Ashish Bhatia

Pulsed laser processing of electrodeposited CuInSe2 photovoltaic absorber thin films

CuInSe 2 (CIS) is commercially processed using energy intensive vacuum processes such as sputtering and thermal evaporation followed by thermal annealing. In order to reduce the cost of fabricating CIS photovoltaic absorber layers we need fast and cheap processing methods. We have investigated the use of non-vacuum electrochemical deposition (ED) followed by ultra violet pulsed laser annealing (UV-PLA). We report here on the results of ns pulsed KrF irradiation of ED CIS films and ED CIS films which were first annealed in a Se atmosphere.

Continuous wave solid phase laser annealing of single-pot electrodeposited CuInSe2 thin films: Effects of Cu/In stoichiometry

CuInSe2 (CISe) is a prototype material for the I–III–VI chalcopyrites such as Cu(In,Ga)(S,Se)2 (CIGSSe) used as absorber layers in thin film photovoltaic cells. These thin film photovoltaic absorber layers are primarily synthesized by vacuum-based techniques in manufacturing. In this work, we investigate non-vacuum film synthesis by electrochemical deposition of CISe from a single bath followed by continuous wave laser annealing (CWLA) using a 1064 nm laser. We find that parameters exist for near-infrared CWLA which result in structural changes without melting and dewetting of the films. While Cu-poor samples show only about 40% reduction in the full width at half maximum (FWHM) of the respective x-ray diffraction (XRD) peaks, identically treated Cu-rich samples indicate a FWHM reduction of more than 70% in the XRD and Raman scattering data for irradiation at 50 W/cm2. This study demonstrates that an entirely solid-phase annealing path driven by intense illumination exists for chalopyrite phase formation ...

Pulsed and continuous wave solid phase laser annealing of electrodeposited CuInSe2 thin films

Cu(In,Ga)Se2 (CIGS) thin film photovoltaic absorber layers are primarily synthesized by vacuum based techniques at industrial scale. In this work, we investigate non-vacuum film synthesis by electrochemical deposition coupled with pulsed laser annealing (PLA) and or continuous wave laser annealing (CWLA) using 1064 nm laser. PLA results indicate that at high fluence (≥100 mJ/cm2) CuInSe2 films melt and dewet on both Mo and Cu substrates. In the submelt PLA regime (≤70 mJ/cm2) no change in XRD results is recorded. However CWLA at 50 W/cm2 for up to 45 s does not result in melting or dewetting of the film. XRD and Raman data indicate more than 80% reduction in full width at half maximum (FWHM) in their respective main peaks for annealing time of 15 s or more. No other secondary phases are observed in XRD or Raman spectrum. These results might help us in setting up the foundation for processing CIGS through an entirely non-vacuum process.

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.

Pulsed laser processing of electrodeposited CuInSe 2 Photovoltaic absorber thin films

In this report we investigate the effects of pulsed laser annealing (PLA) on both as-electrodeposited (ED) and electrodeposited-furnace annealed (EDA) CuInSe2 (CIS) samples by varying the laser fluence (J/cm2) and number of pulses. Results for as-ED samples indicate that liquid CIS-phase formation during PLA with 248 nm laser is to be avoided as liquid CIS dewets on Mo [1] as well as MoSe2. In the case of EDA-PLA samples, scanning electron microscopy (SEM) images suggest no apparent change in surface morphology but photoluminescence (PL) indicates change in PL yield and FWHM after PLA processing, a possible indication of annealing of defect states. The effects of PLA on defects are further explored using deep level transient spectroscopy (DLTS).

Crystallographic study of phases present in CuInSe2 absorber layers produced by laser annealing co-electrodeposited precursors

For the production of high efficiency thin film, Cu(In,Ga)Se2 solar cells, absorber layers with grain sizes of a few hundred nanometers and without detrimental secondary phases are favored. Co-electrodeposition offers a low-cost and material efficient synthesis route, where, in a single step, films containing CuInSe2 are formed. However, the material is nanocrystalline, constitutes multiple phases and has poor photovoltaic properties 1. Therefore a subsequent annealing step is required to produce absorber layers suitable for use in photovoltaic devices. Laser annealing has been demonstrated to improve crystallinity, stimulate atomic diffusion and develop opto-electronic properties when compared to the precursor 2. In this work, high resolution X-ray diffraction was used in order to assess the presence of secondary phases in the absorber layer. All diffractograms of laser annealed films exhibited an additional, unknown peak, measurable through the full depth of the material which is independent of precursor composition, annealing time or laser flux. Evaluation of literature on codeposited CuInSe2, combined with Rietveld refinement suggests a number of possible identities for this peak. The candidates in order of most likely to least likely are structural defects, In2Se3, and CuIn3Se5. We consider the impact that each of these would have on a device formed via this process and thus its success as a new manufacturing route for CuInSe2 solar cells.

Grain growth study of electrochemically deposited CuInSe2 by rapid thermal annealing in sulfur atmosphere

In order to upscale the production of thin film solar cells a cost effective and simple synthesis technique is required. Keeping this in mind we have investigated the effect of electrochemical deposition (ED) and inherently low thermal budget rapid thermal annealing (RTA) processing of CuInSe2 in sulfur atmosphere. X-ray diffraction (θ-2θ) scans indicate increased grain size and improved crystallinity after RTA of ED films. Scanning electron microscopy images (SEM) suggest changes in surface morphology after sulfur incorporation. Raman spectroscopy results and temperature dependent conductivity measurements are also discussed in the paper.

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.

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 annealing in sulfur vapor on electrodeposited CuInSe2 films

In this work we investigate the effects of annealing in sulfur vapor on electrodeposited (ED) CISe films. X-ray diffraction measurement of the as-ED samples show broad peaks in θ-2θ scans. The full width at half maximum (FWHM) of the XRD (112) peak decreases following annealing in sulfur indicating improved crystalline quality and grain growth. Raman spectroscopy shows different dominant vibrational modes for samples with low and high S contents. We have also measured the electrical properties such as J-V, impedance spectroscopy and thermal admittance spectroscopy (TAS) of the samples. The J-V and impedance characteristics suggest deterioration of electrical properties for high S content samples. TAS measurements suggest that high S content causes a distribution of trap states instead of a discrete state in the samples.