Energy-Gap Law for Photocurrent Generation in Fullerene-Based Organic Solar Cells: The Case of Low-Donor-Content Blends.

The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C60 and small amounts of organic donor molecules. We find a pronounced decline of all photovoltaic parameters with decreasing CT state energy. Using a combination of steady-state photocurrent measurements and time-delayed collection field experiments, we demonstrate that the power conversion efficiency, and more specifically, the fill factor of these devices, is mainly determined by the bias dependence of photocurrent generation. By combining these findings with the results from ultrafast transient absorption spectroscopy, we show that blends with small CT energies perform poorly because of an increased nonradiative CT state decay rate and that this decay obeys an energy-gap law. Our work challenges the common view that a large energy offset at the heterojunction and/or the presence of fullerene clusters guarantee efficient CT dissociation and rather indicates that charge generation benefits from high CT state energies through a slower decay to the ground state.

Intercalated vs Nonintercalated Morphologies in Donor-Acceptor Bulk Heterojunction Solar Cells: PBTTT:Fullerene Charge Generation and Recombination Revisited.

In this Letter, we study the role of the donor:acceptor interface nanostructure upon charge separation and recombination in organic photovoltaic devices and blend films, using mixtures of PBTTT and two different fullerene derivatives (PC70BM and ICTA) as models for intercalated and nonintercalated morphologies, respectively. Thermodynamic simulations show that while the completely intercalated system exhibits a large free-energy barrier for charge separation, this barrier is significantly lower in the nonintercalated system and almost vanishes when energetic disorder is included in the model. Despite these differences, both femtosecond-resolved transient absorption spectroscopy (TAS) and time-delayed collection field (TDCF) exhibit extensive first-order losses in both systems, suggesting that geminate pairs are the primary product of photoexcitation. In contrast, the system that comprises a combination of fully intercalated polymer:fullerene areas and fullerene-aggregated domains (1:4 PBTTT:PC70BM) is the only one that shows slow, second-order recombination of free charges, resulting in devices with an overall higher short-circuit current and fill factor. This study therefore provides a novel consideration of the role of the interfacial nanostructure and the nature of bound charges and their impact upon charge generation and recombination.

A thieno[3,2-b][1]benzothiophene isoindigo building block for additive- and annealing-free high-performance polymer solar cells.

A novel photoactive polymer with two different molecular weights is reported, based on a new building block: thieno[3,2-b][1]benzothiophene isoindigo. Due to the improved crystallinity, optimal blend morphology, and higher charge mobility, solar-cell devices of the high-molecular-weight polymer exhibit a superior performance, affording efficiencies of 9.1% without the need for additives, annealing, or additional extraction layers during device fabrication.

A rhodanine flanked nonfullerene acceptor for solution-processed organic photovoltaics.

A novel small molecule, FBR, bearing 3-ethylrhodanine flanking groups was synthesized as a nonfullerene electron acceptor for solution-processed bulk heterojunction organic photovoltaics (OPV). A straightforward synthesis route was employed, offering the potential for large scale preparation of this material. Inverted OPV devices employing poly(3-hexylthiophene) (P3HT) as the donor polymer and FBR as the acceptor gave power conversion efficiencies (PCE) up to 4.1%. Transient and steady state optical spectroscopies indicated efficient, ultrafast charge generation and efficient photocurrent generation from both donor and acceptor. Ultrafast transient absorption spectroscopy was used to investigate polaron generation efficiency as well as recombination dynamics. It was determined that the P3HT:FBR blend is highly intermixed, leading to increased charge generation relative to comparative devices with P3HT:PC60BM, but also faster recombination due to a nonideal morphology in which, in contrast to P3HT:PC60BM devices, the acceptor does not aggregate enough to create appropriate percolation pathways that prevent fast nongeminate recombination. Despite this nonoptimal morphology the P3HT:FBR devices exhibit better performance than P3HT:PC60BM devices, used as control, demonstrating that this acceptor shows great promise for further optimization.

Excited-state dynamics of nitrated push-pull molecules: the importance of the relative energy of the singlet and triplet manifolds.

We present a study of the dynamics following photoexcitation in the first electronic band of NO(2)-para-substituted nitronaphthalenes. Our main goal was to determine the interplay between the nitro group, electron-donating substituents, and the solvent in defining the relative excited-state energies and their photoinduced pathways. We studied 4-nitro-1-naphthylamine and 1-methoxy-4-nitronaphthalene in solution samples through femtosecond fluorescence up-conversion and transient absorption techniques. In all solvents, both compounds have ultrafast fluorescence decays, showing that, similarly to the parent compound 1-nitronaphthalene, these molecules have highly efficient S(1) decay channels. The evolution of the transient absorption signals in the visible region reveals that for the methoxy-substituted compound, independently of solvent polarity, the photophysical pathways are the same as in 1-nitronaphthalene, namely, ultrafast intersystem crossing to an upper triplet state (receiver T(n) state) followed by relaxation into the lowest energy phosphorescent triplet T(1). In contrast, for the amino-substituted nitronaphthalene, the excited-state evolution shows a strong solvent dependence: In nonpolar solvents, the same type of intersystem crossing through an upper receiver triplet state dictates the photochemistry. However, in methanol, where the first singlet excited state shows an important solvent-induced stabilization, we observed typical signals of the repopulation of the electronic ground state in the time scale of less than 1 ps followed by vibrational cooling within S(0). Excited-state calculations at the time-dependent density functional level with the PBE0 functional give an approximate characterization of the states involved and appear to correlate well with the experimental results as they show that the S(1) state of the amino compound is stabilized with respect to upper triplet states only in the polar solvent. These findings sustain and illustrate the recent view that the intersystem crossing channel so prevalent in nitroaromatic compounds is related to an energy coincidence between the pi-pi* first singlet excited state and upper triplet states with n-pi* character. Our results indicate through direct observations that if the S(1) state is sufficiently stabilized, other rapid decay channels like internal conversion to the ground state will minimize the transfer of population to the triplet manifold.

Relaxation in the triplet manifold of 1-nitronaphthalene observed by transient absorption spectroscopy.

Previous phosphorescence and triplet quantum yield determinations indicate that the primary photophysical channel for 1-nitronaphthalene is the formation of its lowest energy triplet state. Also, previous direct measurements of the decay of the fluorescence from this compound indicated that the crossing between the singlet and triplet manifolds is ultrafast (sub-100 fs). In this contribution we present a sub-picosecond transient absorption study of the relaxation of photoexcited 1-nitronaphthalene in methanol and other solvents. Our measurements reveal the time scale in which the fully relaxed T(1) state is formed. We have observed that the spectral evolution associated with this process takes place in time scales from one to a few tens of picoseconds. Specifically, the appearance of the absorption spectrum of T(1) in the visible region is accompanied by the decay of transient signals at wavelengths below 400 nm. Since the fluorescence lifetime of this compound is sub-100 fs, we assigned the picoseconds decaying signals below 400 nm to an intermediate triplet state which acts as a receiver state in the intersystem crossing step and from which the T(1) population accumulates. From the details of the spectral evolution and the effects of different solvents, we also conclude that T(1) formation and vibrational cooling within this state occur in similar time scales of between 1 and 16 ps. Mainly, our results provide direct evidence in support of the participation of an upper triplet state in the mechanism for intersystem crossing in this molecule. This is considered to be common in the photophysics of several nitrated polycyclic aromatic compounds and the most determinant feature of their primary photochemistry.

Excited state intramolecular proton transfer in Schiff bases. Decay of the locally excited enol state observed by femtosecond resolved fluorescence.

Although the late (t>1 ps) photoisomerization steps in Schiff bases have been described in good detail, some aspects of the ultrafast (sub-100 fs) proton transfer process, including the possible existence of an energy barrier, still require experimental assessment. In this contribution we present femtosecond fluorescence up-conversion studies to characterize the excited state enol to cis-keto tautomerization through measurements of the transient molecular emission. Salicylideneaniline and salicylidene-1-naphthylamine were examined in acetonitrile solutions. We have resolved sub-100 fs and sub-0.5 ps emission components which are attributed to the decay of the locally excited enol form and to vibrationally excited states as they transit to the relaxed cis-keto species in the first electronically excited state. From the early spectral evolution, the lack of a deuterium isotope effect, and the kinetics measured with different amounts of excess vibrational energy, it is concluded that the intramolecular proton transfer in the S1 surface occurs as a barrierless process where the initial wave packet evolves in a repulsive potential toward the cis-keto form in a time scale of about 50 fs. The absence of an energy barrier suggests the participation of normal modes which modulate the donor to acceptor distance, thus reducing the potential energy during the intramolecular proton transfer.

Phosphate ester hydrolysis by hydroxo complexes of trivalent lanthanides stabilized by 4-imidazolecarboxylate.

The anion of 4-imidazolecarboxylic acid (HL) stabilizes hydroxo complexes of trivalent lanthanides of the type ML(OH)+ (M = La, Pr) and M2L(n)(OH)(6-n) (M = La, n = 2; M = Pr, n = 2, 3; M = Nd, Eu, Dy, n = 1-3). Compositions and stability constants of the complexes have been determined by potentiometric titrations. Spectrophotometric and (1)H NMR titrations with Nd(III) support the reaction model for the formation of hydroxo complexes proposed on the basis of potentiometric results. Kinetics of the hydrolysis of two phosphate diesters, bis(4-nitrophenyl) phosphate (BNPP) and 2-hydroxypropyl 4-nitrophenyl phosphate (HPNPP), and a triester, 4-nitrophenyl diphenyl phosphate (NPDPP), in the presence of hydroxo complexes of five lanthanides were studied as a function of pH and metal and ligand concentrations. With all lanthanides and all substrates, complexes with the smallest n, that is M2L2(OH)4 for La and Pr and M2L(OH)5 for Nd, Eu, and Dy, exhibited the highest catalytic activity. Strong inhibitory effects by simple anions (Cl-, NO3-, (EtO)2PO2-, AcO-) were observed indicating high affinity of neutral hydroxo complexes toward anionic species. The catalytic activity decreased in the order La > Pr > Nd > Eu > Dy for both diester substrates and was practically independent of the nature of cation for a triester substrate. The efficiency of catalysis, expressed as the ratio of the second-order rate constant for the ester cleavage by the hydroxo complex to the second-order rate constant for the alkaline hydrolysis of the respective substrate, varied from ca. 1 for NPDPP to 10(2) for HPNPP and to 10(5) for BNPP. The proposed mechanism of catalytic hydrolysis involves reversible bridging complexation of a phosphodiester to the binuclear active species followed by attack on the phosphoryl group by bridging hydroxide (BNPP) or by the alkoxide group of the deprotonated substrate (HPNPP).