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Charge generation and recombination dynamics in organic photovoltaic bulk heterojunction films comprising the donor polymer, PDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene}), blended with the fullerene acceptor, PC71BM ([6,6]-phenyl C71-butyric acid methyl ester), have been studied. The charge-carrier generation process was studied using femtosecond transient absorption, and it was found that the efficiency of charge generation is not dominated by geminate recombination but rather is limited by exciton diffusion in the films. Highly sensitive nanosecond transient absorption (ns-TA) and time-resolved microwave conductivity (TRMC) were used to study charge recombination. From ns-TA measurements, we obtained a recombination rate constant of 1 × 10–9 cm3 s–1 and found that charge recombination is limited by the diffusion of charge carriers (Langevin-type recombination). TRMC signals were comparable with ns-TA on shorter time scales. However, in contrast with ns-TA, the TRMC signal contained an additional long-lived component. The fast decay on shorter time scales is attributed to the recombination of the majority of the charge carriers. The long-lived component is assigned to a small population of charge carriers with high mobility, suggesting they are located in isolated, crystalline domains within the bulk heterojunction. These findings are discussed in relation to the morphology of the blend film, fluorescence quenching properties, and device performance including photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) measurements described in our previous publications.

We investigate the photoexcited state dynamics in a donor-acceptor copolymer, poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]- pyrrole-1,4-dione-alt-naphthalene} (pDPP-TNT), by picosecond fluorescence and femtosecond transient absorption spectroscopies. Timeresolved fluorescence lifetime measurements of pDPP-TNT thin films reveal that the lifetime of the singlet excited state is 185 ± 5 ps and that singlet-singlet annihilation occurs at excitation photon densities above 6 × 1017 photons/cm3. From the results of singlet-singlet annihilation analysis, we estimate that the single-singlet annihilation rate constant is (6.0 ± 0.2) × 109cm3 s-1 and the singlet diffusion length is -7 nm. From the comparison of femtosecond transient absorption measurements and picosecond fluorescence measurements, it is found that the time profile of the photobleaching signal in the charge-transfer (CT) absorption band coincides with that of the fluorescence intensity and there is no indication of long-lived species, which clearly suggests that charged species, such as polaron pairs and triplet excitons, are not effectively photogenerated in the neat pDPP-TNT polymer.

We investigate the blend morphology and performance of bulk heterojunction organic photovoltaic devices comprising the donor polymer, pDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1, 4-dione-alt-naphthalene}) and the fullerene acceptor, [70]PCBM ([6,6]-phenyl C71-butyric acid methyl ester). The blend morphology is heavily dependent upon the solvent system used in the fabrication of thin films. Thin films spin-coated from chloroform possess a cobblestone-like morphology, consisting of thick, round-shaped [70]PCBM-rich mounds separated by thin polymer-rich valleys. The size of the [70]PCBM domains is found to depend on the overall film thickness. Thin films spin-coated from a chloroform:dichlorobenzene mixed solvent system are smooth and consist of a network of pDPP-TNT nanofibers embedded in a [70]PCBM-rich matrix. Rinsing the films in hexane selectively removes [70]PCBM and allows for analysis of domain size and purity. It also provides a means for investigating exciton dissociation efficiency through relative photoluminescence yield measurements. Devices fabricated from chloroform solutions show much poorer performance than the devices fabricated from the mixed solvent system; this disparity in performance is seen to be more pronounced with increasing film thickness. The primary cause for the improved performance of devices fabricated from mixed solvents is attributed to the greater donor-acceptor interfacial area and resulting greater capacity for charge carrier generation.

A method is presented that enables the determination of transient absorption in Langmuir films made from a monolayer of spiropyran on water. This is achievable even though the optical pathlength of such a monolayer is <10−9 m. The approach is to monitor reflectivity changes close to the Brewster angle, where the background of reflected light is minimized. This is the key to the sensitivity of the method. Relatively speaking the small changes in reflectivity due to changes in both real and imaginary parts of the refractive index are easier to observe with the intrinsically low backgrounds at the Brewster angle. Notably using Fresnel equations we can show that the real and imaginary parts of the refractive index can be independently assessed for ultrathin films and monolayers using the approach presented.

A method is presented that enables the determination of transient absorption in Langmuir films made from a monolayer of spiropyran on water. This is achievable even though the optical pathlength of such a monolayer is <10−9 m. The approach is to monitor reflectivity changes close to the Brewster angle, where the background of reflected light is minimized. This is the key to the sensitivity of the method. Relatively speaking the small changes in reflectivity due to changes in both real and imaginary parts of the refractive index are easier to observe with the intrinsically low backgrounds at the Brewster angle. Notably using Fresnel equations we can show that the real and imaginary parts of the refractive index can be independently assessed for ultrathin films and monolayers using the approach presented.

Brewster angle reflectometry has been developed as a tool for determining the absorbance and refractive index changes in molecular monolayers containing spiropyran. The method is sensitive to changes in both the real and imaginary parts of the refractive index in the monolayers. It was used to monitor the conversion of spiropyran to merocyanine and the reversal of this reaction when the molecules were immobilised on quartz using silane coupling. An analytical solution of Fresnel formula allowed the transient reflectometry data to be converted into transient absorption information. Absorbances of transients as low as ~10-6 were possible using the current apparatus with a single laser pulse transient measurement. It was found that spiropyran photoconverted to merocyanine with an efficiency of ~0.1. The photochemical reversion of converted merocyanine to spiropyran occurred with efficiencies of 0.03–0.2 and this was probably site dependent. It was found that the thermal conversion from merocyanine to spiropyran was slow and even after 10 min there was no significant thermal reversion. This measurement was possible because the real part of the refractive index of the monolayer could be monitored with time using an off-resonance probe at a wavelength where the merocyanine did not absorb light meaning that the probe did not photobleach the sample. Thus our method also provides a non-intrusive method for probing changes in molecules in thin films.

A series of quartz surfaces were modifed with a series of crosslinkers and functional groups in order to obtain an azide-terminated monolayer, which was then used to immobilize pyrene onto the surface via alkyne-azide \click" chemistry. During the course of the immobilization, different ratios of tert-butyl diphenyl chlorosilane were used to control the distribution and hence the photophysical properties of the pyrene on the surface. The preparative surface reactions and photophysical properties were investigated with contact angle, X-ray photoelectron spectroscopy, UV-visible absorption and emission spectroscopy. High surface coverage was achieved of just under 1molecule per nm2. At this coverage all emission from the pyrene was in the form of excimer emission. Excimer emission dominated at all surface coverages greater than 0.45 molecules per nm2. Below this coverage the monomer emission could also be observed. The conclusions that can be drawn are important for understanding the interactions of neighboring molecules in molecular monolayers. Our results suggest that at high surface coverage a substantial number of the pyrene molecules are already close enough to their neighbors that pairs of them can be directly excited to form excimer with no requirement for diffusion. This can be stated because the long wavelength end of the pyrene absorption and excitation spectra show a broad tail that is assigned to a charge transfer band resulting from an electron being directly transferred from a ground state pyrene to a neighboring pyrene molecule. Furthermore, absorption spectra shifts also indicate that the pyrene molecules undergo some interactions on the surface when they are closely packed.

Self-assembled monolayers of 11-(3’,3’-dimethyl-6,8-dinitrospiro[chromene-2,2’-indoline]-1’-yl) undecanoic acid (amphiphilic spiropyran) at the air–water interface are studied using Brewster angle reflectometry. Transient kinetics of the spiropyran to merocyanine conversion are recorded in a UV-pump, VIS-probe configuration. By varying the probe wavelength using an optical parametric oscillator, we are able to reconstruct absorption spectra of intermediate states with a time-resolution of 10 nanoseconds, limited by the temporal convolution of the two laser pulses. After UV irradiation, spiropyran converts to merocyanine in two stages. The first occurs within a timescale of several tens of nanoseconds and is heavily convoluted with the system response time, whereas the second stage occurs over a few hundred nanoseconds. During the rise time there is a small red shift in the transient absorption spectrum of ∼20 nm. We assign the red shift and the slower kinetics to the isomerization of a merocyanine isomer cis about the central methine bond to those that are trans about the same bond.

A new apparatus for nanosecond-time-resolved Brewster angle reflectometry is described that can be used to measure transient angle-resolved reflectivity changes in thin films andmonolayers in a single pulsed laser shot. In order to achieve this, a cylindrical lens is placed in the probe beam path replacing the goniometer that is usually used for angular scanning in other systems. Using two synchronized nanosecond pulsed lasers in pump-probe configuration it is possible to measure the kinetics of photoinduced conformational changes by altering the delay between pump and probe pulses. The system was used to observe nanosecond time-resolved photodynamics in a spiropyran monolayer at the air-water interface. After UV excitation the spiropyran converted to its merocyanine form in two stages. The first stage occurred with a timescale close to the instrument time resolution (tens of nanoseconds) whereas the second stage occurred over a few hundred nanoseconds.

Raman mapping was used to determine the lipid distribution inside human mesenchymal stem cells during induced adipogenesis by monitoring C-H stretching bands of the fats inside the lipid droplets. By incorporating deuterated glucose into the cell culture medium during induction it was possible to distinguish whether or not downstream metabolites, either in lipid droplets or in the cytoplasm, had been formed before or after the adipogenic cascade, because C-D stretching bands are 1/√2 shifted compared to the C-H bands. Thus, metabolites formed after the initiation of the process displayed both C-H and C-D stretching bands and so were forming during induced adipogenesis rather than prior to it. With the ability to distinguish small putative lipid drops formed by the induction of adipogenesis from those pre-formed in the cell, it was possible to analyze spectral changes occurring in the droplets at the earliest stages of adipogenesis. There were two key findings. Firstly, Raman spectra of lipid droplets evolved over time, suggesting that their composition at the early stages was not the same as at the later stages. Secondly, it was apparent that the proportion of unsaturated fats in droplets was higher at early stages than it was at later stages, suggesting that unsaturated fats arrive in the droplets faster than saturated ones.