AVS2010 Session EN+TF-WeM: CIGS, CZTS and Chalcopyrite Films & Solar Cells

The development of suitable colloidal nanocrystal inks are a key step in the development of low-cost solar cells since they enable the use of fast and inexpensive coating processes such as spray coating and roll coating to form a thin film photoabsorbing layer. Chalcopyrite structure copper indium gallium diselenide (CIGSe) and stannite or kesterite copper zinc tin sulfides (CZTS) are key photoabsorbing materials for thin film solar cells due to their near ideal band gap and their serendipitous defect chemistry (CIGSe) and Earth abundance (CZTS). Although several methods have been reported that describe the synthesis of CIGSe and related nanocrystals, precise control of the composition for these ternary and quaternary compounds has been problematic [1]. We have reported the solution-phase synthesis of stoichiometric chalcopyrite structured CuInSe2 nanocrystals [2], Cu(In,Ga)S2 [3], and the very first synthesis of Cu2ZnSnS4 nanocrystals [4]. The syntheses proceed rapidly from elemental and halide reagents via a simple batch reaction without “hot injection” in a single component coordinating solvent. We have demonstrated the use of these nanocrystals for low-cost solar cells by fabricating devices without using any oxygen-free techniques (after NC synthesis) and employing a scalable roll coating method. The nanocrystal inks are first coated on a back contact (Mo coated sodalime glass in this case). The nanocrystal layer is then easily consolidated into large crystalline domains by a brief thermal treatment in a selenium rich atmosphere to prevent selenium loss or to replace sulfur with selenium. The fabricated cells are robust and increase in efficiency with time, exhibiting similar serendipitous defect chemistry as layers formed by vacuum co-evaporation. We have fabricated solar cells by roll coating CIGS or CZTS nanocrystal-inks over large areas. CIGS devices fabricated by roll coating over large areas with a device architecture of Mo/CIGSSe/CdS/i-ZnO/ITO/Ni/Al are (at the time of the abstract submission) 12.0% efficient under standard AM1.5G illumination. The presentation will focus on the key aspects of the nanocrystal synthesis, ink coating, nanocrystal consolidation, and device fabrication and characterization for both CIGS and CZTS solar cells.

[1] Hillhouse H.W. & Beard M.C., Current Opinion in Colloid & Interface Science, 14, 245 (2009).

[2] Guo, Q.J., Kim, S.J., Kar, M., Shafarman, W.N., Birkmire, R.W., Stach, E.A., Agrawal, R., Hillhouse, H.W., Nano Letters 8, 9, 2982 (2008).

[3] Guo, Q.J., Ford, G.M., Hillhouse, H.W., Agrawal, R., Nano Lett. 9, 8 3060 (2009).

[4] Guo, Q.J., Hillhouse, H.W., Agrawal, R., J. Am. Chem. Soc. 131, 11672 (2009).

CIGS is a promising material for solar cell applications. Uniformity of the polycrystalline composition and structure is an important factor in the solar cell performance. This work is an attempt to study CIGS (non)uniformity systematically as a function of depth, employing independent, complementary high resolution techniques which also reveal inter- and intra-grain compositional, structural, and electronic nonuniformities. In particular, thin cross-sections, prepared by field ion beam (FIB), were studied using TEM-based techniques to achieve higher spatial resolution of the composition than is normally possible. It was found that the Ga/In ratio in the devices drops initially with depth, then rises sharply again near the Mo contact. Furthermore, large variations of this ratio are observed from grain to grain. This variation is even observed within individual single crystal grains along their growth direction. Our measurements confirm the formation of a MoSe₂ phase at the Mo – CIGS interface. The lattice constant, measured by selected area diffraction (SAD), varies with the Ga composition, in agreement with Vegard's law. The extent of compositional variation was found to vary inversely with the temperature of sample preparation. Additionally, our measurements confirm the formation of a MoSe₂ phase at the Mo – CIGS interface.

A series of scanning probe (SPM)-based techniques, including scanning capacitance microscopy (SCM) and conducting probe atomic force microscopy (CP-AFM) were applied to correlate local device performance and doping concentration with the composiitonal (non)uniformity revealed in these TEM results and will be presented here.

Chemical bath deposited CdS thin films are commonly used as buffer layers in CdTe and CIGS photovoltaics because they form a high quality p-n junction with the absorber. However, light absorption by the CdS reduces solar cell efficiency. Cd_1-xZn_xS has a wider band gap than CdS, offering the potential to reduce deleterious absorption of light in the 300-550 nm range and improve current densities by over 2 mA/cm². To maximize energy conversion efficiency, the Cd_1-xZn_xS should be a single-phase ternary alloy and the stoichiometry should be optimized to ideally position the conduction band edge for increased transmissivity while retaining good junction properties.

We report on chemical bath deposition of combinatorial Cd_1-xZn_xS thin films using a continuous flow microreactor. The microreactor uses a sub-millimeter reaction channel, with the substrate acting as one reactor wall. The microreactor behaves like a plug flow reactor whereby the bath composition changes as flowing material is deposited on the substrate. While the bath composition changes with respect to position down the reaction channel, the composition at any position is time-invariant. The graded bath composition results in deposition of graded thin films whose stoichiometry and optoelecronic properties change significantly over length scales of millimeters to centimeters. Spatially resolved characterization of the substrate enables rapid and direct correlation of material properties to growth conditions, which is not possible with a batch reactor where bath composition changes with time.

Graded Cd_1-xZn_xS films grown with 1:200 Cd:Zn bath composition at the inlet had composition that varied from x=0.0 to x=0.17 over a distance of 12 mm on a single substrate. Film stoichiometry was determined by x-ray photoelectron spectroscopy (XPS) mapping. Stoichiometry changes because of differences in speciation and reaction kinetics of the cation species as they flow down the channel. XPS and XRD confirm that CdZnS is a single phase material. Deep level emission in photoluminescence and XPS indicate that O and OH is also incorporated into the film and is bound to Zn, with amount increasing further down the reaction channel. The graded stoichiometry causes the absorption edge to blue-shift by over 80 nm, as determined by UV-vis transmission and reflectance measurements. Blue-shifting band edge and changes in defect density are also seen by photoluminescence. The continuous flow microreactor offers new potential for deposition of graded thin films that act as combinatorial libraries for high throughput screening and accelerated materials discovery.

Inorganic thin film photovoltaics are an attractive technology for achieving large-scale deployment of inexpensive, stable, and efficient solar cells. However, current leading thin-film materials, such as CdTe and CIGS, face production limitations at a global scale as they contain both non-abundant and toxic elements. A material that has gained significant attention is Cu₂ZnSnS₄ (CZTS), which is not constrained by the drawbacks of CdTe and CIGS and has an ideal bandgap for a single junction photovoltaic device of 1.4-1.5 eV. Several groups have studied the fabrication of CZTS solar cells using a variety of methods ranging from sol-gel processing to sputter deposition, with a record efficiency of 9.6% recently reported. In the present work, we report an alternative water-based method for large area deposition of CZTS thin films that does not require expensive or complex equipment. Specifically, thin films of CZTS were fabricated on silicon, glass and molybdenum-coated substrates through a combination of chemical bath deposition, ion exchange and sulfurization heat treatment. The film composition could be controlled through a combination of number and length of chemical bath steps and ion exchange time. The resulting samples were analyzed by scanning electron microscopy (SEM), Auger electron spectroscopy (AES), X-ray diffraction (XRD), Raman spectroscopy, inductively-coupled plasma optical emission spectroscopy (ICP-OES), and diffuse reflectance absorption spectroscopy. XRD, Raman, and UV-Vis optical spectra are consistent with the formation of CZTS. The results show that the process produces thin films of CZTS exhibiting uniform composition, well-defined crystal structure, and good optical properties with a bandgap of 1.45 eV. Complete solar cell devices made with chemical bath-deposited CZTS were fabricated. Measurements on these devices exhibit photovoltaic and rectifying behavior, and the results will be discussed.

Chalcopyrite Cu(In,Ga)Se₂ is one of the most promising absorber materials for high efficiency thin film solar cells with reported conversion efficiency exceeding 20%. The National Solar Technology Roadmap for CIGS PV specifically calls for deposition rates of “30–40 μm/h and <1 μm CIGS absorber thickness” by 2015. This is compared to the current estimate in the Roadmap of “5 μm/h and 1.25–3 μm CIGS absorber thickness.” The comparison translates into a reduction of the absorber synthesis time from ~15 to 36 min to ~1.5 to 2 min. The challenge is to get high throughput and yield with columnar grain growth while retaining the high efficiency. In this work in-situ high temperature X-ray diffraction with and without selenium overpressure is used to determine absorber synthesis mechanisms for various precursors structures. Qualitative analysis of the data gives information on the reaction pathway and quantitative analysis of the data yields rate constants.

This presentation summarizes studies on the selenization of elemental stacked layers of copper, indium, gallium and selenium in two different configurations. In the first configuration (sample A), copper was first deposited on glass/Mo substrates, followed by gallium, indium and selenium. In the other configuration (sample B), gallium was first deposited followed by indium, copper and selenium. ICP results showed that both the samples were copper poor. Reaction pathways were followed with and without selenium overpressure and isothermal soaking experiments were performed to obtain the kinetic parameters using the Avrami growth model to reduce the data. The reaction pathways were similar for both the configurations, showing formation In, Cu₉Ga₄, and Cu₂Se initially, followed by Se crystallization and formation of the intermediates In₄Se₃, CuSe₂,CuSe and CIS, and finally yielding product CuIn_1-xGa_xSe₂. The value of x computed from Vegard’s law yielded 0.35 for sample A and 0.37 for sample B. The activation energy computed from the Avrami model yielded 88.4(±12) and 125.1((±9) kJ/mole for samples A and B, respectively. The decrease in the local Avrami exponents (0.21-0.26) suggests the existence of an inhomogeneous distribution of nuclei during growth or interdiffusion of gallium and indium with simultaneous grain growth. Additional characterization such as SEM and TEM were performed to provide physical and compositional support of the pathway.