Luminescent coupling effects are considered crucial for the performance of multijunction solar cells. We report a novel approach based on small signal measurement, which can directly measure the luminescent coupling efficiency of a multijunction solar cell with different voltage bias. In addition, this method demonstrated the light and voltage dependence of the coupling efficiency, and can potentially lead to a deeper understanding of luminescent coupling effects as well as more effective design of multijunction solar cells.
For solar cells composed of direct bandgap semiconductors such as GaAs, the performance can be significantly improved by utilizing photon recycling and luminescence coupling effects. Accurate modeling with those effects may offer insightful guidance in designing such devices. Previous research has demonstrated different numerical models on photon recycling and luminescent coupling. However, most of those works are based on complicated theoretical derivation and idealized assumptions, which made them hard to implement. In addition, very few works provide method to model both photon recycling and luminescent coupling effects. In this paper, we demonstrate an easy-to-implement but accurate numerical model to simulate those effects in multijunction solar cells. Our numerical model can be incorporated into commonly used equivalent circuit model with high accuracy. The simulation results were compared with experimental data and exhibit good consistency. Our numerical simulation is based on a self-consistent optical-electrical model that includes non-ideal losses in both the single junction and the tandem device. Based on the numerical analysis, we modified the two-diode circuit model by introducing additional current-control-current sources to represent the effects of both photon recycling and luminescence coupling. The effects of photon recycling on the diode equation have been investigated based on detailed-balanced model, accounting for internal optical losses. We also showed the practical limit of performance enhancement of photon recycling and luminescent coupling effects. This work will potentially facilitate the accurate simulation of solar cell with non-ideal effects, and provide more efficient tools for multijunction solar cell design and optimization.
KEYWORDS: Thin film solar cells, Thin films, Solar cells, Oxides, Titanium, Crystals, Silicon solar cells, Silicon, Energy conversion efficiency, Oxygen, Atomic layer deposition
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.
Although the development of a monolithically-integrated, silicon-compatible light source has been traditionally
limited by the indirect band gaps of Group IV materials, germanium-tin (Ge1-xSnx) is predicted to exhibit direct
band gap behavior. In pseudomorphic conditions with materials of smaller lattice constant, the accumulation
of compressive strain in Ge1-xSnx counteracts this behavior to prevent the direct band gap transition. One
possible approach to compensate for this compressive strain is to introduce tensile strain into the system, which
can be achieved by applying an external stressing agent to post-fabricated devices. We describe a suspended
Ge0:922Sn0:078 multiple quantum well microdisk resonator cavity strained by 140 nm of highly compressively stressed
silicon nitride. Raman shifts and photoluminescence redshifts indicate that an additional 0.23-0.30%
strain can be induced in these microdisks with this approach. The ability to tune the optical performance of
these resonator structures by strain engineering has the potential to enable the development of low threshold
Ge1-xSnx-based lasers on Si.
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.
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.
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 $/W and high kW/kg flexible
solar systems.
CMOS image sensors are now widely used in digital imaging systems as pixel size has steadily decreased to allow higher-resolution imaging. When the pixel size scales below 2 &mgr;m, however, microlens performance is significantly affected by diffraction from the edges of the image sensor pixel. This results not only in quantitative performance degradation, but also in a qualitative shift in functionality. We perform a systematic analysis of microlens design during lateral scaling of CMOS image sensor pixels. The optical efficiency and optical crosstalk are calculated with a first-principles finite-difference time-domain (FDTD) method. We find that there are two regimes of operation for three-metal-layer pixels depending on pixel size and wavelength: a refraction-dominated regime for pixel sizes larger than 1.45 &mgr;m and a diffraction-dominated regime for pixel sizes smaller than 1.45 &mgr;m. In the refraction-dominated regime, the microlens can be designed and optimized to perform its concentrating function. In the diffraction-dominated regime, the optimal radii of curvature for microlenses are very large and a flat microlens layer, in fact, becomes the best choice and performance is severely degraded. Under these circumstances, the microlens no longer fulfills its optical function as a focusing element. To extend the functionality of the microlens beyond the 1.45 &mgr;m node, we predict that a one-metal-layer dielectric stack or shorter will be required.
Recent efforts in CMOS image sensor design have focused on reducing pixel size to increase resolution given a fixed package size. This scaling comes at a cost, as less light is incident on each pixel, potentially leading to poor image quality caused by photon shot noise. One solution to this problem is to allow the imaging or objective lens to capture more light by decreasing its f-number. The larger cone of accepted light resulting from a lower f-number, however, can lead to decreased optical efficiency and increased spatial optical crosstalk at the pixel level when the microlens is not able to properly focus the incident light. In this work, we investigate the effects of imaging lens f-number on sub-2µm CMOS image sensor pixel performance. The pixel is considered as an optical system with an f-number, defined as the ratio of the pixel height to width, and we predict the performance of a realistic pixel structure subject to illumination from an objective lens. For our predictions, we use finite-difference time-domain (FDTD) simulation with continuous-wave, diffraction-limited illumination characterized by the f-number of the imaging lens. The imaging lens f-numbers are chosen to maintain resolution and incident optical power as pixel size scales, while the pixel f-number is varied by modifying the height of the pixel structure. As long as pixel f-number is scaled to match the imaging f-number when pixel size is scaled, optical efficiency and crosstalk for on-axis illumination will not be significantly affected down to the 1.2 &mgr;m pixel node. We find the same trend for system MTF, which does not seem to suffer from diffraction effects.
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