Joel N. Duenow

Transparent conducting zinc oxide thin films doped with aluminum and molybdenum

Undoped ZnO, ZnO:Al (0.5, 1, and 2wt% Al2O3), and ZnO:Mo (2wt% Mo) films were deposited by radio-frequency magnetron sputtering. Optimal deposition temperature was found to be ∼200°C for all films. Electron mobilities of 48cm2V−1s−1 were achieved for undoped ZnO films using a sputtering gas with H2∕Ar ratio of 0.3%; corresponding carrier concentrations were ∼3×1019cm−3. A target incorporating 0.5wt% Al2O3 in ZnO yielded films with mobility of 36cm2V−1s−1 and carrier concentration of 3.4×1020cm−3. These films present comparable conductivity and lower free-carrier absorption than films grown from a target containing 2wt% Al2O3. Mo was found to be an n-type dopant of ZnO, though electrical and optical properties were inferior to those of ZnO:Al. Temperature-dependent Hall measurements of ZnO:Al films show evidence of a different scattering mechanism than ZnO:Mo films.

Effects of hydrogen ambient and film thickness on ZnO:Al properties

Undoped ZnO and ZnO:Al (0.1, 0.2, 0.5, 1.0, and 2.0wt.% Al2O3) films were deposited by rf magnetron sputtering. Controlled incorporation of H2 in the Ar sputtering ambient for films grown at substrate temperatures up to 200°C results in mobilities exceeding 50cm2V−1s−1 when using targets containing 0.1 and 0.2wt.% Al2O3. Temperature-dependent Hall measurements show evidence of phonon scattering as the dominant scattering mechanism in these lightly Al-doped films, while ionized impurity scattering appears increasingly dominant at higher doping levels. A combination of compositional and structural analysis shows that hydrogen expands the ZnO lattice normal to the plane of the substrate and desorbs from ZnO at ∼250°C according to temperature-programmed desorption and annealing experiments.

Investigation of ZnO:Al Doping Level and Deposition Temperature Effects on CIGS Solar Cell Performance

Cu(In,Ga)Se 2 (CIGS) photovoltaic cells require a highly conducting and transparent top electrode for optimum device performance. ZnO thin films doped with 2 wt.% Al (ZnO:Al) are commonly used to ensure sufficient conductivity while providing acceptable transparency to the active absorber layers. Deposition of transparent conducting oxide (TCO) films on CIGS cells often is performed at room temperature in the manufacturing process because of production advantages. However, material properties and reproducibility may be poorer at room temperature than at higher temperatures. Maximum mobilities of 2 wt.%-doped ZnO:Al grown at room temperature in pure Ar are typically ∼20-25 cm 2 V -1 s -1 . Relatively high carrier concentration is therefore required to achieve the desired conductivity. This high carrier concentration produces low infrared transmittance due to increased free-carrier absorption. World-record CIGS cells produced at the National Renewable Energy Laboratory (NREL) are known to tolerate photolithographic processing temperatures of ∼100°C, though significant changes in device performance occur beyond 200°C. In this study, we investigate whether ZnO:Al films with superior material properties can be produced at the elevated temperatures consistent with CIGS processing parameters. We examine undoped ZnO and ZnO:Al with doping levels of 0.5, 1, and 2 wt.% Al 2 O 3 for growth at substrate temperatures from 25° to 360°C using radio-frequency magnetron sputtering. For films grown in a 100% Ar ambient, optimal electrical and optical properties are achieved at ∼150°-200°C. Controlled incorporation of H2 in the Ar sputtering ambient at 200°C increases mobility to 48 cm 2 V -1 s -1 for undoped ZnO and 36 cm 2 V -1 s -1 for 0.5 wt.%-doped ZnO:Al. Both values are considerably higher than the 25 cm 2 V -1 s -1 of 2 wt.%-doped ZnO:Al deposited at 200°C in 100% Ar. We have explored whether these higher-mobility films can be exploited in the design of high-quality CIGS devices produced at NREL. Preliminary results show similar open-circuit voltage and only slightly lower short-circuit current compared to devices utilizing the standard 2 wt.%-doped ZnO:Al deposited at room temperature. This suggests that higher-performance devices may result once the TCO thickness is optimized for the higher mobility.

Cathodoluminescence spectrum imaging analysis of CdTe thin-film bevels

We conducted T= 6u2009K cathodoluminescence (CL) spectrum imaging with a nanoscale electron beam on beveled surfaces of CdTe thin films at the critical stages of standard CdTe solar cell fabrication. We find that the through-thickness CL total intensity profiles are consistent with a reduction in grain-boundary recombination due to the CdCl2 treatment. The color-coded CL maps of the near-band-edge transitions indicate significant variations in the defect recombination activity at the micron and sub-micron scales within grains, from grain to grain, throughout the film depth, and between films with different processing histories. We estimated the grain-interior sulfur-alloying fraction in the interdiffused CdTe/CdS region of the CdCl2-treated films from a sample of 35 grains and found that it is not strongly correlated with CL intensity. A kinetic rate-equation model was used to simulate grain-boundary (GB) and grain-interior CL spectra. Simulations indicate that the large reduction in the exciton band intensit...

Recent Results for All-Dry-Processed CdTe/CdS Solar Cells

Several wet-processing steps are used in fabricating high-efficiency CdTe/CdS solar cells. These steps can hinder in-line processing; thus, developing an all-dry processing option is attractive for a manufacturing-friendly process. In this study, we systematically modified the baseline process used in our laboratory to replace CdS deposited by chemical-bath deposition (CBD) with sputter-deposited CdS and Cu-doped graphite paste back-contact with Cu-doped ZnTe deposited by radio-frequency sputtering. In addition to CdTe deposited by close-spaced sublimation, we also used conventionally evaporated CdTe. The results show that replacing only CBD CdS with oxygenated CdS deposited by sputtering produces devices with performance comparable to baseline devices if the front bilayer SnO 2 is replaced by a Cd 2 SnO 4 /ZnSnO alloy. Replacing the graphite paste back-contact with sputter-deposited Cu-doped ZnTe resulted in device performance comparable to baseline devices. Incorporating both dry processing steps gave performance comparable to the devices with sputtered CdS with a SnO 2 front contact. We used capacitance-voltage and minority-carrier lifetime measurements to analyze the factors affecting device performance and we present the results here.

Zno:Al doping level and hydrogen growth ambient effects on CIGS solar cell performance

Cu(In,Ga)Se<inf>2</inf> (CIGS) photovoltaic (PV) cells require a highly conducting and transparent electrode for optimum device performance. ZnO:Al films grown from targets containing 2.0 wt.% Al<inf>2</inf>O<inf>3</inf> are commonly used for this purpose. Maximum carrier mobilities of these films grown at room temperature are ∼20–25 cm²V⁻¹s⁻¹. Therefore, relatively high carrier concentrations are required to achieve the desired conductivity, which leads to free carrier absorption in the near infrared (IR). Lightly doped films (0.05 – 0.2 wt.% Al<inf>2</inf>O<inf>3</inf>), which show less IR absorption, reach mobility values greater than 50 cm²V⁻¹s⁻¹ when deposited in H<inf>2</inf> partial pressure. We incorporate these lightly doped ZnO:Al layers into CIGS PV cells produced at the National Renewable Energy Laboratory (NREL). Preliminary results show quantum efficiency values of these cells rival those of a past world-record cell produced at NREL that used 2.0 wt.% Al-doped ZnO films. The highest cell efficiency obtained in this trial was 18.1%.

Chapter 4:A Review of NREL Research into Transparent Conducting Oxides

Since the 1980s, many of the successes in photovoltaic (PV) research at the US National Renewable Energy Laboratory have been assisted by improved fundamental understanding and advanced synthesis techniques for various transparent conducting oxide (TCO) materials. In this chapter, we review some of these TCO materials, describing not only the degree to which our understanding has improved over the past ∼30 years, but also with a view to indicating where we believe significant advances remain possible. These TCO materials are discussed within the context of their specific PV device application, and include the primary and relevant alloy oxides of indium, zinc, tin and cadmium. Additionally, because the PV application could eventually become the primary use of many of these oxides, the chapter also presents some discussion on issues related to mineral abundance and toxicity.