STATE OF THE ART

Organic solar cells have gained within the last years a growing interest within the research community. Among all organic solar cells, small molecule and polymer based technology are very promising. In particular, polymer based technology has shown wide margins of improvement in terms of energy conversion efficiency [1,2] , reaching values above 10%, actually doubling the performance in the last two years.

Efficiency of organic solar cells

The typical structure of a BHJ-SC comprises a transparent conductive oxide (TCO) of Indium Tin Oxide (ITO) as anodic electrode, a conductive polymer film of PEDOT:PSS as hole transporting layer (HTL), an active layer that is the blend of p and n material (one of the most used is the polymer-fullerene blend P3HT:PCBM), and, finally, a layer of aluminum as metal cathode. This structure has demonstrated good performances with efficiencies up to 5% [3,4] in the case of P3HT:PCBM, with the introduction of new low band-gap polymers and the use of a polymer electrolyte (PFN) as ETL, a record efficiency of 9.2% [5]. For the tandem solar cells, Yang’s group from the University of California at Los Angeles (UCLA) has reported a high-efficiency polymer tandem solar cell, first matching the absorption spectra of the two polymers (8.62% [6]) and then breaking the limit of 10%, with the incorporation of a new infrared-absorbing polymer material provided by Sumitomo Chemical (Japan) into the device (certified by NREL, 10.6%).
Replacing the transparent ITO electrode in organic PV cells by graphene should lead to a number of advantages: the cost and scarcity of ITO due to reduced availability of indium; the potential for work function engineering (the chemical potential could be adjusted by +- 400meV using chemical doping) and finally the potential for fabrication of flexible photovoltaics (in contrast to ITO, graphene is extremely flexible). These advantages would lead to a cheaper production process, while the efficiency could be increased significantly. Currently the major bottleneck for graphene-based transparent electrodes is its relatively poor electrical conductivity: Pristine monolayer graphene has a sheet resistivity of ~5 kΩ/sq, which is more than two orders of magnitude larger than in ITO. However, it has been shown that using high quality graphene grown by chemical vapour deposition (CVD) techniques, chemical adsorbate doping and transfer of four adsorbate-doped layers onto a substrate (4-layer stack), the sheet resistance of the resulting stack can be reduced significantly to 30 Ω/sq at a transparency of 90% [7].
Concerning the use of graphene as electrode in bulk heterojunction polymeric solar cells some devices have been already realized both in a single layer configuration with P3HT:PCBM blend and in a tandem configuration reaching a maximum efficiency of about 3% [8-14].
The devices proposed within the GO-NEXTS project will take into account the results shown in literature to develop graphene-based organic photovoltaic devices with cell efficiency ≥ 14% using doped textured graphene electrodes that act as photonic crystal(s) in order to increase the overall efficiency and performance of bulk heterojunction solar cells.

 

[1] Green, M. A., et al., Prog Photovoltaics (2012) 20(1), 12.
[2] Service, R. F., Science (2011) 332(6027), 293.
[3] M. D. Irwin et al., PNAS 105, 2783 (2008).
[4] M. Reyes-Reyes et al., Org. Lett. 7, 5749 (2005).
[5] Zhicai He, nature photonics, Vol 6, pp 591-595, (2012).
[6] Dou L., et al., Nature Photonics (2012) 6, 180–185.
[7] S.Bae et al Nature Nanotechnology 5, 574–578 (2010).
[8] Xuan Wang et al., Angew. Chem. Int. Ed., 47, 2990 –2992(2008).
[9] Yong Yong Choi, Solar Energy Materials & Solar Cells 96, 281–285 (2012).
[10] Sangchul Lee et al, Nanotechnology 23 344013 (6pp) (2012).
[11] Yu Wang et al., Adv. Mater., 23, 1514–1518(2011).
[12] Yu-Ying Lee et al, ACS Nano, 5, (8), pp 6564–6570 (2011).
[13] S.W. Tong et al, 21, 4430–4435, (2011).
[14] X. Miao et al. Nano Lett. 12, 2745−2750 (2012).

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