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Recent advances in design and processing technology have made possible commercialization of polycrystalline (px)-CdTe as a photovoltaic absorber. Grain boundaries (GBs) are the most prominent struc...

We discuss the optoelectronic property variation between grains and grain boundaries of CdTe polycrystalline thin films using a confocal microscopy system. Single-photon photoluminescence (PL) and time-resolved photoluminescence spectroscopy (PL) is used to map $10 \times 10 \mu \mathrm {m}^{2}$ area at the back surface of CdTe with an optical resolution of 104 nm. TRPL maps show that different grain boundaries have different near-surface lifetimes. Surprisingly, grain-boundaries with high near-surface lifetime are associated with regions of the sample that have low PL yield. This study demonstrates the potential of confocal PL and TRPL mapping to understand carrier lifetime variations in thin films.

One industrially relevant fabrication method for CuInSe2 absorber layers begins with electrodeposition, which is highly resource efficient and easily up-scalable, followed by a high temperature annealing step. Commonly a furnace is used for annealing, although it is possible to reduce the duration of this step by 2–3 orders of magnitude using laser heating. Past work demonstrated that rastering a 1064 nm, continuous wave, neodymium-doped yttrium aluminium garnet laser beam over an electrodeposited precursor film with 1 s dwell time, promoted CuInSe2 formation, grain growth, and gave a photovoltaic device with 1.6% power conversion efficiency. However, this device showed inhomogeneous current collection correlating approximately to the Gaussian flux profile of the laser beam. This work demonstrates how deviations in the incident laser flux on the precursor lead to local temperature variation in the film. The temperature affects the rates of atomic diffusion, grain growth and chemical reactions, with the hottest region having the largest crystallites and a stoichiometric and constant composition through the film depth. Spatially resolved photoluminescence yield shows a positive correlation to the temperature profile caused by the Gaussian flux. It is expected that with a homogeneous laser beam flux, spatial variations in absorber layer properties would be eliminated, leading to uniform device properties.

Photon absorption is a critical process for photovoltaic operation, yet the effect of illumination on semiconductor growth and processing is rarely investigated. In this work we investigate the effect of illumination during the cadmium chloride (CdCl2) annealing treatment of cadmium telluride (CdTe) thin films for photovoltaic applications while keeping the sample temperature constant. Illumination at 808 nm with varying intensity (0, 0.5, 1 and 1.5 W) when incident on CdTe during thermal CdCl2 treatment of CdTe lead to enhancement of grain size and optoelectronic properties of CdTe. However photoassisted etching of the CdTe surface is also observed at illuminations sufficiently low that the sample temperature was barely affected during annealing. Additionally, annealing under high illumination sufficient to drive CdCl2 annealing in the absence of a furnace has no such effect on the CdTe surface. We attempt to reconcile these observations and provide rationale for further experiments.

General rules to reduce models of polycrystalline photovoltaic materials such as amorphous silicon (a-Si), cadmium telluride (CdTe), and Cu(In, Ga)Se 2 (CIGS) to 1D have been sought for decades. However, a key limitation associated with the 1D simulation of thin-film photovoltaics is the absence of grain boundaries (GBs), which are commonly found in polycrystalline solar cells. Depending on the orientation of growth, GBs are categorized as columnar GBs (roughly perpendicular to incident sunlight) and horizontal GBs (perpendicular to incident sunlight). In this work, we investigate whether polycrystalline grain materials can be appropriately treated as an effective medium, using 1D and 2D simulations of CdTe solar cells that are CSS grown and Br:MeOH+CdCl 2 treated. It is found that the grain boundary interface recombination has a marked effect in degrading Voc, which is consistently difficult to capture in a 1D model of an effective medium. In particular, we show that if the initial I-V curve of polycrystalline CdTe does not exhibit a low Schottky barrier, then Br:MeOH-etching the back will not necessarily improve device performance.

The CdCl 2 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 CdCl 2 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: V OC = 800 mV, J SC = 23.34 mA/cm2, and FF= 71 %.

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.

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 high performance of polycrystalline CdTethin 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-situreflectivitymeasurements. 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 processing of the CdTe absorber layer.

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.