LUMINESCENCE INVESTIGATION OF VOLTAGE LOSSES IN CHALCOPYRITE SOLAR CELLS

Chalcopyrite-based solar cells have achieved remarkable power conversion efficiencies of up to 23.6% for absorbers with a band gap near 1.1 eV. However, this efficiency is still around 10 percentage points below the Shockley-Queisser (SQ) theoretical limit. One of the main reasons for this efficiency gap is deficit in the open circuit voltage VOC relative to the SQ-VOC limit.
The SQ predictions are entirely based on an idealized solar cell model that makes several important assumptions. One assumption is that the absorptance A(E) and external quantum efficiency QE(E) is unity for energies higher than band gap and zero in the sub-band gap energy range, meaning that A(E)/QE(E) is a step function. This implies that only photons with energies higher than the band gap are completely absorbed, contributing to the generation of electron-hole pairs, and all photogenerated electron-hole pairs are fully extracted through the contacts. Another key assumption is that charge recombination occurs exclusively through band-to-band radiative recombination mechanism, with non-radiative recombination pathways entirely absent in the device.
However, in practice, the behavior of actual devices deviates significantly from this idealized SQ assumptions. First, instead of exhibiting a sharp step-like transition at the band gap energy, the A(E) and QE(E) spectra of real devices show a gradual increase near the absorption edge, leading to spectral broadening. This broadening causes radiative VOC losses. Second, real solar cells often suffer from incomplete light absorption and charge collection inefficiencies, which further contribute to VOC losses. Finally, unlike the SQ model, along with the radiative band-to-band transition, undesirable non-radiative recombination also occurs within the actual devices, both in the bulk and at interfaces, resulting in detrimental non-radiative VOC losses.
In this thesis, simulations were conducted to evaluate the effects of band gap inhomogeneities, Urbach tails and absorber thickness on the absorption edge broadening and radiative losses. Notably, it was found that band gap distribution along with the reduced absorber thickness have significant contribution to the broadening and radiative losses, while Urbach tails only show a minor effect within the typical range of Urbach energy values observed for CIGSe absorbers.
Moreover, comprehensive experimental analyses are performed to quantitatively evaluate the VOC losses caused by all non-idealities in Cu(In,Ga)Se2 CIGSe solar cell. Two distinct approaches are employed for this purpose. The first method relies on purely optical analysis, where A(E) and photoluminescence (PL) spectra are used to evaluate quasi Fermi level splitting (∆μ) (analogious to VOC) losses of CIGSe absorbers relative to SQ limit. The second method relies on optoelectronic analysis, where VOC losses are extracted through a combination of electroluminescence (EL) and direct QE(E) measurements.
The primary focus of this work is to study the influence of the band gap gradient on the radiative losses in CIGSe solar cells. Experimental findings indicate that absorbers with a graded band gap exhibit stronger broadening at the absorption onset and suffer from 5 -16 meV additional radiative losses compared to the ungraded absorbers. These findings are further supported by a meta-analysis and a review of published data, which consistently show that the presence of a band gap gradient contributes significantly to absorption edge broadening.
This thesis also investigates the impact of alkali ions incorporation on the sub-bandgap Urbach tails and grain boundaries in CIGSe absorbers. PL measurements are used to evaluate the extent of Urbach tails, while Kelvin probe force microscopy (KPFM) is employed to extract the band bending across the grain boundaries. The main objective of these measurements is to determine whether there is any relationship between the reduction in Urbach tails and changes in band bending at grain boundaries. Consistent with previously published results, the PL results show that alkali incorporation reduces the extent of tail states. However, KPFM measurements indicate that decrease in the tail states is not correlated with the changes in the grain boundary band bending. Instead, excitation-dependent PL measurements suggest that the reduction in the Urbach tails is primarily due to the flattening of the electrostatic potential fluctuations within the material.