以昇陳

Publication date: June 2021

Source: Materials Today Energy, Volume 20

Author(s): S.L. Zhang, Y.Z. Shi, K. Wang, X.C. Fan, J. Yu, X.M. Ou, X.H. Zhang

Publication date: February 2021

Source: Nano Energy, Volume 80

Author(s): Dae-Hyung Cho, Sung-Hoon Hong, Woo-Jung Lee, Joo Yeon Kim, Yong-Duck Chung

Molecular packing and thermodynamic properties of D18‐based fullerene‐free organic solar cells are studied. The D18 polymer exhibits strong chain extension in films, which is beneficial to charge transport. Miscibility and other characterizations explain the disparate performance of three systems and the processing procedures.

Abstract

Organic solar cells (OSCs) based on D18:Y6 have recently exhibited a record power conversion efficiency of over 18%. The initial work is extended and the device performance of D18‐based OSCs is compared with three non‐fullerene acceptors, Y6, IT‐4F, and IEICO‐4Cl, and their molecular packing characteristics and miscibility are studied. The D18 polymer shows unusually strong chain extension and excellent backbone ordering in all films, which likely contributes to the excellent hole‐transporting properties. Thermodynamic characterization indicates a room‐temperature miscibility for D18:Y6 and D18:IT‐4F near the percolation threshold. This corresponds to an ideal quench depth and explains the use of solvent vapor annealing rather than thermal annealing. In contrast, D18:IEICO‐4Cl is a low‐miscibility system with a deep quench depth during casting and poor morphology control and low performance. A failure of ternary blends with PC₇₁BM is likely due to the near‐ideal miscibility of Y6 to begin with and indicates that strategies for developing successful ternary or quaternary solar cells are likely very different for D18 than for other high‐performing donors. This work reveals several unique property–performance relations of D18‐based photovoltaic devices and helps guide design or fabrication of yet higher efficiency OSCs.

9‐Fluorenone‐1‐carboxylic acid (FCA) is utilized to prolong the lifetime of photogenerated excitons in a nonfullerene acceptor (IT‐M) approximately twofold, ensuring longer exciton diffusion length and efficiency enhancement in organic photovoltaic devices. The prolongation arises from the discovered intermolecular vibrational coupling between the electronic excited state of IT‐M and the electronic ground state of FCA, thus suppressing the nonradiative decay.

Abstract

Exciton lifetime (τ) is crucial for the migration of excitons to donor/acceptor interfaces for subsequent charge separation in organic solar cells (OSCs); however, obvious prolongation of τ has rarely been achieved. Here, by introducing a solid additive 9‐fluorenone‐1‐carboxylic acid (FCA) into the active layer, which comprises a nonfullerene acceptor, 3,9‐bis(2‐methylene‐((3‐(1,1‐dicyanomethylene)‐6/7‐methyl)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene (IT‐M), τ is substantially prolonged from 491 to 928 ps, together with obvious increases in fluorescence intensity and quantum yield. Time‐resolved transient infrared spectra indicate the presence of an intermolecular vibrational coupling between the electronic excited state of IT‐M and the electronic ground state of FCA, which is first observed here and which can suppress the internal conversion process. IT‐M‐based OSCs display an improved short‐circuit current and fill factor after the addition of FCA. Thus, the power conversion efficiency is increased, particularly for devices with a large donor/acceptor ratio of 1:4, whose efficiency is increased by 56%. This study describes a novel method, which is also applicable to other nonfullerene acceptors, for further improving the performance of OSCs without affecting their morphology and light absorption properties.

Nature Communications, Published online: 05 November 2020; doi:10.1038/s41467-020-19332-5

Designing efficient organic solar cells is limited by the energy required to overcome the mutual Coulomb attraction between electron and hole. Here, the authors reveal long-lived and disorder-free charge-transfer states enable efficient endothermic charge separation in non-fullerene systems with marginal energy offset.

Nature Communications, Published online: 09 November 2020; doi:10.1038/s41467-020-19479-1

Chiral functional materials with circularly polarized luminescence can be used in various applications but rarely reported. Here the authors show, a complex system, which show intense circularly polarized ultraviolet luminescence with large glum value, enabling a chiral UV light triggered enantioselective polymerization.

A non‐fullerene‐acceptor is compared with a fullerene acceptor in organic photodiodes. The eh‐IDTBR‐based devices show 495% higher specific detectivity than the fullerene‐based devices. A response time comparable to that of the silicon photodiode is achieved from the non‐fullerene acceptor‐based devices. The non‐fullerene acceptor‐based devices demonstrate exceptional stability, withstanding external electrical and thermal stress during device operation.

Abstract

Herein, non‐fullerene acceptor‐based organic photodiodes are compared with fullerene acceptor‐based organic photodiodes. The non‐fullerene acceptor, ethylhexyl‐rhodanine‐benzothiadiazole‐coupled indacenodithiophene (eh‐IDTBR)‐based organic photodiodes show a higher detectivity (1.61 × 10¹³ cm Hz^1/2 W⁻¹) and a faster response time (≈2.7 µs) than the fullerene acceptor, [6,6]‐phenyl C₇₁ butyric acid methyl ester (PC₇₁BM)‐based organic photodiodes (3.25 × 10¹² cm Hz^1/2 W⁻¹ and ≈6.24 µs, respectively) owing to the excellent dark current suppression of the carrier injection and the low trap density of eh‐IDTBR. Moreover, the eh‐IDTBR‐based photodetector shows better operational device stability than the fullerene counterpart under electrical and thermal stress. This is corroborated by the morphology and crystallography analyses, both of which reveal that the eh‐IDTBR‐containing photo‐sensitive films remain intact after imposing an external stress. This study elucidates important key advantages of the non‐fullerene acceptor, eh‐IDTBR, and sets a milestone as it compares the non‐fullerene acceptors and fullerene acceptors in organic photodiodes for the first time. This work sets a new record for the performance and stability of solution‐processable non‐fullerene acceptor‐based organic photodiodes.

The relationship between structure design, packing arrangement, and molecular property of organic photovoltaic (OPV) acceptors is explored, in which the 3D network packing originating from non‐covalent intermolecular interactions and aggregation states, is found to promote OPV device performance. This review sheds light on charge transport processes in acceptors and provides a guideline for developing new generation OPV materials.

Abstract

The power conversion efficiency of organic solar cell (OSC) devices has surpassed 18% rapidly. In order to further promote OSC development, it is necessary to understand the packing information at the atomic level to help develop acceptor systems with superior performance. The packing arrangements and intermolecular interactions of these acceptors in the solid state, observed by single crystal X‐ray crystallography, are often used to design materials with expected physicochemical properties. In this review, the chemical structures of acceptors revealed by single crystal X‐ray crystallography are summarized, and the relationship between structural design, packing arrangement, and device properties is discussed. In addition, the concept of “3D network packing” in acceptor systems is proposed, which offers better charge transfer properties in reported chlorinated, fluorinated, brominated, and trifluoromethylated systems, an understanding of 3D network transport also provides guidance in high‐performance materials design. Finally, some current issues related to single crystal studies in OSCs are discussed, with an emphasis on the significance of developing acceptors by understanding and adjusting the aggregation states and intermolecular interactions of materials by single crystal analysis.

In organic light‐emitting diodes based on thermally activated delayed fluorescence of donor–acceptor blends, both the magneto‐electroluminescence response and electron magnetic resonance spectrum are found to be independent of the particular hydrogen isotope (protium or deuterium) present. This shows that the reverse intersystem crossing process from the triplet to singlet states occurs in the exciplex manifold rather than that of the polaron pair.

Abstract

The isotope effect is studied in the magneto‐electroluminescence (MEL) and pulsed electrically detected magnetic resonance of organic light‐emitting diodes based on thermally activated delayed fluorescence (TADF) from donor–acceptor exciplexes that are either protonated (H) or deuterated (D). It is found that at ambient temperature, the exchange of H to D has no effect on the spin‐dependent current and MEL responses in the devices. However, at cryogenic temperatures, where the reverse intersystem crossing (RISC) from triplet to singlet exciplex diminishes, a pronounced isotope effect is observed. These results show that the RISC process is not governed by the hyperfine interaction as thought previously, but proceeds through spin‐mixing in the triplet exciplex. The observations are corroborated by electrically detected transient spin nutation experiments that show relatively long dephasing time at ambient temperature, and interpreted in the context of a model that involves exchange and hyperfine interactions in the spin triplet exciplex. These findings deepen the understanding of the RISC process in TADF materials.

A new DPP‐based alternating D–A copolymer (PD2FCT‐29DPP) is developed for use as a hole‐transport layer. PD2FCT‐29DPP addresses the different requirements for an HTL, offering favorable energetics, near‐infrared absorption, and efficient charge transfer. Therefore, a PD2FCT‐29DPP‐based device achieves a remarkable FF of 70% and the highest PCE of 14.0% among PbS CQD‐SCs.

Abstract

The need for optoelectronic and chemical compatibility between the layers in colloidal quantum dot (CQD) photovoltaic devices remains a bottleneck in further increasing performance. Conjugated polymers are promising candidates as new hole‐transport layer (HTL) materials in CQD solar cells (CQD‐SCs) owing to the highly tunable optoelectronic properties and compatible chemistries. A diketopyrrolopyrrole‐based polymer with benzothiadiazole derivatives (PD2FCT‐29DPP) as an HTL in these devices is reported. The energy level, molecular orientation, and hole mobility of this HTL are manipulated through molecular engineering. By levering the polymer's optical absorption spectrum complementary to that of the CQD active layer, EQE across the visible and near‐infrared regions is maximized. As a result, a PD2FCT‐29DPP‐based device exhibits a fill factor of 70% and approximately 35% efficiency enhancement compared to a PTB7‐based device.

A thermally activated delayed fluorescence emitter, 2tDMG, is designed and synthesized based on the donor (D)/acceptor (A) spatially intramolecular noncovalent interaction. The D/A units are connected via a rigid linker, thereby confining them into a close‐packed coplanar configuration for small singlet–triplet splitting energy. 2tDMG achieves a high external quantum efficiency of 30.8% with a low efficiency roll‐off in evaporation‐processed organic light‐emitting diodes (OLEDs).

Abstract

In this work, two novel thermally activated delayed fluorescence (TADF) emitters, 2tDMG and 3tDMG, are synthesized for high‐efficiency organic light‐emitting diodes (OLEDs), The two emitters have a tilted face‐to‐face alignment of donor (D)/acceptor (A) units presenting intramolecular noncovalent interactions. The two TADF materials are deposited either by an evaporation‐process or by a solution‐process, both of them leading to high OLED performance. 2tDMG used as the emitter in evaporation‐processed OLEDs achieves a high external quantum efficiency (EQE) of 30.8% with a very flat efficiency roll‐off of 7% at 1000 cd m⁻². The solution‐processed OLEDs also display an interesting EQE of 16.2%. 3tDMG shows improved solubility and solution processability as compared to 2tDMG, and thus a high EQE of 20.2% in solution‐processed OLEDs is recorded. The corresponding evaporation‐processed OLEDs also reach a reasonably high EQE of 26.3%. Encouragingly, this work provides a novel strategy to address the imperious demands for OLEDs with high EQE and low roll‐off.

Two well‐regular polymer acceptors (PY‐IT and PY‐OT) with different polymerization sites are developed. For comparison, a random ternary copolymer (PY‐IOT) with the same ratio of the two acceptors is synthesized. All‐polymer solar cells (PSCs) based on PM6:PY‐IT achieve an excellent PCE of 15.05%, which is significantly higher than those based on PY‐OT (10.04%) and PY‐IOT (12.12%).

Abstract

Recent advances in the development of polymerized A–D–A‐type small‐molecule acceptors (SMAs) have promoted the power conversion efficiency (PCE) of all‐polymer solar cells (all‐PSCs) over 13%. However, the monomer of an SMA typically consists of a mixture of three isomers due to the regio‐isomeric brominated end groups (IC‐Br(in) and IC‐Br(out)). In this work, the two isomeric end groups are successfully separated, the regioisomeric issue is solved, and three polymer acceptors, named PY‐IT, PY‐OT, and PY‐IOT, are developed, where PY‐IOT is a random terpolymer with the same ratio of the two acceptors. Interestingly, from PY‐OT, PY‐IOT to PY‐IT, the absorption edge gradually redshifts and electron mobility progressively increases. Theory calculation indicates that the LUMOs are distributed on the entire molecular backbone of PY‐IT, contributing to the enhanced electron transport. Consequently, the PM6:PY‐IT system achieves an excellent PCE of 15.05%, significantly higher than those for PY‐OT (10.04%) and PY‐IOT (12.12%). Morphological and device characterization reveals that the highest PCE for the PY‐IT‐based device is the fruit of enhanced absorption, more balanced charge transport, and favorable morphology. This work demonstrates that the site of polymerization on SMAs strongly affects device performance, offering insights into the development of efficient polymer acceptors for all‐PSCs.