**4. Perovskite-sensitized solar cell**

Perovskite is a term for materials that have a similar crystal structure to calcium titanium oxide (CaTiO3), that is, ABX3 where A and B are cations and X is an anion. A is typically a large cation, such as ethylammonium (CH3CH2NH3 + ) [70], formamidinium (NH2CH═NH2 + ) [71] and methylammonium (CH3NH3 + ) [72]. B is a cation metal of carbon family, such as Ge2+, Sn2+ and Pb2+ and anion X is a halogen (F, Cl, Br and I).

Perovskite cells are typically fabricated with two structures which are mesoporous and planar structures.

#### **4.1. Mesoporous structure**

done via the redox mediators. The redox species in the electrolyte are also responsible for turning the oxidized QD species by donating an electron to the QDs. In QDSSCs, polysulphide

performance and stability [58–60]. Performance of QDSSCs can also be improved by utilization of chemical additives in the polysulphide electrolyte. Park et al. [61] reported that by introducing sodium hydroxide (NaOH) into the polysulphide electrolyte of QDSSCs, *V*oc and *FF*

Due to problems that arise from utilization of liquid electrolytes such as leakage and easy vaporization, researchers have begun to use polymer electrolytes. However, the performance of QDSSCs based on the solid polymer electrolyte [62, 63] is low compared to QDSSCs fabricated with liquid electrolytes. This is because solid state electrolytes suffer from low ionic conductivity. Another alternative to the liquid electrolyte is to use gel polymer electrolytes (GPEs). GPE is very competitive since GPE based QDSSC performance is comparable with QDSSCs fabricated with the liquid electrolyte [64–66]. Kim et al. [65] successfully fabricated CdSe/CdS GPE based QDSSCs with 5.45% efficiency, which is comparable with QDSSCs based on the liquid electrolyte. As the GPE based QDSSCs is comparable with QDSSCs fabricated with the liquid electrolyte, utilization of GPE in QDSSCs will be an advantage in terms of

The counter electrode is another important component in QDSSCs. Electrons from the photoanode are returned to the QD when the electrons react with the redox ions in the electrolyte. In DSSCs, platinum (Pt) is the best material to be used as the CE due to its high stability and high catalytic activity for the triiodide ion to be reduced into the iodide ion.

Hence, researchers look for alternative materials to be used as the CE such as noble metals, carbon based materials and metal chalcogenides [68]. The highest efficiency are presently

Basically, QDSSCs working mechanism is identical with DSSCs. TiO2 is used in the photoanode. Upon light incident, the QD sensitizers absorb photons to excite electrons into its CB (photoexcitation). Electrons in the CB of QDs will be injected to the CB of TiO2 and oxidized QDs will be regenerated by receiving electron from 2 <sup>−</sup> ions in the electrolyte [69]. From CB of TiO2, electrons will leave the photoanode, enter the external circuit and reach the counter

exhibited by QDSSCs utilizing copper sulphide (Cu2S) as the CE (*η* = 9%) [39].

providing stability and overcoming problems that arise from liquid electrolytes.

However, Pt CE does not work for QDSSCs. This is because Pt [67]:

**2.** restrains the charge transfer to polysulphide ions and

2 − are widely utilized by researchers since they can give good

electrolytes with <sup>2</sup> −/

can be increased.

20 Nanostructured Solar Cells

*3.1.3. Counter electrode*

**1.** is not catalytic to the sulphide ion,

**3.** can react with sulphur.

**3.2. Working principle of QDSSC**

The mesoporous structure consists of a transparent conducting oxide (TCO) substrate coated with an oxide semiconductor compact layer, mesoporous metal oxide (e.g. TiO2, Al2O3), perovskite sensitizer, hole conductor and gold conductor.

Kojima et al. [73] reported the first perovskite material (CH3NH3PbBr3 and CH3NH3PbI3) used as a sensitizer in photoelectrochemical cells. The cell consists of mesoporous TiO2 film having 8–12 µm thickness, iodide/triiodide redox couple liquid electrolyte and platinum counter electrode. The band gap CH3NH3PbBr3 is 1.78 eV and that of CH3NH3PbI3 is 1.55 eV. They have reported that the solar cells using CH3NH3PbBr3 and CH3NH3PbI3 sensitizers exhibit the efficiencies of 3.13 and 3.81%, respectively. TiO2 sensitized with orthorhombic (CH3CH2NH3)PbI3 has been reported by Im et al. [70] to have an optical band gap of 2.2 eV. The cell using the (CH3CH2NH3)PbI3 sensitizer and the electrolyte with the iodide/triiodide redox mediator exhibits an efficiency of 2.4%. Based on the work done by Kojima et al. [73], Im et al. [74] have investigated the effect of TiO2 film thickness on perovskite photovoltaic performance. The cell with 8.6 µm thick TiO2 film exhibits an efficiency of 3.37% comparable with that of Kojima et al. [73]. The performance of the cell increases when the TiO2 film thickness decreases. The cell with 3.6 µm thick TiO2 film exhibits an efficiency of 6.2%. Unfortunately, the cell exhibited poor stability due to perovskite decomposition and degraded within minutes. In 2012, the stability of CH3NH3PbI3-sensitized solar cell over 500 h has been reported by Kim et al. [72]. They have substituted the liquid electrolyte that was previously tried by Kojima et al. [73] with a solid state hole transport layer (*spiro*-MeOTAD). Their results also support the work done by Im et al. [74] where the efficiency of the cell increased with the decrease of TiO2 thickness and the highest efficiency of 9.7% observed for the cell having TiO2 thickness of 0.6 µm. Based on the impedance spectroscopy results, they found that the dark current and electron transport resistance increased with the increase in TiO2 film thickness. Koh et al. [71] have synthesized a novel (NH2CH═NH2)PbI3 perovskite with an energy band gap of 1.47 eV. Although the band gap of (NH2CH═NH2)PbI3 is smaller compared to that of CH3NH3PbI3, the efficiency of the cell is only 4.3%. The low efficiency is attributed to the energy level mismatch between TiO2 and the perovskite. The working mechanism of the above perovskite photovoltaics is expected to be similar to DSSC ( **Figure 12a**) where the perovskite absorbs light, injects electrons to the CB of TiO2 and holes to the solid state hole transport material (HTM).

**Figure 12.** Mesoporous structure of perovskite solar cell. (a) Perovskite dot: the structure is similar to DSSC. (b) Meso superstructure: the CB of the oxide semiconductor used is higher than the perovskite material and its surface is coated completely. (c) Inert scaffold: the perovskite fills the pores and makes a thin layer on the top of TiO2.

Lee et al. [75] have constructed a meso superstructure ( **Figure 12b**) of an organometal halide perovskite solar cell. This structure can be obtained by controlling the perovskite precursor concentration. The cell consists of mesoporous n-type TiO2, CH3NH3PbI3Cl and p-type *spiro*-OMeTAD hole conductor. The cell exhibited an efficiency of 7.6%. The efficiency was increased up to 10.9% with the substitution of TiO2 with Al2O3. For the TiO2 based perovskite solar cell, electrons in the CH3NH3PbI3Cl sensitizer is expected to be injected to the CB of TiO2 and transported to the FTO electrode whereas holes will be transferred to the *spiro*-OMeTAD layer. In the case of Al2O3-based perovskite solar cell, electrons will be transferred through the perovskite because Al2O3 has a wider band gap (7–9 eV) and the CB of Al2O3 is higher than CH3NH3PbI3Cl. This shows that the perovskite layer functions as an absorber and n-type component. The authors also reported that the electron diffusion through perovskite is faster than in TiO2 and thus leads to a higher efficiency. The Mesoporous scaffold structure where the perovskite filled up the pores and formed a dense layer on top of mesoporous TiO2 ( **Figure 12c**) has been reported by Heo et al. [76]. For this structure, they have shown that the CH3NH3PbI3 can act both as a light harvester and as a hole conductor which was also previously reported by Etgar et al. [77]. The excitation of CH3NH3PbI3 produced excitons, which was then dissociated via electron injection at the TiO2/CH3NH3PbI3 interface. Injected electrons are transported to the FTO electrode through the TiO2 network and holes are transported through perovskite to HTM and finally arrive at the Au electrode. The highest efficiency reported by Heo et al. [76] was 12% for the cell configuration of FTO/mesoporous TiO2 layer/CH3NH3PbI3/poly-triarylamine/Au. By blending TiO2 nano-particles with nanorods, the efficiency increased up to 15% [78].

#### **4.2. Planar structure**

Kojima et al. [73] reported the first perovskite material (CH3NH3PbBr3 and CH3NH3PbI3) used as a sensitizer in photoelectrochemical cells. The cell consists of mesoporous TiO2 film having 8–12 µm thickness, iodide/triiodide redox couple liquid electrolyte and platinum counter electrode. The band gap CH3NH3PbBr3 is 1.78 eV and that of CH3NH3PbI3 is 1.55 eV. They have reported that the solar cells using CH3NH3PbBr3 and CH3NH3PbI3 sensitizers exhibit the efficiencies of 3.13 and 3.81%, respectively. TiO2 sensitized with orthorhombic (CH3CH2NH3)PbI3 has been reported by Im et al. [70] to have an optical band gap of 2.2 eV. The cell using the (CH3CH2NH3)PbI3 sensitizer and the electrolyte with the iodide/triiodide redox mediator exhibits an efficiency of 2.4%. Based on the work done by Kojima et al. [73], Im et al. [74] have investigated the effect of TiO2 film thickness on perovskite photovoltaic performance. The cell with 8.6 µm thick TiO2 film exhibits an efficiency of 3.37% comparable with that of Kojima et al. [73]. The performance of the cell increases when the TiO2 film thickness decreases. The cell with 3.6 µm thick TiO2 film exhibits an efficiency of 6.2%. Unfortunately, the cell exhibited poor stability due to perovskite decomposition and degraded within minutes. In 2012, the stability of CH3NH3PbI3-sensitized solar cell over 500 h has been reported by Kim et al. [72]. They have substituted the liquid electrolyte that was previously tried by Kojima et al. [73] with a solid state hole transport layer (*spiro*-MeOTAD). Their results also support the work done by Im et al. [74] where the efficiency of the cell increased with the decrease of TiO2 thickness and the highest efficiency of 9.7% observed for the cell having TiO2 thickness of 0.6 µm. Based on the impedance spectroscopy results, they found that the dark current and electron transport resistance increased with the increase in TiO2 film thickness. Koh et al. [71] have synthesized a novel (NH2CH═NH2)PbI3 perovskite with an energy band gap of 1.47 eV. Although the band gap of (NH2CH═NH2)PbI3 is smaller compared to that of CH3NH3PbI3, the efficiency of the cell is only 4.3%. The low efficiency is attributed to the energy level mismatch between TiO2 and the perovskite. The working mechanism of the above perovskite photovoltaics is expected to be similar to DSSC ( **Figure 12a**) where the perovskite absorbs light, injects

22 Nanostructured Solar Cells

electrons to the CB of TiO2 and holes to the solid state hole transport material (HTM).

**Figure 12.** Mesoporous structure of perovskite solar cell. (a) Perovskite dot: the structure is similar to DSSC. (b) Meso superstructure: the CB of the oxide semiconductor used is higher than the perovskite material and its surface is coated

completely. (c) Inert scaffold: the perovskite fills the pores and makes a thin layer on the top of TiO2.

The planar perovskite solar cell architecture is similar to the mesoporous structure except for the mesoporous metal oxide.

**Figure 13.** Planar structure of perovskite solar cell. No mesoporous structure involved.

Lee et al. [75] have shown that the perovskite photovoltaic system can still function without the non-blocking TiO2 layer. Hence, the planar p-i-n and the p-n junction perovskite structures 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 replaced with thinner TiO2 film [81].

## **4.3. Lead free perovskite solar cell**

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+) perovskites of the form, CsGeX3 (X = Cl− , Br− , I− ) with a rhombohedral structure and *R*3*m* 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 photoluminescence signal in the red. Stability is still an issue of concern.
