Guichuan Zhang

High-Performance Ternary Organic Solar Cell Enabled by a Thick Active Layer Containing a Liquid Crystalline Small Molecule Donor

Ternary organic solar cells (OSCs) have attracted much research attention in the past few years, as ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened absorption range, the light harvesting capability of ternary OSCs is still limited because most ternary OSCs use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers. In this paper, we report a highly efficient ternary OSC (11.40%) obtained by incorporating a nematic liquid crystalline small molecule (named benzodithiophene terthiophene rhodanine (BTR)) into a state-of-the-art PTB7-Th:PC71BM binary system. The addition of BTR into PTB7-Th:PC71BM was found to improve the morphology of the blend film with decreased π-π stacking distance, enlarged coherence length, and enhanced domain purity. This resulted in more efficient charge separation, faster charge transport, and less bimolecular recombination, which, when combined, led to better device performance even with thick active layers. Our results show that the introduction of highly crystalline small molecule donors into ternary OSCs is an effective means to enhance the charge transport and thus increase the active layer thickness of ternary OSCs to make them more suitable for roll-to-roll production than previous thinner devices.

An alcohol soluble amino-functionalized organoplatinum(II) complex as the cathode interlayer for highly efficient polymer solar cells

A small-molecule amino-functionalized organoplatinum(II) complex Pt–N has been developed. The unique properties of Pt–N, including a well-defined chemical structure, an excellent solubility in environmentally friendly polar solvents and good electron extraction to high work-function metals, make it a promising candidate for cathode interfacial modification of solution processed multilayer polymer solar cells (PSCs). The resultant PSCs with an ITO/PEDOT:PSS/PTB7:PC71BM/interlayer/Al device configuration exhibited significantly improved efficiencies from 3.62% for an unmodified device to 8.89% by using a Pt–N cathode interlayer, benefiting from the dramatic enhancement in the open circuit voltage (0.53 V for bare Al PSCs to 0.75 V for Pt–N/Al PSCs), the fill factor (43.58% for bare Al PSC to 72.49% for Pt–N/Al PSC), and a slight increase in short-circuit current density. These results indicate that Pt–N is a new promising candidate as the cathode interlayer for highly efficient PSCs.

Dithienosilole-benzothiadiazole-based ternary copolymers with a D1–A–D2–A structure for polymer solar cells

A series of narrow band gap conjugated copolymers comprising two electron-rich (donor, D) and one electron-deficient (acceptor, A) moieties regularly alternating along the polymer backbone were designed and synthesized. The ternary copolymers with the repeating unit in a D1–A–D2–A manner were constructed by copolymerizing a bisstannyled-D1 (D1 = cyclopentadithiophene or benzodithiophene-derivatives) and a dibromo-monomer (Br–A–D2–A–Br, D2 = dithienosilole, A = benzothiadiazole) through a palladium-catalyzed Stille polymerization. It was recognized that the optical properties, frontier molecular orbital energy levels and the photovoltaic performance of the resulting copolymers can be influenced by modifying the copolymerized D1 moiety. By carefully optimizing the electron-donating behaviours and substitutions of the D1 unit, bulk-heterojunction solar cells with a power conversion efficiency of 4.11% were achieved based on an inverted device configuration of ITO/PFN-OX/PBDT-O-ADA:PC71BM/MoO3/Al. These results demonstrated that the construction of regularly alternating narrow band gap conjugated ternary copolymers can be an effective strategy for the development of electron-donating materials for polymer solar cells.

Donor-acceptor-type copolymers based on a naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) scaffold for high-efficiency polymer solar cells.

Four donor-acceptor-type low-bandgap conjugated polymers based on a naphtho[1,2-c:5,6-c]bis(1,2,5-thiadiazole) (NT) acceptor and different donors bridged by a bithiophene spacer have been synthesized through Suzuki or Stille polymerization reactions. Fluorene (F), carbazole (Cz), alkylidene fluorene (AF), and benzodithiophene (BDT) were selected as the donor units to produce a series of new conjugated polymers. Owing to the different electron-donating ability of the donor units, the energy levels, absorption spectra, bandgaps, and carrier mobilities of the resulting polymers were systematically tuned. Bulk-heterojunction-type polymer solar cells based on the new polymers and [6,6]-phenyl-C61 -butyric acid methyl ester (PC61 BM) or [6,6]-phenyl-C71 -butyric acid methyl ester (PC71 BM) were investigated and all of the devices exhibited good photovoltaic performance, with power-conversion efficiencies (PCEs) over 3 %. The best device performance was achieved by PF-C12NT, with an open-circuit voltage (Voc ) of 0.87 V, a short-circuit current density (Jsc ) of 12.19 mA cm(-2) , a fill factor (FF) of 61.36 %, and a PCE of 6.51 % under simulated sunlight (100 mW cm(-2) , AM 1.5G).

Novel donor–acceptor type conjugated polymers based on quinoxalino[6,5-f]quinoxaline for photovoltaic applications

The field of polymer solar cells has undergone tremendous advancement in terms of power conversion efficiency in the past decade. However, there is still an urgent requirement to further enhance device performance for achieving large-scale commercialization. Here, we report the design and synthesis of three novel conjugated polymers alternatively copolymerized by a newly developed 2,3,8,9-tetrakis(3-(alkoxy)phenyl)-6,12-di(thiophen-2-yl)-2,3,8,9-tetrahydroquinoxalino[6,5-f]quinoxaline (DTNQx) acceptor unit and a conventional two-dimensional alkylthienyl-substituted benzodithiophene (BDT) unit. These polymers possess an identical main chain but different side chains. The structure–property relationship is systematically studied. All polymers exhibit an optical bandgap around 1.7 eV and a HOMO energy level around −5.16 eV. Compared with the quinoxaline unit, fused quinoxaline DTNQx leads to a downshifted HOMO energy level in the resulting polymers by 0.04 eV while keeping the bandgap unchanged. By changing the side chain length and by introducing branched side chains, polymer properties including absorption spectra and hole mobilities could be finely tuned. Under optimal conditions, polymer P2 shows the highest hole mobility of 2.7 × 10−4 cm2 V−1 s−1 and the best photovoltaic performance, with Jsc = 9.1 mA cm−2, Voc = 0.88 V, FF = 48%, and PCE = 3.8%.

Synthesis and characterization of π-conjugated copolymers based on alkyltriazolyl substituted benzodithiophene

A series of novel electron-donating building blocks of alkyltriazolyl substituted benzodithiophene were synthesized on the basis of Cu(I)-catalyzed azide–alkyne cycloaddition. The alternating donor–acceptor type of π-conjugated copolymers by using such alkyltriazolyl substituted benzodithiophene as donor units and diketopyrrolopyrrole as acceptor units were synthesized via Suzuki polymerization. The resultant copolymers exhibited good thermal properties and can be easily dissolved in various organic solvents. All copolymers show quite comparable absorption profiles in the range of 300–850 nm with the absorption onset at about 1.4 eV, where the copolymer with longer side chains exhibited a slightly enhanced absorption coefficient. The cyclic voltammetry measurements indicated the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals located at about −5.40 and −3.40 eV, respectively, which were nearly independent of the size of alkyl side chains. Polymer solar cells based on the resultant copolymers as electron-donating materials and PC71BM as the electron-accepting material exhibited moderate photovoltaic performances.

Tailoring π-conjugated dithienosilole–benzothiadiazole oligomers for organic solar cells

A series of donor–acceptor type of π-conjugated oligomers based on dithieno[3,2-b;2′,3′-d]silole as the electron donor and 2,1,3-benzothiadiazole as the electron acceptor were designed and synthesized. It was found that the elongation of the molecular lengths of the chromophores can significantly influence the thermal properties, UV-vis absorption, electrochemical properties, and photovoltaic performances of fabricated organic solar cells. The higher molecular weight chromophore exhibited a narrower band gap compared to lower molecular weight counterparts. Solution processed bulk-heterojunction organic solar cells were fabricated with the inverted device structure of ITO/PFN-OX/oligomer:PCBM/MoO3/Al, in which the best device performance was achieved with a power conversion efficiency of 1.12%. These results indicated that the elongation of the molecular length of π-conjugated small-molecules can be an effective strategy for improving the organic photovoltaic performance of narrow band-gap chromophores.

Efficient device engineering for inverted non-fullerene organic solar cells with low energy loss

In recent years, the use of non-fullerene acceptors in organic solar cells has rapidly advanced with new acceptor materials, which have enabled devices to achieve a power conversion efficiency greater than 13%. In addition to new acceptor materials’ design, device engineering plays an important role in improving the device performance. In this study, we develop effective device engineering strategies, including thermal annealing and interlayer modification, to improve the device performance from 7.39% to 9.39%. With the use of PTB7-Th as the donor and IDT-BT-R as the non-fullerene acceptor, we achieved an efficient non-fullerene organic solar cell based on an inverted device architecture with a power conversion efficiency as high as 9.39%. It is worthy of note that the energy loss of the optimized device is only around 0.5 eV, which can be attributed to weak recombination and the appropriate high energy level of the charge transfer states within the optimized device.

An Open‐Circuit Voltage and Power Conversion Efficiency Study of Fullerene Ternary Organic Solar Cells Based on Oligomer/Oligomer and Oligomer/Polymer

Variations in the open-circuit voltage (Voc ) of ternary organic solar cells are systematically investigated. The initial study of these devices consists of two electron-donating oligomers, S2 (two units) and S7 (seven units), and the electron-accepting [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM) and reveals that the Voc is continuously tunable due to the changing energy of the charge transfer state (Ect ) of the active layers. Further investigation suggests that Voc is also continuously tunable upon change in Ect in a ternary blend system that consists of S2 and its corresponding polymer (P11):PC71 BM. It is interesting to note that higher power conversion efficiencies can be obtained for both S2:S7:PC71 BM and S2:P11:PC71 BM ternary systems compared with their binary systems, which can be ascribed to an improved Voc due to the higher Ect and an improved fill factor due to the improved film morphology upon the incorporation of S2. These findings provide a new guideline for the future design of conjugated polymers for achieving higher performance of ternary organic solar cells.