Acceptors For Organic Solar Cells Research Articles

Unifying Charge Generation, Recombination, and Extraction in Low‐Offset Non‐Fullerene Acceptor Organic Solar Cells

AbstractEven though significant breakthroughs with over 18% power conversion efficiencies (PCEs) in polymer:non‐fullerene acceptor (NFA) bulk heterojunction organic solar cells (OSCs) have been achieved, not many studies have focused on acquiring a comprehensive understanding of the underlying mechanisms governing these systems. This is because it can be challenging to delineate device photophysics in polymer:NFA blends comprehensively, and even more complicated to trace the origins of the differences in device photophysics to the subtle differences in energetics and morphology. Here, a systematic study of a series of polymer:NFA blends is conducted to unify and correlate the cumulative effects of i) voltage losses, ii) charge generation efficiencies, iii) non‐geminate recombination and extraction dynamics, and iv) nuanced morphological differences with device performances. Most importantly, a deconvolution of the major loss processes in polymer:NFA blends and their connections to the complex BHJ morphology and energetics are established. An extension to advanced morphological techniques, such as solid‐state NMR (for atomic level insights on the local ordering and donor:acceptor ππ interactions) and resonant soft X‐ray scattering (for donor and acceptor interfacial area and domain spacings), provide detailed insights on how efficient charge generation, transport, and extraction processes can outweigh increased voltage losses to yield high PCEs.

Subtle Morphology Control with Binary Additives for High-Efficiency Non-Fullerene Acceptor Organic Solar Cells.

Adding an additive is one of the effective strategies to fine-tune active layer morphology and improve performance of organic solar cells. In this work, a binary additive 1,8-diiodooctane (DIO) and 2,6-dimethoxynaphthalene (DMON) to optimize the morphology of PBDB-T:TTC8-O1-4F-based devices is reported. With the binary additive, a power conversion efficiency (PCE) of 13.22% was achieved, which is higher than those of devices using DIO (12.05%) or DMON (11.19%) individually. Comparison studies demonstrate that DIO can induce the acceptor TTC8-O1-4F to form ordered packing, while DMON can inhibit excessive aggregation of the donor and acceptor. With the synergistic effect of these two additives, the PBDB-T:TTC8-O1-4F blend film with DIO and DMON exhibits a suitable phase separation and crystallite size, leading to a high short-circuit current density (Jsc) of 23.04 mA·cm-2 and a fill factor of 0.703 and thus improved PCE.

Designing indenothiophene-based acceptor materials with efficient photovoltaic parameters for fullerene-free organic solar cells.

Non-fullerene small molecular acceptors (NFSMAs) exhibit promising photovoltaic performance which promoted the rapid progress of organic solar cells (OSCs). In this study, an attempt is done to explore indenothiophene-based high-performance small molecular electron acceptors for organic solar cells. We have designed five acceptor molecules (M1-M5) with strong donor moiety indenothiophene linked to five different end-capped group acceptor moieties: diflouro-2-methylene-3-oxo-2,3-dihydroindene-1-ylidene)malononitrile (A1), 1-(dicyanomethylene)-2-methylene-3-oxo-2,3-dihydro-1H-indene-5,6-dicarbonitrile (A2), methyl-6-cyano-3-(dicyanomethylene)-2-methylene-1-oxo-2,3-dihydro-1H-indene-5-carboylate (A3), 2-(6-cyano-5-fluoro-2-methylene-3-oxo-2,3 dihydro-1H-indene-1-ylidene)malononitrile (A4), and (Z)-methyl 3-(benzo [c][1,2,5]thiadiazol-4-yl)-2-cyanoacrylate (A5) respectively. The structure-property relationship was studied and effects of structural modification on the optoelectronic properties of these acceptors (M1-M5) were determined systematically by comparing it with reference molecule R, which is recently reported as excellent non-fullerene-based small acceptor molecule. Among all designed molecules, M5 is proven as a suitable candidate for organic solar cell applications due to better photovoltaic properties including narrow HOMO-LUMO energy gap (2.11 eV), smallest electron mobility (λe= 0.0038eV), highest λmax values (702.82 nm in gas) and (663.09 nm in chloroform solvent) and highest open-circuit voltage (Voc = 1.49V) with respect to HOMOPTB7-Th-LUMOacceptor. Our results indicate that introducing more end-capped electron-accepting units is a simple and effective alternative strategy for the design of promising NFSMAs. This theoretical framework also proves that the conceptualized NFSMAs are superior and thus are recommended for the future construction of high-performance organic solar cell devices. Graphical abstract.

(Invited) Highly Selective and Scalable Fullerene-Cation-Mediated Synthesis Accessing Cyclo[60]Fullerenes with 5-Membered-Carbon-Ring and Their Application to Perovskite Solar Cells

Fullerene and its derivatives have attracted much attention owing to their versatile applicability in the fields of photovoltaics, bioscience, and space science. In thin-film photovoltaics, cyclo[60]fullerenes have been used as electron acceptors in organic solar cells and as electron-transporting layers (ETLs) in perovskite solar cells. In particular, cyclo[60]fullerenes as an over-coating layer of metal oxide ETLs have been one of the most exploited applications in perovskite solar cells, demonstrating a dramatic reduction hysteresis and enhancement of the device charge-dynamics. With respect to such applications, heterocyclo[60]fullerene, which is one of the most abundant types of cyclo[60]fullerene, have shown relatively poor performance compared with full-carbon-ring cyclo[60]fullerenes because of their electrochemical instability. Accordingly, cyclo[60]fullerenes with a full-carbon-ring, such as 3-membered-carbon-rings (e.g. PC61BM) and 6-membered-carbon-rings (e.g. ICBA, MIF),have been the preferred choices for the over-coating layers of metal oxide ETLs. Yet, cyclo[60]fullerenes with a 5-membered-carbon-ring, namely indano[60]fullerenes, have never been demonstrated to date. This is because the existing synthetic methods exhibit low yields and a limited substrate scope of fullerene derivatives. In this work, we report a cyclo[60]fullerene synthesis mediated by fullerene-cation intermediates, which accesses cyclo[60]fullerenes with a 5-membered-carbon-ring in scalable yields. The fullerene-cation-mediated reactions can render more facile and controlled derivatization of fullerenes than conventional fullerene-anion-mediated or fullerene-radical-mediated reactions. This is because the fullerene-cation-mediated reaction harnesses in situ-generated fullerene cations, which are highly reactive due to exceptionally low energy level of the newly formed lowest unoccupied molecular orbitals (LUMOs). Such high reactivity of the intermediates leads to high selectivity, thus a scalable yield with an excellent functional group tolerance. Accordingly, this fullerene-cation-mediated methodology enabled the production of rare fullerenes, indano[60]fullerene derivatives in this work. The synthesis of indano[60]fullerenes showed high yields as expected (the highest ca. 93%). The mechanism of this new synthetic route was investigated and discussed as well. Moreover, we explored the device application of indano[60]fullerenes in perovskite solar cells as 3- or 6-membered-carbon-rings and 5-membered-heteroatom-rings have been widely used in devices. We found that the hydrophobicity, solubility, reorganization energy of fullerene, and the ability to passivate the perovskite interface were crucial in obtaining high power conversion efficiency (PCE). Among different fullerene derivatives synthesized in this work, indano[60]fullerene (5a) gave a PCE of 20.7% when used as the over-coating layer in perovskite solar cells. The obtained PCE was higher than 19.0% and 16.5% of the reference devices in which C60 and PC61BM were used, respectively. Not only was it, the efficiency of 20.7% stands the highest among the reported fullerene over-coating ETL-based NH3CH3PbI3(MAPbI3)-used perovskite solar cells. Figure 1

Synthesis and Photophysical Properties of Bromination the Small-Molecule Acceptor for Organic Solar Cells

At present, the modification of the benzene ring based terminal group IC is widely studied and a series of progress has been made. Our work designed and synthesized a new brominated fused ring small molecule receptor FBDTEH-2Br by introducing bromine atoms into terminal group. On the one hand, the electron-withdrawing effect of bromine enhances the push-pull electron effect between the core electron-donating segment and the end-capping group, making the absorption spectrum of FBDTEH-2Br more suitable. It shows broad absorption in the range of 600–800 nm in solution, the maximum absorption peak of FBDTEH-2Br in solution is 742nm, While for the thin film, the absorption extended broader with an obvious red-shift in the range of 800–900 nm, it is beneficial to higher JSC. The maximum absorption peak is 800nm in the film, which can be explained by it’s nearly planar structure and the good intermolecular π-π stacking in the solid state caused by the dibromide IC. On the other hand, the introduction of Br atoms will also enhance the intermolecular interactions and improve the electron mobility. The optical band gap of FBDTEH-2Br is 1.43 eV. These results demonstrated that due to the bromine atom has a larger size, the end groups of adjacent molecules still exhibit strong interactions with short π-π distance Because of the O···S and Br···S interactions, which is beneficial for the charge to transport along multidirections. Therefore, the introduction of bromine atoms into theterminal group to design and synthesize new brominated small molecule acceptors is expected to obtain better performance in organic solar cell materials.

High Photoelectric Quantum Yield in Donor–Acceptor Bulk Heterojunction Organic Solar Cells

Reasons behind a higher photoelectric quantum yield in donor–acceptor organic solar cells (D-A--OSCs) than in one-component OSCs have been considered: (1) the appearance of an intense charge-transfer (CT) vibronic band in the absorption spectrum; (2) a change in the spectrum of CT states, in which the molecular exciton (ME) zone is located above the lowest charge-transfer state (LCTS); and (3) a longer lifetime of the LCTS as compared to the ME. Cause 1 makes it possible to select the optimal OCS absorption spectrum corresponding to the solar spectrum, and causes 2 and 3 eliminate the need for fast charge separation in the LCTS, for which the rate by ionization in the bulk heterojunction field is sufficient.

An Imine Photocyclization as an Alternative to the Pictet-Spengler Reaction for the Synthesis of AzaBenzannulated Perylenediimide Dyes.

AzaBenzannulated perylenediimide (AzaBPDI) dyes were synthesized in high yields via a new reaction sequence involving an imine condensation followed by visible light-induced photocyclization. The large scope and efficiency of this alternative to the Pictet-Spengler reaction are demonstrated, allowing easy preparation of dimeric AzaBPDI as potential non-fullerene acceptors for organic solar cells.

Effect of fluorination on exciton binding energy and electronic coupling in small molecule acceptors for organic solar cells

A detail study was performed to get deeper understanding of fluorination impact on intermolecular coupling and intermolecular interactions. Results showed that fluorine substitution plays an important role to modify the charge distribution or electrostatic potential as well non-covalent interaction between molecules. Dipole moment difference between ground and first excited states has increased while exciton binding energy has decreased by increasing the fluorine atoms. SM3 with asymmetric fluorine substitution showed higher transfer integral due to stronger attractive interactions. By proper substitution of fluorine atom, π-π stacking distance can be reduced to achieve higher electronic coupling. Substitution of fluorine atoms has increased the reorganization energy and non-linear optical response.

Designing alkoxy-induced based high performance near infrared sensitive small molecule acceptors for organic solar cells

Scientist are dedicated to design and synthesize efficient photovoltaic materials to overcome the energy crises. In this regard, herein, we have designed four new small acceptor molecules namely ( A1, A2, A3 and A4 ) for better performance in organic solar cells. These molecules consist Alkoxy-Induced Naphtho-dithiophene core unit flanked with 2,2-ethylidene-5,6-dicyano-3-oxo-2,3-dihydroinden-1-ylidene-malononitrile ( A1 ), methyl-2-cyano-2,2-ethylidene-5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene-acetate ( A2 ), 5,2-ethylidene-5,6-difluoro-3-oxo-2,3-dihydroinden-1-ylidene-3-methyl-2-thioxothiazolidin-4-one ( A3 ) and 2,5-ethylidene-6-oxo-5,6-dihydrocyclopenta-thiophen-4-ylidene-malononitrile ( A4 ) end-capped acceptor groups. Their optical, electrical and geometries have been compared with reported molecule R. Frontier molecular orbital diagram reveal excellent charge transfer rate, The electron density is shifted from donor to acceptor unit. Among all, A1 exhibits the highest absorption in the visible region (λ max ) at 798 nm in chloroform solvent. The maximum open circuit voltage (2.08 V) is observed for A3 when blended with PTB7-Th donor polymer. All studied molecules have high charge mobilities due to lower reorganization energy values with respect to model molecule R . A1 has the highest electron mobility among all molecules due to lower value of reorganization energy which is 0.0034. Furthermore, all designed molecules show good Solubilities in organic solvent. A2 exhibit high value of dipole moment which reveal good solubilities in fabrication process. We have designed four ( A1, A2, A3 and A4 ) fused ring electron acceptor for their use in OSCs. These molecules contain Alkoxy-Induced Naphtho-dithiophene core unit attached with different acceptor groups to enhance PCE of OSCs. All newly designed molecule have better performance as compared to R . Additionally, A1 exhibits strong absorption in visible region in both gas phase (λ max = 724.1 nm) and solvent (λ max = 798.4 nm) and exhibit lowest energy band gap (1.92 eV). • A1 shows the highest absorption maximum (λ max ) 798.4 nm in chloroform solvent. • The Designed molecules are blended with well-known donor polymer PTB7-Th. • All designed molecules have lower binding energy as compared to reference molecule R. • All newly designed molecules have excellent opto-electronic properties with respect to R.

Indeno[1,2-b]thiophene End-capped Perylene Diimide: Should the 1,6-Regioisomers be systematically considered as a byproduct?

Usually considered as a byproduct, the 1,6-dibrominated PDI has rarely been functionalized for the preparation of electro-active conjugated molecules, particularly in the field of organic photovoltaics. In light of the literature, one can ask oneself: Does a 1,7-isomer based functional molecule systematically perform better than its 1,6-analogue? To answer this question, we report herein the synthesis and direct comparison of two indeno[1,2-b]thiophene (IDT) end-capped perylene diimide regioisomers (PDI) (1,6 and 1,7) used as non-fullerene acceptors in organic solar cells. It turned out that in our case, ie, when blended with the well-known PTB7-Th donor polymer, higher performance was reached for devices made with the 1,6-analogue.

Comparing C60 and C70 as acceptor in organic solar cells: Influence of the electronic structure and aggregation size on the photovoltaic characteristics

The difference in aggregation size of the C60 and C70 fullerenes affect the photovoltaic performance of devices assembled in the so-called bilayer architecture with poly[2,7-(9,9- dioctyl- dibenzosilole)- alt-4,7- bis(thiophen-2-yl)benzo- 2,1,3- thiadiazole] (PSiF-DBT) as the electron donor material. Despite the better performance of the C70 devices, which is related to the high absorption coefficient in the visible range and the superior charge transport properties, the short-circuit current variation upon annealing treatment at 100 °C is approximately twice bigger when the C60 is the acceptor. We attribute this effect to the tendency of C60 in form smaller aggregate domains relatively to the C70. The increased roughness on the polymeric surface after annealing results in an enhanced donor/acceptor contact area and assists the fullerene diffusion deeper inside the polymeric layer. This effect leads to a better mixing between donor and acceptor species and create a interpenetrating layer close to the so-called bulk heterojunction. Since C60 forms smaller aggregates, this mechanism is more pronounced for this molecule. Therefore, a significant variation in the performance of the C60 devices is observed after this kind of treatment. Density Functional Theory calculations of the potential energy of interaction between two fullerene molecules and X-Ray measurements gives evidences to support this idea. In addition, combining spectrally resolved external quantum efficiency measurements with optical modeling our results also indicate the occurrence of the bilayer interfacial mixing for PSiF-DBT/C60.

Boron(iii) β-diketonate-based small molecules for functional non-fullerene polymer solar cells and organic resistive memory devices.

A class of acceptor–donor–acceptor chromophoric small-molecule non-fullerene acceptors, 1–4, with difluoroboron(iii) β-diketonate (BF2bdk) as the electron-accepting moiety has been developed. Through the variation of the central donor unit and the modification on the peripheral substituents of the terminal BF2bdk acceptor unit, their photophysical and electrochemical properties have been systematically studied. Taking advantage of their low-lying lowest unoccupied molecular orbital energy levels (from −3.65 to −3.72 eV) and relatively high electron mobility (7.49 × 10−4 cm2 V−1 s−1), these BF2bdk-based compounds have been employed as non-fullerene acceptors in organic solar cells with maximum power conversion efficiencies of up to 4.31%. Moreover, bistable resistive memory characteristics with charge-trapping mechanisms have been demonstrated in these BF2bdk-based compounds. This work not only demonstrates for the first time the use of a boron(iii) β-diketonate unit in constructing non-fullerene acceptors, but also provides more insights into designing organic materials with multi-functional properties.

An asymmetric small molecule acceptor for organic solar cells with a short circuit current density over 24 mA cm−2

Acceptor Y22 with an asymmetric hexacyclic A–DA′D–A structure achieved a high PCE of 15.4% and a high Jsc of 24.37 mA cm−2, which are among the best values reported for asymmetric acceptor based binary organic solar cells.

Quantifying Voc loss induced by alkyl pendants of acceptors in organic solar cells

In the organic photovoltaic community, extensive efforts have been made in proposing resolutions for depressing Voc loss related to the molecular structures and optoelectronic properties of materials in bulk heterojunctions.

An asymmetric end-capping strategy enables a new non-fullerene acceptor for organic solar cells with efficiency over 10.

Two different terminal groups, rhodanine-flanked benzo[c][1,2,5]thiadiazole (BR) and 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IM2F), were connected to an indaceno[1,2-b:5,6-b']dithiophene (IDT) core to construct a new non-fullerene acceptor (IDTBF). Solar cells based on this acceptor exhibited promising photovoltaic performances with a power conversion efficiency (PCE) of up to 10.43%.

Subnaphthalocyanine triimides: potential three-dimensional solution processable acceptors for organic solar cells

Three-dimensional SubNcTIs are excellent chromophores of solution processable electron acceptors towards high performance organic solar cells.

PDI-based heteroacenes as acceptors for fullerene-free solar cells: importance of their twisted geometry

Two novel PDI-based small molecules with twisted geometry were designed and synthesised as acceptors for organic solar cells.

Aggregation of non-fullerene acceptors in organic solar cells

Various types of molecular stacking can form in NFAs, which influence light absorption and efficiency of organic solar cells.

The role of bulk and interfacial morphology in charge generation, recombination, and extraction in non-fullerene acceptor organic solar cells

This study provides insight into the detailed bulk and interfacial morphological features critical in achieving high PCEs in polymer:NFA OSCs.

A new fluorinated pyran-bridged A-D-A type small molecular acceptor for organic solar cells

A new fluorinated pyran-bridged A-D-A type small molecular non-fullerene acceptor, i-PTIC-F, is designed and synthesized. The optical and electrochemical properties have been well studied and compared with the non-fluorinated molecule, i-PTIC. The photovoltaic performance of i-PTIC-F is characterized using PBDB-T as the donor material. A reasonable power conversion efficiency of 5.68% is achieved with an enhanced Jsc of 14.56 mA cm−2.