Phosphor-converted light emitting diodes (pcLEDs) produce white light through the use of phosphors that convert blue light emitted from the LED chip into green and red wavelengths. Understanding the mechanisms of degradation of the emission spectra and quantum yields of the phosphors used in pcLEDs is of critical importance to fully realize the potential of solid-state lighting as an energy efficient technology. Toward this end, time-resolved photoluminescence spectroscopy was used to identify the mechanistic origins of enhanced stability and luminescence efficiency that can be obtained from a series of carbidonitride red phosphors with varying degrees of substitutional carbon. The increasing substitution of carbon and oxygen in nitrogen positions of the carbidonitride phosphor (Sr2Si5N8–[(4x/3)+z]CxO3z/2:Eu2+) systematically changed the dimensions of the crystalline lattice. These structural changes caused a red shift and broadening of the emission spectra of the phosphors due to faster energy transfer from higher to lower energy emission sites. Surprisingly, in spite of broadening of the emission spectra, the quantum yield was maintained or increased with carbon substitution. Aging phosphors with lowered carbon content under conditions that accurately reflected thermal and optical stresses found in functioning pcLED packages led to spectral changes that were dependent on substitutional carbon content. Importantly, phosphors that contained optimal amounts of carbon and oxygen possessed luminescence spectra and quantum yields that did not undergo changes associated with aging and therefore provided a more stable color point for superior control of the emission properties of pcLED packages. These findings provide insights to guide continued development of phosphors for efficient and stable solid-state lighting materials and devices.Keywords: energy transfer mechanism; phosphor aging; phosphor stability; phosphor-converted LED; time-resolved photoluminescence spectroscopy;

Free carrier dynamics in organo-halide perovskites can directly reveal information about their carrier lifetimes and indirectly reveal information about trap state distributions, both of which are critical to improving their performance and stability. Time-resolved photoluminescence (TRPL) spectroscopy is commonly used to probe carrier dynamics in these materials, but the technique is only sensitive to radiative decay pathways and may not reveal the true carrier dynamics. We used time-resolved infrared (TRIR) spectroscopy in comparison to TRPL to show that photogenerated charges relax into free carrier states with lower radiative recombination probabilities, which complicates TRPL measurements. Furthermore, we showed that trapped carriers exhibit distinct mid-infrared absorptions that can be uniquely probed using TRIR spectroscopy. We used the technique to demonstrate the first simultaneous measurements of trapped and free carriers in organo-halide perovskites, which opens new opportunities to clarify how charge trapping and surface passivation influence the optoelectronic properties of these materials.

AbstractTriplet population dynamics of solution cast films of isolated polymorphs of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) provide quantitative experimental evidence that triplet excitation energy transfer is the dominant mechanism for correlated triplet pair (CTP) separation during singlet fission. Variations in CTP separation rates are compared for polymorphs of TIPS-Pn with their triplet diffusion characteristics that are controlled by their crystal structures. Since triplet energy transfer is a spin-forbidden process requiring direct wavefunction overlap, simple calculations of electron and hole transfer integrals are used to predict how molecular packing arrangements would influence triplet transfer rates. The transfer integrals reveal how differences in the packing arrangements affect electronic interactions between pairs of TIPS-Pn molecules, which are correlated with the relative rates of CTP separation in the polymorphs. These findings suggest that relatively simple computations in conjunction with measurements of molecular packing structures may be used as screening tools to predict a priori whether new types of singlet fission sensitizers have the potential to undergo fast separation of CTP states to form multiplied triplets.

Ultrafast vibrational spectroscopy in the mid-infrared was used to directly probe the delocalization of excitons in two different perylenediimide (PDI) derivatives that are predicted to preclude the formation of excimers, which can act as trap sites for excited state energy in organic semiconductors. We identified vibrational modes within the conjugated C–C stretch modes of PDI molecules whose frequencies reported the interactions of molecules within delocalized excitonic states. The vibrational linewidths of these modes, which we call intermolecular coordinate coupled (ICC) modes, provided a direct probe of the extent of exciton delocalization among the PDI molecules, which was confirmed using X-ray diffraction and electro-absorption spectroscopy. We show that a slip-stacked geometry among the PDI molecules in their crystals promotes delocalized charge-transfer (CT) excitons, while localized Frenkel excitons tend to form in crystals with helical, columnar stacking geometries. Because all molecules possess vibrational modes, the use of ultrafast mid-infrared spectroscopy to measure ICC vibrational modes offers a new approach to examine exciton delocalization in a variety of small molecule electron acceptors for optoelectronic and organic photovoltaic applications.

The multiplication of excitons in organic semiconductors via singlet fission offers the potential for photovoltaic cells that exceed the Shockley–Quiesser limit for single-junction devices. To fully utilize the potential of singlet fission sensitizers in devices, it is necessary to understand and control the diffusion of the resultant triplet excitons. In this work, a new processing method is reported to systematically tune the intermolecular order and crystalline structure in films of a model singlet fission chromophore, 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn), without the need for chemical modifications. A combination of transient absorption spectroscopy and quantitative materials characterization enabled a detailed examination of the distance- and time-dependence of triplet exciton diffusion following singlet fission in these nanocrystalline TIPS-Pn films. Triplet–triplet annihilation rate constants were found to be representative of the weighted average of crystalline and amorphous phases in TIPS-Pn films comprising a mixture of phases. Adopting a diffusion model used to describe triplet–triplet annihilation, the triplet diffusion lengths for nanocrystalline and amorphous films of TIPS-Pn were estimated to be ∼75 and ∼14 nm, respectively. Importantly, the presence of even a small fraction (<10%) of the amorphous phase in the TIPS-Pn films greatly decreased the ultimate triplet diffusion length, suggesting that pure crystalline materials may be essential to efficiently harvest multiplied triplets even when singlet fission occurs on ultrafast time scales.

The electronic properties of organo-halide perovskite absorbers described in the literature have been closely associated with their morphologies and processing conditions. However, the underlying origins of this dependence remain unclear. A combination of inorganic synthesis, surface chemistry, and time-resolved photoluminescence spectroscopy was used to show that charge recombination centers in organo-halide perovskites are almost exclusively localized on the surfaces of the crystals rather than in the bulk. Passivation of these surface defects causes average charge carrier lifetimes in nanocrystalline thin films to approach the bulk limit reported for single-crystal organo-halide perovskites. These findings indicate that the charge carrier lifetimes of perovskites are correlated with their thin-film processing conditions and morphologies through the influence these have on the surface chemistry of the nanocrystals. Therefore, surface passivation may provide a means to decouple the electronic properties of organo-halide perovskites from their thin-film processing conditions and corresponding morphologies.

We investigate the influence that covalent linkage of electron donating and accepting blocks in high performance fully conjugated block copolymer photovoltaics has on charge generation and recombination using ultrafast mid-infrared transient absorption spectroscopy. We show that block copolymer architectures containing a conjugated bridge between the donor and acceptor groups can be used to form ordered mesoscale morphologies that lead to improved photovoltaic performance without enhancing charge recombination. Judicious placement of an electron-rich moiety in the electron accepting block of the block copolymer creates a donor–bridge–acceptor architecture that slows intramolecular charge transfer across the covalent linkage. Charge recombination in such donor–bridge–acceptor block copolymer films proceeds at the same rate as it does in their corresponding homopolymer blends for which the donor and acceptor blocks are not covalently linked, indicating that recombination is dominated by intermolecular charge transfer in both systems. The electrical and morphological properties of functional block copolymer photovoltaics are correlated with their underlying charge generation and recombination kinetics, permitting us to identify key design rules for further improvements in the power conversion efficiency of fully conjugated block copolymer solar cells.

The chemical origins of charge recombination centers in lead-based organohalide perovskites were investigated using a combination of quantitative solution chemistry, X-ray diffraction, and time-resolved photoluminescence spectroscopy. We explored the complex, concentration-dependent solution equilibria among iodoplumbate coordination complexes that have been implicated as potential midgap states in organohalide perovskites. High concentrations of PbI2, PbI3–, and PbI42– were found in precursor solutions that match those used to deposit perovskite films for solar cell applications. We found that the concentration of tetraiodoplumbate PbI42– is uniquely correlated with the density of charge recombination centers found in the final perovskite films regardless of the lead precursor used to cast the films. However, mixed-halide perovskites commonly referred to as CH3NH3PbI3–xClx suppressed the formation of PbI42– in comparison to perovskites that included only iodide, which is consistent with the longer charge carrier lifetimes reported in mixed-halide perovskites. These findings bring a molecular-level view to the chemical origins of charge recombination centers that provides a fundamental basis from which to understand the reported improvement in uniformity of perovskite films and devices deposited using sequential methods. These findings also suggest new approaches to control the formation of defect precursors during the deposition of organohalide perovskite absorbers.

Colloidal quantum dot (CQD) photovoltaics offer a promising approach to harvest the near-IR region of the solar spectrum, where half of the sun’s power reaching the earth resides. High external quantum efficiencies have been obtained in the visible region in lead chalcogenide CQD photovoltaics. However, the corresponding efficiencies for band gap radiation in the near-infrared lag behind because the thickness of CQD photovoltaic layers from which charge carriers can be extracted is limited by short carrier diffusion lengths. Here, we investigate, using a combination of electrical and optical characterization techniques, ligand passivation strategies aimed at tuning the density and energetic distribution of charge trap states at PbS nanocrystal surfaces. Electrical and optical measurements reveal a more than 7-fold enhancement of the mobility-lifetime product of PbS CQD films treated with 3-mercaptopropionic acid (MPA) in comparison to traditional organic passivation strategies that have been examined in the literature. We show by direct head-to-head comparison that the greater mobility-lifetime products of MPA-treated devices enable markedly greater short-circuit current and higher power conversion efficiency under AM1.5 illumination. Our findings highlight the importance of selecting ligand treatment strategies capable of passivating a diversity of surface states to enable shallower and lower density trap distributions for better transport and more efficient CQD solar cells.Keywords: charge trapping; colloidal quantum dot; mobility-lifetime products; photovoltaics; time-resolved infrared spectroscopy

Energetic barriers to charge separation are examined in photovoltaic polymer blends based on regioregular-poly(3-hexylthiophene) (P3HT) and two classes of electron acceptors: a perylene diimide (PDI) derivative and a fullerene (PCBM). Temperature-dependent measurements using ultrafast vibrational spectroscopy are used to directly measure the free energy barriers to charge separation. Charge separation in P3HT:PDI polymer blends occurs through activated pathways, whereas P3HT:PCBM blends exhibit activationless charge separation. X-ray scattering measurements reveal that neither the PDI derivative nor PCBM form highly crystalline domains in their polymer blends with P3HT. The present findings suggest that fullerenes are able to undergo barrierless charge separation even in the presence of structural disorder. In contrast, perylene diimides may require greater molecular order to achieve barrierless charge separation.

We report the observation of vibrational solvatochromism in organic photovoltaic materials. The frequency of the carbonyl (CO) stretch of the methyl ester group of the functionalized fullerene, PCBM, is sensitive to the local phase separated morphology of polymer blends and bilayers incorporating the fullerene. In particular, PCBM molecules at interfaces with conjugated polymers exhibit higher frequency carbonyl stretch vibrations in comparison to molecules imbedded in the interiors of PCBM clusters/layers. The resulting frequency gradient was recently used to examine the dynamics of charge photogeneration in a blend of the conjugated polymer, CN-MEH-PPV, with PCBM. In this contribution, we explore the origin of the frequency shift and show that it arises from variations in the inhomogeneous solvent environments experienced by PCBM molecules as a result of phase separation in the polymer blend.

Mounting evidence suggests that excess energy in charge-transfer (CT) excitonic states facilitates efficient charge separation in organic solar cells. Experimental and theoretical studies have revealed that this excess energy may reside in phonon modes or in electronic coordinates of organic photovoltaic materials that are directly excited by the transition from Frenkel to CT excitons. Despite their strong Coulombic attraction, electron−hole pairs in hot CT excitons are able to undergo activationless separation because the rate of separation competes with thermalization of electronic and nuclear degrees of freedom. We argue that these observations indicate strong coupling of the dynamics of electronic and nuclear coordinates in organic photovoltaic materials. Thus, a nonadiabatic description is needed to properly understand the mechanism of charge photogeneration in organic solar cells. Such a description will support continuing efforts toward the development of low-band-gap organic solar cells that efficiently generate photocurrent with minimal energy losses.

Ultrafast orientational motion and spectral diffusion of the carbonyl stretch vibration of the functionalized fullerene, PCBM, blended with the conjugated polymer, CN-MEH-PPV, are examined with two-dimensional infrared and polarization-resolved IR pump probe spectroscopy. In previous contributions from our group, the carbonyl stretch frequency of PCBM has been used as a local vibrational reporter to measure the temperature dependence of the time scale for dissociation of charge transfer excitons in CN-MEH-PPV:PCBM polymer blends. It was found that the rate of charge separation is independent of temperature, indicating that charge separation occurs through an activationless pathway. This assignment was supported by the observation at room temperature that thermal fluctuations do not give rise to spectral diffusion of the carbonyl stretch vibration on the picosecond and longer time scale. In this contribution, we examine the temperature dependence of the carbonyl vibrational dynamics to determine whether thermal fluctuations might give rise to spectral diffusion at other temperatures. We find that the time scale for fast wobbling-in-cone orientational motion is independent of temperature on the subpicosecond time scale. Similarly, spectral diffusion is not observed on the picosecond and longer time scale at all temperatures examined confirming our earlier interpretation of the frequency shift dynamics exclusively in terms of charge separation. Interestingly, the half angle characterizing the wobbling-in-cone orientational motion does increase at higher temperature due to increased free-volume resulting from thermal expansion of the polymer blend.

The dynamics of free carrier formation following photoinduced electron transfer from the conjugated polymer, CN-MEH-PPV, to the electron-accepting functionalized fullerene, PCBM, are directly measured using ultrafast vibrational spectroscopy. The vibrational frequency of the carbonyl (C═O) stretch of PCBM is sensitive to the location of the molecules relative to the interfaces formed between PCBM clusters and CN-MEH-PPV. The correlation between the carbonyl frequency and the proximity to the interfaces provides the ability to directly measure the escape of electrons from their Coulombically bound radical pairs. The data indicate that the rate of free carrier formation is temperature independent from 200 to 350 K suggesting that excess vibrational energy resulting from the electron transfer reaction enables electrons to escape their Coulombic potentials on ultrafast time scales.

Ultrafast vibrational spectroscopy is used to examine the dynamics of interfacial electron transfer, free-carrier formation, and bimolecular charge recombination and trapping in an organic photovoltaic material. The carbonyl (CO) stretch of the functionalized fullerene, PCBM, is probed as a local vibrational reporter of the dynamics in a blend with a conjugated polymer, CN-MEH-PPV. Ultrafast interfacial electron transfer from CN-MEH-PPV to PCBM occurs on time scales ranging from less than 100 fs to 1 ps. PCBM molecules at interfaces with the polymer have carbonyl vibrations that are higher in frequency compared to the ensemble. The frequency variation results in part from a vibrational Stark shift arising from an interfacial dipole formed by spontaneous charge transfer from the polymer to PCBM. The Stark shift provides a means to observe directly the formation of free carriers through the spectral evolution of the carbonyl stretch. Free carrier formation occurs surprisingly quickly on the 1–10 ps time scale, suggesting that the charges experience a smaller effective Coulombic binding energy than expected. The interfacial dipole decreases the Coulombic binding energy because the negative pole of the dipole repels electrons at the PCBM domain interface. Following free-carrier formation, electrons diffuse within the material and become trapped on the microsecond time scale resulting in the formation of a distinct peak in the vibrational spectra. The time scale of charge trapping corresponds to the carrier lifetime of similar PPV-based polymer blends that have been reported in the literature on the basis of transient photocurrent measurements.

We report the observation of vibrational solvatochromism in organic photovoltaic materials. The frequency of the carbonyl (CO) stretch of the methyl ester group of the functionalized fullerene, PCBM, is sensitive to the local phase separated morphology of polymer blends and bilayers incorporating the fullerene. In particular, PCBM molecules at interfaces with conjugated polymers exhibit higher frequency carbonyl stretch vibrations in comparison to molecules imbedded in the interiors of PCBM clusters/layers. The resulting frequency gradient was recently used to examine the dynamics of charge photogeneration in a blend of the conjugated polymer, CN-MEH-PPV, with PCBM. In this contribution, we explore the origin of the frequency shift and show that it arises from variations in the inhomogeneous solvent environments experienced by PCBM molecules as a result of phase separation in the polymer blend.

Ultrafast vibrational spectroscopy is used to examine the dynamics of interfacial electron transfer, free-carrier formation, and bimolecular charge recombination and trapping in an organic photovoltaic material. The carbonyl (CO) stretch of the functionalized fullerene, PCBM, is probed as a local vibrational reporter of the dynamics in a blend with a conjugated polymer, CN-MEH-PPV. Ultrafast interfacial electron transfer from CN-MEH-PPV to PCBM occurs on time scales ranging from less than 100 fs to 1 ps. PCBM molecules at interfaces with the polymer have carbonyl vibrations that are higher in frequency compared to the ensemble. The frequency variation results in part from a vibrational Stark shift arising from an interfacial dipole formed by spontaneous charge transfer from the polymer to PCBM. The Stark shift provides a means to observe directly the formation of free carriers through the spectral evolution of the carbonyl stretch. Free carrier formation occurs surprisingly quickly on the 1–10 ps time scale, suggesting that the charges experience a smaller effective Coulombic binding energy than expected. The interfacial dipole decreases the Coulombic binding energy because the negative pole of the dipole repels electrons at the PCBM domain interface. Following free-carrier formation, electrons diffuse within the material and become trapped on the microsecond time scale resulting in the formation of a distinct peak in the vibrational spectra. The time scale of charge trapping corresponds to the carrier lifetime of similar PPV-based polymer blends that have been reported in the literature on the basis of transient photocurrent measurements.