As-cast Devices Research Articles

Manipulating Alkyl Inner Side Chain of Acceptor for Efficient As-Cast Organic Solar Cells.

As-cast organic solar cells (OSCs) possess tremendous potential for low-cost commercial applications. Herein, five small-molecule acceptors (A1-A5) are designed and synthesized by selectively and elaborately extending the alkyl inner side chain flanking on the pyrrole motif to prepare efficient as-cast devices. As the extension of the alkyl chain, the absorption spectra of the films are gradually blue-shifted from A1 to A5 along with slightly uplifted lowest unoccupied molecular orbital energy levels, which is conducive for optimizing the trade-off between short-circuit current density and open-circuit voltage of the devices. Moreover, a longer alkyl chain improves compatibility between the acceptor and donor. The in situ technique clarifies that good compatibility will prolong molecular assembly time and assist in the preferential formation of the donor phase, where the acceptor precipitates in the framework formed by the donor. The corresponding film-formation dynamics facilitate the realization of favorable film morphology with a suitable fibrillar structure, molecular stacking, and vertical phase separation, resulting in an incremental fill factor from A1 to A5-based devices. Consequently, the A3-based as-cast OSCs achieve a top-ranked efficiency of 18.29%. This work proposes an ingenious strategy to manipulate intermolecular interactions and control the film-formation process for constructing high-performance as-cast devices.

High Performance As-Cast Organic Solar Cells Enabled by a Refined Double-Fibril Network Morphology and Improved Dielectric Constant of Active Layer.

High performance organic solar cells (OSCs) are usually realized by using post-treatment and/or additive, which can induce the formation of metastable morphology, leading to unfavorable device stability. In terms of the industrial production, the development of high efficiency as-cast OSCs is crucially important, but it remains a great challenge to obtain appropriate active layer morphology and high power conversion efficiency (PCE). Here, efficient as-cast OSCs are constructed via introducing a new polymer acceptor PY-TPT with a high dielectric constant into the D18:L8-BO blend to form a double-fibril network morphology. Besides, the incorporation of PY-TPT enables an enhanced dielectric constant and lower exciton binding energy of active layer. Therefore, efficient exciton dissociation and charge transport are realized in D18:L8-BO:PY-TPT-based device, affording a record-high PCE of 18.60% and excellent photostability in absence of post-treatment. Moreover, green solvent-processed devices, thick-film (300nm) devices, and module (16.60 cm2) are fabricated, which show PCEs of 17.45%, 17.54%, and 13.84%, respectively. This work brings new insight into the construction of efficient as-cast devices, pushing forward the practical application of OSCs.

2,5-dichloro-3,4-diiodothiophene as a versatile solid additive for high-performance organic solar cells

Volatile solid additive (SA) with diverse chemical structures has been proved to be an alternative to prepare high-efficient organic solar cells (OSCs). While, various organic photovoltaic materials raise challenges in selecting suitable SAs during the device optimization. Herein, based on the simple and commonly used thiophene unit, we proposed a perhalogenated 2,5-dichloro-3,4-diiodothiophene (SA-T5) as the SA for the device optimizations. As results, the capacity of SA-T5 in optimizing molecular packings of small-molecular acceptor L8-BO, dimeric acceptor DL8, and polymeric acceptor PY-DT based blend films has been demonstated. Their optimized morphologies assisted by SA-T5 contribute to the suppressed charge recombination and enhanced charge transportation, resulting in their improved short-circuit current density and fill factor, simultaneously. In comparsion with as-cast devices, SA-T5-treated devices showed significantly improved power conversion efficiencies (PCEs) of 18.8 % for PM6:L8-BO, 17.8 % for PM6:DL8, and 17.6 % for PM6:PY-DT, respectively. Furthermore, the potential of SA-T5 in obtaining high-performance ternary OSCs was also confirmed, for example, a 18.1 % efficiency was realized in PM6:DL8:L8-BO based device. To our knowledge, the performance of these SA-T5-treated devices reaches the best results of different acceptor-based OSCs. Importantly, this work provides a rare and successful case to develop versatile SAs for high-performance OSCs.

Binary All-polymer Solar Cells with a Perhalogenated-Thiophene-Based Solid Additive Surpass 18 % Efficiency.

Morphological control of all-polymer blends is quintessential yet challenging in fabricating high-performance organic solar cells. Recently, solid additives (SAs) have been approved to be capable in tuning the morphology of polymer: small-molecule blends improving the performance and stability of devices. Herein, three perhalogenated thiophenes, which are 3,4-dibromo-2,5-diiodothiophene (SA-T1), 2,5-dibromo-3,4-diiodothiophene (SA-T2), and 2,3-dibromo-4,5-diiodothiophene (SA-T3), were adopted as SAs to optimize the performance of all-polymer organic solar cells (APSCs). For the blend of PM6 and PY-IT, benefitting from the intermolecular interactions between perhalogenated thiophenes and polymers, the molecular packing properties could be finely regulated after introducing these SAs. In situ UV/Vis measurement revealed that these SAs could assist morphological character evolution in the all-polymer blend, leading to their optimal morphologies. Compared to the as-cast device of PM6 : PY-IT, all SA-treated binary devices displayed enhanced power conversion efficiencies of 17.4-18.3 % with obviously elevated short-circuit current densities and fill factors. To our knowledge, the PCE of 18.3 % for SA-T1-treated binary ranks the highest among all binary APSCs to date. Meanwhile, the universality of SA-T1 in other all-polymer blends is demonstrated with unanimously improved device performance. This work provide a new pathway in realizing high-performance APSCs.

Molecular insights of non-fused nonfullerene acceptor comprising a different central core for high efficiency organic solar cell

In this study, we applied a strategy which is systematically varying the central’ core in non-fused NFA structure to modulate the optical-electrochemical properties and photovoltaic performance. The optical and structural calculation was analyzed via UV–Vis spectroscopy and DFT calculation. The results exhibit that strong NIR absorption and good planar backbone structure on those non-fused NFAs. Consequently, photovoltaic device based on TT linked acceptor delivered a promising photovoltaic efficiency of 8.37% for as-cast device. These results suggest that the change of central unit is an effective strategy to improve the optoelectronic properties of non-fused NFAs in organic solar cells.

A Two-Step Heating Strategy for Nonhalogen Solvent-Processed Organic Solar Cells Based on a Low-Cost Polymer Donor

Fabrication of high-efficiency organic solar cells (OSCs) using nonhalogen green solvents is highly desired for commercialization. However, the morphology control of a nonhalogen solvent-processed active layer remains challenging because of poor solubility of the commonly used photovoltaic materials in nonhalogen solvents. Herein, we propose a two-step heating strategy to optimize the morphology of a photovoltaic active layer based on a low-cost polymer donor X1 and a small-molecule acceptor (SMA) BTP-eC9 processed from a mixed nonhalogen solvent. First, heating the blend solution can robustly enhance the solubility of the SMA and thus processability of the blend, producing a uniform active layer. Second, further elevating the substrate temperature can shorten the film-formation time and tune the size of the polymer aggregates and SMA crystallites, to form bicontinuous networks in the active layer. As a result, this two-step heating strategy endows the film with improved charge generation, transport, and collection and thus a much higher device efficiency (ca. 4.6 times of that for the as-cast device) in comparison to those from nonoptimal solution and/or substrate temperatures. This work emphasizes the crucial role of regulating processing conditions in fabricating the nonhalogen solvent-processed high-efficiency OSCs.

π-Extension and chlorination of non-fullerene acceptors enable more readily processable and sustainable high-performance organic solar cells

Organic solar cells (OSCs) processed without halogenated solvents and complex treatments are essential for future commercialization. Herein, we report three novel small molecule acceptors (NFAs) consisting of a Y6-like core but with π-extended naphthalene with progressively more chlorinated end-capping groups and a longer branched chain on the Nitrogen atom. These NFAs exhibit good solubilities in non-chlorinated organic solvents, broad optical absorptions, close π-π stacking distances (3.63–3.84 Å), and high electron mobilities (∼10−3 cm2 V−1 s−1). The o-xylene processed and as-cast binary devices using PM6 as the donor polymer exhibit a PCE increasing upon progressive chlorination of the naphthalene end-capping group from 8.93% for YN to 14.38% for YN-Cl to 15.00% for YN-2Cl. Furthermore similarly processed ternary OSCs were fabricated by employing YN-Cl and YN-2Cl as the third component of PM6:CH1007 blends (PCE = 15.75%). Compared to all binary devices, the ternary PM6:CH1007:YN-Cl (1:1:0.2) and PM6:CH1007:YN-2Cl (1:1:0.2) cells exhibit significantly improved PCEs of 16.49% and 15.88%, respectively, which are among the highest values reported to date for non-halogenated solvent processed OSCs without using any additives and blend post-deposition treatments.

Improved ultraviolet stability of fullerene-based organic solar cells through light-induced enlargement and crystallization of fullerene domains

• Ultraviolet stability of fullerene-based solar cells • Impact of thermal annealing on efficiency time evolution under illumination • Morphological and structural analyses of blend layers • Enhanced crystallinity and size of fullerene enriched domains under illumination Organic solar cells (OSCs) are a promising technology with the potential for low-cost manufacturing. However, to translate to economically viable applications, long-term stability is a fundamental requirement. Amongst intrinsic degradation pathways the sensitivity of OSC to ultraviolet (UV) light severely limits their photostability. Here, we focus on the impact of UV on the stability of solar cells based on well-known fullerene-based blends processed with 1,8-diidooctane (DIO) as additive. The post-annealed devices resulting in DIO-free blends are directly compared to as-cast devices containing residual DIO. After a pronounced initial burn-in, as-cast devices demonstrate a self-healing effect leading to stable solar cells under prolonged exposure to UV light. This initial burn-in can be considerably reduced in annealed devices with a suitable heating process, resulting in very stable solar cells under UV-containing light over a long time period. Under UV-free LED light, solar cells are stable, which implies a direct impact of UV on the performance evolution of devices. Advanced characterization techniques were used for in-depth morphological analyses under light exposure to distinguish the observed UV-related processes in the polymer blends. Our results point thus towards the presence of two processes occurring under UV-light within as-cast devices involving fullerenes, one causing a performance degradation and the other allowing a repair tending towards a performance stability. Due to an improved initial crystal order within annealed devices, the process related to the degradation is in the minority. The UV stability of devices can be attributed to the UV light-induced diffusion of fullerenes, leading jointly to the enlargement of the initial existing fullerene domains and to their crystallization under UV light. These results path the way for a better understanding of the stability of efficient normal OSCs under simulated sunlight.

Unfused Acceptors Matching π-Bridge Blocks with Proper Frameworks Enable Over 12% As-Cast Organic Solar Cells.

Emerging unfused-ring acceptors (UFAs) have been explored in pursuit of low-cost high-efficient organic solar cells (OSCs). Assembling unfused building blocks into proper frameworks are challenging for the molecular design of UFAs. The authors report herein four UFAs adopting either dithiophene cyclopentadiene (DTC) or dithieno[3,2-b:2',3'-d]pyrrole (DTP) as π-bridge units with different molecular frameworks for high-efficient as-cast OSCs. All these acceptors exhibit strong near-infrared absorption and narrow optical band gap (Eg opt <1.50eV).DTC-bridged symmetric and DTP-bridged asymmetric UFAs exhibit higher planar conformation as well as suitable miscibility and homogeneous phase separation when blending with polymer donor PBDB-T to promote efficient charge transport in the blends. Their blends with PBDB-T contribute optimal PCE of 12.17% and 11.92% in as-cast OSCs, among the highest values for UFAs based as-cast devices in the literature. Experimental and theoretical simulations systematically reveal the impact of manipulating the molecular framework of UFAs on their conformation, optoelectronic, and photovoltaic performance. The results indicate the matching π-bridge units with molecular frameworks as an attractive approach to design UFAs for high-performance as-cast OSCs.

The synergistic effects of central core size and end group engineering on performance of narrow bandgap nonfullerene acceptors

Understanding the relationship between the molecular structure and photoelectric properties of fused-ring non-fullerene acceptors (NFAs) is of far-reaching significance to the development of organic solar cells (OSCs). Herein, six NFAs based on multiple thiophenes (4 T, 6 T and 8 T) are employed to systematically probe the synergistic effects of extending central core size and terminal fluorination. The absorption results manifest that the molecular absorption is comprehensively affected by molecular crystallinity, planarity, conjugation length and intramolecular electron push–pull effect, simultaneously. The intensity of electron push–pull effect is not only related to the electron-donating ability of central core and the electron-withdrawing ability of end group, but also may be related to the distance between the positive and negative centers. The extension of central core leads to the more planar backbone and stronger crystallinity of NFAs, and less energy loss (Eloss) in its based OSC. Compared with the extension of central core, terminal fluorination has a greater impact on molecular photoelectric properties. The terminal fluorination significantly enhances the push–pull effect, lowers the energy levels, and slightly increases the vibrational relaxation. As a result, the strongest crystallinity and coplanarity of 8TIC-4F lead to a low vibrational relaxation of 0.18 eV, which makes PTB7-Th:8TIC-4F device exhibit a small Eloss of 0.51 eV and a high efficiency of 10.4%. In addition, the fluorinated 6TIC-4F with suitable core size exhibits suitable energy level, absorption, crystallization, and phase separation morphology, making its as-cast device up to 11.61% efficiency. To the best of the authors’ knowledge, the PCE of 11.61% for PTB7-Th:6TIC-4F based device without any treatment is one of the highest values reported for the NAFs with over 1000 nm absorption.

Simple benzothiadiazole-based small molecules as additives for efficient organic solar cells

Multiple strategies have been developed to improve the photovoltaic performance of organic solar cells (OSCs) by fine-tuning the morphology of the active layers, such as introducing functional third components, taking post-treatments and so on. In this work, a series of non-volatile solid additives BT, BTOR, BT2F, BTNO2(2 + 6), and BTNO2(6) were synthesized, by introducing substituent groups into benzothiadiazole units, and the thermal stability, optical and electrochemical properties were systematically studied. These additives were applied to further investigate the effect on the photovoltaic performance of OSCs. The results show that these additives could effectively promote exciton dissociation and charge transfer, reduce bimolecular recombination, and optimize the morphology of the active layer. Significantly, the PBDB-T:ITIC-based devices processed with BT achieved a power conversion efficiency (PCE) of 11.57%, increased by 17% compared to the as-cast devices due to the high and balanced carrier mobility. Enhancement of PCE proved the feasibility of the application of non-volatile solid additives in the non-fullerene-based binary system.

Intermolecular interaction induced spontaneous aggregation enables over 14% efficiency as-cast nonfullerene solar cells

Organic solar cells (OSCs) composed of polymer donors and electron acceptors are facing the challenge to modulate the aggregation-induced morphology in pristine phases and active layers. We herein report our effective chemical modification on thieno[2′',3′':5′,6′]-s-indaceno[2′,1′:4,5]thieno[3,2–b]thieno[2,3–d]pyrrole cored fused-ring electron acceptors (FREAs). By introducing either para-F, -Cl or -methyl onto meta-hexyloxy phenyl side chain, we can finetune the surface electrostatic potentials and aggregation behaviours of the FREAs (IMOF, IMOCl and IMOM) and their intermolecular interactions with polymer donor (PM6). Consequently, F-substituted acceptor IMOF presents higher crystallinity and induces intensified aggregation in blends with PM6. The PM6:IMOF (1:1.2, w/w) as-cast binary OSCs contribute the highest Power conversion efficiency (PCE) of 13.89% and a fill factor of 73.59%. By adding 10 wt% PC71BM, the as-cast ternary devices exhibit an improved PCE of 14.42%. In contrast, the PM6 based binary OSCs with three reference acceptors deliver the maximum PCE of only 12.61%. Such molecular interactions induced aggregation can be further promoted by device optimization via processing additive and thermal annealing. The sidechain engineering to induce intermolecular interactions of FREAs demonstrates as an effective strategy to regulate the molecular aggregation and phase morphology in active layers for high-performance as-cast OSCs.

Improvement in power conversion efficiency of all-polymer solar cells enabled by ultrafast channels for charge dynamics

Despite the rapid breakthrough of polymerized fused-ring small molecular acceptors, developing accessible and inexpensive polymer acceptors for high performance all-polymer solar cells (all-PSCs) and revealing the fundamental work mechanism of all-PSCs is highly desirable. In addition, the isomeric issue of polymerized fused-ring small molecular acceptors still needs to be concerned. In this work, we report an accessible and inexpensive polymer acceptor P2TF by polymerization of 2,2'-((2Z,2Z)-((4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b']dith-iophene-2,7-diyl)bis (me-thanylylidene))bis (3-oxo-2,3-dihydro-1H-indene-2,1-diylide-ne)) dimalononitrile-based small molecular. Molecular electrostatic potentials and permanent dipole moments are evaluated by theoretical calculations, revealing that structural isomerization has a negligible impact on their optoelectronic properties. Femtosecond-resolved transient absorption, device charge dynamic measurements and space-charge limited current method are performed to systematically investigate charge dynamics in all-PSCs. The PM6:P2TF-based as-cast device exhibits poor power conversion efficiency of 4.51%, while the optimized device achieves a superior power conversion efficiency of 10.65%. The significantly improved performance of the optimized device originates from ultrafast channels of charge dynamics for ultrafast charge transfer, efficient charge transport and suppressed charge recombination, which is strongly related to the well-established morphology induced by optimal treatment. Furthermore, the PM6:P2TF also has excellent bending property, indicating its great potential for application in flexible all-PSCs.

Improving the field-effect transistor performance of (E)-1,2-di(thiophen-2-yl)ethenyl-co-naphthalenyl-based polymers by introducing alkoxy sidechains

Three donor-donor type conjugated polymers (namely PTVTN1, PTVTN2 and PTVTN3) are designed and synthesized containing naphthalene unit with different sidechain substitutions. The introducing of alkoxy sidechains on the naphthalene unit could mitigate the polymer backbone twisting through S···O non-covalent interactions and therefore improve the charge carrier mobility. Besides, different sidechain positions also have a significant impact on the molecular orientation and charge carrier mobility. The best field-effect transistor (FET) mobility of 0.014 cm2/Vs is achieved based on PTVTN3 as-cast device, which is due to the favorable edge-on orientation, interdigitation and fine fiber network morphology. This work shows that how sidechains influence the polymer morphology and charge carrier mobility.

Design of All-Fused-Ring Electron Acceptors with High Thermal, Chemical, and Photochemical Stability for Organic Photovoltaics

High-performance donor–acceptor electron acceptors containing 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (INCN)-type terminals are labile toward photooxidation and basic conditions, and ...

Identifying tunneling effects of poly(aryl ether) matrices and boosting the efficiency, stability, and stretchability of organic solar cells

Tunneling effects are one of the most basic theories that have long been overlooked in the bulk-heterojunction (BHJ) layers of organic solar cells (OSCs). Herein, by fabricating BHJ and layer-by-layer (LbL) devices with five different poly(aryl ether)s (PAEs), we reveal a tunneling effect in the active layer of OSCs. The results show that the as-cast devices with 5 wt % and even 30 wt % PAE in PM6:Y6-blends achieve high power conversion efficiencies (PCEs) over 15%. Furthermore, the 5 wt % PAE-based devices with proper post-treatment deliver a PCE of 17.03%. By inserting a thin PAE layer with high dielectric constant, the LbL OSCs (PM6/PAE-237/Y6) yield a 23% improvement PCE compared to one without PAEs. With rational molecular design, we demonstrate that heat-resistant PAE matrices with high stretchability simultaneously improve efficiency, stability, and mechanical flexibility of OSCs. The proposed tunneling effect and PAE matrices strategy have important implications for fabricating efficient, stable, and flexible organic devices.

Achieving 16.68% efficiency ternary as-cast organic solar cells

State-of-the-art organic solar cells (OSCs) often require the use of high-boiling point additive or post-treatment such as temperature annealing and solvent vapor annealing to achieve the best efficiency. However, additives are not desirable in large-scale industrial printing process, while post-treatment also increases the production cost. In this article, we report highly efficient ternary OSCs based on PM6:BTP-ClBr1:BTP-2O-4Cl-C12 (weight ratio=1:1:0.2), with 16.68% power conversion efficiency (PCE) for as-cast device, relatively close to its annealed counterpart (17.19%). Apart from obvious energy tuning effect and complementary absorption spectra, the improved PCE of ternary device is mainly attributed to improved morphological properties including the more favorable materials miscibility, crystallinity, domain size and vertical phase separation, which endorse suppressed recombination. The result of this work provides understanding and guidance for high-performance as-cast OSCs through the ternary strategy.

High performance as-cast P3HT:PCBM devices: understanding the role of molecular weight in high regioregularity P3HT

Four batches of P3HT with varied molecular weight (MW) but constant regioregularity (RR) are investigated. When RR is fixed at 100%, solar cell efficiency is less sensitive to MW with high efficiencies achieved for as-cast devices.

Insights into out-of-plane side chains effects on optoelectronic and photovoltaic properties of simple non-fused electron acceptors

Non-fused ring electron acceptors with varied substituents were developed to construct efficient organic solar cells (OSCs). Out-of-plane rigid substituents such as 2-methylphenyl, 2-tert-butylphenyl and diphenylamine were introduced to the central benzene unit to improve the solubility of non-fused ring acceptors and noncovalent interactions such as O⋯S and N⋯S were used to enhance the planarity of molecular backbones. Blend films based on PBDB-T and these acceptors displayed better morphology without oversized aggregates formed. As-cast devices based on SM1, SM2 and SM3 exhibited power conversion efficiencies of 5.96%, 6.43% and 6.80%, respectively.

Wide bandgap donor polymers containing carbonyl groups for efficient non-fullerene polymer solar cells

Two wide bandgap conjugated polymers (PTCO8 and PTCO1) with different side chains, composed of alkylcarbonyl-substituted thiophene (TOC) as the electron-withdrawing unit and two-dimension benzodithiophene (BDT) as the electron-donating unit, are developed for non-fullerene polymer solar cells (PSCs). Relative to non-carbonyl-substituted control polymer PTC8, PTCO8 and PTCO1 exhibit a red-shifted absorption and deeper-lying highest occupied molecular orbital (HOMO) level. When blended with ITIC, the as-cast PTCO1-based devices yield a power conversion efficiency of up to 9.33% with an open-circuit voltage of 0.95 V, a short-circuit current density of 16.91 mA cm−2, and a fill factor of 57.9%, outperforming those of PTC8-based devices (4.04%) and PTCO8-based devices (5.91%). This work demonstrates that the carbonyl group is a promising electron-deficient moiety to construct wide-bandgap polymers for non-fullerene PSCs. Adjusting the length of side chains is a feasible strategy to improve PSC performance further.