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Three benzodipyrrole (BDP)-based molecules are designed and synthesized as hole transport material for perovskite solar cells. The combination of BDP core and 3-fluorophenyl unit as CB-2 creates highly planar conformation and effective defect passivation via interaction with Pb of the perovskite. Thus, CB-2 achieves improved solar cell performance and excellent long-term storage stability without degradation for over 6 months.

Three benzodipyrrole (BDP)-based organic small molecules with substituted 4-methoxyphenyl (CB-1), 3-fluorophenyl (CB-2), and 3-trifluoromethylphenyl (CB-3) are designed, synthesized, and used as a hole-transporting material (HTM) for perovskite solar cells (PSCs). The electrochemical, optical, thermal, electronic, and optoelectronic properties of the HTMs are characterized to verify their suitability for PSCs. The terminal functional groups of the HTMs having different heteroatoms mainly target effective defect passivation of perovskites. Photoluminescence studies and molecular dynamic simulations reveal that fluorine atoms within CB-2 and CB-3 can contribute to the defect passivation via interaction with Pb of the perovskite. In particular, a highly planar conformation of CB-2 on the perovskite surface can facilitate more efficient hole transfer at the interface. Thus, the PSCs employing CB-2 achieve the highest power conversion efficiency (PCE) of 18.23% while the devices using CB-1 and CB-3 exhibit a lower PCE of 16.78% and 16.74%, respectively. PSCs with the BDP-based HTMs demonstrate excellent long-term storage stability without degradation in their PCEs over 6 months. The highly planar geometry, defect passivation effect, and hydrophobicity of CB-2 show its great potential as an HTM for efficient and stable PSCs.

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.

Exploiting norbornenyl modified terminals endows the fused ring acceptor (SM16) with an unprecedentedly high photoluminescence quantum yield of 8.61%, thus leading to a very low nonradiative voltage loss of 0.145 V when blended with PBDB-T. A power conversion efficiency of 17.1% is achieved by using SM16 as the third component due to the boosting of open-circuit voltage and fill factor.

Abstract

Increasing the photoluminescence quantum yield (PLQY) of narrow bandgap acceptors is of critical importance to suppress the nonradiative voltage loss (ΔV _nr) in organic solar cells (OSCs). Herein, two acceptors, SM16 and SM16-R, with an identical backbone but different terminal groups (norbornenyl modified 1,1-dicyanomethylene-3-indanone and dimethyl substituted 1,1-dicyanomethylene-3-indanone) are designed and synthesized. Compared with SM16-R, SM16 displays better solubility, higher PLQY, and more favorable nanomorphology when blended with polymer donor PBDB-T. PBDB-T:SM16-based OSCs yield a ΔV _nr as low as 0.145 V. Using SM16 as the third component, a high power conversion efficiency of 17.1% is achieved in the ternary OSCs based on PBDB-T:Y14:SM16, considerably higher than that of the binary devices based on PBDB-T:Y14 or PBDB-T:SM16. These results highlight that enhancing the PLQY of low bandgap acceptor via terminal group engineering strategy is highly effective to reduce ΔV _nr of OSCs.

A new end group by extending π-conjugation with a chlorinated thiophene ring is reported. Acceptors with strong luminescence brought about by the end group realizes energy loss mitigation of organic solar cells. Thus, the efficiencies of 17.61% and 18.21% for the binary and ternary devices are exhibited by the chemical modification of the end group.

Abstract

Large energy loss is one of the main limiting factors for power conversion efficiencies (PCEs) of organic solar cells (OSCs). To this effect, the chemical modifications of the famous Y-series nonfullerene acceptor (NFA) BTP-4Cl-BO with a new end group, TPC-Cl, whose π-conjugation is extended through the fusing of 3-(dicyanomethylene)indanone (IC) group with a chlorinated thiophene ring, to synthesize two novel NFAs, BTP-T-2Cl and BTP-T-3Cl are performed. For BTP-T-2Cl with two TPC-Cl groups, the resulting OSC exhibits a modest PCE of 14.89% but an extraordinary low energy loss of 0.49 eV, because its superior electroluminescence quantum efficiency of 0.0606% mitigates significantly the nonradiative loss (0.191 eV). For BTP-T-3Cl with one TPC-Cl group, the corresponding device shows a higher PCE of 17.61% accompanied by a slightly bigger energy loss of 0.51 eV, which can be ascribed to the optimized morphology and/or efficient charge generation. Furthermore, the ternary OSC adopting two NFAs of BTP-T-3Cl and BTP-4Cl-BO achieves an impressive PCE of 18.21% (certified value of 17.9%), which is among the highest values for OSCs to date. The above results demonstrate that expanding end groups of NFAs with electron-donating rings is an effective strategy to realize lower energy losses for OSCs.

This work presents a facile and scalable high-temperature nanomanufacturing of cesium lead iodide (CsPbI₃) nanocrystals (NCs) using a modular flow reactor. The flow chemistry platform integrated with multimodal in situ spectroscopy reveals a detailed understanding of the formation mechanism of CsPbI₃ NCs via the hot-injection synthetic route.

Abstract

Despite the groundbreaking advancements in the synthesis of inorganic lead halide perovskite (LHP) nanocrystals (NCs), stimulated from their intriguing size-, composition-, and morphology-dependent optical and optoelectronic properties, their formation mechanism through the hot-injection (HI) synthetic route is not well-understood. In this work, for the first time, in-flow HI synthesis of cesium lead iodide (CsPbI₃) NCs is introduced and a comprehensive understanding of the interdependent competing reaction parameters controlling the NC morphology (nanocube vs nanoplatelet) and properties is provided. Utilizing the developed flow synthesis strategy, a change in the CsPbI₃ NC formation mechanism at temperatures higher than 150 °C, resulting in different CsPbI₃ morphologies is revealed. Through comparison of the flow- versus flask-based synthesis, deficiencies of batch reactors in reproducible and scalable synthesis of CsPbI₃ NCs with fast formation kinetics are demonstrated. The developed modular flow chemistry route provides a new frontier for high-temperature studies of solution-processed LHP NCs and enables their consistent and reliable continuous nanomanufacturing for next-generation energy technologies.

Publication date: February 2022

Source: Nano Energy, Volume 92

Author(s): Kuntal Maity, Uttam Pal, Hari Krishna Mishra, Prasenjit Maji, Priyabrata Sadhukhan, Zinnia Mallick, Sachindranath Das, Bidya Mondal, Dipankar Mandal

Herein, a series of organic spacer cations with different alkyl chain lengths are introduced onto the three-dimensional perovskite to investigate the chain length effect. It's found that the chain length affects the quality of as-casted two-dimensional perovskite, leading to distinct effects on passivation and charge transfer properties.

While interface modification based on organic spacer cations has been proven to be a viable strategy to boost the performance of 3D perovskite solar cells, the mechanisms behind the accomplished efficiency enhancement and the selection rule for organic spacers are yet to be clarified. Herein, based on representative four ammonium halide salts featuring different chain lengths as spacer cations for the 2D perovskite surface modifier, it is shown that choosing appropriately sized spacers can lead to synergetic mediation on interfacial passivation of point and structural defects and the crystallization of 2D perovskite components in protecting the buried 3D perovskite film. As a result, promoted charge transport and extraction with suppressed charge trapping and recombination are simultaneously realized in the 3D/2D perovskite solar cells. With the optimal spacer cation phenylpropylammonium iodide (PPAI), the resultant 3D/2D devices produce a power conversion efficiency of 22.57% with a fill factor exceeding 0.8. Favorably, the PPAI-treated devices exhibit considerable gains in stability under various external stresses.

(CsFA)PbI_3–x Br _x film is in situ formed atop CsPbI_2.2Br_0.8 perovskite via a facile formamidinium iodide posttreatment method, which can broaden the light absorption range of CsPbI_2.2Br_0.8 and accelerate hole extraction from CsPbI_2.2Br_0.8 to the carbon electrode, thus greatly boosting the efficiency of a hole transporting layer-free carbon-based perovskite solar cell to 15.03% with an ultrahigh fill factor of 0.81.

Hole transporting layer (HTL)-free, all-inorganic CsPbX₃ (X: I, Br, or mixed halides), carbon-based perovskite solar cells (C-PSCs) show promising prospect for photovoltaic application due to their low cost, excellent stability, and theoretical high efficiency. However, the inefficient hole extraction of the carbon electrode and relatively narrow light absorption range of inorganic perovskite limit the power conversion efficiency (PCE) of this kind of PSCs. Herein, these issues are addressed through in situ constructing of an intermediate energy-level perovskite transition layer between CsPbI_2.2Br_0.8 and the carbon electrode via a facile formamidinium iodide (FAI) posttreatment strategy. It is demonstrated that the (CsFA)PbI_3–x Br _x film is in situ formed atop inorganic perovskite due to the ions exchange between FAI and CsPbI_2.2Br_0.8, which can not only broaden the light absorption edge of CsPbI_2.2Br_0.8 from 657 to 680 nm, but also serve as a hole transfer highway between CsPbI_2.2Br_0.8 and the carbon electrode, mainly due to its suitable intermediate energy-level and effective defect passivation. Consequently, the optimized HTL-free C-PSC achieves a champion PCE of 15.03% with an ultrahigh fill factor of 0.81. Besides, the stability of CsPbI_2.2Br_0.8 film (especially under humid environment) and corresponding C-PSC are also improved.

Herein, the advancements of the pure formamidinium (FA) and FA-rich perovskite solar cells (PSCs) with efficiencies exceeding 23% from the viewpoints of composition engineering, solvent engineering, additive engineering, and interface engineering are discussed. Further improvement in voltage and fill factor for the FA-based PSCs is expected to be able to achieve an efficiency over 30%.

To further improve power conversion efficiency (PCE) toward Shockley−Queisser limit efficiency approaching 32% for a single-junction perovskite solar cell (PSC) based on a lead halide perovskite with a bandgap of about 1.45 eV, it is important to improve the open-circuit voltage and fill factor (FF) significantly without sacrificing short-circuit current density. Herein, the advancements of the formamidinium-rich PSCs with PCEs exceeding 23% from the viewpoints of composition engineering, solvent engineering, additive engineering, and interface engineering are summarized and discussed. Based on the lessons, the possible strategies, methods, and research directions are proposed to further improve voltage and FF for ideal PCEs.

C₆H₇NO and C₆H₆FNO additives are first doped into FA_0.75MA_0.25SnI₃ precursors and induce conspicuous increases in power conversion efficiency and stability in FA_0.75MA_0.25SnI₃ perovskite solar cells. The C₆H₆FNO additive with a fluorinated group has a low surface energy, which causes spontaneous migration of themselves to solution–air surface during the spin-coating process and then resulting in more expected initial crystal nucleation and growth from surface.

Stability and efficiency issues are closely related with poor perovskite film quality in perovskite solar cells (PSCs). Herein, 2-aminophenol (C₆H₇NO) and 2-amino-4-fluorophenol (C₆H₆FNO) are introduced to improve film quality of FA_0.75MA_0.25SnI₃, both of which consist of —NH₂ and —OH groups, and the latter also contains —F group. The experimental and theoretical analyses show both —NH₂ and —OH groups interact with I of the SnI₆ ⁴⁻ octahedron via hydrogen bond and fluorinated group with low surface energy causes spontaneous migration of C₆H₆FNO to solution–air surface and induces initial crystal nucleation and growth from surface, both of which contribute to improved film morphology and crystallinity and suppressed Sn²⁺ oxidation via reducing defect-state density and nonradiative recombination. The F atom of C₆H₆FNO facing outward protects FA_0.75MA_0.25SnI₃ from water penetration due to its hydrophobic feature. The C₆H₆FNO-doped PSC acquires a champion efficiency of 9.50% and a long-term stability of >800 h (80% efficiency remained in N₂).

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.

With the development from the printed thin-film devices to thick-film devices, the vertical component distribution are finely modulated by additives for the efficient charge transport, resulting in high-performance large-area thick-film organic solar cells.

Thick active layers in organic solar cells (OSCs) have a great promise of enhancing light absorption and providing pinhole-free films for large-scale fabrication. Since charge carriers in thick films need a longer transporting path in the vertical direction to the electrode than in thin films, modulation of the active layer morphology in thick films is highly required for effective charge transport. Herein, thin-film (≈110 nm) and thick-film (≈300 nm) OSCs based on a PM6:IT-4 F film are fabricated by blade coating with various additive contents. It is found that the optimized thick-film device needs more additives than the optimized thin-film device. The addition of more additives in thick-films promotes vertical component distribution and enhances the crystallization, resulting in efficient charge transport with reduced charge recombination and electron (or hole) accumulation within the thick active layer. These results are also confirmed by PM6:Y6-based devices, in which optimized thin-film and thick-film devices exhibit power conversion efficiency (PCE) of 16.69% and 14.91% at the additive contents of 0.3% and 0.6%, respectively. Encouragingly, thick-film device with 0.6% additive has a narrow distribution of PCE values, and high PCEs of 13.94% and 13.05% are obtained for the large-area (1 cm²) rigid and flexible thick-film OSCs, showing great application prospect.

The grain boundaries in perovskite films play a major role in restricting the performance and stability of perovskite photovoltaics by allowing moisture permeation and ion migration. Herein, Cu₂NiSn(S,Se)₄ nanocrystals effectively passivate the surface and enhance the hole extraction from active layer to hole transport layer, yielding an efficiency of 20.8% with outstanding stability by retaining over 85% of initial performance.

Despite the rapid progress of perovskite materials in emerging perovskite photovoltaic devices, they still suffer from the polycrystalline nature associated with grain boundaries (GBs) which are vulnerable to moisture permeation and/or ion migration. Besides, charge carrier recombination of GBs through defect states plays a crucial role in restricting the performance and stability of perovskite photovoltaics. To address such detrimental issues, quinary kesterite nanocrystals, namely Cu₂NiSn(S,Se)₄ (CNTSSe), having narrow size distribution below 10 nm by a facile hot-casting method are rationally designed and employed as a passivation agent for the GBs/surface of perovskite films. This passivation strategy greatly reduces defect states at perovskite GBs and promotes continuity between adjacent grains, resulting in accelerated hole transport ability and suppressed interfacial recombination. Thereupon, champion power conversion efficiency of 20.8% (20.5 ± 0.3% in average) (Cs_0.05(FA_0.90MA_0.10)_0.95Pb(I_0.90Br_0.10)₃), 18.9% (MAPbI₃), and 18.7% (FAPbI₃) is achieved with a negligible hysteresis and outstanding stability by retaining over 85% of initial performance under ambient conditions with continuous illumination over 900 h. Herein, not only a universal approach to effectively passivate the GBs of the perovskite films by inorganic nanocrystals is presented, but also a deep understanding of detrimental defects on the photovoltaic performance and stability of perovskite solar cells is ensured.

Hydrothermal-synthesized Cu-doped Ga₂O₃ nanocrystals are used as the hole transport material of inverted perovskite solar cells. Cu dopants remarkably improve the performance of devices, which is related to the additional hole transport channels from impurity levels.

In inverted perovskite solar cells (PSCs), metal oxides become kind of promising hole transport layers for their facile synthesis and low cost. For conventional hole transport materials, the valence band match between metal oxides and perovskite layers is usually necessary for the hole extraction process. Ga₂O₃ is an emerging semiconductor material with ultrawide bandgap, but a significant energy level mismatch exists at Ga₂O₃/perovskite interfaces. In this work, Cu-doped Ga₂O₃ (Ga₂O₃:Cu) nanocrystals are synthesized by the hydrothermal method and used as the hole transport material of inverted PSCs for the first time. It is found that Cu dopants can substantially improve the performance of Ga₂O₃ layers, and the efficiency of PSCs is increased from 7.6% to 19.5%. This improvement can be attributed to the additional hole transport channels from impurity levels of Cu dopants, which exactly match with the valence band of perovskite layers. As a consequence, Ga₂O₃:Cu layers can effectively extract holes and inhibit the recombination in perovskite layers. This work also provides an alternative route for the design of hole transport materials.

By incorporating intramolecular S⋅⋅⋅O noncovalent interactions (INIs) for boosting the intrinsic hole mobilities, two simple-structured dopant-free hole-transporting materials (HTMs) were designed and delivered a remarkable efficiency of 21.10 % with decent device stability in inverted perovskite solar cells, demonstrating the great promise of the INI strategy for accessing high-performance dopant-free HTMs.

Abstract

Intramolecular noncovalent interactions (INIs) have served as a powerful strategy for accessing organic semiconductors with enhanced charge transport properties. Herein, we apply the INI strategy for developing dopant-free hole-transporting materials (HTMs) by constructing two small-molecular HTMs featuring an INI-integrated backbone for high-performance perovskite solar cells (PVSCs). Upon incorporating noncovalent S⋅⋅⋅O interaction into their simple-structured backbones, the resulting HTMs, BTORA and BTORCNA, showed self-planarized backbones, tuned energy levels, enhanced thermal properties, appropriate film morphology, and effective defect passivation. More importantly, the high film crystallinity enables the materials with substantial hole mobilities, thus rendering them as promising dopant-free HTMs. Consequently, the BTORCNA-based inverted PVSCs delivered a power conversion efficiency of 21.10 % with encouraging long-term device stability, outperforming the devices based on BTRA without S⋅⋅⋅O interaction (18.40 %). This work offers a practical approach to designing charge transporting layers with high intrinsic mobilities for high-performance PVSCs.

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.

Two polymer acceptors with conjugated thienylene–vinylene–thienylene and unconjugated thienylene–ethyl–thienylene as linkage units are designed. Due to the π-extended coplanar backbone of the polymer acceptor with conjugated linkage unit, enhanced exciton dissociation, superior charge transport, and faster charge extraction are observed in all-polymer solar cells, leading to a high efficiency of 16.13% with superior thickness-insensitivity and long-term stability.

Abstract

Despite the rapid developments in all-polymer solar cells (all-PSCs) due to the progress of polymerized small molecular acceptors (PSMA), the effect of linkage unit conjugation on the polymer acceptor (P_A) is not well understood and P_As with high efficiency, good stability, and thickness-insensitivity are rarely seen. Herein, two novel PSMAs, named PJTVT and PJTET are designed, by incorporating conjugated thienylene-vinylene-thienylene (TVT) and unconjugated thienylene–ethyl–thienylene (TET) units, respectively. Results show that the energy levels, energy losses, and energy offset of the two PSMAs have little difference (<≈0.03 eV). However, due to the π-extended coplanar backbone of PJTVT, when blended with polymer donor JD40, a more ordered π–π stacking and enhanced face-on orientation morphology is observed, which contributes to enhanced exciton dissociation, superior charge transport, and faster charge extraction, leading to a record power conversion efficiency of 16.13% (10.93% for JD40:PJTET). Impressively, the JD40:PJTVT device shows superior thickness-insensitivity and long-term stability, both of which make it an ideal choice for industrialization. These results demonstrate that molecular modulation in the linking unit is a promising strategy to construct PSMAs for high-performance thick-film all-PSCs with superior long-term stability, and shows the superiority of conjugated backbones for PSMAs.

Nonionic cross-linked 1D PbI₂-DPPO located at perovskite grain boundaries can passivate the defects and suppress the ion migration in 3D perovskites, thus significantly improving the intrinsic stability of perovskite films. Consequently, a 1D-PbI₂/3D heterojunction perovskite solar cells mini-module demonstrates a certified stabilized power conversion efficiency of 17.8% and superior long-term stability.

Abstract

Long-term stability has become the major obstacle for the successful large-scale application of perovskites devices. Owing to the ionic nature of metal-halide perovskites, the interfacial ion diffusion can induce irreversible degradation under operational conditions, which presents a great challenge to realize stable perovskite solar modules. Here, a diphenylphosphine oxide compound, ethane-1,2-diylbis(diphenylphosphine oxide) (DPPO) is introduced to coordinate with lead iodide and form a cross-linked 1D Pb₃I₆-DPPO (1D-PbI₂) complex. These judiciously designed cross-linked nonionic low-dimensional lead halide/organic adducts can passivate the defects of perovskite while acting as a robust ion diffusion barrier, thus significantly improving the electronic quality and intrinsic stability of perovskite films. As a result, high-performance inverted (p-i-n) solar modules with a champion efficiency approaching 19% (a certified stabilized efficiency of 17.8%) for active device areas above 17 cm² without the use of antisolvents, accompanied by outstanding operational stability under heat stress and continuous illumination are achieved.

A 10.7% -efficiency Sb₂(S,Se)₃ solar cell is achieved by a solution post-treatment technique, which regulates the energy band alignment and reduces the mismatch of the valence bands between the absorber and hole transport layer.

Abstract

Continuously boosting the power conversion efficiency (PCE) and delving deeper into its functionalities are essential problems faced by the very new antimony selenosulfide (Sb₂(S,Se)₃) solar technology. Here, a convenient and effective solution post-treatment (SPT) technique is used to fabricate high-performance Sb₂(S,Se)₃ solar cells, where alkali metal fluorides are applied to improve the quality of Sb₂(S,Se)₃ films in terms of morphology, crystallinity, and conductivity. In particular, this approach is able to manipulate the S/Se gradient in the films and creates favorable energy alignment which facilitates the carrier transport. As a result, the fill factor and short-circuit current density of Sb₂(S,Se)₃ solar cells (Glass/FTO/Zn(O,S)/CdS/Sb₂(S,Se)₃/Spiro-OMeTAD/Au) based on the SPT strategy are significantly enhanced, achieving a champion efficiency of 10.7%. To date, this conversion efficiency value represents the highest efficiency of all Sb-based solar cells. This study provides an effective post-treatment strategy for improving the quality of Sb₂(S,Se)₃ film which sheds new light on the fabrication of high-efficiency Sb₂(S,Se)₃ solar cells.

A strategy for spontaneous construction of a 2D/3D structure is proposed to prepare inorganic CsPbI_{3− x} Br _x perovskite solar cells. The results reveal that the Ruddlesden–Popper 2D-(MOPEA)₂Pb(Br _x I_{4− x} ) perovskite can effectively enhance the hole extraction efficiency and passivate detrimental surface defects. The power conversion efficiency is significantly increased to 20.31%, making it one of the most efficient inorganic perovskite solar cells.

Abstract

CsPbI_{3− x} Br _x -based organic-free perovskite has emerged as a superstar photovoltaic material not only because of its superior photoelectronic properties but also its outstanding thermal and chemical stability. Unfortunately, the significant energy loss resulting from its nonradiative recombination has become a major obstacle to further improvement of device performance. Here, a 2D/3D multidimensional structure formed spontaneously at room temperature is developed. The results reveal that the formed Ruddlesden–Popper 2D (n = 1) perovskite atop CsPbI_{3− x} Br _x plays an active role in mediating carrier transport, maintaining a long-life charge separation state on the nanosecond time scale and promoting the efficiency of carrier injection into the hole transport layer, and thus enhances the hole extraction efficiency, which greatly reduces severe interfacial nonradiative charge recombination. In addition, the undercoordinated Pb²⁺ is effectively passivated, resulting in significantly reduced surface trap density and prolonged charge lifetime within the perovskite films. Consequently, the combination of the above increases the solar cell efficiency from 19.05% to 20.31%, with an open-circuit voltage raised to 1.23 from 1.17 V, which corresponds to an energy loss reduction from 0.54 to 0.49 eV. Also, the optimized solar cells exhibit better long-term and thermal stability.

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).

Gradient self-doping within the micron scale is established based on an optimized two-step method in printable mesoscopic perovskite solar cells. The difference in the work function of the perovskite enhances the built-in electric field, thus promoting the transport and extraction of photogenerated holes. Reduced carrier recombination losses deliver an average V _OC improvement over 60 mV and a power conversion efficiency of 17.68%.

Abstract

The hole-conductor-free printable mesoscopic perovskite solar cells based on the inorganic scaffolds of mesoporous titania, mesoporous zirconia, and porous carbon have attracted much attention due to their excellent stability and low manufacturing cost. However, in such hole-conductor-free devices, the transport of the photogenerated holes is dominated by the diffusion-assisted charge carrier movement, while the driving force is insufficient. Reinforcing the built-in electric field (BEF) is an effective strategy to promote oriented carrier transport. Herein, by using an optimized two-step deposition method, the BEF is reinforced by creating a work function difference of perovskite (Δµ) in different layers via a gradient self-doping. The enhanced BEF improves the hole transport and extraction, and significantly reduces the carrier recombination losses in the device. As a result, an average open-circuit voltage improvement over 60 mV and a power conversion efficiency of 17.68% are achieved without any additives or complex processes. This strategy provides a new approach toward fabricating highly efficient printable mesoscopic perovskite solar cells with reduced carrier recombination losses.

The similarities and differences in the charge carrier dynamics in organic solar cells and organic–inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated.

Abstract

The charge carrier dynamics in organic solar cells and organic–inorganic hybrid metal halide perovskite solar cells, two leading technologies in thin-film photovoltaics, are compared. The similarities and differences in charge generation, charge separation, charge transport, charge collection, and charge recombination in these two technologies are discussed, linking these back to the intrinsic material properties of organic and perovskite semiconductors, and how these factors impact on photovoltaic device performance is elucidated. In particular, the impact of exciton binding energy, charge transfer states, bimolecular recombination, charge carrier transport, sub-bandgap tail states, and surface recombination is evaluated, and the lessons learned from transient optical and optoelectronic measurements are discussed. This perspective thus highlights the key factors limiting device performance and rationalizes similarities and differences in design requirements between organic and perovskite solar cells.

Two novel small-molecule donors with thioalkyl chains in the para- (P-PhS) and the meta-position (M-PhS) are synthesized to regulate surface tension and molecular packing. An optimized morphology with small domains and ordered packing is simultanously obtained in the M-PhS:BTP-eC9 blend, promoting a record power conversion efficiency (PCE) of 16.2% with excellent (FF × J _sc) in all-small-molecule organic solar cells (ASM-OSCs).

Abstract

In all-small-molecule organic solar cells (ASM-OSCs), a high short-circuit current (J _sc) usually needs a small phase separation, while a high fill factor (FF) is generally realized in a highly ordered packing system. However, small domain and ordered packing always conflicted each other in ASM-OSCs, leading to a mutually restricted J _sc and FF. In this study, alleviation of the previous dilemma by the strategy of obtaining simultaneous good miscibility and ordered packing through modulating homo- and heteromolecular interactions is proposed. By moving the alkyl-thiolation side chains from the para- to the meta-position in the small-molecule donor, the surface tension and molecular planarity are synchronously enhanced, resulting in compatible properties of good miscibility with acceptor BTP-eC9 and strong self-assembly ability. As a result, an optimized morphology with multi-length-scale domains and highly ordered packing is realized. The device exhibits a long carrier lifetime (39.8 μs) and fast charge collection (15.5 ns). A record efficiency of 16.2% with a high FF of 75.6% and a J _sc of 25.4 mA cm⁻² in the ASM-OSCs is obtained. These results demonstrate that the strategy of simultaneously obtaining good miscibility with high crystallinity could be an efficient photovoltaic material design principle for high-performance ASM-OSCs.

This review provides a comprehensive overview of the research progress and challenges associated with thickness-controlled synthesis, stability, surface passivation, doping, optical (linear and nonlinear) properties of both organic and organic–inorganic hybrid lead and lead-free halide perovskite nanoplatelets along with recent progress on their application to light-emitting diodes.

Abstract

Colloidal metal-halide perovskite nanocrystals (MHP NCs) are gaining significant attention for a wide range of optoelectronics applications owing to their exciting properties, such as defect tolerance, near-unity photoluminescence quantum yield, and tunable emission across the entire visible wavelength range. Although the optical properties of MHP NCs are easily tunable through their halide composition, they suffer from light-induced halide phase segregation that limits their use in devices. However, MHPs can be synthesized in the form of colloidal nanoplatelets (NPls) with monolayer (ML)-level thickness control, exhibiting strong quantum confinement effects, and thus enabling tunable emission across the entire visible wavelength range by controlling the thickness of bromide or iodide-based lead-halide perovskite NPls. In addition, the NPls exhibit narrow emission peaks, have high exciton binding energies, and a higher fraction of radiative recombination compared to their bulk counterparts, making them ideal candidates for applications in light-emitting diodes (LEDs). This review discusses the state-of-the-art in colloidal MHP NPls: synthetic routes, thickness-controlled synthesis of both organic–inorganic hybrid and all-inorganic MHP NPls, their linear and nonlinear optical properties (including charge-carrier dynamics), and their performance in LEDs. Furthermore, the challenges associated with their thickness-controlled synthesis, environmental and thermal stability, and their application in making efficient LEDs are discussed.

Three D–D type wide-bandgap donor polymers (PBDTT, PBDTT1Cl, and PBDTT2Cl) are designed and facilely synthesized. Organic solar cells (OSCs) based on PBDTT1Cl exhibit a high power conversion efficiency of 17% and a low nonradiative energy loss of 0.19 eV. In addition, PBDTT1Cl has a very low figure-of-merit and good universality, indicating its potential as a low-cost polymer donor for high-performance OSCs.

Abstract

Three regioregular benzodithiophene-based donor–donor (D–D)-type polymers (PBDTT, PBDTT1Cl, and PBDTT2Cl) are designed, synthesized, and used as donor materials in organic solar cells (OSCs). Because of the weak intramolecular charge-transfer effect, these polymers exhibit large optical bandgaps (>2.0 eV). Among these three polymers, PBDTT1Cl exhibits more ordered and closer molecular stacking, and its devices demonstrate higher and more balanced charge mobilities and a longer charge-separated state lifetime. As a result of these comprehensive benefits, PBDTT1Cl-based OSCs give a very impressive power conversion efficiency (PCE) of 17.10% with a low nonradiative energy loss (0.19 eV). Moreover, PBDTT1Cl also possesses a low figure-of-merit value and good universality to match with different acceptors. This work provides a simply and efficient strategy to design low-cost high-performance polymer donor materials.