Elizabeth A. Lund

Investigation of combinatorial coevaporated thin film Cu2ZnSnS4. I. Temperature effect, crystalline phases, morphology, and photoluminescence

Cu2ZnSnS4 is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325 °C) and high (475 °C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu2ZnSnS4-Cu2SnS3 tie line in the quasi-ternary Cu2S-ZnS-SnS2 phase system, and this effect is attributed to structural similarity between the Cu2ZnSnS4, Cu2SnS3, and ZnS crystalline phases. At 475 °C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling alon...

Investigation of combinatorial coevaporated thin film Cu2ZnSnS4 (II): Beneficial cation arrangement in Cu-rich growth

Cu2ZnSn(S,Se)4 (CZTSSe) is an earth-abundant semiconductor with potential for economical photovoltaic power generation at terawatt scales. In this work, we use Raman scattering to investigate phase coexistence in combinatorial CZTS thin films grown at 325 or 470 °C. The surface of the samples grown at 325 °C is rough except for a prominent specularly reflective band near and along the ZnS-Cu2SnS3 (CTS) tie line in the Cu-Zn-Sn-S quaternary phase diagram. All structurally incoherent secondary phases (SnS2, CuS) exist only as surface phases or are embedded as separate grains, whereas the structurally coherent secondary phase CTS coexists with CZTS in the dense underlying film. In films grown at 325 °C, which are kinetically trapped by the low growth temperature, a change is observed in Cu and Sn site occupancy, evidenced by the shift from cubic-CTS in the Cu-rich region (Cu/Sn > 2) to more tetragonal-CTS in the Sn-rich region (Cu/Sn < 2). For CZTS samples grown at 470 °C, CTS is not observed and regions gro...

Effects of 2 nd Phases, Stress, and Na at the Mo/Cu 2 ZnSnS 4 Interface

Cu 2 ZnSnS 4 (CZTS) is an alternative material to Cu(In,Ga)Se 2 (CIGSe) for use in thin film photovoltaic absorber layers composed solely of commodity elements [1,2]. Thus, if similar material quality and performance can be realized, its use would allow scale-up of terrestrial thin film photovoltaic production unhindered by material price or supply constraints. Here we report on our research on the deposition of CZTS by RF sputtering from a single CZTS target and co-sputtering from multiple binary sources on Mo-coated glass. We find some samples delaminate during post-sputtering furnace annealing in S vapor. Samples on borosilicate glass (BSG) delaminate much more frequently than those on soda-lime glass (SLG). We investigate the influences of the formation of frangible phases such as MoS 2 at the CZTS/Mo interface and residual and thermal mismatch stress on delamination. We implicate fracture in a layer of MoS 2 as the mechanism of delamination between the Mo and CZTS layers using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Wafer curvature measurements show significant (˜400 MPa) deposition stress for minimally optimized Mo deposition; however nearly stress-free Mo layers with good adhesion can be deposited using a multi-step Mo deposition recipe. Co-sputtering CZTS adds 100 MPa of stress on both BSG and SLG, however delamination is nearly absent for samples deposited on low-stress Mo layers. We investigate metallic diffusion barrier layers to prevent the formation of MoS 2 at the interface. Lastly we discuss the importance of removing Mo oxide by sputter etching before CZTS deposition and its effects on adhesion and series resistance.

Modeling Cu2ZnSnS4 (CZTS) solar cells with kesterite and stannite phase variation

Cu2ZnSnS4 (CZTS) may exhibit both kesterite and stannite polymorphs and shows promise as an absorber layer in thin film photovoltaic solar cells to be produced at terawatt scales. This study examines the effects of CZTS polymorphism and inhomogeneous distributions of CZTS polymorphs on device characteristics under scenarios of single phase films, a sinusoidal variation between kesterite and stannite with depth, and single phase films with thin layers of the other polymorph at both interfaces. In general, stannite-only devices are predicted to have higher efficiency than kesterite-only devices and sinusoidal grading results in efficiency between those of the pure phases. However, the device performance is relatively insensitive to the wavelength of the sinusoidal grading and rather is very sensitive to the phase present at the CdS interface. Predicted AM1.5 current-voltage (J-V) curves and descriptive metrics as well as wavelength-resolved quantum efficiencies are reported for all models. Based on these results, we propose control of cation ordering in CZTSSe as a mechanism for device design using bandgap grading and interface engineering without variation of stoichiometry.

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.

Investigating sputtered Cu 2 Si 1−x Sn x S 3 [CSTS] for earth abundant thin film photovoltaics

This study investigates the synthesis of chalcopyrite Cu2Si1−xSnxS3 (CSTS) thin films for photovoltaic solar cell absorber layers. Preliminary results indicate that layered sputtering of Cu, Sn, and Si followed by annealing in a sulfur atmosphere at 500°C does not provide adequate mixing or sulfur incorporation. Annealing/sulfurizing a homogeneous co-sputtered film of Cu, Sn, and S lead to CSTS formation, although low sulfur incorporation and undesired copper sulfide phase formation resolved. Sputtering from sulfide targets may lead to formation of CSTS.

Synthesis of optimized CZTS thin films for photovoltaic absorber layers by sputtering from sulfide targets and sulfurization

Cu 2 ZnSnS 4 (CZTS) is a promising alternative for Cu(In,Ga)Se 2 (CIGS) absorber layers in thin film solar cells and is comprised of commodity elements which will enable scale-up of chalcopyrite panel production unhindered by elemental supplies and costs. Various CZTS synthesis methods, especially sulfurization of stacked metal or metal sulfide layers, are being studied and have led to cell efficiencies up to 6.7% [1]. Here we report our studies of CZTS thin film synthesis via room temperature sputtering from a single CZTS target and co-sputtering from Cu 2 S, ZnS and SnS 2 binary targets, both followed by sulfurization between 500 C - 600 C using either elemental sulfur vapor or in-situ generated H 2 S. Sputtering from sulfur-containing targets is designed to increase the sulfur content in the precursor films to promote stoichiometry. We report on the effects of processing including deposition on soda-lime and borosilicate glasses and deposition of Na-containing layers on film morphology (AFM/SEM), composition (EDS), phase (XRD), grain size (XRD/EBSD), grain boundary structure (EBSD), optical (spectroscopic ellipsometry) and electrical properties. Processing conditions producing desirable Zn-rich/Cu-poor films are identified [1]. The formation of MoSe2 at Mo/CIGS interface is believed to promote Ohmic contacts, but in CZTS we associate excessive formation of frangible MoS 2 with film delamination from Mo/borosilicate glass substrates. Strategies for preventing delamination including adhesion layers are investigated and discussed. P-N junctions are formed with CdS/ZnO using chemical bath deposition and sputtering, and I-V characteristics are reported. Schottky junctions are formed and C-V measurements are used to determine the doping in the CZTS absorber layers.[1] H. Katagiri, et al., MRS Symp. Proc. 1165 1165-M04-01 (2009).

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.

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.