CIGS Cells Research Articles

Effects of Zn Diffusion from (Zn,Mg)O Buffer to CIGS Film on the Performance of Cd-Free Cu(In,Ga)Se2 Solar Cells

A diffusion of Zn into the CIGS film and its effects on the device performance were investigated when the atomic layer deposited (Zn,Mg)O film was used as a buffer layer of CIGS solar cells. SIMS and ICP mass analysis showed that the Zn concentration in the CIGS film was increased with increasing ALD process time. The hole concentration in the CIGS film was decreased by Zn incorporation. It was suggested that open circuit voltage degradation in CIGS cell with Zn incorporation was mainly due to enhanced recombination by Zn defects instead of Fermi-level change. The results suggest that the ALD process should be as short as possible to avoid Zn diffusion. On the other hand, a small amount of Zn incorporation on the surface of the CIGS film by post rapid thermal annealing of a (Zn,Mg)O/CIGS cell at 200°C showed an increased cell performance. However, excess Zn incorporation on the surface of the CIGS film caused degradation of cell performance due to the generation of a new defect. The generation of a deep-level defect and the change of the minority carrier lifetime were characterized by low-temperature PL and time-resolved PL, respectively. The cell performance was correlated with these results.

CIGS Cells and Modules With High Efficiency on Glass and Flexible Substrates

Thin-film solar cells based on Cu(In,Ga)(Se,S) 2 (CIGS) have demonstrated both high efficiencies and a high cost-reduction potential in industrial production. This way, future CIGS module production lines can be profitable even for scales below the GW range. Among the different technologies, only the coevaporation method has demonstrated efficiencies above 20%, approaching the record values of polycrystalline Si cells. The main focus of this contribution is on the new results of the ZSW cell line with efficiencies above 20%, as well as on the mini-module line on glass substrates. Mini modules (10 cm × 10 cm) with efficiencies in the range of 17% give a proof of concept for industrial-sized modules. ZSW is also developing flexible cells and modules, transferring the processes from the glass-based technology. We achieved 18.6% cell efficiency on metal substrates and a 15.4% efficient mini module could be demonstrated with adapted methods of module patterning. In order to develop industrially relevant processes for foils, we are running a roll-to-roll deposition plant. Additionally, we have improved CIGS cell efficiencies with alternative buffers to certified 19.0% for solution-grown Zn(O,S), to 16.4% for sputtered Zn(O,S), and 17.1% for evaporated In 2S 3. Our cells deposited by vacuum-free methods exhibit an efficiency of 8.5% with a nanoparticle-based process.

Impact of solar upconversion on photovoltaic cell efficiency: optical models of state-of-the-art solar cells with upconverters

ABSTRACTCurrent photovoltaic technologies harvest only a fraction of incoming solar energy since they are unable to utilize photons with energies below the cell band gap. Placed behind a solar cell, the upconverter converts transmitted low-energy photons to photons with energies higher than the cell band gap. The higher energy photons are absorbed by the solar cell and contribute to the photocurrent. We developed optical models of several state-of-the-art commercial and research thin-film solar cells incorporating the upconversion layer. We present both analytical models based on published EQE data as well as detailed finite difference time domain (FDTD) models that incorporate absorption in all cell layers. We model the improvement in absorption and overall cell performance of amorphous Si, CIGS, GaAs, CdTe, and Cu2O cells with upconverting layers. We incorporate and discuss the effect of interface texture and different cell layers on the absorption of upconverted photons and make suggestions for improving the overall cell design to get the maximum benefit from upconversion. We estimate that the cell efficiency enhancement can range from 0.5% to up to 5% absolute depending on the cell type and upconversion efficiency. This work connects to the fundamental efficiency limit analysis of narrow-bandwidth solar upconversion by our collaborators [1], but presents concrete optical models of current solar cells and discusses the promise of upconversion for particular applications.

Model for increased efficiency of CIGS solar cells by a stepped distribution of carrier density and Ga in the absorber layer

In this paper, several structures for multilayer Cu (In1−x Ga x ) Se2 (CIGS) thin film solar cells are proposed to achieve high conversion efficiency. All of the modeling and simulations were based on the actual data of experimentally produced CIGS cells reported in the literature. In standard CIGS cells with a single absorber layer, the effects of acceptor density and Ga content on device performance were studied, and then optimized for maximum conversion efficiency. The same procedure was performed for cells with two and three sectioned CIGS absorber layers in which Cu and/or Ga contents were varied within each consecutive section. This produces an internal additional electric field within the absorber layer, which resulted in an increase in carrier collection for longer wavelength photons, and hence, improvement in the conversion efficiency of the cell. An increase of approximately 3% in efficiency is predicted for cells with two layer absorbers. For multilayer cells in which Cu and Ga distribution were stepped simultaneously, the improvement could be approximately 3.5%. This improvement is due to; enhanced carrier collection for longer-wavelength photons, and reduced recombination at the heterojunction and back regions of the cell. These results are confirmed by the physics of the cells.

The role of sodium in photovoltaic devices under high voltage stress: A holistic approach to understand unsolved aspects

The paper reviews the role of sodium in the degradation of PV devices under high voltage stress with the purpose to identify issues still not solved and to introduce a comprehensive method of investigation. If a limited amount of Na diffusion into the CIGS cell structure is beneficial for the cell performance, on the other side when moving in the module structure it could create unpleasant effects, like corrosion and shunts. Soda-lime glass with a concentration of sodium around 13–15% is widely used both as cell substrate and as front layer in PV modules. Glass is not a static material and Na movement is easily activated by different triggering causes (stress, voltage bias, environmental variables). This paper is considered a prelude to further research. For this reason, a methodological approach is outlined, with emphasis on the consideration that PV devices, such as modules, are complex systems of systems. Recalling methodological discussions both in system engineering and in risk analysis, the proposed approach emphasizes the importance to study such complex systems adopting a holistic approach, thus taking into consideration interactions between system components and the surrounding environment. Probabilistic risk analysis (PRA) is suggested as a valid method for representing and understanding the interactions between different system components and environmental factors, while studying system reliability and risks for electricity supply.

Odds Narrow For Thin-Film Solar

A global glut of cheap Chinese-made solar energy parts during 2011 and 2012 was good for firms involved in installing solar energy systems. But it was bad for most everybody else in the solar power industry. Shrinking profit margins took down a number of solar component manufacturers, including many using thin-film technology. The story of Solyndra’s demise is well-known: Despite receiving significant investor backing and government support, the thin-film solar firm went bankrupt in 2011. Less well-known is that several other thin-film players followed Solyndra into oblivion. The companies that remain say they have learned how to survive, but the future looks a lot less bright than it used to. Solyndra made thin-film solar cells out of layers of copper indium gallium selenide, or CIGS as they are known in the industry. Solar modules— connected assemblies of solar cells—made with CIGS cells convert a higher percentage of light into energy ...

Cd-free CIGS solar cells with buffer layer based on the In2S3 derivatives

This study guided by device evaluations was conducted to reveal the reasons for the loss of the photo-generated carriers in CIGS cells with the buffer based on In2S3 derivatives. Chemical bath deposited Inx(OOH,S)y films have been employed as a Cd-free buffer layers. When compared to solar cells with CdS buffer layer, the Cu0.9(In0.7,Ga0.3)Se2.1 (Eg = 1.18 eV) cells with the Inx(OOH,S)y buffer exhibited strong voltage-dependent carrier collection and poor spectral response above 500 nm, presumably, due to energy barrier at the junction. In order to improve the charge collection by upward shift of the conduction band minimum of CIGS absorber, Inx(OOH,S)y/Cu0.9(In0.55,Ga0.45)Se2.1 (Eg = 1.30 eV) solar cells were also fabricated and their spectral responses were examined. When compared to the Cu0.9(In0.7,Ga0.3)Se2.1 cells, the improved spectral response and voltage dependent carrier collection were obtained. Nevertheless, considerable loss in charge collection above 500 nm was still observed. The efficiency reached 9.3% while the Cu0.9(In0.7,Ga0.3)Se2.1 cell exhibited only the efficiency of 3.4%. Finally, CIGS (Eg = 1.18 eV) solar cells with n-ZnO/i-ZnO/Inx(OOH,S)y/CdS/CIGS and n-ZnO/i-ZnO/CdS/Inx(OOH,S)y/CIGS configurations were fabricated. The influence of the TCO/buffer interface on the device characteristics was also addressed by means of comparison between the characteristics of two cells employing different interfaces. A 13.0% efficient cell has been achieved from n-ZnO/i-ZnO/CdS/Inx(OOH,S)y/CIGS configuration. The obtained data suggested that the limitation of the device efficiency was mainly related to the i-ZnO/Inx(OOH,S)y interface. The experimental results provide the knowledge base for further optimization of the interface properties to form high-quality p-n junction in the CIGS solar cells employing the CBD In2S3 buffer layer.

Multiscale Modeling of CdTe Thin Film Deposition Process

ABSTRACTDeposition of semiconductor films is a key process for production of thin-film solar cells, such as CdTe or CIGS cells. In order to optimize photovoltaic properties of the film a comprehensive model of the deposition process should be build, which can relate deposition conditions and film properties. We have developed a multiscale model of deposition of CdTe film in close space sublimation (CSS) process. The model is based on kinetic Monte Carlo method on the rigid lattice, in which each site can be occupied by either Cd or Te atom. The model tabulates the energy of the site as a function of its local environment. These energies were obtained from first-principles calculates and then approximated with analytical formulas. Based on determined energies of each site we performed exchange (diffusion) processes using Metropolis algorithm. In addition the model included adsorption and desorption processes of Cd and Te2 species. The results of the model show that a steady-state structure of the surface layer is formed during film growth. The model can reproduce transition from film deposition to film etching depending on external conditions. Moreover, the model can predict deposition rates for non-stoichiometric gas compositions.

Electrical impact of MoSe2 on CIGS thin-film solar cells

The CIGS solar cell is one of the most promising photovoltaic devices due to the achievement of the highest conversion efficiency (>20%) among all thin-film solar cells. The CIGS cell has a glass/Mo/CIGS/CdS/TCO configuration, and the CIGS-Mo interface is a Schottky barrier to holes. During the sulfurization-after-selenization (SAS) CIGS formation process with H2Se gas, the Mo surface transforms naturally into MoSe2 at the CIGS-Mo interface. In this work, the electrical impact of MoSe2 on CIGS solar cells was investigated. Different CIGS-Mo interfaces were prepared with two CIGS processes. One is SAS, and the other is the sequential-sputtering-selenization CIGS process with Se gas. Formation of MoSe2 is hardly observed in the latter process. Samples were characterized by XRD, the van der Pauw method, reflectance, and visual inspection. Besides, Schottky barrier heights of cells were extracted from J-V-T measurements. For the first time, it was experimentally shown that the existence of thin MoSe2 film can decrease the apparent Schottky barrier height of CIGS solar cells. In addition, 1-dimensional numerical simulation showed that a larger barrier height affects both the fill factor and open-circuit voltage. Therefore, the formation of MoSe2 during the CIGS process should minimize the negative effect of Schottky barrier on solar-cell performances, especially with large Schottky barrier.

Characterization of Electron-Induced Defects in Cu (In, Ga) Se2 Thin-Film Solar Cells using Electroluminescence

ABSTRACTCIGS solar cells were irradiated with 250 keV electrons, which can create only Cu-related defects in the cell, to reveal the radiation defect. The EL image of CIGS solar cells before electron irradiation at 120 K described small grains, thought to be those of the CIGS. After 250 keV electron irradiation of the CIGS cell, the cell was uniformly illuminated compared to before the electron irradiation and the observed grains were unclear. In addition, the EL intensity rose with increasing electron fluence, meaning the change in EL efficiency may be attributable to the decreased likelihood of non-irradiative recombination in intrinsic defects due to electron-induced defects. Since the light soaking effect for CIGS solar cells is reported the same phenomena, the 250 keV electron radiation effects for CIGS solar cells might be equivalent to the effect.

Morphology and Thickness Control of Thin Copper Films Prepared by Electrochemical Deposition onto Mo-sputtered Stainless Steel Substrates

Chalcopyrite thin film solar cells of copper (Cu), indium (In), gallium (Ga) and selenium (Se) (CIGS) are receiving much attention, because they show high efficiency and longterm stability. 1 Methods to prepare CIGS light absorber layers include co-evaporation of elemental sources, 2 physical vapor deposition, 3 printing and electrochemical deposition. 4 Among those, electrochemical deposition is considered as an excellent technique for large area processing with good uniformity, low cost, and high material yield. 5 Electrodeposition of CIGS precursor films can be performed by using either single step or multistep process. Although Bhattacharya et al. have reported the development of a 15.4% efficiency CIGS cell by employing a single-bath electrochemical deposition following physical vapor deposition, 6 no other laboratory has reproduced it. The multistep process can be two or three stage process; some examples of such stacks are like Cu/In/Ga, Cu-Ga/In/Se, Cu/In/Ga-Se, Cu/In-Se/Ga-Se, etc. 7-9 For successful industrialization of CIGS solar cells, control of metal layers is required in morphology, thickness, and production speed. The objective of this study was to fabricate the targeted copper thin film onto Mo/SUS substrates by electrochemical deposition for CIGS thin film solar cell precursor. Morphology and thickness of copper thin film onto the Mo/SUS are very important factors because film’s surface condition can affect electrochemical deposition of the next layers of In, Ga, and Se before thermal selenization process. In this work, we wish to report our preliminary work on the electrochemical conditions that can produce a uniform thin copper metal layer onto the Mo sputtered stainless steel (Mo/SUS) flexible substrate for short time (sec). Mo/SUS substrate has the advantages of weight lighter than soda lime glass, of installation to the building integrated photovoltaic and of roll-to-roll manufacturing leading to substantial cost reductions due to enhanced productivity. All chemicals were analytical grade. Double-distilled water was used throughout. The electrochemical experiment was performed in a freshly prepared 0.1, 0.5 and 1 M CuSO4 baths containing various concentrations of (0.1, 0.5, 1, 1.5 M) H2SO4. Two-electrode configurations were used for deposition of Cu thin film. Hull cell test was performed in order to find the current density. The 100 × 100 mm 2 Mo/

Influence of electrical performance on Cu-related defects generated by 250 keV electron irradiation in Cu (In, Ga) Se2 thin-film solar cells

High-energy electrons are known to induce Cu-related defects in Cu (In, Ga) Se2 material. CIGS cells were irradiated with electrons with the energy of 250keV at temperatures below 150K to avoid the recovery of generated defects due to a thermal annealing effect. The carrier density increased in line with increasing electron fluence. Copper vacancy (VCu) could result in increased carrier density as the shallow acceptor level (VCu) is assumed to be the main defect in the CIGS absorbing layer. In the case of 1MeV electron irradiation, however, a decrease in the carrier density of CIGS solar cells has been reported. The activation energies for the thermal annealing of defects induced by 250keV electrons differ from those induced by 1MeV electrons. The Cu-related defects in CIGS induced by 250keV electron irradiation are not thought to degrade CIGS solar cell output performance.

Effects of ZnO:Al films on CIGS PV modules degraded under accelerated damp heat

For the successful commercial production of CIGS PV modules, a good long-term damp heat stability of solar modules is necessary. The causes for the degradation of encapsulated solar modules are clearly explained. This article investigates the degradation behavior of encapsulated CIGS PV modules combined with the failure analysis of AZO films in CIGS cells. CIGS PV modules were exposed to the damp heat test of the IEC 61646 qualification standard—the modules are discolored from the outside edge to the center. Moreover, each module had its own distinct discolored area. The discolored area of the modules is significantly associated with the degradation of FF and efficiency. To clarify the causes of the degradation of FF and efficiency, failure analysis of AZO films in CIGS cells was performed. Electrical and structural properties as well as observation of the microstructure depending on the position of the AZO layers were characterized to explain the degradation mechanism of the encapsulated CIGS PV modules. It was revealed that the discoloration phenomenon of the modules and the degradation of AZO films were caused by the penetration of water molecules and adsorption of oxygen. By XPS and FT-IR analyses, the causes of degradation were confirmed. It was attributed to the generation of Zn(OH)2 and carboxylic acids by a combination of EVA, AZO and water molecules.

Solution‐processed Cu(In,Ga)(S,Se)2 absorber yielding a 15.2% efficient solar cell

ABSTRACTThe remarkable potential for inexpensive upscale of solution processing technologies is expected to enable chalcogenide‐based photovoltaic systems to become more widely adopted to meet worldwide energy needs. Here, we report a thin‐film solar cell with solution‐processed Cu(In,Ga)(S,Se)2 (CIGS) absorber. The power conversion efficiency of 15.2% is the highest published value for a pure solution deposition technique for any photovoltaic absorber material and is on par with the best nonvacuum‐processed CIGS devices. We compare the performance of our cell with a world champion vacuum‐deposited CIGS cell and perform detailed characterization, such as biased quantum efficiency, temperature‐dependent electrical measurement, time‐resolved photoluminescence, and capacitance spectroscopy. Copyright © 2012 John Wiley & Sons, Ltd.

Modeling low cost hybrid tandem photovoltaics with the potential for efficiencies exceeding 20%

It is estimated that for photovoltaics to reach grid parity around the planet, they must be made with costs under $0.50 per Wp and must also achieve power conversion efficiencies above 20% in order to keep installation costs down. In this work we explore a novel solar cell architecture, a hybrid tandem photovoltaic (HTPV), and show that it is capable of meeting these targets. HTPV is composed of an inexpensive and low temperature processed solar cell, such as an organic or dye-sensitized solar cell, that can be printed on top of one of a variety of more traditional inorganic solar cells. Our modeling shows that an organic solar cell may be added on top of a commercial CIGS cell to improve its efficiency from 15.1% to 21.4%, thereby reducing the cost of the modules by ∼15% to 20% and the cost of installation by up to 30%. This suggests that HTPV is a promising option for producing solar power that matches the cost of existing grid energy.

Growth of Zn doped Cu(In, Ga)Se 2 thin films by RF sputtering for solar cell applications

Cu(In, Ga)Se 2 (CIGS) surface was modified with Zn doping using a magnetron sputtering method. CuInGa:Zn precursor films targeting a CuIn 0.7Ga 0.3Se 2 stoichiometry with increasing Zn content from 0 to 0.8 at% were prepared onto Mo-coated glass substrates via co-sputtering of Cu–Ga alloy, In and Zn targets. The CuInGa:Zn precursors were then selenized with solid Se pellets. The structures and morphologies of grown Zn doped CIGS films were found to depend on the Zn content. At zinc doping level ranging between 0.2 and 0.6 at%, the Zn doping improved the crystallinity and surface morphology of CIGS films. Compared with the performance of the non-doped CIGS cell, the fabricated CIGS solar cell displayed a relative efficiency enhancement of 9–22% and the maximum enhancement was obtained at a Zn content of 0.4 at%.

Theoretical Efficiency Limits for Alternative Solar Cell Device Concepts

As the demand for renewable energy sources is increasing, many alternative concepts have been explored to enrich the prospects. In the solar cell field, multi carrier generation, spectrum manipulation, thermo-photoelectric cells, hot carrier, intermediate band, and many other techniques have been studied as new concepts. In this work, the theoretical limits of multi-carrier generation and multi-interface pre spectrum divided solar cells’ efficiencies are analyzed and discussed in detail. The solar cell market is dominated by single p-n junction devices such as Si, CdTe, and CIGS cells. The theoretical efficiency of such devices is well set by a recent version of the neat Shockley-Queisser (SQ) model. In this model, the single junction cell efficiency depends on the energy gap and cannot exceed 33%. Practically and in support for SQ model, the best lab efficiency is 25% for Si solar cell and it has not changed much since the early 1990s. The alternative device concepts should be able to surpass this limit. For example, a 43% efficient triple junction cell has already been realized. Based on the analysis, more than 80% efficiencies are possible; however, such high efficiencies are achievable only in ideal cases. So, some of the technical and practical difficulties are addressed and discussed. The analysis is based on the classical transport approach and assuming the measured 1.5 AM solar radiation spectrum. We used the reference National Renewable Energy Lab (NREL) measurements. For multi- carrier generation, no distinction is made between multi-exciton generation and carrier multiplications, as the multiplication is assumed ideal as Heaviside step function of the ratio of photon energy over energy gap, where the multiplication happens, is ignored.

Natural resource limitations to terawatt-scale solar cells

The predicted energy demand will reach 28 TW by 2050 and 46 TW by 2100. The deployment of solar cells as a source of electricity will have to expand to a scale of tens of peak terawatts in order to become a noticeable source of energy in the future. Of the current commercial and developmental solar cell technologies, the majority have natural resource limitations that prevent them from reaching a terawatt scale. These limitations include high energy input for crystalline-Si cells, limited material production for GaAs cells, and material scarcity for CdTe, CIGS, dye-sensitized, crystalline-Si, and thin-film Si cells. In this paper, we examine these limitations under the best scenarios for CdTe, CIGS, GaAs, dye-sensitized, and crystalline-Si solar cells. Without significant technological breakthroughs, these technologies combined would meet only a few percentage points (∼2%) of our energy demand in 2100.

Evaluation of external quantum efficiency of a 12.35% tandem solar cell comprising dye-sensitized and CIGS solar cells

A tandem solar cell is constructed by series connection of a semi-transparent dye-sensitized solar cell (DSSC) as a top cell and a Cu(In, Ga)Se 2 (CIGS) solar cell as a bottom cell, where the isolated DSSC and CIGS cells show the conversion efficiency of 8.27% and 11.71%, respectively. The DSSC/CIGS tandem cell exhibits the improved conversion efficiency of 12.35% with photocurrent density of 14.1 mA/cm 2, open-circuit voltage of 1.435 V and fill factor of 0.61. External quantum efficiency (EQE) of the tandem cell is investigated under DC and AC modes. EQE of the isolated DSSC and CIGS cell can be measured by either DC mode or AC mode, whereas EQE for the tandem cell is detected only under AC mode with bias light. Bias light intensity is found to play the crucial role in determining the precise EQE of the tandem cell. At the given chopping frequency as low as 10 Hz, the measured EQE at bias light corresponding to 1 sun intensity is consistent with the simulated EQE data.

Cu(In,Ga)Se 2 solar cells with double layered buffers grown by chemical bath deposition

In based mixture In x(OH,S) y buffer layers deposited by chemical bath deposition technique are a viable alternative to the traditional cadmium sulfide buffer layer in thin film solar cells. We report on the results of manipulating the absorber/buffer interface between the chalcopyrite Cu(In,Ga)Se 2 (CIGS) absorber and CdS or ZnS buffer by addition of a thin In based mixture layer. It is shown that the presence of thin In x(OH,S) y at the CIGS absorber/CdS or ZnS buffer interfaces greatly improve the solar cell performances. The performances of CIGS cells using dual buffer layers composed of In x(OH,S) y/CdS or In x(OH,S) y/ZnS increased by 22.4% and 51.6%, as compared to the single and standard CdS or ZnS buffered cells, respectively.