**3. Thin film based perovskite solar cells**

Any materials which have the crystal structure of calcium titanium oxide (CaTiO3), were known as the perovskite structure and the materials have stoichiometry of ABX3; where "A" is the larger cation, "B" is the smaller cation and "X" is the anion. Each unit cell of ABX3 crystal comprises of corner sharing BX6 octahedra, with the "A" moiety cubo-octahedral cavity. In case of organic-inorganic hybrid perovskites (OIHP), halide anions (I− , Br− , Cl− ) are found at the "X"-site anion instead of oxygen, while monovalent (CH3NH3 + , CH(NH2)2 + ) and bivalent (Pb2+, Sn2+) cations occupy the "A" and "B" sites, respectively. Halide perovskites were first reported by Moller in 1958 for cesium lead halides [59]. Further, it was also observed that small organic molecules with effective radii less than 260 pm [methylammonium (MA), formamidinium (FA), hydrazinium, hydroxylammonium) can also accommodate inside the PbX6 octahedrons. The word "hybrid" indicates that the crystal is made specifically by the combination of "organic" and "inorganic" components. The architecture of OIHPbased solar cell is quite simple and prepared by sandwiching a perovskite absorber layer between the electron transport layer (ETL) and hole transport layer (HTL). A standard OIHP based solar cell device has a structure composed of glass/ transparent conductive oxide (TCO)/TiO2 (ETL)/ mesoporous TiO2 (mp-TiO2)/ perovskite (~500 nm)/ HTL/ metal and a quite high efficiency exceeding 20% can be realized without including complicated processing steps. The operation of the perovskite device is sstraight forward; namely, the photo-electrons and holes created by light absorption are collected in the ETL and HTL, respectively, and the electrons flow through the outer circuit and recombine with holes at the HTL/metal interface. The efficiencies of OIHP-based solar cells have increased all the way from 3.8% in 2009 to 25.5% for single-junction solar cells, and 29.15% for the highest publicly disclosed perovskite/silicon (Si) tandem [60].

The properties of perovskite solar cells were discussed in terms of crystal structure and phase transition, electronic structure, optical properties and electrical properties. One of the interesting aspect of the crystal structure of halide perovskite is the structural flexibility of organic cation. Taking MAPbI3 as an example, the disorder-order

transition of MA+ cation is believed to trigger the phase transition with the decrease of temperature. At high temperature MAPbI3 takes a cubic structure (space group: Pm-3 m; Z = 1). Since MA+ has a lower symmetry of C3v, the orientation of MA+ ion should be disordered to satisfy the Oh symmetry. As the temperature is lowered, tetragonal and orthorhombic phases are realized by an accompanying ordering of methylammonium ion. Structural transition from cubic to tetragonal phase occurs due to the reorientation of MA+ ion, as observed by nuclear magnetic resonance (NMR) studies where lowering the number of disorder states of MA+ was observed from 24 in the cubic phase to 8 in tetragonal phase [61]. Below a critical temperature (tetragonal-orthorhombic phase transition), the MA+ molecule is frozen (only 1 degree of freedom) and the symmetry of MAPbI3 become orthorhombic. Similar crystallographic phase transition can be realized with replacing I by Br and Cl [62].

The band structure of MAPbI3 exhibits a direct bandgap of 1.6 eV at the R point. Calculated band structure suggests conduction band minima (CBM) is dominated by the Pb-*p* orbital, whereas the valence band maxima (VBM) is constituted by I-*p* states mixed with a small amount of Pb-*s* states, which is consistent with the photoemission results [63]. The optical transition of MAPbI3 relies on a direct bandgap *p*-*p* transition, leading to a strong optical absorption coefficient. Strong *s*-*p* antibonding enhances dispersion of the upper valence bands [64], which resulted in small effective masses of electrons (*m\* <sup>e</sup>*) and holes (*mh \**). Further, it is believed that the role of MA+ cation is to maintain the overall charge symmetry and as dictated by the crystal structure of the system [65]. However, it was reported that MA+ cation has an indirect impact on the shape and orbital composition of the band edges. The molecular orientation of MA<sup>+</sup> cation can distort the PbI6 octahedral and affected the cell size and bonding of Pb-I, which modulated the density of states near the band edges [66]. Other halide perovskites also possess similar ways of electronic band structure.

Organic-inorganic hybrid perovskites are direct band gap semiconductor and the direct transition produces large absorption coefficients of the order of 104 –105 cm−1. In the case of perovskite thin films, the optical properties of perovskites are dramatically affected by the quality, composition and morphology of the film [67]. Sizes of the halide anions (X = I, Br, Cl) affected the electronic band structure of the system. Large anion (iodine based materials) showed a smaller bandgap and corresponded the absorption edge at 780 nm; whereas substituting iodine with smaller bromine (chlorine) anion shifts the absorption edge to 535 nm (408 nm) for MA+ based perovskite system [68]. A systematic blue shift of the PL emission peak is observed with the increase of Br concentration in mixed halide perovskite of the type MAPb(I1−xBrx)3. Further, replacing MA with CH(NH2)2 red shifts the absorption spectra by 40 nm, which makes CH(NH2)2PbI3 more suitable for high-performance solar cell applications [69]. Intermediated solid solutions of MASn1−xPbxI3 with x = 0.25 and 0.5 exhibited the smallest band gap of 1.17 eV [70]. Irrespective of bandgap tuning, fundamental understanding of absorption and PL spectra are essential to study the basic photo physical properties of hybrid perovskite. In spite of several optical investigations performed at different temperatures, there have been a lot of ambiguities in the data as well as its interpretations, especially observation of multiple peaks in the photoluminescence (PL) spectrum of organic-inorganic hybrid perovskites. Literature reports excitonic emission, tetragonal inclusion in orthorhombic phase, order-disorder transition, surface-bulk effects are responsible for these multiple PL emissions [71].

Space charge limited current (SCLC) is one of the effective approaches to measure mobility, diffusion length and trap density of hybrid perovskites. Due to the advancement in fabrication techniques, the diffusion length of hybrid perovskite has increased

### *Recent Developments on the Properties of Chalcogenide Thin Films DOI: http://dx.doi.org/10.5772/intechopen.102429*

from 1 to ~10 μm in about 3 years [72]. This improvement reflects the progress that has been recently made in producing samples with better structural order and morphology. Further, it is also observed that the diffusion length has a strong dependence on the grain size of the film. The results showed that more than 1 μm diffusion length has been achieved by realizing films with an average grain size of 2 μm. The perovskite single crystal was found the highest measured diffusion length (10 μm) [73]. Carrier mobility of hybrid perovskite has also been improved over the years and exhibited morphology dependence. Mobility values exceeding 10 cm2 V−1 s−1 have been measured in perovskite film [74] and above 100 cm2 V−1 s−1 in perovskite single crystals. Further, it is also observed that the mobility (and also diffusion length) did not exhibit a strong dependence on the material composition. Further, the dielectric constant (relative permittivity) is a complex number given by, ε = ε / −ε//, where the real part ε / is the charge storage ability and the imaginary part ε // is the energyloss. For MAPbI3, a small ε / is obtained (ε / = 6.5 in experiment, while 5.6 to 6.5 in calculation) at optical frequency and only electronic polarization takes part in dielectric process [75]. With the decrease of frequency, ionic polarization and dipolar polarization (contribution from MA+ dipoles) leading to enhanced ε / (ε / low ~ 60 at 100 KHz).This large dielectric constant facilitates the screening effect of Coulombic attraction between photoexcited electron-hole pairs (excitons), so that they can be separated easily. Also, noncentro symmetric crystal structure in tetragonal and orthorhombic phases proposed OIHP are ferroelectric in nature. It is also believed that ferroelectricity may give rise to hysteresis observed in current-voltage (I-V) curves. However, observation of ferroelectricity in hybrid perovskite is not well justified from polarization-electric field (P-E) hysteresis loop and second harmonic generation experiments. Despite the above controversies, it is of great interest to study the order-disorder transition of hybrid perovskites due to MA+ orientation inside the PbX6 octahedral [76].

Perovskite solar cell has gained attention due to favorable material properties of OIHP, which include a high absorption coefficient with a sharp absorption edge, high photoluminescence quantum yield, long charge carrier diffusion lengths, large mobility, high defect tolerance, and low surface recombination velocity. At the same time, easy solution processability and completely tunable optical bandgap from blue to red regions of wavelength just by mixing the B-site cation (Pb-Sn) and the X-site anion (I-Br-Cl), while maintaining the sharp absorption edge makes the OIHP family a potential candidate for application in multijunction/tandem solar cells. Another strong advantage of hybrid perovskite solar cells is quite high *V*oc, which can be explained by the suppression of the defect formation in the bulk layer as well as at the interfaces. It is observed that OIHP solar cells with Eg < 1.65 eV, the open circuit current (*V*oc) is remarkably high and *V*oc increases with band gap (*E*g) without significant *V*oc loss. In particular, a quite high *V*oc of 1.26 V has been reported for a pure MAPbI3 cell, which is very close to the theoretical limit of 1.32 V, with *V*oc loss of only 60 mV. High absorption coefficient and low nonradiative recombination rate of OIHP solar cell resulted very small short circuit current (Jsc) loss. We know that photovoltaic devices rarely operate at room temperature, and high power output is necessary even under high-temperature operation conditions. It is observed that OIHP solar cell shows the lowest temperature coefficient (TC) of −0.17%, which is far better than other photovoltaic devices. Also nearly 90% efficiency is maintained even at a high operating temperature of 85°C. Further, OIHP-based solar cell exhibited some unique advantages such as low-temperature processes for all sub cells, compatibility with flexible and lightweight applications, low life-cycle environmental impacts and embodied energy, and potentially low fabrication costs.

Although OIHP solar cells produced quite impressive efficiency, they have several limitations too and to overcome these limitations are the major challenge for the commercialization of these devices. One significant drawback of OIHP is degradation of these perovskite materials under a range of environmental factors such as humidity, illumination, oxygen, and thermal stress. OIHP solar cells are ionic crystals, and the presence of H2O leads to the decomposition of the perovskite structures to hexagonalshaped PbI2/hydrate crystals; which can be suppressed by introducing protective (passivation) layers. In case of mixed halide perovskites strong photo-induced phase segregation occurred under illumination and judicial choice of A-site cation can minimize this instability. Further, it is observed that a higher level of performance in OIHP solar cell is hindered by anomalous hysteretic behavior and large discrepancy between the forward and reverse scans put a question on the reproducibility of power conversion efficiency (PCE) of the device. In searching for the possible origins of hysteresis, several explanations such as ion migration, charge trapping/ detrapping, photoinduced capacitive effect, and ferroelectricity have been imposed. Among them, ion migration and ferroelectricity are believed as feasible origins of the hysteresis in transport measurements. Extensive research efforts continue to find the long-term stability of OIHP solar cells.

Another major challenge is the realization of large-area module due to its fabrication limitations. Till now high efficiency of 17.9% has been realized for the large-area module with a size of 30 × 30 cm<sup>2</sup> (aperture area: 802 cm2 ), which was formed by an inkjet printing technology. Thus development of proper fabrication technique is essential to make pinhole free large-area OIHP devices. Also in the large area tandem cells, current matching conditions for the top and bottom cells as well as each sub cell need to be established; which can be improved through technological advances.

High toxicity of heavy metal (lead) is a serious problem which cannot be neglected in OIHP-based solar cells. Although the content of lead (Pb) in OIHP solar panel (~1m<sup>2</sup> ) is only a few hundred milligrams, could be severe problems in environmental impact. As an alternative people are trying to replace Pb2+ with Sn2+; but the efficiency of Sn-based photovoltaic devices are extremely poor. Thus, roof-top application of OIHP modules is difficult and large-area operations as solar farms are more appropriate. Also, encapsulation of photovoltaic module and environmentally friendly 100% recycling programs are essential for OIHP-based solar modules.

The future of perovskite solar cells was highlighted. As discussed earlier, the significantly reduced efficiency upon solar module area scaling-up is still the main challenge to face for the commercialization of OIHP-based solar cell. It is observed that efficiency decreases to 19.6% when the aperture area increases from 0.1 cm2 to about 10 cm2 , and further drops to 17.9% with the area approaching 1000 cm2 , which still lags far behind that of the crystalline silicon cells (26.7% at 79 cm<sup>2</sup> and 24.4% at 13,177 cm2 ). Thus, intensive works should be conducted to precisely control the uniformity of the crystallization process in large-area perovskite films. Also, the fundamental photophysical mechanisms relative to the efficiency loss in OIHP modules should be further studied to understand role of surface and interface. Development of green solvent systems or the solvent-free deposition technology for fabricating large-area perovskite film will be an important research topic in the future. Besides the efficiency, more and more attention need to invest in the long-term stability of OIHP solar modules. Recently, Okinawa Institute of Science and Technology Graduate University in Japan reported over 1100-h operational lifetime for a 10 × 10 cm<sup>2</sup> solar module. Although many research groups and companies claimed that their devices have passed International Electro Technical Commission (IEC) standard test, there

*Recent Developments on the Properties of Chalcogenide Thin Films DOI: http://dx.doi.org/10.5772/intechopen.102429*

are still some stability issues needed to be addressed at the next stage. Thus proper development of encapsulation technology is essential and we believe that a growing number of studies will move to exploit such multifunctional encapsulation materials in the near future. The single-junction OIHP cells with efficiency above 24% and long-term stability can be more cost-effective than tandem cells which may work at a PCE of 27–28%. Thus, more efforts should be made in fabrication and scaling up of single-junction OIHP-based solar cells with high efficiency, high yield, and long-term stability. Development of low-cost large-scale fabrication methods with highly reproducible results is required for commercialization of OIHP-based photovoltaic cells.
