William N. Shafarman

Development of CuInSe2 Nanocrystal and Nanoring Inks for Low-Cost Solar Cells

The creation of a suitable inorganic colloidal nanocrystal ink for use in a scalable coating process is a key step in the development of low-cost solar cells. Here, we present a facile solution synthesis of chalcopyrite CuInSe 2 nanocrystals and demonstrate that inks based on these nanocrystals can be used to create simple solar cells, with our first cells exhibiting an efficiency of 3.2% under AM1.5 illumination. We also report the first solution synthesis of uniform hexagonal shaped single crystals CuInSe 2 nanorings by altering the synthesis parameter.

Bulk and metastable defects in CuIn1−xGaxSe2 thin films using drive-level capacitance profiling

The drive-level capacitance profiling technique has been applied to ZnO/CdS/CuIn1−xGaxSe2/Mo solar cell devices, in order to study properties of defects in the CuIn1−xGaxSe2 film. Properties studied include the spatial uniformity, bulk defect response, carrier density, and light-induced metastable effects. These results indicate that previous estimates of carrier densities, from C–V profiling, may be significantly overestimated. In addition, a defect response previously thought to be located at the interface is observed to exist throughout the bulk material. Finally, an infrared light-soaking treatment is demonstrated to induce metastable changes in the bulk CuIn1−xGaxSe2 film. Hence, the drive-level capacitance profiling technique provides valuable insights into these films. Herein, the technique itself is fully explained, compared to other junction capacitance methods, and its utility is demonstrated using numerical simulation.

DEVICE AND MATERIAL CHARACTERIZATION OF CU(INGA)SE2 SOLAR CELLS WITH INCREASING BAND GAP

Thin‐film solar cells have been fabricated from Cu(InGa)Se2 films which were deposited by four‐source elemental evaporation with [Ga]/([In]+[Ga]) from 0.27 to 0.69 corresponding to a band gap from 1.16 to 1.45 eV. The films were intentionally deposited with no grading of the Ga and In to avoid gradients in their electrical and optical properties. X‐ray diffraction, energy‐dispersive x‐ray spectroscopy, and Auger electron spectroscopy show that the films have uniform composition with no change in structure and morphology. Glass/Mo/Cu(InGa)Se2/CdS/ZnO devices have open‐circuit voltage increasing over the entire band gap range to 788 mV and 15% total area efficiency for band gap less than 1.3 eV, or [Ga]/([In]+[Ga]) less than 0.5. A decrease in device efficiency with higher Ga content is caused primarily by a lower fill factor. Analysis of current–voltage and quantum efficiency measurements show that this results from a voltage‐dependent current collection.

Effect of Ga content on defect states in CuIn1−xGaxSe2 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.

Optical characterization of CuIn1−xGaxSe2 alloy thin films by spectroscopic ellipsometry

Optical constants of polycrystalline thin film CuIn1−xGaxSe2 alloys with Ga/(Ga+In) ratios from 0 to 1 have been determined by spectroscopic ellipsometry over an energy range of 0.75–4.6 eV. CuIn1−xGaxSe2 films were deposited by simultaneous thermal evaporation of elemental copper, indium, gallium and selenium. X-ray diffraction measurements show that the CuIn1−xGaxSe2 films are single phase. Due to their high surface roughness, the films are generally not suitable for ellipsometer measurements. A method is presented in which spectroscopic ellipsometer measurements were carried out on the reverse side of the CuIn1−xGaxSe2 films immediately after peeling them from Mo-coated soda lime glass substrates. A detailed description of multilayer optical modeling of ellipsometric data, generic to ternary chalcopyrite films, is presented. Accurate values of the refractive index and extinction coefficient were obtained and the effects of varying Ga concentrations on the electronic transitions are presented.

High-efficiency solar cells based on Cu(InAl)Se2 thin films

A Cu(InAl)Se2 solar cell with 16.9% efficiency is demonstrated using a Cu(InAl)Se2 thin film deposited by four-source elemental evaporation and a device structure of glass/Mo/Cu(InAl)Se2/CdS/ZnO/indium tin oxide/(Ni/Algrid)/MgF2. A key to high efficiency is improved adhesion between the Cu(InAl)Se2 and the Mo back contact layer, provided by a 5-nm-thick Ga interlayer, which enabled the Cu(InAl)Se2 to be deposited at a 530 °C substrate temperature. Film and device properties are compared to Cu(InGa)Se2 with the same band gap of 1.16 eV. The solar cells have similar behavior, with performance limited by recombination through trap states in the space charge region in the Cu(InAl)Se2 or Cu(InGa)Se2 layer.

Preparation of homogeneous Cu(InGa)Se2 films by selenization of metal precursors in H2Se atmosphere

Homogeneous single phase Cu(InGa)Se2 films with Ga/(In+Ga)=0.25–0.75 were formed by reacting Cu–Ga–In precursor films in H2Se followed by an anneal in Ar. X‐ray diffraction and Auger analysis show that the metal precursors reacted only in H2Se were multiphase films having a layered CuInSe2/CuGaSe2 structure. Solar cells made with the multiphase films have properties similar to CuInSe2 devices. Cells made with the annealed single phase films behave like Cu(InGa)Se2 devices with the band gap expected for the precursor composition.

CuIn1−xAlxSe2 thin films and solar cells

CuIn1−xAlxSe2 thin films are investigated for their application as the absorber layer material for high efficiency solar cells. Single-phase CuIn1−xAlxSe2 films were deposited by four source elemental evaporation with a composition range of 0⩽x⩽0.6. All these films demonstrate a normalized subband gap transmission >85% with 2 μm film thickness. Band gaps obtained from spectroscopic ellipsometry show an increase with the Al content in the CuIn1−xAlxSe2 film with a bowing parameter of 0.62. The structural properties investigated using x-ray diffraction measurements show a decrease in lattice spacing as the Al content increases. Devices with efficiencies greater than 10% are fabricated on CuIn1−xAlxSe2 material over a wide range of Al composition. The best device demonstrated 11% efficiency, and the open circuit voltage increases to 0.73 V.

Incongruent reaction of Cu–(InGa) intermetallic precursors in H2Se and H2S

The reaction pathways to form Cu(InGa)Se2 or Cu(InGa)S2 films at 450°C from metallic precursors were evaluated by reacting Cu–In–Ga films in H2Se or H2S for 10, 30, or 90min and characterizing the phase composition of the resulting films. A starting composition comprising Cu9(In0.64Ga0.36)4 and In phases was detected by x-ray diffraction in Cu–Ga–In precursors annealed at 450°C in an Ar atmosphere. When the precursors were reacted in H2Se, a graded Cu(InGa)Se2 film was formed with a Ga-rich composition and residual Cu–Ga intermetallics at the interface with the Mo back contact. The intermetallic compounds were observed to evolve from Cu9(In0.64Ga0.36)4 to Cu9Ga4 with increasing selenization time. Reaction in H2S formed inhomogeneous Cu(InGa)S2 with Cu–In intermetallics. The results are consistent with thermochemical predictions of the preferential reaction of In with Se, and Ga with S. These reaction preferences can explain the formation of a graded Cu(InGa)Se2 film during reaction in H2Se and provide a r...

Effect of substrate temperature and depostion profile on evaporated Cu(InGa)Se2 films and devices

Abstract This paper addresses the effect of substrate temperature ( T SS ) and deposition profile for Cu(InGa)Se 2 films deposited by multisource elemental evaporation on film structure and solar cell performance. Different temporal Cu flux profiles are utilized to give either a graded deposition incorporating a Cu-rich growth step, with [ Cu ]>[ In ]+[ Ga ] , before achieving the final film composition with [ Cu ] In ]+[ Ga ] , or a uniform deposition with [ Cu ] In ]+[ Ga ] throughout. The Cu(InGa)Se 2 morphology, including quantitative analysis of the grain size distributions, and the performance of completed solar cells are compared at T SS =400 and 550°C. The higher T SS gives larger grains and better device performance with the best devices obtained in this work having efficiencies of 16.4% for 550°C and 14.1% for 400°C. At 550°C, Cu-rich film growth gives bigger grains than a uniform flux process, but there is no difference in the device performance. With T SS =400 °C, there is no significant difference in the grain size with the different flux profiles, but the Cu-rich growth is needed for improved devices.