Se2 Thin Film Solar Cells Research Articles

Enhanced Carrier Collection in Silver Nanoparticle Embedded Zinc Oxide Nanorod Top Electrodes for Thin-Film Photovoltaic Devices

Cu(In, Ga)Se2 solar cells are the representative of high efficient thin-film solar cell with energy conversion efficiency of over 22%. However, nowadays, top electrodes of solar cells gradually become the performance limitation for optimized devices. The Al-doped ZnO thin film is widely used as a transparent top electrode in Cu(In, Ga)Se2, but there are still some techniques and electrical property issues. In this letter, we establish a ZnO/Ag/ZnO:Al multilayer top electrode, composed of ZnO nanorod arrays, Ag nanoparticles, and ZnO:Al thin films. Such multilayer top electrode was applied on Cu(In, Ga)Se2 thin-film solar cells and the devices yield a higher efficiency from 6.79% to 8.52%. Importantly, we also propose that the enhanced electronic carrier concentration in such ZnO/Ag/ZnO:Al top electrode aids the performance of solar cells. Therefore, well-designed ZnO-based top electrodes offer the opportunity to remove the bottleneck of improving the solar cell efficiency.

Formation of a K-In-Se Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers.

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu(In,Ga)Se2 (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated. We find that the alkali-free and NaF-PDT CIGSe surfaces both display the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT, however, leads to the formation of bilayer structure in which a K-In-Se species covers the CIGSe compound that in composition is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber.

The effects of selenium content on Cu(InGa)Se2 thin film solar cells by sputtering from quaternary target with Se-free post annealing

Two kinds of Cu(InGa)Se2(CIGS) absorbers and the corresponding solar cells have been fabricated by sputtering from Se-poor and Se-rich quaternary CIGS targets, respectively. The CIGS thin films fabricated by Se-poor target are mainly composed of chalcopyrite CIGS phase together with small amount of Cu2-xSe compound, while the CIGS thin films fabricated by Se-rich target show single chalcopyrite CIGS phase. The electrical properties of Se-poor films are deteriorated severely due to the formation of Cu2-xSe compared with that of Se-rich film. The highest device efficiency of 12.5% has been achieved in the work with absorber sputtering from Se-rich target with Se-free post annealing. It was found that the key limiting factor to success by this method is sufficient selenium in CIGS absorbers.

Investigation of Defects at Cu(In,Ga)Se<sub>2</sub> Flexible Solar Cells on Macroscopic and Microscopic Level and their Influence on Solar Cell Performance

This paper investigates imperfection issues of Cu (In,Ga)Se2 thin-film solar cell structures and diagnostic methods of the CIGS solar cells. Electroluminescence and thermography are used to localize defect in macroscopic scale. Microstructures found in defective solar cell area are shown using micrographs. Focused ion beam was used to demonstrate that these structures interfere each solar cell layers. It is shown that micro sized defects (voids) behave as extra-stressed conductive channels that can degrade solar cells in module.

Inkjet printed Cu(In,Ga)S2 nanoparticles for low-cost solar cells

Cu(In,Ga)Se2 (CIGSe) thin film solar cells were fabricated by direct inkjet printing of Cu(In,Ga)S2 (CIGS) nanoparticles followed by rapid thermal annealing under selenium vapor. Inkjet printing is a low-cost, low-waste, and flexible patterning method which can be used for deposition of solution-based or nanoparticle-based CIGS films with high throughput. XRD and Raman spectra indicate that no secondary phase is formed in the as-deposited CIGS film since quaternary chalcopyrite nanoparticles are used as the base solution for printing. Besides, CIGSe films with various Cu/(In + Ga) ratios could be obtained by finely tuning the composition of CIGS nanoparticles contained in the ink, which was found to strongly influence the devices performance and film morphology. To date, this is the first successful fabrication of a solar device by inkjet printing of CIGS nanoparticles.

Charge separation effects in time-resolved photoluminescence of Cu(In,Ga)Se2 thin film solar cells

Time-resolved photoluminescence (TRPL) is a powerful tool for investigating the charge carrier dynamics in semiconductors. Many mechanisms may contribute to the PL decay characteristics, such as radiative band-to-band recombination, non-radiative Shockley-Read-Hall recombination, capture and re-emission of minority carriers via trap states, as well as separation of the generated electron-hole pairs by gradients in their electrochemical potentials combined with different conductivities. In Cu(In,Ga)Se2 thin film solar cells charge separation has been shown to dominate the TRPL signal at low injection levels in complete devices whereas the PL decay in CdS-covered absorbers seemed unaffected by charge separation and has been reported to equal that of the bare absorber layer. In this study we investigate the PL decay of complete cells, CdS-covered absorbers and bare absorbers as a function of injection level. We show that the PL decay of not only the complete cell but also the CdS-covered absorber is dominated by charge separation at sufficiently low injection levels. In this injection regime the TRPL signal of the CdS-covered absorber significantly differs from that of the bare absorber, but also at higher injection levels different decay time constants are observed. By means of time-resolved device simulations we analyse the mechanisms of charge separation and their impact on the transient PL decay.

Effect of Three-Stage Growth Modification on a CIGS Microstructure

Cu(In,Ga)Se2 (CIGS) thin-film solar cells from a Global Solar Energy (GSE) production line with 17.7% cell efficiency have been analyzed by a variety of imaging techniques, which include high-resolution electroluminescence and 3-D optical profilometry. The microstructure of supergrown CIGS nodules has been observed by an optical microscope and a scanning electron microscope (SEM). Statistical distribution of the nodule microstructure matched well with the bright spots in electroluminescence. A variation of several orders of magnitude in electroluminescence intensity indicates a wide margin of the quasi-Fermi level splitting potential and a possibility of efficiency improvement. Substrate temperatures of (In,Ga)2Se3 precursor in the three-stage growth process have been modified to eliminate the nodule growth, which resulted in $V_{{\rm OC}}$ increase from the baseline value of 653 to 677 mV for a CIGS optical bandgap of 1.15 eV. Additional measurements, such as conductive-tip atomic force microscopy, energy-dispersive X-ray spectroscopy, and glow-discharge optical emission spectroscopy, are presented, and the results are discussed.

Band gap widening at random CIGS grain boundary detected by valence electron energy loss spectroscopy

Cu(In,Ga) Se2 (CIGS) thin film solar cells have demonstrated very high efficiencies, but still the role of nanoscale inhomogeneities in CIGS and their impact on the solar cell performance are not yet clearly understood. Due to the polycrystalline structure of CIGS, grain boundaries are very common structural defects that are also accompanied by compositional variations. In this work, we apply valence electron energy loss spectroscopy in scanning transmission electron microscopy to study the local band gap energy at a grain boundary in the CIGS absorber layer. Based on this example, we demonstrate the capabilities of a 2nd generation monochromator that provides a very high energy resolution and allows for directly relating the chemical composition and the band gap energy across the grain boundary. A band gap widening of about 20 meV is observed at the grain boundary. Furthermore, the compositional analysis by core-loss EELS reveals an enrichment of In together with a Cu, Ga and Se depletion at the same area. The experimentally obtained results can therefore be well explained by the presence of a valence band barrier at the grain boundary.

Effects of Ga concentration in Cu(In,Ga)Se2 thin film solar cells with a sputtered-Zn(O,S) buffer layer

Cu(In,Ga)Se2 absorbers with several different ratios of Ga/(Ga+In) were deposited by varying the Ga content using co-evaporation. The effects of Ga concentration at the surface of the CIGS on the Zn(O,S)/CIGS photovoltaic performance were investigated. Considering application as the bottom cell in a monolithic tandem structure, the optimum Ga/(Ga+In) ratio of approximately 0.26 led to the best efficiency of 14.56%, and to complete carrier collection at long wavelengths. In addition, the degradation rate in efficiency was lowest at an annealing temperature of 400°C. The reason for losses of other Ga/(Ga+In) ratios (0.33 and 0.36) at longer wavelengths might be due to interface recombination, possibly aided by conduction band offset with the Ga grading shape by varying the Ga concentration. The results suggested that the decrease in efficiency might be due to recombination at the interface between the Zn(O,S) and CIGS layers before/after annealing. The reverse bias effect during quantum efficiency measurements at long wavelengths was used to determine separate optical and electrical losses.

Transparent Conducting Oxide Layer By Thermal Evaporation for Cu(In,Ga)Se2 Solar Cell with Chemical Bath Deposited-ZnS Buffer Layer

Chemical-bath-deposited (CBD) ZnS buffer layer is being steadily studied in Cu(In,Ga)Se2 (CIGS) thin-film solar cell because of their beneficial properties, for examples, good transparency with large direct bandgap, less toxicity and cost effectiveness. CBD-ZnS buffer layer, however, is easily affected during subsequent sputtering process for window layer and the conversion efficiency decreases with sputtering power. The decrease of efficiency in CBD-ZnS/CIGS thin-film solar cells is known to be related to plasma damage originated from high energy negative oxygen ions or neutral particles at the interface. In typical CIGS solar cell, ITO/i-ZnO bilayer deposited by sputtering process has been used as a conventional window layer and then interface properties between window layer and buffer is considered as an important key to determine the cell efficiency. In this paper, to know actual effects of interface property between window layer and buffer on cell performance, we applied the two types of window layer and fabricated CIGS solar cell with CBD-ZnS buffer; (i) typical structure of ITO/i-ZnO/CBD-ZnS/CIGS, and (ii) thermally grown-IZO/i-ZnO/CBD-ZnS/CIGS. To completely exclude plasma damage from sputtering, two types of CIGS solar cell were also fabricated without i-ZnO layer. As a result, the cell performance interestingly shows the opposite tendency depending on presence or absence of i-ZnO layer; cell efficiency with IZO layer is higher than that with ITO layer in the presence of i-ZnO layer, while the former is lower than the latter in the absence of i-ZnO layer. To understand these phenomena, interfacial properties between window and buffer layer are characterized by various analysis tools.

Simulation of metastable changes in time resolved photoluminescence of Cu(In,Ga)Se2 thin film solar cells upon light soaking treatment

Cu(In,Ga)Se2 thin film solar cells are known to show a metastable behavior of their open circuit voltage when exposed to light. Time resolved measurements of the photoluminescence decay after pulsed excitation (TRPL) reveal that also the photoluminescence decay (PL) changes with conditioning, yielding shorter effective lifetimes after annealing under illumination (light soaking) than after annealing in the dark. By using time resolved device simulations we applied a model that explains the TRPL metastable behavior. For this we used a model of capture and reemission of minority charge carriers via two characteristic trap states near the conduction band, while also accounting for changes in trap density and doping concentration caused by light soaking. To further understand the mechanisms behind PL decay we analyse the influence that the charge carrier dynamics play in TRPL decay by a variation of select simulation parameters.

Interfacial quality improvement of Cu(In,Ga)Se2 thin film solar cells by Cu-depletion layer formation

Se irradiation with time, tSe, was introduced after the second stage of a three-stage process to control the Cu2Se layer during Cu(In,Ga)Se2 (CIGS) deposition. Open circuit voltage and fill factor of CIGS solar cells could be improved by introducing Se irradiation. We concluded that the control of the Cu2Se layer led to the formation of a Cu-depletion CIGS layer (CDL), which improved conversion efficiency owing to suppression of interfacial recombination by a valence band offset formed between CIGS and the CDL. Finally, highest efficiency of 19.8% was achieved with tSe of 5 min. This very simple and new technique is promising for the improvement of photovoltaic performance.

Effects of the incorporation of alkali elements on Cu(In,Ga)Se2 thin film solar cells

This study describes in detail the effects of sodium and potassium on Cu(In,Ga)Se2 (CIGS) absorbers and solar cells. We report on the influence of these species on the surface and bulk composition as well as bulk defect structure of CIGS films as revealed by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and photoluminescence (PL). From the XPS studies it is found that Na and K promote oxygen absorption onto the CIGS films. Furthermore, potassium accelerates the formation of indium and gallium oxides on the film surface, making the surface Cu-deficient. Low temperature PL studies suggest that (i) Na and K help passivate non-radiative recombination centers, presumably at the grain boundaries, and (ii) Na further impacts the bulk defect structure inside of CIGS grains, which is not observed with K. This change in bulk defect structure is attributed to the greater diffusivity of Na in CIGS relative to K due to the smaller atomic size. This in-depth study (integration of XPS, SIMS, PL, and device characteristics) reveals that the surface chemistry and the grain boundary passivation have stronger influences on the device performance than the bulk defect structure.

Surface Passivation for Reliable Measurement of Bulk Electronic Properties of Heterojunction Devices.

Quantum efficiency measurements of state of the art Cu(In,Ga)Se2 (CIGS) thin film solar cells reveal current losses in the near infrared spectral region. These losses can be ascribed to inadequate optical absorption or poor collection of photogenerated charge carriers. Insight on the limiting mechanism is crucial for the development of more efficient devices. The electron beam induced current measurement technique applied on device cross-sections promises an experimental access to depth resolved information about the charge carrier collection probability. Here, this technique is used to show that charge carrier collection in CIGS deposited by multistage co-evaporation at low temperature is efficient over the optically active region and collection losses are minor as compared to the optical ones. Implications on the favorable absorber design are discussed. Furthermore, it is observed that the measurement is strongly affected by cross-section surface recombination and an accurate determination of the collection efficiency is not possible. Therefore it is proposed and shown that the use of an Al2 O3 layer deposited onto the cleaved cross-section significantly improves the accuracy of the measurement by reducing the surface recombination. A model for the passivation mechanism is presented and the passivation concept is extended to other solar cell technologies such as CdTe and Cu2 (Zn,Sn)(S,Se)4 .

Non-ionizing energy loss calculations for modeling electron-induced degradation of Cu(In, Ga)Se2 thin-film solar cells**Project supported by the National Natural Science Foundation of China (Grant No. 11547151).

The lowest energies which make Cu, In, Ga, and Se atoms composing Cu(In, Ga)Se2 (CIGS) material displaced from their lattice sites are evaluated, respectively. The non-ionizing energy loss (NIEL) for electron in CIGS material is calculated analytically using the Mott differential cross section. The relation of the introduction rate (k) of the recombination centers to NIEL is modified, then the values of k at different electron energies are calculated. Degradation modeling of CIGS thin-film solar cells irradiated with various-energy electrons is performed according to the characterization of solar cells and the recombination centers. The validity of the modeling approach is verified by comparison with the experimental data.

Reverse-Voltage Bias Induced Shunt Formation in Cu(In,Ga)Se2 Thin Film Solar Cells: an Approach with Three-Dimensional Electro-Thermal Simulations

Reverse-Voltage Bias Induced Shunt Formation in Cu(In,Ga)Se2 Thin Film Solar Cells: an Approach with Three-Dimensional Electro-Thermal Simulations

Investigation of factors limiting efficiency in Cu(In,Ga)Se2 thin film solar cells during rapid evaporation process

Rapid evaporation is crucial in the low-cost manufacturing of Cu(InxGa1-x)Se2 (CIGS) thin-film solar cells. This form of evaporation deteriorates cell performance in an open-circuit voltage and fill factor. Cell performance is strongly dependent on the deposition process and properties of the thin films. With respect to evaporation rates, analyses of electrical properties, film composition, grain structure, and phase characteristics were conducted to investigate limits on the efficiency of evaporated CIGS thin-film solar cells. CIGS solar cells evaporated at different deposition rates were compared. Raman spectroscopy was used to characterize the residual phases deteriorating the cell performance of these solar cells. Sodium (Na) profiles measured using secondary-ion mass spectrometry revealed that with higher CIGS evaporation rates, Na diffusion in the CIGS layers is lower. Rapidly evaporated CIGS led to two features, residual phases of the CIGS remained and Na concentrations near the surface were insufficient. This result implies that a shortfall in the p-type carrier density during rapid evaporation is a critical factor negatively affecting the efficiency of CIGS thin-film solar cells.

Role of Mo:Na layer on the formation of MoSe2 phase in Cu(In,Ga)Se2 thin film solar cells

This study investigated the influence of Mo:Na on the formation of MoSe2 in Cu(In, Ga)Se2 (CIGS) thin film solar cells and its overall effect on the performance of cells with a structure of Ti/Mo:Na/Mo/CIGS/CdS/i-ZnO/ZnOAl/Al. Varying the thickness of the Mo:Na layer enabled the systematic control of the diffusion of Na into the CIGS layer. Experimental results demonstrate that the thickness of MoSe2 phase decreases with an increase in the thickness of the Mo:Na layer. Reducing the thickness of the MoSe2 layer enhanced cell efficiency. Additionally, Ga distribution in the CIGS layer may vary with a change in the thickness of the Mo:Na layer. When Mo:Na/Mo=500/500nm, the Ga content and the thickness of the MoSe2 layer were close to optimal with regard to cell performance. This paper proposes a model to explain the impact of Na doping on the formation of MoSe2 phase formation in polycrystalline CIGS. It appears that the addition of Na leads to the formation of Na2Sex at the grain boundaries in the absorber layer, which reduces the diffusion of Se to form MoSe2.

Influence of Mo–N as diffusion barrier in Mo back contacts for Cu(In,Ga)Se2 solar cells

In a recent article, we have shown that a high selenium amount (selenium partial pressure) during the stacked elemental layer process is beneficial for the Cu(In,Ga)Se2 absorber layer in terms of Ga/In interdiffusion and Ga enhancement at near-surface regions. For highly efficient Cu(In,Ga)Se2 thin film solar cells in the stacked elemental layer process a certain MoSe2 thickness at the interface between Mo and Cu(In,Ga)Se2 is necessary. However, the high selenium supply leads to back contact corrosion, which results in adhesion problems of the film and an increase of the series resistance of the solar cell device. Introduction of a thin diffusion barrier against selenium penetration at the interface between Cu(In,Ga)Se2 absorber layer and Mo back contact is a possible solution which may ensure homogeneous back contact properties while allowing high selenium supply during the stacked elemental layer process. In this work, a new approach for an alternative back contact system is presented which includes a functional Mo–N layer. During the Mo sputtering process, the implementation of a reactive N2 gas process is easily possible. According to structural analysis just a thin self-limited MoSe2 layer with a constant thickness is formed during the stacked elemental layer process at the back contact. It is further demonstrated that a thin Mo–N barrier layer leads to an improved fill factor in the current–voltage characteristic of the cell, whereas thicker barrier layers negatively affect the series resistance of the cell.

Surface morphology control of In2S3 buffer layer by Sn incorporation and its application to cadmium-free Cu(In,Ga)Se2 thin-film solar cells

Sn atoms were incorporated into In2S3 thin-film to modify the morphology of the In2S3 buffer layer for Cd-free Cu(In,Ga)Se2 (CIGS) thin-film solar cells. The incorporation of Sn atoms into the In2S3 films was carried out by adding a SnCl2-dissolved solution to an InCl3-and CH3CSNH2-dissolved solution during chemical bath deposition process. With Sn incorporation into In2S3 film, the surface morphology of In2S3 film was significantly changed showing more uniform and smooth film. Raman spectroscopic analysis shows that the backbone of In2S3 lattice disappear completely with 10% Sn incorporation in the film. In addition to effect of Sn content on the surface morphology of In2S3 film, we found that the morphology of Sn-incorporated In2S3 film strongly depends on the pH value in the reaction solution. At the pH value of 2.53, the film became very dense without pores and secondary phases. The film was completely covered the surface of CIGS substrate. Photoluminescence analysis indicates that the shallow acceptor levels at the CIGS surface might be passivated by Sn doping during chemical bath deposition process. As a result, the efficiency of CIGS solar cell with 4.9%-Sn doped In2S3 buffer was improved from 9.98 to 12.7%.