Cody W. Schlenker

The role of spin in the kinetic control of recombination in organic photovoltaics

In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor–acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron–hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer–fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet (1CT) and spin-triplet (3CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of 3CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of 3CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron–hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.

Charge generation and energy transfer in hybrid polymer/infrared quantum dot solar cells

Conjugated polymers blended with nanocrystal quantum dots are interesting as solution processable active layers for infrared light harvesting in thin film solar cells. We study photocurrent generation processes in hybrid polymer/quantum dot photovoltaics by comparing device performance and photoinduced absorption (PIA) spectra across blends of three different conjugated polymers, poly(2,3-bis(2-(hexyldecyl)-quinoxaline-5,8-diyl-alt-N-(2-hexyldecyl)-dithieno[3,2-b:2′,3′-d]pyrrole) (PDTPQx-HD), poly[(4,4′-bis(3-(2-hexyl-decyl)dithieno[3,2-b:2′,3′-d]pyrrole)-2,6-diyl-alt-(2,5-bis(3-(2-ethyl-hexyl)thiophen-2yl)thiazolo[5,4-d]thiazole)] (PPEHTT), and poly[(4,4′-bis(2-octyl)dithieno[3,2-b:2′3′-d]silole)-2,6-diyl-alt-(2,5-bis(3-octylthiophen-2yl)thiazolo[5,4-d]thiazole)] (PSOTT) with PbS quantum dots. The PIA spectra and device performance provide evidence for long-lived photoinduced charge separation and bulk heterojunction device operation for blends of both PDTPQx-HD and PPEHTT with PbS. In contrast we find that PSOTT/PbS blends can produce viable solar cells without any evidence for long-lived charge transfer in the PIA spectra. Even so, the external quantum efficiency (EQE) spectra of PSOTT/PbS solar cells indicate that the polymer plays a significant role in light harvesting. We use photoluminescence excitation spectroscopy to confirm that the polymer funnels energy to the PbS quantum dots via energy transfer, and speculate that these blends may operate as PbS Schottky diodes sensitized by energy transfer from the semiconducting polymer host.

Modulation of hybrid organic–perovskite photovoltaic performance by controlling the excited dynamics of fullerenes

We present a synergistic approach to modulate organic–perovskite interfaces and their photovoltaic behaviors by tuning the properties of n-contact fullerenes layered atop of perovskite. Fullerenes with excited charge transfer are found to not only suppress fullerene photoluminescence, but also enhance molecular polarization and transport capabilities. This results in optimized perovskite–fullerene contact.

Activationless Multiple-Site Concerted Proton–Electron Tunneling

The transfer of protons and electrons is key to energy conversion and storage, from photosynthesis to fuel cells. Increased understanding and control of these processes are needed. A new anthracene-phenol-pyridine molecular triad was designed to undergo fast photoinduced multiple-site concerted proton-electron transfer (MS-CPET), with the phenol moiety transferring an electron to the photoexcited anthracene and a proton to the pyridine. Fluorescence quenching and transient absorption experiments in solutions and glasses show rapid MS-CPET (3.2 × 1010 s-1 at 298 K). From 5.5 to 90 K, the reaction rate and kinetic isotope effect (KIE) are independent of temperature, with zero Arrhenius activation energy. From 145 to 350 K, there are only slight changes with temperature. This MS-CPET reaction thus occurs by tunneling of both the proton and electron, in different directions. Since the reaction proceeds without significant thermal activation energy, the rate constant indicates the magnitude of the electron/proton double tunneling probability.

Proton-Coupled Electron Transfer from Water to a Model Heptazine-Based Molecular Photocatalyst

To gain mechanistic understanding of heptazine-based photochemistry, we synthesized and studied 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a model molecular photocatalyst chemically related to carbon nitride. On the basis of time-resolved photoluminescence (TR-PL) spectroscopy, we kinetically reveal a new feature that emerges in aqueous dispersions of TAHz. Using global target analysis, we spectrally and kinetically resolve the new emission feature to be blue shifted from the steady-state luminescence, and observe a fast decay component exhibiting a kinetic isotope effect (KIE) of 2.9 in H2O versus D2O, not observed in the steady-state PL. From ab initio electronic-structure calculations, we attribute this new PL peak to the fluorescence of an upper excited state of mixed nπ*/ππ* character. In water, the KIE suggests the excited state is quenched by proton-coupled electron transfer, liberating hydroxyl radicals that we detect using terephthalic acid. Our findings are consistent with recent theoretical predictions that heptazine-based photocatalysts can participate in proton-coupled electron transfer with H2O.

Strategies for Kinetic Control in Organic Solar Cells

Designing organic solar cell materials based on energy level matching sacrifices photovoltage for photocurrent. We demonstrate an alternative strategy of controlling kinetics. We use spectroscopy and x-ray scattering to relate efficiency improvements to materials properties.