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dc.contributor.advisorWolden, Colin Andrew
dc.contributor.authorLi, JiaoJiao
dc.date.accessioned2016-09-21T19:45:01Z
dc.date.accessioned2022-02-03T12:58:17Z
dc.date.available2016-09-21T19:45:01Z
dc.date.available2022-02-03T12:58:17Z
dc.date.issued2016
dc.identifierT 8131
dc.identifier.urihttps://hdl.handle.net/11124/170428
dc.descriptionIncludes bibliographical references.
dc.description2016 Fall.
dc.description.abstractWith an ideal band gap about 1.45 eV and a large absorption coefficient (>104 cm-1) CdTe has emerged as the leading thin film photovoltaic (PV) technology. However an ongoing challenge is forming a high quality ohmic contact with CdTe, which reflects its low carrier concentration and high work function (5.7 eV). To overcome this problem, ZnTe is used as the buffer layer because it has good valence band alignment with CdTe and can be easily doped to form a quasi-ohmic contact. Copper is commonly used to degenerately dope this layer, which narrows the barrier width and permits electron tunneling, creating a quasi-ohmic contact. But excessive copper causes recombination and degradation. With copper having both positive and deleterious effects it is critical to precisely control both its amount and spatial distribution in order to obtain high efficiency. To achieve that, we developed a rapid thermal processing-based process for back contact preparation that de-couples Cu deposition from its re-distribution. The ZnTe:Cu back contact is co-evaporated at low temperature, with little or no interdiffusion. The sample is then subjected to short RTP treatments to activate the junction. RTP was demonstrated as a highly effective approach for reducing back contact barriers in CdTe solar cells contacted with ZnTe:Cu buffer layers, substantially improving both FF (>73%) and VOC (>850mV). It was also successfully deployed on many platforms. We applied our back contact on the CdTe provided by different groups and made by different methods and got an NREL–certified 16.4% flexible CdTe solar cell (current world record for flexible devices). This process has been demonstrated to be beneficial on multiple device structures, as the low thermal budget of RTP facilitates its adoption without impacting the optimization of upstream processing. Further characterizations including the high resolution transmission electron microscopy (HR-TEM) and atom probe tomography (APT) were used to study the evolution of the back contact region during RTP treatment. After activation, the ZnTe layer, initially nanocrystalline and homogenous, transforms into a bilayer structure. Copper, co-evaporated uniformly within ZnTe, is found to dramatically segregate and aggregate after RTP. Analysis of TEM images revealed that Zn accumulates at the edge of these clusters, and three-dimensional APT images confirmed that these are core–shell nanostructures consisting of Cu1.4Te clusters encased in Zn. These changes in morphology and composition are related to cell performance and stability. Au is commonly used as the metallization layer in research labs because of its high work function (5.2 eV), stability in air and, ease of deposition. But Au is not compatible with industrial manufacturing because of its high price so in this work we explore chromium and titanium as practical alternatives. It was found that comparable performance could be obtained with each metal, but that the optimal Cu loading scaled as one would expect based on solubility. Comparisons of J-V and QE behavior among devices produced with insufficient, optimal, and excess Cu dosing are used to provide insight into the role(s) of this critical impurity for device performance. Reliability tests were taken under different stressing conditions to further understand the role of the metallization layers. In addition to the back contact, front contact is also very important to CdTe solar cells. CdS is widely used as the n-type heterojunction partner for CdTe because of their compatibility. However, CdS is not an ideal window layer for CdTe solar cells. Oxygenated cadmium sulfide (CdS:O) is found delivering improved blue response relative to CdS. Our recent study revealed that CdS:O completely transforms into a layer containing cadmium sulfate clusters interspersed among CdS1-yTey nanocrystals during device fabrication. This motivated us to study CdTe solar cells employing pre-formed CdS1-yTey alloy windows without sulfate present. The intrinsic properties of alloys deposited by co-evaporation are evaluated and then used in place of CdS in standard device fabrication. Interestingly we find that device efficiency is nominally unchanged, but there are significant tradeoffs between current collection, fill factor, and open circuit voltage with alloy composition.
dc.format.mediumborn digital
dc.format.mediumdoctoral dissertations
dc.languageEnglish
dc.language.isoeng
dc.publisherColorado School of Mines. Arthur Lakes Library
dc.relation.ispartof2016 - Mines Theses & Dissertations
dc.rightsCopyright of the original work is retained by the author.
dc.subjectrenewable energy
dc.subjectthin films
dc.subjectsolar cells
dc.subjectCdTe
dc.titleDevelopment of ZnTe:Cu contacted CdTe solar cells
dc.typeText
dc.contributor.committeememberOhno, Timothy R.
dc.contributor.committeememberAgarwal, Sumit
dc.contributor.committeememberGorman, Brian P.
thesis.degree.nameDoctor of Philosophy (Ph.D.)
thesis.degree.levelDoctoral
thesis.degree.disciplineChemical and Biological Engineering
thesis.degree.grantorColorado School of Mines


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