A new fullerene-free bulk-heterojunction system for efficient high-voltage and high-fill factor solution-processed organic photovoltaics

Optimizing the Photovoltaic Performance of Organic Solar Cells for Indoor Light Harvesting

Organic solar cells (OSCs) harvesting indoor light are highly promising for emerging technologies, such as Internet of Things. Here, the photovoltaic performance of the PTB7-Th:PC 71 BM solar cells, constructed using "optimized" and "non-optimized" processing conditions, are compared for indoor and outdoor applications. We find that compared to the "optimized" solar cell, the "non-optimized" solar cell is less efficient under simulated solar light illumination (100 mW cm -2 , spectral range 350-1100 nm), due to the significant bimolecular charge carrier recombination losses. However, under simulated indoor illumination (3.28 mW cm -2 , spectral range 400-700 nm), bimolecular recombination is effective suppressed, thus the power conversion efficiency of the "non-optimized" solar cell was increased to 14.7%, higher than that of the "optimized" solar cell (14.2%). These results suggest that the "common" strategy used to optimize the OSCs could be undesired for indoor OSCs. Then, we demonstrate that the difficulty in realizing the desired "morphology" of the active layer for the outdoor OSCs, could be eased for the indoor OSCs, allowing us to realize high-efficiency indoor OSCs using a non-halogenated solvent.

Recent Advances of Furan and Its Derivatives Based Semiconductor Materials for Organic Photovoltaics

The state-of-the-art bulk-heterojunction (BHJ)-type organic solar cells (OSCs) have exhibited power conversion efficiencies (PCEs) of exceeding 18%. Thereinto, thiophene and its fused-ring derivatives play significant roles in facilitating the development of OSCs due to their excellent semiconducting natures. Furan as thiophene analogue, is a ubiquitous motif in naturally occurring organic compounds. Driven by the advantages of furan, such as less steric hindrance, good solubility, excellent stacking, strong rigidity and fluorescence, biomass derived fractions, more and more research groups focus on the furan-based materials for using in OSCs in the past decade. To systematically understand the developments of furan-based photovoltaic materials, the relationships between the molecular structures, optoelectronic properties, and photovoltaic performances for the furan-based semiconductor materials including single furan, benzofuran, benzodifuran (BDF) (containing thienobenzofuran (TBF)), naphthodifurans (NDF), and polycyclic furan are summarized. Finally, the empirical regularities and perspectives of the development of this kind of new organic semiconductor materials are extracted.

Domino C–C/C–O bond formation: palladium-catalyzed regioselective synthesis of 7-iodobenzo[b]furans using 1,2,3-triiodobenzenes and benzylketones

A facile and efficient synthesis of 7-iodobenzo[b]furan derivatives via a highly regioselective tandem α-arylation/intramolecular O-arylation of 5-substituted-1,2,3-triiodobenzenes and benzylketones is described. Remarkably, the α-arylation coupling reactions initiate exclusively at the least sterically-hindered position of the triiodoarene, which results in a highly chemoselective transformation. The highest yields were observed in reactions between electron-poor 1,2,3-triiodoarenes and electron-rich benzylketones, yet the optimized reaction conditions were found to be tolerant to a wide range of different functional groups. This unprecedent synthesis of 7-iodobenzo[b]furans from 1,2,3-triiodobenzenes is scalable, general in scope, and provides easy access to valuable precursors for other chemical transformations.

A facile and unprecedented synthesis of 7-iodobenzo[b]furans via a highly regioselective tandem α-arylation/intramolecular O-arylation is reported that is efficient, scalable and creates versatile precursors for further chemical manipulation.

Efficient and thermally stable organic solar cells based on small molecule donor and polymer acceptor

Efficient organic solar cells (OSCs) often use combination of polymer donor and small molecule acceptor. Herein we demonstrate efficient and thermally stable OSCs with combination of small molecule donor and polymer acceptor, which is expected to expand the research field of OSCs. Typical small molecule donors show strong intermolecular interactions and high crystallinity, and consequently do not match polymer acceptors because of large-size phase separation. We develop a small molecule donor with suppressed π-π stacking between molecular backbones by introducing large steric hindrance. As the result, the OSC exhibits small-size phase separation in the active layer and shows a power conversion efficiency of 8.0%. Moreover, this OSC exhibits much improved thermal stability, i.e. maintaining 89% of its initial efficiency after thermal annealing the active layer at 180 °C for 7 days. These results indicate a different kind of efficient and stable OSCs.

Emissive and charge-generating donor-acceptor interfaces for organic optoelectronics with low voltage losses

Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for organic light-emitting diodes^1,2. Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001% range^3,4. In contrast, the electroluminescence external quantum yield reaches up to 16% in D-A-based organic light-emitting diodes^5-7. Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons^8-11.

Effect of π-bridge units on properties of A–π–D–π–A-type nonfullerene acceptors for organic solar cells

We theoretically designed efficient nonfullerene acceptors (P2 and P5) with lower LUMO energies and higher electron transport abilities for OSCs.

Interfacial electronic structure of Cl6SubPc non-fullerene acceptors in organic photovoltaics using soft X-ray spectroscopies

The significantly high energy gap between a SubPc donor and a Cl₆SubPc acceptor is the origin of highV_OCin OPVs.

Novel benzodithiophene-based polymer acceptors for efficient organic solar cells

Novel polymer acceptors (P2 and P5) exhibiting high light-absorbing capacity, exciton separation ability, and electron mobility have been designed for OSCs.

Design of Diketopyrrolopyrrole (DPP)-Based Small Molecules for Organic-Solar-Cell Applications

After the first report in 2008, diketopyrrolopyrrole (DPP)-based small-molecule photovoltaic materials have been intensively explored. The power conversion efficiencies (PCEs) for the DPP-based small-molecule donors have been improved up to 8%. Furthermore, through judicious structure modification, DPP-based small molecules can also be converted into electron-acceptor materials, and, recently, some exciting progress has been achieved. The development of DPP-based photovoltaic small molecules is summarized here, and the photovoltaic performance is discussed in relation to structural modifications, such as the variations of donor-acceptor building blocks, alkyl substitutions, and the type of conjugated bridges, as well as end-capped groups. It is expected that the discussion will provide a guideline in the exploration of novel and promising DPP-containing photovoltaic small molecules.

High-Efficiency Nonfullerene Polymer Solar Cells with Medium Bandgap Polymer Donor and Narrow Bandgap Organic Semiconductor Acceptor

A nonfullerene polymer solar cell with a high efficiency of 9.26% is realized by using benzodithiophene-alt-fluorobenzotriazole copolymer J51 as a medium-bandgap polymer donor and the low-bandgap organic semiconductor ITIC with high extinction coefficients as the acceptor.

Following the nanostructural molecular orientation guidelines for sulfur versus thiophene units in small molecule photovoltaic cells

In bulk heterojunction (BHJ) organic photovoltaics, particularly those using small molecules, electron donor and/or electron acceptor materials form a distributed network in the photoactive layer where critical photo-physical processes occur. Extensive research has recently focused on the importance of sulfur atoms in the small molecules. Little is known about the three-dimensional orientation of these sulfur atom-containing molecules. Herein, we report on our research concerning the heterojunction textures of the crystalline molecular orientation of small compounds having sulfur-containing units in the side chains, specifically, compounds known as DR3TSBDT that contain the alkylthio group and DR3TBDTT that does not. The improved performance of the DR3TBDTT-based devices, particularly in the photocurrent and the fill factor, was attributed to the large population of donor compound crystallites with a favorable face-on orientation along the perpendicular direction. This orientation resulted in efficient charge transport and a reduction in charge recombination. These findings underscore the great potential of small-molecule solar cells and suggest that even higher efficiencies can be achieved through materials development and molecular orientation control.

1,8-Naphthalimide-Based Planar Small Molecular Acceptor for Organic Solar Cells

Four small molecular acceptors (SM1-4) comprising a central benzene core, two thiophene bridges and two 1,8-naphthalimide (NI) terminal groups were designed and synthesized by direct C-H activation. SM1 has a planar chemical structure and forms H-aggregation as films. By attachment of different substituents on the central benzene ring, the dihedral angles between the two NI end groups of SM1-4 gradually increased, leading to a gradual decrease of planarity. SM1-4 all possess a high-lying LUMO level, matching with wide band gap (WBG) polymer donors which usually have a high-lying LUMO level. When used in OSCs, devices based on SM1 and WBG donor PCDTBT-C12 gave higher electron mobility, superior film morphology and better photovoltaic performance. After optimization, a PCE of 2.78% with a V(oc) of 1.04 V was achieved for SM1 based devices, which is among the highest PCEs with a V(oc) higher than 1 V. Our results have demonstrated that NI based planar small molecules are potential acceptors for WBG polymer based OSCs.

Two new fluorinated copolymers based on thieno[2,3-f]benzofuran for efficient polymer solar cells

Two new copolymers named TBFPF-BT and TBFPF-BO, composed of a fluorine substituted thieno[2,3-f]benzofuran donor unit and benzothiadiazole/benzooxadiazole acceptor unit, were synthesized for photovoltaic applications.

Fullerene-free small molecule organic solar cells with a high open circuit voltage of 1.15 V

A small molecule acceptor named DTBTF was synthesized and a PCE of 3.84% with a V_oc of 1.15 V was achieved.

Enhanced Efficiency in Fullerene-Free Polymer Solar Cell by Incorporating Fine-designed Donor and Acceptor Materials

Among the diverse nonfullerene acceptors, perylene bisimides (PBIs) have been attracting much attention due to their excellent electron mobility and tunable molecular and electronic properties by simply engineering the bay and head linkages. Herein, guided by two efficient small molecular acceptors, we designed, synthesized, and characterized a new nonfullerene small molecule PPDI with fine-tailored alkyl chains. Notably, a certificated PCE of 5.40% is realized in a simple structured fullerene-free polymer solar cell comprising PPDI as the electron acceptor and a fine-tailored 2D-conjugated polymer PBDT-TS1 as the electron donor. Moreover, the device behavior, morphological feature, and origin of high efficiency in PBDT-TS1/PPDI-based fullerene-free PSC were investigated. The synchronous selection and design of donor and acceptor materials reported here offer a feasible strategy for realizing highly efficient fullerene-free organic photovoltaics.

New advances in non-fullerene acceptor based organic solar cells

Non-fullerene organic molecules are alternative and competitive acceptor materials for high-efficiency organic solar cells.