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Perovskite solar cells (PSCs) have gained tremendous research interest recently owing to several advantages, including low material cost, facile solution processability, bandgap tunability, and alluring device efficiency. The organic formamidinium (FA) cation‐based perovskites are mainly considered as one of the potential candidates for charge carrier generation due to their excellent properties, such as bandgap and thermal stability than traditional perovskites. However, the inevitable unfavorable polymorphism (i.e., α to δ) at room temperature still forms the basis for numerous research works to allow the fabrication of a high‐quality absorber and enhances the PSCs performance. The studies to resolve the polymorphism and several contemporary techniques (e.g., passivation strategy) with several recent novel fabrication methods presented in this review form the essence of the improvements in PSCs. The absorber morphology also influences the charge‐transfer behavior and the device's lifetime. Therefore, understanding these properties is essential to improve the absorber quality and avoid many defects. This review focuses on the structure and properties of pure and mixed FA perovskites with various halides, mainly the FA cation's role in the absorber composition. And a comprehensive overview of recent FA cation‐based double, triple, and quadrupole PSCs results with proper scientific explanations to understand the device physics. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of a novel oligomeric cyclic carbazole-based material (PCz1) for an application as hole transporter (HTM) in hybrid perovskite photovoltaic devices are reported. Its preliminary application in Cs0.08FA0.80MA0.12Pb(I0.88Br0.12)3 mixed perovskite solar cell (PSC) leads to a device power conversion efficiency of 18.04% which is comparable to that of the Spiro-OMeTAD reference cells (17.94%). In addition, the novel HTM is proved to act as a good barrier and protect satisfactorily the perovskite surface giving highly stable PSC devices. [ABSTRACT FROM AUTHOR]

The incorporation of large organic ammonium ions renders the crystallization dynamics and layer formation process of halide perovskites complex, difficult to control, and leads to problems of suppressed charge transport with the formation of tiny‐sized grains. In this paper, the use of methylammonium chloride (MACl) and an excess of PbI2 is introduced as a co‐additives in the precursor solution for the control of phenylmethylammonium or benzylammonium (PMA+ spacer) and formamidinium (FA+)‐based quasi‐2D PMA2FAn−1PbnI3n+1 (n = 5) perovskite layers formation. By this method, the morphology of the layer, the inner phase distribution, and the charge transport properties are improved. By employing glow discharge‐optical emission spectroscopy (GD‐OES) and other techniques, it is revealed that the quasi‐2D perovskites prepared in the presence of co‐additives exhibit uniform removal dynamics of the solvent across the film. Furthermore, the grain growth mode, upon thermal annealing, is lateral. It results in large, monolithic grains with low‐trap state density and excellent substrate coverage. Particularly, co‐additives improve the cations dispersion upon the crystallization process, thus suppressing the low‐n phase formed through the aggregation of spacer cations and accelerating the formation of the high‐n phase. [ABSTRACT FROM AUTHOR]

Numerous experiments have demonstrated improvements on the efficiency of perovskite solar cells by introducing plasmonic nanoparticles, however, the underlying mechanisms are still not clear: the particles may enhance light absorption and scattering, as well as charge separation and transfer, or the perovskite's crystalline quality. Eventually, it can still be debated whether unambiguous plasmonic increase of light absorption has indeed been achieved. Here, various optical models are employed to provide a physical understanding of the relevant parameters in plasmonic perovskite cells and the conditions under which light absorption may be enhanced by plasmonic mechanisms. By applying the recent generalized Mie theory to gold nanospheres in perovskite, it is shown that their plasmon resonance is conveniently located in the 650–800 nm wavelength range, where absorption enhancement is most needed. It is evaluated for which active layer thickness and nanoparticle concentration a significant enhancement can be expected. Finally, the experimental literature on plasmonic perovskite solar cells is analyzed in light of this theoretical description. It is estimated that only a tiny portion of these reports can be associated with light absorption and point out the importance of reporting the perovskite thickness and nanoparticle concentration in order to assess the presence of plasmonic effects. [ABSTRACT FROM AUTHOR]

Transient photocurrent is a widely applied characterization technique to probe the charge‐carrier photogeneration and extraction dynamics in perovskite optoelectronic devices. Despite the large number of studies on the properties of perovskite single‐crystals (SCs) photodetectors (PDs), the underlying mechanism that governs the spectral line shape of transient photocurrent is not fully understood. Here, methylammonium lead bromide (MAPbBr3)SC based PDs are used to study the effect of different electrode spacing and bias on the transient photocurrent response under blue and green light irradiation. The observed differences in the spectral line shape of the transient photocurrent are explained using three‐step carrier transport model, which reveals the occurrence of carrier trapping and a recombination process in MAPbBr3 SC. The findings are further corroborated by intensity‐dependent photocurrent and impedance spectroscopy analysis of the resulting PDs. This work provides a basic insight into the origin of the different behavior of transient photocurrent response under variable electrode distance, bias, and irradiance light, which is expected to help to further understand and optimize the performance of perovskite based PDs. [ABSTRACT FROM AUTHOR]

The recent remarkable achievements in terms of efficiency performance and stability of hybrid organic–inorganic perovskite solar cells (PSCs) are notably obtained by optimizing the A‐site cation composition of formamidinium‐based 3D perovskites. As methylammonium chloride is ubiquitously employed in precursor solution for very high efficiency formamidinium‐based PSC, the purpose of the present paper is to unveil the exact role of methylammonium (MA+), chloride, and solvent on the film growth and their fate upon the layer thermal annealing process. Methylammonium is shown to react with formamidinium to form two methyl compounds while its excess is eliminated along with chloride. The final perovskite layer A‐sites are mainly occupied by formamidinium, whereas MA+ content is only 2–3 mol%, and below the 3‐N‐methyl formamidinium one. 1‐N‐methyl formamidinium is detected as traces. Meanwhile, the solvent is homogenously eliminated throughout the layer thickness. Upon thermal aging stress, the layer is degraded from its top with the formation of PbI2. MA+ is rapidly fully eliminated while the stable methyl formamidinium compounds remain in the perovskite layer. [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PSCs) have undergone unprecedented growth in the past decade as an emerging photovoltaic technology. Up till now, the power conversion efficiency of PSCs has exceeded 25% that rivals silicon solar cells and there is still room for further enhancement. However, the development in long‐term stability lags far behind, which remains a great concern for the commercial application in the future. The device instability mainly arises from the functional components, including perovskite film, charge transport layers, and electrodes along with the involved interfaces. As the most widely studied hole transport layer at the current stage, 2,2′,7,7′‐tetrakis(N,N‐di(4‐methoxyphenyl)amino)‐9,9‐spirobifluorene (Spiro‐OMeTAD) helps contribute to the achievement of record efficiency but it weakens the device stability due to the doping‐induced side effects such as hygroscopicity and ion migration. Great efforts are devoted to boosting the stability of Spiro‐OMeTAD while maintaining excellent photovoltaic performance. In this review, the fundamental properties of Spiro‐OMeTAD have been summarized and the recent advances in engineering Spiro‐OMeTAD‐based hole transport layer for the sake of highly efficient PSCs with enhanced longevity are highlighted. In the end, an outlook for the further optimization of Spiro‐OMeTAD is provided and the issues related to large‐scale production are discussed. [ABSTRACT FROM AUTHOR]

The emerging broad range of applications of the glow discharge optical emission spectroscopy (GD‐OES) technique in the field of perovskite solar cells (PSCs) research is reviewed. It can provide a large palette of information by easily and quickly tracking the depth distribution of light to heavy elements. After a discussion of the advantages and the limitations of the technique and a comparison with other analytical techniques, how GD‐OES is employed to give structural information on perovskite solar cells is shown. GD‐OES has allowed the full perovskite film formation process investigation, from the initial precursor layers containing soaking and complexed solvent to the final crystallized 3D perovskite layers. The A‐site elemental cations distribution is followed‐up during the film formation. In addition, this technique gives a deep insight into the action mechanism of additives and their effects on the film formation. It provides fruitful information on optimized light absorbing layers and on the selective contact layers which ensure the charge transport in PSCs. It allows to directly visualize halide ions migration and their blocking by ad‐hoc chemical engineering and to study the films and PSCs ageing. GD‐OES opens new perspectives to explain the final performances of the devices. [ABSTRACT FROM AUTHOR]

An essential role is played by the charge selective layers, in particular, the hole transport layer (HTL) in determining the performance of perovskite solar cells (PSCs). HTL contribution is even more substantial in the inverted PSCs (p‐i‐n) as it also affects the crystallization process of the overlayer perovskite deposition. The influence of HTL on inverted PSCs was probed, the role through admittance spectroscopy quantified, and the photovoltaic performance correlation with the microstructure of the perovskite established. By measuring the low‐frequency impedance responses of inverted PSCs, the photo‐induced charge transfer dynamics in the PSCs were deciphered, revealing negative chemical inductor features at low frequencies in the Nyquist plots. The noted additional recombination pathways are attributed to the ionic vacancies in the interfacial states. The PSCs fabricated with poly(triaryl amine) showed a higher open‐circuit voltage, lower recombination, and higher charge carrier lifetime. The chemical inductance behavior was supported through variable temperature frequency‐dependent capacitance measurements. [ABSTRACT FROM AUTHOR]

Hole-transport material (HTM)-free mesoporous perovskite solar cells (PSCs) with a carbon electrode are of great interest because they can be fabricated using inexpensive carbon materials and simple screen-printing methods. In this work, we investigated the influence of the components on photovoltaic performance of HTM-free mesoporous PSCs with a carbon electrode using electrochemical impedance spectroscopy analysis. The results showed that the series resistance was increased as the thickness of the compact layer and mesoporous ZrO2 layers increased, whereas it decreased with increasing thickness of mesoporous TiO2 and carbon layers. It was also found that charge transfer resistance decreases with increasing thickness of mesoporous TiO2, ZrO2 and carbon layers, and charge recombination resistance is mainly influenced by the compact layer, mesoporous TiO2 and ZrO2 layers, and the chemical capacitance during charge recombination is mainly influenced by the mesoporous TiO2 layer. The efficiency of the device fabricated using the optimized parameters was 15.05%. [ABSTRACT FROM AUTHOR]

Hole transport materials (HTMs) play a requisite role in n–i–p perovskite solar cells (PSCs). The properties of HTMs, such as hole extraction efficiency, chemical compatibility, film morphology, ion migration barrier, and so on, significantly affect PSCs' power conversion efficiencies (PCEs) and stabilities. Up till now, researchers have devoted much attention to developing new types of HTMs as well as promoting pristine HTMs using numerous strategies. In this Review, we summarize the design strategies of various common HTMs for n–i–p PSCs are comprehensively discussed from two separate aspects (additive and non‐additive engineering). Additive engineering generally tunes electronic properties of HTMs while non‐additive engineering basically modifies their steric structures. Critical analysis and comparison between these design strategies are provided, considering the overall PCEs and stabilities of PSCs. Finally, a brief perspective on future promising design strategies for HTMs is given, in order to fabricate efficient and stable n–i–p devices for the commercialization of PSCs. [ABSTRACT FROM AUTHOR]

Polytriarylamine is a popular hole‐transporting materials (HTMs) despite its suboptimal conductivity and significant recombination at the interface in a solar cell setup. Having noted insufficient conjugation among the triarylamine units along the polymer backbone, we inserted a bithiophene unit between two triarylamine units through iron‐catalyzed C−H/C−H coupling of a triarylamine/thiophene monomer so that two units conjugate effectively via four quinoidal rings when the molecule functions as HTM. The obtained triarylamine/bithiophene copolymer (TABT) used as HTM showed a high‐performance in methylammonium lead iodide perovskite (MAPbI3) solar cells. Mesityl substituted TABT forms a uniform film, shows high hole‐carrier mobility, and has an ionization potential (IP=5.40 eV) matching that of MAPbI3. We fabricated a solar cell device with a power conversion efficiency of 21.3 % and an open‐circuit voltage of 1.15 V, which exceeds the performance of devices using reference standard such as poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine (PTAA) and Spiro‐OMeTAD. [ABSTRACT FROM AUTHOR]

Polytriarylamine is a popular hole‐transporting materials (HTMs) despite its suboptimal conductivity and significant recombination at the interface in a solar cell setup. Having noted insufficient conjugation among the triarylamine units along the polymer backbone, we inserted a bithiophene unit between two triarylamine units through iron‐catalyzed C−H/C−H coupling of a triarylamine/thiophene monomer so that two units conjugate effectively via four quinoidal rings when the molecule functions as HTM. The obtained triarylamine/bithiophene copolymer (TABT) used as HTM showed a high‐performance in methylammonium lead iodide perovskite (MAPbI3) solar cells. Mesityl substituted TABT forms a uniform film, shows high hole‐carrier mobility, and has an ionization potential (IP=5.40 eV) matching that of MAPbI3. We fabricated a solar cell device with a power conversion efficiency of 21.3 % and an open‐circuit voltage of 1.15 V, which exceeds the performance of devices using reference standard such as poly[bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine (PTAA) and Spiro‐OMeTAD. [ABSTRACT FROM AUTHOR]

Five donor‐acceptor‐donor (D‐A‐D) hole transport materials (HTMs) are investigated to reveal the effect of core units on the structure‐property relationship. Structurally, the five electron‐withdrawing cores are derivatives of diazosulfide, and the bis(N, N‐bis(4‐[methoxy]phenyl))aniline segment performed as the symmetrical end‐capping. The DFT and TD‐DFT coupled with the Marcus theory are used to look into electronic structures, ground and excited state properties, charge transport mobilities, and charge transfer analyses. It is found that the HTM‐PT shows little differences in planarity, ionization potential, and solubility compared with the original molecule. Additionally, the HTM‐PT exhibits lower frontier molecular orbital (FMOs), smaller bandgap, and red‐shift than HTM‐DP. More importantly, among the four designed HTMs, the HTM‐PT shows a larger hopping rate and mobility. As the charge transfer analyses, the HTM‐PT exhibits a larger intramolecular charger transfer amount and charge transfer distance than the prototypical one, which is conducive to a larger short‐circuit current. Through the evaluation from various aspects, it is reasonable to infer that PT‐core with excellent properties has potential advantages in regulating HTMs. [ABSTRACT FROM AUTHOR]

In an organometallic CH3NH3PbI3 (MAPbI3) perovskite solar cell, we have demonstrated a vastly compact TiO2 layer synthesized by double-step deposition, through a combination of sputter and solution deposition to minimize the electron-hole recombination and boost the power conversion efficiency. As a result, the double-step strategy allowed outstanding transmittance of blocking layer. Additionally, crystallinity and morphology of the perovskite film were significantly modified, provoking enhanced photon absorption and solar cell performance with the reduced recombination rate. Thereby, this straightforward double-step strategy for the blocking layer exhibited 12.31% conversion efficiency through morphological improvements of each layer. [ABSTRACT FROM AUTHOR]

Incorporating multiple cations of the 1A alkali metal column of the periodic table (K+/Rb+/Cs+) to prepare perovskite films is promising for boosting photovoltaic properties but requires a uniform distribution. The effects of NH4Cl additives and alkali metal cations (K+/Rb+/Cs+) on the one‐step formation process of methylammonium‐free, formamidinium‐based, iodide perovskite films are analyzed in a step‐by‐step manner. NH4Cl improves the solubility of PbI2 in solution by forming an intermediate and then favors the perovskite phase formation. Moreover, during the annealing process, this additive is shown to increase grain size, to improve crystallinity and to suppress PbI2 formation. K at low concentration is always homogeneously distributed across the film thickness. On the other hand, Cs is more concentrated at the surface and Rb in the depths of pristine films. With NH4Cl additives, these two alkali metals are more homogeneously distributed because NH4Cl slows down the movement of Cs+ and Rb+, it changes the growth direction of the perovskite film, making the overall crystallization quality improved and the distribution more uniform. It results in perovskite films with large monolithic grains. Combined with a perovskite film surface treatment with n‐propylammonium iodide, a high stabilized power conversion efficiency of 22.04% is reached. [ABSTRACT FROM AUTHOR]

Understanding the device kinetics occurring in the bulk and at the charge selective interfaces is vital for optimizing the performance of perovskite solar cells (PSCs) and their development. Planar PSCs are studied with and without hole transport layer (HTL) and power conversion efficiency of 16.23% is measured with poly(3-hexylthiophene) [P3HT] as HTL, while the PSCs without HTL show significant hysteresis. A comprehensive electrical response of the PSCs is investigated. The bias-dependent electrochemical impedance spectroscopy (EIS) reveals the appearance of the negative capacitance in the low-frequency regime. The modification of the low-frequency PSCs response accompanied by a further decrease in recombination resistance is due to the interfacial interactions between the migrating ions and the metal contact. The doped and undoped P3HT layers impede this interaction which explains the absence of the negative capacitance at lower applied voltages. The investigation provides an understanding of the physical processes behind the negative capacitance. [ABSTRACT FROM AUTHOR]

Herein, the synthesis of methylammonium‐free and bromide‐free CsxFA1−xPbI3 (FA for formamidinium) perovskite (PVK) photovoltaic layers with intrinsic outstanding properties in terms of crystallinity, defect waiving/passivation, and ionic mobility blocking by using a coadditives approach is described. It consists in mixing two chloride additives in the PVK precursor solution: potassium chloride (KCl) and ammonium chloride (NH4Cl). KCl favors the lead iodide (PbI2) precursor solubilization and results in a purer PVK phase. NH4Cl allows the control of the crystallization growth speed, leading to the formation of large crystal grains and well‐crystallized layers. By the glow‐discharge optical emission spectroscopy (GD‐OES) technique, it is directly visualized that potassium incorporated in the whole film blocks the iodide mobility by defect passivation. Also, the reduction (or suppression) of iodide mobility and the reduction (or suppression) of the J–V curve hysteresis is clearly correlated. It is found that blocking the ionic mobility is insufficient to fully stabilize the halide perovskite material which also requires the crystallization monitoring carried out in parallel by the second additive. It resulted in Cs0.1FA0.9PbI3 cells with a stabilized power conversion efficiency (PCE) of 20.02% and with superior stability. [ABSTRACT FROM AUTHOR]

The rise and commercialization of perovskite solar cells (PSCs) is hindered by the toxicity of lead present in the perovskites used as the solar light absorber. To counter this problem, lead (Pb) can be fully (lead‐free) or partially (lead‐less) replaced by diverse elements. The former compounds suffer from poor efficiency and poor stability, whereas the later appear more promising. Herein, a survey of the methods reported in the literature to reduce Pb content in PSCs to fabricate "lead‐less" (also called "lead‐deficient") PSCs is offered. First, the comparison of Sn and Pb elements and the partial replacement of Pb by Sn are developed. Then, its substitution by either Ge, Sr, or other alkaline‐earth‐metals, transition metals, and elements from columns 12, 13, and 15 of the periodic table are detailed. The new families of perovskites based on the insertion of organic cations to replace lead and halogen units, namely the "lead‐deficient" and "hollow" halide perovskites are then presented and discussed. Finally, atypical ways to reduce the toxicity of PSCs are presented: perovskite layer thickness reduction via optimization of photon collection, integration of photonic structures, and usage of recycled lead. The current achievements and the outlook of those strategies are presented and discussed. [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PSCs) have reached their highest efficiency with the state-of-the-art hole-transporting material (HTM) spiro-OMeTAD. However, the relatively low conductivity and high cost of this material significantly limit the commercialization of this promising technology. In this work, a low-cost, asymmetric HTM containing a carbazole core and peripheral triphenylamine moieties, named Cz-3OMeTAD, was designed and synthesized via a facile three-step synthetic procedure and employed successfully in planar CH3NH3PbI3 PSCs. The optical and electrochemical data of the asymmetric HTM Cz-3OMeTAD exhibited appropriate energy levels for efficient interfacial charge transfer with the perovskite. The asymmetric propeller-shaped Cz-3OMeTAD has a relatively high mobility value of 1.42 × 10−3 cm2 V−1 s−1, beneficial for the PSC performance. Moreover, steady-state PL and time-resolved PL measurements indicate successful hole extraction properties of Cz-3OMeTAD from the perovskite at the perovskite/HTM interface. Consequently, the Cz-3OMeTAD-based devices achieved a high efficiency of 16.36% in PSCs under 1 sun conditions, comparable with the efficiency obtained for the reference cell using spiro-OMeTAD. Thus, our result demonstrated the asymmetric carbazole-based material as a new class of HTM for highly efficient PSCs. [ABSTRACT FROM AUTHOR]

Multiple-ring naphthalenediammonium is employed for the first time to overcome the intrinsic instability of β-CsPbI3 perovskite via anchoring the ammonium groups occupying A-site vacancies. It improves charge transport and moisture stability giving out a champion power conversion efficiency of 16.69% for an excellent inorganic perovskite solar cell. [ABSTRACT FROM AUTHOR]

The issue of long‐term stability is one of the main obstacles challenging the progress of perovskite solar cells (PSCs). To alleviate this issue, a thorough understanding of the degradation mechanisms of the device is required. Herein, electrochemical impedance measurements in combination with maximum power point (MPP) tracking are applied to characterize PSCs, aiming to gain an understanding of the charge carrier dynamics in the photoactive bulk and at the contact‐absorber‐interfaces under operation. Electrochemical impedance spectroscopy (EIS) results show that the device charge transport resistance and interface capacitance associated with charge accumulation at the interfaces are both increasing upon continuous operation. This suggests ion migration from the photoactive perovskite layer to the charge transport layer interfaces leaving defects in the bulk. This suggests that reduction of the device performance upon continuous operation is mainly related to the changes in the bulk of the photoactive perovskite film and ions migration. [ABSTRACT FROM AUTHOR]

Nowadays, complex chemistry and precursor solution compositions are developed to stabilize hybrid perovskite films and boost the efficiency of perovskite solar cells (PSCs). In this context, determining the actual composition of these layers, especially in organic cations, and understanding the chemistry behind is challenging. Herein, the introduction of methylammonium (MA+) in formamidinium lead iodide (FAPbI3) 3D perovskite is considered to stabilize the α‐phase, whose quantity must be minimized to reduce the material hydrophilicity and its possible destabilization by degassing. The key effects of methylammonium chloride (MACl) additive on the growth of FA1–xMAxPbI3 perovskite layers are studied. Liquid nuclear magnetic resonance (NMR) is used to analyze the photovoltaic layers. NMR peaks and their origin are identified. The MA and FA content in films actually used in PSCs is reliably measured and prepared over a large additive molar concentration ratio. x is quantified at 0.06 ± 0.01 for pure films, which corresponds to the best entropic compound stabilization. It results in PSCs with a stabilized power conversion efficiency as high as 22.06%. These PSCs are shown to be highly stable under solar irradiation and high moisture. [ABSTRACT FROM AUTHOR]

Solar cells based on organo-metal halide perovskites have gained unprecedented research interest over the last few years due to their low-cost solution processability, high power conversion efficiency, which has recently reached a certified value of 25.2%, and abundance of raw materials. Nevertheless, the best efficiencies remain below the Shockley–Queisser theoretical limit of 32.5% due to several losses arising from either defect traps present in the bulk of the perovskite absorber or at the device heterointerfaces. While bulk defects are detrimental for the device performance by mainly limiting the open circuit voltage, interfacial layers are also crucial. They dictate the charge transfer/transport from the perovskite layer to the collecting electrodes, hence influencing the device photocurrent, but also act as protective barriers against oxygen and moisture penetration. Molecular materials and additives are widely used to improve the bulk properties of perovskite absorbers through the formation of high-quality perovskite films with superior optoelectronic properties, and improved crystallinity, and also of electronically clean interfaces with minimum losses during charge transfer/transport. In this review, we analyze the predominant pathways that contribute to voltage and current losses due to poor interfaces and also due to non-radiative recombination losses arising from inferior perovskite morphology and its inherent polycrystalline and highly defective nature. We then discuss strategies for achieving interfacial organic and inorganic molecular materials for application as electron and hole transport layers in perovskite solar cells with ideal energy levels, high charge mobilities and improved thermal, photo, and structural stability. Moreover, the prerequisites for molecular additives to achieve dimensionality engineering, defect passivation, molecular cross-linking, interfacial energy alignment and electronic doping are thoroughly discussed. Finally, we examine prospects for future research directions and commercialization. [ABSTRACT FROM AUTHOR]

Photoinduced capacitances of perovskite solar cells exhibit peculiarities such as an apparently high capacitance and an inductive feature, so‐called negative capacitance, which are not easily explained with typical carrier dynamics. Consequently, the origins of the photoinduced capacitances in perovskite solar cells have been intensively debated over the past several years. Here, the photoinduced capacitances of perovskite solar cells are analyzed by impedance spectroscopy. The analysis clarifies that the photoinduced capacitances of perovskite solar cells comprise several Debye relaxation‐type capacitance components. Among these components, the photoinduced capacitance in the low‐frequency range is attributed to ionic‐to‐electronic current amplification. However, the photoinduced capacitances in the middle‐ and high‐frequency ranges originate from bipolar injection. The clear elucidation of the origins of the photoinduced capacitances in perovskite solar cells provides comprehensive insights for analyzing properties of perovskites in either the time or frequency domain. [ABSTRACT FROM AUTHOR]

Perovskite solar cells (PSCs) have shown great potential for photovoltaic applications with their unprecedented power conversion efficiency advancement. Such devices generally have a complex structure design with high temperature processed TiO2 as the electron transport layer (ETL). Further careful design of device configuration to fully tap the potentials of perovskite materials is expected. Particularly, for the practical application of PSCs, it is crucial to simplify their device structures thus the associated manufacturing process and cost while maintaining their efficiency to be comparable with the conventional devices. But how simple is simple? ETL-free PSCs promise the simplest structured, thus simple manufacturing processes and low cost large area PSCs in practical applications. They can also help the further exploration of the great potential of perovskite materials and understanding the working principle of PSCs. Within this review, the evolution of the PSC is outlined by discussing the recent advances in the simplification of device configuration and processes for cost effective, highly efficient, and robust PSCs, with a focus on ETLfree PSCs. Their advancement, key issues, working mechanism, existing problems, and future performance enhancements. This review aims to promote the future development of low cost and robust ETL-free PSCs toward more efficient power output. [ABSTRACT FROM AUTHOR]

In perovskite (PSK) solar cells, the selective contacts between interfaces of PSK and charge carrier have an important role in power conversion efficiency (PCE). The active defect sites in the device interfaces control the charge and ionic accumulation that can disturb the operation of devices. In this work, mesoporous PSK solar cells were fabricated and the interfacial defects between polymer HTL and PSK layers were neutralized by modifying HTL. 31% PCE enhancement was achieved by replacing poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) HTL instead of the P3HT HTL in the mesoporous perovskite device. The achieved PCE improvement strongly depends on the interface between PSK and HTM that was studied by optical and electrochemical impedance analyses. Introducing PCBM to P3HT HTL overcame the challenge of the interface defects caused by the non-uniformity of the PSK layer and the inappropriate presence of the pinholes. This architecture reduced the shunt-leakage paths and enhanced the incident photon harvesting, leading to the enhancement of PCE from 10 to 13.14%. These desired effects of P3HT: PCBM were confirmed by enhancement of the recombination resistance and reduction of the charge accumulation at the interfaces. Further, the defect passivation by interface modification reduced the hysteresis behavior of the PSK device. [ABSTRACT FROM AUTHOR]

Perylene diimide‐based small molecules are widely used as intermediates of liquid crystals, owing to their high planarity and electron mobility. In this study, tetrachloroperylene diimide (TCl‐PDI) was used as a small‐molecule replacement for TiO2 as electron‐transporting material (ETM) for planar perovskite solar cells (PVSCs). Among hole‐transporting materials (HTMs) for PVSCs, poly(3‐hexylthiophene) (P3HT) gives the devices the highest stability and reproducibility. Therefore, PVSCs with the structure of indium tin oxide (ITO)/ETM/perovskite/P3HT/MoO3/Ag were used to evaluate the performances of new ETMs. A reference device with compact TiO2 and P3HT gave a reasonable power conversion efficiency (PCE) of 12.78 %, whereas the PVSC with TCl‐PDI as ETM gave an enhanced PCE of 14.73 %, which is among the highest reported values for PVSCs with undoped P3HT as the HTM. Moreover, TCl‐PDI‐based devices displayed higher stability than those based on compact TiO2, owing to the superior perovskite quality. Tetrachloroperylene diimide (TCl‐PDI) has suitable energy levels, high thermal stability, and high electron mobility, and also induces a better crystalline phase of perovskite with large crystalline domains. Owing to these merits, when used as an electron‐transporting material, TCl‐PDI endows perovskite solar cells with better photovoltaic performance and stability. [ABSTRACT FROM AUTHOR]

Further efficiency boost of organic-inorganic perovskite solar cells is hampered by limited knowledge on ion migration, inductive loops, and the relationship between structures and properties in organometal halide perovskites. In this work, in situ alternating current impedance spectroscopy measurements on CH(NH2)2PbBr3 (FAPbBr3) have been carried out under high pressure up to 4.8 GPa. The inductive loop has been discovered at low frequencies and can be tuned dramatically by applying pressure, which is attributed to large FA ion migration in FAPbBr3. Two discontinuous changes are observed in both ionic and electronic resistances around phase transition pressure. The pressure dependent photoresponse of FAPbBr3 has also been studied by in situ photocurrent measurements under high pressure up to 3.8 GPa. It indicates that the photocurrent of FAPbBr3 can be enhanced remarkably at 1.3 GPa and the largest photocurrent value in FAPbBr3 is nearly 10 times larger than that in CH3NH3PbBr3 and about 3 times larger than that in CH3NH3PbI3. [ABSTRACT FROM AUTHOR]

3D and 2D hybrid perovskites, which have been known for more than 20 years, have emerged recently as promising materials for optoelectronic applications, particularly the 3D compound (CH3NH3)PbI3 (MAPI). The discovery of a new family of hybrid perovskites called d-MAPI is reported: the association of PbI2 with both methyl ammonium (MA+) and hydroxyethyl ammonium (HEA+) cations leads to a series of five compounds with general formulation (MA)1−2.48x(HEA)3.48x[Pb1−xI3−x]. These materials, which are lead- and iodide-deficient compared to MAPI while retaining 3D architecture, can be considered as a bridge between the 2D and 3D materials. Moreover, they can be prepared as crystallized thin films by spin-coating. These new 3D materials appear very promising for optoelectronic applications, not only because of their reduced lead content, but also in account of the large flexibility of their chemical composition through potential substitutions of MA+, HEA+, Pb2+ and I− ions. [ABSTRACT FROM AUTHOR]

3D and 2D hybrid perovskites, which have been known for more than 20 years, have emerged recently as promising materials for optoelectronic applications, particularly the 3D compound (CH3NH3)PbI3 (MAPI). The discovery of a new family of hybrid perovskites called d-MAPI is reported: the association of PbI2 with both methyl ammonium (MA+) and hydroxyethyl ammonium (HEA+) cations leads to a series of five compounds with general formulation (MA)1−2.48x(HEA)3.48x[Pb1−xI3−x]. These materials, which are lead- and iodide-deficient compared to MAPI while retaining 3D architecture, can be considered as a bridge between the 2D and 3D materials. Moreover, they can be prepared as crystallized thin films by spin-coating. These new 3D materials appear very promising for optoelectronic applications, not only because of their reduced lead content, but also in account of the large flexibility of their chemical composition through potential substitutions of MA+, HEA+, Pb2+ and I− ions. [ABSTRACT FROM AUTHOR]

Impedance spectroscopy is a powerful electrochemical small-perturbation technique that provides dynamic electrical data in solar cells. This technique has been widely used to characterize dye-sensitized solar cells and perovskite solar cells (PSCs). Physical parameters are normally obtained by fitting to an equivalent circuit, composed of electrical elements which theoretically correspond to physical processes involved in the photoconversion process. A variety of equivalent circuits to model the impedance spectra of PSCs are commonly used by different research groups. In this work, we evaluate their performance and adequacy. We demonstrate the analytical and numerical equivalence of impedance expressions for Voight, matryoshka, and hybrid circuits, which are used to fit a typical impedance spectrum of a PSC and compare the resulting parameters to the empirical values obtained without any equivalent circuit. The numerical equivalence can be demonstrated by using two- and three-component impedance spectra. In contrast, Maxwell-type equivalent circuits reveal parameters that have a more complex relation to empirical values. The presence of inductive effects such as 'loops' and 'negative tails' in impedance spectra are also discussed in terms of negative values of resistances and capacitances. We propose a general protocol to analyze impedance data of PSCs and to extract useful information from them. [ABSTRACT FROM AUTHOR]

The molecular organic Lewis acid bis(pentafluorophenyl)zinc [Zn(C6F5)2] is reported as an efficient p‐type dopant for poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), to be used as hole‐transporting material (HTM) in perovskite solar cells (PSCs) for the first time. To date, the most efficient PSCs use lithium bis(trifluoromethane)sulfonimide lithium salt (LiTFSI) and 4‐tert‐butylpyridine (tBP) as standard additives for HTMs. However, such dopants can induce deleterious effects on device stability. Herein, the effect of the concentration of Zn(C6F5)2 in P3HT HTM on the performance of PSCs is investigated. The P3HT‐based PSCs using a low concentration of the dopant (0.025 mol%) in the HTM layer exhibit the best performance and the highest power conversion efficiency (PCE) of 17.49%, which is almost 3.5% higher than the achieved PCE for pristine P3HT‐based PSCs. The origin of the improved performance for PSCs is further investigated, by studying the conductivity and hole mobility of the thin films based on pristine and doped P3HT. Adding a small amount of Zn(C6F5)2 to P3HT increases its thin‐film hole mobility and its hole extraction ability. [ABSTRACT FROM AUTHOR]

Herein, commercially accessible SnO2 and home‐made TiO2 nanoparticles as a combined electron transporting bilayer (ETBL) are applied to achieve highly efficient planar perovskite solar cells (PSCs). With the formed cascade‐aligned energy levels from the proper stacking of SnO2 and TiO2 layers and the excellent defect‐passivation ability of TiO2, SnO2/TiO2 ETBLs effectively reduce energy loss and inhibit defects formation both at the electron transporting layers (ETL)/perovskite interfaces and within the bulk of perovskite layer as revealed by a comprehensive analysis of photoelectric characteristic analysis, including ultraviolet photoelectron spectroscopy, photoluminescence, and electrochemical impedance spectroscopy. Consequently, the PSC devices acquired a power conversion efficiency (PCE) of 20.50% with a Voc of 1.10 V, a Jsc of 24.2 mA cm−2 and an fill factor of 77%, which are superior to the values of the control device based on single SnO2 layer with a PCE of 18.09% (a 13.3% boosting on PCE). Moreover, there was no degradation after 49 days, indicating the great stability after adding TiO2 layer. Herein, it is demonstrated that the cascaded alignment of energy levels between the electrode and perovskite layer by ETBLs could be an effective approach to improve the photovoltaic performance of the PSCs with excellent long‐term stability. [ABSTRACT FROM AUTHOR]

Recent advances in perovskite solar cells (PSCs) have resulted in greater than 23% efficiency with superior advantages such as flexibility and solution‐processability, allowing PSCs to be fabricated by a high‐throughput and low‐cost roll‐to‐roll (R2R) process. The development of scalable deposition processes is crucial to realize R2R production of flexible PSCs. Gravure printing is a promising candidate with the benefit of direct printing of the desired layer with arbitrary shape and size by using the R2R process. Here, flexible PSCs are fabricated by gravure printing. Printing inks and processing parameters are optimized to obtain smooth and uniform films. SnO2 nanoparticles are uniformly printed by reducing surface tension. Perovskite layers are successfully formed by optimizing the printing parameters and subsequent antisolvent bathing. 2,2′,7,7′‐Tetrakis‐(N,N‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene is also successfully printed. The all‐gravure‐printed device exhibits 17.2% champion efficiency, with 15.5% maximum power point tracking efficiency for 1000 s. Gravure‐printed flexible PSCs based on a two‐step deposition of perovskite layer are also demonstrated. Furthermore, a R2R process based on the gravure printing is demonstrated. The champion efficiency of 9.7% is achieved for partly R2R‐processed PSCs based on a two‐step fabrication of the perovskite layer. Gravure printing for flexible perovskite solar cells is presented. A perovskite layer is successfully printed based on both one‐ and two‐step processes. The all‐printed flexible perovskite solar cells fabricated by sequential gravure printing of hole‐transporting, perovskite, and electron‐transporting layers exhibit 17.2% champion efficiency. Remarkably, partly roll‐to‐roll processed, flexible perovskite solar cells show 9.7% champion efficiency. [ABSTRACT FROM AUTHOR]

Recent advances in perovskite solar cells (PSCs) have resulted in greater than 23% efficiency with superior advantages such as flexibility and solution‐processability, allowing PSCs to be fabricated by a high‐throughput and low‐cost roll‐to‐roll (R2R) process. The development of scalable deposition processes is crucial to realize R2R production of flexible PSCs. Gravure printing is a promising candidate with the benefit of direct printing of the desired layer with arbitrary shape and size by using the R2R process. Here, flexible PSCs are fabricated by gravure printing. Printing inks and processing parameters are optimized to obtain smooth and uniform films. SnO2 nanoparticles are uniformly printed by reducing surface tension. Perovskite layers are successfully formed by optimizing the printing parameters and subsequent antisolvent bathing. 2,2′,7,7′‐Tetrakis‐(N,N‐di‐4‐methoxyphenylamino)‐9,9′‐spirobifluorene is also successfully printed. The all‐gravure‐printed device exhibits 17.2% champion efficiency, with 15.5% maximum power point tracking efficiency for 1000 s. Gravure‐printed flexible PSCs based on a two‐step deposition of perovskite layer are also demonstrated. Furthermore, a R2R process based on the gravure printing is demonstrated. The champion efficiency of 9.7% is achieved for partly R2R‐processed PSCs based on a two‐step fabrication of the perovskite layer. [ABSTRACT FROM AUTHOR]