Absorption In Crystalline Silicon Research Articles

Impact of micro-texturization on hybrid micro/nano-textured surface for enhanced broadband light absorption in crystalline silicon for application in photovoltaics

This paper reports investigation on hybrid micro/nanotextures for enhanced broadband absorption in crystalline silicon(c-Si) for application in photovoltaics. Microscale pyramids are fabricated using sodium hydroxide (NaOH) solution, etched at different durations. Nanowires on pyramids are formed using metal-assisted chemical etching (MACE). With 30 min of NaOH etching, pyramids with 3–5 μm heights (3–7 μm widths) are formed. The pyramids reduce broadband reflection of the c-Si through enhanced scattering. After MACE, nanowires with 450–600 nm heights (30–40 nm widths) are obtained. With the nanowires, weighted average reflection (WAR) for all samples reduces significantly, owing to enhanced light coupling by the nanowires and increased light scattering by the pyramids. The sample with 30 min NaOH etching and MACE demonstrates WAR of 7.5%, which is the lowest broadband reflection achieved in this work. This corresponds to potential Jsc(max) of 38.7 mA/cm2, or 13.5% enhancement compared to the Jsc(max) of the pyramids alone.

Optimized n-type amorphous silicon window layers via hydrogen dilution for silicon heterojunction solar cells by catalytic chemical vapor deposition

A comprehensive study of the microstructures and properties of n-type hydrogenated amorphous silicon (n-a-Si:H) films, deposited by catalytic chemical vapor deposition, for the window layers of silicon heterojunction (SHJ) solar cells is presented. With increasing hydrogen-to-silane dilution ratio (RH), the deposited films first become dense, after which they loosen. With further increases in RH, the films tend to crystallize with native post-oxidization. The doping efficiencies of phosphorus in the various n-a-Si:H films are similar, but the upper surface doping levels of the films are affected by RH. The post-oxidized n-a-Si:H film is more transparent at short wavelengths than a dense film deposited at low RH, exhibiting an external quantum efficiency gain of 20% at 300 nm. Finally, a higher efficiency and short-circuit current density (Jsc) are obtained with the post-oxidized n-type a-Si:H window layer; a Jsc gain of 0.25 mA/cm2 and an efficiency increase of 0.36% were achieved for the optimized SHJ solar cell. At the device level, a dense intrinsic a-Si-H passivated layer is beneficial for suppressing fill-factor (FF) deterioration. The natively post-oxidized n-a-Si:H window layer is a potential choice for improving Jsc by apparently enhancing light absorption in crystalline silicon at short wavelengths.

Experimental Determination of the Uncertainty of the Absorption Coefficient of Crystalline Silicon

Based on a combined analysis of spectroscopic ellipsometry, reflectance and transmittance measurements as well as spectrally resolved luminescence measurements and spectral responsivity measurements, we present data of the coefficient of band-to-band absorption of crystalline silicon at 295K in the wavelength range 250 – 1450nm. A systematic measurement uncertainty analysis according to the “Guide to the Expression of Uncertainty in Measurements” (GUM) is carried out for each method, showing that the relative uncertainty of the absorption coefficient data so determined is of the order of 0.3% at 300nm, 1% at 900nm, 10% at 1200nm and 180% at 1450nm. The data are consolidated by comparison of measurements carried out independently at different institutions. The uncertainty of solar cell energy conversion predictions by means of simulations due to the uncertainty of the absorption coefficient data is shown to be of the order of 0.1% relative.

Uncertainty analysis for the coefficient of band-to-band absorption of crystalline silicon

We analyze the uncertainty of the coefficient of band-to-band absorption of crystalline silicon. For this purpose, we determine the absorption coefficient at room temperature (295 K) in the wavelength range from 250 to 1450 nm using four different measurement methods. The data presented in this work derive from spectroscopic ellipsometry, measurements of reflectance and transmittance, spectrally resolved luminescence measurements and spectral responsivity measurements. A systematic measurement uncertainty analysis based on the Guide to the expression of uncertainty in measurement (GUM) as well as an extensive characterization of the measurement setups are carried out for all methods. We determine relative uncertainties of the absorption coefficient of 0.4% at 250 nm, 11% at 600 nm, 1.4% at 1000 nm, 12% at 1200 nm and 180% at 1450 nm. The data are consolidated by intercomparison of results obtained at different institutions and using different measurement approaches.

Uncertainty of the coefficient of band-to-band absorption of crystalline silicon at near-infrared wavelengths

We present data of the coefficient of band-to-band absorption of crystalline silicon at 295 K in the wavelength range from 950 to 1350 nm and analyze its uncertainty. The data is obtained from measurements of reflectance and transmittance as well as spectrally resolved photoluminescence measurements and spectral response measurements. A rigorous measurement uncertainty analysis based on an extensive characterization of our setups is carried out. We determine relative uncertainties of 4% at 1000 nm, increasing to 22% at 1200 nm and 160% at 1300 nm, and show that all methods yield comparable results.

Electrical and optical characterization of crystalline silicon/porous silicon heterojunctions

We have investigated the photovoltage and photocurrent spectra of crystalline silicon/porous silicon heterojunctions. The porous silicon layers were prepared using anodic etching of p-type crystalline silicon at a current density of 25 mA/cm 2. From the spectral dependence of the photovoltage and photocurrent, we suggest that the photovoltaic properties of the junction are dominated by absorption in crystalline silicon only. We have also studied the effect of increase in the thickness of porous silicon layers on these spectra. We find that the open-circuit voltage of the devices increases, but the short-circuit current decreases with an increase in the thickness of the porous silicon layers. We propose a qualitative explanation for this trend, based on the increase in the series and the shunt resistance of these devices. The effect of hydrogen passivation on the junction properties by exposing the devices to hydrogen plasma is also reported.

Absorption coefficient of silicon: An assessment of measurements and the simulation of temperature variation

The absorption coefficient of crystalline silicon is an important input for designing solar cells and extracting recombination parameters from device measurements. Since many contradicting measurements have been published, an assessment of data is given based on device measurements and on the discussion of experiments in the literature. An absorption coefficient for the ultraviolet to infrared spectral range is proposed, based on the results of three groups. These data can be described semiempirically by the theory of direct and indirect band transitions. This formulation enables the determination of the optical absorption in crystalline silicon at an arbitrary temperature. The interpretation and design of solar cells for operation at temperatures above room temperature is now possible.

Effect of nonequilibrium charge carriers on picosecond pulse absorption in crystalline silicon

Near band-edge light-induced absorption of picosecond pulses in crystalline silicon is studied. It is shown that along with free-carriers absorption, one should take into account also the interband absorption nonlinearity due to the band filling and the screening. The developed theoretical model fits well the experimental results.

Orientational disorder in amorphous silicon probed by XANES (X-ray Absorption near Edge Structure)

The difference between the K-edges XANES (X-ray Absorption Near Edge Structure) spectra of crystal and amorphous silicon is discussed. We show that the multiple scattering signal gives a large contribution to the total absorption in crystalline silicon in an energy range of about 70eV. The double scattering term probing the triplet distribution function gives the major contribution to the XANES. We show that the difference between the crystalline and amorphous absorption spectra is due mainly to the crystalline multiple scattering signal which is quenched in the amorphous spectrum. The suppression of the signal due to the triplet distribution function in the amorphous silicon is assigned to the large orientational disorder.