Heterostructures For Solar Cells Research Articles

Tight binding modeling of CdSe/ZnS and CdZnS/CdS II–VI heterostructures for solar cells: Role of d-orbitals

We present a semiempirical tight binding model with sp 3 d 5 basis set to investigate the alloy composition and strain effects on the electronic band structure of group II–VI heterostructures for solar cells. The model Hamiltonian includes nearest neighbor interactions and spin-orbit splitting of p-states. Bond lengths and atomic energies of cation and anion forming compound semiconductors are taken as nonlinear function of composition. Using this scheme, we investigated the composition and strain effects on electronic band structure in Cd 1 − x Zn x S/CdS and CdSe/ZnS heterostructures. We found that the sp 3 d 5 TB model accurately reproduces the band gaps and both the valence band and conduction band dispersion curves at Γ, X and L high symmetry points at the edge of Brillouin zone. A good agreement is found between model predictions and experiment for nonlinear variation of band gaps at Γ, X and L symmetry points.

Defect study on the indium-gallium alloy system of copper chalcopyrites performed on solar cell heterostructures

We report on defect spectroscopy on Cu(In,Ga)Se2 based solar cells with varying gallium content. Our investigations using deep level transient and admittance spectroscopy do not reveal a pronounced qualitative difference in the defect spectra for the case of absorbers containing indium and gallium. Therefore, we conclude that there exists no detrimental defect in the bulk material that generally prohibits one to achieve an increase in efficiency even for solar cells based on absorbers with a gallium to gallium plus indium ratio (GGI) larger than 0.3. The boundary compositions with CuInSe2 and CuGaSe2 absorbers (i.e., GGI=0 and GGI=1, respectively) show additional trap signals. Additionally, a peak width analysis was performed for two defect signals that occur for all absorber compositions. The relative peak width was found to be independent of GGI whereas the activation energies show some correlation with the gallium content of the absorber layer.

Quantum dot integration in heterostructure solar cells

Abstract InAs self-assembled quantum dots (SA-QDs) were incorporated into GaAlAs/GaAs heterostructure for solar cell applications. The structure was fabricated by molecular beam epitaxy on p-GaAs substrate. After the growth of GaAs buffer layer, multi-stacked InAs QDs were grown by self-assembly with a slow growth rate of 0.01 ML/s, which provides high dot quality and large dot size. Then, the structure was capped with n-GaAs and wide band gap n-GaAlAs was introduced. One, two or three stacks of QDs were sandwiched in the p–n heterojunction. The contribution of QDs in solar cell hetero-structure is the quantized nature and a high density of quantized states. I–V characterization was conducted in the dark and under AM1 illumination with 100 mW/cm2 light power density to confirm the solar cell performance. Photocurrent from the QDs was confirmed by spectral response measurement using a filtered light source (1.1-μm wavelength) and a tungsten halogen lamp with monochromator with standard lock-in technique. These experimental results indicate that QDs could be an effective part of solar cell heterostructure. A typical I–V characteristic of this yet-to-be-optimized solar cell, with an active area of 7.25 mm2, shows an open circuit voltage Voc of 0.7 V, a short circuit current Isc of 3.7 mA, and a fill factor FF of 0.69, leading to an efficiency η of 24.6% (active area).

An inverted a-Si:H/c-Si hetero-junction for solar energy conversion

A new structure to circumvent the problem of light absorption in dead layers (i.e., highly doped emitters) is investigated: an inversion of the conventional a-S:H/c-Si solar cell hetero-structure, i.e., illumination of the backside, the c-Si part of the junction. This has the advantage that only a small part of the excitation light reaches the a-Si:H emitter. However, in the simplest configuration the lateral collection of excess majority carriers is less easy and a higher series resistance therefore limits the current. The longer path of excess-charge carrier pairs to the junction also causes higher demands on the minority carrier lifetime than in the conventional configuration. It will be shown that a thin (70nm) Si3N4 film offers an efficient electrical passivation as an antireflective coating. The first inverted Si3N4/n c-Si/p a-Si:H solar cells were characterized by a moderate efficiency of about 11%. Most detrimental appeared to be the high series resistance of the device. Modelling enabled the causes of this effect to be localized and new ways of contacting the c-Si base decreasing the series resistance will be proposed. Also the contacting of the c-Si part of the junction will be discussed in view of an optimal absorption of the incident radiation.

Semiempirical tight binding modeling of Cd based II–VI heterostructures for solar cells

Realization of the full potentials of group II–VI heterostructures for photovoltaic device applications require reliable and precise predictive band structure models that are consistent with the fundamental principles of solid state physics. In this article, the electronic band structure of Cd based group II–VI ternary/binary heterostructures are calculated using the second nearest neighbor (2 nn) sp 3 s ⁎ semi-empirical tight binding formalism including the spin-orbit coupling. Using this scheme, we investigated the interface strain effects on band structure, such as band gaps and band offsets, in pseudomorphic Cd 1− x Zn x Te / CdTe and Cd 1− x Zn x S / CdS heterostructures for the entire composition range (0 ≤ x ≤ 1). It is gratifying to report that there is excellent agreement between the model predictions and experiment. The model should be useful in designing the group II–VI based heterostructure solar cells for optimum performance.

Comparison of amorphous/crystalline heterojunction solar cells based on n- and p-type crystalline silicon

In this work, we have investigated the difference and the potentiality of amorphous crystalline silicon (c-Si) heterojunctions based on both n-type and p-type c-Si substrate. Experimental comparison of solar cell performances realized on different doping type crystalline wafer have been evaluated and discussed. We have analyzed with the aid of a numerical model, useful for multilayer structure description, the transport mechanism and the influence of interface defect density on the behavior of both kind of solar cell heterostructures. Differences in the technological steps needed to device formation have been considered for high efficiency solar cell. Finally, an explanation of the difference in the role played by the defect density at the interface for both kind of structure has been proposed.

High-resolution work function imaging of single grains of semiconductor surfaces

The size reduction of modern electronic devices creates a growing demand for characterization tools to determine material properties on a nanometer scale. The Kelvin probe force microscope is designed to obtain laterally resolved images of the sample’s work function. Using a setup in ultrahigh vacuum, we were able to distinguish work function variations for differently oriented crystal facets of single grains on a semiconductor surface. For the tetragonal solar cell material CuGaSe2 the experiments demonstrate differences as low as 30 meV between (102) and (111) oriented surfaces and up to 255 meV between (1̄1̄2̄) and (110) surfaces. This influences the band bending of solar cell heterostructures and consequently also the solar power conversion efficiency.

Diffusions in (In,Se)–Cu(In,Ga)Se2/SnO2 thin film structures

In this article we study (In,Se)–Cu(In,Ga)Se2/SnO2 thin film heterostructures for solar cells, the light coming through SnO2. The Cu(In,Ga)Se2 layers were grown by close-spaced vapor transport and the (In,Se) layers by close-spaced sublimation. We first clear up the main results obtained in the fabrication and characterization of the samples. Then, the diffusion mechanisms appearing in these structures are studied, from secondary ion mass spectroscopy studies. Copper diffusion is probably the key of the creation of a p–n junction lying near SnO2 and responsible for the photovoltaic effect that is observed.