Richard J. Matson

Cu (In,Ga)Se2 thin films and solar cells prepared by selenization of metallic precursors

CuIn(1−x)GaxSe2 (CIGS) thin films with Ga ratio, x, ranging from 0.55 to 0.75 were grown on Mo/glass substrates by the selenization of metallic precursors in a H2Se atmosphere. Without a postdeposition annealing step the films were found to have a highly graded composition that became Ga rich near the absorber/Mo interface. A high‐temperature annealing step promoted diffusion of Ga to the surface region of the films. These absorbers were used to fabricate glass/Mo/CIGS/CdS/ZnO thin‐film solar cells with open‐circuit voltages ranging from 0.4 to 0.74 V and efficiencies approaching 12%. Devices, as well as the absorber layers, were characterized.

EFFECT OF NITRIC-PHOSPHORIC ACID ETCHES ON MATERIAL PROPERTIES AND BACK-CONTACT FORMATION OF CDTE-BASED SOLAR CELLS

Forming a low-resistance contact to p-type CdTe is a critical issue for successful commercialization of CdTe-based photovoltaic devices. One solution to this problem has been to incorporate surface pretreatments to facilitate contact formation. In this article, the effects of a nitric–phosphoric (NP) acid pretreatment on material properties and device performance are investigated for polycrystalline CdTe-based devices. We demonstrate that the NP acid pretreatment, when applied to CdTe thin films, forms a thick, highly conductive Te layer on the back surface of the film and on exposed grain boundaries. When etched under optimal conditions, this results in CdS/CdTe devices with reduced series resistance and enhanced performance. On the other hand, we find that the NP etch preferentially etches grain boundaries. Overetching can result in complete device failure by forming shunt paths that extend to the heterointerface. Therefore, carefully controlling the etch concentration and duration is critical to optimi...

14.1% CuIn1 − x Ga x Se2‐Based Photovoltaic Cells from Electrodeposited Precursors

The authors have fabricated 14.1% efficient CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} (CIGS) based devices from electrodeposited precursors. As-deposited precursors are Cu-rich films and are polycrystalline in nature. Additional In, Ga, and Se were added to the precursor films by physical evaporation to adjust the final composition to CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2}. Addition of In and Ga and also selenization at high temperature are very crucial for obtaining high-efficiency devices. The X-ray analysis of the as-deposited precursor film indicates the presence of CIGS and Cu{sub 2}Se phases. The X-ray analysis of the film after adjusting the composition of the final film shows only the CIGS phase. The films/devices have been characterized by inductively coupled plasma spectrometry, Auger electron spectroscopy, X-ray diffraction, electron-probe microanalysis, current-voltage characteristics, capacitance-voltage, and spectral response.

12.3% Efficient CuIn1 − x Ga x Se2‐Based Device from Electrodeposited Precursor

Of the emerging materials for solar cell applications, CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} (CIGS) is a leading candidate and has received considerable attention in recent years. Copper-indium-gallium-selenium (Cu-In-Ga-Se) precursor thin films have been prepared by electrodeposition techniques on molybdenum substrates. The films have been characterized by inductively coupled plasma spectrometry, Auger electron spectroscopy, x-ray diffraction, electron probe microanalysis, current-voltage characteristics, spectral response, and electron-beam-induced current. Additional In or Cu, Ga, and Se have been added to the electrodeposited precursor film by physical evaporation to adjust the final composition to CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2}, and allowed to crystallize at 550 C. A ZnO/CdS/CuIn{sub 1{minus}x}Ga{sub x}Se{sub 2} device fabricated using electrodeposited Cu-In-Ga-Se precursor layers resulted in an efficiency of 12.3%.

Development of rf sputtered, Cu-doped ZnTe for use as a contact interface layer to p-CdTe

Cu-doped ZnTe films deposited by rf-magnetron sputtering have been analyzed with the intention to use this material as a contact interface in CdS/CdTe thin-film photovoltaic solar-cell devices. It is observed that unless careful attention is made to the pre-deposition conditioning of the ZnTe target, the electrical resistivity of thin films (∼70 nm) will be significantly higher than that measured on thicker films (∼1.0 μm). It is determined that N contamination of the target during substrate loading is likely responsible for the increased film resistivity. The effect of film composition on the electrical properties is further studied by analyzing films sputtered from targets containing various Cu concentrations. It is determined that, for targets fabricated from stoichiometric ZnTe and metallic Cu, the extent of Zn deficiency in the film is dependent on both sputtering conditions and the amount of metallic Cu in the target. It is observed that the carrier concentration of the film reaches a maximum value of ∼3 × 1020 cm−3 when the concentrations of Te and (Zn+Cu) are nearly equal. For the conditions used, this optimum film stoichiometry results when the concentration of metallic Cu in the target is ≈6 at.%.

DEPENDENCE OF MATERIAL PROPERTIES OF RADIO-FREQUENCY MAGNETRON-SPUTTERED, CU-DOPED, ZNTE THIN FILMS ON DEPOSITION CONDITIONS

Because of its ability to be doped highly p type, and because its valence‐band discontinuity with CdTe is small, ZnTe is believed to have an important use as an interface layer for electrical contacts to crystalline and thin‐film CdTe devices. Although this and other potential uses have been identified, relatively little is known about the fundamental properties of ZnTe in its thin‐film form, and even less is known about possible effects specific to sputter deposition. In this study, we investigate the effects of deposition conditions on the compositional, electrical, optical, and structural properties of thin‐film ZnTe produced by rf‐magnetron sputtering. Results indicate that, as is often observed when sputtering high‐vapor‐pressure compounds, rf‐sputtered ZnTe films are slightly Zn deficient. Although this may be undesirable when producing high‐resistivity undoped films, it may be advantageous when producing low‐resistivity p‐type films because it may encourage incorporation of substitutional Group I d...

Quantitative incorporation of sodium in CuInSe/sub 2/ and Cu(In,Ga)Se/sub 2/ photovoltaic devices

Sodium was deliberately introduced into CuInSe/sub 2/ and Cu(In,Ga)Se/sub 2/ photovoltaic solar cells in a controlled manner. The amount of sodium added was varied in order to pinpoint the range of sodium concentrations in the CuIn(Ga)Se/sub 2/ film for optimal performance. Films were analyzed using secondary ion mass spectroscopy and induced-coupling plasma spectroscopy to quantify the sodium concentration. The results are compared with the calculation. Finished devices show improvements in open-circuit voltage, fill factor and hole density for sodium concentrations in the range of approximately 0.05 to 0.5 atomic percent.

CIGS films via nanoparticle spray deposition: attempts at densifying a porous precursor

This report summarizes our efforts to produce large-grained CIGS materials from porous nanoparticle precursor films. In our approach, a nanoparticle colloid is first prepared via low-temperature solution synthesis. Next, a thin precursor film is prepared by spray deposition of this colloid onto a heated substrate. Post-processing of these precursors is performed to induce phase formation and grain growth. Various colloids have been sprayed onto substrates such as molybdenum/glass and CdS/SnO/sub 2//glass. Post-processing treatments under controlled gas ambients have been performed according to both standard and rapid-thermal annealing (RTA) approaches. Morphology, phase formation, and chemical composition of these nanoparticle-derived films have been characterized using SEM, XRD, Raman, and AES. Although the majority of the approaches employed did not produce large-grained materials, CIGS colloid sprayed onto CdS/SnO/sub 2//glass substrates heated at 225/spl deg/C produced films with a dense morphology as-deposited.

Microstructural study of sputter-deposited CdTe thin films

In this investigation, cadmium telluride (CdTe) thin films are deposited by radio frequency (rf)‐magnetron sputtering and analyzed structurally. Structural variations with processing parameters are assessed using x‐ray diffraction and scanning electron microscopy. Results indicate that the microstructure and stress of the films are sensitive to the deposition conditions and CdCl2/annealing procedures. Films deposited at room temperature are columnar type, demonstrating a mixture of zinc blende (cubic) and wurtzite (hexagonal) phases. These films form with the preferential column axis along the cubic [111] direction. In contrast, films deposited at elevated substrate temperatures consist of a closely packed array of cubic‐phase polycrystalline grains. The CdCl2/annealing treatment appears to reorganize CdTe microstructures from a columnar to a granular structure, and changes the preferred orientation. The treatment also appears to reduce the residual stress that accumulates during film formation and thereb...

Nanoparticle colloids as spray deposition precursors to CIGS photovoltaic materials

Cu-In-Ga-Se nanoparticle colloids have been used as precursors in the spray deposition of photovoltaic films. Precursor colloid was prepared by reaction of the metal iodides in pyridine with sodium selenide in methanol at reduced temperature according to one of two routes: synthesis of each of the component binary selenides (Type I) followed by physical mixing of the isolated particles; or a one-pot synthesis with all the metal iodides reacting together in one flask to form a mixed-metal Cu-In-Ga-Se colloid (Type II). The constituent nanoparticles in these colloids were analyzed by TEM and XRD and were determined to be amorphous as-synthesized. Crystalline phase formation of these nanoparticles was observed by XRD after a thermal treatment. These precursor colloids were sprayed onto Mo-coated glass substrates at elevated temperatures. The nanoparticle-derived Cu-In-Ga-Se films were characterized by SEM and XRD prior to being finished into CIGS solar cell devices according to standard NREL protocol. I–V ch...