Chun-Sheng Jiang

Innovative dual function nc-SiOx:H layer leading to a >16% efficient multi-junction thin-film silicon solar cell

We present our development of n-type nano-structured hydrogenated silicon oxide (nc-SiOx:H) as a dual-function layer in multi-junction solar cells. We optimized nc-SiOx:H and attained a conductivity suitable for a doped layer and optical property suitable for an inter-reflection layer. We tested the effectiveness of the dual-function nc-SiOx:H layer by replacing the normal n layer between the middle and the bottom cells in an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure. A significant gain in the middle cell current density of ∼1.0 mA/cm2 is achieved. We further optimized the component cells and the triple-junction structures and attained an initial active-area cell efficiency of 16.3%.

Employing Lead Thiocyanate Additive to Reduce the Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells.

Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.

Synthesis and photovoltaic effect of vertically aligned ZnO/ZnS core/shell nanowire arrays

A vertically aligned ZnO/ZnS core/shell nanowire array with type II band alignment was directly synthesized on an indium-tin-oxide glass substrate and the photovoltaic effect of the nanowire array was investigated. The epitaxial relationship, wurtzite (0001) matching zinc-blende (111), was observed in the ZnO/ZnS nano-heterostructure. ZnS coating is found to quench the photoluminescence of ZnO nanowires but enhance the photocurrent with faster response in the photovoltaic device, indicating improvement in charge separation and collection in the type II core/shell nanowire.

Hydrogen dilution profiling for hydrogenated microcrystallinesilicon solar cells

The structural properties of hydrogenated microcrystalline silicon solar cells are investigated using Raman, x-ray diffraction, and atomic force microscopy. The experimental results showed a significant increase of microcrystalline volume fraction and grain size with increasing film thickness. The correlation between the cell performance and the microstructure suggests that the increase of grain size and microcrystalline volume fraction with thickness is the main reason for the deterioration of cell performance as the intrinsic layer thickness increases. By varying the hydrogen dilution in the gas mixture during deposition, microstructure evolution has been controlled and cell performance significantly improved.

Direct evidence of a buried homojunction in Cu(In,Ga)Se2 solar cells

The built-in electrical potential of Cu(In,Ga)Se2 (CIGS) solar cells was measured quantitatively and resolved spatially using scanning Kelvin probe microscopy. Profiles of the electrical potential along cross sections of the device demonstrate that the p–n junction is a buried homojunction, and the p/n boundary is located 30–80 nm from the CIGS/CdS interface in the CIGS film. The built-in electric field terminates at the CIGS/CdS interface, indicating that the CdS and ZnO layers of the device structure are inactive for the collection of photoexcited carriers.

Carrier separation and transport in perovskite solar cells studied by nanometre-scale profiling of electrical potential

Organometal–halide perovskite solar cells have greatly improved in just a few years to a power conversion efficiency exceeding 20%. This technology shows unprecedented promise for terawatt-scale deployment of solar energy because of its low-cost, solution-based processing and earth-abundant materials. We have studied charge separation and transport in perovskite solar cells—which are the fundamental mechanisms of device operation and critical factors for power output—by determining the junction structure across the device using the nanoelectrical characterization technique of Kelvin probe force microscopy. The distribution of electrical potential across both planar and porous devices demonstrates p–n junction structure at the TiO2/perovskite interfaces and minority-carrier diffusion/drift operation of the devices, rather than the operation mechanism of either an excitonic cell or a p-i-n structure. Combining the potential profiling results with solar cell performance parameters measured on optimized and thickened devices, we find that carrier mobility is a main factor that needs to be improved for further gains in efficiency of the perovskite solar cells.

Synthesis of band-gap-reduced p-type ZnO films by Cu incorporation

p-type ZnO thin films with significantly reduced band gaps were synthesized by heavy Cu incorporation at room temperature and followed by postdeposition annealing at 500°C in air for 2h. All the films were synthesized by rf magnetron sputtering on F-doped tin oxide-coated glass. The p-type conductivity was confirmed by Mott-Schottky plots and illuminated I-V analysis. The Cu+1 acceptor states (at substitutional sites) and their band-gap reduction were demonstrated by UV-visible absorption and x-ray excited valence band measurements.

Lattice-mismatched GaAsP Solar Cells Grown on Silicon by OMVPE

We report on lattice-mismatched GaAs_0.7P_0.3 solar cells grown on silicon substrates. This composition of GaAs_0.7P_0.3 has a band gap of about 1.7 eV and is well suited as the top junction of a III-V/Si two-junction tandem solar cell. Using a thin, high-quality GaP nucleation layer, a lattice-matched GaN_0.02P_0.98 buffer layer, and a compositionally graded GaAs_xP_1-x buffer layer, the threading dislocation densities was reduced to less than 10⁸ cm^-2 in the active region. The efficiencies of these single-junction cells without any antireflection coatings were as high has 9.8% under the AM1.5G spectrum. The quality of these solar cells based on V_oc is comparable to the best III-V solar cells ever grown on Si substrates with a III-V buffer

Measurement of built-in electrical potential in III–V solar cells by scanning Kelvin probe microscopy

We report on direct measurements of the built-in electrical potential in III–V semiconductor-based solar cell devices by using scanning Kelvin probe microscopy. Potential profiles on cross sections of the devices were measured quantitatively and spatially resolved in open and short circuits, under and without illuminations, with selective photon energies matching band gaps of the junctions. The measurements provide valuable information about the electrical properties of the devices, and are useful for understanding the performance of solar cells. On a GaInP2-single junction cell, two potential features were measured and were assigned to the p-n junction and the potential barrier at the interface between the GaInP2 base layer and the GaAs substrate. The potential on the p-n junction is photoactive, and that on the GaInP2/GaAs interface is photoinactive. On a GaInP2/GaAs tandem cell, two potential features were measured near the top and the bottom p-n junctions. When the sample was illuminated by light with...

Deposition and properties of CBD and CSS CdS thin films for solar cell application

Abstract We deposited cadmium sulfide (CdS) thin films using the chemical-bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The films were then treated in CdCl 2 vapor at 400 °C for 5 min. The CSS CdS films had hexagonal structure, and good crystallinity. The CdCl 2 treatment did not produce major changes, but there was a decrease in the density of planar defects. The untreated CBD CdS films had cubic structure and poorer crystallinity than the CSS films. After the CdCl 2 treatment, these films recrystallized to the hexagonal phase, resulting in better crystallinity and a lower density of planar defects. The conformal coverage and the presence of bulk oxygen are the key issues in making the CBD films more suitable for photovoltaic applications.