Dennis S. Pruzan

Temperature dependence of equivalent circuit parameters used to analyze admittance spectroscopy and application to CZTSe devices

We present a device physics and equivalent circuit model for admittance spectroscopy of CZTSe based photovoltaic devices. The experimental variations of the capacitance and conductance in the depletion width are reproduced for state of the art coevaporated CZTSe devices. We will show that simple Arrhenius analysis of the main capacitance step seen in CZTSe results in erroneous values for the dominant acceptor energy. We will also show that the bulk resistivity in the quasi-neutral region (QNR), even in the presence of the dominant acceptor freezeout, cannot account for the observed increase in series resistance which is responsible for the temperature dependent frequency shift of the capacitance step. Thus, we suggest that dopant freezeout must affect another component of the lumped series resistance such as a non-Ohmic back contact.

Effects of back contact resistance, depletion width and relaxation time distributions in admittance spectroscopy of CZTSe devices

We present an updated equivalent circuit model based in the physics of heterojunction devices and apply it in the interpretation of admittance spectroscopy from CZTSe photovoltaic devices. We investigate the effects of fundamental device parameters on capacitance-frequency profiles and show that the widely-used derivative analysis method is only accurate for secondary capacitance steps, which may not be easily distinguished from the main junction capacitance-frequency step. We investigate whether a distribution of depletion widths or other temperature dependencies in the capacitance can account for the behavior of the main capacitance step and find that only a strong temperature-dependent series resistance can explain the frequency shift of the main capacitance step. For the samples studied, this resistance appears to be non-monotonic. We discuss the validity of this result within a model of thermionic emission over a back contact having a distribution of barrier heights.

Minority carrier electron traps in CZTSSe solar cells characterized by DLTS and DLOS

We report observations of minority carrier interactions with deep levels in 6-8% efficient Cu2ZnSn(S, Se)4 (CZTSSe) devices using conventional and minority deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS). Directly observing defect interactions with minority carriers is critical to understanding the recombination impact of deep levels. In devices with Cu2ZnSn(S, Se)4 nanoparticle ink absorber layers we identify a mid-gap state capturing and emitting minority electrons. It is 590±50 meV from the conduction band mobility edge, has a concentration near 1015/cm3, and has an apparent electron capture cross section ~10-14 cm2. We conclude that, while energetically positioned nearly-ideally to be a recombination center, these defects instead act as electron traps because of a smaller hole cross-section. In CZTSe devices produced using coevaporation, we used minority carrier DLTS on traditional samples as well as ones with transparent Ohmic back contacts. These experiments demonstrate methods for unambiguously probing minority carrier/defect interactions in solar cells in order to establish direct links between defect energy level observations and minority carrier lifetimes. Furthermore, we demonstrate the use of steady-state device simulation to aid in the interpretation of DLTS results e.g. to put bounds on the complimentary carrier cross section even in the absence its direct measurement. This combined experimental and theoretical approach establishes rigorous bounds on the impact on carrier lifetime and Voc of defects observed with DLTS as opposed to, for example, assuming that all deep states act as strong recombination centers.

A method for depositing CdTe from aqueous solution

We present a novel method for depositing crystalline cadmium telluride (CdTe) thin films from aqueous solutions. The films were deposited via a spin-coating technique by direct conversion alternating layers of Cd and Te precursor solutions. Powder x-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images of the films show crystallization and preferential orientation of the grains to the (111) following a post-deposition thermal annealing in the presence of cadmium chloride (CdCl2). This result is a promising first step towards the fabrication of a CdTe-based photovoltaic device using a solution-deposited absorber layer.

Chemical bath deposition and laser annealing: A low cost fast process for depositing CdTe thin films

We demonstrate a low cost solution based process for polycrystalline CdTe deposition followed by a fast laser annealing step. A chemical bath deposition (CBD) process using a Cd(OH)2 and a Te precursor solution to form polycrystalline CdTe thin films via ion-exchange reaction. This is followed by a fast laser annealing process by a 248nm excimer laser to improve the grain size of the deposited CdTe layers. X-ray diffraction analysis shows that the CBD process favors the growth of (111) CdTe. Additionally the laser treatment improves the crystalline quality of the films as is evidenced by a decreased FWHM of the (111) XRD peaks of the laser treated samples.

Comparison and interpretation of admittance spectroscopy and deep level transient spectroscopy from co-evaporated and solution-deposited Cu 2 ZnSn(S x , Se 1−x )4 solar cells

Cu2ZnSn(S, Se)4 (CZTSe) is an earth-abundant semiconductor with potential for economical thin-film photovoltaic devices. Short minority carrier lifetimes contribute to low open circuit voltage and efficiency. Deep level defects that may contribute to lower minority carrier lifetimes in kesterites have been theoretically predicted, however very little experimental characterization of these deep defects exists. In this work we use admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) to characterize devices built using CZTSSe absorber layers deposited via both coevaporation and solution processing. AS reveals a band of widely-distributed activation energies for traps or energy barriers for transport, especially in the solution deposited case. DLTS reveals signatures of deep majority and minority traps within both types of samples.

Sub-100 nm resolution 3-D tomography of CZTSe using transmission X-ray Microscopy

Transmission X-ray Microscopy (TXM) is a powerful technique with a theoretical resolution down to 30 nm that can be used to fill the mesoscale gap between the atomic scale resolution of atom probe tomography (APT) and macroscale resolution of energy dispersive spectroscopy (EDS). For this study, thin film solar cells based on Cu2ZnSnSe4 (CZTSe) absorber layers with Zn/Sn ratios of 1.0 and 1.4 were characterized using element-specific TXM and 3-D tomographic reconstruction. The resulting data are 3-D concentration fields for Cu, Zn, Sn, and Se. From these data we analyze compositional fluctuations at a previously inaccessible combination of sampling volume and resolution.