**2. Working mechanism and device architectures of PSCs**

According to the operating principle of PSCs and the obtained information on the working mechanism, they are still insufficient for now [18, 19]. There are different approaches to figure out a suitable mechanism for the working principles of PSC. The principal mechanism of PSCs like (i) light absorption, (ii) charge separation, (iii) charge transport, and (iv) charge collection are essential to address because they are general SC parameters during conversion of sunlight into electricity.

In this regard, the choice of photon harvesters is the first step for the specification of the physical structure of an SC. Hence, investigation of PV parameters of perovskite has priority during the design engineering. This is optimum in terms of theoretical understanding for energy conversion mechanism [1, 20]. It is known that the structure of organic-inorganic halide exhibits both electron and whole transport properties together. Hence, PSCs can be engineered as *p-n* junction architecture or *p-i-n* junction structure. The two layouts can be described as a p-i-n junction if the light harvester or perovskite is an intrinsic semiconductor, whereas in p-n junction, the light harvester has *n-*type or *p-*type property. This junction is capable to carry electrons or holes to the perovskite harvester [1, 9–11, 21].

The typical structures of PSCs are engineered depending on two structures such as mesoporous and planar. **Figure 3** demonstrates the schematic architecture for both mesoporous and planar type PSCs. The first structure consists of a mesoporous type metal oxide layer like TiO2 or Al2 O3 accompanied by perovskite sensitizer. The second structure contains a perovskite film sandwiched between electron and hole transporting layers. Herein, the photo-generated charge carrier takes place in perovskite which further inject to the TiO2 and finally collect at transparent conductive oxide glass [22–25].

To manage efficient charge collection, these processes should be much faster than the recombination rate occurred from (4) to (8). This charge carrier and light management will further

Analyzing the device architecture of the PSC, it is very important to understand the factors that limit the photocurrents. The devices must minimize parasitic losses while the suitable thickness of photo-absorber such as organic-inorganic trihalide should have a better capability of incident photon harvesting. The enhancement in photocurrent density (Jsc) from 11 to 21 mA cm−2 has been achieved for the PSCs in 2 years [27, 28]. Later on, utilizing 1.6 eV energy band gap of perovskite materials in PSCs a Jsc of 22 mA cm−2 was obtained [29]. Researches are focused on understanding the photocurrent losses occurring in the PSCs. In this regard, internal quantum efficiency (IQE) has been confirmed as one of the major losses. Additionally, thin

losses and parasitic absorption [30]. Crystallinity enhancement has been shown pleasurable to minimize IQE losses. Consequently, yielding photocurrents of 23 mA cm−2 [31]. Tuning the energy band gap via Tin (Sn) based perovskites; the photocurrent density has been improved from 25 to 26.9 mA cm−2 [32, 33]. However, the unstable Sn-based perovskites are crucial for their quantum efficiencies. This newly emerging field needs further insights to achieve the

The enhancement of open-circuit voltage (Voc) depends on the tunable energy band gap in PSCs. Thermodynamic limit of Voc relates theoretical efficiency limit. The reciprocity between absorption and emission have been shown avoidable due to the radiative recombination, this

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/spiro-OMeTAD/Au can cause reflection/transmission

Pathways Towards High-Stable, Low-Cost and Efficient Perovskite Solar Cells

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(band gap ≈1.6 eV) [34]. The broad absorption

benefit high power conversion efficiency.

films in device stack such as FTO/TiO2

proposed theoretical photocurrents.

returns to the Voc limit of 1.33 V for CH3

**3.2. Open-circuit voltage**

**3.1. Photocurrent density**

**3. Paramount photovoltaic parameters of PSCs**

**Figure 4.** Trends of charge generation, charge transportation and recombination.

In comparison, the two architectures follow the same charge transport rate. However, the mesoporous PSCs display higher recombination rates [26]. On the other hand, the planar PSCs are suitable for the field of flexible solar cells since they do not need high sintering temperature. The trends of generation and recombination of charge carrier in PSCs are depicted in **Figure 4**. The charge generation rate and charge movement take place from (1) to (3).

**Figure 3.** Schematic illustration of perovskite solar cells architectures, c-ETL; compact electron transporting layer, c-HTL; compact hole transport layer, m; mesoporous, TCO; transparent conductive oxide.

**Figure 4.** Trends of charge generation, charge transportation and recombination.

To manage efficient charge collection, these processes should be much faster than the recombination rate occurred from (4) to (8). This charge carrier and light management will further benefit high power conversion efficiency.
