Chen Weijie

A low-cost thiophene-based hole-transporting material, triarylamine-substituted bithiophene (BT-4D), is used as a hole-transporting material in perovskite solar cells with comparable photovoltaic performance to that of 2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene. The solar cell using BT-4D demonstrates exceptional long-term stability by retaining 98% of its initial power conversion efficiency after 1186 h under continuous 1-sun illumination in an inert atmosphere.

Abstract

Triarylamine-substituted bithiophene (BT-4D), terthiophene (TT-4D), and quarterthiophene (QT-4D) small molecules are synthesized and used as low-cost hole-transporting materials (HTMs) for perovskite solar cells (PSCs). The optoelectronic, electrochemical, and thermal properties of the compounds are investigated systematically. The BT-4D, TT-4D, and QT-4D compounds exhibit thermal decomposition temperature over 400 °C. The n-i-p configured perovskite solar cells (PSCs) fabricated with BT-4D as HTM show the maximum power conversion efficiency (PCE) of 19.34% owing to its better hole-extracting properties and film formation compared to TT-4D and QT-4D, which exhibit PCE of 17% and 16%, respectively. Importantly, PSCs using BT-4D demonstrate exceptional stability by retaining 98% of its initial PCE after 1186 h of continuous 1 sun illumination. The remarkable long-term stability and facile synthetic procedure of BT-4D show a great promise for efficient, stable, and low-cost HTMs for PSCs for commercial applications.

Ascertaining heat energy transfer is essential for the design of organic materials for energy conversion. For Y-series molecules, an extended backbone framework together with advantageous morphologies and suppressed structural disorder trigger more efficient heat diffusion properties. Higher thermal diffusivities enable better spread of phonons to relieve the thermal stress of organic semiconductor devices, leading to enhanced device thermal durability.

Abstract

Efficient heat transfer is beneficial to heat dissipation and the thermal durability of organic solar cell (OSCs). In this regard, heat transfer properties of organic semiconductors within OSCs should play important roles, but their thermal properties are rarely explored. Here, heat diffusion properties of Y-series non-fullerene acceptors processing different DA′D framework, named BZ4F-5, BZ4F-6, and BZ4F-7 are probed; it is found that backbone rings extension from five- to six- and seven-membered-fused rings trigger longer phonon mean free path and higher thermal diffusivities (D) in their pristine solid films and bulk heterojunction blends. Particularly, the correlation between the thermal transport properties in Y-series acceptors and their backbone geometry, molecule stacking, and thin-film crystallinity is demonstrated. More importantly, both organic thin-film transistors and OSCs confirm that thermal durability of organic semiconductor devices correlated with the thermal properties of their active layer. Although BZ5F-6 and BZ4F-7 based devices possess similar device performance at room temperature, superior heat dissipation in BZ4F-7 molecule endows it with enhanced device lifetime. These results contribute to critical design criteria for future molecular optimization in photovoltaic and optoelectronic devices.

The same coordination chemistry of Ag⁺ and Cu⁺ in dimethyl sulfoxide solution results in the successful fabrication of solid solution (Ag _x ,Cu_{1− x} )₂ZnSnS₄ (x = 0≈1). The novel Ag incorporation strategy significantly reduces band tailing, and a champion kesterite solar cell with an efficiency of 12.5% and a record low open circuit voltage (V _oc) loss (V _oc/V _oc ^SQ of 64.2%) is achieved with 5% Ag incorporation.

Abstract

The large open-circuit voltage deficit (V _oc,def) is the key issue that limits kesterite (Cu₂ZnSn(S,Se)₄, [CZTSSe]) solar cell performance. Substitution of Cu⁺ by larger ionic Ag⁺ ((Ag,Cu)₂ZnSn(S,Se)₄, [ACZTSSe]) is one strategy to reduce Cu–Zn disorder and improve kesterite V _oc. However, the so far reported ACZTSSe solar cell has not demonstrated lower V _oc,def than the world record device, indicating that some intrinsic defect properties cannot be mitigated using current approaches. Here, incorporation of Ag into kesterite through a dimethyl sulfoxide (DMSO) solution that can facilitate direct phase transformation grain growth and produce a uniform and less defective kesterite absorber is reported. The same coordination chemistry of Ag⁺ and Cu⁺ in the DMSO solution results in the same reaction path of ACZTSSe to CZTSSe, resulting in significant suppression of Cu_Zn defects, its defect cluster [2Cu_Zn + Sn_Zn], and deep level defect Cu_Sn. A champion device with an efficiency of 12.5% (active area efficiency 13.5% without antireflection coating) and a record low V _oc,def (64.2% Shockley–Queisser limit) is achieved from ACZTSSe with 5% Ag content.

This work presents a facile, low-cost, and upscalable process for depositing a mesoporous NiO _x (mp-NiO _x ) layer based on a polymer templating approach by spin-coating. Herein, this templated mp-NiO _x film is employed as a hole-transport layer in inverted perovskite solar cells with an outstanding efficiency larger than 20%. The beneficial effects of this mp-NiO _x layer, such as negligible hysteresis and reduced recombination losses are demonstrated.

Abstract

Despite the outstanding role of mesoscopic structures on the efficiency and stability of perovskite solar cells (PSCs) in the regular (n–i–p) architecture, mesoscopic PSCs in inverted (p–i–n) architecture have rarely been reported. Herein, an efficient and stable mesoscopic NiO _x (mp-NiO _x ) scaffold formed via a simple and low-cost triblock copolymer template-assisted strategy is employed, and this mp-NiO _x film is utilized as a hole transport layer (HTL) in PSCs, for the first time. Promisingly, this approach allows the fabrication of homogenous, crack-free, and robust 150 nm thick mp-NiO _x HTLs through a facile chemical approach. Such a high-quality templated mp-NiO _x structure promotes the growth of the perovskite film yielding better surface coverage and enlarged grains. These desired structural and morphological features effectively translate into improved charge extraction, accelerated charge transportation, and suppressed trap-assisted recombination. Ultimately, a considerable efficiency of 20.2% is achieved with negligible hysteresis which is among the highest efficiencies for mp-NiO _x based inverted PSCs so far. Moreover, mesoscopic devices indicate higher long-term stability under ambient conditions compared to planar devices. Overall, these results may set new benchmarks in terms of performance for mesoscopic inverted PSCs employing templated mp-NiO _x films as highly efficient, stable, and easy fabricated HTLs.

The fabrication of fully printed and cost-efficient perovskite solar cells in ambient air–as required for an industrial scalable process is reported. Through multi-objective optimization, fully printed carbon electrode perovskite solar cells and modules are obtained, providing a stable power conversion efficiency of 18.1% and 15.3%, respectively, which is the highest performance of fully printed perovskite devices reported so far.

Abstract

Scalable deposition processes at low temperature are urgently needed for the commercialization of perovskite solar cells (PSCs) as they can decrease the energy payback time of PSCs technology. In this work, a processing protocol is presented for highly efficient and stable planar n–i–p structure PSCs with carbon as the top electrode (carbon-PSCs) fully printed at fairly low temperature by using cheap materials under ambient conditions, thus meeting the requirements for scalable production on an industrial level. High-quality perovskite layers are achieved by using a combinatorial engineering concept, including solvent engineering, additive engineering, and processing engineering. The optimized carbon-PSCs with all layers including electron transport layer, perovskite, hole transport layer, and carbon electrode which are printed under ambient conditions show efficiencies exceeding 18% with enhanced stability, retaining 100% of their initial efficiency after 5000 h in a humid atmosphere. Finally, large-area perovskite modules are successfully obtained and outstanding performance is shown with an efficiency of 15.3% by optimizing the femtosecond laser parameters for the P2 line patterning. These results represent important progress toward fully printed planar carbon electrode perovskite devices as a promising approach for the scaling up and worldwide application of PSCs.

For a solar cell, the spatial distribution of minority carriers plays a key role in determining the recombination flux. By introducing a front surface gradient to push the minority carriers away from the defect-rich surface, a high open-circuit voltage (93% of the Shockley–Queisser limit) and power conversion efficiency (22.36%) are archieved for perovskite solar cells without defect passivation.

Abstract

The recombination flux in a solar cell is determined by not only recombination centers, but also the spatial distribution of minority carriers. For halide perovskite solar cells (PSCs), although there has been a tremendous amount of work focusing on defect passivation, the issue of carrier distribution is not as well studied as for other types of solar cells. Here in this work, with the incorporation of perovskite quantum dots, the concept of the front surface gradient in PSCs using a solution process is successfully realized. Evidenced by multiple characterization techniques, the minority carriers are pushed away from the defect-rich surface by the gradient of valence band maximum, which effectively reduces surface recombination without compromising photocurrent. As a result, the normal structured hybrid PSCs and MAPbI₃ cells exhibit open-circuit voltages exceeding 93% and 90% of their respective Shockley–Queisser limits, and the power conversion efficiencies reach 22.36% and 20.53%, respectively.

Lead halide perovskites are unstable in water due to the high water solubility of the A-site cations present in them. We introduce the intermolecular cation-π interactions between the A-site organic cations in 1D hybrid lead bromide perovskites. The cation-π interactions make these 1D perovskites completely stable in water.

Abstract

Optoelectronically active hybrid lead halide perovskites dissociate in water. To prevent this dissociation, here, we introduce long-range intermolecular cation-π interactions between A-site cations of hybrid perovskites. An aromatic diamine like 4,4′-trimethylenedipyridine, if protonated, can show a long-range cation-π stacking, and therefore, serves as our A-site cation. Consequently, 4,4′-trimethylenedipyridinium lead bromide [(4,4′-TMDP)Pb₂Br₆], a one-dimensional hybrid perovskite, remains completely stable after continuous water treatment for six months. Mechanistic insights about the cation-π interactions are obtained by single-crystal X-ray diffraction and nuclear magnetic resonance spectroscopy. The concept of long-range cation-π interaction is further extended to another A-site cation 4,4′-ethylenedipyridinium ion (4,4′-EDP), forming water-stable (4,4′-EDP)Pb₂Br₆ perovskite. These water-stable perovskites are then used to fabricate white light-emitting diode and for light up-conversion through tunable third-harmonic generation. Note that the achieved water stability is the intrinsic stability of perovskite composition, unlike the prior approach of encapsulating the unstable perovskite material (or device) by water-resistant materials. The introduced cation-π interactions can be a breakthrough strategy in designing many more compositions of water-stable low-dimensional hybrid perovskites.

A robust route simultaneously allows effective defect passivation and reduced energy difference between the valence band edge of the perovskite and the highest occupied molecular orbital of the hole transport layer (HTL) via the judicious placement of strongly polar molecules at the perovskite/HTL interface.

Abstract

The ability to passivate defects and modulate the interface energy-level alignment (IEA) is key to boost the performance of perovskite solar cells (PSCs). Herein, we report a robust route that simultaneously allows defect passivation and reduced energy difference between perovskite and hole transport layer (HTL) via the judicious placement of polar chlorine-terminated silane molecules at the interface. Density functional theory (DFT) points to effective passivation of the halide vacancies on perovskite surface by the silane chlorine atoms. An integrated experimental and DFT study demonstrates that the dipole layer formed by the silane molecules decreases the perovskite work function, imparting an Ohmic character to the perovskite/HTL contact. The corresponding PSCs manifest a nearly 20 % increase in power conversion efficiency over pristine devices and a markedly enhanced device stability. As such, the use of polar molecules to passivate defects and tailor the IEA in PSCs presents a promising platform to advance the performance of PSCs.

Nature Materials, Published online: 10 June 2021; doi:10.1038/s41563-021-01029-9

Publication date: 21 July 2021

Source: Joule, Volume 5, Issue 7

Author(s): Yao Wu, Jing Guo, Wei Wang, Zhihao Chen, Zeng Chen, Rui Sun, Qiang Wu, Tao Wang, Xiaotao Hao, Haiming Zhu, Jie Min

Nature Energy, Published online: 07 June 2021; doi:10.1038/s41560-021-00843-4

Current high-efficiency lead chalcogenide colloidal quantum dot solar cells still have high V _oc loss, which places great restrictions on the performance enhancement. The origin of V _oc loss from solar absorber and interface is discussed in detail and various strategies for reducing the loss are summarized. Moreover, promising research directions are provided to further improve the solar cell performance.

Abstract

Lead chalcogenide colloidal quantum dot solar cells (CQDSCs) have received considerable attention due to their broad and tunable absorption and high stability. Presently, lead chalcogenide CQDSC has achieved a power conversion efficiency of ≈14%. However, the state-of-the-art lead chalcogenide CQDSC still has an open-circuit voltage (V _oc) loss of ≈0.45 V, which is significantly higher than those of c-Si and perovskite solar cells. Such high V _oc loss severely limits the performance improvement and commercialization of lead chalcogenide CQDSCs. In this review, the V _oc loss is first analyzed via detailed balance theory and the origin of V _oc loss from both solar absorber and interface is summarized. Subsequently, various strategies for improving the V _oc from the solar absorber, including the passivation strategies during the synthesis and ligand exchange are overviewed. The great impact of the ligand exchange process on CQD passivation is highlighted and the corresponding strategies to further reduce the V _oc loss are summarized. Finally, various strategies are discussed to reduce interface V _oc loss from charge transport layers. More importantly, the great potential of achieving performance breakthroughs via various organic hole transport layers is highlighted and the existing challenges toward commercialization are discussed.

A universality strain-regulation approach—crosslinking-enabled strain-regulating crystallization (CSRC)—is introduced to eliminate intrinsic tensile strain in perovskite film, which significantly boosts the perovskite solar cells’ (PSCs) stability and performance. The CSRC approach precisely modulates the perovskite film strain through synchronous cooperation of perovskite crystallization manipulation and in situ chemical crosslinking process, as showcased with several types of crosslinking agents.

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Abstract

α-Formamidinium lead triiodide (α-FAPbI₃) represents the state-of-the-art for perovskite solar cells (PSCs) but experiences intrinsic thermally induced tensile strain due to a higher phase-converting temperature, which is a critical instability factor. An in situ crosslinking-enabled strain-regulating crystallization (CSRC) method with trimethylolpropane triacrylate (TMTA) is introduced to precisely regulate the top section of perovskite film where the largest lattice distortion occurs. In CSRC, crosslinking provides in situ perovskite thermal-expansion confinement and strain regulation during the annealing crystallization process, which is proven to be much more effective than the conventional strain-compensation (post-treatment) method. Moreover, CSRC with TMTA successfully achieves multifunctionality simultaneously: the regulation of tensile strain, perovskite defects passivation with an enhanced open-circuit voltage (V _OC = 50 mV), and enlarged perovskite grain size. The CSRC approach gives significantly enhanced power conversion efficiency (PCE) of 22.39% in α-FAPbI₃-based PSC versus 20.29% in the control case. More importantly, the control PSCs’ instability factor—residual tensile strain—is regulated into compression strain in the CSRC perovskite film through TMTA crosslinking, resulting in not only the best PCE but also outstanding device stability in both long-term storage (over 4000 h with 95% of initial PCE) and light soaking (1248 h with 80% of initial PCE) conditions.

Fabrication of phase-pure films of 2D perovskites using a novel, simple, and scalable method, referred to as the phase-selective method, is demonstrated. Phase-purity is enabled by the presence of sub-micrometer-sized seeds in the precursor-solution that preserves the memory of the dissolved single-crystals. A photovoltaic efficiency of 17.1% with a V _OC of 1.20 V and stability T _97.5 = 800 h at MPP is reported.

Abstract

2D perovskites are a class of halide perovskites offering a pathway for realizing efficient and durable optoelectronic devices. However, the broad chemical phase space and lack of understanding of film formation have led to quasi-2D perovskite films with polydispersity in perovskite layer thicknesses, which have hindered device performance and stability. Here, a simple and scalable approach is reported, termed as the “phase-selective method”, to fabricate 2D perovskite thin films with homogenous layer thickness (phase purity). The phase-selective method involves the dissolution of single-crystalline powders with a homogeneous perovskite layer thickness in desired solvents to fabricate thin films. In situ characterizations reveal the presence of sub-micrometer-sized seeds in solution that preserve the memory of the dissolved single crystals and dictate the nucleation and growth of grains with an identical thickness of the perovskite layers in thin films. Photovoltaic devices with a p–i–n architecture are fabricated with such films, which yield an efficiency of 17.1% enabled by an open-circuit voltage of 1.20 V, while preserving 97.5% of their peak performance after 800 h under illumination without any external thermal management.