Chen Weijie

Hydrophobic organic ammonium halides (Cl, Br, and I) are used for the modification of inorganic CsPb(I_0.75Br_0.25)₃ perovskite solar cells. Benefiting from their passivation effects and hydrophobic long alkyl chain, the modified devices exhibit enhanced efficiency and stability. Among them, the hexadecyltrimethylammonium chloride (CTAC)-modified device shows the best performance with a power conversion efficiency (PCE) of 18.05%.

Inorganic cesium lead halide perovskite solar cells are promising candidates for next-generation photovoltaic applications. However, their phase instability and relatively low efficiency hinder their commercialization. Herein, hydrophobic organic ammonium halides (Cl, Br, and I) are rationally used for the modification of inorganic CsPb(I_0.75Br_0.25)₃ perovskite solar cells. Benefiting from their passivation effects and hydrophobic long alkyl chain, the modified devices exhibit enhanced efficiency and stability. Among them, the hexadecyltrimethylammonium chloride (CTAC)-modified device shows the best performance with a power conversion efficiency (PCE) of 18.05%. Furthermore, a gradient triple anion inorganic perovskite CsPb(I_0.75Br_0.25)_3−x Cl _x layer is formed in situ during the CTAC modification, which demonstrates better phase stability than CsPb(I_0.75Br_0.25)₃. As a result, the modified device also shows excellent stability, maintaining 94% of the initial efficiency after 35 days in N₂ atmosphere.

Organic planar heterojunctions comprising a nonfullerene small molecular acceptor (NFA) and a polymer donor are successfully fabricated by transferring polymer films on top of water to NFA layers, demonstrating ultrafast charge transfer. They are rendered into bilayer organic solar cells, achieving a power conversion efficiency of up to 7.47% and high stability in ambient air.

Herein, planar heterojunctions comprising a nonfullerene small molecular acceptor (NFA) and a polymer donor are demonstrated by transferring polymer films on a water surface on top of NFA layers. So far, most solution-processed layer-by-layer architectures have been reported as sequentially deposited bulk heterojunctions or pseudo-bilayers because mixed regions at the donor/acceptor interface are inevitable in these methods. By virtue of the unique properties of conjugated polymers such as hydrophobicity and spontaneous film formation on a water surface, the fabrication of NFA/polymer bilayer nanostructures is clearly demonstrated by dramatically simplified methods. These bilayers are successfully rendered into bilayer organic solar cells achieving a power conversion efficiency of up to 7.47%. This reflects that these bilayers have appropriate morphological and optoelectrical properties to be operated as photoactive layers in photovoltaic devices. Further, ultrafast charge transfer from the polymer donor to the NFA and fast carrier mobility are investigated by transient-absorption spectroscopy and photoinduced charge-extraction measurements. Fast carrier dynamics are observed, which are essential for the efficient harvest of excitons in photovoltaic devices. It is believed that the formation of planar heterojunctions on water can offer technical diversity for the fabrication methods of the photovoltaic devices.

Herein, a semitransparent flexible MAPbBr₃ perovskite solar cell is demonstrated to be the roof of a greenhouse. It demonstrates a power conversion efficiency (PCE) of 7.67% with an average transmittance of ≈60% in the range of 540–760 nm.

Perovskite photovoltaics (PV) is an emerging thin-film solar energy technology that is advantageous over the currently dominant crystalline silicon PV in terms of its adjustable bandgap with sub-bandgap transparency, potential flexibility, and more rapid continuous roll-to-roll manufacturing, showing promise for unique niche applications. Herein, methylammioun lead tribromide (MAPbBr₃) is utilized in a semitransparent flexible solar cell with a transparent electrode using a sandwiched MoO₃/Au/MoO₃ (MAM) multilayer to harvest around 80% of the visible light region. Through design of the thickness of the MAM multilayer, the reflected light loss is significantly reduced, thereby improving the light transmittance in the visible light region to maximize the photosynthetic yield. The semitransparent flexible device exhibits a power conversion efficiency (PCE) of 7.67% (the highest efficiency of MAPbBr₃-based semitransparent flexible devices), and the opaque rigid MAPbBr₃ solar cell shows a PCE of 9.73% with a high open-circuit voltage of 1.629 V. Optical measurement demonstrates that the flexible cell without metal electrode shows over 77% transparency in the 540–1100 nm range, whereas the overall semitransparent cell shows an average transmittance of 60% in the 540–760 nm range, which is perfect for greenhouse vegetation to not only act as protective coverage but also provide practical output power.

Efficient ternary thick-film organic photovoltaics (OPVs) are fabricated using PM6 as the donor and BP4T-4F and BP3T-4F with symmetric and asymmetric structures as alloyed acceptors. The power conversion efficiency (PCE) of ternary OPVs is slightly decreased from 16.91% to 16.03% for active layer thickness 100–300 nm, exhibiting an excellent PCE tolerance on active layer thickness.

High-performance organic photovoltaics (OPVs) with relatively thick active layers are essential for large-scale production. Herein, series of OPVs with different active layer thicknesses are fabricated using PM6 as the donor and BP4T-4F and BP3T-4F with symmetric and asymmetric structures as acceptors. With the active layer thickness increasing from 100 to 300 nm, the power conversion efficiency (PCE) of BP3T-4F-based binary OPVs is slightly decreased from 15.37% to 14.40%, while the PCEs of BP4T-4F-based binary OPVs are markedly decreased from 16.89% to 14.99%. The two kinds of binary OPVs exhibit distinct PCEs and thickness tolerance features, which may be recombined into ternary OPVs using compatible BP3T-4F and BP4T-4F as alloyed acceptors. The ternary OPVs exhibit a slightly decreased PCE from 16.91% to 16.03% along with active layer thickness from 100 to 300 nm, benefiting from the well-optimized phase separation in ternary active layers. It is worth highlighting that the fill factor (FF) of 71.47% is achieved in ternary thick-film OPVs. The PCE of 16.03% and FF of 71.47% should be among the highest values among OPVs with 300 nm thick active layers.

Incorporation of thus far unreported 1D-Tpy₂Pb₃I₆ and 2D-TpyPb₃I₆ perovskites allows to form a multidimension coupled 1D-2D-3D perovskite, which is proved to effectively release the residual strain whilst improving the carriers transport ability. Accordingly, the as-fabricated 1D-2D-3D hybrid CsPbI₂Br perovskite solar cells demonstrate high power conversion efficiency whilst extraordinary stability within 1066 h in ambient atmosphere.

Abstract

Despite the rapid development of CsPbI _x Br_{3− x} (0 ≤ x ≤ 3) inorganic perovskite solar cells, associated with their superior thermal stability, their low moisture stability limits their commercial deployment. In this study, 1D-2D-3D multidimensional coupled perovskites are prepared by means of an in situ self-integration approach. This pioneering method allows incorporating thus far unreported 1D-Tpy₂Pb₃I₆ and 2D-TpyPb₃I₆ (Tpy; terpyridine) perovskites. Heterojunction perovskites demonstrate superior stability against water in comparison with control 3D CsPbI₂Br, which is related to the hydrophobicity of low-dimension (LD) perovskites. Remarkably, the spontaneous involvement of LD perovskites can adjust/reconstruct the interfacial structure. This modification allows releasing the residual strain, establishing effective charge transfer channels that increase the carrier transport ability. Accordingly, 1D-2D-3D hybrid CsPbI₂Br perovskite solar cells demonstrate a stabilized power conversion efficiency as high as 16.1%, which represents a very significant improvement, by a factor of 43%, with respect to control 3D CsPbI₂Br perovskite solar cell. Equally importantly, the multidimensional coupled perovskite solar cells exhibit extraordinary stability, well above 1000 h in ambient atmosphere.

The in situ optical spectra reveal a significantly prolonged crystallization window during the perovskite deposition via additive strategy. Finer thickness gradient by n values in the direction orthogonal to the substrate leads to more efficient charge transport between quantum wells and suppressed charge recombination in the additive-treated film. Finally, the power conversion efficiency of 14.4% is obtained.

Abstract

New structural type of 2D AA′ _{n −1}M _n X_{3 n +1} type halide perovskites stabilized by symmetric diammonium cations has attracted research attention recently due to the short interlayer distance and better charge-transport for high-performance solar cells (PSCs). However, the distribution control of quantum wells (QWs) and its influence on optoelectronic properties are largely underexplored. Here effective phase-alignment is reported through dynamical control of film formation to improve charge transfer between quantum wells (QWs) for 2D perovskite (BDA)(MA) _{n -1}Pb _n I_{3 n +1} (BDA = 1,4-butanediamine, 〈n〉 = 4) film. The in situ optical spectra reveal a significantly prolonged crystallization window during the perovskite deposition via additive strategy. It is found that finer thickness gradient by n values in the direction orthogonal to the substrate leads to more efficient charge transport between QWs and suppressed charge recombination in the additive-treated film. As a result, a power conversion efficiency of 14.4% is achieved, which is not only 21% higher than the control one without additive treatment, but also one of the high efficiencies of the low-n (n ≤ 4) AA′ _{n −1}M _n X_{3 n +1} PSCs. Furthermore, the bare device retains 92% of its initial PCE without any encapsulation after ambient exposure for 1200 h.

Asymmetry and halogenation are employed to design a fused-ring non-fullerene electron acceptor, and demonstrate the synergistic effect of tuning optoelectronic properties and enhancing molecular stacking, leading to the highest device efficiency.

Abstract

Fused-ring non-fullerene electron acceptors (NFAs) boost the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Asymmetric and halogenated NFAs have drawn increasing attention in recent years due to their unique optoelectronic properties. Starting from the symmetric NFA ITCC-M, this work systematically designs and synthesizes an asymmetric counterpart ITCC-M-2F, halogenated counterpart ITCC-Cl, and asymmetric and halogenated counterpart IDTT-Cl-2F. Among these NFAs, IDTT-Cl-2F shows the shallowest lowest unoccupied molecular orbital energy level, broader absorption range, and the tightest molecular packing. As a result, when blended with the donor PBDB-T-2Cl, IDTT-Cl-2F-based OSCs yield the highest PCE of 13.3% with an open-circuit voltage of 0.96 V, short-circuit current of 19.20 mA cm^–2, and fill factor of 71.1%, which is the highest PCE of OSCs employing 2-(2-chloro-6-oxo-5,6-dihydro-4H-cyclopenta[b]thiophen-4-ylidene) malononitrile (ClIC) unit terminated NFA. The results demonstrate the synergistic effect of asymmetry and halogenation toward tuning of the optoelectronic properties of NFAs for high performance OSCs.

A dissymmetric backbone and selenophene substitution on the central core was employed to synthesize dissymmetric A-DA′D-A NF-SMAs. Their detailed single-crystal packing were revealed successfully. The dissymmetric A-WSSe-Cl:PM6 device presented an impressive PCE of 17.51 %, which is the highest values for selenophene-based and the dissymmetric NF-SMAs in binary PSCs.

Abstract

A dissymmetric backbone and selenophene substitution on the central core was used for the synthesis of symmetric or dissymmetric A-DA′D-A type non-fullerene small molecular acceptors (NF-SMAs) with different numbers of selenophene. From S-YSS-Cl to A-WSSe-Cl and to S-WSeSe-Cl, a gradually red-shifted absorption and a gradually larger electron mobility and crystallinity in neat thin film was observed. A-WSSe-Cl and S-WSeSe-Cl exhibit stronger and tighter intermolecular π–π stacking interactions, extra S⋅⋅⋅N non-covalent intermolecular interactions from central benzothiadiazole, better ordered 3D interpenetrating charge-transfer networks in comparison with thiophene-based S-YSS-Cl. The dissymmetric A-WSSe-Cl-based device has a PCE of 17.51 %, which is the highest value for selenophene-based NF-SMAs in binary polymer solar cells. The combination of dissymmetric core and precise replacement of selenophene on the central core is effective to improve J _sc and FF without sacrificing V _oc.

Featuring a fused tricyclic core, an organic small molecule was intentionally synthesized to reduce defects density and improve hole transportation in perovskite devices via π-Pb²⁺ interactions, confirmed by multiple characterizations and simulation.

Abstract

Molecular doping is an of significance approach to reduce defects density of perovskite and to improve interfacial charge extraction in perovskite solar cells. Here, we show a new strategy for chemical doping of perovskite via an organic small molecule, which features a fused tricyclic core, showing strong intermolecular π-Pb²⁺ interactions with under-coordinated Pb²⁺ in perovskite. This π-Pb²⁺ interactions could reduce defects density of the perovskite and suppress the nonradiative recombination, which was also confirmed by the density functional theory calculations. In addition, this doping via π-Pb²⁺ interactions could deepen the surface potential and downshift the work function of the doped perovskite film, facilitating the hole extraction to hole transport layer. As a result, the doped device showed high efficiency of 21.41 % with ignorable hysteresis. This strategy of fused tricyclic core-based doping provides a new perspective for the design of new organic materials to improve the device performance.

A new non-fullerene acceptor named BTP1O-4Cl-C12 which contains chlorinated end groups, extended inner side chains and asymmetric alkyl and alkoxy outer side chains is reported. These modifications help BTP1O-4Cl-C12-based devices achieve high efficiency of 17.1% and show its potential application in ternary organic solar cells.

Abstract

Chemical modifications of non-fullerene acceptors (NFAs) play vital roles in the development of high efficiency organic solar cells (OSCs). In this work, on the basis of the previously reported molecule named Y6-1O, chlorination and inner side-chain engineering are adopted to endow the corresponding devices with higher open-circuit voltage (V _OC) and short-circuit current density (J _SC) as well as good morphology for high fill factor (FF). As a result, the molecule named BTP1O-4Cl-C12 can help achieve a higher power conversion efficiency (PCE) of 17.1% than that of Y6-1O (16.1%). Furthermore, the following comparisons between BTP1O-4Cl-C12 and the two symmetric acceptors named BTP2O-4Cl-C12 and BTP-4Cl-C12 demonstrate the effect of asymmetric alkoxy substitution on the outer side chains, which not only achieves a balance between V _OC and J _SC, but also help obtain appropriate morphology for efficient charge dissociation and suppressed charge recombination. Therefore, the asymmetric BTP1O-4Cl-C12 can achieve a higher PCE compared to the symmetric BTP2O-4Cl-C12 and BTP-4Cl-C12. The work not only reports an excellent NFA for high-performance OSCs, but also puts forward a series of methods for consecutive chemical modifications on Y-series acceptors, which can be further applied to boost the PCE of OSCs to a higher level.

In situ nonmetallic SO₄ ²⁻ anions and modulated bulk sulfur vacancies are introduced into ZnIn₂S₄ photoanodes for photoelectrochemical water splitting. The SO₄ ²⁻ groups help to reduce the oxygen evolution reaction (OER) overpotential, thus enhancing OER activity. In addition, the control of sulfur vacancies can promote the bulk charge separation and alleviate surface carrier recombination.

Abstract

Severe charge recombination and slow surface water oxidation kinetics seriously limit the practical application of ZnIn₂S₄ photoanodes for photoelectrochemical water splitting. Herein, an in situ strategy to introduce sulfate (SO₄ ²⁻) anions and controlled bulk sulfur vacancies (S_v) into a ZnIn₂S₄ photoanode is developed, and its PEC performance is remarkably enhanced, achieving a photocurrent density of 3.52 mA cm⁻² at 1.23 V versus reversible hydrogen electrode (V _RHE) and negatively shifted onset potential of 0.01 V _RHE in phosphate buffer without a sacrificial agent under AM 1.5G illumination. The experimental characterizations and density functional theory calculations reveal that the SO₄ ²⁻ groups enhance the oxygen evolution reaction kinetics, while bulk S_v improves the bulk carrier separation. The remarkable bulk carrier separation efficiency of 75.01% and surface carrier injection efficiency of 79.69% are achieved at 1.23 V _RHE. This work provides a new route to design efficient photoanodes by the simultaneous manipulation of metal-free anions and sulfur vacancies.

Ethylenediamine (EDA) coating changes the p-type tin-lead perovskite to n-type, increases the built-in potential, and decreases the open-circuit voltage (V _oc) loss in perovskite solar cells. With Br inclusion into the lattice and passivation by EDA, the highest power conversion efficiency of 21.74% and Voc of 0.86 V is achieved using Cs_0.025FA_0.475MA_0.5Sn_0.5Pb_0.5I_2.975Br_0.025 perovskite film with a bandgap of 1.25 eV.

Abstract

Tin-lead perovskite solar cells (PSCs) show inferior power conversion efficiency (PCE) than their Pb counterparts mainly because of the higher open-circuit voltage (V _oc) loss. Here, it is revealed that the p-type surface of perovskite transforms to n-type, based on post-treatment by a Lewis base, ethylenediamine. This approach forms a graded band structure owing to the rise of the Fermi-energy level at the surface of the perovskite layer, and increases the built-in potential from 0.56 to 0.76 V, which increases the V _oc by more than 100 mV. It is demonstrated that EDA can lower the defect density (Sn⁴⁺ amount) by screening perovskite against oxygen, and by bonding with undercoordinated Sn on the surface. This study further explores the role of Br anion inclusion in the perovskite lattice from the viewpoint of reducing the lattice strain and Urbach energy. Finally, a high V _oc of 0.86 V is obtained, corresponding to a voltage deficit of 0.39 V, using a perovskite absorber with a bandgap of 1.25 eV and the highest PCE (21.74%) reported so far for Sn-Pb PSCs is achieved.

Publication date: 16 June 2021

Source: Joule, Volume 5, Issue 6

Author(s): Qingshun Dong, Min Chen, Yuhang Liu, Felix T. Eickemeyer, Weidong Zhao, Zhenghong Dai, Yanfeng Yin, Chen Jiang, Jiangshan Feng, Shengye Jin, Shengzhong (Frank) Liu, Shaik M. Zakeeruddin, Michael Grätzel, Nitin P. Padture, Yantao Shi

The involvement of guanidinium in perovskite bulk film and CH₃O-PEABr passivation on the perovskite surface synergistically suppresses the trap states. The charge carrier lifetimes of perovskite films increase by tenfold and fivefold to 981 ns and 8.02 µs at the crystal surface and in its bulk, respectively. The decreased nonradiative recombination loss translates to a record efficiency of 40.1%.

Abstract

Perovskite solar cells exhibit not only high efficiency under full AM1.5 sunlight, but also have great potential for applications in low-light environments, such as indoors, cloudy conditions, early morning, late evening, etc. Unfortunately, their performance still suffers from severe trap-induced nonradiative recombination, particularly under low-light conditions. Here, a holistic passivation strategy is developed to reduce traps both on the surface and in the bulk of micrometer-thick perovskite film, leading to a record efficiency of 40.1% under 301.6 µW cm⁻² warm light-emitting diode (LED) light for low-light solar-cell applications. The involvement of guanidinium into the perovskite bulk film and 2-(4-methoxyphenyl)ethylamine hydrobromide (CH₃O-PEABr) passivation on the perovskite surface synergistically suppresses the trap states. The charge carrier lifetimes of the perovskite film increase by tenfold and fivefold to 981 ns and 8.02 µs at the crystal surface and in its bulk, respectively. The decreased nonradiative recombination loss translates to a high open-circuit voltage (V _oc) of 1.00 V, a high short-circuit current (J _sc) of 152.10 µA cm⁻², and a fill factor (FF) of 79.52%. Note that this performance also stands as the highest among all photovoltaics measured under indoor light illumination. This work of trap passivation for micrometer-thick perovskite film paves a way for high-performance, self-powered IoT devices.

An overview of recent progress on emerging Pb-free semiconductors containing lone-pair ns² cations is provided, with the purpose of providing valid insights for discovering auspicious optoelectronic materials other than lead halide perovskites. The issues hindering performance enhancement and the design strategies of novel ns²-cation-based optoelectronic semiconductors are discussed.

Abstract

Lead halide perovskites have emerged in the last decade as advantageous high-performance optoelectronic semiconductors, and have undergone rapid development for diverse applications such as solar cells, light-emitting diodes , and photodetectors. While material instability and lead toxicity are still major concerns hindering their commercialization, they offer promising prospects and design principles for developing promising optoelectronic materials. The distinguished optoelectronic properties of lead halide perovskites stem from the Pb²⁺ cation with a lone-pair 6s² electronic configuration embedded in a mixed covalent–ionic bonding lattice. Herein, we summarize alternative Pb-free semiconductors containing lone-pair ns² cations, intending to offer insights for developing potential optoelectronic materials other than lead halide perovskites. We start with the physical underpinning of how the ns² cations within the material lattice allow for superior optoelectronic properties. We then review the emerging Pb-free semiconductors containing ns² cations in terms of structural dimensionality, which is crucial for optoelectronic performance. For each category of materials, the research progresses on crystal structures, electronic/optical properties, device applications, and recent efforts for performance enhancements are overviewed. Finally, the issues hindering the further developments of studied materials are surveyed along with possible strategies to overcome them, which also provides an outlook on the future research in this field.

Nature Energy, Published online: 03 June 2021; doi:10.1038/s41560-021-00859-w

A “two-in-one” strategy is applied to form an acceptor alloy for fine-tuning the donor/acceptor energy alignment and blend morphology. Enhanced hole transfer and suppressed charge recombination in the alloy acceptor consisting of AQx-3 and Y6 enable a power conversion efficiency of over 18%, which is the highest documented for ternary organic solar cells utilizing two nonfullerene acceptors.

Abstract

The trade-off between the open-circuit voltage (V _oc) and short-circuit current density (J _sc) has become the core of current organic photovoltaic research, and realizing the minimum energy offsets that can guarantee effective charge generation is strongly desired for high-performance systems. Herein, a high-performance ternary solar cell with a power conversion efficiency of over 18% using a large-bandgap polymer donor, PM6, and a small-bandgap alloy acceptor containing two structurally similar nonfullerene acceptors (Y6 and AQx-3) is reported. This system can take full advantage of solar irradiation and forms a favorable morphology. By varying the ratio of the two acceptors, delicate regulation of the energy levels of the alloy acceptor is achieved, thereby affecting the charge dynamics in the devices. The optimal ternary device exhibits more efficient hole transfer and exciton separation than the PM6:AQx-3-based system and reduced energy loss compared with the PM6:Y6-based system, contributing to better performance. Such a “two-in-one” alloy strategy, which synergizes two highly compatible acceptors, provides a promising path for boosting the photovoltaic performance of devices.

The strategies for high-performance blue perovskite light-emitting diodes (PeLEDs) are described, with the main focus on the optimization of the optical and electrical properties of perovskite nanocrystals and quasi-2D perovskites. Meanwhile, the strategies for efficient 3D mixed-halide perovskite and lead-free perovskite blue LEDs are also briefly introduced, as well as the challenges and future directions of blue PeLEDs.

Abstract

Substantial progress has been made in blue perovskite light-emitting diodes (PeLEDs). In this review, the strategies for high-performance blue PeLEDs are described, and the main focus is on the optimization of the optical and electrical properties of perovskites. In detail, the fundamental device working principles are first elucidated, followed by a systematical discussion of the key issues for achieving high-quality perovskite nanocrystals (NCs) and quasi-2D perovskites. These involve ligand optimization and metal doping in enhancing the carrier transport and reducing the traps of perovskite NCs, as well as the perovskite phase modulation and defect passivation in improving energy transfer and emission efficiency of quasi-2D perovskites. The strategies for efficient 3D mixed-halide perovskite and lead-free perovskite blue LEDs are then briefly introduced. After that, other strategies, including effective charge transport layer, efficient perovskite emission system, and effective device architecture for high light outcoupling efficiency, are further discussed to boost the blue PeLED performances. Meanwhile, the testing standard of blue PeLED lifetime is suggested to enable the direct comparisons of the device operational stability. Finally, challenges and future directions for blue PeLEDs are addressed.