As-cast Organic Solar Cells Research Articles

As-cast organic solar cells with 19.44% efficiency enabled by introducing functional fullerene derivative to facilitate charge carrier generation and transport

As-cast organic solar cells with 19.44% efficiency enabled by introducing functional fullerene derivative to facilitate charge carrier generation and transport

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.

Side-Chain Engineering in ITIC Skeleton Enabling As-Cast Organic Solar Cells with Reduced Energy Loss and Improved Vertical Phase Distribution

Side-Chain Engineering in ITIC Skeleton Enabling As-Cast Organic Solar Cells with Reduced Energy Loss and Improved Vertical Phase Distribution

Terthiophene based low-cost fully non-fused electron acceptors for high-efficiency as-cast organic solar cells

Two fully non-fused electron acceptors based on terthiophene have been synthesized and a PCE of 11.2% is achieved.

High-Efficiency As-Cast Organic Solar Cells Based on an Asymmetric Acceptor

High-Efficiency As-Cast Organic Solar Cells Based on an Asymmetric Acceptor

Simple unfused acceptors with optimal naphthalene isomerization enabling 10.72% as-cast organic solar cells

The isomerization strategy is adopted to modulate the molecular packing, optoelectronic and morphological properties of simple unfused acceptors (SUAs) for high-efficiency as-cast organic solar cells (OSCs). We report herein three SUAs (DNO15T, DNO26T and DNO27T) by flanking different naphthalene isomers with thiophene and dithieno[3,2-b:2′,3′-d]pyrrole as the donating cores. The SUAs exhibit narrow optical band gap (ca. 1.62 ∼ 2.04 eV) but largely different absorption profiles. The isomerization exerts great impact in molecular geometry and π-π stacking of SUAs as well as their morphology and photovoltaic properties when blending with polymer donor PBDB-T. The SUA featuring high coplanarity and good compatibility with PBDB-T affords the optimal phase separation morphology, photon harvesting and charge mobility in binary blends. The as-cast binary OSCs deliver a power conversion efficiency of 10.72% and open-circuit voltage of 0.89 V, competing with most reported SUAs based best-performing devices. Our study provides a good guidance for the design of SUAs via optimizing regional isomerization to achieve high-performance as-cast OSCs.

Alkyl side chain engineering enables high performance as-cast organic solar cells of over 17% efficiency

Achieving high-performance as-cast OSCs is crucial for industrialization in the future, owing to the advantages of better stability, environmental-friendly, and decreasing production cost. In this regard, we synthesized an A-DA′D-A type acceptor, Y6-eC6-BO, by shortening the straight alkyl side-chains on the thiophene position from C11 to C6 as well as lengthening the branched alkyl side-chains on the pyrrole position of Y6 to achieve a stronger crystallization and better miscibility than Y6. As a result, the corresponding chloroform-processed as-cast PM6: Y6-eC6-BO OSC showed a high PCE of 17.33%, which was one of the highest efficiencies of as-cast OSCs. And the as-cast PM6:Y6-eC6-BO OSCs processed from o-xylene displayed a PCE of 16.38%, as far as we know, this is among the highest efficiencies of non-halogenated-solvent processed as-cast OSCs. These results demonstrated tailoring the alkyl side-chain of NFAs is a feasible and simple approach to achieve high performance as-cast OSCs and provides guideline in molecular design in the future.

Aminonaphthalimide-Based Molecular Cathode Interlayers for As-Cast Organic Solar Cells.

A series of imide-based small molecules, namely NA, NAA, and NEA with simple structures, were designed and synthesized by introducing different amine side-chains into the benzene unit of imide, which were used as cathode interfacial materials in organic solar cells (OSCs). The amine side-chain substitution positions were systematically investigated with these small-molecule imides. Compared with NA without amide chains-NAA, and NEA, with 3-dimethylaminopropylamine and ethylenediamine chains, respectively-show bathochromic shifts in absorption, decreased band gaps, and higher highest occupied molecular orbital (HOMO) energy levels. A power conversion efficiency (PCE) of 15.04 % was obtained with the NEA-based as-cast OSCs with a high open-circuit voltage and fill factor for PM6 : Y6 blend and the maximum PCE of 15.80 % was reached for as-cast PM6 : Y6 : IT-M ternary OSCs. NEA exhibits better conductivity, higher electron mobility, and stronger the capability of lower work function of cathode among three molecules, affording OSCs with better photovoltaic performance. Additionally, these three molecules show excellent thermal stability both in solution and in films at 150 °C. The results indicate that imide-based small molecules are promising cathode interfacial materials for commercial OSCs.

Small-molecule acceptors with long alkyl chains for high-performance as-cast nonfullerene organic solar cells

Abstract Three fused-ring small-molecule electron acceptors, IDTC16-IC, IDTC16-Th, and IDTC16-4F, were designed and synthesized by introducing indacenodithiophene (IDT) as the electron-donating core and 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (IC), fluorinated IC, and a thiophene-based unit as the electron-withdrawing end group. Here, instead of the commonly used n-hexyl or n-hexylphenyl side chains, n-hexadecyl peripheral substituents were employed at the IDT core to study the influence of alkyl groups on photovoltaic performance of the nonfullerene acceptors. The introduction of flexible n-hexadecyl group endowed the three acceptors with excellent solubility in common organic solvents. All the three acceptors presented strong absorption ranging from 450 nm to 720 nm in solution with high molar extinction coefficients. As a result, the as-cast organic solar cells (OSCs) based on IDTC16-IC and the wide bandgap polymer donor PM6 exhibited a power conversion efficiency (PCE) of 5.12%. The OSCs based on PM6:IDTC16-Th and PM6:IDTC16-4F showed much better photovoltaic performance with PCEs of 8.76% and 8.55%, respectively. The PCE values were improved to 5.89%, 9.09%, and 9.42% for the PM6:IDTC16-IC, PM6:IDTC16-Th, and PM6:IDTC16-4F OSCs, respectively, with the addition of the solvent additive 1,8-diiodooctane. These findings demonstrate that the combination of alkyl chains at the fused rings and fluorination or aromatic structure change of the terminal groups leads to greatly enhanced photovoltaic performance of nonfullerene acceptors through improving the photophysical, molecular orbital, and film morphological properties.

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.

An unfused-ring acceptor with high side-chain economy enabling 11.17% as-cast organic solar cells.

Side-chain engineering on nonfullerene acceptors (NFAs) is crucial for modulating their solubility and crystallinity as well as packing behaviours in active layers to pursue high-performance organic solar cells (OSCs). High weight ratios of side chains are generally used by NFAs for the desired device efficiencies. Side-chain economy has seldom been discussed despite increased cost and difficulties in synthesis when optimizing the molecular design. Herein, we introduce 7H-dibenzo[c,g]carbazole (DCB) as an electron-donating core to design unfused-ring acceptors (UFAs) with a dramatically low weight ratio of side chains. DCB-4F has thus been designed and compared with the carbazole cored analogue (CB-4F). The unique conformation of the DCB core endows DCB-4F with higher solubility (8.2 mg mL-1 in chloroform) compared to CB-4F (2.2 mg mL-1) when using the same side chains. Featuring a lowest unoccupied molecular orbital (LUMO) level of -3.86 eV and an optical bandgap of 1.55 eV, the DCB-4F film exhibits an absorption profile (maximum 667 nm) complementary to polymer donor PM6. The PM6:DCB-4F as-cast OSCs deliver a power conversion efficiency (PCE) of 9.56% with a high open-circuit voltage (VOC) of 1.00 V. By adding 10 wt% PC71BM into the casting solutions, a greatly improved PCE of 11.17% is readily achieved, which is one of the highest PCEs for as-cast single-junction UFA-based devices. The PM6:DCB-4F based blends show homogeneous nano-fiberous morphology and higher hydrophobicity. The design of conformation-tuned NFAs using sterically hindered DCB-like cores is promising to achieve highly efficient as-cast OSCs.

Fused thienobenzene-thienothiophene electron acceptors for organic solar cells

Abstract Small molecule ladder-type heteroarenes IHBT-2F is designed and synthesized with strong electron-donating and molecular packing properties, where the central unit, fused thienobenzene-thienothiphene (IHBT), is attached with the strong electron-deficient 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC) as the end group. The counterpart IDBT-2F with indancenodibenzothiophene (IDBT) mainchain is sythesized for comparison, in which thieno[3,2-b]thiophene (TT) core of IHBT is replaced by benzene core. Relative to benzene-core IDBT-2F, TT-core IHBT-2F shows a much higher highest occupied molecular orbital energy level (IHBT-2F: − 5.46 eV; IDBT-2F: –5.72 eV) and significantly redshifted absorption, due to the π-donor capability of the sulfur atom, the larger π-conjugation and stronger intermolecular π–π stacking. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and IHBT-2F acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 8.74%, which is much higher than that of PTB7-Th: IDBT-2F (6.73%).

A chlorinated low-bandgap small-molecule acceptor for organic solar cells with 14.1% efficiency and low energy loss

A new acceptor-donor-acceptor (A-D-A) type small-molecule acceptor NCBDT-4Cl using chlorinated end groups is reported. This new-designed molecule demonstrates wide and efficient absorption ability in the range of 600–900 nm with a narrow optical bandgap of 1.40 eV. The device based on PBDB-T-SF:NCBDT-4Cl shows a power conversion efficiency (PCE) of 13.1% without any post-treatment, which represents the best result for all as-cast organic solar cells (OSCs) to date. After device optimizations, the PCE was further enhanced to over 14% with a high short-circuit current density ( J sc) of 22.35 mA cm−2 and a fill-factor (FF) of 74.3%. The improved performance was attributed to the more efficient photo-electron conversion process in the optimal device. To our knowledge, this outstanding efficiency of 14.1% with an energy loss as low as 0.55 eV is among the best results for all single-junction OSCs.

Effect of Isomerization on High-Performance Nonfullerene Electron Acceptors.

We design and synthesize two isomeric fused-ring electron acceptors, FNIC1 and FNIC2, which have the same end-groups and side-chains, but isomeric fused-nine-ring cores. Subtle changes in the two isomers influence their electronic, optical, charge-transport, and morphological properties. As compared with FNIC1, FNIC2 film exhibits a red-shifted absorption peak at 794 nm (752 nm for FNIC1), larger electron affinity of 4.00 eV (3.92 eV for FNIC1), smaller ionization energy of 5.56 eV (5.61 eV for FNIC1), and higher electron mobility of 1.7 × 10-3 cm2 V-1 s-1 (1.2 × 10-3 cm2 V-1 s-1 for FNIC1). The as-cast organic solar cells based on PTB7-Th:FNIC2 blends exhibit a power conversion efficiency (PCE) of 13.0%, which is significantly higher than that of PTB7-Th:FNIC1-based devices (10.3%). Semitransparent devices based on PTB7-Th:FNIC2 blends exhibit PCEs varying from 9.51% to 11.6% at different average visible transmittance (AVT, 20.3- 13.6%), significantly higher than those of PTB7-Th:FNIC1-based devices (7.58-9.14% with AVT of 20.2- 14.7%).

Fused Tris(thienothiophene)-Based Electron Acceptor with Strong Near-Infrared Absorption for High-Performance As-Cast Solar Cells.

A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600-950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 105 m-1 cm-1 , low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10-3 cm2 V-1 s-1 . Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.