Myths and reality of HPbI3 in halide perovskite solar cells

All-inorganic perovskites have a special place in halide perovskite family because of their potential for better stability. However, the representative cesium lead iodide (CsPbI3) is metastable and spontaneously converts to the non-perovskite structure at room temperature. Here, we demonstrate that what appears to be all-inorganic CsPbI3 stabilized in its perovskite form using the purported intermediate known as hydrogen lead iodide (HPbI3) is, in fact, the hybrid perovskite cesium dimethylammonium lead iodide (Cs1−xDMAxPbI3, x = 0.2 to 0.5). Thus, many of the reported all-inorganic perovskites are actually still hybrid organic-inorganic perovskites, as strongly evidenced by a wide battery of experimental techniques presented here. Solar cells based on the representative composition Cs0.7DMA0.3PbI3 can achieve an average power conversion efficiency of 9.27 ± 1.28% (max 12.62%). These results provide an alternative angle to look at previous results pertaining all-inorganic CsPbI3 while the DMA cation is now revealed as an alternative A site cation.

H alide perovskites have unique properties such as tunable band gaps, long diffusion lengths, high optical absorption coefficients, and low exciton binding energies [1][2][3][4][5][6][7][8] . Halide perovskite solar cells have achieved a record power conversion efficiency (PCE) of 22.7% 9 , which are a great candidate for the next generation of photovoltaics. High-performance perovskite solar cells typically employ organic-inorganic hybrid Pb-based perovskites as light absorbers such as methylammonium lead iodide (MAPbI 3 ), formamidinium lead iodide (FAPbI 3 ), and their mixtures, due to their excellent optical and electrical properties [10][11][12][13][14][15][16] . However, the intrinsic thermal and light instabilities of organic cations of MA and FA present serious hurdles in the further development and commercialization of long-term operating devices 17 . The all-inorganic perovskite analog cesium lead iodide (CsPbI 3 ) could present an important way forward, since it possesses a relatively wider band gap with large band widths and especially a significantly better thermal stability compared with the organic cations-based perovskites [17][18][19][20] . However, the black CsPbI 3 perovskite is metastable and it spontaneously converts to the undesired yellow polytype (δ-phase, NH 4 CdCl 3 structuretype) at room temperature 5,18,21,22 . Yellow δ-CsPbI 3 has a wide band gap of E g = 2.82 eV and has poor transport properties 18 . By contrast, black CsPbI 3 has a narrow band gap of around 1.7 eV and it is suitable for solar cell applications 21,23 . Therefore, much effort has been devoted to improving the phase-stability of the black-phase CsPbI 3 perovskites so that highly efficient allinorganic solar cells can be made 18,[24][25][26][27][28][29][30][31][32][33][34][35] . In one approach, researchers used pre-synthesized CsPbI 3 nanocrystals to make solar cells. For example, Protesescu et al. demonstrated that the black CsPbI 3 perovskite (γ-phase) could be stabilized as nanocrystals 36 , due to the large contribution of surface energy. Subsequent work by Luther et al. demonstrated that colloidal CsPbI 3 quantum dots synthesized by a hot-injection method can be phase-stable for several months in ambient air and the solar cells achieved a PCE as high as 13.43% 27,28 . In another approach, You et al. used dimethylformamide (DMF)/dimethyl sulfoxide (DMSO) solvent followed by high-temperature annealing process to obtain the black α-phase CsPbI 3 33 . These solar cells yielded a record PCE of 15.7% and showed a good light-soaking stability under the protection of N 2 -filled glovebox. Much more commonly, the use of hydriodic acid (HI) 18,26 or the so-called hydrogen lead iodide (HPbI 3 ) precursor 31,37 , produced by HI treatment, are the most used approaches to stabilize the black α-CsPbI 3 . The use of HPbI 3 was first reported as an intermediate in the synthesis of FAPbI 3 solar cells 38 , and was prepared by mixing PbI 2 and HI in DMF. Eperon et al. subsequently reported that HI can be used directly as an additive in DMF solutions of CsPbI 3 to stabilize the black α-CsPbI 3 films at room temperature 18 . The planar device made from this black CsPbI 3 film achieved a PCE of 2.9%. Recently, Zhang et al. reported the formation of black CsPbI 3 by a low-temperature deposition using the presynthesized PbI 2 ·xHI, which was also made from DMF, PbI 2 , and HI 31 . Combined with the use of larger ethylenediammonium (en) cation, the champion α-CsPbI 3 solar cell yielded a PCE of 11.8% with enhanced stability and good reproducibility 31 . More recently, Jiang et al. also used HPbX 3 (X = I, Br) as the precursor together with phenylethylammonium (PEA) cation to fabricate low-dimensional α-CsPbI 3 and the solar cells achieved a high PCE of 12.4% 37 . Despite the apparent success of the HPbI 3 methodology, the mechanism of the stabilization of the black CsPbI 3 by HI and HPbI 3 remains unclear. The claim alone that HPbI 3 even exists is extraordinary and there is no structural or spectroscopic validation for the existence of this compound. From the chemistry point of view, HPbI 3 would be a solid acid with HI fragment in it and intuitively is unlikely to exist. This is because the HI molecule itself is not known to engage in binding to metals and it is likely that cannot form a stable structure 39 and will readily dissociate to PbI 2 and HI. Thus, the claim that HPbI 3 is a real compound deserves closer attention and we set out to investigate it in the context of the present manuscript.
Here, we report that HPbI 3 does not exist and instead what was believed to be it is in fact a compound of DMAPbI 3 (DMA = dimethylammonium, (CH 3 ) 2 NH 2 + ), where DMA is a decomposition product of the acidic hydrolysis of DMF catalyzed by HI. We demonstrate that the black CsPbI 3 films deposited from DMF containing HI/HPbI 3 and reported to be all-inorganic black phase of CsPbI 3 are not. Instead, they are the mixed-cation perovskite phase of cesium dimethylammonium lead iodide (Cs 1-x DMA x PbI 3 , x = 0.2 to 0.5). The α-Cs 1-x DMA x PbI 3 perovskite films have similar characteristics with the black γ-CsPbI 3 films arising from HI addition, but they exhibit better charge transport due to the superior band structure characteristics (larger bandwidth, smaller band gap) of the cubic phase. Our bestperforming solar cell based on α-Cs 0.7 DMA 0.3 PbI 3 perovskite absorber achieves a remarkable PCE of 12.62%. Our results reveal the existence of α-Cs 0.7 DMA 0.3 PbI 3 a black 3D perovskite which is a great absorber for the fabrication of high efficiency solar cells.

Results
Phase diversity of CsPbI 3 . The structure of halide perovskites which have a general formula of AMX 3 can be only stabilized by limited cations, according to the rule of tolerance factor, t 39,40 . Where A, M, and X represent a nonbonding univalent cation, an octahedrally coordinated bivalent metal ion, and a monoanionic halide ion, respectively 5,41 . A cubic structure of perovskite materials typically has a suitable t value ranged from 0.9 to 1.0, which is defined by the equation of t = (r A + r X )/√2(r M + r X ). r A , r M , and r X represent the ionic sizes of A, B, and X, respectively 42 . Different A cations have different ionic sizes, only cubic MAPbI 3 could be stable at room temperature. Both FAPbI 3 and CsPbI 3 normally adopt the yellow phase at room temperature, due to the too large and too small size of A cations, respectively 5,42 . Specifically, CsPbI 3 can adopt two structures: 21 the NH 4 CdCl 3 -type yellow phase (δ-phase) stable at room temperature (Fig. 1a) and the black cubic CaTiO 3 -type (α-phase) structure stable above 300°C (Fig. 1b). The perovskite structure can be kinetically stabilized at room temperature, where it adopts the black orthorhombic γ-phase (bandgap of around 1.73 eV) but it converts within hours to the δ-phase 5,42,43 . Thus, CsPbI 3 is typically stabilized by adjusting the t via formation of solid solutions such as partial replacement of I with smaller Br anions to form CsPbI 3−x Br x compositions, which exhibit a wider band gap [44][45][46][47][48][49][50][51][52][53][54] , or by substituting the A-site with larger organic cations such as FA to form Cs 1−x FA x PbI 3 solid solutions 55 . DMA (2.72 Å) is very similar in shape to FA (2.53 Å) but has a slightly larger effective radius owing to the conjugated bonding in the latter (Fig. 1c) 56,57 . Because of this, pure DMAPbI 3 , which has a large t = 1.026 and is not able to form the perovskite structure, unlike FAPbI 3 . Pure DMAPbI 3 , which is identical to what previously reported as HPbI 3 , by inspection of the reported and simulated powder diffraction patterns (PXRD), has a 1D structure assembled by facesharing [PbI 6 ] 4− octahedra 58 , and it is isostructural with yellow FAPbI 3 5 . Unlike FAPbI 3 , DMAPbI 3 cannot be isolated as a black perovskite due to the too large size of DMA which leads to a prohibitively large t, even at elevated temperatures. On the other hand, the effective radius of Cs cation is only 1.88 Å and pure CsPbI 3 has a low tolerance factor of t = 0.851 which is below the ideal range of cubic structure (Fig. 1d) 17,19 . However, mixing Cs with DMA, i.e., Cs 1-x DMA x PbI 3 , can on average bring the effective tolerance factor within the desirable region, thus enabling the stabilization of a black perovskite phase, namely Cs 1−x DMA x PbI 3 (x = 0.2 to 0.5), having a t = 0.904 (x = 0.3) that allows it to adopt the cubic α-phase (Fig. 1d), similarly to Cs 1−x FA x PbI 3 .
Cs 1-x DMA x PbI 3 film properties. This has indeed turned out to be the case in our thin films, where the nominal Cs 0.7 DMA 0.3 PbI 3 composition is able to adopt the perovskite phase. As shown in Fig. 2a, the Cs 0.7 DMA 0.3 PbI 3 film without HI addition shows a black color, indicting the successful fabrication of black-phase perovskite structure with lattice constant a = 6.272 Å and Pm-3m space group. On the contrary, the pristine CsPbI 3 and DMAPbI 3 films deposited on FTO/PEDOT substrates are yellow and paleyellow color (Fig. 2a), respectively, because they are nonperovskite NH 4 CdCl 3 structure-type 56 . For comparison, we also fabricated CsPbI 3 film with HI addition (see details in Methods). Similar to the previously reported results 18,25,26,59 , the CsPbI 3 film prepared with HI addition also shows a black color and a shiny surface (Fig. 2a), indicative of the formation of the similar black-phase perovskite structure. The color of the films is in good agreement with the band gaps of the respective compounds as determined by optical absorption measurements. Fig. 2b shows that the Cs 0.7 DMA 0.3 PbI 3 film has an absorbance onset at around 730 nm, indicating a band gap of 1.7 eV. The CsPbI 3 film with HI addition shows a similar absorption but slightly blue-shifted from 730 nm to 710 nm. As expected, the pure CsPbI 3 and DMAPbI 3 films only show an absorption at around 440 nm and 390 nm, respectively, due to the wide band gaps of these yellow phases 18 . Photoluminescence (PL) measurements reveal the same trend. As shown in Fig. 2c, the Cs 0.7 DMA 0.3 PbI 3 film has a PL emission at around 724 nm. The black CsPbI 3 film with HI addition shows a blue-shift PL peak at around 704 nm, consistent with the trend of UV-vis absorption spectra. Both CsPbI 3 and DMAPbI 3 films only show negligible PL emission, due to the indirect nature of the band gaps.
X-ray diffraction (XRD) patterns on the thin-films confirm the structure description discussed above. As shown in Fig. 2d, the main peaks of the neat yellow CsPbI 3 film locates at 9.9°, 13.2°, 22.8°etc. are assigned to the orthorhombic (Pnma) yellow δ-phase (Supplementary Fig. 1) 18 . The XRD pattern of the DMAPbI 3 film only shows two main reflections at 2θ = 11.8°and 20.4° (Fig. 2d), due to the preferred orientation, corresponding to the hexagonal (P6 3 /mmc) yellow phase ( Supplementary Fig. 2). The In addition, the CsPbI 3 film with HI also reveals a weak reflection at 2θ = 9.9°, which corresponds to the yellow δ-phase, indicating that the conversion to the black phase is incomplete.
To further prove that DMA cation is actually present in the black CsPbI 3 films, we measured the proton nuclear magnetic resonance ( 1 H NMR) spectrum of the powder obtained from scratching away the Cs 0.7 DMA 0.3 PbI 3 films and dissolving it in dimethyl sulfoxide-d 6 (DMSO-d 6 ) (Fig. 2e). The signals of -NH 2 + -(singlet) and -CH 3 (doublet) protons are located at Remarkably, the NMR spectrum of the DMSO-d 6 solution of the powder obtained from scratching away the CsPbI 3 film with HI addition also shows the same two peaks in the 1:3 for -NH 2 +and -CH 3 ratio ( Supplementary Fig. 7).
The chemical mechanism and reality of so-called HPbI 3 . Thus, it transpires from the NMR results that the DMA cation is produced via a side reaction (of DMF with HI) and then gets incorporated in the CsPbI 3 films. Since only CsI and PbI 2 were dissolved in the DMF solvent, the only possible source of DMA in the precursor solution is DMF itself, which is known to slowly hydrolyze to DMA and formic acid (HCOOH) [60][61][62][63][64][65] . Addition of HI significantly accelerates the hydrolysis according to Eq. 1, heavily shifting the equilibrium towards the dissociation products, thus generating significant amounts of DMA in the mixture 66 .
In order to further demonstrate this reaction, we synthesized the DMAI from the reaction of DMF and HI directly (see Methods). As Fig. 2e illustrate, DMAI can indeed be readily formed by the reaction between DMF and HI, with 1 H-NMR spectra and XRD patterns confirming its identity in comparison with the commercial DMAI compound (Supplementary Figs. 6 and 8). Note that the two peaks at δ = 7.22 ppm and δ = 6.18 ppm shown in Fig. 2e are assigned to the -PH 2protons of hypophosphorous acid (H 3 PO 2 ) which presents as a stabilizer in concentrated HI aqueous solutions. Therefore, this is the reason why 1 H-NMR spectra of Cs 0.7 DMA 0.3 PbI 3 , CsPbI 3 with HI, DMAI, and DMF + HI samples are identical. Most importantly, it explains why the UV-vis absorption, PL, XRD, and 1 H-NMR results between the intentionally made Cs 0.7 DMA 0.3 PbI 3 film and the DMF derived CsPbI 3 with HI addition films are so similar. Note that the red shift (around 20 nm) of PL and UV-vis spectra of Cs 0.7 DMA 0.3 PbI 3 with respect to the CsPbI 3 sample with HI is because the former (which contains significant amounts of DMA) adopts the cubic α-CsPbI 3 phase, whereas the latter (containing only small amounts of DMA) stabilizes the orthorhombic γ-phase (as would be expected by the effective tolerance factor discussed above).
The hydrolysis of DMF in the presence of acid such as HI as a film fabrication tool has great implications in the fabrication of perovskite films and solar cells. Since HI addition has been reported to alter the phase transition temperature and improve the film morphology of MAPbI 3 67 , and also was identified as the critical reagent for stabilizing the perovskite phase of CsPbI 3 , the nature and exact composition of these crystalline films will need to be re-examined 18,25,26,59 . In addition, HPbI 3 , obtained from   the reaction of HI, DMF, and PbI 2 , has been widely used for making stable FAPbI 3 38 , CsPbI 3 31,37,53 , and MAPbI 3 solar cells 68 , which actually does not exist and should be reformulated to DMAPbI 3 . It is confirmed by the XRD patterns of our DMAPbI 3 film (Fig. 2d and Supplementary Fig. 2) and the reported HPbI 3 38 , showing the exact same reflections. Given that the use of HPbI 3 in a serendipitous manner, these systems also need to be revisited and studied carefully under the understanding of the DMF hydrolysis mechanism. It is important to re-evaluate these systems counting the time as a synthetic parameter since the preparation will significantly vary over time as different amounts of DMA are produced at different times of the reactions and therefore the composition of the films will vary accordingly.
In a previous report, we demonstrated that HI addition can alter the MAPbI 3 phase transition temperature 67 . A room temperature phase transition from tetragonal to cubic was observed for MAPbI 3 films treated with high HX (X = Cl, Br, I) concentrations. In our original hypothesis, we and others speculated that the reduction of the crystallite size induced by the acid were responsible for this phase transition 18,67 . HI can improve the perovskite film morphology and eliminate the hysteresis of MAPbI 3 solar cells 69,70 . However, in light of the results reported here, we can now revise our original concept and ascribe the compositional phase transition to the presence of DMA, induced by the presence of acid. Note that the concept of the role of acid expands beyond the HX acids, as other acids such as H 3 PO 2 71 appear to have a similar effect. Therefore, based on the above results, all those compositionally-induced phase transitions may be attributed to the introduction of the in-situ forming DMA cation in the perovskite lattice and it applies equally well in the MAPbI 3 and CsPbI 3 systems. Note that FAPbI 3 is excluded since FA is already too large to stabilize the perovskite structure. Now that we have established that the so-called all-inorganic CsPbI 3 films processed with HI and HPbI 3 are in fact Cs 1x DMA x PbI 3 films, we return to the study of the latter and their behavior and performance in complete solar cells. The CsPbI 3 film with HI addition was prepared by a conventional one-step method and using DMF as solvent 18 , actually is unintentionally-made Cs 1 −x DMA x PbI 3 but we still referred it as HI-treated CsPbI 3 in the following discussion. A small amount of HI aqueous solution was added into the CsPbI 3 precursor after all materials were dissolved in DMF solvent. All other intentionally-made Cs 1−x DMA x PbI 3 (x = 0 to 0.5) films were fabricated by CsI, PbI 2 , and presynthesized DMAI using a solvent-engineering method 14,72 with a mixture of DMF and DMSO as the solvent. Scanning electron microscopy (SEM) images show that the yellow-phase CsPbI 3 film has a rough surface with big grains and pin-holes, Fig. 3a. The morphology of the neat yellow DMAPbI 3 film is even worse, showing some long and big cracks (Fig. 3b). Fig. 3c shows the onestep method prepared CsPbI 3 film treated with HI which clearly has much smoother surface, still containing some pin holes. The best film quality was obtained from the CsPbI 3 film with DMA cation i.e., Cs 0.7 DMA 0.3 PbI 3 . As shown in Fig. 3d, the Cs 0.7 DMA 0.3 PbI 3 film has a smooth surface, big grains and few pinholes. The average grain size of the Cs 0.7 DMA 0.3 PbI 3 film is around 500 nm, which is comparable with the film thickness. This is desirable since big grains facilitate charge transfer and reduce the recombination at grain boundaries 14  Effects of DMA cation on solar cell performance. We then investigated the effects of various absorbers on device performance. Fig. 4a shows the photocurrent density-voltage (J-V) curves of the representative solar cells based on the yellow CsPbI 3 , the pale-yellow DMAPbI 3 , the black HI-treated CsPbI 3 , and the black Cs 0.7 DMA 0.3 PbI 3 absorbers, measured under a reverse voltage scan. The performance of the solar cells based on different absorbers shows huge difference. As expected, the solar cells with yellow CsPbI 3 and DMAPbI 3 absorbers show very poor efficiencies. As shown in Fig. 4a, the solar cell employed the yellow CsPbI 3 film achieved a very low PCE of 0.002% with an opencircuit voltage (V oc ) of 0.05 V, a short-circuit current density (J sc ) of 0.21 mA cm −2 , a fill factor (FF) of 22.63%. A similarly low PCE of 0.001% with a V oc of 0.03 V, a J sc of 0.30 mA cm −2 , and an FF of 14.23% was obtained for the solar cell employing the yellow DMAPbI 3 film. Both CsPbI 3 and DMAPbI 3 devices were almost short-circuited, due to the poor film quality, non-perovskite structure, and limited light absorption due to their wide band gaps. The device performance of the so-called black CsPbI 3 was significantly enhanced after adding HI into the perovskite precursor. The HI-treated CsPbI 3 solar cell achieved a V oc of 0.82 V, a J sc of 9.56 mA cm −2 , an FF of 61.56%, and a PCE of 4.84%, which is comparable with other reports 18,31 . The enhanced performance can be mainly attributed to the better morphology and the unintended incorporation of DMA cation which can stabilize the Cs 1−x DMA x PbI 3 perovskite as a black phase. Nevertheless, adding HI into the perovskite precursor makes it difficult to control the amount of DMA in the final film in a consistent manner, so after realizing the role of HI in the film formation the main focus was given in the easily reproducible fabrication of devices containing controlled amounts of DMA. As expected, the solar cells based on the designed Cs 0.7 DMA 0.3 PbI 3 absorber yielded much better performance. A significantly enhanced PCE of 10.39% with a high V oc of 1.03 V, a J sc of 15.43 mA cm −2 , an FF of 65.61% was achieved for an optimized device, with both the high-quality of the films and narrow band gap contributing to the decent performance of the Cs 0.7 DMA 0.3 PbI 3 solar cells.
We find that the amount of DMA cation incorporated in the perovskite structure of CsPbI 3 has a huge effect on the material properties. Unlike the HI addition to DMF which results in the uncontrollable generation of DMA, it is easy to fabricate Cs 1x DMA x PbI 3 films using precise DMA ratios. First, the film morphology was affected by different amounts of DMA. As shown in Supplementary  Fig.  9, Cs 0.8 DMA 0.2 PbI 3 , Cs 0.7 DMA 0.3 PbI 3 , Cs 0.6 DMA 0.4 PbI 3 , and Cs 0.5 DMA 0.5 PbI 3 films show different morphology. All films are compact and have smooth surface with Cs 0.7 DMA 0.3 PbI 3 film having the biggest grains. The Cs 0.6 DMA 0.4 PbI 3 , and Cs 0.5 DMA 0.5 PbI 3 films are compact and have smaller grains. In addition, these films show similar band baps and PL peaks but different absorption intensities (Supplementary Fig. 10). In particular, the Cs 0.7 DMA 0.3 PbI 3 film shows the strongest absorption at long wavelength range. Supplementary Fig. 11 shows that the different films also have different XRD results. The diffraction intensities of the cubic Cs 0.6 DMA 0.4 PbI 3 and Cs 0.5 DMA 0.5 PbI 3 films are weaker while the extra peak at 2θ = 11.8°indicates that DMAPbI 3 is present as a second phase, suggesting that the substitution of DMA for Cs is limited. Cs 0.7 DMA 0.3 PbI 3 and Cs 0.8 DMA 0.2 PbI 3 show the exclusive formation of the black α-CsPbI 3 phase along with the former orienting preferentially along both (100) and (110) planes and the latter exclusively along the (110) planes. From the combined data, around 30% of DMA seems to be the optimum amount to make high-quality perovskite film. This is also reflected in the device performance of the corresponding solar cells based on the CsPbI 3 absorbers with various amounts of DMA. Supplementary Fig. 12 shows the J-V curves of the solar cells using the Cs 0.8 DMA 0.2 PbI 3 , Cs 0.7 DMA 0.3 PbI 3 , Cs 0.6 DMA 0.4 PbI 3 , and Cs 0.5 DMA 0.5 PbI 3 absorbers. The device performance first increases and then decreases as the DMA amount increases. As expected, the solar cell derived from the best-quality Cs 0.7 DMA 0.3 PbI 3 film achieved the highest performance (Supplementary Table 1). Figure 4b shows the J-V curves of the best-performing Cs 0.7 DMA 0.3 PbI 3 solar cell measured under different voltage scan directions. This solar cell achieved a V oc of 0.99 V, a J sc of 16.65 mA cm −2 , an FF of 76.49%, and therefore a high PCE of 12.62% when measured under the reverse voltage scan. A similar PCE of 12.31% with a V oc of 0.99 V, a J sc of 16.44 mA cm −2 , and an FF of 75.71% was achieved for the solar cell measured under the forward voltage scan, suggesting small hysteresis behavior of our devices. External quantum efficiency (EQE) measurement was taken to confirm the high J sc obtained from the J-V curves. The EQE spectrum of the solar cell based on the Cs 0.7 DMA 0.3 PbI 3 absorber is shown in Fig. 4c. The cell shows  Furthermore, we also added the DMA cation into solutions of CsPbI 2 Br and CsPbIBr 2 perovskites. Supplementary Fig. 13a-b shows UV-vis absorption and XRD pattern of a Cs 0.7 DMA 0.3 I 2 Br film coated on a FTO/PEDOT:PSS substrate. The Cs 0.7 DMA 0.3 I 2 Br film shows an absorption onset at around 685 nm, according to a band gap of around 1.8 eV (Supplementary Fig. 13a). The Cs 0.7 DMA 0.3 I 2 Br film has good crystalline quality and shows peaks at 14.7°, 20.5°, 29.5°, and 41.6°( Supplementary Fig. 13b), which can be indexed to the (100), (110), (200), and (220) planes. Supplementary Fig. 13c shows the J-V curve of a solar cell based on the CsPbI 2 Br absorber. The Cs 0.7 DMA 0.3 I 2 Br solar cell achieved a PCE of 5.24%, a V oc of 1.02 V, a J sc of 12.39 mA cm −2 , and an FF of 41.67%. The lower J sc is consistent with the wider band gap compared to Cs 0.7 DMA 0.3 PbI 3 . The Cs 0.7 DMA 0.3 PbIBr 2 film has a more blue-shifted absorption onset at around 620 nm, according to a wider band gap of around 2.0 eV, Supplementary Fig. 14a. The Cs 0.7 DMA 0.3 PbIBr 2 solar cell achieved a PCE of 2.80%, a V oc of 1.11 V, a J sc of 8.55 mA cm −2 , and an FF of 29.57% ( Supplementary  Fig. 14c). A much lower J sc is obtained from the Cs 0.7 DMA 0.3 PbIBr 2 cell because of the much wider band gap. Higher performance for the Cs 1-x DMA x PbI 2 Br and Cs 1−x DMA x PbIBr 2 cells is anticipated after further device optimization.

Discussion
We have demonstrated that small cation DMA can be formed in suite by the HI induced decomposition of DMF and can subsequently stabilize the black perovskite phase of CsPbI 3 . However, the films are not really the all-inorganic CsPbI 3 phase but the Cs 1x DMA x PbI 3 (x = 0.2 to 0.5) cubic perovskite solid solution. Cs 0.7 DMA 0.3 PbI 3 has a band gap of around 1.7 eV and can enable high efficiency solar cells. In the context of this work, we realize that DMA is likely present in many other devices reported in the past, since it can be inadvertently created from the degradation of the commonly used DMF solvent and then incorporated to some extend in the perovskite structures as an A cation. This DMF decomposition process is accelerated by acidic hydrolysis, and must be taken into account when considering the role of the solvent degradation in device performance. DMF hydrolysis therefore explains the effects of the widely used HI addition and HPbI 3 methods to stabilize the purported all-inorganic black CsPbI 3 perovskite. Based on above results, we show that HPbI 3 does not exist while at the same time it is actual in situ formation of the DMA cation, which should widely exist in the perovskites processed with DMF, HI, and HPbI 3 . We anticipate that the DMA cation, which now has been revealed as a viable A-site cation, may be unsuspectingly occurring not only in "CsPbI 3 films" but also in films of other perovskite families, for example, pure MA, MA/FA, Cs/FA, and Cs/MA/FA mixed-cation perovskites. We suggest that NMR spectroscopy be used for such films by dissolving them in pure DMF or DMSO to check for the presence of the DMA cation.

Methods
Device fabrication. For the DMAI synthesis, DMF was slowly added into the HI solution with continue stirring at 0°C for 30 min. Water and excess DMF in solution was removed by low-vacuum rotary evaporation and then a white polycrystalline powder (DMAI powder) was obtained. Pre-cleaned FTO substrates were treated with UV-Ozone for 30 min. PEDOT:PSS films were coated on the FTO substrates with a spin-rate of 4000 rpm for 30 s and then annealing for 30 min at 150°C. Perovskite films were deposited on FTO/PEDOT:PSS substrates in a N 2filled glovebox. Neat CsPbI 3 , neat DMAPbI 3 , and Cs 1−x DMA x PbX 3 (X = I, Br) films were prepared by a solvent-engineering method 14 at a spin rate of 4000 rpm for 60 s. 0.7 ml of diethyl ether was quickly dropped onto the rotating substrates during the spin-coating process. CsPbI 3 films with HI addition were prepared by a one-step method at a spin rate of 2000 rpm for 60 s 18 . The neat CsPbI 3 precursor was prepared by dissolving 260 mg of CsI (99.999%, Sigma-Aldrich) and 461 mg of PbI 2 (beads, 99.999%, Sigma-Aldrich) in 0.8 ml of DMF (anhydrous, 99.8%, Sigma-Aldrich) and 0.2 ml of DMSO (anhydrous,≥ 99.9%, Sigma-Aldrich). The neat DMAPbI 3 precursor was prepared by dissolving 173 mg of DMAI and 461 mg of PbI 2 in 0.8 ml of DMF and 0.2 ml of DMSO. The Cs 1−x DMA x PbI 3 precursor was prepared by dissolving (1−x) × 260 mg of CsI, x × 173 mg of DMAI, and 461 mg of PbI 2 in 0.8 ml of DMF and 0.2 ml of DMSO. The CsPbI 3 precursor with HI addition was prepared by dissolving 260 mg of CsI and 461 mg of PbI 2 in 2 ml of DMF and then adding 3.3 vol% HI (57 wt.% in H 2 O, 99.95%, Sigma-Aldrich) into the dissolved solution. All the films were annealed for 2 min at 60°C and then 5 min at 100°C on a hot plate. To complete the devices, a thin C 60 (20 nm), a thin 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) (5 nm), and a Ag (80 nm) films were sequentially thermal evaporated on top of the absorber layers using a metal mask. The active area of the solar cells was 0.09 cm 2 .
Film and device characterization. 1 H-NMR spectra were measured with Bruker Avance III 600 MHz system with BBI probe. The morphology of the films and devices was examined on a high-resolution field emission SEM (Hitachi SU8030). XRD patterns of the films were characterized by a Rigaku Miniflex600 pXRD (Cu Kα graphite, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter. PL spectra were taken on a Horiba LabRAM HR Evolution confocal Raman microscope spectrometer (600 g mm −1 diffraction grating) equipped with a diode continuouswave laser (473 nm, 25 mW) and a Synapse charge-coupled device camera. UV-vis absorption spectra of the films were measured with a Shimadzu UV-3600 UV-vis NIR spectrometer operating in the 200-2000 nm region at room temperature. EQE spectrum was obtained on an Oriel model QE-PV-SI instrument equipped with a NIST-certified Si diode. J-V curves were recorded using a Keithley model 2400 instrument under AM1.5 G simulated irradiation with a standard solar simulator (Abet Technologies). J-V curves were measured from 1.5 V to −0.2 V (reverse) or from −0.2 V to 1.5 V (forward) with an integration time of 16.67 ms and a voltage step of 11.4 mV. The light intensity of the solar simulator was calibrated by a NREL-certified monocrystalline silicon solar cell.