**5. Key results**

As shown in the **Figure 4**; scanning electron microscopy (SEM) image of the semitransparent cell; Absorption spectra of Rb-doped and Rb-free perovskite and external quantum efficiency (EQE) of the semi-transparent perovskite cell and filtered silicon cell put together with the absorption and transmittance of the semi-transparent perovskite cell. It is observed that the integrated current from the EQE of semitransparent cell is 18.2 mA/cm<sup>2</sup> , and the integrated current from the filtered silicon cell is 18.7 mA/cm<sup>2</sup> . These values are good for marching the two cells. These results are similar with those of the currents determined from *J*–*V* characteristics. In the **Figure 4c**, *J*–*V* characteristics of the silicon cell with and without filter, and reverse, forward scan and steady state efficiency of the semi-transparent perovskite cell. The current density versus voltage (*J*–*V*) characteristic for this tandem solar cell is shown.

The application of detailed balance limit calculations that used in single junction solar cells can also be extended to tandem or multi-junction solar cells. This was first done on detailed balance calculation for tandem solar cells by De Vos [7]. Then latter Green gave a general description of the detailed balance theory for multi-junction solar cells [16]. For this work, this theory is applied to perovskite/silicon tandem solar cells. Typical tandem solar cell can be used either as a two terminal (2-T) or fourterminal (4-T) devices. Plots of tandem configurations (a) 2-T, (b) 4-T and (c) spectrum splitting and the energy conversion efficiency of two and four-terminal tandem solar cells are provided in **Figure 5a**–**c**.

4-T device is the case where the incoming radiation is split into two diodes which are electrically separate. The conversion efficiency of more than 40% can be realized

#### **Figure 4.**

*(a) X-sectional (SEM) image of the semi-transparent tandem cell (b) Rb-doped and Rb-free perovskite absorption spectra. (c) EQE of the PVSK cell and filtered silicon cell and their* J*–*V *characteristics [23].*

by several combinations of band-gaps in PVSK. The overall electrical energy conversion efficiency is determined from the sum of output power generated by both diodes independently. The incoming radiation is split among the two solar cells that form the tandem structure. In monolithic tandem solar cell tittled 2-Terminal (2-T), all the layers corresponding to the two sub-cells are assembled straight on top of another subcell i.e. **Figure 5a**. In four terminal (4-T), the sub-cell structures are constructed separately, and then mechanically stacked top cell onto bottom cell (**Figure 5b**). This configuration enable independent optimization of each sub-cell. The optical splitting tandem solar cell termed as 4-T optical operates with optical spectrum filter to split the light spectrum to each sub-cell. The sub-cells merely function independently without any integration, which makes the selection of sub-cells more flexible. **Figure 6a** shows the conversion efficiency of a two-terminal device or a serial connected tandem solar cell that is determined by the current at zero applied voltage. The total short-circuit current is equal to the current at zero applied voltage of the bottom solar cell if the short-circuit current of the bottom cell is smaller than the short-circuit current of the top cell. The total short-circuit current is determined by the short-circuit current of the top cell if the short-circuit current is larger than the short-circuit current of the bottom cell. Ideally the short-circuit current of a tandem solar is said to matched if the short-circuit current of the top cell and the bottom cell is equal or almost equal. By matching the bandgaps of the bottom and top solar cells; the energy conversion efficiency of a tandem solar cell is maximized. With proper

#### **Figure 5.**

*Two-terminal and four-terminal configuration for tandem cells and light splitting.*

combination of band gaps of the top cell; and bottom c-Si solar cell known, the twoterminal tandem solar cell's short-circuit current is matched. There is a possibility of the two-terminal tandem solar cells reaching the energy conversion efficiencies of the four terminal tandem solar cells. The relationship for maximum energy conversion efficiency is determined by the equation; *E*G\_top = 0.5 *E*G\_bot + 1.14 eV; where *E*g-top is energy gap for the top cell; *E*g-bottom is the energy gap for the bottom cell. For PVSK/c-Si tandem solar cell, maximum energy conversion efficiency is achieved when the bandgap of the top diode is about 1.725 eV. This gives the maximum energy conversion efficiency of about 43% for this arrangement. Using perovskite (MAPbI3) as the base absorber with a bandgap of about 1.6 eV; the maximum energy conversion efficiency of a perovskite/silicon tandem solar cell is approximately 33%. By using the bottom solar cell of bandgap 0.9 eV; and the top cell of perovskite with a bandgap of 1.6 eV, a higher energy conversion efficiency of approximately 44% is achieved.

### **6. Discussion**

It is observed that perovskites have gained considerable attention as a photovoltaic material [26–28]. From its inception in 2009, the energy conversion efficiency of single-junction PSC has been increasing to over 22% [34–38]. It is true that, perovskites are a promising material system for the implementation of tandem or multijunction solar cells. For the case of perovskite/c-Si tandem solar cells, there is a possibility of reaching high energy conversion efficiencies while potentially maintaining low fabrication and maintenance cost.

Perovskite solar cells are so named because they use a class of crystal structure similar to that found in the mineral known as perovskite. They are structured

**Figure 6.** *The energy conversion efficiency of two and four-terminal tandem solar cells.*

compounds, commonly hybrid organic-inorganic lead halide-based materials. A layered approach is used in preventing lead leakage in PSC. There is a recently developed process (on–device sequestration approach) that can be easily incorporated with the PSC configurations. Acting as anti-reflecting agent; a transparent lead absorbing film is applied to front conducting glass. The breakthrough in both device architecture and module manufacturing of Si-Perovskite tandem technology will lead to successful commercialization of these devices. Currently, the best devices in this technology with high efficiencies are developed using laboratory scale processes that include spincoating and anti-solvent dropping methods for PVSK. These techniques are not strongly formed and economical for large scale manufacturing, since their substandard solvent coverage will lead to poor quality pinhole PVSK layers. The alternative to this challenge for perovskite solar cell, is a transition of fabrication processes to large scale deposition techniques such as blade coating, solution processes, printing and spray coating are recommended.

The recombination layers between Si and Perovskite cells should be impenetrable and free from pinholes which is challenging for solution processes involved. The blade coating process requires flat surfaces; but with the existence of textured structure of

c-Si cell, this approach is quite complicated. In addition to these fabrication issues, the tandem technology currently shows higher material cost over the process cost. The use of expensive organic transport materials on perovskite cells is a long-standing moisture stability problem and needs an alternative as inorganic PSC. The wearing down of perovskite and leaching of ionic lead (Pb) out of the cell leads to serious environmental hazards and hinder their entry into commercial sector. Although a lot of investigation is concentrated towards replacing Pb2+ with dual cations, vacancies and all possible transition metal ions, these alternatives in solar cell devices have so far not been successful. These issues are hindrance for further advancement in the efficiency for the tandem solar cells.
