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CuInGaSe2 (CIGS) based compound semiconductors are among the leading materials for photovoltaic applications. Pulsed current electrochemical deposition is employed in this study to fabricate CIGS films wherein deposition is carried out from an electrolyte consisting chlorides of copper, indium, gallium and selenous acid at a pH around 2.2. The process also uses trisodium citrate which plays the role of complexing agent and aids in narrowing the reduction potentials of elements thereby easing the optimization to obtain desired composition. The precursor films are annealed in Ar at 550 °C and are characterized comprehensively. Results unveil that CIGS films possess compact agglomerated particle like morphology with an average particle size of ≈500 nm, copper-poor stoichiometric composition, and tetragonal chalcopyrite structure devoid of any unwanted secondary phases. Optical studies yield the bandgap to be ≈1.2 eV. Photoelectrochemical performance ascertains the photoactivity of CIGS under illumination. The low-cost pulse electrodeposition for obtaining pure quality CIGS films, which is also scalable and can be a great choice for fabrication of economic solar cells.

In order to analyze why the CIGS (CuInGaSe2) - based solar cells efficiency decrease for wide band gap (high Ga content), we have performed a series of samples by PVD using the three stage process. The average compositions of our solar cells range from x = 0 to x = 0.88, as measured by X-ray diffraction. An important feature of this process is to create a double Ga gradient into the absorber, which contributes to improve efficiencies, and this has a major impact on the determination of the sample composition from x-ray diffraction data. We have developed a model in order to assess this impact and question the validity of the compositions extracted from x-ray data. This model allows to get some information about gradient shapes. Using our model, we have obtained some insights on the evolution of the gallium gradient in samples with increasing Ga content, and we determine that this gradient is less pronounced when increasing the amount of gallium. It is a well known fact that this gradient assists the extraction of photocreated carriers, and the modifications of the gallium profile that we have determined may explain, for some part, the degradation of solar cells efficiency for high gallium compositions.

Photovoltaic (PV) energy generation has become one of the key pillars of the shift to a renewable energy future. Current devices, under favorable conditions, can already undercut the price per kWh electricity of other technologies on the market. Further reduction in the cost of installed PV systems and increase in solar module conversion efficiency will improve the affordability even more and will substantially aid in wider market penetration and enhance the volume of PV installations. Currently the PV market is dominated by silicon wafer based solar cells, but alternative technologies offer some distinctive advantages, making them interesting for numerous applications. Thin film technologies, as for example based on Cu(In,Ga)Se2 (CIGS) compound semiconductors with high optical absorption coefficient, are becoming important due to lower material and energy requirements for processing of high conversion efficiency solar cells. Inherent advantages are large area depositions with low production costs, and the possibilities for construction of lightweight, flexible devices with roll-to-roll manufacturing processes. The highest efficiency of single-junction CIGS solar cells is approaching the thermodynamic limit, making the use of alternative concepts such as concentration or multijunction (tandem-) devices the next logical step for further increase in efficiency beyond the Shockley-Queisser limit (S-Q limit). Especially the multi-junction technology, in which the thermodynamic losses are reduced by stacking of solar cells with different band gaps, decreasing thermalization of charge carriers excited with energies above the band gap, is a promising approach for enhanced utilization of the solar spectrum, yielding improved efficiency. Such devices, based on epitaxial layers of III-V compounds have already demonstrated remarkably high efficiencies beyond the S-Q limit. However, these devices grown on rather expensive single crystal wafers and with small size are prohibitively pricey for low cost terrestrial solar electricity generation. On the other hand, multi-junction solar cell technology based on polycrystalline thin films is an attractive option for large area, low cost production, provided adequately high efficiencies are achieved. In this context, two-junction tandem devices, developed by stacking a semitransparent large band gap solar cell of 1.6-1.7 eV on top of a low band gap (~1.0 eV) bottom cell, is a viable option. Earlier attempts in this direction were not so successful, but with the rise of perovskite thin film solar cells as a compatible high efficiency wide band gap (>1.6 eV) top cell and CIGS with a tunable band gap as bottom cell, the prospect for all thin film tandem devices with efficiencies beyond the single-junction limitations has opened. Such all thin film devices hold the potential for the low cost production necessary for large scale terrestrial application. This thesis focuses on the development of high efficiency narrow bandgap (1.0 eV) CIGS solar cells for application in all thin film tandem devices. While for CIGS with band gap of around 1.15 eV efficiencies of over 23 % have been demonstrated, cells with a narrow band gap close to 1.0 eV only reach 15.0 %. The efficiency of these narrow band gap cells are limited by charge carrier recombination, leading to low open circuit voltage (VOC) and reduced fill factor. For solar cell efficiency enhancement it is necessary to investigate the underlying reasons contributing to the deficits in PV parameters and develop processes to overcome the limiting factors. An option to reduce recombination within the solar cell is the implementation of a band gap grading as discussed in Chapter 3. The increase of the band gap at the location of highest recombination leads to a reduction in diode current, and therefore an increase in VOC. To keep the band gap of 1.0 eV a substantial part of the absorber needs to be Ga free. As the primary source of recombination is not obvious, different gradings (realized by a change in the Ga to In ratio) are implemented and compared. A single grading with increased band gap (higher Ga/In ratio) towards the front of the absorber shows no significant improvement on photovoltaic parameters. Any gain in VOC is offset by losses in current due to reduced charge collection, mainly visible for long wavelength photons and probably a result of the upwards bending in the conduction band. A single backgrading (higher Ga/In ratio towards the back electric contact) on the other hand leads to substantial improvements in performance ( from 12.0 % to 16.1 %). It is shown that the collection of photo-generated charge carriers improves and recombination is reduced. Measurements of the effective lifetime by time resolved photo-luminescence are carried out, showing an increase from approximately 20 ns to 100 ns when comparing ungraded with back-graded absorbers. By selectively changing the recombination speed at the back contact, strong differences in the behavior of cells with and without a band gap widening towards the back are observed. The results support that considerable recombination at the back contact is present in pure CIS solar cells, and that the single Ga back-grading approach is effective at suppressing this loss channel. In Chapter 4 the alkali treatment of CIS based solar cells is investigated. Alkali elements are known to strongly influence doping and passivation in CIGS solar cells. It is shown that the amount of sodium necessary to reach sufficient doping levels for high performance CIS solar cells is not achieved using the processes developed for CIGS. This may be based on insufficient Na diffusion into the grain, as those cells generally show larger grains than their CIGS counter parts, and since alkali migration energies in CIS are reported to be higher compared to those in CGS. If CIS cells are grown on soda lime glass without any diffusion barrier and additionally receive post deposition treatment (PDT) with NaF they still show low apparent doping concentration and poor PV performance ( = 10.9 %). However, additional annealing at ~ 370 C substrate temperature after PDT is shown to solve this problem, leading to an increase in apparent doping levels close to 1016 cm−3 and cell efficiency of 15.0 %. The application of an additional heavy alkali PDT, specifically RbF, is shown to lead to further improvements in cell efficiency. Changes at the front interface due to the PDT allow a decrease of buffer layer thickness, leading to a higher photo current (approximately + 1.0 mAcm−2). In addition, reduced recombination and the resulting increase in lifetime leads to additional gains in VOC, resulting in considerably improved device performance, up to an efficiency of 18.0 %. Further efficiency improvement is achieved by investigating the effect of close to stoichiometric compositions of Cu to group III elements as described in Chapter 5. The sub-stoichiometric Cu composition of state-of-the-art CIGS absorbers leads to a high concentration of detrimental defects. The defect density within the absorbers is reduced by approaching a stoichiometric Cu composition. Improvements in the defect density are identified by the decrease of Urbach energy from 20 to 16 mV and an increase in doping is observed for cells with almost stoichiometric Cu content. Cells with high, and especially stoichiometric Cu composition tend to be limited by recombination at the front interface, leading to a decrease of VOC of about 20 mV. Using the modified absorber surface after heavy alkali PDT, these losses are suppressed. Based on these improvements, a narrow band gap cell with record breaking 19.2 % efficiency and an open circuit voltage of 609 mV is achieved. Throughout the whole thesis the suitability of these cells for tandem devices with semitransparent perovskite top cells is investigated by 4-terminal tandem measurements. The improvements achieved in this work led to CIS based solar cells that not only show outstanding single cell performance, but also enable highly efficient tandem devices up to 25.0 %. They outperform state-of-the-art single junction CIGS and perovskite cells while showing prospects for further efficiency improvement. Due to the low band gap of the CIS absorber the current density from the bottom cell is high enough to produce current matched tandem devices with high efficient perovskite top cells (19.2 to 18.6 mAcm−2 in 4-terminal configuration), and also monolithic two-terminal configurations are feasible in the future.

Thin films of CuGaSe were deposited on n-Si (1 0 0) by rf magnetron sputtering from a stoichiometric CuGaSe2 target. The objective of this study was to characterize the thin film/Si heterojunction for potential photovoltaic applications, evaluate possible candidates for metal contacts and to establish whether heteroepitaxial growth could be achieved, particularly as the mismatch of lattice parameters corresponding to the base of the copper gallium selenide (CGS) tetragonal cell is quite close to that of Si, with a 2.9% mismatch. For this study, Si substrates were prepared by the standard Radio Corporation of America (RCA) cleaning procedure immediately followed by the deposition of CGS by sputtering at a substrate temperature of 600°C. The deposited thin-film stoichiometry and morphology were characterized by Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). Rutherford back scattering (RBS) analysis indicated a thin-film composition of Cu1Ga1Se1 indicating that the films were Se deficient, although channeling was not observed. The polycrystalline nature of the deposited thin film was established by cross-sectional TEM. An estimated 1.5-nm thick layer likely to be SiO x was observed at the CGS–Si interface. It is believed that this interfacial layer prevented heteroepitaxy CGS on Si. Additionally, circular metal contacts were deposited on the thin films and characterized by capacitance and current–voltage measurements. It was observed that Al and Ag contacts were rectifying, from which the thin-film carrier density was estimated to be ~5 × 1015 and ~7.68 × 1015 cm−3 with Al and Ag contacts, respectively. Au, Pt, W and Cr were ohmic, and Mo and Ni provided semi-ohmic contacts to CGS films.

The aim of this PhD thesis work is to develop an innovative process for growing Cu(In,Ga)Se2 thin films, starting from precursors obtained by new materials, like Indium Selenide In2Se3 and InSe, Gallium Selenide Ga2Se3 and GaSe and Copper, suitable for producing in a reproducible way high-efficiency solar cells. Lo scopo di questo lavoro di Tesi di Dottorato di Ricerca è sviluppare un processo innovativo di crescita di film sottili di Cu(In,Ga)Se2, partendo da precursori ottenuti con nuovi materiali, quali Seleniuro di Indio In2Se3 e InSe, Seleniuro di Gallio Ga2Se3 e GaSe e Rame, adatto alla produzione di celle solari ad alta efficienza in modo riproducibile.

Copper Indium Gallium Selenide has become one of the most highly promising absorber materials for thin film solar cells due to its exceptional semiconductor characteristics. Various attempts have been made in the recent years to scale up the production of these films. In this review the difficulty in scaling up of the existing technologies, the necessity of nano-particles of CIGS for solar cells, how the charge separation in this nano scale photovoltaic (PV) materials occurs which help in absorption of radiation, and the electro-deposition route, a low cost one, produces thin film solar cells are analyzed. The envisaged pulse electro-deposition route will not only produce the required nano-scale CIGS films but also a route to large scale economical production of thin films 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.

Cu(In,Ga)Se2 and CdTe heterojunction solar cells grown on rigid (glass) or flexible foil substrates require p-type absorber layers of optimum optoelectronic properties and n-type wide-bandgap partner layers to form the p–n junction. Transparent conducting oxide and specific metal layers are used for front and back electrical contacts. Efficiencies of solar cells depend on various deposition methods as they control the optoelectronic properties of the layers and interfaces. Certain treatments, such as addition of Na in Cu(In,Ga)Se2 and CdCl2 treatment of CdTe have a direct influence on the electronic properties of the absorber layers and efficiency of solar cells. Processes for the development of superstrate and substrate solar cells are reviewed. Copyright © 2004 John Wiley & Sons, Ltd.