Peter A. Hersh

Direct write metallization for photovoltaic cells and scaling thereof

Atmospheric solution processing can help toward a significant cost reduction of photovoltaics. We investigate the use of direct write deposition approaches for deposition of metallization for a variety of solar cell materials. We are studying inkjet printing and aerosol spraying of metal contacts for Si, CIS/CIGS and organic photovoltaics. We have developed metal organic decomposition inks for metals such as: silver, nickel, copper and aluminum. All of these can be deposited in lines with 30–40 µm width and conductivities close to that of bulk metals. For silicon photovoltaics materials have been developed to facilitate Ohmic contact formation through an anti reflection coating. Initial research has been focusing on small cells, but in order to transfer the technology to production it has to be demonstrated on large area cells as well. For this the Atmospheric Processing Platform (APP) was developed at NREL. This platform allows us to scale the deposition of the developed inks and processing to large area (Up to 157 mm × 157 mm) and prototype contact patterns. The APP consists of several deposition, processing and characterization units, most located in a controlled environment. The atmospheric deposition tools in the APP are: inkjet printing, aerosol spraying and ultrasonic spraying. A rapid thermal processing unit is integrated for thermal processing. XRF and XRD can be accessed without leaving the controlled environment to determine the composition and structure of the deposited material. Sputter deposition and evaporation are also part of the APP, even though these techniques are not atmospheric. Details of the individual platforms in the APP will be given together with results of direct write contacts on large area cells.

Spray deposition of high quality CuInSe 2 and CdTe films

A number of different ink and deposition approaches have been used for the deposition of CuInSe2 (CIS), Cu(In,Ga)Se2 (CIGS), and CdTe films. For CIS and CIGS, soluble precursors containing Cu, In, and Ga have been developed and used in two ways to produce CIS films. In the first, In-containing precursor films were sprayed on Mo-coated glass substrates and converted by rapid thermal processing (RTP) to In2Se3. Then a Cu-containing film was sprayed down on top of the In2Se3 and the stacked films were again thermally processed to give CIS. In the second approach, the Cu-, In-, and Ga-containing inks were combined in the proper ratio to produce a mixed Cu-In-Ga ink that was sprayed on substrates and thermally processed to give CIGS films directly. For CdTe deposition, ink consisting of CdTe nanoparticles dispersed in methanol was prepared and used to spray precursor films. Annealing these precursor films in the presence of CdCl2 produced large-grained CdTe films. The films were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM). Optimized spray and processing conditions are crucial to obtain dense, crystalline films.

Direct-write contacts: Metallization and contact formation

Using direct-write approaches in photovoltaics for metallization and contact formation can significantly reduce the cost per watt of producing photovoltaic devices. Inks have been developed for various materials, such as Ag, Cu, Ni and Al, which can be used to inkjet print metallizations for various kinds of photovoltaic devices. Use of these inks results in metallization with resistivities close to those of bulk materials. By means of inkjet printing a metallization grid can be printed with better resolution, i.e. smaller lines, than screen-printing. Also inks have been developed to deposit transparent conductive oxide films by means of ultrasonic spraying.

Using amorphous zinc-tin oxide alloys in the emitter structure of CIGS PV devices

The typical CIGS device structure employs a molybdenum back contact and a CdS/ZnO/ZnO:Al emitter structure. In this work the undoped ZnO is replaced with amorphous zinc-tin oxide alloys (ZTO). Varying composition and deposition method of the ZTO can provide a wide range of band gap (3.3-3.9eV) and work function (4.3-5.2eV), while remaining amorphous. The flexibility of the ZTO provides the opportunity to tune the bands to optimize band-edge and Fermi level alignment. Devices demonstrated to date with ZTO alloy composition have yielded a maximum efficiency of 11.9% with an average of 11.3%, which is very similar to comparable devices with undoped ZnO that have a maximum efficiency of 12.0% with an average of 11.3%. On going optimization may further improve the results.

Solution-deposited CIGS thin films for ultra-low-cost photovoltaics

We describe the production of photovoltaic modules with high-quality large-grain copper indium gallium selenide (CIGS) thin films obtained with the unique combination of low-cost ink-based precursors and a reactive transfer printing method. The proprietary metal-organic inks contain a variety of soluble Cu-, In- and Ga- multinary selenide materials; they are called metal-organic decomposition (MOD) precursors, as they are designed to decompose into the desired precursors. Reactive transfer is a two-stage process that produces CIGS through the chemical reaction between two separate precursor films, one deposited on the substrate and the other on a printing plate in the first stage. In the second stage, these precursors are rapidly reacted together under pressure in the presence of heat. The use of two independent thin films provides the benefits of independent composition and flexible deposition technique optimization, and eliminates pre-reaction prior to the synthesis of CIGS. In a few minutes, the process produces high quality CIGS films, with large grains on the order of several microns, and preferred crystallographic orientation, as confirmed by compositional and structural analysis by XRF, SIMS, SEM and XRD. Cell efficiencies of 14% and module efficiencies of 12% were achieved using this method. The atmospheric deposition processes include slot die extrusion coating, ultrasonic atomization spraying, pneumatic atomization spraying, inkjet printing, direct writing, and screen printing, and provide low capital equipment cost, low thermal budget, and high throughput.

Inkjet printed contacts for use in photovoltaics

Using direct-write approaches in photovoltaics for metallization and contact formation can significantly reduce the cost per watt of producing photovoltaic devices. Inks have been developed for various materials, such as Ag, Cu, Ni and Al, which can be used to inkjet print metallizations for various kinds of photovoltaic devices. Use of these inks results in metallization with resistivity close to those of bulk materials. By means of inkjet printing a metallization grid can be printed with better resolution, i.e. smaller lines, than screen-printing. For metallization on top of silicon photovoltaics also an ink has been developed that will facilitate the burn-through of the contact through the anti-reflection coating. Using this burn-through material may reduce the firing temperature by more than 100°C compared to conventional contact technology.

Rapid reactive transfer printing of CIGS photovoltaics

We demonstrate photovoltaic integrated circuits (PVIC) with high-quality large-grain Copper Indium Gallium Selenide (CIGS) obtained with the unique combination of low-cost ink-based or Physical Vapor Deposition (PVD) based nanoengineered precursor thin films and a reactive transfer printing method. Reactive transfer is a two-stage process relying on chemical reaction between two separate precursor films to form CIGS, one deposited on the substrate and the other on a printing plate in the first stage. In the second stage, these precursors are brought into intimate contact and rapidly reacted under pressure in the presence of an electrostatic field while heat is applied. The use of two independent thin films provides the benefits of independent composition and flexible deposition technique optimization, and eliminates pre-reaction prior to the synthesis of CIGS. High quality CIGS with large grains on the order of several microns, and of preferred crystallographic orientation, are formed in just several minutes based on compositional and structural analysis by XRF, SIMS, SEM and XRD. Cell efficiencies of 14% and module efficiencies of 12% have been achieved using this method. When atmospheric pressure deposition of inks is utilized for the precursor films, the approach additionally provides further reduced capital equipment cost, lower thermal budget, and higher throughput.

Solution deposited precursors and rapid optical processing used in the production of CIGS solar cells

In this paper we use the combination of solution deposited liquid precursors and rapid optical processing (ROP) to make CIGS. The ROP process takes less than 1 minute of heating to convert the precursor stack to CIGS. Device made with ROP rival performance of device processed using field assisted simultaneous synthesis and transfer FASST® processing.

Solution-based precursors in conjunction with rapid optical processing for high-quality hybrid CIGS

HelioVolt Corporation is currently developing Copper Indium Gallium Selenide (CIGS) products using a solution-based deposition of precursor films followed by rapid optical processing (ROP) to make CIGS. The ROP process takes less than 1 minute of heating to convert the precursor stack to CIGS. Device made with ROP rival performance of device processed using field assisted simultaneous synthesis and transfer (FASST®) processing.

Field assisted simultaneous synthesis and transfer FASST ® method used in conjunction with liquid precursors to produce CIGS solar cells

Field assisted simultaneous synthesis and transfer FASST method is used in conjunction with liquid precursors to produce CIGS solar cells. Performance of CIGS cells using liquid precursors is on par with the performance of cells produced from PVD precursors. The FASST process has yielded up to 14% efficiency devices.