Continuous Wave Photoinduced Absorption Research Articles

Comparing the loss of charge carrier generation with the loss of V OC at low temperatures in organic bulk-heterojunction blends

Abstract We estimate the temperature dependence of charge carrier generation in P3HT:ICBA and PTB7:PCBM using continuous-wave photoinduced absorption. For P3HT:ICBA, the generation rate increases fivefold from 79 K to ca 250 K, above which it saturates. We compare this to the temperature dependence of open-circuit voltage (V OC) which at low temperatures shows a decrease with decreasing temperatures, in disagreement with the typically expected linear rise. This voltage loss is too large to be explained by the decrease of generation with decreasing temperature. For PTB7:PCBM the dominant species has a generation rate increasing from 90 to 150 K, after which it decreases. For PTB7:PCBM there is no decrease in V OC at low temperatures. Our findings support the lack of correlation between the decrease in V OC and a decrease in generation rate. We conclude that the loss of V OC does not originate in charge carrier dynamics in the bulk material, but is due to contact effects.

Recognizing and correcting for errors in frequency-dependent modulation spectroscopy

This work shows how to acquire reliable data from frequency dependent continuous-wave modulation spectroscopy. We demonstrate this through the example of continuous-wave photoinduced absorption (cwPA), a characterization technique useful for studying long-lived photoexcitations in thin-film solar cell materials. Experimental errors arising at moderate frequencies in modulation spectroscopy are identified and corrected for. Limitations of the detectors and electronics are seen to cause both signal loss and phase shifts. Imperfect charge collection in the detector leads to wavelength-dependent correction factors, while phase shifts caused by the experimental setup call for frequency-dependent corrections. The methods outlined in this work act as a guide to avoid pitfalls in setting up modulation spectroscopy measurements and correcting for limitations.

Interchain and intrachain triplets in poly(3-thienylene vinylene) derivatives

We are reporting the optical study of two types of poly (3-thienylene vinylene) (PTV) derivatives with linear and bulk side chain configurations. The first one, poly(3-dodecylthienylenevinylene) with controlled regioregularity (PTV-CR) with an alkyl side group, showed a high degree of crystallinity, whereas the second one, imide-PTV with an electron deficient imide side group, is amorphous. Continuous wave-photoinduced absorption revealed different features of the long-lived triplet absorptions, with the geminately generated triplet–triplet pair in imide-PTV bearing more intrachain characteristics with fast recombination kinetics (<10 μs). On the other hand, the isolated triplet in PTV-CR generated by intersystem crossing had more interchain characteristics with slow recombination kinetics (ms). The decay of the triplet–triplet pair in imide-PTV is monomolecular even at a high pump intensity of 240 mW/cm2, whereas the isolated triplets in PTV-CR recombine bimolecularly at a very low pump intensity of 10 mW/cm2. Grazing incidence X-ray diffraction, doping induced absorption, and various dependences of the photoinduced absorption spectrum have corroborated the finding that reduced interchain interaction favors the formation of a correlated triplet–triplet pair, a precursor to two independent triplets formed via fission.

Long Lived Photoexcitation Dynamics in π-Conjugated Polymer/PbS Quantum Dot Blended Films for Photovoltaic Application.

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in a blend of PbS quantum dot and regio-regular poly (3-hexylthiophene) (RR-P3HT). The charge transfer from RR-P3HT to PbS as well as from PbS to RR-P3HT were observed after changing the capping ligand of PbS from a long chain molecular to a short one. Therefore, PbS could be used to extend the working spectral range in hybrid solar cells with a proper capping ligand. However, we found that the recombination mechanism in the millisecond time region is dominated by the trap/defects in blended films, while it improves to a bimolecular recombination partially after ligand exchange. Our results suggest that passivating traps of nanocrystals by improving surface ligands will be crucial for relevant solar cell applications.

Simultaneous Monitoring of Photoinduced Absorption Signals and Short-Circuit Photocurrent during Photoexcitation in Dye-Sensitized Solar Cells

Continuous-wave photoinduced absorption (PIA) measurements were performed simultaneously with photovoltage or photocurrent (PC) measurements to monitor slow transfer processes of electron carriers in operating dye-sensitized solar cells (DSCs). Time-resolved simultaneous experiments demonstrate that the rise curve of PC during photoexcitation is delayed against that of PIA signals. The delay is analyzed by a model that considers effects of electron transfer on PIA signals and shown to be determined by transfer time of electrons over the TiO2 film. On the contrary, no delay is found between the decay curves at excitation–off of PIA signals and PC, suggesting that the transfer kinetics of TiO2 electron varies after the electrons reach the transparent electrode. These spectral features indicate that the time-resolved PIA signals of TiO2 electron under short-circuit conditions are primarily determined by its slow transfer process and that the simultaneous PIA experiments are effective to study the slow electr...

Method for characterizing bulk recombination using photoinduced absorption

The influence of reaction order and trap-assisted recombination on continuous-wave photoinduced absorption measurements is clarified through analytical calculations and numerical simulations. The results reveal the characteristic influence of different trap distributions and enable distinguishing between shallow exponential and Gaussian distributions and systems dominated by direct recombination by analyzing the temperature dependence of the in-phase and quadrature signals. The identifying features are the intensity dependence of the in-phase at high intensity, PAI∝IγHI, and the frequency dependence of the quadrature at low frequency, PAQ∝ωγLF. For direct recombination, γHI and γLF are temperature independent, and for an exponential distribution, they depend on the characteristic energy Ech as γHI=1/(1+Ech/kT) and γLF=kT/Ech, while a Gaussian distribution shows γHI and γLF as functions of I and ω, respectively.

Excited-states spectroscopies and its magnetic field effect of π-conjugated polymer-fullerene blends with below-gap excitation

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long-lived polarons in blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT) with below-gap excitation. Compared to above-gap excitation, the polarons exhibited thermally activated and multi-trap limited bimolecular recombination too, but with narrower distribution profile in polaron states in millisecond time regime. However, magnetophoto-induced absorption (MPA) of polarons presented much weaker magnetic field effect in below-gap excitation (∼0.5mT) than in above-gap excitation (order of ∼10mT), although the underline mechanism is same as spin mixing of polaron pairs by the hyperfine interaction at small field region.

Optical Properties of Heterojunction between Hybrid Halide Perovskite and Charge Transport Materials: Exciplex Emission and Large Polaron

The photoluminescence from exciplex states at lower energy than the bandgap of CH3NH3PbI3 (MAPbI3) was observed in both MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM bilayer films. At the same time, using continuous wave photoinduced absorption (PIA) spectroscopy, we observed the PIA band at 0.14 eV, which was due to the positive or negative polarons in perovskite for MAPbI3/Spiro-OMeTAD and MAPbI3/PCBM film, respectively. The binding energies of both kinds of polarons are about ∼40 meV, with the radius of phonon cloud of about 5.9 nm. Our finding may supply direct spectroscopic evidence for the existence of exciplex states in heterojunction of hybrid halide perovskite and charge transport materials as well as consequent large polarons in perovskite.

Continuous-wave photoinduced absorption study on trapped carriers in bulk-heterojunction solar cells connected to load

By using continuous-wave photoinduced absorption spectroscopy, we investigate hole-transport processes in polymer-based bulk-heterojunction solar cells at various operating points between the open- and short-circuit conditions. The results reveal that the detrapping time constant of holes decreases as the short-circuit condition is approached. This decrease may be explained by an acceleration of detrapping that is caused by an internal electric field. This interpretation is supported by a numerical simulation based on rate equations, considering a low density of a single trap level. Since the detrapping signal disappears at short-circuit, we estimate the associated trap levels to be relatively shallow.

Long lived photoexcitation dynamics in π-conjugated polymer and fullerene blended films

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long lived polarons in blend of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT), and in blend of PCBM and 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene) (MEH-PPV). In millisecond time regime, delocalized polarons (DP) and localized polarons (LP) in RR-P3HT/PCBM as well as polarons in MEH-PPV/PCBM all exhibit dispersive bimolecular recombination process which was limited by the trap states, with the average lifetimes of those polarons inverse proportional to the square root of pump intensity (I). The recombination in RR-P3HT/PCBM was weak temperature dependence with small thermal activation energy, Δ for DPs and LPs of 25meV and 13meV, respectively; in contrast, Δ for polarons in amorphous MEH-PPV/PCBM was ∼160meV. Furthermore, we proved that the values of Δ for both of LP and DP increase, as well as the relatively intensity ratio of DP and LP decreases, in an intentionally degraded RR-P3HT/PCBM film. Overall, it is demonstrated that steady state photomodulation technique with thermal-activated-recombination analysis can be applied to evaluate polymer (dis)order in bulk heterojunction films.

Continuous-wave photoinduced absorption studies in polythiophene and fullerene blended thin films

The generation and recombination dynamics of photocarriers (polarons) in poly(3-hexylthiophene) and $[6,6]$-phenyl-${\mathrm{C}}_{61}$-butyric acid methyl ester blended thin films have been investigated by means of continuous-wave photoinduced absorption spectroscopy. Continuous-wave photoinduced absorption spectroscopy has the advantage of being used to investigate long-lived excited species with lifetimes that range from microseconds to milliseconds. In this time range, three kinds of excited species are recognized in the blends: trapped, mobile, and delocalized polarons. From the modulation-frequency dependence, the mean lifetimes of the three types of polarons at low temperature were estimated. It is known that polarons are not generated directly by the pump beam but rather through Coulombically bound radical pairs. We have found that this two-step model for polaron generation can be recognized on a microsecond time scale by continuous-wave photoinduced absorption measurements.

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.

Confinement-Dependent Below-Gap State in PbS Quantum Dot Films Probed by Continuous-Wave Photoinduced Absorption

We have measured continuous-wave (cw) photoinduced absorption (PA) spectra of PbS quantum dot (QD) films with four different sizes over a spectral range of 0.25-0.5 eV at T=10 K. The PA spectrum shows a strong asymmetric IR absorption peak (IR-PA). Both the peak position and shape of this IR-PA indicate distinct confinement dependence. Combining with results of interband transitions and Stokes shift, we assign this IR-PA to a transition from a well-defined below-gap state to the second excitonic level (1P). By measuring the frequency dependence of this IR-PA, we estimate the lifetime of this below-gap state to be around several microseconds. Possible interpretations of the origin of this below-gap state are given. This transition could potentially be used to monitor photogenerated charge transfer in such QD systems.

Long-Lived Charged States in Single-Walled Carbon Nanotubes

We present continuous wave photoinduced absorption spectroscopy of single-walled carbon nanotubes dispersed in a polymer matrix. The spectrum is dominated by a modulation of the absorption line shape, predominantly of large diameter tubes, that we assigned to electroabsorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation points to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.

Photoinduced absorption in an alternating polyfluorene copolymer for photovoltaic applications

We present a detailed study of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)- alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3-benzo-thiadiazole)], and its blends with the fullerene derivative [6,6]-phenyl-C 61-butyric acid methyl ester, using continuous wave photoinduced absorption (PA) techniques. We also present the use of phase information from the PA measurements for estimating the number of different photoexcitation types present in the PA spectra as well as their lifetimes. In all blends the PA spectra show a broad high-energy PA band ranging from ∼1 to 2 eV as well as a low-energy band at ∼0.35 eV. We find two kinds of excitations present at 80 K, polarons showing dispersive recombination with lifetimes of roughly a millisecond, and a second kind of photoexcitations tentatively assigned to coulombically bound intrachain polaron pairs showing practically non-dispersive recombination with a lifetime of around 20 μs.

Photophysical studies on nanostructured PPV-systems

We have used continuous wave photoinduced absorption (PIA) to probe the (spin-1/2) polaron and (spin-1) triplet excitations in bulk poly( para-phenylene vinylene) (PPV) films and isolated PPV chains incorporated into a self-assembled matrix ordered at the nanometer scale (nano-PPV). The ordering is a result of the lyotropic liquid crystalline character of the matrix material. We investigated the influence of the chain isolation on the transition energies and the effect of the resulting one-dimensionality on the excitation dynamics.