Fluorination-substitution effect on all-small-molecule organic solar cells

Due to the strong crystallinity and anisotropy of small molecules, matched molecular photoelectric properties and morphologies between small molecules and non-fullerene acceptors are especially important in all-small-molecule organic solar cells(OSCs).Introducing fluorine atoms has been proved as an effective strategy to achieve a high device performance through tuning molecular energy levels, absorption and assembly properties. Herein, we designed a novel benzodithiophene-based small molecule donor BDTF-CA with deep highest occupied molecular orbital(HOMO) energy level. All-small-molecule OSCs were fabricated by combing non-fullerene acceptor IDIC with different fluorine-atom numbers. Two or four fluorine atoms were introduced to the end-capped acceptor of IDIC, which are named as IDIC-2 F and IDIC-4 F, respectively. With the increase of fluorination from IDIC to IDIC-4 F, the open circuit voltage(V_(oc)) of the devices decreased, while hole and electron mobilities of the active layers increased by one order of magnitude. Contributed to the most balanced V_(oc), short-circuit current(J_(sc)) and fill factor(FF), the device based on BDTF-CA/IDIC-2 F achieved the highest power conversion efficiency of 9.11%.     

A low-energy-gap organic dye for high-performance small-molecule organic solar cells

A novel donor-acceptor-acceptor (D-A-A) donor molecule, DTDCTB, in which an electron-donating ditolylaminothienyl moiety and an electron-withdrawing dicyanovinylene moiety are bridged by another electron-accepting 2,1,3-benzothiadiazole block, has been synthesized and characterized. A vacuum-deposited organic solar cell employing DTDCTB combined with the electron acceptor C(70) achieved a record-high power conversion efficiency (PCE) of 5.81%. The respectable PCE is attributed to the solar spectral response extending to the near-IR region and the ultracompact absorption dipole stacking of the DTDCTB thin film.     

Development of small-molecule materials for high-performance organic solar cells

With the rapid development in recent years, small-molecule organic solar cell is challenging the dominance of its counterpart, polymer solar cell. The top power conversion efficiencies of both single and tandem solar cells based on small molecules have surpassed 9%. In this mini review, achievements of small molecules with impressive photovoltaic performance especially reported in the last two years were highlighted. The relationship between molecular structure and device performance was analyzed, which draws some rules for rational molecular design. Five series of p- and n-type small molecules were selected based on the consideration of their competitiveness of power conversion efficiencies.     

Enhancing the performances of all-small-molecule ternary organic solar cells via achieving optimized morphology and 3D charge pathways

With the emergence of non-fullerene acceptors (NFAs), the power conversion efficiencies (PCEs) of all-small-molecule organic solar cells (ASM-OSCs) have been significantly improved. However, due to the strong crystallinities of small molecules, it is much more challenging to obtain the ideal phase separation morphology and efficient charge transport pathways for ASM-OSCs. Here, a high-efficiency ternary ASM-OSC has been successfully constructed based on H11/IDIC-4F system by introduction of IDIC with a similar backbone as IDIC-4F but weak crystallinity. Notably, the addition of IDIC has effectively suppressed large-scale phase aggregation and optimized the morphology of the blend film. More importantly, the molecular orientation has also been significantly adjusted, and a mixed face-on and edge-on orientation has formed, thus establishing a more favorable three-dimensional (3D) charge pathways in the active layer. With these improvements, the enhanced short-circuit current density (J_SC) and fill factor (FF) of the ternary system have been achieved. In addition, because of the high lowest unoccupied molecular orbital (LUMO) energy level of IDIC as well as the alloyed structure of the IDIC and IDIC-4F, the promoted open circuit voltage (V_OC) of the ternary system has also been realized.     

High-efficiency all-small-molecule organic solar cells based on an organic molecule donor with an asymmetric thieno[2,3-f] benzofuran unit

Two p-type small molecules BDTT-TR and TBFT-TR with benzo[1,2-b′:4,5-b′]dithiophene(BDT) and thieno[2,3-f]benzofuran(TBF) as central core units are synthesized and used as donors in all-small-molecule organic solar cells(all-SMOSCs) with a narrow-bandgap small molecule Y6(2,2′-((2 Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3'′:4',5′]thieno[2′,3′: 4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1 H-indene-2,1-diylidene))dimalononitrile) as the acceptor. In comparison to BDTT-TR with centrosymmetric BDT as the central unit, TBFT-TR with asymmetric TBF as the central unit shows red-shifted absorption, higher charge-carrier mobility and better charge pathway in blend films. The power conversion efficiency(PCE) of the all-SMOSCs based on TBFT-TR:Y6 reaches 14.03% with a higher short-circuit current density of 24.59 m A cm-2 and a higher fill factor of72.78% compared to the BDTT-TR:Y6 system. The PCE of 14.03% is among the top efficiencies of all-SMOSCs reported in the literature to date.     

Self-doped n-type small molecular electron transport materials for high-performance organic solar cells

Two naphthalene diimide (NDI) and perylene diimide (PDI) based n-type water/alcohol soluble small molecules (NFN and PFP) are designed and utilized as electron transport layers (ETLs) for organic solar cells (OSCs). NFN and PFP are synthesized by using Sonogashira coupling from alkynyl modified fluorene with mono-bromo substituted NDI and PDI. Density functional theory study results of NFN and PFP show that they possess excellent planarity due to the employment of triple bonds as connection units. Moreover, it was shown by electron paramagnetic resonance study that both NFN and PFP possess obvious self-doping behaviors, which may effectively enhance their charge transporting capability as ETLs in OSCs. Power conversion efficiencies of 8.59% and 9.80% can be achieved for OSCs with NFN and PFP as ETLs, respectively. The higher power conversion efficiency (PCE) of PFP based photovoltaic device is originated from the stronger doping property and higher mobility of PFP.     

Optimal bulk-heterojunction morphology enabled by fibril network strategy for high-performance organic solar cells

A bicontinuous network formed spontaneously upon film preparation is highly desirable for bulk-heterojunction(BHJ) organic solar cells(OSCs). Many donor-acceptor(D-A) type conjugated polymers can self-assemble into polymer fibrils in the solid state and such fibril-assembly can construct the morphological framework by forming a network structure, inducing the formation of ideal BHJ morphology. Our recent works have revealed that the fibril network strategy(FNS) can control the blend morphology in fullerene, non-fullerene and ternary OSCs. It has been shown that the formation of fibril network can optimize phase separation scale and ensure efficient exciton dissociation and charge carriers transport, thus leading to impressive power conversion efficiencies(PCEs) and high fill factor(FF) values. We believe that FNS will provide a promising approach for the optimization of active layer morphology and the improvement of photovoltaic performance, and further promote the commercialization of OSCs.     

Molecular design strategies for high-performance organic electrochemical transistors

Organic electrochemical transistors (OECTs) utilize ion flow from the electrolyte to modulate the electrical conductivity of the whole bulk organic semiconductor channel. With the characteristic of mixed ionic-electronic conducting in the entire volume, OECTs exhibit high transconductance and act as good transducers, particularly in bioelectronics. To gain high-performance OECTs, developing novel high-performance polymeric semiconductors is important. In this article, operation principles, performance evaluations, and polymerization methods are first discussed. We then analyze the molecular design strategies for high-performance OECT materials and highlight the characteristics and effects of backbone design and side chain engineering. Finally, we discuss some neglected and unsolved issues and provide an outlook for the OECTs research and development.     

Synergistic effect of solvent and solid additives on morphology optimization for high-performance organic solar cells

Science China Chemistry - Controlling the photoactive layer morphology towards nanoscale bi-continuous donor/acceptor interpenetrating networks is a key issue to build high-performance organic...     

Oxoammonium enabled secondary doping of hole transporting material PEDOT:PSS for high-performance organic solar cells

Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate(PEDOT:PSS) is one of the most widely used hole transporting materials in organic solar cells(OSCs). Multiple strategies have been adopted to improve the conductivity of PEDOT:PSS, however,effective strategy that can optimize the conductivity, work function, and surface energy simultaneously to reach a better energy alignment and interface contact is rare. Here, we demonstrate that oxoammonium salts(TEMPO~+X~-) with different counterions can act as facile and novel dopants to realize secondary doping of PEDOT:PSS. The effective charge transfer process achieved between TEMPO~+X~- and PEDOT:PSS results in enhanced carrier density and improved conductivity of PEDOT:PSS. Moreover,different counterions of TEMPO~+X~- can tune the work function and surface energy of PEDOT:PSS, enabling improved device performances. The resulting device with PM6:Y6 as the active layer shows a high power conversion efficiency(PCE) over 16%.Moreover, this doping strategy can also be applied to other conjugated polymers such as poly(3-hexylthiophene). This work provides a promising strategy to tune the properties of conjugated polymers through doping, thus effectively boosting the performance of organic solar cells.     

Printable SnO2 cathode interlayer with up to 500 nm thickness-tolerance for high-performance and large-area organic solar cells

The printable electrode interlayer with excellent thickness tolerance is crucial for mass production of organic solar cells(OSCs)by solution-based print techniques. Herein, high-quality printable SnO_2 films are simply fabricated by spin-coating or bladecoating the chemical precipitated SnO_2 colloid precursor with post thermal annealing treatment. The SnO_2 films possess outstanding optical and electrical properties, especially extreme thickness-insensitivity. The interfacial electron trap density of SnO_2 cathode interlayers(CILs) are very low and show negligible increase as the thicknesses increase from 10 to 160 nm,resulting in slight change of the power conversion efficiencies(PCEs) of the PM6:Y6 based OSCs from 16.10% to 13.07%. For blade-coated SnO_2 CIL, the PCE remains high up to 12.08% even the thickness of SnO_2 CIL is high up to 530 nm. More strikingly, the large-area OSCs of 100 mm~2 with printed SnO_2 CILs obtain a high efficiency of 12.74%. To the best of our knowledge, this work presents the first example for the high-performance and large-area OSCs with the thickness-insensitive SnO_2 CIL.     

Furan-containing conjugated polymers for organic solar cells

Development of organic semiconductors is one of the most intriguing and productive topics in material science and engineering. Many efforts have been made on the synthesis of aromatic building blocks such as benzene, thiophene and pyrrole due to the facile preparation accompanied by the intrinsic environmental stability and relatively efficient properties of the resulting polymers. In the past, furan has been less explored in this field because of its high oxidation potential. Recently, furan has attracted obsession due to its weaker aromaticity, the greater solubilities of furan-containing π-conjugated polymers relative to other benzenoid systems and the accessibility of furan-based starting materials from renewable resources. This review elaborates the advancements of organic photovoltaic polymers containing furan building blocks. The uniqueness and advantages of furan-containing building blocks in semiconducting materials are also discussed.     

A-D-A-D-A-type oligothiophenes for vacuum-deposited organic solar cells

Novel A-D-A-D-A-type oligothiophenes incorporating electron-withdrawing benzo[c][1,2,5]thiadiazole (BTDA) as core and trifluoroacetyl (TFA) as terminal acceptor groups have been developed. Vacuum-processed planar heterojunction organic solar cells incorporating these new oligomers as donor and C(60) as acceptor showed very high open circuit voltages up to 1.17 V, resulting in power conversion efficiencies of 1.56% under AM1.5G conditions.     

Triaryl linked donor acceptor dyads for high-performance dye-sensitized solar cells

The effect of changing substituents of organic dyes for their performance on dye-sensitized solar cells (DSSCs) is examined. These dyes consist of an aromatic amine donor group, a cyanoacrylic acid acceptor group, and a triaryl spacer group, while they are linked together by consecutive palladium catalyzed coupling reactions. These materials exhibit strong charge transfer absorption bands in the UV/vis region. Their redox potential levels were estimated by cyclic voltammetry, and found to suit well to the charge flow in DSSCs. Adding electron-donating substituents on the phenyl groups of aromatic amines increased the electron density on the donor groups, therefore reduced the HOMO/LUMO band gap. These dyes were chemisorbed on the surface of nanocrystalline TiO₂, and fabricated into DSSCs through standard operations. For a typical device the maximal monochromatic incident photon-to-current conversion efficiency (IPCE) can reach to 80%, with a short-circuit photocurrent density (J_sc) 16.34 mA cm⁻², an open-circuit photovoltage (V_oc) 0.68 V, and fill factor (FF) 0.55, which corresponds to an overall conversion efficiency of 6.05%.     

New tetrazole-based organic dyes for dye-sensitized solar cells

A series of new metal-free organic dyes that contain donors with triphenylamine or its derivatives and tetrazole-based acceptors were synthesized and characterized by photophysical, electrochemical, and theoretical computational methods. They were applied in nanocrystalline TiO_2 solar cells(DSSCs). It is found that the introduction of diphenylamine units as antennas in the as-synthesized dyes could improve photovoltaic performance compared with phenothiazine and carbazole units as antennas in DSSCs. The dye with(2H-tetrazol-5-yl) acrylonitrile electron acceptor also displayed the highest solar-to-electrical energy conversion efficiency.     

Novel pyrenoimidazole-based organic dyes for dye-sensitized solar cells

A novel class of organic dyes containing pyrenoimidazole donors, cyanoacrylic acid acceptors, and oligothiophene π-linkers has been synthesized and characterized. The electro-optical properties of these dyes can be tuned by changing the conjugation length of the π-linkers. A dye containing terthiophene in the conjugation pathway exhibited a solar energy-to-electricity conversion efficiency of 5.65%.     

An easily accessible isoindigo-based polymer for high-performance polymer solar cells

A new, low-band-gap alternating copolymer consisting of terthiophene and isoindigo has been designed and synthesized. Solar cells based on this polymer and PC(71)BM show a power conversion efficiency of 6.3%, which is a record for polymer solar cells based on a polymer with an optical band gap below 1.5 eV. This work demonstrates the great potential of isoindigo moieties as electron-deficient units for building donor-acceptor-type polymers for high-performance polymer solar cells.     

Biopolymer passivation for high-performance perovskite solar cells by blade coating

Thin films of perovskite deposited from solution inevitably introduce large number of defects,which serve as recombination centers and are detrimental for solar cell performance.Although many small molecules and polymers have been delicately designed to migrate defects of perovskite films,exploiting credible passivation agents based on natural materials would offer an alternative approach.Here,an ecofriendly and cost-effective biomaterial,ploy-L-lysine(PLL),is identified to effectively passivate the defects of perovskite films prepared by blade-coating.It is found that incorporation of a small amount(2.5 mg mL^-1)of PLL significantly boosts the performance of printed devices,yielding a high efficiency of 19.45% with an increase in open-circuit voltage by up to 100 mV.Density functional theory calculations combined with X-ray photoelectron spectroscopy reveal that the functional groups(-NH2,-COOH)of PLL effectively migrate the Pb-I antisite defects via Pb-N coordination and suppress the formation of metallic Pb in the blade-coated perovskite film.This work suggests a viable avenue to exploit passivation agents from natural materials for preparation of high-quality perovskite layers for optoelectronic applications.     

Novel organic dyes for efficient dye-sensitized solar cells

Two novel metal-free organic dyes containing thienothiophene and thiophene segments have been synthesized. Nano-crystalline TiO2 dye-sensitized solar cells were fabricated using these dyes as light-harvesting sensitizers, and a high solar energy-to-electricity conversion efficiency of 6.23% was achieved.     

Light harvesting tetrafullerene nanoarray for organic solar cells

A light absorbing pi-conjugated oligomer-tetrafullerene nanoarray has been synthesized and its photophysical study reveals the presence of an intramolecular energy transfer. A phototovoltaic device fabricated from this nanoarray and poly(3-hexylthiophene) shows an external quantum efficiency of 15% at 500 nm.