Ligang Yuan

The lack of selectivity and energy alignment of the charge transport layers in perovskite solar cells induce a mismatch between the external open‐circuit voltage and the internal quasi‐Fermi level splitting due to enhanced interface recombination. This limits the maximum open‐circuit voltage potentially achievable and results in its saturation at high illumination intensities.

Abstract

Today's perovskite solar cells (PSCs) are limited mainly by their open‐circuit voltage (V _OC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity‐dependent measurements of the quasi‐Fermi level splitting (QFLS) and of the V _OC on the very same devices, including pin‐type PSCs with efficiencies above 20%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the V _OC is generally lower than the QFLS, violating one main assumption of the Shockley‐Queisser theory. This has far‐reaching implications for the applicability of some well‐established techniques, which use the V _OC as a measure of the carrier densities in the absorber. By performing drift‐diffusion simulations, the intensity dependence of the QFLS, the QFLS‐V _OC offset and the ideality factor are consistently explained by trap‐assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the V _OC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the V _OC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS‐V _OC relation is of great importance.

The power conversion efficiency of perovskite colloidal quantum dot (CQD) solar cells is improved using a conjugated small molecule, ITIC. The carrier dynamics of this unique perovskite CQD/ITIC system are investigated, showing an effective carrier transfer from the perovskite CQDs to the ITIC, which provides an additional driving force for charge separation in perovskite CQDs photovoltaic devices and boosts the efficiency up to 12.7%.

Abstract

Halide perovskite colloidal quantum dots (CQDs) have recently emerged as a promising candidate for CQD photovoltaics due to their superior optoelectronic properties to conventional chalcogenides CQDs. However, the low charge separation efficiency due to quantum confinement still remains a critical obstacle toward higher‐performance perovskite CQD photovoltaics. Available strategies employed in the conventional CQD devices to enhance the carrier separation, such as the design of type‐Ⅱ core–shell structure and versatile surface modification to tune the electronic properties, are still not applicable to the perovskite CQD system owing to the difficulty in modulating surface ligands and structural integrity. Herein, a facile strategy that takes advantage of conjugated small molecules that provide an additional driving force for effective charge separation in perovskite CQD solar cells is developed. The resulting perovskite CQD solar cell shows a power conversion efficiency approaching 13% with an open‐circuit voltage of 1.10 V, short‐circuit current density of 15.4 mA cm⁻², and fill factor of 74.8%, demonstrating the strong potential of this strategy toward achieving high‐performance perovskite CQD solar cells.

Halide‐ and nonhalide‐based guanidinium salts are explored to study the impact of counterions supplied along with the guanidinium cation on the photophysical properties of perovskite films and photovoltaic performance of perovskite solar cells.

The impacts of halide and nonhalide sources of guanidinium cations, including guanidinium chloride (GCl) ((NH₂)₃CCl) and guanidinium thiocyanate (GTC) ((NH₂)₃CSCN), are comparatively analyzed on the structural, morphological, and photophysical properties of (CsMAFA)PbBr _x I_3 − x (x = 0.17) (MA = methylammonium, FA = formamidinium) perovskite films. X‐ray diffraction (XRD) reveals that the formation of photoinactive phases depends on the nature of counterions (halide vs nonhalide). Furthermore, morphological analysis shows that with the addition of guanidinium salts, the apparent grain size decreases due to the enhancement of nucleation density and/or slow growth of perovskite structures. More importantly, the introduction of GCl leads to the fabrication of perovskite solar cells (PSCs), yielding a photovoltage as high as 1.16 V (1.1 V for reference). In contrast, the introduction of GTC minimally affects the photovoltage, underlining the significance of counterions in improving the photovoltage of PSCs. The present preliminary results of the density functional theory based theoretical investigation related to the effect of G cation on the structure of the perovskite system is presented. In summary, the insights gained through structural and morphological characterization helps to understand the critical role of counterions of guanidinium salts in PSCs.

The highest efficiency of 9.1% on electrodeposited Cu₂ZnSnSe₄ (CZTSe) thin‐film solar cells is successfully achieved, benefitting from the reinforced intermetallic diffusion driven by Cu‐promoted Zn electrodeposition. The dense alloyed prefabricated layer with a reversed elemental distribution is conducive to the post‐selenization, making the CZTSe absorber layer pinhole‐free, large‐grained, and resulting in its outstanding photovoltaic performance.

Electrodeposition (ED) presents a substantially technical advantage over other methods, conducted with a simply equipped nonvacuum green process. Zn deposition is recognized as the biggest challenge in the process of electrodepositing CuZnSn‐based photovoltaic materials, due to the fiercely competitive reaction with H₂O. Herein, Cu‐promoted Zn deposition from a preferred Cu/Sn/Zn stacked electrodeposition process is proposed. Intermetallic diffusion is confirmed to be positively reinforced during Zn electrodeposition, leading to a dense prefabricated alloyed layer with a reversed elemental distribution. Thus, a high‐quality Cu₂ZnSnSe₄ (CZTSe) absorber layer with up–down microsized grains is achieved by a two‐step sequence selenization, thereby reducing the V _oc deficit and space‐charge‐region (SCR) recombination. With these sequential positive effects, a power conversion efficiency of 9.1% is achieved, accompanied with a depressed V _oc deficit of 556 mV, to be the highest efficiency on electrodeposited CZTSe‐based devices. Additional processes of antireflective coating or surface treatments are expected to further improve the efficiency. This work highly contributes to the progress of high‐performance Cu‐kesterite solar cells by the low‐cost green electrodeposition process.

The oxygen‐stabilizing effect of [6,6]‐phenyl‐11 C₆₁‐butyric acid‐(3,4,5‐tris(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)phenyl)methanol ester (PCBB‐OEG) is investigated and it is found that the excellent electron transfer/extraction of PCBB‐OEG can reduce the generation of superoxides and enhance the stability of perovskite solar cells (pero‐SCs). The resulting pero‐0.1/[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM):PCBB‐OEG‐based pero‐SC delivers a high power conversion efficiency of 20.49% as well as high long‐term stability under ambient atmosphere at ≈50% humidity.

Poor stability is one of the main limiting factors for the commercialization of perovskite solar cells (pero‐SCs). The degradation of perovskite films is usually triggered by the reaction of the perovskite active layer with the superoxide when exposed in ambient atmosphere, which is not prevented by simple encapsulation. Herein, an oxygen‐stabilizing effect is found by utilizing a hydrophilic [6,6]‐phenyl‐C₆₁‐butyric acid‐(3,4,5‐tris(2‐(2‐(2‐methoxyethoxy)ethoxy)ethoxy)phenyl)methanol ester (PCBB‐OEG) as a dopant of the perovskite film and electron‐transporting layer (ETL). PCBB‐OEG accelerates photoelectron transport in perovskite films and enhances the electron‐extracting ability of ETL. This process is demonstrated to efficiently prevent the reaction between electrons and oxygen to form a superoxide. Hence, the presence of PCBB‐OEG in the perovskite film improves its stability against oxygen. The stability and efficiency of pero‐SCs are further improved by doping PCBB‐OEG in [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) ETL. As a result, the p‐i‐n pero‐SCs with PCBB‐OEG as an additive in both the perovskite active layer and ETL show the best power conversion efficiency of 20.49%. Importantly, the related device retains 98% of this initial efficiency after 60 days of storage in ambient atmosphere without encapsulation.

Flexible planar heterojunction perovskite solar cells are constructed with an architecture of polyethylene‐2,6‐naphthalate/indium tin oxide/SnO₂/perovskite/spiro‐OMeTAD/Ag and fabricated via a combination of roll‐to‐roll microgravure printing and slot‐die coating under ambient conditions with a relative humidity of ≈40%, leading to a power conversion efficiency (PCE) up to 10.56% and an average PCE of 9.97%.

It is highly desirable to develop large‐scale, low‐cost fabrication processes for flexible perovskite solar cells (f‐PSCs) under ambient conditions for accelerating their potential commercialization. Roll‐to‐roll (R2R) printing technology enables high‐output manufacturing and is well suited for commercially processing f‐PSCs. Herein, triple‐cation f‐PSCs are developed with a planar heterojunction structure consisting of polyethylene‐2,6‐naphthalate/indium tin oxide/SnO₂/perovskite/spiro‐OMeTAD/Ag via a combination of R2R microgravure printing and slot‐die coating under ambient conditions with a relative humidity of ≈40%. A mixture of isopropanol and water is used to dilute an as‐purchased SnO₂ colloid solution and modify the contact between the electron‐transport layer (ETL) and substrate, leading to a smooth morphology of the R2R‐printed ETL SnO₂ layer. Furthermore, suitable intrinsic organic salt additives and the N₂ gas blowing‐assisted process are introduced to effectively improve the crystallization of the perovskite, resulting in a high‐quality perovskite film via R2R. After the optimization, the f‐PSCs based on the R2R‐printed ETL SnO₂ and the perovskite film under an ambient condition show a power conversion efficiency (PCE) of up to 10.56% and an average PCE of 9.97%. This study provides a potential strategy for commercially fabricating f‐PSCs via a scalable and efficient R2R printing process.

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.

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.

An interconnected SnO₂ thin film (composed of presynthesized SnO₂ nanocrystals interconnected by amorphous phase SnO _x ) is proposed as an electron transport layer for efficient flexible perovskite solar cells. The interconnected SnO₂ thin film enables fast electron extraction from the perovskite layer and retards nonradiative charge carrier recombination. Corresponding flexible solar cells demonstrate a power conversion efficiency as high as 16.29%.

This study reports on interconnected SnO₂ electron transport layers (composed of presynthesized SnO₂ nanocrystals interconnected by amorphous phase SnO _x ) processed at low temperature (120 °C) for highly efficient flexible perovskite solar cells. Herein, the amorphous phase SnO _x serves as an effective binder to connect the SnO₂ nanocrystals to obtain ultra‐smooth electron transport layers. Further characterization of the charge carrier kinetics at the perovskite/electron transport layer interface confirms that the interconnected SnO₂ nanocrystals layer facilitates electron extraction and retards nonradiative charge carrier recombination. Consequently, a power conversion efficiency of 16.29% is achieved for flexible perovskite solar cells using the interconnected SnO₂ electron transport layer on indium tin oxide/polyethylene terephthalate substrates.

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%.

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.

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.

A facile method is reported for preparing α‐CsPbI₃ perovskite films at room temperature by introducing ascorbic acid (AA) in the CsPbI₃ precursor solution. The champion device not only showed a high efficiency of 11.44% but also had excellent stability, retaining more than 76% of its initial efficiency after aging in ambient conditions for 250 h without encapsulation.

The all‐inorganic α‐CsPbI₃ perovskite with superb thermal stability and suitable band gap for light harvesting has been considered as a promising candidate for efficient perovskite solar cells (PSCs). However, the photoactive black α‐CsPbI₃ is thermodynamically unstable and transforms spontaneously into nonphotoactive yellow δ‐phase at room temperature. Herein, a facile method is reported to prepare α‐CsPbI₃ perovskite films with high stability at room temperature by mixing a small amount of ascorbic acid (AA) in the CsPbI₃ precursor solutions. It is revealed that the interaction of AA with the CsPbI₃ precursors could effectively inhibit the rapid crystallization of CsPbI₃ and reduce the size of the coordination colloidal, and thus decrease the grain size of CsPbI₃ for preparing long‐term stable α‐CsPbI₃ films. The PSCs based on the AA‐stabilized CsPbI₃ films exhibit reproducible photovoltaic performance with a champion efficiency of up to 11.44% and stable output of 11.30%, along with excellent stability, retaining more than 76% of its initial efficiency after aging in ambient conditions for 250 h without encapsulation. Most importantly, such low‐cost, solution‐processable inorganic PSCs with high performance also show promising potential for large‐scale preparation.

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.

Hysteresis in the dark, attributable to bias induced degradation of the p‐type interface, is investigated and eliminated in NiO‐based inverted perovskite solar cells. Enhanced stability to forward bias is obtained with the introduction of a low‐temperature hybrid magnesium‐based interlayer.

Abstract

In perovskite solar cells (PSCs), the interfaces are a weak link with respect to degradation. Electrochemical reactivity of the perovskite's halides has been reported for both molecular and polymeric hole selective layers (HSLs), and here it is shown that also NiO brings about this decomposition mechanism. Employing NiO as an HSL in p–i–n PSCs with power conversion efficiency (PCE) of 16.8%, noncapacitive hysteresis is found in the dark, which is attributable to the bias‐induced degradation of perovskite/NiO interface. The possibility of electrochemically decoupling NiO from the perovskite via the introduction of a buffer layer is explored. Employing a hybrid magnesium‐organic interlayer, the noncapacitive hysteresis is entirely suppressed and the device's electrical stability is improved. At the same time, the PCE is improved up to 18% thanks to reduced interfacial charge recombination, which enables more efficient hole collection resulting in higher V _oc and FF.

Spiro‐OMeTAD(TFSI)₂ is successfully employed in the fabrication of highly efficient n–i–p perovskite solar cells as a p‐dopant in the absence of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. With this approach, the proportion of [spiro‐OMeTAD]⁺ is precisely controlled, and the spiro‐OMeTAD(TFSI)₂‐doped devices show a remarkably improved long‐term stability and well‐retained hole‐transporting material (HTM) morphology after aging for 300 h.

Abstract

To date, the most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) hole‐transporting material (HTM), which achieves optimum conductivity with the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. However, this additive along with its oxidation process leads to poor reproducibility and is detrimental to stability. Herein, a dicationic salt spiro‐OMeTAD(TFSI)₂, is employed as an effective p‐dopant to achieve power conversion efficiencies of 19.3% and 18.3% (apertures of 0.16 and 1.00 cm²) with excellent reproducibility in the absence of LiTFSI and air exposure. As far as it is known, these are the highest‐performing n–i–p PSCs without LiTFSI or air exposure. Comprehensive analysis demonstrates that precise control of the proportion of [spiro‐OMeTAD]⁺ directly provides high conductivity in HTM films with low series resistance, fast hole extraction, and lower interfacial charge recombination. Moreover, the spiro‐OMeTAD(TFSI)₂‐doped devices show improved stability, benefitting from well‐retained HTM morphology without forming aggregates or voids when tested under an ambient atmosphere. A facile approach is presented to fabricate highly efficient PSCs by replacing LiTFSI with spiro‐OMeTAD(TFSI)₂. Furthermore, this study provides an insight into the relationship between device performance and the HTM doping level.

The factors affecting the V _OC in 2D perovskite cells with different [PbI₆]⁴⁻ layer sheets (n = 2–4) are elucidated. Nonradiative recombination at the perovskite/C₆₀ interface is found to dominate except for the n = 2 system where the bulk recombination determines the properties of the cell. Substantial V _OC gains through suppression of interfacial recombination at the top interface are expected.

Abstract

2D Ruddlesden–Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite‐based cells. Herein, 2D (CH₃(CH₂)₃NH₃)₂(CH₃NH₃) _{n −1}Pb _n I_{3 n +1} perovskite cells with different numbers of [PbI₆]⁴⁻ sheets (n = 2–4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open‐circuit voltage (V _OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C₆₀ interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi‐Fermi level splitting matches the device V _OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.

An interconnected SnO₂ thin film (composed of presynthesized SnO₂ nanocrystals interconnected by amorphous phase SnO _x ) is proposed as an electron transport layer for efficient flexible perovskite solar cells. The interconnected SnO₂ thin film enables fast electron extraction from the perovskite layer and retards nonradiative charge carrier recombination. Corresponding flexible solar cells demonstrate a power conversion efficiency as high as 16.29%.

This study reports on interconnected SnO₂ electron transport layers (composed of presynthesized SnO₂ nanocrystals interconnected by amorphous phase SnO _x ) processed at low temperature (120 °C) for highly efficient flexible perovskite solar cells. Herein, the amorphous phase SnO _x serves as an effective binder to connect the SnO₂ nanocrystals to obtain ultra‐smooth electron transport layers. Further characterization of the charge carrier kinetics at the perovskite/electron transport layer interface confirms that the interconnected SnO₂ nanocrystals layer facilitates electron extraction and retards nonradiative charge carrier recombination. Consequently, a power conversion efficiency of 16.29% is achieved for flexible perovskite solar cells using the interconnected SnO₂ electron transport layer on indium tin oxide/polyethylene terephthalate substrates.

Spiro‐OMeTAD(TFSI)₂ is successfully employed in the fabrication of highly efficient n–i–p perovskite solar cells as a p‐dopant in the absence of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. With this approach, the proportion of [spiro‐OMeTAD]⁺ is precisely controlled, and the spiro‐OMeTAD(TFSI)₂‐doped devices show a remarkably improved long‐term stability and well‐retained hole‐transporting material (HTM) morphology after aging for 300 h.

Abstract

To date, the most efficient perovskite solar cells (PSCs) employ an n–i–p device architecture that uses a 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenyl‐amine)‐9,9′‐spirobifluorene (spiro‐OMeTAD) hole‐transporting material (HTM), which achieves optimum conductivity with the addition of lithium bis(trifluoromethane)sulfonimide (LiTFSI) and air exposure. However, this additive along with its oxidation process leads to poor reproducibility and is detrimental to stability. Herein, a dicationic salt spiro‐OMeTAD(TFSI)₂, is employed as an effective p‐dopant to achieve power conversion efficiencies of 19.3% and 18.3% (apertures of 0.16 and 1.00 cm²) with excellent reproducibility in the absence of LiTFSI and air exposure. As far as it is known, these are the highest‐performing n–i–p PSCs without LiTFSI or air exposure. Comprehensive analysis demonstrates that precise control of the proportion of [spiro‐OMeTAD]⁺ directly provides high conductivity in HTM films with low series resistance, fast hole extraction, and lower interfacial charge recombination. Moreover, the spiro‐OMeTAD(TFSI)₂‐doped devices show improved stability, benefitting from well‐retained HTM morphology without forming aggregates or voids when tested under an ambient atmosphere. A facile approach is presented to fabricate highly efficient PSCs by replacing LiTFSI with spiro‐OMeTAD(TFSI)₂. Furthermore, this study provides an insight into the relationship between device performance and the HTM doping level.