Unveiling the effects of post-deposition treatment with different alkaline elements on the electronic properties of CIGS thin film solar cells

Abstract

Thin film solar cells with a Cu(In,Ga)Se2 (CIGS) absorber layer achieved efficiencies above 20%. In order to achieve such high performance the absorber layer of the device has to be doped with alkaline material. One possibility to incorporate alkaline material is a post deposition treatment (PDT), where a thin layer of NaF and/or KF is deposited onto the completely grown CIGS layer. In this paper we discuss the effects of PDT with different alkaline elements (Na and K) on the electronic properties of CIGS solar cells. We demonstrate that whereas Na is more effective in increasing the hole concentration in CIGS, K significantly improves the pn-junction quality. The beneficial role of K in improving the PV performance is attributed to reduced recombination at the CdS/CIGS interface, as revealed by temperature dependent J-V measurements, due to a stronger electronically inverted CIGS surface region. Computer simulations with the software SCAPS are used to verify this model. Furthermore, we show that PDT with either KF or NaF has also a distinct influence on other electronic properties of the device such as the position of the N1 signal in admittance spectroscopy and the roll-over of the J-V curve at low temperature. In view of the presented results we conclude that a model based on a secondary diode at the CIGS/Mo interface can best explain these features.

Engineering Solar Cell Absorbers by Exploring the Band Alignment and Defect Disparity: The Case of Cu‐ and Ag‐Based Kesterite Compounds

“…This intrinsic defect problem has hindered the further progress of Cu-based kesterite solar cells, the record effi ciency is only 12.7%, [ 30 ] still much lower than those of the CdTe and CIGS cells despite they have similar band gaps. In contrast, the type inversion (through forming n-type Cu-poor ordered vacancy compounds or Cd Cu doping near the CdS/CIGS interface) and thus a large band bending have been observed in CIGS solar cells, [ 28,[31][32][33][34][35] which makes their maximum V oc much higher than that of the Cu 2 ZnSn(S,Se) 4 solar cells, as shown in Figure 1 c,d. It is thus critical to suppress the formation of Cu Zn and make the type inversion possible at the p-n junction interface if one intends to overcome the V oc defi cit of Cu 2 ZnSn(S,Se) 4 …”

Injection Current Barrier Formation for RbF Postdeposition‐Treated Cu(In,Ga)Se2‐Based Solar Cells

“…[5][6][7][8][9]26,30] Additionally, as presented here for a RbF PDT, with the introduction of a KF PDT, a blocking of the diode current was observed. [15] Recently, Malitckaya et al calculated the bandgap energies of AlkInSe 2 (Alk = Li, Na, K, Rb, Cs) secondary phases, which are expected to segregate on the surface of the CIGS absorber for K, Rb, and Cs. [31] It was found that KInSe 2 and RbInSe 2 both have a bandgap of ≈2.5 eV.…”

First‐Principles Modeling of Point Defects and Complexes in Thin‐Film Solar‐Cell Absorber CuInSe2

“…The high-efficiency Cu-poor CIGSe absorbers are prepared under a selenium-rich atmosphere. [32] The experimental conditions correspond to the boundary between the CuInSe 2 and Se stability regions in the Table 5 for all relevant charge states. Defects which have in the formation energy plots a negative charge at the VBM are shallow acceptors and defects becoming negative slightly above the VBM are deeper acceptors.…”