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See detailOptical investigation of voltage losses in high-efficiency Cu(In,Ga)Se2 thin-film solar cells
Wolter, Max UL

Doctoral thesis (2019)

The increases in power conversion efficiencies up to 23.35 % in thin-film Cu(In,Ga)Se2 (CIGS) solar cells in recent years can mainly be ascribed to the alkali post-deposition treatment (PDT). The latter ... [more ▼]

The increases in power conversion efficiencies up to 23.35 % in thin-film Cu(In,Ga)Se2 (CIGS) solar cells in recent years can mainly be ascribed to the alkali post-deposition treatment (PDT). The latter consists of an additional treatment step after absorber growth where alkali elements, such as sodium (Na) or rubidium (Rb), are injected into the absorber. While the beneficial effects of the alkali PDT, attributed partly to a reduction of voltage losses, are undeniable, it is not yet entirely clear what underlying mechanisms are responsible. To clarify the specific influence of the alkali PDT on the voltage of the CIGS solar cells, photoluminescence (PL) spectroscopy experiments were conducted on state-of-the-art CIGS absorbers having undergone different alkali PDTs. Photoluminescence allows the investigation of possible voltage losses on the absorbers through the analysis of optoelectronic quantities such as the absorption coefficient, the quasi-Fermi level splitting (QFLS), electronic defects, and potential fluctuations. Mainly due to a smooth surface and a band gap minimum inside the bulk, the PL spectra of state-of-the-art CIGS absorbers are distorted by interference fringes. To remove the interference fringes at room temperature, an experimental method, which revolves around the measurement of PL under varying angles, is developed in this thesis. In addition, to enable PL experiments even at low temperatures, an auxiliary polystyrene-based scattering layer is conceptualized and deposited on the surface of the absorbers. With the influence of the interference fringes under control, the quasi-Fermi level splitting can be measured on bare and CdS-covered absorbers. The results reveal an improvement of the QFLS in absorbers that contain Na with an additional increase being recorded in absorbers that also contain Rb. The improvement of the QFLS is present in both bare and CdS-covered absorbers, indicating that the beneficial effect of the alkali PDT is not only occurring on the surface but also inside the bulk. To identify possible origins of the QFLS increase, various PL-based experiments were performed. At room temperature, spatially-resolved PL measurements on the microscopic scale do not reveal any optoelectronic inhomogeneities in state-of-the-art CIGS absorbers. Defect spectroscopy at low temperatures also does not reveal the presence of deep-level trap states. Through temperature- and excitation-dependent PL experiments, a reduction of electrostatic potential fluctuations is observed in absorbers that contain Na with a stronger reduction witnessed in absorbers that contain Rb as well. The extraction of the absorption coefficient through PL measurements at room temperature reveals a reduction of band tails with alkali PDT that empirically correlates to the measured increase in the QFLS. This correlation might indicate that the band tails, through non-radiative recombination, may be the origin of the performance-limiting voltage losses. In combination with reports from literature, it is suggested that the beneficial effect of the light alkali PDT (Na) is mainly a doping effect i.e. an increase in the QFLS through an increase in the hole carrier concentration. The beneficial effect of the heavier alkali PDT (Rb) is attributed partly to a surface effect but mainly to a grain boundary effect, either through a reduction in band bending or a reduction of non-radiative recombination through tail states. Finally, the various voltage losses in state-of-the-art CIGS solar cells are compared to the best crystalline silicon device, revealing almost identical losses. This shows that the alkali PDT enables the fabrication of high-efficiency CIGS solar cells that show, in terms of voltage, identical performance. To bridge the gap between CIGS and the even better performing GaAs, the results of this thesis suggest that grain boundaries are crucial in this endeavour. [less ▲]

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See detailTime-resolved photoluminescence on double graded Cu(In,Ga)Se2 – Impact of front surface recombination and its temperature dependence
Weiss, Thomas UL; Carron, Romain; Wolter, Max UL et al

in Science and Technology of Advanced Materials (2019), 20

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See detailInfluence of Sodium and Rubidium Postdeposition Treatment on the Quasi-Fermi Level Splitting of Cu(In,Ga)Se2 Thin Films
Wolter, Max UL; Bissig, Benjamin; Avancini, Enrico et al

in IEEE Journal of Photovoltaics (2018)

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See detailCorrecting for interference effects in the photoluminescence of Cu(In,Ga)Se2 thin films
Wolter, Max UL; Bissig, Benjamin; Reinhard, Patrick et al

in Physica Status Solidi C. Current Topics in Solid State Physics (2017), 14, no 6

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