Roll to Roll Flexible Microgroove Based Photovoltaics. John Topping Chief Scientist Big Solar Limited

Transcription

1 Roll to Roll Flexible Microgroove Based Photovoltaics John Topping Chief Scientist Big Solar Limited Big Solar Limited, Washington Business Centre 2 Turbine Way, Sunderland SR5 3NZ John@powerroll.solar Telephone: +44 (0) Mobile: Abstract: The concept and development of a non-horizontally integrated photovoltaic design is discussed that aims to produce low-cost, roll-to-roll solar cells. A roll-to-roll, 3D embossed architecture is examined that utilizes the internal geometry of embossed grooves. This allows for traditional top and bottom contacts to be made within the groove in the horizontal plane. Secondary patterns also allow for horizontal cell-tocell and module-to-module electrical interconnections. This process is achieved via off axis vacuum deposition, and prepares the substrate for device completion via a suitable photo-absorber being applied to fill the groove. Cascades of grooves are linked in series to give a desired voltage multiplication, with series linked cascades connected in parallel for high current applications. Results of copper indium diselenide (CuInSe2) based devices have demonstrated the voltage multiplication. This solution to 3D interconnections allows many photo-absorbers to be used; with PbS, perovskite and P3HT/PCBM having all been used to fabricate devices. This demonstrates the ability for a low cost, roll-to-roll process that can produce PV at less than 7c per Watt. Extended abstract Introduction The global photovoltaics (PV) market continues to grow rapidly. Big Solar Limited (BSL) is developing a next generation PV product, Power Roll (PR). PR is the world s only horizontally integrated solar cell. This unique architecture allows PR to exploit roll to roll manufacturing techniques widely used in other sectirs, enabling it to be made using an ultra-low cost, simple four-step manufacturing process. This ultra-low manufactured cost enables the generation of renewable electricity at a cost comparable to (or cheaper than) hydro-carbon energy sources and some 50%. PR is also ultra-lightweight, 98% lighter than other PV, and together with a simplified electrical interconnect system reduces the cost of installation, which will enable it to penetrate the widest range of global markets.

2 Design benefits PR is a highly innovative and unique horizontally integrated PV cell. Despite global R&D activities in solar, our extensive literature reviews, patent searches and filings, feedback from research partners, and competitor research has revealled that there is no comparable design in the market. Since the initial patent application in 2011, BSL has filed patents covering a further 11 patent families with 4 of the patent families now having granted patents across some 41 separate countries. The PR technology has been subjected to independent scientific scrutiny through Prof. Brian Korgel from the Centre of Next Generation Photovoltaics, University of Austin, Texas and Prof. Andrew Watt from Oxford University-both confirming that they consider the technology highly innovative. The technical advantages of PR compared to other PV technologies include: The built in groove design illuminates the active semiconductor directly through the top encapsulation material creating an ohmic contact at the vertical facing groove walls. There this means that the expensive Transparent conductor oxide (TCO) layer is not required. TCO is required in all other solar cells comprising up to 10-20% of total cost. Each series of grooves is connected via another separate novel feature which is turn connects the cascades of grooves in parallel. This allows the current to be transferred without any further process steps, eliminating the need for a cell stringing process (in Si PV) or TCO patterning and selective coating. The impact of shading or other defects is reduced and the electrical design of PR also reduces the potential for short circuits. The unique groove design can accommodate a wide range of semiconductors and has already been proven with CIS, Quantum Dots and Perovskites. The design is compatible with high speed and continuous R2R manufacturing using simple production processes that are widely used in other sectors. The voltage of a PR module can be varied depending on how many grooves are connected in series. Market opportunity BSL will initially target two core markets: commercial rooftops and off-grid installations for developing countries although there are many more potential available markets. As the development journey increases BSL will be seeking commercial partners to explore and exploit a range of markets. The lightweight and flexible form factor of PR will allow PV to be deployed on commercial rooftops including non-load bearing structures. Remote areas in developing countries will be accessible due to the ultra-low cost and ease of

3 transportation. Other markets sectors, currently dominated by Si PV (e.g. ground mounted PV and domestic PV) as well as military/consumer devices will be explored in the longer term. The commercial rooftop sector, with 2.5 billion m2 of south facing rooftops in the UK alone presents a clear market opportunity. Development of this market has been constrained by the heavy weight nature (~12kg/m2) and high cost of silicon (Si) PV (the current market leader). Si PV solutions are still more expensive than carbon based energy sources and without generous subsidies, mass take has been limited. Some lightweight thin film PV products are available, but because of the high cost of manufacture (typically 3-6X Si PV), they have only penetrated niche markets. The global PV market has expanded rapidly from 7GW in 2009 to +60GW in 2015 (compound annual growth rate (CAGR) of +40%). Ground mounted and domestic roof top applications have seen the largest growth within the overall PV market however typically still requiring some form of government incentive. The market is forecast by to grow to >100GW p.a by 2020 at a CAGR of 27%, representing a > 10bn market for PR. Method It is proposed that using grooves with selective vacuum coated side walls as shown in Figure 1a can, with the introduction of a suitable photovoltaic absorber into the coated grooves, produce a cascade of photovoltaic cells connected in series. This form of cell production is designed to utilize the roll to roll fabrication systems currently used in the packaging and flexible electronics industries. Figure 1a: Schematic of filled selective coated grooves with an equivalent circuit The selective coating is obtained by off axis coating through either source repositioning or source shielding, this restricts the coating angle to the sides of the

4 coating drum in roll to roll coating systems. This restriction ensuring that the bottom of the grooves remains uncoated and thus the grooves remain open circuit ready for the introduction of the solar absorber materials. This isn t sufficient for a solar product system however how all the cascades are connected is also important we need connect the cascades together in parallel so as to fix the operating voltage and to increase the operating current. Figure 1b shows have this can be achieved with a simple feature produced by demetalization or similar. Figure 1b: Using a demetalization feature as shown the cascades are both separated and connected leaving the sides of the grooved substrate as the positive and negative connections for the entire interconnected groove cascade. [Patented design.] The solar absorbers have their own characteristics and need to be introduced into grooves with the appropriate widths to maximize efficiencies and with the correct contact materials. Initially this work was carried out in a bell jar vacuum coater fitted with a rotating drum system as shown in Figure 2. This system proved that such devices could be fabricated and allowed for some investigation of material systems and optimum coating geometries. The bell jar system was limited in vacuum coating of some materials as it was a thermal evaporation system only. It was sufficient however to use bell jar coating to produce samples for proof of concept and to develop knowhow on the interconnection of the cells in cascades. It was understood early in the investigation that if this was to be a roll to roll system the vacuum coated substrate that must be 100% functional, as slitting or sampling the working from non working regions would eliminate most if not all of the economic benefit from the process. One of the strengths of the groove based design is that there needs to be quite a lot of defects in a cascade to render it unusable. The cascade design has a high degree of interconnectivity and uncoated (open circuit) regions can simply be bypassed and any short circuits only effect a small portion of the overall cascade. This is due to the intrinsic resistance limiting nature of the

5 design whereby the further from a defect you are, the internal resistances of the groove vacuum coatings (the ohmic conductors) limit how much current can flow from the unaffected or working areas to the defect region. This is an effective selflimiting or mitigation benefit that comes from the groove geometry. Figure 2: Edwards 610 Bell jar coater with rotating drum shown The Bell jar system allowed for the determination of coating throw distances and for the investigation of process open circuit yields. Early samples produce with the assistance of Sheffield University s Prof. D. Lidzeys group on organic solar cells required grooves to be fabricated with only a few hundred nanometer widths. These devices were fabricated using simple contacts for the P3ht/Pcbm materials that would be coated at Sheffield. Figure 3 shows a cross section of a working groove based device that was among the first independent validations of the groove concept.

6 Figure 3: FIB cross section of a working organic device the contacts were aluminium on one side and tungsten oxide on the other. This was then spin coated with P3HT/PCBM. The cross section shown in figure 3 did produce a working PV device a typical IV curve of a working device with P3HT/PCBM is shown in Figure 4. Figure 4: IV of organic device fabricated at Sheffield University The IV curve shown is not under a Am1.5 simulator it is simply under the low power LED source that was in the glove box, but as the outcome of this study was does the technology have validity this was sufficient to extend the study and seek further resources to develop same. The next step in the development involved working with the Centre for next generation Photovoltaics (NGPV). Here we used their CIS (copper indium diselenide) nanoparticle inks and their lab scale deposition systems to develop test cells. With this increased effort and through multiple design and groove iterations we obtained a number of working devices with continually improving PCE. Figures 5 and 6 show a working device performance and the SEM of an example device. Figure 5: IV curve of working device that showed a nominal efficiency of 3%

7 Figure 6: SEM of the device shown in figure 5 showing the regions for imperfect coating. Structure of device :- Aluminium/Cadmium Sulphide/CIS/Gold Figures 5 and 6 illustrate to some degree the robustness of the groove when cascaded design. The SEM shows many regions of no or imperfect coating but sufficient well coated regions exist to still make a working device with the beginnings of useful efficiency. Improved development capabilities The result from the NGPV enabled a fundamental step change in our development capabilities. In particular our new roll to roll vacuum coater from Emerson & Renwick figure 7 provides the capability to produce samples with higher repeatability, consistency and performance at significantly increase scale. A single 10 metre run in the Coater provides the equivalent number of samples that could be produced in 6-8 months using our old equipment.

8 Figure 7: Emerson and Renwick Vacuum ebeam system showing winding chamber One of the major objectives of the Coater was the ability to deposit materials to replace CdS (Cadmium Sulphide, n-type semiconductor but toxic) and gold (high work function metal but expensive) historically used in the production of our record devices. With the replacement of CdS with Titanium Dioxide, Zinc Oxide, Zinc sulphide and mixes of these we have been able to use the same grooves over a number of materials for the solar absorber. Materials currently being investigated are: CIS, PbS and perovskite. The use of the groove design for Perovskite is quite interesting we are using only more air stable systems and to date we have achieved 3.5% on a system that is capable of making a +13% standard design cell. In figure 8 we show the IV curve of a perovskite filled 4 groove device. This achieved a PCE of 3.5% and a VOC of 0.8, this low VOC is something we have observed befor in CIS devices and we know how we can imporve this as its due in part to overcoating of the grooves that introduces an external shunt resistance or shunt path.

9 Figure 8: Perovskite filled 4 groove device with contact materials of Ti0x and Mo Conclusions Although working devices have been fabricated usinga number of absorber systems the increase in production yeild and uniformity offered with the new roll to roll coater has not yet been fully evaluated. The overall groove concept has been validated and with some help from our current and future development partners we are now investigating a range of other issues including encapsulation, material combinations, other asborbers, lifetime and other factors..