Characterization of Defects-Location in Hydrogenated Microcrystalline Silicon Thin Films and Its Influence on Solar Cell Performance

Shuichi HiZA, Akira YAMADA, and Makoto KONAGAI

 

Purpose of the study

By using both experiment and numerical analysis to study hydrogenated microcrystalline silicon (Si:H) based solar cells. And discuss the effect of grain-growth on the defect-fromation and on the performance of solar cells.

 

Methods

Intrinsic Si:H thin films of varying of thickness were prepared by HW-CVD. Then surfaces of the prepared films were observed by scanning electron microscopy (SEM) and crystallinity was estimated from Raman spectra. Numerical study was carried out using the AMPS-1D device simulator, which was based on Poison’s equation and electron and hole continuity equations.

 

Key findings

1. Each large grain in Si:H grewithout collisions with neighbors to a thickness of 50nm assuming a typical cone-shaped growth of the grain from a nucleus. In this region, the grain size increased almost linearly with the thickness.  
2. The calculated open circuit voltage Voc function of the assumed density of the Gaussian state as the gap state in numerical analysis was showed. The linear relations between Voc and the logarithm of thesumed density of the gap staes were found independent of the assumed capture cross-sections.
3. The correlation of the surface area of large grains can be greatly affected by the shape of the grains in terms of the total surface area of the large grains.  
4. Defects in Si:H material are located mainly at the boundaries between the large grains in which hundreds of nano-sized crystallites are contained. Moreover, each nano-sized crystallite which forms a large grain seems to have no defects at the surface.

Optical Gain and Stimulated Emission in Nanocrystal Quantum Dots

V. I. Klimov, A. A. Mikhailovsky, Su Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H. –J. Eisler, M. G. Bawendi.

OCTOBER 2000 VOL 290 SCIENCE

Purpose of the study

Examine the competing dynamical processes involved in optical amplification and lasing in the nanocrystal quantum dots. Demonstrate the feasibility of nanocrystal quantum dot lasers.   

Methods

This report uses femtosecond (fs) transient absorption (TA) and time-resolved PL to investigate the dynamical processes leading to buildup and decay of the optical gain. It shows that there are intrinsic mechanisms that complicate the development of stimulated emission in strongly confined QDs but do not inherently prevent it.. 

Key findings

1. In very small QDs, the spacing of the electronic states is much greater than the available thermal energy (strong confinement), which result in a lasing threshold that is temperature-insensitive at an excitation level of only one electron-hole (e-h) pair per dot on average..
2. Strong quantum confinement in nanocrystal QDs results in a large splitting of band-edge states and in an enhancement of instrinsic nonradiative Auger recombination, which dominates optical gain loss in QDs.
3. Both electron and hole intraband relaxations in nanocrystal QDs are sufficiently fast to successfully compete with the Auger effect.
4. Spectral energy can be controlled by facile manipulation of QD size and semiconductor composition.
5. The stimulated emission can only be observed if its buildup time is faster than the gain relaxation.

Useful terms: ( from http://en.wikipedia.org/wiki)

Quantum dot: a semiconductor whose excitations are confined in all three spatial dimensions.

Auger recombination: An electron and electron hole (electron-hole pair) can recombine giving up their energy to an electron in the conduction band, increasing its energy.

Interplay between Auger and Ionization Processes in Nanocrystal Quantum Dots

Robert M. Kraus, Pavlos G. Lagoudakis, Josef Muller, Andrey L. Rogach, John M. Lupton, and Jochen Feldmann, Dimitri V. Talapin and Horst Weller

J.Phys. Chem. B, Vol. 109, No.39, 2005

 

Purpose of the study

Study the interplay between Auger effects and ionization processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots. Explain that spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence decay rate on excitation density.

 

Methods

In the experiment, Time-resolved PL measurements were performed on 130-nm thick films of dispersed NQDs (average interparticle distance ~30 nm) embedded in polystyrene using time correlated single-photon counting of 1.2-ns resolution. The NQDs were excited nonresonantly with 130-fs pulses at 400nm at room temperature under vacuum conditions.

Then a systematical model is proposed in the quantized Auger regime describing these experimental observations and providing an estimate of the Auger assisted ionization rates.

 

Key findings

1.   Cooperative nonradiative effects dominate the fluorescence decay dynamics in the high-excitation-density regime. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the PL decay rate with excitation density. A kinematic analysis suggests that ionization processes, which lead to the occupation of dark, nonemissive nanocrystal states, are accelerated in the presence of Auger recombination (which occurs on much faster time scales).

2.   The distribution of NQD sizes in an ensemble impedes a quantitative interpretation of the recombination dynamics. Spectral filtering of the PL can substantially reduce the size distribution of NQDs involved in the dynamics. And Cooperative nonradiative effects, such as Auger recombination and phonon assisted tunneling dominate the PL dynamics of NQDs.

3.   Auger recombination becomes progressively faster as the number of e-h pairs N in the NQD increases.

4.   The intriguing increase in the PL decay rate above the average population of one e-h pair can be described by the limit of strong electronic confinement.

 

Useful terms: (From http://en.wikipedia.org)

Auger recombination: An electron and e-h (electron-hole pair) can recombine giving up their energy to an electron in the conduction band, increasing its energy.

Ionization: Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions.

 

Structural and Magnetic Properties of Amorphous and Nanocrystalline CoFeSiB Thin Films

Yoon, J.; Park, S.-Y.; Jo, Y.; Jung, M.-H.; You, C.-Y.; Kim, T.; Youn Hwang, J.; In Yim, H.; Roh Rhee, J.; Sun Chun, B.; Song Kim, Y.; Keun Kim, Y.

Nanotechnology, IEEE Transactions on Volume 7, Issue 4

 

Purpose of the study

Examine the two samples, amorphous Co₇₄Fe₄Si₄B₈ (denoted as sample-A) and nanocrystalline Co₇₈Fe₂Si₁₂B₈ (denoted as sample-N) thin films to study the structural, magnetic, and transport properties of CoFeSiB thin films with different Co compositions.

 

Methods

The CoFeSiB films were prepared by using a six-target dc magnetic sputtering system. Co chips were added to control Co composition in films, wich was confirmed by energy dispersive X-ray spectroscopy (EDX) and inductively coupled plasma-atomic emission spectrometry (ICP-AES). The transition from amorphous to nanocrystalline occurred at more than 8 chips. The microstructure, magnetization curves, temperature dependence and anisotropic magnetoresistance have been sequentially compared and analyzed between two samples.

 

Key findings

1.  With increasing Co composition, the structure changes from amorphous phase to a nanocrystalline Co phase surrounded by an amorphous matrix.Average size of nanocrystalline clusters is enlarged with respect to the Co composition.

2.   Due to lack of crystalline anistropy, the coercivity of sample-A is quite low.

3.   Sample-NC represents a small square hysteresis loop at low fields, a linear response of the magnetization to a higher field and a large saturation field. It can be explained qualitatively considering the three magnetic interaction terms: soft ferromagnetism of the amorphous CoFeSiB phase, rather than hard ferromagnetism of the nanocrystalline Co phase, and an antiferromagnetic interaction at the interface between the nanocrystalline and amorphous phases.

4.   Monotonic decrease in Neel temperature with increasing magnetic field manifests the antiferromagnetic exchange coupling in sample-NC.

 

Keywords (from http://en.wikipedia.org)

1.   Amorphous- no long-range order of thepositions of the atoms.

2.  Antiferromagnetic exchange coupling-the antiferromagnet or annealed in an aligning magnetic field, causing the surface atoms of the ferromagnet to align with the surface atoms of the antiferromagnet. This provides the ability to “pin” the orientation of a ferromagnetic film, which provides one of the main uses in so-called spin valves.