Chen Weijie

Here, monolithic perovskite/PbS quantum dots tandem solar cells are developed using interface engineering, light management techniques, and a device with a stabilized efficiency of 17.1%, and excellent stability is achieved.

Abstract

Here, highly efficient and stable monolithic (2-terminal (2T)) perovskite/PbS quantum dots (QDs) tandem solar cells are reported, where the perovskite solar cell (PSC) acts as the front cell and the PbS QDs device with a narrow bandgap acts as the back cell. Specifically, ZnO nanowires (NWs) passivated by SnO₂ are employed as an electron transporting layer for PSC front cell, leading to a single cell PSC with maximum power conversion efficiency (PCE) of 22.15%, which is the most efficient NWs-based PSCs in the literature. By surface passivation of PbS QDs by CdCl₂, QD devices with an improved open-circuit voltage and a PCE of 8.46% (bandgap of QDs: 0.92 eV) are achieved. After proper optimization, 2T and 4T tandem devices with stabilized PCEs of 17.1% and 21.1% are achieved, respectively, where the 2T tandem device shows the highest efficiency reported in the literature for this design. Interestingly, the 2T tandem cell shows excellent operational stability over 500 h under continuous illumination with only 6% PCE loss. More importantly, this device without any packaging depicts impressive ambient stability (almost no change) after 70 days in an environment with controlled 65% relative humidity, thanks to the superior air stability of the PbS QDs.

A high-performance piezoelectric nanogenerator (PENG) based on the composite fibers combined polyvinylidene difluoride (PVDF) with CsPbBr₃ nanocrystals is constructed by in situ growth method. The PENG reveals a density of short-circuit current (I _sc) of 170 µA cm⁻², which is 4.86 times higher than that of lead halide perovskites counterpart ever reported. Moreover, the PENG exhibits fundamentally improved thermal/water/acid-base stabilities.

Abstract

Inorganic lead halide perovskite has become an emerging material for modern photoelectric and electronic nanodevices due to its excellent optical and electronic properties. In view of its huge dielectric and electrical properties, inorganic CsPbBr₃ perovskite is introduced into the piezoelectric nanogenerator (PENG). Based on one-step electrospinning of solutions containing CsPbBr₃ precursors and polyvinylidene difluoride (PVDF), in situ growth of CsPbBr₃ nanocrystals in PVDF fibers (CsPbBr₃@PVDF composite fibers) with highly uniform size and spatial distribution are synthesized. The CsPbBr₃@PVDF composite fibers based PENG reveals an open-circuit voltage (V _oc) of 103 V and a density of short-circuit current (I _sc) of 170 µA cm⁻², where the V _oc is comparable to the state-of-the-art hybrid composite piezoelectric nanogenerators (PENGs) and the density of I _sc is 4.86 times higher than that of lead halide perovskites counterpart ever reported. Moreover, CsPbBr₃@PVDF composite fibers based PENG exhibits fundamentally improved thermal/water/acid–base stabilities. This study suggests that the CsPbBr₃@PVDF composite fiber is a good candidate for fabricating high-performance PENGs, promising application potentials in mechanical energy harvesting and motion sensing technologies.

Highly efficient and stable γ-CsPbI₃ perovskite solar cells (PSCs) can be obtained using a simple inorganic additive strategy by regulating the nucleation and crystallization process of the CsPbI₃ film. Improved grain boundaries and interfacial contact of the CsPbI₃ film lead to significant suppression in the non-radiative recombination, which shows better efficiency and remarkable stability in all-inorganic PSCs.

Abstract

All-inorganic perovskite cesium lead iodide (CsPbI₃) exhibits excellent prospects for commercial application as a light absorber in single-junction or tandem solar cells due to its outstanding thermal stability and proper bandgap. However, the device performance of CsPbI₃-based perovskite solar cells (PSCs) is still restricted by the unsatisfactory crystal quality and severe non-radiative recombination. Herein, inorganic additive ammonium halides are introduced into the precursor solution to regulate the nucleation and crystallization of the CsPbI₃ film by exploiting the atomic interaction between the ammonium group and the Pb–I framework. The grain boundaries and interfacial contact of the CsPbI₃ film have been improved, which leads to significant suppression in the non-radiative recombination and an enhancement in the charge transport ability. With these benefits, a high efficiency of 18.7% together with an extraordinarily high fill factor of 0.83–0.84 has been achieved, comparable to the highest records reported so far. Moreover, the cell exhibits ultra-high photoelectrical stability under continuous light illumination and high bias voltage with 96% of its initial power-conversion efficiency being sustained after 2000 h operation, even superior to the world-champion CsPbI₃ solar cell. The findings are promising for the development and application of all-inorganic PSCs using a simple inorganic additive strategy.

The intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) response of perovskite solar cells is accurately resolved, demonstrating that features commonly observed at high frequency in literature reports are due to artefacts caused by limitations of standard instruments. The time dependence of the IMPS/IMVS response shows clear links to effects of ion migration on the electric field and interfacial recombination rates within devices.

The complete interpretation of small perturbation frequency-domain measurements on perovskite solar cells has proven to be challenging. This is particularly true in the case of intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements in which the high frequency response is obscured by instrument limitations. Herein, a new experimental methodology capable of accurately resolving the high frequency response—often observable in the second and third quadrants of the complex plane—of a range of perovskite devices is demonstrated. By combining single-frequency IMPS/IMVS measurements, it is able to construct the time dependence of the IMPS/IMVS response of these devices during their initial response to illumination. This reveals significant negative photocurrent/photovoltage signals at high frequency while devices reach steady state, which is in keeping with observations made from comparable time-domain transient measurements. These techniques allow the underlying interfacial recombination and ion migration processes to be assessed, which are not always evident using steady-state measurements. The ability to study and mitigate these processes is vital in optimizing the real-world operation of devices.

The structure–properties relationships of hole transport materials (HTMs) with fused and unfused core units are investigated. The HTM with a fused core exhibits a high hole mobility and an efficient charge extraction capability, leading to a remarkable efficiency of 19.63% when it is used as an HTM without any dopant, much higher than unfused HTM-based devices (10.03%).

Replacing the dominating and dopant-needing 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spiro-bifluorene (spiro-OMeTAD) with dopant-free organic hole transport materials (HTMs) in n-i-p structured perovskite solar cells (PSCs) is a big challenge. Herein, a class of conjugated organic semiconductor materials, namely, ZT-H1 and ZT-H2, with unfused and fused core units, respectively, are successfully designed and synthesized for dopant-free HTMs. It is found that the HTM ZT-H1 exhibits a hole mobility of 7.08 × 10⁻⁵ cm² V⁻¹ s⁻¹, which is improved to 5.16 × 10⁻⁴ cm² V⁻¹ s⁻¹ for HTM ZT-H2 due to the enlarged molecular planarity of ZT-H2, leading to efficient intermolecular π–π interaction. Further investigation indicates that ZT-H2 is more fit to facilitating hole extraction, restraining charge recombination, and guaranteeing long-term stability of the devices. Consequently, a planar n-i-p structured device using ZT-H2 as HTM without any dopants exhibits a remarkable efficiency of 19.63%, which is much higher than that of ZT-H1-based devices (10.64%). Importantly, ZT-H2-based devices are much more stable than the control devices using ZT-H1 or spiro-OMeTAD as the HTM. The findings reveal that the fused central core unit with extended π-conjugation is an efficient strategy for rationally designing dopant-free HTMs toward stable and efficient PSCs.

Two novel triarylamine-based donor-acceptor copolymers featuring an imide-functionalized backbone are developed. Benefiting from the good energy level alignment, appropriate film morphology, and most importantly, improved hole mobility, the pristine PTTI-TPA based inverted perovskite solar cells achieve a high power conversion efficiency of up to 21% with negligible hysteresis and substantial stability.

Abstract

Dopant-free hole-transporting layers (HTLs) are highly desired for realizing efficient and stable perovskite solar cells (PVSCs), but only very few of them can enable power conversion efficiencies (PCEs) over 20%. Herein, two imide-functionalized triarylamine-based donor-acceptor (D-A) type copolymers, PBTI-TPA and PTTI-TPA, are developed and applied as dopant-free HTLs in inverted PVSCs. The combination of a classic redox-active triphenylamine donor unit and an electron-withdrawing oligothiophene imide co-unit with rigid and planar backbone furnishes the two polymers with quasi-planar backbone, suitable frontier molecular orbital (FMO) energy levels, favorable thermal stability, appropriate film morphology, and passivation effect. More importantly, the greatly improved hole mobility renders them as promising HTLs for PVSCs. As a result, the undoped PTTI-TPA-based inverted PVSCs deliver a remarkable PCE up to 21% as well as negligible hysteresis and substantial long-term stability, outperforming the devices based on PBTI-TPA and PTAA. The performance also represents one of the highest PCEs reported to date for PVSCs based on dopant-free polymeric HTLs. The results highlight the great potentials of oligothiophene imides for constructing donor-acceptor polymeric HTLs for enabling high-performance dopant-free PVSCs.

The self-doping effect on the light stability of perovskite solar cells (PSCs) is systemically investigated through various opto-electrical characterizations. Although both PSCs with Pb-rich and Pb-deficient conditions exhibit similar initial performance, the Pb-rich PSC degrades relatively quickly under light illumination even without H₂O and O₂, resulting in the shift of the defect state associated with the formation of deep-level defects.

Abstract

Although there have been significant advances in the stability of perovskite solar cells through encapsulation techniques to remove extrinsic degradation factors, such as moisture and oxygen, irreversible photo-degradation originating from intrinsic defects is still challenging and remains elusive. Herein, the photo-aging mechanism due to intrinsic defects is investigated in nitrogen-filled conditions, excluding extrinsic degradation factors. Devices with similar power conversion efficiencies (PCE) of 21%, but with different Fermi levels in the perovskite films, via controlling the self-doping effect, have been investigated. Opto-electronic investigations and depth profiles of the elemental constituents show that after photo-aging, strain relaxation in the perovskite lattice and a Fermi level shift towards conduction band edge are observed, implying the formation of new defect states in Pb-rich devices. Furthermore, thermal admittance spectroscopy measurement of the devices suggests that the formation of the deep-traps in the perovskite leads to irreversible degradation. Thin-film solar cells that are relatively Pb-deficient (FA-rich) exhibit improved long-term stability, retaining over 90% of their initial PCE during 500 h of continuous 1-Sun illumination. This study suggests passivation of the Pb-I related antisite defects near the grain boundaries and the interface is crucial for the fabrication of solar cells with enhanced long-term stability.

A low-dimensional perovskite layer is constructed between the perovskite absorber and carbon electrode in printable triple-mesoscopic perovskite solar cells by posttreatment. By forming graded type-II band alignment, the device performance is significantly enhanced, delivering a power conversion efficiency of 17.47% with an open-circuit voltage of 1.02 V.

Abstract

Printable hole-conductor-free perovskite solar cells (PSCs) have attracted intensive research attention due to their high stability and simple manufacturing process. However, the cells have suffered severe potential loss in the absence of the hole transporting layer. The dimensionality of the perovskite absorber in the mesoporous carbon electrodes by conducting post-treatments is reduced. The low-dimensional perovskites possess wide-bandgaps and form type-II band alignment, favoring directional charge transportation and thus enhancing the device performance. For the cells using MAPbI₃ (MA = methylammonium) as the light absorber, the open-circuit voltage (V _OC) is significantly enhanced from 0.92 to 0.98 V after posttreatment, delivering an overall efficiency of 16.24%. For the cells based on FAPbI₃ (FA = formamadinium), a high efficiency of 17.47% is achieved with V _OC of 1.02 V, which are both the highest reported values for printable hole-conductor-free PSCs. This strategy provides a facile method for tuning the energy level alignment for mesoscopic perovskite-based optoelectronics.

A strategic implementation of indolo[3,2,1-jk]carbazole units into polycyclic heteroaromatics is proposed, making use of not only the multi-resonance for narrowband emission but also the enhanced electronic coupling of para-positioned nitrogen atoms to narrow energy gaps. The corresponding emitters show narrowband deep-blue electroluminance and high efficiencies when assisted by a sensitizer with thermally activated delayed fluorescence.

Abstract

Multiple-resonance (MR) organic emitters bearing small full-width at half-maximum (FWHMs) are of general interest in organic light-emitting diodes. Indolo[3,2,1-jk]carbazole (ICz) embedded MR-fluorophors have demonstrated extremely small FWHMs, yet in the violet region with low electroluminescence efficiency. Herein, a strategic implementation of ICz subunits into MR fluorophors is proposed by taking advantage of the synergetic effect of para-positioned nitrogen atoms to enhance electronic coupling to decrease emitting energy gap. Deep blue emitters peaking at 441 and 447 nm with FWHMs of only 18 and 21 nm are thereof obtained, respectively, accompanied by ≈90 % photo-luminance quantum yields. With the assistance of a thermally activated delayed fluorescence sensitizer to recycle excitons, the corresponding narrowband electroluminescent devices show unprecedent high maximum external quantum efficiencies of 32.0 % and 34.7 % with CIE_y of 0.10 and 0.085, respectively.

We constructed a series of noncovalently fused-ring electron acceptors (NFREAs) with S⋅⋅⋅O noncovalent intramolecular interactions. Combining the strategies of noncovalent conformational locks and π-extended end-group engineering, a record PCE of 14.53 % in labs and a certified PCE of 13.8 % for NFREAs based devices were achieved.

Abstract

Noncovalently fused-ring electron acceptors (NFREAs) have attracted much attention in recent years owing to their advantages of simple synthetic routes, high yields and low costs. However, the efficiencies of NFREAs based organic solar cells (OSCs) are still far behind those of fused-ring electron acceptors (FREAs). Herein, a series of NFREAs with S⋅⋅⋅O noncovalent intramolecular interactions were designed and synthesized with a two-step synthetic route. Upon introducing π-extended end-groups into the backbones, the electronic properties, charge transport, film morphology, and energy loss were precisely tuned by fine-tuning the degree of multi-fluorination. As a result, a record PCE of 14.53 % in labs and a certified PCE of 13.8 % for NFREAs based devices were obtained. This contribution demonstrated that combining the strategies of noncovalent conformational locks and π-extended end-group engineering is a simple and effective way to explore high-performance NFREAs.

The integration and encapsulation of perovskite nanocrystals into multifunctional metal–organic frameworks results in new features and paves the way to promising applications. This Minireview summarizes the recent progress of perovskite NC@MOF composites from the perspective of synthetic chemistry, functional mechanisms, and potential applications.

Abstract

As an emerging optical material, perovskite nanocrystals (NCs) exhibit excellent optoelectronic properties and show great potential for various optoelectronic applications. However, the inherent inferior stability against moisture, oxygen, light and heat limit their practical application. As well, the exploration and development of perovskite NCs with novel properties and functions are new challenges. To achieve these goals, the integration and encapsulation of perovskite NCs with multifunctional metal–organic frameworks (MOFs) to form perovskite NC@MOF composites, is a promising strategy for enhancing the stability and broadening the application scope. In this minireview, we summarize and discuss the synthesis strategies and functional mechanisms of perovskite NC@MOF composites, along with applications of light emitting diodes (LED), information security, photocatalysis, sensing, and detection. We further briefly point out the current challenges as well as the future opportunities for the emerged composite materials.

Inorganic CsPbI₃ perovskite is well-known for its resistance to organic cation substitution. Here, a specific organic cation of tetrabutylammonium (TBA⁺) with strong ionic binding to the Pb-I octahedral framework can effectively intercalate into CsPbI₃ and substitute the Cs⁺ cation. A post-synthesis TBAI passivation then leads to in situ formation of TBAPbI₃ layer to heal CsPbI₃ perovskite with lower defect density and enhanced stability.

Abstract

The in situ formation of reduced dimensional perovskite layer via post-synthesis ion exchange has been an effective way of passivating organic-inorganic hybrid perovskites. In contrast, cesium ions in Cs-based inorganic perovskite with strong ionic binding energy cannot exchange with those well-known organic cations to form reduced dimensional perovskite. Herein, we demonstrate that tetrabutylammonium (TBA⁺) cation can intercalate into CsPbI₃ to effectively substitute the Cs cation and to form one-dimensional (1D) TBAPbI₃ layer in the post-synthesis TBAI treatment. Such TBA cation intercalation leads to in situ formation of TBAPbI₃ protective layer to heal defects at the surface of inorganic CsPbI₃ perovskite. The TBAPbI₃-CsPbI₃ perovskite exhibited enhanced stability and lower defect density, and the corresponding perovskite solar cell devices achieved an improved efficiency up to 18.32 % compared to 15.85 % of the control one.

Publication date: 21 April 2021

Source: Joule, Volume 5, Issue 4

Author(s): Di Yan, Andres Cuevas, Jesús Ibarra Michel, Chun Zhang, Yimao Wan, Xinyu Zhang, James Bullock

A self-sufficient micrometer-level vacuum growth chamber is proposed for MAPbBr₃ microcrystals to effectively prevent water and oxygen, and to greatly improve the thermal and optical stability by the reduction of deep level trap states.

Abstract

Perovskite materials and their optoelectronic devices have attracted intensive attentions in recent years. However, it is difficult to further improve the performance of perovskite devices due to the poor stability and the intrinsic deep level trap states (DLTS), which are caused by surface dangling bonds and grain boundaries. Herein, the CH₃NH₃PbBr₃ perovskite microcrystal is encapsulated by a dense Al₂O₃ layer to form a microenvironment. Through optical measurement, it is found that the structure of perovskite can be healed by itself even under high temperature and long-time laser illumination. The DLTS density decreases nearly an order of magnitude, which results in 4–14 times enhancement of light emission. The observation is ascribed to the micron-level environment, which serves as a self-sufficient high-vacuum growth chamber, where the components of the perovskite are completely retained when sublimated and the decomposed atoms can re-arrange after thermal treatment. The modified structure showing high thermal stability is able to maintain excellent optical and lasing stability up to 2 years. This discovery provides a new idea and perspective for improving the stability of perovskite and can be of practical interest for perovskite device application.

A patterned blade coating strategy is investigated to print non-fullerene based devices, with a PCE of 15.93%. The designed patterned blade enhances fluid flow to optimize morphology evolution kinetics for achieving an optimized morphology and device reproducibility. In addition, the versatility in large-area devices and other systems is also shown.

Abstract

Morphology evolution kinetics at multi-scale regime is a challenging problem which is critical for industrial fabrication of high-performance organic solar cells (OSCs). An innovative strategy utilizing a patterned blade to print non-fullerene (NF) based devices in ambient conditions is demonstrated. A specially designed patterned blade with micro-cylinder arrays exhibit a reasonable control over the fluid flow at high extensional and shear strain rate to enhance lateral mass transport during blade-coating. Comparison of patterned and normal blade in printing polymer:NF blend film at different speeds reveals interesting avenues to optimize the blend films morphology. Patterned blade printed PM6:Y6 films yield a PCE of 15.93% as compared to 14.55% from a normal blade. Through in situ and ex situ morphology characterization techniques, the use of patterned blades induce conformational changes in PM6 chains, enabling Y6 to crystallize faster and more efficiently. Such improved blend morphology enables favorable charge transfer and transport to realize superior device performance. A lower stick-slip effect at the macro-scale with the patterned blade results in a smoother film promoting device reproducibility. Applications in efficient large-scale devices, confirming the choice of patterned blade design are reported. The efforts collaborating device engineering, morphology evolution kinetics would enable reproducibility and eased commercialization of OSCs at large scale.

Publication date: 21 April 2021

Source: Joule, Volume 5, Issue 4

Author(s): Hsin-Hsiang Huang, Qi-Han Liu, Hsinhan Tsai, Shreetu Shrestha, Li-Yun Su, Po-Tuan Chen, Yu-Ting Chen, Tso-An Yang, Hsin Lu, Ching-Hsiang Chuang, King-Fu Lin, Syang-Peng Rwei, Wanyi Nie, Leeyih Wang

Publication date: 21 April 2021

Source: Joule, Volume 5, Issue 4

Author(s): Tianhao Wu, Xiao Liu, Xinhui Luo, Xuesong Lin, Danyu Cui, Yanbo Wang, Hiroshi Segawa, Yiqiang Zhang, Liyuan Han

A novel kinetic‐trapping method that is compatible with solution‐processing is presented. The method is demonstrated by fabricating an ultraviolet‐emitting perovskite alloy: Cs_{1− x} Rb _x PbCl₃. The method involves heating a precursor solution to provide it with sufficient thermal energy to form the alloy state. The method yields a material with superior stability and optical properties compared to simple precursor mixing.

Abstract

Engineering halide perovskites through alloying allows synthesis of materials having tuned electronic and optical properties; however, synthesizing many of these alloys is hindered by the formation of demixed phases arising due to thermodynamically unstable crystal structures. Methods have been developed to make such alloys, such as solid‐phase reactions, chemical vapor deposition, and mechanical grinding; but these are incompatible with low‐temperature solution‐processing and monolithic integration, precluding a number of important applications of these materials. Here, solvent‐phase kinetic trapping (SPKT), an approach that enables the synthesis of novel thermodynamically unfavored perovskite alloys, is developed. SPKT is used to synthesize Cs_{1− x} Rb _x PbCl₃ and report the first instance of ultraviolet emission in polycrystalline perovskite thin films. SPKT leads to materials exhibiting superior thermal and photostability compared to non‐kinetically trapped materials of the same precursors. Transient absorption spectroscopy of the kinetically trapped material reveals improved optical properties: greater absorption, and longer ground‐state bleach lifetimes. SPKT may be applied to other perovskites to realize improved material properties while benefiting from facile solution‐processing.

Chirality dependence of a novel optoelectronic phenomenon, circular photogalvanic effect, is observed in a pair of 2D organic–inorganic hybrid perovskites with introduced chiral molecules. The sign reversal of zero‐bias photocurrents between enantiomers indicates the chirality‐dependent spin‐polarized bands induced by spin–orbit coupling.

Abstract

The control of the optoelectronic properties of 2D organic–inorganic hybrid perovskite (2D‐OIHP) lead halides is an increasingly prevalent topic. Herein, the observation of the circular photogalvanic effect (CPGE) in new enantiomorphic 2D‐OIHP lead iodides is reported, which are synthesized as a first OIHP‐related system belonging to a chiral space group by incorporating organic chiral cations into the inorganic layers of lead iodides. The CPGE is an optoelectronic phenomenon associated with the spin–orbit coupling of heavy atoms in noncentrosymmetric systems. Owing to the CPGE, light‐helicity‐dependent steady photocurrents are generated without an external bias voltage under the irradiation of circularly polarized light. Furthermore, the sign reversal of the CPGE photocurrent depending on the chirality of the designed 2D‐OIHP lead iodides is observed. This result indicates formation of the theoretically predicted radial spin‐polarized texture in k‐space of chiral systems owing to spin‐momentum locking. Hence, chiral 2D‐OIHP lead halides can be a promising platform for engineering opto‐spintronic functionalities.

Ternary organic solar cells are fabricated, achieving a significant J _SC boost by virtue of an optimized crystalline feature with the formation of a eutectic mixture with better acceptor crystalline fibrils. The optimal morphology suppresses energetic disorder and recombination and increases charge transfer and transport, yielding a high efficiency of 17.84% with significant current boost.

Abstract

The intrinsic electronic properties of donor (D) and acceptor (A) materials in coupling with morphological features dictate the output in organic solar cells (OSCs). New physical properties of intimate eutectic mixing are used in nonfullerene-acceptor-based D–A₁–A₂ ternary blends to fine-tune the bulk heterojunction thin film morphology as well as their electronic properties. With enhanced thin film crystallinity and improved carrier transport, a significant J _SC amplification is achieved due to the formation of eutectic fibrillar lamellae and reduced defects state density. Material wise, aligned cascading energy levels with much larger driving force, and suppressed recombination channels confirm efficient charge transfer and transport, enabling an improved power conversion efficiency (PCE) of 17.84%. These results reveal the importance of utilizing specific material interactions to control the crystalline habit in blended films to form a well-suited morphology in guiding superior performances, which is of high demand in the next episode of OSC fabrication toward 20% PCE.