planar heterostructure [29]. It has been stated that if inverted planar heterostructure PSC is made by mixing methylammonium bromide as an additive in lead acetate and methylammonium iodide solution, maximum PCE of 18.3% is achieved [30].
2.5 Properties
2.5.1 High Optical Absorption
The absorber layer is generally referred to as the heart of the solar cell. The material of this absorber is made of organic inorganic hybrid perovskite. This absorber material has a direct band gap with high absorption coefficient. These properties have permitted to be used as very thin compact absorber materials for gathering light which have thickness in the range of hundreds of nanometers. Whereas the thickness of the film for the traditional solar cell (silicon and germanium) is the range of one micrometer to hundreds of micrometers.
Out of the three generations of solar cell, the first-generation absorber is an indirect bandgap semiconductor material. In this type of semiconductor, the minimum value of conduction band and maximum value of valence band does not lie for the same value of k. Because of this, the momentum of the electron in conduction and valence band is different.
In indirect bandgap semiconductor, transition of photon is horizontal in nature; hence, the momentum of photon is comparatively larger. So a large change of momentum is required for electron-hole pair recombination, hence momentum is not conserved here as k value for both the bands is different. Hence, a large change in momentum is required for recombination of the electron-hole pair as compared with direct bandgap semiconductors. So there is a prohibition for transition of electrons between these two levels and the transition becomes weak because of the participation of lattice vibrations during excitation. Hence, the probability of transition is lower in indirect band gap material than those in direct band gap material. Thus, the thickness of the absorber material of indirect bandgap semiconductor is way more than that of direct band gap semiconductor material for absorbing identical number of photons per unit area [32]. Because of this, for achieving alike efficiency, there is a rise in cost of production of indirect bandgap semiconductor material solar cells as compared with direct band gap semiconductor material solar cells.
Figure 2.7 Transitions in direct and indirect semiconductor [33].
Second-generation absorber material (GaAs) and perovskite (CH3NH3PBI3) have direct band gap; hence, they have high optical absorption properties compared to silicon. However, their electronic configuration of both the absorbers is not the same as shown in Figure 2.7. The lower part of conduction of gallium arsenide is obtained from the delocalized s orbitals, whereas the lower part of CH3NH3PBI3 mainly consists of Pb p bands. The p orbital is less dispersed as compared to s orbitals [33]. Hence, in GaAs, the transition efficiency is moderate. There is a high probability of transition from intra-atomic Pb s to Pb p, because of this, in CH3NH3PBI3, the transition probability between the two bands is almost equivalent to GaAs [33]. Therefore, the optical absorptions are stronger in halide perovskites than GaAs.
2.5.2 High Open-Circuit Voltage
The most distinct property of the PSC technology is their high open circuit voltage, which is the maximum voltage that a solar cell can generate [1]. As from Figure 2.8, we can observe that the best PSC has open circuit voltage greater than 1.1V. The loss-in-potential is around 450 meV, which is quite low. There is energy loss of 250 to 300 meV due to thermodynamic constraints, depending on the bandgap) [34].
Figure 2.8 Open-circuit voltage (Voc) versus optical band gap (Eg). Reprinted with permission from [1]. Copyright {2013} American Chemical Society.
Every data related to GaAs, Si, CIGS, CdTe, nanocrystalline Si (nc-Si), amorphous Si (a-Si), copper zinc tin sulfide/selenide (CZTSSe), organic photovoltaics (OPVs), and DSSCs were provided by the solar cell efficiency tables of Green et al. [35].
CdTe solar cell was the most successful solar cell commercially till 2015 with 19.6% efficiency with losses of approximately 0.59eV. The basic losses of CdTe solar cells are high as compared with perovskites cells [35]. The recombination rate (nonradiative) in perovskite absorber material is lower than polycrystalline film semiconductor [32]. High output voltage is one of the important factors which is responsible for high power conversion efficiency. We know that perovskite is placed in a very good position compared with the other materials for making solar cells. Voltage at maximum power point (Vmpp) is to be considered as the most relevant voltage for considering maximum efficiency [36]. Because of the manufacturing defects, there is high series resistance, which greatly affects the Vmpp. On optimization of solar cells the Vmpp will improve significantly [1].
2.5.3 Low Recombinations
In the recombination process, the excited electrons in the conduction band deexcite in the valence band and occupy the hole. This process can also be termed as electron hole pair recombination. This process competes with the process of separation of the excitation to electrons and holes and their collection at the electrodes. Because of this, there is a decline in the efficiency and current of the solar cell [32].
Recombinations are of two types: radiative and nonradiative recombination. When an electron deexcites from conduction band to valence band it releases a photon to release energy called radiative recombination. Whereas in nonradiative recombination, when an electron deexcites the energy is released in the form of heat. It causes harm to the performance of the cell. The device gets heated up during nonradiative recombinations due to which efficiency gets decreased.
Diffusion length is defined as the average length a carrier moves between its generation/formation and its recombination. On the basis of diffusion length parameters, the semiconductor material can be assessed for solar cell applications. Semiconductor materials have a shorter diffusion length and higher recombinations because they are heavily doped. If the diffusion length is higher, then the longer will be the lifetime of recombinations, the better the collection of carriers at the electrode. CH3NH3PBI3−xClx has a diffusion length of more than 1 micron. This diffusion length is almost three times the thickness of the film in solar cells [37].
For constructing planar heterojunction solar devices this characteristic is very important. As the diffusion length of perovskite(CH3NH3PBI3) is only a hundred nanometer, so for transportation of charges to terminals, a nanoparticle system of a mesoporous TiO2 is required [38].
2.5.4 Tunable Bandgap
For designing the solar cell, it is necessary that the light absorber is absorbing the maximum amount of the sunlight. To achieve this, we have to tune the band gap of the absorber. So here perovskite has been greatly advantageous as a light absorber because its bandgap is tunable/controllable. The perovskite has a structure of ABX3. So the bandgap of the perovskite material (absorber) can easily be regulated by altering the organic cation (A) or the metal atom (B) or the halide (X).
2.5.4.1 Organic Cation (A)
