Additive Modulated Perovskite Microstructures for High Performance Photodetectors

Organic-inorganic hybrid perovskites have been widely used as light sensitive components for high-efficient photodetectors due to their superior optoelectronic properties. However, the unwanted crystallographic defects of perovskites typically result in high dark current, and thus limit the performance of the device. Herein, we introduce a simple route of microstructures control in MAPbI3 perovskites that associates with introducing an additive of 3,3,4,4-benzophenonetetracarboxylic dianhydridean (BPTCD) for crystallization adjustment of the perovskite film. The BPTCD additive can facilitate the formation of high-quality perovskite film with a compact and nearly pinhole-free morphology. Through characterizing the molecular interactions, it was found that the carbonyl groups in BPTCD is the key reason that promoted the nucleation and crystallization of MAPbI3. As a result, we obtained high-efficient and stable perovskite photodetectors with low dark current of 9.98 × 10−8 A at −0.5 V, an on/off ratio value of 103, and a high detectivity exceeding 1012 Jones over the visible region.

Micromachines 2020, 11, x 3 of 13 The current density-voltage-luminance (J-V-L) characteristics were tested in dark and under a white light (100 mW/cm 2 ) with a Keithley 4200 source. Quantum efficiency test system (Zolix SC 100, Beijing, China) was used to obtain the EQE spectra then detectivity was calculated based on the obtained EQE results. The absorption spectra were acquired on a Horiba 320 detector. The surface morphology and the cross-section view of the perovskite film were characterized by scanning electron microcopy (SEM, FEI Inspect F50, FEI Company, Eindhoven, The Netherlands). Surface morphologies of active layers were characterized by atomic force microscope (AFM, AFM 5500, Agilent, Tapping Mode, Chengdu, China). The crystalline structures were characterized by X-ray diffraction (XRD, D2 PHASER, Karlsruhe, Germany). The Thermo Scientific Escalab 250Xi with an ultraviolet photoelectron spectroscopy (UPS) system was used to measure the energy level of the perovskite layers. Fourier-transform infrared (FTIR) measurement was conducted with a FTIR spectrometer (Thermo Cientific, Nicole−10, Waltham, MA, USA). The impedance spectra were measured by Agilent precision impedance analyzer 4294A. All the measurements were performed in air at room temperature without encapsulation.  [12,13,36]. Such results indicate that the orthorhombic crystal structure exists in both pure perovskite and the BPTCD added perovskite films [37]. However, the intense diffraction peaks of the BPTCD added perovskite film demonstrate greater crystallinity. We note here that the enhancements of crystallinity are favorable to reduce the traps, suppress the recombination of charge carries and consequently lower the dark current.  Additionally, the pure perovskite film exhibits a weak diffraction peak at 12.5 • , which corresponds to the (001) plane of unconverted PbI 2 . The existence of PbI 2 is typically due to the inefficient reaction between the PbI 2 and MAI precursor or decomposition of MAPbI 3 induced by ambient exposure [37][38][39]. Owing to the passivation function of the PbI 2 , PbI 2 residues in the perovskite film could lower the opportunities of charge carrier recombination, thus lead to an improved photocurrent (I ph ) and EQE of PePDs [16,38].  To demonstrate the effect of the BPTCD additive upon device performances, the I ph and I d of PePDs were tested under white light irradiation and dark condition, respectively. The semi-log I-V characteristics of these PePDs are shown in Figure 2a and Figure S1. Typically, a low I d is a key factor for high performance PePDs [12,13,16,40]. Here the control device with pure perovskite film shows a high I d value of 2.93 × 10 −6 A at −0.5 V, while the I d for those PePDs with BPTCD additive reveal much lower values. The PePDs with 1, 2, 3 and 4 wt.% BPTCD show I d values of 6.73 × 10 −8 A, 1.06 × 10 −7 A, 9.98 × 10 −8 A and 7.06 × 10 −8 A at −0.5 V, respectively. Nevertheless, the I ph of the control device is slightly higher than that of the BPTCD added devices. The control device exhibits an I ph of 4.74 × 10 −4 A at −0.5 V while the I ph of the devices with 1−4 wt% BPTCD additive are 8.75 × 10 −5 A, 1.78 × 10 −4 A, 2.33 × 10 −4 A and 5.63 × 10 −5 A at −0.5 V respectively. Since the most important parameter for PePDs is D*, which is taken into account the contribution from both photocurrent (on current) and dark current (off current). The on/off current ratio of the devices is studied as shown in Figure 2b and Figure S1b. It can be seen that BPTCD added devices have much higher on/off current ratio (over 10 3 at −0.5 V) than the control device. In addition, when the concentration of BPTCD additive increases to 4 wt.%, the on/off current ratio decreases significantly. Although the device with 4 wt.% BPTCD additive shows a relatively low I d , its I ph is much lower than other devices as well.

Results
To further investigate the effect of the different concentration of BTPCD additives on photo-gain, the EQEs of the PePDs were calculated and analyzed. As displayed in Figure 2c and Figure S1, the control device shows a relatively higher EQE value exceeding 40 % in the Visible region, which is benefited from a better device performance under light condition proved by the above results of I-V characteristics. The BPTCD added devices also exhibit efficient photo response in the Visible region.
The EQE values of devices slightly increase with the increase of BPTCD concentration from 1 wt.% to 3 wt.%, and then decrease when the concentration is enhanced to 4 wt.%. Furthermore, according to the EQE spectra of the PePDs, we can estimate the D* of the devices over the Visible region based on the equations as follows: where c is the speed of light in a vacuum, q is the elementary charge of the electron. The calculated D* results at a bias of −0.1 V are shown in Figure 2d and Figure S1. Apparently, compared to the control device, the BPTCD added devices exhibit overwhelming D* values exceeding 10 12 Jones over the entire Visible region. In particular, the device with 3 wt.% BPTCD additive achieves the highest D* value of 4.55 × 10 12 Jones at a wavelength of 685 nm, where the enhancement of D* is attributed to the suppressed of I d . In addition, the photo response of the white light in PePDs with 3 wt.% BPTCD as a function of time was measured and shown in Figure 2e. The photocurrent of these devices is consistent and repeatable, which indicated good stability of the device. The spike liked cure of the initial part of on current is due to the time precision limit of our test equipment. Meanwhile, the temporal photo-response of the PePDs with 3 wt.% BTPCD was also measured ( Figure 2f). The rise and fall times are defined as the times for the transient current rising from 10% to 90% and decreasing from 90% to 10% of the maximum output current, respectively. The rise and fall times for the PePD with 3 wt.% BPTCD were approximately 850 ms and 800 ms, respectively, which was the detection limit of our equipment. The overall device performance of our PePDs with BPTCD and comparison with recent reported PePDs are summarized in Tables 1 and 2.  To disclose the reason for the suppressed I d in BPTCD added devices, the morphology of perovskite films with various concentrations of BPTCD additives was characterized by SEM ( Figure 3 and Figure S2). The pure perovskite film exhibits a relatively poor morphology with uneven grain sizes and many interior pinholes. These defects of the pure perovskite film can lead to a high leak current of the PePDs under the dark condition [33,37], which is also proved by I-V curve shown in Figure 2. After introducing BPTCD additive, the morphology of perovskite film changes significantly: the grain sizes become smaller and much more compact, thus resulting in a dense perovskite film. More significantly, the 4 wt.% BPTCD added perovskite film exhibits the smallest perovskite grain size, and the highest film coverage ratio. The improvement of film density can reduce the leak current of the devices, responsible for a suppressed I d . Atomic force microscopy (AFM) was also employed to study the morphology of perovskite films. As displayed in Figure S3, the perovskite film with 3 wt.% BPTCD shows smaller grain size and smoother surface than the pure perovskite. Larger grain size of pure perovskite film may lead to larger undulation thus rougher surface than perovskite film with 3 wt.% BPTCD. Benefited from smooth surface of perovskite film with 3 wt.% BPTCD, the contact interface property between perovskite film and PCBM film can be improved to suppress the leak current, resulting in low dark current. However, as shown in Figure S1, photocurrents of the devices with BPTCD additives decrease. Especially in the case of 4% BPTCD, the photocurrent is decrease by nearly one order compared to the control device. This phenomenon could be attributed to the decrease of grain size of MAPbI 3 microstructure that would also lower the performance of the PePDs under light irradiation by unexpected loss of photo-induced charge carriers [39]. Benefited from the small-sized perovskite grains for 4 wt.% BPTCD added device, I d decreased obviously, however, the I ph also decreases, thus cause a performance deterioration. Therefore, a scrupulous balance between the grain size and film density should be considered for achieving high performance PePDs.
To illuminate the molecular interaction between BPTCD and PbI 2 , FTIR measurement was employed. As reported by Bi et al. [38], the C=O group can interact with PbI 2 and trigger heterogeneous nucleation of MAPbI 3 , thus improve the crystallinity of the perovskite film. Similar results were also reported by Peng et al. [42], such that the C=O groups are responsible for the passivation of perovskite film via Lewis-base electronic passivation of Pb 2+ ions, which reduces trap density and enhances the morphology of perovskite film. Here the used BPTCD as an additive is rich of C=O groups, which may share the similar effect on the improvement of perovskite film quality. As displayed in Figure 4a, the stretching vibration of C=O groups in the pure BPTCD shows at 1749 cm −1 , while it shifts to 1743 cm −1 with the addition of PbI 2 . This result indicates a weakening of the C=O bond derived from molecular interaction between BPTCD and the PbI 2 precursor, which is consistent with the previous reports [38,42]. The weakening of the C=O bond is indicative of the formation of an intermediate BPTCD-PbI 2 adduct, which can be expected to improve the crystallinity of the perovskite film [38,43]. Therefore, in this work, the BPTCD additive plays a role to favor the heterogeneous nucleation of the MAPbI 3 , lower the energy barrier of nucleation and consequently to generate a high density of nuclei, leading to a more compact and smaller sized MAPbI 3 microstructure [38,43,44]. To illuminate the molecular interaction between BPTCD and PbI2, FTIR measurement was employed. As reported by Bi et al. [38], the C=O group can interact with PbI2 and trigger heterogeneous nucleation of MAPbI3, thus improve the crystallinity of the perovskite film. Similar results were also reported by Peng et al. [42], such that the C=O groups are responsible for the passivation of perovskite film via Lewis-base electronic passivation of Pb 2+ ions, which reduces trap density and enhances the morphology of perovskite film. Here the used BPTCD as an additive is rich of C=O groups, which may share the similar effect on the improvement of perovskite film quality. As displayed in Figure 4a, the stretching vibration of C=O groups in the pure BPTCD shows at 1749 cm −1 , while it shifts to 1743 cm −1 with the addition of PbI2. This result indicates a weakening of the C=O bond derived from molecular interaction between BPTCD and the PbI2 precursor, which is consistent with the previous reports [38,42]. The weakening of the C=O bond is indicative of the formation of an intermediate BPTCD-PbI2 adduct, which can be expected to improve the crystallinity of the perovskite film [38,43]. Therefore, in this work, the BPTCD additive plays a role to favor the heterogeneous nucleation of the MAPbI3, lower the energy barrier of nucleation and consequently to generate a high density of nuclei, leading to a more compact and smaller sized MAPbI3 microstructure [38,43,44].  The interfacial properties in the photodetectors dominate the charge extraction and transportation, thus influence the performance of PePDs [37]. Here, the UPS measurement (with a He I of 21.2 eV) of perovskite films on the ITO substrates was carried out to characterize the change of energy levels of the perovskite films with and without BPTCD additive (Figure 4b). The high binding The interfacial properties in the photodetectors dominate the charge extraction and transportation, thus influence the performance of PePDs [37]. Here, the UPS measurement (with a He I of 21.2 eV) of perovskite films on the ITO substrates was carried out to characterize the change of energy levels of the perovskite films with and without BPTCD additive (Figure 4b). The high binding energy cutoff region is shown in the left panel, while the onset region is in the right panel. The perovskite films with and without BPTCD additive share nearly the same high binding energy cutoff and the work function is estimated to be 3.92 eV. However, the valence band minimum (VBM) position of the pure perovskite film occurs at 1.38 eV below Femi level (E f ) while the VBM position of perovskite film with BPTCD additive shifts to 1.62 eV below E f , indicating a deeper VBM energy level (Figure 4c). This energy shift could lead to a mismatch of energy levels between the perovskite film and PEDOT:PSS (HOMO of 4.9 eV), which hinders the hole injection into the active layer due to the energy barrier and therefore suppress the I d . However, this unmatched energy level could lead to the suppress of I ph as well.
To study the influence of the trap density in the perovskite film on the dark current, the electron-only devices (ITO/PCBM/MAPbI 3 /PCBM/Ag) were fabricated to analyze the trap density by using the space charge limited current (SCLC) method. The I-V curves of the devices with and without the BPTCD additive are shown in Figure 4d. The linear I-V plot indicates an Ohmic response at low bias, and the current increase nonlinearly when the bias voltage exceeds the trap-filled limit voltage (V TFL ), demonstrating that all the available trap states are filled by the injected charge carriers [45]. The onset voltage V TFL is linearly proportional to the density of trap states η t , which follows as Equation (3): where e is the elementary charge of the electron (e = 1.6 × 10 −19 C), L is the perovskite film thickness, ε is the relative dielectric constant of MAPbI 3 (here we use 32 [45]), ε 0 is the vacuum permittivity (ε 0 = 8.854 × 10 −12 F/m), and η t is the trap state density of perovskite film. The V TFL of the MAPbI 3 film with and without BPTCD additive can be identified in Figure 4d, respectively. The thickness of perovskite film with and without BPTCD additive is 431 nm and 397 nm, respectively, derived from the cross-section SEM images in Figure 3c,d. The electron trap state density of perovskite film with BPTCD additive is estimated as 1.01 × 10 16 cm −3 , which is much lower than the perovskite film without BPTCD additive (1.50 × 10 16 cm −3 ). This decrease of trap density is due to the Lewis-base nature of the oxygen atoms in the C=O groups on BPTCD, and Lewis base material can passivate the defects induced by Pb 2+ as the recombination centers on the surface and grain boundaries of perovskite films [42]. Therefore, dark leakage current originated from defect could be significantly lowered by the passivation of BPTCD additive.
To further investigate the influence of BPTCD additive on the electrical properties of PePDs, the impedance of the devices was measured by an impedance analyzer under dark condition. The measured electrical parameters of R 1 , R 2 , R 3 , C 1 , CPE 1 -T and CPE 1 -P are listed in Table 3 through the fitting curves. In this circuit model (Figure 5a inset), the R 1, R 2 , and R 3 correspond to the device series resistance, the interfacial resistance, and recombination resistance, respectively [37]. From the corresponding Nyquist plots, the R 1 of the pristine and BPTCD added PePDs are similar. The R 2 of BPTCD added devices is about two folds larger than that of the pristine devices. This result accounts for the great reduction of the leak current of BPTCD added devices [40,46,47] as depicted in the I-V curves in Figure 2a. The compact morphology, low trap density and relativity unmatched energy level between the active layer and PEDOT:PSS layer are responsible for the enhancement of R 2 . Meanwhile, a minor decrease of R 3 in the control device is observed, which suggests that the pristine perovskite film with larger grain size can suppress charge recombination more effectively under light condition, resulting in a higher EQE and I ph of the control device [48]. value after operating 400 s, while the control device only maintains 47% Iph. The enhancement of device stability is attributed to the high-quality perovskite film with decreased defects, since the defects at grain boundaries typical provide charge accumulation sites and infiltration pathways for water vapor in air [49]. This is responsible for the irreversible moisture-induced degradation of perovskite, thus degrades the long-term stability of the devices. As mentioned above, the perovskite film with BPTCD shows much lower tarp density than the pristine perovskite film, indicating more stable crystal structure and enhanced long-term stability of PePDs.   Finally, to examine the contribution of BPTCD additive toward the stability of PePDs, we performed a long-term photocurrent measurement on the PePDs with and without BPTCD additive. As shown in Figure 5b, the I ph of the PePD with BPTCD additive can maintain 85% of the original value after operating 400 s, while the control device only maintains 47% I ph . The enhancement of device stability is attributed to the high-quality perovskite film with decreased defects, since the defects at grain boundaries typical provide charge accumulation sites and infiltration pathways for water vapor in air [49]. This is responsible for the irreversible moisture-induced degradation of perovskite, thus degrades the long-term stability of the devices. As mentioned above, the perovskite film with BPTCD shows much lower tarp density than the pristine perovskite film, indicating more stable crystal structure and enhanced long-term stability of PePDs.

Conclusions
In summary, we developed a simple route for controlling the microstructure of MAPbI 3 by using BPTCD as an additive, and realized high performance PePDs based on the well controlled MAPbI 3 film. The results showed that the interaction between C=O groups in BPTCD and the PbI 2 in perovskite precursors could favor the heterogeneous nucleation of the MAPbI 3 , thus lower the energy barrier of nucleation and facilitate the growth of MAPbI 3 crystal structures. Moreover, the BPTCD additive could down-shift the VBM level of the perovskite film and therefore contributes to the reduction of I d . As a result, the I d of the devices was significantly suppressed by nearly two orders of magnitude compared to the control device but kept I ph almost unchanged. Hence, with an optimal concentration of BPTCD additive (3 wt.%), the PePD exhibits a high D* value exceeding 10 12 Jones over the Visible region with a maximum D* value of 4.55 × 10 12 Jones at 685 nm at a bias of −0.1 V. Moreover, the PePDs with 3 wt.% BPTCD shows much greater stability comparing to the control devices. This work demonstrates a facile and low-cost method for tuning the MAPbI 3 microstructure to obtain high quality perovskite film and open a novel route to realize high performance PePDs.