Transparent, flexible solar cells | MIT News

Think about a future wherein photo voltaic cells are throughout us — on home windows and partitions, cell telephones, laptops, and extra. A brand new versatile, clear photo voltaic cell developed at MIT is bringing that future one step nearer.

The system combines low-cost natural (carbon-containing) supplies with electrodes of graphene, a versatile, clear materials comprised of cheap and considerable carbon sources. This advance in photo voltaic expertise was enabled by a novel technique of depositing a one-atom-thick layer of graphene onto the photo voltaic cell — with out damaging close by delicate natural supplies. Till now, builders of clear photo voltaic cells have usually relied on costly, brittle electrodes that are likely to crack when the system is flexed. The power to make use of graphene as an alternative is making potential really versatile, low-cost, clear photo voltaic cells that may flip nearly any floor right into a supply of electrical energy.

Photovoltaic photo voltaic cells fabricated from natural compounds would supply a wide range of benefits over immediately’s inorganic silicon photo voltaic cells. They’d be cheaper and simpler to fabricate. They’d be light-weight and versatile relatively than heavy, inflexible, and fragile, and so can be simpler to move, together with to distant areas with no central energy grid. And so they could possibly be clear. Many natural supplies soak up the ultraviolet and infrared elements of daylight however transmit the seen half that our eyes can detect. Natural photo voltaic cells might due to this fact be mounted on surfaces throughout us and harvest vitality with out our noticing them.

Researchers have made vital advances over the previous decade towards creating clear natural photo voltaic cells. However they’ve encountered one persistent stumbling block: discovering appropriate supplies for the electrodes that carry present out of the cell.

“It’s uncommon to seek out supplies in nature which can be each electrically conductive and optically clear,” says Professor Jing Kong of the Division of Electrical Engineering and Pc Science (EECS).

Probably the most broadly used present choice is indium tin oxide (ITO). ITO is conductive and clear, nevertheless it’s additionally stiff and brittle, so when the natural photo voltaic cell bends, the ITO electrode tends to crack and elevate off. As well as, indium is pricey and comparatively uncommon.

A promising various to ITO is graphene, a type of carbon that happens in one-atom-thick sheets and has exceptional traits. It’s extremely conductive, versatile, strong, and clear; and it’s comprised of cheap and ubiquitous carbon. As well as, a graphene electrode could be simply 1 nanometer thick — a fraction as thick as an ITO electrode and a much better match for the skinny natural photo voltaic cell itself.

Graphene challenges

Two key issues have slowed the wholesale adoption of graphene electrodes. The primary drawback is depositing the graphene electrodes onto the photo voltaic cell. Most photo voltaic cells are constructed on substrates comparable to glass or plastic. The underside graphene electrode is deposited straight on that substrate — a activity that may be achieved by processes involving water, solvents, and warmth. The opposite layers are then added, ending with the highest graphene electrode. However placing that prime electrode onto the floor of the so-called gap transport layer (HTL) is hard.

“The HTL dissolves in water, and the natural supplies slightly below it are delicate to just about something, together with water, solvents, and warmth,” says EECS graduate scholar Yi Track, a 2016-2017 Eni-MIT Energy Fellow and a member of Kong’s Nanomaterials and Electronics Group. Because of this, researchers have usually continued in utilizing an ITO electrode on the highest.

The second drawback with utilizing graphene is that the 2 electrodes have to play totally different roles. The convenience with which a given materials lets go of electrons is a set property known as its work perform. However within the photo voltaic cell, simply one of many electrodes ought to let electrons movement out simply. Because of this, having each electrodes made out of graphene would require altering the work perform of one among them so the electrons would know which approach to go — and altering the work perform of any materials will not be easy.

A easy graphene switch

For the previous three years, Kong and Track have been working to resolve these issues. They first developed and optimized a course of for laying down the underside electrode on their substrate.

In that course of, they develop a sheet of graphene on copper foil. They then switch it onto the substrate utilizing a way demonstrated by Kong and her colleagues in 2008. They deposit a layer of polymer on prime of the graphene sheet to assist it after which use an acidic resolution to etch the copper foil off the again, ending up with a graphene-polymer stack that they switch to water for rinsing. They then merely scoop up the floating graphene-polymer stack with the substrate and take away the polymer layer utilizing warmth or an acetone rinse. The outcome: a graphene electrode resting on the substrate.

However scooping the highest electrode out of water isn’t possible. In order that they as an alternative flip the floating graphene-polymer stack right into a sort of stamp, by urgent a half-millimeter-thick body of silicon rubber onto it. Greedy the body with tweezers, they elevate the stack out, dry it off, and set it down on prime of the HTL. Then, with minimal warming, they will peel off the silicon rubber stamp and the polymer assist layer, leaving the graphene deposited on the HTL.

Initially, the electrodes that Track and Kong fabricated utilizing this course of didn’t carry out nicely. Assessments confirmed that the graphene layer didn’t adhere tightly to the HTL, so present couldn’t movement out effectively. The apparent options to this drawback wouldn’t work. Heating the construction sufficient to make the graphene adhere would harm the delicate organics. And placing some sort of glue on the underside of the graphene earlier than laying it down on the HTL would stick the 2 layers collectively, however would find yourself as an added layer between them, lowering relatively than growing the interfacial contact.

Track determined that including glue to the stamp may be the way in which to go — however not as a layer beneath the graphene.

“We thought, what occurs if we spray this very gentle, sticky polymer on prime of the graphene?” he says. “It could not be in direct contact with the outlet transport layer, however as a result of graphene is so skinny, maybe its adhesive properties may stay intact by the graphene.”

To check the thought, the researchers integrated a layer of ethylene-vinyl acetate, or EVA, into their stamp, proper on prime of the graphene. The EVA layer may be very versatile and skinny — kind of like meals wrap — and might simply rip aside. However they discovered that the polymer layer that comes subsequent holds it collectively, and the association labored simply as Track had hoped: The EVA movie adheres tightly to the HTL, conforming to any microscopic tough options on the floor and forcing the high-quality layer of graphene beneath it to do the identical.

The method not solely improved efficiency but additionally introduced an sudden aspect profit. The researchers thought their subsequent activity can be to discover a approach to change the work perform of the highest graphene electrode so it might differ from that of the underside one, guaranteeing easy electron movement. However that step wasn’t crucial. Their method for laying down the graphene on the HTL truly adjustments the work perform of the electrode to precisely what they want it to be.

“We received fortunate,” says Track. “Our prime and backside electrodes simply occur to have the proper work features on account of the processes we use to make them.”

Placing the electrodes to the take a look at

To see how nicely their graphene electrodes would carry out in follow, the researchers wanted to include them into functioning natural photo voltaic cells. For that activity, they turned to the photo voltaic cell fabrication and testing amenities of their colleague Vladimir Bulović, the Fariborz Maseeh (1990) Professor of Rising Expertise and Affiliate Dean for Innovation for the College of Engineering.

For comparability, they constructed a collection of photo voltaic cells on inflexible glass substrates with electrodes fabricated from graphene, ITO, and aluminum (a normal electrode materials). The present densities (or CDs, the quantity of present flowing per unit space) and energy conversion efficiencies (or PCEs, the fraction of incoming solar energy transformed to electrical energy) for the brand new versatile graphene/graphene gadgets and the usual inflexible ITO/graphene gadgets had been comparable. They had been decrease than these of the gadgets with one aluminum electrode, however that was a discovering they anticipated.

“An aluminum electrode on the underside will mirror among the incoming mild again into the photo voltaic cell, so the system general can soak up extra of the solar’s vitality than a clear system can,” says Kong.

The PCEs for all their graphene/graphene gadgets — on inflexible glass substrates as nicely as versatile substrates — ranged from 2.8 p.c to 4.1 p.c. Whereas these values are nicely under the PCEs of present business photo voltaic panels, they’re a big enchancment over PCEs achieved in prior work involving semitransparent gadgets with all-graphene electrodes, the researchers say.

Measurements of the transparency of their graphene/graphene gadgets yielded additional encouraging outcomes. The human eye can detect mild at wavelengths between about 400 nanometers and 700 nanometers. The all-graphene gadgets confirmed optical transmittance of 61 p.c throughout the entire seen regime and as much as 69 p.c at 550 nanometers. “These values [for transmittance] are among the many highest for clear photo voltaic cells with comparable energy conversion efficiencies within the literature,” says Kong.

Versatile substrates, bending habits

The researchers observe that their natural photo voltaic cell could be deposited on any sort of floor, inflexible or versatile, clear or not. “If you wish to put it on the floor of your automotive, as an example, it received’t look dangerous,” says Kong. “You’ll have the ability to see by to what was initially there.”

To display that versatility, they deposited their graphene-graphene gadgets onto versatile substrates together with plastic, opaque paper, and translucent Kapton tape. Measurements present that the efficiency of the gadgets is roughly equal on the three versatile substrates — and solely barely decrease than these made on glass, seemingly as a result of the surfaces are rougher so there’s a higher potential for poor contact.

The power to deposit the photo voltaic cell on any floor makes it promising to be used on shopper electronics — a area that’s rising quickly worldwide. For instance, photo voltaic cells could possibly be fabricated straight on cell telephones and laptops relatively than made individually after which put in, a change that will considerably cut back manufacturing prices.

They’d even be well-suited for future gadgets comparable to peel-and-stick photo voltaic cells and paper electronics. Since these gadgets would inevitably be bent and folded, the researchers subjected their samples to the identical remedy. Whereas all of their gadgets — together with these with ITO electrodes — could possibly be folded repeatedly, these with graphene electrodes could possibly be bent way more tightly earlier than their output began to say no.

Future targets

The researchers are actually working to enhance the effectivity of their graphene-based natural photo voltaic cells with out sacrificing transparency. (Growing the quantity of energetic space would push up the PCE, however transparency would drop.) In accordance with their calculations, the utmost theoretical PCE achievable at their present degree of transparency is 10 p.c.

“Our greatest PCE is about 4 p.c, so we nonetheless have some approach to go,” says Track.

They’re additionally now contemplating how greatest to scale up their photo voltaic cells into the large-area gadgets wanted to cowl whole home windows and partitions, the place they may effectively generate energy whereas remaining nearly invisible to the human eye.

This analysis was supported by the Italian vitality firm Eni S.p.A. as a part of the Eni-MIT Alliance Photo voltaic Frontiers Heart. Eni is a Founding Member of the MIT Vitality Initiative.

This text appeared within the Spring 2017 concern of Vitality Futures, the journal of the MIT Vitality Initiative.

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