Perovskite Solar Cells: Passivation Techniques
|Written by Mary O'Kane, a PhD student in Perovskite Solar Cells at the University of Sheffield, in collaboration with Ossila Ltd.|
What is Passivation?
Perovskite solar cells (PSCs) have demonstrated impressive device metrics, including open-circuit voltages (Voc) of up to 1.2V1. However, in order for PSCs to achieve their theoretical best efficiencies, all non-essential recombination pathways should be eliminated2. Considering that the defect density of perovskites is quite high3, the amount of non-radiative recombination is remarkably low.
Passivation refers to making a material “passive”. In physical chemistry, this generally refers to ensuring a material has less interaction with its environment. With PSCs, there are two main types of passivation layers (the first type will be the main focus of this article):
- Chemical - which passivate by filling trap states
- Physical - to isolate layers of the PSC from the external environment.10
As perovskites are usually solution-processed, there are many chances for defects – or trap states – to form during their crystallisation4,5. Two types of traps are mentioned in this article: vacancies and interstitials.
Iodide interstitials appear to act as non-radiative recombination centres6 and can reduce the Voc of PSCs. This is an interstitial iodine atom that inserts itself between two of the iodine molecules that form the iodide octahedral (as shown in Figure 1b).
Vacancies occur when the perovskite crystal structure is missing an element (an iodide vacancy is shown in Figure 1c). These defects do not have a huge impact on device performance, especially when compared to other solar cells (such as Cadmium Telluride)3. It is suspected that the band energies of most defects are very close to (or within) the band edges of the perovskite, and so will not have a massive effect on optoelectronic properties.
Migration Through the Perovskite Layer
Vacancies and interstitials can also migrate through the perovskite layer. This migration of defects is thought to be responsible for the hysteresis seen in J-V sweep. It can lead to charge build-up at the interfacial layers – affecting charge transport5. Research shows that the bulk of charge recombination centres (or at least the ones that are most detrimental to device performance) are localised to the surface of perovskite crystals5,7,8. These can act as traps for separated electrons or holes, reducing charge extraction (see Figure 2). By passivating these surface traps, charge-carrier lifetime can be increased and PSC performance can be improved.
How Can You Measure Defect Density?
Ultimately, trap states affect device performance. Therefore, device metrics and J-V curves are a good indication of whether passivation techniques are successfully reducing defect density. High Voc losses are seen when surface recombination is prevalent5. Defect passivation at layer interfaces should show an improvement in Voc 11.
A high defect density can also encourage vacancy-assisted ion migration within the perovskite crystal. This can lead to a build-up of ions at interfacial barriers of the device, which has complex effects on device performance5. One consequence thought to result from this is the hysteresis effect seen in J-V curves. It has been shown that hysteresis can be reduced by passivating vacancies in the perovskite.
One thing to examine when exploring passivation techniques is steady-state or time-resolved photoluminescence (PL) data. From steady-state PL, quenching effects can be seen. PL-quenching refers to a decrease of photoluminescence (as shown in Figure 3). If charge-transport layers are in contact with the perovskite layer, PL-quenching will occur if there is good charge extraction between them12. The passivation of surface defects will lead to an improvement in charge extraction, which results in increased PL-quenching.
Average charge-carrier lifetime can be determined from time-resolved PL (TRPL). With TRPL, perovskite layers are exposed to light until they are fully excited. Then, the photoluminescence is measured over a very short period of time. From this exponential decay, carrier lifetimes can be calculated.
External Radiative Efficiency
A perfect solar cell should also be a good emitter at Voc, as this indicates that there is no occurrence of unwanted recombination. The external radiative efficiency (ERE) is defined as the fraction of dark recombination current that results in light emission2,13. This is essentially used as a measure of non-radiative recombination, with the best-performing cells being at around 1%.
The above methods are all indirect ways of measuring trap-state density. Thermally-stimulated current has been shown to identify electronic trap states directly in perovskite materials.14 First, the device is cooled to very low temperatures, then illuminated to create charge carriers. While being held at these low temperatures, the carriers can relax inside the density of states and occupy trap states. The temperature is then slowly raised, and the carriers are released from their traps and produce current. Shallow traps are released at low temperatures. The deeper the trap, the higher the temperature before it is released.
Where Can Passivation Be Used?
It has been shown that there are noticeable non-recombination losses at interfaces between the charge transport and perovskite layers,15 and at grain boundaries within the perovskite.5 There are many passivation techniques that are being investigated at this time, but a few different approaches are outlined here.
Passivation Between the Electron-Transport Layer and Perovskite
TiO2 is a common electron-transport material (ETL) and has a high trap-state density. These surface traps can be partially passivated by introducing an interlayer (such as Phenyl-C61-butyric acid methyl ester (PCBM)), between the perovskite layer and the ETL.10 This has been shown to reduce defect density in the TiO2, reduce hysteresis effects and make the device more stable under UV radiation.
Another passivation technique on TiO2 is to use a self-assembled monolayer (SAM) of organic molecules. These monolayers usually contain nitrogen atoms, which are thought to form hydrogen-bonds with the methylamine groups of the perovskite,10,12 leading to increased film stability.
Defects at grain boundaries within the perovskite film can encourage trap-assisted recombination. It has been shown that a passivation layer (such as PCBM) placed on top of a perovskite layer can infiltrate the perovskite layer. This can help passivate grain boundaries,9,10 as shown in Figure 4.
Grain boundaries and TiO2 traps10 can been passivated by introducing a layer of PbI2 around the perovskite. The exact process of how this works is still being debated, but it has been suggested that the band misalignment introduced by having a PbI2 passivation layer decreases the likelihood of change recombination.15 Figure 5 illustrates several theories on the mechanisms behind this process. It has been suggested that by introducing PbI2 interlayers between perovskite and TiO2 ETLs, recombination due to trap states is intercepted. It has also been suggested that band misalignment introduced by having a PbI2 passivation layer decreases the likelihood of change recombination between the perovskite and hole-transport layer (HTL).10,16
It has also been suggested that excess methylammonium iodide (MAI) could self-assemble into a passivating layer around the perovskite if PbI2 is deficient in the solutions.10 The use of excess MAI can produce perovskites with higher Voc and fill factor. Along with time-resolved photoluminescence measurements, Son et al concludes that MAI reduces trap states at the grain perovskite grain boundaries.17
Perovskite Film Surface Passivation
It has been suggested that the deposition of a fullerene layer (such as PCBM) on top of the perovskites can reduce trap density by up to two orders of magnitude.17 PL data shows that this layer increases carrier lifetime, and infers that surface recombination is reduced due to the passivation of PbI3 anti-site traps.
Introducing Lewis bases (e.g. iodopentafluorobenzene (IPFP)) on top of the perovskite has also been proven to passivate defects. Under-coordinated halide anions can act as electron traps. Lewis bases bind to these uncoordinated halogen ions – effectively screening the charge, thus reducing recombination.9,18
Reports show that small molecule or polymer interlayers with electron-rich side groups can bond with uncoordinated Pb ions,5 thus reducing defect density. These polymer layers can also protect the perovskite from extrinsic factors (such as moisture or oxygen penetration).10
During Perovskite Deposition
Usually, anything added to the perovskite precursor is considered an additive rather than a passivation technique. However, F-PDI introduced in the solvent quenching phase of perovskite formation has been reported to chelate with Pb2+ at grain boundaries, effectively passivating the perovskite layer as it forms.19
PSCs already show fantastic device performance without requiring much effort to reduce defect density. However, if PSCs are to achieve their full potential, any factors that can lead to non-radiative recombination should be addressed. Defect passivation is therefore an important topic in perovskite research.
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