awn

Graphite carbon nitride (g‐C₃N₄) with various functional groups is developed and serves as an additive in perovskite solar cells (PSCs). This functional g‐C₃N₄ not only improves charge transport but also passivates the defects on grain boundaries in PVSCs. These features enhance the power conversion efficiency from 17.85% to 20.08% and demonstrate a feasible new concept of additives with multifunctions.

Passivation strategies are considered as one of the most efficient methods to suppress nonradiative recombination of organic–inorganic lead halide perovskite solar cells (PSCs), leading to tremendous photovoltaic performance. An innovative 2D polymer, graphitic carbon nitride (g‐C₃N₄), as well as various organic groups (amino, sulfonic, nitrato, and hydroxy group), are widely used as passivation agents, according to the previous reports. Anchoring g‐C₃N₄ and the aforementioned organic groups as additives in perovskite can both heal charged defects around the grain boundaries by passivating the charge recombination center. In addition, the crystalline quality can also be enhanced by the incorporation of g‐C₃N₄, leading to improved conductivity of perovskite light absorber films that is beneficial for benign charge extraction efficiency. Inspired by the underlining mechanisms, a series of novel passivation molecules, functionalized g‐C₃N₄ (F‐C₃N₄) with assorted organic groups, is designed herein, yielding a champion power conversion efficiency (PCE) of 20.08% for NO₃‐C₃N₄‐based p‐i‐n structure PSC, in comparison with that of PSC without passivation (17.85%). These findings present an efficient strategy to understand and design multiple facets of applications of novel passivation molecules to further improve the PCE of PSCs.

The in situ reduction of parasitic Sn⁴⁺ to Sn²⁺ by metallic tin powder effectively reduces Sn⁴⁺ content and thereby decreases the trap density of the perovskite films, giving rise to a remarkably long charge carrier lifetime and favorable energy‐level alignment at the interfaces. Consequently, a high power conversion efficiency of 20.7% is achieved for low‐bandgap Pb–Sn‐alloyed perovskite solar cells.

Although the theoretical power conversion efficiency (PCE) of low‐bandgap Pb–Sn‐alloyed perovskite solar cells (PSCs) is higher than that of its conventional pure Pb counterpart, its device performance currently has been severely restricted by the large open‐circuit voltage (V _oc) loss. Herein, it is discovered that the Sn⁴⁺‐induced trap states of the perovskite film can be effectively suppressed by introducing excess Sn powder into the precursor solution (FASnI₃) to reduce the Sn⁴⁺ content. As a result, the average charge carrier lifetime of the perovskite film increases remarkably from 115 to 701 ns due to the suppressed nonradiative recombination, and the energy levels have up‐shifted by about 0.27 eV, rendering a more favorable energy‐level alignment at the interface. Ultimately, the champion PSCs using a low‐bandgap (FASnI₃)_0.6(MAPbI₃)_0.4 perovskite film with Sn⁴⁺ reduction show a high V _oc of 0.843 V corresponding to a V _oc loss as low as 0.397 eV and a high fill factor of 80.34%, leading to an impressive PCE of 20.7%, which is one of the few instances of a PCE over 20% for low‐bandgap mixed Pb–Sn PSCs to date.

Band‐Gap Engineering

In article number 1900304, Jingjing Chang and co‐workers summarize the various reported bandgap engineering strategies. The two most widely used strategies including impurity and pressure as well as their underlying mechanisms are reviewed comprehensively. In addition, intermediate band and external electric field for bandgap structure tuning are also discussed. Moreover, future research directions are outlined to guide further investigation.

Moisture Stability

In article number 1900345, Hong Lin and co‐workers show that the addition of N719 dye molecules effectively alleviates the moisture sensitivity of organic perovskite materials by suppressing the water migration into the perovskite structure. It is verified by later formation of the intermediate hydrate products and decreased reaction rate of macroscopic exposition to polar solvents. With better efficiency, the long‐term stability of the device is greatly boosted.

In article number 1900245, Jae‐Joon Lee, Ashraful Islam, and co‐workers introduce a post‐deposition vapor annealing process, assisted by methylammonium chloride vapor to fabricate stable, homogeneous pin‐hole‐free FASnI₃ perovskite absorber films with low crystal defects and low surface recombination over a relatively large area. Inverted planar Pb‐free perovskite solar cells fabricated with a 1.02 cm² aperture area show a maximum power conversion efficiency of 6.33% with high reproducibility and stability.

An excellent antioxidant, tea polyphenol (TP), is introduced to a CsPb_0.5Sn_0.5I₂Br precursor solution to obtain high‐efficiency inorganic PSCs. TP can not only retard the oxidation of Sn²⁺ but also regulate the formation of Pb/Sn binary perovskite films, leading to a reduced density of defects. The corresponding device demonstrates power conversion efficiency as high as 8.10% with high stability.

Implementation of inorganic perovskite compounds and reduction toxicity of lead are important for developing sustainable and renewable photovoltaic power generation. The inorganic Pb/Sn binary metal halide perovskites offer a perfect opportunity for tuning optical bandgap and thus hold significant potential in emerging technologies such as solar cells. However, an easy oxidation of Sn²⁺ to Sn⁴⁺ has become one of the main issues for achieving efficient and stable Sn‐based perovskite solar cells (PSCs). Herein, an effective method for stabilizing CsPb_0.5Sn_0.5I₂Br is proposed to realize high‐efficiency PSCs via antioxidant tea polyphenol (TP). It is found that TP can not only slow down the oxidation of Sn²⁺ but also regulate perovskite film crystallization during the formation of perovskite films via coordination interaction, leading to a reduced density of defects and an enlarged open‐circuit voltage. The resultant perovskite solar cell using CsPb_0.5Sn_0.5I₂Br (TP) with an all‐inorganic mesoscopic framework of FTO/c‐TiO₂/m‐TiO₂/Al₂O₃/NiO/carbon achieves an impressive power conversion efficiency of 8.10% with high stability.

Organic amine cation, GA⁺ is intentionally incorporated in MA_0.7FA_0.3PbI₃ perovskite to stiffen the inorganic Pb–I lattice, restrain the formation of iodine vacancies defects, and reduce ion diffusion. Solar cells based on this component engineering and PFN‐Br interfacial strategy demonstrate an enhanced power conversion efficiency value over 21% for SnO₂‐based planar perovskite solar cells and excellent thermal stability.

Tin oxide (SnO₂) offers its advantages in widespread applications that require efficient carrier transport. However, the usages of SnO₂ in organic solar cells are hindered because of dangling bonds on the surface of SnO₂. Herein, PFN‐Br as an interfacial layer to tailor the work function of SnO₂ is adopted, making it an ideal candidate for interfacial material in organic electronics. Meanwhile, such an efficient SnO₂/PFN‐Br electron transport layer (ETL) can also be applied to perovskite devices and achieve competitive efficiency. To eliminate current–voltage hysteresis and improve poor thermodynamic stability of perovskite solar cells (PSCs), 5 mol% of guanidinium iodide (GAI) into the (MA) _x (FA)_1 − x PbI₃ precursor solution is incorporated, enabling the formation of triple‐cation perovskite films with excellent optoelectronic quality and stability. The combination of an SnO₂/PFN‐Br ETL and GAI doping strategy finally delivers power conversion efficiencies over 21% and negligible current–voltage hysteresis in planar PSCs. These improvements arise from the strong hydrogen bonding caused by the incorporation of GA⁺. It can stiffen the inorganic Pb–I lattice of the unit cell and restrain the formation of iodine vacancies defects. Moreover, the strong hydrogen bonding can immobilize iodide ion and thus enhance the thermal stability of the corresponding device.

The photostability of perovskite films and solar cells under red, green, and blue (RGB) light illumination in medium vacuum that belongs to near space is systemically investigated as a promising power source mounted on spacecrafts. It is found that RGB light induces different degradations of perovskites from morphological, chemical, structural, and device performance points of view.

Hybrid perovskite solar cells with a high specific power have great potential to become promising power sources mounted on spacecrafts in space applications. However, there is a lack of study on their photostability as light absorbers in those conditions. Herein, the stability of the perovskite films and solar cells under red, green, and blue (RGB) light illumination in medium vacuum that belongs to near space is explored. The perovskite active layers exhibit different degradations from morphological, chemical, and structural points of view. This is attributed to the strong coupling between photoexcited carriers and the crystal lattice and the diversity of RGB light absorption in the perovskite films. Device characterizations reveal that the efficiency loss of perovskite solar cells results from not only perovskite degradation, but also the photoexcited carriers reducing the energy barrier of ion migration and accelerating the migration to generate more deep‐level trap defects. Moreover, comparative devices suggest that the well encapsulation can weaken the effect of vacuum on stability.

Spectrally and spatially resolved absorptivity at sub‐bandgap wavelengths of perovskite materials, extracted from their luminescence spectra, is employed to study degradation across perovskite and perovskite/silicon tandem solar cells. The absorptivity is demonstrated to reflect real degradation in the perovskite film and is much more robust and sensitive than its luminescence spectral peak position, representing its optical bandgap and intensity.

Abstract

Instability in perovskite solar cells is the main challenge for the commercialization of this solar technology. Here, a contactless, nondestructive approach is reported to study degradation across perovskite and perovskite/silicon tandem solar cells. The technique employs spectrally and spatially resolved absorptivity at sub‐bandgap wavelengths of perovskite materials, extracted from their luminescence spectra. Parasitic absorption in other layers, carrier diffusion, and photon smearing phenomena are all demonstrated to have negligible effects on the extracted absorptivity. The absorptivity is demonstrated to reflect real degradation in the perovskite film and is much more robust and sensitive than its luminescence spectral peak position, representing its optical bandgap, and intensity. The technique is applied to study various common factors which induce and accelerate degradation in perovskite solar cells including air and heat exposure and light soaking. Finally, the technique is employed to extract the individual absorptivity component from the perovskite layer in a monolithic perovskite/silicon tandem structure. The results demonstrate the value of this approach for monitoring degradation mechanisms in perovskite and perovskite/silicon tandem cells at early stages of degradation and various fabrication stages.

Crystallizing perovskites on an ionic liquid‐modified SnO₂ substrate causes a shift of the perovskite Fermi level toward the conduction band and decreases the density of trap states in the perovskite. This results in a reduction of nonradiative recombination losses and, consequently, improved solar cell efficiencies.

Abstract

Halide perovskites are currently one of the most heavily researched emerging photovoltaic materials. Despite achieving remarkable power conversion efficiencies, perovskite solar cells have not yet achieved their full potential, with the interfaces between the perovskite and the charge‐selective layers being where most recombination losses occur. In this study, a fluorinated ionic liquid (IL) is employed to modify the perovskite/SnO₂ interface. Using Kelvin probe and photoelectron spectroscopy measurements, it is shown that depositing the perovskite onto an IL‐treated substrate results in the crystallization of a perovskite film which has a more n‐type character, evidenced by a decrease of the work function and a shift of the Fermi level toward the conduction band. Photoluminescence spectroscopy and time‐resolved microwave conductivity are used to investigate the optoelectronic properties of the perovskite grown on neat and IL‐modified surfaces and it is found that the modified substrate yields a perovskite film which exhibits an order of magnitude lower trap density than the control. When incorporated into solar cells, this interface modification results in a reduction in the current–voltage hysteresis and an improvement in device performance, with the best performing devices achieving steady‐state PCEs exceeding 20%.

Vacuum‐assisted growth control (VAGC) allows growing micrometer‐sized and pinhole‐free low‐bandgap (E _G ≈1.27 eV) perovskite thin films. VAGC exhibits promising reproducibility and potential in fabrication of larger active‐area solar cells up to 1 cm². The efficient low‐bandgap perovskite solar cells fabricated by VAGC enable efficient four‐terminal all‐perovskite tandem solar cells with power conversion efficiencies up to 23%.

Abstract

All‐perovskite multijunction photovoltaics, combining a wide‐bandgap (WBG) perovskite top solar cell (E _G ≈1.6–1.8 eV) with a low‐bandgap (LBG) perovskite bottom solar cell (E _G < 1.3 eV), promise power conversion efficiencies (PCEs) >33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum‐assisted growth control (VAGC) of solution‐processed LBG perovskite thin films based on mixed Sn–Pb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well‐established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge‐carrier lifetime. The improved optoelectronic characteristics enable high‐performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four‐terminal all‐perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active‐area solar cells up to 1 cm².

Bulky cation addition to perovskite solar cells is demonstrated as an effective means of significantly improving device performance. Detailed structural characterization reveal additives are located at surfaces and grain boundaries, resulting in suppression of nonradiative recombination. Judicious cation selection results in MAPI‐based perovskite cells with a power conversion efficiency >20% and MAPBrI cells with a V _oc of 1.22 V.

Abstract

The origin of performance enhancements in p‐i‐n perovskite solar cells (PSCs) when incorporating low concentrations of the bulky cation 1‐naphthylmethylamine (NMA) are discussed. A 0.25 vol % addition of NMA increases the open circuit voltage (V _oc) of methylammonium lead iodide (MAPbI₃) PSCs from 1.06 to 1.16 V and their power conversion efficiency (PCE) from 18.7% to 20.1%. X‐ray photoelectron spectroscopy and low energy ion scattering data show NMA is located at grain surfaces, not the bulk. Scanning electron microscopy shows combining NMA addition with solvent assisted annealing creates large grains that span the active layer. Steady state and transient photoluminescence data show NMA suppresses non‐radiative recombination resulting from charge trapping, consistent with passivation of grain surfaces. Increasing the NMA concentration reduces device short‐circuit current density and PCE, also suppressing photoluminescence quenching at charge transport layers. Both V _oc and PCE enhancements are observed when bulky cations (phenyl(ethyl/methyl)ammonium) are incorporated, but not smaller cations (Cs/MA)—indicating size is a key parameter. Finally, it demonstrates that NMA also enhances mixed iodide/bromide wide bandgap PSCs (V _oc of 1.22 V with a 1.68 eV bandgap). The results demonstrate a facile approach to maximizing V _oc and provide insights into morphological control and charge carrier dynamics induced by bulky cations in PSCs.

The aprotic butyldimethylsulfonium‐driven MAPbI₃ perovskite shows a much more pronounced effect on the improvement of moisture stability compared to the protic butylammonium (BA)‐based counterpart. The BA having a potential hydrogen donor, which exists on the surface and/or grain boundaries, is vulnerable to H₂O‐induced degradation initiators, resulting in the faster hydration followed by the irreversible degradation of perovskites.

Abstract

Many organic cations in halide perovskites have been studied for their application in perovskite solar cells (PSCs). Most organic cations in PSCs are based on the protic nitrogen cores, which are susceptible to deprotonation. Here, a new candidate of fully alkylated sulfonium cation (butyldimethylsulfonium; BDMS) is designed and successfully assembled into PSCs with the aim of increasing humidity stability. The BDMS‐based perovskites retain the structural and optical features of pristine perovskite, which results in the comparable photovoltaic performance. However, the fully alkylated aprotic nature of BDMS shows a much more pronounced effect on the increase in humidity stability, which emphasizes a generic electronic difference between protic ammonium and aprotic sulfonium cation. The current results would pave a new way to explore cations for the development of promising PSCs.

A new doping strategy is developed for poly(3‐hexylthiophene) (P3HT) as the hole transport layer (HTL) in triple‐cation/double‐halide mesoscopic perovskite solar cells (PSCs), achieving efficiencies of 19.25% and 13.3% on lab‐scale and large‐area module, respectively. Promising stability results after 1500 h air exposure (relative humidity ≈ 60%, r.t.), more than 500 h at 85 °C, and 100 h of continuous light soaking.

Abstract

As the hole transport layer (HTL) for perovskite solar cells (PSCs), poly(3‐hexylthiophene) (P3HT) has been attracting great interest due to its low‐cost, thermal stability, oxygen impermeability, and strong hydrophobicity. In this work, a new doping strategy is developed for P3HT as the HTL in triple‐cation/double‐halide ((FA_1−x−yMA_xCs_y)Pb(I_1−xBr_x)₃) mesoscopic PSCs. Photovoltaic performance and stability of solar cells show remarkable enhancement using a composition of three dopants Li‐TFSI, TBP, and Co(III)‐TFSI reaching power conversion efficiencies of 19.25% on 0.1 cm² active area, 16.29% on 1 cm² active area, and 13.3% on a 43 cm² active area module without using any additional absorber layer or any interlayer at the PSK/P3HT interface. The results illustrate the positive effect of a cobalt dopant on the band structure of perovskite/P3HT interfaces leading to improved hole extraction and a decrease of trap‐assisted recombination. Non‐encapsulated large area devices show promising air stability through keeping more than 80% of initial efficiency after 1500 h in atmospheric conditions (relative humidity ≈ 60%, r.t.), whereas encapsulated devices show more than >500 h at 85 °C thermal stability (>80%) and 100 h stability against continuous light soaking (>90%). The boosted efficiency and the improved stability make P3HT a good candidate for low‐cost large‐scale PSCs.

Film fabrication environment and anti‐solvent properties strongly influence the microstructure evolution of perovskite films. An ambient fabrication environment induces anisotropies in crystallization. The choice of antisolvent is critical to alleviating these anisotropies. The key is to induce uniform spherulitic crystallization to achieve robust pinhole‐free films possessing grains, crystallinity, crystallographic phases, and crystallite orientations unaffected by the processing environment.

Abstract

The influence of precursor solution properties, fabrication environment, and antisolvent properties on the microstructural evolution of perovskite films is reported. First, the impact of fabrication environment on the morphology of methyl ammonium lead iodide (MAPbI₃) perovskite films with various Lewis‐base additives is reported. Second, the influence of antisolvent properties on perovskite film microstructure is investigated using antisolvents ranging from nonpolar heptane to highly polar water. This study shows an ambient environment that accelerates crystal growth at the expense of nucleation and introduces anisotropies in crystal morphology. The use of antisolvents enhances nucleation but also influences ambient moisture interaction with the precursor solution, resulting in different crystal morphology (shape, size, dispersity) in different antisolvents. Crystal morphology, in turn, dictates film quality. A homogenous spherulitic crystallization results in pinhole‐free films with similar microstructure irrespective of processing environment. This study further demonstrates propyl acetate, an environmentally benign antisolvent, which can induce spherulitic crystallization under ambient environment (52% relative humidity, 25 °C). With this, planar perovskite solar cells with ≈17.78% stabilized power conversion efficiency are achieved. Finally, a simple precipitation test and in situ crystallization imaging under an optical microscope that can enable a facile a priori screening of antisolvents is shown.

A dopant‐free hole transporting material (HTM) named DMZ, is synthesized and applied in inverted planar perovskite solar cells (PSCs). High power conversion efficiency (PCE) (18.61%) and stable‐enhanced PSCs devices are achieved and after storage for nearly 560 h, 90% of the maximum PCE is retained in air with a relative humidity ≈ 45%–50% without any encapsulation.

Abstract

A new hole transporting material (HTM) named DMZ is synthesized and employed as a dopant‐free HTM in inverted planar perovskite solar cells (PSCs). Systematic studies demonstrate that the thickness of the hole transporting layer can effectively enhance the morphology and crystallinity of the perovskite layer, leading to low series resistance and less defects in the crystal. As a result, the champion power conversion efficiency (PCE) of 18.61% with J _SC = 22.62 mA cm⁻², V _OC = 1.02 V, and FF = 81.05% (an average one is 17.62%) is achieved with a thickness of ≈13 nm of DMZ (2 mg mL⁻¹) under standard global AM 1.5 illumination, which is ≈1.5 times higher than that of devices based on poly(3,4‐ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT:PSS). More importantly, the devices based on DMZ exhibit a much better stability (90% of maximum PCE retained after more than 556 h in air (relative humidity ≈ 45%–50%) without any encapsulation) than that of devices based on PEDOT:PSS (only 36% of initial PCE retained after 77 h in same conditions). Therefore, the cost‐effective and facile material named DMZ offers an appealing alternative to PEDOT:PSS or polytriarylamine for highly efficient and stable inverted planar PSCs.

A simple perovskite solar cell architecture, which is based on dopant‐free electron and hole conductors and carbon back contact deposited at room temperature, is demonstrated. The resulting architecture leads to the fabrication of cheap and highly efficient perovskite solar cells exhibiting unprecedented long‐term operational and UV stability thus hold immense potential for large‐scale deployment.

Abstract

Today's perovskite solar cells (PSCs) mostly use components, such as organic hole conductors or noble metal back contacts, that are very expensive or cause degradation of their photovoltaic performance. For future large‐scale deployment of PSCs, these components need to be replaced with cost‐effective and robust ones that maintain high efficiency while ascertaining long‐term operational stability. Here, a simple and low‐cost PSC architecture employing dopant‐free TiO₂ and CuSCN as the electron and hole conductor, respectively, is introduced while a graphitic carbon layer deposited at room temperature serves as the back electrical contact. The resulting PSCs show efficiencies exceeding 18% under standard AM 1.5 solar illumination and retain ≈95% of their initial efficiencies for >2000 h at the maximum power point under full‐sun illumination at 60 °C. In addition, the CuSCN/carbon‐based PSCs exhibit remarkable stability under ultraviolet irradiance for >1000 h while under similar conditions, the standard spiro‐MeOTAD/Au based devices degrade severely.

In article number 1900245, Jae‐Joon Lee, Ashraful Islam, and co‐workers introduce a post‐deposition vapor annealing process, assisted by methylammonium chloride vapor to fabricate stable, homogeneous pin‐hole‐free FASnI₃ perovskite absorber films with low crystal defects and low surface recombination over a relatively large area. Inverted planar Pb‐free perovskite solar cells fabricated with a 1.02 cm² aperture area show a maximum power conversion efficiency of 6.33% with high reproducibility and stability.

Self‐Recrystallization

In article number 1900302, Keiko Waki and co‐workers discover the ability of functionalized carbon nanotubes to make use of the self‐recrystallization nature of perovskites. As a result, junctions between perovskite layers and electrodes are reconstructed, leading to a reduction of charge transfer resistance and a significant improvement in the conversion efficiency, for instance from 3.21% to 11.19%, after 77 days of storage in ambient air.

This study reports a simultaneous contact and grain‐boundary passivation strategy in planar perovskite solar cells using SnO₂‐KCl composite as the electron transport layer. When applied to perovskite solar cells employing a composition of (FAPbI₃)_0.95(MAPbBr₃)_0.05, this strategy increases the open‐circuit voltage from 1.077 to 1.137 V and the corresponding efficiency from 20.2% to 22.2%.

Abstract

The performance of perovskite solar cells is sensitive to detrimental defects, which are prone to accumulate at the interfaces and grain boundaries of bulk perovskite films. Defect passivation at each region will lead to reduced trap density and thus less nonradiative recombination loss. However, it is challenging to passivate defects at both the grain boundaries and the bottom charge transport layer/perovskite interface, mainly due to the solvent incompatibility and complexity in perovskite formation. Here SnO₂‐KCl composite electron transport layer (ETL) is utilized in planar perovskite solar cells to simultaneously passivate the defects at the ETL/perovskite interface and the grain boundaries of perovskite film. The K and Cl ions at the ETL/perovskite interface passivate the ETL/perovskite contact. Meanwhile, K ions from the ETL can diffuse through the perovskite film and passivate the grain boundaries. An enhancement of open‐circuit voltage from 1.077 to 1.137 V and a corresponding power conversion efficiency increasing from 20.2% to 22.2% are achieved for the devices using SnO₂‐KCl composite ETL. The composite ETL strategy reported herein provides an avenue for defect passivation to further increase the efficiency of perovskite solar cells.

The crystallization process of perovskite inside a mesoscopic scaffold is revealed and a possible model of crystallization is proposed. By applying a solvent evaporation controlled crystallization method, ideal crystallization in the mesoscopic structure is achieved. As a result, a stabilized power conversion efficiency of 16.26% based on a printable mesoscopic perovskite solar cell is achieved.

Abstract

Controlling the crystallization of organic–inorganic hybrid perovskite is of vital importance to achieve high performing perovskite solar cells. The growth mechanism of perovskites has been intensively studied in devices with planar structures and traditional structures. However, for the printable mesoscopic perovskite solar cells, it is difficult to study the crystallization mechanism of perovskite owing to the complicated mesoporous structure. Here, a solvent evaporation controlled crystallization method to achieve ideal crystallization in the mesoscopic structure is provided. Combining results of scanning electron microscope and X‐ray diffraction, it is found that adjusting the evaporation rate of solvent can control the crystallization rate of perovskite and a model for the crystallization process during annealing in mesoporous structures is proposed. Finally, a homogeneous pore filling in the mesoscopic structure without any additives is successfully achieved and a stabilized power conversion efficiency of 16.26% using ternary‐cation perovskite absorber is realized. The findings will provide better understanding of perovskite crystallization in printable mesoscopic perovskite solar cells and pave the way for the commercialization of perovskite solar cells.

Oxo‐functionalized graphene/dodecylamine is used to solve ion migration in cesium‐formamidinium‐methylammonium triple cation‐base perovskites. The ultra‐thin two‐dimensional network structure can wrap the crystals and reduce the ion migration of the perovskite film. The resulting devices deliver a power conversion efficiency of 21.1%, and a remarkable fill factor of 81%, with reduced hysteresis and improved long‐term stability.

Abstract

Mixed cation/halide perovskites have led to a significant increase in the efficiency and stability of perovskite solar cells. However, mobile ionic defects inevitably exacerbate the photoinduced phase segregation and self‐decomposition of the crystal structure. Herein, ultrathin 2D nanosheets of oxo‐functionalized graphene/dodecylamine (oxo‐G/DA) are used to solve ion migration in cesium (Cs)‐formamidinium (FA)‐methylammonium (MA) triple‐cation‐based perovskites. Based on the superconducting carbon skeleton and functional groups that provide lone pairs of electrons on it, the ultrathin 2D network structure can fit tightly on the crystals and wrap them, isolating them, and thus reducing the migration of ions within the built‐in electric field of the perovskite film. As evidence of the formation of sharp crystals with different orientation within the perovskite film, moiré fringes are observed in transmission electron microscopy. Thus, a champion device with a power conversion efficiency (PCE) of 21.1% (the efficiency distribution is 18.8 ± 1.7%) and a remarkable fill factor of 81%, with reduced hysteresis and improved long‐term stability, is reported. This work provides a simple method for the improvement of the structural stability of perovskite in solar cells.

A strategy for introducing the additive 1,4,7,10,13,16‐hexaoxacyclooctadecane (18C6) into the triple cation perovskite precursor solution is demonstrated, and its influence in precursor and perovskite crystals is thoroughly investigated with simultaneous experimental and theoretical methods. It is found that the formation of the 18C6/Pb complex plays a significant role in the enhanced precursor stability and defect passivation effect within the crystal surface.

Abstract

Triple cation perovskites (Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃) have received lots of attention owing to the excellent stability and photovoltaic performance. However, the development toward efficient solar cells has been significantly impeded by its intrinsic precursor instability, as well as defective crystal surface. Herein, a strategy for introducing the additive of 1,4,7,10,13,16‐hexaoxacyclooctadecane (18C6) in the precursor solution, rendering an excellent stability of more than one month, and the defect passivation effect on the crystal surface are demonstrated. In those perovskite solar cells, a power conversion efficiency of 20.73% has been achieved with a substantially improved open‐circuit voltage and fill factor. As evidenced by the density functional theory calculations, the fundamental reason relating to the enhanced performance is found to be the interaction effect between the 18C6 and cations, and in particular the formation of the 18C6/Pb complex. This finding represents an alternative strategy for achieving a stable precursor solution and efficient perovskite solar cells.

High‐efficiency organic solar cells are achieved through the use of a new electron acceptor AQx‐2 with a quinoxaline‐containing fused core. The increase in performance is attributed to the optimized phase separation morphology that significantly boosts hole transfer and suppresses geminate recombination. The power conversion efficiency of these devices, 16.4%, is the highest certified value for binary organic solar cells.

Abstract

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene‐electron‐acceptor‐based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx‐2 with quinoxaline‐containing fused core and PBDB‐TF as donor. The significant increase in photovoltaic performance of AQx‐2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx‐1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure–morphology–property relationship is established. The stronger π–π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx‐2‐based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next‐generation high‐performance OSCs.

An effective composite electron transport layer (ETL) is fabricated using carboxylic‐acid‐ and hydroxyl‐rich red‐carbon quantum dots to dope low‐temperature solution‐processed SnO₂. The electron mobility of SnO₂ is dramatically increased by ≈20 times from 9.32 × 10⁻⁴ to 1.73 × 10⁻² cm² V⁻¹ s⁻¹. A planar perovskite solar cell based on this novel SnO₂ ETL demonstrates an outstanding improvement in efficiency up to 22.77%.

Abstract

An efficient electron transport layer (ETL) plays a key role in promoting carrier separation and electron extraction in planar perovskite solar cells (PSCs). An effective composite ETL is fabricated using carboxylic‐acid‐ and hydroxyl‐rich red‐carbon quantum dots (RCQs) to dope low‐temperature solution‐processed SnO₂, which dramatically increases its electron mobility by ≈20 times from 9.32 × 10⁻⁴ to 1.73 × 10⁻² cm² V⁻¹ s⁻¹. The mobility achieved is one of the highest reported electron mobilities for modified SnO₂. Fabricated planar PSCs based on this novel SnO₂ ETL demonstrate an outstanding improvement in efficiency from 19.15% for PSCs without RCQs up to 22.77% and have enhanced long‐term stability against humidity, preserving over 95% of the initial efficiency after 1000 h under 40–60% humidity at 25 °C. These significant achievements are solely attributed to the excellent electron mobility of the novel ETL, which is also proven to help the passivation of traps/defects at the ETL/perovskite interface and to promote the formation of highly crystallized perovskite, with an enhanced phase purity and uniformity over a large area. These results demonstrate that inexpensive RCQs are simple but excellent additives for producing efficient ETLs in stable high‐performance PSCs as well as other perovskite‐based optoelectronics.