Presentation Details
| Polycrystalline CdSeTe has Distinct Band Tail and Carrier Trap Spatial Distributions, Indicating Defect Origins and Device Impacts Darius Kuciauskas1, Brian Good2, Mahisha Amarasinghe2, Marco Nardone3. 1Loughborough University, Loughborough, United Kingdom.2First Solar Inc., Perrysburg, OH, USA.3Bowling Green State University, Bowling Green, OH, USA |
Abstract
To improve passivation, high-efficiency CdTe solar cells use Se alloying, making polycrystalline absorber ternary and graded CdSeTe semiconductor. This can increase band tails (Urbach energies Eu) and can introduce carrier trapping, which is quantified by lower carrier mobility. Band tails and electronic traps in CdSeTe have distinct radiative emission signatures. By taking advantage of this feature, we use hyperspectral photoluminescence (PL) imaging to find that PL emission attributed to traps is stronger near the grain boundaries (GBs), supporting the critical role of grain growth and Se diffusion not only for passivation, but also for trapping. In contrast, band tail spectral broadening (expressed as Eu increase) is already present on the <300 nm spatial scale and its distribution is more uniform. For loss analysis, CdSeTe band tails are similar to corresponding features in perovskites and CIGS, except that band tails in CdSeTe can span a broader energy range. Carrier traps are unique for CdSeTe, requiring minimization of these features in semiconductor fabrication. Metrology based on radiative emission and carrier dynamics can be used in monitoring improvements in radiative voltage (Eu metrics from PL spectra) and mobility (trapping effects from TRPL). Our results were obtained for CdSexTe1-x with reduced band tails (Eu = 11 meV for x = 0, 0.1 and 21 - 19 meV for x = 0.2, 0.3) and low As dopant incorporation (»5E16 cm-3). Future work is needed to understand defect distributions and device impacts for a broader range of absorber compositions and doping variations. It is likely that targeted and precise control of Se and Se defect distributions will unlock performance gains due to improved radiative voltage and mobility.
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No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author.