From breakthroughs in cell photoelectric conversion efficiency and device stability issues, to large-area modular fabrication, to the diverse applications of flexible wearable and translucent cells, perovskite solar cells ushered in a milestone in the past year. develop.

Breakthrough in photoelectric conversion efficiency
Organic-inorganic hybrid perovskite cell materials have excellent photovoltaic properties such as suitable and tunable band gaps, strong solar spectral absorption, long carrier transport distances, and high defect state tolerance.

In the past year, the photoelectric conversion efficiency of various device structures of perovskite solar cells has made great breakthroughs, and it has been able to match the efficiency of commercial standard silicon-based cells.

Compared with the formal structure originally developed for perovskite cells, trans-structured devices have the advantages of low temperature processing, no obvious hysteresis effect, simple preparation process, and can be combined with traditional solar cell materials in a stacked structure. The interfacial non-radiative recombination mostly causes its photoelectric conversion efficiency to be relatively low, which limits its development.

Zhu Rui's research group from Peking University and researchers from the University of Oxford and the University of Surrey have jointly developed a solution-processable secondary growth technology for perovskite crystals.

As shown in the figure above, the quality of the perovskite film has been greatly improved, and the open circuit voltage and photoelectric conversion efficiency of perovskite solar cells based on the trans-planar structure have been improved.

For the first time, they proposed a "guanidine-assisted secondary growth" method, which pioneered the control of the semiconducting properties of perovskite materials, thereby greatly reducing the energy loss of non-radiative recombination in the device, which is crucial for the compensation of open circuit voltage.

By tuning the band gap width of the perovskite material, a high open-circuit voltage of 1.21 V was achieved for the first time in a trans-structured device, thereby achieving the highest laboratory photoelectric conversion efficiency of 21.51%. Certified by the China Institute of Metrology, the photoelectric conversion efficiency of the battery is as high as 20.90%, which is currently the highest photoelectric conversion efficiency based on the trans structure, laying the foundation for the further development of the laminated composite structure battery.

In order to further explain the interfacial non-radiative recombination of battery devices, Neher's group from the University of Potsdam in Germany and Unold's group from Swansea University in the United Kingdom realized the original non-radiative recombination of organic semiconductor interface materials through transient and photoluminescence imaging techniques. bit observation.

The study uses steady-state and time-resolved photoluminescence (PL and TRPL) spectroscopy to explain the causes of non-radiative recombination losses at the interface of perovskite cells. It is found that the loss of quasi-Fermi level splitting and the loss of additional free energy at the interface are both It occurs at the interface between the perovskite and the electrode, which further illustrates the importance of the optimization of the device interface layer compared to the optimization of the bulk phase of the perovskite material.

In this study, a conjugated polyelectrolyte material was introduced between the triphenylamine derivative (PTAA) and the perovskite layer, and an ultrathin LiF material was also introduced into the electron transport layer. The photoelectric conversion efficiency of the battery chip exceeds 20%, and there is no hysteresis. This work points out an effective way to increase the open-circuit voltage of the trans-structure.

In the past year, important progress has also been made in the photoelectric conversion efficiency of single-cell perovskite cells.

The Seo team of the Korea Institute of Chemical Technology has developed a new fluorine-terminated hole transport material for the formal device structure, replacing the expensive traditional spirofluorene (spiro-OMeTAD) material.

The study found that the good energy level matching and glass transition temperature make the transport layer material show higher photoelectric conversion efficiency and stability than spiro-OMeTAD, and the cell device efficiency based on this material reaches 23.2%. In addition, the cell exhibits excellent stability, maintaining an initial photoelectric conversion efficiency of 95% at 60 °C for 500 h.

At the end of 2018, the team of You Jingbi from the Institute of Semiconductors of the Chinese Academy of Sciences further refreshed the conversion efficiency of perovskite cells. It was certified by the US Renewable Energy Laboratory and its efficiency exceeded 23.7%, which is currently the highest single-cell efficiency.

In addition, Sahli and others of the Swiss Federal Institute of Technology in Lausanne combined silicon-based materials with the highest commercialization rate to develop a tandem structure battery combining silicon and perovskite.

Using both perovskite and silicon materials can simultaneously exploit their spectral absorption advantages and broaden the overall spectral range of the device. In this study, scientists creatively adopted a micron "pyramid" interface structure, which can better capture light, thereby further increasing the photoelectric conversion efficiency to 25.2%. The method has a simple preparation process, can be transformed on an existing production line, and exhibits excellent application potential.

Stability issues resolved

The stability of perovskite battery materials and devices is a bottleneck restricting the development and application of batteries. With the deepening of research, more and more researches focus on the stability of materials. Under the premise of obtaining high-performance photoelectric conversion efficiency, how to improve the stability of cells in different environments has become a hot issue in this field.

The Saliba team of the Swiss Federal Institute of Technology in Lausanne found that the methylamine molecule of the perovskite material is one of the important reasons for the instability of the phase structure. Under the conditions of moisture stability and thermal stability, the methylamine molecule is very easy to decompose. Based on this Look for more stable materials from an angle.

The study found that the use of Rb and Cs inorganic cations can effectively prepare perovskite films without methylamine molecules; and by adjusting the concentration of cations, the perovskite crystal can be achieved without Br element, which greatly enhances the perovskite material. Band gap controllability. The band gap of the RbCsFAPbI3 perovskite material prepared by using stable formamidine molecules to replace methylamine molecules is 1.53 eV.

Based on the above results, the researchers finally achieved a stable cell device with a photoelectric conversion efficiency of 20.35% by optimizing electron and hole transport. This study proposes a new method to stably fabricate perovskite cells with low processing temperature during fabrication, which can be applied to flexible battery structures.

The above studies show that it is an effective way to solve the stability problem of perovskite materials from the perspective of molecular structure design and crystal growth of perovskite materials.

At the same time, the team of Grötzel Michael of the Swiss Federal Institute of Technology in Lausanne, aiming at the problem of the instability of methyl amine molecules in perovskite materials and the difficulty of large-scale application of anti-solvent membrane preparation methods, proposed a multifunctional molecular design strategy, which can solve the above two problems at the same time. big problem.

The study found that the instability of methylamine molecules is mainly manifested in the grain boundaries and interfaces of polycrystalline perovskite films, resulting in non-radiative recombination of carriers, and easy to generate perovskite material ion migration and external infiltration such as water and oxygen. .

The team designed a variety of small-molecule additives to modify perovskite materials for the FA0.9Cs0.1PbI3 perovskite structure. Functionalizing hydrophobic aromatic groups by introducing thiol groups and amine groups can suppress the A-site cation vacancy defect and increase the grain size and passivate the surface interface.

Its unique tautomeric molecular form can simultaneously play a role in assisting crystallization and reducing defects, and the fabricated perovskite cells have greatly improved photovoltaic performance and excellent working stability. The 1cm2 battery prepared by the researchers has an efficiency of over 20%, and this preparation method does not require an anti-solvent process, making it easier to achieve large-area preparation than traditional methods.

In the research to solve the battery stability problem, there is also a part of the research work focused on the device interface.

The Luther team of the National Renewable Energy Laboratory of the United States proposed an interface control strategy that can simultaneously meet the requirements of battery stability during long-term practical use of water, oxygen, and light.

Taking the conventional formal perovskite structure as the research object, the researchers systematically investigated the reasons for the poor stability of the original structure from the interface of the device.

The study found that the choice of interface material is very important. The traditional Spiro-OMeTAD material cannot protect the stability of the battery in all aspects. The new EH44/MoOx composite hole transport layer is used to replace the Spiro-OMeTAD layer. The unpackaged device can still maintain 94% of the original photoelectric conversion efficiency within 1000h under simulated illumination conditions of 1000 h. The results of this study reveal the influence of the interface on the device stability and provide a reliable solution.

Based on the method of improving the stability of the device interface layer, the Sargent research group of the University of Toronto, Canada proposed that the strain relaxation generated by the interaction between cations and halides can effectively suppress the formation of perovskite vacancies.

The essence of this method is that the doping of halogen elements can greatly improve the defect formation energy, while suppressing the decomposition and ion migration of perovskite.

The study found that the elemental doping of Cd and Cl can inhibit the formation of atomic vacancies, greatly improve the stability of the device in a high humidity environment, and relax the perovskite battery requirements for device packaging materials and processing technology, which is suitable for commercialization. Use provides new ideas.

Breakthrough in large-area modular preparation

To achieve commercial application of perovskite solar cells, standard modular fabrication must be achieved. From small-area device chips in the laboratory to large-area modules, problems such as large-area film formation processes, increased defect state density, and module series attenuation are often encountered.

Researchers have made a series of important breakthroughs in the large-area modular fabrication of perovskite solar cells.

Usually, the tandem patterning process of solar cell modules is complex and requires high precision, which often causes obvious performance loss during the preparation of perovskite cells.

Lee's group at the Gwangju Institute of Science and Technology in South Korea has developed an electrochemical patterning technique that solves the problem of connecting perovskite chips in series.

In this work, the researchers cleverly exploited the ionic conduction properties of perovskite materials to induce metallic silver nanoelectrodes to construct a tandem module structure through ionic conduction.

This research is entirely based on low temperature and all-solution processing, which can be applied to batteries of various substrates. The module based on the planar structure of 9cm2 exhibits a photoelectric conversion efficiency of 14%, and more importantly, its geometric fill factor is as high as 94.1%.

In addition, there has also been a breakthrough in the printing of the planar structure battery interface layer. The commercial SnO2 printing ink will cause corrosion of the printing die, and it is impossible to prepare a large-area smooth SnO2 interface layer.

The research group of Cheng Yibing from Wuhan University of Technology cleverly solved this problem by doping KOH into commercial SnO2 printing ink.

KOH doping can effectively adjust the acidity and alkalinity of the ink to meet the needs of protecting the printing die. In addition, K+ in the SnO2 interface layer can promote the growth of perovskite nuclei to prepare high-quality perovskite films, while K+ can fill the crystal vacancies of perovskite and passivate at the interface layer. Through the application of this strategy, a large-area high-quality SnO2 thin film can be prepared by slit extrusion printing, and a 16.07cm2 hysteresis-free flexible perovskite solar cell module can be prepared from the SnO2 interface layer, and the photoelectric conversion efficiency exceeds 15%.

Following the preparation of perovskite crystals by the soft film overlay method in 2017, the printing process technology for the preparation of large-area modules has also made important progress in 2018.

The perovskite films prepared by the traditional doctor blade printing method are prone to produce island or ring patterns on the surface of the film due to surface tension or thermal convection, which in turn affects the crystal quality and photoelectric conversion efficiency of perovskite.

Huang Jinsong of the University of Nebraska-Lincoln in the United States realized the rapid preparation of large-area high-quality perovskite thin films by adding surfactants to the perovskite precursor ink.

This study shows that adding a small amount of surfactant soybean lecithin to the perovskite precursor can form a solvent evaporation flow opposite to the Maragni flow direction, thereby realizing the regulation of the hydrodynamics and drying process during the blade coating process. , and the surfactant can greatly improve the ductility of the ink and improve the quality of the large-area film.

The researchers propose that this method can realize the rapid and large-area preparation of perovskite thin films, and the ions in the surfactant can also play a role in the interface passivation of the crystal, which is beneficial to improve the efficiency and stability of the device. Based on this method, a solar cell module with a photoelectric conversion efficiency of 14.6% and an area of ​​57 cm2 can be prepared, and it can work continuously and stably for 20 days.

For the stability study of perovskite battery modules, Song Yanlin's team from the Institute of Chemistry, Chinese Academy of Sciences proposed to introduce pure-phase two-dimensional (2D) perovskite into the grain boundaries of traditional three-dimensional (3D) perovskite crystals to achieve Highly oriented 2D-3D perovskite lateral heterojunction structure.

This structure can effectively overcome the limitation of quantum size effect in the process of carrier transport, inhibit the non-radiative recombination of carriers from the grain boundary, prevent water and oxygen from corroding the film from the grain boundary, and the photoelectric conversion efficiency of the prepared solar cell more than 21%.

Further by adjusting the wettability of the ink during the printing process, the photoelectric conversion efficiency of the prepared battery module remained above 90% of the initial value after a 3000h decay test. This 2D-3D planar perovskite structure satisfies the high efficiency and high stability of the battery module at the same time, and has strong practical application value.

Flexible wearable and translucent battery applications are more diverse

The low-temperature fabrication characteristics of perovskite cells are very suitable for the fabrication of flexible and translucent perovskite cells. In the past 10 years, a variety of low-temperature techniques such as one-step, two-step, vacuum film-forming, soft-film lamination, and solvent annealing have been developed to improve the photoelectric conversion efficiency of flexible and translucent perovskite solar cells and stability.

The research team of Liu Shengzhong from Shaanxi Normal University conducted research on the growth of perovskite crystals on flexible substrates and found that the use of dimethyl sulfide additives can effectively control the growth rate of perovskite crystals, thereby improving the crystal quality.

The mechanism is that the lone pair of electrons of the S atom in dimethyl sulfide can combine with the empty orbital of Pb to interact to form a complex. During the crystallization of the precursor, the slow dissociation of dimethyl sulfide from the complex reduces the growth rate of perovskite crystals.

The high-quality perovskite film prepared by the dimethyl sulfide additive method was applied to flexible devices, and the photoelectric conversion efficiency was significantly improved to 18.4%, which is the highest efficiency of flexible perovskite solar cells at present. In addition, this flexible structure exhibits good bending resistance, maintaining 86% of the original efficiency after 5000 bends at a radius of curvature of 4 mm

The research group of Song Yanlin from the Institute of Chemistry, Chinese Academy of Sciences further developed flexible perovskites to prepare high-performance wearable battery modules.

Inspired by the crystallization mechanism and structure of nacre in nature, the research group improved the quality of perovskite crystals on flexible substrates by introducing an amphiphilic elastic crystalline matrix, which solved the problem of perovskite materials' brittleness.

The research shows that the vertical parallel structure growth of perovskite crystals can be achieved by adjusting the doping amount, which eliminates the inhibitory effect of lateral grain boundaries on carrier transport. At the same time, the formed elastic "brick-mud" structure achieves a breakthrough in mechanical stability, and realizes the stretchable function of the flat film for the first time.

Through this biomimetic crystallization and structural design, the photoelectric conversion efficiency of the prepared 1cm2 flexible perovskite solar cell exceeded 15%, and the third-party certified efficiency of the 56cm2 large-area cell module was as high as 7.9%. The solar cell module has the advantages of high photoelectric conversion efficiency, stable performance, and strong wearable fit, and can meet the power supply requirements of various wearable battery products.

In the past year, perovskite materials have also made breakthroughs in the field of smart glass. Among them, translucent perovskite solar cells show excellent performance, which can provide services such as shading, lighting, and energy supply for smart windows. However, such translucent devices are often unable to change color, and the light transmittance sometimes cannot meet the needs of indoor lighting.

Peidong Yang's team at the University of California, Berkeley, accidentally discovered cesium lead iodine bromide pure inorganic perovskite materials in experiments, which have excellent thermal and environmental stability.

More importantly, the crystal structure of this type of perovskite can efficiently undergo a reversible transition between the low-temperature phase and the high-temperature phase, and its electrical properties can be recovered. Cesium lead iodine bromide can complete the transformation from low-T phase to high-T phase by heating, and the material also changes from colorless and transparent to orange-red, and has good reversibility - it can be high in water vapor at room temperature. Efficiently transform from high-T phase to low-T phase.

The study found that the photovoltaic performance of the devices in the high-T phase and the low-T phase is very different. The CsPbIBr2-based battery device can achieve a power conversion efficiency (PCE) of 5.57% in the high-T phase; for the same battery device, when the perovskite is converted to the low-T phase, the PCE is only 0.11%. For different perovskite compositions, the highest PCE is around 7%.

By controlling conditions such as temperature and humidity, the transformation of perovskite high temperature phase (low transmittance) to low temperature phase (high transmittance) can be effectively achieved, and thermochromic smart windows can be successfully fabricated. With high thermal stability and fully reversible color and performance, the device is expected to be used in architectural glass, automotive windows, information display screens, and more.

Since the advent of perovskite solar cells, they have shown strong commercial application prospects due to their simple preparation process and low cost.

In the past year, researchers have made important breakthroughs in laboratory photoelectric conversion efficiency and stable power output. At present, the research based on the stability of battery modules is not systematic enough, and there is a lack of standards for module efficiency and stability testing. Further research is needed from the perspective of application.

Compared with silicon cells, perovskite solar cells show more diverse application potential. It is believed that through the efforts of scientists, portable perovskite solar cell products will enter daily life in the future.