Chen Weijie

Ultrathin 2D perovskite shell is fabricated to passivate the CsPbI₃ nanocrystals, in which organic cation guanidinium is introduced onto the host surface. The 2D perovskite nanoshell on CsPbI₃ nanocrystals can effectively repair the defects without weakening the charge transport between adjacent nanocrystals. Solar cells based on the core/shell nanocrystals can achieve a champion power conversion efficiency (PCE) of up to 15.53%.

Abstract

All-inorganic lead halide perovskite nanocrystals (NCs) emerge as a rising star in photovoltaic fields on account of their excellent optoelectronic properties. However, it still remains challenging to further promote photovoltaic efficiency due to the susceptible surface and inevitable vacancies. Here, this work reports a 3D/2D core/shell perovskite heterojunction based on CsPbI₃ NCs and its performance in solar cells. The guanidinium (GA⁺) rich 2D nanoshells can significantly passivate surface trap states and lower the capping ligand density, resulting in improved photoelectric properties and carrier transport and diminished nonradiative recombination centers via the hydrogen bonds from amino groups in GA⁺ ions. Consequently, an outstanding power conversion efficiency (PCE) of up to 15.53% is realized, substantially higher than the control device (13.77%). This work highlights the importance of surface chemistry and offers a feasible avenue to achieve high-performance perovskite NCs-based optoelectronic devices.

Publication date: 9 March 2023

Source: Chem, Volume 9, Issue 3

Author(s): Xuehan Chen, Jing Huang, Feng Gao, Bo Xu

Herein, high-performance perovskite solar cells with significantly enhanced short-circuit current (J _SC), open-circuit voltage (V _OC), and fill factor (FF) by incorporating the Ce³⁺ into the PTAA layer are reported, which are thermal-induced transferred onto the perovskite lattice and surface layer to suppress the bulk/surface defects, enhance crystallinity, and suppress nonradiative recombination.

Hybrid organic–inorganic perovskites have attracted significant attention due to their remarkable optoelectronic properties and the feasibility of cost-effective, high-throughput manufacturing of perovskite solar cells (PSCs). The present p–i–n PSCs have poor film-forming ability on poly[bis(4-phenyl)(2,4,6-trimethylphenyl)-amine] (PTAA) film, resulting in a great number of defects within perovskite. Herein, the cerium ion (Ce³⁺) into the PTAA layer is successfully incorporated, which is thermal-induced transferred onto the perovskite lattice and surface layer, by replacing Pb²⁺ in partial or uncoordinated metalsites and Ce³⁺–Ce⁴⁺ ion pair to suppress the bulk/surface defects, leading to a significantly enhanced power conversion efficiency (PCE). Systemically studies demonstrate that thermal-induced Ce^3+/4+-doped MAPbI_2.91Br_0.09 thin film possesses superior film morphology with a uniform surface, suppressed nonradiative recombination, and enhanced crystallinity due to fewer bulk/surface defects. Moreover, PSCs by MAPbI_2.91Br_0.09/PTAA:xCe^3+/4+ (x = 0.5 wt%) thin film exhibit suppressed charge carrier recombination and shorter charge carrier extraction time. As a result, PSCs by MAPbI_2.91Br_0.09/PTAA:xCe^3+/4+ (x = 0.5 wt%) thin film exhibit PCE of 21.32% with significantly increased fill factor (FF) of 81.17% and long-term stability. All these results indicate that the approach provides a facile way to incorporate rare-earth ions into perovskites to boost the performance of PSCs.

Porous organic cage (POC) is introduced between tin dioxide and perovskite to spontaneously reconstitute the buried interface. Through polydentate chelation and host-guest interaction between POC with SnO₂ and perovskite, reduced trap-state density, enhanced charge extraction and suppressed ion migration are achieved, exhibiting a superior PCE of 24.13% and conspicuous improved long-term stability.

Abstract

In perovskite solar cells (PSCs), the buried interface containing high concentrations of defects is critical for efficient charge extraction toward high-performance device. Herein, porous organic cage (POC) is introduced between tin dioxide and perovskite to spontaneously reconstitute the buried interface. Through the chemical linkage formed by polydentate chelation of POC with SnO₂ and perovskite, the buried interface achieves greatly reduced defect density and enhanced carrier extraction. More importantly, it is found that iodide ions in aged devices to migrate down to the electron transport layer and even invade the ITO electrode, changing the work function of ITO. This detrimental effect can be well resolved by POC since the host-guest interaction of POC can effectively suppress the iodide ions trying to migrate downward. As a result, the PSC fabricated by POC-restructured strategy yields a superior PCE of 24.13%. Moreover, the unencapsulated PSCs exhibit conspicuous improved long-term stability and retain 93% of its initial efficiency after 5000 h in ambient condition.

Formamidine acetate (FAAc) has a strong COSn coordination with Sn²⁺, which can stabilize the lattice structure, minimize defect states, and suppress the oxidation of Sn²⁺. Benefiting to this coordination ability, it not only leads to large-size colloidal clusters in the precursor but also slows down the crystallization process and improves the crystallinity of tin halide perovskite films. The device with FAAc achieves an increased PCE from initially 9.84% to 12.43%, and it can maintain 94% of its initial value for 2000 h in N₂ atmosphere.

Abstract

Tin halide lead-free perovskite solar cells (TPSCs) have received tremendous research interest recently due to their nearly ideal bandgap, broad light absorption, non-toxicity, and environmental friendliness. However, the uncontrollable crystallization process and the facile oxidation of Sn²⁺ limit the further increase of power conversion efficiency (PCE). To solve these problems, a series of acetates are introduced into the perovskite precursor solution to regulate the crystallization process. It is revealed that formamidine acetate (FAAc) has strong COSn coordination with Sn²⁺ compared with acetic acid (HAc) and methylammonium acetate (MAAc), which can stabilize the lattice structure, minimize defect states and suppress the oxidation of Sn²⁺. Meanwhile, benefiting from this coordination ability, it not only leads to large-size colloidal clusters in precursor but also slows down the crystallization process and improves the crystallinity of tin halide perovskite films. The device with FAAc achieved an increased PCE from initially 9.84% to 12.43%, and it could maintain 94% of its initial value for 2000 h in N₂ atmosphere. This work provides a feasible strategy for depositing high-quality tin perovskite films with low defect density and lattice distortion, which will be crucial for related photovoltaics and other optoelectronic devices.

A new family of low-dimensional Ruddlesden-Popper perovskites (GABA)₂MA _n−1Pb _n I_3n+1 (n=1, 2, 3, 4, MA⁺=CH₃NH₃ ⁺) with inhibited dielectric confinement were constructed with γ-aminobutyric acid (GABA) as the spacer cation. The hydrogen bonds between the spacers link the adjacent spacing sheets, enabling the charges localized in the van der Waals gap. The power conversion efficiency of (GABA)₂MA₄Pb₅I₁₆ solar cells with excellent carrier transport properties is up to 18.73 %.

Abstract

Low-dimensional Ruddlesden-Popper (LDRP) perovskites still suffer from inferior carrier transport properties. Here, we demonstrate that efficient exciton dissociation and charge transfer can be achieved in LDRP perovskite by introducing γ-aminobutyric acid (GABA) as a spacer. The hydrogen bonding links adjacent spacing sheets in (GABA)₂MA₃Pb₄I₁₃ (MA=CH₃NH₃ ⁺), leading to the charges localized in the van der Waals gap, thereby constructing “charged-bridge” for charge transfer through the spacing region. Additionally, the polarized GABA weakens dielectric confinement, decreasing the (GABA)₂MA₃Pb₄I₁₃ exciton binding energy as low as ≈73 meV. Benefiting from these merits, the resultant GABA-based solar cell yields a champion power conversion efficiency (PCE) of 18.73 % with enhanced carrier transport properties. Furthermore, the unencapsulated device maintains 92.8 % of its initial PCE under continuous illumination after 1000 h and only lost 3 % of its initial PCE under 65 °C for 500 h.

Benzylphosphonic acid (BnPA)/pentafluorobenzylphosphonic acid (F₅BnPA) mixture can form an ordered self-assembly monolayer on indium tin oxide (ITO) due to the strong arene–perfluoroarene interaction, thus achieving an efficiency of 18.0% for hole-transport layer (HTL)-free polymer solar cells. The device shows much better stability compared with devices with BnPA- or F₅BnPA-modified ITO, and is comparable to ITO/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-based device.

Self-assembly monolayers (SAMs) of small molecules are attractive alternatives of traditional transporting materials to reconcile interfaces with tunable interface properties in polymer solar cells (PSCs). Herein, it is found that benzylphosphonic acid (BnPA)/pentafluorobenzylphosphonic acid (F₅BnPA) mixture could form an ordered SAM on indium tin oxide (ITO) due to the strong arene–perfluoroarene interaction, thus the hole-transport-layer-free PSCs based on poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b′]-dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PM6): BO-4Cl achieves a power conversion efficiency (PCE) of 18.0%. The high performance is attributed to the improved energy level alignment, excellent carrier-extraction ability, and reduced recombination. The device also shows much better stability compared with the devices based on BnPA- or F₅BnPA-modified ITO, and shows comparable stability to the device based on ITO/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). Furthermore, the device with an area of 1.05 cm² shows a PCE of 15.3%, which is among the highest reported values. Herein, the potential of SAMs is highlighted for highly stable and high-performance PSCs toward commercialization.

Spirobifluorene-based hole-transporting materials are fundamental for the next-generation solar cells. Herein, the readers are guided through the most recent advances and how novel approaches based on spirobifluorene-core must be explored to advance toward new efficient systems for solar cells.

Organic–inorganic halide perovskite solar cells (PSCs) and organic solar cells (OSCs) attract great attention as alternative renewable photovoltaic technology. The state-of-the-art spiro-OMeTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) is the most successful hole-transport material (HTM) employed in PSCs, whereas solution-processed inverted OSCs generally use poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Recently, various types of spirobifluorene-based organic small molecules are reported to overcome the known disadvantages of spiro-OMeTAD, such as the complex synthetic route, high synthetic cost, and requirement for hygroscopic dopants to improve the charge-carrier mobility and device performance. Examples of spirobifluorene-based molecules are also reported as alternative HTMs in inverted OSCs to exceed the drawbacks of PEDOT:PSS, such as acidity and batch-to-batch reproducibility. These features significantly limit spiro-OMeTAD and PEDOT:PSS for large-scale application in the future. Herein, an overview of recent developments in spirobifluorene organic small molecules as HTM in PSCs and OSCs is provided by focusing on synthetic and electrical features. Finally, the further research directions are discussed to develop novel spirobifluorene-based HTMs for the realization of reliable and long-term stable photovoltaic devices.

An acceptor CH7 with the extended conjugation central unit and long-branched side chains is reported and a 25.2 cm² organic solar module based on PM6:CH7 and processed from eco-friendly solvent o-xylene demonstrated high efficiency and decent stability.

It is a challenge to fabricate organic solar modules with the combination of high efficiency, good stability, and green solvent treatment. To address the issue, active layer materials still play crucial roles. Herein, a non-fullerene acceptor CH7 with the extended conjugation central unit and long-branched side chains is reported for the fabrication of high-performance large-area modules. The long-branched alkyl chains can ensure CH7 to have good solubility in non-halogen solvent o-xylene (OX). Meanwhile, the steric hindrance of long-branched alkyl chains can suppress molecular excessive aggregation. The inverted structure prototype device based on PM6:CH7 and processed with OX showed a promising power conversion efficiency (PCE) of 17.49% mainly due to the favorable active layer morphology. Based on the small area device results, processed from OX, a 25.2 cm² module is fabricated and demonstrates a high PCE of 14.42% and good photo stability with maintaining 93% of its initial efficiency after 500 h continuous illumination. Moreover, the module also shows decent thermal stability, maintaining with 82% of its original efficiency after the thermal stress at 65 °C for 500 h.

The additive 2,6-diaminopyridine (TNPD) not only effectively induces crystal growth and ultimately passivates the resulting perovskite films via bidentate anchoring, but also releases the micro-strain generated during the film growth. TNPD-involved devices attain the best power conversion efficiency (PCE) of 20.35% (the control PCE =17.28%).

Abstract

The power conversion efficiency (PCE) of tin–lead perovskite solar cells (PSCs) is normally lower than that of Pb cells, mainly due to greater open circuit voltage (V _OC) losses. Herein, the additive 2,6-diaminopyridine (TNPD) is designed to anchor on the surface of the perovskite precursor colloid as nucleating agent to modulate the growth of Pb–Sn perovskites. It is observed that the TNPD not only effectively induces crystal growth during the nucleation stage, remaining on the crystal surface and ultimately passivating the resulting perovskite films, but also releases the micro-strain generated during the film growth. Furthermore, TNPD could lower the defect density (Sn⁴⁺ amount) by screening the perovskite against oxygen and by synergistically bonding with undercoordinated Sn/Pb on the surface. Finally, a high V _OC of 0.85 V is obtained, corresponding to a voltage deficit of 0.41 V using a perovskite absorber with a bandgap of 1.26 eV, and a high PCE (20.35%) reported so far for Pb–Sn PSCs. Moreover, the stability of the TNPD-incorporated device is significantly improved, and the PCE maintains 50% of the initial value after about 1000 h storage in glovebox without encapsulated, in comparison to that of the control device (about 700 h, maintaining 30% of the initial value).

Inspired by the traditional Chinese dragon shape, a flexible tandem perovskite solar cell with a theoretical photoelectric conversion efficiency of 34.83% is designed for the first time using the bionic concept and simulation method combined with the finite-element method and wave optics theory. Herein, a practical way is presented to design high-performance flexible tandem photovoltaic devices.

Flexible tandem solar cells (FTSCs) are a kind of photovoltaic device with enormous potential application value that truly integrates low cost and high efficiency. Perovskite materials have immediately become a dazzling new star in the field of tandem solar cells and flexible solar cells (FSCs) owing to their groundbreaking optoelectronic properties. However, compared with rigid solar cells, the photoelectric conversion efficiency of FSCs still has considerable room for improvement, which is mainly limited by the optical loss of the device. Herein, inspired by the shape of the Chinese dragon, various types of two-terminal (2T) all-perovskite FTSCs are constructed utilizing bionic concepts and the finite-element method and wave optics theory are coupled to study the device's performance. The optical performance of flexible photovoltaic devices is influenced by structure size parameters. In addition, the dependence of the 2T all-perovskite FTSCs on the light incident angle is also investigated. Finally, the total photocurrent (J _ph) value of the device reaches 37.63 mA cm⁻² (J _top = 18.82 mA cm⁻², J _bottom = 18.81 mA cm⁻²). The optical properties of all-perovskite FTSCs are demonstrated in the simulation results with ultralow surface reflection and it is found extremely insensitive to the incident angle of light. The designed 2T all-perovskite FTSCs provide a practical route to realize high-efficiency, low-cost photovoltaic modules.

Cost-effective self-assembled molecules bearing anchoring groups are developed and applied as hole transporting layer in p–i–n perovskite solar cells. The donor–acceptor type self-assembled molecule has strong interactions with indium doped tin oxide substrate and perovskite, resulting in a power conversion efficiency of over 23% and a lifetime of over 2000 h at 80% efficiency (T₈₀) under maximum power point tracking.

Abstract

The self-assembled hole transporting molecules (SAHTMs) bearing anchoring groups have been established as the hole transporting layers (HTLs) for highly efficient p–i–n perovskite solar cells (PSCs), yet their stability and engineering at the molecular level remain challenging. A topological design of highly anisotropic aligned SAHTM-based HTLs for operationally stable PSCs that exhibit exceptional solar-to-electric power conversion efficiencies (PCEs) is demonstrated. The judiciously designed multifunctional self-assembled molecules comprise the donor–acceptor subunit for hole transporting and the phosphonic acid group for anchoring, realizing face-on π-stacking parallel to the transparent conductive oxide substrate. The high affinity of SAHTMs to the multi-crystalline perovskite thin film benefits passivating the perovskite buried interface, strengthening interfacial contact while facilitating interfacial hole transfer. Consequently, highly efficient p–i–n PSC devices are obtained with a champion PCE of 23.24% and outstanding operational stability toward various environmental factors including long-term full sunlight soaking at evaluated temperatures. Perovskite solar modules with a champion efficiency approaching 20% are also fabricated for an active device area above 17 cm².

A synchronous optimization strategy is realized via simultaneously introducing PbS QDs into SnO₂ electron transport layer and employing Eu:PbS QDs film with hydrophobic chain ligands as the NIR light-absorping layer and hole transporting layer of devices simultaneously. The successful synchronous optimization greatly elevates all photovoltaic parameters, reaching a maximum power conversion efficiency of 23.27% with lower hysteresis.

Abstract

Colloidal lead sulfide (PbS) quantum dots (QDs), which possess quantum confinement effect and processing compatibility with perovskite, are regarded as an excellent material for optimizing perovskite solar cells (PSCs). However, the existing PSCs optimized by PbS QDs are still facing the challenges of poor performance of the charge transport layers, low utilization in the near-infrared (NIR) region, and unsuitable energy level alignment, which limit the improvement of power conversion efficiency (PCE). Herein, a synchronous optimization strategy is realized via simultaneously introducing PbS QDs into SnO₂ electron transport layer and employing rare-earth-doped PbS QDs (Eu:PbS QDs) film with hydrophobic chain ligands as the NIR light-absorping layer and hole transport layer (HTL) of devices. PbS QDs effectively decrease the density of trap states by passivating defects. Eu:PbS QDs film with adjustable bandgap is employed as an absorption layer to broaden the NIR spectral absorption. The well-matched energy level between Eu:PbS QDs layer and perovskite layer implies efficient hole transfer at the interface. The successful synchronous optimization greatly elevates all photovoltaic parameters, reaching a maximum PCE of 23.27%. This PCE is the highest for PSCs utilizing PbS QDs material in recent years. The optimized PSCs retain long-term moisture and light stability.

Two A-D-A type 2D small molecules with longitudinal conjugate extension using triphenylamine groups as side chains are successfully developed. The 2D small molecules show a dominant face-on orientation and better hole transport mobility. A champion efficiency of 21.44% is achieved using the dopant-free 2D small molecule hole transport material in perovskite solar cells along with improved stability.

Abstract

Developing dopant-free hole transport materials (HTMs) to replace Spiro-OMeTAD is a challenging but urgent issue for commercialization of state-of-the-art n-i-p structured perovskite solar cells (PSCs). Here, this work proposes an effective two-dimensional conjugate engineering strategy to tune molecular stacking orientation and improve the hole mobility of dopant-free small molecule HTMs. For the first time, triphenylamine (TPA) groups are incorporated as side chains of benzo [1,2-b:4,5-b′]dithiophene (BDT) unit to extend the longitudinal conjugate, achieving two donor-acceptor-acceptor type 2D small molecules, namely XF2 and XF3, which show a dominant face-on orientation and better hole transport mobility than the linear small molecule XF1. The incorporation of alkoxy Lewis base groups makes XF3 a more effective defect passivator for perovskite surfaces. As a result, the PSCs using pristine XF3 HTM show a dramatically improved efficiency of 20.59% along with improved long-term stability compared to that of XF1 HTM (power conversion efficiency (PCE) = 18.84%). A champion efficiency of 21.44% is achieved through device engineering for dopant-free XF3-based PSCs. The results show that the building block with longitudinal conjugate extension in small molecules plays an essential role in the face-on orientation morphology and elucidates a key design rule for the dopant-free small molecule HTMs for high-performance PSCs.

Tin-based perovskite solar cells (PSCs) experience fast crystallization and spontaneous oxidation, which act as the dominant factors causing device efficiency deterioration. Such phenomena are found to be related to the inherent properties of tin. By constructing low-dimensional structures, the resulting devices possess improved stability and performance, highlighting the challenges and the potential for replacing lead-based perovskite.

Abstract

Due to its outstanding optoelectronic properties, halide perovskite solar cells (PSCs) power conversion efficiency has rapidly grown to 25.7%. Nonetheless, lead poisoning is a significant hurdle to the deployment of perovskite solar cells (PSCs). Tin is the most alternative with the most potential due to its similar electric and electronic properties to lead and its less hazardous nature. Yet, the performance of Sn-based PSCs lags significantly below that of Pb-based PSCs due to the Sn (II)'s easy oxidation to Sn (IV). Incorporating large-sized organic cations to form quasi-two-dimensional (2D) structured-tin perovskites increases the stability of the PSC. In addition, the hydrophobic group of the quasi-2D structure inhibits moisture and oxygen from penetrating the absorber layers. This review analyzes and evaluates the characteristics and performance of quasi-2D Sn-based perovskites such as Ruddlesden–Popper, Dion–Jacobson, and alternating cation interlayer (ACI). This work further proposes alternative strategies to improve the efficiency and stability of tin-based PSCs, including constructing new mixed 2D/3D perovskite structures, enhancing the transmission capacity, novel organic cations, and fabricating new ACI perovskite structures and controlling perovskite strain.

An organic photovoltaic catalyst (Y6CO) with a central steric-hindrance-free carbonyl group, achieves efficient Pt⁰ in situ deposition through σ-π anchoring, leading to significantly enhanced utilization of the Pt cocatalyst and photocatalytic hydrogen evolution of Y6CO/Pt based nanoparticles under simulated solar light illumination.

Abstract

Efficient in situ deposition of metallic cocatalyst, like zero-valent platinum (Pt), on organic photovoltaic catalysts (OPCs) is the prerequisite for their high catalytic activities. Here we develop the OPC (Y6CO), by introducing carbonyl in the core, which is available to σ-π coordinate with transition metals, due to the high-energy empty π* orbital of carbonyl. Y6CO exhibits a stronger capability to anchor Pt species and reduce them to metallic state, resulting in more Pt⁰ deposition, relative to the control OPC without the central σ-π anchor. Single-component and heterojunction nanoparticles (NPs) employing Y6CO show enhanced average hydrogen evolution rates of 230.98 and 323.22 mmol h⁻¹ g_[OPC] ⁻¹, respectively, under AM 1.5G, 100 mW cm⁻² for 10 h, and heterojunction NPs yield the external quantum efficiencies of ca. 10 % in 500–800 nm. This work demonstrates that σ-π anchoring is one efficient strategy for integrating metallic cocatalyst and OPC for high-performance photocatalysis.