Chen Weijie

A new nonhalide lead source, lead formate (Pb(HCOO)₂), is used to prepare perovskite solar cells. Using Pb(HCOO)₂ forms PbI₂ domain "walls" bordering perovskite grains, leading to an ultralong carrier lifetime (≈1.7 microseconds) and a record‐high power conversion efficiency (>20%) for MAPbI₃ based devices produced from nonhalide lead sources.

Abstract

Usage of nonhalide lead sources for fabricating perovskite solar cells (PSCs) has recently attracted increasing attention as a promising route toward realizing high quality PSC devices. However, the unique role of nonhalide lead sources in improving perovskite film morphology and PSC performance has largely remained unexplored, impeding broader application of these materials. Here, it is demonstrated that by using a new nonhalide lead source, lead formate (Pb(HCOO)₂), good control of perovskite film morphology can be achieved. With the usage of lead formate, PbI₂ can nicely border the perovskite grain boundaries (GBs) and form domain “walls” that segregate the individual perovskite crystal domains. The PbI₂ at the GBs lead to significant improvement in film quality and device performance through passivating the defects at the perovskite GBs and suppressing lateral carrier diffusion. An impressive carrier lifetime at the microsecond scale (τ ₂ = 1714 ns) is achieved, further with an optimal power conversion efficiency of 20.3% for the resulting devices. This work demonstrates a promising and effective method toward fabricating high‐quality perovskites and high‐efficiency PSCs.

Lead‐free perovskite‐inspired materials (PIMs) provide a particularly attractive route to low‐toxicity indoor photovoltaics (IPV). Two exemplar PIMs, bismuth oxyiodide (BiOI) and Cs₃Sb₂Cl _x I_{9‐ x} , deliver an IPV efficiency of 4–5%, and can power thin‐film‐transistor electronics. Loss analyses and calculations of the optically limited efficiency reveal that further efficiency increases are possible, encouraging future efforts for the exploration of PIMs for powering Internet of Things (IoT) devices.

Abstract

With the exponential rise in the market value and number of devices part of the Internet of Things (IoT), the demand for indoor photovoltaics (IPV) to power autonomous devices is predicted to rapidly increase. Lead‐free perovskite‐inspired materials (PIMs) have recently attracted significant attention in photovoltaics research, due to the similarity of their electronic structure to high‐performance lead‐halide perovskites, but without the same toxicity limitations. However, the capability of PIMs for indoor light harvesting has not yet been considered. Herein, two exemplar PIMs, BiOI and Cs₃Sb₂Cl _x I_{9‐ x} are examined. It is shown that while their bandgaps are too wide for single‐junction solar cells, they are close to the optimum for indoor light harvesting. As a result, while BiOI and Cs₃Sb₂Cl _x I_{9‐ x} devices are only circa 1%‐efficient under 1‐sun illumination, their efficiencies increase to 4–5% under indoor illumination. These efficiencies are within the range of reported values for hydrogenated amorphous silicon, i.e., the industry standard for IPV. It is demonstrated that such performance levels are already sufficient for millimeter‐scale PIM devices to power thin‐film‐transistor circuits. Intensity‐dependent and optical loss analyses show that future improvements in efficiency are possible. Furthermore, calculations of the optically limited efficiency of these and other low‐toxicity PIMs reveal their considerable potential for IPV, thus encouraging future efforts for their exploration for powering IoT devices.

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

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

Current density–voltage (J–V) hysteresis in perovskite solar cells (PSCs) is a major challenge in this field. Herein, the possible origins and factors of J–V hysteresis behavior in PSCs are focused and the strategies to suppress the hysteresis are summarized. Finally, insights on the future development of the J–V hysteresis in PSCs are also provided.

The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has exceeded 25%, showing great potential in the photovoltaic field. However, PSCs often show anomalous current density–voltage (J–V) hysteresis behavior in the forward and reverse scanning directions, which makes it impossible to accurately evaluate the performance of PSCs. Therefore, it is necessary to clearly understand the mechanism of hysteresis and suppress the hysteresis. Herein, the J–V hysteresis behavior in PSCs and strategies to suppress hysteresis is focused: first, the various factors that affect J–V hysteresis in PSCs are summarized. And the mechanism behind the various possible origins of hysteresis and the challenges encountered are explored. Then, the strategies to suppress or eliminate the hysteresis are summarized, including optimizing the perovskite light‐absorbing layer, improving the performance of the carrier transport layer and interface engineering. Finally, insights on the future development of the hysteresis are also provided.

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

Abstract

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

As crystallized powders, hybrid perovskites (HP) show highly sensitive mechanochromic luminescence properties. Composites consisting of monolayered 2D and 3D HP components exhibit reversible tunable color emission upon mechanical strain: bright green emission originates from the 3D HP after efficient energy funneling from the multi‐layered 2D HP produced at the 2D/3D interface by the mechanical treatment.

Abstract

Hybrid perovskite (HP) materials are of interest in photovoltaics and lighting applications. Here we report that hybrid perovskite composites, as crystallized powders, can behave as intelligent materials showing highly sensitive and reversible mechanochromic luminescence (MCL). Composites consisting of monolayered 2D HP and 3D HP components exhibit reversible tunable color emission upon mechanical strain. The bluish‐whitish emission of the 2D HP turns into orange in the composite owing to an energy transfer process. The bright green emission, observed as soon as the composite is slightly crushed, originates from the 3D HP after efficient energy funneling from the multi‐layered 2D HP produced at the 2D/3D interface by the mechanical treatment. Besides highlighting the key role of the interfaces in light emission of HP, our findings pave the way for hybrid perovskites as highly sensitive MCL smart materials for mechanosensors, security papers, or optical storage applications.

An ionic liquid, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF₆), was used to passivate a perovskite to decrease the defects of Pb‐cluster and Pb‐I antisite, thereby reducing the energy barrier between the perovskite and hole transport layer. A perovskite solar cell attained a 23.25 % efficiency with a high stability due to hydrophobic DMIMPF₆.

Abstract

Surface defects have been a key constraint for perovskite photovoltaics. Herein, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF₆) ionic liquid (IL) is adopted to passivate the surface of a formamidinium‐cesium lead iodide perovskite (Cs_0.08FA_0.92PbI₃) and also reduce the energy barrier between the perovskite and hole transport layer. Theoretical simulations and experimental results demonstrate that Pb‐cluster and Pb‐I antisite defects can be effectively passivated by [DMIM]⁺ bonding with the Pb²⁺ ion on the perovskite surface, leading to significantly suppressed non‐radiative recombination. As a result, the solar cell efficiency was increased to 23.25 % from 21.09 %. Meanwhile, the DMIMPF₆‐treated perovskite device demonstrated long‐term stability because the hydrophobic DMIMPF₆ layer blocked moisture permeation.

An ionic liquid, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF₆), was used to passivate a perovskite to decrease the defects of Pb‐cluster and Pb‐I antisite, thereby reducing the energy barrier between the perovskite and hole transport layer. A perovskite solar cell attained a 23.25 % efficiency with a high stability due to hydrophobic DMIMPF₆.

Abstract

Surface defects have been a key constraint for perovskite photovoltaics. Herein, 1,3‐dimethyl‐3‐imidazolium hexafluorophosphate (DMIMPF₆) ionic liquid (IL) is adopted to passivate the surface of a formamidinium‐cesium lead iodide perovskite (Cs_0.08FA_0.92PbI₃) and also reduce the energy barrier between the perovskite and hole transport layer. Theoretical simulations and experimental results demonstrate that Pb‐cluster and Pb‐I antisite defects can be effectively passivated by [DMIM]⁺ bonding with the Pb²⁺ ion on the perovskite surface, leading to significantly suppressed non‐radiative recombination. As a result, the solar cell efficiency was increased to 23.25 % from 21.09 %. Meanwhile, the DMIMPF₆‐treated perovskite device demonstrated long‐term stability because the hydrophobic DMIMPF₆ layer blocked moisture permeation.

Publication date: February 2021

Source: Nano Energy, Volume 80

Author(s): Priyanshu Goel, Shashank Sundriyal, Vishal Shrivastav, Sunita Mishra, Deepak P. Dubal, Ki-Hyun Kim, Akash Deep

Aided by theoretical calculation, a multifunctional 2,2‐difluoropropanediamide (DFPDA) molecule that bears carbonyl, amino, and fluorine groups is first introduced into the perovskite precursor, serving as a crystal growth mitigator, grain boundaries passivator, and surface protection material. With the help of the combined effects of multifunctional groups in DFPDA, the perovskite cells deliver an efficiency of 22.21% and improved stability.

Abstract

With a certified efficiency as high as 25.2%, perovskite has taken the crown as the highest efficiency thin film solar cell material. Unfortunately, serious instability issues must be resolved before perovskite solar cells (PSCs) are commercialized. Aided by theoretical calculation, an appropriate multifunctional molecule, 2,2‐difluoropropanediamide (DFPDA), is selected to ameliorate all the instability issues. Specifically, the carbonyl groups in DFPDA form chemical bonds with Pb²⁺ and passivate under‐coordinated Pb²⁺ defects. Consequently, the perovskite crystallization rate is reduced and high‐quality films are produced with fewer defects. The amino groups not only bind with iodide to suppress ion migration but also increase the electron density on the carbonyl groups to further enhance their passivation effect. Furthermore, the fluorine groups in DFPDA form both an effective barrier on the perovskite to improve its moisture stability and a bridge between the perovskite and HTL for effective charge transport. In addition, they show an effective doping effect in the HTL to improve its carrier mobility. With the help of the combined effects of these groups in DFPDA, the PSCs with DFPDA additive achieve a champion efficiency of 22.21% and a substantially improved stability against moisture, heat, and light.

Spintronics holds great potential for next-generation high-speed and low–power consumption information technology. Recently, lead halide perovskites (LHPs), which have gained great success in optoelectronics, also show interesting magnetic properties. However, the spin-related properties in LHPs originate from the spin-orbit coupling of Pb, limiting further development of these materials in spintronics. Here, we demonstrate a new generation of halide perovskites, by alloying magnetic elements into optoelectronic double perovskites, which provide rich chemical and structural diversities to host different magnetic elements. In our iron-alloyed double perovskite, Cs₂Ag(Bi:Fe)Br₆, Fe³⁺ replaces Bi³⁺ and forms FeBr₆ clusters that homogenously distribute throughout the double perovskite crystals. We observe a strong temperature-dependent magnetic response at temperatures below 30 K, which is tentatively attributed to a weak ferromagnetic or antiferromagnetic response from localized regions. We anticipate that this work will stimulate future efforts in exploring this simple yet efficient approach to develop new spintronic materials based on lead-free double perovskites.

An electrospray printing technique is developed to continuously print the TiO₂ electron transport layer, perovskite layer, and carbon layer, enabling a cost‐effective device. The electrospray technique is capable of printing uniform, compact, and high adhesion layers with controllable dimensions and patterns. This work demonstrates a fully printed low‐cost solar cell and provides a feasible process for perovskite solar cells to initial industrialization.

Abstract

With the power conversion efficiencies of perovskite solar cells (PSCs) exceeding 25%, the PSCs are a step closer to initial industrialization. Prior to transferring from laboratory fabrication to industrial manufacturing, issues such as scalability, material cost, and production line compatibility that significantly impact the manufacturing remain to be addressed. Here, breakthroughs on all these fronts are reported. Carbon‐based PSCs with architecture fluorine doped tin oxide (FTO)/electron transport layer/perovskite/carbon, that eliminate the need for the hole transport layer and noble metal electrode, provide ultralow‐cost configuration. This PSC architecture is manufactured using a scalable and industrially compatible electrospray (ES) technique, which enables continuous printing of all the cell layers. The ES deposited electron transport layer and perovskite layer exhibit properties comparable to that of the laboratory‐scale spin coating method. The ES deposited carbon electrode layer exhibits superior conductivity and interfacial microstructure in comparison to films synthesized using the conventional doctor blading technique. As a result, the fully ES printed carbon‐based PSCs show a record 14.41% power conversion efficiency, rivaling the state‐of‐the‐art hole transporter‐free PSCs. These results will immediately have an impact on the scalable production of PSCs.

Lead‐free perovskite‐inspired materials (PIMs) provide a particularly attractive route to low‐toxicity indoor photovoltaics (IPV). Two exemplar PIMs, bismuth oxyiodide (BiOI) and Cs₃Sb₂Cl _x I_{9‐ x} , deliver an IPV efficiency of 4–5%, and can power thin‐film‐transistor electronics. Loss analyses and calculations of the optically limited efficiency reveal that further efficiency increases are possible, encouraging future efforts for the exploration of PIMs for powering Internet of Things (IoT) devices.

Abstract

With the exponential rise in the market value and number of devices part of the Internet of Things (IoT), the demand for indoor photovoltaics (IPV) to power autonomous devices is predicted to rapidly increase. Lead‐free perovskite‐inspired materials (PIMs) have recently attracted significant attention in photovoltaics research, due to the similarity of their electronic structure to high‐performance lead‐halide perovskites, but without the same toxicity limitations. However, the capability of PIMs for indoor light harvesting has not yet been considered. Herein, two exemplar PIMs, BiOI and Cs₃Sb₂Cl _x I_{9‐ x} are examined. It is shown that while their bandgaps are too wide for single‐junction solar cells, they are close to the optimum for indoor light harvesting. As a result, while BiOI and Cs₃Sb₂Cl _x I_{9‐ x} devices are only circa 1%‐efficient under 1‐sun illumination, their efficiencies increase to 4–5% under indoor illumination. These efficiencies are within the range of reported values for hydrogenated amorphous silicon, i.e., the industry standard for IPV. It is demonstrated that such performance levels are already sufficient for millimeter‐scale PIM devices to power thin‐film‐transistor circuits. Intensity‐dependent and optical loss analyses show that future improvements in efficiency are possible. Furthermore, calculations of the optically limited efficiency of these and other low‐toxicity PIMs reveal their considerable potential for IPV, thus encouraging future efforts for their exploration for powering IoT devices.

Through investigation of the underlying thermodynamic and kinetic aspects of non‐fullerene acceptor crystallization, the importance of diffusion coefficients and melting enthalpies in controlling the crystal growth rates is demonstrated, and it is revealed and that differences in halogenation can drastically change crystallization kinetics and device stability.

Abstract

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non‐fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room‐temperature (RT) diffusion coefficients of 3.4 × 10⁻²³ and 2.0 × 10⁻²² are measured for ITIC‐2Cl and ITIC‐2F, two state‐of‐the‐art non‐fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC‐2F and ITIC‐2Cl. The differences as observed by secondary‐ion mass spectrometry, differential scanning calorimetry (DSC), grazing‐incidence wide‐angle X‐ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC‐4F and ITIC‐4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.

Publication date: January 2021

Source: Nano Energy, Volume 79

Author(s): Ivan S. Zhidkov, Danil W. Boukhvalov, Azat F. Akbulatov, Lyubov A. Frolova, Larisa D. Finkelstein, Andrey I. Kukharenko, Seif O. Cholakh, Chu-Chen Chueh, Pavel A. Troshin, Ernst Z. Kurmaev

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

Abstract

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

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

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

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

Abstract

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