Solar Solutions for the Future

Abstract

The energy conversion efficiency and limits of perovskite/silicon solar cells are investigated. The influence of a layered approach in preventing lead leakage in perovskite solar cells is discussed. The highest efficiency perovskite tandem to date was achieved by pairing a perovskite top cell with a Si bottom cell in a four-terminal configuration, yielding 26.4%. Perovskite cell integrated with crystalline silicon cell to form a tandem solar device has shown high performance above the single pn-junction silicon devices. Although sufficient work and different strategies have been applied to increase efficiency in these devices, the tandem application has achieved efficiency of 29% in a short period.

1. Introduction

The growing demand of energy, has forced researchers to look for cheap alternative sources of energy. Among these cheap sources energy under investigations, photovoltaics is one of them. The International Energy Agency (IEA) has noted declining prices of photovoltaics and estimates solar systems in general to supply 5% of global electricity consumption in 2030. These estimates are likely to rising to 16% by 2050. This can only be achieved by increasing the global production of solar energy to 4600 GW by 2050 [1]. At the same time solar and wind energy is expected to contribute up to 50% of total energy generated. Reduced costs of energy generation and improvements in performance will lead to more penetration of solar energy across the globe. These coupled with more research, will make solar energy cheaper than fossil fuel in the near future. Using innovative new designs, researchers are continuously working to improve photovoltaic energy systems. This work is geared towards reducing the power generation cost by combining several materials in photovoltaic cells. The best cell efficiency of 39.2% has been demonstrated from multi-junction solar cells. However, it should be observed that this is applied in space technologies that have complex fabrication processes. Currently the theoretical efficiency of crystalline silicon (c-Si) based solar cells is approximately 26.7% and for thin film technology it is ∼23.4%. These solar cell technologies outlined above are mature, and already in production. However, the complexed of the production process and energy consumption needs to be reduced.

Currently inexpensive and easy to fabricate solar cells are under investigation by researchers. Most of these solar cell technologies are categorized as emerging PV technologies. Examples include; organic solar cells, dye-sensitized solar cells, quantum dot solar cells; perovskite solar cells (PSC). PSC shows very promising results and can favorably compete with c-Si solar cells in terms of efficiency. Perovskite was discovered in the Ural Mountain, in Russia, and basically describes compounds having crystal structures like calcium titanium oxide (CaTiO₃). The materials being used for newly developed PSC have their structure in the form of ABX structure where A, and B are cations and X is the bonding anion. In comparison, with traditional Si cells PSC absorbs sunlight with a hundred times thinner active layer. This ABX structure allows for variation in the energy gap by mixing and matching scenario where we can have multiple compounds by substituting any of the constituents. Within a decade of its inception, PSC has already overtaken thin film technology such as cadmium telluride (CdTe) or copper-indium-gallium-selenide (CIGS) and reached the level of c-Si solar cells. Already an efficiency of 25.2% for single junction PSC has been realized by a team led by Prof. Michael Saliba at KRICT2 whereas the theoretical efficiency for these PSC’s are about 31%. When Perovskite solar cell is in tandem with other solar cell technologies, they give varying efficiencies. CIGS-perovskite tandem cells have shown efficiencies up to 21.5%, whereas, when combined with c-Si technology the perovskite-c-Si tandem cells have shown efficiencies up to 28% that shows better performance in comparison with a single pn-junction of silicon; and also single junction of PSC. The efficiency of the perovskite/c-Si tandem solar cells are in the range of 32.8% for gallium-indium-phosphide/gallium arsenide (GaInP/GaAs) tandem solar cells. This type of solar cells is realized through intensified processes and expensive manufacturing techniques. With more research the efficiency of the Perovskite/silicon tandem cell can get towards 30%. Although PV technology is not the most widely used energy source; due to low energy conversion efficiency and high initial system cost; in comparison to non-renewable energy sources; it is a fast growing energy source in the power sector [2, 3, 4]. So far, solar modules from c-Si single-junction solar cells have their conversion efficiency in the lab as 26.3% [5], while the upper theoretical energy conversion efficiency of a solar cell with a bandgap of 1.14 eV (e.g., silicon) is about 33.5% [6]. Multi-junction solar cell approach has been used to increase the theoretical limits of single-junction solar cells [7, 8, 9, 10, 11, 12, 13]. It has been shown that series connection of tandem solar cells can reach conversion efficiency in excess of 40% if proper material combination is selected for the top and bottom solar cell [6, 7]. Higher than 40% can be attained if the relationship of the cells is E_{G_top} = 0.5 × E_{G_bot} + 1.14 eV, where E_{G_top} and E_{G_bot} are the bandgaps of the top and bottom diode absorbers. This equation holds true when the bottom cell bandgap is between 0.8.5 and 1.2 eV; hence several material combinations can be identified. One of the materials suitable as the bottom solar cell is c-Si with a bandgap of 1.14 eV. This has led to many activities focusing on the development of tandem solar cell using a c-Si bottom solar cell. With c-Si bottom solar cell; the highest conversion efficiency can be achieved if the bandgap of the top-cell is about 1.7 eV.

When combining well established c-Si solar cell technology with other material systems or fabrication process; a number of crucial aspects must be considered. Although amorphous silicon has a fav0urable bandgap of 1.7 eV, its tail states hinder it in being applied on the formation of tandem cell with high values of open circuit voltage. This is a condition for attaining high efficiency in a tandem structure [14, 15, 16, 17, 18]. Also silicon oxide/c-Si based quantum dot and quantum well have been researched for the top solar cell material. Un-successfully the two materials have shown comparatively low conversion efficiency. Although, compound semiconductors have been researched as potential top solar cell absorber material; high fabrication temperatures, the lattice mismatch between silicon and compound semiconductors, and the fabrication cost are the biggest hindrances to its application. In the last decade; the perovskite material system has attracted a lot of research either for single-junction solar cells or as material for perovskite/silicon tandem solar cells [18, 19, 20, 21, 22, 23, 24, 25]. This has shown high energy conversion efficiencies, with open-circuit voltages close to the theoretical limit of silicon material [26, 27, 28, 29, 30, 31, 32, 33]. In addition, a variety of deposition methods at low temperatures can be used in the fabrication of a perovskite top solar cell on a c-Si bottom solar cell. Conversion efficiencies above 20% have been realized with perovskite single-junction solar cells [34, 35, 36, 37, 38].

The perovskite/silicon tandem solar cells have both high and low band gap material in a single device; enabling the device to be active in both long and short wavelength regions of the electromagnetic spectrum, where each wavelength region can effectively be converted to electric power resulting in high efficiencies. Also perovskite SCs (solar cells) have unique properties like high absorption coefficient, variable band-gap, high defect tolerance, high open circuit voltage, abundant availability of its constituent elements and easy processability. Perovskite SCs can use the high energy blue and green light much more efficiently than silicon SCs. We note that perovskite/silicon tandem solar cells with high efficiencies is only possible if the perovskite top solar cell and the silicon bottom solar cell operate at a value very near to their theoretical limit. It is worth noting that, perovskite/silicon tandem solar cells with certified energy conversion efficiencies above 27% have been achieved [39]. The realization of solar cells with higher energy conversion efficiencies approaching or even exceeding 30% is feasible in the near future.

2. The basics of photovoltaic cells

A solar cell is an electrical device which converts the energy of sunlight directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon. The solar cells structure consists of either a p-n junction or p-i-n junction [4, 40]. Initially the incident radiation directed on the surface are absorbed resulting in the creation of electron/hole pairs. Then these photo generated electron/hole pairs are separated by the electric field and subsequently collected at the terminals. The charge collection of the photo generated charges occurs due to diffusion, drift or the combination of both transport processes to the contacts of the solar cell. Figure 1 provides an overview of different solar cells. Note that schematic figures are not to scale.

Figure 1.

Typical schematic diagrams of (a) homo-junction solar cell of c-Si, (b) hetero junction solar cell for a c-Si and amorphous silicon, (c) homo-junction thin-film solar cell for amorphous silicon, (d) hetero junction thin-film solar cell for perovskite, and (e) tandem solar cell of perovskite/silicon with layers showing different materials.

Figure 1a above is a typical diagram of a c-Si homo-junction solar cell. It is assumed that absorption of high energy photons is done in the whole of p-n junction. Electron/hole pairs that are produced are predominantly transferred by charge diffusion process. Hetero junction solar cell that consists of a c-Si absorber and amorphous silicon contact layers is shown in Figure 1b. The hetero junction structure when compared with traditional homo-junction solar cells, there is a minimization of optical losses, particularly in the emitter. As a result we have high short-circuit current density and high open-circuit voltages. Generally, amorphous silicon p- and n-layers are used to form electrical contacts. The main charge transport mechanism in c-Si is charge diffusion; since it has high diffusion length. The p-i-n structure is currently applied in many thin-film solar cells. In this arrangement the intrinsic material layer is placed between the p-type material and n-type material. The charge collection process is done by a drifting mechanism of the electron/hole pairs to the contacts. Figure 1c shows a typical homo-junction of an amorphous silicon thin-film solar cell. A heterojunction thin-film solar cell is shown in Figure 1d. A perovskite layer is used as an absorber of the incident photon. Different kinds of materials for electron transporting/hole blocking layers are being researched for a possible contact layer. Similarly other materials are under investigation for hole transporting/electron blocking layers. For example, transparent conductive oxides (TCO) are being used as contact layers. Perovskites can be applied to conventional silicon, thus combining the strengths of both material classes: Silicon, in this case, utilizes sunlight in the red and infrared range of the solar spectrum efficiently, while perovskites are good at converting blue light. "If the materials, i.e. perovskites on silicon, are stacked on top of each other, the efficiencies of already commercial silicon cells can be increased considerably. This tandem idea has the potential to herald a solar revolution. Basically the critical parameter to characterizing a solar cell is the energy conversion efficiency. This is typically given by the ratio of the electrical output power density to the optical input power density. The standard optical input spectrum of air mass 1.5 is normally used [41]. The relationship of the efficiency with the physics of solar cell parameters involves short-circuit current density, fill factor, and open-circuit voltage. The short-circuit current density is obtained when the applied voltage is equal to zero, such that J(V = 0) = J_sc, while the open circuit voltage is also obtained when the current is equal to zero, i.e., J(V = V_oc) = 0. The maximum power density output for a typical solar cell is given by the product of V_mp and J_mp i.e., (V_mp × J_mp), where V_mp and J_mp are the voltage and current density at the maximum power point (MPP). These two parameters are derived from the current–voltage characteristic, of a solar cell.

$$\eta =\frac{V_{\mathrm{mp}}\times J_{\mathrm{mp}}}{P_{\text{in}}}=\frac{V_{\mathrm{oc}}\times J_{\mathrm{sc}}\times \mathit{FF}}{P_{\text{in}}}$$E1

Hence the fill factor can be calculated by Eq. (2):

$$\mathrm{FF}=\frac{V_{\mathrm{mp}}\times J_{\mathrm{mp}}}{V_{\mathrm{oc}}\times J_{\mathrm{sc}}}$$E2

An ideal solar cell can be described by Eq. (3):

$$J\left(v\right)=J_{\mathrm{o}}\left\{exp\left(\frac{\mathit{qV}}{K{T}_{\text{cell}}}\right)-1\right\}-J_{\mathrm{sc}}$$E3

From this Eq. (3); q is the elementary charge, V is the applied voltage, k is the Boltzmann constant, T_cell is the solar cell temperature, and J₀ is the saturation current density. The open-circuit voltage of the solar cell can be determined by using Eq. (4):

$$V_{\mathrm{oc}}=\frac{k{T}_{\text{cell}}}{q}ln\left(\frac{J_{\mathrm{sc}}}{J_{\mathrm{o}}}+1\right)\cong \frac{k{T}_{\text{cell}}}{q}ln\left(\frac{J_{\mathrm{sc}}}{J_o}\right)$$E4

Understanding the fundamental limits in the energy conversion process of solar cells and determining a potential upper limit of the energy conversion efficiency is important in developing high-efficiency solar cells [42].

3. Solar cell conversion efficiency limit

The maximum conversion efficiency is the theoretical energy conversion limit of a semi-conductor –based solar cell. In deriving the limit we shall assume that the solar cell is described by a single-junction solar cell, which consists of a semiconductor with a constant bandgap. The light beam with photo energies equal or greater than the bandgap is absorbed, while photons with energies smaller than the bandgap are not absorbed. All the photo generated electron/hole pairs are assumed to be collected at contacts. Therefore the recombination of electron/hole pairs is not considered but only thermalization and absorption losses are taken into consideration. Thermalization losses occur for energies larger than the bandgap while absorption losses occur for photon energies smaller than the bandgap [42]. The absorbed photon flux density of the sun by the solar cell, is given by Eq. (5) [6, 42]:

$$F_{\text{cell}}\left(T=T_{\mathrm{sun}}\right)=\frac{2\pi }{h^3{c}^2}{\int }_{E_g}^{\infty }\frac{E^2\mathrm{d}E}{exp\left(\frac{E}{k{T}_{\mathrm{sun}}}\right)-1}$$E5

where h, c, k, and E_g are Planck’s constant, speed of light, Boltzmann constant, and energy bandgap of the photovoltaic material. The photon flux can be approximated by Eq. (6):

$$F_{\text{cell}}\left(T=T_{\mathrm{sun}}\right)=\frac{2\pi }{h^3{c}^2}{\int }_{E_{\mathrm{g}}}^{\infty }exp\left(-\frac{E}{k{T}_{\mathrm{sun}}}\right)E^2\mathrm{d}E={\int }_{E_{\mathrm{g}}}^{\infty }{\varnothing }_{\mathrm{sun}}\mathrm{d}E$$E6

where ϕ_sun is the blackbody radiation flux of the sun, which is given by Eq. (7):

$${\varnothing }_{\mathrm{sun}}=\frac{2\pi }{h^3{c}^2}\times E^2\times exp\left(\frac{-E}{k{T}_{\mathrm{sun}}}\right)$$E7

The photocurrent density of the solar cell is given by J = q × F_cell (T = T_sun). The electrical output power density of the solar cell is calculated by Eq. (8):

$$P_{\text{out}}=J\times V=q\times F_{\text{cell}}\left(T=T_{\mathrm{sun}}\right)\times \frac{E_{\mathrm{g}}}{q}=F_{\text{cell}}\left(T=T_{\mathrm{sun}}\right)\times E_{\mathrm{g}}$$E8

The input sun power density is given by Eq. (9) [42]:

$$P_{\text{in}}=\frac{2\pi }{h^3{c}^2}{\int }_{E_{\mathrm{g}}}^{\infty }\frac{E^3\mathrm{d}E}{exp\left(\frac{E}{k{T}_{\mathrm{sun}}}\right)-1}\cong \frac{2{\pi }^5{\left(k{T}_{\mathrm{sun}}\right)}^4}{15h^3{c}^2}$$E9

From the above Eqs. (8) and (9), the energy conversion efficiency of a solar cell can be determined by η = P_out/P_in.

Using the blackbody spectrum at T = 6000 K and AM 1.5 global spectrum, the solar cell gives a maximum conversion efficiency of 44% and 49%, respectively, for an ideal bandgap of 1.1 eV as shown in Figure 2.

Figure 2.

Ultimate conversion efficiency and Shockley-Queisser limit of single-junction solar cells as a function of the bandgap. A blackbody spectrum at 6000 K and an AM 1.5G spectrum were used for the calculations.

4. Methodology

Multi-level approach is employed for effective designing of tandem perovskite/silicon solar cell. This approach includes improving the performance of individual layers in each cell; examining the charge transport between each layer when they are stacked, and finally efficient light in-coupling between top and bottom cells (Figure 3).

Figure 3.

a and b: (Left). Schematic of a perovskite-silicon tandem solar cell, together with the absorption spectrum of both perovskite.

4.1 Perovskite (PVSK) film processing

There are several methods in the fabrication of perovskite (PVSK) films employed as active layers in solar cells; e.g. spin coating, dip coating, gas-quenching (GQ), thermal co-evaporation or vapor phase conversion. Of all these methods, spin coating is most preferred, since it can control the production of consistent films with high quality when compared with the other techniques. It is easy to realize PVSK films on a flat substrate using the above methods generally. For textured substrate however the challenge remains, when using solution-processed spin coating. Consequently a combination of sequential co-evaporation and spin-coating was developed as a hybrid two-step deposition method. That forms a conformal PVSK layer on the micro-size pyramids of textured mono-crystalline silicon.

4.1.1 PVSK layer on flat substrates

Under PVSK processed on flat substrate substrates two techniques are discussed; GQ and solution spin-coating. GQ method was developed at the same time with the solvent quenching method. Instead of using anti-solvent, in the GQ method, a flow of nitrogen gas was employed to facilitate the evaporation of precursor solution during one-step coating. By annealing, smooth films with densely packed grains are attained. Recently, many studies related to efficient multi-cation and multi-anion PVSK solar cells have been reported. Such a method has been widely used in fabricating PVSK layer in tandem devices. Adopting the GQ method, in 2020, McGehee and his team reported a 1.67 eV wide-band gap PVSK that consisted of triple-halide alloys of chlorine, bromine, and iodine. In addition the realizing the PVSK, the top cell showed improvement in carrier lifetime and charge-carrier mobility comparing to controlled ones. The reason for improvement was attributed to the enhanced solubility of chlorine by replacing iodine with bromine to shrink the lattice parameter. Of great importance, light-induced phase segregation in PVSK films was significantly suppressed. The conversion efficiency of 27% with an area of 1 cm², for this tandem cell was achieved in the laboratory. The cells had an improved stability with less than 4% degradation after 1000 h of MPP operation at around 60°C.

In the solution spin coating technique, a lot of work has also been done in the fabrication of PVSK films. By using this method, in 2016, a team led by Rech fabricated monolithic tandem cells with 18% conversion efficiency [43]. In 2019, Chen et al. combine two additives, MACl and MAH₂PO₂ in PVSK precursor, which significantly improve the morphology of the wide bandgap (1.74–1.70 eV) PVSK films, resulting in a high tandem V_OC of 1.80 V and improved conversion 25.4%. In 2020, Shin and his team used the solution spin coating method to develop stable PVSK solar cells with a band gap of about 1.7 eV and conversion efficiency of 20.7%. The fabricated solar cells were tested and found to be stable under extreme conditions. Those cells fabricated could retain nearly 80% of their initial efficiency after 1000 h under continuous illumination. In controlling both structural and electrical properties of the PVSK, anion engineering for materials is undertaken i.e. phenethylammonium (PEA)-based 2D additives is found to be important. Under this method high efficiency of 26.7% in a monolithic 2T wide gap PVSK/Si tandem solar cell was realized by combining spectral responses of the top and bottom diodes.

4.1.2 PVSK layer on textured c-Si

Combination of two-step deposition method: Currently, a single-side texturing arrangement of monolithic PVSK/Si tandem devices is predominantly common. Texturing the back side is done to enhance light trapping properties of the solar cell. In comparison with a double-side polished c-Si device, that has their front surface flat-polished in order to be conformable with the solution based PVSK manufacturing process; light trapping property in such an arrangement is not perfect. Consequently, there is a need to build high conversion efficient tandem cells by using double-side textured c-Si approach. This technique has been used by Ballif and his team to come up with a two-step deposition method, where sequential co-evaporation and spin-coating processes are applied. This resulted into conformal PVSK absorber layers on the micrometer-sized pyramids of textured monocrystalline Si. This type of arrangement resulted in a high current density of 19.5 mA cm⁻². The team achieved a conversion efficiency of 25.2% after texturing of the c-Si bottom cell in the micrometer range pyramids. This process reduced the primary reflection loss, enhancing light trapping properties in device.

PVSK on textured c-Si: Solution processed PVSK on textured c-Si has several limitations. These include uncovered Si peaks, shunt paths, and poor charge collection in films with variable thickness, etc. The covering of pyramidal peaks by PVSK of good quality has been reported by a team led by Sargent [44]. The improvement of drift and diffusion of photo-generated carriers in these films enhanced charge collection. In this approach, they used PVSK of wide-bandgap solar cells with a bottom cell of pyramidal-textured c-Si. This approach resulted into improvement of depletion width, in the PVSK and enhancement of carrier collection. In addition to increasing the carrier diffusion length, they used a passivator on the PVSK rough surfaces. And this passivation suppresses the undesired phase segregation. These attributes resulted into PVSK/c-Si cells achieving a conversion efficiency of 25.7% and good thermal stability at 85°C and MPP tracking at 40°C. Blade-coated PVSK on textured silicon with pyramids less than 1 μm in height has been reported by Huang–s group in 2020. Similarly a conformal hole transport layer and perovskite layer that fully covers the textured silicon solar cell were fabricated using nitrogen-assisted blading process. This perovskite/silicon tandem device achieved a conversion efficiency of 26% on textured silicon [45].

In conclusion, several deposition techniques for PVSK film have been advanced and widely researched on. A tandem cell of good quality of polycrystalline films can also be fabricated on both flat and textured substrates. In PVSK/Si tandem solar cells, there are no technical difficulties for flat c-Si. But for textured c-Si, it still appears to be challenging to get conformal films with uniform thickness via sequential co-evaporation and spin-coating methods, in particular for textured monocrystalline Si with large micrometer-sized pyramids. This calls for more investigation and development of more convenient and efficient deposition methods for PVSK films on the textured substrate.

5. Key results

As shown in the Figure 4; scanning electron microscopy (SEM) image of the semi-transparent 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 semi-transparent cell is 18.2 mA/cm², and the integrated current from the filtered silicon cell is 18.7 mA/cm². 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.

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].

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 four-terminal (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.

Figure 5.

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

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 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 sub-cell 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 combination of band gaps of the top cell; and bottom c-Si solar cell known, the two-terminal 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.

Figure 6.

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

6. Discussion

It is observed that perovskites have gained considerable attention as a photovoltaic material [26, 27, 28]. From its inception in 2009, the energy conversion efficiency of single-junction PSC has been increasing to over 22% [34, 35, 36, 37, 38]. It is true that, perovskites are a promising material system for the implementation of tandem or multi-junction 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 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 spin-coating 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.

7. Conclusion

Perovskite silicon tandem solar cell perhaps has a very high potential to reach low level cost of electricity.

To achieve highly efficient and reliable tandem perovskite/silicon solar cells, a multi-level approach is required. This includes improving the performance of individual layers in each cell. Examining the charge transport in each layer when they are stacked, and finally efficient-light in-coupling between top and bottom cells. One of the critical conditions for high device efficiency is the proper choice of the bandgap for the top perovskite cell in the tandem. Optical losses due to parasitic absorption losses resulting from both inefficient intermediate reflecting layers and inefficient absorption in the top cell have to be addressed. Low absorption coefficient of silicon bottom cell reduces the light absorption and reflects in efficiency of tandem solar cell. Stable and efficient light management is necessary for further improvement in the device performance.

The research on replacing highly pure expensive Si with recycled left-over multi-crystalline Si in tandem cells without much compromise in efficiency will upscale low-cost tandem cells. Si heterojunction solar cells still exist as the favorite bottom cell over homo-junction cells. Efficient doping with hydrogen passivates and stabilizes silicon and its heterojunction cells. Theoretically it is possible to achieve higher efficiency from the tandem cells of 31.2% at AM 1.5G and 44% out of space.