The realization of the perovskite/perovskite tandem solar cell (PPTSC) is an alternative approach to achieve a high PCE with some advantages, such as simple bandgap tuning of perovskite absorbers, cost-effective and low-temperature monolithic production processes for both the sub-cells, and use of inexpensive precursor materials on flexible substrates [66,97,125,126].

Interestingly, a detailed balance theory has demonstrated that the PCE of TSC may go beyond 45% using optimum material bandgaps (~1.1 eV and ~1.73 eV) [10,127,128], allowing the prediction of a great advantage in using a monolithic perovskite-based design, as the bandgap of these materials is easily tunable. A key challenge is to save the quality of the lower cell while processing the upper cell. As for SHJ and CIGS, perovskite bottom cells also require processing temperatures below 200 °C, and therefore all contact layers need to be processed at low temperatures. This has been realized in both n–i–p [126,129,130,131] and p–i–n [125,132,133,134,135,136] sub-cell architectures, the latter one being more frequently used because it allows a better efficiency to be obtained. The first example of PPTSC used two analogue sub-cells with similar absorbing materials: the MA lead iodide (MAPbI₃) and the MAPbBr₃. They were connected with a 2 μm thick doped HTL by a lamination procedure [137]. Despite the demonstration of a V_OC as high as 2.2 V (that corresponds to the sum of the two sub-cell V_OC), the PCE was limited due to the spectral properties of the absorbing layers, which in turn greatly reduced the overall attainable current density. The PCE was further reduced by the low conductivity of the HTL layer due to its thickness. The HTL transport properties have been improved by Li-TFSI additives that act as hole conductors such as quasi-solid electrolytes because the additives provide Li/Li⁺ redox shuttle [138]. Selecting proper materials with complementary bandgaps, Lin et al. [132] built a maximum PCE of 24.8% by monolithically fabricating all-perovskite PPTSC. They used 1.22 eV and 1.77 eV bandgap perovskites as bottom and top sub-cells, respectively, demonstrating the perovskite flexibility in terms of bandgap [132]. A schematic cross-section of the proposed planar PPTSC is shown in Figure 1. The front contact ETL consists of 50 nm Al-doped ZnO (AZO) followed by a perovskite absorber layer. A double layer of NiO/AZO is utilized as an HTL, where a very thin NiO is used with a 50 nm thick AZO film. Furthermore, the combined NiO/AZO layer works as a tunneling junction, facilitating efficient charge carrier transportation [139,140,141,142]. Finally, a 100 nm Al is used as a back reflector. Concerning the absorbing layer, the top cell consists of a wide bandgap (Eg~1.72 eV) Cs_yFA_1−yPb(I_xBr_1−x)₃ perovskite with a composition ratio of 0.1, while a low bandgap (Eg~1.16 eV) MASn_xPb_1−xI₃ perovskite with a composition factor of 0.85 is utilized as a bottom cell. As the overall efficiency strongly depends on the electrodes’ transport qualities, some effort has been devoted to improving the ETL layer also. This latter usually consists of a mesoporous–titanium dioxide thin film, which, however, can allow non-radiative recombination at the ETL perovskite interface, worsening the performance. It has been demonstrated that a thin layer of polyacrylic-acid-stabilized tin (IV) oxide quantum dots (paa-QD-SnO₂) on the compact titanium dioxide enhances light capture and widely suppresses nonradiative recombination. Using paa-QD-SnO₂ as the electron-selective contact enabled PSCs (0.08 cm²) with a PCE of 25.7% (certified 25.4%) and allowed an increase of the PSC areas. Namely, areas of 1.20 and 64 cm² were obtained with PCEs of 23.3, 21.7 and 20.6%, respectively [30].

Another possible strategy proposed to improve the ETL transport properties consists of introducing iodine-doped g-C₃N₄ (ICN) as an additive via a glass-assisted annealing route into ZnTiO₃. The ETL thus obtained has a better crystalline quality and a more adequate alignment of energy levels, and, when used, results in a better PCE [143]. Some authors have focused their attention on the carrier diffusion length of the perovskite layer that should be many times the absorber thickness to guarantee good PV performance and sufficient charge transport in PSCs [144,145]. Unfortunately, diffusion length is mainly limited by grain surface traps, the number of which is very large compared with the number of traps distributed within the grain [146,147,148,149].

Hence, grain surface passivation has been proposed as a promising route to improve carrier diffusion and PCE [150,151,152], as this strategy overcomes the limits due to the diffusion length, allowing a thicker absorption layer (>1 μm) that, in turn, improves the photons catching. 4-Trifluoromethyl-phenylammonium (CF3-PA) has been used as a strong perovskite surface passivator capable of increasing the diffusion length by up to 5 μm and skyrocketing the PCE to over 26%.CF3-PA passivated tandem devices maintained 90% of their initial PCE after 600 h of operation at the maximum power point under 1 sun illumination in ambient conditions, exhibiting better performance when compared with unpassivated devices [153]. Another well-known problem of PPTSC concerns the low V_OC, since Br concentrations higher than 20% lead to an increase in the density of traps. In addition, photoinduced halide segregation effects and poor energy alignment with the charge transport layers are possible [154,155]. Finally, the interface with the charge transport layers can give rise to localized states within the perovskite bandgap, which, acting as recombination centers, reduce the quantum yield [156,157,158]. The origin of this phenomenon has been attributed to several causes including band misalignment [158], energy-level pinning [154], and halide migration from the perovskite into the transport layer [159]. A typical example of the above issue is the case high performance PPTSC using fullerene ETL that is considered one of the worst offenders regarding the induction of trap states.

Surface passivation using organic phenethylammonium iodide (PEAI) halide salt [160] or butylammonium iodide (BA) [161] and 1,3-propane-diammonium iodide [162] is a possible strategy proposed to improve PCE by reducing trap states. Recently, bifacial solar cells have attracted the attention of researchers as they can achieve a higher PCE since they can absorb reflected and scattered light (albedo) that achieves the cell’s back-surface (Figure 2). The main difference consists of using or not using a reflecting layer as the bottom electrode. The possibility of making semitransparent PPTSC represents a great opportunity for the development and application of high-efficiency bifacial cells. In fact, the tandem cells, which can be manufactured using the thin film technology, are excellent candidates because they simultaneously offer a high PCE and a structure that easily allows the lower cell to integrate transparent bottom electrodes, making the cell active also towards the light that comes from below. According to numerical simulation, the benefits of tandem architecture and bifacial design can be inherited by bifacial tandem cells, allowing for higher thermodynamic efficiency than monofacial tandems and single-junction designs [163,164].

In 2022, Li and co-workers [165] developed the first bifacial all-perovskite tandem by using TCO as a rear electrode, starting from an already well known monofacial configuration. The flexibility in designing the perovskite bandgap allowed optimization of the PCE under different realistic illumination and albedo conditions, a PCE > 25% under AM1.5G 1 sun illumination to be obtained. Moreover, in field tests, it is possible to gain a power density as high as 26 mW·cm⁻² [166]. The cell architecture also needs to be investigated; indeed, the thickness of an absorber in a 2T bifacial tandem configuration strongly depends upon the magnitude of albedo due to the current-matching constraint between the sub-cells, while the 4T configuration can work better for a wide range of albedo (Figure 3) [167].