Abstract
Thin film materials for photovoltaics such as cadmium telluride (CdTe), copper-indium diselenide-based chalcopyrites (CIGS), and lead iodide-based perovskites offer the potential of lower solar module capital costs and improved performance to microcrystalline silicon. However, for decades understanding and controlling hole and electron concentration in these polycrystalline films has been extremely challenging and limiting. Ionic bonding between constituent atoms often leads to tenacious intrinsic compensating defect chemistries that are difficult to control. Device modeling indicates that increasing CdTe hole density while retaining carrier lifetimes of several nanoseconds can increase solar cell efficiency to 25%. This paper describes in-situ Sb, As, and P doping and post-growth annealing that increases hole density from historic 1014 limits to 1016–1017 cm−3 levels without compromising lifetime in thin polycrystalline CdTe films, which opens paths to advance solar performance and achieve costs below conventional electricity sources.
Highlights
Polycrystalline CdTe solar cells offer an example of where overcoming hole density limitations can lead to important technology shifts
Present-generation CdTe solar cells are based on a front wall superstrate configuration in which p-type CdTe or CdTe1−xSex alloy is deposited at rates of microns/min onto glass coated with transparent conducting films, which serve as the n-type junction partner, and possibly an n-type buffer layer such as CdS or MgZnO
Doping without impurities in CdTe is difficult, CdTe is often used in applications where very high resistivity is desired due to its strong tendency to compensate
Summary
Polycrystalline CdTe solar cells offer an example of where overcoming hole density limitations can lead to important technology shifts. Cells are completed by a back surface preparation to facilitate formation of a low-resistance electrical contact Empirical refinement of this fabrication approach has produced CdTe films which repeatedly only reach mid-1014 cm−3 acceptor concentration. While it is possible to dope polycrystalline thin films by ex-situ diffusion, studies indicate that diffusion along grain boundaries is orders of magnitude greater than substitutional bulk diffusion, or the fast interstitial diffusion that occurs only with P13,14 This makes it challenging to obtain 1016 cm−3 doping with the uniformity, recombination rates, and processing temperatures currently required for practical solar technology. Activation of the dopants by short thermal annealing and subsequent quench cooling extends the hole density to >1016 cm−3
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