Yani Chen

Solution processed organic thermoelectrics: towards flexible thermoelectric modules

Organic semiconductor materials have advantages of low cost, light weight, mechanical flexibility and low-temperature solution processability over large areas, enabling the development of personal, portable, and flexible thermal modules. This review article summarizes the recent progress made in the area of organic thermoelectrics (TEs), including organic molecular structures, devices, characterization methods, and approaches to improve the performance. We begin with the discussion of each TE parameter and particularly their correlations in organic TEs. Then the TE applications of molecular organic semiconductors, poly(3,4-ethylenedioxythiophene), polymer nanostructures and molecular junctions are reviewed. Next we turn to highlight the nanocomposites of polymers and carbon nanotubes or nanocrystals, which lead to enhanced TEs. Interestingly, the merging of TEs and photovoltaics offers a new direction towards a great capability of electric energy output. Critical challenges of organic TE materials include stability, sample preparation and measurement techniques, which are also discussed. Finally, the relationships among organic semiconductor structures, hybrid composites, doping states, film morphology and TE performance are revealed, and a viable avenue is envisioned for synergistic optimization of organic TEs.

Structure and Growth Control of Organic–Inorganic Halide Perovskites for Optoelectronics: From Polycrystalline Films to Single Crystals

Recently, organic–inorganic halide perovskites have sparked tremendous research interest because of their ground‐breaking photovoltaic performance. The crystallization process and crystal shape of perovskites have striking impacts on their optoelectronic properties. Polycrystalline films and single crystals are two main forms of perovskites. Currently, perovskite thin films have been under intensive investigation while studies of perovskite single crystals are just in their infancy. This review article is concentrated upon the control of perovskite structures and growth, which are intimately correlated for improvements of not only solar cells but also light‐emitting diodes, lasers, and photodetectors. We begin with the survey of the film formation process of perovskites including deposition methods and morphological optimization avenues. Strategies such as the use of additives, thermal annealing, solvent annealing, atmospheric control, and solvent engineering have been successfully employed to yield high‐quality perovskite films. Next, we turn to summarize the shape evolution of perovskites single crystals from three‐dimensional large sized single crystals, two‐dimensional nanoplates, one‐dimensional nanowires, to zero‐dimensional quantum dots. Siginificant functions of perovskites single crystals are highlighted, which benefit fundamental studies of intrinsic photophysics. Then, the growth mechanisms of the previously mentioned perovskite crystals are unveiled. Lastly, perspectives for structure and growth control of perovskites are outlined towards high‐performance (opto)electronic devices.

Efficient and Balanced Charge Transport Revealed in Planar Perovskite Solar Cells

Hybrid organic-inorganic perovskites have emerged as novel photovoltaic materials and hold great promise for realization of high-efficiency thin film solar modules. In this study, we unveil the ambipolar characteristics of perovskites by employing the transport measurement techniques of charge extraction by linearly increasing voltage (CELIV) and time-of-flight (TOF). These two complementary methods are combined to quantitatively determine the mobilities of hole and electron of CH3NH3PbI3 perovskite while revealing the recombination process and trap states. It is revealed that efficient and balanced transport is achieved in both CH3NH3PbI3 neat film and CH3NH3PbI3/PC61BM bilayer solar cells. Moreover, with the insertion of PC61BM, both hole and electron mobilities of CH3NH3PbI3 are doubled. This study offers a dynamic understanding of the operation of perovskite solar cells.

Interfacial engineering by using self-assembled monolayer in mesoporous perovskite solar cell

A self-assembled monolayer of 4-aminobenzoic acid (PABA) is prepared on the top of a mesoporous TiO2 electron transfer layer. Through the full characterization of PABA-SAM modified TiO2 and the above-prepared perovskite layer with Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), contact angle and photoluminescence, it was clear that the introduction of the SAM improved the interfacial compatibility and therefore the quality of the perovskite, thus leading to fewer traps, better carrier transport and good energy matching between the TiO2 layer and the perovskite layer. The power conversion efficiency (PCE) of the perovskite device with interfacial modification was enhanced to 10.58% compared with that of the device without SAM.

2D Ruddlesden–Popper Perovskites for Optoelectronics

Conventional 3D organic-inorganic halide perovskites have recently undergone unprecedented rapid development. Yet, their inherent instabilities over moisture, light, and heat remain a crucial challenge prior to the realization of commercialization. By contrast, the emerging 2D Ruddlesden-Popper-type perovskites have recently attracted increasing attention owing to their great environmental stability. However, the research of 2D perovskites is just in their infancy. In comparison to 3D analogues, they are natural quantum wells with a much larger exciton binding energy. Moreover, their inner structural, dielectric, optical, and excitonic properties remain to be largely explored, limiting further applications. This review begins with an introduction to 2D perovskites, along with a detailed comparison to 3D counterparts. Then, a discussion of the organic spacer cation engineering of 2D perovskites is presented. Next, quasi-2D perovskites that fall between 3D and 2D perovskites are reviewed and compared. The unique excitonic properties, electron-phonon coupling, and polarons of 2D perovskites are then be revealed. A range of their (opto)electronic applications is highlighted in each section. Finally, a summary is given, and the strategies toward structural design, growth control, and photophysics studies of 2D perovskites for high-performance electronic devices are rationalized.

Bendable n-Type Metallic Nanocomposites with Large Thermoelectric Power Factor

Highly bendable n-type thermoelectric nanocomposites are successfully developed by embedding metallic Ni nanowires within an insulating poly(vinylidene fluoride) (PVDF) matrix in solution. These nanocomposites exhibit an abnormal decoupling of the electrical conductivity and Seebeck coefficient as a function of Ni contents. A maximum power factor of 220 µW m-1 K-2 and ZT of 0.15 can thus be obtained with 80 wt% Ni at 380 K.

Nonvolatile chlorinated additives adversely influence CH3NH3PbI3 based planar solar cells

We demonstrate that nonvolatile CaCl2 additives can significantly improve the film morphology of CH3NH3PbI3. Unlike those volatile chlorinated additives, a small portion of Cl anions from CaCl2 seem to enter into the CH3NH3PbI3 crystal, yet most insulating CaCl2 remains within the perovskite film, which is detrimental to perovskite solar cells.

One-Step Chemical Synthesis of ZnO/Graphene Oxide Molecular Hybrids for High-Temperature Thermoelectric Applications

ZnO as high-temperature thermoelectric material suffers from high lattice thermal conductivity and poor electrical conductivity. Al is often used to n-dope ZnO to form Zn1-xAlxO (AZO). Owing to very limited Al solubility (less than 2 atom %) in AZO, however, electrical conductivity is difficult to improve further. Moreover, such a low concentration of Al dopants can hardly reduce the thermal conductivity. Here, we propose slightly adding chemically reduced graphene oxides (rGOs) to AZO in various contents to modulate the carrier concentration and simultaneously optimize the electrical and thermal conductivities. Such nanocomposites with rGO embedded in AZO matrix are formed on the molecular level by one-step solution chemistry method. No obvious changes are found in crystalline structures of AZO after introducing rGOs. The rGO inclusions are shown to uniformly mix the AZO matrix that consists of compacted nanoparticles. In such AZO/rGO hybrids, Zn2+ is captured by the rGO, releasing extra electrons and thus increasing electron density, as confirmed by Hall measurements. The phonon-boundary scattering at the interface between AZO and rGO remarkably reduces the lattice thermal conductivity. Therefore, a respectable thermoelectric figure of merit of 0.28 at 900 °C is obtained in these nanocomposites at the rGO content of 1.5 wt %, which is 8 times larger than that of pure ZnO and 60% larger than that of alloyed AZO. This work demonstrates a facile wet chemistry route to produce nanostructured thermoelectric composites in which electrical conductivity can be greatly increased while largely lowering thermal conductivity, collectively enhancing the thermoelectric performance.

Hot-Injection Synthesis of Cu-Doped Cu2ZnSnSe4 Nanocrystals to Reach Thermoelectric zT of 0.70 at 450 °C

As a new class of potential midrange temperature thermoelectric materials, quaternary chalcogenides like Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) suffer from low electrical conductivity due to insufficient doping. In this work, Cu-doped CZTSe nanocrystals consisting of polygon-like nanoparticles are synthesized with sufficient Cu doping contents. The hot-injection synthetic method, rather than the traditional one-pot method, in combination with the hot-pressing method is employed to produce the CZTSe nanocrystals. In Cu-doped CZTSe nanocrystals, the electrical conductivity is enhanced by substitution of Zn(2+) with Cu(+), which introduces additional holes as charge carriers. Meanwhile, the existence of boundaries between nanoparticles in as-synthesized CZTSe nanocrystals collectively results in intensive phonon-boundary scatterings, which remarkably reduce the lattice thermal conductivity. As a result, an average thermoelectric figure of merit of 0.70 is obtained at 450 °C, which is significantly larger than that of the state-of-the-art quaternary chalcogenides thermoelectric materials. The theoretical calculations from the Boltzmann transport equations and the modified effective medium approximation are in good agreement with the experimental data.

Correlating Molecular Structures with Transport Dynamics in High-Efficiency Small-Molecule Organic Photovoltaics

Efficient charge transport is a key step toward high efficiency in small-molecule organic photovoltaics. Here we applied time-of-flight and organic field-effect transistor to complementarily study the influences of molecular structure, trap states, and molecular orientation on charge transport of small-molecule DRCN7T (D1) and its analogue DERHD7T (D2). It is revealed that, despite the subtle difference of the chemical structures, D1 exhibits higher charge mobility, the absence of shallow traps, and better photosensitivity than D2. Moreover, charge transport is favored in the out-of-plane structure within D1-based organic solar cells, while D2 prefers in-plane charge transport.