Re‐evaluating the Role of Sterics and Electronic Coupling in Determining the Open‐Circuit Voltage of Organic Solar Cells

Graham, Kenneth R.; Erwin, Patrick; Nordlund, Dennis; Vandewal, Koen; Li, Ruipeng; Ndjawa, Guy Olivier Ngongang; Hoke, Eric T.; Salleo, Alberto; Thompson, Mark E.; McGehee, Michael D.; Amassian, Aram

doi:10.1002/adma.201301319

Cited by 95 publications

(165 citation statements)

References 44 publications

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“…In a BHJ solar cell the effective bandgap at the donor-acceptor interface is termed the charge transfer state energy. 21 In some metal-free organic dyes used in DSC the extent of internal recombination becomes a concern, 22 and much more so when the absorber is an inorganic QD, which displays complex internal electronic dynamics, often governed by recombination at surface states. Effective passivation of the QD absorber greatly improves the internal quantum efficiency (IQE) in QD sensitized solar 5 cells.…”

Section: Charge Separationmentioning

confidence: 99%

Classification of solar cells according to mechanisms of charge separation and charge collection

Kirchartz

Bisquert

Mora‐Seró

et al. 2015

Phys. Chem. Chem. Phys.

116

108

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In the last decade, photovoltaics (PV) has experienced an important transformation. Traditional solar cells formed by compact semiconductor layers have been joined by new kinds of cells that are constituted by a complex mixture of organic, inorganic and solid or liquid electrolyte materials, and rely on charge separation at the nanoscale. Recently, metal organic halide perovskites have appeared in the photovoltaic landscape showing large conversion efficiencies, and they may share characteristics of the two former types. In this paper we provide a general description of the photovoltaic mechanisms of the single absorber solar cell types, combining all-inorganic and hybrid and organic cells into a single framework. The operation of the solar cell relies on a number of internal processes that exploit internal charge separation and overall charge collection minimizing recombination. There are two main effects to achieve the required efficiency, first to exploit kinetics at interfaces, favouring the required forward process, and second to take advantage of internal electrical fields caused by a built-in voltage and by the distribution of photogenerated charges. These principles represented by selective contacts, interfaces and the main energy diagram, form a solid base for the discussion of the operation of future types of solar cells. Additional effects based on ferroelectric polarization and ionic drift provide interesting prospects for investigating new PV effects mainly in the perovskite materials.

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Section: Charge Separationmentioning

confidence: 99%

Classification of solar cells according to mechanisms of charge separation and charge collection

Kirchartz

Bisquert

Mora‐Seró

et al. 2015

Phys. Chem. Chem. Phys.

116

108

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“…Our recent studies on pentacene/C 60 complexes 12,13 show that the CT energies calculated with default-ω LRC functionals could be as large as 3 eV (i.e., about twice the experimental values 14 ) and strongly depend on the choice of the functional. Upon tuning the range-separation parameter, all LRC functionals give comparable results for both local and CT excited states.…”

mentioning

confidence: 99%

Description of the Charge Transfer States at the Pentacene/C₆₀ Interface: Combining Range-Separated Hybrid Functionals with the Polarizable Continuum Model

Zheng

Brédas

Coropceanu

2016

J. Phys. Chem. Lett.

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Density functional theory (DFT) approaches based on range-separated hybrid functionals are currently methods of choice for the description of the charge-transfer (CT) states in organic donor/acceptor solar cells. However, these calculations are usually performed on small-size donor/acceptor complexes and as result do not account for electronic polarization effects. Here, using a pentacene/C60 complex as a model system, we discuss the ability of long-range corrected (LCR) hybrid functionals in combination with the polarizable continuum model (PCM) to determine the impact of the solid-state environment on the CT states. The CT energies are found to be insensitive to the interactions with the dielectric medium when a conventional time-dependent DFT/PCM (TDDFT/PCM) approach is used. However, a decrease in the energy of the CT state in the framework of LRC functionals can be obtained by using a smaller range-separated parameter when going from an isolated donor/acceptor complex to the solid-state case.

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“…[35] However, as demonstrated in these publications, care has to be taken to disentangle the effect of orientation dependent ionization energies and electron affinities on the one hand, and the actual differences in electronic couplings on the other hand. [17] Moreover, the importance of long-range electrostatic interactions should not be underestimated as well. [36] As already mentioned, D-A interfaces can be more complex than discussed above.…”

Section: Reducing the Ct Strengthmentioning

confidence: 99%

“…[7,15] Though the microscopic origin for the low EL quantum efficiencies has not been clarified yet, empirically, a linear relationship q · V OC = E CT − 0.6 eV was obtained over a wide range of energies covered by both polymeric as well as molecular materials. [14,16,17] Compared to the bandgapvoltage offset in Si or GaAs solar cells (0.3-0.4 eV) the recombination loss between V OC and E CT in OPVs is significantly higher. Moreover, one has to be aware that in many cases an additional loss comes from the fact that the driving force ΔE CT is non-negligible.…”

Section: Introductionmentioning

confidence: 99%

Energy Losses in Small‐Molecule Organic Photovoltaics

Linderl

Zechel

Brendel

et al. 2017

Advanced Energy Materials

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processible PV technology has recently appeared as well. [5] Comparing the different technologies in terms of PCE, which is specified as the product of the short-circuit current density j SC , the open-circuit voltage V OC and the fill factor FF divided by the incoming light intensity under standard AM 1.5G illumination conditions, OPVs can well compete with their inorganic counterparts in terms of j SC or, more precisely, the external quantum efficiency and also with minor trade-off in FF, but clearly suffer from lower V OC at a given energy gap E g of the light absorbing material. While this socalled bandgap-voltage offset can be as low as 0.3-0.4 eV in Si and GaAs [6] and only a little larger in perovskite cells, [7] OPV cells exhibit energy losses of at least 0.6 eV -in many cases, however, this offset can approach and even exceed 1 eV. [8] This is currently one of the main bottlenecks toward making OPVs competitive with inorganic PV cells.In this research news, we provide the required background information on the appearance of energy losses in OPV cells, by which we mean the difference between the equivalent of the optical gap and the measured open-circuit voltage that is frequently also denoted as voltage loss, and discuss recent progress toward better understanding their origin and strategies to reduce them. To keep focused, we will mainly address small molecules as active organic semiconductors, which are being processed into thin films by vacuum deposition techniques. Compared to frequently studied π-conjugated polymers, the synthesis of small molecules is more reproducible. Moreover, a rigorous purification of small molecules is easier, which gives the opportunity to reproducibly investigate well-defined systems. The application of vacuum deposition techniques prevents the use of solvents, which as a third component in wet chemical processing can strongly influence the morphology. [9] Thus, active layers of small molecules prepared by vacuum deposition methods mark a well-controlled model system for fundamental studies such as the origin of energy losses in OPV devices. However, we expect that most of the findings can be transferred to solution-processed OPVs as well, which have considerably higher complexity in terms of local morphology and phase behavior. Excitonic Organic Solar CellsIn order to properly address energy losses in OPVs it is useful to look at their working principles in more detail (see also [10] ).

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Re‐evaluating the Role of Sterics and Electronic Coupling in Determining the Open‐Circuit Voltage of Organic Solar Cells

Cited by 95 publications

References 44 publications

Classification of solar cells according to mechanisms of charge separation and charge collection

Classification of solar cells according to mechanisms of charge separation and charge collection

Description of the Charge Transfer States at the Pentacene/C₆₀ Interface: Combining Range-Separated Hybrid Functionals with the Polarizable Continuum Model

Energy Losses in Small‐Molecule Organic Photovoltaics

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Re‐evaluating the Role of Sterics and Electronic Coupling in Determining the Open‐Circuit Voltage of Organic Solar Cells

Cited by 95 publications

References 44 publications

Classification of solar cells according to mechanisms of charge separation and charge collection

Classification of solar cells according to mechanisms of charge separation and charge collection

Description of the Charge Transfer States at the Pentacene/C60 Interface: Combining Range-Separated Hybrid Functionals with the Polarizable Continuum Model

Energy Losses in Small‐Molecule Organic Photovoltaics

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Description of the Charge Transfer States at the Pentacene/C₆₀ Interface: Combining Range-Separated Hybrid Functionals with the Polarizable Continuum Model