A model of the dependence of photovoltaic properties on effective diffusion length in polycrystalline silicon

Measurement of the minority carrier diffusion length (L) can be performed by measurement of the rate of decay of excess minority carriers with the distance (x) of an electron beam excitation source from a p-n junction or Schottky barrier junction perpendicular to the surface in an SEM. In an ideal case, the decay is exponential according to the equation, I = Ioexp(−x/L), where I is the current measured at x and Io is the maximum current measured at x=0. L can be obtained from the slope of the straight line when plotted on a semi-logarithmic scale. In reality, carriers recombine not only in the bulk but at the surface as well. The result is a non-exponential decay or a sublinear semi-logarithmic plot. The effective diffusion length (Leff) measured is shorter than the actual value. Some improvement in accuracy can be obtained by increasing the beam-energy, thereby increasing the penetration depth and reducing the percentage of carriers reaching the surface. For materials known to have a high surface recombination velocity s (cm/sec) such as GaAs and its alloys, increasing the beam energy is insufficient. Furthermore, one may find an upper limit on beam energy as the diameter of the signal generation volume approaches the device dimensions.

Download Full-text

Some Comparisons of Ag Deposits on Ge and Si(111)

MRS Proceedings ◽

10.1557/proc-280-55 ◽

1992 ◽

Vol 280 ◽

Cited By ~ 3

Author(s):

F. L. Metcalfe ◽

J. A. Venables

Keyword(s):

Crystal Growth ◽

Surface Diffusion ◽

Secondary Electron ◽

Diffusion Length ◽

Effective Diffusion ◽

Rate Equations ◽

Kinetic Rate ◽

Annealing Conditions ◽

Electron Imaging

ABSTRACTCrystal growth and surface diffusion have been studied in the Ag/Ge(lll) system using UHV-SEM based techniques, biassed secondary electron imaging (b-SEI), micro-AES and RHEED. Ag was deposited through and past a mask of holes held close to the substrate at 300<Td< 775K. Under certain conditions, the Ag patches were observed to split into two regions corresponding to the √3×√3R30° (hereafter √3) and a lower coverage 4×4 structure, each of which were easily observable using b-SEI. These patch widths were measured as a function of Td, and of annealing times at temperatures Ta, and effective diffusion coefficents extracted. The diffusion length of adatoms over the 4×4 structure is larger than that over the √3 structure. These observations are modelled using kinetic rate equations, and the results are compared with previous studies of Ag/Si(111). We find that energies characterising processes on top of the √3 layers of both systems are very similar, but that processes involved in the formation of the layers are quite different. The coverage of the √3 Ag/Ge(111) layer is close to 1 ML for all Td studied, unlike √3 Ag/Si(111). where it depends on deposition and annealing conditions.

Download Full-text

Grain Boundary Structures and Properties in Polycrystalline Silicon

MRS Proceedings ◽

10.1557/proc-5-155 ◽

1981 ◽

Vol 5 ◽

Cited By ~ 1

Author(s):

Y.S. Tsuo ◽

J.B. Milstein ◽

T. Surek

Keyword(s):

Grain Boundaries ◽

Polycrystalline Silicon ◽

Physical Vapor Deposition ◽

Growth Parameters ◽

Defect Structures ◽

Photovoltaic Properties ◽

Structures And Properties ◽

Silicon Materials ◽

Silicon Sheet ◽

Ribbon Growth

ABSTRACTThe method of preparation of polycrystalline silicon can have a strong influence on the types and distributions of grain boundaries, and thereby influence the electrical properties of devices made from such materials. Examples of methods employed in the preparation of polycrystalline silicon for solar cell applications include directional solidification (Czochralski pulling and various casting techniques), ribbon growth techniques (ribbon-to-ribbon, edgedefined film-fed growth, low-angle silicon sheet growth, edge supported pulling, silicon-on-ceramic), chemical and physical vapor deposition (CVD and PVD) on silicon and foreign substrates, recrystallization techniques (laser, electron beam), and others such as graphoepitaxy and electrodeposition. This paper reviews the important morphological features such as grain size and defect structures of the various polycrystalline silicon materials and the influence of growth parameters on these features. The effects of grain boundaries on the electrical and photovoltaic properties of various polycrystalline silicon materials will also be discussed.

Download Full-text

Molecular Multilayer Organic Solar Cells with Large Excitonic Diffusion Length

MRS Proceedings ◽

10.1557/proc-0965-s11-01 ◽

2006 ◽

Vol 965 ◽

Author(s):

Seunghyup Yoo ◽

William J Potscavage ◽

Benoit Domercq ◽

Sung-Ho Han ◽

Dean Levi ◽

...

Keyword(s):

Thin Films ◽

Solar Cells ◽

Performance Improvement ◽

Organic Solar Cells ◽

Diffusion Length ◽

Collection Efficiency ◽

Blocking Layer ◽

Photovoltaic Properties ◽

Optical Functions ◽

Carrier Collection

ABSTRACTWe report on the photovoltaic properties of organic solar cells based on pentacene and C60 thin films. A peak external quantum efficiency (EQE) of 69 % at a wavelength of λ = 668 nm is achieved upon optimization of the exciton blocking layer (EBL) thickness. Complex optical functions of pentacene films are measured as a function of wavelength by spectroscopic ellipsometry and used to analyze the EQE spectra. Detailed analysis of the EQE spectra indicate that the pentacene layers exhibit large excitonic diffusion lengths of ∼70 nm and that the performance improvement in EQE can be attributed to the influence of the thickness of the EBL layer on the carrier collection efficiency.

Download Full-text

Improved efficiency of polymer solar cells by plasmonically enhanced photon recycling

Sustainable Energy & Fuels ◽

10.1039/c9se00215d ◽

2019 ◽

Vol 3 (10) ◽

pp. 2597-2603 ◽

Cited By ~ 6

Author(s):

Seok Ho Cho ◽

Sung-Min Lee ◽

Kyung Cheol Choi

Keyword(s):

Solar Cells ◽

Gold Nanorods ◽

Diffusion Length ◽

Polymer Solar Cells ◽

Effective Diffusion ◽

Photon Radiation ◽

Simple Route ◽

Photon Recycling

A simple route to enhance the efficiency of polymer solar cells is presented by exploiting plasmonically assisted photon recycling. Embedded gold nanorods promote the photon radiation from excitons, and hence improve the effective diffusion length of excitons.

Download Full-text