New Hopes for Organic Photovoltaics
Outside of the mainstream silicon and thin-film based solar PV offerings, a group of alternative solar PV technologies have long captivated researchers' interest. Among them is OPV which, although it trails more mainstream technologies in raw power conversion efficiency offers some other intriguing capabilities that envision success in some significant market opportunities: from building-integrated systems such as facades, to power-charging devices in greatly varying styles from finger-size strips up to awnings.
OPV cells have continued to draw focus of much research because of the allure of their core attributes: they are lightweight, flexible, inexpensive, highly tunable, and potentially disposable. The main advantage of organic materials is the claimed ability to produce photovoltaic devices using techniques that can enable low-cost, high-throughput manufacturing such as roll-to-roll (R2R). Besides processing simplicity, organic semiconductor materials have a very high absorption coefficient that allows the use of thin films while still absorbing a sufficient portion of the solar spectrum.
What OPV however has signally failed to do is make any money for investors and the big question hanging over OPV is whether it ever will!
Since our last report in early 2015 the assets of Konarka, then the industry's leading company, were essentially absorbed by Belectric, with Konarka shareholders getting nothing out of the deal. Several other OPV suppliers have gone quiet (such as Solarmer), though also seem to be still tending to their R&D (NanoFlex née Global Photonic Energy) and the latest company to stop OPV projects is BASF. All in all, OPV's journey to commercialization has been longer than hoped and rocky for some.
What seems to have kept hope alive is that the underpinning OPV technologies and materials have seen some significant improvements in the past 12-24 months, from conversion efficiency to lifetimes. They do seem quite closer to achieving levels perceived as necessary for commercialization—and perhaps even knocking on the doorstep, if one believes the most optimistic views.
OPV Struggling But Good Long-Term Potential
Those various capabilities of OPV—reduction in material used, flexibility, low-cost manufacturing techniques—have led to the implication that organic semiconductors have the potential to make a significant impact in certain PV markets. But it is hard to deny that OPV continues to have the lingering whiff of the science project about it:
First of all, it has proven difficult to devise optimal material sets and chemical synthesis processes that can produce efficiencies much above 10 percent in commercial cells. The best OPV cells produced in a lab top out between 11 percent-13.2 percent, led by Germany's Heliatek. But that company's highest OPV efficiency on a pilot line is around 6-7 percent. No OPV cells with double-digit efficiency are in commercialization.
Moreover, the roll-to-roll production technique as envisaged by Heliatek is still taking shape. The company has reported an OPV film with 40 percent transparency and a 7.8 percent conversion rate (March 2014), and in March 2016 it is still the same. Modules from Heliatek’s roll-to-roll production show efficiencies of up to 6.8 percent on the active area of 1033 sq. centimeters with a fill factor of 65.4 percent. (October 2014). It also claims to have made tandem modules in an R2R line with 5 percent efficiency on the active module area. Yet again these are not yet achieved in a production environment.
OPV, DSC, CIGS: Where They Stand
Further muddying the waters for OPV is its comparison to other alternative solar PV technologies that are also relegated to niche status at the moment, but offer many of the same benefits as OPV:
Dye-sensitized solar cells (DSC): Historically, DSC and OPV have been mentioned in tandem since they both utilize organic materials; moreover they have generally offered similar benefits (flexibility, potential low-cost processes and production) compared with performance results (lower efficiencies and lifetimes, small-scale activity) that would relegate them to the same non-mainstream end markets outside the Si and thin-film PV worlds.
Yet, n-tech Research sees DSC as having pulled ahead of OPV towards commercialization in many ways: conversion efficiency (up to 15 percent for cells in labs), renewed investment activities (for example, helping G24 Power and Exeger), and pilot projects (several going back to 2012, in the U.S. and Europe).
Add to this the recent clamor over perovskites, which we might think of as "next-generation DSC," promising many of the same benefits as “traditional” DSC and OPV, with achieved efficiency of 21.02 percent in 2016.
A great deal of research is being directed at perovskites, and DSC leader Dyesol has fully shifted to commit to this technology. Yet the question is: will markets be willing to dial back the commercial timelines by another decade to get perovskites ready for market, when OPV and others are right now approaching that doorstep?
Thin-film CIGS: Also in the mix is thin-film copper-indium-gallium-(di)selenide (CIGS), which has been associated with BIPV since early on. While CIGS shares some challenges with OPV and DSC in field performance (lifetimes and encapsulation), the trajectory for CIGS conversion efficiency is vastly higher than those other two, already approaching the low-20 percent range enjoyed by silicon-based PV.
Key Technical Challenges and Probable Solutions for OPV
From a technology perspective, OPV has a number of high-priority areas to focus on, and possible pathways to solve them:
Improve module lifetime, with creation of new low-cost barrier materials. Organic semiconductors are susceptible to moisture and oxygen, and for long-term stability OPV modules need a robust encapsulation system. This is a tractable engineering challenge that has been solved in other organic optoelectronic arenas; OLEDs being the obvious example. To be competitive in commercial markets, OPV module manufacturers need to devise better encapsulation technologies that ensure at least 10-year module lifetimes—and ideally at least 20 years or more in some contexts such as BIPV.
Raise conversion efficiency levels, either through multijunction structures (which are more complex and costly) or through better single-junction designs. Multiple junctions extend the absorption range of a PV cell for more efficient light harvesting, although these are more complex and expensive to manufacture. An alternative strategy would be to extend the range of single junctions using better concepts, such as complementary absorbing acceptor-donor pairs.
Replace costly components such as the current transparent conducting electrode materials (i.e. ITO) with lower-cost alternatives. ITO has been the dominant transparent conductor used in OPV, largely because it has been readily available with acceptable performance, allowing OPV companies to focus on improvements in other parts of the technology. On the other hand, ITO requires post-processing and/or intermediate layers, and flexibility is an issue, both of which add complexity and extra costs.
Ultimately, we see OPVs turning to polymers (notably PEDOT:PSS), silver nanowires (as seen already in a pairing between Armor and Cambrios although Cambrios has since gone out of business and rebooted) and metal grids.
Reduce cost and manufacturing complexity by developing large-area monolithic sub-module architectures. Reducing the transparent conducting electrode (TCE) sheet resistance can lead to larger monolithic active areas, thus simplifying manufacturing complexity compared to the current serially connected thin-strip architectures. At the same time, this can allow for greater current-voltage module flexibility, reduce the amount of interconnection metal, and improve yields by eliminating processes such as laser scribing.
Reduce defect density and improve manufacturing yield by developing “thick junction” cells. In order to be solution processable, OPVs that are thin-film structures (with junction less than 200 nm) face the challenge of increased defect density (because of larger area) and loss of fill factor (because of shorting). This is also true to a somewhat lesser extent for vacuum-evaporated systems.
New junction materials with better electrical properties—higher mobilities and lower bimolecular recombination coefficients—can lead to the formation of thicker junctions while maintaining fill factor. Such a development will enable manufacturers to adopt larger monolithically active areas with lower defect densities. Further, the formation of thicker junctions (400–500 nm) should be enough to leverage the low-cost benefits of solution processing techniques.
We also note that AC integration of OPV technology will require new inverter and power electronics—not only to connect to the conventional grid, but also to fully utilize some unique benefits of OPVs such as low-light performance and better efficiency yields in hotter environments.
Markets for OPV: Decisions, Decisions
Another thing that hasn't really changed since n-tech Research’s early 2015 OPV report is the dual nature of OPV's perceived end-market strategies: off-grid charging applications and grid-connected systems, mainly building-integrated (BIPV) and building-applied (BAPV).
Purely power-generation sectors such as utility power plants or even commercial/rooftop arrays are not viewed as viable markets anytime soon if at all, as they are rather impregnably held by c-Si solar PV technologies (and some thin-film) offering significantly better LCOE. What also seems unchanged is that OPV companies face a choice in their market strategies:
Niche products. Energy harvesting cells, portable solar chargers (including new sectors such as wearable electronics), and visible and near-IR photodetection are areas where very low (single-digit) power conversion efficiencies are acceptable, when balanced with other capabilities such as thinness/flexibility and low cost that OPV can promise.
Importantly, a few years of lifetime is often sufficient in many of these applications, to last as long as whatever they're attached to, be it a pack or table awning.
BIPV. Efficiency requirements may not be as lax as in niche areas listed above, but there is still a trade-off between conversion efficiencies and other desirable factors such as low weight, flexibility, able to be integrated into a building's design instead of bolted via racking on the roof, e.g. curtain walls, semi-transparent windows.
What does have a higher priority is longer lifetimes; currently pushing 10 years, but in recognition that 20+ years is most desirable to really open up these markets. There is also the consideration to adhere to building codes, which vary from region to region.
Automotive products. Another end-market that has a conceivable play for OPV is automotive, leveraging a highly transparent flavor of OPV to pair with vision glass requirements. n-tech is somewhat bearish on this market for now; not coincidentally this sector also has sniffed around organic lighting (OLED), an evolving pairing which can help us think about where OPV might similarly (or might not) find success in this sector. However we note that thin-film CIGS, a competitor to OPV in most every application that OPV needs to go, appears to be making inroads into automotive as well, given recent comments by Hanergy.
There are different and strong opinions among OPV companies about which is the right end-market strategy to pursue. Most suppliers are adamant that BIPV is where the necessary volumes are to generate real revenues and profits for OPV, and higher field performance will happen soon enough to reward those with a little more patience. On the other hand, some others think the efficiencies and lifetimes clearly are too low today to really gain traction in BIPV, so they'll go after solar charging applications instead where applications—if not profit margins—seem broadest.
It seems to n-tech that the volumes anticipated from BIPV, once performance improvements can be realized (and suppliers we talked with are quite confident this will happen), is the most likely way to get costs down to attract wide enough business, so this seems to be the broader attractive play.
In any case, it seems to us that we're still maybe five to ten years out from OPV truly establishing a strong foothold in BIPV as its main addressable market. That includes the more optimistic plans of Heliatek, which is arguably at the head of the class getting OPV to commercial scale—and we can't envision OPV being a viable sector with a single supplier at scale.Back