Liuyanfeng

Due to the unprecedented rapid increase of their power conversion efficiency, hybrid organic–inorganic perovskites CH₃NH₃PbX₃ (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature-dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (>27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH₃NH₃ ⁺ cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of ε_stat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low-temperature orthorhombic state is antipolar. No indications of ferroelectricity are found.

Charge carrier screening in the methylammonium lead halides is the major mechanism assuring long charge carrier lifetime. The high dielectric response is shown to stem from lattice as well as dipole orientation contributions both acting independently at different frequencies. The resulting screening is more effective than a single mechanism and acts on electronic carriers as well as charged defects.

The performance of all-polymer solar cells (all-PSCs) is often limited by the poor exciton dissociation process. Here, the design of a series of polymer donors (P1–P3) with different numbers of fluorine atoms on their backbone is presented and the influence of fluorination on charge generation in all-PSCs is investigated. Sequential fluorination of the polymer backbones increases the dipole moment difference between the ground and excited states (Δµ_ge) from P1 (18.40 D) to P2 (25.11 D) and to P3 (28.47 D). The large Δµ_ge of P3 leads to efficient exciton dissociation with greatly suppressed charge recombination in P3-based all-PSCs. Additionally, the fluorination lowers the highest occupied molecular orbital energy level of P3 and P2, leading to higher open-circuit voltage (V_OC). The power conversion efficiency of the P3-based all-PSCs (6.42%) outperforms those of the P2 and P1 (5.00% and 2.65%)-based devices. The reduced charge recombination and the enhanced polymer exciton lifetime in P3-based all-PSCs are confirmed by the measurements of light-intensity dependent short-circuit current density (J_SC) and V_OC, and time-resolved photoluminescence. The results provide reciprocal understanding of the charge generation process associated with Δµ_ge in all-PSCs and suggest an effective strategy for designing π-conjugated polymers for high performance all-PSCs.

An efficient approach for achieving high-performance all-polymer solar cells (all-PSCs) is demonstrated by controlling the dipole moment of polymers (P1–P3) via sequential fluorination on polymer backbones. P3-based all-PSCs with large dipole moments produce a greatly enhanced power conversion efficiency of 6.42%, which is well-correlated with efficient charge generation including improved exciton dissociation efficiency, reduced charge recombination, and enhanced lifetime of excitons.

Two novel wide bandgap copolymers based on quinoxalino[6,5-f]quinoxaline (NQx) acceptor block, PBDT–NQx and PBDTS–NQx, are successfully synthesized for efficient nonfullerene polymer solar cells (PSCs). The attached conjugated side chains on both benzodithiophene (BDT) and NQx endow the resulting copolymers with low-lying highest occupied molecular orbital (HOMO) levels. The sulfur atom insertion further reduces the HOMO level of PBDTS–NQx to −5.31 eV, contributing to a high open-circuit voltage, V_oc, of 0.91 V. Conjugated n-octylthienyl side chains attached on the NQx skeletons also significantly improve the π–π* transitions and optical absorptions of the copolymers in the region of short wavelengths, which induce a good complementary absorption when blending with the low bandgap small molecular acceptor of 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene. The wide absorption range makes the active blends absorb more photons, giving rise to a high short-circuit current density, J_sc, value of 15.62 mA cm⁻². The sulfur atom insertion also enhances the crystallinity of PBDTS–NQx and presents its blend film with a favorable nanophase separation, resulting in improved J_sc and fill factor (FF) values with a high power conversion efficiency of 11.47%. This work not only provides a new fused ring acceptor block (NQx) for constructing high-performance wide bandgap copolymers but also provides the NQx-based copolymers for achieving highly efficient nonfullerene PSCs.

Two novel wide bandgap copolymers based on quinoxalino[6,5-f]quinoxaline (NQx) acceptor block, PBDT–NQx and PBDTS–NQx, are successfully synthesized for efficient nonfullerene polymer solar cells. These new polymers exhibit high absorption at short wavelength, matching well with low bandgap acceptors, and have deep highest occupied molecular orbital (HOMO) levels, allowing their use in highly efficient nonfullerene solar cells with up to 11.47% power conversion efficiency.

Surface-functionalized monolayers are obtained by a new approach based on coupled exfoliation and surface modification, reported by Yuya Oaki and co-workers in article number 1601014. The original surface molecules on the monolayer surface are exchanged to the guest ones during the exfoliation process. The current approach can be applied to surface modification of a variety of two-dimensional nanomaterials.

A new fluorinated nonfullerene acceptor, ITIC-Th1, has been designed and synthesized by introducing fluorine (F) atoms onto the end-capping group 1,1-dicyanomethylene-3-indanone (IC). On the one hand, incorporation of F would improve intramolecular interaction, enhance the push–pull effect between the donor unit indacenodithieno[3,2-b]thiophene and the acceptor unit IC due to electron-withdrawing effect of F, and finally adjust energy levels and reduce bandgap, which is beneficial to light harvesting and enhancing short-circuit current density (J_SC). On the other hand, incorporation of F would improve intermolecular interactions through CF···S, CF···H, and CF···π noncovalent interactions and enhance electron mobility, which is beneficial to enhancing J_SC and fill factor. Indeed, the results show that fluorinated ITIC-Th1 exhibits redshifted absorption, smaller optical bandgap, and higher electron mobility than the nonfluorinated ITIC-Th. Furthermore, nonfullerene organic solar cells (OSCs) based on fluorinated ITIC-Th1 electron acceptor and a wide-bandgap polymer donor FTAZ based on benzodithiophene and benzotriazole exhibit power conversion efficiency (PCE) as high as 12.1%, significantly higher than that of nonfluorinated ITIC-Th (8.88%). The PCE of 12.1% is the highest in fullerene and nonfullerene-based single-junction binary-blend OSCs. Moreover, the OSCs based on FTAZ:ITIC-Th1 show much better efficiency and better stability than the control devices based on FTAZ:PC₇₁BM (PCE = 5.22%).

Single-junction binary-blend nonfullerene polymer solar cells based on fluorinated acceptor ITIC-Th1 afford power conversion efficiency of 12.1%, which is much higher than those of nonfluorinated ITIC-Th (8.88%) and PC₇₁BM (5.22%) counterparts under the same condition. Moreover, the nonfullerene devices exhibit better thermal stability than the fullerene devices.

A series of metal acetylacetonates produced by a full low-temperature (below 100 °C) process are successfully employed to obtain both “multistable” and high-performance planar-inverted perovskite solar cells. All the three kinds of champion cells in small area exhibit over 18% in conversion-efficiency with negligible hysteresis, along with a conversion efficiency above 16% for planar PSCs in an aperture area of over 1 cm².

The photoactive layer of bulk-heterojunction organic solar cells, in a thickness range of tens to hundreds of nanometers, comprises phase-separated electron donors and acceptors after solution casting. The component distribution in the cross-section of these thin films is found to be heterogeneous, with electron donors or acceptors accumulated or depleted near the electrode interfaces. This vertical stratification of the photovoltaic blend influences device metrics through its impact on charge transport and recombination, and consequently plays an important role in determining the power conversion efficiency of photovoltaic devices. Here, different techniques, e.g., surface analysis and sputter-assisted depth-profiling, reflectivity modeling, and 3D imaging, that have been employed to characterize vertical stratification in bulk-heterojunction photovoltaic blends are reviewed. The origins of vertical stratification are summarized, including thermodynamics, kinetics, surface free energy, and selective dissolubility. The impact of correct and wrong vertical stratification to device metrics of solar cells are highlighted. Examples are then given to demonstrate how desired vertical stratification can be controlled with properly aligned device architecture to enable solar cells with high efficiency.

Vertical stratification of the electron donor and acceptor components in bulk-heterojunction photovoltaic blends can happen during their film-formation process after solution casting and the subsequent post-treatment process, and can significantly influence device efficiency of organic solar cells through its impact on the effective charge transport and collection.

Hybrid organic/inorganic perovskite solar cells (PSCs) have shown great potential in meeting the future challenges in energy and environment. Solvent-vapor-assisted posttreatment strategies are developed to improve the perovskite film quality for achieving higher efficiency. However, the intrinsic working mechanisms of these strategies have not been well understood yet. This study identifies an MA₂Pb₃I₈(DMSO)₂ intermediate phase formed during the annealing process of methylammonium lead triiodide in dimethyl sulfoxide (DMSO) atmosphere and located the reaction sites at perovskite grain boundaries by observing and rationalizing the growth of nanorods of the intermediate. This enables us to propose and validate an intermediate-assisted grain-coarsening model, which highlights the activation energy reduction for grain boundary migration. Leveraging this mechanism, this study uses MABr/DMSO mixed vapor to further enhance grain boundary migration kinetics and successfully obtain even larger grains, leading to an impressive improvement in power conversion efficiency (17.64%) relative to the pristine PSCs (15.13%). The revelation of grain boundary migration-assisted grain growth provides a guide for the future development of polycrystalline perovskite thin-film solar cells.

MA₂Pb₃I₈(DMSO)₂ intermediate is identified and tracked during the methylammonium lead triiodide thin-film anneal process under dimethyl sulfoxide (DMSO) solvent vapor, which helps to reduce the activation energy of perovskite grain boundary migration. Leveraging this mechanism, an MABr/DMSO mix vapor anneal method is developed to further facillitate grain goundary migration to achieve 17.64% efficiency in NiO-based inverted perovskite solar cell.