JIALINGBO

Highly efficient hole transport layer-free low bandgap mixed Pb–Sn perovskite solar cells are realized using a binary additive system composed of CuSCN and GlyHCl. The improved charge transport and suppressed nonradiative recombination across the hole extractive interface have a marked impact on device performance, achieving the highest efficiency reported to date of 20.1%.

Abstract

The development of high-performance hole transport layer (HTL)-free perovskite solar cells (PSCs) with a simplified device structure has been a major goal in the commercialization of PSCs due to the economic advantage of low manufacturing cost. Unfortunately, low bandgap (E _g) mixed Pb–Sn perovskites, which have promising utility for constructing efficient all-perovskite tandem solar cells, have rarely been explored in simplified HTL-free device configurations. In this study, efficient band bending and defect engineering at the interface between perovskite and indium tin oxide (ITO) are realized via a binary additive system using copper thiocyanate (CuSCN) and glycine hydrochloride (GlyHCl). Using mixed Pb–Sn perovskites decorated with crystalline p-type CuSCN, the energy level alignment at the hole extractive interface is modulated in favor of hole extraction, simultaneously increasing hole mobility. Suppressed nonradiative carrier recombination in the perovskite bulk, or across the charge extractive interface, is further achieved by GlyHCl without disturbing the efficient hole transfer characteristics. Notably, a more optimized band alignment is achieved at the hole extractive interface with the addition of GlyHCl. The HTL-free mixed Pb–Sn PSC shows an efficiency up to 20.1% under forward bias with negligible hysteresis, comparable to state-of-the-art high-performance full-structured mixed Pb–Sn PSCs.

A poly(methyl methacrylate) (PMMA) interlayer between the perovskite and CuPc improves the insufficient electron blocking due to the low LUMO energy level of CuPc. In addition, the PMMA layer strongly interacts with the perovskite, greatly reducing the defect density responsible for non-radiative recombination. Eventually, PMMA significantly improves the efficiency of PSCs fabricated using CuPc as a hole conductor.

Abstract

The use of inexpensive, highly efficient, and long-term stable hole-transporting layers (HTLs) while facilitating the fabrication process has become a critical issue for PSC commercialization. Among organic HTLs, copper phthalocyanine (CuPc) has been increasingly studied owing to its low cost and excellent thermal stability. Nevertheless, CuPc has a low energy level in the conduction band, resulting in low efficiency due to a poor electron barrier. In this study, an efficient and stable PSC is fabricated by combining CuPc with an ultrathin poly(methyl methacrylate) (PMMA) interlayer, which is deposited on a [(FAPbI₃)_0.95(MAPbBr₃)_0.05] absorption layer (here, FAPbI₃ and MAPbBr₃ denote formamidinium lead triiodide and methylammonium lead tribromide, respectively). PMMA in perovskite has been found to reduce perovskite surface defects and series resistance as well as the electronic barrier to HTL. The optimum concentration of PMMA allows for the fabrication of the PSC with a PCE of 21.3%, which is the highest PCE for PSCs featuring metal phthalocyanines as the HTL reported to date. The stability of the encapsulated PSC exceeds 80% after 760 h at 85 °C under 85% RH conditions.

A molecularly engineered cyclobutane-based hole-transporting material synthesised using simple and green-chemistry-inspired protocols achieves an impressive efficiency of 21 % in perovskite solar cells and over 19 % in perovskite solar module with an active area of 30.24 cm².

Abstract

Hybrid lead halide perovskite solar cells (PSCs) have emerged as potential competitors to silicon-based solar cells with an unprecedented increase in power conversion efficiency (PCE), nearing the breakthrough point toward commercialization. However, for hole-transporting materials, it is generally acknowledged that complex structures often create issues such as increased costs and hazardous substances in the synthetic schemes, when translated from the laboratory to manufacture on a large scale. Here, we present cyclobutane-based hole-selective materials synthesized using simple and green-chemistry inspired protocols in order to reduce costs and adverse environmental impact. A series of novel semiconductors with molecularly engineered side arms were successfully applied in perovskite solar cells. V1366-based PSCs feature impressive efficiency of 21 %, along with long-term operational stability under atmospheric environment. Most importantly, we also fabricated perovskite solar modules exhibiting a record efficiency over 19 % with an active area of 30.24 cm².

Publication date: March 2022

Source: Nano Energy, Volume 93

Author(s): Jiachen Xi, Yeyong Wu, Weijie Chen, Qilong Li, Jiajia Li, Yunxiu Shen, Haiyang Chen, Guiying Xu, Heyi Yang, Ziyuan Chen, Na Li, Jian Zhu, Yaowen Li, Yongfang Li

A trifluoroethoxyl functionalized hole transport material, Spiro-4TFETAD, is designed and synthesized. Spiro-4TFETAD shows lower highest occupied molecular orbital level, improved hole mobility, conductivity, and hydrophobicity, as well as effectiveness in minimizing perovskite decomposition, compared to Spiro-OMeTAD. Spiro-4TFETAD-based perovskite solar cells exhibit a power conversion efficiency up to 21.11% with excellent stability, which are superior to those of Spiro-OMeTAD-based devices.

It is crucial to finely optimize the properties of hole transport materials (HTMs) to improve the performance and stability of perovskite solar cells (PSCs). Herein, a new spiro-based HTM (Spiro-4TFETAD) is developed by replacement of partial methoxy groups in Spiro-OMeTAD with trifluoroethoxy substituents. Spiro-4TFETAD has lower highest occupied molecular orbital level, higher thermal stability (T _g = 140 °C), hole mobility (2.04 × 10⁻⁴ cm²V⁻¹ s⁻¹), and better hydrophobicity with respect to Spiro-OMeTAD. The PSCs using Spiro-4TFETAD achieve a power conversion efficiency of 21.11% and excellent humidity resistance. It maintains an average 83% of their initial power conversion efficiency values even in high relative humidity of 60% without encapsulation and 82% of its initial performance after 100 h continuous illumination at the maximum power point. The superior performance underscores the promising potential of the trifluoroethoxyl molecular design in preparing new HTMs toward highly efficient and stable PSCs.

Polyacrylonitrile which has C≡N groups was introduced to passivate the uncoordinated lead cations in perovskite films. The coordination of C≡N with the lead cation was much stronger than that of the normally used C=O group. It could also reduce the I/Pb ratio at the film surface. The device efficiency was improved from 21.58 % to 23.71 %, with the open-circuit voltage enhanced from 1.12 V to 1.23 V.

Abstract

In solution-processed organic–inorganic halide perovskite films, halide-anion related defects including halide vacancies and interstitial defects can easily form at the surfaces and grain boundaries. The uncoordinated lead cations produce defect levels within the band gap, and the excess iodides disturb the interfacial carrier transport. Thus these defects lead to severe nonradiative recombination, hysteresis, and large energy loss in the device. Herein, polyacrylonitrile (PAN) was introduced to passivate the uncoordinated lead cations in the perovskite films. The coordinating ability of cyano group was found to be stronger than that of the normally used carbonyl groups, and the strong coordination could reduce the I/Pb ratio at the film surface. With the PAN perovskite film, the device efficiency improved from 21.58 % to 23.71 % and the open-circuit voltage from 1.12 V to 1.23 V, the ion migration activation energy increased, and operational stability improved.

An aerosol-assisted crystallization method to prepare high-quality, pure α-FAPbI₃ films at only 100 °C without chemical additives is reported. Remarkably improved phase stability of the α-FAPbI₃ films and their applications in solar cells are demonstrated. The overriding mechanism of stabilizing α-FAPbI₃ is shown to be relaxation of residual tensile strains in the films.

Abstract

Formamidinium lead triiodide (FAPbI₃) is attractive for photovoltaic devices due to its optimal bandgap at around 1.45 eV and improved thermal stability compared with methylammonium-based perovskites. Crystallization of phase-pure α-FAPbI₃ conventionally requires high-temperature thermal annealing at 150 °C whilst the obtained α-FAPbI₃ is metastable at room temperature. Here, aerosol-assisted crystallization (AAC) is reported, which converts yellow δ-FAPbI₃ into black α-FAPbI₃ at only 100 °C using precursor solutions containing only lead iodide and formamidinium iodide with no chemical additives. The obtained α-FAPbI₃ exhibits remarkably enhanced stability compared to the 150 °C annealed counterparts, in combination with improvements in film crystallinity and photoluminescence yield. Using X-ray diffraction, X-ray scattering, and density functional theory simulation, it is identified that relaxation of residual tensile strains, achieved through the lower annealing temperature and post-crystallization crystal growth during AAC, is the key factor that facilitates the formation of phase-stable α-FAPbI₃. This overcomes the strain-induced lattice expansion that is known to cause the metastability of α-FAPbI₃. Accordingly, pure FAPbI₃ p–i–n solar cells are reported, facilitated by the low-temperature (≤100 °C) AAC processing, which demonstrates increases of both power conversion efficiency and operational stability compared to devices fabricated using 150 °C annealed films.

Nature Energy, Published online: 16 December 2021; doi:10.1038/s41560-021-00953-z

A successive surface treatment of alkali fluoride and organic ammonium salt for wide bandgap perovskite that synergistically passivates their surface defects is developed for highly efficient stacked tandem applications.

Abstract

Superior bandgap tunability enables solution-processed halide perovskite a promising candidate for multi-junction photovoltaics (PVs). Particularly, optically coupling wide-gap perovskite by stacking with commercially available PVs such as silicon and CIGS (also known as 4-terminal tandem) simplifies the technology transfer process, and further advances the commercialization potential of perovskite technology. However, compared with matured PV materials and the phase-pure FAPbI₃, wide-gap perovskite still suffers from huge voltage deficits. Here, the authors take advantage of the synergetic effect behind a sequential fluoride and organic ammonium salt surface passivation strategy to control non-radiative energy losses, and obtained a 17.7% efficiency in infrared-transparent wide-gap perovskite solar cells (21.1% for opaque device), and achieved efficiencies of over 25% when stacked with commercial Si and CIGS products with original PCEs of 18–20% under a 4-terminal working condition.

3-sulfopropyl methacrylate potassium salt (SPM) has a multifunctional effect on the crystallization and passivation of perovskite film. The devices passivated by SPM achieve comprehensive efficiency with ≈22% photovoltaic (PV) efficiency and 10.7% electroluminescent (EL) quantum efficiency (under an injection current of short-circuit photocurrent). The reciprocity between PV and EL is correlated.

Abstract

Integration of photovoltaic (PV) and electroluminescent (EL) functions and/or units in one device is attractive for new generation optoelectronic devices but it is challenging to achieve highly comprehensive efficiency. Herein, perovskite solar cells (PSCs) are fabricated, assisted by 3-sulfopropyl methacrylate potassium salt (SPM) additive to tackle this issue. SPMs not only induce large grain size during the film formation but also produce a secondary phase of 2D K₂PbI₄ to passivate the grain boundaries (GBs). In addition, its sulfonic acid group and potassium ion can coordinate to lead ion and fill the interstitial defects, respectively. Thus, SPM reduces the defective states and suppresses nonradiative recombination loss. As a result, planar PSC delivers a power conversion efficiency of ≈22%, with a maximum open-circuit voltage (V _oc) of 1.20 V. The V _oc is 94% of the radiative V _oc limit (1.28 V), higher than the control device (V _oc of 1.12 V). In addition, the reciprocity between PV and EL is also correlated to quantify the energy losses and understand the device physics. When operated as a light-emitting diode, the maximum EL external quantum efficiency (EQE_EL) is up to 12.2% (EQE_EL of 10.7% under an injection current of short-circuit photocurrent), thus leading to high-performance PV/EL dual functions.

Biguanide hydrochloride (BGCl) is used at the tin oxide/perovskite interface for simultaneous interfacial modification and crystallization control. Assisted by Lewis coordination, electrostatic coupling and hydrogen bond anchoring between BGCl and its neighbors, better energetic alignment, reduced interfacial defects, and homogeneous perovskite crystallites are achieved, yielding an impressive power conversion efficiency of 24.4% and long-term device stability.

Abstract

Interfacial modification, which serves multiple roles, is vital for the fabrication of efficient and stable perovskite solar cells. Here, a multifunctional interfacial material, biguanide hydrochloride (BGCl), is introduced between tin oxide (SnO₂) and perovskite to enhance electron extraction, as well as the crystal growth of the perovskite. The BGCl can chemically link to the SnO₂ through Lewis coordination/electrostatic coupling and help to anchor the PbI₂. Better energetic alignment, reduced interfacial defects, and homogeneous perovskite crystallites are achieved, yielding an impressive certified power conversion efficiency (PCE) of 24.4%, with an open-circuit voltage of 1.19 V and a drastically improved fill factor of 82.4%. More importantly, the unencapsulated device maintains 95% of its initial PCE after aging for over 500 h at 20 °C and 30% relative humidity in ambient conditions. These results suggest that the incorporation of BGCl is a promising strategy to modify the interface and control the crystallization of the perovskite, toward the attainment of highly efficient and stable perovskite solar cells as well as other perovskite-based electronics.

This study reports on novel solution-processed fullerene derivatives, namely indene-C60-propionic acid butyl ester and indene-C60-propionic acid hexyl ester, as the interlayers in narrow-bandgap perovskite solar cells as well as tandem solar cells. Their effects on the performance and non-radiative recombination in the devices are systematically studied.

Abstract

Interfacial engineering is the key to high-performance perovskite solar cells (PSCs). While a wide range of fullerene interlayers are investigated for Pb-based counterparts with a bandgap of >1.5 eV, the role of fullerene interlayers is barely investigated in Sn-Pb mixed narrow-bandgap (NBG) PSCs. In this work, two novel solution-processed fullerene derivatives are investigated, namely indene-C60-propionic acid butyl ester and indene-C60-propionic acid hexyl ester (IPH), as the interlayers in NBG PSCs. It is found that the devices with IPH-interlayer show the highest performance with a remarkable short-circuit current density of 30.7 mA cm⁻² and a low deficit in open-circuit voltage. The reduction in voltage deficit down to 0.43 V is attributed to reduced non-radiative recombination that the authors attribute to two aspects: 1) a higher conduction band offset of ≈0.2 eV (>0 eV) that hampers charge-carrier-back-transfer recombination; 2) a decrease in trap density at the perovskite/interlayer/C₆₀ interfaces that results in reduced trap-assisted recombination. In addition, incorporating the IPH interlayer enhances charge extraction within the devices that results in considerable enhancement in short-circuit current density. Using a NBG device with an IPH interlayer, a respectable power conversion efficiency of 24.8% is demonstrated in a four-terminal all-perovskite tandem solar cell.

A rylene-fullerene hybrid, S-Fuller-PMI, is doped into PM6/Y6 blend film to construct ternary organic solar cells. The formation of dual-acceptor alloys gives rise to optimized phase separation morphology. The power conversion efficiency and fill factor of PM6:Y6:S-Fuller-PMI devices are significantly enhanced to 16.17% and 0.77, respectively. The enlarged entropy effectively boosts the long-term photothermal stability of ternary organic solar cells (OSCs).

Abstract

Molecular carbon imides, especially extended perylene diimides (PDIs) have been the best wide-band-gap nonfullerene acceptors. Despite their excellent photothermal/chemical stability, flexible reaction sites, and unique photoelectronic properties, there is still a lack of fundamental understanding of their molecular characteristics as a third component. Here, generations of PDIs with distinctive molecular architecture, are deliberately screened out as the third component to PM6:Y6. Only a rylene-fullerene hybrid, S-Fuller-PMI, surprisingly boosts the fill factor (FF) of ternary organic solar cells (OSCs) to 0.77 from 0.72 for PM6:Y6 binary ones, and therefore the power conversion efficiency (PCE) of ternary cells is enhanced from 15.3% to 16.2%. Compared with highly-flexible rylene dimer and rigid multimer, S-Fuller-PMI exhibits higher electron mobility, favorable surface tension, and, therefore tailored compatibility with Y6. These formed Y6:S-Fuller-PMI alloys play as a morphological controller to improve charge separation and transport process. Simultaneously, the suppressed photothermal-induced traps, along with inherent enlarged entropy effect, endow the ternary OSCs still with ≈70% of initial PCE even after 500 h continuous illumination, whereas only 53% is left in their binary counterparts. These results provide new insight into the molecular design principle for distinctive molecular carbon imides as the third component for efficient and durable PM6:Y6-based OSCs.

Various diammonium spacer cations are used to construct 2D/3D perovskite. The mechanism of molecular configuration-induced regulation of crystal orientation and carrier dynamics is investigated. 2D/3D perovskite solar cells based on 2,2′-(ethylenedioxy)bis(ethylamine) achieve a device efficiency of 22.68 % and excellent moisture stability, retaining 82 % of initial efficiency after aging at 50±5 % relative humidity for 1560 h.

Abstract

The effects from the molecular configuration of diammonium spacer cations on 2D/3D perovskite properties are still unclear. Here, we investigated systematically the mechanism of molecular configuration-induced regulation of crystallization kinetic and carrier dynamics by employing various diammonium molecules to construct Dion-Jacobson (DJ)-type 2D/3D perovskites to further facilitating the photovoltaic performance. The minimum average Pb-I-Pb angle leads to the smallest octahedral tilting of [PbX₆]⁴⁻ lattice in optimal diammonium molecule-incorporated DJ-type 2D/3D perovskite, which enables suitable binding energy and hydrogen-bonding between spacer cations and inorganic [PbX₆]⁴⁻ cages, thus contributing to the formation of high-quality perovskite film with vertical crystal orientation, mitigatory lattice distortion and efficient carrier transportation. As a consequence, a dramatically improved device efficiency of 22.68 % is achieved with excellent moisture stability.

In addition to the molecular design of the backbone, side chain engineering is another fundamental method for polymer modification. A benzo[1,2-b:4,5-b′]dithiophene (BDT)-benzodithiophene-4,8-dione copolymer PBDB-Cz is developed by employing carbazole as the conjugated side chain of BDT, which exhibits outstanding superior hole transport properties over its thiophene and alkoxy counterparts when used in n–i–p perovskite solar cells.

Abstract

In conventional n–i–p perovskite solar cells (PVSCs), electron donor (D)–acceptor (A) polymers have been found to be potential substitutes for doped spiro-based small molecule hole-transporting materials (HTMs) due to their excellent performance in hole mobility, film formability, and stability. Herein, a benzo[1,2-b:4,5-b′]dithiophene (BDT)-benzodithiophene-4,8-dione (BDD) copolymer PBDB-Cz is developed by employing carbazole as the conjugated side chain of BDT. PBDB-O and PBDB-T with alkoxy and thiophene as the side chain of BDT, respectively, are also synthesized and studied for comparison. The synergistic effect of the carbazole side chain and the BDT-BDD backbone to promote hole transport properties is found in PBDB-Cz. The carbazole side chain enhances both coplanarity and interaction of polymer chains, while simultaneously deepening energy levels and improving the hole mobility of the polymeric HTM. Consequently, PBDB-Cz outperforms two counterparts, exhibiting a promising power conversion efficiency (PCE) of 22.06%. Notably, the PBDB-Cz also improves the device stability, and the devices can retain more than 90% of their initial PCEs after being stored at ambient conditions for 100 days. To the best of the authors’ knowledge, this is the first report to incorporate carbazole into D–A polymeric HTM by side chain engineering.

A novel and low-cost phenothiazine-based self-assembly monolayer is designed and employed at the hole-transporting layer in a p-i-n perovskite solar cell, yielding a high efficiency over 22% along with an impressive operational stability over 100 h. This feature mainly originates from its well-aligned energy level match with the perovskite and efficient interfacial defect passivation.

Abstract

Recent advances in perovskite solar cells (PSCs) performance have been closely related to improved interfacial engineering and charge selective contacts. Here, a novel and cost-competitive phenothiazine based, self-assembled monolayer (SAM) as a hole-selective contact for p-i-n PSCs is introduced. The molecularly tailored SAM enables an energetically well-aligned interface with the perovskite absorber, with minimized nonradiative interfacial recombination loss, thus dramatically improving charge extraction/transport and device performance. The resulting PSCs exhibit a power conversion efficiency (PCE) of up to 22.44% (certified 21.81%) with an average fill factor close to 81%, which is among the highest efficiencies reported to date for p-i-n PSCs. The new SAM also demonstrates the outstanding operational stability of the PSC, with increasing PCE from 20.3% to 21.8% during continuous maximum power point tracking under a simulated 1 sun illumination for 100 h. The reported findings highlight the great potential of engineered SAMs for the fabrication of stable and high performing PSCs.

The buried interface between the perovskite and the electron transport layer is crucial for the further improvement of efficiency and stability of perovskite solar cells. Herein, the SnO₂/perovskite buried interface is enhanced by cesium modification. The CsI-SnO₂ complex facilitates growth of perovskite films and suppresses the carrier recombination. The champion efficiency of modified devices reaches 23.3% with excellent UV stability.

Abstract

The buried interface between the perovskite and the electron transport layer (ETL) plays a vital role for the further improvement of power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). However, it is challenging to efficiently optimize this interface as it is buried in the bottom of the perovskite film. Herein, a buried interface strengthening strategy for constructing efficient and stable PSCs by using CsI-SnO₂ complex as an ETL is reported. The CsI modification facilitates the growth of the perovskite film and effectively passivates the interfacial defects. Meanwhile, the gradient distribution of Cs⁺ contributes to a more suitable band alignment with the perovskite, and the incorporation of Cs⁺ into the perovskite at the bottom interface enhances the resistance against UV illumination. Eventually, a significantly improved PCE up to 23.3% and a much-enhanced UV stability of FAPbI₃-based PSCs are achieved. This work highlights the importance of cesium-enhanced interfaces and provides an effective approach for the simultaneous realization of highly efficient and UV-stable perovskite solar cells.

A polymeric hole-transport material (HTM) based on the phenanthrocarbazole derivative PC6 features a two-dimensionally conjugated phenanthrocarbazole and S-O noncovalent conformational locking. Perovskite solar cells employing PC6 as a dopant-free HTM afforded an excellent power conversion efficiency (PCE) of 22.2 % and long-term stability.

Abstract

Adequate hole mobility is the prerequisite for dopant-free polymeric hole-transport materials (HTMs). Constraining the configurational variation of polymer chains to afford a rigid and planar backbone can reduce unfavorable reorganization energy and improve hole mobility. Herein, a noncovalent conformational locking via S–O secondary interaction is exploited in a phenanthrocarbazole (PC) based polymeric HTM, PC6, to fix the molecular geometry and significantly reduce reorganization energy. Systematic studies on structurally explicit repeats to targeted polymers reveals that the broad and planar backbone of PC remarkably enhances π–π stacking of adjacent polymers, facilitating intermolecular charge transfer greatly. The inserted “Lewis soft” oxygen atoms passivate the trap sites efficiently at the perovskite/HTM interface and further suppress interfacial recombination. Consequently, a PSC employing PC6 as a dopant-free HTM offers an excellent power conversion efficiency of 22.2 % and significantly improved longevity, rendering it as one of the best PSCs based on dopant-free HTMs.

Methyl 3-sulfamoyl-2-thiophenecarboxylate (MSTC), containing sulfonamides and carbonyl groups, is doped into a two-step precursor as an effective additive engineering strategy. MSTC can coordinate with PbI₂ or FAI precursor through coordination bonding, hydrogen bonding, and collaboration bonding, enhancing the performance effectively. The champion power conversion efficiency of solar cells is increased from 19.19% to 22.14%, with improved stability.

Intrinsic defects are key factors that would affect the performance and stability of perovskite solar cells (PSCs). Herein, a sulfonamides additive, methyl 3-sulfamoyl-2-thiophenecarboxylate (MSTC), is introduced into the PbI₂ or FAI/MACl/MABr precursor solution, to prepare high-quality PSCs with a two-step method. After the addition of MSTC, all the devices show enhanced performance. With optimized MSTC incorporated into PSCs, the champion power conversion efficiency (PCE) of the PSCs is increased from 19.19% to 22.14%, and the stability is also improved. The MSTC-FAI based device can still maintain 89% of its initial PCE compared to 68% of the control one after 15 days in ambient condition under relative humidity of 40–50% at room temperature in dark. Test results reveal that amido group in MSTC would coordinate with PbI₂ or FAI through hydrogen bonding (NH···I), thus effectively enhancing the performance of devices. Nevertheless, the sulfonyl and carbonyl groups in MSTC would coordinate with the FAI precursor through chemical bond of COS and COC. And with the hydrogen bonding connection between MSTC and FAI, the inherent defects in the MSTC-FAI based device are effectively suppressed, leading to the enhanced photovoltaic performance.

Phenethylamine halides (PEAX) coated on perovskite layers either form 2D perovskites or dipole moments. High-performance perovskite solar cells are realized mainly due to the formation of dipole moments caused by PEAX leading to high open-circuit voltages. This implies that direct contact of PEAX with the perovskite layer is not necessary for further improvements.

Abstract

Surface modification of 3D hybrid perovskites using 2D perovskites, such as phenethylamine halides (PEAX), increases the overall power conversion efficiency (PCE) and stability of perovskite solar cells (PSCs). The effect is based on a surface passivation phenomenon where PEAX is in direct contact with the perovskite and hole transport layer (HTL). However, it is herein observed that the PCE of PSCs containing PEAX increases significantly when they are not in direct contact with either the bottom layers (perovskites) or top layers (HTLs). Moreover, the highest PCE (>22%) is obtained for the PSCs when PEAX is not in contact with HTLs by using poly(methyl methacrylate) (PMMA). Photoemission measurements reveal that the shift of the highest occupied molecular orbital of the hole transporting material (a donor-acceptor-donor molecule synthesized for the study) to a deeper level results in an increased hole transfer at the perovskite/HTL interface leading to an improved device performance. It is proposed that PEAX acts as dipoles aligned between perovskite and HTL resulting in a shift in the energy levels. The combination of PEAX/PMMA at the interface enables high open-circuit voltage (1.19V) close to the Shockley–Queisser limit for the triple-cation (Cs-MA-FA) perovskites (bandgap, 1.51 eV).

Layered zirconium phosphate-phosphonomethylglycine is exfoliated in thin nanosheets and employed as additive for MAPbI₃ perovskite films. Protected films show enhanced stability toward humidity and UV irradiation if compared to the crude perovskite. The presence of the additive also avoids the tetragonal to cubic phase transition normally observed in MAPbI₃ crystals.

Abstract

Methylammonium lead iodide perovskite (MAPbI₃) is today considered the most promising component for highly efficient third generation solar cells. However, the lifetime of the solar devices is strongly affected by the stability of the MAPbI₃ films toward humidity, UV irradiation, and temperature. The search for efficient protective additives to be used for building up composite perovskite films with enhanced stability is a topic of great interest in the scientific community. In the present paper, a layered zirconium phosphate-phosphonate based on N,N-phosphonomethylglycine, exfoliated in thin nanosheets (NS), as additive for the stabilization of MAPbI₃ crystalline films toward humidity, UV-irradiation, and temperature changes is applied. Notably, the additive is extremely efficient in preventing degradation of the perovskite film, preserving the optical and structural properties, and avoiding the phase transitions normally observed due to temperature increase.

The perovskite solar cell (PSC) has emerged rapidly in the field of flexible photovoltaics. A self-healing polysiloxane (SHP) polymer with pyridine-based heterocyclic structures and plenty of dynamic hydrogen bonds was utilized to passivate and heal the cracks at grain boundaries. A champion efficiency of 19.50 % was achieved and the PSC with SHP recovered 80 % of original efficiency after self-healing for 2 h in ambient atmosphere.

Abstract

Polymer doping is a significant approach to precisely control nucleation and crystal growth of perovskites and enhance electronic quality in perovskite solar cells (PSC) prepared in air. Here, a brand-new self-healing polysiloxane (SHP) with dynamic 2,6-pyridinedicarboxamide (PDCA) coordination units and plenty of hydrogen bonds was designed and incorporated into perovskite films. PDCA units, showing strong intermolecular Pb²⁺-N_amido, I⁻-N_pyridyl, and Pb²⁺-O_amido coordination interactions, were expected to enhance crystallinity and passivate the grain boundary. In addition, abundant hydrogen bonds in SHP afforded the self-healing of cracks at grain boundaries for fatigue PSCs. Significantly, the doped device demonstrated a champion efficiency of 19.50 % with inconspicuous hysteresis, almost rivaling those achieved in control atmosphere. This strategy of heterocyclic-based macromolecular doping in PSCs will pave a way for realizing efficient and durable crystalline semiconductors.

2D Perovskites

In article number 2102236, Jong Hyeok Park, Nam-Gyu Park, Hyunjung Shin and co-workers extract photo-generated holes from 3D bulk perovskites of FAPbI₃ through cyclohexylamonium-based 2D perovskites. 2D/3D perovskite heterojunction structures are successfully fabricated and the efficient extraction of holes is expressed in this cover picture.

New starburst molecules, bearing the carbazole moiety both as a central core and as peripheral groups, are synthesized and evaluated as hole transporting materials for perovskite solar cells. Their power conversion efficiencies range from 16.3% to 19.0%. Compounds bearing short aliphatic chains show exceptionally high glass transition temperatures, resulting in superior thermal stability compared with that of spiro-OMeTAD.

Five new star-shaped carbazole-based molecules are successfully synthesized from low-cost, commercially available reagents via a simple one-step synthesis route. All carbazole derivatives comprise a 3,6-diaminocarbazole core with carbazole peripheral groups substituted at the 2- or 3-positions and various aliphatic side chains. These molecules are evaluated as hole transporting materials to replace 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) in perovskite solar cells. Power conversion efficiencies of the devices with these carbazole hole transporting layers reach 19.0%, comparable with 19.7% obtained with the spiro-OMeTAD-based device. The thermal and operational stability of the candidate molecules are found to depend on the side chain substituents. Two candidate molecules with ethyl side chains show superior thermal stability compared with that of the reference solar cells prepared with spiro-OMeTAD.