A tandem organic solar cell (OSC) is a valid structure to widen the photon response range and suppress the transmission loss and thermalization loss. In the past few years, the development of low‐bandgap materials with broad absorption in long‐wavelength region for back subcells has attracted considerable attention. However, wide‐bandgap materials for front cells that have both high short‐circuit current density (JSC) and open‐circuit voltage (VOC) are scarce. In this work, a new fluorine‐substituted wide‐bandgap small molecule nonfullerene acceptor TfIF‐4FIC is reported, which has an optical bandgap of 1.61 eV. When PBDB‐T‐2F is selected as the donor, the device offers an extremely high VOC of 0.98 V, a high JSC of 17.6 mA cm−2, and a power conversion efficiency of 13.1%. This is the best performing acceptor with such a wide bandgap. More importantly, the energy loss in this combination is 0.63 eV. These properties ensure that PBDB‐T‐2F:TfIF‐4FIC is an ideal candidate for the fabrication of tandem OSCs. When PBDB‐T‐2F:TfIF‐4FIC and PTB7‐Th:PCDTBT:IEICO‐4F are used as the front cell and the back cell to construct tandem solar cells, a PCE of 15% is obtained, which is one of best results reported to date in the field of organic solar cells.
Benefiting from low cost and simple synthesis, polythiophene (PT) derivatives are one of the most popular donor materials for organic solar cells (OSCs). However, polythiophene‐based OSCs still suffer from inferior power conversion efficiency (PCE) than those based on donor–acceptor (D–A)‐type conjugated polymers. Herein, a fluorinated polythiophene derivative, namely P4T2F‐HD, is introduced to modulate the miscibility and morphology of the bulk heterojunction (BHJ)‐active layer, leading to a significant improvement of the OSC performance. The Flory–Huggins interaction parameters calculated from the surface energy and differential scanning calorimetry results suggest that P4T2F‐HD shows moderate miscibility with the popular nonfullerene acceptor Y6‐BO (2,2′‐((2Z,2′Z)‐((12,13‐bis(2‐butyloctyl)‐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‐1H‐indene‐2,1‐diylidene))dimalononitrile), while poly(3‐hexylthiophene) (P3HT) is very miscible with Y6‐BO. As a result, the P4T2F‐HD case forms desired nanoscale phase separation in the BHJ film while the P3HT case forms a completely mixed BHJ film, as revealed by transmission electron microscopy (TEM) and grazing‐incidence wide‐angle X‐ray scattering (GIWAXS). By optimizing the cathode interface and the morphology of the P4T2F‐HD:Y6‐BO films processed from nonhalogenated solvents, a new record PCE of 13.65% for polythiophene‐based OSCs is demonstrated. This work highlights the importance of controlling D/A interactions for achieving desired morphology and also demonstrates a promising OSC system for potential cost‐effective organic photovoltaics.
Materials by design are expected to deliver optimized performance for well-defined applications. [1,2] In most cases, the performance of a material depends on the chemical structure and kinetic processes that control the morphology formed, making material design a challenge to directly relate chemical structure to performance. Examples are seen in semiconducting polymers. While the conjugated backbone provides optoelectronic function, side chains, substitution groups, and molecular geometry are factors that dictate how the material will assemble. [3][4][5][6][7][8] When making multicomponents blends, as in the active layer of bulk heterojunction organic photovoltaics (OPV), elucidation of the structure of conjugated polymers and their interactions with fullerenes or other type of acceptors is essential. [9][10][11][12][13] Optimization of a material is, more often than not, a search in a multidimensional space. As seen with the PTBx series of electron donors, the quinoid resonance backbone improved solar light harvesting. [14] Side chain optimization and fluorine atom regioregularity tuning further improved the performance of the materials. [15,16] Donor-acceptor (D-A) conjugated polymers have had tremendous success in generating high power conversion efficiencies in organic solar cells, but required a fine tailoring of the chemical structure to ensure suitable energy levels and to enable intramolecular charge transfer (ICT). [17,18] The D-A conjugated polymers are commonly prepared in a one-pot synthesis through a condensation of A2+B2 intermediates, which inevitably leads to geometric defects if asymmetric monomers are used. Monofluorinated monomers, such as thieno[3,4-b] thiophene (FTT), benzothiadiazole (FBT), and pyridalthiadiazole are well-known high performance units. [16,[19][20][21] However it is essential to be able to minimize or eliminate batch-to-batch variations in the geometry and conformation of the backbone to reliably establish a structure-property relationship, so as to enable the generation of materials by design.Here we present the synthesis of unidirectional, high regioregular conjugated polymers using 5-fluoro-2,1,3-benzothiadiazole (FBT) asymmetric unit. By unidirectional we mean that positioning of the fluorine atom is always at the same position relative to the direction of the chain, which, as will be discussed, orients the dipole in each unit in the same direction. Consequently, the dipole moment would accumulate along the The chemical structure of conjugated polymers plays an important role in determining their physical properties that, in turn, dictates their performance in photovoltaic devices. 5-Fluoro-2,1,3-benzothiadiazole, an asymmetric unit, is incorporated into a thiophene-based polymer backbone to generate a hole conducting polymers with controlled regioregularity. A high dipole moment is seen in regioregular polymers, which have a tighter interchain stacking that promotes the formation of a morphology in bulk heterojunction blends with improved power conversion efficiencie...
Polythiophenes (PTs) are promising donor materials for the industrialization of polymer solar cells (PSCs) due to the merits of easy synthesis, low cost, and large-scale producibility. The rapid progress of non-fullerene acceptors requires the development of new PTs for use in non-fullerene PSCs. In this work, we present a set of PTs with different degrees of backbone fluorination (P6T-F00, P6T-F50, P6T-F75, and P6T-F100) to investigate the effect of fluorination on the photovoltaic properties of PTs in non-fullerene PSCs. Upon increasing fluorine content, the PTs tend to have higher crystallinity, higher absorption coefficients, and enhanced relative dielectric constants. When blended with a non-fullerene acceptor EH-IDTBR, the blend films show increased photoluminescence quenching efficiency, reduced charge recombination loss, and extended charge carrier lifetime along with increasing fluorine content of PTs. These positive factors collectively result in dramatically improved power conversion efficiency from 4.3% for P6T-F00:EH-IDTBR to 7.3% for P6T-F100:EH-IDTBR, which is superior to the champion binary non-fullerene PSCs based on P3HT. Our results demonstrate that PTs are promising donor materials for non-fullerene PSCs via backbone fluorination.
Two donor–acceptor (D–A) conjugated polymers composed of the same ratio of 5‐fluorobenzothiadiazole and thiophene subunits are synthesized through different routes, providing a precisely regioregular (2TRR) and a random (2TRA) polymer structures. Detailed structural analyses indicate that the backbone of regioregular 2TRR has only one donor segment of bithiophene, while the backbone of random 2TRA consists of three different donor segments: thiophene, bithiophene, and terthiophene (in a ratio of 0.16:0.68:0.16). Synergetic contributions from these segments allow the “tetrapolymer” 2TRA to achieve more favorable film morphology and a higher hole‐mobility relative to 2TRR. Consequently, the random polymer 2TRA achieves a substantially higher power conversion efficiency (8.8%) than the regioregular polymer 2TRR (5.1%). Notably, the “tetrapolymer” 2TRA is readily synthesized from two monomers, rather than through complex conventional preparation required for similar multipolymers. These findings provide a novel route toward the design and synthesis of multipolymeric materials and demonstrate their potential advantages in high‐performance organic electronic applications.
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