7. Conclusion

6. Applications for energy harvesting devices

92 Graphene Materials - Structure, Properties and Modifications

Figure 16. Band gap modification by surface modification of reduced graphene oxide [12].

and rGO; and Fermi level of 4.5 eV for graphene [23].

tion, especially for its use in solar cells.

Energy harvesting is the process of capturing minute amounts of energy from one or more of these naturally occurring energy sources, accumulating them and storing them for further use. Graphene has the potential to address several scientific challenges, ranging from the need for more efficient alternative energy technologies. The unique properties of graphene previously mentioned along the present chapter shows the characteristics for energy-harvesting applica-

Since graphene monolayers were analyzed and obtained by micromechanical exfoliation in 2004 [22], graphene, as well as graphene oxide and reduced graphene oxide, has been added to the list of materials with potential application in OPVs in their different components, either in substitution of a previously used material or forming synergies that lead to an improvement in the devices. This is because of the three types of materials that have very different band gaps, from zero to 3 eV; HOMO values from 6.6 to 4.8 eV; LUMO values of 3.6 to 4 eV for GO

Based on the previously described characteristics of the graphene, it is expected that the first applications designed for it are in [1] substitution of the indium-tin oxide (ITO) electrodes, since this material is fragile, expensive, and with a transmittance of 90% [1]. It has been The vast properties of graphene open endless possibilities for very different purposes. However, some applications require characteristics achieved through graphene derivatives such as graphene oxide, reduced graphene oxide, and reduced graphene oxide functionalized. Tailoring of the properties and surface chemistry of graphene lays on the control of its performance in a polymer matrix and a device.

Therefore, the capability of controlling these properties is of great importance. Moreover, a functionalization that makes reduced graphene oxide soluble in organic solvents must be successfully achieved, due to the varieties of applications and specific needs of the different energy harvesting devices' demands.

The chemical reduction of graphene oxide has been carried out by innocuous agents such as ascorbic acid, fructose, and glucose, controlling the band gap value of the resulting materials.

A process to decrease the band gap, by means, reducing the oxygen content in graphene oxide, i.e. rGO, was established. GO was synthesized through a modified Hummers method, and it has been found that NH4OH, glucose, fructose, and ascorbic acid at pH 10 can be used to modify the band gap. The XPS C1s and UV-vis spectra indicate the gradual elimination of oxygen groups and restitution of graphitic structure, ensuing to a decreased value of band gap.

Ascorbic acid at pH 10 is more effective and fast for sp<sup>2</sup> restitution, resulting in an optical band gap of 1.55 eV. The chemical reduction by fructose and glucose at pH 10 is slower which could be preferred in order to have a more precise control of band gap, but the total sp<sup>2</sup> restoration is lower than that achieved by the ascorbic acid, nevertheless, it has the lowest band gap 1.15 eV with fructose at pH 10.

For application in organic solar cells, low band gaps are important because the PCE depends strongly on the effective generation, diffusion, and dissociation of the exciton. Because most of the solar energy is in the visible spectrum, a band gap in these values means more absorption of light and hence more exciton generation. Another aspect to consider is that restitution of sp<sup>2</sup> domains entails a better exciton diffusion, because of the high carrier mobility. Finally, considering the band gap as the energy difference between the HOMO and LUMO, a low value is advantageous because the difference of the LUMOs, both in the acceptor and donor molecules or polymers, must be adequate for the exciton bonding dissociation and therefore increase the PCE.

### Acknowledgements

The authors are beholden for the technical support and facilities at CIMAV Monterrey, as well as to the Mexican National Research Council CONACyT for the scholarship of the students involved in this work. Lilia Magdalena Bautista Carrillo and Luis Gerardo Silva Vidaurri for UV–Vis spectra and XPS spectra, respectively, are also acknowledged.
