Insights into post-growth doping and proposals for CdTe:In photovoltaic devices

Abstract

This paper is motivated by the potential advantages of higher doping and lower contact barriers in CdTe photovoltaic devices that may be realized by using n- type rather than the conventional p-type solar absorber layers. We present post-growth doping trials for indium in thin polycrystalline CdTe films using diffusion of indium metal and with indium chloride. Chemical concentrations of indium up to 1019 cm-3 were achieved and the films were verified as n-type by hard x-ray photoemission. Post growth chlorine treatment (or InCl3) was found to compensate the n-doping. Trial structures comprising CdS/CdTe:In verified that the doped absorber structures performed as expected both before and after chloride treatment, but it is recognized that this is not an optimum combination. Hence in order to identify how the advantages of n-type absorbers might be fully realized in future work, we also report simulations of a range of p-n junction combinations with n-CdTe, a number of which have the potential for high Voc.

n-type CdTe:In for photovoltaics: in situ doping, type verification and compensation effects

“…This indicates that chloride processing acts to compensate the n-type doping from indium, and while the Fermi level remains above mid-gap, the material is close to being intrinsic. Moreover, these results are broadly in agreement with our earlier findings HAXPES for films that had been post-growth doped by diffusing in indium metal: the doping gave Fermi levels that were high in the gap implying n-type conduction, but as for the present samples, it was compensated by chloride processing [4].…”

“…For all samples the time dependent photoluminescence decay was fitted to a two-component exponential 2. (Data for the undoped films is also shown in Thomas et al [4]).…”

“…The core levels of Cd, Te, O and In were used to gain chemical information while the positions of the Fermi levels in the band gap were estimated by measuring the valence band edge relative to the Fermi level of a gold calibration specimen. Further details of the methods are reported for work on CdTe [4] and Sb 2 Se 3 [5]. Both indium-doped and, doped/MgCl 2 -treated samples were measured, these being freshly prepared in the normal laboratory environment and transferred to the synchrotron in a vacuum suitcase system.…”

“…The depth scale was estimated by assuming the characteristic escape depth of the photoelectrons to be twice the inelastic mean free path for photoelectrons having the average energies for the Cd and Te 3d 5/2 and 3d 3/2 emissions [6]. Details of the method may be found in [4]. It may be seen from the figure that the Fermi level position (E F ) in the as-grown sample is above mid-gap, and increases from 1.10 eV to 1.28 eV above the valence band maximum (VBM) deeper into the sample.…”

“…TRPL lifetimes extracted from a double exponential fit to the data in figure 7. (Data for the undoped films is from Thomas et al [4]). where A accounts for the background, B 1 and τ 1 are for charge separation and B 2 and τ 2 are for minority carrier recombination (the latter being more sensitive to doping and point defects).…”