Quasi Fermi level splitting of Cu-rich and Cu-poor Cu(In,Ga)Se2 absorber layers

Abstract

The quasi Fermi level splitting is measured for Cu(In,Ga)Se2 absorber layers with different copper to (indium + gallium) ratios and for different gallium contents in the range of 20%–40%. For absorbers with a [Cu]/[In + Ga] ratio below one, the measured quasi Fermi level splitting is 120 meV higher compared to absorbers grown under copper excess independent of the gallium content, contrary to the ternary CuInSe2 where the splitting is slightly higher for absorber layers grown under copper excess. Possible explanations are the gallium gradient determined by the secondary ion mass spectrometry measurement which is less pronounced towards the surface for stoichiometric absorber layers or a fundamentally different recombination mechanism in the presence of gallium. Comparing the quasi Fermi level splitting of an absorber to the open circuit voltage of the corresponding solar cell, the difference for copper poor cells is much lower (60 meV) than that for copper rich cells (140 meV). The higher loss in V OC in the case of the Cu-rich material is attributed to tunneling enhanced recombination due to higher band bending within the space charge region.

High‐performance low bandgap thin film solar cells for tandem applications

“…All characterizations were performed after adding the ARC layer. For the extraction of the quasi‐Fermi‐level splitting (qFLS), photoluminescence (PL) spectra were recorded on CdS‐covered absorbers in a home‐built calibrated setup at room temperature under continuous monochromatic illumination with 660‐nm wavelength . The spectral and absolute corrected PL spectra are converted into energy space.…”

Silver-Doped Cu2SnS3Absorber Layers for Solar Cells Application

“…With Ag, the PL yield is always higher than pure CTS and has a maximum at Ag/(Cu + Ag) = 2%. A higher PL yield indicates larger quasiFermi level splitting, which is a measure for V oc [29]. Fig.…”

Absorption Coefficient of a Semiconductor Thin Film from Photoluminescence

“…For thin-film investigation with a calibrated PL, the spectral variation of Θ is often disregarded by simply setting Θ ¼ 1 in Eq. (13) [26,[29][30][31]. Here, we compare two approximations for Θ from the perspectives of QFLS and absorption coefficient measurement.…”

“…However, for a real PL measurement, the signal in the high absorption range is limited by noise, which depends on the experimental setup, sample, and measurement conditions. But a 2-to 3-order-of-magnitude difference between the PL peak maximum and the noise level is typically reported [31,[35][36][37][38][39][40]. Thus, random noise is introduced so that the high-energy side of the PL signal can be resolved over intensities spanning 3 orders of magnitude [the blue crosses in Fig.…”