Flex PV modules by monolithic series connection during thin film dep

The implementation of an in situ series connection in roll-to-roll deposition tools opens a new avenue for low cost mass production …

by Rainer Merz, Markus B. Schubert, Jürgen H. Werner, Universität Stuttgart, Institut für Physikalische Elektronik, Stuttgart, Germany

The implementation of an in situ series connection in roll-to-roll deposition tools opens a new avenue for low cost mass production of flexible solar modules with no need for laser patterning, nor breaking the vacuum during in-line processing.

Flexible photovoltaic (PV) modules based on thin film solar cells promise cost advantages over rigid competitors due to the lower cost of substrates and production equipment at very high production volume. Cost-effective encapsulation and monolithic series connection are demanding challenges for bringing such promises closer to reality. This article presents a technique for the in situ series connection of flexible thin film solar cells into PV modules, without any laser scribing, nor breaking of the vacuum process flow. Applying the wire-shading technique to a laboratory-scale solar cell process, verifies the concept by producing an amorphous silicon (aSi)-based module comprising ten single cells on 40cm2. The interconnection loss F<15%, and a total area module efficiency ¿mod=3%, are in close agreement with expectations deduced from the reference cell parameters. High open-circuit voltage and fill factor of the modules prove the feasibility of the in situ series connection technique, and open a pathway for integrating this method into large scale roll-to-roll production equipment.

Large-volume production

Very recently, large-volume production of aSi-based PV modules started on glass sheets with an area >5m2 [1,2]. Plasma enhanced vapor deposition (PECVD) grows silicon-based thin film solar cells either in so-called superstrate configurations [1-5], often denoted as p-i-n (according to the deposition sequence), or in substrate configurations [6,7], denoted as n-i-p. While the n-i-p sequence mostly employs opaque substrates such as steel foil [7], the p-i-n sequence requires highly transparent substrates such as glass plates [1-5]. The incident light enters the solar cell structure through the p-type doped layer to facilitate hole collection.

In addition to optimized cell deposition tools and processes, mass production of thin film modules needs scribing or patterning techniques for electrically connecting single solar cells in series to form photovoltaic modules [8]. Common processes for patterning the layers into stripes are laser or mechanical scribing. Those technologies require additional equipment and interrupt the production flow for scribing back contact, semiconductor layers, and front contact in three different steps (often denoted as P1, P2, P3) outside the vacuum.

Roll-to-roll manufacturing of thin film solar modules offers a pathway to further cost reduction beyond the use of very large glass plates. The cost of foil substrates can be significantly lower. However, low-temperature processing of solar cells on flexible plastic foils, scribing and interconnecting them into modules, and their long-term encapsulation, are still demanding challenges [9].


Figure 1. In situ series connection of n-i-p aSi solar cells by successive shading of the layer depositions. Wire shift #1 after n-µcSi deposition opens access to the back contact; i-aSi isolates the cells; wire shift #2 before p-µcSi deposition forms series connection with negligible resistance RS ? 0 O of the p-n tunnel junction.
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For large scale production of flexible thin film modules, M. Izu and S. R. Ovshinsky [6] introduced roll-to-roll deposition on stainless steel in 1982 that developed into the as yet most successful aSi-based roll-to-roll technology [7]. M. Yano, K. Suzuki, K. Nakatani, H. Okaniwa presented first aSi-based solar cells from a roll-to-roll process on polyethylene terephthalate (PET) with an efficiency ¿=9% in 1987 [10].

In this article, we present a process for the in situ series connection (ISSC) of flexible thin film photovoltaic modules that avoids the need of breaking the vacuum during module manufacturing. This in situ patterning method easily integrates into in-line roll-to-roll fabrication of most kinds of thin film solar modules.

For testing the potential of this in situ series connection, we present photovoltaic modules from a single, aSi-based n-i-p stack grown on polyethylene-naphthalate (PEN) foil at a substrate temperature Tsub=160°C. On a thermally evaporated Ag back contact with a sputtered ZnO buffer layer, PECVD deposits n-type doped microcrystalline silicon (µcSi), undoped (intrinsic) hydrogenated aSi (i-aSi), p-type µcSi, and a final sputtering process adds a transparent conductive oxide (TCO) as a front contact. The incident light enters the cell structure through the TCO and p-µcSi layers for absorption and generation of electron-hole pairs in the i-aSi absorber. The thin p- and n-type layers generate an electric field across the 400nm thick i-aSi absorber layer. This electric field separates electrons and holes in the drift-controlled n-i-p structure, and the addition of the single cell voltages via ISSC finally yields the module output voltage VPV.

In situ series connection

Monolithic series connection is used to add the output voltages of single thin film solar cells to raise the operating voltage of photovoltaic modules for a low-loss PV system layout. The common laser scribing techniques work well on high-temperature stable glass substrates, but face more difficulties on flexible polymer foils, resulting in increased area loss because of the wider scribe lines and distances, shunting problems etc. Therefore, we proposed to implement the monolithic series connection on flexible substrates in situ by appropriate masking during the various deposition steps [11].

Simultaneously with layer deposition, masking wires aligned along the slightly bent substrate foil form the scribe lines, and two subsequent shifts of the masking wires implement the monolithic series connection of single cell stripes into completed PV modules [11], with no interruption of vacuum processing nor the need for extra equipment.

Figure 1 demonstrates the principle of the in situ series connection. The schematic shows three neighboring solar cells consisting of sequentially deposited Ag back contact with ZnO buffer layer, n-i-p diode and ZnO front contact, with the electrical series-connection into a PV module. In an optimized processing sequence [11], the masking wires first pattern the back contact into stripes during silver evaporation, ZnO sputtering, and n-µcSi PECVD, completely avoiding lateral electrical connections between the adjacent stripes. After n-µcSi deposition, the wire shift #1 masks the back contact and the thin, low ohmic n-µcSi layer for i-aSi deposition. The intrinsic i-aSi absorber with its low conductivity si-aSi ? 10-10 (Ocm)-1 isolates the back contacts of adjacent cells from each other. For establishing the series connection, wire shift #2 opens access to the n-µcSi layer, and at the same time patterns the front contact layers into adjacent stripes. Monolithic series connection forms while the top p-µcSi layer connects to the bottom n-µcSi during p-µcSi deposition. Sputtering of Al-doped zinc oxide (ZnO) as front TCO completes the cell structure. The interconnection of the single cells via the p-n tunnel junction within the interconnection gap on principle introduces an additional series resistance RS, which is negligible at RS ? 0 O [11].

Experimental results

For proving the concept of ISSC, we designed a special substrate holder for manufacturing PV modules in an existing cluster tool (MV Systems Inc., Golden, CO, USA) which is capable of coating 6-inch substrates by stationary PECVD and sputter depositions, but without the capability of dynamic roll-to roll coating. The special substrate holder for stationary ISSC [11] clamps the polyethylene-naphthalate (PEN) foil over a bent surface to enable consecutive patterning of the single layers by the masking wires, as well as easy and well-reproducible wire shifts. The mechanics of our current ISSC holder limits the module area to a width wmod<13cm and a length Lmod<7cm.


Figure 2. a) Layout of the special substrate holder for proving the ISSC concept. Springs and tension bar clamp the shading-wire assembly onto the surface of the substrate foil during deposition (further details are in Fig. 3); and b) a completed ISSC module comprising 10 aSi solar cells with a cell width wC = 8.8mm, and a total area efficiency ¿mod = 3%.
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Figure 2a presents a schematic drawing of the special ISSC holder for in situ patterning of the single layers in our stationary deposition setup. The design of the holder enables patterning during evaporation, plasma and sputter deposition. Wire guides position the wires parallel to each other with a wire distance wd. The springs and the tension bar tightly clamp the masking wires onto the surface of the substrate foil. For implementing the wire shifts #1 and #2 indicated in Fig. 1, the ISSC holder allows for lifting up the wires and shifting the wire guides.

Figure 2b shows an experimental ISSC module with ten aSi-based solar cells of width wmod=10cm and length Lmod=4cm. The area loss due to the interconnection gaps with a width wG=1.2mm each, reduces the total area Atot=40cm² to the active area Aact=35.2cm². The bright lines discernible in the photograph result from the combination of three successive patterning steps, performed with two wire shifts by a distance ws=0.5mm between the patterning lines. The open circuit voltage VOC=9V of the resulting PV module proves that ISSC works without introducing any shunt path associated with the patterning lines. The fill factor FFmod=46.2% of the module is close to FFref=47% of the reference cells, demonstrating successful interconnection with no additional resistive loss.

The drop from the reference cell efficiency ¿ref=3.3% to the total area module efficiency ¿mod=3% results from the module’s total-to-active area ratio Atot/Aact and from resistive losses caused by the TCO. The measured short circuit current ISC=29mA is close to the expected ISC, calc=JSC Aact/10 cells=28.1mA when taking into account the interconnection gap wG. Calculating the interconnection loss F = (PG+PTCO+Ppn)/Pmax with respect to the upper limit of the output power Pmax=¿cell Atot 100mWcm-2 yields Fmin=15% for a wire shift ws=0.5mm and an optimized wire distance wd=10mm [12]. The calculation takes into consideration the loss PG=VMPP JMPP wG L due to the interconnection gap when operating the solar cells at their maximum power point voltage VMPP with the corresponding output current density JMPP. The ZnO resistivity ¿ZnO=7×10-6Ocm causes a power loss PTCO = ¿ZnO dZnO (JMPP)² (wC³L/3) for a ZnO thickness dZnO=400nm. The resistive loss PRS = (JMPP)² RS L of the tunnel junction is negligible.


Figure 3. Cross-sectional view of the ISSC substrate holder depicted in Fig. 2. a) Wire guides, springs and tension bar position and tightly attach the masking wires onto the surface of the substrate foil. b) Intimate mechanical contact between shading wire and substrate surface assures perfect patterning, even during plasma deposition of aSi and µcSi. c) The wire assembly is lifted for performing the wire shifts #1 and #2. d) Summary of the shading-wire shift and re-positioning sequence for implementing ISSC in a side view to the setup of Figs. 3a-3c.
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Figure 3a presents a cross-sectional view of the ISSC substrate holder. The tool bends the flexible substrate over a slightly curved surface. Wire guides assure close mechanical contact between the masking wires and the substrate surface. Introducing the tool into a standard parallel-plate PECVD setup according to Fig. 3b, high quality aSi and µcSi layers grow with simultaneously forming interconnection gaps. The wire shifts before (#1) and after (#2) i-aSi deposition proceed according to Fig. 3c. When lifting the wire assembly, the wires are free-standing but held in position by the wire guides. A small movement of the wire guides simultaneously shifts all masking wires by a distance wS. Figure 3d summarizes the complete wire-shifting process.

Large-scale roll-to-roll production

The wire-shifting sequence of Figs. 1 and 3d easily translates into a concept for large-scale roll-to-roll deposition with either dynamic control of the positions of the different masking-wire assemblies for each chamber, or with fixed wire positions in the different deposition chambers.


Figure 4. In-line deposition sequence for roll-to-roll production of flexible ISSC modules without breaking the vacuum. Position offsets of the masking wire assemblies from chamber to chamber enable monolithic series connection according to the scheme of Fig. 1.
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Figure 4 sketches one example of an in-line ISSC deposition setup for manufacturing flexible PV modules without breaking the vacuum. Instead of the manual wire shifts #1 and #2 performed in Sect. 3 for proving the concept of ISSC, the shading-wire assemblies can be mounted with small lateral offsets before, during, and after i-layer deposition in a roll-to-roll production setup. The arrangement of chambers and the deposition sequence correspond to our experimental n-i-p layer stack presented in Fig. 1. Shading wire assemblies at such fixed positions pattern the single layers during film deposition according to the ISSC scheme of Fig. 1, resulting in a complete ?endless? PV module being delivered to the take-up roll. Neither laser patterning, nor interruptions of the vacuum processing are needed. Different mounting points re-position and control the masking wires according to their specific target position in the different chambers. The indicated offset between the scribe lines before, during, and after i-layer deposition is not to scale, but represents the principle of operation. Further investigations address details of the wire-shading processes, such as the effects of synchronous, no movement, or opposite movement, of the shading wires in the direction of the substrate foil motion.

Conclusion

This article presents an in situ series connection technique for low-cost production of flexible thin film solar modules. Appropriate wire-shading during deposition of the constituting layers of a solar cell stack yields a complete monolithic series connection of single cell stripes into an ?endless? PV module. A proof-of-concept experiment using a cluster tool for stationary rather than roll-to-roll deposition produces a thin film module consisting of ten series-connected single aSi cells. Module fill factor FFmod=46.2% and total area efficiency ¿mod=3% are in good agreement with the values deduced from single cell performance. Neither shunting, nor additional resisitive losses deteriorate the module performance.

Acknowledgment

We gratefully acknowledge financial support by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) under project no. 0325029.

References

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Rainer Merz is working towards his PhD as research staff at the Universität Stuttgart, Institut für Physikalische Elektronik (IPE), Stuttgart, Germany; rainer.merz@ipe.uni-stuttgart.de. Markus B. Schubert holds a Dipl.-Ing. and a Dr.-Ing. degree, both obtained from the Faculty of Electrical Engineering at U. Stuttgart, and is a group leader at IPE. Jürgen H. Werner received his PhD from the Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany. He is a professor at the Universität Stuttgart and director of the IPE.

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