Processing additives are used in organic photovoltaic systems to optimize the active layer film morphology. However, the actual mechanism is not well understood. Using X-ray scattering techniques, we analyze the effects of an additive diiodooctane (DIO) on the aggregation of a high-efficiency donor polymer PTB7 and an acceptor molecule PC(71)BM under solar cell processing conditions. We conclude that DIO selectively dissolves PC(71)BM aggregates, allowing their intercalation into PTB7 domains, thereby optimizing both the domain size and the PTB7-PC(71)BM interface.

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.

Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.

No abstract

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.

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.

Exciton dissociation is a key step for the light energy conversion to electricity in organic photovoltaic (OPV) devices. Here, excitonic dissociation pathways in the high-performance, low bandgap "in-chain donor-acceptor" polymer PTB7 by transient optical absorption (TA) spectroscopy in solutions, neat fi lms, and bulk heterojunction (BHJ) PTB7:PC 71 BM (phenyl-C 71 -butyric acid methyl ester) fi lms are investigated. The dynamics and energetics of the exciton and intra-/intermolecular charge separated states are characterized. A distinct, dynamic, spectral red-shift of the polymer cation is observed in the BHJ fi lms in TA spectra following electron transfer from the polymer to PC 71 BM, which can be attributed to the time evolution of the hole-electron spatial separation after exciton splitting. Effects of fi lm morphology are also investigated and compared to those of conjugated homopolymers. The enhanced charge separation along the PTB7 alternating donor-acceptor backbone is understood by intramolecular charge separation through polarized, delocalized excitons that lower the exciton binding energy. Consequently, ultrafast charge separation and transport along these polymer backbones reduce carrier recombination in these largely amorphous fi lms. This charge separation mechanism explains why higher degrees of PCBM intercalation within BHJ matrices enhances exciton splitting and charge transport, and thus increase OPV performance. This study proposes new guidelines for OPV materials development.

We report synthesis and characterizations of two novel series of polymers, namely the PBTZ and PBIT series. The PBTZ1 polymer was synthesized as a copolymer of 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b′]dithiophene (BDT) along with 2,5-bis(2-ethylhexyl)-3,6-bisthiazol-2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TzDPP), while PBTZ2 was a copolymer of TzDPP and 2-(1-butylheptyl)thieno[3,4-d]thiazole (TTz). The PBIT series based on dithienopyrrolobenzothiadiazole (DPBT), and BDT was also synthesized. The PBIT series of polymers showed enhanced ground and excited state dipole moments (μg and μe) when compared to the previously reported PBB3 polymer, while PBTZ1 showed the largest dipole change (1.52 D) from ground to excited state (Δμge) in respective single polymer units. It was found that the power conversion efficiencies of the polymer series were strongly correlated to Δμge. The results reported demonstrate the utility of the calculated parameter Δμge of single units of the polymers to predict the performance of donor–acceptor copolymers in photovoltaic devices. We rationalize this result based on the large degree of polarization in the excited state, which effectively lowers the Coulomb binding energy of the exciton in the excited state and leads to faster charge separation kinetics, thus facilitating the full separation of electron and hole.