The reaction of metallic precursors has become the primary method of industrial manufacturing for Cu(In,Ga)Se2. Commonly used Cu3Ga sputter targets have thus far dictated that the relative Ga composition of these precursors is Ga/(In+Ga) ≈ 0.25. Cu-In-Ga precursors are prepared with varying DC sputtering conditions and Ga compositions ranging from 0 ≤ Ga/(In+Ga) ≤ 0.75. The phase evolution and morphology of these precursors is characterized using x-ray diffraction (XRD) and scanning electron microscopy, including in situ annealing of precursors during XRD measurements. It is observed that the Ga composition of as-deposited precursors affects phase evolution with annealing. Consistent morphology changes were not observed with changing Ga, however, film morphology was controlled by adjusting In sputter conditions.

Sulfur is demonstrated to be an effective and promising surface passivation element for both n- and p-type Si wafers after reacting in dilute hydrogen sulfide gas (2 - 6% in argon) at 550°C. Effective minority carrier lifetimes of > 2 ms and > 0.25 ms are achieved for n- and p-type Si wafers, respectively, comparable to the industry standard thermal oxide and atomic layer deposited aluminum oxide passivation level. Surface characterization by x-ray photoelectron and emission spectroscopy reveals that sulfur is primarily bonded in a sulfide environment.

CIGS absorbers are developed using Se-capped precursor films with increased Ga composition. Films are reacted using a rapid thermal process, which uses reaction times on the order of minutes. It is observed that Ga profiles can be controlled by adjusting the Se availability in the reaction. Additionally, it is shown that CIGS grain size increases with Ga content; however, morphology variability may be increased. Two different processes for introducing S to the reaction are investigated.

The precursor reaction method for the fabrication of Cu(In,Ga)Se 2 (CIGS) solar cells potentially allows for low-cost fabrication and scalable processing for manufacturing. Additionally, this process has yielded record efficiencies in lab scale experiments. Thus far, research on the precursor reaction method has been restricted to relatively low Ga compositions in CIGS (Ga/(In+Ga) ≈ 25%). By increasing the ratio of Ga, it is possible to increase bandgap and thus increase open circuit voltage. This work focuses on developing and characterizing the precursor reaction process for use with the higher Ga regime, with the goal of improving open circuit voltage and efficiency. It is shown that with increased Ga ratio, the rate of conversion from precursor to absorber phases is decreased. Additionally, increased Ga improves film adhesion at increased selenization temperatures as well as improving film morphology.

Copper indium gallium diselenide, Cu(InGa)Se-2 (CIGS) solar cells have achieved efficiencies of 22.3% at the cell level and 17.5% at the module level. CIGS-based modules are also in the early stages of commercialization, with >1 GW annual production capacity. The most common method for producing CIGS in industry is via precursor reaction, which consists of depositing Cu-In-Ga precursor films and reacting them with gas-phase H2Se at 450-550 degrees C for 60 min, or longer, and is commonly called selenization. Recently, interest has been growing in selenization by Rapid Thermal Processing (RTP), which is characterized by rapid temperature ramping, approximately 550 degrees C or higher temperature reactions, and improved process throughput. However, it has been difficult to commercialize RTP for CIGS film production in part because implementing a rapid, linear temperature ramp in the reacting thin film is complicated by two intrinsic process characteristics: (i) the temperature of the CIGS film cannot usually be measured directly; and (ii), the process is significantly nonlinear due to the dominance of radiative heat transfer at high temperatures. In this paper, we present the design and modeling, construction, and successful operation of a pilot-scale RTP selenization reactor utilizing a novel temperature control system.Our two-fold approach to the unique temperature control challenges involves the design and implementation of (i) a first-principles, model-based observer to estimate the desired surface temperature; and (ii) a specialized controller to enable effective tracking of the desired linear temperature ramp set point. Our experimental results demonstrate that the control system is effective in tracking rapid temperature ramps accurately, with performance limited only by the physical constraints of the experimental system. (C) 2016 Elsevier Ltd. All rights reserved.

The control of composition and bandgap in chalcopyrite thin-film absorber layers formed by a metal precursor reaction is addressed. Two processes using reaction with either H2Se or H2S as the final step of a three-step reaction process were compared as follows: a three-step H2Se/Ar/H2S reaction and a three-step H2Se/Ar/H2Se reaction. In both processes, significant Ga homogenization was obtained during the second-step Ar anneal, but the third-step selenization resulted in Ga depletion near the Cu(InGa)Se-2 surface, whereas the third-step sulfization did not. Solar cells were fabricated using absorbers formed using each method, and the surface Ga depletion significantly affected device performances. The solar cell incorporating the sulfization yielded a better device performance, with an efficiency of 14.4% (without an anti-reflection layer) and an open-circuit voltage of 609mV. The bandgap control in the metal precursor reaction is discussed in conjunction with the device behavior. Copyright (c) 2014 John Wiley & Sons, Ltd.

There are a number of requirements in the design of evaporation sources for the inline deposition of Cu(InGa)Se2 absorber layers. These include evaporation rates ≫10g/hr without anomalous behavior such as spitting, spatial composition uniformity across the area of thousands or tens of thousands of consecutive substrates, and low capital and maintenance costs. To meet these requirements, both quantitative and qualitative design considerations must be made, including thermal and flow modeling, fabrication methods, and material properties such as thermal and chemical compatibilities.

Cu(In,Ga)(Se,S) 2 (CIGSS) absorbers with thicknesses from 1.9 to 0.25 µm have been grown using a three-step selenization/Ar-anneal/sulfization reaction ofCu-In-Ga metal precursors. Material characterization revealed changes in orientation, apparent grain size, and formation of voids at the Mo/CIGSS interface with reduced thickness. Even with absorber thickness decreased to 0.25 µm and lateral compositional nonuniformity, V OC and fill factor were nearly sustained, while J SC decreased due to incomplete absorption. With the 0.25-µm-thick absorber layer, an efficiency of 9.1% (without AR coating) with V OC = 612 mV, J SC = 21.0 mA/cm2, and FF = 71.1% was obtained.

Cu(In,Ga)(Se,S)2 (CIGSS) absorbers with thicknesses from 1.9 to 0.25 μm have been grown using a three-step selenization/Ar-anneal/sulfization reaction of Cu-In-Ga metal precursors. Material characterization revealed changes in orientation, apparent grain size, and formation of voids at the Mo/CIGSS interface with reduced thickness. Even with absorber thickness decreased to 0.25 μm and lateral compositional nonuniformity, VOC and fill factor were nearly sustained, while JSC decreased due to incomplete absorption. With the 0.25-μm-thick absorber layer, an efficiency of 9.1% (without AR coating) with VOC = 612 mV, JSC = 21.0 mA/cm2, and FF = 71.1% was obtained.

A three-step H 2 Se/Ar/H 2 S reaction is used to process Cu-In-Ga metal precursors to form Cu(In,Ga)(Se,S) 2 films over 10 × 10 cm 2 substrates. The 1 st selenization step gives fine microstructure with Ga accumulation near the Mo back contact, primarily in a Cu 9 Ga 4 phase. Significant grain growth with homogenous through-film Ga distribution is obtained by the 2 nd Ar annealing step. The 3 rd sulfization step completes the reaction process and incorporates S near the Cu(In,Ga)Se 2 surface. The resulting films show good adhesion and yielded devices with η = 14.8% and V OC = 612 mV.

Modification of Ga depth profiles in Cu(InGa)Se2 films on polyimide substrates has coincided with device efficiencies approaching 19% at temperatures at least 100°C lower than on glass substrates. In the present study, (AgCu)(InGa)Se2 films with nominal bandgaps Eg ≈ 1.4 eV have been deposited on polyimide substrates using various three-stage deposition processes to modify Ga depth profiles. Devices were characterized by J-V and quantum efficiency measurements for comparison to their Ga profiles. The highest-efficiency device had η = 17.9% with device parameters VOC = 744 mV, JSC = 32.2 mA/cm2, and FF = 74.7%.

Control of the through-film composition and adhesion are critical issues for Cu(In,Ga)Se2 (CIGS) and/or Cu(In,Ga)(Se,S)2 (CIGSS) films formed by the reaction of Cu–In–Ga metal precursor films in H2Se or H2S. In this work, CIGSS films with homogenous Ga distribution and good adhesion were formed using a three-step reaction involving: (1) selenization in H2Se at 400 °C for 60 min, (2) temperature ramp-up to 550 °C and annealing in Ar for 20 min, and (3) sulfization in H2S at 550 °C for 10 min. The 1st selenization step led to fine grain microstructure with Ga accumulation near the Mo back contact, primarily in a Cu9(In1−xGax)4 phase. The 2nd Ar anneal step produces significant grain growth with homogenous through-film Ga distribution and the formation of an InSe binary compound near the Mo back contact. The 3rd sulfization step did not result in any additional change in Ga distribution or film microstructure but a small S incorporation near the CIGSS film surface and complete reaction of InSe to form CIGSS were observed. The three-step process facilitates good control of the film properties by separating different effects of the reaction process and a film growth model is proposed. Finally, CIGSS solar cells with the three-step reaction were fabricated and devices with efficiency = 14.2% and VOC = 599 mV were obtained.

The compositional distribution of Ga and S in Cu(InGa)(SeS)2 films fabricated by a simultaneous selenization and sulfization process was systematically investigated. At low H2Se/H2S reaction temperature (490°C), most Ga remains at the back of the film adjacent to the Mo back contact. However, the Ga/III ratios at the top and bottom of the Cu(InGa)(SeS)2 layer monotonically increase and decrease with reaction temperatures, respectively. At T>550°C, homogeneous distribution of elemental Ga and In through film is achieved. Further increase of the reaction temperature (e.g., T>550°C) causes phase segregation on the surface of the Cu(InGa)(SeS)2 film confirmed by XRD, GIXRD and EDS analysis.

AgCu(InGa)Se2 alloy absorber layers with various Ga/(Ga+In) and Ag/(Ag+Cu) ratios were deposited using multi-source elemental evaporation and analyzed by glancing incidence x-ray diffraction and energy dispersive x-ray spectroscopy. All films exhibit satellite chalcopyrite reflections in the x-ray diffraction pattern and films with 0.5 ≤ Ga < 1 and Ag > 0.5 have additional reflections consistent with an ordered defect phase which is limited to the near-surface region of the film. X-ray photoelectron spectroscopy results show that all films have low (Ag+Cu)/Se ratios near the surface, consistent with an ordered defect compound. Films with 0 < w < 1 have (Ag+Cu)/Se and (Ag+Cu)/(Ga+In) ratios at the surface close to the (AgCu)(InGa)5Se8 ordered defect phases. Additionally the near-surface region of (AgCu)(InGa)Se2 films contains a higher Ag/(Ag+Cu) ratio than the bulk and the Ag(InGa)Se2 film contains excess Ag near the surface.

We have examined the electronic properties of (Ag1−xCux)(In1−yGay)Se2 (ACIGS) alloys over a wide range of compositions to assess whether such alloys might allow one to achieve larger values of VOC at larger band gaps compared to the Cu(In1−yGay)Se2 (CIGS) alloys. Our studies employed junction capacitance techniques such as drive level capacitance profiling (DLCP) and transient photocapacitance (TPC) spectroscopy, as well as temperature dependent J–V measurements. The TPC spectra revealed not only that the band gap did indeed increase as the Ag-fraction was increased, but also that the bandtailing (or Urbach energies) in all ACIGS samples were substantially smaller than for CIGS samples of corresponding band gaps. This indicates that the Ag alloying somehow reduces the degree of disorder present. The DLCP measurements indicated very low free carrier densities, on the order of 1014cm−3, as well as evidence of defects located at the CdS/ACIGS junction. Temperature-dependent I–V measurements revealed a distinct “kink” in the VOC vs T characteristics, suggesting a transition from an interface-trap limited regime to a bulk-limited regime. At temperatures below 250K, the VOC increased by up to 0.1V as the sample was light soaked. This suggests that the interface traps limiting the VOC can be passivated by exposure to light.