Fluorinated Photovoltaic Materials for High‐Performance Organic Solar Cells

Over the past decade, organic solar cells (OSCs) have achieved a dramatic boost in their power conversion efficiencies from about 6 % to over 16 %. In addition to developments in device engineering, innovative photovoltaic materials, especially fluorinated donors and acceptors, have become the dominant factor for improved device performance. This minireview highlights fluorinated photovoltaic materials that enable efficient OSCs. Impressive OSCs have been obtained by developing some important molds of fluorinated donor and acceptor systems. The molecular design strategy and the matching principle of fluorinated donors and acceptors in OSCs are discussed. Finally, a concise summary and outlook are presented for advances in fluorinated materials to realize the practical application of OSCs.     

Side‐Chain Engineering of Benzodithiophene‐Fluorinated Quinoxaline Low‐Band‐Gap Co‐polymers for High‐Performance Polymer Solar Cells

A new series of donor–acceptor co‐polymers based on benzodithiophene and quinoxaline with various side chains have been developed for polymer solar cells. The effect of the degree of branching and dimensionality of the side chains were systematically investigated on the thermal stability, optical absorption, energy levels, molecular packing, and photovoltaic performance of the resulting co‐polymers. The results indicated that the linear and 2D conjugated side chains improved the thermal stabilities and optical absorptions. The introduction of alkylthienyl side chains could efficiently lower the energy levels compared with the alkoxyl‐substituted analogues, and the branched alkoxyl side chains could deepen the HOMO levels relative to the linear alkoxyl chains. The branched alkoxyl groups induced better lamellar‐like ordering, but poorer face‐to‐face packing behavior. The 2D conjugated side chains had a negative influence on the crystalline properties of the co‐polymers. The performance of the devices indicated that the branched alkoxyl side chains improved the V_oc, but decreased the J_sc and fill factor (FF). However, the 2D conjugated side chains would increase the V_oc, J_sc, and FF simultaneously. For the first time, our work provides insight into molecular design strategies through side‐chain engineering to achieve efficient polymer solar cells by considering both the degree of branching and dimensionality.     

Donor–Acceptor Oligoimides for Application in High‐Performance Electrical Memory Devices

Two new oligoimides, OI(APAP-6FDA) and OI(APAN-6FDA) , which consisted of electron‐donating N‐(4‐aminophenyl)‐N‐phenyl‐1‐aminopyrene ( APAP ) or N‐(4‐aminophenyl)‐N‐phenyl‐1‐aminonaphthalene ( APAN ) moieties and electron‐accepting 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride ( 6FDA ) moieties, were designed and synthesized for application in electrical memory devices. Such devices, with the indium tin oxide (ITO)/oligoimide/Al configuration, showed memory characteristics, from high‐conductance Ohmic current flow to negative differential resistance (NDR), with corresponding film thicknesses of 38 and 48 nm, respectively. The 48 nm oligoimide film device exhibited NDR electrical behavior, which resulted from the diffusion of Al atoms into the oligoimide layer. On further increasing the film thickness to 85 nm, the OI(APAP-6FDA) film device showed a reproducible nonvolatile “write once read many” (WORM) property with a high ON/OFF current ratio (more than ×10⁴). On the other hand, the device that was based on the 85 nm OI(APAN-6FDA) film exhibited a volatile static random access memory (SRAM) property. The longer conjugation length of the pyrene unit compared to that of a naphthalene unit was considered to be responsible for the different memory characteristics between these two oligoimides. These experimental results suggested that tunable switching behavior could be achieved through an appropriate design of the donor–acceptor oligoimide structure and controllable thickness of the active memory layer.     

Extended Conjugated Donor–Acceptor Molecules with E‐(1,2‐Difluorovinyl) and Diketopyrrolopyrrole (DPP) Moieties toward High‐Performance Ambipolar Organic Semiconductors

Two diketopyrrolopyrrole (DPP)‐based donor–acceptor (D–A) conjugated molecules, DPP‐F and DPP‐2F, which contain E‐(1,2‐difluorovinyl) moieties, are reported. The LUMO energies of DPP‐F and DPP‐2F were estimated to be ?3.49 and ?3.70 eV, respectively, based on their redox potentials and absorption spectral data; these values were clearly lowered because of the incorporation of electron‐withdrawing E‐(1,2‐difluorovinyl) moieties. Organic field‐effect transistors (OFETs) with thin films of DPP‐F and DPP‐2F were successfully fabricated with conventional techniques. Based on the respective transfer and output characteristics measured in an inert atmosphere, thin films of DPP‐2F display ambipolar semiconducting behavior with hole and electron mobilities reaching 0.42 and 0.80 cm² V^?1 s^?1, respectively. The as‐prepared OFET of DPP‐2F already shows high hole and electron mobilities that are not influenced remarkably by thermal annealing. For thin films of DPP‐F, only p‐type semiconducting behavior was observed in both an inert atmosphere and air, and the hole mobility increased to 0.1 cm² V^?1 s^?1 after thermal annealing. XRD and AFM studies were performed with thin films of DPP‐F and DPP‐2F after annealing at different temperatures.     

Benzimidazole/Pyridoimidazole‐Based Organic Sensitizers for High‐Performance Dye‐Sensitized Solar Cells

A new series of benzimidazole ( BIm )‐based dyes ( SC32 and SC33 ) and pyridoimidazole‐( PIm ) based dyes ( SC35, SC36N and SC36 ) were synthesized as sensitizers for dye‐sensitized solar cells (DSSCs). The N‐substituent and C‐substituent at the BIm and PIm cores were found to be the dominating factor in determining the electronic properties of the dyes and their DSSCs performance. The efficiency of BIm ‐based dyes ( SC35 and SC36 ) was found to be higher than that of the PIm ‐based dyes ( SC32 and SC33 ) due to better light harvesting. The C‐substituents in SC36 , a 4‐hexylloxybenzene and a hexyl chain, are beneficial to dark current suppression, and hence SC36 achieves the best efficiency of 7.38 % (≈85 % of N719 ). The two BIm dyes have better cell efficiencies than their congeners with a bithiophene entity between the BIm and the anchor due to better light harvesting of the former.     

HOMO Stabilisation in π‐Extended Dibenzotetrathiafulvalene Derivatives for Their Application in Organic Field‐Effect Transistors

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB‐TTF) central core and a 2,1,3‐chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB‐TTF, have been synthesised as active materials for organic field‐effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron‐withdrawing 2,1,3‐chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution‐processed single‐crystal transistors exhibit high performance with a hole mobility up to 0.04 cm² V^?1 s^?1 as well as good ambient stability.     

Dipolar Compounds Containing Fluorene and a Heteroaromatic Ring as the Conjugating Bridge for High‐Performance Dye‐Sensitized Solar Cells

A novel series of dipolar organic dyes containing diarylamine as the electron donor, 2‐cyanoacrylic acid as the electron acceptor, and fluorene and a heteroaromatic ring as the conjugating bridge have been developed and characterized. These metal‐free dyes exhibited very high molar extinction coefficients in the electronic absorption spectra and have been successfully fabricated as efficient nanocrystalline TiO₂ dye‐sensitized solar cells (DSSCs). The solar‐energy‐to‐electricity conversion efficiencies of DSSCs ranged from 4.92 to 6.88 %, which reached 68–96 % of a standard device of N719 fabricated and measured under the same conditions. With a TiO₂ film thickness of 6 μm, DSSCs based on these dyes had photocurrents surpassing that of the N719‐based device. DFT computation results on these dyes also provide detailed structural information in connection with their high cell performance.     

High‐Performance Förster Resonance Energy Transfer (FRET)‐Based Dye‐Sensitized Solar Cells: Rational Design of Quantum Dots for Wide Solar‐Spectrum Utilization

High‐performance Förster resonance energy transfer (FRET)‐based dye‐sensitized solar cells (DSSCs) have been successfully fabricated through the optimized design of a CdSe/CdS quantum‐dot (QD) donor and a dye acceptor. This simple approach enables quantum dots and dyes to simultaneously utilize the wide solar spectrum, thereby resulting in high conversion efficiency over a wide wavelength range. In addition, major parameters that affect the FRET interaction between donor and acceptor have been investigated including the fluorescent emission spectrum of QD, and the content of deposited QDs into the TiO₂ matrix. By judicious control of these parameters, the FRET interaction can be readily optimized for high photovoltaic performance. In addition, the as‐synthesized water‐soluble quantum dots were highly dispersed in a nanoporous TiO₂ matrix, thereby resulting in excellent contact between donors and acceptors. Importantly, high‐performance FRET‐based DSSCs can be prepared without any infrared (IR) dye synthetic procedures. This novel strategy offers great potential for applications of dye‐sensitized solar cells.     

Efficient Hole‐Transporting Materials with Triazole Core for High‐Efficiency Perovskite Solar Cells

Efficient hole‐transporting materials (HTMs), TAZ‐[MeOTPA]₂ and TAZ‐[MeOTPATh]₂ incorporating two electron‐rich diphenylamino side arms, through direct linkage or thiophen bridges, respectively, on the C3‐ and C5‐positions of a 4‐phenyl‐1,2,4‐triazole core were synthesized. These synthetic HTMs with donor–acceptor type molecular structures exhibited effective intramolecular charge transfer for improving the hole‐transporting properties. The structural modification of HTMs by thiophene bridging might increase intermolecular π–π stacking in the solid state and afford a better spectral response because of their increased π‐conjugation length. Perovskite‐based cells using TAZ‐[MeOTPA]₂ and TAZ‐[MeOTPATh]₂ as HTMs afforded high power conversion efficiencies of 10.9 % and 14.4 %, respectively, showing a photovoltaic performance comparable to that obtained using spiro‐OMeTAD. These synthetically simple and inexpensive HTMs hold promise for replacing the more expensive spiro‐OMeTAD in high‐efficiency perovskite solar cells.     

Metal‐Free Sensitizers for Dye‐Sensitized Solar Cells

This review focuses on our work on metal‐free sensitizers for dye‐sensitized solar cells (DSSCs). Sensitizers based on D?A′?π?A architecture (D is a donor, A is an acceptor, A′ is an electron‐deficient entity) exhibit better light harvesting than D?π?A‐type sensitizers. However, appropriate molecular design is needed to avoid excessive aggregation of negative charge at the electron‐deficient entity upon photoexcitation. Rigidified aromatics, including aromatic segments comprising fused electron‐excessive and ‐deficient units in the spacer, allow effective electronic communication, and good photoinduced charge transfer leads to excellent cell performance. Sensitizers with two anchors/acceptors, D(–π–A)₂, can more efficiently harvest light, inject electrons, and suppress dark current compared with congeners with a single anchor. Appropriate incorporation of heteroaromatic units in the spacer is beneficial to DSSC performance. High‐performance, aqueous‐based DSSCs can be achieved with a dual redox couple comprising imidazolium iodide and 2,2,6,6‐tetramethylpiperidin‐N‐oxyl, and/or using dyes of improved wettability through the incorporation of a triethylene oxide methyl ether chain.

     

A Solution‐Processable Donor–Acceptor Compound Containing Boron(III) Centers for Small‐Molecule‐Based High‐Performance Ternary Electronic Memory Devices

A novel small‐molecule boron(III)‐containing donor–acceptor compound has been synthesized and employed in the fabrication of solution‐processable electronic resistive memory devices. High ternary memory performances with low turn‐on (V_Th1=2.0 V) and distinct threshold voltages (V_Th2=3.3 V), small reading bias (1.0 V), and long retention time (>10⁴ seconds) with a large ON/OFF ratio of each state (current ratio of “OFF”, “ON1”, and “ON2”=1:10³:10⁶) have been demonstrated, suggestive of its potential application in high‐density data storage. The present design strategy provides new insight in the future design of memory devices with multi‐level transition states.     

Dye‐Incorporated Polynaphthalenediimide Acceptor for Additive‐Free High‐Performance All‐Polymer Solar Cells

All‐polymer solar cells (all‐PSCs) can offer unique advantages for applications in flexible devices, and naphthalene diimide (NDI)‐based polymer acceptors are the widely used polymer acceptors. However, their power conversion efficiency (PCE) still lags behind that of state‐of‐the‐art polymer solar cells, due to low light absorption, suboptimal energy levels and the strong aggregation of the NDI‐based polymer acceptor. Herein, a rhodanine‐based dye molecule was introduced into the NDI‐based polymer acceptor by simple random copolymerization and showed an improved light absorption coefficient, an up‐shifted lowest unoccupied molecular orbital level and reduced crystallization. Consequently, additive‐free all‐PSCs demonstrated a high PCE of 8.13 %, which is one of the highest performance characteristics reported for all‐PSCs to date. These results indicate that incorporating a dye into the n‐type polymer gives insight into the precise design of high‐performance polymer acceptors for all‐PSCs.     

Synthesis and Characterization of Donor–π‐Acceptor‐Based Porphyrin Sensitizers: Potential Application of Dye‐Sensitized Solar Cells

New porphyrin sensitizers based on donor–π‐acceptor (D‐π‐A) approach have been designed, synthesized, characterized by various spectroscopic techniques and their photovoltaic properties explored. N,N′‐Diphenylamine acts as donor, the porphyrin is the π‐spacer, and either carboxylic acid or cyanoacryclic acid acts as acceptor. All compounds were characterized by using ¹H NMR spectroscopy, ESI‐MS, UV–visible emission spectroscopies as well as electrochemical methods. The presence of aromatic groups between porphyrin π‐plane and acceptor group push the absorption of both Soret and Q‐bands of porphyrin towards the red region. The electrochemical properties suggests that LUMO of these sensitizers above the TiO₂ conduction band. Finally, the device was fabricated using liquid redox electrolyte (I^?/I₃^?) and its efficiency was compared with that of a leading sensitizer.     

1‐Alkyl‐1H‐imidazole‐Based Dipolar Organic Compounds for Dye‐Sensitized Solar Cells

A series of donor–π–acceptor‐type organic dyes based on 1‐alkyl‐1H‐imidazole spacers 1 , 2 , 3 , 4 , 5 have been developed and characterized. The two electron donors are at positions 4 and 5 of the imidazole, while the electron‐accepting cyanoacrylic acid is incorporated at position 2 by a spacer‐containing heteroaromatic rings, such as thiophene and thiazole. Detailed investigation on the relationship between the structure, spectral and electrochemical properties, and performance of DSSC is described here. Dye‐sensitized solar cells (DSSCs) using dyes as the sensitizers exhibit good efficiencies, ranging from 3.06 to 6.35 %, which reached 42–87 % with respect to that of N719‐based device (7.33 %) fabricated and measured under similar conditions. Time‐dependent density functional theory (TDDFT) calculations have been performed on the dyes, and the results show that both electron donors can contribute to electron injection upon photo‐excitation, either directly or indirectly by internal conversion to the lowest excited state.     

Modification of D–A–π–A Configuration toward a High‐Performance Triphenylamine‐Based Sensitizer for Dye‐Sensitized Solar Cells: A Theoretical Investigation

In an attempt to shed light on how the addition of a benzothiadiazole (BTD) moiety influences the properties of dyes, a series of newly designed triphenylamine‐based sensitizers incorporating a BTD unit as an additional electron‐withdrawing group in a specific donor–acceptor–π‐acceptor architecture has been investigated. We found that different positions of the BTD unit provided significantly different responses for light absorption. Among these, it was established that the further the BTD unit is away from the donor part, the broader the absorption spectra, which is an observation that can be applied to improve light‐harvesting ability. However, when the BTD unit is connected to the anchoring group a faster, unfavorable charge recombination takes place; therefore, a thiophene unit was inserted between these two acceptors, providing redshifted absorption spectra as well as blocking unfavorable charge recombination. The results of our calculations provide valuable information and illustrate the potential benefits of using computation‐aided sensitizer design prior to further experimental synthesis.     

Structure–Performance Correlations of Organic Dyes with an Electron‐Deficient Diphenylquinoxaline Moiety for Dye‐Sensitized Solar Cells

The high performances of dye‐sensitized solar cells (DSSCs) based on seven new dyes are disclosed. Herein, the synthesis and electrochemical and photophysical properties of a series of intentionally designed dipolar organic dyes and their application in DSSCs are reported. The molecular structures of the seven organic dyes are composed of a triphenylamine group as an electron donor, a cyanoacrylic acid as an electron acceptor, and an electron‐deficient diphenylquinoxaline moiety integrated in the π‐conjugated spacer between the electron donor and acceptor moieties. The DSSCs based on the dye DJ104 gave the best overall cell performance of 8.06 %; the efficiency of the DSSC based on the standard N719 dye under the same experimental conditions was 8.82 %. The spectral coverage of incident photon‐to‐electron conversion efficiencies extends to the onset at the near‐infrared region due to strong internal charge‐transfer transition as well as the effect of electron‐deficient diphenylquinoxaline to lower the energy gap in these organic dyes. A combined tetraphenyl segment as a hydrophobic barrier in these organic dyes effectively slows down the charge recombination from TiO₂ to the electrolyte and boosts the photovoltage, comparable to their Ru^II counterparts. Detailed spectroscopic studies have revealed the dye structure–cell performance correlations, to allow future design of efficient light‐harvesting organic dyes.     

Diketopyrrolopyrrole‐Based and Tetracyano‐Bridged Small Molecules for Bulk Heterojunction Organic Solar Cells

Research on bulk heterojunction organic solar cells has rapidly grown over the past two decades, and device performance has reached power conversion efficiencies over 13 %. In this focus review, we highlight design strategies used for the development of diketopyrrolopyrrole‐ and tetracyano‐based molecular donors. We also describe how tetracyano‐bridged non‐fullerene acceptors can be developed by a click‐type [2+2]‐cycloaddition–electrocyclic ring‐opening reaction of acetylene‐bridged small molecules with tetracyanoethylene by simple modification.     

Ternary Donor–Insulator–Acceptor Systems for Polymer Solar Cells

A series of block copolymers with fixed length of the semiconductor‐block poly(3‐butylthiophene) (P3BT) and varying length of the insulator‐block polystyrene (PS) are synthesized. These copolymers are blended with phenyl‐C61‐butyric acid methyl ester (PCBM) for the bulk heterojunction photoactive layers. With appropriate insulator‐block length and donor–acceptor ratio, the power conversion efficiency increases by one order of magnitude compared with reference devices with pure P3BT/PCBM. PS blocks improve the miscibility of the active layer blends remarkably. The P3BT‐b‐PS crystallizes as nanorods with the P3BT core covered with the PS‐block, which creates a nanoscale tunneling barrier between donor and acceptor leading to more efficient transportation of charge carriers in the semiconductors.