Toward High Efficiency Polymer Solar Cells: Influence of Local Chemical Environment and Morphology

Abstract

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...

Toward High Efficiency Polymer Solar Cells: Rearranging the Backbone Units into a Readily Accessible Random Tetrapolymer

“…As shown in Figure , both polymers were prepared via the Stille polycondensation reaction using 5‐fluorobenzothiadiazole (FBT) and thiophene units under microwave‐assisted heating. To synthesize the regioregular polymer 2TRR , the monomer M1 contains each of the reactive functional groups on a single precursor, thus ensuring strict ordering of the asymmetric D–A structural units within the polymer backbone . A related random polymer 2TRA can be synthesized with a dibrominated FBT‐thiophene monomer ( M2 ) and the distannylated thiophene monomer ( M3 ).…”

“…This is surprising; in many cases, the regioregular polymer has a better mobility relative to the random counterpart. [10,15b]…”

Machine learning for accelerating the discovery of high-performance donor/acceptor pairs in non-fullerene organic solar cells

“…In addition to material design and synthesis, which involves new route, structure characterization, material purification, etc., the process of material optimization and evaluation is also uncertain. On one hand, the efficient device performance is achieved under the synergistic effects of donor/acceptor (D/A) combinations, including complementary absorption 8 , matched energy levels 9 , favorable blend morphology with nanoscale phase separation 10 , high and balanced charge transport property [11][12][13] , etc. On the other hand, adjusting processing parameters such as D/A ratio, processing solvent and solvent additive, pre-and post-treatments, selection of film forming technology and so on [14][15][16][17][18] , and constructing device architectures via interface engineering and device engineering [19][20][21] , are also tedious and laborious work.…”

Wide Band Gap Polymer Based on Indacenodithiophene and Acenaphthoquinoxaline for Efficient Polymer Solar Cells Application

“…One of the most successful ways to design a conjugated polymer with good photovoltaic performance is the donor–acceptor (D–A) approach, where the polymer backbone consists of an electron-rich donor unit and an electron-deficient acceptor unit alternatively [ 23 , 24 , 25 , 26 , 27 , 28 ]. Meanwhile, the morphological optimization of the active layer in PSCs also plays a key role to improve the photovoltaic performance by facilitating the charge generation, separation, and transport within the device [ 29 , 30 , 31 , 32 ]. Various methods such as thermal annealing, solvent annealing, and adding additives have been used to control the morphology to achieve favorable phase separation.…”