNP-ZFO/TiO₂ showed a markedly greater photocurrent density of 0.795 mA cm⁻² in comparison to the pristine TiO₂, which had a value of 0.260 mA cm^{− 2}. The UCNP-ZFO/TiO₂ had a photon-to-current efficiency that was 4.1 times greater than TiO₂. Additionally, its photoanode exhibited the lowest charge transfer resistance and the largest charge density compared to the other photoelectrodes. Trang Nguyen Thi Thuy et al. [21] detailed the development of TiO₂ thin films enhanced with an upconversion (UC) phosphor for solar water splitting. The study investigated how varying the UC phosphor concentration affected the morphology and photoelectrochemical (PEC) performance of the samples. YF₃:Yb³⁺, Tm³⁺ showed notable upconversion emits UV photons when exposed to IR light. Adding UC phosphor to a TiO_2, thin layer improved the performance of the photoelectrochemical system. A photoelectrode with 7 wt.% UC phosphors showed the maximum photocurrent of 0.073 mA cm⁻² at 1.23 V compared to other samples. Qianfan Jian et al. [22] had reported displayed plasmonic-enhanced water splitting photoanode using hematite-lanthanide. NaYF₄:Er,Yb upconversion nanoparticles nanocomposites that captured photons below the bandgap of hematite. Hematite, a conventional semiconductor oxide photoelectrode, absorbs only UV and visible light from the sun spectrum, resulting in a 40% loss of infrared energy. The NaYF₄:Er,Yb upconversion process converts photons from 980 nm to (510–570) nm, falling under the bandgap of hematite. The sample containing only hematite and UCNPs showed minimal photocurrent signal due to the UCNPs’ very poor upconversion efficiency. Conversely, the sample containing hematite and AuNDs but lacking UCNPs exhibited a modest yet detectable photocurrent at the identical potential. Plasmonic materials found interact with semiconductors by light scattering, hot electron injection (HEI), and plasmonic resonance-induced energy transfer (PRIET). The author suggested two-pathway method, involving plasmonic-enhanced upconversion process and hot electron injection procedure. Yuxiang Zhu et al. [23] reported the photocatalytic conversion of N₂ to NH₃ using NaYF₄:Yb,Tm (NYF) upconversion nanoparticles (NPs) attached to carbon nitride nanotubes with nitrogen vacancies (NYF/NV-CNNTs) in water under near-infrared (NIR) light exposure. Nanoflowers with a particle size of approximately 20 nanometers were evenly spread throughout the surface of nitrogen-vacancy carbon nanotubes. The NYF/NV-CNNTs containing 15 wt % NYF showed the greatest NH₃ synthesis yield of 1.72 mmol L⁻¹ gcat⁻¹ with an apparent quantum efficiency of 0.50% under NIR light. This activity was almost three times more than that of the naked CNNTs under UV-filtered solar light. Hannah Kwon et al. [24] reported a tri-doped β-NaYF₄:Yb³⁺,Tm³⁺,Gd³⁺ upconversion (UC) nanorods, which were incorporated into a carbon-doped mesostructured TiO₂ hybrid film with triblock copolymer P123, serving as a mesoporous template and carbon source. The novel material’s photoactivity was demonstrated by the breakdown of nitrobenzene, a typical organic waste. UC nanorod-embedded C-doped TiO₂ showed enhanced absorption throughout a wide spectrum from UV to NIR, resulting in a significant improvement in nitrobenzene degradation (83%) within 3 hours under light exposure, compared to pristine TiO₂ (50%). This study demonstrates a synergistic connection between the impact of an innovative photon-trapping TiO₂ structure, enhanced NIR light-capturing effectiveness with UC nanorod integration, and a concurrent reduction in band gap energy and heightened absorption of visible light through C-doping of the oxide lattice. Anushree A. Chilkalwar et al. [25] demonstrated that the combined effect of surface plasmon resonance of Au nanoparticles and upconversion properties of (Yb,Er) NYF greatly improves broadband absorption and the formation and transmission of photogenerated charges. Gold acts as a plasmonic nanoparticle on (Yb,Er) NYF-TiO₂, serving the dual purpose of absorbing solar energy and demonstrating exceptional catalytic properties. The (Yb,Er) NYF-TiO₂/Au heterostructure exhibited a consistent and notably high hydrogen generation rate of 350 μmol h⁻¹ under solar AM 1.5, roughly 3.1 times more than Au/TiO₂ (Degussa P-25) in donor-assisted photocatalytic hydrogen generation testing. Ramireddy Boppella et al. [26] showed 3D composite photoanode containing near-infrared responsive upconversion nanocrystals (UCNs) such as NaYF4:Yb³⁺-based UCNs doped with Er³⁺ or Tm³⁺ ions, and visible-responsive plasmonic gold nanoparticles (NPs) within 3D titanium dioxide inverse opal nanostructures (Au/UCN/TiO₂). It showed enhanced solar energy absorption across the ultraviolet-visible-near-infrared spectrum. Their hybrid nanostructure, consisting of Au/Er-UCN/TiO₂, showed a tenfold enhancement in photocurrent density when exposed to UV-vis-NIR light compared to a pure TiO₂ sample. Tm-doped upconverting nanoparticles (UCN) generated a higher photocurrent compared to the Er-doped UCN. This result supports the alignment between the UV emission bands of Tm-UCN (345 and 361 nm) and the absorption band of TiO₂ (360 nm). Kuang Feng et al. [27] have developed core-shell or core-shell-shell shaped upconversion nanoparticles (UCNPs) to act as “nano-transducers” that transform NIR light into visible light, enabling NIR light to be indirectly absorbed by visible-responsive photocathodes. Upon introducing NaYF₄:Yb_20%/Er_2% @NaYF₄, NaYF₄@NaYbF₄:Tm_1%, and NaYbF₄:Er_60%@NaYF₄ core-shell UCNPs into the buffer solution, the Cu₂ZnSnS₄ (CZTS)-based photocathode demonstrated a significant photocurrent density of −4 mA cm⁻² at 0 V_RHE when exposed to 980 nm irradiation. A new experimental approach was demonstrated to enhance the use of long or short-wavelength lights for CZTS photocathode. The report detailed the utilization of a CZTS-based photocathode with a buffer solution containing UCNPs to facilitate water splitting processes under NIR light exposure. The device generated 16 micromoles of hydrogen over a 3-hour period. The concentration of H₂ increased steadily at a fixed rate of 0.09 μmol min⁻¹. Wei Gao et al. [28] demonstrated an effective photocatalyst consisting of RGO, CdS, and an upconversion component (NaYF₄-Yb³⁺/Er³⁺) for efficient water splitting to produce hydrogen. A cooperative catalyst was developed by merging an upconversion component, NaYF₄-Yb³⁺/Er³⁺, with a visible light catalyst for 980 nm NIR-driven water splitting. An optimal hydrogen evolution rate of 4.54 μmol g⁻¹ h⁻¹ was attained when exposed to 980 nm NIR irradiation. The 980 nm laser energy-to-hydrogen conversion efficiency (LTH) was 0.011%. Wei Gao et al. [29] displayed the catalytic production of hydrogen using a photocatalyst made of semiconductor CdS and an upconversion component NaYF₄-Yb³⁺/Er³⁺ (NYF) under infrared light. The 980 nm laser irradiation turned the input infrared light into visible light, which was then used to stimulate CdS for the production of hydrogen and oxygen. The maximum rate of hydrogen evaporation achieved is 3.38 μmol g⁻¹ h⁻¹. An attempt has been made to achieve 0.008% AQE under NIR light exposure. Amit Kumar Verma et al. [30] demonstrated hydrogen production from aqueous methanol using a TiO₂ upconversion (CeF₃:Ho³⁺) nanosystem (CHT) under visible light exposure. CeF₃:Ho³⁺ nanoparticles were incorporated into TiO₂in situ by a polyol reduction process with triethanolamine as the reducing agent. The CeF₃:Ho³⁺-incorporated TiO₂ nanosystem (CHT) demonstrated increased photocatalytic activity and showed a peak hydrogen evolution rate of 79.85 μmol h⁻¹ under visible light. The solar-to-hydrogen conversion efficiency of 1.37% and the apparent quantum efficiency of 4.00% were reported for the photocatalyst system. Wei Gao et al. [31] demonstrated visible light-induced water splitting by combining reduced graphene oxide (RGO) with CaTiO₃ and Pr³⁺-Y₂SiO₅, leading to enhanced charge transfer inside the CaTiO₃/Pr³⁺-Y₂SiO₅/RGO catalyst. UV-responsive nano-CaTiO₃ was activated by upconverting visible light into UV light using the upconversion luminescent material Pr³⁺-Y₂SiO₅ (Pr³⁺-YSO) to generate charge pairs. The efficient transfer of charge to the hydrogen-producing site Pt by RGO was successfully accomplished. The reverse recombination reaction of hydrogen and oxygen was notably hindered by removing newly produced oxygen from the reaction mixture with an artificial gill. The anodic quantum efficiency (AQE) of reduced graphene oxide (RGO)/Praseodymium (III) ion-coordinated thioglycolic acid (CTYS)/Platinum (Pt) for hydrogen evolution was recorded 0.003% at a wavelength of 400 nm. In Table 2, we summarize the UC photoluminescence properties of a few upconversion materials available for splitting purposes.

Sr. No.	Up-conversion material	Up-conversion photoluminescence property	Ref.
1	Yb,Er)-doped YF3 and (Yb,Tm)-doped YF3 upconversion glass-ceramics as substrate	Standard photoluminescence (PL) methods with a 980 nm laser were used to study the UC light emission properties of GC-Er and GC-Tm. GC-Tm had emitted blue light at around 455 nm and 480 nm, as well as UV light at around 348 nm and 365 nm. On the other hand, GC-Er mostly emitted green light at around 550 nm.	[19]
2	YF3:Yb^3+, Tm^3+	The YF3:Yb^3+, Tm^3+ microrods exhibited upconversion emission spectra when excited at a wavelength of 980 nm, ranging from 320 to 440 nm. The YF3:Yb^3+, Tm^3+ upconversion emission peaks were identified as resulting from the ^1I6 → ^3F4 and ^1D2 → ^3H6 transitions in Tm^3+.	[21]
3	UCNPs NaYF4: Er, Yb UCNPs	The process of upconversion converted photons from a wavelength of 980 nm to a range of 510 nm–570 nm, which falls inside the bandgap of hematite.	[22]
4	NaYF4:Yb^3+,Tm^3+,Gd^3+ Nanorods	The tri-doped UCN system emitted intense blue light visible to the naked eye when stimulated by a 980 nm laser. The photoluminescence spectra of UCN showed peaks at 450, 474, 645, 695, and 802 nm, corresponding to Tm^3+ transitions of ^1D2 → ^3F4, ^1G4 → ^3H6, ^1G4 → ^3F4, ^3F3 → ^3H6, and ^3H4 → ^3H6.	[24]
5	(Yb,Er)NaYF4	Under excitation at 980 nm (NIR), (Yb,Er)NYF produced four emission bands in the upconversion emission spectra. The UV region at 408 nm exhibited blue emission, corresponding to the transition from ^2H9/2 to ^4I15/2. Green emission in the visible area was observed (520 and 540 nm), which corresponds to the transitions from ^2H11/2 to I15/2 and from ^4S3/2 to ^4I15/2. The red emission was produced by the transition of erbium ions from the ^4F9/2 to ^4I15/2 state, resulting in light at 654 nm.	[25]
6	Hexagonal phase NaYF4:Yb^3+,Er^3+/Tm^3+ (80:18:2)	Tm-UCN, when exposed to radiation, produced two peaks in the blue spectrum at 451 and 476 nm, and three peaks in the red to near-infrared spectrum at 646, 697, and 743 nm. The emissions listed were attributed to electronic transitions from ^1D2 to ^3F4, ^1G4 to ^3H6, ^1G4 to ^3F4, ^3F2 to ^3H6, and ^3F3 to ^3H6. The Er-UCN displayed three distinct peaks at 525, 541, and 655 nm corresponding to transitions from ^4S3/2, ^2H11/2, and ^4F9/2 to the ^4I15/2 level, respectively.	[26]
7	NaYF4–Yb^3+/Er^3+	When exposed to 980 nm irradiation, the green light emission was observed from the excited states ^2H11/2 and ^4S3/2to the ground state ^4I15/2 of Er^3+, whereas the blue emission was found associated with the excitation route from ^2H9/2 to ^4I15/2 of Er3+.	[28]
8	NaYF4–Yb^3+-Er^3+	When excited at 975 nm, the NYF exhibited a prominent emission peak at 650 nm and a less intense peak at 450 nm.	[29]
9	CeF3: Ho^3+	The photoluminescence upconversion spectra were recorded by applying wavelengths from 400 to 700 nm, resulting in observed upconversion responses within the visible range. The upconversion reactions were caused by the energy levels of 460 nm (^5I8–^5F1), 480 nm (^5I8–^5F3), 535 nm (^5I8–^5F4), and 640 nm (^5I8–^5F5) present in rare earth Ho(III) ions.	[30]
10	Pr^3+-Y2SiO5 (Pr^3+-YSO)	Pr^3+-YSO, Pr^3+- CTYS/Pt, and RGO/Pr^3+-CTYS/Pt effectively converted visible light wavelengths of 447 nm, 488 nm, 515 nm, and 540 nm into UV emission wavelengths ranging from 253 nm to 371 nm.	[31]

Table 2.

The up-conversion material integrated with semiconductors with their up-conversion photoluminescence property for splitting applications is tabulated.

2.3 Carbon or graphene-based upconverter integrated semiconductor system

Graphene and carbon quantum dots exhibit impressive optical and electrical properties. Several groups have utilized them as upconversion materials. While the exact mechanism is challenging to determine, these materials demonstrate exceptional upconversion capabilities. We have summarized a few of these reports (Table 3). Ensafi et al. [32] developed a novel form of graphene quantum dots known as nitrogen-doped graphene quantum dots (N-GQDs). The N-GQDs exhibited a strong tendency to generate light by photoluminescence and upconversion emission. It was observed that the addition of nitrogen-doped graphene quantum dots (N-GQDs) enhanced the photoelectrochemical (PEC) performance of the TiO₂ photoanode. The photo electrocatalytic (PEC) efficiency of TiO₂ was evaluated with and without the inclusion of upconversion N-GQDs. The upconversion N-GQDs displayed a high quantum yield of 51.5% and, when paired with TiO₂ nanoarrays, showed an enhanced photocurrent density of 3.0 mA.cm⁻² at 1.23 V vs. RHE. Zhao Liang et al. introduced hydrogenated TiO₂ nanorod arrays adorned with carbon quantum dots to improve the effectiveness of photoelectrochemical water splitting, as detailed in their publication [33]. The limited ability to absorb light and collect charges due to its large energy gap is acknowledged as a significant obstacle in developing highly efficient TiO₂ photoanodes. To overcome these intrinsic constraints, the group has developed a photoanode made of TiO₂ nanoarrays that underwent hydrogenation treatment and were embellished with carbon quantum dots (CQDs). The study showed that hydrogenation treatment leads to the formation of oxygen vacancies, which effectively prevents the recombination of photoinduced carriers. The decorated CQDs acted as electron reservoirs to absorb photoinduced electrons and enhanced the absorption of solar light through their upconversion action. The newly made photoanodes showed a significant water splitting photocurrent density of approximately 3.0 mA.cm⁻² at 1.23 V compared to the reversible hydrogen electrode under simulated sunshine. This value was almost six times higher than that of the original TiO₂ photoanodes. Van Dien Dang et al. [34] described the photoelectrochemical (PEC) breakdown of trimethoprim and hydrogen evolution. The accomplishment was realized utilizing a photoanode composed of N-doped carbon dots (NCD) mixed with g-C₃N₄/αFe₂O₃ (CNFO) shell/core nanocomposite. The NCD@CNFO photoanode exhibited a photocurrent density of 3.07 mA.cm⁻² at 1.6 V relative to the normal hydrogen electrode (NHE). This value was found four times more than CNFO and fifteen times greater than intact αFe₂O₃. The PEC system demonstrated the concurrent creation of hydrogen and the decomposition of TMP, leading to a hydrogen generation rate of 550 μmol cm⁻² h⁻¹. The high efficiency labeled a photoelectrochemical (PEC) system using carbon dots/semiconductor hybrid catalysts to break down antibiotics and generate hydrogen through photocatalysis in wastewater. The article provided a different approach to dealing with issues with environmental pollution and the energy problem. NCD efficiently absorbed longer wavelengths (600-800 nm) from the light source and released shorter wavelengths (300-450 nm) because of its upconversion capabilities. As a result, the absorption efficiency of the NCD@CNFO nanocomposite was found improved. Yiqing Deng et al. [35] produced upconversion carbon quantum dots (CQDs) by a hydrothermal method involving L-glutamic acid (L-Glu) and m-phenylenediamine (MPD). The CQDs were combined with commercial nano-TiO₂ to produce CQDs/TiO₂ composites for photocatalytic destruction of methyl orange. The fluorescence spectra showed that the generated CQDs effectively converted visible light at approximately 600 nm into UV light at 350 nm. In photocatalysis experiments, a composite of carbon quantum dots (CQDs) and titanium dioxide (TiO₂) with a molar ratio of L-Glu to TiO₂ of 1:1 showed the best performance in breaking down methyl orange (MO). When exposed to a 600 nm light source, CQDs/TiO2 exhibited 70.56% degradation of MO (40 ppm) within 6 hours, a degradation rate equal to its 78.75% under 365 nm UV light. The degradation process was discretely associated with the presence of H⁺ and ^• OH under upconverted visible light, proposed by free radical scavenging studies and electron spin resonance (ESR) testing.

Sr. No.	UC material/semiconductor	Synthesis conditions	Activity (photocurrent density)	Reference
1	N-GQDs/TiO2	produced by carbonizing precursors of tetraethylenepentamine (TEPA) and citric acid (CA) hydrothermally under pressure and at a high temperature of 180°C.	3.0 mA.cm^−2 (at 1.23 V vs. RHE in 1 M KOH solution); eight times better than unmodified TiO2	[32]
2	CQDs/TiO2, CQDs-H/TiO2	Ascorbic acid and ethanol precursor materials are used in hydrothermal synthesis at 160°C.	3.0 mA.cm^−2 (at 1.23 V vs. RHE in 1 M KOH solution), 3.3 mA.cm^−2 (at 0.8 V vs. Ag/AgCl)	[33]
3	NCD-gC3N4/α-Fe3O4	Using ethylenediamine and citric acid as precursors, NCD was produced in an oil bath at 120°C. The NCD@CNFO nanocomposite was developed by using the EDC/NHS process to conjugate the carboxylic groups of NCD with the amine groups of the 3-aminopropyl)-triethoxysilane (APTES) coated CNFO.	95% TMP degradation (at 0.1 M N2SO4), 3.07 mA.cm^−2 (at 1.6 V vs. NHE in 0.1 M N2SO4) 550 μmol cm^−2 h^−1 Hydrogen generation	[34]
4	CQDs/TiO2	Precursor materials L-glutamic acid (L-Glu) and m-phenylenediamine (MPD) were prepared hydrothermally at 200°C.	70.56% of MO (40 ppm) degradation in 6 h at 600 nm light source, 78.75% under 365 nm.	[35]

Table 3.

Summarizes the graphene-based up-conversion materials available for photo splitting cells.

3. Conclusion

The book chapter offered a succinct and lucid rationale for the importance of sub-bandgap harvesting. Upconversion integration is a proposed strategy for collecting all sub-bandgap photons. However, limited research has been conducted in the field of solar splitting or fuel generation by upconversion. This book chapter offers a systematic examination of upconversion integrated semiconductor systems. These systems include graphene-based, inorganic lanthanide-based, and organic triptycene annihilation-based systems developed for solar cell splitting. Graphene-based upconversion materials are highly prospective candidates for efficient solar energy conversion due to their considerable specific surface area, high electrical conductivity, and tunable properties. Inorganic upconversion materials composed of rare earth elements are known for converting low-energy photons from the solar spectrum into higher-energy photons due to their exceptional photostability and efficient upconversion. In contrast, organic upconversion materials provide several benefits, including cost-effectiveness, straightforward processing, and the capacity to customize optical characteristics via molecular engineering. While organic materials may have a lower upconversion efficiency than their inorganic counterparts, ongoing research endeavors are focused on surmounting these constraints in order to fully exploit the capabilities of organic materials in the area of solar energy conversion. Through ongoing research and development, scientists anticipate uncovering novel materials that exhibit superior upconversion properties and demonstrate enhanced performance in practical applications. Upconversion materials are crucial for advancing toward a sustainable energy future. This technology holds great promise in addressing the pressing global issues surrounding energy and the environment. To increasingly capture sub-bandgap photons, we require a substantial amount of active participation in order to develop a significantly more effective upconversion semiconductor integrated system. This chapter provides guidance to those who wish to do research on sub-bandgap photon harvesting for solar splitting applications.

Acknowledgments

The authors would like to thank the Department of Science and Technology (DST/TMD-EWO/AHFC-2021/2021/111). The authors are grateful for the invaluable help provided by Rajiv Gandhi Institute of Petroleum Technology.

Conflict of interest

The authors declare no conflict of interest.