Transient Photoinduced Absorption Research Articles

17.3% efficiency CsPbI2Br solar cells by integrating a Near-infrared absorbed organic Bulk-heterojunction layer

All-inorganic perovskite solar cells (PSCs) have gained extensive attention owing to the excellent intrinsic thermal stability, but their power conversion efficiencies (PCEs) still lag behind those of organic–inorganic hybrid PSCs due to their relatively wide bandgap. To extend the absorption of CsPbI2Br based all-inorganic PSCs, in this work, a low bandgap organic active layer of PM6:Y6 is integrated on the top of CsPbI2Br layer. Compared with CsPbI2Br PSCs using PM6 as hole transport layer, the integrated solar cells (ISCs) obtained an extended photo-response range to 950 nm and improved short-circuit current density (JSC) from 14.25 to 15.44 mA cm−2, as well as promoted PCE from 15.31% to 17.33%. Furthermore, the photoluminescence (PL), time-resolved-photoluminescence (TRPL), photoinduced transient absorption spectroscopy (TAS), transient photocurrent (TPC), and transient photovoltage (TPV) results demonstrate the effective carriers’ bidirectional transport and suppressed carriers’ recombination at CsPbI2Br/PM6:Y6 interface, which is the basis for ISCs to obtain high efficiency. Our findings demonstrate that ISC is a promising strategy to extend the photo-response range and improve the performance of CsPbI2Br based PSCs, and give a deeper understanding of carrier transfer in ISCs.

Bidirectional Photochemistry of Antarctic Microbial Rhodopsin: Emerging Trend of Ballistic Photoisomerization from the 13-cis Resting State.

The decades-long ultrafast examination of nearly a dozen microbial retinal proteins, ion pumps, and sensory photoreceptors has not identified structure–function indicators which predict photoisomerization dynamics, whether it will be sub-picosecond and ballistic or drawn out with complex curve-crossing kinetics. Herein, we report the emergence of such an indicator. Using pH control over retinal isomer ratios, photoinduced transient absorption is recorded in an inward proton pumping Antarctic microbial rhodopsin (AntR) for 13-cis and all-trans retinal resting states. The all-trans fluorescent state decays with 1 ps exponential kinetics. In contrast, in 13-cis it decays within ∼300 fs accompanied by continuous spectral evolution, indicating ballistic internal conversion. The coherent wave packet nature of 13-cis isomerization in AntR matches published results for bacteriorhodopsin (BR) and Anabaena sensory rhodopsin (ASR), which also accommodate both all-trans and 13-cis retinal resting states, marking the emergence of a first structure–photodynamics indicator which holds for all three tested pigments.

Disentangling Light- and Temperature-Induced Thermal Effects in Colloidal Au Nanoparticles.

We present temperature-dependent (from room temperature to 80 °C) absorption spectra of Au/SiO2 core–shell nanoparticles (NPs) (core diameter: ∼25 nm) in water in the range from 1.5 to 4.5 eV, which spans the localized surface plasmon resonance (LSPR) and the interband transitions. A decrease in absorption with temperature over the entire spectral range is observed, which is more prominent at the LSPR. These changes are well reproduced by theoretical calculations of the absorption spectra, based on the experimentally measured temperature-dependent real (ε1) and imaginary (ε2) parts of the dielectric constant of Au NPs and of the surrounding medium. In addition, we model the photoinduced response of the NPs over the entire spectral range. The experimental and theoretical results of the thermal heating and the simulations of the photoinduced heating are compared with the ultrafast photoinduced transient absorption (TA) spectra upon excitation of the LSPR. These show that while the latter is a reliable monitor of heating of the NP and its environment, the interband region mildly responds to heating but predominantly to the population evolution of charge carriers.

Small-Polaron Hopping and Low-Temperature (45–225 K) Photo-Induced Transient Absorption in Magnesium-Doped Lithium Niobate

A strongly temperature-dependent photo-induced transient absorption is measured in 6.5 mol% magnesium-doped lithium niobate at temperatures ranging from 45 K to 225 K. This phenomenon is interpreted as resulting from the generation and subsequent recombination of oppositely charged small polarons. Initial two-photon absorptions generate separated oppositely charged small polarons. The existence of these small polarons is monitored by the presence of their characteristic absorption. The strongly temperature-dependent decay of this absorption occurs as series of thermally assisted hops of small polarons that facilitate their merger and ultimate recombination. Our measurements span the high-temperature regime, where small-polaron jump rates are Arrhenius and strongly dependent on temperature, and the intermediate-temperature regime, where small-polaron jump rates are non-Arrhenius and weakly dependent on temperature. Distinctively, this model provides a good representation of our data with reasonable values of its two parameters: Arrhenius small-polaron hopping’s activation energy and the material’s characteristic phonon frequency.

Photoinduced Reversible Modulation of Fluorescence of CdSe/ZnS Quantum Dots in Solutions with Diarylethenes.

Steady-state absorption and fluorescence spectra, fluorescence decay kinetics of CdSe/ZnS quantum dots (QD) with photochromic diarylethenes (DAE) in toluene have been studied. Two kinds of QDs emitting at 525 and 600nm were investigated and DAE were selected to ensure good overlap of their photoinduced absorption band with QDs emission spectra. It has been found that photochromic molecules form complexes with QD which results in partial fluorescence quenching. A reversible modulation of QDs emission intensity which correlates with magnitude of transient photoinduced absorption band of the diarylethenes during photochromic transformations has been demonstrated.

Expanding the Light Harvesting of CsPbI2Br to Near Infrared by Integrating with Organic Bulk Heterojunction for Efficient and Stable Solar Cells.

All-inorganic perovskite (CsPbX3, X = Br or I) solar cells demonstrate superior stability, while the power conversion efficiency (PCE) lags behind the organic-inorganic hybrid counterparts mainly due to the limitation of narrow absorption bands. To broaden their absorption spectrum and improve their PCE, all-inorganic perovskite/organic integrated solar cells utilizing CsPbI2Br as an ultraviolet-visible light absorber and PBDTTT-E-T:IEICO as a near-infrared light absorber are demonstrated in this work. The integrated solar cells exhibit a broadened photoresponse to over 900 nm, attributed to the integration of PBDTTT-E-T:IEICO. The additional absorption enhances the short-circuit current density from 14.78 to 15.98 mA/cm2, resulting in greatly improved PCE of 14.03% for integrated solar cells, much higher than that of the control perovskite solar cells (12.53%) and organic solar cells (7.51%). An in-depth understanding of the charge-transfer dynamic process in the CsPbI2Br/PBDTTT-E-T:IEICO film is comprehensively analyzed by photoinduced transient absorption spectroscopy. Furthermore, the air stability and thermal stability of the integrated solar cells are greatly enhanced. For unencapsulated integrated solar cells, the PCE still preserves 95% of its initial value after aging for 300 h in an ambient environment and retains about 90% of its original value even after aging at 85 °C for 180 h in nitrogen.

Transient Magnetic Field Effect of Photoexcitations in Donor-Acceptor Organic Semiconductors.

We report transient photoinduced absorption (t-PA) and magnetic field ( B)-dependent t-PA (t-MPA( B)) in a pristine low band gap π-conjugated copolymer composed of donor and acceptor moieties, namely, the poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thi-eno[3,4- b]thiophenediyl]]) (or PTB7) used in photovoltaic applications. Unlike traditional π-conjugated polymers in which the primary photoexcitations are singlet excitons (SE), in pristine PTB7 we find at short times coexistence of two primary photoexcitation species, namely, SE and triplet-triplet (TT) pair. Both species are photogenerated directly from the ground state and are spin-correlated. Although the TT pair decomposes into two separate triplet excitons (TEs) in ∼100 ps, the separated TE spins are still entangled up to ∼6 μs. At longer times, the t-MPA( B) response of the surviving TEs shows transient narrowing effect, which is attributed to a distribution of the TE size.

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.

Photoluminescence in Organometal Halide Perovskites: Free Carrier Versus Exciton

The photoexcitation dynamics in CH3NH3PbI3 thin films has been studied by nanosecond time-resolved photoluminescence (PL) and transient photoinduced absorption. Under excitation intensity from ∼0.1 to ∼100 μJ/cm2, free carrier is the dominant photoexcitation species. However, in the intermediate excitation intensity from ∼1 to ∼10 μJ/cm2, a small fraction of photoexcitations are excitons, which, however, are responsible for PL within this excitation intensity range. The branching ratio between excitons and free carriers at 2 μJ/cm2 was estimated to be 17%. Furthermore, the spectral narrowing PL was found at the low temperature after the formation of excitons, indicating that laser action could be due to excitonic emission. Our findings provide fundamental insights into the optical properties of organometal halide perovskites with direct implications for optoelectronics.

Efficient Extraction of Trapped Holes from Colloidal CdS Nanorods.

Cadmium Sulfide (CdS) nanostructures have been widely applied for solar driven H2 generations due to its suitable band gap and band edge energetics. For an efficient photoreduction reaction, hole scavenging from CdS needs to compete favorably with many recombination processes. Extensive spectroscopic studies show evidence for hole trapping in CdS nanostructures, which naturally leads the concern of extracting trapped holes from CdS in photocatalytic reactions. Here, we report a study of hole transfer dynamics from colloidal CdS nanorods (NRs) to adsorbed hole acceptor, phenothiazine (PTZ), using transient absorption spectroscopy. We show that >99% of the holes were trapped (with a time constant of 0.73 ps) in free CdS NRs to form a photoinduced transient absorption (PA) feature. In the presence of PTZ, we observed the decay of the PA feature and corresponding formation of oxidized PTZ(+) radicals, providing direct spectroscopic evidence for trapped hole transfer from CdS. The trapped holes were extracted by PTZ in 3.8 ± 1.7 ns (half-life) to form long-lived charge separated states (CdS(-)-PTZ(+)) with a half lifetime of 310 ± 50 ns. This hole transfer time is significantly faster than the slow conduction band electron-trapped hole recombination (half lifetime of 67 ± 1 ns) in free CdS NRs, leading to an extraction efficiency of 94.7 ± 9.0%. Our results show that despite rapid hole trapping in CdS NRs, efficient extraction of trapped holes by electron donors and slow recombination of the resulting charge-separated states can still be achieved to enable efficient photoreduction using CdS nanocrystals.

Molecular packing and electronic processes in amorphous-like polymer bulk heterojunction solar cells with fullerene intercalation.

The interpenetrating morphology formed by the electron donor and acceptor materials is critical for the performance of polymer:fullerene bulk heterojunction (BHJ) photovoltaic (PV) cells. In this work we carried out a systematic investigation on a high PV efficiency (>6%) BHJ system consisting of a newly developed 5,6-difluorobenzo[c]125 thiadiazole-based copolymer, PFBT-T20TT, and a fullerene derivative. Grazing incidence X-ray scattering measurements reveal the lower-ordered nature of the BHJ system as well as an intermixing morphology with intercalation of fullerene molecules between the PFBT-T20TT lamella. Steady-state and transient photo-induced absorption spectroscopy reveal ultrafast charge transfer (CT) at the PFBT-T20TT/fullerene interface, indicating that the CT process is no longer limited by exciton diffusion. Furthermore, we extracted the hole mobility based on the space limited current (SCLC) model and found that more efficient hole transport is achieved in the PFBT-T20TT:fullerene BHJ as compared to pure PFBT-T20TT, showing a different trend as compared to the previously reported highly crystalline polymer:fullerene blend with a similar intercalation manner. Our study correlates the fullerene intercalated polymer lamella morphology with device performance and provides a coherent model to interpret the high photovoltaic performance of some of the recently developed weakly-ordered BHJ systems based on conjugated polymers with branched side-chain.

P-type Mesoscopic nickel oxide/organometallic perovskite heterojunction solar cells.

In this article, we present a new paradigm for organometallic hybrid perovskite solar cell using NiO inorganic metal oxide nanocrystalline as p-type electrode material and realized the first mesoscopic NiO/perovskite/[6,6]-phenyl C61-butyric acid methyl ester (PC61BM) heterojunction photovoltaic device. The photo-induced transient absorption spectroscopy results verified that the architecture is an effective p-type sensitized junction, which is the first inorganic p-type, metal oxide contact material for perovskite-based solar cell. Power conversion efficiency of 9.51% was achieved under AM 1.5 G illumination, which significantly surpassed the reported conventional p-type dye-sensitized solar cells. The replacement of the organic hole transport materials by a p-type metal oxide has the advantages to provide robust device architecture for further development of all-inorganic perovskite-based thin-film solar cells and tandem photovoltaics.

Nongeminate recombination in neat P3HT and P3HT:PCBM blend films

The slow decay of charge carriers in polymer–fullerene blends measured in transient studies has raised a number of questions about the mechanisms of nongeminate recombination in these systems. In an attempt to understand this behavior, we have applied a combination of steady-state and transient photoinduced absorption measurements to compare nongeminate recombination behavior in films of neat poly(3-hexyl thiophene) (P3HT) and P3HT blended with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM). Transient measurements show that carrier recombination in the neat P3HT film exhibits second-order decay with a recombination rate coefficient that is similar to that predicted by Langevin theory. In addition, temperature dependent measurements indicate that neat films exhibit recombination behavior consistent with the Gaussian disorder model. In contrast, the P3HT:PCBM blend films are characterized by a strongly reduced recombination rate and an apparent recombination order greater than two. We then assess a number of previously proposed explanations for this behavior including phase separation, carrier concentration dependent mobility, non-encounter limited recombination, and interfacial states. In the end, we propose a model in which pure domains with a Gaussian density of states are separated by a mixed phase with an exponential density of states. We find that such a model can explain both the reduced magnitude of the recombination rate and the high order recombination kinetics and, based on the current state of knowledge, is the most consistent with experimental observations.

Ultrafast Dynamics of Photogenerated Electrons in CdS Nanocluster Multilayers Assembled on Solid Substrates: Effects of Assembly and Electrode Potential

The ultrafast dynamics of photogenerated electrons in multilayer assemblies of CdS nanoparticles prepared on quartz and indium-tin oxide (ITO) substrates were followed by femtosecond (fs) visible-pump/mid-IR probe spectroscopy. Based on the observation of the photoinduced transient absorption spectra in the broad mid-IR range at the multilayer assembly of CdS nanoparticles, the occupation and fast relaxation of higher electronic states (1P(e)) were clarified. As compared with the electron dynamics of isolated (dispersed in solution) nanoparticles, the decay of photoexcited electrons in the multilayer assembly was clearly accelerated probably due to both electron hopping and scattering during interparticle electron tunneling. By using an ITO electrode as a substrate, the effect of the electric field on the photoelectron dynamics in the multilayer assembly was also investigated in situ. Both the amplitude and lifetime of photoexcited electrons gradually reduced as the potential became more positive. This result was explained by considering the reduction of the interparticle tunneling probability and the increase in the electron-transfer rate from the CdS nanoparticle assembly to the ITO electrode.

Ultrafast Charge Photogeneration in Semiconducting Carbon Nanotubes

We show that excitons are not the unique outcome of photoexcitation in single-walled carbon nanotubes (SWNTs). Our experiments of transient photoinduced absorption suggest that charge carriers are formed with quantum yield of a few percent and that such species strongly affect the long-lived transient spectrum. Photogenerated charge carriers induce strong local electric fields that shift by the Stark effect the second subband exciton absorption in SWNTs, resulting in a characteristic derivative shape of the transient absorption spectra.

Dynamic Monte Carlo modeling of exciton dissociation in organic donor-acceptor solar cells

A general dynamic Monte Carlo model for exciton dissociation at a donor-acceptor interface that includes exciton delocalization and hot charge separation is developed to model the experimental behavior observed for the poly(3-hexylthiophene):fullerene system and predict the theoretical performance of future materials systems. The presence of delocalized excitons and the direct formation of separated charge pairs has been recently measured by transient photo-induced absorption experiments and has been proposed to facilitate charge separation. The excess energy of the exciton dissociation process has also been observed to have a strong correlation with the charge separation yield for a series of thiophene based polymer:fullerene systems, suggesting that a hot charge separation process is also occurring. Hot charge separation has been previously theorized as a cause for highly efficient charge separation. However, a detailed model for this process has not been implemented and tested. Here, both conceptual models are implemented into a dynamic Monte Carlo simulation and tested using a simple bilayer donor-acceptor system. We find that exciton delocalization can account for a significant reduction in geminate recombination when compared to the traditional, bound polaron pair model. In addition, the hot charge separation process could further reduce the geminate recombination, but only if the hot charge mobility is several orders of magnitude larger than the standard charge mobility.

Spectroscopic investigation of excitons, photocarriers, and bias-induced carriers in regioregular poly(3-alkylthiophene)

Excitons, photocarriers, and bias-induced carriers in regioregular poly(3-alkylthiophene) (P3AT) films were investigated using a variety of spectroscopic techniques, such as continuous-wave- (cw-) and femtosecond (fs-) transient photoinduced absorption (PIA) and device modulation (DM) spectroscopies. Comparison between the cw PIA and DM spectra of the poly(3-hexylthiophene) (P3HT) film reveals that photocarriers generated by cw photoexcitation are dominated by localized polarons, whereas charge injection primarily creates delocalized polarons. The photoexcitation intensity dependence of the cw PIA signals demonstrates that both localized and delocalized photogenerated polarons obey a bimolecular recombination process under cw photoexcitation. The proportion of polarons generated by cw photoexcitation in the P3HT film is estimated to be 1/(1$+$0.137 ${I}^{1/2}$) for $I$ (mW/cm${}^{2}$) by comparison of the spectral intensities between the DM and cw PIA spectra. DM spectroscopy reveals that both localized and delocalized polarons rapidly increase at a rise point of the bias current. The cw PIA bleaching spectra of P3AT films are shown to vary with the length of the alkyl sidechains of P3AT and be almost reproduced using a single Huang-Rhys factor. The Huang-Rhys factors obtained reveal a trend that the electron-phonon couplings of final exciton migration sites become strong with the increase in the interlamella distance in the P3AT film, which indicates that interlamella couplings affect the electron-phonon couplings of the migration sites located at two-dimensional lamellas.

Evidence for excimer photoexcitations in an orderedπ-conjugated polymer film

We report pressure-dependent transient picosecond and continuous-wave photomodulation studies of disordered and ordered films of 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene). Photoinduced absorption (PA) bands in the disordered film exhibit very weak pressure dependence and are assigned to intrachain excitons and polarons. In contrast, the ordered film exhibits two additional transient PA bands in the midinfrared that blueshift dramatically with pressure. Based on high-order configuration interaction calculations we ascribe the PA bands in the ordered film to excimers. Our work brings insight to the exciton binding energy in ordered films versus disordered films and solutions. The reduced exciton binding energy in ordered films is due to new energy states appearing below the continuum band threshold of the single strand.

W-band transient EPR and photoinduced absorption on spin-labeled fullerene derivatives

We investigated by W-band (94 GHz) transient electron paramagnetic resonance (TREPR) and photoinduced absorption (PIA) spectroscopy two fullerene derivatives bearing a nitroxide radical unit. After pulsed laser photoexcitation of the molecules in liquid toluene solution, complex EPR spectra are recorded, with lines in absorption and emission. The intrinsic higher spectral and temporal resolution of the W-band frequency leads to the assignment of all the lines in the spectrum and the determination of the sign and the absolute value of the exchange coupling between the fullerene in its photoexcited triplet state (S(T) = 1) and the radical (S(R) = 1/2). The two compounds with different fullerene-nitroxide spacers show opposite-ferromagnetic and antiferromagnetic-exchange couplings. The time evolution of the spectra and the polarization of the lines are interpreted in terms of several possible spin polarization mechanisms. The EPR measurements are complemented with PIA experiments.

Ultrafast transient photoinduced absorption in Ge core and Ge/GeO2 core/shell nanoparticles: effect of interface passivation

Transient photoinduced absorption in Ge core and Ge/GeO2 core/shell nanoparticles (NPs) has been studied in visible spectral range. Two distinctive relaxation processes were observed: the fast relaxation with lifetime of several picoseconds corresponds to the carriers transfer from the excitation region to the surrounding interface or defect-related states, and the slow relaxation (tens of picoseconds) is correlated with the carriers through the interface-related states toward the indirect lowest energy states. Time-resolved studies show that the passivation of GeO2 shell significantly reduces the density of interface state and suppresses the rate of carrier trapping in Ge/GeO2 core/shell structure, which provides an alternative method to modulate the optical properties in NPs.