Beyond the Adiabatic Limit: Charge Photogeneration in Organic Photovoltaic Materials

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

Influence of Acceptor Structure on Barriers to Charge Separation in Organic Photovoltaic Materials

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

Time-Resolved Infrared Spectroscopy Directly Probes Free and Trapped Carriers in Organo-Halide Perovskites

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

Approaching Bulk Carrier Dynamics in Organo-Halide Perovskite Nanocrystalline Films by Surface Passivation

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

Molecular Origins of Defects in Organohalide Perovskites and Their Influence on Charge Carrier Dynamics

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 PbI₄^2– were found in precursor solutions that match those used to deposit perovskite films for solar cell applications. We found that the concentration of tetraiodoplumbate PbI₄^2– 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 CH₃NH₃PbI_3–<i>x</i>Cl_<i>x</i> suppressed the formation of PbI₄^2– 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

Dynamic Exchange During Triplet Transport in Nanocrystalline TIPS-Pentacene Films

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

Electron–Phonon Coupling and Resonant Relaxation from 1D and 1P States in PbS Quantum Dots

Observations of the hot-phonon bottleneck, which is predicted to slow the rate of hot carrier cooling in quantum confined nanocrystals, have been limited to date for reasons that are not fully understood. We used time-resolved infrared spectroscopy to directly measure higher energy intraband transitions in PbS colloidal quantum dots. Direct measurements of these intraband transitions permitted detailed analysis of the electronic overlap of the quantum confined states that may influence their relaxation processes. In smaller PbS nanocrystals, where the hot-phonon bottleneck is expected to be most pronounced, we found that relaxation of parity selection rules combined with stronger electron–phonon coupling led to greater spectral overlap of transitions among the quantum confined states. This created pathways for fast energy transfer and relaxation that may bypass the predicted hot-phonon bottleneck. In contrast, larger, but still quantum confined nanocrystals did not exhibit such relaxation of the parity selection rules and possessed narrower intraband states. These observations were consistent with slower relaxation dynamics that have been measured in larger quantum confined systems. These findings indicated that, at small radii, electron–phonon interactions overcome the advantageous increase in energetic separation of the electronic states for PbS quantum dots. Selection of appropriately sized quantum dots, which minimize spectral broadening due to electron–phonon interactions while maximizing electronic state separation, is necessary to observe the hot-phonon bottleneck. Such optimization may provide a framework for achieving efficient hot carrier collection and multiple exciton generation

Tuning the Dielectric Properties of Organic Semiconductors via Salt Doping

Enhancing the dielectric permittivity of organic semiconductors may open new opportunities to control charge generation and recombination dynamics in organic solar cells. The potential to tune the dielectric permittivity of organic semiconductors by doping them with redox inactive salts was explored using a combination of organic synthesis, electrical characterization, and time-resolved infrared spectroscopy. The addition of the salt, LiTFSI (lithium bis­(trifluoro-methyl-sulfonyl)­imide), to a conjugated polymer specifically designed to incorporate ions into its bulk phase increased the density of holes and enhanced the static dielectric permittivity of the polymer blend by more than an order of magnitude. The frequency and phase dependence of the real dielectric function demonstrates that the increase in dielectric permittivity resulted from dipolar motion of bound ion pairs or clusters of ions. Interestingly, the increases in the hole density and dielectric permittivity were associated with enhancement of the hole mobility by 2 orders of magnitude relative to the undoped polymer. The charge recombination lifetime also increased by an order of magnitude in the blend with a fullerene electron acceptor when ions were added to the polymer. The findings indicate that ion doping enables organic semiconductors with large increases in low frequency dielectric permittivity and that these changes result in improved charge transport and suppressed charge recombination on the microsecond time scale

Molecular Rectification in Conjugated Block Copolymer Photovoltaics

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

Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission

Singlet fission is an excitation multiplication process in molecular systems that can circumvent energy losses and significantly boost solar cell efficiencies; however, the nature of a critical intermediate that enables singlet fission and details of its evolution into multiple product excitations remain obscure. We resolve the initial sequence of events comprising the fission of a singlet exciton in solids of pentacene derivatives using femtosecond transient absorption spectroscopy. We propose a three-step model of singlet fission that includes two triplet-pair intermediates and show how transient spectroscopy can distinguish initially interacting triplet pairs from those that are spatially separated and noninteracting. We find that the interconversion of these two triplet-pair intermediates is limited by the rate of triplet transfer. These results clearly highlight the classical kinetic model of singlet fission and expose subtle details that promise to aid in resolving problems associated with triplet extraction