Ideal diode equation for organic heterojunctions. II. The role of polaron pair recombination

Giebink, Noel C.; Lassiter, Brian E.; Wiederrecht, Gary P.; Wasielewski, Michael R.; Forrest, Stephen R.

doi:10.1103/physrevb.82.155306

Cited by 78 publications

(97 citation statements)

References 27 publications

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“…Insight into polaron pair and charge recombination dynamics at the donor-C 70 interface in a solar cell is provided by the analysis of Giebink, et al 23,24 The active layer morphology, energy level offsets between D and A, and charge transfer at heterojunctions play important roles in determining the short-circuit current (J SC ), open-circuit voltage (V OC ), and the fill factor (FF) of OPV cells. [23][24][25][26][27] In steady-state, the J-V characteristics are governed primarily by polaron-pair and free charge recombination dynamics at the D-A junction.…”

Section: Theorymentioning

confidence: 99%

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Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

et al. 2015

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We investigate dipolar donor materials mixed with a C 70 acceptor in an organic photovoltaic (OPV) cell. Dipolar donors that have donor-acceptor-acceptor (d-a-a') structure result in high conductivity pathways due to close coupling between neighboring molecules in the mixed films.We analyze the charge transfer properties of the dipolar donor:C 70 mixtures and corresponding neat donors using a combination of time-resolved electroluminescence from intermolecular polaron pair states and conductive tip atomic force microscopy, from which we infer that dimers of the d-a-a' donors tend to form a continuous network of nanocrystalline clusters within the blends. Additional insights are provided by quantum mechanical calculations of hole transfer coupling and hopping rates between donor molecules using nearest neighbor donor packing motifs taken from crystal structural data. The approximation using only nearest neighbor interactions leads to good agreement between donor hole hopping rates and the conductive properties of the donor:C 70 blends. This represents a significant simplification from requiring details of the nano-and mesoscale morphologies of thin films to estimate their electronic 2 characteristics. Using these dipolar donors, we obtain a maximum power conversion efficiency of 9.6 ± 0.5 % under 1 sun, AM1.5G simulated illumination for an OPV comprised of an active layer containing a dipolar donor mixed with C 70 .3

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Section: Theorymentioning

confidence: 99%

“…[23][24][25][26][27] In steady-state, the J-V characteristics are governed primarily by polaron-pair and free charge recombination dynamics at the D-A junction. 23 Thus:…”

Section: Theorymentioning

confidence: 99%

Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

et al. 2015

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“…The diode equation also gives a good empirical fit to experiment in many cases. However, it has been criticized on the grounds that the physical quantities used to derive the Shockley model are not the most appropriate ones to describe an organic solar cell and that some phenomena require a different derivation, which is more closely tied to the fundamental physics of organic solar cells [5,6], and that the Shockley model cannot describe certain phenomena, such as the S-shaped current-voltage curves sometimes observed in organic solar cells [5,6,108,109]. Either in the context of the Shockley model or, better yet, in terms of some improved model, it is tempting to try to calculate first principles parameters for use in the diode equation for comparison against experiment and, in particular, to seek a better understanding of how charge separation occurs near the donor-acceptor interface, both in the dark and when illuminated.…”

Section: Discussionmentioning

confidence: 99%

“…In contrast to the latter, which is well enough understood [1], that fabless manufacturing may be used for circuit design using modeling software, such as SPICE [2], and then be outsourced to a semiconductor foundry for actual fabrication, organic electronics is a rapidly growing, but much less well understood [3][4][5][6][7][8] technology. One example is organic solar cells [9][10][11][12], which offer the advantage of being relatively inexpensive to manufacture, flexible and printable.…”

Section: Introductionmentioning

confidence: 99%

Assessment of Density-Functional Tight-Binding Ionization Potentials and Electron Affinities of Molecules of Interest for Organic Solar Cells Against First-Principles GW Calculations

et al. 2015

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Ionization potentials (IPs) and electron affinities (EAs) are important quantities input into most models for calculating the open-circuit voltage (V oc ) of organic solar cells. We assess the semi-empirical density-functional tight-binding (DFTB) method with the third-order self-consistent charge (SCC) correction and the 3ob parameter set (the third-order DFTB (DFTB3) organic and biochemistry parameter set) against experiments (for smaller molecules) and against first-principles GW (Green's function, G, times the Computation 2015, 3 617 screened potential, W) calculations (for larger molecules of interest in organic electronics) for the calculation of IPs and EAs. Since GW calculations are relatively new for molecules of this size, we have also taken care to validate these calculations against experiments. As expected, DFTB is found to behave very much like density-functional theory (DFT), but with some loss of accuracy in predicting IPs and EAs. For small molecules, the best results were found with ∆SCF (∆ self-consistent field) SCC-DFTB calculations for first IPs (good to ± 0.649 eV). When considering several IPs of the same molecule, it is convenient to use the negative of the orbital energies (which we refer to as Koopmans' theorem (KT) IPs) as an indication of trends. Linear regression analysis shows that KT SCC-DFTB IPs are nearly as accurate as ∆SCF SCC-DFTB eigenvalues (± 0.852 eV for first IPs, but ± 0.706 eV for all of the IPs considered here) for small molecules. For larger molecules, SCC-DFTB was also the ideal choice with IP/EA errors of ± 0.489/0.740 eV from ∆SCF calculations and of ± 0.326/0.458 eV from (KT) orbital energies. Interestingly, the linear least squares fit for the KT IPs of the larger molecules also proves to have good predictive value for the lower energy KT IPs of smaller molecules, with significant deviations appearing only for IPs of 15-20 eV or larger. We believe that this quantitative analysis of errors in SCC-DFTB IPs and EAs may be of interest to other researchers interested in DFTB investigation of large and complex problems, such as those encountered in organic electronics.

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“…However, the carrier dynamics in the OPV cells are still not fully understood because of the complexity of the interactions between each organic layer. By using intensity-modulated photocurrent spectroscopy (IMPS), Forrest et al reported that the donoracceptor interface interactions in p-n heterostructured OPV cells was governed by the separation of polaron pairs and the carrier recombination rate [6]. Iwamoto et al have analyzed the carrier accumulation behavior using a combination of electric-field-induced second harmonic generation and impedance spectroscopy (IS), which indicated that the Maxwell Wagner effect was present at the interface between pentacene and C 60 [7].…”

Section: Introductionmentioning

confidence: 99%

Carrier Dynamics of p-n Heterojunction Organic Photovoltaic Cells Analyzed by a Novel Graphic Representation of Impedance Spectroscopy

Maeda¹,

Tokairin²,

Ikeda³

et al. 2015

AMPC

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The carrier dynamics in organic photovoltaic (OPV) cells were investigated by impedance spectroscopy. We introduced a novel impedance spectrum representation called dynamic modulus plot (DMP), which allowed us to observe the layer-to-layer carrier injection behavior graphically. In this work, the impedance responses were characterized in the N,N'-diphenyl-N,N'-di-m-tolyl-4,4'-diaminobiphenyl (TPD)/C 60 p-n heterostructured OPV cells against applied voltages. The dependence of impedance responses on the layer thickness revealed a constant internal electric field that disturbed the carrier transport within the OPV cells. We applied this technique to new donor materials, in which thiophene units were inserted to the center of TPD. By increasing the number of thiophene units in TPD the fill-factor (FF) improved from 33% to 59%, which increased the power conversion efficiency (PCE). Based on the DMP analysis, we assigned the improvement in device performance to the reduction of the internal electric field.

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Ideal diode equation for organic heterojunctions. II. The role of polaron pair recombination

Cited by 78 publications

References 27 publications

Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

Charge transport and exciton dissociation in organic solar cells consisting of dipolar donors mixed withC70

Assessment of Density-Functional Tight-Binding Ionization Potentials and Electron Affinities of Molecules of Interest for Organic Solar Cells Against First-Principles GW Calculations

Carrier Dynamics of p-n Heterojunction Organic Photovoltaic Cells Analyzed by a Novel Graphic Representation of Impedance Spectroscopy

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