Lianjie Zhang

Low Band‐Gap Conjugated Polymers with Strong Interchain Aggregation and Very High Hole Mobility Towards Highly Efficient Thick‐Film Polymer Solar Cells

Absorption spectra of polymer FBT-Th4 (1,4) (M n = 46.4 Kg/mol, E g = 1.62 eV, and HOMO = -5.36 eV) indicate strong interchain aggregation ability. High hole mobilities up to 1.92 cm(2) (V s)(-1) are demonstrated in OFETs fabricated under mild conditions. Inverted solar cells with active layer thicknesses ranging from 100 to 440 nm display PCEs exceeding 6.5%, with the highest efficiency of 7.64% achieved with a 230 nm thick active layer.

Largely Enhanced Efficiency with a PFN/Al Bilayer Cathode in High Efficiency Bulk Heterojunction Photovoltaic Cells with a Low Bandgap Polycarbazole Donor

Polymeric photovoltaic cells (PVCs) have attracted considerable attention over the past several years because of their unique advantages of low cost, light weight, and great potential for the realization of fl exible and large-area devices. [ 1 , 2 ] Typically, bulk heterojunction (BHJ) PVCs, a promising device confi guration for high power conversion effi ciency (PCE), involve the use of a phase-separated blend of an electron-donating conjugated poly mer and an electron-accepting fullerene derivative as the active layer. [ 3 − 10 ] Judicious design of a polymer donor and selection of a fullerene acceptor have realized high effi ciency BHJ PVCs with PCE values over 5%. [ 11 − 16 ] Tremendous efforts have also been made to optimize the active layer formation and the device confi guration. Many optimization methods, such as using different solvents to fabricate the active layer, [ 4 ] thermal annealing of the active layer or the device, [ 17 ] fi lm forming speed, [ 18 ] the addition of additives to the active layer, [ 19 ] the use of an optical spacer, [ 15 ]

Hydrophilic poly(triphenylamines) with phosphonate groups on the side chains: synthesis and photovoltaic applications

Two triphenylamine-based homopolymers PTPA-EP and PTPA-PO3Na2, comprising diethyl phosphonate and sodium phosphonate end groups on side chains, respectively, were synthesized. The UV-vis absorption and photoluminescence (PL) properties of the PTPA-EP and PTPA-PO3Na2 are mainly determined by the conjugated poly(triphenylamine) main chain. The PTPA-EP and PTPA-PO3Na2 possess comparable HOMO levels of around −5.03 eV. The PTPA-EP, with better solubility than PTPA-PO3Na2 in hydrophilic solvents, was utilized as cathode interlayer to construct efficient bulk-heterojunction photovoltaic cells with a low bandgap poly(2,7-carbazole) (PCDTBT) as the polymer donor and [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) as the acceptor. The work function of ITO was shifted to −4.3 eV by PTPA-EP, which matches well with the LUMO level of PC71BM for good electron extraction. Inverted solar cells with a device configuration of ITO/PTPA-EP/active layer/MoO3/Al exhibited a power conversion efficiency (PCE) of 4.59%, which is a good efficiency among inverted solar cells with an organic interlayer on an ITO cathode. The PCE shows a 79% increase in comparison to that of a bare ITO cathode, though the efficiency is lower than 5.13% for an inverted solar cell with an inorganic ZnO interlayer on ITO. Moreover, a conventional solar cell with a device configuration of ITO/PEDOT:PSS/active layer/PTPA-EP/Al could show a better PCE of 5.27%. The results indicate that PTPA-EP is a promising new cathode interlayer for high efficiency inverted and conventional solar cells.

Low band gap conjugated polymers combining siloxane-terminated side chains and alkyl side chains: side-chain engineering achieving a large active layer processing window for PCE > 10% in polymer solar cells

Alternating and random conjugated copolymers with a siloxane-terminated side chain for a repeating unit based on 5,6-difluoro[2,1,3]benzothiadiazole (FBT) and quarterthiophene (4T) were synthesized, among which side-chain random copolymers PFBT4T-C5Si-50% and PFBT4T-C5Si-25% with low contents of 50% and 25% siloxane-terminated side chains, respectively, in conjunction with alkyl side chains were found to be more suitable for optoelectronic applications due to good film-forming in solution processing. Grazing incidence X-ray diffraction (GIXD) indicated that the siloxane-terminated side chain could induce PFBT4T-C5Si-50% and PFBT4T-C5Si-25% with face-on orientations, giving high 3-D hole transport in neat films as supported by a high hole mobility up to 2.46 cm2 V−1 s−1 in field-effect transistors and an SCLC hole mobility up to 5.9 × 10−2 cm2 V−1 s−1 in hole-only devices. Fast SCLC hole and electron transports were seen for their bulk-heterojunction (BHJ) blend films with PC71BM as the acceptor, due to the retention of a polymer face-on orientation. The BHJ blend film of PFBT4T-C5Si-25% showed lower film surface roughness, more balanced hole and electron transport, and relatively smaller phase separation when compared with PFBT4T-C5Si-50%, as evidenced by atomic force microscopy (AFM), transmission electron microscopy (TEM), SCLC, and resonant soft X-ray scattering (RSoXS) measurements. The PFBT4T-C5Si-25%-based PSCs with 270, 420, and 600 nm thick active layers exhibited outstanding power conversion efficiencies (PCEs) of 10.39%, 11.09%, and 10.15%, respectively, readily offering a high thickness tolerance to achieve an unprecedented wide active layer processing window for PCE > 10%. This is also the first PCE of more than 10% achieved by an active layer of a 600 nm thickness level in PSCs. Another notable feature is very high fill factors of more than 74% and 71% being achieved for very thick active layers of 420 and 600 nm, respectively. The results suggest that side-chain engineering through the incorporation of a partial siloxane-terminated side chain is a unique handle to afford new photovoltaic polymers with enhanced vertical carrier transport towards application in roll-to-roll processing of PSCs.

An extended π-conjugated area of electron-donating units in D–A structured polymers towards high-mobility field-effect transistors and highly efficient polymer solar cells

Two D–A conjugated polymers, FBT-DThDT-1T and FBT-DThDT-TT, using 5,6-difluoro-2,1,3-thiadiazole (FBT) as the electron-accepting unit, and terthiophene or 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene as the electron-donating unit, respectively, were synthesized. Among them, the first batch of FBT-DThDT-TT with relatively low molecular weight (MW) can be denoted as FBT-DThDT-TT-L and the second batch of FBT-DThDT-TT with much higher MW can be denoted as FBT-DThDT-TT-H. FBT-DThDT-1T possesses a low FET hole mobility of 2.6 × 10−3 cm2 (V s)−1 and a poor power conversion efficiency (PCE) of 0.91% in inverted polymer solar cells (i-PSCs) under the illumination of AM1.5G, 100 mW cm−2 light. Compared with FBT-DThDT-1T, FBT-DThDT-TT with extended π-conjugation bears a TT replacing the middle thiophene of terthiophene on the backbone, which would increase the coplanarity of the polymer and thus facilitate both intermolecular packing and charge transport. FBT-DThDT-TT shows strong interchain aggregation in a room temperature solution, its absorption spectra in a room temperature solution and in a thin film were almost identical. The field-effect transistors based on FBT-DThDT-TT-L and FBT-DThDT-TT-H show improved hole mobilities of 0.38 and 0.20 cm2 (V s)−1, respectively. The i-PSCs based on FBT-DThDT-TT-L show a better PCE of 3.47%, and the i-PSCs based on FBT-DThDT-TT-H with a higher MW exhibit the best PCE up to 7.78%, with highly improved absorption capacity and miscibility with PC71BM. Moreover, with a 355 nm thick active layer, a PCE of 6.72% with a high FF of 67.8% is still obtained for FBT-DThDT-TT-H-based devices. The impressive results make FBT-DThDT-TT a promising candidate for applications of large-scale solution-processable PSCs.

Low band-gap benzodithiophene-thienothiophenecopolymers: the effect of dual two-dimensional substitutions on optoelectronic properties

Two new conjugated copolymers, PBDT-T6-TTF and PBDT-T12-TTF, were derived from a novel 4-fluorobenzoyl thienothiophene (TTF). In addition, two types of benzodithiophene (BDT) units with 2,3-dihexylthienyl (T6) and 2,3-didodecylthienyl (T12) substituents, respectively, were successfully synthesized. The effect of the dual two-dimensional (2D) substitutions of the building blocks upon the optoelectronic properties of the polymers was investigated. Generally, the two polymers exhibited good solubility and broad absorption, showing similar optical band gaps of ∼1.53 eV. However, PBDT-T6-TTF with its shorter alkyl chain length possessed a larger extinction coefficient in thin solid film. The highest occupied molecular orbital (HOMO) level of PBDT-T6-TTF was located at −5.38 eV while that of PBDT-T12-TTF was at −5.51 eV. In space charge-limited-current (SCLC) measurement, PBDT-T6-TTF and PBDT-T12-TTF displayed respective hole mobilities of 3.0×10−4 and 1.6×10−5 cm2 V−1s−1. In polymer solar cells, PBDT-T6-TTF and PBDT-T12-TTF showed respective power conversion efficiencies (PCEs) of 2.86% and 1.67%. When 1,8-diiodooctane (DIO) was used as the solvent additive, the PCE of PBDT-T6-TTF was remarkably elevated to 4.85%, but the use of DIO for the PBDT-T12-TTF-blend film resulted in a lower PCE of 0.91%. Atomic force microscopy (AFM) indicated that the superior efficiency of PBDT-T6-TTF with 3% DIO (v/v) should be related to the better continuous phase separation of the blend film. Nevertheless, the morphology of the PBDT-T12-TTF deteriorated when the 3% DIO (v/v) was added. Our results suggest that the alkyl-chain length on the 2D BDT units play an important role in determining the optoelectronic properties of dual 2D BDT-TT-based polymers.

A Highly Crystalline Wide-Band-Gap Conjugated Polymer toward High-Performance As-Cast Nonfullerene Polymer Solar Cells

A new wide-band-gap conjugated polymer PBODT was successfully synthesized that showed high crystallinity and was utilized as the active material in nonfullerene bulk-heterojunction polymer solar cells (PSCs). The photovoltaic devices based on the as-cast blend films of PBODT with ITIC and IDIC acceptors showed notable power conversion efficiencies (PCEs) of 7.06% and 9.09%, with high open-circuit voltages of 1.00 and 0.93 V that correspond to low energy losses of 0.59 and 0.69 eV, respectively. In the case of PBODT:ITIC, lower exciton quenching efficiency and monomolecular recombination are found for devices with small driving force. On the other hand, the relatively higher driving force and suppressed monomolecular recombination for PBODT:IDIC devices are identified to be the reason for their higher short-circuit current density (Jsc) and higher PCEs. In addition, when processed with the nonchlorinated solvent 1,2,4-trimethylbenzene, a good PCE of 8.19% was still achieved for the IDIC-based device. Our work shows that such wide-band-gap polymers have great potential for the environmentally friendly fabrication of highly efficient PSCs.

Para-Azaquinodimethane: A Compact Quinodimethane Variant as an Ambient Stable Building Block for High-Performance Low Band Gap Polymers

Quinoidal structures incorporating expanded para-quinodimethane (p-QM) units have garnered great interest as functional organic electronic, optical, and magnetic materials. The direct use of the compact p-QM unit as an electronic building block, however, has been inhibited by the high reactivity conveyed by its biradical character. Herein, we introduce a stable p-QM variant, namely p-azaquinodimethane (p-AQM), that incorporates nitrogen atoms in the central ring and alkoxy substituents on the periphery to increase the stability of the quinoidal structure. The succinct synthesis from readily available precursors leads to regio- and stereospecific p-AQMs that can be readily integrated into the backbone of conjugated polymers. The quinoidal character of the p-AQM unit endows the resulting polymers with narrow band gaps and high carrier transport mobilities. The study of a series of copolymers employing different numbers of thiophene units revealed an unconventional trend in band gaps, which is distinct from the widely adopted donor-acceptor approach to tuning the band gaps of conjugated polymers. Theoretical calculations have shed light on the nature of this trend, which may provide a unique class of conjugated polymers with promising optical and electronic properties.

Hydrophilic Conjugated Polymers with Large Bandgaps and Deep-Lying HOMO Levels as an Efficient Cathode Interlayer in Inverted Polymer Solar Cells

Two hydrophilic conjugated polymers, PmP-NOH and PmP36F-NOH, with polar diethanol-amine on the side chains and main chain structures of poly(meta-phenylene) and poly(meta-phenylene-alt-3,6-fluorene), respectively, are successfully synthesized. The films of PmP-NOH and PmP36F-NOH show absorption edges at 340 and 343 nm, respectively. The calculated optical bandgaps of the two polymers are 3.65 and 3.62 eV, respectively, the largest ones so far reported for hydrophilic conjugated polymers. PmP-NOH and PmP36F-NOH also possess deep-lying highest occupied molecular orbital levels of -6.19 and -6.15 eV, respectively. Inserting PmP-NOH and PmP36F-NOH as a cathode interlayer in inverted polymer solar cells with a PTB7/PC71 BM blend as the active layer, high power conversion efficiencies of 8.58% and 8.33%, respectively, are achieved, demonstrating that the two hydrophilic polymers are excellent interlayers for efficient inverted polymer solar cells.

Solution-processed small molecules with ethynylene bridges for highly efficient organic solar cells

Two acceptor–donor–acceptor (A–D–A) conjugated molecules DPP-E-BDT and DPP-E-BDT-T, using diketopyrrolopyrrole (DPP) as the A-unit, ethynylene bridge flanked benzo-[1,2-b:4,5-b′]dithiophene (E-BDT) as the central D-unit, and different 4,8-substitutions on the BDT were synthesized by Sonogashira coupling reactions for solution-processed organic solar cells (OSCs). The insertion of electron-withdrawing ethynylene bridges (sp hybridization) in the DPP-E-BDT and DPP-E-BDT-T molecules leads to planar and enlarged aromatic skeletons, larger band gaps, and deeper HOMO levels. 4,8-Dithienyl substitution on BDT in DPP-E-BDT-T results in additional conjugation extension to give a slightly smaller band gap compared to the 4,8-dialkoxy substitution in target molecules. Bulk heterojunction solar cells using DPP-E-BDT and DPP-E-BDT-T as the donor materials and fullerene acceptor showed a high open-circuit voltage of 0.89 V and moderate current densities of 10.9 mA cm−2. Besides, quite high fill factors (73.6%) could be obtained. Power conversion efficiencies (PCE) of 7.12% were obtained for DPP-E-BDT-T blends, which is the highest efficiency among small molecules based on DPP and BDT units. In active layer fabrication, 1,8-diiodooctane (DIO) was used as a solvent additive and subsequent thermal annealing treatment was also employed. We saw that these combined treatments led to balanced hole and electron transports, with values around 1.2 × 10−4 cm2 V−1 s−1 for the active layers. These results demonstrated that ethynylene bridges in small molecule donors are quite useful, both in tuning the electronic structure and in defining the thin film morphology, thus would be a promising method to enhance photovoltaic performances of the resulting materials.