Ayasofya Müzesi

Taha Toros Arşivi, Dosya No: 102-Camilerİstanbul Kalkınma Ajansı (TR10/14/YEN/0033) İstanbul Development Agency (TR10/14/YEN/0033

Triplet Exciton Dynamics in Fluorene−Amine Copolymer Films

Effect of aromaticity on triplet exciton dynamics was studied by transient absorption spectroscopy for two fluorene-based random copolymers with different aromatic amine, poly­(9,9′-di-<i>n</i>-octylfluorene-<i>ran</i>-<i>N</i>,<i>N</i>′-bis­(4-<i>n</i>-butylphenyl)-<i>N</i>,<i>N</i>′-diphenyl-1,4-benzenediamine) (F8-PDA) and poly­(9,9′-di-<i>n</i>-octylfluorene-<i>ran</i>-<i>N</i>,<i>N</i>′-bis­(4-<i>t</i>-butylphenyl)-<i>N</i>,<i>N</i>′-diphenyl-9,10-anthracenediamine) (F8-ADA). On a time scale of nanoseconds, triplet exciton was efficiently formed in F8-PDA through the intersystem crossing (ISC) from singlet exciton with a rate constant of 2.0 × 10⁸ s^–1. On the other hand, the ISC was not efficient in F8-ADA, resulting in efficient fluorescence emission. On a time scale of micro- to milliseconds, F8-PDA exhibited bimolecular triplet exciton decay due to triplet–triplet annihilation (TTA), but the TTA was negligible in F8-ADA, indicating that triplet excitons can diffuse freely in F8-PDA while they are trapped at ADA units in F8-ADA. The difference in the ISC efficiency and triplet exciton diffusion is discussed in terms of the aromaticity of the amine units. On the basis of these analyses, we discuss a strategy for further improvement in the efficiency of fluorene–amine copolymer based light-emitting diodes

Intermixed Donor/Acceptor Region in Conjugated Polymer Blends Visualized by Conductive Atomic Force Microscopy

The charge-transport characteristics of phase-separated blend films of poly­(3-hexyl­thiophene) (P3HT; electron donor) and poly­[2,7-(9,9-didodecyl­fluorene)-<i>alt</i>-5,5-(4′,7′-bis­(2-thienyl)-2′,1′,3′-benzo­thiadiazole)] (PF12TBT; electron acceptor) were visualized by conductive atomic force microscopy (C-AFM). The C-AFM hole-current images clearly showed two phases: an electrically conductive region assigned to the P3HT-rich donor domain and a nonconductive region assigned to the PF12TBT-rich acceptor domain. The hole current in the conductive region was small compared with that of a neat P3HT film with similar thickness, indicating that the P3HT-rich domain contained a large fraction of PF12TBT as a minor component. Thermal annealing initially increased the hole current throughout the P3HT-rich domain because of reorganization of the P3HT chains from their as-cast configurations. Further annealing increased the hole current mainly in the middle of the P3HT-rich domain, but it decreased the hole current in the boundary areas close to the PF12TBT-rich domain owing to the presence of an intermixed region with a gradient of the P3HT/PF12TBT composition ratio. After annealing at temperatures above the glass-transition point of PF12TBT, the widths of the intermixed regions decreased to ∼30 nm as phase separation proceeded with decomposition of the intermixed region. Such variations in the intermixed region, which were electrically resolved by the C-AFM, accounted for the temperature dependence of the photovoltaic properties of P3HT/PF12TBT blend solar cells

Ternary Blend Hybrid Solar Cells Incorporating Wide and Narrow Bandgap Polymers

Ternary hybrid solar cells based on zinc oxide with wide bandgap poly­(3-hexylthiophene) (P3HT) and narrow bandgap poly­[2,3-bis­(3-octyloxyphenyl)­quinoxaline-5,8-diyl-<i>alt</i>-thiophene-2,5-diyl] (PTQ1) exhibit improved photovoltaic performance compared to that of individual binary hybrid solar cells. The increase in the photocurrent is partly due to the complementary absorption bands, which can extend the light-harvesting range from visible to near-infrared regions, and partly due to efficient energy transfer from P3HT to PTQ1, by which P3HT excitons are more efficiently collected at the PTQ1/ZnO interface and hence convert to charge carriers effectively. Furthermore, the improvement in the fill factor may be due to efficient hole transfer from PTQ1 to P3HT with higher hole mobility, and thereby, hole polarons are more efficiently collected on the electrode

Role of Interfacial Charge Transfer State in Charge Generation and Recombination in Low-Bandgap Polymer Solar Cell

The charge carrier dynamics in blend films of poly­[2,6-(4,4-bis­(2-ethylhexyl)-4<i>H</i>-cyclopenta­[2,1-<i>b</i>;3,4-<i>b</i>′]­dithiophene)-<i>alt</i>-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) was studied by transient absorption spectroscopy in order to address the origin of limited external quantum efficiency (EQE) of this solar cell compared to that of a benchmark solar cell composed of regioregular poly­(3-hexythiphene) (RR-P3HT) and PCBM. Upon photoexcitation, PCPDTBT singlet excitons promptly convert to the interfacial charge transfer (CT) state that is a Coulombically bound charge pair of PCPDTBT polaron and PCBM anion at the heterojunction with almost 100% efficiency in a picosecond. In other words, the exciton diffusion efficiency η_ED and charge transfer efficiency η_CT are 100% in this blend, which are higher than and comparable to those of the RR-P3HT/PCBM solar cell, respectively. On a time scale of nanoseconds, 70% of the PCPDTBT bound polarons are dissociated into free charge carriers, and the others recombine geminately to the ground state through the CT state. The charge dissociation efficiency η_CD = 70% is lower than that of RR-P3HT/PCBM solar cells. The PCPDTBT dissociated polarons recombine bimolecularly on a time scale of nano- to microseconds with a charge lifetime of ∼10^–7 s, which is shorter than that observed for RR-P3HT/PCBM blends. In summary, the lower charge dissociation efficiency and shorter charge lifetime are the limiting factors for the photovoltaic performance of PCPDTBT/PCBM solar cells. Furthermore, the origin of such limitation is also discussed in terms of the charge dissociation and recombination through the interfacial CT state in PCPDTBT/PCBM blends

Polymer/Polymer Blend Solar Cells Improved by Using High-Molecular-Weight Fluorene-Based Copolymer as Electron Acceptor

The highest power conversion efficiency (PCE) of 2.7% has been achieved for all-polymer solar cells made with a blend of poly­(3-hexylthiophene) (P3HT, electron donor) and poly­[2,7-(9,9-didodecylfluorene)-<i>alt</i>-5,5-(4′,7′-bis­(2-thienyl)-2′,1′,3′-benzothiadiazole)] (PF12TBT, electron acceptor). The PCE of the P3HT/PF12TBT solar cells increases from 1.9% to 2.7% with an increase in the molecular weight (<i>M</i>_w) of PF12TBT from 8500 to 78 000 g mol^–1. In a device with high-molecular-weight PF12TBT, efficient charge generation is maintained even at high annealing temperatures because of the small phase separation on the length scale of exciton diffusion due to an increase in the glass transition temperature (<i>T</i>_g) and a reduced diffusional mobility of the PF12TBT chains above <i>T</i>_g. On the other hand, efficient charge transport is also achieved through the formation of interconnected networks of PF12TBT-rich domains, which is facilitated by the high molecular weight of PF12TBT, and the ordering of P3HT chains in P3HT-rich domains, which is a result of high-temperature annealing. Thus, when high-molecular-weight PF12TBT is used, an optimal blend morphology that supports efficient charge generation as well as charge transport can be obtained by thermal annealing, and consequently, the highest PCE reported so far for an all-polymer solar cell is achieved

One-Dimensional Singlet Exciton Diffusion in Poly(3-hexylthiophene) Crystalline Domains

Singlet exciton dynamics in crystalline domains of regioregular poly­(3-hexylthiophene) (P3HT) films was studied by transient absorption spectroscopy. Upon the selective excitation of crystalline P3HT at the absorption edge, no red shift of the singlet exciton band was observed with an elapse of time, suggesting singlet exciton dynamics in relatively homogeneous P3HT crystalline domains without downhill relaxation in the energetic disorder. Even under such selective excitation conditions, the annihilation rate coefficient γ­(<i>t</i>) was still dependent on time, γ­(<i>t</i>) ∝ <i>t</i>^–1/2, which is attributed to anisotropic exciton diffusion in P3HT crystalline domains. From the annihilation rate coefficient, the singlet exciton diffusion coefficient <i>D</i> and exciton diffusion length <i>L</i>_D in the crystalline domains were evaluated to be 7.9 × 10^–3 cm² s^–1 and 20 nm, respectively. The origin of the time-dependent exciton dynamics is discussed in terms of dimensionality

Measurement of Exciton Diffusion in a Well-Defined Donor/Acceptor Heterojunction based on a Conjugated Polymer and Cross-Linked Fullerene Derivative

We designed a well-defined donor/acceptor heterojunction for measuring exciton diffusion lengths in conjugated polymers. To obtain an insoluble electron acceptor layer, a new cross-linkable fullerene derivative (bis-PCBVB) was synthesized by functionalizing [6,6]-diphenyl-C₆₂-bis­(butyric acid methyl ester) (bis-PCBM) with two styryl groups. The spin-coated bis-PCBVB film was cross-linked in situ by heating at 170 °C for 60 min. Surface characterizations by UV–visible absorption, atomic force microscopy, and photoelectron yield spectroscopy revealed that a smooth and solvent-resistant film (<i>p</i>-PCBVB) was obtained. In bilayer films with a donor conjugated polymer, poly­[2,7-(9,9-didodecylfluorene)-<i>alt</i>-5,5-(4′,7′-bis­(2-thienyl)-2′,1′,3′-benzothiadiazole)] (PF12TBT), spin-coated on top of the <i>p</i>-PCBVB acceptor layer, the photoluminescence (PL) of the PF12TBT was effectively quenched. This is because the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the <i>p</i>-PCBVB film are nearly the same as those of the parent bis-PCBM spin-coated film. On the basis of the PL quenching results, the exciton diffusion length and exciton diffusion coefficient in the PF12TBT were evaluated to be 11 nm and 9.8 × 10^–4 cm² s^–1, respectively

Charge Transport in Intermixed Regions of All-Polymer Solar Cells Studied by Conductive Atomic Force Microscopy

Charge transport in intermixed regions of all-polymer solar cells based on a blend of poly­(3-hexyl­thiophene) (P3HT; electron donor) with poly­[2,7-(9,9-didodecyl­fluorene)-<i>alt</i>-5,5-(4′,7′-bis­(2-thienyl)-2′,1′,3′-benzo­thiadiazole)] (PF12TBT; electron acceptor) was studied by conductive atomic force microscopy (C-AFM). For a blend film fabricated from a chlorobenzene solution, intermixed regions were detected between the P3HT-rich and PF12TBT-rich domains. The overall hole current in the intermixed regions remained almost constant, both before and after thermal annealing at 80 °C, but it increased in the P3HT-rich domains. For a blend film fabricated from a chloroform solution, the entire observed area constituted an intermixed region, both before and after thermal annealing. The overall hole current in this film was significantly improved following thermal annealing at 120 °C. These finely mixed structures with efficient charge transport yielded a maximum power conversion efficiency of 3.5%. The local charge-transport properties in the intermixed region, as observed via C-AFM, was found to be closely related to the photovoltaic properties, rather than the bulk-averaged properties or topological features

Charge Generation and Recombination in Fullerene-Attached Poly(3-hexylthiophene)-Based Diblock Copolymer Films

The charge generation and recombination dynamics in fullerene-attached poly­(3-hexythiophene) (P3HT)-based diblock copolymer were studied in comparison with those in blend films of P3HT and a fullerene derivative (PCBM) in order to understand the potential advantage of diblock copolymer-based polymer solar cells. Upon photoexcitation, P3HT singlet excitons are promptly converted to P3HT polarons with a time constant of ∼30 ps in both P3HT-PCBM diblock copolymer and P3HT/PCBM blend films. This similar charge generation dynamics is indicative of analogous phase-separated morphology both in these films on a scale of nanometers. After the charge generation, a part of polarons in disorder phases geminately recombine to the ground state in diblock copolymer films, while no geminate recombination is observed in blend films. This geminate recombination loss is probably due to defects of phase-separated structures in diblock copolymer films. On the other hand, charge carrier lifetime is as long as 15 μs in diblock copolymer films. Such a long carrier lifetime may result in a relatively high fill factor in P3HT-PCBM copolymer films. Finally, we discuss the overall device performance in terms of phase-separated structures