Two-dimensional halide perovskites are exciting new semiconductors that show great promising in low cost and high performance optoelectronics devices. However, the weak chemical bonding of halide perovskites makes them chemically, thermally, and mechanically unstable. To address this critical issue and move forward for commercialization, deeper fundamental insights regarding the degradation mechanism and better stabilization strategies have to be achieved. In this talk, I will present a new molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks as the surface capping ligands. Then, I will talk about the applications of these materials in high performance and highly stable solar cells and thermoelectric conversion devices.
Semiconductor quantum wells and superlattices, which are usually fabricated through metal-organic chemical vapor deposition or molecular beam epitaxy, are key building blocks in modern optoelectronics. The ability to simultaneously realize defect-free epitaxial growth and to individually fine-tune the chemical composition and band structure of each layer is essential for achieving the desired performance. Such structures are challenging to realize using organic or hybrid materials because of the difficulty of controlling the materials growth. In this talk, I will present a molecular approach to the synthesis of high-quality organic-inorganic hybrid perovskite quantum wells through incorporating widely tunable organic semiconducting building blocks. By introducing sterically tailored groups into the molecular motif, the strong self-aggregation of the conjugated organic molecules can be suppressed, and single crystalline organic-perovskite hybrid quantum wells (down to one mono-layer thick) and superlattices can be easily obtained via one-step solution-processing. Energy transfer and charge transfer between adjacent organic and inorganic layers are extremely fast and efficient, owing to the atomically-flat interface and ultra-small interlayer distance. The 2D hybrid perovskite superlattices are surprisingly stable, due to the protection of the bulky hydrophobic organic groups. Finally, we demonstrate the applications of these materials in high performance solar cells and field effect transistors.
Tandem solar cells provide an effective way to harvest a broader spectrum of solar radiation by combining two or more
solar cells with different absorption ranges. However, for polymer solar cells (PSCs), the performance of tandem devices
lags behind single-layer cells mainly due to the lack of a high-performance low-bandgap polymer with appropriate
spectral response range. Here, we demonstrate a novel low bandgap conjugated polymer (~1.44 eV) specifically suitable for tandem structure. In the single-layer device, power conversion efficiency (PCE) of 6.5% was achieved. When the polymer was applied to tandem solar cells, we demonstrated a NREL certified PCE of 8.62% . Further optimization on materials and devices of this system has lead to record breaking efficiency of 10.6%. Furthermore, the tandem devices show excellent stability due both to the intrinsic stability of the polymer and the advanced device structure.