Investigation of the effect of potassium on Cu(In,Ga)Se2 layers and solar cells

Local Electrical Response in Alkaline-Doped Electrodeposited CuInSe2/Cu Films

“…Therefore, the effect of Na and Cs will overcome the effect of the deleterious Cu 2 Se phase through progressive (Cu 2 Se) 1−x (Cu 2 O) x formation on the Cu 2 Se surface, as the relative intensities of the Cu 2 O and Cu 2 Se/CIS peaks in the Cs-and Na-doped films indicate. In the case of K, the observed formation of metallic phases within the film as well as the proposed KInSe 2 compound [9] would deteriorate any photocurrent response, and also there are reports that mention that, in contrast to Na-doping, the beneficial effect of which has been related to V Cu capture and thus Cu mass transport [19], K creates electron traps, particularly on Cu-poor CIS films [20], which would also deteriorate the photocurrent response, as actually observed. case of the Cs-doped film it is evident that it is mostly formed during growth.…”

“…The peak intensity in CIS:K shows a decrease with respect to CIS:Na, which could be attributed to the reported influence of K on elemental interdiffusion when added during growth [6]. On the other hand, the XRDs presented in Figure 2B display peaks corresponding to Cu2O at 29.5°, 36.4° and 42.3°, respectively, in all the patterns, but particularly evident in the Cs-doped sample; elementary In is particularly strong in the K-doped sample and a small peak corresponding to CuIn at 34.5° is also evident in the K sample, and both could be attributed to a less complete reaction because the above-mentioned K-dependent hindering of elementary diffusion [9,10]. The presence of Cu2O would arise from the early oxide formed before deposition, but in the In this region, the patterns present a wide peak associated with tetragonal CIS near 26 • as well as a shoulder associated with triclinic Cu 2 Se at 25.3 • .…”

“…The maximum contents of detectable alkaline ions (Na, K and Cs) were Na 0.25 at.%, K 2.80%, and Cs 0.28%, respectively. The higher amount of K is intriguing, even though K segregation to the grain boundaries has been reported; another explanation for the abnormally high K contents would be the formation of a KInSe2 phase which has been suggested as responsible for the detriment in the electrical properties of K-doped Cu(In,Ga)Se2 films [6,9,10]. This assumption could be related to the increment in the Se/Cu ratio in the K-doped film together with the K increase at the film top observed by EDS.…”

“…No particular tendency related to ion sizes is observed because of the large dispersion in the I-V curves (see Figures S1-S5 in Supplementary information); however, the large values of n indicate the prevalence of tunneling conduction mechanisms rather than thermoionic emission caused by the uneven distribution of dopants incorporated during film growth, as well as by the local variations of surface-accumulated charges observed in the electrical images. Future work is intended to "load" the dopants on the Cu substrate before film growth as reported in [9], although the effect of Cu 2 O formation must be assessed as well, as it has been suggested that it could have a beneficial effect in back contacts in CdTe solar cells. …”

“…In the case of Li, a small contribution to the valence or conduction band states was found, although an increase in the band gap was attributed to effects on Se p-orbitals due to Li ionicity [8]. For K-doping in Cu(In,Ga)Se 2 , a reduced minority carrier lifetime, and poorer and temperature-dependent collection of photogenerated charge carriers were observed [6], although recent data contradict these observations [9,10]: K-doping in Cu(In,Ga)Se 2 (CIGS) films, not yet introduced during deposition or by a postdeposition treatment, increases the cell efficiency by reducing Cu at the CIGS surface upon formation of In and Ga oxides, although detrimental effects of a proposed KInSe 2 compound on the electrical properties have been mentioned in [9]. Finally, in a study of anodic p-Cu 2 O films doped with Li, Na, K and Cs [11], a shift of the acceptor level position together with an increase of the carrier density and electrical conductivity and a reduction in the photocurrent response with the dopant ion size was observed.…”