Real-time observation of the buildup of polaron in α-FAPbI3.

The formation of polaron, i.e., the strong coupling process between the carrier and lattice, is considered to play a crucial role in benefiting the photoelectric performance of hybrid organic-inorganic halide perovskites. However, direct observation of the dynamical formation of polarons occurring at time scales within hundreds of femtoseconds remains a technical challenge. Here, by terahertz emission spectroscopy, we demonstrate the real-time observation of polaron formation process in FAPbI3 films. Two different polaron resonances interpreted with the anharmonic coupling emission model have been studied: P1 at ~1 THz relates to the inorganic sublattice vibration mode and the P2 at ~0.4 THz peak relates to the FA+ cation rotation mode. Moreover, P2 could be further strengthened than P1 by pumping the hot carriers to the higher sub-conduction band. Our observations could open a door for THz emission spectroscopy to be a powerful tool in studying polaron formation dynamics in perovskites.

Ion-Electron Coupling Enables Ionic Thermoelectric Material with New Operation Mode and High Energy Density.

Ionic thermoelectrics (i-TE) possesses great potential in powering distributed electronics because it can generate thermopower up to tens of millivolts per Kelvin. However, as ions cannot enter external circuit, the utilization of i-TE is currently based on capacitive charge/discharge, which results in discontinuous working mode and low energy density. Here, we introduce an ion-electron thermoelectric synergistic (IETS) effect by utilizing an ion-electron conductor. Electrons/holes can drift under the electric field generated by thermodiffusion of ions, thus converting the ionic current into electrical current that can pass through the external circuit. Due to the IETS effect, i-TE is able to operate continuously for over 3000 min. Moreover, our i-TE exhibits a thermopower of 32.7 mV K-1 and an energy density of 553.9 J m-2, which is more than 6.9 times of the highest reported value. Consequently, direct powering of electronics is achieved with i-TE. This work provides a novel strategy for the design of high-performance i-TE materials.

Volatile Solid Additive-Assisted Sequential Deposition Enables 18.42% Efficiency in Organic Solar Cells.

Morphology optimization of active layer plays a critical role in improving the performance of organic solar cells (OSCs). In this work, a volatile solid additive-assisted sequential deposition (SD) strategy is reported to regulate the molecular order and phase separation in solid state. The OSC adopts polymer donor D18-Cl and acceptor N3 as active layer, as well as 1,4-diiodobenzene (DIB) as volatile additive. Compared to the D18-Cl:N3 (one-time deposition of mixture) and D18-Cl/N3 (SD) platforms, the D18-Cl/N3(DIB) device based on DIB-assisted SD method exhibits a finer phase separation with greatly enhanced molecular crystallinity. The optimal morphology delivers superior charge transport and extraction, offering a champion power conversion efficiency of 18.42% with significantly enhanced short-circuit current density (Jsc ) of 27.18 mA cm-2 and fill factor of 78.8%. This is one of the best performances in binary SD OSCs to date. Angle-dependent grazing-incidence wide-angle X-ray scattering technique effectively reveals the vertical phase separation and molecular crystallinity of the active layer. This work demonstrates the combination of volatile solid additive and sequential deposition is an effective method to develop high-performance OSCs.

Oxidation of Spiro-OMeTAD in High-Efficiency Perovskite Solar Cells.

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (spiro-OMeTAD), as an organic small molecule material, is the most commonly employed hole transport material (HTM) in perovskite solar cells (PSCs) because of its excellent properties that result in high photovoltaic performances. However, the material still suffers from low conductivity, leading to the necessary use of dopants and oxidative processes to overcome this issue. The spiro-OMeTAD oxidation process is highlighted in this review, and the main parameters involved in the process have been studied. Furthermore, the best alternatives aiming to improve the spiro-OMeTAD electrical properties have been discussed. Lastly, this review concludes with suggestions and outlooks for further research directions.

Simultaneous Interfacial Modification and Crystallization Control by Biguanide Hydrochloride for Stable Perovskite Solar Cells with PCE of 24.4.

Interfacial modification, which serves multiple roles, is vital for the fabrication of efficient and stable perovskite solar cells. Here, a multifunctional interfacial material, biguanide hydrochloride (BGCl), is introduced between tin oxide (SnO2 ) and perovskite to enhance electron extraction, as well as the crystal growth of the perovskite. The BGCl can chemically link to the SnO2 through Lewis coordination/electrostatic coupling and help to anchor the PbI2 . Better energetic alignment, reduced interfacial defects, and homogeneous perovskite crystallites are achieved, yielding an impressive certified power conversion efficiency (PCE) of 24.4%, with an open-circuit voltage of 1.19 V and a drastically improved fill factor of 82.4%. More importantly, the unencapsulated device maintains 95% of its initial PCE after aging for over 500 h at 20 °C and 30% relative humidity in ambient conditions. These results suggest that the incorporation of BGCl is a promising strategy to modify the interface and control the crystallization of the perovskite, toward the attainment of highly efficient and stable perovskite solar cells as well as other perovskite-based electronics.

Role of Ions in Hydrogels with an Ionic Seebeck Coefficient of 52.9 mV K-1.

Ionic thermoelectric (i-TE) material with mobile ions as charge carriers has the potential to generate large thermal voltages at low operating temperatures. This study highlights the role of ions in i-TE hydrogels employing a poly(vinyl alcohol) (PVA) polymer matrix and a number of ion providers, e.g., KOH, KNO3, KCl, KBr, NaI, KI, and CsI. The relationship between the intrinsic physical parameters of the ion and the thermoelectric performance is established, indicating the ability to influence the hydrogen bond by the ion is a crucial factor. Among these i-TE hydrogels, the PVA/CsI hydrogel exhibits the largest ionic Seebeck coefficient, reaching 52.9 mV K-1, which is the largest of all i-TE materials reported to date. In addition, our work demonstrates the influence of ions on polymer configuration and provides an avenue for ion selection in the Soret effect in ionic thermoelectrics.