Ligang Yuan

This review aims at presenting a comprehensive overview of the latest progress on perovskite solar cells (PSCs), especially the strategies toward enhancing their near‐infrared light harvesting. An in‐depth understanding of the working mechanism of tandem solar cells (TSCs) and integrated perovskite/organic solar cells (IPOSCs) is presented, and the recent developments of perovskite/Si, perovskite/Cu(In_1–x , Ga _x )Se₂ TSCs, and IPOSCs are further highlighted.

The emerging perovskite materials present great opportunities for cost‐saving and efficient photovoltaic devices. However, perovskite solar cells (PSCs) suffer from the limitation of short optical absorption edge, resulting in most of the near‐infrared (NIR) light being wasted. Recently, strategies toward broadening the NIR spectra response and further improve the power conversion efficiency of PSCs have attracted extensive attention. In this review, the unique features of perovskite materials are first introduced; subsequently, the current developments of organic–inorganic hybrid PSCs and all‐inorganic PSCs are highlighted. Then, a detailed summary of the strategies toward enhancing the NIR light harvesting of PSCs, namely, perovskite/Si and perovskite/Cu(In_1–x , Ga _x )Se₂ tandem solar cells (TSCs) and the integrated perovskite/organic solar cells (IPOSCs), is presented. After an in‐depth understanding of the working mechanism of TSCs and IPOSCs, a comprehensive overview about their recent developments, key detrimental factors restricting their further performance enhancement, and feasible countermeasures to conquer these scientific and technological problems are given. In the end, the perspectives on the related materials and devices are addressed.

The location and distribution of fullerenes in the perovskite:fullerene hybrid phase are confirmedly visualized by the conductive atomic force microscopy and Kelvin probe force microscopy measurements. Macroscopic current hysteresis originating from the influxes of all nanoscopic grain boundary current is avoided in perovskite solar cells based on the hybrid perovskite:fullerene phases.

In perovskite solar cells (PSCs), hybrid perovskite:fullerene phases are proposed to suppress macroscopic current hysteresis behavior by alleviating ion migration. However, the understanding of how fullerenes exactly alleviate the current hysteresis and what is the influence of fullerenes in such hybrid phases are still unclear from a microscopic viewpoint. Herein, the intentional incorporation of fullerene into perovskite is used to examine how fullerene exactly reduces the macroscopic current hysteresis. The location and distribution of fullerenes in the hybrid phase are confirmedly visualized using conductive atomic force microscopy and Kelvin probe force microscopy measurements. Fullerenes located at grain boundaries function as a source of beneficial effect on choking the channels of ion migration and also as the electron traps that compromise the photocarrier extraction. Macroscopic current hysteresis originating from the influxes of all nanoscopic grain boundary current signals is avoided in PSCs based on the hybrid perovskite:fullerene phases. These results not only provide a strong correlation between nanoscopic and macroscopic current hysteresis behaviors but also clearly clarify how fullerenes play a role in reducing the current hysteresis in hybrid phases and thus prototype devices.

Efficient Zn‐SnO _x electron transport layers (ETLs) by the low‐temperature (100 °C) electron beam (E‐beam) method are prepared. Doping Zn²⁺ in SnO₂ improves conductivity, suppresses charge recombination, and optimizes the energy level structure of SnO₂, leading to an improved power conversion efficiency from 18.95% to 20.16%. The low‐temperature preparation of ETLs and the excellent performance of devices present great commercial potential for future applications.

Perovskite solar cells (PSCs) attract tremendous interest due to their feasibility, high power conversion efficiency (PCE), light weight, and flexible architecture. However, some challenges are still present for cheap mass fabrication in commercial applications. Herein, efficient Zn‐SnO _x electron transport layers (ETLs) are used by the low‐temperature (100 °C) electron beam (E‐beam) method. Doping Zn²⁺ in SnO₂ improves conductivity, suppresses charge recombination, and optimizes the energy level structure of SnO₂, leading to an improved PCE from 18.95% to 20.16%. More importantly, the PCE of the modified device is more than 80% of its initial values for 800 h in ambient air with a relative humidity of ≈40%. The flexible device exhibits a PCE of 15.25% and remains at an initial PCE of 83% after 100 bending cycles. The efficient and flexible PSCs are potentially used as wearable energy power sources. The low‐temperature preparation of ETL and the excellent performance of devices present great commercial potential for future applications.

Herein, Pb‐site doping in organic–inorganic hybrid perovskite (OIH‐LHP) and inorganic CsPbX₃‐based materials is discussed, elucidating the functions of doping on lead halide perovskite (LHP) crystallization, optoelectronic property, and stability. Perspectives for further investigation are also presented.

Although great success has been achieved in perovskite solar cells (PSCs), it still suffers from several drawbacks in terms of stability and higher efficiency. Doping as an effective method to modify the optical and electronic properties of the materials is extensively studied in lead halide perovskites (LHPs). Herein, Pb‐site doping in organic–inorganic hybrid perovskites (OIH‐LHPs) and inorganic CsPbX₃‐based materials is discussed. Doping has three functions toward PSCs: participating in the crystalline process, modifying the energy states in LHPs, and contributing to the stability of PSCs. Issues about further improvements are raised, and perspectives for further investigation are presented.

The essential advances in perovskite semiconductor‐based radiation detectors, mainly X‐ray and γ‐ray detectors, are reviewed. The promising properties of lead halide perovskites and recent advancements in material preparation, device design, and material improvement for radiation detector applications are discussed. A brief outlook for the further development of lead halide perovskite‐based radiation detectors is also provided.

Research interest in lead halide perovskites has shown a spurt of growth in the last few years due to their high absorption coefficient, large carrier mobility, and long diffusion length. Besides their wide applications in solar cells, LEDs, lasers, and photodetectors, lead halide perovskites are demonstrated as excellent candidate materials for radiation detectors with comparable performance to commercial Si and CdZnTe (CZT) detectors. Herein, the essential results on perovskite semiconductor‐based radiation conductors are summarized. Furthermore, a brief outlook for the further development of lead halide perovskites‐based radiation detectors is proposed.

Planar perovskite solar cells (PSCs) are promising photovoltaic devices accompanied with the obvious advantages of easy fabrication and scalability. Herein, a method toward obtaining high‐quality FA_1–x MA _x PbI₃ film‐based planar PSCs by sequential deposition of chlorobenzene and methylammonium chloride is proposed. A champion power conversion efficiency (PCE) of 18.57% is achieved and 91% of the initial PCE is maintained after 500 h.

Planar perovskite solar cells (PSCs) are promising photovoltaic devices accompanied with the obvious advantages of easy fabrication and scalability. To achieve highly efficient and stable PSCs, the morphology control of perovskite crystallization is crucial. Herein, a novel method toward obtaining high‐quality FA_1–x MA _x PbI₃ films by spin coating methylammonium chloride (MACl) and chlorobenzene (CB) in different sequential processes on the top of substrates is proposed. Controlling the nucleation process is beneficial for the formation of a homogeneous nucleus at the nucleation stage, leading to highly ordered seed crystals and an ultrasmooth perovskite film. As determined by photoluminescence and time‐resolved photoluminescence spectroscopy, the defects and the associated charge recombination are notably reduced by the high crystalline quality of perovskite film. Finally, a champion power conversion efficiency (PCE) of 18.57% is achieved and 91% of the initial PCE is retained after 500 h. The devices are stored in an ambient condition with 20% relative humidity (RH) at 30 °C in the dark.

Self‐assembled monolayers (SAMs) in perovskite solar cells are summarized comprehensively herein. SAMs play significant roles such as boosting the optoelectronic properties and improving perovskite stability. An overview of SAM modification in perovskite solar cells and state‐of‐the‐art applications is provided. Finally, the remaining challenges and outlooks for future research are presented.

Perovskite solar cells (PSCs) are considered as potential candidates for next‐generation energy harvesting due to their advantages. A classic PSC has two charge transport layers (CTLs) above and below a perovskite layer, and these CTLs largely influence charge extraction and transport. Thus, an interface inevitably forms between the CTL and perovskite layer, and if the CTL and perovskite do not form a compact contact, these interfaces can become a nonradiative recombination center, which can degrade device efficiency and stability. Accordingly, interface engineering is considered an effective way to alleviate this issue. Herein, an overview of interface engineering methods on PSCs is provided, particularly with regard to types of self‐assembled monolayers and their roles in device energy level alignment and passivation effects.

Vacuum codeposition can be used to fabricate mixed tin–lead formamidinium iodide perovskite. The three precursor thermal sources are combined with an additional source to incorporate tin fluoride as an additive, which improves film formation and reduces tin oxidation. The vacuum‐deposited perovskite films are integrated in devices with >14% photovoltaic efficiency as a proof of concept.

The tunability of the optoelectrical properties upon compositional modification is a key characteristic of metal halide perovskites. In particular, bandgaps narrower than those in conventional lead‐based perovskites are essential to achieve the theoretical efficiency limit of single‐absorber solar cells, as well as develop multijunction tandem devices. Herein, the solvent‐free vacuum deposition of a narrow bandgap perovskite based on tin–lead metal and formamidinium cation is reported. Pinhole‐free films with 1.28 eV bandgap are obtained by thermal codeposition of precursors. The optoelectrical quality of these films is demonstrated by their use in solar cells with a power conversion efficiency of 13.98%.

A major drawback of hybrid perovskites is the lack of long‐term stability. This is related to the degradation of organic cations. Light‐induced degradation of CH₃NH₃PbI₃ extends from ambient temperatures down to 5 K. Illumination of thin films and single crystals at T = 5 K causes the formation of localized states that can be annealed at T ≥ 15 K.

In a period of only a few years, the power conversion efficiency of organic–inorganic perovskite solar cells has surpassed a value of 24.2%. However, a major drawback is the lack of long‐term stability, which is partially related to the dissociation of organic cations under prolonged illumination. This degradation mechanism is not limited to ambient temperatures. At low temperatures (T = 5 K), illumination of methyl ammonium lead iodide (CH₃NH₃PbI₃) thin films with a photon energy of E _ph = 3.4 eV results in the formation of localized trap states located about 100 meV within the bandgap. These light‐induced defects are metastable, and annealing at T ≥ 15 K removes the localized states. Defect creation is not limited to polycrystalline perovskites but is also observed in single‐crystal CH₃NH₃PbI₃. The experimental data are discussed in terms of a two‐level model where the metastable state is separated from the annealed state by an energy barrier.

Here, studies on regulation of the interfacial charge balance in SnO₂‐based planar perovskite solar cells are reported. SnO₂ with optimum thickness exhibits enhanced charge balance. Moreover, trap‐assisted carrier recombination is significantly suppressed by using diethylenetriaminepentaacetic acid as a passivator. As a result, the champion device demonstrates a promising efficiency of 21.28% with negligible hysteresis and much improved environmental stability.

Control of dynamics at the electron transport layer–perovskite interface, such as charge transfer and recombination, is essential in achieving high‐efficiency planar perovskite solar cells (PSCs). Herein, it was observed that the trade‐off between unfavorable electron transport of a thick SnO₂ film and serious electron recombination at thin SnO₂ film/perovskite interfaces is essential for the performance of SnO₂‐based planar PSCs. The optimized efficiency of devices beyond 20% is obtained by using a two‐step deposition of SnO₂. Moreover, trap‐assisted carrier recombination is significantly suppressed by using the diethylenetriaminepentaacetic acid passivator via the formation of coordination with undercoordinated Sn and Pb²⁺ ions. As a result, the champion device demonstrates a promising efficiency of 21.28% with negligible hysteresis and much improved environmental stability, i.e., retaining 98% of the initial efficiency under ambient atmosphere over 1000 h.

Carbon‐electrode based perovskite solar cells (CPSCs) are well known for their low cost and sound stability. However, the highest power conversion efficiency of these devices is only about 70% of that demonstrated by metal electrode‐based PSCs, leaving a gap of about 30%. Bulk engineering and interface engineering is helpful in narrowing the gap. Herein, these two strategies are summarized for CPSCs.

Carbon electrodes have been adopted widely in perovskite solar cells (PSCs). Due to its suitable work function (though not high enough), the carbon electrode itself could extract photogenerated holes and has helped to achieve a power conversion efficiency of ≈16% in the absence of hole‐transporting material. Meanwhile, due to the inert chemical nature and the micrometer‐sized film thickness (≈10 μm), carbon electrodes can prolong the stability of PSCs. These merits are appealing for the commercialization of PSCs. However, the efficiency of carbon‐electrode PSCs is relatively low. A gap of ≈30% remains when comparing with PSCs using evaporated metal films as the electrode. Herein, the progresses in the efficiency of the four kinds of carbon‐electrode based PSCs (mesoscopic, embedment, planar, and quasi‐planar) are reviewed and compared to metal‐electrode based PSCs. Then, the role of bulk engineering and interface engineering in the progress of efficiency is discussed. Finally, outlooks are described in accordance with the discussions.

Bi₂Te₃ nanoplates with a tunable energy structure are introduced in inorganic perovskite solar cells (PSCs), accelerating hole transport by the matched band alignment. Confirmed by systematic measurements, charge recombination is largely suppressed due to lower trap density and higher carrier mobility. The optimal PSC with Bi₂Te₃ exhibits highly decreased V _OC loss and enhanced long‐term stability over 50 days.

To solve the thermal instability issue of organic–inorganic hybrid perovskites, all‐inorganic perovskite solar cells (PSCs) have been featured in the spotlight. However, their power conversion efficiencies (PCEs) are far from satisfactory due to the substantially radiative and nonradiative recombination of charge carriers in the common‐structured devices. Herein, bismuth telluride (Bi₂Te₃) nanoplates are designed as an interlayer between cesium lead halide (CsPbBrI₂) and 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,90‐spirobifluorene (Spiro‐OMeTAD) to reduce the notorious trap states and charge recombination. Confirmed by systematic electrochemical and photoelectrical techniques, the Bi₂Te₃ interlayer optimizes hole extraction and transport efficiency because of the matched band level structure and drastically reduces trap defect densities. Prolonged effective lifetime and shorter diffusion time induced by the Bi₂Te₃ interlayer reveal less electron–hole recombination and more efficient carrier transport, which lead to a larger photocurrent and less open circuit voltage loss of PSCs. The all‐inorganic PSCs with the optimal Bi₂Te₃ interlayer exhibit a highly enhanced PCE of 11.96%. Moreover, Bi₂Te₃ also acts as a blocking layer for the migration of iodide ions, silver, and moisture, resulting in a considerable device stability of more than 70% of initial PCE after 50 days without extra encapsulation. This low‐cost and facile method for efficient and stable all‐inorganic PSCs offers great promise as a next‐generation renewable energy source.

Sn‐based perovskite solar cells (PSCs) with 6.33% power conversion efficiency are fabricated with an aperture area of 1 cm² by introducing a post‐deposition vapor annealing method. The fabricated Sn‐based PSCs show promising stability, both under dark and maximum power‐point tracking conditions.

Sn‐based perovskite solar cells (PSCs) are promising alternatives to replacing toxic Pb‐based PSCs, which have shown a rapid rise in photovoltaic applications in the past 1 year. However, the reported Sn‐based PSCs are often fabricated with a small aperture area (typically 0.02–0.1 cm²) because forming homogeneous pinhole‐free continuous films over a large surface area is still challenging. Herein, a post‐deposition vapor annealing (PDVA) process assisted by methylammonium chloride vapor is presented that enables the fabrication of stable, homogeneous pinhole‐free FASnI₃ perovskite absorber films with low crystal defects and low surface recombination over a relatively large area up to 1.02 cm². Inverted planar solar cells fabricated with a 1.02 cm² aperture area show a maximum power conversion efficiency of 6.33% with high reproducibility and stability. The shelf‐lifetime stability test shows that the PSCs retain 90% of their performance for more than 1000 h when stored in a N₂‐filled glove box and under dark conditions. The preliminary light‐soaking stability tests under continuous illumination and maximum power‐tracking conditions are relatively promising. This study marks an important step toward the up scaling of Sn‐based PSCs.

High‐performance perovskite solar cells with an average power conversion efficiency of 21.4% are achieved based on mixed 2D/3D perovskites with induced phenylethylammonium iodide and exhibit an ultrahigh fill factor (83.6%). The unencapsulated device exhibits enhanced operational stability under continuous simulated sunlight illumination and outstanding air stability after 1000 h of storage under ambient air conditions.

2D/3D perovskite heterostructures or composites are recognized as efficient strategies to improve the stability of perovskite solar cells. Herein, a novel solution process to develop 2D/3D perovskites with modulated diffusion passivation by introducing phenylethylammonium iodide (PEAI) and N,N‐dimethylformamide (DMF) additive, which could effectively enhance device performance and long‐term stability, is demonstrated. Compared with conventional devices, the device with PEAI and DMF solvent additive treatment exhibit enhanced charge transport, improved charge extraction, and suppressed nonradiative carrier recombination. The solar cells with an optimal 2D/3D perovskite passivation treatment exhibit an extremely high fill factor of 83.6% and an average power conversion efficiency of 21.4% (21.3% using integrated photocurrent from the incident photon‐to‐current efficiency spectra) based on the NiO _x hole transport layer. Furthermore, the unencapsulated device exhibits excellent stability under continuously simulated sunlight illumination and outstanding air stability after 1000 h of storage under ambient air conditions.

In article no. 1900192, Ai Ping Chen, Shuang Yang, Yu Hou, and co‐workers employ a versatile alkaline earth metals doping strategy to engineer the electronic structure of NiO_x contacts for inverted planar perovskite solar cells, in which the champion device demonstrates a power conversion efficiency of 19.49% with a high open circuit voltage of 1.14 V. Alkaline earth metals doping can significantly optimize the electrical properties by deepening the valence band maximum and enhancing the hole conductivity.

Black phosphorus quantum dots (BPQDs)‐assisted growth of a perovskite film is reported. Serving as heterogeneous nucleation centers, the BPQDs assist in the crystallization of the perovskite film, achieving perovskite films with higher crystallinity and less defects. Consequently, the perovskite solar cells made with BPQDs achieve a maximum power conversion efficiency of 20% and an encouraging improved thermal stability.

Crystallinity and trap‐state density of a perovskite film play a critical role in the performance of corresponding perovskite solar cells (PVSCs). Herein, liquid‐phase‐exfoliated black phosphorus quantum dots (BPQDs) are incorporated into the perovskite precursor solution as additives to direct the formation of the perovskite film, i.e., methylammonium lead iodide (MAPbI₃). It is found that the perovskite films made with BPQDs have higher crystallinity and less nonradiative detects compared with the pristine ones, leading to longer carrier lifetime and higher carrier collection efficiency. Time‐of‐flight secondary‐ion mass spectra and surface density calculation of BPQDs reveal that the improvement of the perovskite film quality may be related to the heterogeneous nucleation of the perovskite film at the BPQDs. PVSCs using MAPbI₃ films made with BPQDs achieve a maximum power conversion efficiency of 20.0% and an encouraging thermal stability of T ₈₀ = 100 h at 100 °C. Both values are remarkably higher than the devices with pristine perovskite films. Therefore, this work demonstrates the potential of the 2D materials quantum dots‐assisted growth method for high‐performance PVSCs.

Propane‐1,3‐diammonium cations are first adopted to construct cesium–formamidinium (Cs–FA) perovskite solar cells (PSCs) with an efficiency of 18.1% and much enhanced device stability, and the opposing effects induced by the diammonium cation are resolved.

Incorporating diammonium cations, which electrostatically connect the adjacent inorganic slabs ([PbI₆]⁴⁻), into 3D perovskite is recently proposed to develop high‐performance perovskite solar cells (PSCs). However, due to limited studies, the effects of these organic cations on the perovskite structural and optoelectronic properties are yet to be understood. Herein, a diammonium cation, propane‐1,3‐diammonium (PDA), is first proposed to modulate the cesium–formamidinium (Cs–FA)‐mixed cation perovskite. By increasing the PDA content, the efficiency of the Cs_0.15FA_{0.85 − x} PDA _x PbI₃ PSC first increases and then drastically decreases. The highest power conversion efficiency (PCE) of 18.10% obtained by Cs_0.15FA_0.83PDA_0.02PbI₃ is superior to that of the Cs_0.15FA_0.85PbI₃ (16.82%). Through systematic investigations, it is revealed that the PDA content–dependent efficiency is attributed to a competition between the enhanced defect passivation and emerged excitonic effect with an increased PDA content. Moreover, the encapsulated Cs_0.15FA_0.83PDA_0.02PbI₃ device exhibits almost 1.5 times increased stability than the Cs_0.15FA_0.85PbI₃ counterpart, with 83% of its initial efficiency retained after 500 h exposure, under continuous light soaking at 60 °C in ambient air. This study provides a practical strategy to enhance the device stability without sacrificing the efficiency and deepens our understanding on effects of diammonium cation incorporated in 3D perovskite.

A versatile alkaline earth metals doping strategy is utilized to engineer the electronic structure of NiO _x contacts for inverted planar perovskite solar cells, which demonstrates a power conversion efficiency of 19.49% with a high open‐circuit voltage of 1.14 V. Enhanced charge extraction and conductivity are responsible for the high‐performance devices.

Organometallic halide perovskite solar cells (PSCs) are rapidly evolving as the promising photovoltaic technologies with high record efficiency over 24%. The inorganic p‐type semiconductor NiO _x is extensively used as important hole transport layers for the realization of stable and hysteresis‐free solar cells due to their good electronic properties, facile fabrication, and excellent chemical endurance. However, the critical issues of NiO _x films including poor intrinsic conductivity and mismatched band alignment limit further improvement of the device performance. Herein, it is demonstrated that a versatile alkaline earth metal (Mg, Ca, Sr, and Ba) doping strategy can effectively engineer the electronic properties of NiO _x contacts in inverted planar PSCs. Alkaline earth metal doping can deepen valence band maximum and enhance the hole conductivity of NiO _x films, which better aligns the energy band in solar cells. The champion device based on Sr‐doped NiO _x films attains a power conversion efficiency of 19.49% with a high open‐circuit voltage (V _OC) of 1.14 V for NiO _x ‐based CH₃NH₃PbI₃ devices. The resulted device shows negligible hysteresis and high stability as well. This finding provides a systematic doping strategy to further improve the performance of inverted planar PSCs.

A liquid crystal (LC) molecule (4′‐heptyl‐4‐biphenylcarbonitrile) is first used as a “binding agent” to connect grain boundaries of perovskite. The crystal orientation of perovskite grains is controlled and the electron transport process is accelerated after treating with LC; these are reflected by the significant improvement in power conversion efficiency and high fill factor. Remarkably, the LC greatly contributes to the humid‐stability of perovskite solar cells.

Hybrid perovskites have rapidly emerged as highly promising optoelectronic materials for perovskite solar cells (PSCs), whereas solution‐processed perovskite films usually contain a large amount of grain‐boundary network, which is unbeneficial for efficient film function, including charge transport and environmental stability. Herein, a liquid crystal (LC) molecule is first used as a “binding agent” to connect grains and fill grain boundaries of perovskite. The LC molecule (4′‐heptyl‐4‐biphenylcarbonitrile) interacts with PbI₂ to control the crystal orientation for fine and oriented perovskite grains, which accelerates electron transport and enhances environmental stability. Consequently, compared with the pristine devices, the power conversion efficiency of the LC‐based device increases from 17.14% to 20.19% with a high fill factor (over 80%). Remarkably, the LC‐based PSCs retain 92% of their initial efficiency at 25 °C, and a relative humidity of 70% after 500 h, whereas the control samples are almost degraded completely under the same conditions.

Pb(SCN)₂ functions at the grain boundaries and pinholes to in situ polish the perovskite film surface. A 425 nm‐thick CsPbI₂Br film with high crystalline, smooth, and uniform surface morphology is obtained, with an efficiency of 10.44% for a low cost and stable carbon‐based perovskite solar cell processed under low‐temperature (150 °C).

Improvement in stability and an economical processing technique are the main aspects of the commercialization of perovskite solar cells (PSCs). In this study, a 425 nm‐thick CsPbI₂Br film with a high crystalline, smooth, and uniform surface morphology is obtained by Pb(SCN)₂ passivating the grain boundaries under low temperature (150 °C). The results of a series of electrochemical analyses, including space‐charge‐limited‐current (SCLC), open‐circuit voltage decay (OCVD), electrical impedance spectroscopy (EIS), intensity‐modulated photocurrent spectroscopy (IMPS), and intensity‐modulated photovoltage spectroscopy (IMVS), demonstrate that the trap density of the CsPbI₂Br film is greatly reduced with Pb(SCN)₂, which effectively inhibits the interface recombination and promotes charge transport in CsPbI₂Br PSC. Efficiencies of 12.22% and 10.44% are achieved for low‐temperature‐processed CsPbI₂Br planar‐architecture PSCs with ITO/SnO₂/CsPbI₂Br/ poly(3‐hexylthiophene) (P3HT)/Ag and ITO/SnO₂/CsPbI₂Br/carbon, respectively. This low‐cost, high‐efficiency carbon‐based inorganic PSC shows potential industrial application, especially for tandem solar cells.

An ingenious surface chlorination treatment method is used to passivate the interface defects of perovskite/zinc oxide (ZnO), which effectively reduces the interface charge recombination loss and improves the poor interface chemical characteristics. Thus, the fabricated zinc oxide–chlorine (ZnO–Cl)‐based device achieves an enhanced efficiency and suppressed hysteresis, as well as strengthened stability in perovskite solar cells.

Defect states on the zinc oxide (ZnO) surface cause severe interfacial charge recombination and perovskite decomposition during device operation, which inevitably leads to efficiency loss and poor device stability, making the usage of ZnO in perovskite solar cells (PSCs) problematic. Herein, a simple and effective method of inorganic chlorination treatment is used to passivate the surface defects of the ZnO electron transport layer. It is shown that chlorine (Cl) effectively fills the oxygen vacancy defects of ZnO, suppressing charge recombination and facilitating charge transport at the perovskite/ZnO interface. Therefore, the resulting CH₃NH₃PbI₃‐based device achieves an enhanced power conversion efficiency with suppressed hysteresis. Meanwhile, the chlorination of the ZnO surface protects the perovskite layer from decomposition, thus improving device stability. Herein, an ingenious method is developed to further improve the device performance of ZnO‐based PSCs and useful guidance is provided for the development of other perovskite optoelectronics, especially those with ZnO as the charge transport layer.

Various strategies related to light management and photocarrier collection are developed to enhance perovskite solar cell performance. The exploration of novel plasmonic nanostructures with predesigned size and shape is needed in the field. Herein, a bioinspired nanostructure of Au nanorod–nanoparticle dimers with structural darkness is used to enhance the light harvesting and performance of perovskite solar cells.

Hybrid perovskites have recently attracted enormous attention for photovoltaic applications, and various strategies related to light management and photocarrier collection are developed to enhance their performance. As an effective route toward near‐field light enhancement, metal nanostructures with subwavelength dimensions can couple incident photons with conduction electrons, giving rise to localized surface plasmon resonances. However, efficiency enhancements through plasmonic routes are limited to the short wavelength range corresponding to metal extinction wavelength. Thus, the exploration of novel plasmonic nanostructures with predesigned sizes and shapes is needed to advance this field. Herein, for the first time, a bioinspired nanostructure of Au nanorod–nanoparticle dimers with structural darkness is exploited to enhance the light harvesting and performance of perovskite solar cells. Differing from conventional metallic nanoparticles, biometric nanoparticles introduce geometric singularity to the system, providing a broadband response for energy harvesting. By embedding the core–shell gold dimers in the perovskite solar cells, a notable enhancement of broadband light absorption is observed, and sequentially, the efficiency of perovskite solar cells increases by 16%.

A systematic study of structurally similar fullerene derivatives shows that even minor modifications in their structure have a strong impact on their performance as electron transport layer (ETL) materials for perovskite solar cells. The best ETL significantly improves ambient stability of the devices for >800 h presumably due to an optimal size/shape of the solubilizing addend enabling compact molecular packing.

It is known that the operation lifetime of perovskite solar cells can be extended by orders of magnitude if properly selected hole‐transport and electron transport layers provide good isolation for the perovskite absorber preventing evaporation of volatile species (e.g., photoinduced) from the active layer and blocking the diffusion of aggressive moisture and oxygen from the surrounding environment. Herein, a systematic study of a family of structurally similar fullerene derivatives as electron transport layer (ETL) materials for p‐i‐n perovskite solar cells is presented. It is shown that even minor modifications of the molecular structure of the fullerene derivatives have a strong impact on their electrical performance and, particularly, ambient stability of the devices. Indeed, an optimally functionalized fullerene derivative applied as an ETL enables stable operation of perovskite solar cells when exposed to air for >800 h, which is manifested in retention of 90% of the original photovoltaic performance. In contrast, the reference devices with phenyl‐C₆₁‐butyric acid methyl ester as the ETL degraded almost completely within less than 100 h of air exposure. Most probably, the side chains of the best‐performing fullerene ETL materials are filling the gaps between the carbon spheres, thus preventing the diffusion of oxygen and moisture inside the device.

Diboron‐treated SnO₂ exhibits some Sn³⁺ species, which serve as electron donors with more n‐type nature, resulting in the higher Fermi level on the surface of SnO₂, promoting electron extraction and reducing carrier recombination in the electron transport layer (ETL)/perovskite interface. A power‐conversion efficiency of 22.04% is obtained in an n‐i‐p structure perovskite solar cell.

Energy‐level modulation between perovskite and carrier transport layers to obtain a promoted carrier extraction and reduced charge recombination is an effective way to achieve high‐efficiency perovskite solar cells. Here, diboron is used as an effective interfacial modifier between SnO₂ and perovskite. By taking advantage of the higher Fermi level on the surface of SnO₂ after diboron treatment, a power‐conversion efficiency of 22.04% in a solar cell device based on two‐step solution‐processed planar n‐i‐p structure is obtained. With the help of thorough characterizations, it is argued that the diboron‐treated SnO₂ exhibits some Sn³⁺ species, which serve as electron donors with a more n‐type nature, promoting electron extraction and reducing carrier recombination in the electron transport layer (ETL)/perovskite interface. Further analysis speculates that the formation of surface diboron–oxygen Lewis pair induces a reducing state of diboron complexes, resulting in the spontaneous electron redistribution and the formation of Sn³⁺−O^–• species. This provides an effective chemical approach to tune the energy alignment between the oxide ETL and absorber.

The notorious fine‐grained bottom layer in solution‐processed CuInSe₂ thin films is converted into a working absorber upon Na addition, leading to a significant increase in the collection length and finally short current density of the devices.

Na has been reported to increase the efficiency of not only vacuum‐processed but also the solution‐processed Cu(In,Ga)(Se,S)₂ (CIGSSe)‐type solar cells. However, the physical mechanism underlying the improvement is significantly different depending on the nature of the processes, as exemplified by the experimental observation that the short circuit current density (J _SC) of the solution‐processed devices reportedly increases upon Na addition, whereas that of the vacuum‐processed devices remains the same or even decreases. A systematic study is conducted to elucidate the reason for this Na‐induced J _SC increase in the solution‐processed CuInSe₂ (CISe) devices. In the amorphous nanoparticle‐based route, upon Na addition, the previously fine‐grained bottom layer near the CISe/Mo interface is transformed into a large‐grained layer, presumably because of the Na–Se liquid flux‐assisted sintering. At the same time, Na also induces phase homogenization in the bottom layer in which the mixed‐phases are converted to single‐phase CISe upon Na addition. These morphological and phase transformations are found to be directly related to the improved collection of the minority carriers generated in this region, which suggests to be the main reason for the observed J _SC increase.

A comprehensive theoretical analysis of two‐terminal and four‐terminal perovskite/copper indium gallium selenide (CIGS) tandem solar cells is investigated from optical and electrical aspects. According to different optical absorptions, the current matching points of different halide components are obtained. Under the condition of current matching, an optimal performance up to 31.13% can be obtained by using two‐terminal CH₃NH₃PbI₂Br/CIGS tandem structure.

Perovskite/copper indium gallium selenide (CIGS) tandem solar cells represent an attractive configuration to obtain ultrahigh efficiency. A detailed theoretical analysis is crucial for further improving the performance of tandem solar cells. Herein, four‐terminal and two‐terminal perovskite/CIGS tandem solar cells are intensively researched. For four‐terminal perovskite/CIGS tandem solar cell, the optimal thicknesses of CH₃NH₃PbI₃ and CIGS are 0.5 and 3 μm, respectively, according to the simulation result. Reducing the thickness of TiO₂ and Spiro‐OMeTAD can minimize the short‐wavelength parasitic absorption and long‐wavelength parasitic absorption, respectively. Meanwhile, using antireflection coating, such as 100 nm MgF₂, is beneficial to increase the photon absorption. For two‐terminal perovskite/CIGS tandem solar cells, the thicknesses of perovskite and CIGS are tuned to meet the current matching. To further improve the efficiency of two‐terminal tandem cells, FTO thickness is reduced to minimize reflection, and the optimal doping concentration of CIGS (1 × 10¹⁸ cm⁻³) is used. In addition, results show that the quality of perovskite films should be improved by enlarging the grain size to decrease the trap states at grain boundary. Finally, the optimal efficiency of two‐terminal CH₃NH₃PbI₂Br/CIGS tandem solar cells reaches 31.13%.

Perovskite solar cells are very promising for their high efficiency and solution‐process feasibility. Herein, some fabrication methods for gaining a high‐quality perovskite layer with long‐term stability are reviewed. These approaches significantly enhance the stability of perovskites, which makes it applicable for commercialization. However, these methods have some issues and it still leaves much room for further optimization.

Organic–inorganic hybrid perovskites (OIHPs) are one of the hottest fields on account of their immense potential for photovoltaics. As one of the most promising OIHPs, formamidinium (FA)‐based perovskites have been developed very fast in the past few years. The power conversion efficiency (PCE) has reached certified 24.2%, which is comparable with that of monocrystalline silicon solar cells. However, the easy formation of nonperovskite δ‐phase formamidinium lead triiodide (FAPbI₃) at a low temperature needs to be solved when fabricating a high‐quality light absorber layer. Several strategies have been used to avoid the formation of δ‐phase FAPbI₃ and improve phase stability in recent years such as tolerance factor adjustment, dimensional engineering, addictive processing, interfacial modification, defects passivation, and in situ growth. These approaches can enhance the phase stability to some extent; however, their contribution to long‐term stability and especially their real mechanism is still unknown. Herein, the relationships among the tolerance factors, the structure of FAPbI₃, and the phase transition phenomenon are summarized. In addition, various methodologies and potential mechanisms for stabilizing α‐phase FAPbI₃ at room temperature (RT) are discussed. In conclusion, a series of challenges in the popular processings of perovskite solar cells and their corresponding solutions that help achieve commercialization faster are summarized.

Calculations of Shockley–Queisser limits for perovskite solar cells under artificial light sources reveal the existence of an unusual zone, in which the bandgaps (E _g) of commonly used perovskite materials are too small to harvest photonic energy efficiently. Accordingly, increasing the value of E _g of the perovskite solar cell, by incorporating Br⁻ ions, improves the power conversion efficiencies under indoor lighting conditions.

Abstract

Indoor photovoltaics (IPVs) are attracting renewed interest because they can provide sustainable energy through the recycling of photon energy from household lighting facilities. Herein, the Shockley–Queisser model is used to calculate the upper limits of the power conversion efficiencies (PCEs) of perovskite solar cells (PeSCs) for two types of artificial light sources: fluorescent tubes (FTs) and white light–emitting diodes (WLEDs). An unusual zone is found in which the dependence of the PCEs on the bandgap (E _g) under illumination from the indoor lighting sources follows trends different from that under solar irradiation. In other words, IPVs exhibiting high performance under solar irradiation may not perform well under indoor lighting conditions. Furthermore, the ideal bandgap energy for harvesting photonic power from these indoor lighting sources is ≈1.9 eV—a value higher than that of common perovskite materials (e.g., for CH₃NH₃PbI₃). Accordingly, Br⁻ ions are added into the perovskite films to increase their values of E _g. A resulting PeSC featuring a wider bandgap exhibits PCEs of 25.94% and 25.12% under illumination from an FT and a WLED, respectively. Additionally, large‐area (4 cm²) devices are prepared for which the PCE reaches ≈18% under indoor lighting conditions.

Herein, a simple cation exchange growth (CEG) method is demonstrated that replaces the organic MA⁺ cation with Cs⁺ to produce a high‐quality black γ‐phase CsPbI₃ perovskite device, enhancing both power conversion efficiency and stability. As a result, the device fabricated using the optimized CEG method yields efficiency up to 14.1%.

Abstract

Inorganic lead halide perovskites have attracted attention due to their tolerance to higher processing temperature and higher bandgap suitable for tandem solar cell application. Not only do they improve cell stability and efficiency, they also reveal many interesting and un‐anticipated material qualities. This work reports a simple cation exchange growth (CEG) method for fabricating inorganic high‐quality cesium lead iodide (CsPbI₃) by adding methylammonium iodide (MAI) additive in the precursor. X‐ray diffraction results reveal a multi‐stage film formation process whereby i) MAPbI₃ perovskite first formed that acts as a perovskite template for ii) subsequent ion exchange whereby the MA⁺ ions in the MAPbI₃ are replaced by Cs⁺ (as temperature ramps up) and iii) form g‐phase perovskite CsPbI₃. Optical microscopy, photoluminescence, and electrical characterizations reveal that the CEG process produces high‐quality film with better absorption, uniform and dense film with better interface, lower defects, and better stability. Using the CEG approach, the power conversion efficiency of the best CsPbI₃ solar cell is significantly increased up to 14.1% for the device fabricated using 1.0 m MAI additive. The outcome is beneficial for further improvement of inorganic perovskite solar cells and their application in perovskite‐silicon tandem devices.

Vacuum‐deposited methylammonium lead iodide can adopt a perovskite structure with a stable cubic lattice at room temperature. Reducing the metallic salt evaporation rate leads to a tetragonal phase structure. This room‐temperature cubic perovskite circumvents the tetragonal to cubic phase transition resulting at ≈55 °C, and leads to photovoltaic devices with efficiencies above 19%.

Abstract

Methylammonium lead triiodide (MAPI) has emerged as a high‐performance photovoltaic material. Common understanding is that at room temperature, it adopts a tetragonal phase and it only converts to the perfect cubic phase around 50–60 °C. Most MAPI films are prepared using a solution‐based coating process, yet they can also be obtained by vapor‐phase deposition methods. Vapor‐phase‐processed MAPI films have significantly different characteristics than their solvent‐processed analogous, such as relatively small crystal‐grain sizes and short excited‐state lifetimes. However, solar cells based on vapor‐phase‐processed MAPI films exhibit high power‐conversion efficiencies. Surprisingly, after detailed characterization it is found that the vapor‐phase‐processed MAPI films adopt a cubic crystal structure at room temperature that is stable for weeks, even in ambient atmosphere. Furthermore, it is demonstrated that by tuning the deposition rates of both precursors during codeposition it is possible to vary the perovskite phase from cubic to tetragonal at room temperature. These findings challenge the common belief that MAPI is only stable in the tetragonal phase at room temperature.