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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...

The degradation of methyl-ammonium lead iodide (MAPbI3) upon exposure to air is a potentially limiting effect for large scale MAPbI3 photovoltaic production. Here, we report a systematic study on effects of air-exposure on the structural and optical properties of MAPbI3 thin films. The X-ray diffraction studies indicate a shrinking volume of MAPbI3 upon air-exposure as the material decomposes back to its precursors, methyl amine (CH3NH2) and lead iodide (PbI2). However, the photoluminescence (PL) yield and carrier lifetime measured with time-resolved photoluminescence, show an increasing trend upon air-exposure. These phenomena can be explained by self-passivation of MAPbI3 grains by PbI2 layer that reduces the number of non-radiative recombination centres at the grain boundaries. However, this process is not self-limiting and it eventually leads to a film that has completely reverted back to its precursor state. It is shown that this conversion of MAPbI3 film back to its precursors is also accelerated by exposure to laser illumination. Furthermore, we report unusual variation of PL intensity on a shorter time scale of a few seconds in all the films used for the experiment. The variations are found to follow different trends in the encapsulated samples as compared to the un-encapsulated samples. We propose that the decomposition followed by the ionic diffusion through film is responsible for such unusual behaviours.

We report the growth of p -type As-doped, Cd-rich CdTe single crystals using metallic Cd as the solvent in the traveling-heater method. We investigate the growth process from Cd solution in terms of the solid-liquid interface shape and the effects of As incorporation on p -type doping. The resulting CdTe crystals have Cd-rich composition which enhances p -type doping. The As doping efficacy was measured for As concentrations by the combination of inductively coupled plasma mass spectrometry, capacitance-voltage measurements. The p -type doping concentration varied from 6 × 10 15 to 8 × 10 16 cm −3 with increasing As concentration, with an apparent doping limit just below 10 17 cm −3 .

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 precursor's 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.

The effect of Indium (In) doping on CdTe thin film solar cells was investigated. CdTe thin films were deposited using the elemental vapor transport (EVT) technique under various Cd/Te gas phase ratios and In vapor concentrations. TCO/Buffer/n-CdTe/p-ZnTe solar cells have been fabricated and characterized. Indium doping has been found to affect the device performance. Capacitance-voltage (C-V) measurements revealed that net n-type doping increased with In concentration during the EVT deposition. Spectral response (SR) measurements indicated an enhanced carrier collection at higher In concentrations. The open-circuit voltage ($\text{V}_{\mathbf {OC}}$) and fill factor (FF) of the devices was found to increase with increased n-type doping and therefore builtin voltage. To-date n-type CdTe solar cells yielded devices with $\text{V}_{\mathbf {OC}} \quad =$ 730 mV, $\text{J}_{\mathbf {SC}} \quad =$ 20 mA/cm$^{\mathbf {2}} {, \mathbf {FF}} =$textbf 62 % and efficiency near 9 %.

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.

The CdTe solar cell back contact interface gets activated by means of thermal annealing. Depending on the back contact (BC) material the annealing time can vary between 20 - 60 minutes. In this study fast contact anneal times - $< 90$ sees - are investigated using a 60 Watt dual diode 808nm laser. Two types of back contacts are used: (a) Cu-Graphite, and (b) MoN/Mo-Cu. Laser power density (LPD) and Cu concentration were varied to study their effect on device performance. Capacitance-voltage characteristics revealed a correlation between LPD and the net doping concentration in CdTe. Higher Cu concentration was found to be detrimental to the device performance, possibly due to excess Cu reaching the CdTe/CdS interface as indicated by secondary ion mass spectroscopy (SIMS) measurements. Solar cells exhibited open-circuit voltage (Voc), short-circuit current (Jsc), and fill-factor (FF) similar to the baseline thermally anneal devices. To-date the LPD was optimized with best cell parameters being: $\mathrm{V_{oc}} ={830} \ \mathrm{mV}, \ \mathrm{J_{sc}} ={22} \ \mathrm{mA/cm}^{2}$, and $\mathrm{FF}=70\%$, for laser anneal time of 90 seconds.

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 %.

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 (CdCl 2 ). This result is a promising first step towards the fabrication of a CdTe-based photovoltaic device using a solution-deposited absorber layer.

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.

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.

One method for producing CuInSe 2 (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 (P Se ) during annealing. Higher P Se results in increased grain size and drastically increased photoluminescence yield. The introduction of an elevated P Se in the laser annealing chamber enabled the fabrication of the first known CuInSe 2 photovoltaic device using electrodeposition followed by laser annealing which gave 1.6% efficiency.

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.