A crucial issue facing polymer-based solar cells is how to manage the energetics of the polymer/fullerene blends to maximize short-circuit current density and open-circuit voltage at the same time and thus the power conversion efficiency. Here we demonstrate that the use of a naphthobisoxadiazole-based polymer with a narrow bandgap of 1.52 eV leads to high open-circuit voltages of approximately 1 V and high-power conversion efficiencies of ∼9% in solar cells, resulting in photon energy loss as small as ∼0.5 eV, which is much smaller than that of typical polymer systems (0.7–1.0 eV). This is ascribed to the high external quantum efficiency for the systems with a very small energy offset for charge separation. These unconventional features of the present polymer system will inspire the field of polymer-based solar cells towards further improvement of power conversion efficiencies with both high short-circuit current density and open-circuit voltage.

Singlet exciton diffusion plays a central role in the photovoltaic conversion in organic photovoltaics (OPVs). Upon light absorption, singlet excitons are promptly generated in organic materials instead of charge carriers because the dielectric constant (εr) is small (∼3-4), which is in sharp contrast to inorganic and perovskite solar cells. In order to convert to charge carriers, excitons need to diffuse into an interface between electron donor and acceptor materials before deactivating to the ground state. Therefore, fundamental understanding of exciton diffusion dynamics is one of the most important issues to further improve OPVs. We highlight recent leading studies in this field and describe several approaches for efficient exciton harvesting at the interface in OPVs.

Singlet exciton dynamics in crystalline domains of regioregular poly(3-hexylthiophene) (P3HT) films was studied by transient absorption spectroscopy. Upon the selective excitation of crystalline P3HT at the absorption edge, no red shift of the singlet exciton band was observed with an elapse of time, suggesting singlet exciton dynamics in relatively homogeneous P3HT crystalline domains without downhill relaxation in the energetic disorder. Even under such selective excitation conditions, the annihilation rate coefficient γ(t) was still dependent on time, γ(t) ∝ t(-1/2), which is attributed to anisotropic exciton diffusion in P3HT crystalline domains. From the annihilation rate coefficient, the singlet exciton diffusion coefficient D and exciton diffusion length LD in the crystalline domains were evaluated to be 7.9 × 10(-3) cm(2) s(-1) and 20 nm, respectively. The origin of the time-dependent exciton dynamics is discussed in terms of dimensionality.

We examined the mechanisms underlying the free carrier generation in a very topical PM6/Y6 organic solar cell. We observed slow yet efficient spatial charge dissociation driven by downhill energy relaxation through the interfacial energy cascade.

The resistance switching properties in Pt∕Ni–O∕Pt and Pt∕Co–O∕Pt synthesized by the magnetron sputtering have been investigated. The oxygen partial pressure during sputtering and the post-thermal process are crucial to forming of the trilayer. By investigating x-ray photoemission spectroscopy spectra, the increase of initial resistance in Ni–O was caused by the variation of the stoichiometry, while that in the Co–O was accompanied by the phase transformation between CoO and Co3O4. The resistance switching in Pt∕Co–O∕Pt and Pt∕Ni–O∕Pt exhibits the analogous electrode area and temperature dependences. As a result of the I-V measurements at the elevated temperature, the assistance of Joule heating in the reset process is implied.

Excited-state dynamics in poly[4,6-(dodecyl-thieno[3,4-b]thiophene-2-carboxylate)-alt-2,6-(4,8-dioctoxylbenzo[1,2-b:4,5-b]dithiophene)] (PTB1) was studied by transient absorption spectroscopy. Upon photoexcitation at 400 nm, an additional transient species is promptly generated along with singlet excitons and survives up to nanoseconds, while singlet excitons disappear completely. In order to assign the long-lived species, we measured transient absorption spectra over the wide spectral range from 900 to 2500 nm. As a result, we found that the long-lived species is ascribed not to polarons but to triplet excitons, which is formed through the ultrafast singlet fission (SF). We discuss the ultrafast SF mechanism in push-pull low-bandgap polymer PTB1 films on the basis of the excited-state dynamics under various excitation wavelengths and intensities.

Rapid, long-range charge separation in polymer-fullerene organic solar cells (OSCs) enables electrons and holes to move beyond their Coulomb capture radius and overcome geminate recombination. Understanding the nature of charge generation and recombination mechanisms in efficient, nonfullerene-acceptor-based OSCs are critical to further improve device performance. Here we report charge dynamics in an OSC using a perylene diimide (PDI) dimer acceptor. We use transient absorption spectroscopy to track the time evolution of electroabsorption caused by the dipolar electric field generated between electron-hole pairs as they separate after ionization at the donor-acceptor interface. We show that charges separate rapidly (<1 ps) and that free charge carriers are generated very efficiently (∼90% quantum yield). However, in the PDI-based OSC, external charge extraction is impaired by faster nongeminate decay to the ground state and to lower-lying triplet states.

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