A lot of study was done after high-efficiency perovskite solar cell materials were introduced in solid-state devices in 2012. The efficiency went from 10% in 2012 to over 23% in 2018, which is the verified solar-to-electricity power conversion efficiency (PCE). This progress was made possible by earlier work on dye-sensitized, organic, and thin film solar cells, as well as the special visual and physical features of perovskite solar cell materials.
Time resolved optical spectroscopy methods can look at the optical signatures of these things and how they change over time on time scales ranging from femtoseconds to seconds. Photo-induced time resolved optical spectroscopy works by shining light on a sample to start certain reactions and then looking at how these reactions change the material’s absorption or emission spectra. For instance, in a contact-free perovskite film, the rate of electron and hole recombination can be guessed from the time resolved photoluminescence (PL) spectrum. This is done by keeping track of the material’s emission at wavelengths close to the band edge after photoexcitation.
When there is a perovskite film on a contact material, the same measurement will rely on how the PL is quenched by carriers moving out of the film and back contact recombination. It is possible to guess and study how these visual fingerprints change over time by looking at how charge carriers move and what kind of heterojunction contact material is used.
In order to fully understand photovoltaic processes, it is necessary to carefully plan the setup, use the right amount of perturbing light for the solar cell to work, be in the right time frame, and think about and explain any interference that might happen. Studies and descriptions of solar cell devices, especially those that use dyes to detect light, have been done using these methods for a long time.
Fundamental processes within the perovskite film
The way solar cells work during photovoltaic action is affected by basic processes in the perovskite film. These include the film’s ability to absorb incoming photons, heat up charge carriers, create free charge carriers and separate charges, move charges towards contacts, and pick out electrons and holes at charge selective contacts. An external quantum efficiency (EQE) can be used to describe these processes. Where LHE is the light harvesting efficiency, λ is the frequency, βsep and βtrans are the quantum efficiency of charge separation and movement, and Πcoll is the quantum efficiency of charge gathering.
While the device is photovoltaic, it goes through a lot of unwanted processes. These include bound charge carriers or excitons and the recombination of charges. Other processes include trapping of the charges, flaw migration, and band filling effects. The device’s working state is set by how these effects interact with each other. This has a direct connection to the quasi-Fermi levels and, by extension, the device’s open circuit voltage.
In a solar cell, the photovoltage is the difference between the quasi-Fermi levels for electrons and holes when different electrodes are lit up. It is less than the difference in energy between the band ends in the photo-absorber. This energy loss between the band gap and the photovoltage (loss) can be seen as the cost of not having an electric field between the holes and electrons, as well as energy loss from materials that aren’t perfectly made. This loss can be shown in Eq. (6.2), where φrec is the ratio of the non-radiative recombination rates to the radiative recombination rates. Voc is the open circuit voltage, Eg is the band gap, T is the temperature, c is the speed of light in vacuum, h is Planck’s constant, n is the refractive index, jgen is the rate of photon absorption in the AM1.5 spectrum, α is the absorption coefficient, Πrec is the ratio of the non-radiative recombination rates to the radiative recombination rates, and L is the minority carrier diffusion length.
Processes at open circuit condition
The rate at which electrons are created (G) and recombined (R) in perovskite solar cells is the same. This changes the steady state density of electrons and holes at the valence band (VB) and conduction band (CB), which makes the photovoltage higher. The rate of production is affected by the light-absorbing layer’s steady-state absorption spectrum, the number of empty states at the top VB, the transition chance, and the optical transition dipole strength. What these factors are?
Eq. 6.4 shows the relationship between the open circuit voltage of a device and the number of charge carriers present. Here, e is the basic charge and Nc/v is the total number of charge carriers that are available. The loss of the effective number of charge carriers through recombination can be broken down into interface recombination, bulk recombination, and trap assisted recombination. These can all be further broken down into radiative and non-radiative recombination, with their ratios shown in Eq. 6.2.
Recombination rates for perovskite solar cells depend on the number density, energy states, and position of traps in the film. When you do photoinduced absorption spectroscopy, you should keep in mind that the reaction to light depends on how bright the light is and how long the film has been exposed to it. Radiant recombination can be reused to create charge carriers, but non-radiative recombination through subband gap states makes the device less efficient than what would be expected from theory. An average recombination rate (k) that is inversely related to carrier life time (ϱ) can be used to describe how photogenerated carriers change over time.
Processes at short circuit condition
The short circuit current density (Jsc) at a certain wavelength λ is directly related to the light harvesting efficiency (LHE) and the charge collection efficiency (Φcol). These are found by looking at how charges move within layers and how they move at surfaces. τtrans and τrec show the recombination duration and transfer time of charge carriers, respectively. LHE is based on the light absorber’s absorption spectrum (A(λ)). The electron-phonon interactions greatly affect the transport time in a single crystal of MAPbI3 perovskite. The way the device reacts to a short circuit can depend on how the film has been illuminated in the past, and this can be talked about in terms of trap generation and destruction, contact with polarisability domains, and photo-induced trap movement. Short circuit current levels in world record mixed perovskite solar cells are already close to the theoretical maximum with close to 100% internal quantum efficiencies. This is because they are very good at absorbing light and conducting charge.
Devices under working conditions
The diode equation tells us how to figure out the current-voltage relationship for solar cell devices. It takes into account the whole device’s resistance (R), which is made up of series, ionic movement, transport, and shunt resistances. The fill factor (FF) and how well the device converts power are affected by the slope of the current voltage graph. The motion directly affects the transport resistance in the perovskite film. On the other hand, the shunt resistance is based on the lives of the electrons and the different ways they recombine.
When these processes are running, they are affected by many things, including flaws moving around, catching electrons, and band filling effects. Table 6.1 shows the time scales for some important events that happen in perovskite solar cells. These events include instantaneous excitation, exciton breakdown, band filling, recombination, transport, charge transfer, the sharp effect, thermalisation, and electron-phonon scattering time.
Characterisation methods for perovskite solar cell materials are very important for finding the right factors for improving both the materials and the devices. This part is mostly about the MAPbI3 hybrid perovskite, which is one of the perovskite materials that has been studied the most. The method would be the same for high-efficiency perovskite materials that are mixed with cations and halides.
Light absorption and charge separation kinetics
Any photons that come in with more energy than the band gap can move electrons from the valence band (VB) to the conduction band (CB), creating a hole in the valence band. Excited electrons heat up and cool down to the lowest empty states in the CB (VB). They then give off more energy as phonons that reach the lattice. For electrons to be excited in materials with indirect band gaps, energy and motion must be kept the same. This means that less light can pass through them than in materials with direct band gaps.
There are a lot of different band gaps for metal halogen-based perovskite solar cell materials. They range from less than 1.2 eV (for CH3NH3Sn0.5Pb0.5I3) [18] to more than 4 eV (for CH3NH3BaI3) [19]. The gap is being tuned by metal, monovalent cation, anion exchange, and a mixed method, which is still being studied. The band gap and absorption coefficient are very important for the effectiveness of the device in both stand-alone and paired uses. They affect the colour, clarity, efficiency per mass, and cost of the material.
The main way that MAPbI3 changes from VB to CB is through a charge transfer from a halide to a metal. In blue and UV light, density functional theory calculations show that there is also a charge transfer from iodide localised electrons to the organic cation [20]. In this case, an artificial network with a negative charge and an organic molecule with a positive charge can move towards a state with lower charges. This can cause CH3NH2 to form along with a free H1, which can then couple to I2 and spread more easily through the material as HI.
It heats up in different ways at different wavelengths of light (near band edge 760 nm and blue light 420 nm, for example). Researchers have recently looked into how the excitation wavelength affects the movement of ions in perovskite devices. They found that the amount of released phonons using near bandgap light (red light) and light absorbing deeper in the band structure (blue light) affects the current voltage behaviour and hysteresis of the device.
The newly created pairs of electrons and holes could be drawn together by Columbic forces, which would cause excitons to form. The lead perovskite’s high dielectric constant is one of the main features that causes high dielectric screening of Columbic forces. At room temperature, thermal energy can overcome these forces, which is good for the device’s high solar efficiency.
Excitonic fingerprint wavelengths are close to the edge of the band and generally show up in the absorption spectrum as sharp peaks close to the edge of the band. Many different things leave their visual marks near the band edge, so there is no easy and direct way to find excitonic peaks in the UV-vis spectrum at room temperature. There are several ways to figure out the exciton binding energy, such as applying Elliot’s time-resolved photo-induced optical spectroscopy theory to the band edge curve, thermally driven PL quenching, or thermal widening of the absorption start. Exciton dynamics are more important in materials with higher excitons binding energy, like 2D perovskites and lead perovskite based on bromide.
Charge recombination, transfer and transport kinetics
Researchers have used photoluminescence spectra and the way photon recombination peaks change over time to learn a lot about how charges move around in perovskite solar cell materials. As the lifetime and transport time of electrons in the film are only a few microseconds, and ions can move around, it’s important to be careful when using methods from new generation solar cells with longer electronic life times, like dye sensitised and polymer solar cell technologies. Electron-hole recombination is usually broken down into three types: trap supported (linear term), free charge carrier (second order), and Auger (third order).
Density functional theory predictions and experiments have been used to figure out what kind of traps are in MAPbI3. The results show that there aren’t any deep traps in the bandgap. The reactions that lead to deep traps don’t seem to have a big effect on the performance of high-efficiency solar devices made from MAPbI3 and mixed perovskites. For the MAPbI3 perovskite, defect levels of about 109 to 1014 have been found. In polycrystalline materials and perovskites made from other metals, on the other hand, the trap density can be much greater, which could control the recombination rates.
The time-dependent PL spectra in a thin film are mostly caused by the exchange of free electrons and holes when there isn’t a charge-selective layer present. In the presence of a charge selective layer, electron-hole coupling and the movement and collection of charge carriers towards the contacts are in competition with each other. The Pl peak’s fading time to 1/e of its starting value gives a good idea of the lifetimes of the associated particles, which can be used to guess the diffusion length of carriers in the film.
There may be phase segregation in mixed perovskite solar cells, especially if a lot of halogens are used that have very different lead-halogen bond lengths. This can change how the PL spectrum changes over time. These days, the main perovskite PL spectrum’s time-dependent decay can’t just be attributed to electron-hole recombination. The change in the spectra can also be linked to a phase splitting process.
A full study about how temperature affects mono-, bi-, and Auger recombination was published in [28]. It used instantaneous PL readings at different excitation fluencies and pump-probe spectroscopy to look at the data and fit it to Eq. 6.6. Ultrafast transient absorption spectroscopy was used to study how charges move through MAPbI3 films with different scaffolds and between normal TiO2 and spiroOMeTAD charge selective layers.
Stark effects, defects and defect migration in perovskite solar cells
Stark effects
Changes in photo-induced near band edge optical absorption tell us a lot about the local electric field and the features of the material. This helps us understand how different processes happen inside the device after it is excited. The band filling effect (Burstein-Moss) is one that happens close to the band edge. It bleaches the photo-induced absorption spectrum by making the optical band gap bigger by filling up states with electrons and holes. These effects depend on how bright the light is and can last from femtoseconds to nanoseconds. They can also overlap with features of the Stark effect on frequency scales from GHz to THz. Kinetics of band filling effects can be used to study how charges build up and recombine in the perovskite film, as well as how bands change.
The optical Stark effect is the change in wavelength that happens when there is an electric field present. This effect can be caused by light or by electric fields applied from outside the system. We can look at the effect by looking at the small frequency shift v of a certain optical transition caused by the electric field E. This shift is linked to the change in the dipole moment between the ground-state and the excited-state and the change in polarisability. The change in absorption (A) that was observed in the experiment is related to the electric field (E).
De Angelis and his colleagues were the first to report seeing a Stark effect in perovskite solar cells in 2014. The perovskite solar cell materials’ photo-induced absorption spectra (PIA spectra) have been found to work like the Stark effect. PIA lets you change the system under lighting with small or large amplitude pulsed lights. You can then measure how the material absorbs light when the illumination is on and off, and finally you can get a delta absorption spectrum.
The monitor in the PIA setup is very accurate and sensitive, which lets us measure changes in absorption up to 1027 times. The PIA Stark spectrum is very sensitive and can find unique near-band edge fingerprints of the perovskite material. This means that it can be used to study how phases separate when light shines on mixed anion perovskite films.
On the other hand, the changed electric fields can be used outside of the photo-induced electric fields as well. In this case, the same PIA setup can be used without changing the excitations. Instead, an electric field can be added to the sample, turning the method into electro-adsorption spectroscopy.
A study by Wu et al. looked into the second harmonic electro-reflectance spectra of MAPbI3 and FAPbI3.
Dielectric relaxation
During photovoltaic action, perovskite solar cell materials go through structural changes. These changes are energetically positive and include ionic movement, trap formation, destruction, reorientation of dipolar cations, and changes at the interface. These changes can slow things down or make them work better. For instance, tilting metal halogen octahedra can change how much light they absorb, and turning dipolar cations around can change the band gap and the movement of charge carriers. The movement of ions changes the way ions are distributed in the material, which in turn changes how charges combine locally and how stable the device is. Stark spectroscopy can be used to study these changes in structure while the solar system is working. The Stark effects in MAPbI3 have mostly been used to study the charges at the surfaces of dye-sensitized solar cells and the electric fields that form when the structures change. The material’s dielectric response is based on the dipole moment of the single-valent cation. This can block local electric fields after photoexcitation, which makes the Stark effect weaker. This happens because of the one-valent cation’s rapid dielectric response and the way the PbI6 octahedra in perovskite films are distorted in some places.
Relevance to defects
In perovskite solar cell materials and systems, defect generation, ion migration, and photo-induced defect migration are all very important. They change the effects of current voltage feedback, how well photovoltaics work, how stable they are, and basic photophysical processes. In the photoluminescence (PL) or absorption spectrum, defects can leave visual marks that can change photo-induced Stark effects and charge recombination in a roundabout way. Light can make flaws that can be fixed in the dark, creating flaws that are actively made and healed. Getting rid of flaws after illumination depends on how much air is available for the film during lighting. Defects are very important to the solar action of the device because they determine the highest power and efficiency. They can move around inside the film, which can lead to things like current-voltage feedback, and build up at the edges, which can cause degradation. For gadget stability, it’s good to have the right kind of covering or thin, solid layers in between, like two-dimensional perovskites. More research needs to be done to figure out how flaw movement affects the security and operation of a device in the workplace. Photo-induced absorption spectroscopy studies can tell us about changes in the local field and have been used to look into the effects of heating, dipolar response, ion and flaw movement in materials used in perovskite solar cells.
Comparison with other solar cell technologies
It is possible to use stark spectroscopy to learn about many different types of solar cell materials, such as perovskite solar cells, dye-sensitized solar cells, organic solar cells, and quantum-confined systems. But it’s hard to make comparisons because the time factors and sources of spectral changes in different material classes are not the same. When light hits dye-sensitized solar cells, photo-injected electrons and counter ions from the solution create electric fields that are perpendicular to the dye. This makes it possible to study the Helmholtz layer capacitance and dye saturation in microseconds to milliseconds. This range of time constants in perovskite solar cells is from nanoseconds to several seconds and is linked to octahedral bending and ion movement. There are also stark traits that can add to the photo-induced absorption range. In organic solar cells, Stark spectroscopy describes the electric fields that form when charges separate at the surfaces using very fast, time-resolved optical spectroscopy in the fs range. Stark spectroscopy is a useful tool that hasn’t been fully studied yet for perovskite solar cell materials, so it needs to be looked into more.
Electron-phonon interactions and polarons in CH3NH3PbI3 perovskites
During thermalisation, charge carriers (hot carriers) cool down to the CB and VB edges by giving the structure extra energy through interactions between electrons (holes) and phonons. For many matter systems, the time scale that matters is sub or a few ps. It takes longer for carriers to cool down in perovskite films than in silicon and CIGS films. This is because lead-halide perovskites naturally contain heavier elements. Researchers have used photo-induced absorption spectroscopy to find out how long it takes for carriers to cool down in flat MAPbI3 films. These films cool down one order of magnitude more slowly than GaAs films. If you record the time resolved transient absorption spectrum or PL spectra at various temperatures, you can learn about how the carriers cool down over time.
Frost et al. have shown that this type of photoexcitation likely involves a shift of energy to big polaronic states through electron (hole)-phonon interactions. It was found that the rate of thermalisation cooling for MAPbI3 films is 78 meVps21 by fitting the data to the Fro¨hlisch model. These cooling rates are important for hot-carrier solar cells that can possibly go beyond the Schockley-Queisser limit in certain photovoltaic conditions.
A lot of people have used the Drude model to explain how motion works in perovskite solar cells. Based on the Drude model, π 5 ne2τm} can be used to describe the carriers’ static conductivity (π). The charge carrier mobilities for perovskite solar cell materials are thought to be around 10 cm2 V21 s-21. This is higher than the numbers for CIGS and amorphous Si solar cells, which are usually around 0.5 and 1 cm2 V21 s-21, respectively.
The way electrons interact with polaron-based lattice distortions can be a big part of how perovskite solar cells work as photovoltaic solar cells. This is shown in part by the effects on the electronic current, where defects and defect migration have been studied in terms of device hysteresis, ionic movement dependent electron hole recombination, and the rate at which the current decays.
Summary and outlook
This study talks about the basic ideas behind photo-induced time resolved optical spectroscopy in hybrid perovskite solar cells. It focusses on the basic processes and how they affect the performance of the photovoltaic system. It is possible to learn a lot about these processes by looking at the spectrum reaction before and after received photon energies are changed into charge carriers. The main basic processes in perovskite solar cells can take anywhere from a few femtoseconds to several seconds, and they depend a lot on the chemical make-up and crystal quality of the material. Some things that only happen in perovskite solar cells are local phase segregation, photo-induced trap formation, ion movement, and carrier dynamics that rely on the amount of light that hits the cell. These things need extra attention when characterising the device. The study also shows how important photo-induced absorption spectroscopy is as a Stark spectroscopy that looks at how changes in local electric fields affect absorption. More research into how carrier cooling works and how electrons and phonons interact can help us understand basic ideas, usable mobilities, and PCEs for real-world gadget uses. The report also talks about how defects form and change when light hits them, which is very important for improving devices and making perovskite solar cells available to the public in the future.
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