Jaemin Kong

Stable Graphene-Two-Dimensional Multiphase Perovskite Heterostructure Phototransistors with High Gain

Recently, two-dimensional (2D) organic-inorganic perovskites emerged as an alternative material for their three-dimensional (3D) counterparts in photovoltaic applications with improved moisture resistance. Here, we report a stable, high-gain phototransistor consisting of a monolayer graphene on hexagonal boron nitride (hBN) covered by a 2D multiphase perovskite heterostructure, which was realized using a newly developed two-step ligand exchange method. In this phototransistor, the multiple phases with varying bandgap in 2D perovskite thin films are aligned for the efficient electron-hole pair separation, leading to a high responsivity of ∼105 A W-1 at 532 nm. Moreover, the designed phase alignment method aggregates more hydrophobic butylammonium cations close to the upper surface of the 2D perovskite thin film, preventing the permeation of moisture and enhancing the device stability dramatically. In addition, faster photoresponse and smaller 1/f noise observed in the 2D perovskite phototransistors indicate a smaller density of deep hole traps in the 2D perovskite thin film compared with their 3D counterparts. These desirable properties not only improve the performance of the phototransistor, but also provide a new direction for the future enhancement of the efficiency of 2D perovskite photovoltaics.

Three-Phase Morphology Evolution in Sequentially Solution-Processed Polymer Photodetector: Toward Low Dark Current and High Photodetectivity

Sequentially solution-processed polymer photodetectors (SSP PPDs) based on poly(3-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) are fabricated by depositing the top layers of PC71BM from an appropriate cosolvent of 2-chlorophenol (2-CP)/o-dichlorobenzene (ODCB) onto the predeposited bottom layers of P3HT. By adjusting the ratio of 2-CP/ODCB in the top PC71BM layers, the resulting SSP PPD shows a decreased dark current and an increased photocurrent, leading to a maximum detectivity of 1.23 × 1012 Jones at a wavelength of 550 nm. This value is 5.3-fold higher than that of the conventional bulk heterojunction PPD. Morphology studies reveal that the PC71BM partially penetrates the predeposited P3HT layer during the spin-coating process, resulting in an optimal three-phase morphology with one well-mixed interdiffusion P3HT/PC71BM phase in the middle of the bulk and two pure phases of P3HT and PC71BM at the two electrode sides. We show that the pure phases form high Schottky barriers (>2.0 eV) at the active layer/electrodes interface and efficiently block unfavorable reverse charge carrier injection by significantly decreasing the dark current. The interdiffussion phase enlarges the donor-acceptor interfacial area leading to a large photocurrent. We also reveal that the improved performance of SSP PPDs is also due to the enhanced optical absorption, improved P3HT crystallinity, increased charge carrier mobilities, and suppressed bimolecular recombination.

PEOz-PEDOT:PSS Composite Layer: A Route to Suppressed Hysteresis and Enhanced Open-Circuit Voltage in a Planar Perovskite Solar Cell

The successful commercialization of perovskite solar cells (Pvs-SCs) calls for the need to find low-temperature processable interlayers with outstanding charge-transport features. In this work, we strategically blend poly(2-ethyl-2-oxazoline) (PEOz) with PEDOT:PSS as the modified hole transport layer (HTL) to achieve high-efficiency P-I-N CH3NH3PbI3 Pvs-SCs. The PEOz-PEDOT:PSS HTL exhibits enhanced features over the conventional layer including the following: (1) promoting perovskite with enlarged grain sizes to decrease the perovskite layers recombination, (2) increasing the work function of the HTL, and (3) decreasing the noncapacitive current in Pvs-SCs. Remarkably, we demonstrate a 17.39% power conversion efficiency with very low hysteresis and high Voc values of 1.075 V for Pvs-SCs with PEOz-PEDOT:PSS.