Metal halide perovskite solar cells (PSCs) are a promising competitor to traditional silicon photovoltaics, with their power conversion efficiency (PCE) already exceeding 25%1-3. The lightweight perovskite thin films are especially suitable for flexible devices, which are applicable in novel fields, such as vehicle-integrated photovoltaics, aerial vehicles, and other stretchable electronics applications4-10. Although in a very short time, there is a rapid increase in the PCE of flexible perovskite solar cells (FPSCs) from 2.62% to 21.73%11,12, the performance of FPSCs still lies behind that of their rigid counterparts.
The quality of the crystalline perovskite films and thus the performance of FPSCs strongly depends on the flexible substrate5,6, which influences the nucleation density, crystal orientations, and crystal growth rates of the perovskite film7. A rough surface means more nucleation sites and faster crystal growth rates, which will result in low coverage of defects-rich perovskite films13,14. Another negative factor can be the thermal expansion coefficient mismatch between perovskite and substrates, which will inevitably form lattice strain, deep level defects, and even cracks in perovskite films during the thermal annealing process7,15,16. The most widespread substrates in FPSCs, polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), suffer from high surface roughness and prominent thermal shrinkage. Therefore it is challenging to obtain high-quality perovskite films on flexible substrates17-19.
Device-quality crystalline perovskite films usually have a larger grain size20-22, which is achieved by delaying nucleation and promoting crystal growth according to the LaMer model23. Some strategies have been proven to effectively control the nucleation and crystal growth of device-quality crystalline perovskite films on rigid substrates, including adding additives, such as Cesium (Cs) Bromide (Br-), chloride (Cl-), ionic liquid, thiocyanate (SCN-) anions24-28, solvent coordination complexes29, or formate anions (HCOO-)30,31 and so on. To improve the efficiency of FPSCs, researchers have made great efforts, including optimizing the flexible substrate, the transport layer material, and preparation process8-10,32-34, using lower preparation temperature14,15,35,36, interface passivation37,38 and doping39-41. Up to now, the performance of FPSCs is still not very high.
Flexible substrates are not the only challenging surfaces for the growth of perovskite films. The perovskite/silicon tandem solar cells (TSCs) are promising candidates to surpass the single-junction solar cells efficiency limit42-45. One of the biggest challenges for enabling the high-efficiency of TSCs is the compatibility of perovskite crystal growth with micron-sized pyramidal textured substrates46,47. Currently, the highest PCE of TSCs based on the textured substrate has reached up to 27.4%48, but with rational manipulation of the perovskite crystal growth and passivation of deep level traps, more efficient tandem solar cells can be achieved.
In this work, we introduce Cadmium Acetate (CdAc2) as a perovskite precursor solution additive to manipulate the perovskite crystal growth, achieving excellent optoelectronic properties on the flexible substrate and textured silicon substrate. Using density functional theory (DFT) calculations, we confirm that CdAc2 coordinates with lead iodide (PbI2), forming an intermediate phase and slowing down the perovskite crystal growth rate, which eventually results in a better crystallinity of the final perovskite film. The grain boundaries are also passivated by the crosslinking structure of Ac-, resulting in the excellent thermal durability of the FAPbI3 film. As a result, we demonstrate that with the synergistic effect of Cd ion doping and Ac-ion-assisted crystallization, the intrinsic defect density of perovskite polycrystalline thin film is vastly reduced, and the carrier lifetime is increased to over 6 μs. We achieved PCEs of 24.46% and 22.78% for single-junction PSCs on the rigid and flexible substrates. The latter is the highest reported for flexible PSCs to date. We also applied the CdAc2 in perovskite/silicon tandem solar cells and obtained an efficiency of 29.25% and a certified efficiency of 23.07% with aperture areas of 0.5003 and 11.8792 cm2, respectively. Operational stability showed that un-encapsulated TSCs based on CdAc2 maintain 109.78% of their initial efficiency after continuous one-sun illumination for 300 hours at 45 oC in the N2 atmosphere.
Formation of nano-hole array PbI2 and FAPbI3 films
The FPSCs in this work are based on FAPbI3 and are fabricated by using a two-step solution method. In the two-step sequential deposition, the quality of the final perovskite films strongly depends on the PbI2 films. In general, a compact crystalline PbI2 film tends to form on a flexible substrate. In this work, we developed an effective method to create PbI2 films with arrays of nano-sized pinholes at step one by introducing CdAc2 into PbI2 precursor solution. These pinholes allow for improved diffusion and intercalation of the organic ammonium cations in the second step of the deposition and finally produce perovskites with large and uniform grains.
To gain insights into how CdAc2 may influence the microscopic structure of the perovskites, we carried out several density functional theory (DFT) calculations (details in SI notes 1,2). Our results show that Cd2+ can be incorporated in the PbI2 lattice, substituting Pb2+, and Ac- can bind at the surfaces of PbI2, replacing I-. Because Ac- is a larger and more asymmetric ion, the direct consequence of the latter substitution is an increased distance between the PbI2 layers and weakened binding between each layer by 0.05 eV/formula unit (Fig. 1a). As shown in supplementary Fig. 1, the introduction of Ac leads to a significant decrease in the peak intensity of PbI2. This finding suggests that less densely packed PbI2 films are more likely to form when Ac- is added to PbI2 precursor solution, which may stimulate the formation of pinholes.
The presence of pinholes can potentially facilitate the diffusion and intercalation of FAI in the PbI2 film, where the perovskite is formed, resulting in more uniform perovskite films, as illustrated in Figs. 1b and c. Our calculations show that during the reactions of FAI with Ac--doped PbI2 films the Ac- is readily replaced by I- to produce a clean PbI2 surface. At the next stage, the perovskite formation, the role of the Cd becomes important, as the transformation from a face-sharing structure in PbI2 to a corner-sharing structure in perovskite requires the breaking of some of the metal-iodine bonds in the MI2 layer and the formation of new metal-iodine bonds for the 3D perovskite. The Cd incorporated in the PbI2 lattice structure (in the first step) has direct consequences for the kinetics of the overall transformation. In the Cd-doped PbI2 structure, creating an I vacancy near a Cd site (to facilitate ion transport) costs 0.02 eV/formula unit more energy than near a Pb site, whereas in the Cd-doped perovskite structure, it costs 0.03 eV/formula unit less energy. This indicates that the incorporation of Cd in the lattice increases the activation barriers for the reaction of converting the layered structure to a 3D perovskite structure. This can lead to a slower crystallization process and result in larger and more uniform perovskites grains.
To further elucidate how CdAc2 affects the crystallization of the perovskite films, grazing incidence X-ray diffraction (GIXRD) with various X-ray incident angles was employed, as shown in Figs. 1b and c. From the GIXRD, it is be found that the crystal orientation is significantly different at the surface and bottom of the perovskite film with CdAc2. It is clearly shown that the perovskite film with CdAc exhibits a prominent (111) orientation at the bottom, relative to other crystal planes’ peaks, which indicates that the (111) crystals grew faster than differently oriented crystals with the presence of CdAc2. The (111) plane orientation is parallel to the substrate and favorable perovskite lattices49. Keith P. McKenna50 found the (111) plane is extremely stable in pure formamidinium lead iodide and presents relatively small barriers (< 100 meV) to transport carriers51. Furthermore, the XRD patterns in supplementary Figs. 2 and 3 show that the (111) orientation peak of FAPbI3 was enhanced with CdAc2 on both rigid and flexible substrates. The prominent (111) orientation with lower trap density enables efficient PSCs on both rigid and flexible substrates as will be demonstrated in the following.
Enhanced optical and electrical properties of FAPbI3 films
To gain insight into the properties of our perovskite films, we carried out confocal photoluminescence (PL) mapping measurements to investigate the morphologies and optoelectronic properties of the perovskite films with and without CdAc2 (Fig. 2a). We observed an enhanced photoluminescence performance with better uniformity and stronger intensity for the perovskite with CdAc2. As shown in Fig. 2b, the PL peak wavelength distribution of the CdAc2 treated perovskite film is obviously narrower than the reference one, indicating a more uniform film. Moreover, the CdAc2-treated perovskite film exhibits higher PL mapping intensity than the control film, showing better optical properties (Fig. 2c). This can also be confirmed by time-resolved photoluminescence (TRPL) measurements (Fig. 2d), in which the CdAc2-treated perovskite film exhibits a much longer carrier lifetime (6514 ns) than the control film (Supplementary Table S1), underlining the benefit of CdAc2 on the perovskite film by suppressed non-radiative recombination. To further investigate the charge transfer properties of perovskite films, steady-state PL and absorption spectra were also conducted. As shown in Fig. 2e, the emission intensity of the perovskite film is enhanced dramatically with the incorporation of CdAc2, indicating a decrease of non-radiative recombination, consistent with PL mapping and TRPL results. The ultraviolet-visible (UV-Vis) absorption spectra are slightly enhanced. We attribute this to the larger grain size and the decreased defect density.
The low defect density can be explained by two factors: (i) the better intercalation of FAI in the pinhole-rich PbI2 films in the first step and slower crystallization (effect of Cd-) in the second step have overall led to large and high-quality perovskites and (ii) the Ac- species on the grain boundaries passivate the undercoordinated Pb2+, preventing further degradation, i.e. the formation and diffusion of I vacancy defects. Such passivation effect is verified by additional DFT calculations, where the relative binding strength of Ac- (compared to I-) to Pb is calculated by moving the target anion on the PbI2-terminated surface away at a distance of ~4 Å (Supplementary Fig. 4). The energies show that Ac- has high binding energy equally to recently reported HCOO-2, and is about 1 eV higher than that of I- (Supplementary Fig. 5).
To further investigate the influence of the CdAc2 on the properties of perovskite film, we carried out the X-ray Photoelectron Spectroscopy (XPS). In the Pb 4f spectra of both perovskite films (Fig. 2f), we can observe two additional peaks (located at 136.4 and 141.3 eV) for the control perovskite film, which correspond to metallic lead and originate from the unintended losses of iodine, leading to deep-level defects and non-radiative recombination. Fortunately, it seems that the formation of metallic Pb can be suppressed by the addition of CdAc2. Besides, it can be estimated from Fig. 2f and supplementary Fig. 6 that the ratio of Pb/I is 1:2.26 and 1:2.63 in the perovskite film without and with CdAc2 doping, respectively, indicating that the iodine vacancies on the surface of the perovskite film are reduced with the addition of CdAc2. The metallic lead is widely perceived as contributing to deep-level defects and result in non-radiative recombination, which also can be found in the control perovskite film from supplementary Fig. 7. The presence of CdAc2 succeeded in preventing the formation of metallic Pb. We also confirm the effect of CdAc2 on PbI2 by 13C nuclear magnetic resonance (NMR) spectra (Fig. 2g). With the addition of CdAc2, the peak of C=O moved from 167.2 ppm to 167.6 ppm, which indicates a strong interaction between CdAc2 (C=O group) and Pb2+. Fig. 2h presents the TGA spectra of the PbI2 solution and the different adducts. The dimethyl sulfoxide (DMSO) adduct of PbI2/DMF decomposes rapidly at 80 °C, while PbI2/DMF-DMSO-CdAc2 decomposes at 139°C. It indicates that the PbI2/DMF-DMSO-CdAc2 is more stable than the PbI2/DMF-DMSO due to the strong molecular interaction between CdAc2 and PbI2.
Photovoltaic performance and stability of flexible perovskite solar cells
Based on the above results, we conducted a series of experiments fabricating rigid and flexible perovskite solar cells with the structure of ITO/SnO2/Perovskite/Spiro-OMeTAD/Au (Fig. 3a). The FAPbI3 active layer was deposited by a two-step method, with CdAc2 added to the precursor solution. The cross-sectional SEM image shows that the thickness of the perovskite absorber layer is about 750-nm with grain throughout the film, indicating high crystallinity (Figs. 3b and c). The element distribution in FPSCs (Supplementary Figs. 8 and 9), indicates that Cd is uniformly distributed in the perovskite film. Combined with the benefits of effectively decreasing the trap density with the addition of CdAc2 as discussed in the previous section, with the good transmittance of the flexible PET substrate used in this work (Supplementary Fig. 10), we achieved the champion current density voltage (J-V) curves of FPSCs. As shown in Fig. 3d, the control FPSCs exhibited a PCE of 19.86% with JSC of 24.32 mA/cm2, VOC of 1.096 V, and fill factor (FF) of 74.48% under standard AM1.5 solar illumination. The CdAc2-treated FPSCs reached the champion PCE of 22.78%, along with a JSC of 24.63 mA/cm2, VOC of 1.167 V, and an FF of 79.27%, which is the highest PCE reported for flexible PSCs to date (Supplementary Fig. 3e and Table 2). More details for the statistical photovoltaic performance of FPSCs without passivation are shown in supplementary Fig. 11. Moreover, a steady PCE output of 22.7% was demonstrated by maximum power point (MPP), as shown in supplementary Fig. 12. The corresponding external quantum efficiencies (EQE) of the devices are exhibited in Fig. 3f. The CdAc2-treated device shows an integrated JSC of 24.75 mA/cm2, slightly higher than the control device, matching J-V curves. The bandgap of the perovskite is 1.53 eV and remains constant regardless of the presence of CdAc2, as shown in supplementary Fig. 13. A champion PCE of 24.46% was achieved on the rigid substrate by adding CdAc2, with a JSC of 25.75 mA/cm2, a VOC of 1.169 V, and a fill factor (FF) of 81.24% (Supplementary Fig. 14).
To provide more solid evidence on the charge recombination process, we measured the dependence of the VOC on the light intensity for the control and CdAc2-treated FPSCs. The slope deviation of VOC versus the natural logarithm of light intensity from kBT/q indicates the occurrence of non-radiative trap assisted recombination (monomolecular recombination), which has been reported to be a dominant recombination pathway in PSCs. As shown in Fig. 3g, the control FPSCs exhibited a slope of 1.91 KBT/e, while the CdAc2-treated FPSCs showed a much smaller slope of 1.32 KBT/e. This indicates that defect-assisted non-radiative recombination was effectively suppressed by using CdAc2, which is in accordance with the enhanced VOC and FF. The lower leakage dark current (Supplementary Fig. 15) also improved charge transport and decreased recombination loss in the CdAc2-treated FPSCs. To quantify the trap density in perovskite films, we performed space-charge-limited-current (SCLC) of corresponding perovskite films (Fig. 3h). As a result, the perovskite film with CdAc2 has a smaller VTFL than that of the control film, indicating that films with CdAc2 can suppress the formation of defects.
To evaluate the carrier transport characteristics and output voltage in FPSCs, capacitance-voltage (C-V) measurements were further performed, and the relevant Mott-Schottky curves were presented in Fig. 3i. The built-in potential (1.166 V) of the CdAc2-treated FPSCs is extracted to be much larger than that of the control device (1.054 V), which means increased separation of photo-generated carriers and enhanced output voltage of FPSCs. A larger recombination resistance is also beneficial to suppress non-radiative recombination (Supplementary Fig. 16).
Although tremendous progress has been made, long-term stability is still the critical hurdle to commercialization, especially the mechanical performance of FPSCs for their stretchable applications. On that account, we primarily investigated the mechanical stability of un-encapsulated FPSCs with and without CdAc2 in this study. As depicted in supplementary Fig. 17, after 10000 cycles bend test with a bending radium 3 mm, the performance of FPSCs can still maintain about 80% of the initial value. However, the PCE for the control FPSCs decreased significantly, only providing about 11% of the original PCE after the 1800 cycles bend test. Furthermore, we also conducted cold and hot cycle tests. After 24 cold and hot cycles, the CdAc2 treated FPSCs provide 87.28% of their initial efficiency, while the control device only maintains 21.59% of its initial efficiency after 13 cold and hot cycles. Based on these results, both bending stability and thermal stability were enhanced by the beneficial effect of the CdAc2.
Efficient monolithic perovskite/silicon tandem solar cells based on a textured substrate
To further highlight the positive effect of this strategy for perovskite crystallization improvement, we fabricated perovskite/silicon TSCs based on a textured substrate, where the crystal growth would be more complicated than the polished one. The perovskite used here was Cs0.22(FA0.76MA0.02)0.78Pb(I0.85Br0.15)3 with a wide bandgap (1.68-eV), which is deposited by one-step method with the addition of CdAc2. We noticed the quality of wide-bandgap perovskite with CdAc2 has also been significantly improved from SEM, XRD, and TRPL in supplementary Figs. 18-20. The J-V characteristics and EQE of wide bandgap single-junction PSCs without and with CdAc2 treatment are shown in supplementary Fig. 21. It is worth mentioning that the pin structure single-junction PSCs shows a high voltage of 1.24 V. The schematic structure of the textured TSCs is shown in Fig. 4a (ITO/HTL/perovskite/C60/SnO2/IZO/Ag/PDMS), among which ITO (20 nm) served as a recombination layer. The polydimethylsiloxane (PDMS) as light management antireflective foils with excellent optical properties was used in this structure52. Figs. 4b and c show the SEM top-view images of the perovskite films based on textured silicon with and without CdAc2, both of them offer a compact film and full coverage on the textured silicon wafer (Fig. 4d). But the CdAc2 treated perovskite film exhibits a much larger grain size. Fig. 4e shows the J-V characteristic of the champion perovskite/silicon TSCs with 29.25%, which is the highest efficiency reported for perovskite/silicon TSCs based on textured silicon by spin-coating method (Supplementary Table 3). The statistics of the PCE of TSCs without and with CdAc2 demonstrated that the CdAc2-based TSCs are more reproducible and have a higher Voc and FF (Supplementary Fig. 22). In Fig. 4f, the EQE of the tandem device is presented. The extracted current density of the top and bottom cells are 20.05 mA/cm2 and 19.85 mA/cm2, respectively, which match well with J-V curve. In addition, a large-area perovskite/silicon TSC of the same design with an active area of 11.8792 cm2 was successfully fabricated. A PCE of 23.23% was achieved with the CdAc2 device (Certified with a PCE of 23.07% in supplementary Fig. 23 and Table 4), whereas that of the control device was 19.05% in Fig. 4g. We also examined the long-term operational stability of un-encapsulated TSCs with CdAc2, which was evaluated under one sun continuous irradiation at maximum power point (MPP) tracking in the N2 atmosphere. As shown in Fig. 4h, the CdAc2 treated TSCs maintain 109.78% of the initial efficiency after 300 h. We ascribe the high stability of the TSCs to two main reasons: i) the substitution of Cd indeed slow down the crystallization rate and suppresses the formation of I vacancy and phase segregation53. ii) Moreover, as discussed earlier that our DFT calculations show that the Ac anions coordinate strongly with Pb2+ and passivates I vacancy on the grain boundaries of the perovskite films. This evident coordination may also help slow down the perovskite growth process, thus resulting in a larger grain size of the perovskite film with much less trap density in TSCs.