Impact of RbF and NaF Postdeposition Treatments on Charge Carrier Transport and Recombination in Ga‐Graded Cu(In,Ga)Se₂ Solar Cells

Abstract

Two key strategies for enhancing the efficiency of Cu(In,Ga)Se2 solar cells are the bandgap gradient across the absorber and the incorporation of alkali atoms. The combined incorporation of Na and Rb into the absorber has brought large efficiency gains compared to Na‐containing or alkali‐free layers. Here, transient absorption spectroscopy is employed to study the effect of NaF or combined NaF+RbF postdeposition treatments (PDT) on minority carrier dynamics in different excitation volumes of typical composition‐graded Cu(In,Ga)Se2 solar cells. Electron lifetimes are found to be highly dependent on the film composition and morphology, varying from tens of nanoseconds in the energy notch to only ≈100 ps in the Ga‐rich region near the Mo‐back contact. NaF PDT improves recombination lifetimes by a factor of 2–2.5 in all regions of the absorber, whereas the effectiveness of the RbF PDT is found to decrease for higher Ga‐concentrations. Electron mobility measured in the absorber region with large grains is promoted by both alkali PDTs. The data suggest that NaF PDT passivates shallow defect states (Urbach tail) throughout the Cu(In,Ga)Se2 film (including the interior of large grains), whereas the additional RbF PDT is effective at grain boundary surfaces (predominantly in regions with medium to low Ga‐concentrations).

Precise Alkali Supply during and after Growth for High‐Performance Low Bandgap (Ag,Cu)InSe2 Solar Cells

“…As mentioned above, Rb preferentially segregates at GBs and passivates deep defects at surfaces, [52] reducing detrimental recombination and elevating carrier lifetime. [13,29,55,56] Moreover, the formation of RbInSe 2 at the CdS interface and grain surfaces is reported to affect the charge transport due to a higher bandgap of RbInSe 2 [57] and thus suppresses non-radiative recombination. [56] Unlike NaF precursor devices, we do not observe a clear correlation between morphology and carrier lifetime.…”

Cu1–xCsxInSe2: Candidate Absorber for Higher Conversion Efficiency and Broader Band Photodetector

“…17 About a decade ago, the implementation of heavy alkali elements into the absorber film and at its interfaces initiated a gradual increase in the PCE of copper based solar cells from about 20% to 23%. 21−24 Furthermore, combining Rb-PDT (postdeposition treatment) with other dopants facilitates defect passivation, 18 downshifts the valence band maximum, 19 and improves the power conversion efficiency 22 and device performance. 20 Obviously, the effects of doping may vary depending on the dopant used and specific parameters of the solar cells and the photodetectors.…”

“…These doping elements can passivate the defects at the grain boundary and interior, which improves the hole carrier concentration and minority carrier lifetime . About a decade ago, the implementation of heavy alkali elements into the absorber film and at its interfaces initiated a gradual increase in the PCE of copper based solar cells from about 20% to 23%. − Furthermore, combining Rb-PDT (postdeposition treatment) with other dopants facilitates defect passivation, downshifts the valence band maximum, and improves the power conversion efficiency and device performance …”

Non-Vacuum Design of СuGa$_{x}$In$_{1-x}$Se$_{2}$ Films for Solar Energy Applications

“…Copper indium (gallium) (di)selenide (CIS), CuIn(Ga)Se2, is an important semiconductor for the production of thinfilm solar cells [21][22][23]. It is characterised by a high optical absorption index and long-term stability [24,25], which allows it to be used as a reliable absorbing layer.…”