Solution-processable Bulk-heterojunction Devices Research Articles

Donor-acceptor-donor modelled donor targets based on indoline and naphthalene diimide functionalities for efficient bulk-heterojunction devices

Two new organic semiconductors based on indoline and naphthalene diimide building blocks are reported for solution-processed bulk-heterojunction devices. Both materials, coded as W5 and W6, were based on the donor–acceptor–donor structural format, where indoline was used as a terminal donor and the naphthalene diimide unit was the central acceptor. Both W5 and W6 were designed to be the structural analogues where a phenyl ring was used as a π-linker in the former, and the latter comprised an additional thiophene ring. The π-linkers were induced between the donor (indoline) and acceptor (naphthalene diimide) functionalities. As a result of adding a thiophene ring in W6, its thin film demonstrated improved light-harvesting properties together with a narrower optical band gap. The photovoltaic properties were evaluated using solution-processed, bulk-heterojunction devices and W6 afforded a promising power conversion efficiency of 6.71% when tested as a donor material together with the conventional acceptor ([6,6]-phenyl-C71-butyric acid methyl ester (PC71BM)). The efficiency exhibited by W6: PC71BM is approximately 50% higher than the device based on the W5 blend (4.49%). To our knowledge, W5 and W6 are the first reported examples in the literature where indoline and naphthalene diimide units are linked together, and the optoelectronic and photovoltaic properties have been altered with the incorporation of a thiophene ring within the investigated molecular backbone.

A series of V-shaped small molecule non-fullerene electron acceptors for efficient bulk-heterojunction devices

Two simple semiconducting acceptor-acceptor1-donor-acceptor (A-A1-D-A) modular, small molecule non-fullerene electron acceptors, 2-(4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)-3-(6-((4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)ethynyl)-9-octyl-9H-carbazol-3-yl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (NDICz-5) and 2-(4-(3,3-dicyano-2-(4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)-1-(6-((4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)ethynyl)-9-octyl-9H-carbazol-3-yl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (NDICz-6), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of NDICz-5 and NDICz-6 were directly compared with those of a structural analogue, 7,7'-(((9-octyl-9H-carbazole-3,6-diyl)bis(ethyne-2,1-diyl))bis(4,1-phenylene))bis(2-hexadecylbenzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone) (NDICz-4), which was designed based on an A-D-A format. All of these new materials were designed to be V-shaped and comprised an electron rich carbazole donor core (D) together with electron deficient naphthalene diimide terminal core (A). In the simple A-D-A system, naphthalene diimide was the terminal functionality, whereas in the A-A1-D-A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A1 units by keeping D/A units constant. The inclusion of A1 was mainly done to induce cross-conjugation within the molecular backbone and hence to improve light-harvesting. The physical and optoelectronic properties were characterized by ultraviolet–visible, thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited energy levels complementing those of the conventional donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with NDICz-4, NDICz-5 and NDICz-6 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: NDI-Cz6-based device showed an impressive enhanced power conversion efficiency of 7.58%, an increase of around two-fold with respect to as-cast blend, after solvent vapor annealing using carbon disulfide. Not only are the results among the best in the current literature using the conventional donor polymer P3HT, but clearly demonstrate that A-A1-D-A type, V-shaped small molecules are promising non-fullerene acceptors in the research field of organic solar cells.

Generating a three-dimensional non-fullerene electron acceptor by combining inexpensive spiro[fluorene-9,9′-xanthene] and cyanopyridone functionalities

A spiro[fluorene-9,9′-xanthene]-functionalized non-fullerene acceptor A1 [D : A1 = 1 : 1.2; P3HT(D) = 5.84%, PTB7(D) = 7.21%].

An efficient non-fullerene acceptor based on central and peripheral naphthalene diimides.

Through the coupling of central and terminal naphthalene diimide functionalities, a unique non-fullerene electron acceptor, coded as N10, was designed, synthesized, characterized and applied in solution-processable bulk-heterojunction devices. The target N10 displayed good solubility, excellent thermal stability and energy levels complementing those of the conventional donor polymer poly(3-hexyl thiophene) (P3HT). An excellent power conversion efficiency of 7.65% was obtained in simple BHJ devices (P3HT : N10 1 : 1.2), which is the highest observed so far for NDI core-based non-fullerene acceptors.

Donor–acceptor–acceptor-based non-fullerene acceptors comprising terminal chromen-2-one functionality for efficient bulk-heterojunction devices

Two simple semiconducting donor–acceptor–acceptor (D–A1–A) modular, small molecule, non-fullerene electron acceptors, 2-(4-(diphenylamino)phenyl)-3-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (P2) and 2-(4-(3,3-dicyano-1-(4-(diphenylamino)phenyl)-2-(4-((2-oxo-2H-chromen-3-yl)ethynyl)phenyl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (P3), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of P2 and P3 were directly compared with those of a structural analogue, 3-((4-((4-(diphenylamino)phenyl)ethynyl)phenyl)ethynyl)-2H-chromen-2-one (P1), which was designed based on a D–A format. All of these new materials comprised an electron rich triphenylamine (TPA) donor core (D) and electron deficient chromen-2-one terminal core (A). In the simple D–A system, TPA and chromenone were the terminal functionalities, whereas in the D–A1–A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A1 units by keeping the D/A units constant. The inclusion of A1 was primarily done to induce cross-conjugation within the molecular backbone and hence to generate low band gap targets. The physical and optoelectronic properties were characterized by ultraviolet–visible (UV–Vis), thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited broadened absorption spectra, for instance panchromatic absorbance in case of P3, excellent solubility and thermal stability, and energy levels matching those of the conventional and routinely used donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with P1, P2 and P3 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: P3-based device showed an impressive enhanced power conversion efficiency of 4.21%, an increase of around two-fold with respect to the efficiency of the best P3HT: P1-based device (2.28%). Our results clearly demonstrate that the D–A1–A type small molecules are promising non-fullerene electron acceptors in the research field of organic solar cells.

Small molecular non-fullerene acceptors based on naphthalenediimide and benzoisoquinoline-dione functionalities for efficient bulk-heterojunction devices

Through the conjunction of naphthalenediimide and benzoisoquinoline-dione functionalities, two novel, solution-processable non-fullerene electron acceptors, 2,7-dioctyl-4,9-bis(2-octyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (coded as NDI-N1) and 2,7-dioctyl-4,9-bis(5-(2-octyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)thiophen-2-yl)benzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (coded as NDI-N2), were designed, synthesized, and used as active components for bulk-heterojunction photovoltaic devices. The new chromophores were based on the acceptor–acceptor–acceptor module where naphthalenediimide unit served as the central acceptor flanked by terminal benzoisoquinoline-dione acceptor functionality. The optoelectronic and photovoltaic properties of NDI-N1 and NDI-N2 were directly compared. Solution-processable bulk-heterojunction devices were fabricated using NDI-N1 and NDI-N2 as non-fullerene electron acceptor materials. Studies on the photovoltaic properties revealed that the best poly(3-hexylthiophene) (P3HT): NDI-N2-based device showed an impressive enhanced power conversion efficiency of 4.04%, around 40% increase with respect to the efficiency of the best NDI-N1-based device (2.91%). It is justifiable to mention that the device outcome reported herein provides strong support and incentive for the current research strategy, and that the conjoint use of potential acceptor building blocks, such as naphthalenediimide and benzoisoquinoline-dione, can indeed generate efficient non-fullerene acceptors.

Enhancing the efficiency of solution-processable bulk-heterojunction devices via a three-dimensional molecular architecture comprising triphenylamine and cyanopyridone

Through the employment of a three-dimensional molecular geometry, a novel, solution-processable small molecular organic chromophore was designed, synthesized and characterized for application in bulk-heterojunction solar cells. The new chromophore, [(5Z,5′Z,5″Z)-5,5′,5″-(((nitrilotris(benzene-4,1-diyl))tris(thiophene-5,2-diyl))tris(methanylylidene))tris(1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile)] (coded as 3D), was based on a donor–acceptor (D–A) module where a simple triphenylamine unit served as an electron donor, cyanopyridone as an electron acceptor, and a thiophene unit as π-bridge embedded between the donor and acceptor functionalities. The optoelectronic and photovoltaic properties of 3D were directly compared with those of a structural analogue, [(Z)-5-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile)], namely 1D. The three-dimensional (otherwise termed as non-linear) design, compared to one-dimensional (or linear) design, demonstrated (1) an enhancement of light-harvesting ability by about 50%; (2) an increase in wavelength of the longest wavelength absorption maximum of thin film (633 nm vs. 581 nm) and (3) a narrower optical band-gap (1.53 eV vs. 1.65 eV). Solution-processable bulk-heterojunction devices were fabricated with 3D and 1D as donor materials. Studies on the photovoltaic and hole charge mobility properties revealed that the best 3D- [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)-based device showed an impressive enhanced power conversion efficiency of 4.76%, an increase of greater than two-fold with respect to the efficiency of the best 1D-based device (2.11%), and a higher hole mobility of the order of 3.4 × 10−4 cm2 V−1 s−1. Not only is 3D the first reported example in the literature where cyanopyridone acceptor functionality has been used to generate a three-dimensional donor molecule but the power conversion efficiency reported here is the highest value reported so far for non-linear, star-shaped molecular architectures. These results clearly illustrated that the use of a three-dimensional geometry helped to extend molecular conjugation, thus enhancing the light-harvesting ability and short-circuit current density, while further improving the bulk-heterojunction device performance.

Naphthalene diimide-based non-fullerene acceptors for simple, efficient, and solution-processable bulk-heterojunction devices

A novel, NDI-based non-fullerene electron acceptors (R1, R2) for solution-processable bulk-heterojunction, displayed good thermal stability and afforded 2.24% power conversion efficiency.

A non-fullerene electron acceptor based on central carbazole and terminal diketopyrrolopyrrole functionalities for efficient, reproducible and solution-processable bulk-heterojunction devices

A novel, solution-processable non-fullerene electron acceptor displayed excellent solubility, thermal stability, and afforded 2.30% power conversion efficiency with a high open-circuit voltage (1.17 V).

Organic donor materials based on Bis(arylene ethynylene)s for bulk heterojunction organic solar cells with high V(oc) values.

A series of purely organic small molecules 1-4 based on bis(arylene ethynylene)s with various electron-rich aromatic bridges, such as benzene, cyclopentadithiophene (CDT), and diphenyl(p-tolyl)amine, were synthesized by a Sonogashira coupling reaction. Their optical, electronic, and electrochemical properties were fully characterized. The photovoltaic properties of these molecules as donor materials and [6,6]-phenyl-C71 -butyric acid methyl ester as an acceptor material in solution-processed bulk heterojunction devices were studied. Among them, CDT-bridged molecule 2 exhibited the best photovoltaic performance and achieved a power conversion efficiency of 3.28 %. In addition, the photovoltaic efficiencies of these molecules are higher than their corresponding platinum-containing counterparts, probably owing to their stronger light absorption and improved open-circuit voltage.

A solution-processable electron acceptor based on diketopyrrolopyrrole and naphthalenediimide motifs for organic solar cells

A novel, solution-processable small molecular electron acceptor (HP1) based on diketopyrrolopyrrole and naphthalenediimide fragments was designed and synthesized via a Stille coupling reaction, characterized by spectroscopic means, and exhibited excellent solubility and thermal stability. HP1 exerted strong and very broad absorption tailing into the near infra-red region, with appropriate energy levels matching with the archetypal electron donor, poly(3-hexylthiophene) (P3HT), and afforded 1.02% power conversion efficiency with a high open-circuit voltage (1.05V) when tested in solution-processable bulk-heterojunction devices.

A-D-A-Type Oligothiophenes Containing Benzothiadiazole Terminal Units for Small Molecule Organic Solar Cells

AbstractThe electron-deficient, fused-heterocyclebenzo[c][1,2,5]thiadiazole (BTDA) is investigated as acceptor group in A-D-A-type oligothiophenes in order to correlate their relative acceptor strength with opto-electronic and photovoltaic properties. In this respect, two novel BDTA-capped oligothiopheneswere synthesized and characterized by optical and electrochemical measurements. They showed broad absorptions in the visible spectrum and HOMO-LUMO energies well suited for organic solar cells. The attachment of terminal BTDA acceptor units to the conjugated oligothiophene backbone resulted in a hypsochromic shift in UV-Vis absorption and larger band gap in comparison to previously reported analogous dicyanovinylene (DCV)-substituted oligothiophenes indicating that BDTA is a weaker acceptor than DCV. Vacuumprocessed m-i-p (metal-intrinsic-p-doped)-type bilayer solar cells using these co-oligomers as donor and C60 as acceptor gave moderate power conversion efficiencies of around 1.0%. Bulk-heterojunction (BHJ) solar cells prepared by solution-processing using fullerene PC61BM as acceptor generated slightly lower efficiencies of 0.9%, whichwere increased to 1.5% by using the higher fullerene PC71BM. It was found that the cell efficiencies were mostly limited by the low photocurrent densities due to moderate light absorption in the bilayer devices and low fill factors coming from inefficient charge transport in the solutionprocessed BHJ devices.