Reversibility of (Ag,Cu)(In,Ga)Se2 electrical properties with the addition and removal of Na: Role of grain boundaries

Abstract

The Na content of (Ag,Cu)(In,Ga)Se2 films was cyclically adjusted using a novel method involving cycles of water rinsing at 60 °C followed by heating in air at 200 °C to remove Na and evaporation of NaF to re-introduce Na back into the film. The low temperatures and short heating times ensure that Na is removed only from grain boundaries while leaving grain interiors unaffected. Cross-grain conductivity and Seebeck coefficient were measured during this removal procedure and both measurements decreased when Na was removed and both recovered upon the re-addition of Na, consistent with an increase in compensating donor defects in the absence of Na. These results demonstrate that Na reversibly affects the electrical properties of grain boundaries. We propose that Na reversibly passivates donor-like defects such as InCu double donors at grain boundaries.

Impact of Wide-Ranging Nanoscale Chemistry on Band Structure at Cu(In, Ga)Se2 Grain Boundaries

“…If alkalis reside on Cu sites, then their contribution to charge carriers should be benign; yet, their incorporation has long been shown to increase p-type conductivity 55 , 56 . Long-standing theories that describe the phenomenon relate the presence of alkalis to the reduction of n-type defects: (In, Ga) Cu or V Se 26 , 57 – 59 . However, there has been little experimental verification.…”

Na‐Diffusion Enhanced p‐type Conductivity in Cu(In,Ga)Se2: A New Mechanism for Efficient Doping in Semiconductors

“…In the mechanism that assumes Na (K) at the grain boundary, it was proposed that the Na passivates the donorlike defects at grain boundaries, such as In Cu [5,18,19] or Se vacancies (in conjunction with O 2 ), [20,21] thus increasing the net hole concentration. [22,23] This grain-boundary mechanism induced by K doping, and provides new strategies for efficient p-type or n-type doping in semiconductors.…”

Secondary phase formation and surface modification from a high dose KF‐post deposition treatment of (Ag,Cu)(In,Ga)Se2solar cell absorbers

“…[5][6][7][8] Similarly to CIS absorbers, addition of alkali metal elements may also have positive effects on Ag-alloyed CIGS absorbers. Na-doping seemingly passivates the In Cu double donor defects 2,9,10 and allows increased electrical performance as compared with Na-free devices, which exhibit a lower voltage and in some cases also a visible roll-over behavior in the IV measurements. 7 Similar beneficial effects from other alkali metals on ACIGS, as seen for Cu (In,Ga)Se 2 (CIGS), are presently investigated.…”