Robert W. Birkmire

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.

CuInSe2 for photovoltaic applications

The properties and most successful methods for producing CuInSe2 films for solar‐cell applications are reviewed and the production, analysis, and performance of photovoltaic devices based on CuInSe2 are discussed. The most successful methods for depositing thin CuInSe2 films for high‐efficiency solar cells are three‐source elemental evaporation and selenization of Cu/In layers in H2Se atmospheres. Devices based on CuInSe2 have achieved the highest conversion efficiencies for any nonepitaxial thin‐film solar cell, 14.1% for a small cell and 10.4% (aperture efficiency) for a 3916‐cm2 (4 sq. ft) device. Furthermore, high‐efficiency devices have been produced by several groups and have shown no evidence of degradation of performance with time. The internal quantum efficiency is remarkably close to 100%, although various losses prevent making use of all of the generated carriers. The high performance results, in part, from the very‐high‐absorption coefficient of CuInSe2, which is of the order of 105 cm−1 for photons with energies slightly above 1 eV. Models of the operation of CuInSe2/CdS heterojunctions have begun to explain the processes limiting the device performance. The success of the models is based, in part, on the large amount of data which has accumulated on CuInSe2 in spite of the relatively short time it has been extensively studied.

Phases, morphology, and diffusion in CuInxGa1−xSe2 thin films

CuInxGa1−xSe2 thin films, with various Ga/(Ga+In) ratios, suitable for solar cells were processed by selenizing stacked Cu, Ga, and In precursor layers in a H2Se reactor in the temperature range of 400–500 °C. Cu/Ga/In and Cu/In/Ga precursors were obtained by sequential sputtering of the elemental layers. The Cu/Ga/In and Cu/In/Ga precursors, and the selenized films were characterized by scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, and Auger electron spectroscopy. The precursors contained only binary and elemental phases in the as-deposited condition and after annealing. The selenized films had a nonuniform distribution of Ga and In. The surface of the selenized films were In rich, while the Mo/film interface in these films was Ga rich. The selenized films with Ga/(Ga+In) ratios greater than 0.25 contain graded Ga and In compositions, and the selenized films with Ga/(Ga+In) ratios less than 0.6 contain a phase-separated mixture of CuInSe2 and CuGaSe2 with the CuInSe2 ne...

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.

Recrystallization and sulfur diffusion in CdCl2‐treated CdTe/CdS thin films

The role of CdCl2 in prompting recrystallization, grain growth and interdiffusion between CdS and CdTe layers in physical vapor-deposited CdS/CdTe thin-film solar cells is presented. Several CdTe/CdS thin-film samples with different CdTe film thicknesses were treated in air at 415°C for different times with and without a surface coating of CdCl2. The samples were characterized by scanning electron microscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffractometry and optical absorption. The results show that CdCl2 treatment enhances the recrystallization and diffusion processes, leading to a compositional variation within the CdTe layer due to diffusion of sulfur from the CdS. The highest sulfur concentrations observed after 30 min treatments with CdCl2 at 415°C are near the solubility limit for sulfur in CdTe. The compositional distributions indicated by x-ray diffraction measurements of samples with different CdTe thickness show that the S-rich CdTe1−xSx region lies near the CdTe/CdS interface. A multiple-step mixing process must be inferred to account for the diffraction profiles obtained.

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.

Controlling Growth Chemistry and Morphology of Single-Bath Electrodeposited Cu ( In , Ga ) Se2 Thin Films for Photovoltaic Application

Single-bath electrodeposition of polycrystalline Cu(In,Ga)Se 2 thin films for photovoltaic applications is described. Cu(In,Ga)Se 2 was deposited onto Mo electrodes from low concentration aqueous baths containing CuCl 2 , InCl 3 , GaCl 3 , and H 2 SeO 3 . Buffering the solutions to pH ∼ 2.5 stabilized bath chemistry and improved Cu(In,Ga)Se 2 film composition. Bath concentrations were shown to affect composition of deposited films, with a bath [Se 4+ ]/[Cu 2+ ] ratio of 1.75 required to maintain suitable deposited Se and Cu levels, while [In 3+ ] could be adjusted to control deposited In and Ga. Deposited films initially exhibited significant cracking, which was prevented by lowering the [Se 4+ ] in the bath, and contained Cu 2-x Se as secondary phases, resembling cauliflower florets, embedded in the film surfaces. The formation of these secondary phases was overcome by pretreating the Mo electrodes with a short 1 min deposition from the Cu(In,Ga)Se 2 bath. This, coupled with a multipotential deposition regime, led to growth of smooth, compact, crack-free films of near stoichiometric values. Mechanisms of film growth and morphology control are discussed. All as-deposited films exhibit low crystallinity, and for device processing require recrystallization by annealing in an H 2 Se atmosphere. Promising preliminary results of electrodeposited Cu(In,Ga)Se 2 devices are presented.

Interdigitated back contact silicon heterojunction solar cell and the effect of front surface passivation

This letter reports interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells which combine the performance benefits of both back contact and heterojunction technologies while reducing their limitations. Low temperature (<200°C) deposited p- and n-type amorphous silicon used to form interdigitated heteroemitter and contacts in the rear preserves substrate lifetime while minimizes optical losses in the front. The IBC-SHJ structure is ideal for diagnosing surface passivation quality, which is analyzed and measured by internal quantum efficiency and minority carrier lifetime measurements. Initial cells have independently confirmed efficiency of 11.8% under AM1.5 illumination. Simulations indicate efficiencies greater than 20% after optimization.

Analysis of post deposition processing for CdTe/CdS thin film solar cells

A post-deposition process for optimizing the efficiency of thin film CdTe/CdS solar cells deposited by physical vapor deposition has been developed and the effects of the individual process steps on the materials and device properties have been analyzed. A 400 °C heat treatment with CdCl2 restructures the CdTe resulting in enhanced grain size and crystallographic reorientation. Structural and optical measurements indicate interdiffusion of sulfur and tellurium during the heat treatment resulting in formation of a CdSxTe1−x layer with a narrower band gap than CdTe. Bifacial current-voltage and quantum efficiency analysis of the CdTe devices at various stages of the optimization process shows the evolution of the device from a p-i-n structure to a heterojunction. A chemical treatment improves the open circuit voltage (Voc) and Cu/Au contact to the CdTe. The optimization process can be applied to cells using CdTe and CdS deposited by different methods.