Photovoltaic devices can be described using photovoltage and photocurrent transients, which tell us a lot about their features, like how long charge carriers last. For these methods, you use pulsed light to mess with the gadget and record the photovoltage or photocurrent in either the time or frequency domain. This is especially helpful for dye-sensitized solar cells (DSCs) and perovskite solar cells (PSCs) that change their qualities depending on the amount of light they receive.
As the light strength changes in DSCs, the photocurrent and photovoltage transients show a nonlinear reaction of electron transport and recombination. Small-modulation methods are the best way to study these processes because they keep the amounts of charge carriers fixed and keep them in trap states while measurements are being taken. When the modulation amplitude changes, the reaction amplitude should change linearly with it. Time constants, on the other hand, should not change with the modulation amplitude.
Perovskite solar cells are complicated systems that can conduct both electricity and ions. The selective contacts can be flat or mesoporous, and the performance can change depending on how they are used. Small-modulation transient photocurrent and photovoltage methods can also be used for PSCs to learn about how charges move, build up, and mix again. PSCs can also give us useful information when we use large modulation transient methods and turn off all the lights.
Small modulation transient techniques
Transient photo-voltage technique (TPV)
We use Transient Photo-Voltage (TPV), a method that lets us see how carriers recombine in solar cells over time, to study dye-sensitized and organic solar cells. It works by using a quick and small change in the incoming light to excite the device. This changes the open-circuit voltage (VOC) of the solar cell, which is directly linked to a small change in the quasi-Fermi level.
In a TPV, a light source that maintains a steady and stable VOC shines on the solar cell all the time. The solar cell is kept in open circuit, which means that no current can flow through the contacts. It is also linked to an oscilloscope, which can record how the voltage changes over time. Once the VOC is steady, an extra short-lived laser wave is used to excite the solar cell and cause a small change in the VOC. The change in the VOC (ΔV) is related to the photo-generated carriers caused by the laser flash. The solar cell is in open-circuit, so the “extra” photo-generated carriers have to come back together. This registers the transient to the starting VOC.
The Voc decay method, which is also used to measure carrier recombination lifetime, is very different from TPV. The goal of the TPV method is to cause a small change in the solar cell’s Fermi level. The VOC decay, on the other hand, keeps track of the full decay from a certain light strength until the VOC decays to zero. The main difference between the two methods is that TPV decays things in a single exponential way, while VOC decays things in multiple exponential ways because it fits more complicated dynamics.
Transient photo-current decay (TPC) and differential capacitance (DC)
When small-modulation techniques are used to measure charge density in solar cells, differential capacitance analysis (DC) is a useful tool. Before we can measure DC, we need to find the Transient Photo-Current decays (TPC). This will help us figure out how much charge is in the solar cell. Charge Extraction (CE), which is a big modulation tool, is another way to find out the charge density. The DC method is better than the CE method because it can be used even in systems with high recombination rates, where the TPV decay and the CE decay have similar dynamics and it is not possible to get all the charges out.
The current reaction of a solar cell when it is held in short circuit is called TPC decays. It is recorded in the same way that TPV is. The setting for this experiment is a lot like the setup for TPV, but the device is held in short-circuit and hooked up to a small resistor. The laser pulse changes the current in the device, which can be seen on the monitor as a voltage drop across the resistor. This voltage drop is easily turned into a transient current by using Ohm’s law. We measure this short-term current and add it up over time to get the amount of photo-generated charges (Δq) that the pulse causes.
However, if charge exchange is slower than CE, then the outcomes of both DC and CE methods should be the same. The current reaction of a solar cell when it is held in short circuit is called TPC decays. It is recorded in the same way that TPV is. However, this method isn’t always useful because it only works when charge carrier losses at short-circuit situations are very small.
Before starting to use TPC to do Differential Capacitance analysis, you need to make sure of three different test criteria: 1. The short-circuit current in a solar cell must follow a power law (JSC = LI±); 2. The rate of TPC decay must be the same under all light conditions; and 3. The rate of TPC decay must be faster than TPV decay, which means that charge is being collected faster than it is being recombined.
To sum up, TPC and figuring out differential capacitance are different ways to use the CE technique. This is especially useful when CE decays seem to be slower than their TPV counterparts, since TPC is faster than TPV.
Square-wave modulation for photovoltage and photocurrent transients (SW-PVT and SW-PCT)
For quick and easy study of charge movement and recombination in Direct Semiconducting (DSC) solar cells, square-wave (SW) controlled stimulation methods have been created. The frequency-modulated techniques, intensity-modulated photocurrent, and photovoltage methods (IMPS and IMVS), which were created earlier, are not as good as these. Low-cost tools like LEDs for lighting and digital capture boards for recording transients can be used for SW methods.
One time constant is taken from SW-PCT and SW-PVT for most simple DSC systems. This time constant is known as the electron transport time for photocurrent transients and the electron lifetime for photovoltage transients. There are two step reaction functions in the SW response signal that can be looked at individually. The rise and fall times and step sizes should be pretty much the same if the variation is small enough, usually less than 10% of the base light strength.
The photocurrent reaction time in DSCs shows how long it takes for the electron to move from the mesoporous TiO2 to the FTO pin. The speeds found in PSCs show recombination processes, which are different from DSCs because they involve electrons and holes. The time constant that was found can be thought of as the solar cell’s RC time constant. However, in an open circuit, the capacitor can only discharge through recombination.
There was early work on PSCs that used a structure that was a lot like a solid-state DSC. The trends in small modulation SW photocurrent/voltage transients were also very similar. It was found that carrier lifetimes and travel times got shorter as the strength of the base light went up. It’s interesting that the amount of perovskite coverage in the mesoporous structure has a big effect on the time constants that were found. The lowest coverage led to very short carrier lifetimes and slow transport, while the best coverage led to two orders of magnitude longer carrier lifetimes at the same VOC and one order of magnitude shorter carrier transport times.
If you think that the perovskite is a better conductor of electrons than the mesoporous TiO2, you can understand the results for the transfer time. Different tests of carrier mobilities and conductivities prove this to be true. It’s interesting that the movement of carriers seems to follow the same pattern as in DSC. This suggests that in PSC with mesoporous TiO2 contact, the electrons in the mesoporous TiO2 are charged to balance out the charge of the perovskite layer, either by ions or holes.
As shown in a groundbreaking study by Snaith and colleagues, things change when the mesoporous TiO2 is taken away or changed with a different mesoporous material that doesn’t react with anything. We discovered that carrier transport was faster when mesoporous TiO2 wasn’t present. This shows that perovskite is a good conductor of electrons.
A study on perovskite solar cells discovered that the short-term reaction is almost always two-phase. Using fast modulation (2 kHz), the rise and fall of these cells are absolutely monoexponential. When slow modulation (2 Hz) is used, on the other hand, 60% of the rise/fall intensity comes from the fast process and 40% comes from the slow process. This number stays the same no matter how bright the bias light is. The fast process speeds up as the light intensity and VOC go up, but the slow process doesn’t seem to change with the light intensity or VOC. The fast process is probably connected to the exchange of “free” charge carriers. The slow process, on the other hand, is probably connected to the movement of ions in the perovskite solar cell. At the brightest light levels, the fast time constant seems to stay the same, which could be because the instrument’s reaction is limited. The study also looked into photovoltage and photocurrent transient methods for perovskite solar cell materials, mainly FTO/TiO2(dense)/TiO2(meso)/(FAPbI3)0.85(MAPbBr3)0.15/spiroMeOTAD/Au.
Intensity-modulated photocurrent and photovoltage (IMPS and IMVS)
Researchers have used intensity-modulated photocurrent and photovoltage methods (IMPS and IMVS) to look at the surfaces between semiconductors and electrolytes and DSCs. To get the solar cell excited, CW or LED laser light sources with a rippled frequency of 1% to 10% are used. The signal that is made is then looked at by a lock-in amplifier or frequency response analyser that is tuned to the modulation frequency. This gives information about the signal’s intensity and phase shift with respect to the stimulus. A normal experiment tests a lot of different frequencies that are spread out in a logarithmic way.
The solar cell is held in IMPS under potentiostatic conditions, which are usually short-circuit conditions. In IMVS, the solar cell is held under galvanostatic conditions, which are usually open-circuit conditions. The IMPS/IMVS spectra that are made are analysed using a complex mathematical model that is similar to those used in electrochemical impedance spectroscopy (EIS). For a relatively simple response, the frequency (f) where the highest (or lowest) point in the imaginary part of the response can be used to directly find the time constant ϱ.
Several study groups have measured the IMPS and IMVS of perovskite solar cells. There are different types of IMPS spectrums depending on how the device is set up. The high frequency process is the device’s RC-time, and the lower frequency process is charge movement and recombination. IMVS makes it easy to find out the recombination lifetime without having to fit data to a complicated equivalent circuit.
Large modulation techniques
VOC rise and decay
A lot of people use the VOC decline transition to look at how charge carriers recombine in photovoltaics devices. For DSCs, you can figure out the electron lifetime by looking at the slope of the VOC transient. This can be done without light by adding a starting potential and then watching the VOC fade. This method works for PSC devices, but it’s harder to figure out how it works.
The VOC decline transient relation was found for carrier lifetime in a thin layer solar cell. This relation is based on two mobile charge carriers whose amounts are higher than the material’s doping density. The fast part is when the geometry capacitance is discharged across the recombination resistance. The slow part is when the ionic charge relaxes.
Checking the rise and fall of VOC with different amounts of light shows that both are biphasic, with a fast part happening in the microsecond to millisecond range and a slow part happening in the second to minute range. On a semi-logarithmic plot, the slow part of the VOC decline looks almost like a straight line, which means that the dynamics are first order.
What kind of light is used to excite has a big impact on voltage decay transients. For example, blue light excitation causes a lot more of the slow tail in the VOC decay than red light excitation. This impact happens because of the extra energy compared to the bandgap of perovskite, which can cause more ions to move around. But we don’t fully understand this effect or how it changes depending on the perovskite’s make-up yet.
Charge extraction (CE)
The charge extraction (CE) method is a quick and simple way to measure the VOC of a solar cell. It keeps the VOC stable at a certain place on the IV curve. Once the VOC is even, the solar cell is turned into a short-circuit (V = 5.0 V), and the light source is turned off as well. There is a short-circuit current because the solar cell discharges through the contacts when the circuit is closed. It is important for VOC stabilisation to be high in perovskite solar cells because of the ion movement process.
In the CE setup, the solar cell is put in front of a group of white LEDs. The light can be changed to different levels to match the VOC that is used to track the TPV decays. The VOC stabilisation is an important factor in perovskite solar cells because of the ion movement process. Often, it takes tens of seconds for some devices to reach this level of stability. There is a short circuit through a circuit with a small resistance (usually 50 Ohms), and the LEDs are turned off. The analyser is also hooked up to the solar cell so that the voltage change can be seen over time.
By integrating the transient voltage over time and using Ohm’s rule, the voltage drop can be turned into charge. It’s important to note that CE takes out all the charge that is on the solar cell at a certain voltage, including ionic, carrier, and geometrical charge. It is important that the charge extraction happens faster than the carrier recombination so that no charge is lost before the cell short-circuits.
The CE method and the DC method don’t give the same results for perovskite solar cells. The CE method gives much higher charge values at the same voltage for the solar cell than the DC method. The charge difference between the two methods is because of the different ways that the charges can be extracted, which is caused by the ionic migration process happening at the perovskite in operando conditions.
It’s likely that the CE experiment gets rid of both the photo-generated carriers (electrons and holes) and the charges that go along with the movement of ions when the solar cell is exposed to light until the VOC balances. This doesn’t mean, though, that the CE can be used to measure how the ions in the perovskite solar cell change positions when the voltage goes from open-circuit to short-circuit. It shows how important it is to find ways to separate the different charges that are present in perovskite solar cells when they are working, paying attention to their nature, such as how they are connected to the movement of ions, the shape of the charge, or the effect of light-induced carrier formation (electrons and holes).
Current interrupt voltage (CIV)
The current interrupt method is like charge extraction in that it takes energy from a solar cell device. Boschloo and his colleagues created it so that DSCs could find out about the fermilevel of mesoporous TiO2 when there is a short circuit. For the first time, O’Regan et al. used this method on perovskite solar cells. In the monitoring process, the device is first lit up when there is a short circuit and then turned off when there is an open circuit. As the charges that are still there move towards selected electrodes, they create a potential that quickly disappears because the charges are recombined. In dye-sensitized solar cells, the highest potential found is a rough indicator of how the quasi-fermi level splits in the device when it is illuminated by SC. This picture (Fig. 7.21), which shows CIV for an inverted-structure perovskite solar cell, shows the highest voltage that was measured a few milliseconds after the switch was made. The current interrupt voltage is thought to come from the photoinduced gradient of ions in the perovskite solar cell, which forms when the cell is lit up and there is a short circuit.
Conclusions
Photovoltage and photocurrent transients are important ways to describe perovskite solar cells because they show how carriers move, build up, and mix with other carriers, as well as how ions move. To study charge recombination, small modulation photovoltage methods and large large modulation VOC decay can be used. Free charge carriers make up fast parts, while ionic flow in the perovskite makes up slow parts. When a cell’s geometric capacitor is discharged, fast time constants show it. On the other hand, slow time constant processes have high activation energies. Photocurrent transients help us learn about how carriers move and how charges build up in the perovskite solar cell.
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