Chen Weijie

Nature Communications, Published online: 03 April 2023; doi:10.1038/s41467-023-37486-w

Growth mechanism of thermally evaporated γ-CsPbI₃ is elucidated in aspect of perovskite formation and stabilization. The kinetic energies of evaporated molecules and the substrate thermo synergistically provide the formation energy. Also, the small grain size reduces the Gibbs free energy to make it stabilize. By adjusting the grain size, the optimal efficiency is 12.75% without any additives or high temperature.

Abstract

Cesium lead triiodide (CsPbI₃) inorganic perovskite possesses excellent thermal stability and matched bandgap for silicon-based tandem photovoltaics. The solution method with high-temperature annealing process for CsPbI₃ film preparation creates challenges to scalable application and conformal growth on the textured silicon. Although additives can decrease the annealing temperature, it will introduce undesired organic components and increase material cost. Thermal co-evaporation for CsPbI₃ has intrinsic advantages to overcome these issues, but the vague growth mechanism impedes the photovoltaic device development. In this study, γ-CsPbI₃ films are directly obtained through co-evaporation at 50 °C without any additives or high-temperature post-annealing. Focusing on the molecular thermodynamic calculations, it is proposed that the unique kinetic energy of evaporated molecules and the in-situ substrate thermal energy synergistically provide the energy prerequisite for γ-CsPbI₃ formation. Furthermore, the γ phase stabilization is clarified by the crystal grain size effect with regard to the Gibbs free energy difference between the γ and δ phases, which is adjusted through substrate temperature and evaporation rate. The obtained p-i-n device realizes an efficiency of 12.75%, which is the highest value for the thermally evaporated γ-CsPbI₃ photovoltaics at low temperature without additives. This study deepens the understanding of thermal evaporation process, benefiting to high-performance CsPbI₃-textured silicon tandem photovoltaics.

Publication date: July 2023

Source: Journal of Energy Chemistry, Volume 82

Author(s): Qiaoyan Ma, Jufeng Qiu, Yuzhao Yang, Fei Tang, Yilin Zeng, Nanxi Ma, Bohao Yu, Feiping Lu, Chong Liu, Andreas Lambertz, Weiyuan Duan, Kaining Ding, Yaohua Mai

The carrier transport layer plays a critical role in high-efficiency and stable perovskite solar cells (PSCs). Herein, CsPbBr₃ quantum dots-sensitized mesoporous TiO₂ (CPB–TiO₂) is developed, which enhances the perovskite crystallinity, phase stability, and reduces the nonradiative carrier recombination in PSCs. As a result, the CPB–TiO₂ significantly improves the power conversion efficiency of FAPbI₃ PSCs from 23.11% to 24.46%.

The carrier transport layer plays a critical role in high efficiency and stable perovskite solar cells (PSCs). Herein, CsPbBr₃ quantum dots-sensitized mesoporous TiO₂ (CPB–TiO₂) electron transport layer for fabricating high-quality perovskite films with enhanced phase stability is developed. The CPB–TiO₂ layer enhances the perovskite crystallinity, phase stability, and reduces the nonradiative carrier recombination in PSCs. As a result, the CPB–TiO₂ significantly improves the power conversion efficiency of FAPbI₃ PSCs from 23.11% to 24.46% and reduces the device hysteresis. The CPB–TiO₂–FAPbI₃ device shows enhanced stability, retaining 90% of its initial efficiency after 500 h operation at maximum power point tracking under 1 sun illumination, whereas the control device degrades to 80% of initial performance in the first 200 h. In addition, this strategy is applicable to CsPbI₃ perovskite, which provides a new and general strategy for preparing high-efficiency PSCs.

Rubidium iodide as an additive improves properties of methylammonium-free tin-lead perovskite films, including larger grain size. It further reduces nonradiative recombination losses, increases carrier lifetime, reduces defect densities, and suppresses Sn (IV) vacancy formation. Therefore, the open-circuit voltage of RbI-containing tin-lead perovskite solar cells increases by 61 mV on average with best power conversion efficiencies of up to 20.12%.

Abstract

Outstanding optoelectronic properties of mixed tin-lead perovskites are the cornerstone for the development of high-efficiency all-perovskite tandems. However, recombination losses in Sn-Pb perovskites still limit the performance of these perovskites, necessitating more fundamental research. Here, rubidium iodide is employed as an additive for methylammonium-free Sn-Pb perovskites. It is first investigated the effect of the RbI additive on the perovskite composition, crystal structure, and element distribution. Quasi-Fermi level splitting and transient photoluminescence measurements reveal that the RbI additive reduces recombination losses and increases carrier lifetime of the perovskite films. This finding is attributed to an approximately ten-fold reduction in the defect density following RbI treatment, as probed using constant final state yield photoelectron spectroscopy. Additionally, the concentration of Sn vacancies is also reduced, and the perovskite film becomes less p-type both in the bulk and at the interface towards the electron contact. Thus, the conductivity for electrons increases, improving carrier extraction. As a result, the open-circuit voltage of RbI-containing solar cells improves by 61 mV on average, with the best efficiency >20%. This comprehensive study demonstrates that RbI is effective at reducing recombination losses and carrier trapping, paving way for a new approach to Sn-Pb perovskite solar cell research.

An additive-assisted two-step annealing process was developed for high-performance CsSnI₃ perovskite solar cells (PSCs). The carbazide (CBZ) can slow down crystallization for dense coverage at 80 °C. After further annealing at 150 °C, the uncoordinated CBZ reduces Sn⁴⁺ to Sn²⁺ with few deep traps. The efficiency of CsSnI₃:CBZ is 11.21%, which is the highest performance for CsSnI₃-based PSCs to date.

Abstract

Inorganic CsSnI₃ with low toxicity and a narrow bandgap is a promising photovoltaic material. However, the performance of CsSnI₃ perovskite solar cells (PSCs) is much lower than that of Pb-based and hybrid Sn-based (e.g., CsPbX₃ and CH(NH₂)₂SnX₃) PSCs, which may be attributed to its poor film-forming property and the deep traps induced by Sn⁴⁺. Here, a bifunctional additive carbazide (CBZ) is adapted to deposit a pinhole-free film and remove the deep traps via two-step annealing. The lone electrons of the NH₂ and CO units in CBZ can coordinate with Sn²⁺ to form a dense film with large grains during the phase transition at 80 °C. The decomposition of CBZ can reduce Sn⁴⁺ to Sn²⁺ during annealing at 150 °C to remove the deep traps. Compared with the control device (4.12%), the maximum efficiency of the CsSnI₃:CBZ PSC reaches 11.21%, which is the highest efficiency of CsSnI₃ PSC reported to date. A certified efficiency of 10.90% is obtained by an independent photovoltaic testing laboratory. In addition, the unsealed CsSnI₃:CBZ devices maintain initial efficiencies of ≈100%, 90%, and 80% under an inert atmosphere (60 days), standard maximum power point tracking (650 h at 65 °C), and ambient air (100 h), respectively.

Diazabicyclic electroactive ionenes with high density of interfacial dipoles are developed as interlayers in organic solar cells. Devices containing the optimal electroactive ionene interlayer give efficiency up to 18.43 % and excellent operational stability with exceptional interlayer thickness tolerance over 100 nm.

Abstract

Electroactive ionenes combining caged-shaped diazabicyclic cations and aromatic diimides were developed as interlayers in organic solar cells (OSCs). These ionenes reduce the work-function of air-stable metal electrodes (e.g., Ag, Cu and Au) by generating strong interfacial dipoles, and their optoelectronic and morphological characters can be modulated by aromatic diimides, leading to high conductivity and good compatibility with active layers. The optimal ionene exhibits superior charge-transport, desirable crystallinity, and weak visible-absorption, boosting the efficiency of benchmark PM6 : Y6-based OSCs up to 17.44 %. The corresponding normal devices show excellent stability at maximum power point test under one sun illumination for 1000 h. Replacing Y6 with L8-BO promotes the efficiency to 18.43 %, one of the highest in binary OSCs. Notably, high efficiencies >16 % are maintained as the interlayer thickness increasing to 105 nm, the best result with interlayer-thickness over 100 nm.

A facile and effective co-solvent engineering strategy is applied to obtain high quality perovskite film and achieve optimal PCE over 20% and 16% for devices with small area and 5 × 5 cm module, respectively.

Abstract

The pinhole-free and defect-less perovskite film is crucial for achieving high efficiency and stable perovskite solar cells (PSCs), which can be prepared by widely used anti-solvent crystallization strategies. However, the involvement of anti-solvent requires precise control and inevitably brings toxicity in fabrication procedures, which limits its large-scale industrial application. In this work, a facile and effective co-solvent engineering strategy is introduced to obtain high- quality perovskite film while avoiding the usage of anti-solvent. The uniform and compact perovskite polycrystalline films have been fabricated through the addition of co-solvent that owns strong coordination capacity with perovskite components , meanwhile possessing the weaker interaction with main solvent . With those strategies, a champion power conversion efficiency (PCE) of 22% has been achieved with the optimal co-solvent, N-methylpyrrolidone (NMP) and without usage of anti-solvent. Subsequently, PSCs based on NMP show high repeatability and good shelf stability (80% PCE remains after storing in ambient condition for 30 days). Finally, the perovskite solar module (5 × 5 cm) with 7 subcells connects in series yielding champion PCE of 16.54%. This strategy provides a general guidance of co-solvent selection for PSCs based on anti-solvent free technology and promotes commercial application of PSCs.

A highly efficient and stable ternary system by introducing a new asymmetric electron acceptor SSe-NIC into the MPhS-C2:BTP-eC9 host system is developed, which exhibits a record-high PCE of 18.02% and demonstrates better operational stability as compared to the host system.

Abstract

Using a combinatory blending strategy is demonstrated as a promising path for designing efficient organic solar cells (OSCs) by boosting the short-circuit current density and fill factor. Herein, a high-performance ternary all-small molecule OSC (all-SMOSCs) using a narrow-bandgap alloy acceptor containing symmetric and asymmetric molecules (BTP-eC9 and SSe-NIC) and a wide-bandgap small molecule donor MPhS-C2 is reported. Introducing the synthesized SSe-NIC into the MPhS-C2:BTP-eC9 host system can broaden the absorption spectrum, modulate energy offsets, and optimize the molecular packing of the host materials. After systematically optimizing the weight ratio of MPhS-C2:BTP-eC9:SSe-NIC, a champion efficiency of 18.02% is achieved. Impressively, the ternary system not only delivered a broad composition tolerance with device efficiencies over 17% throughout the whole blend ratios, but also exhibited less non-geminate recombination and energy loss, and better-light-soaking stability than the corresponding binary systems. This work promotes the development of high-performance ternary all-SMOSCs and heralds their brighter application prospects.

The newly developed cathode interlayer, rhodamine-functionalized dodecahydro-closo-dodecaborate derivates, enhanced the performance and stability of inverted perovskite solar cells (PSCs), by passivating interface traps and improving interfacial properties, achieving a power conversion efficiency of 21.08%, a fill factor of 83.37% and good stability against moisture erosion and self-corrosion. This new interlayer holds promise for preparation of high-performance inverted PSCs.

Abstract

Efficient modification of the interface between metal cathode and electron transport layer are critical for achieving high performance and stability of the inverted perovskite solar cells (PSCs). Herein, a new alcohol-soluble rhodamine-functionalized dodecahydro-closo-dodecaborate derivate, RBH, is developed and applied as an efficient cathode interlayer to overcome the (6,6)-phenyl-C₆₁ butyrie acid methyl ester (PCBM)/Ag interface issues. By introducing RBH cathode interlayer, the functions of the interface traps passivation, interfacial hydrophobicity enhancement, interface contact improvement as well as built-in potential enhancement are realized at the same time and thus correspondingly improve the device performance and stability. Consequently, a power conversion efficiency (PCE) of 21.08% and high fill factor of 83.37% are achieved, which is one of the highest values based on solution-processed MAPbI₃/PCBM heterojunction PSCs. Moreover, RBH can act as a shielding layer to slow down moisture erosion and self-corrosion. The PCE of the RBH devices still maintain 84% for 456 h (85 °C @ N₂), 87% for 360 h (23 °C @ relative humidity (RH) 35%) of its initial PCE value, while the control device can only maintain ≈23%, 58% of its initial PCE value under the same exposure conditions, respectively.

Nature, Published online: 29 March 2023; doi:10.1038/s41586-023-05992-y

Nature, Published online: 28 March 2023; doi:10.1038/s41586-023-06006-7

A novel ionic liquid of dimethylammonium bis(trifluoromethanesulfonyl)imide (DMATFSI) is designed and synthesized for stabilizing the 2,2′,7,7′-tetrakis (N, N-di-p-methoxyphenyl-amine) 9,9′-spirobifluorene (spiro-OMeTAD), as well as passivating the perovskite surface, where the synergistic ability of DMATFSI added into spiro-OMeTAD further simplifies the fabrication of solar cells. Finally, the corresponding device with DMATFSI-based spiro-OMeTAD shows over 23% efficiency, as well as high operational stability.

Abstract

Perovskite solar cells (PSCs) with n-i-p structures often utilize an organic 2,2′,7,7′-tetrakis (N, N-di-p-methoxyphenyl-amine) 9,9′-spirobifluorene (spiro-OMeTAD) along with additives of lithium bis(trifluoromethanesulfonyl)imide salt (LiTFSI) and tert-butylpyridine as the hole transporting layer (HTL). However, the HTL lacks stability in ambient air, and numerous defects are often present on the perovskite surface, which is not conducive to a stable and efficient PSC. Therefore, constructive strategies that simultaneously stabilize spiro-OMeTAD and passivate the perovskite surface are required. In this work, it is demonstrated that a novel ionic liquid of dimethylammonium bis(trifluoromethanesulfonyl)imide (DMATFSI) could act as a bifunctional HTL modulator in n-i-p PSCs. The addition of DMATFSI into spiro-OMeTAD can effectively stabilize the oxidized spiro-OMeTAD⁺ cation radicals through the formation of spiro-OMeTAD⁺TFSI⁻ because of the excellent charge delocalization of the conjugated CF₃SO₂ ⁻ moiety within TFSI⁻. In addition, DMA⁺ cations could move toward the perovskite from the HTL, resulting in the passivation of defects at the perovskite surface. Accordingly, a power conversion efficiency of 23.22% is achieved for PSCs with DMATFSI and LiTFSI co-doped spiro-OMeTAD. Moreover, benefiting from the improved ion migration barrier and hydrophobicity of the HTL, still retained nearly 80% of their initial power conversion efficiency after 36 days of exposure to ambient air.

There is a great challenge in improving exciton utilization efficiency on top of acceptor layer in layer-by-layer all-polymer solar cells (LbL all-PSCs) due to the limited exciton diffuse distance and impossible energy transfer from acceptor to donor. The exciton utilization efficiency in PY-IT layers near electrode can be improved by incorporating appropriate PM6 in PY-IT layer, resulting in the enhanced power conversion efficiencies from 16.04% to 17.45% in LbL all-PSCs with 10 wt% PM6 in PY-IT layer.

Abstract

Layer-by-layer all-polymer solar cells (LbL all-PSCs) are prepared with PM6 and PY-IT by using sequential spin coating method. The exciton dissociation efficiency in acceptor layer near electrode is rather low due to the limited exciton diffuse distance and impossible energy transfer from narrow bandgap acceptor to wide bandgap donor. In this study, less PM6 is incorporated into PY-IT layer to enhance exciton dissociation in PY-IT layer near electrode. A power conversion efficiency (PCE) of 17.45% is achieved in the LbL all-PSCs incorporating 10 wt% PM6 into PY-IT layer, which is much larger than 16.04% PCE of PM6/PY-IT-based LbL all-PSCs. Over 8% PCE enhancement can be realized by incorporating 10 wt% PM6 into PY-IT layer, which is attributed to the enhanced exciton utilization efficiency in PY-IT layers near electrode. The enhanced exciton utilization efficiency in PY-IT layer can be confirmed from the quenched photoluminescence (PL) emission in PY-IT:PM6 films. Meanwhile, charge transport in acceptor layers can be optimized by incorporating less PM6, as confirmed from the optimized molecular arrangement. This study indicates that the strategy of incorporating less donor into acceptor layer has great potential in fabricating efficient LbL all-PSCs by improving exciton utilization efficiency in acceptor layer near electrode.

A brand-new passivator, Boc-S-4-methoxy-benzyl-l-cysteine (BMBC), is demonstrated to post-treat CsPbI_3−xBr_x perovskite films. The synergistic effects of an energetic electron donor group and multiple Lewis-based functional groups in BMBC are purposefully designed to passivate majority of defects. Benefiting from a hydrophobic tert-butyl group in BMBC, the protection against water invasion is thus significantly enhanced. This study provides a useful understanding and rational design regarding the influence of the molecular configuration on the passivation efficacy in inorganic perovskite-type photovoltaics.

Abstract

Surface–defect-triggered non-radiative charge recombination and poor stability have become the main roadblock to continued improvement in inorganic perovskite solar cells (PSCs). Herein, the main culprits are identified on the inorganic perovskite surface by first-principles calculations, and to purposefully design a brand-new passivator, Boc-S-4-methoxy-benzyl-l-cysteine (BMBC), whose multiple Lewis-based functional groups (NH, S and CO) to suppress halide vacancies and coordinate with undercoordinated Pb²⁺ through typical Lewis baseacid reactions. The tailored electron-donating methoxyl group (CH₃O–) can cause an increased electron density on the benzene ring, which strengthens the interaction with undercoordinated Pb²⁺ via electrostatic interactions. This BMBC passivation can reduce the surface trap density, enlarge grains, prolong the charge lifetime, and cause a more suitable energy-level alignment. In addition, the hydrophobic tert-butyl in butoxycarbonyl (Boc-) group ensures that BMBC is uniformly covered and prevents harmful aggregation through steric repulsion at the perovskite/hole–transporting layer (HTL) interface, thus providing a hydrophobic umbrella to resist moisture invasion. Consequently, the combination of the above increases the efficiency of CsPbI_3−xBr_x PSC from 18.6% to 21.8%, the highest efficiency for this type of inorganic metal halide PSCs so far, as far as it is known. Moreover, the device exhibits higher environmental and thermal stability.

A new approach was proposed to design and synthesize efficient carbonyl additives to improve perovskite device performance. After systematic research, we find that molecular dipole plays an important role in determining the passivation effect of the additive molecule. After optimization, the companion efficiency of the perovskite device is 23.20 % (0.09 cm²), 20.18 % (14 cm²) and it can maintain long-term stability under harsh conditions.

Abstract

Carbonyl functional materials as additives are extensively applied to reduce the defects density of the perovskite film. However, there is still a lack of comprehensive understanding for the effect of carbonyl additives to improve device performance. In this work, we systematically study the effect of carbonyl additive molecules on the passivation of defects in perovskite films. After a comprehensive investigation, the results confirm the importance of molecular dipole in amplifying the passivation effect of additive molecules. The additive with strong molecular dipole possesses the advantages of enhancing the efficiency and stability of perovskite solar cells (PSCs). After optimization, the companion efficiency of PSCs is 23.20 %, and it can maintain long-term stability under harsh conditions. Additionally, a large-area solar cell module-modified DLBA was 20.18 % (14 cm²). This work provides an important reference for the selection and designing of efficient carbonyl additives.

The polymer donor D18 and the acceptor L8-BO are selected to fabricate highly efficient layer-by-layer organic solar cells. By simply controlling the solution temperature, pre-aggregation state of polymer donor in solution and therefore, its solid-state film microstructure morphology are effectively fine-tuned. Finally, a champion power conversion efficiency of 18.02% can be obtained with simultaneously improved short-circuit current density, open-circuit voltage, and fill factor.

The fabrication of organic solar cells (OSCs) by a layer-by-layer (LBL) method has attracted growing attention in recent years. As already known, the pre-aggregates of conjugated polymers in solution have a profound impact on their microstructure morphology in films. Herein, by simply controlling the solution temperature and annealing processes, the pre-aggregation behavior of D18 polymer in solution can be fine-tuned and the microstructure of D18 bottom layer is well manipulated. The optimized D18 bottom layer can effectively regulate L8-BO upper-layer-forming suitable networks for efficient charge transportation. In addition, a vertical phase separation with a special D/D:A/A structure (P-i-N-type component distribution) is also formed. As a result, compared to the 16.43% power conversion efficiency (PCE) of the bulk heterojunction devices, such control enables bilayer OSC devices based on the polymer D18 and L8-BO to deliver an enhanced PCE of 18.02% with simultaneously improved short-circuit current density, open-circuit voltage, and fill factor. It is also demonstrated in these results that the LBL deposition process utilizing the pre-aggregation of polymer and its fiber-network-forming ability is a very promising approach to improve charge dynamics, suppress carrier recombination, and fabricate highly efficient OSCs.