Highly Efficient and Stable Self-Powered Perovskite Photodiode by Cathode-Side Interfacial Passivation with Poly(Methyl Methacrylate)

For several years now, organic–inorganic hybrid perovskite materials have shown remarkable progress in the field of opto-electronic devices. Herein, we introduce a cathode-side passivation layer of poly(methyl methacrylate) (PMMA) for a highly efficient and stable self-powered CH3NH3PbI3 perovskite-based photodiode. For effective noise–current suppression, the PMMA passivation layer was employed between a light-absorbing layer of CH3NH3PbI3 (MAPbI3) perovskite and an electron transport layer of [6,6]-phenyl-C61-butyric acid methyl ester. Due to its passivation effect on defects in perovskite film, the PMMA passivation layer can effectively suppress interface recombination and reduce the leakage/noise current. Without external bias, the MAPbI3 photodiode with the PMMA layer demonstrated a significantly high specific detectivity value (~1.07 × 1012 Jones) compared to that of a conventional MAPbI3 photodiode without a PMMA layer. Along with the enhanced specific detectivity, a wide linear dynamic response (~127 dB) with rapid rise (~50 μs) and decay (~17 μs) response times was obtained. Furthermore, highly durable dynamic responses of the PMMA-passivated MAPbI3 photodiode were observed even after a long storage time of 500 h. The results achieved with the cathode-side PMMA-passivated perovskite photodiodes represent a new means by which to realize highly sensitive and stable self-powered photodiodes for use in developing novel opto-electronic devices.


Methods
Pre-patterned 80-nm-thick indium tin oxide (ITO, 20 Ω/sq) layers on glass substrates were used as transparent anodes for the photodiodes. The ITO substrates used here were ultrasonically cleaned with ethanol, a detergent, and deionized (DI) water, after which they underwent an ultraviolet ozone treatment in an oven for 10 min.
For the NiO x HTL, a precursor solution with 29 mg of Ni(NO 3 ) 2 ·6H 2 O dissolved in mixed solvents of EG and IPA at a 6:4 volume ratio was prepared by stirring for 3 h. The precursor solution was then spin-coated onto the ITO substrate at 2600 rpm for 40 s, followed by 60 min of annealing at 300 • C to form the a 10-nm-thick NiO x HTL. Subsequently, the substrates coated with the NiO x layer were placed in a nitrogen-filled glovebox.
To fabricate the perovskite light-absorbing active layers of MAPbI 3 , an antisolventassisted spin-coating method was used [25,26]. The perovskite precursor solution was prepared by dissolving PbI 2 and MAI at a 1:1 molar ratio in a mixed polar solvent of DMF and DMSO at an 8:2 volume ratio, with the precursor solution then stirred overnight. In the nitrogen-filled glovebox, the precursor solution was spin-coated onto the NiO x -coated substrate at 4400 rpm for 30 s. During the spinning process, anhydrous CB or CB containing 0.05 wt% of PMMA was dropped onto the perovskite precursor-coated substrate as an antisolvent at a dripping delay time of 5-8 s after the spinning process had started. When the dripping time was changed from 5-8 s, the film quality of the fabricated perovskite layer was degraded and the film became inhomogeneous and hazy. Hence, in this study, we set the dripping time to 5-8 s to fabricate a uniform and homogeneous perovskite precursor layer without or with a cathode-side PMMA passivation layer. The perovskite precursor layer was then dried at room temperature for 5 min and subsequently annealed at 100 • C for 20 min to crystallize the MAPbI 3 . The thicknesses of the MAPbI 3 layers studied here were nearly identical to each other (~250 nm), and the thickness of the PMMA layer was estimated to be less than 5 nm.
Thereafter, PCBM 60 in CB and a colloidal suspension of ZnO nanoparticles were subsequently spin-coated above the coated layers, resulting in a uniform 50-nm-thick PCBM 60 layer and a 20-nm-thick layer of ZnO as ETLs. The substrates were then transferred to a vacuum chamber for thermal evaporation of the 12-nm-thick BCP layer and 70-nmthick Al cathode layer, subsequently at a base pressure below 2.0 × 10 −6 torr. Thus, a photodiode with the structure of [ITO/NiO x /MAPbI 3 /PMMA/PCBM 60 /ZnO/BCP/Al] was fabricated. All fabricated photodiodes had an active area of 6 mm 2 .

Characterization
The surface morphology of the MAPbI 3 perovskite layer was analyzed with a scanning electron microscope (Inspect F50, FEI Company, Hillsboro, OR, USA). ImageJ software (National Institutes of Health, Bethesda, MD, USA) was used to analyze the grain size distributions in the MAPbI 3 perovskite layers from the observed SEM images.
The UV-Visible optical adsorption spectra were studied using an UV-Vis spectrometer (Agilent 8453 Diode Array UV-VIS, Agilent Technologies, Inc., Santa Clara, CA, USA). The contact angles of the fabricated MAPbI 3 films were measured with a contact angle goniometer (Ossila, London, UK).
A monochromatic light source of a 637 nm diode laser (model COMPACT-100G-637-A, 100 mW, maximum internal modulation frequency: 50 kHz, World Star Tech, Markham, OT, Canada) was used to assess the performance of the photodetector. Keithley source meters (models 2400 and 2636, Tektronix, Inc. Beaverton, OR, USA) were used to assess the current-versus-voltage (J-V) characteristics. An incident photon-to-current conversion efficiency (IPCE) measurement system (ORIEL IQE-200, Newport, Irvine, CA, USA) was used to measure the spectral responsivity, R(λ), of the photodiode.
The noise current levels of the photodiode were obtained from the fast Fourier transform of the measured dark currents of the photodiode as a function of time using the 2636 Keithley source meter operated at a sampling rate of 1 kHz. The 3 dB cutoff bandwidths of the photodiodes were estimated from the logarithmic transform of their normal-ized photo responses, measured as a function of the modulation frequency of the irradiated light using the 637 nm laser system.

Characteristics of MAPbI 3 Layers with a PMMA Passivation Layer
A schematic illustration of the passivation layer of the PMMA on a MAPbI 3 perovskite layer is shown in Figure 1a. As illustrated in the figure, PMMA dissolved in an antisolvent can simply be spin-coated during the formation of a 250-nm-thick MAPbI 3 perovskite lightabsorbing layer. To verify the formation of the PMMA passivation layer on the perovskite layer, first, we observed the contact angles of DI water on the fabricated films ( Figure 1b). As shown in the figure, the water contact angle on the NiO x /MAPbI 3 /PMMA perovskite film (sample) was 37.4 • , significantly higher than that (~26.8 • ) of the NiO x /MAPbI 3 perovskite film (reference). This increase in the water contact angle mainly stems from the intrinsic hydrophobicity of the PMMA layer in the sample film, providing evidence of the simple and reliable fabrication method of the PMMA passivation layer on the perovskite layer. Thus, such a hydrophobic PMMA passivation layer may affect the growth of the grains of the underlying MAPbI 3 perovskite by tailoring the nucleation of the perovskite crystal growth [3,13,27]. It is noteworthy that the coated PMMA polymers mainly existed at the perovskite/ETL interface, with some of the polymer possibly present inside the perovskite layer, similar to an earlier conjugated polymer that formed an interfacial passivation layer [20].  To assess the effect of the PMMA passivation layer on the growth of grains for the MAPbI3 perovskite films, scanning electron microscopy (SEM) was utilized. The surface morphologies obtained from the SEM observations are shown in Figure 1c. As shown in these SEM images, the surfaces of both perovskite films were fairly smooth and showed well-packaged grains. However, compared to the reference film without a passivation layer, it is clear that the introduction of the PMMA layer affected the increment of the grain size while also decreasing the number of grain sites in the sample perovskite film.
For an additional quantitative analysis, ImageJ software was used to adjust the contrast of the SEM images of the MAPbI3 perovskite layers without/with a PMMA To assess the effect of the PMMA passivation layer on the growth of grains for the MAPbI 3 perovskite films, scanning electron microscopy (SEM) was utilized. The surface morphologies obtained from the SEM observations are shown in Figure 1c. As shown in these SEM images, the surfaces of both perovskite films were fairly smooth and showed well-packaged grains. However, compared to the reference film without a passivation layer, it is clear that the introduction of the PMMA layer affected the increment of the grain size while also decreasing the number of grain sites in the sample perovskite film.
For an additional quantitative analysis, ImageJ software was used to adjust the contrast of the SEM images of the MAPbI 3 perovskite layers without/with a PMMA passivation layer ( Figure 1c) and to identify the borders of the grains in the SEM images. Then, utilizing the black and white threshold of each adjusted SEM image, it was possible to estimate the range of the domain sizes of the surrounding grains. Histograms of the domain size distributions for MAPbI 3 perovskite layers without/with a PMMA passivation layer are presented in Figure 1d. As shown in this figure, the average grain size of the reference film was~151 nm, while that of the sample film was~196 nm, clearly demonstrating that the average grain size of the MAPbI 3 layer was increased significantly due to the presence of the PMMA passivation layer. From these SEM studies, it was confirmed that the generation of unnecessary MAPbI 3 nucleation could be significantly suppressed by the coordinate bonding of the carbonyl of the PMMA polymer with the uncoordinated Pb ions of MAPbI 3 [28][29][30]. Thus, the PMMA passivation layer enables an increase in the MAPbI 3 grain size while also mitigating film defects such as spikes and/or pinholes [31,32].
Apart from the increased grain size of the MAPbI 3 perovskite layer, the PMMA passivation layer can effectively cover defects and grain boundaries in the perovskite layer, possibly providing an effective passivation route for interface defects between the MAPbI 3 layer and the adjacent ETL [33,34]. Moreover, due to the improved interface quality of the MAPbI 3 layer stemming from the PMMA, the reduction in the recombination loss of charge carriers can improve the charge carrier extraction from the MAPbI 3 layer to the charge transport layers [31,32,[34][35][36]. Thus, given these properties of the MAPbI 3 layer in the sample films, we hold that the introduction of a PMMA passivation layer can effectively improve the film and interface qualities of the MAPbI 3 layer, likely leading to an improvement in the optoelectronic performance of perovskite devices, even without any critical changes in the optical characteristics such as the optical absorption capabilities ( Figure 1e) [25]. To confirm this, the optical absorption characteristics of the reference and sample perovskite films were also investigated, as shown in Figure 1e. In this figure, strong absorption in the visible range of 450~700 nm for the reference and sample films could be observed. Notably, the sample film showed an optical absorption nearly identical to that of the reference perovskite film, proving that the PMMA passivation layer did not significantly change or deteriorate the optical absorption properties of the perovskite layer. Thus, by introducing the PMMA passivation layer onto the perovskite layer, modification of the interface between the MAPbI 3 layer and the ETL can be anticipated.

Characterization of Perovskite Photodiodes without and with a PMMA Passivation Layer
Inspired by the desirable passivation effect of PMMA described above, we fabricated MAPbI 3 photodiodes with the device structure illustrated in Figure 2a. As shown in the figure, ITO acts as a transparent anode, with NiO x used as a HTL, MAPbI 3 as a perovskite lightabsorbing layer, and PMMA as the cathode-side passivation layer. PCBM 60 , ZnO nanoparticles and BCP were utilized as the ETLs and Al was used as the top cathode electrode.
To evaluate the device performance of the fabricated MAPbI 3 photodiodes without (reference) and with a PMMA passivation layer (sample), the dark-current densities (J dark ) were investigated as a function of the applied voltage (J dark -V). As indicated in Figure 2b, excellent diode characteristics with large rectifying ratios (RRs) and low leakage current densities were found in these photodiodes. Due to the suppression of the leakage currents by the PMMA passivation layer, a somewhat higher RR value, 6.1 × 10 5 (at 1.0 V), for the sample was obtained compared to that (5.3 × 10 5 ) of the reference, indicative of the useful film properties of the MAPbI 3 perovskite films, as above-mentioned. It is also noteworthy that the observed RR values were one order higher than the corresponding values in the literature [13]. Similarly, the PMMA passivation layer in the sample led to a reduced average J dark value of 5.1 × 10 −7 mA/cm 2 at zero applied voltage, lower than that (7.3 × 10 −7 mA/cm 2 ) of the reference without a PMMA layer, providing evidence of the suppressed leakage currents caused by the improved interfacial quality of the perovskite light-absorbing layer. and the ETL can be anticipated.

Characterization of Perovskite Photodiodes without and with a PMMA Passivation Layer
Inspired by the desirable passivation effect of PMMA described above, we fabricated MAPbI3 photodiodes with the device structure illustrated in Figure 2a. As shown in the figure, ITO acts as a transparent anode, with NiOx used as a HTL, MAPbI3 as a perovskite light-absorbing layer, and PMMA as the cathode-side passivation layer. PCBM60, ZnO nanoparticles and BCP were utilized as the ETLs and Al was used as the top cathode electrode. To evaluate the device performance of the fabricated MAPbI3 photodiodes without (reference) and with a PMMA passivation layer (sample), the dark-current densities (Jdark) were investigated as a function of the applied voltage (Jdark-V). As indicated in Figure 2b, excellent diode characteristics with large rectifying ratios (RRs) and low leakage current densities were found in these photodiodes. Due to the suppression of the leakage currents by the PMMA passivation layer, a somewhat higher RR value, 6.1 × 10 5 (at 1.0 V), for the sample was obtained compared to that (5.3 × 10 5 ) of the reference, indicative of the useful film properties of the MAPbI3 perovskite films, as above-mentioned. It is also noteworthy that the observed RR values were one order higher than the corresponding values in the literature [13]. Similarly, the PMMA passivation layer in the sample led to a reduced average Jdark value of 5.1 × 10 −7 mA/cm 2 at zero applied voltage, lower than that (7.3 × 10 −7 mA/cm 2 ) of the reference without a PMMA layer, providing evidence of the suppressed Furthermore, for an in-depth examination of the perovskite photodiodes presented here, their trap-filled limit voltages (V TFL s) were estimated from the J dark -V curves on a loglog scale (Figure 2c) [13,20,32,37,38]. The obtained V TFL values for the reference and sample were~0.49 and 0.47 V, respectively. Based on the V TFL values, the trap density states (n trap ) can also be estimated using the space-charge-limited current model, as follows [38,39]: In this equation, q is the elementary charge, L is the thickness of the perovskite layer, while ε 0 and ε p are the dielectric constants of the vacuum and perovskite, respectively. The estimated n trap values were around 1.30 × 10 15 cm −3 for the reference and 1.25 × 10 15 cm −3 for the sample, indicating that the PMMA layer in the sample could effectively passivate defects at the interface between the perovskite layer and the ETL.
Subsequently, we measured the photocurrents as a function of the bias voltage in the range of ±1.2 V (I light -V) for several input power levels (Ps) of incident laser light with a wavelength of λ = 637 nm, as shown in Figure 3a. In the figure, the open-circuit potential (V OC ) and the short-circuit current (I SC ) increased considerably as the input power of the incident light was increased. For a detailed comparison, the built-in potentials (V bi s) of the reference and sample were obtained using the Shockley diode model equation, expressed as follows, where k B is the Boltzmann's constant; T is the temperature; n is the ideality factor; and J 0 is the reverse-saturated current density [13,[40][41][42]. The estimated values of V bi and n were approximately 0.58 V and 1.49, respectively, for the sample, slightly higher than the V bi value of approximately 0.57 V and the n value of about 1.47 for the reference. As the thickness of the PMMA layer was considered to be less than~5 nm, it is clear that the PMMA passivation layer did not degrade the built-in potential despite its intrinsic insulating properties. It should be noted that the short-circuit current I SC and built-in potential V bi began to decrease noticeably as the thickness of each functional layer, especially the PMMA layer or the MAPbI 3 layer, was changed from its optimized thickness.
value of approximately 0.57 V and the n value of about 1.47 for the reference. As the thickness of the PMMA layer was considered to be less than ~5 nm, it is clear that the PMMA passivation layer did not degrade the built-in potential despite its intrinsic insulating properties. It should be noted that the short-circuit current ISC and built-in potential Vbi began to decrease noticeably as the thickness of each functional layer, especially the PMMA layer or the MAPbI3 layer, was changed from its optimized thickness.
where κ is a proportional constant and θ is the power law index, the trap states existing in the MAPbI 3 perovskite light-absorbing layer studied here can be analyzed [13,20,43]. We obtained θ values for both the reference and sample using the power law above with the best fitted parameter values, determining an θ value of 0.999 for the sample, which is much closer to θ = 1.0 for an ideal photodiode compared to the value of θ = 0.984 for the reference without a PMMA layer. Hence, the cathode-side PMMA passivation layer can effectively decrease the number of trap states and reduce the second-order recombination loss under a short-circuit condition (i.e., self-powered condition). Next, based on the photocurrent data in Figure 3b, the responsivity at the wavelength of λ (R λ ) for the reference and sample were estimated using the equation.
where I PH denotes the net photocurrent [7,9,[12][13][14]20]. The responsivity R 637 value at the wavelength λ = 637 nm of incident light under zero bias voltage as obtained here was approximated as 360 mA/W for the sample with the PMMA passivation layer, higher than that (~352 mA/W) of the reference at zero bias voltage.
To evaluate the photodiode performance over a wide light intensity range, we also calculated the linear dynamic range (LDR) of the photodiodes using the equation [7,9,12,14,20]: Even at zero bias voltage, a notably higher LDR value was realized,~127 dB, for the sample compared to that (~124 dB) of the reference. These high values of R λ and LDR are clear evidence of the excellent photoelectric conversion ability and remarkably good linearity over a wide range of incident light intensity for the sample with the cathode-side PMMA passivation layer. It should also be noted that the sample device showed a relatively high LDR value compared to those of previous perovskite thin-film photodetectors and industrial silicon photodetectors (90~120 dB). Given the nearly ideal value of θ and the high values of R λ and LDR, the cathode-side PMMA passivation layer can serve to realize trap-less perovskite photodiodes.
For a further evaluation of the photodiode performance, the R λ spectra were also measured using an IPCE system in the self-powered condition, as presented in Figure 4a. This figure indicates that the peak R λ value of~401 mA/W for the sample was notably higher than that (386 mA/W) for the reference and higher than those of the MAPbI 3based photodiodes in the literature [9,13,25]. This finding clearly shows that the improved interface of the MAPbI 3 layer for efficient charge extraction is crucial when attempting to realize high responsivity from perovskite photodiodes. Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of 14 somewhat lower than that (0.96 pA Hz −1/2 ) of the reference. This indicates that the proposed cathode-side PMMA passivation layer clearly suppresses the noise level by passivating unwanted film defects at the interface between the MAPbI3 layer and the PCBM60 ETL. Next, the obtained values of in and Rλ were applied to assess the noise equivalent power (NEP) in an effort to determine the minimum detectable incident optical power using the relationship of [7,9,11,12,14,45,46]: The obtained minimum NEP value for the sample was ~230 fW/Hz 1/2 , notably less than that (~2520 fW/Hz 1/2 ) for the reference at zero bias voltage. This clearly indicates that the cathode-side PMMA passivation layer can enhance the low-power-detection ability of the MAPbI3 photodiode compared to a non-passivated MAPbI3 photodiode.
Based on the NEP values presented above, the specific detectivity D * values of the photodiodes were also evaluated under the self-powered condition in order to evaluate their weak-signal detection capacities. This was carried out with the following equation, where and are the active area of the device and the 1 Hz specific bandwidth, respectively [7,[9][10][11][12][13][14]20,[44][45][46][47][48][49]. In the self-powered condition, the estimated specific detectivity D * spectra for the reference and sample using Equation (7) are plotted in Figure  4c. As shown in this figure, while the peak value of D * was ~0.97 × 10 11 Jones for the reference, the peak value of D * for the sample was ~1.07 × 10 12 Jones, which is more than eleven times greater for weak-signal detection compared to the reference. Hence, the suppression of unnecessary leakage/noise currents by the cathode-side PMMA passivation layer clearly increased the photodetector performance outcomes. Additionally, the D * value of 1.07 × 10 12 Jones for the sample was conspicuously higher than those (D * value ~0.81 × 10 12 Jones) of the MAPbI3 photodiodes in the literature with an anode-side PMMA passivation layer at the interface between the MAPbI3 perovskite light-absorbing layer and the NiOx HTL; this was also comparable to those of commercial silicon photodetectors [14,20,25,47].
As another comparison, we also estimated the conventional simplified specific detectivity D * values of the reference and sample based on the shot noise ( , ), as estimated from the dark-current density ( ) with the simple relationship of * ~ / 2 [7,9,13,14,48,49]. The estimated peak value of the simplified D * for the Subsequently, we analyzed the noise current (i n ) of the photodiodes. At an ambient temperature, the dark currents (I dark ) were measured over time with no voltage applied. Subsequently, the dark currents were fast Fourier transformed and plotted as a function of the frequency, as shown in Figure 4b [7,10,11,20,44], where the photodiodes exhibited white noise unrelated to the frequency in the ranges observed here. Additionally, as shown in the figure, the noise level of each photodiode was evaluated at a bandwidth of 1 Hz. The estimated noise level of the sample was~0.09 pA Hz −1/2 at a bandwidth of 1 Hz, somewhat lower than that (0.96 pA Hz −1/2 ) of the reference. This indicates that the proposed cathodeside PMMA passivation layer clearly suppresses the noise level by passivating unwanted film defects at the interface between the MAPbI 3 layer and the PCBM 60 ETL.
Next, the obtained values of i n and R λ were applied to assess the noise equivalent power (NEP) in an effort to determine the minimum detectable incident optical power using the relationship of [7,9,11,12,14,45,46]: The obtained minimum NEP value for the sample was~230 fW/Hz 1/2 , notably less than that (~2520 fW/Hz 1/2 ) for the reference at zero bias voltage. This clearly indicates that the cathode-side PMMA passivation layer can enhance the low-power-detection ability of the MAPbI 3 photodiode compared to a non-passivated MAPbI 3 photodiode.
Based on the NEP values presented above, the specific detectivity D * values of the photodiodes were also evaluated under the self-powered condition in order to evaluate their weak-signal detection capacities. This was carried out with the following equation, where A and B are the active area of the device and the 1 Hz specific bandwidth, respectively [7,[9][10][11][12][13][14]20,[44][45][46][47][48][49]. In the self-powered condition, the estimated specific detectivity D * spectra for the reference and sample using Equation (7) are plotted in Figure 4c. As shown in this figure, while the peak value of D * was~0.97 × 10 11 Jones for the reference, the peak value of D * for the sample was~1.07 × 10 12 Jones, which is more than eleven times greater for weak-signal detection compared to the reference. Hence, the suppression of unnecessary leakage/noise currents by the cathode-side PMMA passivation layer clearly increased the photodetector performance outcomes. Additionally, the D * value of 1.07 × 10 12 Jones for the sample was conspicuously higher than those (D * value~0.81 × 10 12 Jones) of the MAPbI 3 photodiodes in the literature with an anode-side PMMA passivation layer at the interface between the MAPbI 3 perovskite light-absorbing layer and the NiO x HTL; this was also comparable to those of commercial silicon photodetectors [14,20,25,47].
As another comparison, we also estimated the conventional simplified specific detectivity D * values of the reference and sample based on the shot noise (i n,s ), as estimated from the darkcurrent density (J dark ) with the simple relationship of D * ∼ R λ / √ 2eJ dark [7,9,13,14,48,49]. The estimated peak value of the simplified D * for the reference was~2.5 × 10 13 Jones. In contrast, the peak value of simplified D * for the sample showed an increase to~4.5 × 10 13 Jones. It should also be noted that the simplified D * value for the sample was significantly higher compared to earlier values for other MAPbI 3 thin-film-based photodiodes previously reported in the literature [25,48,49]. However, such simplified D * values may be overestimated because the shot noise levels were lower than their measured noise current values, as indicated by the dotted lines in Figure 4b [49].
To assess the dynamic characteristics of the photodiodes studied here, the temporal responses were investigated at zero bias voltage by irradiating monochromatic light (λ = 637 nm, P = 190 µW) modulated at a frequency of 2 kHz (Figure 5a). In Figure 5a, the measured temporal responses of the sample clearly indicated higher photocurrents than those of the reference, as expected. The response times of the rise (τ r ) and decay (τ d ) times were determined as the time intervals required to change the photocurrent signal amplitudes between 10% (I 10 ) and 90% (I 90 ) up the rising and decay edges of the signal curves, respectively; τ r and τ d for the sample were~50 and~17 µs, respectively, similar to those (~49 and~18 µs) of the reference. Moreover, the obtained response times for the sample and reference were considerably shorter than those of MAPbI 3 -based photodiodes in the literature [13].
Nanomaterials 2023, 13, x FOR PEER REVIEW 10 of 14 reference was ~2.5 × 10 13 Jones. In contrast, the peak value of simplified D * for the sample showed an increase to ~4.5 × 10 13 Jones. It should also be noted that the simplified D * value for the sample was significantly higher compared to earlier values for other MAPbI3 thinfilm-based photodiodes previously reported in the literature [25,48,49]. However, such simplified D * values may be overestimated because the shot noise levels were lower than their measured noise current values, as indicated by the dotted lines in Figure 4b [49].
To assess the dynamic characteristics of the photodiodes studied here, the temporal responses were investigated at zero bias voltage by irradiating monochromatic light (λ = 637 nm, P = 190 μW) modulated at a frequency of 2 kHz (Figure 5a). In Figure 5a, the measured temporal responses of the sample clearly indicated higher photocurrents than those of the reference, as expected. The response times of the rise (τr) and decay (τd) times were determined as the time intervals required to change the photocurrent signal amplitudes between 10% (I10) and 90% (I90) up the rising and decay edges of the signal curves, respectively; τr and τd for the sample were ~50 and ~17 μs, respectively, similar to those (~49 and ~18 μs) of the reference. Moreover, the obtained response times for the sample and reference were considerably shorter than those of MAPbI3-based photodiodes in the literature [13]. Next, the 3 dB cutoff bandwidths (f-3dB) for the photodiodes were measured. These values were f-3dB = ~1.40 × 10 4 and ~1.33 × 10 4 Hz for the reference and sample, respectively (Figure 5b). These dynamic photo responses of the photodiodes indicate that the limitation of the response bandwidths in the devices may stem mainly from the RC time constants of the devices, implying that additional improvements in the response speed can be achieved by greater optimization of the device architecture [45]. Additionally, it should be noted that as the modulation frequency applied to the irradiated laser light was increased, the observed rise and decay times began to decrease continuously in the frequency range investigated here, as shown in the inset of Figure 5b.
Finally, we investigated another important functionality of the PMMA passivation Next, the 3 dB cutoff bandwidths (f -3dB ) for the photodiodes were measured. These values were f -3dB =~1.40 × 10 4 and~1.33 × 10 4 Hz for the reference and sample, respectively (Figure 5b). These dynamic photo responses of the photodiodes indicate that the limitation of the response bandwidths in the devices may stem mainly from the RC time constants of the devices, implying that additional improvements in the response speed can be achieved by greater optimization of the device architecture [45]. Additionally, it should be noted that as the modulation frequency applied to the irradiated laser light was increased, the observed rise and decay times began to decrease continuously in the frequency range investigated here, as shown in the inset of Figure 5b.
Finally, we investigated another important functionality of the PMMA passivation layer based on its excellent hydrophobicity, as shown in Figure 1b. Herein, the storage stability of the photodiodes was measured in terms of the photocurrent flowing through the photodiode devices. Given that the storage lifetime is closely linked to water permeation into the functional layers, particularly the MAPbI 3 active layer, measuring the temporal responses of the stored photodiodes can provide information pertaining to the degradation mechanisms in these devices [50].
To measure the storage stability, the devices in this study were stored in a nitrogen glovebox between successive measurements of their temporal responses at a regular time interval of approximately 48 h. Temporal response measurements were also taken at zero bias at room temperature. Figure 5c presents the representative temporal response characteristics for the photodiodes stored for a total storage time of~500 h. As shown in the figure, the photocurrent of the reference was reduced significantly to~91.9% of its original value after being stored for approximately 500 h. In contrast, the sample exhibited a relatively negligible decrement (~0%) of the photo-response, even after the long storage time used here. Thus, as clearly shown in the figure, the sample with the cathode-side PMMA passivation layer is much more stable than the reference without a PMMA layer, indicating once again that the PMMA passivation layer significantly enhances the storage stability of MAPbI 3 perovskite photodiodes. Additional details related to the long-term and/or continuous stability of the photodiode with the PMMA passivation layer, together with a more stable Ag, Cu, or Au cathode instead of the Al cathode used here, will be discussed elsewhere.

Discussion
The observations above highlight the effects of the cathode-side PMMA passivation layer; it not only improved the quality of the film but also decreased the leakage/noise currents through the MAPbI 3 perovskite layers, thus providing a new paradigm of a high, fast, and durable performance of MAPbI 3 perovskite-based photodiodes. Additional developments of organic/inorganic perovskite layers and/or the introduction of other novel functional layers will result in further improvements in the device performance for self-powered, highly sensitive, and stable solution-processable perovskite photodiodes with a cathode-side PMMA passivation layer.

Conclusions
In this study, we demonstrated that the introduction of a cathode-side PMMA passivation layer between an absorber layer of MAPbI 3 and the ETL effectively suppressed the leakage/noise currents and in turn improved the performance of the self-powered solution-processable MAPbI 3 photodiodes. With the cathode-side PMMA passivation layer, the interface quality of the MAPbI 3 perovskite layer was shown to be improved, and the film defects were reduced. Moreover, we noted an increase in the grain size and effective decrements in the recombination losses as well as the noise currents. Consequently, even at zero bias voltage, the MAPbI 3 photodiode with the PMMA passivation layer exhibited a notably high specific detectivity D * of~1.07 × 10 12 Jones. Moreover, a significantly low NEP of~230 fW/Hz 1/2 and wide LDR of~127 dB were achieved. The MAPbI 3 photodiode with the PMMA passivation layer revealed excellent device performance compared to that of a conventional MAPbI 3 photodetector without a passivation layer. A rapid rise response time of 50 µs and decay response times of 17 µs were also obtained, as were highly durable dynamic responses of the photodiode, even after a long storage time of nearly 500 h. These findings provide clear evidence of the remarkably improved device performance of a perovskite photodiode with a noise-reducing PMMA passivation layer at the interface between the MAPbI 3 perovskite light-absorbing layer and the ETL. Thus, a cathode-side PMMA passivation layer in a perovskite photodiode, as presented here, can enhance the applicability of interface-engineered perovskites in photodiodes, imaging sensors, and various low-energy-consuming light-detecting devices. Data Availability Statement: Data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.