Surface Passivation and Detrimental Heat-Induced Diffusion Effects in RbF-Treated Cu(In,Ga)Se2 Solar Cell Absorbers.

Alkali postdeposition treatments of Cu(In,Ga)Se2 absorbers with KF, RbF, and CsF have led to remarkable efficiency improvements for chalcopyrite thin film solar cells. However, the effect of such treatments on the electronic properties and defect physics of the chalcopyrite absorber surfaces are not yet fully understood. In this work, we use scanning tunneling spectroscopy and X-ray photoelectron spectroscopy to compare the surface defect electronic properties and chemical composition of RbF-treated and nontreated absorbers. We find that the RbF treatment is effective in passivating electronic defect levels at the surface by preventing surface oxidation. Our X-ray photoelectron spectroscopy (XPS) data points to the presence of chemisorbed Rb on the surface with a bonding configuration similar to that of a RbInSe2 bulk compound. Yet, a quantitative analysis indicates Rb coverage in the submonolayer regime, which is likely causing the surface passivation. Furthermore, ab initio calculations confirm that RbF-treated surfaces are less prone to oxidation (in the form of Ga, In, and Se oxides) than bare chalcopyrite surfaces. In addition, elemental diffusion of Rb along with Na, Cu, and Ga is found to occur when the samples are annealed under ultrahigh vacuum conditions. Magnetic sector secondary ion mass spectrometry measurements indicate that there is a homogeneous spatial distribution of Rb on the surface both before and after annealing, albeit with an increased concentration at the surface after heat treatment. Depth-resolved magnetic sector secondary ion mass spectrometry measurements show that Rb diffusion within the bulk occurs predominantly along grain boundaries. Scanning tunneling and XPS measurements after subsequent annealing steps demonstrate that the Rb accumulation at the surface leads to the formation of metallic Rb phases, involving a significant increase of electronic defect levels and/or surface dipole formation. These results strongly suggest a deterioration of the absorber-window interface because of increased recombination losses after the heat-induced diffusion of Rb toward the interface.

Electronic Structure of the CdS/Cu(In,Ga)Se2 Interface of KF- and RbF-Treated Samples by Kelvin Probe and Photoelectron Yield Spectroscopy.

Ambient-pressure Kelvin probe and photoelectron yield spectroscopy methods were employed to investigate the impact of the KF and RbF postdeposition treatments (KF-PDT, RbF-PDT) on the electronic features of Cu(In,Ga)Se2 (CIGSe) thin films and the CdS/CIGSe interface in a CdS thickness series that has been sequentially prepared during the chemical bath deposition (CBD) process depending on the deposition time. We observe distinct features correlated to the CBD-CdS growth stages. In particular, we find that after an initial CBD etching stage, the valence band maximum (VBM) of the CIGSe surface is significantly shifted (by 180-620 mV) toward the Fermi level. However, VBM positions at the surface of the CIGSe are still much below the VBM of the CIGSe bulk. The CIGSe surface band gap is found to depend on the type of postdeposition treatment, showing values between 1.46 and 1.58 eV, characteristic for a copper-poor CIGSe surface composition. At the CdS/CIGSe interface, the lowest VBM discontinuity is observed for the RbF-PDT sample. At this interface, a thin layer with a graded band gap is found. We also find that K and Rb act as compensating acceptors in the CdS layer. Detailed energy band diagrams of the CdS/CIGSe heterostructures are proposed.

Proton Radiation Hardness of Perovskite Tandem Photovoltaics.

Monolithic [Cs0.05(MA0. 17FA0. 83)0.95]Pb(I0.83Br0.17)3/Cu(In,Ga)Se2 (perovskite/CIGS) tandem solar cells promise high performance and can be processed on flexible substrates, enabling cost-efficient and ultra-lightweight space photovoltaics with power-to-weight and power-to-cost ratios surpassing those of state-of-the-art III-V semiconductor-based multijunctions. However, to become a viable space technology, the full tandem stack must withstand the harsh radiation environments in space. Here, we design tailored operando and ex situ measurements to show that perovskite/CIGS cells retain over 85% of their initial efficiency even after 68 MeV proton irradiation at a dose of 2 × 1012 p+/cm2. We use photoluminescence microscopy to show that the local quasi-Fermi-level splitting of the perovskite top cell is unaffected. We identify that the efficiency losses arise primarily from increased recombination in the CIGS bottom cell and the nickel-oxide-based recombination contact. These results are corroborated by measurements of monolithic perovskite/silicon-heterojunction cells, which severely degrade to 1% of their initial efficiency due to radiation-induced recombination centers in silicon.

Well-Controlled Dielectric Nanomeshes by Colloidal Nanosphere Lithography for Optoelectronic Enhancement of Ultrathin Cu(In,Ga)Se2 Solar Cells.

Ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells pose challenges of incomplete absorption and back contact recombination. In this work, we applied the simple collodial nanosphere lithography and fabricated 2D SiO2 nanomeshes (NMs), which simultaneously benefit ultrathin CIGSe solar cells electrically and optically. Electrically, the NMs are capable of passivating the back contact recombination and increasing the minimum bandgap of absorbers. Optically, the parasitic absorption in Mo as a main optical loss is reduced. Consequently, the SiO2 NMs give rise to an increase of 3.5 mA/cm2 in short circuit current density (Jsc) and of 57 mV in open circuit voltage increase (Voc), leading to an absolute efficiency enhancement as high as 2.6% (relatively 30%) for CIGSe solar cells with an absorber thickness of only 370 nm and a steep back Ga/[Ga + In] grading.

Investigation of the potassium fluoride post deposition treatment on the CIGSe/CdS interface using hard X-ray photoemission spectroscopy - a comparative study.

The impact of the potassium fluoride post deposition treatment on CIGSe chalcopyrite absorbers is investigated by means of depth resolved hard X-ray photoemission spectroscopy of the near surface region. Two similar, slightly Cu-poor CIGSe absorbers were used with one being treated by potassium fluoride prior to the chemical bath deposition of an ultrathin CdS layer. The thickness of the CdS layer was chosen to be in the range of about 10 nm in order to allow the investigation of the CIGSe/CdS interface by the application of hard X-rays, increasing the information depth up to 30 nm. Besides strong intermixing on both samples, an increased Cu depletion of the KF treated absorber was observed in combination with an increased accumulation of Cd and S. In addition, a general shift of about 0.15 eV to higher binding energies of the CIGSe valence band at the absorber surface as well as the CIGSe and CdS related core levels was measured on the KF treated sample. This phenomenon is attributed to the impact of additional cadmium which acts as donor and releases further electrons into the conduction band of the absorber. Finally, the electrons accumulate at the CdS surface after having passed the interface region. This additional surface charge leads to a pronounced shift in the photoemission spectra as observed on the KF treated CIGSe absorber compared to the non-treated absorber.

Correlating the Local Defect-Level Density with the Macroscopic Composition and Energetics of Chalcopyrite Thin-Film Surfaces.

The unusual defect chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films is a main issue for a profound understanding of recombination losses in chalcopyrite thin-film solar cells. Especially, impurity-driven passivation of electronic levels due to point defects segregating at the surface and at grain boundaries is extensively debated. By combining current imaging tunneling spectroscopy with photoelectron spectroscopy, the local defect-level density and unusual optoelectronic grain-boundary properties of this material are correlated with the macroscopic energy levels and surface composition. Vacuum annealing of different CIGSe materials provides evidence that Na diffusion from the glass substrate does not affect the surface defect passivation or grain-boundary properties of standard Cu-poor materials. Furthermore, we find no major impact on the observed thermally activated dipole compensation or the accompanying change in surface band bending (up to 0.6 eV) due to Na. In contrast, Cu-rich CIGSe shows an opposing surface defect chemistry with only minor heat-induced band bending. Our results lead to a comprehensive picture, where the highly desirable type inversion at the p/n interface in standard chalcopyrite thin-film solar cells is dominated by band bending within the CIGSe absorber rather than the result of Na impurities or an n-type defect phase segregating at the interface. This is in accordance with recent studies suggesting a surface reconstruction as the origin for Cu depletion and band-gap widening at the surface of chalcopyrite thin films.