Geminate Pair Dissociation Research Articles

Coherent and Incoherent Contributions to Charge Separation in Multichromophore Systems

In organic materials, exciton dissociation into free charges requires overcoming an electron–hole Coulomb interaction that exceeds the thermal energy and may still be large after charge transfer at a donor/acceptor interface. We analyze the factors affecting efficiency of charge separation and subsequent removal of electrons and holes from such a donor–acceptor interface and suggest strategies for optimizing these processes. Energy transfer, charge separation, and charge transfer in the vicinity of the donor–acceptor interface are studied within a common theoretical framework based on a quantum master equation, for a model system with realistic excitation energies and electronic couplings. We find that enhancing the efficiency of both charge transfer from the donor to the acceptor and of charge removal from the donor–acceptor interface requires an intricate balance between the extent of electronic delocalization throughout the material and rates of energy dissipation. For very large exciton binding energies, cascade charge separation in systems with more than one donor and one acceptor species, such as molecular polyads, is found to greatly facilitate the dissociation of geminate pairs. Our calculations predict charge separation on subpicosecond timescales for several parameter combinations, leading to design principles for enhancing charge separation in multichromophore systems.

Modeling geminate pair dissociation in organic solar cells: high power conversion efficiencies achieved with moderate optical bandgaps

We propose a model for geminate electron–hole dissociation in organic photovoltaic (OPV) cells and show how power conversion efficiencies greater than those currently achieved might be realized via design strategies employing moderate optical bandgaps and enhanced charge delocalization near the donor–acceptor interface. Applying this model to describing geminate electron–hole dissociation via charge transfer (CT) states, we find good agreement with recently published high-efficiency experimental data. The optimal bandgap for current-generation organic active layer materials is argued to be ∼1.7 eV – significantly greater than in previous analyses, including the Shockley–Queisser approach based upon non-excitonic solar cell dynamics. For future higher efficiency OPVs, the present results show that the optimal bandgap should be slightly lower, ∼1.6 eV. Finally, these results support design strategies aimed at enhancing mobility near the donor–acceptor interface and reducing the electron–hole binding energy, rather than striving to further reduce the bandgap.

Electronic Structure and Geminate Pair Energetics at Organic–Organic Interfaces: The Case of Pentacene/C60 Heterojunctions

AbstractOrganic semiconductors are characterized by localized states whose energies are predominantly determined by electrostatic interactions with their immediate molecular environment. As a result, the details of the energy landscape at heterojunctions between different organic semiconductors cannot simply be deduced from those of the individual semiconductors, and they have so far remained largely unexplored. Here, microelectrostatic computations are performed to clarify the nature of the electronic structure and geminate pair energetics at the pentacene/C60 interface, as archetype for an interface between a donor molecule and a fullerene electron acceptor. The size and orientation of the molecular quadrupole moments, determined by material choice, crystal orientation, and thermodynamic growth parameters of the semiconductors, dominate the interface energetics. Not only do quadrupoles produce direct electrostatic interactions with charge carriers, but, in addition, the discontinuity of the quadrupole field at the interface induces permanent interface dipoles. That discontinuity is particularly striking for an interface with C60 molecules, which by virtue of their symmetry possess no quadrupole. Consequently, at a pentacene/C60 interface, both the vacuum‐level shift and geminate pair dissociation critically depend on the orientation of the pentacene π‐system relative to the adjacent C60.

Monte Carlo modeling of geminate recombination in polymer-polymer photovoltaic devices

A Monte Carlo model is used to examine geminate pair dissociation in polymer-polymer photovoltaic devices. It is found that increasing one or both carrier mobilities aids geminate separation yield eta(GS) particularly at low fields. This, in turn, leads to improved maximum power output from polymer-polymer blend photovoltaics, even when carrier mobilities are unbalanced by a factor of 10. The dynamic behaviors of geminate charges that eventually separate and recombine are examined for the first time. It is shown that geminate pairs in a bilayer become effectively free when separated by approximately 4 nm, which is far smaller than the thermal capture radius of 16 nm here. This may lead one to expect that eta(GS) would not be limited by the separation allowed by the morphology once the domain size has increased above 4 nm. In fact it is found that eta(GS) in a blend improves continuously as the average domain size increases from 4 to 16 nm. We show that although a small degree of separation may be available in a blend, the limited number of possible routes to further separation makes charge pairs in blends more susceptible to recombination than charge pairs in a bilayer.

Field and temperature dependence of the photocurrent in polymer/fullerene bulk heterojunction solar cells

The photocurrent in polymer/fullerene blends is characterized as a function of bias at temperatures ranging from 125to300K. Assuming a constant generation rate and bimolecular recombination, the results are numerically modeled within the drift-diffusion approximation. Bimolecular recombination is found to be a dominant factor in the field dependence of the photocurrent in the entire measured voltage range. Inclusion of field dependent geminate pair dissociation and recombination via the Onsager expressions gives a much stronger field dependence than experimentally observed. From the temperature dependence of the extracted mobilities, we can simultaneously estimate the broadening of the transporting highest occupied and lowest unoccupied molecular orbital levels.

Thermally stimulated luminescence versus thermally stimulated current in organic semiconductors

A unified model of hopping thermally stimulated luminescence (TSL) and thermally stimulated current (TSC) in disordered organic semiconductors and polymers is suggested. The differences between photoinduced TSL and TSC, measured on the same sample, are explained by the fact that metastable geminate pairs of charges rather than free carriers are primarily photogenerated in these materials. Since the Coulomb interaction does not prevent carriers from recombining into excitons, the rate-limiting step in the TSL kinetics is carrier release from traps. However, the Coulomb binding energy strongly affects the dissociation of geminate pairs into free charge carriers and, therefore, it changes the apparent activation energy of TSC.

Exciton dissociation in conjugated polymers

AbstractIn conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field‐assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field‐independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.

Transient photoconductivity in a thin film of a poly-phenylenevinylene-type conjugated polymer

Transient photocurrents in a 150 nm thick film of a copolymer of a phenyl-substituted poly-phenylenevinylene were observed upon excitation by a 10 ns laser flash. The signal is due to a superposition of time-dependent generation of mobile holes and their motion. The essential quantity considered was the number of holes transported in either the neat sample or a film doped with trinitrofluorene as a function of electric field, temperature, photon dose at different photon energies, and polarity of the irradiated electrode. Based upon complementary spectroscopic evidence it has been argued that the rate limiting step for charge generation is the dissociation of geminate pairs forming an electron at an either unintentional or intentional electron acceptor and hole at an adjacent chain. The field and temperature dependence can be fitted by a recent theory by Arkhipov et al. that differs from the conventional Onsager description.

Charge carrier photogeneration yield in amorphous materials with long-range potential fluctuations

Random intrinsic electrostatic potentials cause local intrinsic electric fields that must affect the Onsager probability of geminate pair dissociation. If the intrinsic fluctuating field is strong enough the dissociation yield could be close to unity even at weak external electric fields and at low temperatures. An analytic model describing the effect of intrinsic field and potential fluctuations on the carrier photogeneration yield is formulated and compared to observed behavior in a-Si:H and a-Se.

Porous Silicon in Solvents: Constant-Lifetime PL Quenching and Confirmation of Dielectric Effects

The time-resolved photoluminescence (PL) of luminescent porous silicon has been investigated in-situ when porous silicon is immersed in chemically inert organic solvents. PL transients are recorded upon square-wave modulation of the excitation light. They are fairly well fitted with exponential curves. The obtained lifetimes lie in the 1–100 μs range, and decrease with increasing emission energy. At a given energy, the lifetimes exhibit a weak decrease when porous silicon is immersed in a solvent, whilst the PL intensity decreases by more than three orders of magnitude when the dielectric constant of the solvent increases from 2 to 20. The weak variation of the lifetime indicates that the PL quenching necessarily involves a decrease of the radiative recombination probability. This is accounted for in a model where the solvent-induced dielectric screening favours geminate pair dissociation.

Electron-Transfer Process in Layered Photoreceptors Containing Azo Compounds

Abstract Photoinduced electron transfer (ET) is the most essential process for carrier photogeneration in organic optoelectronic devices. In azo compound-based layered photoreceptors, carrier photogeneration is sensitized by hole transport material (HTM) incorporation. We investigated this process to elucidate the highly sensitizing mechanism. First, the photoinduced ET efficiency and overall quantum efficiency were measured and compared for the layered photoreceptor and the carrier generation layer. The result that the HTM enhances the photoinduced ET implies that the HTM works catalytically to diminish the activation energy. Although such extrinsic ET takes place independently of an electric field, the subsequent geminate pair dissociation depends on the electric field. Next, the energy-gap dependence of the ET was investigated by employing more than 50 photoreceptors to vary the energy gap over a wide range. The measured efficiency was plotted against the energy gap, in which an inverted region was not observed.

Temperature dependence of charge carrier creation in poly(p-phenylene vinylene) [PPV

The photocurrent decays measured in poly(p-phenylene vinylene) [PPV] from room temperature (RT) down to 77 K in the presence of air traces have been studied. We have observed long-lived decays, between RT and ~200 K, which can be fitted to the well known Kohlrausch's law: ip(t) = ip(0) exp-(t/τ)β characteristic of dispersive transport. A detailed temperature dependence study has been carried out to investigate the behavior of the exponent β and the relaxation time τ in order to understand the origin of the observed complex transport mechanism. At lower temperature, especially below 160 K, a fast component is detected alone, where the slow one is frozen. The associated interpretation is based on a recent work of simulated yield of geminate pair dissociation in an energetically random disorder. Modulated experiments show that both components are superimposed at enough high temperature. In our model, the long-lived contribution is interpreted as being extrinsic due to dissociation of polaron pairs assisted by electronegative defects and the fast one as being intrinsic due to dissociation of excitons assisted by the intrinsic energetic disorder, created by the distribution of conjugation length in PPV.

Charge injection into light-emitting diodes: Theory and experiment

An analytic theory is presented for dark injection from a metallic electrode into a random hopping system, e.g., a conjugated polymer or a molecularly doped polymer. It encompasses injection of a charge carrier from the Fermi level of the electrode into tail states of the distribution of hopping states of the dielectric followed by either return of the charge carrier to the electrode or diffusive escape from the attractive image potential. The latter process resembles Onsager-type geminate pair dissociation in one dimension. The theory yields the injection current as a function of electric field, temperature and energetic width of the distribution of hopping states. At high electric fields it resembles that the current calculated from Fowler-Nordheim tunneling theory although tunneling transitions are not included in the theory. Good agreement with experimental data obtained for diode structures with conjugated polymers as a dielectric is found.

Optical charge carrier generation in a conjugated polymer

Stationary photoconduction was measured on 100 nm thick films of poly(methylimino-1,4-phenylene-1-phenyl-1,2-ethenylene-2,5-dimethoxy-1,4-phenylene-2-phenyl-1,2-ethenylene-1,4-phenylene) sandwiched between indium tin oxide and aluminium electrodes. Bulk photoionization was observed upon irradiating through the positively biased aluminium electrode. For photon energies 2.6 eV < hv < 3.2 eV either defect catalyzed or intrinsic dissociation of relaxed singlet excitations prevails. For hv > 3.2 eV the carrier yield increases with excess photon energy due to intrinsic photoionization. The functional dependencies on electric field and photon energy are in accord with the 3D version of Onsager's theory of geminate pair dissociation implying a geminate pair distance that increases with hv. As an ionizing event, coupling between the non-relaxed initial Franck-Condon state with an mAg state is involved. Further increase of the yield correlates with the onset of optical absorption to the third optically allowed Bu state.

Optical charge carrier generation in a conjugated polymer

Stationary photoconduction was measured on 100 nm thick films of poly(methylimino-1,4-phenylene-1-phenyl-1,2-ethenylene-2,5-dimethoxy-1,4-phenylene-2-phenyl-1,2-ethenylene-1,4-phenylene) sandwiched between indium tin oxide and aluminium electrodes. Bulk photoionization was observed upon irradiating through the positively biased aluminium electrode. For photon energies 2.6 eV < hv < 3.2 eV either defect catalyzed or intrinsic dissociation of relaxed singlet excitations prevails. For hv > 3.2 eV the carrier yield increases with excess photon energy due to intrinsic photoionization. The functional dependencies on electric field and photon energy are in accord with the 3D version of Onsager's theory of geminate pair dissociation implying a geminate pair distance that increases with hv. As an ionizing event, coupling between the non-relaxed initial Franck-Condon state with an mAg state is involved. Further increase of the yield correlates with the onset of optical absorption to the third optically allowed Bu state.

Photoconduction in thin films of a ladder-type poly-para-phenylene

Stationary photoconductivity has been measured on typically 100 nm thick films of a methyl-substituted ladder-type poly-para-phenylene sandwiched between indium–tin oxide (ITO) and aluminium (Al) electrodes. Three contributions to photocarrier production have been identified: (1) hole injection from the ITO and the Al-anode via dissociation of singlet excitons; (2) intrinsic carrier production via relaxed S 1 excitons; and (3) intrinsic photogeneration via non-relaxed electronic states excited by photons with an energy of 1.1 eV above the S 1←S 0 0–0 absorption band. The intrinsic photoionization yield increases strongly with electric field but is virtually independent of temperature, at variance with the prediction of the conventional Onsager theory of geminate pair dissociation.

Intrinsic and extrinsic photocarriers in polyparaphenylenevinylene

We report on photoconductivity in polyparaphenylenevinylene films under continuous wave illumination in the presence of air traces. Near room temperature the photocurrent buildup is extremely slow, steady state not being reached after 1 h illumination. When light is turned off the photocurrent decay kinetics is slower and slower and it takes about 24 h to reach thermal equilibrium. The instantaneous lifetime follows a power-law time dependence ${\ensuremath{\tau}}_{\mathrm{inst}}{(t)=i}_{p}(t)/|{\mathrm{di}}_{p}(t)/dt|\ensuremath{\propto}{t}^{\ensuremath{\alpha}},$ $0\ensuremath{\prec}\ensuremath{\alpha}\ensuremath{\prec}1$ and the decay kinetics can be accurately fitted to a stretched exponential relaxation law ${i}_{p}{(t)=i}_{p}(0)\mathrm{exp}\ensuremath{-}(t/\ensuremath{\tau}{)}^{\ensuremath{\beta}}$ where $\ensuremath{\beta}=1\ensuremath{-}\ensuremath{\alpha}.$ When temperature is decreased down to $\ensuremath{\approx}255\mathrm{K}$ the exponent \ensuremath{\beta} and the relaxation time \ensuremath{\tau} are found to be temperature dependent, \ensuremath{\beta} being linearly dependent upon $T:$ $\ensuremath{\beta}{=T/T}_{0}$ with ${T}_{0}=1175\mathrm{K}.$ The nonlinear decrease of \ensuremath{\beta} between 255 and 190 K is interpreted as a transport mode transition at ${T}_{t}\ensuremath{\approx}225\mathrm{K}.$ When temperature is further decreased in the range 160\char21{}77 K the slow component is frozen and leaves room to a fast signal that rapidly reaches steady state and is slightly dependent upon temperature. The magnitude of the slow photocurrent is proportional to the square root of the light intensity whereas that of the fast one is proportional to the light intensity. Modulated photocurrent studies allowed us to show that the fast signal exists in the whole temperature range. Examination of the temperature dependence of the fast photocurrent at low temperature led us to interpret the fast signal as being an intrinsic photocurrent due exciton dissociation. Such an interpretation rests on the recent work of Albrecht and B\assler on the yield of geminate pair dissociation in an energetically random hopping system with built-in energetic disorder. The slow component is interpreted as being extrinsic due to dissociation of polaron pairs through interaction with oxygenated defects to create positive polarons. Recombination and the functional form of the extrinsic component decay is attributed to the dispersive diffusion of the positive polarons in a random distribution of negatively charged defects.

Intrinsic Photoconduction in PPV-Type Conjugated Polymers

Steady state photoconduction experiments have been performed on thin films of derivatives of polyphenylenevinylene contacted in sandwich configuration. The use of a silicon monoxide blocking layer allows the separation of intrinsic from extrinsic photocurrents. Intrinsic photogeneration increases with electric field, temperature, and photon energy in agreement with the 3D version of Onsager's theory of geminate pair dissociation adapted to energetically random hopping systems. The thermalization distance, which determines the escape rate, increases with the effective conjugation length of the chain segments. The average energy required to dissociate a confined on-chain excitation is 0.4 eV.

Extrinsic and intrinsic dc photoconductivity in a conjugated polymer

Steady-state photoconduction was studied in typically 100-nm-thick films of a polyphenylenevinylene derivative with ether linkages in the backbone. Samples were prepared in sandwich configuration between indium tin oxide (ITO) and Al electrodes. Forward photocurrents measured upon irradiating through the positively biased ITO electrode decreased by typically three orders of magnitude when a 8--10-nm-thick silicon monoxide (SiO) layer had been deposited on top of the ITO to prevent photoinjection of positive charge carriers. Insertion of a SiO layer can thus be used as a tool to separate extrinsic from intrinsic sources of optical charge-carrier generation. Analysis of the dependences of extrinsic and intrinsic photocurrents on temperature, electric field, and photon energy indicate that they can be described in terms of Onsager's theory of geminate pair dissociation in either one or three dimensions.

Geminate pair dissociation in random organic systems

The temperature dependences of (i) intrinsic photogeneration, (ii) photoinjection of holes from an indium tin oxide (ITO) electrode via excited state dissociation at the interface and (iii) dark injection from an ITO anode have been studied for a variety of random organic systems. They approach activated behavior in the high-temperature limit while at lower temperature, typically T < 200 K, the yield becomes temperature-independent and, concomitantly, exceeds the yield expected for energetically ordered systems. The effect is in accord with recent simulation work on geminate pair dissociation in energetically disordered hopping systems.