**4. Transparent flexible solar cells**

A flexible transparent solar cell was first developed at MIT as shown in **Figure 4**. Researchers have developed a novel technique using graphene to prepare solar cells on surfaces of glass, plastic, paper, and tape. This device contains carbon-based organic materials with graphene electrodes and transparent material. This device involves a layer of graphene on the solar cell. The flexible transparent solar cells would be lightweight and cheap technology as well as low-cost materials [23]. Organic materials absorb the UV and IR components of the solar spectrum but transmit visible light. The most affordable option is to use indium tin oxide (ITO) coated flexible substrate. But ITO is brittle and may break during deposition on flexible substrate. A one-atom thick layer of graphene can be developed as alternative to ITO in transparent solar cells. This alternative material is flexible, highly conductive, and transparent. The most important thing is that it is made up of inexpensive carbon material [24].

There are some problems during the deposition of graphene electrodes on solar cells. The first problem is depositing 1 atom layer thick of graphene. A bottom layer of graphene is deposited directly on the substrate and then the top layer. The fabrication of graphene on the top layer in solar cells as a hole transport layer is very tricky. The other main problem is preparing graphene electrodes for different applications. One electrode should let electrons flow out easily in solar cells. So, here both graphene electrodes have different work functions. Therefore, changing the work function is not an easy task. A layer of ethylene-vinyl acetate (EVA) can be incorporated into the graphene layer to make the device more flexible. This would differ both graphene electrodes to work for different purpose [25].

In practical, a transparent solar cell was developed using graphene, ITO, and aluminum materials. The performance of this solar cell was lower than solar cells with one aluminum electrode. Aluminum electrode on the bottom reflects some incoming light back into solar cell and increases the absorbance of sunlight more than a

**Figure 4.** *Transparent solar cell on flexible substrate developed at MIT.*

### *Transparent Solar PV Panels DOI: http://dx.doi.org/10.5772/intechopen.110062*

transparent cell. The PCE for graphene/graphene devices was comparable to existing commercial solar panels, about 4.1% with transparency of about 61%.

The organic solar cell has the advantage to deposit on any type of surface, such as rigid or flexible, transparent, etc. The graphene/graphene devices have been demonstrated on various flexible substrates such as plastic, opaque paper, translucent kapton tape, etc. The performance was nearby in all these devices [26].
