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A facile and effective additive strategy is devised by introducing bifunctional guanidine sulfamate (GuaSM) molecules into perovskite (PVK) layer. The synergistic effect of the SM⁻ anions and the Gua⁺ cations are demonstrated, which effectively reduces the trap density and the recombination in PVK, so that the photovoltaic performance and stability of the perovskite solar cells are improved noticeably.

Abstract

High efficiency and good stability are the challenges for perovskite solar cells (PSCs) toward commercialization. However, the intrinsic high defect density and internal nonradiative recombination of perovskite (PVK) limit its development. In this work, a facile additive strategy is devised by introducing bifunctional guanidine sulfamate (GuaSM; CH₆N₃ ⁺, Gua⁺; H₂N−SO₃ ⁻, SM⁻) into PVK. The size of Gua⁺ ion is suitable with Pb(BrI)₂ cavity relatively, so it can participate in the formation of low‐dimensional PVK when mixed with Pb(BrI)₂. The O and N atoms of SM⁻ can coordinate with Pb²⁺. The synergistic effect of the anions and cations effectively reduces the trap density and the recombination in PVK, so that it can improve the efficiency and stability of PSCs. At an optimal concentration of GuaSM (2 mol%), the PSC presents a champion power conversion efficiency of 21.66% and a remarkably improved stability and hysteresis. The results provide a novel strategy for highly efficient and stable PSCs by bifunctional additive.

Dimethylammonium (DMA) is incorporated in controlled, incremental amounts into the A‐site of CsPbI₃ perovskite materials. Confirming that the stabilization afforded from the DMA iodide precursors in CsPbI₃ perovskites comes from an alloy of the A‐site with an organic cation. The limit to DMA incorporation is ≈25%, making a Cs_0.75DMA_0.25PbI₃ material that is more stable than neat CsPbI₃.

All‐inorganic perovskite materials are attractive alternatives to organic–inorganic perovskites because of their potential for higher thermal stability. Although CsPbI₃ is compositionally stable under elevated temperatures, the cubic perovskite α‐phase is thermodynamically stable only at >330 °C and the low‐temperature perovskite γ‐phase is metastable and highly susceptible to non‐perovskite δ‐phase conversion in moisture. Many methods have been reported which show that the incorporation of acid (aqueous HI) or “HPbI₃”—recently shown to be dimethylammonium lead iodide (DMAPbI₃) —lowers the annealing temperature required to produce the black, perovskite phase of CsPbI₃. Herein, the optical and crystallographic data presented show that dimethylammonium (DMA) can successfully incorporate as an A‐site cation to replace Cs in the CsPbI₃ perovskite material. This describes the stabilization and lower phase transition temperature reported in the literature when HI or HPbI₃ is used as precursors for CsPbI₃. The Cs–DMA alloy only forms a pure‐phase material up to ≈25% DMA; at higher concentrations, the CsPbI₃ and DMAPbI₃ begin to phase segregate. These alloyed materials are more stable to moisture than neat CsPbI₃, but do not represent a fully inorganic perovskite material.

Bearing high hole mobility and appropriate energy levels, an organic small molecule 2,7‐bis(4‐octylphenyl)naphtho[2,1‐b:6,5‐b 0] difuran (C₈‐DPNDF) is introduced as a dopant‐free hole transporting material in inverted perovskite solar cells. The device with C₈‐DPNDF as HTM shows a decent power conversion efficiency of 17.5% and can keep 92% of its initial value after 30 days in ambient air.

Hole transport material (HTM) is a significant constituent in perovskite solar cells (PSCs). However, HTM generally is not utilized in its pristine form but with dopants (such as lithium salt, tert‐butyl pyridine, F₄‐TCNQ), which accelerates device degradation and leads to poor stability. Therefore, dopant‐free HTM is highly desirable to fabricate stable devices. Herein, a fused furan organic small molecule (C₈‐DPNDF) is introduced as a dopant‐free HTM in inverted PSCs. As a potential HTM candidate, C₈‐DPNDF shows excellent properties, such as high hole mobility, matched energy level with perovskite, and resistance to perovskite precursor solution. As a result, the device based on C₈‐DPNDF as HTM shows a power conversion efficiency (PCE) of 17.5%, compared with 17.1% of the control device based on classic poly(bis(4‐phenyl)(2,4,6‐trimethylphenyl)amine) (PTAA) as the HTM. In addition, the unencapsulated device based on C₈‐DPNDF as HTM keeps 92% of its initial PCE after 30 days of storage in ambient air with a relative humidity of ≈40%. This finding is expected to pave the way toward stable and highly efficient inverted PSCs based on dopant‐free HTMs.

Perovskite solar cells with the 2D passivation layer display excellent photovoltaic performance and superior stability via introducing hydrophobic alkyl molecules with polyfunctional groups.

The charges stuck in trap sites hinder charge transport and lead to V _oc below the radiative limit, which seriously restrict the performance and stability of organic–inorganic halide perovskite solar cells (PSCs). Chemical passivation is an effective method to reduce defects and suppress nonradiative recombination. Herein, a new passivation molecule l‐cysteine methyl ester hydrochloride (CME) with thiol and ester groups is designed to modify the interface between the perovskite layer and hole transport layer (HTL). It reveals that thiol possesses outstanding moisture resistance and ester suppresses nonradiative recombination by coordinating with undercoordinated Pb²⁺. Furthermore, the 2D modified layer at the grain boundaries and surface passivates surface defects and promotes hole extraction. As a result, the CME device achieves the highest PCE of 20.33% with an enhanced open‐circuit voltage (V _oc) of 1.11 V. Due to the barrier of highly hydrophobic 2D perovskites, the modified devices show excellent stability while exposed to humidity and high‐temperature environment. A facile and effective strategy to design organic molecular structures with polyfunctional groups to passivate trap‐assisted nonradiative recombination at the surface and grain boundaries is provided.

Nature Communications, Published online: 05 November 2020; doi:10.1038/s41467-020-19332-5

Designing efficient organic solar cells is limited by the energy required to overcome the mutual Coulomb attraction between electron and hole. Here, the authors reveal long-lived and disorder-free charge-transfer states enable efficient endothermic charge separation in non-fullerene systems with marginal energy offset.

A comparative study is reported on the Brønsted acid and Lewis acid doping of poly(3‐hexylthiophene) with a largely soluble molecular dopant, tris(pentafluorophenyl)borane. The Brønsted acid doping enables the formation of unconventional type II polymorph of the polymer, thereby leading to drastic increases in electrical and thermoelectric properties and excellent air stabilities via simple one‐step solution mixing.

Abstract

Molecular doping is essential for improving the thermoelectric properties of conjugated polymers, but dopants of low solubility either restrict the formation of high quality films or complicate fabrication steps. Although a highly soluble molecular dopant, tris(pentafluorophenyl)borane (BCF), has been sporadically studied, its potential has not yet been fully explored. Herein, particularly intriguing effects of Brønsted acid doping with BCF‐water complexes for poly(3‐hexylthiophene) (P3HT) are reported, which can facilitate substantial increases in electrical and thermoelectric properties with remarkable doping stabilities. Interestingly, a unique polymorph of P3HT with interdigitated alkyl chains (called type II) is observed in the Brønsted acid doping with BCF‐water complexes. Moreover, the doped P3HT shows conformational change to the quinoid structure, enabling increased backbone planarity. As a result, the Brønsted acid‐doped P3HT films exhibit outstanding electrical conductivities, thermoelectric power factors, and figure‐of‐merit of up to 33.0 S cm⁻¹, 28.3 µW m⁻¹ K⁻², and 0.034, respectively. These values are at least an order of magnitude higher than those of P3HT films doped with a conventional molecular dopant, 7,7,8,8‐tetracyano‐2,3,5,6‐tetrafluoroquinodimethane. The Brønsted acid doping with BCF‐water complexes also affords excellent air stabilities of P3HT films, which potentially provides a strong comparative advantage over existing highly reactive salt‐type dopants, such as FeCl₃.

Nature Energy, Published online: 02 November 2020; doi:10.1038/s41560-020-00714-4

Ultrathin solar cells attract interest for their relatively low cost and potential novel applications. Here, Massiot et al. discuss their performance and the challenges in the fabrication of ultrathin absorbers, patterning of light trapping structures and ensuring efficient charge-carrier collection.

A dissymmetric fused‐ring acceptor BTIC‐2Cl‐γCF₃ with chlorine and trifluoromethyl end groups give a power conversion efficiency (PCE) of over 17 % which is the highest among polymer solar cells processed by halogen‐free solvents. Dissymmetric chlorination and trifluoromethylation is a practical approach towards a low band‐gap acceptor for eco‐compatible processed photovoltaic applications.

Abstract

To elevate the performance of polymer solar cells (PSC) processed by non‐halogenated solvents, a dissymmetric fused‐ring acceptor BTIC‐2Cl‐γCF₃ with chlorine and trifluoromethyl end groups has been designed and synthesized. X‐ray crystallographic data suggests that BTIC‐2Cl‐γCF₃ has a 3D network packing structure as a result of H‐ and J‐aggregations between adjacent molecules, which will strengthen its charge transport as an acceptor material. When PBDB‐TF was used as a donor, the toluene‐processed binary device realized a high power conversion efficiency (PCE) of 16.31 %, which improved to 17.12 % when PC71ThBM was added as the third component. Its efficiency of over 17 % is currently the highest among polymer solar cells processed by non‐halogenated solvents. Compared to its symmetric counterparts BTIC‐4Cl and BTIC‐CF₃‐γ, the dissymmetric BTIC‐2Cl‐γCF₃ integrates their merits, and has optimized the molecular aggregations with excellent storage and photo‐stability, and also extending the maximum absorption peak in film to 852 nm. The devices exhibit good transparency indicating a potential utilization in semi‐transparent building integrated photovoltaics (ST‐BIPV).

Herein, novel Ruddlesden–Popper Cs₂PbI₂Cl₂ nanosheets are synthesized and creatively employed as a multifunctional interface optimization material to improve the performance of CsPbI₂Br solar cells. Based on the heterostructured NSs/CsPbI₂Br/NSs inorganic film, an efficiency of 16.65% is obtained, which is one of the best reported for CsPbI₂Br solar cells, along with much‐enhanced UV, air, and thermal stabilities.

Abstract

Inorganic CsPbI₂Br perovskite solar cells (PSCs) have gained enormous research interest due to their excellent thermal and light stabilities. However, their unsatisfactory power‐conversion efficiency and poor intrinsic phase stability remain roadblocks to their further development. Herein, Cs₂PbI₂Cl₂ nanosheets (NSs) with the Ruddlesden–Popper (RP) structure are synthesized, and an NSs/CsPbI₂Br/NSs heterostructure is employed to enhance both the stability and efficiency of CsPbI₂Br solar cells. The novel Cs₂PbI₂Cl₂ NSs can not only passivate the top and bottom surfaces of the perovskite film and top surface of the TiO₂ film but also enhance the stability of the perovskite film. Based on the heterostructured NSs/CsPbI₂Br/NSs inorganic perovskite film, the efficiency of the CsPbI₂Br PSCs is improved from 15.02% to 16.65%. Moreover, the unencapsulated CsPbI₂Br devices with the NSs/CsPbI₂Br/NSs heterostructure sustain over 90% of their original efficiencies after being exposed to ambient conditions (≈25 °C and ≈35% RH) for 648 h. Both the UV‐light‐soaking stability (100 mW cm⁻¹ 365 nm UV light) and thermal stability (T = 85 °C) of the optimized devices are dramatically improved in comparison with their counterparts with only a 3D active layer. Therefore, this work promotes the application of RP inorganic perovskite nanocrystals in a range of perovskite optoelectronic devices.

A newly conceived n‐type small molecule (Y‐Th2) is incorporated as an efficient additive in perovskite solar cells, achieving simultaneous improvements in device performance and stability. Y‐Th2 effectively passivates defects in perovskite crystals by Lewis acid–base interactions and intermolecular hydrogen bonds, obtaining high‐quality perovskite film. The inverted structure device exhibits a power conversion efficiency of 21.5% with notably enhanced operational stability.

Abstract

Significant efforts have been devoted to modulating the grain size and improving the film quality of perovskite in perovskite solar cells (PSCs). Adding materials to the perovskite is especially promising for high‐performance PSCs, because the additives effectively control the crystal structure. Although the additive engineering approach has substantially boosted the efficiency of PSCs, instability of the perovskite film has remained a primary bottleneck for the commercialization of PSCs. Herein, a newly conceived bithiophene‐based n‐type conjugated small molecule (Y‐Th2) is introduced to PSCs, which simultaneously enhances the performance and stability of the cell. The Y‐Th2 effectively passivates the defect states in perovskite through Lewis acid–base interactions, increasing the grain size and quality of the perovskite absorber. An inverted PSC containing the Y‐Th2 additive achieves a power conversion efficiency of 21.5%, versus 18.3% in the reference device. The operational stability is also considerably enhanced by the improved hydrophobicity and intermolecular hydrogen bonds in the perovskite.

An anion/cation synergy strategy is proposed by the incorporation of ZnI₂ in CsPbI₃ quantum dots (QDs) to improve the stability and photoelectric properties. The obtained Zn:CsPbI₃ QDs show lower defect state density and enhanced structural stability. Perovskite quantum dot solar cells fabricated with Zn:CsPbI₃ QDs exhibit a champion power conversion efficiency over 16%.

Abstract

All‐inorganic CsPbI₃ quantum dots (QDs) have shown great potential in photovoltaic applications. However, their performance has been limited by defects and phase stability. Herein, an anion/cation synergy strategy to improve the structural stability of CsPbI₃ QDs and reduce the pivotal iodine vacancy (V _I) defect states is proposed. The Zn‐doped CsPbI₃ (Zn:CsPbI₃) QDs have been successfully synthesized employing ZnI₂ as the dopant to provide Zn²⁺ and extra I⁻. Theoretical calculations and experimental results demonstrate that the Zn:CsPbI₃ QDs show better thermodynamic stability and higher photoluminescence quantum yield (PLQY) compared to the pristine CsPbI₃ QDs. The doping of Zn in CsPbI₃ QDs increases the formation energy and Goldschmidt tolerance factor, thereby improving the thermodynamic stability. The additional I⁻ helps to reduce the V _I defects during the synthesis of CsPbI₃ QDs, resulting in the higher PLQY. More importantly, the synergistic effect of Zn²⁺ and I⁻ in CsPbI₃ QDs can prevent the iodine loss during the fabrication of CsPbI₃ QD film, inhibiting the formation of new V _I defect states in the construction of solar cells. Consequently, the anion/cation synergy strategy affords the CsPbI₃ quantum dot solar cells (QDSC) a power conversion efficiency over 16%, which is among the best efficiencies for perovskite QDSCs.

Highly efficient ternary all‐polymer solar cells (PSCs) based on an ultranarrow bandgap polymer acceptor are realized. The optimized ternary all‐PSCs achieve a full coverage of solar spectrum, yielding an excellent power conversion efficiency of 12.1% with a remarkable short‐circuit current density of 21.9 mA cm⁻².

Abstract

Developing organic solar cells (OSCs) based on a ternary active layer is one of the most effective approaches to maximize light harvesting and improve their photovoltaic performance. However, this strategy meets very limited success in all‐polymer solar cells (all‐PSCs) due to the scarcity of narrow bandgap polymer acceptors and the challenge of morphology optimization. In fact, the power conversion efficiencies (PCEs) of ternary all‐PSCs even lag behind binary all‐PSCs. Herein, highly efficient ternary all‐PSCs are realized based on an ultranarrow bandgap (ultra‐NBG) polymer acceptor DCNBT‐TPC, a medium bandgap polymer donor PTB7‐Th, and a wide bandgap polymer donor PBDB‐T. The optimized ternary all‐PSCs yield an excellent PCE of 12.1% with a remarkable short‐circuit current density of 21.9 mA cm⁻². In fact, this PCE is the highest value reported for ternary all‐PSCs and is much higher than those of the corresponding binary all‐PSCs. Moreover, the optimized ternary all‐PSCs show a photostability with ≈68% of the initial PCE retained after 400 h illumination, which is more stable than the binary all‐PSCs. This work demonstrates that the utilization of a ternary all‐polymer system based on ultra‐NBG polymer acceptor blended with compatible polymer donors is an effective strategy to advance the field of all‐PSCs.

Perpendicular crystal orientation and orderly n‐phase distribution in quasi‐2D perovskite films are simultaneously achieved by F‐substitution in phenethylammonium (PEA⁺), leading to an impressive 18.10%‐efficiency of perovskite solar cells with n = 4. Meanwhile, the horizontal crystal orientation and random n‐phase distribution are obtained in perovskite films based on PEA and (Cl/Br)‐substituted PEA, respectively.

Abstract

Halide substitution in phenethylammonium spacer cations (X‐PEA⁺, X = F, Cl, Br) is a facile strategy to improve the performance of PEA based perovskite solar cells (PSCs). However, the power conversion efficiency (PCE) of X‐PEA based quasi‐2D (Q‐2D) PSCs is still unsatisfactory and the underlying mechanisms are in debate. Here, the in‐depth study on the impact of halide substitution on the crystal orientation and multi‐phase distribution in PEA based perovskite films are reported. The halide substitution eliminates n = 1 2D perovskite and thus leads to the perpendicular crystal orientation. Furthermore, nucleation competition exists between small‐n and large‐n phases in PEA and X‐PEA based perovskites. This gives rise to the orderly distribution of different n‐phases in the PEA and F‐PEA based films, and random distribution in Cl‐PEA and Br‐PEA based films. As a result, (F‐PEA)₂MA₃Pb₄I₁₂ (MA = CH₃NH₃ ⁺, n = 4) based PSCs achieve a PCE of 18.10%, significantly higher than those of PEA (12.23%), Cl‐PEA (7.93%) and Br‐PEA (6.08%) based PSCs. Moreover, the F‐PEA based devices exhibit remarkably improved stability compared to their 3D counterparts.

Novel interface polarization induced field‐effect passivation based on amorphous transition metal oxide is developed for efficient and ambient‐air‐stable perovskite solar cells. Comprehensive insights into the interaction between the field‐effect passivation, interface polarities, and the performance of the device have been elucidated in detail.

Abstract

Organolead halide hybrid perovskite solar cells (PSCs) have become a shining star in the renewable devices field due to the sharp growth of power conversion efficiency; however, interfacial recombination and carrier‐extraction losses at heterointerfaces between the perovskite active layer and the carrier transport layers remain the two main obstacles to further improve the power conversion efficiency. Here, novel field‐effect passivation has been successfully induced to effectively suppress the interfacial recombination and improve interfacial charge transfer by incorporating interfacial polarization via inserting a high work function interlayer between perovskite and holes transport layer. The charge dynamics within the device and the mechanism of the field‐effect passivation are elucidated in detail. The unique interfacial dipoles reinforce the built‐in field and prevent the photogenerated charges from recombining, resulting in power conversion efficiency up to 21.7% with negligible hysteresis. Furthermore, the hydrophobic interlayer also suppresses the perovskite decomposition by preventing the moisture penetration, thereby improving the humidity stability of the PSCs (>91% of the initial power conversion efficiency (PCE) after 30 d in 65 ± 5% humidity). Finally, several promising research perspectives based on field‐effect passivation are also suggested for further conversion efficiency improvements and photovoltaic applications.

Two well‐regular polymer acceptors (PY‐IT and PY‐OT) with different polymerization sites are developed. For comparison, a random ternary copolymer (PY‐IOT) with the same ratio of the two acceptors is synthesized. All‐polymer solar cells (PSCs) based on PM6:PY‐IT achieve an excellent PCE of 15.05%, which is significantly higher than those based on PY‐OT (10.04%) and PY‐IOT (12.12%).

Abstract

Recent advances in the development of polymerized A–D–A‐type small‐molecule acceptors (SMAs) have promoted the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs) over 13%. However, the monomer of an SMA typically consists of a mixture of three isomers due to the regio‐isomeric brominated end groups (IC‐Br(in) and IC‐Br(out)). In this work, the two isomeric end groups are successfully separated, the regioisomeric issue is solved, and three polymer acceptors, named PY‐IT, PY‐OT, and PY‐IOT, are developed, where PY‐IOT is a random terpolymer with the same ratio of the two acceptors. Interestingly, from PY‐OT, PY‐IOT to PY‐IT, the absorption edge gradually redshifts and electron mobility progressively increases. Theory calculation indicates that the LUMOs are distributed on the entire molecular backbone of PY‐IT, contributing to the enhanced electron transport. Consequently, the PM6:PY‐IT system achieves an excellent PCE of 15.05%, significantly higher than those for PY‐OT (10.04%) and PY‐IOT (12.12%). Morphological and device characterization reveals that the highest PCE for the PY‐IT‐based device is the fruit of enhanced absorption, more balanced charge transport, and favorable morphology. This work demonstrates that the site of polymerization on SMAs strongly affects device performance, offering insights into the development of efficient polymer acceptors for all‐PSCs.