Carl Poelking

Impact of mesoscale order on open-circuit voltage in organic solar cells

Structural order in organic solar cells is paramount: it reduces energetic disorder, boosts charge and exciton mobilities, and assists exciton splitting. Owing to spatial localization of electronic states, microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at the mesoscale. Long-range electrostatic interactions nevertheless probe this ordering, making local properties depend on the mesoscopic order. Using a technique developed to address spatially aperiodic excitations in thin films and in bulk, we show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. Optimal use of long-range ordering also rationalizes the acceptor-donor-acceptor paradigm for molecular design of donor dyes. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation.

Band structure engineering in organic semiconductors

Organic solar cells tuned by blending Electrical engineers can finetune the energetics of rigid photovoltaics and transistors by blending different semiconducting materials. However, its hard to apply this tuning protocol to the flexible class of carbon-based semiconductors. Schwarze et al. now show that continuous band energy tuning is indeed possible by varying the blend ratios of certain organic phthalocyanines and their fluorinated or chlorinated derivatives (see the Perspective by Ueno). They demonstrated the effect, which they attribute to quadrupolar interactions, in model solar cells. Science, this issue p. 1446; see also p. 1395 Quadrupolar interactions enable continuous energetic tuning of organic semiconductor blends. A key breakthrough in modern electronics was the introduction of band structure engineering, the design of almost arbitrary electronic potential structures by alloying different semiconductors to continuously tune the band gap and band-edge energies. Implementation of this approach in organic semiconductors has been hindered by strong localization of the electronic states in these materials. We show that the influence of so far largely ignored long-range Coulomb interactions provides a workaround. Photoelectron spectroscopy confirms that the ionization energies of crystalline organic semiconductors can be continuously tuned over a wide range by blending them with their halogenated derivatives. Correspondingly, the photovoltaic gap and open-circuit voltage of organic solar cells can be continuously tuned by the blending ratio of these donors.

Design Rules for Organic Donor-Acceptor Heterojunctions: Pathway for Charge Splitting and Detrapping

Organic solar cells rely on the conversion of a Frenkel exciton into free charges via a charge-transfer state formed on a molecular donor-acceptor pair. These charge-transfer states are strongly bound by Coulomb interactions and yet efficiently converted into charge-separated states. A microscopic understanding of this process, though crucial to the functionality of any solar cell, has not yet been achieved. Here we show how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority carrier trapping across a donor-acceptor interphase. Comparing a variety of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a trade-off between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices.

Electrostatic phenomena in organic semiconductors: fundamentals and implications for photovoltaics

This review summarizes the current understanding of electrostatic phenomena in ordered and disordered organic semiconductors, outlines numerical schemes developed for quantitative evaluation of electrostatic and induction contributions to ionization potentials and electron affinities of organic molecules in a solid state, and illustrates two applications of these techniques: interpretation of photoelectron spectroscopy of thin films and energetics of heterointerfaces in organic solar cells.

Morphology and Charge Transport in P3HT: A Theorist’s Perspective

Poly(3-hexylthiophene) (P3HT) is the fruit fly among polymeric organic semiconductors. It has complex self-assembling and electronic properties and yet lacks the synthetic challenges that characterize advanced donor–acceptor-type polymers. P3HT can be used both in solar cells and in field-effect transistors. Its morphological, conductive, and optical properties have been characterized in detail using virtually any and every experimental technique available, whereas the contributions of theory and simulation to a rationalization of these properties have so far been modest. The purpose of this review is to take a snapshot of these results and, more importantly, outline directions that still require substantial method development.

Long-Range Embedding of Molecular Ions and Excitations in a Polarizable Molecular Environment

We present a method for evaluating electrostatic and polarization energies of a localized charge, charge transfer state, or exciton embedded in a neutral molecular environment. The approach extends the Ewald summation technique to polarization effects, rigorously accounts for the long-range nature of the charge-quadrupole interactions, and addresses aperiodic embedding of the charged molecular cluster and its polarization cloud in a periodic environment. We illustrate the method by evaluating the density of states and ionization energies in thin films and heterostructures of organic semiconductors. By accounting for long-range mesoscale fields, we obtain the ionization energies in both crystalline and mesoscopically amorphous systems with high accuracy.

Effect of Mesoscale Ordering on the Density of States of Polymeric Semiconductors

A multiscale simulation scheme, which incorporates both long-range conformational disorder and local molecular ordering, is proposed for predicting large-scale morphologies and charge transport properties of polymeric semiconductors. Using poly(3-hexylthiophene) as an example, it is illustrated how the energy landscape and its spatial correlations evolve with increasing degree of structural order in mesophases with amorphous, uniaxial, and biaxial nematic ordering. It is shown that the formation of low-lying energy states in more ordered systems is mostly due to larger (on average) conjugation lengths and not due to electrostatic interactions. The proposed scheme is general and can be applied to a wide range of polymeric organic materials.

Design rules for organic D-A heterojunctions: pathway for charge splitting(Conference Presentation)

Organic solar cells rely on the conversion of a Frenkel exciton into free charges via a charge transfer state formed on a molecular donor-acceptor pair. These charge transfer states are strongly bound by Coulomb interactions, and yet efficiently converted into charge-separated states. A microscopic understanding of this process, though crucial to the functionality of any solar cell, has not yet been achieved. Here we show how long-range molecular order and interfacial mixing generate homogeneous electrostatic forces that can drive charge separation and prevent minority-carrier trapping across a donor-acceptor interphase. Comparing a variety of small-molecule donor-fullerene combinations, we illustrate how tuning of molecular orientation and interfacial mixing leads to a tradeoff between photovoltaic gap and charge-splitting and detrapping forces, with consequences for the design of efficient photovoltaic devices.