Organoboron molecules and polymers for organic solar cell applications

Organic solar cells (OSCs) are emerging as a new photovoltaic technology with the great advantages of low cost, light-weight, flexibility and semi-transparency. They are promising for portable energy-conversion products and building-integrated photovoltaics. Organoboron chemistry offers an important toolbox to design novel organic/polymer optoelectronic materials and to tune their optoelectronic properties for OSC applications. At present, organoboron small molecules and polymers have become an important class of organic photovoltaic materials. Power conversion efficiencies (PCEs) of 16% and 14% have been realized with organoboron polymer electron donors and electron acceptors, respectively. In this review, we summarize the research progress in various kinds of organoboron photovoltaic materials for OSC applications, including organoboron small molecular electron donors, organoboron small molecular electron acceptors, organoboron polymer electron donors and organoboron polymer electron acceptors. This review also discusses how to tune their opto-electronic properties and active layer morphology for enhancing OSC device performance. We also offer our insight into the opportunities and challenges in improving the OSC device performance of organoboron photovoltaic materials.

BODIPY-Based Molecules, a Platform for Photonic and Solar Cells

The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-based molecules have emerged as interesting material for optoelectronic applications. The facile structural modification of BODIPY core provides an opportunity to fine-tune its photophysical and optoelectronic properties thanks to the presence of eight reactive sites which allows for the developing of a large number of functionalized derivatives for various applications. This review will focus on BODIPY application as solid-state active material in solar cells and in photonic devices. It has been divided into two sections dedicated to the two different applications. This review provides a concise and precise description of the experimental results, their interpretation as well as the conclusions that can be drawn. The main current research outcomes are summarized to guide the readers towards the full exploitation of the use of this material in optoelectronic applications.

BODIPY-Based Semiconducting Materials for Organic Bulk Heterojunction Photovoltaics and Thin-Film Transistors

The rapid emergence of organic (opto)electronics as a promising alternative to conventional (opto)electronics has been achieved through the design and development of novel π-conjugated systems. Among various semiconducting structural platforms, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) π-systems have recently attracted attention for use in organic thin-films transistors (OTFTs) and organic photovoltaics (OPVs). This Review article provides an overview of the developments in the past 10 years on the structural design and synthesis of BODIPY-based organic semiconductors and their application in OTFT/OPV devices. The findings summarized and discussed here include the most recent breakthroughs in BODIPYs with record-high charge carrier mobilities and power conversion efficiencies (PCEs). The most up-to-date design rationales and discussions providing a strong understanding of structure-property-function relationships in BODIPY-based semiconductors are presented. Thus, this review is expected to inspire new research for future materials developments/applications in this family of molecules.

Rational design of near-infrared dyes based on boron dipyrromethene derivatives for application in organic solar cells

With the aim to further improve the light-absorption efficiency of organic solar cells (OSCs), we have designed a series of novel pyrrolopyrrole boron dipyrromethene (BODIPY) derivatives by replacing the sulfur atom and introducing different fused aromatic heterocycle end-caps. The optical, electronic, and charge transporting properties of the designed molecules have been systematically investigated by applying density functional theory (DFT) and time-dependent DFT (TD-DFT) methodologies. The calculated the frontier molecular orbital (FMO) energies and spectral properties showed that the designed molecules exhibit narrower band gaps and strong absorption in the red/near-infrared (NIR) region, which led to the higher light-absorbing efficiency. Furthermore, the calculated reorganization energies show that the designed molecules are expected to be promising candidates for hole and/or electron transport materials. The results reveal that the designed molecules can serve as high-efficiency red/NIR-active donor materials as well as hole and/or electron transport materials in OSC applications.

Precise determination of the orientation of the transition dipole moment in a Bodipy derivative by analysis of the magnetophotoselection effect

Precise determination, in isotropic samples, of the electronic transition dipole moment orientation in the molecular frame by exploiting magnetophotoselection effects.

Aza-BODIPY Derivatives Containing BF(CN) and B(CN)2 Moieties

Two novel aza-BODIPY derivatives are synthesized, with the fluorine atoms in the BF₂ moiety replaced by cyano groups. The introduction of cyano groups changes the phenyl substituents on the 3,5 positions from parallel to antiparallel. The HOMO/LUMO energy levels are stabilized gradually upon increasing the number of cyano groups, and the photophysical properties show corresponding shifts. With high thermal stability, the derivatives can be purified by sublimation and used to prepare vacuum-deposited thin films. This research extends the aza-BODIPY family with cyano-substituted derivatives.

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.

Synthesis and Spectroscopic Investigation of Diketopyrrolopyrrole - Spiropyran Dyad for Fluorescent Switch Application

We report the synthesis and characterization of a new fluorescent dyad SP-DPP-SP(9) via efficient palladium-catalyzed Sonogashira coupling of prop-2-yn-1-yl 3-(3',3'dimethyl-6-nitrospiro[chromene-2,2'-indolin]-1'-yl)propanoatespiropyran, SP(8), a well known photochromic accepter, with 3,6-bis(5-bromothiophen-2-yl)-2,5-bis((R)-2-ethylhexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione, DPP(4), a highly fluorescent donor. Under visible light exposure the SP unit is in a closed hydrophobic form, whereas under UV irradiation it converts to a polar, hydrophilic open form named Merocyanine (MC), which is responsible for functioning of photo-switch application. The photochemistry pertaining to fluorescence switch, 'on/off' behaviour, of model dyad SP-DPP-SP(9) is experimentally analyzed in solution as well as in solid state in polymer matrices by photoluminescence(PL) and absorption spectroscopy. After absorption of UV light the spiropyran unit of the dyad under goes the rupture of the spiro C-O bond leading to the formation of MC. The absorption band of MC fairly overlaps to the fluorescence of DPP unit resulting quenching of fluorescence via fluorescence resonance energy transfer from exited DPP unit to ground state MC. In contrary, the fluorescence of DPP is fully regained upon transformation of MC to SP by exposure to visible light or thermal stimuli. Hence, the fluorescence intensity of dyad 9 is regulated by reversible conversion among the two states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the MC form of SP and the DPP unit. Conversely, these scrutiny of the experiment express that the design of dyad 9 is viable as efficient fluorescent switch molecule in many probable commercial applications, such as, logic gates and photonic and optical communications.