Potassium fluoride postdeposition treatment with etching step on both Cu-rich and Cu-poor $\mathrm{CuInS}{\mathrm{e}}_2$ thin film solar cells

Recent progress in the power conversion efficiency of $\mathrm{Cu}\left(\mathrm{In},\mathrm{Ga}\right)\mathrm{S}{\mathrm{e}}_2$ thin film solar cells has been achieved by an alkali postdeposition treatment. This treatment has been shown to change the surface composition and structure as well as the bulk properties. To investigate the relative importance of those two effects we study the impact of the treatment on Cu-rich and Cu-poor $\mathrm{CuInS}{\mathrm{e}}_2$, which show a different influence of interface recombination without the treatment. We develop a potassium postdeposition treatment that can be applied to Cu-rich material, where an additional etching step is necessary. The same postdeposition treatment with etching step is applied to Cu-poor material. In both cases we observe an increase of the power conversion efficiency and open circuit voltage. Comparing the increase in open circuit voltage to the increase in quasi-Fermi level splitting indicates that the improvement in Cu-poor solar cells is mostly due to changes in the bulk, whereas in Cu-rich solar cells both the bulk and the interface are improved. The improvement of the interface is corroborated by temperature dependent current-voltage characteristics, which show that the dominating recombination path in Cu-rich solar cells moves from the interface to the bulk after treatment and by admittance spectroscopy, which shows that the treatment removes a 200 meV deep defect. Photoluminescence spectroscopy shows that even in Cu-rich material the alkali treatment creates a Cu-poor surface, which in this case cannot be created by diffusion of Cu into the bulk, but is grown during the treatment.

Figure 1

Scanning electron microscope images of a “Cu-rich” grown absorber after etching (a) and the same absorber after an 8-min-long KF PDT (b).

Figure 2

IV characteristics of an untreated “Cu-rich” CIS solar cell and three KF treated cells where the alkali deposition lasted for 4, 8, and 12 min. The inset of the figure represents a wider voltage axis in the reverse range.

Figure 3

(a) EQE measurements, (b) Reflection measurements for “Cu-rich” CIS solar cell with and without KF PDT.

Figure 4

Temperature dependence of the open circuit voltage for an untreated and treated “Cu-rich” solar cells. A linear fit (dotted line) is used to extract the activation energy. The bandgap (${\mathrm{E}}_{\mathrm{G}}$) is represented by a dashed double-dotted line.

Figure 5

Capacitance-voltage measurements of an untreated “Cu-rich” solar cell and three cells with different KF deposition times during the PDT. The apparent doping is extracted from the inverse slope of a linear fit at (short dotted lines) small forward bias. The fitting range for all four cells is indicated by the short dashed lines.

Figure 6

PL emission spectra measured at 10 K of a “Cu-rich” grown absorber (a) and an absorber with KF PDT measured at high excitation (b) and at low excitation (c). The red lines depict fitting functions used (dashed – asymmetric gauss profile representing a Cu-poor phase, solid – DA transition plus phonon replica representing a “Cu-rich” phase). For better visibility, the peak around 1.03 eV is magnified by a factor of 100.

Figure 7

Admittance measurements for “Cu-rich” CIS solar cells (a) without KF PDT and (b) with a 4-min KF PDT for temperatures between 50 and 300 K. The Arrhenius plot indicates the activation energy of the main capacitance step for absorbers (c) without treatment and (d) with 4-min treatment.

Figure 8

Electrical characterization of a Cu-poor CIS sample before and after a 6-min KF PDT in terms of (a) IV, (b) EQE, and (c) IV(T). The short dotted lines indicate the extrapolated activation energy to zero Kelvin. The dashed double-dotted line indicates the bandgap of Cu-poor cells. (d) CV measurements. The short dotted lines indicate the fit from where the apparent doping is extracted. The short dashed lines (black) indicate the region of fitting.