1.2 Explanation of the work carried per WP
1.2.1 WP1
The FAPbI3-based low bandgap (~1.54 eV) PSC by two-step evaporation and then spin coating of organohalide solution was successfully fabricated. Not only the performance of the devices improved by optimization of the process parameters, but also the Voc and FF of the devices are among the highest values in all FAPbI3-based perovskites (Fig. 1(d)). The crystalline structure of the films show that the film mainly consists of perovskite phase. Moreover, the preferential growth occurred towards (001) plane, reducing the (011) planes, which is crucial for improving the stability confirmed by TEM analysis (Fig. 1(a)). The morphology and surface roughness of the films show that the domain size of about 1 µm and surface roughness of about 67 nm are formed, which demonstrate the high quality of the samples and high light trapping ability of the surface (Fig. 1(c)). PL of the sample reveals around 1% PLQY, 1.16 eV QFLS, and 0.11 V non-radiative losses in the films, illustrating high quality of the films (Fig.1(b)). The devices exhibit efficiencies of >20% with Jsc of >23 mA/cm2, Voc of >1050 mV, FF of >84%, and MPPT stability of >20% (Fig. 1(e,f)).
1.2.2 Work package 2
The high bandgap perovskites with the composition of CsFAPb(IBrCl)3 were successfully prepared. The XRD pattern as well as TEM analyses show that the samples mainly consist of perovskite phase. The films are also highly oriented along (001) direction, which is beneficial for charge transport and recombination reduction (Fig. 2(a)). SEM micrograph of the film also demonstrate that the film has average domain size of 450 nm (Fig. 2(b)). The films also show high PLQY value of >1% demonstrating QFLS of around 1.23 eV and non-radiative losses of just 0.11 V, revealing high film quality with low recombination centers (Fig. 2 (c)). The efficiency of 19.14% is obtained. The values of FF for these devices reach to >85%, which is one the highest values for perovskite devices (Fig. 2(d)). The MPPT stability of the device is also very high as shown in inset Fig. 2(f).
1.2.3 Work package 3
Tandem devices based on low and high bandgap PSC on SHJ were successfully fabricated. The FAPbI3-based and CsFAPb(IBrCl)3 PSCs with bandgap of 1.5 and 1.8 eV, respectively, were considered for simulation of current matching. The optimum thicknesses for these two PSCs on top of SHJ for achieving current matching is 1100 and 300 nm, respectively (Fig. 3(a)). The simulated EQE for triple junction device also show 14 mA/cm2 for the highest current density in triple junction device, which with considering accumulated Voc of around 3.03 V, and FF of around 82%, simulated efficiency of 34.78% would be obtained (Fig. 3(b)). The SEM micrograph of the PSC/SHJ double junction tandem on textured SHJ bottom cell is also shown in Fig. 3(d). As can be seen, the perovskite layer is fully covered on the textured surface of the bottom cell. The efficiency of this tandem reaches to >24% with the Voc and FF values of 1.8 V and >83%, respectively (Fig. 3(e)). In terms of high bandgap PSC/SHJ tandem, the efficiency of >27% was obtained with Voc and FF of 1.83 V and 77%, respectively (not shown). The MPPT stability of the low bandgap perovskite/SHJ tandem device is also good as shown in the Fig. 3(h). In conclusion, the devices show great efficiency with both low and high bandgap perovskites. The simulation also reveals great potential of triple junction devices. The results of this project pave the way to realize high potential efficiency of the triple junction devices in real life.
