Conversion of Solar Energy Into Electricity
Solar Spectrum and Photovoltaic Performance Parameters
The sun is the planet's most abundant and sustainable energy source, delivering nearly 120,000 terawatts (TW) of energy annually, which is over 10,000 times greater than current global energy demand Solar energy, in the form of photons, varies across different wavelengths influenced by factors like latitude, time of day, and atmospheric conditions, collectively referred to as the solar spectrum.
The solar spectrum illustrates the power of solar incidence per area per wavelength (W m −2 nm −1), known as irradiance, with a bandwidth of 1 nm The designations AM0, AM1.0, and AM1.5 represent solar spectra derived from various ASTM standards tailored for specific applications Notably, AM1.0 and AM1.5 spectra, calculated per ASTM G173, serve as reference standards for terrestrial applications, while the AM0 spectrum, based on ASTM E490, is utilized for space-related applications.
The solar spectrum, represented in W m −2 nm −1 according to AM0 and AM1.5 standards, is crucial for the development of photovoltaic cells By taking into account geographic and atmospheric variables such as the angle of incidence and air density, accurate spectral calculations can be made This laboratory reproduction of the solar spectrum is essential for comparing and certifying the performance of photovoltaic devices from various manufacturers, as well as those still in the development phase.
The performance of illuminated solar cells is evaluated through key photovoltaic parameters, including power output per illuminated area (P out, in W/cm²), open-circuit voltage (V oc, in V), short-circuit current density (J sc, in mA/cm²), fill factor (FF), and overall conversion efficiency (η) These parameters are fundamentally influenced by the incident light power on the cell (P in), as described in equation 1.1.
The equation P = h * c / λ (1.1) describes the relationship between photon energy and wavelength, where h represents the Planck constant (4.14 × 10^−15 eV·s), c denotes the speed of light (3.0 × 10^8 m/s), and Φ(λ) indicates the flux of photons, adjusted for reflection and absorption prior to reaching the cell (in cm^−2 s^−1 per Δλ).
The open-circuit voltage is the voltage measured between the terminals of an illuminated solar cell when they are not connected, indicating infinite resistance In contrast, the short-circuit current density occurs when the terminals are connected to a load with zero resistance, which increases with the intensity of incident light due to a higher number of photons and electrons Typically, current is expressed as current density (J), calculated as current per unit area, as it tends to rise with the solar cell's active area When a load is connected, the current decreases while a voltage develops as the electrodes charge, with the resultant current represented by the integral Pin=∫ λ hc λΦ 0 ( λ )d λ.
Figure 1.2 illustrates the various forms of solar radiation that reach the Earth and the corresponding standards for calculating the solar spectrum Solar cells, typically composed of a p-n junction (a junction between p-type and n-type semiconductors) or a d-A junction (formed by an electron donor and acceptor), can be modeled as diodes The short-circuit current generated by photon absorption is superimposed with a dark current, which is the voltage produced by the load flowing in the opposite direction For an ideal diode, the dark current density (J_dark) is defined by specific parameters.
The equation J = J0 (qV/kBT) describes the current density at absolute zero, where J0 represents the current density at 0 K, q is the electron charge (1.6 × 10 −19 C), V is the voltage across the cell's electrodes, kB is the Boltzmann constant (8.7 × 10 −5 eV/K), and T is the absolute temperature The overall current can be understood as a combination of both the short-circuit current and the dark current.
The open-circuit voltage occurs when the current (J) is zero, indicating that the currents are balanced and no current flows through the cell, representing the open-circuit condition The resulting expression for this condition is provided in reference [16].
The performance of a solar cell is evaluated through an experimental current-voltage (J × V) curve, as illustrated in Fig 1.3 This assessment occurs under standard operating conditions, specifically with illumination that meets established standards.
AM1.5, under an irradiating flux of 100 Wãcm −2 and a temperature of 25°c.
FIGURE 1.3 Current density–voltage (J × V) curve of an illuminated solar cell
The maximum power generated by an illuminated solar cell occurs within the voltage range of V = 0 (short circuit) to V = V oc (open circuit), represented as V max, with the corresponding current density denoted as J max The conversion efficiency (η) of the solar cell is defined as the ratio of the maximum power output to the incident power, expressed by the formula η = J max V max / P in.
An ideal solar cell exhibits a J × V curve that is rectangular in shape, indicating maximum and constant current density (J sc) up to the open-circuit voltage (V oc) However, real-world solar cells deviate from this ideal behavior, as not all incident power is converted into energy To quantify this deviation, the fill factor (FF) is introduced, which measures how closely a photovoltaic cell approaches its ideal performance.
By definition, FF ≤ 1 Thus, the overall conversion efficiency can be expressed using the
FF value [16]: η = J V FF P sc oc in
Quantum efficiency is a crucial performance parameter that quantifies the number of electrons generated by each incident photon of wavelength λ that can perform work It is categorized into internal and external classifications External quantum efficiency (eQe), also referred to as incident photon to current efficiency (Ipce), is calculated by dividing the number of electrons collected by the cell's electrode under short-circuit conditions by the number of incident photons Its value is determined by equation 1.10.
(1.10) since the Ipce value depends on the wavelength of the incident radiation, an Ipce ver- sus wavelength curve corresponds to the cell’s spectral response, also known as the action
The Ipce40 IscλPin spectrum of a solar cell measures its internal quantum efficiency (IQe), defined as the ratio of electrons collected by the cell's electrode in a short-circuit configuration to the number of photons that effectively enter the cell This metric does not account for all incident photons, as some are lost due to reflection or absorption prior to charge separation in the absorption layer Both efficiencies are positive and capped at 100%.
A solar cell functions as a power generator, represented by an equivalent circuit that includes a diode, dark and short-circuit currents, and two resistances: series resistance (R s ) and parallel resistance (R p ) The series resistance reflects the nonideal conductive behavior of the cell, while the parallel resistance accounts for leakage current due to insulation issues Ideally, R s is 0 and R p is infinite The current expression can be refined by incorporating these resistances into the equivalent circuit model.
(1.11) where A is the active area of the cell in centimeter square.
Operating Principles of a Solar Cell
A solar cell typically comprises an absorbing material situated between two electrodes, which can be made from semiconductors or dyes, and may vary in structure as monocrystalline, polycrystalline, nanocrystalline, or amorphous This absorbing material is crucial as it collects solar light and must possess a bandgap energy (E g) that aligns with the solar spectrum The process of separating charges into individual carriers and their subsequent transport can be facilitated by the absorber, while the electrodes, made from conductive materials with differing work functions, must include at least one transparent electrode to allow light to pass through.
The photovoltaic conversion process can be divided into four sequential stages [16]:
1 light absorption causes an electron transition in the cell’s absorbing material from the ground state to the excited state;
2 the excited state is converted into a pair of separate charge carriers, one negative and the other positive;
FIGURE 1.4 Equivalent circuit of a solar cell the ground state.
The mechanisms of light absorption and charge separation in solar cells rely on the electron structure and morphology of the absorber In traditional inorganic semiconductor solar cells, like silicon and gallium arsenide (GaAs), these processes occur solely within the absorber material The band separation energy of silicon (1.1 eV) and GaAs (1.42 eV) enables the collection of nearly 70% of solar radiation When photons are absorbed, electrons are promoted to the conduction band, creating corresponding gaps in the valence band, a phenomenon observed in both direct-gap and indirect-gap semiconductors The generated charge carriers are then separated at the semiconductor's junction, formed at the interface of p-type and n-type doping regions, even when insulated by a third intrinsic layer This junction is created during the manufacturing of the absorbing film, where a thin semiconductor layer undergoes a second doping process, resulting in distinct doping regions separated by the junction interface.
A built-in potential forms at the junction region due to varying electron affinities in adjacent areas This potential is sufficiently strong to separate charge carriers into distinct entities: electrons and holes.
Figure 1.5 illustrates the energy levels of p-type and n-type semiconductors before and after junction formation In Figure 1.5A, the energy levels of the isolated semiconductors are aligned to vacuum, highlighting the relative positions of their bands and the Fermi levels, E F, which correspond to their work functions, φ W1 for n-type material 1 and φ W2 for p-type material 2 When the semiconductors come into contact, as shown in Figure 1.5B, an abrupt interface is established, creating a potential step due to the differing electron affinities Figure 1.5C depicts the steady state where a single Fermi level exists at a specific temperature, while the Fermi levels of each material maintain their positions relative to their respective voltages, V n and V p This unified Fermi level necessitates an electrostatic potential across the junction, resulting in the deformation of the valence and conduction bands, as well as the local vacuum level The potential energy difference at the junction acts as the driving force for charge separation after light absorption, with the high electron mobility of the absorbing film facilitating efficient transport of the separated charges to the electrodes.
Solar cells utilizing organic absorbers, such as dyes and conjugated polymers, generate excitons—electron-hole pairs created through light absorption These excitons are neutral and can only move via diffusion To effectively separate these excitons, donor-acceptor (D-A) architectures are employed, similar to the p-n junctions found in traditional semiconductors In this setup, the donor material captures photons to create excitons, while the acceptor material, possessing high electron affinity, facilitates their separation due to the differing electron affinities of the two materials.
Figure 1.5 illustrates the electron energy levels in n and p semiconductors at three stages: before contact, during contact, and after contact when in thermodynamic equilibrium This depiction emphasizes the importance of controlling the nanoscale morphology of the interfaces The subsequent section will delve into the structure and functionality of these semiconductor cells.
Organic Solar Cells
Organic solar cells (OSCs) utilize organic semiconductors, including small organic dyes and conjugated polymer molecules, for their absorber/donor functions While coordination compounds like phthalocyanines and metalloporphyrins can also serve this purpose, they are not strictly organic Fullerenes are the most commonly used acceptors, although other materials such as polymers, small conjugated molecules, carbon nanotubes, and metal oxide nanoparticles are also viable options The initial OSC models emerged in the 1980s, featuring organic pigment films sandwiched between metal electrodes, but their efficiency was below 0.1% The introduction of the donor-acceptor (d–A) junction in bilayer configurations, along with the use of different dyes, improved efficiency to nearly 1% The advent of bulk heterojunction solar cells in the early 2000s, which combined conjugated polymers and fullerenes, further enhanced performance, leading to current conversion efficiencies of 12% for small molecule-based cells and 9.2% for those based on conjugated polymers.
In the near future, efficiency values of organic cells are anticipated to reach 15% However, it is widely agreed that efficiency plays a secondary role in the adoption of organic cells; the primary focus is on achieving a lower watt/hour cost for these devices Figure 1.6 illustrates the structures of absorbers used in organic solar cells (OSCs) and highlights the photovoltaic parameters of the cells produced under optimized conditions.
The introduction of conjugated polymers has significantly advanced the field of organic solar cells (OSCs) due to their unique optoelectronic properties and ability to be processed in solution at room temperature Utilizing fast printing techniques like roll-to-roll and screen printing, the manufacturing costs of these cells are notably reduced Additionally, the ability to deposit polymer films on plastic substrates enables the large-scale production of flexible solar cells, promoting their widespread application in building-integrated photovoltaics Conjugated polymers consist of a backbone of sp² carbon atoms with alternating single and double bonds, leading to a delocalization of π electrons along the polymer chain, which contributes to their distinct electronic properties.
The structural formula of absorbing materials for organic solar cells (OSCs) is illustrated in Figure 1.6, highlighting key photovoltaic parameters under optimized conditions The materials include Copper phthalocyanine, MDMO-PPV, P3HT, NP7, BisDMO-PFTDBT, HXS-1, PCDTBT, and PDTSPTBT These materials are characterized as semiconductors due to their valence and conduction bands Notably, achieving conduction levels comparable to metals requires doping the polymer.
In fact, most conjugated polymers have separation energies between bands above 2.0 eV
The absorption of solar light in conjugated polymers is significantly limited due to their bandgap of 620 nm Additionally, these materials exhibit low electron mobility, typically under 0.01 cm² V⁻¹ s⁻¹, which is substantially lower than that of traditional semiconductors like silicon, whose mobility can be up to 10,000 times greater However, conjugated polymers possess an exceptionally high optical absorption coefficient, around 10⁵ cm⁻¹, allowing ultrathin films (less than 100 nm thick) to effectively capture most incident photons and offset the challenges posed by low mobility.
The operation of organic solar cell systems (OSCS) relies on the photoinduced transfer of electrons from a donor material to an acceptor with high electron affinity In this process, a conjugated polymer acts as the donor, absorbing energy and transferring photoexcited electrons to a fullerene, which serves as the electron acceptor.
The process of electron transfer between dyes or conjugated polymers and fuller- enes occurs on a much smaller time scale (on the order of femtoseconds) than that of
FIGURE 1.8 OCS module manufactured by Konarka From http://www.konarka.com/
FIGURE 1.7 Structural formula of some fullerenes used in OSCs 1, [6,6]-Phenyl-C61-butyric acid methyl ester (PC61BM);
The compound 2, [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) plays a significant role in competitive processes like photoluminescence, occurring on the order of nanoseconds The energy levels of the donor and acceptor materials facilitate efficient electron transfer The donor-acceptor (d–A) junction in organic solar cells (OSCs) can be formed in two primary configurations: (A) a plane or bilayer heterojunction, where a fullerene layer is placed atop a donor layer, similar to a p–n junction in inorganic semiconductors, and (B) a bulk heterojunction, where fullerene is blended with a conjugated polymer throughout the film This nanoscale morphology of the blend enhances the photovoltaic conversion by significantly increasing the interfacial area of the d–A junction.
The photovoltaic performance of organic solar cells (OSCs) is significantly influenced by the energy levels of the electron donor and acceptor materials, as well as the work function of the electrodes The open-circuit voltage (V_oc) can reach its maximum when it equals the energy difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor Research shows that in OSCs utilizing conjugated polymers, there is an inverse linear correlation between V_oc and the first reduction voltage (LUMO) of fullerene compounds, indicating that lower LUMO values lead to higher V_oc Additionally, V_oc is directly related to the oxidation voltage (HOMO) of the polymer The interfaces between the active layer and the electrodes also affect cell voltage; for instance, indium tin oxide (ITO) used as an anode has a work function of 4.7 eV, which is slightly lower than the HOMO levels of many dyes and polymers To minimize the energy barrier at the electrode-active layer interface, the ITO work function can be enhanced through plasma treatment or by applying thin layers of conductive polymers like the PEDOT:PSS complex or polyaniline The cathode typically consists of a metal with a low work function, such as aluminum.
The photoinduced electron transfer process between p-phenylene-vinylene (PPV) molecules and fullerene (C60) is illustrated in Figure 1.9, highlighting the energy levels of each material This schematic is adapted from the comprehensive overview of organic solar cells by H Hoppe and N.S Sariciftci, published in the Journal of Materials Research.
The current density produced by a solar cell is determined by the charge collection at the electrodes, particularly in organic solar cells (OSCs), where it results from the multiplication of three key efficiencies: photon absorption (η abs), exciton dissociation (η diss), and carrier collection by the electrodes (η out).
The absorption efficiency depends on the absorption spectrum of the absorbent ma- terial and on optical effects, especially interference, inside the device [38] As mentioned, Jsc=ηabs⋅ηdiss⋅ηout
The schematic in Figure 1.10 illustrates the OSC configurations for bilayer heterojunction (A) and bulk heterojunction (B) Conjugated polymers, along with small conjugated molecules and coordination compounds, exhibit a high absorption coefficient The OSC architecture incorporates a thick glass substrate next to a highly reflective metal electrode, creating interference conditions that can disrupt the alignment between the electrooptical field's maximum amplitude and the donor-acceptor interface, ultimately diminishing photon collection efficiency.
The efficiency of exciton dissociation is significantly influenced by the donor-acceptor interface in bulk heterojunction organic solar cells (OSCs) To optimize performance, it is crucial to control the nanoscale morphology, ensuring minimal phase segregation and maintaining a donor-acceptor interface size that aligns with the exciton diffusion length.
Films composed of a 1:4 (m/m) MdMo-ppV:c 60 blend, deposited via spin coating on an ITO substrate, exhibit varying morphologies depending on the solvent used in the deposition solutions When toluene is employed, it leads to the development of globules of different sizes.
The use of chlorobenzene in the preparation of organic solar cells (OSCs) leads to a smoother film surface with smaller-scale segregation of approximately 20 nm, in contrast to the more irregular surface created by toluene, which results in clustered structures around 200 nm This morphological difference significantly impacts conversion efficiency, with OSCs made using chlorobenzene exhibiting an efficiency that is three times greater than those prepared with toluene.
The efficiency of charge collection is influenced by the pathways taken by carriers within the active layer, with material mobility playing a crucial role Most conjugated polymers exhibit low mobility due to their disordered solid-state structure, leading to conduction primarily through the hopping of localized charge carriers According to the Poole–Frenkel model, carrier mobility is proportional to the electric field, as described by the equation à(E) = à₀ exp(γE), where à₀ represents the mobility at absolute zero and γ is a temperature-dependent preexponential factor For effective carrier movement towards the electrodes, the electric field must be sufficiently strong, necessitating maximized work function differences between electrodes and minimized active layer thickness, ideally below 100 nm.
The FF and the overall conversion efficiency of a solar cell depend, in addition to V oc and
Dye-Sensitized Solar Cells
The development of dye-sensitized solar cells (DSSCs) began in the mid-1980s, marking a significant advancement in solar technology Research indicated that colloidal titanium dioxide (TiO2) particles, measuring 20 nm in diameter, could be effectively sensitized to visible light using a ruthenium-based luminescent complex, specifically tris(2,2′-bipyridine-4,4′-dicarboxylate) chloride of ruthenium(II).
The study revealed that the photoluminescence of the complex was entirely suppressed when immobilized on oxide nanoparticles, indicating that photoexcited electrons were transferred to the semiconductor's conduction band This principle was later utilized in device applications, leading to significant advancements in the dye-sensitized solar cell (DSSC) field Currently, the overall conversion efficiency of these cells exceeds 11%.
A typical dye-sensitized solar cell (DSSC) comprises a photoanode, a counter-electrode, and an electrolyte that separates the two electrodes The photoanode is made from a mesoporous, nanocrystalline semiconductor film, typically titanium dioxide (TiO2), zinc oxide (ZnO), or tin dioxide (SnO2), which is deposited on a transparent electrode like fluorine-doped tin oxide (FTO) This semiconductor is then photosensitized using various dyes, including ruthenium(II) complexes, quantum dots such as CdSe and PbS, synthetic organic dyes, or natural extracts.
The J-V curves of organic solar cells (OSCs) under AM1.5 irradiation at 100 mW/cm² are presented, showcasing the performance differences between two types of fullerenes Additionally, the structural formula of the PTB1 polymer, utilized as an absorber/donor in these solar cells, is included This information is adapted from the research by Y Liang and colleagues, highlighting advancements in semiconducting polymers for enhanced solar cell efficiency, as published in the Journal of the American Chemical Society.
V OC : 720 mV; J SC : 18.2 mA.cm –2
V OC : 736 mV; J SC : 20.9 mA.cm –2 FF: 0.72; η: 11.1%
V OC : 826 mV; J SC : 17.0 mA.cm –2 FF: 0.72; η: 10.1%
V OC : 700 mV; J SC : 15.9 mA.cm –2
V OC : 747 mV; J SC : 15.1 mA.cm –2 FF: 0.69; η: 8.0%
FIGURE 1.12 Structural formula of the main dyes used in dye-sensitized solar cells (DSSCs) and their respective photovoltaic parameters (for cells in optimized conditions) 11, N3; 12, N749; 13, N719; 14, Z907; 15, K19; 16, YD2-o-C8;
The electrolyte in dye-sensitized solar cells (DSSCs) typically includes a redox pair, such as I/I3-, cobalt(II/III) complexes, or TeMpo/TeMpo+ Alternatives like ionic liquids, polymer electrolytes, and p-type semiconductors, including conjugated polymers, can also be utilized The counter-electrode is generally a transparent FTO electrode, often enhanced with a thin platinum layer, although materials like graphite and conductive polymers may serve as substitutes The operating principle of DSSCs involves the photoexcited dye injecting electrons into the semiconductor's conduction band, which are then collected by the transparent electrode to generate anodic current The dye is regenerated by electrons from the electrolyte's redox pair, which in turn are replenished by electrons from the external circuit captured by the counter-electrode This cyclic and regenerative process distinguishes DSSCs from organic solar cells (OSCs), as energy absorption and charge transport occur separately.
FIGURE 1.12 (cont.) elements in the device, a feature that widens the options of different absorbing materials and electrodes.
The efficiency of dye-sensitized solar cells (DSSCs) in converting light to electricity relies on effective electron coupling between the excited state of the photosensitizer and the conduction band of the semiconductor This is primarily achieved through the use of ruthenium(II)-based dyes in combination with TiO2 mesoporous films, resulting in optimal conversion values Additionally, polypyridine complexes of ruthenium(II) exhibit strong absorption characteristics, enhancing the overall performance of DSSCs.
Figure 1.13 illustrates the schematics of a dye-sensitized solar cell (DSSC), featuring a side view that highlights the oxidation-reduction voltages of the dye and mediator, alongside the semiconductor's conduction band position in relation to the normal hydrogen electrode (NHE) Additionally, an amplified side view of the photoanode showcases the semiconductor nanoparticles that are sensitized with dye molecules, as adapted from M Gratzel's work on recent advancements in sensitized mesoscopic solar cells published in Acc Chem Res 42.
In 2009, research highlighted the significant role of metal-to-ligand charge transfer (MLCT) transitions in visible light absorption These complexes not only exhibit intense absorption but also possess energy levels that facilitate rapid electron transfer to semiconductors, occurring in timescales ranging from nanoseconds to femtoseconds This process is notably quicker than the recombination processes, similar to organic solar cells (OSCs).
The voltage generated by an illuminated dye-sensitized solar cell (DSSC) is determined by the difference between the Fermi level of electrons in TiO2 nanoparticles and the redox potential of the electrolyte For numerous photosensitizers attached to TiO2 nanoparticles, the electron injection driving force is 0.15 eV, while the energy required to regenerate the photosensitizer is 0.6 eV This energy imbalance can be mitigated by using photosensitizers with redox potentials closer to that of the iodide/triiodide pair Additionally, enhancing the dye's absorption range into the infrared can further improve DSSC efficiency Notably, DSSCs utilizing two ruthenium(II) complex photosensitizers, N3 and N749, exhibit a high external quantum efficiency (EQE) of up to 80%, with N749 extending its spectral response into the near infrared, allowing for broader solar spectrum utilization and a 1.5-fold increase in conversion efficiency compared to N3 Current DSSC photoelectrochemical parameters are promising, with short-circuit current densities (Jsc) around 20 mA cm², open-circuit voltages (Voc) between 0.7 and 0.86 V, and fill factors (FF) ranging from 0.65 to 0.8 under AM1.5 solar irradiation.
Ruthenium complexes are the main photosensitizers used in dsscs However, porphy- rin-based dsscs (structure 16, Fig 1.12) have conversion efficiencies on the order of 12%
Metal-free dyes, derived from natural sources, serve as effective photosensitizers For synthetic organic dyes intended for use in dye-sensitized solar cells (DSSCs), it is essential that they possess electron-donating units, electron-conducting units, and anchoring groups.
The Incident Photon to Current Efficiency (IPCE) curves illustrate the performance of photosensitizers derived from ruthenium(II) complexes, specifically N3 and N749, alongside pure mesoporous TiO2 films Accompanying these curves, the chemical structures of the photosensitizers are presented, highlighting their significance in the analysis of photovoltaic efficiency.
Reproduced with permission from M Gratzel, Recent advances in sensitized mesoscopic solar cells, Acc Chem Res 42
Berries such as blackberry, raspberry, and açaí are rich in anthocyanins, flavonoids that combat free radicals and serve as effective photosensitizers in dye-sensitized solar cells (DSSCs) The anthocyanin structure features hydroxyl groups that can bond with the TiO2 surface, allowing for electron injection into the semiconductor's conduction band upon photoexcitation A study evaluating the stability of DSSCs sensitized with blackberry extract found that its performance is comparable to that of DSSCs sensitized with N3, provided the device is properly sealed.
The mesoporous and nanocrystalline layer of a semiconductor oxide, along with the photosensitizer, are crucial components of dye-sensitized solar cells (DSSCs) Typically, nanoparticles are synthesized using the sol-gel method followed by hydrothermal growth, achieving sizes of 20–25 nm Once produced, these nanoparticles are mixed into a paste with a stabilizing agent like poly(ethylene oxide) and applied to a transparent electrode The electrode is then sintered in an oven, which facilitates electrical connectivity among the particles throughout the mesoscopic structure This mesoscopic architecture significantly enhances the semiconductor film's surface area, leading to increased absorption of the photosensitizer and superior light absorption compared to compact films Titanium dioxide (TiO2) in its anatase form is the most commonly utilized semiconductor in DSSCs, characterized by its bipyramidal shape and exposed facets.
(101) direction, which is the direction with the lowest surface energy [51] Fig 1.15 shows the morphology of a Tio2 mesoporous film used in dsscs In general, the thickness of a
FIGURE 1.15 Scanning electron microscopy micrograph of a TiO 2 mesoporous film Reproduced with permission from
A.O.T Patrocinio, et al., Layer-by-layer TiO 2 films as efficient blocking layers in dye-sensitized solar cells, J Photochem
In the study of photobiology, it is observed that in semiconductors, electrons become mobile through diffusion once traps are filled Notably, the electron diffusion coefficient is variable and tends to increase with higher light intensity.
Electron transport in semiconductors is facilitated by the movement of cations in the electrolyte, which prevents the buildup of noncompensated spatial charges that could impede electron flow Additionally, the mesoporous film features a terminal layer approximately 5 micrometers thick, composed of larger particles ranging from 400 to 500 nanometers, designed to enhance the optical path and improve light collection in the red spectrum.
Photoelectrochemical Cells for the Production of Solar Fuels
Solar cells convert solar energy into electricity, making them a viable source of clean and affordable energy However, the challenge of energy production at night arises due to the sun's intermittent nature, with only about 6 hours of usable solar irradiation daily To make solar energy a primary energy source, effective storage solutions are essential Nature offers insights through photosynthesis, where plants convert solar energy, carbon dioxide, and water into high-energy chemical compounds like carbohydrates Researchers are exploring methods to artificially replicate photosynthesis, enabling the conversion of solar energy into fuels that can be utilized in power plants or fuel cells at any time The direct conversion of solar light into solar fuels involves transforming solar energy into chemical bonds using photocatalysts, providing a sustainable energy solution capable of storing significant amounts of energy indefinitely The most studied artificial photosynthesis techniques include water-splitting to produce molecular hydrogen and CO2 reduction to generate methanol or hydrocarbons.
The reaction 2H2O + 4hν → 2H2 + O2 (ΔGo = 4.92 eV, n = 4) highlights the role of platinum as an electrocatalyst in enhancing hydrogen evolution and water oxidation reactions However, due to platinum's status as a noble and costly metal, it is essential to explore the development of more affordable photocatalytic systems derived from sustainable sources.
A photoelectrochemical cell for artificial photosynthesis primarily consists of two electrodes, one or both made from photoactive materials, along with an electrolyte that connects the electrodes to the external circuit The photoactive electrode serves as a photocatalyst, absorbing solar energy and facilitating essential oxidation-reduction reactions for water splitting or CO2 reduction Various semiconductor materials can function as photocatalysts in these reactions, with hematite (α-Fe2O3) being particularly noteworthy due to its theoretical light-to-hydrogen conversion efficiency of 16.8%.
Pure TiO2 is recognized as an excellent photocatalyst when activated by UV light, but it requires dye photosensitization to effectively harness visible and infrared light Various photocatalytic systems, including those based on metal complexes and biocatalysts like enzymes (notably hydrogenases and dehydrogenases), have been explored for applications in water splitting and CO2 reduction Typically, noble metals and their oxides, such as Pt, Pd, RuOx, and IrOx, serve as cocatalysts to enhance the efficiency of O2 and H2 evolution reactions However, the high cost and limited availability of these materials pose significant challenges for the large-scale production of photoelectrochemical cells utilizing such electrocatalysts.
In aqueous media with a pH of 0, the energy levels of inorganic semiconductors are depicted, highlighting the oxidation potential of water at 1.23 V and the reduction potential of protons at 0 V, as illustrated in Figure 1.16 This information is adapted from the work of P.D Tran et al in their 2012 study on recent advancements in hybrid photocatalysts for solar fuel production, published in Energy & Environmental Science.
One of the leading photoelectrochemical cell prototypes for hydrogen production through water photodecomposition utilizes photoanodes made from nanostructured hematite films Hematite, recognized as the most stable crystalline form of iron oxide, plays a crucial role in this innovative technology.
Hematite exhibits semiconductor properties with a band gap of 2.1 eV, demonstrating stability under operational conditions and affordability A schematic representation of a hematite-based photo-electrochemical cell illustrates its functioning during water photooxidation Upon light absorption, electrons are excited to the conduction band, while the valence band vacancies facilitate oxygen evolution Subsequently, these conduction band electrons flow through an external circuit to the cathode, where they reduce protons to produce hydrogen gas The electrolyte used can be an aqueous sodium hydroxide solution with a pH of approximately 13.
The hematite conduction band level is inadequate for proton reduction to hydrogen, necessitating an overvoltage for photooxidation to commence This overvoltage can be provided by connecting a photovoltaic cell in series with the photooxidation cell Additionally, hematite faces limitations such as a low absorption coefficient, which requires thick films (400–500 nm) for complete light absorption, and low conductivity of the primary charge carriers.
A schematic representation of a photoelectrochemical cell designed for water oxidation is illustrated, featuring a hematite photoanode The diagram highlights the energy levels of the semiconductor alongside the water oxidation and hydrogen ion reduction reactions, all referenced to the Normal Hydrogen Electrode (NHE) potential This information is adapted from the work of K Sivula et al., which discusses advancements in solar water splitting utilizing alpha-Fe2O3 photoelectrodes.
The oxygen evolution reaction facilitated by hematite is notably slow; however, the introduction of a passivating layer like Al2O3 and a cobalt(II) or amorphous cobalt(II) phosphate layer effectively eliminates surface states and enhances the reaction, reducing the overvoltage to approximately 0.1–0.3 V Additionally, optimizing morphology and incorporating dopants such as silicon significantly enhance light absorption and improve the conductivity of the electrodes Hematite films that are doped with silicon and exhibit a columnar morphology demonstrate these advancements effectively.
Recent advancements in "cauliflower-shaped" nanostructured columns have achieved the highest solar light conversion efficiency into hydrogen at 2.1% A micrograph of a hematite photoanode with this morphology reveals a photocurrent density of 2.3 mA cm−2 at the water oxidation potential of 1.23 V, marking a significant milestone Surface functionalization with cobalt (co(OH)2) boosts the photocurrent to 2.7 mA cm−2 and improves the initiation of photooxidation by approximately 80 mV Studies indicate that nanostructured films of pure hematite can be highly effective if they possess a preferential crystallographic orientation for enhanced charge transport Furthermore, the integration of coordination compounds based on ruthenium, cobalt, and nickel can sensitize semiconductor electrodes and act as cocatalysts for the oxygen evolution reaction, paving the way for innovative methods to enhance the conversion efficiency of hematite-based cells.
Ru II (Mebimpy)(oH 2 )] 4+ is a complex designed to mimic the biological process of photosynthesis, featuring distinct modules for various functions The chromophore, which is directly bonded to the electrode, plays a crucial role in sensitization by absorbing light and injecting electrons into the semiconductor These electrons are subsequently transferred to the external circuit, reaching the cathode Additionally, the catalytic center, located further from the electrode, facilitates the oxidation of water molecules into O2 and H+, with the electrons generated in this reaction being redirected to the chromophore.
The 2,2′-bipyrazine unit facilitates electron transfer to the cathode for the reduction reaction 2H⁺ + 2e⁻ → H₂, demonstrating stability in electrochemical studies for up to 28,000 cycles without deterioration However, prolonged operation under oxidizing conditions leads to the degradation of organic ligands, limiting these systems' competitiveness for large-scale applications, though they allow controlled implementation of certain photosynthesis reactions Cobaloxime, a cobalt(III)-based complex, serves as an effective catalyst for hydrogen evolution, particularly in acidic and organic environments; however, its stability remains low when immobilized on substrates for photoelectrodes Currently, only a few photoanodes utilizing cobalt complexes have been developed for water splitting, with one promising design incorporating an ITO electrode sensitized with perylene and layered with cobalt phthalocyanine, functioning in conjunction with platinum.
A cross-section of a hematite film, grown through chemical vapor deposition on a fluorine-doped tin oxide (FTO) substrate, demonstrates its structure and functionality The J × V curves illustrate the performance of the hematite film both in darkness and under simulated illumination (AM1.5), highlighting the effects of cobalt modification This photoanode exhibits the capability to efficiently split water at pH 11 when exposed to visible light, achieving a notable voltage of 0.4 V (Ag/AgCl) and a remarkable yield of 3500 cycles per hour.
Conclusions and Perspectives
Solar cells and photoelectrochemical cells present viable solutions to meet global energy demands while addressing the environmental issues linked to fossil fuels To effectively implement these scientific advancements, coordinated efforts among the scientific community, industry, and government are essential for the benefit of society.
The conversion of solar light into usable energy relies on a sequence of rapid processes, including light absorption, charge transfer, separation, and transport at the atomic and molecular levels Nanotechnology plays a crucial role in this field, as nanomaterials serve as building blocks for creating advanced systems with exceptional capabilities for solar energy capture and charge transport The emergence of various semiconductors, both organic and inorganic, along with unique properties at the nanoscale, enhances the efficiency of third-generation solar cells, such as organic solar cells (OSCs) and dye-sensitized solar cells (DSSCs) Additionally, the development of flexible and potentially disposable devices made from cost-effective materials and simpler manufacturing methods presents exciting opportunities for the future of solar energy technology.
The complex structure of [(4,4′-((HO) 2 P(O)CH 2 ) 2 bpy) 2 Ru II (bpm)Ru II (Mebimpy)(OH 2 )] 4+ has been immobilized on an electrode, as detailed in research by J.J Concepcion et al Despite the exclusive features of new solar cells, there is a need for materials and devices with improved efficiency and stability for market introduction Historically, crystalline silicon solar cells were deemed impractical due to high costs, yet their ideal photovoltaic properties have led to their widespread availability A similar trajectory is anticipated for third-generation solar cells In the realm of artificial photosynthesis and solar fuel production, early-stage developments show promise, particularly with advances in third-generation cells and the photochemistry of coordination compounds and semiconductors While conversion efficiencies remain modest, encouraging breakthroughs with hematite photoelectrodes, a low-cost and abundant material, are noteworthy Additionally, molecular photocatalytic systems offer a sophisticated platform for studying biological processes, paving the way for the creation of more efficient energy conversion devices.
DSSCs dye-sensitized solar cells
FTO Fluorine-doped tin oxide
HOMO Highest occupied molecular orbital
IPCE Incident photon to current efficiency
ITO Indium-doped tin oxide
J sc short-circuit current density
LUMO lowest occupied molecular orbital
MDMO-PPV poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]
MLCT Metal-to-ligand charge transfer
PC61BM [6,6]-phenyl-c61-butyric acid methyl ester
PC71BM 2, [6,6]-phenyl-c71-butyric acid methyl ester
PEDOT:PSS poly(3,4-ethylenedioxytiophene)-poly(styrenesulfonic acid)
Pout power produced per illuminated area
PSS poly(styrenesulfonic acid) spiro-OMeTAD N2,N2,N2′,N2′,N7,N7,N7′,N7′-octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-
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Copyright © 2017 Elsevier Inc All rights reserved.
DEPARTMENT OF ELECTRONIC SYSTEMS ENGINEERING, POLYTECHNIC SCHOOL OF THE USP, SÃO PAULO, BRAZIL
2.1 Organic Materials for Nanoelectronics: Insulators and Conductors 35 2.1.1 Techniques for Making Organic Films 38
2.2 Process of Charge Transport in Organic Devices 40
2.3 Organic Thin Film Transistors 42 2.3.1 Structure of the TFT 42 2.3.2 Modeling of the Characteristic Curves 44
2.4 Organic Light–Emitting Diodes 56 2.4.1 Structure of Thin Films in OLEDs and Typical Materials Used 56 2.4.2 Electrooptic Characterization of OLEDs 57
Organic materials suitable for electronic applications include small molecules like oligomers and carbon-based polymers Carbon, with six electrons arranged as 1s² 2s² 2p², exhibits four valence electrons and a moderate electronegativity, favoring covalent bonding through electron sharing The sp² hybridization typical of organic semiconductors is exemplified by ethylene (or ethene), where each carbon atom forms σ bonds in the plane of carbon and hydrogen atoms, while π bonds exist above and below this plane, contributing to a highly delocalized electronic structure.
π-Conjugated molecules are defined by their alternating single and double bonds, as illustrated in Fig 2.1B–E These molecules feature overlapping pz atomic orbitals that form molecular orbitals known as ligands (π) and antiligands (π*) The electrons in the ligand orbital, referred to as the highest occupied molecular orbital (HOMO), possess lower energy, making them susceptible to excitation due to the typically weaker π bonds This excitation allows electrons to transition to a higher energy level, known as the lowest unoccupied molecular orbital (LUMO), facilitated by the delocalization of π orbitals across neighboring atoms The significant energy gap between the HOMO and LUMO levels exceeds the thermal energy (kBT), where kB is Boltzmann’s constant.
The temperature (T) influences the formation of a forbidden band, or bandgap, in materials In this context, the highest occupied molecular orbital (HOMO) can be compared to the valence band (VB), while the lowest unoccupied molecular orbital (LUMO) corresponds to the conduction band (CB) in inorganic semiconductors or insulators Conjugated polymers can display electrical conductivity characteristics that range between those of insulators and conductors, depending on chemical modifications and the level of doping The concept of utilizing polymers for their conductive properties has emerged relatively recently.
In 1977, Shirakawa and colleagues discovered that trans-polyacetylene, when doped with iodine, demonstrated a remarkable conductivity of 10³ S cm⁻¹ This groundbreaking finding led to the Nobel Prize in Chemistry being awarded to Heeger, McDiarmid, and Shirakawa for their contributions to the field.
Since the year 2000, there has been a growing interest in synthesizing various organic materials with unique properties, particularly polymers featuring a π-electron structure Notable examples include polyaniline (pAni), polypyrrole (ppy), polythiophene (pT), polyfuran (pFu), poly(p-phenylene) (ppp), and polycarbazole (pcz), all of which have been synthesized and evaluated for their performance in electronic devices.
FIGURE 2.2 Chemical structure of monomers from the principal semiconducting polymers (A) Polyaniline (PAni), (B) polypyrrole (PPy), (C) polythiophene (PT), (D) polyfuran (PFu), (E) poly(p-phenylene) (PPP), and (F) polycarbazole (PCz).
Carbon sp² hybridization is illustrated in the ethylene molecule, with organic materials categorized into small molecules like pentacene and fullerene, and polymers such as trans- and cis-polyacetylene Notably, pentacene and fullerene exhibit electron mobilities exceeding those of hydrogenated amorphous silicon, reaching approximately 6 cm² V⁻¹ s⁻¹ While these values are lower than the 100 cm² V⁻¹ s⁻¹ seen in monocrystalline silicon, they are adequate for the production of electronic devices.
Organic semiconductors offer a significant advantage over inorganic semiconductors due to their ability to synthesize materials by modulating bandgap and energy levels, resulting in diverse optical properties and photon absorption/emission capabilities across various wavelengths These materials are foundational for devices like light-emitting diodes (LEDs) and organic solar cells (OSCs), making them highly suitable for applications in mobile devices, large-scale screens, ambient lighting, and flexible, transparent displays Additionally, organic materials can be utilized in passive devices such as resistors, capacitors, and diodes, as well as in organic thin film transistors (OTFTs), which are essential for developing electronic circuits.
Field effect transistors (FETs) serve multiple functions, including logic gates, processors, and memory systems The integration of organic thin-film transistors (OTFTs) in radio frequency identification (RFID) devices is set to transform automatic identification systems across commercial and industrial sectors Additionally, polymeric electronic memories are being explored as a viable alternative due to their compatibility with polymeric electronic circuits and potential for miniaturization at the nanoscale.