Sudhajit Misra

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.

Sub-bandgap laser annealing of room temperature deposited polycrystalline CdTe

We investigate how post-deposition laser annealing can be used to improve structural and electronic quality of room-temperature deposited CdTe. We use continuous-wave, 1064 nm laser light to anneal CdTe solar cell stacks prior to back contact deposition. Sub-bandgap optical absorption measurements by photothermal deflection spectroscopy show a reduction of sub-bandgap defects due to the annealing process. Since the 1064 nm light is only partially absorbed, in situ monitoring of the transmitted light during laser annealing gives real-time information about changes in the material. These results reveal an evolution of electronic defect annealing and surface roughness modification with laser exposure time. This hypothesis is supported by electron microscopy. Two distinct annealing regimes emerge: one at low laser power where electronic defect annealing saturates after about one minute exposure and another at high power where structural defects are annealed after several minutes exposure. Temperatures reached during laser annealing are estimated by finite element modeling of the thermal transport due to heat generation from optical absorption.

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.

Near infrared laser annealing of CdTe and in-situ measurement of the evolution of structural and optical properties

The high performance of polycrystalline CdTe thin film solar cells is enabled by annealing in the presence of Cl. This process is typically carried out for tens of minutes resulting in reduction of defect states within the bandgap among other beneficial effects. In this work, we investigate laser annealing as a means of rapidly annealing CdTe using a continuous wave sub-bandgap 1064 nm laser. The partial transmission of the beam allows us to monitor the annealing process in-situ and in real time. We find that optoelectronic and structural changes occur through two distinct kinetic processes resulting in the removal of deep defects and twinned regions, respectively. A multilayer optical model including surface roughness is used to interpret both the in-situ transmission as well as ex-situ reflectivity measurements. These experiments demonstrate beneficial material changes resulting from sub-bandgap laser-driven CdCl2 treatment of CdTe in minutes, which is an important step towards accelerating the processi...

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.

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.

Near infrared laser CdCl 2 heat treatment for CdTe solar cells

The CdCl2 heat treatment (HT) is one of the most critical steps in the fabrication of high efficiency CdTe solar cells. In this study a laser based CdCl2 treatment is presented. A high power near infrared (808 nm) diode laser was used for laser annealing (LA) CdTe cells. The effect of laser power density (LPD) and annealing time on the devices was studied using photoluminescence (PL), current-voltage and spectral response measurements. PL spectra exhibited a correlation between the LPD and the defect density in CdTe. CdS thinning and narrowing of the CdTe bandgap was observed with increasing LPD and longer anneal times. All solar cell parameters, open-circuit voltage (VOC), short-circuit current (JSC), and fill-factor (FF), improved as a result of the laser annealing compared to as-deposited cells. The LPD was optimized with best cell parameters obtained to-date being: VOC= 800 mV, JSC= 23.34 mA/cm2, and FF= 71 %.The CdCl2 heat treatment (HT) is one of the most critical steps in the fabrication of high efficiency CdTe solar cells. In this study a laser based CdCl2 treatment is presented. A high power near infrared (808 nm) diode laser was used for laser annealing (LA) CdTe cells. The effect of laser power density (LPD) and annealing time on the devices was studied using photoluminescence (PL), current-voltage and spectral response measurements. PL spectra exhibited a correlation between the LPD and the defect density in CdTe. CdS thinning and narrowing of the CdTe bandgap was observed with increasing LPD and longer anneal times. All solar cell parameters, open-circuit voltage (VOC), short-circuit current (JSC), and fill-factor (FF), improved as a result of the laser annealing compared to as-deposited cells. The LPD was optimized with best cell parameters obtained to-date being: VOC= 800 mV, JSC= 23.34 mA/cm2, and FF= 71 %.

Laser annealing in thin-film chalcogenide photovoltaics

Thin-film photovoltaic (TFPV) solar cells have become a viable alternative to established crystalline silicon technologies in terms of both cost and efficiency. However, to compete with conventional electricity generation, manufacturing costs need to be further reduced. One of the bottlenecks in cost is thermal annealing of the semiconductor light-absorbing layer. The two leading chalcogenide TFPV technologies are based on polycrystalline copper indium gallium diselenide (CIGSe) or cadmium telluride (CdTe) absorber layers, 2–4 m thick, with appropriate front and back contact layers deposited on a glass substrate. When furnace annealing is used to improve the structural properties of the film, it takes a huge thermal budget to heat both the glass substrate (a poor thermal conductor) and the absorber layer. This makes the process both timeand cost-intensive. In recent years, our group has investigated the use of laser annealing in both CdTe and CIGSe. Laser annealing has the potential to increase throughput and thus reduce manufacturing costs compared with standard thermal processes, and may also unlock unique property regimes because of transient and nonequilibrium kinetics. Choosing the right wavelengths and dwell times of the incident laser makes it possible to selectively heat the TFPV device stack. This allows processing in a transient regime, which in turn enables heating to higher temperatures and thus shorter processing times than with conventional furnace annealing. In this article, we review some general principles and results from our collaborative work.1–9 For timescales longer than 1ns, heat will flow through the entire thin-film stack, but it may take 100s or more to establish a steady-state 1D temperature profile through a glass substrate. At <1ns timescales, light absorption and thus annealing can be targeted within a thin-film stack. This further reduces the thermal budget, which potentially can reduce cost and increase manufacturing throughput. Figure 1 shows the bandgaps (Egap) of common TFPV materials and lasers suitable for annealing the Figure 1. Common thin-film photovoltaic (TFPV) materials and lasers (in blue lettering) suitable for laser annealing. Al: Aluminum. Cd: Cadmium. Cu: Copper. CIGS: Copper indium gallium diselenide. Ga: Gallium. Ge: Germanium. In: Indium. Mg: Magnesium. Mn: Manganese. S: Sulfur. Se: Selenium. Sn: Tin. Te: Tellurium. Zn: Zinc. TCO: Transparent conductive oxide. FTO: Fluorine-doped tin oxide. ITO: Indium tin oxide. ZnO: Zinc oxide. KrF: Krypton fluoride. XeCl: Xenon chloride. Nd:YAG: Neodymium-doped yttrium aluminum garnet. Ti:Saph: Titanium-doped sapphire. EGAP: Bandgap of the material. a0: Lattice parameter (spacing). !: Harmonic component of laser.

Continuous-wave laser driven post deposition chlorine treatment of CdTe

Post-deposition laser processing of CdTe has potential for reducing manufacturing costs of CdTe PV modules. In this work we demonstrate the applicability of lasers to thermally processing CdTe modules by using a continuous wave Nd:YAG 1064 nm laser to drive CdCl2 treatment of CdTe. The data show that, similar to traditional annealing, laser annealing for only 15 seconds at higher than typical temperatures, the annealed CdTe displays an increase in A-center peak (1.435 eV) in photoluminescence yield. These indicate that chlorine activation of the absorber layer occurs in a mere 15 s and demonstrates the potential of accelerating the post-deposition CdCl2 treatment of CdTe by utilizing laser annealing at higher temperatures.