Helene J. Meadows

Laser processing for thin film chalcogenide photovoltaics: a review and prospectus

Abstract. We review prior and on-going works in using laser annealing (LA) techniques in the development of chalcogenide-based [CdTe and Cu(In,Ga)(S,Se)2] solar cells. LA can achieve unique processing regimes as the wavelength and pulse duration can be chosen to selectively heat particular layers of a thin film solar cell or even particular regions within a single layer. Pulsed LA, in particular, can achieve non-steady-state conditions that allow for stoichiometry control by preferential evaporation, which has been utilized in CdTe solar cells to create Ohmic back contacts. Pulsed lasers have also been used with Cu(In,Ga)(S,Se)2 to improve device performance by surface-defect annealing as well as bulk deep-defect annealing. Continuous-wave LA shows promise for use as a replacement for furnace annealing as it almost instantaneously supplies heat to the absorbing film without wasting time or energy to bring the much thicker substrate to temperature. Optimizing and utilizing such a technology would allow production lines to increase throughput and thus manufacturing capacity. Lasers have also been used to create potentially low-cost chalcogenide thin films from precursors, which is also reviewed.

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.

The importance of Se partial pressure in the laser annealing of CuInSe 2 electrodeposited precursors

One method for producing CuInSe2 (CISe) absorber layers is electrodeposition followed by annealing. Replacing the commonly used furnace annealing step with a laser can reduce annealing times by 2-3 orders of magnitude: from 30 minutes to 1 s. However, laser processing has, to date, not resulted in absorber layers which can form functioning final devices. One reason is due to Se loss during annealing even on these short timescales. We show how this Se loss is reduced by using a background partial pressure of Se (PSe) during annealing. Higher PSe results in increased grain size and drastically increased photoluminescence yield. The introduction of an elevated PSe in the laser annealing chamber enabled the fabrication of the first known CuInSe2 photovoltaic device using electrodeposition followed by laser annealing which gave 1.6% efficiency.

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.

Laser annealing of electrodeposited CuInSe2 semiconductor precursors: experiment and modeling

Laser annealing can reduce the annealing time required to form Cu(In,Ga)(S,Se)2 (CIGSe) thin films for use in thin film photovoltaics to a single second timescale, if not faster. In this work, we use microstructural characterization coupled with modeling of the optical and thermal properties to understand the laser annealing of three types of electrodeposited precursor stacks for the CIGSe parent compound CuInSe2. The precursor films are: stacked elemental layers Cu/In/Se, stacked binary selenides In2Se3/Cu2−xSe, and a single layer of coelectrodeposited Cu–In–Se. Conceptually, these stacks are ordered in terms of decreasing stored chemical and interfacial potential free energy, consideration of which predicts that the formation of large grained CuInSe2 from the stacked elemental layers would be the most exothermic and thus most rapid process. However we find that microstructural details of the electrodeposited films such as void fraction present in the stacked binary selenides dramatically alter the heat and mass flow. Additionally, modeling of the optical absorption within the elemental stacked precursor suggests extremely localized heating at the In/Se interface resulting in significant Se loss. Despite its lower chemical potential energy, the coelectrodeposited CuInSe2 precursors more uniform optical absorption of near-bandgap light coupled with its compact, low void fraction microstructure of nano-sized grains results in the most optimal recrystallization and compositional homogenization via interdiffusion. Furthermore this annealed layer formed a working device with a short circuit current density of 23 mA cm−2. This combined modeling and experimental investigation underscores the need to consider practical micro- and nanostructure-dependent properties as well as the optical absorption and not simply thermodynamics when designing accelerated two step deposition and annealing processes for compound semiconductors.