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Bastola, Ebin

Publications and source records attributed to Bastola, Ebin.

7 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Comprehensive Study of Carrier Recombination in High‐Efficiency CdTe Solar Cells Using Transient Photovoltage

Cadmium telluride (CdTe) solar cells represent a commercially successful photovoltaic technology, with an annual production capacity approaching 20 GW. However, improving the open-circuit voltage (V OC ) remains challenging. This study aims to deepen the understanding of charge carrier recombination in CdTe solar cells and to explore alternative dynamical characterization methods that address the limitations found in conventionally used time-resolved photoluminescence for CdTe solar cells. Transient photovoltage and transient photocurrent techniques are utilized to investigate charge carrier dynamics under conditions resembling real-world solar cell operation. The results reveal that an effective nonradiative recombination lifetime of 580 ns dominates the charge dynamics at VOC values below 850 mV. Above this threshold, radiative recombination becomes significant, with a radiative recombination coefficient of 1.1 × 10 −9 cm 3 s −1 . Additionally, the stationary charge carrier density at 1 sun is determined to be around 1 × 10 14 cm −3 . By accurately determining both radiative and nonradiative recombination, this work provides a comprehensive understanding of carrier dynamics in high-performing CdTe devices and paves the way for improving the V OC and performance of CdTe solar cells.

CdTe solar cells↗
DOE OSTI2024

The effect of remnant CdSe layers on the performance of CdSeTe/CdTe photovoltaic devices

Thin film CdTe-based photovoltaic devices have achieved high efficiency above 22 %. The recent improvement in efficiency is due to Se alloying in the CdTe absorbers to form a CdSeTe/CdTe structure. The subsequent band gap grading increases the short circuit current density. The Se can be introduced by depositing a precursor thin film of either CdSe or a CdSeTe alloy and then diffusing the Se into the CdTe during the high temperature cadmium chloride activation process. Using CdSe is preferred because it is easier to control the Se concentration. However, during fabrication of the CdSeTe/CdTe devices, the CdSe thickness needs to be precisely controlled to prevent the retention of a CdSe remnant layer after the activation treatment. Retention of a remnant CdSe layer causes a dramatic reduction in device efficiency. In this work, we show that the reduction in efficiency is caused by a number of factors. The remnant CdSe layer is n-type which moves the position of the p-n junction. Also, it is widely thought that the CdSe remnants are photo-inactive. In this work, we clarify that the individual CdSe grains are actually highly photo-active. However, the grain sizes in the CdSe remnant and the adjacent CdSeTe layer are very small resulting in a high grain boundary area. Although the grain boundaries are passivated with chlorine, cathodoluminescence imaging and electrical measurements show that this is only partially effective. Also, EQE measurements show that the remnant CdSe causes parasitic absorption. Overall, the remnant CdSe layer causes a reduction in short circuit current density and device efficiency. The thickness of the CdSe precursor layer and the cadmium chloride activation process conditions must be precisely optimised to ensure that all the CdSe is consumed and inter-diffused to form the CdSeTe alloy for highest efficiency devices.

CdSe remnant↗
DOE OSTI2023

Understanding and Advancing Bifacial Thin Film Solar Cells under Dual Illumination

There is a great deal of interest in increasing the energy yield from thin film solar cells by implementing bifacial operation. However, deleterious band bending and high interface recombination at the back transparent electrode can cause problems. Herein, it is investigated how bifacial thin film devices perform when illuminated through the front, back, and simultaneously through both interfaces using numerical modeling. It is shown that the downward band bending near the back interface is reduced during illumination when the carrier concentration is low. This effect is not found when the doping is relatively high, but the minority carrier distribution is still modified. Under either condition, the power generated under bifacial illumination exceeds the sum of the power generated when the illumination is solely from the back or the front. It is also shown that the back-illuminated device performance is independent of the angle at which the light enters the back of the device, which provides accommodation for scattered light. Finally, the power enhancement is calculated for bifacial devices relative to front-illuminated devices, and is shown that any significant loss in frontside power generation is difficult to overcome with back illumination under real-world albedo conditions.

CdTe↗
DOE OSTI2022

Bifacial CdTe Solar Cells with Copper Chromium Oxide Back-Buffer Layer

Bifacial solar cells have the potential to maintain energy output higher than monofacial devices under unfavorable weather conditions. A transparent back-buffer layer which can passivate the interface and improve the minority carrier lifetime is critical in CdTe-based bifacial devices. Herein, solution-processed Cu x Cr y O z as a promising back-buffer for CdTe/CdS solar cells is demonstrated. The carrier lifetimes measured at the front and back of the device are 31.2 and 3.1 ns, respectively, which correspond to an increase of ≈38% and 138%, respectively, compared to the reference device. This dramatic improvement in lifetime results in a 100% increase in short-circuit current measured with backside illumination. The best bifacial device has efficiencies 7.6% and 12.5%, respectively, from back and front illumination, yielding a bifaciality factor of 0.60.

14 SOLAR ENERGY↗
DOE OSTI2022

Reduced Recombination and Improved Performance of CdSe/CdTe Solar Cells due to Cu Migration Induced by Light Soaking

The performance of CdTe solar cells has advanced impressively in recent years with the incorporation of Se. Instabilities associated with light soaking and copper reorganization have been extensively examined for the previous generation of CdS/CdTe solar cells, but instabilities in Cu-doped Se-alloyed CdTe devices remain relatively unexplored. In this work, we fabricated a range of CdSe/CdTe solar cells by sputtering CdSe layers with thicknesses of 100, 120, 150, 180, and 200 nm on transparent oxide-coated glass and then depositing CdTe by close-spaced sublimation. After CdCl 2 annealing, Cu-doping, and back metal deposition, a variety of analyses were performed both before and after light soaking to understand the changes in device performance. The device efficiency was degraded with light soaking in most cases, but devices fabricated with a CdSe layer thickness of 120 nm showed reasonably good efficiency initially (13.5%) and a dramatic improvement with light soaking (16.5%). Here, the efficiency improvement is examined within the context of Cu ion reorganization that is well known for CdS/CdTe devices. Low-temperature photoluminescence data and V oc versus temperature measurements indicate a reduction in nonradiative recombination due to the passivation of defects and defect complexes in the graded CdSe x Te 1–x layer.

CdSexTe1−x↗