M. L. Keshtov

New conjugated alternating benzodithiophene-containing copolymers with different acceptor units: synthesis and photovoltaic application

Two new alternating low band gap D–A copolymers with different acceptor structures of 4,8-bis-(5-bromothiophene-2-yl)-benzo[1,2,5]thiadiazole (P1) and 4,8-dithiophene-2-yl-benzo[1,2-c;4,5-c′]-bis-[1,2,5]thiadiazole (P2) and a common BDT donor segment have been synthesized under Stille reaction conditions and characterized. The polymers showed good solubility, broad absorption bands and optical band gaps of 1.62 eV and 1.16 eV for P1 and P2, respectively. Bulk heterojunction (BHJ) polymer solar cells based on P1 and P2 as electron donors and fullerene derivatives (PC60BM and PC70BM) as acceptor were fabricated and their photovoltaic response was investigated. The overall power conversion efficieny (PCE) achieved for BHJ solar cells based on P1:PC60BM, P2:PC60BM, P1:PC70BM and P2:PC70BM blends cast from THF solvent is about 2.17%, 0.80%, 3.45% and 1.19%, respectively. The higher PCE for the device based on P1 has been attributed to the high value of hole mobility for P1 as compared to P2 and a larger driving force i.e. LUMO–LUMO offset, for photo-induced charge transfer for P1:PCBM BHJ active layer. The PCE has been further increased up to 5.30% and 1.58% for P1:PC70BM and P2:PC70BM blends cast from DIO/THF solvent, which is attributed to the improved crystallinity and a more balanced charge transport in the device.

Synthesis and characterization of a low band gap quinoxaline based D–A copolymer and its application as a donor for bulk heterojunction polymer solar cells

A new alternating copolymer P comprising of benzo[1,2-b;4,5,b′]dithiophene (BDT) derivative and 4,9-bis-(5-bromothiophene-2-yl)-6,7-di-(2-ethylhexyl)-[1,2,5]thiadiazolo[3,4-g]quinoxaline (DTQx) derivative electron, donating and electron withdrawing units, respectively, has been synthesized by Stille reaction. The copolymer was characterized by TGA, UV-visible absorption and cyclic voltammetry. The optical band gap of P was calculated from the onset wavelength of absorption to be about 1.38 eV. The copolymer P was used as electron donor along with PC60BM or PC70BM as electron acceptors for the fabrication of bulk heterojunction solar cells with configurations of ITO/PEDOT:PSS/P:PC60BM or PC70BM/Al. The power conversion efficiencies (PCE) of the copolymer solar cells blended with PC60BM and PC70BM as electron acceptors, spin cast from THF solvent, were 2.10% and 3.26%, respectively. The PCE device based on the P:PC70BM blend processed from DIO/THF was enhanced up to 4.47%. After optimizing the device parameters, such as blend ratio of P to PCBM and the choice of processing solvent, power conversion efficiency reaches as high as 5.12% for P:PC70BM blend, when processed from DIO/THF solvent, a blend ratio of 1 : 2 w/w and DMSO doped PEDOT:PSS buffer layer is used.

Thienopyrazine or dithiadiazatrindene containing low band gap conjugated polymers for polymer solar cells

Four new low-band-gap alternating copolymers (P-1, P-2, P-3 and P-4) based on electron-rich benzodithiophene and newly developed electron-deficient units, thienopyrazine or dithiadiazatrindene derivatives, were synthesized by Stille polycondensation. All polymers exhibit good solubility in common organic solvents and a broad absorption band in the visible to near-infrared regions. The film optical band gaps of the polymers are in the range of 1.28–2.07 eV and the highest occupied molecular orbital (HOMO) energy levels are in the range of −4.99 eV to −5.28 eV. Bulk heterojunction polymer solar cells (PSCs) of the polymers were fabricated with phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor material, and a power conversion efficiency of 0.80% was realized with P-1 as donor material.

Small molecule carbazole-based diketopyrrolopyrroles with tetracyanobutadiene acceptor unit as a non-fullerene acceptor for bulk heterojunction organic solar cells

Herein, we investigated the photovoltaic properties of carbazole-based diketopyrrolopyrroles with tetracyanobutadiene acceptor units as highly efficient non-fullerene acceptors together with a D–A conjugated polymer, P, as a donor for polymer solar cells. After optimization, i.e. donor to acceptor weight ratio and solvent vapour annealing, the polymer solar cells exhibited power conversion efficiencies of up to 4.86% and 7.19% for DPP7 and DPP8 as acceptors, respectively. These results indicate that slight changes in the chemical structure of the small acceptor molecule significantly increase the efficiency of the device. The design and synthesis of these non-fullerene acceptors with broader absorption spectra extended towards the near infrared region may be key for the further development of high-performance and cost-effective solution-processed organic solar cells.

Unprecedented low energy losses in organic solar cells with high external quantum efficiencies by employing non-fullerene electron acceptors

In order to realize high photocurrent generation in the low-energy region of the solar spectrum, two conjugated A–D–A oligomers, PY-1 and DCI-2 comprising a central dithieno[3,2-b:2′,3′-d]pyrrole donor (D) unit and end-capping acceptors (A) 1-butyl-4-methyl-2,6-dioxopyridine-3-carbonitrile (PY) or 3-dicyanomethyleneindan-1-one (DCI) have been synthesized and characterized. The oligomers showed strong absorptions in the red and near-IR region in solution and in the solid state. As a consequence of the strong electron-accepting character, the absorption band of DCI-2 in thin films is significantly red-shifted compared to that of PY-1 resulting in a low optical energy gap of 1.23 eV. In optimized solution-processed bulk-heterojunction solar cells using a polymeric donor P1, the new non-fullerene acceptor DCI-2 provides an excellent power conversion efficiency of 6.94% which is noticeably higher than that of PY-1-based devices (PCE = 4.89%). Most importantly, a high open-circuit voltage (VOC) of ∼0.8 V with unprecedented energy losses, between the optical energy gap and the open-circuit voltage, between 0.39 and 0.43 eV concomitant with excellent external quantum efficiencies of 69%@880 nm in the NIR-regime have been achieved for DCI-2-based devices.

Efficient Polymer Solar Cells with High Open-Circuit Voltage Containing Diketopyrrolopyrrole-Based Non-Fullerene Acceptor Core End-Capped with Rhodanine Units

Herein we report the synthesis of a novel A-D-A-D-A non-fullerene small-molecule acceptor (NFSMA) bearing a diketopyrrolopyrrole (DPP) acceptor central core coupled to terminal rhodanine acceptors via a thiophene donor linker (denoted as MPU1) for use in non-fullerene polymer solar cells (PSCs). This NFSMA exhibits a narrow optical band gap (1.48 eV), strong absorption in the 600-800 nm wavelength region of the solar spectrum, and a lowest unoccupied energy level of -3.99 eV. When the mixture of a medium band gap D-A copolymer P (1.75 eV) was used as donor and MPU1 as acceptor, the blend film showed a broad absorption profile from 400 to 850 nm, beneficial for light harvesting efficiency of the resulted polymer solar cell. After optimization of the donor-to-acceptor weight ratios and concentration of solvent additive, the P-MPU1-based PSC exhibited a power conversion efficiency of 7.52% (Jsc= 12.37 mA/cm2, Voc = 0.98 V, and fill factor = 0.62), which is much higher than that for a P3HT-MPU1-based device (2.16%) prepared under identical conditions. The higher value for the P-MPU1-based device relative to the P3HT-MPU1-based one is related to the low energy loss and more balanced charge transport in the device based on the P donor. These results indicate that alteration of the absorption spectra and electrochemical energy levels of non-fullerene acceptors, and appropriate selection of the polymer donor with complementary absorption profile, is a promising means to further boost the performance of PSCs.

Design and synthesis of new ultra-low band gap thiadiazoloquinoxaline-based polymers for near-infrared organic photovoltaic application

Two D–A copolymers, F1 and F2, with fluorene and thiazole units were substituted, respectively, on a thiadiazoloquinoxaline (TDQ) unit to enhance the electron-accepting strength of TDQ. The copolymers were synthesized by a cross-coupling Stille reaction and their optical and electrochemical properties were examined, which revealed that they have ultra-low band gaps and absorption in the near-infrared. These copolymers were employed as donors along with PC71BM as an electron acceptor for the fabrication of solution-processed bulk heterojunction (BHJ) polymer solar cells. After the optimization of the donor-to-acceptor weight ratio and the solvent additive (4 v% DIO as solvent additive), devices with F1:PC71BM and F2:PC71BM displayed power conversion efficiencies (PCEs) of 5.80% and 3.32%, respectively. Although F2 possesses a broader absorption profile compared with F1, the lower value of PCE for the F2-based device was attributed to the low LUMO offset between F2 and PC71BM, which limited the exciton dissociation. The abovementioned results indicate that these copolymers can be utilized for ternary BHJ and tandem solar cells to achieve a high PCE.

Tuning the optoelectronic properties for high-efficiency (>7.5%) all small molecule and fullerene-free solar cells

A new non-fullerene small molecule acceptor, MPU2, that incorporates dicyano-rhodanine moieties on the edges of thiophene–diketopyrrolopyrrole was designed and prepared. In films MPU2 showed a broad absorption in the 550–850 nm region with an optical energy gap of 1.59 eV. MPU1 and MPU2 were used as small molecule acceptors to construct solar cells. DTS(QxHTh2)2, which has a D1–A–D2–A–D1 structure and complementary absorption in the 470–700 nm region of the solar spectrum, was selected as the electron-donor. All small molecule solution-processed bulk heterojunction organic solar cells were prepared using these small molecules as acceptors along with a small molecule donor. In these devices, MPU2 showed higher power conversion efficiency than MPU1. The morphology of the active layer was improved by applying thermal annealing and vacuum drying methodologies. The devices for which the latter methodology was used showed 6.28% (MPU1) and 7.76% (MPU2) power conversion efficiencies (PCEs), with the latter value being one of the highest PCEs for all small molecule fullerene-free solar cells. The results of this study show that vacuum drying treatment of the active layer is better than thermal annealing to achieve high PCE values.

Symmetrical and unsymmetrical triphenylamine based diketopyrrolopyrroles and their use as donors for solution processed bulk heterojunction organic solar cells

Two small molecules DPP3 (D–π–A) and DPP4 (D–π–A–π–D) with triphenylamine (TPA) donors and diketopyrrolopyrrole (DPP) acceptors linked with ethyne linkers were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. Their photonic, electronic, thermal and computational properties were investigated. The red shift in the electronic absorption spectra of DPP4 as compared to DPP3 is related to extended conjugation and increased donor–acceptor interaction. We have used DPP3 and DPP4 as electron donors along with PC71BM as an electron acceptor for solution processed bulk heterojunction organic solar cells. The solar cells prepared from DPP3:PC71BM and DPP4:PC71BM (1 : 2) processed from chloroform (CF) exhibit a power conversion efficiency (PCE) of 2.23% (Jsc = 6.74 mA cm−2, Voc = 0.92 V and FF = 0.36) and 3.05% (Jsc = 8.26 mA cm−2, Voc = 0.88 V and FF = 0.42), respectively. The higher PCE of the device with DPP4 compared to DPP3 was demonstrated as to the higher hole mobility and broader IPCE spectra. The devices based on DPP3:PC71BM and DPP4:PC71BM processed with solvent additive (1 v% DIO, 1,8-diiodooctane) showed PCE values of 4.06% and 5.31%, respectively. The device optimization results from the improvement of the balanced charge transport and better nanoscale morphology induced by the solvent additive.

Synthesis, photophysical, and electrochromic properties of new triarylamino-containing polyphenylquinoxalines

New triarylamino-containing bis(α-diketones) are synthesized. On the basis of these compounds, a series of electrochromic organosoluble polyphenylquinoxalines with glass-transition temperatures of 224–315°C are prepared. All polymers intensively fluoresce in solutions and thin films with maxima at 535–600 and 530–560 nm, respectively. Cyclic voltammograms of polyphenylquinoxalines exhibit reversible redox properties in the range E 1/2 = 0.92–1.25 eV. It is shown that, after 15 cycles, all polymers preserve high stability and reversibility of electrochromic characteristics, but their color changes from yellow (neutral form) to wine red (oxidized form).