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Development and application of novel thermomechanical cleave technique for CDTE solar cells
McGott, Deborah L.
McGott, Deborah L.
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2021
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2023-04-14
Abstract
Cadmium telluride (CdTe) has emerged as the leading commercial thin-film photovoltaic (PV) technology with over 25 GW installed capacity. One of the main advantages of CdTe is its near-ideal bandgap of ∼1.5 eV, which should allow for an open-circuit voltage (VOC ) of over 1.1 V in devices. However, VOC has stagnated at ∼850 mV, making the improvement of VOC a primary focus in CdTe research. Modeling suggests that as the bulk continues to improve, the front interface becomes a bottleneck for performance. Because CdTe is stan- dardly grown in the superstrate configuration, however, the front interface is buried under microns of material and difficult to access. Moreover, the high temperatures and harsh chemical environments used during CdTe growth can cause substantial changes to the front interface which make it difficult to optimize. In this work, a novel thermomechanical cleave technique, or delamination, was developed to directly access the front interface while main- taining surface chemistry. This is done by applying a stressor layer (e.g., polymer, epoxy) to the back of completed superstrate device stacks, then thermally shocking the system at low temperatures. This causes the stressor, which has a relatively high coefficient of ther- mal expansion, to compress quickly and separate the thin film from the glass substrate. In this way, the conditions required for state-of-the-art device growth (high temperatures, reactive environments) can be decoupled from final lightweight (i.e., plastic) packaging. In the first portion of this work, a theoretical framework was developed to identify key ma- terial properties and process variables that can be tuned to control delamination. Using this understanding, delamination was demonstrated using lightweight polymers, over large areas (up to 3x3”, limited only by the size of available growth chambers), for varied architec- tures (CdS/CdTe, MgyZn1−yO/CdSexTe1−x), defect chemistries (Cu-, As-doped CdTe), and technologies (CdTe, CuIn1−xGaxSe2) from six different institutions including international (Swansea) and industrial (First Solar) partners.
The second portion of this work was focused on the application of the cleave technique; namely, the study and modification of the front interface, as well as reconstruction of the front contact. Previous work using the cleave technique has shown that a two-dimensional (2D) layered structure, CdCl2, naturally forms at the front interface during standard device processing. In this work, it is shown that CdCl2 passivates (i.e., reduces recombination at) the front interface and improves VOC. Moreover, it was found, through an extensive literature search and collaborations, that two additional leading polycrystalline thin-film technologies, CuIn1−xGaxSe2 and perovskites, have analogous 2D layers at absorber surfaces, suggesting a possible secret to their shared success. Attributes of successful 2D passivating materials and design strategies for their incorporation are presented with the ultimate goal of enabling the next generation of polycrystalline thin-film solar cells. To study recombination at the front interface more effectively, a time-resolved photoluminescence (TRPL) system with two excitation wavelengths - 670 nm (standard, probes surface and bulk) and 405 nm (primarily probes surface due to shallow absorption depth) - was built as a part of this thesis work. Using this TRPL system, two major recent advances in CdTe solar cells were studied: the incorporation of Se to form graded CdSexTe1−x (CST) and the replacement of CdS with MgyZn1−yO (MZO). It was found that x = 0.2 Se was required to obtain the lifetime improvements that are commonly associated with Se incorporation, and this change was primarily seen in the bulk. Additionally, evidence for trapping at the MZO/CST interface was observed, indicating that MZO does not provide sufficient passivation at the front interface.
During delamination, the CdTe absorber is separated from the front contact; thus, to make a completed device, the front contact must be reconstructed. In this case, the emitter can be engineered directly with properties that will not evolve during subsequent high- temperature processing. Concurrently, the complication of growing high-quality absorber material in the substrate configuration (i.e., using low temperatures) is eliminated. The combination of these two has enabled record efficiencies for substrate CdTe devices (15%) in this work. By applying the reconstruction process to absorbers that were either Cu- or As-doped, the elevated importance of front interface quality in highly-doped (As) devices was also revealed. Thus, the novel thermomechanical cleave technique and dual wavelength TRPL system developed in this thesis have enabled several significant new insights regarding the important front interface in CdTe-based solar cells, and have provided a platform for the continued investigation and improvement of this photovoltaic technology.
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