Thomas Paul Weiss

The band gap of Cu2ZnSnSe4: Effect of order-disorder

The order-disorder transition in kesterite Cu2ZnSnSe4 (CZTSe), an interesting material for solar cell, has been investigated by spectrophotometry, photoluminescence (PL), and Raman spectroscopy. Like Cu2ZnSnS4, CZTSe is prone to disorder by Cu-Zn exchanges depending on temperature. Absorption measurements have been used to monitor the changes in band gap energy (Eg) of solar cell grade thin films as a function of the annealing temperature. We show that ordering can increase Eg by 110 meV as compared to fully disordered material. Kinetics simulations show that Eg can be used as an order parameter and the critical temperature for the CZTSe order-disorder transition is 200 ± 20 °C. On the one hand, ordering was found to increase the correlation length of the crystal. But on the other hand, except the change in Eg, ordering did not influence the PL signal of the CZTSe.

Admittance spectroscopy in kesterite solar cells: Defect signal or circuit response

Unlike Cu(In,Ga)Se2 based solar cells, Cu2ZnSn(S,Se)4 solar cells show a strong increase in series resistance with decreasing temperature. In this study we deduce the series resistance from temperature dependent current-voltage measurements on a 5.5% efficient Cu2ZnSnSe4 solar cell. By applying a simple circuit model an increasing series resistance with decreasing temperature alone results in a capacitance step within the C-f profile. We show that this step needs to be distinguished from a step caused by a defect state or a carrier freeze-out. Consequently, the deduced activation energy is strongly distorted by the circuit response.

Different Bandgaps in Cu

We present a high-temperature Cu

_2

_2

ZnSnSe

ZnSnSe

_4

_4

: A High Temperature Coevaporation Study

coevaporation study, where solar cells with a power conversion efficiency of 7.1% have been achieved. The process is monitored with laser light scattering in order to follow the incorporation of the Sn into the film. We observe the segregation of ZnSe at the Mo/CZTSe interface. Optical analysis has been carried out with photoluminescence and spectrophotometry. We observe strong band tailing and a bandgap, which is significantly lower than in other reported efficient CZTSe absorbers. The photoluminescence at room temperature is lower than the bandgap due to the existence of a large quantity of tail states. Finally, we present effects of low-temperature postannealing of the absorbers on ordering of the Cu/Zn atoms in CZTSe and solar cell parameters. We observe strong changes in all solar cell parameters upon annealing. The efficiency of the annealed devices is significantly reduced, although ordering is improved compared with ones made from nonannealed absorbers.

Direct Evaluation of Defect Distributions From Admittance Spectroscopy

Evaluating interfering capacitance steps in admittance spectroscopy for solar cell defect analysis is still a problem which needs to be solved. While the common analysis developed by Walter et al.[1] is capable of extracting defect distributions from the capacitance data, it results in erroneous defect densities in the presence of overlapping capacitance steps. We derive an expression for the capacitance step caused by defects with a density of states distributed in energy. By adding several of these defect distributions, interfering capacitance steps can be described. Thus, it is possible to fit the entire capacitance spectrum simultaneously for all temperatures. We apply the presented method to Cu2ZnSnSe4 -based solar cells with power conversion efficiencies between 5% and 7%. Comparing the obtained defect parameters with the ones obtained by the method from Walter et al. reveals that the Walter method overestimates the defect densities in the case of overlapping capacitance steps.

Detection of a MoSe2 secondary phase layer in CZTSe by spectroscopic ellipsometry

We demonstrate the application of Spectroscopic Ellipsometry (SE) for identification of secondary phase MoSe2 in polycrystalline Cu2ZnSnSe4 (CZTSe) samples. A MoSe2 reference sample was analyzed, and its optical constants (e1 and e2) were extracted by SE analysis. This dataset was implemented into an optical model for analyzing SE data from a glass/Mo/CZTSe sample containing MoSe2 at the back side of the absorber. We present results on the n and k values of CZTSe and show the extraction of the thickness of the secondary phase MoSe2 layer. Raman spectroscopy and scanning electron microscopy were applied to confirm the SE results.

Impact of annealing on electrical properties of Cu2ZnSnSe4 absorber layers

Reported growth processes for kesterite absorber layers generally rely on a sequential process including a final high temperature annealing step. However, the impact and details for this annealing process vary among literature reports and little is known on its impact on electrical properties of the absorber. We used kesterite absorber layers prepared by a high temperature co-evaporation process to explicitly study the impact of two different annealing processes. From electrical characterization it is found that the annealing process incorporates a detrimental deep defect distribution. On the other hand, the doping density could be reduced leading to a better collection and a higher short circuit current density. The activation energy of the doping acceptor was studied with admittance spectroscopy and showed Meyer–Neldel behaviour. This indicates that the entropy significantly contributes to the activation energy.