CdTe thin films doped by Cu and Ag - a comparison in substrate configuration solar cells

Abstract

One of the main problems to be solved in order to raise the efficiency of CdTe solar cells is the low acceptor concentration in polycrystalline CdTe layers which is limiting the open circuit voltage. The commonly used acceptor dopant Cu not only forms rather deep acceptor defects but is also likely to limit the net acceptor concentration due to the formation of compensating donor type defects. In this work, Ag is examined as a possible candidate for improving acceptor concentration in CdTe thin films. Hole density in CdTe layers with varying elemental Ag concentration follows a similar trend as previously reported for Cu doping. In finished solar cells efficiencies up to 11.6 % could be reached using Ag as acceptor dopant. Measurements of hole density, barrier height, space charge region width and depth dependent elemental distribution indicate the similarity of the Ag and Cu doping process.

Structural and electronic properties of CdTe1-xSex films and their application in solar cells

“…This is comparable to previous results, where the formation of acceptor defects (Cu’ Cd ) was assumed to be responsible for increasing carrier concentration at low Cu content, and the additional formation of compensating donors (Cu • i ) at higher Cu content reduced the net p-type conductivity [8]. The mobility has a minimum value in the range of (0.5–2)×10 15 copper atoms/cm 2 , with an increase for higher copper values.…”

“…First-principle calculations indicate that Cu on a Cd site (Cu’ Cd ) forms deep acceptors [6,7]. However, copper also forms compensating donors on interstitial sites (Cu • i ), and as a result the achievable hole density is limited [6,8]. At low amounts of added copper, the formation of acceptors (Cu’ Cd ) increases the p-type conductivity.…”

“…At higher amounts of added copper, donor defects (Cu • i ) are formed in addition to the acceptors, reducing the net p-type conductivity. As a consequence, excessive copper amounts are detrimental to the resistivity and the hole density [8]. Additionally, the majority of Cu in solar cells is segregated at grain boundaries [5].…”

Proton irradiation of CdTe thin film photovoltaics deposited on cerium‐doped space glass

“…This shows the reason for the large drop observed in J sc where the EQE was suppressed across most of the spectrum with a spike in the EQE near the band edge of CdTe >775 nm. This EQE characteristic is consistent with conversion to n‐type across some of the CdTe absorber layer thickness, creating a buried junction.…”

Review of CdTe1−xSex Thin Films in Solar Cell Applications

“…A possible explanation is that compensating donors, such as copper on interstitial sites (Cu i ), are formed as well, and that the formation energy of these defects is also dependent on x. If the electronic and/or structural effects of CdTe 1−x Se x alloying facilitate the formation of compensating donors more than the formation of acceptor defects, the result is a limit to the achievable hole density [8,49,50].…”