Chen Weijie

Three vacuum-deposited donor–acceptor–acceptor (d–a–a') small molecule donors are studied with different side chains attached to an asymmetric heterotetracene donor block for use in high efficiency organic photovoltaics (OPVs). The donor with an isobutyl side chain yields the highest crystal packing density compared to molecules with 2-ethylhexyl or n-butyl chains, leading to the largest absorption coefficient and short circuit current in an OPV. It also exhibits a higher fill factor, consistent with its preferred out-of-plane molecular π–π stacking arrangement that facilitates charge transport in the direction perpendicular to the substrate. A power conversion efficiency of 9.3 ± 0.5% is achieved under 1 sun intensity, AM 1.5 G simulated solar illumination, which is significantly higher than 7.5 ± 0.4% of the other two molecules. These results indicate that side chain modification of d–a–a' small molecules offers an effective approach to control the crystal packing configuration, thereby improving the device performance.

Three vacuum-deposited donor–acceptor–acceptor's small molecule donors with different alkyl chain configurations (R₁–R₃) are synthesized and characterized to understand the side chain effect on organic photovoltaic (OPV) performance. The donor with an isobutyl (R₃) chain yields the highest crystal packing density and largest short circuit current among the three molecules. Its preferred face-on molecular stacking orientation on the substrate leads to the highest fill factor. The optimized OPV structure achieves a power conversion efficiency (PCE) = 9.3 ± 0.5%.

Recent years have seen a substantial efficiency improvement for a variety of solar cell technologies as well as the rise of a new class of photovoltaic absorber materials, the metal-halide perovskites. Conversion efficiencies that are coming closer and closer to the thermodynamic limits require a physical description of the corresponding solar cells that is compatible with those limits. This progress report summarizes recent work on the interdependence of basic material properties of semiconductor materials with their efficiency potential as photovoltaic absorbers. The connection of the classical Shockley–Queisser approach, with the band gap energy as the only parameter, to a more general radiative limit and to situations where nonradiative recombination dominates is discussed. The authors delineate a consistent loss analysis that enables a quantitative comparison between different solar cell technologies. In a next step, bulk material properties that influence the photovoltaic performance of a semiconductor like absorption coefficient, densities of states of the free carriers, or phonon energies are considered. It is shown that variations of these properties have a big influence on the optimized design of a solar cell but not necessarily on the achievable efficiency.

This progress report explains how microscopic properties of solar cell absorber materials affect properties such as absorption coefficient, mobility, and charge carrier lifetime and how these properties affect photovoltaic performance. The report provides the necessary theoretical background to describe solar cells on different levels of abstraction which helps our understanding of what makes some materials good solar cell materials.

A novel double perovskite Sr₂FeMo_2/3Mg_1/3O_{6− δ} is prepared and characterized as an anode material for solid oxide fuel cells (SOFCs). X-ray diffraction refinement reveals that Mg and Mo cations locate separately in two different B sites (B and B′ in A₂BB′O₆) while Fe occupies both B and B′ sites, forming the lattice structure with the form of Sr₂(Mg_1/3Fe_2/3)(Mo_2/3Fe_1/3)O_6−δ. The inactive element Mg doping not only endows the material with excellent redox structural stability but also triggers the creation of antisite defects in the crystal lattice, which provide the material with excellent electrochemical activity. The anode performance of Sr₂FeMo_2/3Mg_1/3O_6−δ is characterized in an La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ electrolyte supported cell with La_0.58Sr_0.4Fe_0.8Co_0.2O_3−δ cathode. A peak power density of 531, 803, 1038, and 1316 mW cm⁻² at 750, 800, 850, and 900 °C, respectively, is achieved in humidified H₂. The Sr₂FeMo_2/3Mg_1/3O_6−δ shows suitable thermal expansion coefficient (16.9(2) × 10⁻⁶ K⁻¹), high electrical conductivity, and good tolerance to carbon deposition and sulfur poisoning. First-principle computations demonstrate that the presence of Fe_BOFe_B′ bonds can promote the easy formation and fast migration of oxygen vacancies in the lattice, which are the key to affecting the anode reaction kinetics. The excellent overall performance of Sr₂FeMo_2/3Mg_1/3O_6−δ compound makes it a promising anode material for SOFCs.

Double perovskite Sr₂FeMo_2/3Mg_1/3O_6−δ shows very interesting structural/catalytic properties, excellent redox stability and high electrochemical performance. These excellent properties are related to the discovered anti-site defects, which is triggered by Mg doping for Mo and created by the occupancy of Fe at both B and B′-sites. The anti-site defects are found to be beneficial to ionic conductivity and electrochemical catalytic activity.