Serdar Aksu

Electrodeposition of In–Se and Ga–Se Thin Films for Preparation of CIGS Solar Cells

An approach was developed for electrochemical codeposition of In-Se and Ga-Se films with high repeatability and controllable In/Se and Ga/Se molar ratios to prepare precursor layers for copper indium gallium diselenide (CIGS) film formation by two-stage processes. Full potential of complexation was used for the first time by using aqueous electroplating solutions containing complexing agents at alkaline regime. It was found that tartrate and citrate were suitable complexing agents to solubilize In and Ga ions at high pH, respectively. Because no appreciable complexation of Se occurred, Se reduction potential could be independently controlled by the amount of dissolved Se. Use of alkaline solutions also reduced hydrogen bubble generation and related defects.

Status of electroplating based CIGS technology development

CIGS is the leading thin film PV material in terms of it capability to yield high efficiency solar cells. Co-evaporation method already yielded solar cells with close to 20% efficiency. Despite this success, however, commercialization of CIGS has not been aggressive. One reason for this is the fact that CIGS is a complex material. The other reason is the difficulty of scaling up this technology while keeping the cost structure competitive. SoloPower has developed a low cost electrodeposition-based CIGS technology for large scale roll-to-roll manufacturing. The substrate is a flexible metallic foil. CIGS absorber layers are formed through annealing and thermal activation of electrodeposited precursor layers. Top and bottom contacts are formed by roll-to-roll sputtering approaches. The technique has excellent ability to control the composition of the deposited layers over large are substrates. After forming a roll of solar cells, devices are cut and then packaged in module structures. Flexible solar cells with an area of over 100 cm2 were fabricated with over 12% efficiency. Over 1 m2 area modules with an efficiency of 10% were also fabricated.

Recent advances in electroplating based CIGS solar cell fabrication

Electrodeposition based CIGS technology developed by SoloPower is highly attractive route for preparation of precursor layers due to its low cost, efficient materials utilization and scalability to high-volume manufacturing. Several aqueous electroplating solutions both in acidic and alkaline regimes are formulated and optimized for achieving stable plating solutions for roll-to-roll electrodeposition that give adherent and high quality films with controllable molar compositions. An optimized rapid thermal annealing approach was developed to achieve device-quality CIGS absorber layers. In this paper, we discuss various electrodeposition approaches for CIGS precursor formation and present a summary of the recent results for SoloPowers flexible cells.

Indoor and outdoor testing of low weight flexible CIGS modules

Some of the critical advantages of the thin films are being light and flexible. These attributes of the thin films can be advantageously realized when thin film CIGS material is formed on flexible substrates such as stainless steel foils. However due to moisture sensitivity of the CIGS solar cells, commercial flexible CIGS modules that can meet the markets requirements and obtain formal certifications have been elusive. In this presentation, work will be presented that will describe the significant progress made in the field of flexible low weight CIGS modules. A significant milestone was reached when the formal certifications to UL 1703, IEC 61646, and IEC 61730 standards were obtained for the first time for flexible CIGS modules. These modules have excellent power levels reaching a 12.1 aperture efficiency as measured at NREL. Many accelerated tests including the damp humidity test (85C and 85% humidity) were performed in excess of 5000 hours. The results show that there is very little change in the Pmax of the modules and that the total power loss after 5000 hours of testing is less than 10%. Further accelerated indoor tests including temperature cycling and humidity freeze will be discussed and the results will be compared against the rigid CIGS modules. After obtaining the formal certifications, the modules were deployed outdoors and grid connected. The flexible CIGS modules perform very well with performance ratios exceeding 0.85. Additional indoor and outdoor test results will be presented.

Electrodeposition of Novel Precursor Structures for Efficient Copper Indium Gallium Selenide (CIGS) Films

CIGS is one of the most advanced absorber materials for thin film solar cells due to its direct bandgap, high absorption coefficient, and ability to yield good quality devices. CIGS-based solar cells have yielded the highest conversion efficiencies of all thin film solar cells, reaching up to about 20%. One of the techniques used to form CIGS layers is a two-stage approach which involves deposition of a precursor layer on a substrate followed by a high temperature activation step that converts the precursor layer into solar cell grade CIGS. Although a variety of techniques such as evaporation and sputtering have been employed to prepare precursor layers, electrodeposition is especially attractive due to its low cost, efficient utilization of raw materials, and scalability to high-volume manufacturing. A wide range of processing approaches employing electrodeposition has been explored for CIGS film formation during the last two decades. However, most of these concentrated on acidic electrolyte compositions. Moreover, they were generally aimed at growing CIS layers rather than CIGS films. This is partly due to the fact that addition of Ga into electrodeposited films is challenging because of the high negative plating potential of Ga compared to Cu, In and Se. Such high plating potential gives rise to excessive hydrogen evolution on the cathode surface during plating of the films out of aqueous electrolytes. Hydrogen evolution reduces the plating efficiency of Ga and causes defects such as pinholes in the grown layers since the small gas bubbles stick to the surface of the growing film and prevent proper deposition at that location. Development of specialized electrolytes with long term stability and ability to control the crucially important Cu/(In+Ga) and Ga/(Ga+In) molar ratios are of utmost importance for the successful industrial application of electrochemistry to CIGS film growth. Using SoloPower’s patented electroplating process, conversion efficiencies close to14% for small area cells and about 12.5 % for cells with area over 120 cm were demonstrated. In addition, flexible solar modules with aperture efficiencies reaching to 12% have been fabricated. 2,3

Electrochemical Co-deposition of In-Se and Ga-Se Thin Films for Preparation of CIGS Solar Cells

Electrochemical co-depositions of indium (In) with selenium (Se) and gallium (Ga) with selenium were carried out to obtain high-quality In-Se and Ga-Se films. The approach utilized full potential of complexation for the first time by using aqueous electroplating solutions containing complexing agents at alkaline regime. Complexing agents were employed to solubilize In and Ga ions at high pH and to bring their reduction potentials down, closer to that of Se. Since no appreciable complexation occurred between Se and the complexing agents, Se reduction potential could be independently controlled by the amount of dissolved Se. Tartrate and citrate were determined to be suitable complexing agents for In and Ga. By optimizing the concentrations of the metal salts, the complexing agents, the selenium source, pH and the electrodeposition current density, it was possible to obtain adherent and smooth In-Se and Ga-Se films with high repeatability and controllable In/Se and Ga/Se molar ratios.