Direct measurement of Urbach tail and gap state absorption in CuGaSe2 thin films by photothermal deflection spectroscopy and the constant photocurrent method

Abstract

The applicability of photothermal deflection spectroscopy (PDS) and the constant photocurrent method (CPM) to chemical vapor deposited and physical vapor deposited CuGaSe2 (CGSe) thin films is investigated. Process dependent variations in direct band-gap energies, Urbach tail widths, bulk and surface defect densities of the films are given as a function of composition. Differences in the PDS and CPM absorption spectra of more than one order of magnitude are found. Using the standard a-Si theory of PDS and CPM analysis, a two-layer system consisting of a defect-rich near-surface layer and a less defective bulk layer is found in device-quality films. Optical improvements due to the Ga+Se annealing step as a second stage of the growth process result in a reduced bulk defect density while the near-surface defect density remains unchanged.

Pinning effect on the band gap modulation of crystalline BexZn1−xO alloy films grown on Al2O3(0001)

“…This is commonly associated with a decrease of the crystalline quality. 24,25 Changes in the optical band gap energies, taken from transmittance spectra, exhibited a non-linear dependence on the Be mole fraction (Figure 1b . A large increase in bowing parameters was observed as the Be concentration increased.…”

Growth of Cu2SnS3 thin films by a two-stage process: structural, microstructural and optical properties

“…The onset of fundamental absorption edge occurs at *1550 nm. The spectral transmittance is quite low at higher wavelengths (non-absorbing region) partly due to scattering losses, as the film is not so specular and partly due to absorption by film inhomogeneities resulting in band to defect type transitions [37]. In the absorbing region, the spectral transmittance does not exhibit a steep fall, and two regions R-I (*1550-1150 nm) and R-II (*1150-750 nm) with different slopes can be seen in Fig.…”

A Comprehensive Perspective on the Fabrication of CuGaSe2/Si Tandem Solar Cells

“…The other methods for CGSe deposition include electrodeposition, [ 73 ] epitaxial method, [ 74,75 ] and other vapor deposition methods. [ 76,77 ] The CGSe films formed by these methods are not yet proven to be the best in device performance. An ideal CGSe thin‐film growth demands much effort in exploring new deposition methods.…”