Se2-based Thin Film Solar Cells Research Articles

Observation and manipulation of CIGSe phase formation in a two stage sequential process

The formation process of the absorber layer in Cu(In,Ga)Se2-based (CIGSe) thin film solar cells is complex and still not understood in detail. Specifically, the Ga gradient of CIGSe solar cells has a strong impact on the efficiency. In a common sequential process, the gradient cannot be adjusted directly by a variation of the sputtered metal precursor layers. Therefore, CIGSe solar cells were produced in a two stage sequential process. The selenization was further split into two stages at different selenization temperatures in order to manipulate the phase formation. We observed that a variation of the selenization temperature in the first stage leads to absorber layers with significant changes in the Ga distribution and show that the formation of the CIGSe phase starts in the center of the deposited precursor layers and not at any interface to the back contact or at the surface.

Optimization of Intrinsic ZnO Thickness in Cu(In,Ga)Se2-Based Thin Film Solar Cells.

The typical structure of high efficiency Cu(InGa)Se2 (CIGS)-based thin film solar cells is substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al(AZO) where the sun light comes through the transparent conducting oxide (i.e., i-ZnO/AZO) side. In this study, the thickness of an intrinsic zinc oxide (i-ZnO) layer was optimized by considering the surface roughness of CIGS light absorbers. The i-ZnO layers with different thicknesses from 30 to 170 nm were deposited via sputtering. The optical properties, microstructures, and morphologies of the i-ZnO thin films with different thicknesses were characterized, and their effects on the CIGS solar cell device properties were explored. Two types of CIGS absorbers prepared by three-stage co-evaporation and two-step sulfurization after the selenization (SAS) processes showed a difference in the preferred crystal orientation, morphology, and surface roughness. During the subsequent post-processing for the fabrication of the glass/Mo/CIGS/CdS/i-ZnO/AZO device, the change in the i-ZnO thickness influenced the performance of the CIGS devices. For the three-stage co-evaporated CIGS cell, the increase in the thickness of the i-ZnO layer from 30 to 90 nm improved the shunt resistance (RSH), open circuit voltage, and fill factor (FF), as well as the conversion efficiency (10.1% to 11.8%). A further increas of the i-ZnO thickness to 170 nm, deteriorated the device performance parameters, which suggests that 90 nm is close to the optimum thickness of i-ZnO. Conversely, the device with a two-step SAS processed CIGS absorber showed smaller values of the overall RSH (130–371 Ω cm2) than that of the device with a three-stage co-evaporated CIGS absorber (530–1127 Ω cm2) ranging from 30 nm to 170 nm of i-ZnO thickness. Therefore, the value of the shunt resistance was monotonically increased with the i-ZnO thickness ranging from 30 to 170 nm, which improved the FF and conversion efficiency (6.96% to 8.87%).

Coevaporated KInSe2: A Fast Alternative to KF Postdeposition Treatment in High-Efficiency Cu(In,Ga)Se2Thin Film Solar Cells

Cu(In,Ga)Se2-based thin film solar cells have reached 22.3% energy conversion efficiency. This outstanding level of performance has been made possible by the use of a so-called potassium fluoride postdeposition treatment (KF-PDT) after the absorber synthesis. Such a treatment, consisting of evaporating KF under Se atmosphere, has been suggested to enhance the formation of a KInSe2 surface layer. In this paper, we propose an alternative to the standard KF-PDT, in which we simultaneously evaporate KF and In under Se atmosphere. We compare by X-ray photoemission spectroscopy the modified absorber surfaces in both cases and discuss the advantages of this alternative in terms of robustness and rapidity of the process.

Enhanced Conversion Efficiencies of Cu(In,Ga)Se2-Based Thin Film Solar Cells Using New Precursors by Sputtering

Enhanced Conversion Efficiencies of Cu(In,Ga)Se2-Based Thin Film Solar Cells Using New Precursors by Sputtering

Light soaking induced doping increase and sodium redistribution in Cu(In,Ga)Se2-based thin film solar cells

Light-induced metastabilities of Cu(In,Ga)Se2 (CIGS)-based thin film solar cells have been extensively researched for many years. It is commonly observed that junction capacitance and p-doping level in CIGS absorbers increase considerably after light soaking (LS). In this work, we focus on the LS behaviors of cells with different minority carrier lifetimes (τn). Experiments show that high efficiency cells with long τn lose open circuit voltage (Voc) and fill factor (FF) upon LS, whereas low efficiency cells with short τn lose less or even gain Voc and FF. The sodium content measured with glow discharge optical emission spectroscopy (GD-OES) increases in the region close to the CdS/CIGS interface with LS and may contribute to the observed LS behaviors. The change in electrical parameters is explained with simulations, which relate the Voc and FF changes to a reduced recombination rate in the space charge region due to the light-induced doping increase. The simulations also suggest that cells with higher n-doping in the CdS are less sensitive to changes in interface recombination rate and doping of CIGS, which agrees with the hypothesis that the CdS buffer deposition is important for the LS behavior.

Impact of DC-power during Mo back contact sputtering on the alkali distribution in Cu(In,Ga)Se2-based thin film solar cells

DC-sputtered Mo back contact layers were deposited on soda-lime glass (SLG) with different power densities applied on the Mo target to study its influence on the photovoltaic performance of Cu(In,Ga)Se2-based (CIGSe) solar cell. CIGSe absorber was then deposited simultaneously on these SLG/Mo, following the 3-stage process. These devices have good but different photovoltaic performance (>16% efficiency without MgF2 coating). To find a material origin, secondary ion mass spectroscopy (SIMS) profiles were carried out on complete cells, revealing that Na and K content and distribution in each layer depend on the deposition conditions of the back contact. Even before the CIGSe deposition and despite similar morphologies, Na content can vary 10-fold from one Mo layer to another. The same applies to the absorber; when grown on a different Mo they present the same grain boundary density but different alkali contents in bulk or at interfaces. This has an influence on the compositional grading in absorber, confirmed by X-ray diffraction and SIMS.

Defect spectroscopy of Cu(In,Ga)Se2-based thin film solar cells on polyimide substrate

Cells made on a polyimide substrate, with sodium supplied at different stages of the absorber deposition, have been studied by defect spectroscopy methods. In all of the cells, admittance spectroscopy measurements revealed the presence of two defect levels. Discussion on the origin of the observed levels, based on the correlation with free carrier concentration profiles and photocurrent results, is presented. We attribute one of the levels as belonging to interface defects, and the second one to a bulk deep defect. A distinctive feature of defects' sensitivity to a hole concentration, which leads to unusual behaviour of defect parameters is discussed.

Preparation and characterization of ZnS thin films by the chemical bath deposition method

ZnS thin films prepared on quartz substrates by the chemical bath deposition (CBD) method with three type temperature profile processes have been investigated by X-ray diffraction, scanning electron microscope, energy dispersive X-ray analysis and light transmission. One is a 1-step growth process, and the other is 2-steps growth and self-catalyst growth processes. The surface morphology of CBD-ZnS thin films prepared by the CBD method with the self-catalyst growth process is flat and smooth compared with that prepared by the 1-step and 2-steps growth processes. The self-catalyst growth process in order to prepare the particles of ZnS as initial nucleus layer was useful for improvement in crystallinity of ZnS thin films prepared by CBD. ZnS thin films prepared by CBD method with self-catalyst growth process can be expected for improvement in the conversion efficiency of Cu(InGa)Se2-based thin film solar cells by using it for the buffer layer.

Incorporation of Cu in Cu(In,Ga)Se2-based Thin-film Solar Cells

of 11.1%. The other prepared with a 3-step process had nearly the same In-Ga and Cu concentrations and showed a conversion eciency of 15.5%. We discuss the incorporation of Cu in the two types of thin-film solar cells.

Buffer layers in Cu(In,Ga)Se 2 solar cells and modules

We report the technical progress that has occurred in the last decade in the development of Cd-free buffer layers for Cu(In,Ga)Se 2-based thin-film solar cells and modules. In different laboratories, films based on ZnS, ZnSe, ZnO, (Zn,Mg)O, In(OH) 3, In 2S 3, In 2Se 3, InZnSe x , SnO 2, and SnS 2 were deposited on differently processed absorbers and tested as an alternative to the traditional CdS buffer. The deposition methods used are: chemical bath deposition (CBD), atomic layer deposition (ALD), metal organic chemical vapour deposition (MOCVD), ion layer gas reaction (ILGAR), sputtering, thermal evaporation, and electrodeposition (ED). Several processes demonstrated efficiencies comparable with the CBD CdS standard. The highest total-area cell efficiency of 18.6% was achieved by NREL and AGU with a CBD ZnS-based buffer layer on a three-stage-process absorber. Showa Shell fabricated several 30×30 cm 2 modules with efficiencies up to 14.2% using a CBD ZnS-based buffer layer on Cu(In,Ga)(Se,S) 2 absorbers grown after the selenisation/sulphurisation of precursor metal films. ZSW fabricated 30×30 cm 2 modules with an ALD In 2S 3 buffer on in-line evaporated Cu(In,Ga)Se 2 absorbers with a maximum aperture-area efficiency of 12.9%.

Structural and chemical investigations of CBD- and PVD-CdS buffer layers and interfaces in Cu(In,Ga)Se2-based thin film solar cells

It is known that high-efficiency thin film solar cells based on Cu(In,Ga)Se2 (CIGS) can be obtained using CdS buffer layers grown by chemical bath deposition (CBD). The highest efficiencies achieved with CdS buffer layers produced by physical vapor deposition (PVD) are significantly lower. To find reasons for this difference, structural and chemical properties of CBD- and PVD-CdS buffer layers and their interfaces with CIGS were investigated by means of bright-field (BF-TEM), high-resolution (HR-TEM) and energy-filtered transmission electron microscopy (EF-TEM), and also by energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM). PVD-CdS grains were shown to be clearly larger than the CBD-CdS grains. Also, a large defect density was detected at the PVD-CdS/CIGS interface, which is attributed to the larger lattice mismatch than at the CBD-CdS/CIGS interface. Cu diffusion from CIGS into CdS was found for the CBD- and the PVD-CdS sample. The PVD-CdS/CIGS interface turned out to be quite abrupt, whereas the CBD-CdS/CIGS interface is rather diffuse. The differences in efficiencies of solar cells with CBD- and PVD-CdS buffer layers can partly be explained by referring to the higher defect density and the probable absence of an inversion of the near-interface region from p- to n-type at the PVD-CdS/CIGS interface.

98/01421 Quality control and characterization of Cu(In,-Ga)Se 2-based thin-film solar cells by surface photovoltage spectroscopy : Kronik, L. et al. Solar Energy Materials and Solar Cells, 1998, 51, (1), 21–34

98/01421 Quality control and characterization of Cu(In,-Ga)Se 2-based thin-film solar cells by surface photovoltage spectroscopy : Kronik, L. et al. Solar Energy Materials and Solar Cells, 1998, 51, (1), 21–34

Quality control and characterization of Cu(In,Ga)Se 2-based thin-film solar cells by surface photovoltage spectroscopy

Surface photovoltage spectroscopy (SPS) has been used for quality control of ZnO/CdS/ Cu(In,Ga)Se 2 (CIGS) thin-film solar cells. The results show that SPS makes it possible to detect “hard failures” following CIGS deposition, and both “hard” and “soft” failures following CdS deposition and following ZnO deposition. In addition, a semi-quantitative screening of CdS/CIGS and ZnO/CdS/CIGS samples is possible. Hence, SPS is suggested as a useful tool for in-line monitoring of CIGS-based solar cell production lines. Moreover, SPS is shown to yield important new information regarding CIGS-based solar cells: (a) A deep gap state is found in samples of superior performance. (b) As opposed to the CdS CIGS structure, a marked decrease in the open-circuit voltage upon Na contamination in ZnO CIGS structures is found.

Influence of Na and H2O on the surface properties of Cu(In,Ga)Se2 thin films

The influence of humidity on the electronic structure of Na-containing polycrystalline Cu(In,Ga)Se2 thin films on soda-lime glass substrates has been investigated by x-ray and UV photoemission as well as by Auger electron spectroscopy. Different interactions between coadsorbed Na, H2O, and the Cu(In,Ga)Se2 surface are revealed at low temperatures and upon annealing at room temperature. Both, reversible and irreversible interactions such as a H2O-induced reduction of the Na surface content and the formation of a Na–O–Cu(In,Ga)Se2 complex are observed. Our findings can be correlated with the influence of ambient conditions on Cu(In,Ga)Se2 solar cell adsorbers and demonstrate the importance of adequate encapsulation of Na-containing Cu(In,Ga)Se2-based thin-film solar cells.