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Herein, we describe the synthesis of fluorinated polythienothiophene-co-benzodithiophenes (PTBFs) and the characterization of their physical properties, especially their performance in solar cells. Fluorination of the polymer backbone lowered both the HOMO and LUMO energy levels and simultaneously widened the energy bandgap of the polymer (0.1-0.2 eV). Incorporation of fluorine into the various positions of the polymer backbone significantly affected the solar cells' power conversion efficiency from 2.3% to 7.2%. Detailed studies revealed that the polymer containing mono-fluorinated thienothiophene gave the best solar cell performance. Perfluorination of the polymer backbone led to poor compatibility with PC(71)BM molecules, thus poor solar energy conversion efficiency. This is possibly due to the enhanced self-organization properties of the polymer chains and the fluorophobicity effect. Furthermore, it was found that perfluorination of the polymer backbone resulted in poor photochemical stability against singlet oxygen attack. Theoretical studies indicated that the internal polarization caused enhancement of the negative charge density on thienothiophene rings, which rendered them vulnerable to [2+4] cycloaddition reaction with singlet oxygen.

A new low band gap copolymer PBB3 containing [6,6']bi[thieno[3,4-b]thiophenyl]-2,2'-dicarboxylic acid bis-(2-butyloctyl) ester (BTT) and 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b']dithiophene (BDT) units was synthesized and tested for solar cell efficiency. PBB3 showed a broad absorbance in the near-IR region with a substantially red-shifted (by more than 100 nm) λ(max) at 790 nm as compared to the PTB series of polymers, which have been previously reported. The PBB3 polymer also showed both a favorable energy level match with PCBM (with a LUMO energy level of -3.29 eV) and a favorable film domain morphology as evidenced by TEM images. Despite these seemingly optimal parameters, a bulk heterojunction (BHJ) photovoltaic device fabricated from a blend of PBB3 and PC(71)BM showed an overall power conversion efficiency (PCE) of only 2.04% under AM 1.5G/100 mW cm(-2). The transient absorption spectra of PBB3 showed the absence of cationic and pseudo charge transfer states that were observed previously in the PTB series polymers, which were also composed of alternating thienothiophene (TT) and BDT units. We compared the spectral features and electronic density distribution of PBB3 with those of PTB2, PTB7, and PTBF2. While PTB2 and PTB7 have substantial charge transfer characteristics and also relatively large local internal dipoles through BDT to TT moieties, PTBF2 and PBB3 have minimized internal dipole moments due to the presence of two adjacent TT units (or two opposing fluorine atoms in PTBF2) with opposite orientations or internal dipoles. PBB3 showed a long-lived excitonic state and the slowest electron transfer dynamics of the series of polymers, as well as the fastest recombination rate of the charge-separated (CS) species, indicating that electrons and holes are more tightly bound in these species. Consequently, substantially lower degrees of charge separation were observed in both PBB3 and PTBF2. These results show that not only the energetics but also the internal dipole moment along the polymer chain may be critical in maintaining the pseudocharge transfer characteristics of these systems, which were shown to be partially responsible for the high PCE device made from the PTB series of low band gap copolymers.

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.

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High‐performance inverted solar cells using various low‐bandgap polymers are achieved by using excellent metal oxide nanoparticles including ZnO and TiO2:Cs nanoparticles as an electron‐transport layer (ETL). Power conversion efficiencies (PCE) as high as 7.3% are achieved with inverted single cells based on PTB7:PC71BM. The results give a guideline for high‐efficiency and stable inverted single‐junction and tandem polymer solar cells.

Perovskite solar cells (PSCs) based on organic monovalent cation (methylammonium or formamidinium) have shown excellent optoelectronic properties with high efficiencies above 22%, threatening the status of silicon solar cells. However, critical issues of long-term stability have to be solved for commercialization. The severe weakness of the state-of-the-art PSCs against moisture originates mainly from the hygroscopic organic cations. Here, rubidium (Rb) is suggested as a promising candidate for an inorganic-organic mixed cation system to enhance moisture-tolerance and photovoltaic performances of formamidinium lead iodide (FAPbI 3 ). Partial incorporation of Rb in FAPbI 3 tunes the tolerance factor and stabilizes the photoactive perovskite structure. Phase conversion from hexagonal yellow FAPbI 3 to trigonal black FAPbI 3 becomes favored when Rb is introduced. The authors find that the absorbance and fluorescence lifetime of 5% Rb-incorporated FAPbI 3 (Rb 0.05 FA 0.95 PbI 3 ) are enhanced than bare FAPbI 3 . Rb 0.05 FA 0.95 PbI 3 -based PSCs exhibit a best power conversion efficiency of 17.16%, which is much higher than that of the FAPbI 3 device (13.56%). Moreover, it is demonstrated that the Rb 0.05 FA 0.95 PbI 3 film shows superior stability against high humidity (85%) and the full device made with the mixed perovskite exhibits remarkable long-term stability under ambient condition without encapsulation, retaining the high performance for 1000 h. Figure 6. Fluorescence lifetime imaging microscopy (FLIM) images of a) FAPbI 3 and b) Rb 0.05 FA 0.95 PbI 3 films deposited on glass. c) Time-resolved photoluminescence (TRPL) spectra of FAPbI 3 and Rb 0.05 FA 0.95 PbI 3 films. Scale bar: 2 µm.

Molecular packing structures and photoinduced charge separation dynamics have been investigated in a recently developed bulk heterojunction (BHJ) organic photovoltaic (OPV) material based on poly(thienothiophene-benzodithiophene) (PTB1) with a power conversion efficiency (PCE) of >5% in solar cell devices. Grazing incidence X-ray scattering (GIXS) measurements of the PTB1:PCBM ([6,6]-phenyl-C(61)-butyric acid methyl ester) films revealed pi-stacked polymer backbone planes oriented parallel to the substrate surface, in contrast to the pi-stacked polymer backbone planes oriented perpendicular to the substrate surface in regioregular P3HT [poly(3-hexylthiophene)]:PCBM films. A approximately 1.7 times higher charge mobility in the PTB1:PCBM film relative to that in P3HT:PCBM films is attributed to this difference in stacking orientation. The photoinduced charge separation (CS) rate in the pristine PTB1:PCBM film is more than twice as fast as that in the annealed P3HT:PCBM film. The combination of a small optical gap, fast CS rate, and high carrier mobility in the PTB1:PCBM film contributes to its relatively high PCE in the solar cells. Contrary to P3HT:PCBM solar cells, annealing PTB1:PCBM films reduced the device PCE from 5.24% in the pristine film to 1.92% due to reduced interfacial area between the electron donor and the acceptor. Consequently, quantum yields of exciton generation and charge separation in the annealed film are significantly reduced compared to those in the pristine film.