Yangsen Kang

High-efficiency nanostructured window GaAs solar cells.

Nanostructures have been widely used in solar cells due to their extraordinary optical properties. In most nanostructured cells, high short circuit current has been obtained due to enhanced light absorption. However, most of them suffer from lowered open circuit voltage and fill factor. One of the main challenges is formation of good junction and electrical contact. In particular, nanostructures in GaAs only have shown unsatisfactory performances (below 5% in energy conversion efficiency) which cannot match their ideal material properties and the record photovoltaic performances in industry. Here we demonstrate a completely new design for nanostructured solar cells that combines nanostructured window layer, metal mesa bar contact with small area, high quality planar junction. In this way, we not only keep the advanced optical properties of nanostructures such as broadband and wide angle antireflection, but also minimize its negative impact on electrical properties. High light absorption, efficient carrier collection, leakage elimination, and good lateral conductance can be simultaneously obtained. A nanostructured window cell using GaAs junction and AlGaAs nanocone window demonstrates 17% energy conversion efficiency and 0.982 V high open circuit voltage.

Bias-dependence of luminescent coupling efficiency in multijunction solar cells

In this work we describe the dependence of luminescent coupling efficiency on the bias-voltage in multijunction solar cells. We combine a theoretical derivation and Sentaurus simulations to show that the luminescent coupling efficiency has a significant dependence on the bias voltage, and such dependence is mainly due to the change in the ratio between radiative and non-radiative recombination currents at different bias voltage. We further show that such change is due to the variation in the recombination rate distribution in the cell. In addition to showing the necessity of including bias-dependence in luminescent coupling modeling, this work also demonstrates the importance of including a bias-dependent luminescent coupling efficiency to accurately model multijunction solar cells.

Design and fabrication of nano-pyramid GaAs solar cell

We demonstrate a genetic method to fabricate large-area nano-structure III-V solar cells with conformal epitaxial growth on pre-patterned substrate. The design, simulation, fabrication, and characterization of a nano-structure gallium arsenide (GaAs) solar cell device are presented. The optical simulation illustrates that the nano-pyramid array is able to suppress the reflection and enhance the absorption in a wide spectrum range. The IV characterization shows that the short circuit current of the nano-pyramid GaAs solar cell with 200 nm thick junction is as high as 18.5 mA/cm2, which is more than triple of the planar cell. Our results suggest this nano-structure thin film absorber could significantly reduce epitaxial growth cost and increase yield, thus provides a pathway towards high-efficiency and low-cost solar cells.

GaAs thin film nanostructure arrays for III-V solar cell applications

State of art III-V multi-junction solar cells have demonstrated a record high efficiency of 43.5%. However, these cells are only applicable to high concentration systems due to their high cost of substrates and epitaxial growth. We demonstrate thin film flexible nanostructure arrays for III-V solar cell applications. Such nanostructure arrays allow substrate recycling and much thinner epitaxial layer thus could significantly reduce the cost of traditional III-V solar cells. We fabricate the GaAs thin film nanostructure arrays by conformally growing GaAs thin film on nanostructured template followed by epitaxial lift-off. We demonstrate broadband optical absorption enhancement of a film of GaAs nanostructure arrays over a planar thin film with equal thickness. The absorption enhancement is about 300% at long wavelengths due to significant light trapping effect and about 30% at short wavelengths due to antireflection effect from tapered geometry. Optical simulation shows the physical mechanisms of the absorption enhancement. Using thin film nanostructure arrays, the III-V solar system cost could be greatly reduced, leading to low

Ultra-thin film nanostructured gallium arsenide solar cells

/W and high kW/kg flexible solar systems.

Titanium oxide electron-selective layers for contact passivation of thin-film crystalline silicon solar cells

State-of-the-art III-V cells have reached the highest energy conversion efficiency among all types of solar cells. However, these cells are not applicable to widespread terrestrial solar energy system yet due to the high cost of epitaxial growth. Ultra-thin film absorbers with advanced light management is one of the most promising solutions to drive down the cost. In this paper, we present an ultra-thin film nano-window gallium arsenide (GaAs) solar cell design. This ultrathin cell consists of a nano-structured Al0.8Ga0.2As window layer on the front side to reduce the reflection and to trap the light, and a metal reflector on the back side to further increase the light path. The 300 nm thick GaAs cell with Al0.8Ga0.2As nano-window shows a broad band absorption enhancement from visible to near infrared (NIR), achieving a spectrally averaged absorption of 94% under normal incidence. In addition, this cell shows excellent angular absorption properties, achieving over 85% spectral averaged absorption at up to 60 degree off normal incidence. Meanwhile, this structure with planar junction and nano-window has solved the issue of low fill factor and low open-circuit voltage in nano-structured GaAs solar cell. A nano-window cell with a 3 μm thick GaAs junction demonstrated an open circuit voltage of 0.9V.

Efficiency enhancement of gallium arsenide photovoltaics using solution-processed zinc oxide nanoparticle light scattering layers

In crystalline silicon (c-Si) solar cells, carrier selective contacts are among the remaining issues to be addressed in order to reach the theoretical efficiency limit. Especially in ultra-thin-film c-Si solar cells with small volumes and higher carrier concentrations, contact recombination is more critical to the overall performance. In this paper, the advantages of using TiOX as electron-selective layers for contact passivation in c-Si solar cells are analyzed. We characterize the metal/TiOX/n-Si electron-selective contact with the contact recombination factor J0c and the contact resistivity ρc for the first time. Experimental results show that both J0c and ρc decrease after the insertion of TiOX. In addition, the effect of post-deposition rapid-thermal-annealing (RTA) at different temperatures is also evaluated. The best J0c of 5.5 pA/cm2 and the lowest ρc of 13.6 mΩ·cm2 are achieved after the RTA process. This work reveals the potential of TiOX as an electron-selective layer for contact passivation to enable high-efficiency ultra-thin c-Si solar cells with a low cost.

A new electro-absorption modulator structure based on Ge/SiGe coupled quantum wells for on-chip optical interconnects

We demonstrate a high-throughput, solution-based process for subwavelength surface texturing of a III-V compound solar cell. A zinc oxide (ZnO) nanoparticle ink is spray-coated directly on top of a gallium arsenide (GaAs) solar cell. The nanostructured ZnO films have demonstrated antireflection and light scattering properties over the visible/near-infrared (NIR) spectrum. The results show a broadband spectral enhancement of the solar cell external quantum efficiency (EQE), a 16% enhancement of short circuit current, and a 10% increase in photovoltaic efficiency.

Nanostructured Dielectric Layer for Ultrathin Crystalline Silicon Solar Cells

In this paper, a novel electro-absorption modulation mechanism based on coupled-quantum-wells (CQWs) is proposed and demonstrated. Compared to a quantum-confined-stark-effect (QCSE) modulator with multiple fully decoupled single-QWs, the newly designed CQW modulator has two sub-quantum-wells partially coupled with a small barrier in between. Modulation is based on the change of electron and hole wave-function overlap in the CQWs, which requires a small bias electric field of <10 kV/cm) compared to the operation of a typical QCSE modulator which requires >50 kV/cm bias electrical field. Theoretically, the power consumption of this new CQW modulator can be lower than 20 fJ/bit and the speed can be higher than 10 Gbps, which outperforms the best Ge/SiGe QCSE modulator that has been previously demonstrated. A proof-of-concept Ge/SiGe CQW modulator based on this novel modulation mechanism was designed and fabricated. Instead of a traditional PIN diode structure, the new CQW modulator uses a PIP structure.

Luminescent coupling effects in double-junction solar cells

Nanostructures have been widely used in solar cells due to their extraordinary photon management properties. However, due to poor pn junction quality and high surface recombination velocity, typical nanostructured solar cells are not efficient compared with the traditional commercial solar cells. Here, we demonstrate a new approach to design, simulate, and fabricate whole-wafer nanostructures on dielectric layer on thin c-Si for solar cell light trapping. The optical simulation results show that the periodic nanostructure arrays on dielectric materials could suppress the reflection loss over a wide spectral range. In addition, by applying the nanostructured dielectric layer on 40 μm thin c-Si, the reflection loss is suppressed to below 5% over a wide spectra and angular range. Moreover, a c-Si solar cell with 2.9 μm ultrathin absorber layer demonstrates 32% improvement in short circuit current and 44% relative improvement in energy conversion efficiency. Our results suggest that nanostructured dielectric layer has the potential to significantly improve solar cell performance and avoid typical problems of defects and surface recombination for nanostructured solar cells, thus providing a new pathway towards realizing high-efficiency and low-cost c-Si solar cells.