The path towards a high-performance solution-processed kesterite solar cell ☆

CdS/CdTe/Au thin film solar cells have been fabricated on different transparent conducting oxide (TCO) substrates/front contacts to study the influence of these different TCOs on the performance of the devices. The TCOs used were ZnO, ZnO:Al and SnO2:F. Under dark condition, all three device structures of the type glass/TCO/n-CdS/n-CdTe/Au n–n heterojunction + Schottky barrier, show interesting rectifying behaviors with rectification factors (RF) in the range (102.5—105.0), Schottky barrier heights (Φ
B) greater than (0.69–0.81) eV, diode ideality factors (n) in the range (1.85–2.12), reverse saturation current densities (J
0) in the range (3.18 × 10−6–3.18 × 10−8) A cm−2, series resistances (R
s) in the range (507–1114) Ω and shunt resistances (R
sh) in the range (0.84–271) MΩ. The device structures glass/SnO2:F/n-CdS/n-CdTe/Au and glass/FTO/ZnO:Al/n-CdS/n-CdTe/Au show the best performance with equal J
0 of 3.18 × 10−8 A cm−2, equal Φ
B > 0.81 eV, RF of 104.9 and 105.0, n value of 2.01 and 2.12, R
s of 615 Ω and 507 Ω and R
sh of 197 and 271 MΩ respectively. The device structure with ZnO shows the least performance. Under AM1.5 illumination, the device structure glass/SnO2:F/n-CdS/n-CdTe/Au shows the best solar cell performance with open-circuit voltage of 630 mV, short-circuit current density of 23.5 mAcm−2, fill factor of 0.44 and conversion efficiency of 6.5%, and is followed by the device structure with ZnO:Al showing a conversion efficiency of 6.0%. Suggested energy band diagrams of the devices as well as possible reasons for the observed trends in performance are presented and discussed.

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An investigation of the influence of different transparent conducting oxide substrates/front contacts on the performance of CdS/CdTe thin-film solar cells

DYNAMICS OF VISCOELASTIC COLLOIDAL SUSPENSIONS Dipl. Phys. Rick Dannert University of Luxembourg, 2016 Supervisor: Prof. Dr. Roland Sanctuary Co-Supervisor: Dr. Jorg Baller The influence of different types of nanoparticles on the dynamics of glass forming matrices has been studied by small oscillatory shear rheology. Experimental measurements reveal that besides the glass transition process of the matrix an additional relaxation process occurs in presence of nanoparticles. The latter is identified as the macroscopic signature of the microscopic temporal fluctuations of the intrinsic stress and is called Brownian relaxation. Besides the fact that Brownian relaxation has so far not been observed in colloidal suspensions with a matrix exhibiting viscoelastic behaviour in the frequency range of the experimental probe, the study reveals another important feature to be highlighted: the evolution of the Brownian relaxation times depends non-monotonously on the filler concentration. This finding challenges the use of the classical Peclet-time as a characteristic timescale for Brownian relaxation. Literature defines the Peclet-time as the specific time needed by a particle to cover –via self-diffusiona distance comparable to its own size. As a main result it will be shown that after replacing the particle size which is relevant for the Peclet time by the mean interparticle distance depending on the filler content the non-monotonic evolution of the relaxation times can be fully described. Moreover, the introduction of the new characteristic length scale allows to include data from literature into the phenomenological description.

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Docteur de l'université du luxembourg

Ultrathin tunnel oxides (SiOx) were grown on silicon, pretreated with 5% hydrofluoric acid, using high pressure, low temperature oxidation. These oxides were characterized using variable illumination current-voltage (Voc - Jsc) measurements on semi-transparent metal gate MIS diodes. The open circuit voltage (Voc), short circuit current (Jsc), ideality factor (n) and reverse saturation current (Jo) are studied as a function of oxidation time. The interface state density Dits for the HF treated sample was found to decrease from 2 X 1012 cm-2eV-1 to 9.21 X 1011 cm-2eV-1. Highly reproducible, good quality ultrathin oxides were obtained by pre-treatment of the wafer in hydrofluoric acid.

Ultrathin oxide (SiOx) grown on HF-treated silicon

An experimental study has been made of metal-silicon solar cells, with thin Ag, Au, Cu, Fe and In electrodes. No intentional interfacial layers were introduced and the silicon surfaces were chemically prepared in such a way as to minimise the residual oxide layer. The characteristics of the devices were taken at various illumination levels and the effect of barrier height, series resistance and n-value on the open-circuit voltage and the fill factor were studied. Comparison between theoretical predictions and the experimental results show that Voc/Jsc measurements that provide the n-values appropriate for the expression for the open-circuit voltage also provide a reliable method for experimental barrier-height determination under illumination and that the above ‘true’ n-value should also be used in fill-factor calculations.

P. Panayotatos

Papers

Use of VOC/JSC measurements for determination of barrier height under illumination and for fill-factor calculations in Schottky-barrier solar cells