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Abstract Four wide bandgap donor-acceptor (D-A) conjugated polymers, i.e. P-diF2T, P-TT, P-BDT and P-NDT, with bis(5-(2-decyltetradecyl)-[2,3′-bithiophen]-2′-yl)arylenes as D-units and 2-propyl-5,6-difluorobenzo[ d ][1,2,3]triazole (ffTAZ) as A unit were synthesized for photovoltaic applications. In P-diF2T, P-TT, P-BDT and P-NDT, 3,3′-difluoro-2,2′-bithiophene (diF2T), thieno[3,2- b ]thiophene (TT), benzo[1,2- b :4,5- b ']dithiophene (BDT) and naphtho[1,2- b :5,6- b ']dithiophene (NDT) were used as aryl central units, respectively, to adjust the properties of the polymers. The optical bandgap is 1.83 eV for P-diF2T, 1.85 eV for P-TT, 1.90 eV for P-BDT and 2.04 eV for P-NDT. All four polymers show temperature-dependent aggregation behavior in solution, and their absorption spectra are complementary with that of non-fullerene acceptor (NFA) 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(5-hexylthiophen-2-yl)-dithieno[2,3- d :2′,3′- d ′]- s -indaceno[1,2- b :5,6- b ′]dithiophene (ITIC-Th). Polymer solar cells (PSCs) based on these polymers and ITIC-Th were fabricated and characterized. Power conversion efficiencies (PCEs) higher than 7% were demonstrated for all polymers. The devices based on P-diF2T exhibited the highest PCE up to 10.08% with a high fill factor (FF) of 0.741. The good photovoltaic performance of P-diF2T is attributed to the synergies of the low-lying highest occupied molecular orbital (HOMO) energy level of the polymer and the favorable microstructures and morphology and thereby high and balanced charge carrier mobilities of the blend films. This study suggested that adjusting aromatic D units together with using alkylthiophene as side chains is an effective approach to design donor polymers for high performance PSCs based on NFA. Graphical abstract Image 1 Highlights • Alkylthiophene was used as side chains to construct wide bandgap D-A conjugated polymers with ffTAZ as A unit. • With four different D units, the physicochemical and photovotaic properties and mophologies of the polymers were studied. • A high PCE up to 10.08% was achieved for PSC devices based on P-diF2T and ITIC-Th, with a high FF of 0.741. [ABSTRACT FROM AUTHOR]

The proper crystallinity and a balance between H-aggregation and J-aggregation are both necessary for the best performance of small molecule organic solar cells. It is well known that J-aggregation is favorable to form more excitons due to its narrower bad-gap, leading to a high photocurrent. While H-aggregation has a higher lowest excited level which could provide stronger driving force for exciton dissociation. However, for 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2- b :4,5- b ′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[ c ][1,2,5]thiadiazole) ( p -DTS(FBTTh 2 ) 2 )/[6,6]-phenyl-C 71 -butyric acid methyl ester (PC 70 BM) blend system, the crystallinity is very low and p -DTS(FBTTh 2 ) 2 is mainly in J-aggregation. Thus, increasing H-aggregation of p -DTS(FBTTh 2 ) 2 is an effective way to improve photovoltaic efficiency of p -DTS(FBTTh 2 ) 2 /PC 70 BM system. In this paper, it was found that solvent vapor annealing (SVA) was an effective method to improve the crystallinity and the H-aggregation of p -DTS(FBTTh 2 ) 2 in blend film. Due to the prolonged time for self-organization, the crystallinity of p -DTS(FBTTh 2 ) 2 was enhanced. In addition, molecular motion ability of p -DTS(FBTTh 2 ) 2 was controlled and p -DTS(FBTTh 2 ) 2 could adjust its structure to form H-aggregation. As a result, the ratio of H/J could be tuned from 0.71 to 1.47 and the power conversion efficiencies (PCE) was improved from 2.64% to 6.63%. [ABSTRACT FROM AUTHOR]

A singly-linked perylene bisimide dimer (di-PBI) and a low band gap polymer based on dithienocarbazole and isoindigo moieties (P(IID-DTC)) are selected as electron acceptor and donor respectively to fabricate non-fullerene based polymer solar cells. The combination of di-PBI acceptor and P(IID-DTC) donor provides well overlapped absorption profile with solar light in the visible region. The P(IID-DTC):di-PBI blend morphology and its effect on charge transport and recombination have been investigated in detail. The dominant bimolecular recombination in commonly processed P(IID-DTC):di-PBI blend results in a low power conversion efficiency of merely 1%. We find that CH 2 Cl 2 vapor annealing can effectively improve bi-continuous phase separation and boost the electron transport by more than two orders of magnitude due to the detrap of di-PBI molecules from amorphous P(IID-DTC) matrix. More importantly, the dominant bimolecular recombination in the P(IID-DTC):di-PBI blend films is strongly suppressed. Finally, the power conversion efficiency of non-fullerene P(IID-DTC):di-PBI blend solar cell is dramatically improved to 2.95%. [ABSTRACT FROM AUTHOR]

Abstract: Tandem polymer photovoltaic cells with the subcells having different absorption characteristics in series connection are widely investigated to enhance absorption coverage over the solar spectrum. Herein, we demonstrate efficient tandem polymer photovoltaic cells with the two stacked subcells comprising different band-gap conjugated polymer and fullerene derivative bulk heterojunction in parallel connection. A semitransparent metal layer combined with inorganic semiconductor compounds is utilized as the intermediate electrode of the two stacked subcells to create the required built-in potential for collecting photo-generated charges. The short-circuit current of the stacked cell is the sum of the subcells and the open-circuit voltage is similar to the subcells. [Copyright &y& Elsevier]