CuInSe2 Surface Research Articles

Crystallization Modulation and Holistic Passivation Enables Efficient Two-Terminal Perovskite/CuIn(Ga)Se2 Tandem Solar Cells

HighlightsIntegrating perovskite solar cells onto irregular rough CuIn(Ga)Se2 (CIGS) surfaces remains a challenge; new strategy was explored to develop monolithic perovskite/CIGS tandem solar cell by manipulating the crystallization of perovskite.Surface reconstruction and field-effect passivation are developed synergistically to issue complex interface relationship between perovskite and C60.The champion power conversion efficiency （PCE） of 24.6% realized, providing significant commercial opportunities for all thin-film-based perovskite/CIGS tandem cells.

The Atomic Structure of Polar Cuinse2 (001) Surface

Through the first-principles calculations, we studied the atomic structure of polar CuInSe2 (001) surface. It is revealed that polar CuInSe2 (001) surface has different atomic structure for different atom-terminated surface models. Before and after relaxation, the atomic structure of different atom-terminated surface models is various. Bond lengthens are also changed.

Effect of sodium diffusion on the properties of CIGS solar absorbers prepared using elemental Se in a two-step process

The influence of Na diffusion from various glass substrates during a high-temperature slenization process on the microstructure and morphology of two-step formed CIGS absorber layers is investigated. In order to minimise the CIGS absorber formation time, elemental Se vapour is used to prepare the CIGS absorber. The grain sizes of the CIGS films are found to increase with increasing sodium in the glass substrates (extra clear glass, soda-lime glass, borosilicate glass). TiN and SiN thin films are used as diffusion barrier layers inserted between the glass substrate and the Mo rear conatct to tune the Na diffusion from the soda-lime glass. The interdiffusion between the In-rich CuInSe2 surface layer and the Ga-rich CuGaSe2 layer is promoted by the barrier layer, leading to larger CIGS grains. Efforts are also taken to understand the differences in Na diffusion (from the glass substrates) and their effects on the MoSe2 intermediate layer formation during the high-temperature CIGS absorber formation processes. We find that excess amounts of Na and Se are essential for the MoSe2 growth. The excessive Na in the form of Na2Sex at the CIGS/Mo interface works as a Se source and catalyses the MoSe2 formation. The Se flow in the two-step CIGS formation process must be sufficiently high to obtain high-efficiency CIGS solar cells.

Transient and modulated charge separation at CuInSe2/C60 and CuInSe2/ZnPc hybrid interfaces

Spectral dependent charge transfer and exciton dissociation have been investigated at hybrid interfaces between inorganic polycrystalline CuInSe2 (untreated and Na-conditioned) thin films and organic C60 as well as zinc phthalocyanine (ZnPc) layers by transient and modulated surface photovoltage measurements. The stoichiometry and electronic properties of the bare CuInSe2 surface were characterized by photoelectron spectroscopy which revealed a Cu-poor phase with n-type features. After the deposition of the C60 layer, a strong band bending at the CuInSe2 surface was observed. Evidence for dissociation of excitons followed by charge separation was found at the CuInSe2/ZnPc interface. The Cu-poor layer at the CuInSe2 surface was found to be crucial for transient and modulated charge separation at CuInSe2/organic hybrid interfaces.

Interface formation of CuInSe2 (112) and ZnO deposited by atomic layer deposition

Interfaces of epitaxial CuInSe2 (112) surfaces to ZnO have been prepared by Atomic Layer Deposition (ALD) and are analyzed in situ by photoelectron spectroscopy at BESSY. LEED data show a (0001) oriented ZnO film for thicker films despite the large lattice mismatch. In contrast to results obtained from ZnO-MOMBE (Metal-Organic Molecular Beam Epitaxy) preparation where an inherent ZnSe interface layer of 2nm is formed, for ALD a ZnIn2Se4 interface layer is observed which is introducing a different band alignment as in the MOMBE case.

Schottky diode properties of CuInSe2 films prepared by a two-step growth technique

CuInSe2 films were synthesized by a thermal evaporation method on both glass and Mo coated glass substrates. To obtain Al/CuInSe2/Mo Schottky diode, Al metal was evaporated on the upper surface of CuInSe2 as a front contact and electrical properties of the structure were analyzed. The structural and optical behaviors of the CuInSe2 thin films were also investigated. The X-ray diffraction studies showed that the chalcopyrite CuInSe2 was obtained with a preferential orientation in the (112) plane with lattice parameters a and c as 0.577 and 1.161nm, respectively. Mo back contact layer had a preferential orientation in the (110) plane. Scanning electron microscopy equipped with energy dispersive spectroscopy revealed an irregular and rough surface morphology with Cu-rich protruding regions. Optical studies showed the existence of three different band gaps, which were determined as 1.06, 1.17 and 1.39eV, respectively. From the Hall Effect measurements, we determined the carrier concentration of CuInSe2 films as 4.0×1017cm−3. The electrical properties of the CuInSe2 films were further studied by fabricating Al/p-CuInSe2/Mo structures and obtaining their forward and reverse bias current–voltage characteristics in a wide temperature range of 100–300K, in steps of 25K. A thorough analysis of the forward bias current–voltage characteristics based on thermionic emission theory showed that the zero bias barrier height increases while series resistance and ideality factor decreases with an increase in temperature. After a barrier height inhomogeneity correction, the Richardson constant and mean barrier height were found to be 34.71A/cm2K2 and 0.72eV, respectively.

Stoichiometry and surface reconstruction of epitaxial CuInSe2(112) films

CuInSe2(112) films were grown on GaAs(111)A substrates by molecular beam epitaxy. The resulting surface stoichiometry was deduced by consideration of results from various surface analytic techniques. The obtainable Cu/In stoichiometry range in XPS was 0.4–1.2, where 1.2 marks the onset of Cu2−xSe phase segregation at the surface and 0.4 corresponds to the copper-depleted surface with ordered defect compound (ODC) composition. For the stoichiometric CuInSe2(112) surface, a c(4×2) reconstruction of the zinc blende surface periodicity is observed in the LEED pattern, with three rotational domains present on the flat GaAs(111) substrate. With the use of stepped (111) substrates, domain formation could be suppressed. By comparison of the LEED data and concentration depth profiles from angle-resolved XPS, two types of surface reconstructions could be distinguished. According to surface energy calculations in the literature, these correspond to surfaces stabilized by either CuIn or 2VCu defects. The surface of copper-poor CuIn3Se5 shows no reconstruction of the zinc blende order.

Atomic-scale characterization of the CdS/CuInSe2 interface in thin-film solar cells

Elemental mixing at the CdS/CuInSe2 interface of a thin-film solar cell was studied by means of atom probe tomography. A Cu-depleted and Cd-doped region (∼2 nm in width) was detected at the CuInSe2 surface, proving the existence of a buried p-n homojunction within the CuInSe2 absorber layer. Furthermore, CdS was found to infiltrate open pores existing in CuInSe2 during the chemical bath deposition. This could explain why chemical bath deposition of CdS leads to higher solar cell efficiencies compared to physical vapor deposition of CdS.

Terahertz radiation from nonstoichiometric CuInSe2 films excited by femtosecond laser pulses

We report on the observation of efficient terahertz radiation from the surface of CuInSe2 excited by femtosecond laser pulses. Terahertz radiation emitted by polycrystalline CuInSe2 layers manufactured by using electrodeposition technology was as powerful as the signals radiated by single-crystalline semiconductor surfaces. It has been found that terahertz radiation efficiency is critically dependent on the stoichiometry of the CuInSe2 layers. The results of a double-pulse excitation experiment have indicated that terahertz radiation from the photoexcited surfaces of CuInSe2 samples was caused by the presence of a built-in electrical field at those surfaces.

Band structure investigation of chalcopyrite CuInSe2(001) by angle-resolved photoelectron spectroscopy

AbstractClean and ordered chalcopyrite CuInSe2 surfaces are a precondition for the study of the electronic structure by angle-resolved photoelectron spectroscopy. The preparation of welldefined CuInSe2(001) surfaces by the combination of molecular beam epitaxy and a selenium capping and decapping process is described. The surface structure of CuInSe2 epilayers with different bulk composition is compared and analysed by low-energy electron diffraction.Employing near-stoichiometric surfaces, the valence electronic structure of CuInSe2 was investigated by angle-resolved photoelectron spectroscopy at the synchrotron source BESSY 2. This is the first study of the valence band structure of a copper chalcopyrite semiconductor material by photoelectron spectroscopy. The valence band dispersion along τT, i.e. the [001] direction, was investigated by a variation of the excitation energy from 10 to 35 eV under normal emission, and the band dispersion along τT, i.e. the [110] direction, was analysed by angular scans with hv = 13 eV.The valence bands derived from antibonding and bonding Se4p-Cu3d hybrid orbitals, nonbonding Cu3d states and In-Se hybrid states are clearly indentified. The strongest dispersion is found for the topmost valence band with a bandwidth of ∼0.7 eV from τ to T. From τ to N, the observed dispersion was 0.5 eV. The experimental valence bands are discussed in relation to calculated band structures in the literature.

Cd doping at the CuInSe2/CdS heterojunction

The chemical composition of the CuInSe2/CdS heterojunction interface is investigated by angle resolved x-ray photoelectron spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy in combination with selective etching of CdS. We demonstrate that ∼0.8 monolayer of Cd is incorporated into the first 1–3 atomic layers of the CuInSe2. This is accompanied by significant Cu depletion with respect to In in the same region. The results suggest that CdCu defects heavily dope CuInSe2 surface n type and cause the observed large band bending on the CuInSe2 side of the heterojunction.

Cu depletion at the CuInSe2 surface

The chemical composition of the (112)B surface of epitaxial CuInSe2 thin films is investigated by angle resolved x-ray photoelectron spectroscopy. Results show that a severe Cu depletion exists in the top 1–2 atomic layers. No bulk second phase is found at the surface. The source of this depletion and its relation to the Cd doping at the CdS/CuInSe2 interface are discussed.

Induced photopleochroism of CuInSe2 structures prepared byheat treatment and deposition of CdS and In2O3 thin films

Photosensitive structures have been prepared by heat treatment ofp- and n-CuInSe2 polycrystalline substrates at500 °C in vacuum or air and subsequent deposition of CdS andIn2O3 thin films on the CuInSe2 surface. Thephotosensitivity spectra of the structures have been measured innatural and linearly polarized radiation. The photosensitivity isanalysed in relation to the structure preparation conditions.Induced polarization photosensitivity was observed, andpeculiarities of this phenomenon caused by the antireflectioneffect in thin films of the prepared structures are discussed. Aconclusion is made that the polarization photoelectricspectroscopy can be used for diagnostics of photovoltaic solarcells.

Effects of Na on the electrical and structural properties of CuInSe2

We found theoretically that Na has three effects on CuInSe2: (1) If available in stoichiometric quantities, Na will replace Cu, forming a more stable NaInSe2 compound having a larger band gap (higher open-circuit voltage) and a (112)tetra morphology. The ensuing alloy NaxCu1−xInSe2 has, however, a positive mixing enthalpy, so NaInSe2 will phase separate, forming precipitates. (2) When available in small quantities, Na will form defect on Cu site and In site. Na on Cu site does not create electric levels in the band gap, while Na on In site creates acceptor levels that are shallower than CuIn. The formation energy of Na(InCu) is very exothermic, therefore, the major effect of Na is the elimination of the InCu defects and the resulting increase of the effective hole densities. The quenching of InCu as well as VCu by Na reduces the stability of the (2VCu−+InCu2+), thus suppressing the formation of the “Ordered Defect Compounds.” (3) Na on the surface of CuInSe2 is known to catalyze the dissociation of O2 into atomic oxygen that substitutes Se vacancy (shallow donors), converting them into OSe. We find, however, that OSe is an (isovalent) deep rather than shallow acceptor. We also find that having removed the donors, O atoms in CuInSe2 form Cu2O and In2O3 compounds, and phase separate, forming precipitates at the surfaces and grain boundaries. Our results are compared with previous models and provide new insights into the defect physics of Na in CIS.

Fermi-level-dependent defect formation in Cu-chalcopyrite semiconductors

Valence-band photoelectron spectroscopy of CuInSe2, CuInS2, and CuGaSe2 surfaces and interfaces give evidence for the formation of Cu vacancies when the Fermi level moves upwards in the band gap due to contact formation. The effect might be a key issue in understanding basic properties of solar cell devices based on these materials.

Photoelectron spectroscopy of chalcopyrites and Zn based II–VI semiconductor heterostructures

We have investigated CuInSe2 surfaces as well as the interfaces of the systems ZnSe/CuInSe2 and ZnS/CuInSe2 by vacuum ultraviolet photoelectron spectroscopy with synchrotron radiation. The CuInSe2 substrates are prepared with a hydrogen plasma and show high quality surfaces suitable for further growth of ZnSe in order to determine the valence-band offset of ZnSe/CuInSe2. At the surface of CuInSe2 Se atoms have been replaced by Te or S atoms, so that CuInTe2 or CuInS2 surfaces were formed. We determined the valence-band offsets of CuInTe2/CuInSe2 and CuInS2/CuInSe2 to be ΔEvb(CuInTe2/CuInSe2)=(0.85±0.05)eV and ΔEvb(CuInS2/CuInSe2)=(0.87±0.05)eV. ΔEvb(ZnSe/CuInSe2)=−(0.5±0.1)eV is in good agreement with values from literature. Furthermore we have examined the interface ZnS/CuInS2 and obtained the valence-band offset ΔEvb(ZnS/CuInS2)=−(2.3±0.1)eV.

Interaction of Atomic Hydrogen with the Surface of Cu-llII-V12 Chalcopyrite Semiconductors-A Method for Controlled Stoichiometry Variation

ABSTRACTTo introduce atomic hydrogen into Cu-chalcopyrite samples, low energy broad beam ion implantation into heated targets was used. In addition to the expected hydrogen diffusion into the sample from the implanted thin surface layer, another hydrogen-related effect was observed. As demonstrated for single crystalline CuInSe2, at target temperatures above 150°C a surface layer becomes In-rich. Two phases, the CuInSe2 α-phase and a In-rich phase similar to the reported ordered vacancy compounds (β-phase) coexist, as detected by Raman spectroscopy. The depth profile and the thermal stability of this hydrogen-related compositional variation are investigated. Based on the results we discuss a model of this effect, involving a copper in-diffusion, caused by the hydrogenation due to filling of Cu vacancies and possible substitution of Cu by H. The observed β-phase-like compound is unstable against annealing above 180°C, but can be restored in a thermal cycling process. Annealing for 1.5 h at 400°C removes it completely and restores the asgrown CuInSe2 surface. A decrease of the hydrogen concentration in the surface layer due to redistribution and out-diffusion, followed by the recovery of the Cu content, might be responsible for this behaviour.

X-ray photoemission analysis of chemically treated I–III–VI semiconductor surfaces

Device-grade thin-film CuInSe2 was subjected to various chemical treatments commonly used in photovoltaic device fabrication to determine the resulting microscopic surface composi- tion/morphology and the effect on II–VI CuInSe2 heterojunction formation. HCl (38%), Br–MeOH (<1% Br), (NH4)2S, and NH4OH/thiourea solutions were used separately to modify the surface chemistry of the CuInSe2 polycrystalline films. Scanning electron microscopy was used to evaluate the resultant surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and Cu 2p, In 3d, Ga 2p, and Se 3d core lines were used to evaluate the chemistry of the chemically treated surfaces. CdS overlayers were then deposited on these chemically treated surfaces. Photoemission measurements were acquired to determine the resultant heterojunction valence-band discontinuity between the CdS and the chemically modified CuInSe2 surface.

Partial density of states in the CuInSe2 valence bands

The valence band spectra of a vacuum cleaved CuInSe2(011) surface were measured with synchrotron radiation at photon energies between 16 and 95 eV. The strong dependence of the photoionization cross section of atomic levels between 28 and 60 eV is used to divide the valence band emissions into contributions from Se 4p and Cu 3d states in order to map the respective partial density of states. The derived partial density of Cu 3d states to the total valence band density of states is around 50% in the upper part of the valence band and about 75% at its maximum corresponding to non-bonding Cu d states.

Chemical interaction of Na with cleaved (011) surfaces of CuInSe2

To study the beneficial effect of sodium-containing substrate material on the photovoltaic properties of thin-film CuInSe2 solar cells the chemical interaction of Na with CuInSe2 has been investigated by synchrotron excited photoelectron spectroscopy. A clean CuInSe2 (011) surface was prepared by cleaving an oriented single crystal in UHV. The cleaved surface exhibits an electron affinity of χ≊4.6 eV. The Se 3d level shows a surface core level shift of −0.4 eV. Na was sequentially deposited in UHV in small steps from a commercial dispenser source. Initially ionized Na adsorbs on the surface leading to a shift of the surface Fermi level by 0.3 eV closer to the conduction band. Removal of Cu from the surface is observed. For higher Na deposition a chemical reaction of Na with the CuInSe2 surface occurs leading to the formation of metallic indium and Na2Se.