Abstract
1. Introduction
The simplest solar cell is a sandwich of at least four materials (front electrode, p- and n-semiconductors, and back electrode) with at least five interfaces (three between materials, and front and back surfaces). As solar cells are developed, they will involve more layers and interfaces: for instance, the best Si solar cell consists of p–n junction layers and passivasive interfaces[
Interface engineering has played a significant role in mature photovoltaic technologies (Fig. 1). For example, the Si solar cell with a record efficiency of 26.7% PCE was achieved by extensive interface passivations: an n-type crystalline Si (c-Si) was passivated by p+ heterojunction layer stack (i.e., a-Si:H layer deposition followed by p:a-Si:H layer deposition) from one side and n+ heterojunction layer stack (i.e., a-Si:H layer deposition followed by n-type thin-film Si layer deposition) from the other side[
Figure 1.(Color online) Efficiency evolution of different solar cells. Interface engineering has recently played an increasingly important role in obtaining a higher efficiency for each cell.
For the emerging perovskite photovoltaics, halide perovskite now undergoes rapid advance to a higher efficiency, and a better stability and scalability[
However, it is known that the interface is formed between two heteromaterials and usually spans over only several atomic layers[
2. The role of interfaces for perovskite solar cells
It is known that interface of a perovskite solar cell impacts the performance of the device (e.g., the excitation information, separation, and recombination). Furthermore, the degradation of a device is also highly sensitive to the interface. An experimental point of view showed that interfacial defects in PSCs were relevant with such these key issues, such as instability[
2.1. Oxygen infiltration
Interfaces in PSCs affect their chemical stability[
2.2. Humidity corrosion
Compared to the oxygen penetration at the interface, moisture diffusion along the interface seems to be more detrimental. A great deal of research has reported performance degradation due to interface-related moisture corrosion[
2.3. Electronic behavior
As a key part of multilayer optoelectronic devices, the interfaces in perovskite solar cells mainly play a role in charge transfer extraction. An efficiency gap may result from nonradiative recombination (Fig. 2), energy mismatch and optical losses at the interface. During the transfer of charge between perovskite and CTL, a recombination of the carrier would occur. Interface engineering is required to reduce interfacial recombination losses[
Figure 2.(Color online) (a) Perovskite crystal structure, Schottky defect, Frenkel defect and ion migration through interfaces. (b) Schematic illustration of photo-generation and, diffusion and transfer of charges at interfaces, trap-assisted nonradiative recombination (due to intrinsic defects and impurities at interfaces) and back transfer and interface recombination. (c) Energy band alignment of some typical materials used in perovskite solar cells.
For charge transfer extraction, it is necessary to have a suitable energy-level alignment between transport layers and perovskite. This is vital to solar cell parameters, such as photovoltage and fill factor (FF)[
3. Interface characterization
Because the interfaces are ultrathin—probably within several atoms—and are deeply buried inside the device, it is difficult to carry out direct measurements on these parts. However, because of its importance, some characterization methods have been developed. In this part, we focus on some important characterization process that can reflect the morphological, optical, and electrical information of interfaces. We note that this is not an exhaustive list of characterization tools, but purposely covers the most relevant ones that might be useful for direct or indirect investigation of interfaces.
3.1. Morphology and composition characterization
Modern scanning electron microscopes (SEMs) have resolutions down to 1 nm. In SEMs, the fast incoming electrons supply energy to the atomic outer-shell electrons in the specimen, which is sufficient for the atomic electron to be released as a “secondary electron” (SE) whose images mainly show the surface structure (topography) of the specimen[
Figure 3.(Color online) Interface material characterization methods. (a) Cross-section SEM image of PSCs showing excess of PbI2 at interfaces. Reproduced with permission from Ref. [
Although obtained by optical measurements, photoluminescence (PL) mapping of a perovskite film can reflect the morphological and compositional information of the film, especially when the perovskite films contain multi-phases that show different in-situ PL properties. Different PL intensity and PL wavelengths usually indicate different phase distributions. Therefore, this characterization could be used to study the phase and crystallization properties in the hybrid. Zhao et al. studied the mixture of a perovskite solution and PbI2, and proved that the perovskite solution behaved as dispersed seed that showed distinct PL phenomenon in the PbI2 matrix. When fabricating the perovskite film by a typical two-step method, the as-existed perovskite seed benefited the crystallization of perovskite with large size. The entire process can be clearly demonstrated by in-situ PL mapping properties of the films (Fig. 3(b))[
An atomic force microscope (AFM) is a useful instrument with high atomic resolution that can detect the morphological properties of various materials and samples in the nanometer scale. Compared with a conventional microscope, AFM has the advantage of observing the sample surface with high magnification under atmospheric conditions. It can be used to detect the three-dimensional image of a sample’s surface. The roughness calculation, thickness, step width, block diagram and granularity analysis can also be obtained for the 3D morphology image. For example, Methawee Nukunudompanichet al. used different methods to prepare TiO2 electron transport layers and used AFM to characterize the surface roughness. The growth mechanism of perovskite grain on ETLs with different roughness was thus obtained based on the preliminary AFM characterization results at the interfaces (Fig. 3(c))[
A transmission electron microscope (TEM) can be used to observe fine structures smaller than 0.2 μm that cannot be seen under an optical microscope. These structures are called submicroscopic structures or ultrastructures. Because electrons are easily scattered or absorbed by objects, the penetration is low, and the density and thickness of samples will affect the final imaging quality. Therefore, ultrathin perovskite sections with a thickness within several tens of nanometers should be prepared. For interface characterization by TEM, usually cross-sectional sample containing interface part should be prepared and precisely targeted during TEM measurements. Like SEM, TEM is also a direct way to “see” the interface. For example, by high resolution TEM (HRTEM), the interfacial region could be tested to get useful information, such as thickness, location and, if aided by high-angle annular dark-field (HAADF) scanning TEM, the content of a certain atom (Fig. 3(d))[
X-ray photoelectron spectroscopy (XPS) is a very sensitive tool to characterize the composition of a film[
Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is another useful method to measure the chemical distribution of the nanocomposite, the interface degradation and ion migration in the device[
When considering TOF-SIMS, one’s attention should be paid to the choice of tracking makers because some markers like benzene rings are not easy to detect. For example, when proving the existence of surface ultrathin modifiers, we found TOF-SIMS had difficulties in detecting characteristic group of benzene ring in molecules such as Spiro-OMeTAD or polyTPD, while the fluorine atom instead can be easily tracked[
3.2. Optical characterization
From this analysis, it can be seen that the morphology and component of the junction/interface part could be directly investigated by some professional measurements. However, it is not that easy to directly investigate the optoelectronic properties of the interfaces themselves. Optical characterization on the stacked layers usually aims at the influence of the interfaces on the physical properties of an adjacent perovskite layer. In this part, we show some optical characterizations that can reflect the role of interfaces.
PL, including steady state and time resolved PL (TRPL) spectra, is a commonly used method. Typically, PL characterization on perovskite films with buffer layers (contain heterojunction interfaces) can demonstrate the influence of interfaces, crystallization quality (defect density) of the entire perovskite film or the contact region. Buffer layer-induced contact passivation or enhanced crystallization (that means less defect states or grain boundaries) usually enables an enhanced PL intensity and elongated PL lifetime, while a blue shift of the peak is indicative of the decrease in spontaneous nonradiative recombination from the trap states[
Figure 4.(Color online) (a) Steady state and (b) time resolved photoluminescence (PL) spectra of perovskite films with different back contact layers. Reproduced with permission from Ref. [
In our opinion, if a charge (either holes or electrons) acceptor buffer layer with passivasive surface is adopted to form contact with perovskite, then one should pay special attention to the conclusion made from PL and TRPL results because both passivation effect (cause enlarged PL intensity and TRPL lifetime) and charge transfer effect (cause decreased PL intensity and TRPL lifetime) exist in this contact[
Besides the photoluminescence spectrum, transient absorption spectroscopy (TAS) is a powerful technique that allows studying carrier dynamics both in bulk film and at interfaces. The change in light absorption or reflection is recorded by adjusting the time interval between the pump pulse and the probe pulse arriving at the sample. In addition, TAS provides indirect information on contact interfaces by measuring the absorption behaviors of perovskite film itself[
For the interface contact for solar cell application, energy-level alignments at the interface play a critical role in charge dynamics. An ultraviolet photoelectron spectrometer (UPS) is frequently used to evaluate this key parameter. Because the energy of UV excitation light source is low, it can only ionize the valence electrons and valence band electrons in the outer electronic orbit of atoms, and can distinguish the vibrational energy levels of molecules. The penetration/detection depth during UPS measurement is as small as several (typically within 5 nm) nanometers. Therefore, to uncover the band alignment at the interface of two layers, one needs to carry out separate UPS measurement on each layer (Fig. 4(e))[
To enhance optical properties of perovskite films, effective light management is useful for increasing light absorption. An artificial structure with textured surface was thus developed to form contact with perovskite film[
3.3. Electrical characterization
As optoelectronic devices, perovskite solar cells mainly work on charge transfer and extraction at interfaces. Electrical characterization on these processes is of great importance. Compared to the main indirect optical measurements, electrical characterization on perovskite layers with heterojunction interfaces have proven to be more straightforward to get information of interfaces. In this part, we analyze some typical electrical methods that can directly reflect the properties of interfacial contact.
Transient photocurrent (TPC) characterization is a typical process to depict the charge transfer extraction at the perovskite interfaces. In a short circuit condition, the largest photocurrent output will drop after the transient light excitation disappears. The faster the photocurrent drops, the faster and more efficient the charges transfer extraction will be[
Figure 5.(Color online) (a) Transient photocurrent spectra of perovskite films with different contact thin layers. Reproduced with permission from Ref. [
In the open circuit condition, however, the neat photocurrent output is zero due to complete charge recombination—either via intrinsic electron/hole recombination or via trap state induced nonradiative recombination. In this model, photovoltage is usually tracked to evaluate the interfacial charge recombination because defect induced charge nonradiative recombination is much faster than the intrinsic recombination[
Electrochemical impendence spectrum (EIS) a similar characterization that can also reflect the interfacial charge dynamics at open circuit conditions. By varying the bias voltage applied on the perovskite solar cells, different charge behaviors can be modulated. EIS measurements are commonly adopted in dye-sensitized solar cells to show three distinct charge processes in the solar cells: charge collection by electrode (the fastest, high frequency), the interfacial charge transfer between sensitizer dye and nanoparticles (TiO2) (the modest, middle frequency), and charge transport in the liquid electrolyte (the slowest, low frequency)[
We have noted that, regarding the two typical charge behaviors (i.e., charge transfer separation/extraction and charge transfer recombination), the corresponding electrical characterization has mainly been carried out at either short circuit condition (charge transfer separation/extraction) or open circuit condition (charge transfer recombination). Considering that the electrochemical impendence at zero bias is usually too large to be regularly and precisely determined, some important interfacial information relating to charge transfer extraction might have been ignored in the previous literature. This speculation is yet to be uncovered.
As a facile I–V measurement, space-charge-limited current (SCLC) characterization has usually been adopted to evaluate some new HTLs or ETLs that can enable interfacial passivation except for its basic charge acceptor role[
Considering that the organic/inorganic hybrid perovskite materials is volatile and decomposable under exposure to strong optical, thermal, electrical or humidity stimulation, the optical and electrical measurements mentioned here have not contained very strong optical radiation or large electric fields. However, some characterization with very high space resolution ratio (which usually needs a focused stimulation source, such as an electron beam) can provide detailed measurements on precisely targeted interfaces. The electron beam induced current (EBIC) process was developed to measure the current mapping of cross-section including interfaces because we know that electron beam from sources like SEM chamber has high space resolution ratio[
4. Challenges and outlook
Interface modulation has played an increasingly important role in performance improvement and commercialization of perovskite solar cells, in which characterization of this critical component should be fully and persuasively developed. Our analysis of the presently reported interface-related techniques allows us to summarize several detailed problems that should be given attention when carrying out measurements on interfaces.
It was found that some interface-related conclusions were not persuasive based on inadequate characterizations. A typical example is the PL and TRPL research of the perovskite/buffer layer heterojunction, where improved charge transfer and contact passivation coexist. Additional experiments are needed to make two distinct effects clear: charge transfer caused a decrease in PL intensity and TRPL lifetime, and passivation induced enhancement in these two factors. Some characterization methods (e.g., electrochemical impedance spectrum) originated from other photovoltaic techniques. Therefore, their direct adoption for solid state perovskite thin-film solar cells might cause confusion when analyzing detailed charge dynamics at interfaces because there is no remarkable difference in the speed (frequency) of charge transfer and transport at different parts of a perovskite solar cell. A precise and stable impedance equipment with high time resolution is required to tell different kinetics apart.
Considering the intrinsic chemical and phase stability of perovskite, another challenge is to keep the perovskite phase as it is during various measurement conditions. Usually, perovskite samples would be tested in ambient air with a certain humidity and oxygen that will deteriorate the chemical and phase stability of perovskite materials. Optoelectronic measurements under intense light, high temperature or large electric field would also cause degradation, iron migration or phase variation in perovskite, which would influence the accuracy of measurement results correlated with interfaces.
A direct and more reliable characterization strategy is required to better serve the working mechanism of interface-related modulation. Simultaneous measurements on in-situ morphology together with intrinsic optoelectronic properties seem attractive in revealing interfacial species behaviors at a precise location. This might also require a robust sample preparation process that should present the real interface part while keeping its original properties. In addition, this research field would benefit more from a deeper study on how to characterize some interesting dynamical process when a solar cell is under work, such as electron transfer induced potential variation, field induced trap filling, or even light scattering by nanostructured interfaces.
5. Conclusions
Interface-related characterization techniques have played a major role in understanding the physical and chemical properties and evolution of PSCs. In this review, we have illustrated the importance of interface-characterization methods for PSC research. These techniques allow us to obtain insights of the nano- and microscale details of perovskite surfaces and interfaces. While some characterization techniques indeed provide insightful understanding of the interfaces, new strategic and reliable methods are still needed. This requires a thorough and full understanding of the nature of the interfaces, such as composition, morphology, defects, and stability. We have no doubt that these versatile tools will continue to enable the exploration of questions related to the micro and nano structures in PSCs, bringing these materials beyond the state-of-the-art.
Acknowledgements
This publication is based in part on the work supported by the Science and Technology Development Project of Henan Province (grant no. 202300410048), the Intelligence Introduction Plan of Henan Province in 2021 (CXJD2021008), the Postdoctoral Fund of China (grant no. FJ3050A0670111), the Henan University Fund, and the Canada Research Chairs Supplement Fund and New Frontiers in Research Fund (NFRF).
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