Recently, solution-treatable all-inorganic metal perovskites have attracted extensive attention and research in solar cells, (1−3) laser diodes, (4−6) and photodetectors (PDs) (7−15) due to their superior photothermal stability and outstanding photoelectric properties, such as strong optical absorption, long-lived carriers, and high defect tolerance. (16,17) However, abundant defects including halogen ion defects, Pb²⁺-related defects, oxygen vacancy (O_v), (18) and a chemisorbed hydroxyl group (−OH) exist in the interface of the perovskite film, and the electron transport layer (ETL) will lead to nonradiative recombination loss and ion migration, thus deteriorating the performance of the device. (19−22) The issues mentioned above are substantial obstacles in the improvement of responsivity, detection capabilities, and stability of perovskite PDs (PPDs), thus further preventing their practical applications and commercialization. (8−12)

The most commonly employed method to mitigate these defects involves passivating single surface defects on the perovskite layer. In contrast, the recently reported dual-interface defect passivation in both the perovskite layer and ETL has been proven to be more effective in improving the photovoltaic performance and stability of PPDs; hence, it attracted extensive attention. (21−27) Numerous efforts have been dedicated to exploring suitable Lewis base (such as –NH₂, –COOH, and halides) passivators to heal the deep-level trap-state defects at the ETL/perovskite heterojunction interface. (26−31) Due to the strong hydrogen bonding ability and high electron density, guanidine compounds (containing –C═NH and −NH_x–) have been extensively applied to passivate defects and modulate perovskite crystallization. (30,32) For instance, Zhuang, Wu, and Zhang et al. added guanidinium sulfate, guanidine hydrochloride, and aminoguanidine hydrochloride onto the surface of SnO₂, respectively, as buried interface modifiers. (30,31,33) Although the above-mentioned guanidine Lewis base molecules have shown noticeable modification on the SnO₂/perovskite interface defects, these molecules are toxic and not easily accessible, which is unfavorable for the scalable fabrication of perovskite devices. Furthermore, the effect of individual functional groups of these guanidine compounds on interface modulation has not been systematically investigated. Therefore, a series of guanidine salts containing different anions were applied in the previous research since the most efficient passivators were not clear. (30) Until now, the defect passivation principles with guanidine compounds remain indistinct, and the selection or application of these molecules still lacks theoretical guidance. Therefore, the investigation is significant in selecting highly efficient passivators for fabricating the desired PPD devices.

Gua was intuitively selected as the dual-interface passivator of perovskite and SnO₂ because of its multiple functional groups (−COOH, –NH₂, –CH═N, and –NH−) and easy accessibility. (34) Moreover, the excellent solubility of Gua in SnO₂ solution (S > 10 mg mL^–1, Figure S1a), high thermal stability (T_d > 280 °C, Figure S1b), and its high boiling point (BP > 340 °C, Figure S1c) make it processable and durable in device fabrication. Additionally, the SnO₂ film modified with Gua maintained the same high light transmittance (Figure S2) as the unmodified film, signifying that it did not influence the light absorption of the perovskite. Before applying it practically in a perovskite device, the electrostatic potential (ESP) (35) and dipole moment (μ) of Gua were calculated to evaluate its potential interaction ability with defects in the perovskite and ETL (SnO₂) film. As shown in Figure 1a, the deep color areas which indicate high electron cloud density of the functional groups and large dipole moment (μ = 3.66 D) mean strong interaction with uncoordinated ions in SnO₂/perovskite. (36) Additionally, the delocalization of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in different spatial regions of Gua further demonstrates its merit in charge transfer (37) according to the calculated Frontier orbital in Figure 1b. Based on the calculations above, it can be expected that the functional groups in Gua may act as Lewis bases to passivate defects like uncoordinated Pb²⁺ and X^–, vacancy defects (V_O and V_Sn), and free −OH groups on the interface of SnO₂ and perovskite as shown in schematic Figure 1c. Then, the performance of the corresponding PPD devices can be improved by enhancing charge transfer and reducing of nonradiative recombination. (38) Therefore, Gua was practically applied as a dual-interface defect passivator of n–i–p planar perovskite PDs. In Figure 1d, Gua was dissolved in SnO₂ solution and spin-coated onto the FTO substrate. After annealing and further coating with the perovskite film, the PD was fabricated with the architecture of FTO/SnO₂ (-Gua)/CsPbIBr₂/P3HT/Ag. Here, the concentration of Gua in SnO₂ solution was confirmed as 1 mg/mL by measuring the conductivity (dark current) (24) of an ITO/SnO₂ (-Gua)/Ag structure with different Gua amounts (Figure S3).

During the preparation of the PD, the SnO₂ and perovskite films with and without treatment of Gua were characterized by atomic force microscopy (AFM) in Figure 2a–f and scanning electron microscopy (SEM) in Figure S4, respectively. In Figure 2a,b,e, the Gua-treated SnO₂ exhibits a smaller root-mean-square (RMS) roughness (36.2 nm), an average height (107.92 nm), and a standard deviation (0.578 nm) compared to the untreated sample (RMS: 38.7 nm, average height: 113.39 nm, and standard deviation: 0.744 nm). The decreased surface roughness and enhanced uniformity of the SnO₂-Gua film facilitate the subsequent growth of the perovskite film, promoting the decreasing of RMS (from 28.5 to 24.5 nm), average height (from 125.44 to 98.26 nm), and standard deviation (0.115–0.067 nm) in Figure 2c,d,f. Moreover, the grain size of the perovskite film was prominently increased from 422.4 to 582.6 nm in Figures 2g,h. The improved uniformity of the perovskite film can also be verified by a reduced water contact angle from 4.8 to 3.6° in Figure S5. The X-ray diffraction (XRD) patterns were employed to characterize the crystallinity of the PVK film. The SnO₂/PVK film exhibits two characteristic diffraction peaks located at 15.93 and 31.10° (Figure 2i), corresponding to the (110) and (220) crystallographic planes, respectively. After the treatment of Gua, the two diffraction peaks are significantly boosted, and the full width at half-maximum is reduced (Figure S6a), suggesting that the Gua-buried SnO₂ modification is facilitating the crystallinity of the top perovskite film. (39) As shown in Figure 2j, the smoother surface, larger crystal size, and more homogeneous size distribution of the PVK-Gua film compared to the pristine sample can be primarily attributed to Lewis acid–base and hydrogen bond interactions between Gua and perovskite. First, Pb²⁺ which acts as a prototypical Lewis acid with empty orbitals forms a coordination bond with the Lewis base functional groups (–C═O and –NH₂) in Gua. Second, the –NH₂ group in Gua molecules can form hydrogen bonds with iodide ions, hence inhibiting the migration of halogen ions. (40,41) The strong Lewis acid–base interactions and hydrogen bonds increased the nucleation activation energy and slowed the growth of perovskite crystals, thus improving the crystal size and surface smoothness. (40,41) Besides the smooth morphology, the energy-dispersive spectrometry (EDS) mapping of Sn, O, C, and N elements in Figure S7 also demonstrates the homogeneous distribution of Gua on the SnO₂/PVK film. (39) The optimized surface uniformity of SnO₂/PVK and large perovskite grain size always means fewer defects and nonradiative recombination centers, thereby facilitating efficient carrier transport and enhancing device performance. (24,36)

Figures 3a and S6c,d show the UV–vis absorption spectra and corresponding Tauc plot images of PVK and PVK-Gua films, respectively. Although the optical bandgap of the PVK film remains almost unchanged, the absorbance of the PVK-Gua film was distinctly enhanced due to the larger grain size. (9,20) In Figure S8 and Table S1, the PVK on FTO-Gua shows higher steady-state photoluminescence (PL) intensity and longer PL lifetime than the PVK on the bare FTO substrate, indicating improved quality of the PVK film due to Gua treatment. However, the PL and time-resolved PL (TRPL) of SnO₂-PVK and SnO₂-Gua-PVK films in Figure 3b and c show an opposite trend. The PL intensity of SnO₂-Gua-PVK is significantly attenuated compared to that of SnO₂–PVK, indicating that the Gua-modified SnO₂ film exhibits superior electron extraction and transport capability from the PVK-active layer than the pristine SnO₂. (24) In Figure 3c and Table S2, the SnO₂-Gua-PVK film exhibits a shorter average carrier lifetime (τ_ave) of 36.02 ns than that of SnO₂-PVK (τ_ave = 55.05 ns). The short PL lifetime further confirmed the high efficiency of the SnO₂-Gua ETL film in extracting electrons from the perovskite layer. (42) Thus, the high-quality SnO₂ film with reduced oxygen vacancy defects and the surface-adsorbed hydroxyl (−OH) can be responsible for the enhanced electron extraction and transport ability of Gua-SnO₂, as the similar efficacy observed on the multifunctional anion and 1,3-thiazole-2,4-diammonium-treated SnO₂/PVK interfaces. (30,38) Subsequently, the space–charge-limited current (SCLC) technique was employed to estimate the trap density (N_d) in perovskite films with a device configuration of FTO/SnO₂(/-Gua)/perovskite/PC₆₁BM/Ag. The logarithmic form dark current–voltage curves of PVK and PVK-Gua are shown in Figure 3d,e. N_d can be given by $N_{\mathrm{d}}=\frac{2{\epsilon }_0\epsilon V_{\mathrm{T}\mathrm{F}\mathrm{L}}}{e{L}^2}$, (9,34) where ε₀ is the vacuum permittivity, ε is the relative dielectric constant of CsPbIBr₂, V_TFL is the trap-filled limit voltage, e is the elementary charge, and L is the thickness of the film. From the above equation, the N_d of PVK reduced from 5.29 × 10¹⁵ to 4.31 × 10¹⁵ cm^–3 after the modification of Gua in SnO₂. At the same time, in the devices with the hole-only (Figure 3g,h), the N_d of PVK reduced from 5.38 × 10¹⁵ to 3.52 × 10¹⁵ cm^–3 after the modification of Gua in PEDOT/PSS. Compared to the pristine device, the diminished N_d of SnO₂-Gua-PVK can be attributed to the less Pb²⁺ and X^– interface defects because of Gua passivation and lower grain boundary density because of larger perovskite grain size. In addition, the electron mobility (μ_e) of both films (Figure 3f) can be calculated via the Mott–Gurney formula: (9) $\mu =\frac{8L^3{J}_{\mathrm{D}}}{9\epsilon {\epsilon }_0{V}^2}$, where J_D is the current density and V is the applied voltage. In Figure 3f, the electron mobility of the Gua-modified device (5.57 × 10^–3 cm² V^–1 s^–1) is much higher than that of the control one (4.25 × 10^–3 cm² V^–1 s^–1). In addition, the hole mobilities of the control group and the Gua-modified device were also elaborately calculated to be 3.31 and 5.69 × 10^–3 cm² V^–1 s^–1, respectively. From the aforementioned data, both electron mobility and hole mobility in the Gua-modified devices have been remarkably enhanced, with hole mobility showing particularly significant improvement. The ratio of hole to electron mobility in the Gua-modified perovskite devices is 1.02, which is closer to unity than that of the control group (0.78), indicating a more balanced carrier transport within the Gua-modified perovskite. The enhanced electron and hole mobility confirmed the charge transport promotion of Gua between SnO₂ and perovskite, which is consistent with the structure characterization in Figure 2 and is conducive to the fast response of the PDs. (9) The reduced N_d and enhanced μ_e can be further supported by the attenuated internal series resistance (R_s) and charge recombination resistance (R_rec) shown in the Nyquist plots (Figure 3i). (34)

To verify the aforementioned defect passivation of Gua, the DFE between Gua and typical defects was calculated by the density functional theory (DFT) simulations. Figure 4a,b shows the model of common defects in perovskite including interstitial defects (Br_i and I_i), antisite defects (Br_Pb, Pb_Br, I_Pb, and Pb_I), and vacancy defects (V_Br, V_Pb, and V_I). In Figure 4c,d, the much lower defect DFE on the (100) plane than that on the (001) plane indicates that perovskite defects predominantly occur on the (100) planes as interstitial Br (Br_i) and I (I_i) defects. Then, the charge density difference of the CsPbIBr₂ (100)/Gua model is plotted to reveal the interface charge transport dynamics (Figure 4e). For the (100) plane, the yellow region indicates electron accumulation, while the green region represents electron depletion. Specifically, the Pb²⁺ in CsPbIBr₂ interacts with Gua as electron acceptors, while N and carbonyl oxygen (–C═O) units are electron donors. The electron depletion region is primarily located on the –NH₂ and –C═NH functional group. (43) The strong charge transfers between Gua and PVK further confirm the effective passivation of Gua on uncoordinated Pb²⁺, and the presence of –NH₂ and –C═NH is more effective in this passivation process than the presence of –COOH.

The ADE between different Gua functional groups (–COOH, –C═NH, and –NH₂) and then main interface defects (Pb²⁺, Sn⁴⁺, and –OH on SnO₂ surface) were calculated to reveal their specific interactions. The adsorption model for interactions between Pb²⁺, Sn⁴⁺, and –OH and these Gua functional groups is shown in Figure 5a–c, respectively. The corresponding calculated ADEs are summarized in Figures 5d–f. The ADE between –C═NH and Pb²⁺ (E_ad = −0.713 eV) is lower than that of −NH₂/Pb²⁺ (E_ad = −0.587 eV) and –COOH/Pb²⁺ (E_ad = −0.053 eV), implying the superior effectiveness of –C═NH on passivating Pb²⁺ defects. (34,37,38,44) The simulated interaction model between Gua and SnO₂ is depicted in Figures 5b,c and S9, and the corresponding calculated results are displayed in Figure 5e,f, respectively. The ADE between –NH₂ and –OH on the SnO₂ surface (E_ad = −0.422 eV) is stronger than that of –C═NH/–OH (E_ad = −0.362 eV) and –COOH/–OH (E_ad = −0.177 eV), which could validly alleviate nonradiative recombination at ETL/perovskite contact and slow down the perovskite decomposition. (45,46) The reduced interaction between –C═NH and –OH can be attributed to the simultaneous adsorption of –C═NH by –OH and Sn⁴⁺ after the system approached the lowest total energy. In addition, the ADE between –C═NH and Sn⁴⁺ on the SnO₂ surface (E_ad = −1.466 eV) is much stronger than that of –NH₂/Sn (E_ad = −0.927 eV) and –COOH/Sn (E_ad = −0.511 eV), indicating that –C═NH has the strongest interaction with Sn⁴⁺to passivate the Sn-related defects. (38,44)

The above-simulated results of ADE calculation and defect passivating effectiveness are well demonstrated by XPS of SnO₂/PVK (Figure 5g–i). Figure 5i shows the XPS of Sn 3d in the SnO₂ film modified by Gua and control SnO₂ films, respectively. Compared to the control sample, both Sn 3d_3/2 and Sn 3d_5/2 in the SnO₂-Gua film have a left shift of 0.12 eV toward high binding energy, (38,42,47) demonstrating the inhibiting effect of Gua on Sn⁴⁺ defects, which is consistent to the high ADE of Gua functional groups in Figure 5e. Figure 5g shows the characteristic peaks of Pb 4f (Pb 4f_5/2 and Pb 4f_7/2) in the control and Gua-modified perovskite films. The two peaks of the Gua-treated perovskite film undergo approximately 0.11 and 0.07 eV shifts toward lower binding energy compared to the control sample, indicating effective Pb²⁺ defect passivation of Gua functional groups and well agreement with the calculated ADE in Figure 5d. Similarly, (36−38) a right shift of 0.08 eV also occurs at the core-level spectra of I 3d after the Gua modification (Figure 5h). (37,38) The downward shift of I 3d binding energy indicates the receiving of electrons and hindering of I atom aggregation. (37,38)

Moreover, in the XPS of SnO₂-Gua and Gua (Figure S10), N 1s shows a larger right shift of 0.28 eV than O 1s (a left shift of 0.2 eV). The larger XPS shift indicates a stronger interaction between –C═NH and SnO₂ than that of –COOH, further verifying the highest adsorption energy of –C═NH in Figure 5e. In the Fourier transform infrared spectroscopy (FTIR) of Gua (Figure S11), SnO₂-Gua, and Pb²⁺-Gua, –C═NH exhibits an obvious shift in both SnO₂-Gua and Pb²⁺-Gua films. However, the –COOH peak has only a weak shift in SnO₂-Gua, and the –NH peak has almost no deviation in the two films. The shift of the FTIR peak is mainly attributed to the chemical coordination interaction between Gua and SnO₂ or Pb²⁺, further demonstrating the dominant role of –C═NH on passivating defects on SnO₂ and perovskite films among the other functional groups. (38)

After modification of the SnO₂ and perovskite film by Gua, the photosensitive properties of PPDs with the configuration FTO/SnO₂(-Gua)/perovskite/P3HT/Ag were investigated. Figure S12 shows that the IP of the SnO₂-Gua PPD device exhibits a more linear increase with rising light intensity compared to the pristine device. However, in Figure 6a, the dark current (I_D) of the SnO₂/Gua-based PPD device decreases slightly to 1.11 × 10^–8 A/cm², compared to 1.43 × 10^–8 A/cm² for the control PPD device without an external bias voltage, which is consistent with the noise current (n_i) spectrogram in Figure 6b for both PPD devices. To provide a realistic comparison and avoid instrument limitations, the dark currents of the control group and the Gua-modified devices under −0.02 and a −1 V bias were 5.68 × 10^–7, 1.04 × 10^–4, 1.55 × 10^–9, and 5.14 × 10^–5 A/cm², respectively. (48) In addition, a similar dark current trend in the control group and the Gua-modified device is observed at bias voltages of 0.02 and 1 V, with measured values of 1.09 × 10^–7 and 8.28 × 10^–6 A/cm for the control group and 1.81 × 10^–8 and 1.32 × 10^–6 A/cm² for the Gua-modified device, respectively. It further confirms that the Gua compound can effectively reduce the dark current in PPD devices. In Figure 6c, the devices based on PVK-Gua show a higher external quantum efficiency (EQE) spectrum than the control device because of their larger crystal grain size and stronger optical absorption. Based on the EQE spectrum, the responsivity (R) for quantifying the detection efficiency of PPDs can be calculated as $R=\frac{\mathrm{E}\mathrm{Q}\mathrm{E}\lambda q}{hc}$, (9,49) where λ is the wavelength, q is the elementary charge, h? is Planck constant, and c is the lightspeed in the vacuum. In Figure 6d, the R-value of PVK-Gua is higher than control PPDs in the wavelength range from 300 to 580 nm and achieves a maximum R of 0.30 and 0.27 A/W at 0 V bias, respectively. Then, the specific detectivity (D*) for the measuring of the weakest light detection capability can be obtained by D* = R $\sqrt{A/(2e{I}_{\mathrm{d}})}$, (50) where A is the effective photoactive area for the PPD devices. In Figure 6e, the low I_d and high R in PVK-Gua PPD create a dramatically high D* of 1.32 × 10¹³ Jones compared to the control one (4.02 × 10¹² Jones). The high specific detectivity is very competitive among the recently reported all-inorganic perovskite PPDs (Table S3).

The linear dynamic range (LDR) which is conducted to assess the limits of a detector in detecting strong and weak light can be given by $\mathrm{L}\mathrm{D}\mathrm{R}=20?\log ?\frac{I_{\mathrm{u}\mathrm{p}\mathrm{p}\mathrm{e}\mathrm{r}}}{I_{\mathrm{l}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}}}$, (51) where I_upper is the upper limit of I_P at which the photoresponse of the PPDs deviates from linearity, and I_lower is the lower limit. In Figures 6f and S13, the photocurrent of the device treated by Gua exhibits a much higher LDR of 86 dB than the control device (50 dB) in broad light intensity response ranges from 0.02 to 500 mW/cm², enabling flexible and accurate detection of weak and strong light. Then, the I–T curves of PPD devices were obtained by the 405 nm optical laser with different intensities irradiated, as depicted in Figures S14–S17, and the corresponding rise and fall time are summarized in Tables S5 and S6. Both PPD devices exhibit preferable on/off characteristics (1–500 mW/cm²). However, the Gua-modified PPD device still shows a clear time-resolved photocurrent curve at a low light intensity of 0.02 mW/cm². In comparison, the photocurrent curve already falls into chaos at 0.05 mW/cm² (Figure 6g). Moreover, the PPD device based on Gua shows an 8.8 times higher on/off ratio (Figure 6h) than the control devices due to the high-quality perovskite film. Meanwhile, the Gua-modified PPD device also possesses a faster response with a T_rise/T_fall of 46.84/77.57 μs than that of the pristine device (a T_rise/T_fall of 142.02/156.84 μs) as depicted in Figure S18. This faster response time can be attributed to the reduced trap density and more favorable band matching in the modified PPD device (Figures 3, S19, and S20), allowing charges to move more efficiently through the material. The above results indicate that the low N_d of perovskite and efficient carrier extraction are realized after the Gua treatment, thus contributing to faster photoresponsivity and higher detectivity. Furthermore, based on the analysis of the frequency response characteristics, the 3 dB cutoff frequencies (f_3dB) of the unmodified and Gua-modified PDs are calculated to be approximately 3.7 kHz and 20 kHz (Figure S21), respectively. The enhanced f_3dB properties, which stem from defect reduction, indicate that the modified device’s operating bandwidth has been significantly improved. (52)

The response speed is a critical factor that indicates the tracking capability of PDs for high-frequency optical signals. (49) Fast and consistent responses to light are essential for enhancing the application of PPDs in imaging and optical communication domains. (53) A mechanical rotating chopper is utilized to adjust the frequency of the incident light coming from a 405 nm visible laser for irradiation. Figures 7a,b and S22 illustrate the photocurrent response data of both the control and the Gua-engineered PDs at different frequencies. The Gua-engineered PPD device (Figure 7b) demonstrates a stable and consistent transient response across a broad response time range compared with the control device (Figure 7a) at 3990 Hz for 405 nm detection. In addition, the control PPD device even at the lower frequency (Figure S22) cannot demonstrate a preferable square response signal compared to the Gua-modified device. Additionally, stable PPD devices with long-term durability are crucial for future commercial applications. The PPD devices were exposed to air with an ambient humidity of 45% and illuminated by a 405 nm laser to characterize their stability (Figure 7c). The Gua-modified PPD device maintained 90% of its initial performance, even after enduring 900 s of intense laser irradiation (500 mW/cm²), which is higher than the 85% performance retention observed in the pristine device. Then, the unencapsulated devices were exposed to air for 240 h and measured with the same strong laser above to demonstrate their long-term stability. As shown in Figure 7d, the Gua-modified PPD device still maintains 95.4% of its initial response. In comparison, it is only 81.4% for the control device, manifesting the excellent durability of the Gua-modified PPD device under humid and strong irradiative conditions. This stability is comparable to that reported in recent studies on perovskite solar cells, where unpackaged devices exposed to an environment with approximately 50% to 60% humidity for 400 h demonstrated the ability to retain 92.7% of their initial performance. (54) Previous studies have indicated that excessive oxygen vacancy defects at the buried interface of ETL could significantly accelerate the deterioration of perovskite devices. (52) The observed enhanced stability in Gua-modified devices can be attributed to the reduction of oxygen vacancy defects on the SnO₂ interface facilitated by the buried Gua compound, thus substantially bolstering the environmental durability of the device. Gua can not only reduce the dual-interface defects of SnO₂ and perovskite films and improve the device performance of PPDs but also enhance their long-term stability greatly. Furthermore, the halogen element distribution of both unencapsulated devices after 10 days of storage is also tested (Figure S23). The Gua-modified devices exhibited fewer pinholes and less severe bromine element enrichment, indicating the effectiveness of Gua modification in enhancing device stability.