Steady-state Photoinduced Absorption Research Articles

Measuring Competing Recombination Losses in a Significantly Reduced Langevin System by Steady-State Photoinduced Absorption and Photocurrent Spectroscopy

Understanding and disentangling photophysical properties of long-lived photoexcitations in bulk heterojunction (BHJ) solar cells, which contribute mostly to photocurrent, provide essential guidelines to their improvement. However, to construct improved physical models, their rational design relies on reliable measurement techniques for charge recombination. Here, we combine photocurrent and photoinduced absorption spectroscopy (PCPIA) to directly probe the free carrier concentration and investigate loss mechanisms of long-lived excitations in nearly 10% efficient PPDT2FBT/PC70BM BHJ solar cells under steady-state operational conditions. From the PCPIA data obtained under open-circuit and short-circuit conditions, the absorption cross section and the concentration of photoexcitations are obtained. This material system exhibits an exceptionally low bimolecular recombination rate, about 300 times smaller than the diffusion-controlled electron and hole encounter rate. Furthermore, we observe that the fill factor is limited by losses originating from long-lived photoexcitations undergoing dispersive bimolecular recombination.

Long-lived-correlated triplet-pair state in an imide substituted poly-thienylene vinylene-based π-conjugated polymer

We report on the photophysical properties of a soluble thienylene-vinylene π-conjugated polymer, namely imide poly-thienylene vinylene. Ultrafast pump probe spectroscopy reveals that a broad photoinduced absorption (PA) band is immediately photogenerated along with a narrower PA from the photoinduced singlet excitons. The broad PA is susceptible to magnetic field and thus is assigned to a correlated triple pair state. This feature shows a long lifetime persisting into the μs time domain, in contrast to the singlet exciton PA, which quickly decays on a 10-ps timescale. The steady-state PA spectrum is identical to the transient PA spectrum of the triplet pair state but shows a magnetic field response that is typical to isolated triplet excitons.

Singlet fission of hot excitons in π-conjugated polymers.

We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≈2E(T) (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2E(T), which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(ω) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=E(g) (≈2.25 eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2A(g) exciton, dominates the triplet photogeneration in PDA polymer as E(g)>2E(T). The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously.

Generation and Recombination Kinetics of Optical Excitations in Poly(3-dodecylthienylenevinylene) with Controlled Regioregularity

Regioregularity in π-conjugated polymers is known to have desirable effects on chain organization in the solid state leading to improved characteristics for electronic device applications. In this report, we studied the photophysics of a new series of π-conjugated polymers, namely, poly(3-dodecylthienylenevinylene), with controlled regioregularity (PTV-CR), using the steady state photoinduced absorption (PIA), doping-induced absorption (DIA), thermally modulated absorption (TMA), and photoluminescence (PL) techniques in a broad spectral range, as well as Grazing incidence X-ray diffraction (GIXRD). PTV-CR has a low band gap of 1.65-1.70 eV from the most random PTV-5:5 to regular PTV-0:10. We have found that regioregularity affects optical properties of PTV-CR in different ways than in the case of well-studied poly(3-alkylthiophene). We discovered that even the most random PTV-5:5 shows submicrometer crystalline order, and there is no correlation between regioregularity and crystallinality. We found that PTV is intrinsically dark due to the reverse order of 1B(u) and 2A(g) excitons, with PL quantum efficiency ranging from 10(-5) to 2 × 10(-4). Spectroscopic data confirm the existence of long-lived neutral (triplets) and charged (polaron) excitations, and thus PTV is a unique nondegenerate ground state π-conjugated polymer which is intrinsically nonluminescent. The recombination mechanism and lifetime of such long-lived excitations are investigated through the pump intensity and pump frequency dependencies as well as temperature dependence of the sample. Increasing regioregularity is seen to reduce the thermal responsivity and intersystem crossing in these films. The novel, intrachain character of these effects and their implications for use of PTV derivatives for photovoltaic application are discussed.

Characterization of the Pore Filling of Solid State Dye Sensitized Solar Cells with Photoinduced Absorption Spectroscopy

Near steady-state photoinduced absorption (PIA) and UV-Vis absorption spectroscopy are used to characterize the pore filling of spiro-MeOTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene) into the nanoparticulate TiO2electrode of a solid-state dye-sensitized solar cell (ssDSC). The volumetric ratio of filled to unfilled pore volumes, as well as the optical signature of interacting chemical species, that is, the hole-transfer yield (HTY), are investigated. PIA spectroscopy is used to measure the HTY, relative to the amount of spiro-MeOTAD present, without needing to determine the extinction coefficients of the dye and spiro-MeOTAD cation species. The Beer-Lambert law is used to relate the relative PIA signal to the penetration length of the hole-conductor in the TiO2film. For the sample thickness range of1.4–5 μm investigated here, the optimum characteristic penetration length is determined to be3.1+0.46 μm, which is compared to 1.4 μm for the 200 mgmL−1concentration of spiro-MeOTAD conventionally used. Therefore, doubling the effective penetration of spiro-MeOTAD is necessary to functionalize all the dye molecules in a ssDSC.

A novel 2,7-linked carbazole based “double cable” polymer with pendant perylene diimide functional groups: preparation, spectroscopy and photovoltaic properties

The preparation and chemical analysis of a ‘double-cable’ conjugated polymer comprising a backbone of alternating 2,7-linked carbazole repeat units with covalently attached perylene diimide (PDI) substituents and dithienyl repeat units is presented. The backbone of the new “double-cable” polymer P1 acts as an electron donor while the pendant PDI substituents act as electron acceptors. In order to investigate the influence of the PDI moieties on the polymer backbone as well as to elucidate their photophysical interaction, three reference-compounds were also prepared and analysed: a polymer with the same backbone as that of P1 but without PDI substituents (P2) and two PDI derivatives with branched alkyl side chains (PDI 2 with 12-tricosanyl substituents and PDI 1 with 3-pentyl substituents). We find that polymer P1 shows pronounced electron transfer from the polymer backbone to the PDI chromophore units covalently bound to it, resulting in highly efficient quenching of excitons and strong suppression of fluorescence in solutions and in thin films. The existence of long-lived polaronic states resulting from exciton dissociation on P1 is confirmed using steady state photo-induced absorption spectroscopy. Despite the improved exciton quenching yield shown by P1 over a blend of P2/PDI, photovoltaic devices fabricated from P1 have a low external quantum efficiency (EQE) of around 0.43% at 410 nm; a value that is smaller than that from a conventional BHJ device of P2/PDI which is found to have a peak external quantum efficiency of 3.7% at 463 nm. We tentatively ascribe the reduced EQE of the double-cable polymer to geminate recombination of charge-carriers as a result of poor charge transport and a complete lack of donor acceptor phase separation.

Steady-state photoinduced absorption of CdSe/CdS octapod shaped nanocrystals

Colloidal branched nanocrystals have been attracting increasing attention due to evidence of an interesting relationship between their complex shape and charge carrier dynamics. Herein, continuous wave photoinduced absorption (CW PIA) measurements of CdSe/CdS octapod-shaped nanocrystals are reported. CW PIA spectra show strong bleaching due to the one-dimensional (1D) CdS pod states (480 nm) and the zero-dimensional (0D) CdSe core states (690 nm). The agreement with previously reported ultrafast pump-probe experiments indicates that this strong bleaching signal may be assigned to state filling. Additional bleaching features at 520 and 560 nm are characterized by a longer lifetime and are thus ascribed to defect states, localized at the pod-core interface of the octapod, showing that some of the initially photogenerated carriers get quickly trapped into these long-lived defect states. However, we remark that a relevant part of electrons remain untrapped: this opens up the opportunity to exploit octapod shaped nanocrystals in photovoltaics applications, as electron acceptor materials, considering that several efficient hole extracting materials are already available for the realization of a composite bulk heterojunction.

Molecular Packing Effects on the Optical Spectra and Triplet Dynamics in Oligofluorene Films

We report on the triplet spectra and dynamics in two types of oligomeric films deposited by two different techniques: thermal evaporation and spin coating. The different molecular arrangement in both films is manifested in a red shift of the absorption, PL, and T1-Tn absorption spectra of the sublimated film relative to the spin-coated one. Triplet recombination dynamics studied with steady-state photoinduced absorption (PA) spectroscopy follow a dispersive bimolecular recombination model away from the trap filling regime. We obtained values for the triplet bimolecular recombination ratio (beta) of 3.4 x 10 (-14) and 1.1 x 10 (-15) cm3 s (-1) for evaporated and spin-coated film, respectively, the difference being attributed to diverse molecular arrangement in both films.

Steady state photoinduced absorption of PbS quantum dots film

We have measured steady state photoinduced absorption (PIA) of PbS quantum dot (QD) film in an energy range of interband electronic transitions at low temperature. PIA spectra show both positive and negative PIAs corresponding to five consecutive interband transitions. The close resemblance between PIA spectrum and the second derivative of the linear optical absorption strongly suggests that these steady state PIAs may be caused by photoinduced local electric field, i.e., the linear Stark effect. We postulate this local electric field be generated by the trapped charges under laser excitations.

Two-step field-induced singlet dissociation in a fluorene trimer

We time resolve the event of singlet separation into free carriers in an oligofluorene using femtosecond field assisted pump-probe technique. Steady state photoinduced absorption (PA) measurements have also been employed to identify triplet absorption $({T}_{1}\ensuremath{-}{T}_{n})$ in the spectral region of $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. As a result of an applied electric field of $2.2\phantom{\rule{0.3em}{0ex}}\mathrm{MV}∕\mathrm{cm}$ singlets are observed to undergo a dissociation process consisting of first singlet splitting into bound polaron pairs and second further dissociation of polaron pairs into free carriers. Subsequent free carrier recombination yields a new population of singlets and triplets. Our results suggest that nongeminate recombination leads to the formation of 35% singlets and 65% triplets.

Photoinduced Electron Dynanics in A Phosphate Heterostructure Dispersed with C 60 -Nanoclusters

Development of a series of lead phosphate heterostructures dispersed with C 60 -nanoclusters which upon 230-400 nm UV irradiation show a phenomenon of electron transfer from the divalent lead centre (Pb +2 ) of the lead phosphate network to the incorporated fullerene, has been reported. Studies of the steady state photoinduced absorption and photoinduced ESR spectra showed formation of Pb +3 type of hole and C 60 -nanocluster anions in the irradiated composite. The charge separated state thus formed has life time ranging from few hours to days, which is higher than those hitherto reported for various cased donor acceptor systems. An analysis of the carrier-dynamics suggests that the photoinduced migration of electrons from the hole-sites to the fullerene sites involves two steps : 1) Trapping of the electrons released from the Pb-sites by the UV, within the phosphate-network and 2) Detrapping and tunneling of these electrons from the trap sites to the fullerene sites by the electron-scavenging effect of the latter.

Photoinduced charge transfer in composites of conjugated polymers and semiconductornanocrystals

Photoinduced electron transfer in composites of CdSe and InP nanocrystals (NC) with aconjugated polymer [2-methoxy-5-(3-,7-dimethyl-octyloxy)-1, 4-phenylene vinylene](OC1C10-PPV) is studied by means of light-induced electron spin resonance (LESR), photoluminescence(PL) as well as quasi steady-state photoinduced absorption (PIA) spectroscopy. Thequenching of PL, the occurrence of two new optical absorption bands, and the formation oflight-induced paramagnetic species on the polymer chain are altogether interpreted as anelectron transfer between a donor polymer and acceptor NC in the excited state. In particularthe LESR provides evidence of an electron transfer rather than energy transfer due to anoverlap of absorption and emission bands of NC and the conjugated polymer. Theg-factor of the signal is consistent with the cationic origin ofOC1C10-PPV radicals. Additionally, the LESR signal is sensitive to NC surface coating being smaller in blends with thetri-n-octylphosphineoxide–tri-n-octylphosphine (TOPO–TOP) coated CdSe surface and completely absent in blendswith TOPO–TOP–InP. LESR probes predominantly positive polarons on theconjugated polymer chain generated at room temperature in the course of anelectron transfer to NC. This is very different from the situation in blends ofOC1C10-PPV with fullerenes, another promising photovoltaic acceptor, where the photogenerated cationand anion were both observed in the LESR. In PIA we found contributions of both polymerpolarons and electrons on the NC. The recombination kinetics shows a broad distributionof lifetimes, which is characteristic for dispersive (diffusive) recombination processes withfractional power frequency dependence.

TiO2 sensitized with an oligo(p-phenylenevinylene) carboxylic acid: a new model compound for a hybrid solar cell

The interaction of an acid-functionalized oligo(p-phenylenevinylene) (OPV3-COOH) with nanocrystalline TiO2 has been studied as a model for semiconducting polymer–inorganic material hybrid solar cells. Langmuir adsorption studies show that the monolayer coverage of OPV3-COOH on nc-TiO2 results in 35% filling of the nanopores. Photoluminescence quenching and near steady-state photoinduced absorption spectroscopy demonstrate that an efficient forward photoinduced electron transfer occurs from OPV3-COOH to TiO2. Photovoltaic cells based on OPV3-COOH/nc-TiO2 and a liquid electrolyte as a medium for hole transport provide an incident photon-to-electron conversion efficiency of 55%, comparable to the values obtained with the best ruthenium dyes, and show that the majority of the photogenerated charges can be collected.

Long‐Lived Photoinduced Charge Separation in (Zinc, Lead) Phosphate Glass–C60 Composites

A group of (zinc, lead) phosphate glass–C60 composites that display long‐lived photoinduced electron transfer from the Pb2+ center to the fullerene upon 230–400 nm UV irradiation has been developed. Both the steady state photoinduced absorption and photoinduced electron spin resonance spectra of the irradiated system support this view. Analysis of the carrier dynamics suggests that the electrons migrate from the hole sites to the fullerene sites in a tunneling mode.

Photo-induced electron transfer from a dithieno thiophene-based polymer to TiO 2

A spectroscopical study of composites made with a new conjugated polymer, polydithienothiophene, and nanocrystalline TiO 2 is carried out by optical absorption, photoluminescence and steady state photo-induced absorption spectroscopy. Sintered nanoporous TiO 2 films have been stained by dipping the inorganic semiconductor films into dithienothiophene polymer solutions. The optical absorption spectra indicate that this polymer is well absorbed into nanoporous TiO 2 films, showing a remarkable interaction with the inorganic semiconductor as compared to other alkyl substituted thiophene based conjugated polymers. The photoluminescence quenching and the steady state photo-induced absorption spectra evidence that TiO 2 acts as an efficient electron acceptor toward the polymer in the excited state. These features indicate that this conjugated polymer is a promising material to develop new hybrid organic/inorganic photovoltaic cells.

Recombination of electronic excitations in regioregular poly(3-dodecylthiophene)

We use optical spectroscopy to study the recombination kinetics of long lived electronic excitations in a thin film of regioregular poly(3-dodecylthiophene). We have measured the steady-state photo-induced absorption spectrum, as well as the time-resolved decay of the transient photo-induced absorption in the ms time domain as a function of temperature. The transient decay of these absorptions seems to follow two hyperbolic decay components. We discuss one possible model that could give rise to such decay components.

Photoexcitation of poly(p-phenylene sulfoxide): signatures of triplet excitons

Near steady-state photoinduced absorption spectroscopy has been used for the initial study of millisecond photoexcitations in poly(p-phenylene sulfoxide) (PPSx). A photoinduced absorption (PA) band at ∼2.3eV is observed. We conclude, based on the transition energy, and frequency and intensity dependence studies of this band, that this PA feature is associated with the formation of triplet excitons in PPSx. The PA spectrum of PPSx is compared with PA spectra previously reported for the leucoemeraldine base (LEB) form of polyaniline and poly(p-phenylene sulfide) (PPS). Ring-torsional polarons were identified as the dominant excitations in LEB and PPS. We suggest that the sulfoxide unit is one factor inhibiting the production of polarons in PPSx.

Interaction of singlet excitons with polarons in wide band-gap organic semiconductors: A quantitative study

The steady-state photoinduced absorption (PA), photoluminescence (PL), PL-detected magnetic resonance (PLDMR), and PA-detected magnetic resonance (PADMR) of poly- and oligo-(para-phenylenes) films is described. In particular, the excitation density (laser power) ${N}_{0}$ dependence of the PA, PL, and PLDMR signals is analyzed by means of a rate equation model, which describes the dynamics of singlet excitons (SE's) and polarons in all three experiments quantitatively with the same set of parameters. The model is based on the observations that mobile SE's are quenched by trapped and free polarons and that the spin-$\frac{1}{2}$ magnetic resonance conditions reduce the total polaron population. Since the sublinear ${N}_{0}$ dependences of the positive (PL-enhancing) spin-$\frac{1}{2}$ PLDMR and the polaron PA band are essentially the same, we conclude that PLDMR is due to a reduced quenching of SE's by polarons. The agreement between the model, the current results, and results from other spectroscopic techniques provides strong evidence for this quenching mechanism. This also suggests that it is a very significant process in luminescent \ensuremath{\pi}-conjugated materials and organic light-emitting devices. Consequently, the quenching mechanism needs to be taken into account, especially at high excitation densities, which is of great importance for the development of electrically pumped polymer laser diode structures.

Spectroscopical evidences of photoinduced charge transfer in blends of C 60 and thiophene-based copolymers with a tunable energy gap

We report a study of the photoinduced charge transfer in the blends of buckminsterfullerene (C 60) and thiophene-based copolymers with tunable energy gap. The photoluminescence and steady state photoinduced absorption spectra evidence the electron transfer from the photoexcited copolymer to C 60. The efficiency of the charge transfer depends on the degree of miscibility of the two components which is easily controlled by the copolymer composition. In the blends obtained with a copolymer with an high energy absorption onset, the spectral signatures of the photoinduced hole transfer to the polymer, arising from the selective excitation of the C 60, are also detected.

New evidence of long-lived photoexcited charged states in thin films of PDA-4BCMU

We report results of steady-state photoinduced absorption experiments on PDA-4BCMU grown on silica substrate. We observe three photoinduced absorption bands at 0.82, 0.95 and 1.39 eV whose relaxation dynamics indicate that the first two are due to charged states while the third is originated by triplet excitons. The photogeneration of both charged and neutral states in PDA-4BCMU previously observed in polycarbazolyldiacetylenes, indicates that this is a general feature of polydiacetylenes. The relative weight of one excited species over the other one is influenced by the molecular and supramolecular organization of the polymer