**Acknowledgements**

are possible to construct. **Figure 13** shows an example of the p-i-n junction perovskite solar cell, which consists of an n-type compact metal oxide thin layer, intrinsic perovskite layer and p-type HTM layer. This structure has been demonstrated by Liu et al. [79] using n-type TiO2 compact layer, perovskite CH3NH3PbI3-xClx and p-type *spiro*-MeOTAD. They used vapour deposition technique to deposit the perovskite layer and reported an efficiency of 15%. Murugadoss et al. [80] have reported an efficiency of 8.38% for the CH3NH3PbI3 perovskite solar cell using SnO2 as the compact layer and the CuSCN as hole conductor. The first hole conductor free perovskite solar cell with an efficiency of 5.5% was reported by Etgar et al. [77]. The cell configuration was FTO/compact TiO2/TiO2 nanosheet/Perovskite/Au. A year later, the efficiency increased to 8% as reported by the same group after the TiO2 nanosheet has been

Perovskite cells have shown a high efficiency of 21%. The perovskite material is very absorptive and moisture sensitive. The main problems are stability and lifetime. Perovskite solar cells are even less stable than organic polymer photovoltaics. Lead is also poisonous and has to be substituted by some other friendlier materials, like Sn. These are among the main challenges faced by researchers. The absorption of tin halide perovskite has been reported up to 1000 nm [82]. By partially substituting lead with tin (CH3NH3Sn*x*Pb1−*x*I3), the band gap can be reduced by increasing the Sn concentration. Hao et. al [83] has reported an efficiency of 7.37% for CH3NH3Sn0.25Pb0.75I3 and 5.44% for CH3NH3SnI3 perovskite solar cell. Germanium (Ge2+)

> , Br− , I−

symmetry is another candidate for perovskite photovoltaics. However, the maximum efficiency of 3.2% is still far below the performance of CH3NH3PbI3 perovskite. Orthorhombic (C4H9NH3)2GeI4 is another variation of Ge-perovskite. This material shows a photolumines-

The third-generation-sensitized solar cells have proved that they have the potential to compete with the conventional silicon based photovoltaics. The use of cheap materials with high performance make third-generation-sensitized solar cells a bright candidate as a future photovoltaic technology compared to other third-generation solar cells. The sensitized photovoltaic started with the emergence of DSSC using mesoporous nanocrystalline TiO2 sensitized with the ruthenium based dye molecule. Since then, the molecular engineering of the dye molecules are extensively studied to improve the DSSC performance. The sensitizer used in the photovoltaic device evolved from organic (dye) to inorganic (quantum dot) and hybrid organic-inorganic (perovskite) sensitizer. The tuneable energy band gap of quantum dots enables them to produce multiple electron-hole pairs per photon. The progress in the performance of perovskite solar cells is very promising. In the beginning, the efficiency of the perovskite solar cell was less than 4%. The efficient reached around 20% within less than 10 years. However, the stability and toxicity issues of lead have to be solved before they can be

) with a rhombohedral structure and *R*3*m*

replaced with thinner TiO2 film [81].

24 Nanostructured Solar Cells

**4.3. Lead free perovskite solar cell**

perovskites of the form, CsGeX3 (X = Cl−

**5. Summary**

cence signal in the red. Stability is still an issue of concern.

Authors thank University of Malaya, Malaysian Ministry of Higher Education (MOHE) and Malaysian Ministry of Science, Technology and Innovation (MOSTI) for the UMRG grant no. RP003-13AFR, PRGS grant no. PR001-2014A and Science fund project no. 03-01-03-SF0995.
