Non-fullerene Polymer Solar Cells Research Articles

Two new A-D-A type small molecule acceptors based on C2v-symmetric dithienocyclopentaspiro[fluorene-9,9′-xanthene] core for polymer solar cells

Abstract A C2v-symmetric core, dithienocyclopentaspiro[fluorene-9,9′-xanthene], was used as the central block for the first time to design and synthesize A-D-A type small molecule acceptors for nonfullerene polymer solar cells (PSCs), and two new small molecule acceptors of TSFX-2F and TSFX-4F were synthesized based on the C2v-symmetric core. The two TSFX-based acceptors show high thermal stability, strong absorption in the wavelength region of 550–750 nm and appropriate energy levels. The PSCs with the broad bandgap polymer J71 as donor and TSFX-2F as acceptor demonstrated power conversion efficiency (PCE) of 9.42% with open circuit voltage (Voc) of 0.89 V, short circuit current density (Jsc) of 15.27 mA cm−2 and fill factor (FF) of 69.30%, while the PSC based on J71:TSFX-4F shows a PCE of 8.47% with Voc of 0.83 V, Jsc of 15.48 mA cm−2 and FF of 66.16%. The higher Voc of the PSC based on J71: TSFX-2F is benefitted from the up-shifted LUMO energy level of the TSFX-2F acceptor, and its higher FF can be ascribed to the higher and more balanced hole and electron mobilities of the J71: TSFX-2F active layer. This work demonstrates that the new C2v-symmetric building block is a promising central D-unit for the design and synthesis of new structured norfullerene acceptors for high-performance PSCs.

An N-type Naphthalene Diimide Ionene Polymer as Cathode Interlayer for Organic Solar Cells

Polymer solar cells (PSCs) based on non-fullerene acceptors have the advantages of synthetic versatility, strong absorption ability, and high thermal stability. These characteristics result in impressive power conversion efficiency values, but to further push both the performance and the stability of PSCs, the insertion of appropriate interlayers in the device structure remains mandatory. Herein, a naphthalene diimide-based cathode interlayer (NDI-OH) is synthesized with a facile three-step reaction and used as a cathode interlayer for fullerene and non-fullerene PSCs. This cationic polyelectrolyte exhibited good solubility in alcohol solvents, transparency in the visible range, self-doping behavior, and good film forming ability. All these characteristics allowed the increase in the devices’ power conversion efficiencies (PCE) both for fullerene and non-fullerene-based PSCs. The successful results make NDI-OH a promising cathode interlayer to apply in PSCs.

BODIPY-modified terpolymer donors for efficient fullerene- and nonfullerene-polymer solar cells

For the first time, BODIPY was employed as a comonomer to develop efficient terpolymer donors resulting in synergetic PCE improvements in both fullerene and nonfullerene polymer solar cells.

Dithienobenzoxadiazole-based wide bandgap donor polymers with strong aggregation properties for the preparation of efficient as-cast non-fullerene polymer solar cells processed using a non-halogenated solvent

The efficient as-cast polymer solar cells processed by non-halogenated solvent are achieved through new dithienobenzoxadiazole-based wide bandgap donor polymers with strong aggregation property.

Synergy strategy to the flexible alkyl and chloride side-chain engineered quinoxaline-based D–A conjugated polymers for efficient non-fullerene polymer solar cells

We are developing both copolymers with quinoxaline (Qx) as acceptor units by fine tuning the side chains with halogen atom chlorine (Cl) and flexible alkyl engineering for efficient non-fullerene polymer solar cells.

Wide-bandgap donor polymers based on a dicyanodivinyl indacenodithiophene unit for non-fullerene polymer solar cells.

A wide-bandgap polymer donor with improved efficiency plays an important role in improving the photovoltaic performance of polymer solar cells (PSCs). In this study, two novel wide-bandgap polymer donors, PBDT and PBDT-S, were designed and synthesized based on a dicyanodivinyl indacenodithiophene (IDT-CN) moiety, in which benzo[1,2-b:4,5-b′]dithiophene (BDT) building blocks and IDT-CN are used as electron-sufficient and -deficient units, respectively. In our study, the PBDT and PBDT-S polymer donors exhibited similar frontier-molecular-orbital energy levels and optical properties, and both copolymers showed good miscibility with the widely used narrow-bandgap small molecular acceptor Y6. Non-fullerene polymer solar cells (NF-PSCs) based on PBDT:Y6 exhibited an impressive power conversion efficiency of 10.04% with an open circuit voltage of 0.88 V, a short-circuit current density of 22.16 mA cm−2 and a fill factor of 51.31%, where the NF-PSCs based on PBDT-S:Y6 exhibited a moderate power conversion efficiency of 6.90%. The enhanced photovoltaic performance, realized by virtue of the improved short-circuit current density, can be attributed to the slightly enhanced electron mobility, higher exciton dissociation rates, more efficient charge collection and better nanoscale phase separation of the PBDT-based device. The results of this work indicate that the IDT-CN unit is a promising building block for constructing donor polymers for high-performance organic photovoltaic cells.

Induced crystallization of sol–gel-derived zinc oxide for efficient non-fullerene polymer solar cells

We demonstrate a novel strategy for induced crystallization of zinc oxide (ic-ZnO) that shows increased crystallinity relative to sol–gel derived zinc oxide, which is also proved to reduce the interfacial charge recombination efficiently in PSCs.

Importance of interface engineering between the hole transport layer and the indium-tin-oxide electrode for highly efficient polymer solar cells

Over 17% efficiency non-fullerene polymer solar cells were achieved by modifying the interface between ITO and a PEDOT:PSS hole transport layer using a conjugated polyelectrolyte (WPFSCz-).

Non-fullerene polymer solar cells based on quinoxaline units with fluorine atoms

In this study, low bandgap polymers containing a monofluoroquinoxaline (FQx) or a difluoroquinoxaline (DFQx) as the electron-deficient unit and 2-thiophenylbenzo[1,2-b:4,5-b']dithiophene as the electron-rich unit were synthesized by Stille polymerization. The polymers exhibit satisfactory thermal stability and a broad absorption band from 300 to 650 nm. The HOMO/LUMO energy levels of PB-FQx and PB-DFQx were − 5.62/− 3.64 and − 5.70/− 3.63 eV, respectively. We found that for a high number of fluorine atoms in the conjugated backbone, the HOMO energy levels were deeper, which led to increased VOC. The PB-DFQx:ITIC-based PSC exhibited the best power conversion efficiency of 6.31%, with its VOC of 0.99 V, JSC of 10.61 mA/cm2, FF of 60%.

Design of Thienothiophene-Based Copolymers with Various Side Chain-End Groups for Efficient Polymer Solar Cells.

Three two-dimensional donor–acceptor conjugated copolymers consisting of a benzo[1,2-b:4,5-b′]dithiophene derivative and thieno[3,2-b]thiophene with a conjugated side chain were designed and synthesized for use in bulk heterojunction (BHJ) or nonfullerene polymer solar cells (PSCs). Through attaching various acceptor end groups to the conjugated side chain on the thieno[3,2-b]thiophene moiety, the electronic, photophysical, and morphological properties of these copolymers were significantly affected. It was found that the intermolecular charge transfer interactions were enhanced with the increase in the acceptor strength on the thieno[3,2-b]thiophene moiety. Moreover, a better microphase separation was obtained in the copolymer: PC71BM or ITIC blend films when a strong acceptor end group on the thieno[3,2-b]thiophene moiety was used. As a result, BHJ PSCs based on copolymer:PC71BM blend films as active layers exhibited power conversion efficiencies from 2.82% to 4.41%, while those of nonfullerene copolymer:ITIC-based inverted PSCs ranged from 6.09% to 7.25%. These results indicate the side-chain engineering on the end groups of thieno[3,2-b]thiophene unit through a vinyl bridge linkage is an effective way to adjust the photophysical properties of polymers and morphology of blend films, and also have a significant influence on devices performance.

Aldol Condensation‐Polymerized n‐Doped Conjugated Polyelectrolytes for High‐Performance Nonfullerene Polymer Solar Cells

Doping provides an efficient strategy to control the electronic properties of organic semiconductors. However, compared with the widely reported p‐type doping protocols, n‐type doping of organic semiconductors still remains a challenge. Herein, a series of novel n‐doped conjugated polyelectrolytes (CPEs) with high doping levels and conductivity are designed. These CPEs are synthesized via a facile, metal‐free, and high‐yield aldol condensation protocol from bis‐isatin and bis‐oxindole monomers. The designed CPEs possess a n‐type electron‐deficient and rigid conjugated backbone, resulting in easy charge delocalization, enhanced n‐doping behaviors, and high conductivity. The evolution on the counterions of these CPEs further alters their n‐doping behaviors, charge transporting properties, and work function tunability, etc. By using these CPEs as electron transport materials (ETMs) for nonfullerene polymer solar cells (NF–PSCs), high power conversion efficiencies (PCEs) over 16% can be achieved when PM6:Y6 is used as the active component. Moreover, these CPEs can enable efficient NF–PSCs even if their thicknesses are up to 60 nm, indicating the potential of these CPEs as thickness‐insensitive ETMs for the fabrication of large‐area NF–PSCs.

Wide bandgap donor polymers containing carbonyl groups for efficient non-fullerene polymer solar cells

Two wide bandgap conjugated polymers (PTCO8 and PTCO1) with different side chains, composed of alkylcarbonyl-substituted thiophene (TOC) as the electron-withdrawing unit and two-dimension benzodithiophene (BDT) as the electron-donating unit, are developed for non-fullerene polymer solar cells (PSCs). Relative to non-carbonyl-substituted control polymer PTC8, PTCO8 and PTCO1 exhibit a red-shifted absorption and deeper-lying highest occupied molecular orbital (HOMO) level. When blended with ITIC, the as-cast PTCO1-based devices yield a power conversion efficiency of up to 9.33% with an open-circuit voltage of 0.95 V, a short-circuit current density of 16.91 mA cm−2, and a fill factor of 57.9%, outperforming those of PTC8-based devices (4.04%) and PTCO8-based devices (5.91%). This work demonstrates that the carbonyl group is a promising electron-deficient moiety to construct wide-bandgap polymers for non-fullerene PSCs. Adjusting the length of side chains is a feasible strategy to improve PSC performance further.

ZnO/Ti3C2Tx monolayer electron transport layers with enhanced conductivity for highly efficient inverted polymer solar cells

MXenes, a novel intriguing family of two-dimensional (2D) transition metal carbides and nitrides, have a wide spectrum of applications owning to their unique optical and electronic properties. Herein, we use Ti3C2Tx, a representative of MXenes, as an additive in zinc oxide (ZnO) to fabricate novel ZnO/Ti3C2Tx nanohybrid composite film. The addition of Ti3C2TX nanosheets constructs new electron transport pathways between the ZnO nanocrystals, and passivates the surface of ZnO by forming the Zn-O-Ti bonding on the ZnO surface. The novel ZnO/Ti3C2Tx nanohybrid film exhibits excellent photoelectric characteristics, and is used as electron transport layers (ETLs) in fullerene and non-fullerene polymer solar cells for the first time. As a result, the power conversion efficiency (PCE) of the photovoltaic devices based on PBDB-T:ITIC with the ZnO/Ti3C2Tx ETLs is 12.20%, up from 10.56% for the corresponding device utilizing pristine ZnO as ETL, a relative increase of 15.53%. Moreover, PM6:Y6 based IPSCs achieve a champion PCE of 16.51% from 14.99% for the reference device, suggesting the good applicability of the ZnO/Ti3C2Tx ETL. The enhancement of PCE is mainly due to the increased transfer and collection of charges in IPSCs. More interestingly, devices based on ZnO/Ti3C2TX composite ETL display relatively good stability compared with the control device. The layered Ti3C2TX should be responsible for such enhancement.

A 2,5-difluoro benzene-based low cost and efficient polymer donor for non-fullerene solar cells

Although the latest progress in the power conversion efficiency (PCE) of non-fullerene polymer solar cells (NF-PSCs) has proved their potential as next-generation solar technologies, the development of efficient wide-bandgap (WBG) donor polymers having a low-cost and easily scalable molecular design is still lagging in terms of number and diversity. In this contribution, we report a new WBG polymer donor, PBDT-2FBnT, based on alternative benzodithiophene as the donor unit and 2,5-difluoro benzene (2FBn) as the acceptor unit, which was prepared by a facile three-step process with an overall yield over 80% using cheap raw materials. Benefitting from the incorporation of an electron-deficient 2FBn unit, PBDT-2FBnT demonstrated lower frontier energy levels with an optical bandgap of 2.12 eV. Additionally, the combination of 2FBn with an adjacent two thiophene bridge in the polymer backbone significantly minimized the steric hindrance via non-covalent F···S and CH···F inter/intramolecular interactions, thereby promoting highly coplanar geometry for effective molecular packing and charge transport. Consequently, optimized blends of PBDT-2FBnT with an ITIC acceptor delivered complementary panchromatic absorption, well-aligned energy levels, higher charge carrier mobilities, and well-distributed nano-fibrillar morphology, thereby leading to a remarkable efficiency of 9.5% with a good trade-off between corresponding photovoltaic parameters. Thus, the high photovoltaic performance of PBDT-2FBnT together with simple preparation can provide a promising way for the future development of low-cost PSCs, and we can foresee further PCE improvements of the PBDT-2FBnT structure via synergistic variation of donor and acceptor material.

Highly efficient, green-solvent processable, and stable non-fullerene polymer solar cells enabled by a random polymer donor

Abstract Rapid development of non-fullerene polymer solar cells (NF–PSCs) with high efficiency have gained wide research attention recently, however, high-performance large-area NF–PSCs are still lacking since most developed NF–PSCs use a thin active layer around 100 nm, which is not compatible with the fabrication of large-area devices. Herein, we fabricate high-performance and green-solvent processed NF–PSCs with thick active layer using a random copolymer (2TRA) and non-fullerene acceptors. Compared to its highly-crystallized analogue (2TRR), 2TRA with rearranged backbone enables better blend morphology, higher exciton generation rate, more efficient excitons dissociation, and less recombination in NF–PSCs. With O-IDTBR and IEICO-4F as non-fullerene electron acceptors, highly-efficient photovoltaic devices with PCE of 10.08% and 12.06% are achieved. Moreover, 2TRA-based NF–PSCs show good compatibility with multiple processing solvents, low sensitivity to the active layer thickness (up to 300 nm), and excellent photostability. The combined advantages of 2TRA-based solar cell devices indicate that 2TRA is a promising polymer donor for the application in large-area PSC modules through roll-to-roll technology, and the rearrangement design on polymer donors could be useful to design high-performance polymers for practical application.

Black phosphorous quantum dots as an effective interlayer modifier in polymer solar cells

Black phosphorus quantum dots (BPQDs), as 2D van der Waals crystals, possess remarkable electronic and optoelectronic properties. Their excellent ultra-high mobility and thickness-dependent tunable direct bandgap can meet well the needs of polymer solar cells (PSCs) for adjustable energy levels in different active layer systems. The formation of better energy alignment in the devices is significantly beneficial to the charge transfer, exciton dissociation and reduce charge recombination, which can lead to improved power conversion efficiency (PCE). Herein, it is introduced to modify hole transport layer (HTL) as interfacial modifier (IM) in fullerene and non-fullerene PSCs. The cascade band structure is formed successfully between the anode and polymer donor. In PTB7-Th:PC71BM system, PCE is increased from 8.12% to 9.11%. In PM6:IT-4F system, PCE is enhanced from 11.65% to 12.81%. BPQDs are utilized as HTL IM in conventional device in our study, which can open up a promising route for photovoltaic research.

The effect of halogenation on PBDTT-TQxT based non-fullerene polymer solar cells – Chlorination vs fluorination

The rapid advancement in the development of non-fullerene acceptors has led to single-junction polymer solar cells with efficiencies over 18%. Even with these novel acceptor materials, the choice of the donor polymer remains important. Tuning of the donor and acceptor compatibility in terms of absorption, frontier orbital energy levels, mixing enthalpy and charge carrier mobility is routinely performed by side chain variation. Fluorination presents an additional powerful approach to optimize these parameters. Although significantly less studied, chlorination can give rise to similar effects, while donor-acceptor phase separation due to fluorophobic interactions is less of an issue. Moreover, from a material synthesis point of view, the introduction of chlorine groups is in many cases much more straightforward. In this work, we present a series of push-pull type benzo[1,2-b:4,5-b']dithiophene-alt-quinoxaline donor polymers and compare the behavior of the non-halogenated, fluorinated and chlorinated derivatives in polymer solar cells when combined with small molecule and polymer type non-fullerene acceptors. The solar cell efficiencies vary from 2.4 to 8.4%, elucidating the large impact of these small structural variations. Best results are achieved for the chlorinated donor polymer, affording a high open-circuit voltage, balanced charge carrier mobilities and favorable donor-acceptor interactions. Combined with the easier synthesis of chlorinated materials, this suggests that more emphasis should be put on chlorination as a valuable approach to tune the properties of organic semiconductors for solar cell blends (and other optoelectronic applications).

Wide Band-gap Two-dimension Conjugated Polymer Donors with Different Amounts of Chlorine Substitution on Alkoxyphenyl Conjugated Side Chains for Non-fullerene Polymer Solar Cells

In this study, wide bandgap (WBG) two-dimensional (2D) copolymer donors (DZ1, DZ2, and DZ3) based on benzodithiophene (BDT) on alkoxyphenyl conjugated side chains without and with different amounts of chlorine atoms and difluorobenzotriazole (FBTZ) are designed and synthesized successfully for efficient non-fullerene polymer solar cells (PSCs). Three polymer donors DZ1, DZ2, and DZ3 display similar absorption spectra at 300–700 nm range with optional band-gap (Egopt) of 1.84, 1.92, and 1.97 eV, respectively. Compared with reported DZ1 without chlorine substitution, it is found that introducing chlorine atoms into the meta-position of the alkoxyphenyl group affords polymer possessing a deeper the highest occupied molecular orbital (HOMO) energy level, which can increase open circuit voltage (VOC) of PSCs, as well as improve hole mobility. Non-fullerene bulk heterojunction PSCs based on DZ2:MeIC demonstrate a relatively high power conversion efficiency (PCE) of 10.22% with a VOC of 0.88 V, a short-circuit current density (JSC) of 17.62 mA/cm2, and a fill factor (FF) of 68%, compared with PSCs based on DZ1:MeIC (a PCE of 8.26%) and DZ3:MeIC (a PCE of 6.28%). The results imply that adjusting chlorine atom amount on alkoxyphenyl side chains based on BDT polymer donors is a promising approach of synthesizing electron-rich building block for high performance of PSCs.

Shorter alkyl chain in thieno[3,4-c]pyrrole-4,6-dione (TPD)-based large bandgap polymer donors – Yield efficient non-fullerene polymer solar cells

Typically, conjugated polymers are composed of conjugated backbones and alkyl side chains. In this contribution, a cost-effective strategy of tailoring the length of alkyl side chain is utilized to design high-performing thieno[3,4-c]pyrrole-4,6-dione (TPD)-based large bandgap polymer donors PBDT-BiTPD(Cχ) (χ = 48, 52, 56), in which χ represents the alkyl side chain length in term of the total carbon number. A combination of light absorption, device, and morphology examinations make clear that the shorter alkyl side chains yield (i) higher crystallinity and more predominant face-on crystallite orientation in their neat and BHJ blend films, (ii) higher charge mobilities (6.7 × 10−4 cm2 V−1 s−1 for C48 vs. 3.2 × 10−4 cm2 V−1 s−1 for C56), and negligible charge recombination, consequently, (iii) significantly improved fill-factor (FF) and short current (JSC), while almost the same open circuit voltage (VOC) of ca. 0.82 V in their corresponding BHJ devices. In parallel, as alkyl side chain lengths decrease from C56 to C48, power conversion efficiencies (PCEs) increased from 7.8% for C56 to 11.1% for C52, and further to 14.1% for C48 in their BHJ solar cells made with a narrow bandgap non-fullerene acceptor Y6. This systematic study declares that shortening the side chain, if providing appropriate solubility in device solution processing solvents, is of essential significance for developing high-performing polymer donors and further improving device photovoltaic performance.

A Novel Wide‐Bandgap Polymer with Deep Ionization Potential Enables Exceeding 16% Efficiency in Ternary Nonfullerene Polymer Solar Cells

AbstractTernary strategies have attracted extensive attention due to their potential in improving power conversion efficiencies (PCEs) of single‐junction polymer solar cells (PSCs). In this work, a novel wide bandgap polymer donor (Egopt ≈ 2.0 eV) named PBT(E)BTz with a deep highest occupied molecular orbital (HOMO) level (≈−5.73 eV) is designed and synthesized. PBT(E)BTz is first incorporated as the third component into the classic PBDB‐T‐SF:IT‐4F binary PSC system to fabricate efficient ternary PSCs. A higher PCE of 13.19% is achieved in the ternary PSCs with a 5% addition of PBT(E)BTz over binary PSCs (12.14%). Similarly, addition of PBT(E)BTz improves the PCE for PBDB‐T:IT‐M binary PSCs from 10.50% to 11.06%. The study shows that the improved PCE in ternary PSCs is mainly attributed to the suppressed charge carrier recombination and more balanced charge transport. The generality of PBT(E)BTz as a third component is further evidenced in another efficient binary PSC system—PBDB‐TF:BTP‐4Cl: an optimized PCE of 16.26% is realized in the ternary devices. This work shows that PBT(E)BTz possessing a deep HOMO level as an additional component is an effective ternary PSC construction strategy toward enhancing device performance. Furthermore, the ternary device with 5% PBT(E)BTz displays better thermal and light stability over binary devices.