Material characterization of hybrid solar cells

Relating to the configuration of hybrid SCs like HJ SCs or dye-sensitized SCs, various materials have been suggested by research groups. The BHJ devices were characterized by an interpenetrating network of donor and acceptor materials, providing a large interface area where photo-induced excitons could efficiently dissociate into separated electrons and holes. However, the interpenetrating network cannot be easily formed in the blended mixture. In addition, the organic materials are not good in carrier transport. Thus, the power conversion efficiency is still limited by the low dissociation probability of excitons and the inefficient hopping carrier transport (Huang et al., 2009, as cited in Sirringhaus et al.,1999; Shaw et al.,2008). Semiconductor nanostructures are used to be combined with the organic materials to provide not only a large interface area between organic and inorganic components for exciton dissociation but also fast electron transport in semiconductors. Therefore, many research groups combined organic materials with semiconductor nanostructures to overcome the drawbacks of the organic solar cells. Many inorganic nanowire (NW) had been experimented for this purpose, including CdTe, CdS, CdSe, ZnO, and TiO2 NWs (Huang et al., 2009, as cited in Kang & Kim, 2006). Totally, BHJ hybrid SCs itself have been demonstrated in various inorganic materials such as CdSe nanodots, nanorods and tetrapods, TiO2, ZnO, ZnS nanoparticles, CuInS2, CuInSe2, CuPc, CdS, SnS, CIS, PbSe or PbS nanocrystals, HgTe ncs, Si NWs, Si ncs (Chandrasekaran et al., 2010 as cited in Kwong et al.,2004, Arici et al., 2004 Greenham et al., 1996 Qi et al., 2005; Choudhury et al., 2005), etc. which act as acceptors and polymer materials acting as donors are P3HT, PPHT, P3OT, P3BT, P3MeT (Lin et al., 2006), MDMO-PPV, MEH-PPV, MOPPV, etc. (Chandrasekaran et al., 2010). However, CdTe, CdS, and CdSe materials are harmful to the environment, while ZnO and TiO2 have a bandgap higher than 3eV and so cannot effectively absorb the solar spectrum. To overcome this, SiNWs are suitable for this application because they are environmental friendly and have high absorption coefficient in the infrare dregion (Huang et al., 2009).

J. Haung et al. (2009) reported the fabrication of the SiNW/P3HT:PCBM blend hybrid SCs using the SiNW transfer technique.

Подпись:image440SiNW P3HT:PCBM PEDOT:PSS ITO

Their investigation showed that after introducing the SiNWs, the Jsc increases from 7.17 to 11.61 mA/ cm2 and r| increases from 1.21% to 1.91% (Haung et al., 2009). This increase is due to this fact that the NWs act as a direct path for transport of charge without the presence of grain boundaries.( Movla et al., 2010b).

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Fig. 5. The current density-voltage characteristics for the SCs with and without the SiNWs under simulated AM1.5 illumination. Reprinted with permission from Solar Energy Materials & Solar Cells Vol.93, Huang, J. et al. Well-aligned single-crystalline silicon nanowire hybrid solar cells on glass, pp. 621-624 © 2009, Elsevier.

More precisely, the results clearly indicate that combination of the SiNWs and P3HT:PCBM blend is an attractive route to obtain high Jsc and efficiencies by improving the optical absorption, dissociation of excitons, and the electron transport. Silicon wafer is commercially available and cheap. SiNWs can be fabricated at low temperature from solution processing without any vacuum equipment or high-temperature processing. In addition, this transfer method for SiNWs is simple and fast. It is not a laborious way. This method is suitable for plastic SCs because it can be processed fast, is cheap and simple ( Haung et al., 2009).

Similar work was done by G. Kalita et al. (2009) for demonstrating hybrid SCs using Si NWs and polymer incorporating MWNTS. This fabricated device with the structure of Au/P3OT+O-MWNTS/n-Si NWs marked a conversion efficiency of 0.61% (Bredol et al., 2009). Another study was done by C. Y. Liu et al. (2009) about fabricating the hybrid SCs on blends of Si ncs and P3HT (Liu et al., 2009). Also, V. Svrcek et al. (2009), investigated the photoelectric property of BHJ SC based on Si-ncs and P3HT. They came into conclusion that I-V characteristic enhanced when BHJ was introduced into TiO2 nanotube (nt). The arrangement of Si-ncs/P3HT BHJ within ordered TiO2 nt perpendicular to the contact facilitated excition separation and charge transfer along nts (Chandrasekaran et al., 2010, as cited in Svrcek et al., 2009).

A new approach for hybrid metal-insulator-semiconductor (MIS) Si solar cells is adopted by the Institute of Fundamental Problems for High Technology, Ukrainian Academy of Sciences. In this technique, the porous silicon layers are created on both sides of single crystal wafers by chemical etching before an improved MIS cell preparation process. The porous Si exhibits unique properties. It works like a sunlight concentrator, light scattering diffuser and reemitter of sunlight as well as an electrical isolator in the multilayer Si structure. The most important advantage of using porous Si in SCs is its band gap which behaves as a direct band gap semiconductor with large quantum efficiency and may be adjusted for optimum sunlight absorption. Employing a specific surface modification, porous Si improves the PV efficiency in UV and NIR regions of solar spectra (Tuzun et al., 2006, as cited in Tiris et al., 2003). In this approach, due to high quality starting materials and rapid low-temperature (<800 0C) processing a high minority carrier life time is attainable; this, in turn, gives rise to a high photogenerated current collection. Therefore, the SCs with efficiencies above 15% have been obtained under AM1.5 condition (under 100 mW/cm2 illumination at 25 0C) (Tuzun et al., 2006).

Another study was reported by J. Ackermann et al. (2002) about the growth of quaterthiophene (4T), a linear conjugated oligomer of thiophene behaving as a p-type semiconductor, on n-doped GaAs and Si substrates to form hybrid HJ SCs. This study shows that in the case of Si as substrate there is almost defect-free high ordered films with grain sizes of several micrometers up to a film thickness of 250 mm (Ackermann et al., 2002).

K. Yamamoto et al. (2001) investigated a-Si/poly-Si hybrid (stacked) SC paying attention to the stabilized efficiency, since a-Si has photo degradation, while a poly-Si is stable. This tandem cell exhibited a stabilized efficiency of 11.3%. Also, this research group prepared three-stacked cell of a-Si:H/poly-Si/poly-Si(triple), which will be less sensitive to degradation by using the thinner a-Si. This triple cell showed a stabilized efficiency of 12%( Yamamoto et al., 2001) . Their reasons for applying poly-Si are (i) high growth rate (ii) large area and uniform deposition at the same time and (iii) monolithic series interconnection. In addition, for enhancing the absorption, they suggested natural surface texture and a back reflector (See Fig.6.) (Yamamoto et al., 2001).

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Fig. 6. Schematic view of thin film poly-Si solar cell with natural surface texture and enhanced absorption with back reflector structure. Reprinted with permission from Solar Energy Materials & Solar Cells Vol.93, Huang, J. et al. Well-aligned single-crystalline silicon nanowire hybrid solar cells on glass, pp. 621-624 © 2009, Elsevier.

Apart from BHJ SCs, solid state dye-sensitized SCs can experience various materials. Cul, CuBr, CuSCN, MgO (Tennakone et al.,2001) can be replacements for liquid crystals as inorganic p-type semiconductors. Also, 2,2′,7,7′-tetranis(N, N-di-p-methoxyphenyl – amine)9,9′-spirobifluorene (OMETAD) can replace the liquid crystals as organic p-type semiconductor due to their low cost processability (Bach et al.,1995). Poly (3 alkylthiophenes) were used to replace the liquid electrolyte by Sicot et al. (Sicot et al., 1991) and Gebeyehu et al. (Gebeyenha et al.,2002a,2002b) as conjugated polymers although high molecular weight polymers cast from solution, do not penetrate into the pores of the nanoparticles. A polymeric gel electrolyte is considered as a compromise between liquid electrolytes and hole conductors in quasi solid state dye-sensitized SCs (Gunes & Sariciftci,2008, as cited in Nogueira et al.,2004;Murphy,1998; Megahed & Scosati,1995) . A mixture of NaI, ethylene carbonate, propylene carbonate and polyacrylonitrile was reported by Cao et al. [55]. Poly (vinylidenefluoride – co-hexafluoropropylene) (PVDF-HFP) used to solidify 3-methoxypropionitrile (MPN) was utilized by Wang et al. (Wang et al.,2004 ). The last efficiency for dye-sensitized reported by Sharp Corporation is about 10.4 which stands in lower rank in comparison with crystalline Si showing efficiency of approximately 25% ( Green et al., 2011).