Xiaoqiang Li

18.5% efficient graphene/GaAs van der Waals heterostructure solar cell

The honeycomb connection of carbon atoms by covalent bonds in a macroscopic two-dimensional scale leads to fascinating graphene and solar cell based on graphene/silicon Schottky diode has been widely studied. For solar cell applications, GaAs is superior to silicon as it has a direct band gap of 1.42 eV and its electron mobility is six times of that of silicon. However, graphene/GaAs solar cell has been rarely explored. Herein, we report graphene/GaAs solar cells with conversion efficiency (Eta) of 10.4% and 15.5% without and with anti-reflection layer on graphene, respectively. The Eta of 15.5% is higher than the state of art efficiency for graphene/Si system (14.5%). Furthermore, our calculation points out Eta of 25.8% can be reached by reasonably optimizing the open circuit voltage, junction ideality factor, resistance of graphene and metal/graphene contact. This research strongly support graphene/GaAs hetero-structure solar cell have great potential for practical applications.

Interface designed MoS2/GaAs heterostructure solar cell with sandwich stacked hexagonal boron nitride

MoS2 is a layered two-dimensional semiconductor with a direct band gap of 1.8u2009eV. The MoS2/bulk semiconductor system offers a new platform for solar cell device design. Different from the conventional bulk p-n junctions, in the MoS2/bulk semiconductor heterostructure, static charge transfer shifts the Fermi level of MoS2 toward that of bulk semiconductor, lowering the barrier height of the formed junction. Herein, we introduce hexagonal boron nitride (h-BN) into MoS2/GaAs heterostructure to suppress the static charge transfer, and the obtained MoS2/h-BN/GaAs solar cell exhibits an improved power conversion efficiency of 5.42%. More importantly, the sandwiched h-BN makes the Fermi level tuning of MoS2 more effective. By employing chemical doping and electrical gating into the solar cell device, PCE of 9.03% is achieved, which is the highest among all the reported monolayer transition metal dichalcogenide based solar cells.

Graphene/h-BN/GaAs sandwich diode as solar cell and photodetector.

In graphene/semiconductor heterojunction, the statistic charge transfer between graphene and semiconductor leads to decreased junction barrier height and limits the Fermi level tuning effect in graphene, which greatly affects the final performance of the device. In this work, we have designed a sandwich diode for solar cells and photodetectors through inserting 2D hexagonal boron nitride (h-BN) into graphene/GaAs heterostructure to suppress the static charge transfer. The barrier height of graphene/GaAs heterojunction can be increased from 0.88 eV to 1.02 eV by inserting h-BN. Based on the enhanced Fermi level tuning effect with interface h-BN, through adopting photo-induced doping into the device, power conversion efficiency (PCE) of 10.18% has been achieved for graphene/h-BN/GaAs compared with 8.63% of graphene/GaAs structure. The performance of graphene/h-BN/GaAs based photodetector is also improved with on/off ratio increased by one magnitude compared with graphene/GaAs structure.

Gate tunable monolayer MoS2/InP heterostructure solar cells

We demonstrate monolayer molybdenum disulfide (MoS2)/indium phosphide (InP) van der Waals heterostructure with remarkable photovoltaic response. Furthermore, benefiting from the atomically thin and semiconductor nature of MoS2, we have designed the gate tunable MoS2/InP heterostructure. Applied with a top gate voltage, the Fermi level of MoS2 is effectively tuned, and the barrier height at the MoS2/InP heterojunction correspondingly changes. The power conversion efficiency of MoS2/InP solar cells has reached a value of 7.1% under AM 1.5G illumination with a gate voltage of +6u2009V. The tunable MoS2/InP heterostructure may be promising for highly efficient solar cells.

Suppression of boron–oxygen defects in p-type Czochralski silicon by germanium doping

We have demonstrated the impact of germanium (Ge) doping on the boron–oxygen (B–O) defects in p-type Czochralski (CZ) silicon. It is found that germanium can effectively suppress the formation of B–O defects, whereby the reduction percentage of B–O defect concentration increases with the Ge content. The efficiency of Ge-doped CZ silicon solar cell and the power output of corresponding module both exhibit a significantly lower loss. Based on the fact of a relatively lower concentration of O2i existing in GCZ silicon, it is believed that the suppression of B–O defects is a result of Ge improving the diffusion barrier of Oi.

Enhanced monolayer MoS2/InP heterostructure solar cells by graphene quantum dots

We demonstrate significantly improved photovoltaic response of monolayer molybdenum disulfide (MoS2)/indium phosphide (InP) van der Waals heterostructure induced by graphene quantum dots (GQDs). Raman and photoluminescence measurements indicate that effective charge transfer takes place between GQDs and MoS2, which results in n-type doping of MoS2. The doping effect increases the barrier height at the MoS2/InP heterojunction, thus the averaged power conversion efficiency of MoS2/InP solar cells is improved from 2.1% to 4.1%. The light induced doping by GQD provides a feasible way for developing more efficient MoS2 based heterostructure solar cells.

Graphene/h-BN/ZnO van der Waals tunneling heterostructure based ultraviolet photodetector.

We report a novel ultraviolet photodetector based on graphene/h-BN/ZnO van der Waals heterostructure. Graphene/ZnO heterostructure shows poor rectification behavior and almost no photoresponse. In comparison, graphene/h-BN/ZnO structure shows improved electrical rectified behavior and surprising high UV photoresponse (1350AW(-1)), which is two or three orders magnitude larger than reported GaN UV photodetector (0.2~20AW(-1)). Such high photoresponse mainly originates from the introduction of ultrathin two-dimensional (2D) insulating h-BN layer, which behaves as the tunneling layer for holes produced in ZnO and the blocking layer for holes in graphene. The graphene/h-BN/ZnO heterostructure should be a novel and representative 2D heterostructure for improving the performance of 2D materials/Semiconductor heterostructure based optoelectronic devices.

Graphene/CdTe heterostructure solar cell and its enhancement with photo-induced doping

We report a type of solar cell based on graphene/CdTe Schottky heterostructure, which can be improved by surface engineering as graphene is atomic thin. By coating a layer of ultrathin CdSe quantum dots onto graphene/CdTe heterostructure, the power conversion efficiency is increased from 2.08% to 3.10%. Photo-induced doping is mainly accounted for this enhancement, as evidenced by field effect transport, Raman, photoluminescence, and quantum efficiency measurements. This work demonstrates a feasible way of improving the performance of graphene/semiconductor heterostructure solar cells by combining one dimensional with two dimensional materials.

Two dimensional graphene nanogenerator by coulomb dragging: Moving van der Waals heterostructure

Harvesting energy from environment is the current focus of scientific community. Here, we demonstrate a graphene nanogenerator, which is based on moving van der Waals heterostructure formed between graphene and two dimensional (2D) graphene oxide (GO). This nanogenerator can convert mechanical energy into electricity with a voltage output of around 10 mV. Systematic experiments reveal the generated electricity originates from the coulomb interaction induced momentum transfer between 2D GO and holes in graphene. 2D boron nitride was also demonstrated to be effective in the framework of moving van der Waals heterostructure nanogenerator. This investigation of nanogenerator based on the interaction between 2D macromolecule materials will be important to understand the origin of the flow-induced potential in nanomaterials and may have great potential in practical applications.

Modeling and Simulation of Graphene-Gated Graphene-GaAs Schottky Junction Field-Effect Solar Cell for Its Performance Enhancement

Modeling and simulation of graphene-gated graphene-GaAs Schottky junction field-effect solar cell is performed using in-house developed algorithm based on finite-difference method, where Poisson and drift-diffusion equations are solved in an appropriate way. Our algorithm is verified by comparing the simulated J-V curve of solar cell with the previous experimental one. The carrier generation rate is calculated with light multireflection in the solar cell excluded, due to large optical absorption coefficient of GaAs, and the radiative recombination in GaAs leads to ~5% light generated carrier loss. The charge transfer effect in graphene is investigated, which can affect open circuit voltage, and in particular for the semiconductor substrate with very low hole mobility. It is further numerically demonstrated that: (1) both open circuit voltage VOC and short circuit current JSC of the solar cell are governed by doping concentration through affecting Schottky barrier height and depletion width; (2) the value of VOC can be effectively adjusted by decreasing the thickness of gate oxide; however, it may result in oxide breakdown or reliability problem; and (3) there is tunability inertia for VOC with multilayer junction graphene implemented, but it can provide low series resistance.