Novel Approaches to Wide Bandgap CuInSe2 Based Absorbers

This project targeted the development of high performance wide bandgap solar cells based on thin film alloys of CuInSe2 to relax constraints on module design and enable tandem solar cell structures. This addressed goals of the Solar Energy Technologies Program for Next Generation PV to develop technology needed for higher thin film module efficiency as a means to reduce costs. Specific objectives of the research project were: 1) to develop the processes and materials required to improve the performance of wide bandgap thin film solar cells based on alloys of CuInSe2, and 2) to provide the fundamental science and engineering basis for the material, electronic, and device properties required to effectively apply these processes and materials to commercial manufacture. CuInSe2-based photovoltaics have established the highest efficiencies of the thin film materials at both the cell and module scales and are actively being scaled up to commercialization. In the highest efficiency cells and modules, the optical bandgap, a function of the CuInSe2-based alloy composition, is relatively low compared to the optimum match to the solar spectrum. Wider bandgap alloys of CuInSe2 produce higher cell voltages which can improve module performance and enable the development of tandem solar cells to boost the overall efficiency. A focus for the project was alloying with silver to form (AgCu)(InGa)Se2 pentenary thin films deposited by elemental co-evaporation which gives the broadest range of control of composition and material properties. This alloy has a lower melting temperature than Ag-free, Cu-based chalcopyrite compounds, which may enable films to be formed with lower defect densities and the (AgCu)(InGa)Se2 films give improved material properties and better device performance with increasing bandgap. A comprehensive characterization of optical, structural, and electronic properties of (AgCu)(InGa)Se2 was completed over the complete compositional range 0 ≤ Ga/(In+Ga) ≤ 1 and 0 ≤ Ag/(Ag+Cu) ≤ 1. Evidence of improved material quality includes reduced sub-bandgap optical absorption, sharper bandtails, and increased grain size with Ag addition. The Ag alloying was shown to increase the range of bandgaps over which solar cells can be fabricated without any drop-off in performance. With bandgap greater than 1.6 eV, in the range needed for tandem solar cells, (AgCu)(InGa)Se2 gave higher efficiency than other CuInSe2-based alloys. Using a simple single-stage co-evaporation process, a solar cell with 17.6% efficiency using a film with bandgap = 1.3 eV was achieved, demonstrating the viability of (AgCu)(InGa)Se2 for high efficiency devices. With a three-stage co-evaporation process for (AgCu)(InGa)Se2 deposition a device with efficiency = 13.0 % and VOC = 890 mV with JSC = 20.5 mA/cm2, FF = 71.3% was achieved. This surpasses the performance of other wide bandgap CuInSe2-based solar cells. Detailed characterization of the electronic properties of the materials and devices including the application of advanced admittance-based easements was completed

Effect of Ga Content on Defect States in CuIn\u3csub\u3e1-x\u3c/sub\u3eGa\u3csub\u3ex\u3c/sub\u3eSe\u3csub\u3e2\u3c/sub\u3e Photovoltaic Devices

Defects in the band gap of CuIn1-xGaxSe2 have been characterized using transient photocapacitance spectroscopy. The measured spectra clearly show response from a band of defects centered around 0.8 eV from the valence band edge as well as an exponential distribution of band tail states. Despite Ga contents ranging from Ga/(In+Ga)=0.0 to 0.8, the defect bandwidth and its position relative to the valence band remain constant. This defect band may act as an important recombination center, contributing to the decrease in device efficiency with increasing Ga content

Characterization and device performance of (AgCu)(InGa)Se2 absorber layers

The study of (AgCu)(InGa)Se2 absorber layers is of interest in that Ag-chalcopyrites exhibit both wider bandgaps and lower melting points than their Cu counterparts. (AgCu)(InGa)Se2 absorber layers were deposited over the composition range 0 < Ag/(Ag+Cu) < 1 and 0.3 < Ga/(In+Ga) < 1.0 using a variety of elemental co-evaporation processes. Films were found to be singlephase over the entire composition range, in contrast to prior studies. Devices with Ga content 0.3 < Ga/(In+Ga) <0.5 tolerated Ag incorporation up to Ag/(Ag+Cu) = 0.5 without appreciable performance loss. Ag-containing films with Ga/(In+Ga) = 0.8 showed improved device characteristics over Cu-only control samples, in particular a 30-40% increase in short-circuit current. An absorber layer with composition Ag/(Ag+Cu) = 0.75 and Ga/(In+Ga) = 0.8 yielded a device with VOC = 890 mV, JSC = 20.5mA/cm2, fill factor = 71.3%, and η = 13.0%

Electron drift-mobility measurements in polycrystalline CuIn1-xGaxSe2 solar cells

We report photocarrier time-of-flight measurements of electron drift mobilities for the p-type CuIn1-xGaxSe2 films incorporated in solar cells. The electron mobilities range from 0.02 to 0.05 cm^2/Vs and are weakly temperature-dependent from 100–300 K. These values are lower than the range of electron Hall mobilities (2-1100 cm2/Vs) reported for n-type polycrystalline thin films and single crystals. We propose that the electron drift mobilities are properties of disorder-induced mobility edges and discuss how this disorder could increase cell efficiencies

Characterizing the effects of silver alloying in chalcopyrite CIGS solar cells with junction capacitance methods

A variety of junction capacitance-based characterization methods were used to investigate alloys of Ag into Cu(In1-xGax)Se2 photovoltaic solar cells over a broad range of compositions. These alloys show encouraging trends of increasing VOC with increasing Ag content, opening the possibility of wide-gap cells for use in tandem device applications. Drive level capacitance profiling (DLCP) has shown very low free carrier concentrations for all Ag-alloyed devices, in some cases less than 1014 cm-3, which is roughly an order of magnitude lower than that of CIGS devices. Transient photocapacitance spectroscopy has revealed very steep Urbach edges, with energies between 10 meV and 20 meV, in the Ag-alloyed samples. This is in general lower than the Urbach edges measured for standard CIGS samples and suggests a significantly lower degree of structural disorder

Novel Approaches to Wide Bandgap CuInSe2 Based Absorbers

This project targeted the development of high performance wide bandgap solar cells based on thin film alloys of CuInSe2 to relax constraints on module design and enable tandem solar cell structures. This addressed goals of the Solar Energy Technologies Program for Next Generation PV to develop technology needed for higher thin film module efficiency as a means to reduce costs. Specific objectives of the research project were: 1) to develop the processes and materials required to improve the performance of wide bandgap thin film solar cells based on alloys of CuInSe2, and 2) to provide the fundamental science and engineering basis for the material, electronic, and device properties required to effectively apply these processes and materials to commercial manufacture. CuInSe2-based photovoltaics have established the highest efficiencies of the thin film materials at both the cell and module scales and are actively being scaled up to commercialization. In the highest efficiency cells and modules, the optical bandgap, a function of the CuInSe2-based alloy composition, is relatively low compared to the optimum match to the solar spectrum. Wider bandgap alloys of CuInSe2 produce higher cell voltages which can improve module performance and enable the development of tandem solar cells to boost the overall efficiency. A focus for the project was alloying with silver to form (AgCu)(InGa)Se2 pentenary thin films deposited by elemental co-evaporation which gives the broadest range of control of composition and material properties. This alloy has a lower melting temperature than Ag-free, Cu-based chalcopyrite compounds, which may enable films to be formed with lower defect densities and the (AgCu)(InGa)Se2 films give improved material properties and better device performance with increasing bandgap. A comprehensive characterization of optical, structural, and electronic properties of (AgCu)(InGa)Se2 was completed over the complete compositional range 0 ≤ Ga/(In+Ga) ≤ 1 and 0 ≤ Ag/(Ag+Cu) ≤ 1. Evidence of improved material quality includes reduced sub-bandgap optical absorption, sharper bandtails, and increased grain size with Ag addition. The Ag alloying was shown to increase the range of bandgaps over which solar cells can be fabricated without any drop-off in performance. With bandgap greater than 1.6 eV, in the range needed for tandem solar cells, (AgCu)(InGa)Se2 gave higher efficiency than other CuInSe2-based alloys. Using a simple single-stage co-evaporation process, a solar cell with 17.6% efficiency using a film with bandgap = 1.3 eV was achieved, demonstrating the viability of (AgCu)(InGa)Se2 for high efficiency devices. With a three-stage co-evaporation process for (AgCu)(InGa)Se2 deposition a device with efficiency = 13.0 % and VOC = 890 mV with JSC = 20.5 mA/cm2, FF = 71.3% was achieved. This surpasses the performance of other wide bandgap CuInSe2-based solar cells. Detailed characterization of the electronic properties of the materials and devices including the application of advanced admittance-based easements was completed

In-situ post-deposition thermal annealing of co-evaporated Cu(InGa)Se2 thin films deposited at low temperatures

The effects of deposition temperature and in-situ post-deposition annealing on the microstructure of coevaporated Cu(InGa)Se2 thin films and on the performance of the resulting solar cell devices have been characterized. Films were deposited at substrate temperatures of 150°C, 300°C and 400°C. Films were also deposited at these temperatures and then annealed in-situ at 550°C for 10 minutes. In as -deposited films without annealing, additional XRD reflections that may be due to a polytypic modification of the chalcopyrite phase were observed. Films deposited at 150°C were Se-rich. Post-deposition annealing caused microstructural changes in all films and improved the resulting solar cells. Only films deposited at 400°C, however, yielded high-efficiency devices after post-deposition annealing that were equivalent to devices made from films grown at 550°C. Films originally deposited at 300°C yielded devices after post-deposition annealing with VOC close to that of devices made from films grown at 550°C, despite smaller grain size

A stochastic model of solid state thin film deposition: Application to chalcopyrite growth

Developing high fidelity quantitative models of solid state reaction systems can be challenging, especially in deposition systems where, in addition to the multiple competing processes occurring simultaneously, the solid interacts with its atmosphere. In this work, we develop a model for the growth of a thin solid film where species from the atmosphere adsorb, diffuse, and react with the film. The model is mesoscale and describes an entire film with thickness on the order of microns. Because it is stochastic, the model allows us to examine inhomogeneities and agglomerations that would be impossible to characterize with deterministic methods. We demonstrate the modeling approach with the example of chalcopyrite Cu(InGa)(SeS)2 thin film growth via precursor reaction, which is a common industrial method for fabricating thin film photovoltaic modules. The model is used to understand how and why through-film variation in the composition of Cu(InGa)(SeS)2 thin films arises and persists. We believe that the model will be valuable as an effective quantitative description of many other materials systems used in semiconductors, energy storage, and other fast-growing industries