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Distinction of different alcohols as solvents

The perovskite films were fabricated by a two-step sequential deposition method based on previous work15,23. As depicted in Fig.1a, our process combines co-evaporation and blade-coating techniques to meet the requirements for large-area fabrication of the perovskite films. Supplementary Fig.2 shows the deposition of an inorganic framework on both glass and textured silicon substrates. It is worth noting that the second step was implemented in air to match the realistic production environment. However, ethanol and isopropyl alcohol, which are widely used as solvents of the organic salt in the second step, confront two major challenges in the natural environment: firstly, these solvents readily absorb environmental moisture38; secondly, the rapid evaporation rate of the solution will affect the film uniformity. Consequently, these challenges often result in inhomogeneous and poor perovskite films, adversely affecting the PCE and stability of the devices.

a Schematic of the hybrid two-step deposition method. b Physical parameters of different alcohols. c Images of organic salts used different alcohols and after exposed to air for 1h. d Images of perovskite films after blade-coating organic salts without gas-quenching and annealing. The direction of blade-coating is from left to right.

To address this issue, we carried out analysis and study on various alcohols with different saturated vapor pressures and polarities including ethyl alcohol (EA), isopropanol (IPA), n-butanol (nBA) and n-pentanol (nPA). The images of different solutions after adding organic salts to the alcohol are shown in Supplementary Fig.3. For ease of expression, we refer to the following solutions, films and devices fabricated with ethanol as EA solution, EA film and EA device, as well as for IPA, nBA and nPA. As the carbon chain is lengthened, both the polarity of alcohols and the saturated vapor pressure decrease, as illustrated in Fig. 1b41. The saturated vapor pressure reflects the evaporation speed of the solvent, while the dielectric constant is positively related to the polarity of the solvent. Following the principle that like dissolves like42, the mutual solubility of alcohols and waterand thus their capacity to absorb moistureis dictated by their polarity difference. Given waters high polarity, alcohols with greater polarity are more soluble in water, leading to increased water absorption.

To investigate the impact of moisture on these different alcohol solutions in air, we exposed a measured amount of each solution to open air and observed the changes. In air environment, moisture absorption leads to rapid oxidation of I to I2, manifesting as a yellowing of the solution43,44. As shown in Fig.1c, the EA and IPA solutions turned from colorless to light yellow after one hour of exposure, while nBA and nPA solutions exhibited no significant color change, underscoring the protective effect of low polarity solvents against moisture interference. Furthermore, we compared the films after blade-coating without gas quenching and annealing on the same substrate (glass/inorganic frame) and documented the changes photographically. Figure1d illustrates that EA and IPA volatilize fastly and completely after blade-coating, in contrast to nBA and nPA films, which shows a gradual darkening. This shift signifies a decrease in volatilization rate with increasing carbon chain length, affecting perovskite crystallization dynamics. However, the slower volatilization rates results in the lingering of residual organic salts, which continues to undergo dissolution-recrystallization reactions with the perovskite45. This leads to localized accumulations of organic salts, as evidenced in Supplementary Figs.4 and 5.

Supplementary Fig.6 displays images of perovskite films fabricated using different alcohols, both in N2 and air environments. These images corroborate the notion that moisture positively affects the crystallization rate of perovskite films46, as inferred from the observable color changes. To further evaluate the effect of the volatilization rate of the solutions on the perovskite films formation, we compared the morphology and the structure of perovskite films by using a scanning electron microscope (SEM) and X-ray diffraction (XRD). Supplementary Fig.7 reveals a pronounced PbI2 signal in EA films before annealing, leading to a substantial amount of PbI2 at the bottom of the perovskite layer (Fig.2a and e). This indicates that the conversion from the inorganic framework to perovskite is incomplete. Such findings suggest that the delay of solvent volatilization rate is conducive to prolonging the reaction of inorganic frameworks with organic salt solutions in terms of promoting the transformation of the inorganic framework into perovskite.

ad Top-view and cross-sectional SEM images. e XRD patterns of perovskite films after annealing. f PL spectra of perovskite films with the emission from the glass side. g Time-resolved PL transients of perovskite films. For TRPL, double exponentials were used for fitting the curves. hj PL mapping of perovskite films with the active area for 1.5cm1.5cm.

Comparatively, perovskite films fabricated in air environment exhibit a heightened PbI2 signal (Supplementary Fig.8 and Fig.2e), demonstrating that the moisture absorbed during fabrication prompts the decomposition of perovskite films upon air annealing. Specifically, the IPA films show a strong PbI2 diffraction peak located at 12.6 (Fig.2e), which stemmed from the decomposition of perovskite films after air annealing at 35% relative humiditya finding consistent with SEM image of the IPA films (Fig.2b). Impressively, the nBA films exhibited the lowest intensity of PbI2 peak in Fig.2e, with minimal residual PbI2 particles observed on the surface (Fig.2c), indicating negligible perovskite decomposition. However, a strong PbI2 signal was found in the nPA films with the solvent volatilization rate further slowed down (Fig.2e), which was attributed to the destruction of perovskite structure by residual solution (Fig.2d). Despite the low polarity of the nPA, the reduction in solution evaporation rate inadvertently introduces excessive H2O into the perovskite films, exacerbating degradation during annealing34. The UV-vis spectra and Tauc-plots of perovskite films fabricated using various alcohols are shown in Supplementary Figs.9 and 10. Additionally, the UV-vis spectra of the inorganic framework are detailed in Supplementary Fig.9. These results elucidate both the polarity and evaporation rate of the solvent have a joint effect on H2O absorption levels. In this view, nBA emerges as the optimal solvent for our specific requirements.

To discern the impact of different alcohol solvents on the defect density of perovskite layers, we performed steady-state photoluminescence (PL) measurements on samples with the configuration of glass/perovskite. As shown in Fig.2f, for EA films, the PL emission peak of the glass side exhibited a blue shift by several nanometers relative to the others. This shift indicates a residual amount of PbI2 at the bottom of the perovskite, owing to the incomplete conversion of PbI2. Notably, the nBA films exhibited the highest PL intensity, surpassing both IPA and nPA films. This enhancement is attributed to the enlarged grain size and effective elimination of PbI2, which in turn reduces the density of grain boundaries and suppresses the non-radiative recombination. In addition, the time-resolved photoluminescence (TRPL) measurements further supported these findings, with the lifetime of each sample recorded at 136.3, 146.6, 350.7 and 142.9ns, respectively (Fig.2g). These results highlight the superior performance of nBA in minimizing non-radiative recombination within the perovskite bulk.

We then performed PL mapping test to investigate the homogeneity of the films, as shown in Fig.2hj. Given the significant amount of PbI2 in EA filmswhich will notably passivate the defects and enhance the PL signal strength (as detailed in Supplementary Fig.11)EA films were excluded from this part of the analysis. The nBA and nPA films demonstrated superior uniformity compared to the IPA films, a trait ascribed to their lower saturated vapor pressure. This characteristic, combined with the solvents extended chain length, leads to slower volatilization, while reduced polarity further restricts water ingress into the films. Both factors contribute to a diminished crystallization rate of perovskite, yielding films with enhanced homogeneity47. However, the slow volatilization rate of the solvent allows the residual solution to continue interacting with the perovskite through dissolution-crystallization reactions. This process tends to produce a non-optically active -phase and creates voids within the bulk48, culminating in a diminished PL mapping signal in nPA films. Overall, the nBA films demonstrated less non-radiative recombination and superior uniformity, making them conducive to the scale-up fabrication of perovskite films.

We fabricated the single-junction perovskite solar cells with an architecture of Glass/ITO/NiO/SAM/1.68 eV-perovskite/C60/SnOx/Cu. The schematic structure is shown in Fig.3a while the detailed photovoltaic parameters of the devices with an active area of 0.049cm2 applying EA, IPA, nBA and nPA are summarized in Supplementary Table3 and Fig.3b. For further comparison, we constructed devices under two distinct conditions: an N2 environment and ambient air, with their respective photovoltaic parameters detailed in Supplementary Fig.12. Devices fabricated in air exhibit smaller VOC compared to those fabricated in N2 glove box, which can be attributed to moisture-induced films deterioration. More notably, air-fabricated devices generally suffered from pronounced efficiency losses, except for those using nBA solvent. This exception highlights nBAs resilience to air exposure during fabrication, with such devices achieving the highest conversion efficiency. In our champion devices, nBA devices displayed distinct advantages in VOC, JSC, and FF with a narrower distribution proving its higher repeatability, as shown in Fig.3b. According to Fig.3c and Supplementary Table4, the improvement of nBA devices in VOC and JSC compared with IPA groups was attributed to the lower non-radiative recombination loss and parasitic absorption caused by PbI2 in the surface and bulk, which was also beneficial to the cells' light stability (as shown in Supplementary Fig.13). The integrated JSC value from the external quantum efficiency (EQE) curve in Fig.3d was calculated to be 20.81 and 20.99mAcm2, respectively, corresponding well with the values obtained from JV measurements. Compared with the IPA devices, the nBA displayed improved charge collection, particularly between 400 and 600nm, due to the larger grain sizes minimizing recombination49. In order to prove the influence of uniformity on the performance of large-area devices, we compared the JV curves of devices with a 1.044cm2 aperture area fabricated by IPA and nBA (Fig.3e and Supplementary Figs.16 and 17), and the specific data are shown in Supplementary Table5. The nBA devices outperformed the IPA counterparts in terms of FF and JSC, attributed to superior uniformity. Additionally, from the EQE spectra of eight cells with a small area of 0.049cm2 (Supplementary Figs.14 and 15), we observed that the nBA devices exhibited a much narrower distribution of the corresponding integrated current. Furthermore, we fabricated PSCs with an area of 1.044cm2, producing 15 devices per type. The histogram of their PCE was displayed in the inset of Fig.3e. Moreover, we compared the photovoltaic parameters of devices fabricated by IPA and nBA in different humidity, and the XRD of films were shown as well (Supplementary Figs.18 and 19), which proved that nBA hinders the effect of moisture during the fabrication of devices.

a Schematic architecture of single junction. b Photovoltaic parameters for IPA and nBA devices. c JV curves of the champion opaque devices (0.049 cm2 aperture area). d EQE spectra of the champion device. e JV curves of the champion opaque devices (1.044cm2 aperture area); PCE distributions of 15 devices for each sample are shown inset. f QFLS values extracted from the PL spectra for neat perovskite, HTL/perovskite and HTL/perovskite/ETL. g EL spectra for IPA and nBA perovskite devices. h VOC evolution as a function of light intensities for the IPA and nBA perovskite devices.

We then carried out photoluminescence quantum yield (PLQY) measurements to quantify the quasi-Fermi level splitting (QFLS) in the neat perovskite layers and the stacks by different layers (Fig.3f)50,51,52. The implied VOC values estimated from the PLQY measurements were in good agreement with the values obtained from the JV results. The above results suggested that replacing IPA with nBA could promote the conversion of PbI2, thereby synergistically mitigating the non-radiative recombination losses both in the bulk and in the interface between hole-transport-layer (HTL) and perovskite. The VOC, indicative of the recombination rate within devices, was assessed through the EQE at short circuit current conditions, effectively modeling the device as a light-emitting diode36. Furthermore, under the injection current of 21mAcm2 (equal to short circuit current Jph), the electroluminescence (EL) efficiencies of IPA and nBA devices were 0.1% and 0.4% (Fig.3g), corresponding to the VOC loss of 0.180 and 0.144V, respectively. This result is almost consistent with the JV results, that is, the IPA and nBA devices showed a VOC of around 1.20V and 1.22V. To further study the carrier recombination behavior, we investigated the dependence of the VOC on the light intensity53, as shown in Fig.3h. The semilogarithmic relationship displayed follows the expression with a slope = nkT/q log10e, where n is the diode quality factor. The IPA and nBA devices exhibited n values of 1.633 and 1.481, respectively, indicating reduced trap-assisted recombination in the nBA device.

Considering the need for thicker perovskite layers when fabricating on textured silicon, relevant characterizations for perovskite films on both glass and textured silicon substrates were conducted, as shown in Supplementary Figs.2026. However, the limited solvent penetration depth of IPA led to a significant amount of unreacted PbI2 in the underlying layer and further degraded the performance of the devices. While complete conversion of the inorganic framework to perovskite is achievable through adjustments in parameters like quenching gas pressure and blade-coating rate54,55, such modifications can detract from film uniformity and device performance, as evidenced in Supplementary Figs.2730. Consequently, parameter tuning was not utilized to fully convert IPA films in tandem devices.

Specifically, the illustration of the tandem device is demonstrated in Fig.4a and a broader area showing the top-view as well as cross-sectional SEM images of the bottom SHJ is seen in Supplementary Fig.31. The performance of SHJ cell with and without semitransparent perovskite as a filter is shown in Supplementary Fig.32 and Supplementary Table6. It can be clearly seen from Fig.4b that the textured surface with pyramid sizes of 23m was well-covered by the conformally coated perovskite films as well as other functional layers. The corresponding device performance is depicted in Fig.4c and d; a tandem solar cell with an active area of 1.044cm2 achieved a champion PCE of 29.4% (VOC=1.83V, JSC=20.45mAcm2 and FF=78.63%) under reverse scan and the stabilized PCE was observed to be 28.8%. Moreover, an independently certified efficiency of 28.7% was tested from Fraunhofer ISE (shown in Supplementary Fig.33).

a Schematic diagram of perovskite/SHJ tandem solar cell. b Cross-sectional SEM images of perovskite/SHJ (average pyramid size is 23m) tandem for nBA devices. c JV curves of the tandem device (1.044cm2 aperture area); the digital photo of a device is shown in the inset. d MPP tracking of the tandems; PCE distributions of 16 individual tandem devices for each type is shown in the inset. e EQE spectra of a current-matched fully textured monolithic perovskite/SHJ tandem cell. f JV curves of the tandem device (16 cm2 aperture area); the digital photo of a device is shown in the inset.

As shown in Fig.4d, the integrated JSC value of the front and back subcell from EQE spectra (Fig.4e) was 20.62 and 20.51mAcm2, respectively, which was in good agreement with the JSC value determined from the JV measurements considering the loss caused by Ag grid. We further evaluated the operation stability of encapsulated tandem solar cells by measuring the maximum power output under 1-sun-equivalent illumination in ambient air with a relative humidity of 3050%. The encapsulated device retained 96.8% of its initial PCE after 780h of maximum power point (MPP) tracking (Supplementary Fig.34).

To validate the applicability of our approach for scalable fabrication, we applied blade-coating to produce perovskite films on a 36cm glass substrate. Subsequent steady PL and XRD tests conducted on samples from different regions of perovskite films (Supplementary Figs.35 and 36) demonstrated superior uniformity in nBA films compared to IPA films. Furthermore, we fabricated 36 cm2 perovskite/silicon tandem cells (aperture area, 16 cm2) and achieved a conversion efficiency of 26.3% (VOC=1.815V, JSC=18.54mAcm2, FF=78.31%), which is among the highest PCE of large-area perovskite/silicon tandem cells11. The consistency of the EQE spectra at different regions suggested that the film exhibited excellent uniformity (Supplementary Fig.37).

For the further development of perovskite/silicon tandem solar cells, scaling up the size of the perovskite films to M6 (166mm*166mm) becomes essential, a goal that proves challenging with blade-coating due to issues with film uniformity. Therefore, slot-die coating, an expandable technology that allows continuous liquid injection, emerges as the preferable future method49,56. Critical to this method is the complete conversion of the inorganic framework into perovskite films, achievable through careful adjustment of precursor solution concentration, the injection rate of the precursor solution, the rate of slot-die, gap distances between the blade and substrate, and quenching gas pressure. Digital photographs of the perovskite films fabricated under these conditions are depicted in Supplementary Figs.3840. With the optimum slot-die coating parameters (1mL/min, 100 m and 30 PSI), we achieved perovskite films with excellent homogeneity (Supplementary Fig.41). The optimal device delivered a PCE of 25.9% for 16cm2 (VOC=1.823V, JSC=18.50mAcm2, FF=76.63%), as shown in Supplementary Fig.42. These results are anticipated to surpass the efficiency of devices fabricated by the blade-coating in the future.

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Solvent engineering for scalable fabrication of perovskite/silicon tandem solar cells in air - Nature.com