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Abstract
In this study, a perovskite is integrated with an ultra-thin Cu/Cu2O (CCO) composite film, a transparent material with high mobility, to achieve a double-side and low-voltage operable photodetector. Compared to photodetectors that utilize metal electrode with perovskite, the use of CCO significantly enhances the photocurrent (from nA up to mA). It acts as a large-scale hole transport layer. The photodetector exhibits high responsivities of 6.79 AW−1 [illuminated via the fluorine-doped tin oxide (FTO) side] and 10.23 AW−1 (illuminated via CCO side). The detectivities obtained at both illuminated sides are as high as over 1011 Jones. Additionally, the Cu/Cu2O-covered perovskite effectively prevents the reaction of perovskite in the interface. This work reveals that the perovskite/CCO heterojunction photodetector can be considered a promising candidate for applications in bifacial-illuminated and flexible/wearable optoelectronic technologies.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photodetectors (PDs) are key components of optoelectronic systems such as optical communications [1,2], space and defense technology [3,4], and biochemical detection [5]. Simple processed, large-area, and low-power driving PDs are of great interest for the upcoming development of novel optoelectronic technologies. In addition, PDs with bifacial structure are highly attractive because of their potential applications in various areas including smart displays, window-integrated electronic circuit and sensors, and flexible and wearable optoelectronic systems [6–9].
For these purposes, the development of all transparent metal oxide PDs has been reported in several studies [10–16]. Metal oxides are employed due to their advantages of easy deposition, high stability, and resistant to deterioration from the influence of external environment [17–21]. However, metal oxides usually have a large band gap. Consequently, the fabrication of all transparent metal oxide photodetectors will limit the detection to only the short-wavelength regime of the ultraviolet section. In addition, such PDs typically require high power as the driving force due to insufficient light-sensitive properties.
Currently, hybrid organic—inorganic perovskites such as CH3NH3PbX3 (X = Cl, Br, and I) have been regarded as promising light-sensitive materials owing to their direct bandgap, ambipolar transport, long exciton diffusion length, high carrier mobility, and high absorption coefficient [22–27]. Recently, bifacial perovskite solar cells have also been developed [28–30]. Although there have been many researchers that demonstrated the fabrication of pristine perovskite-based photodetectors, the reported photocurrents are still very low (nA–μA) [31–35]. Some researchers integrated perovskites with two-dimensional (2D) materials to enhance photocurrents, which exhibited very high responsivities [36]. However, the preparation of 2D materials is difficult and usually requires a high growth temperature. The size of the prepared 2D material is generally small that limits the detection area of the photodetector. Therefore, some research integrated perovskites with metal oxides (MOs) [25,37–41]. Although the responsivities are far less than those integrated with 2D materials, the process can be simplified and can be deposited over a large area. In these PDs, the active layers (perovskites) are usually deposited above 2D or MOs layer to improve the device performance. However, there are several corresponding problems. First, the active layer is easily exposed to the ambient atmosphere, resulting in the degradation of perovskite, and thus poor device stability. Furthermore, the perovskite active layer directly contacts the metal electrodes such as silver and gold, which can result in severe interdiffusion [42–44]. The products of interdiffusion and chemical reaction (such as AgI or AuI) are responsible for the poor stability of these devices.
To address the above problems, we prepare perovskite-based PDs with Cu/Cu2O (CCO) nanocomposites deposited on the top of perovskite. Cu2O (cuprous oxide) is a well-known p-type semiconductor with good hole mobility [45,46] and has also been used in solar cell and photodiode, such as, exhibited efficient charge carrier separation and excellent visible light photoresponse in Cu2O nanowires [47]; efficiency enhancement of ZnO/Cu2O solar cells with well oriented and micrometer grain sized Cu2O films [48]; preparation of cubic Cu2O nanoparticles wrapped by reduced graphene oxide for the efficient removal of rhodamine B [49]. In addition to a long charge diffusion length (~10 μm) [50,51], Cu2O has a well matching energy level with perovskite [52,53]. However, owing to high resistivity, Cu2O cannot directly be used as an electrode. Here, we report the use of an ultra-thin Cu/Cu2O composite film as a hole transport layer for perovskite-based PDs. Owing to the presence of a small amount of incomplete oxidized metal Cu, the introduction of Cu/Cu2O does not increase the driving force, but effectively transmits the photogenerated carriers, thereby increasing photocurrent to 11.32 mA and 7.11 mA at 1 V bias when light illumination was incident on the fluorine-doped tin oxide (FTO) side and Cu/Cu2O side, respectively. The photodetector exhibits not only high responsivities of 6.79 AW−1 (illuminated via the FTO side) and 10.23 AW−1 (illuminated via CCO side), but also large detectivities of ~1011 Jones to detect weak light signals. This high-performance and double-side operable perovskite/(Cu/Cu2O) photodetector appears to have great potential in connection with future novel optoelectronic systems.
2. Results and discussion
In our previous studies, we deposited CCO on an organic hole transport material Spiro-OMeTAD [2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-bifluorene] as a p-type transparent-conductive-oxide for efficient bifacial-illuminated perovskite solar cells [28]. In the present work, we have removed the organic hole transport material and directly deposited CCO on the perovskite layer as a hole transport layer, forming a CCO/perovskite heterojunction photodetector. The lack of an organic hole transport material simplifies the process steps and also significantly reduces costs. Considering that this film will be in direct contact with the perovskite, a Cu/Cu2O film grown at a high oxygen flow rate (OFR) is selected to avoid the adverse effects of the metal on the perovskite. In addition, the film grown at a higher OFR presents higher carrier mobility, which is an important feature for the high performance of photodetector [54,55]. The X-ray diffraction (XRD) patterns of pristine perovskite and perovskite covered with CCO are shown in Fig. 1(a). The sample with CCO shows the same perovskite crystalline diffraction peaks as the sample without CCO. To check whether CCO has been deposited on the perovskite, specifically at angle 2θ ranging from 36.3°~36.6°, longer scanning time was selected for detailed inspection. The diffraction peak of perovskite/CCO at 36.52° is manifested corresponding to the Cu2O (111) plane (JCPDS card number: 01-077-0199), which is not observed in the perovskite sample [see inset of Fig. 1(a)]. Additionally, for the perovskite without covered CCO, the intensity of PbI2 (lead iodide) is much enhanced when the sample is placed in atmosphere for seven days, as shown in Fig. 1(b). In comparison, PbI2 peak does not strengthen for the CCO-capped sample. This indicates that CCO has a good protective effect on perovskite.
Fig. 1 (a) XRD spectra of the pristine perovskite (Cs-FAMAPbIBr) film without Cu/Cu2O and that covered with Cu/Cu2O and (b) XRD of the Cu/Cu2O-capped sample placed in atmosphere for seven days as well as the reference sample of perovskite without Cu/Cu2O capped for comparison. The inset of Fig. 1(a) shows the XRD spectrum of the selected 2θ range (blue frame) obtained at longer scanning time.
Figures 2(a) and 2(b) show the atomic force microscope (AFM) of the pristine perovskite and CCO-covered perovskite, respectively. From the analysis of AFM data (Table 1), the root mean square roughness (Rq) and average roughness (Ra) of perovskite sample are 22.5 nm and 18.1 nm, respectively. When the Cu/Cu2O film is deposited on the perovskite, Rq and Ra are 17.7 nm and 14.1 nm, respectively. As indicated by the AFM images, the morphology of perovskite shows a large-grain crystal structure, and that of the CCO-covered perovskite is much flattened.
Fig. 2 The AFM topography images of (a) perovskite sample and (b) Cu/Cu2O-covered perovskite sample.
Table 1. Root mean square roughness (Rq) and average roughness (Ra) of perovskite sample and Cu/Cu2O-covered perovskite sample.
We then studied the optical properties of perovskite without and with CCO. Figure 3(a) shows the photoluminescence (PL) spectra of perovskite without and with CCO layer. Perovskite with CCO presents a significant PL quenching effect in comparison to perovskite without CCO. Typically, a strong PL quenching effect indicates efficient charge transfer from the photoactive layer to the charge transport layer [56,57]. Compared to the pristine perovskite, the absorbance of perovskite with CCO is the same at wavelength between 550 nm and 800 nm (Fig. 3(b)). Wavelength less than 550 nm is much increased, making the overall absorbance of perovskite with CCO higher than that of pristine perovskite.
Fig. 3 (a) Photoluminescence spectra of the perovskite and perovskite with Cu/Cu2O layer—the inset is a magnified spectrum of perovskite with Cu/Cu2O — and (b) UV-vis absorption spectra of the perovskite and perovskite with Cu/Cu2O layer.
We further investigated the energy level of the CCO composite film using ultraviolet photoelectron spectroscopy (UPS). The work function of the CCO film is estimated by measuring ultraviolet photoemission. Figure 4(a) shows the measured cut-off spectra of secondary photoelectrons of UPS. To detect secondary photoelectrons on the lower kinetic energy side, a negative bias of 5 V was applied on the sample during the measurements. The onset of secondary photoelectrons was evaluated by applying linear extrapolation to the measured spectra. Consequently, the work function of the CCO film was determined to be 4.5 eV. The band alignment and the relative position of the Fermi level to the valence band maximum (∆EVB) of the CCO film are probed, as shown in Fig. 4(b). The inset of Fig. 4(b) shows the magnification of UPS spectra around the valence band region (marked by blue frame). The Au metal was employed to calibrate the Fermi energy level position (EF) to 0 eV, and ∆EVB of the CCO film was obtained as 0.55 eV. Therefore, the energy level of the CCO film can be further calculated with a valence band maximum (VBM) of −5.05 eV.
Fig. 4 (a) Cut-off spectrum for secondary photoelectrons of the Cu/Cu2O film. The kinetic energy for secondary photoelectrons was calibrated by applying a bias of −5 V on the substrate. (b) UPS spectra of the valence band region near the Fermi level for Au and the Cu/Cu2O film. (c) Schematic illustration of the energy level alignment for perovskite-based photodetector employing Cu/Cu2O carrier transport electrode.
The UV-Vis spectrum can be used to calculate the band gaps of semiconductor materials by plotting (αhν)2 versus photon energy (hν), where α is the optical absorption coefficient. UV-vis spectroscopy with wavelength of 300–900 nm was conducted to determine the transmission of the CCO film, commercial fluorine-doped tin oxide (FTO) glass, and indium tin oxide (ITO) glass, as shown in Fig. 5(a). It is worth mentioning that the transmission of the CCO film of thickness 4 nm is higher than the commercial FTO glass and comparable to the ITO glass. Figure 5(b) presents the curve of (αhν)2 versus hν that was obtained from the transmission spectrum. The band gap (Eg) determined from the intersection is ~2.2 eV [Fig. 5(b)] [28], and the location of the conduction band minimum (CBM) at ∼-2.85 eV was identified. The energy level diagram of the photodetector device can be further plotted, as shown in Fig. 4(c). The result shows that the VBM of CCO matches well with perovskite and can effectively transport holes. The CBM of CCO was higher than perovskite, which means CCO can serve as the electron block layer, hindering the electron recombination on the top electrode.
Fig. 5 (a) Transmission spectra of the Cu/Cu2O film, FTO glass and ITO glass. (b) (αhν)2 versus hν curves for the Cu/Cu2O film and the band gap determined by the intersection with photon energy [28].
3. Double-side operable photodetector
In this work, Cu/Cu2O was directly deposited on the top of a multicomponent perovskite that was synthesized by mixing cation and halide, and was denoted as Cs-FAMAPbIBr (where Cs: cesium; FA: formamidinium, (HC(NH2)2+); MA: methyl ammonium, CH3NH3+) [58]. The architecture of the photodetector was composed of FTO/Cs-FAMAPbIBr (perovskite)/(Cu/Cu2O)/Au (inset of Fig. 6). The fabrication process of perovskite-based photodetectors is schematically illustrated in Appendix Fig. 10.
Fig. 6 Current-voltage (I-V) curves of perovskite-based photodetector with Cu/Cu2O measured under dark and bright conditions, which were illuminated via (a) Cu/Cu2O side and (b) FTO side, respectively. The insets show the architecture of the PD. (c) and (d) are magnified views of the I-V curves from (a) and (b), respectively.
Figure 6 shows the current-voltage (I-V) curves of perovskite-based PDs with a CCO layer measured under dark and illumination conditions, and the latter is under laser irradiation with a light density of 1.528 mW/cm2 and wavelength of 632 nm. The magnified view of the I-V curves is plotted in Figs. 6(c) and 6(d). The I-V curves do not pass the zero point, suggesting that the device can function in a self-powered mode without external power supply. The photocurrents at the applied bias of 1 V were 11.32 mA and 7.11 mA when the device was illuminated from the CCO and FTO sides, respectively. Owing to the difference in transmission [see Fig. 5(a)], the light source of the same intensity makes the photocurrent generated by illuminating the CCO side [Fig. 6(a)] higher than that the FTO side [Fig. 6(b)]. For the reference PD without CCO, the photocurrent at the applied bias of 1 V was 25 nA, as shown in Fig. 7(a). Comparison of I-V curve of the PDs without and with CCO measured under bright conditions from FTO sides, the corresponding photocurrent is enhanced from 25 nA to 7.11 mA at the applied bias of 1 V (Fig. 7(b)), which could be resulted from the alignment of energy level between perovskite and Cu/Cu2O, as shown in Fig. 4(c). The perovskite acts as both a light harvester and an ambipolar transporter of electrons and holes. Since the photogenerated electron-hole pairs in the perovskite could recombine within a few picoseconds [59], significant loss of the photocurrent is expected.
Fig. 7 (a) Current-voltage (I-V) curve of pristine perovskite photodetector measured under dark and bright conditions. (b) I-V curve of the photodetectors without and with Cu/Cu2O measured under bright conditions from FTO sides. (c) Responsivity and (d) detectivity of pristine perovskite photodetector as a function of different voltage under light intensity of 1.528 mW/cm2.
The responsivity (R) and detectivity (D*) are important figure of merits for photodetectors. R is defined as where Jph is the photocurrent and Llight is the intensity of the incident light [60]. The responsivities of the perovskite/CCO PD can be estimated as 6.79 A/W and 10.23 A/W when the device is illuminated via the FTO and CCO sides, respectively, at a voltage bias of 1 V [Figs. 8(a) and 8(b)]. Remarkably, the obtained responsivities are much higher than our reference sample: perovskite/Au photodetector, which is ∼32 μA/W (at 1 V illuminated from FTO side), as shown in Fig. 7(c). D* is estimated as where q is the electronic charge and Jd is the dark current [60], indicating the ability of PD to detect weak light signals. Although the introduction of CCO increases the dark current (from nA to mA), perovskite/CCO exhibits a large D* [∼3.0 × 1011 Jones at 1 V (CCO side, Fig. 8(c)), and ∼1.6 × 1011 Jones at 1 V (FTO side, Fig. 8(d))] that is at least one order of magnitude larger than that of perovskite/Au PD [∼7.3 × 109 Jones at 1 V, Fig. 7(d)]. In addition, compared to most of the all transparent metal oxide-based PDs (Table 2), the proposed perovskite/CCO PD exhibits high responsivity and detectivity. The results clearly indicate the importance of fabricating the hybrid structure of perovskite/CCO to achieve high performance and bifacial photodetector.
Fig. 8 Responsivities and detectivities of perovskite/CCO photodetector as a function of different voltages under light intensity of 1.528 mW/cm2, illuminated via (a), (c) Cu/Cu2O side and (b), (d) FTO side, respectively.
Table 2. Comparison of performance of perovskite-based photodetector with metal oxides and other transparent metal oxide photodetectors.
We trace the I-V curves of perovskite-based PDs without and with a CCO layer. The result is illustrated in Fig. 9. For the PDs stored in dark for 7 days, the photocurrent of pristine perovskite PD drops sharply, pink dotted line as shown in Fig. 9(a), and the photocurrent is almost the same with the dark current (Fig. 9(b)). In contrast, PD with Cu/Cu2O has a small decrease in photocurrent [11.32 mA to 10.3 mA at 1 V (CCO side), Fig. 9(c); 7.11 mA to 6.68 mA at 1 V (FTO side), Fig. 9(d)]. The results demonstrate that Cu/Cu2O composite film is not only a promising transparent carrier transport electrode for bifacial-illuminated PDs but also serves as a protection layer for the perovskite active layer against the water vapor in the air.
Fig. 9 Stability test of I-V curves of (a) pristine perovskite-based photodetector and with Cu/Cu2O measured under dark and bright conditions, which were illuminated via (c) Cu/Cu2O side and (d) FTO side, respectively. (b) Magnified view of I-V curve from Fig. 9(a).
4. Conclusions
In this study, an efficient and double-side operable perovskite-based photodetector using an inorganic Cu/Cu2O (CCO) as a hole transport layer is reported. The CCO exhibits superior charge transfer capability, high transparency, and energy level that match well with perovskite, making it suitable for use in bifacial perovskite-based optoelectronic devices. The introduction of Cu/Cu2O effectively separates electron-hole pairs generated by perovskite, leading to enhanced photocurrent of up to 11.32 mA and 7.11 mA at a bias voltage of 1 V when the device is illuminated from the CCO and fluorine-doped tin oxide (FTO) sides, respectively. The enhanced photocurrent results in high responsivities: 10.23 A/W illumination from the CCO side and 6.79 A/W illumination from the FTO side. In addition, the obtained detectivities of ∼3.0 × 1011 and 1.6 × 1011 Jones, illumination from the CCO and FTO sides, respectively, are large. The perovskite/(Cu/Cu2O) heterojunction-based photodetector enables light detection from both front and rear sides with good photodetection performance, suggesting it a good candidate for bifacial-operable photodetectors that may find important applications such as flexible/wearable optoelectronic devices.
5. Experimental
5.1. Device fabrication
First, the 2 cm2 FTO glasses were etched with zinc powder and hydrochloric acid (1 M) to house two electrodes, which were cleaned via sonication in sequence using acetone and ethanol for 15 min, followed by treatment in a UV-ozone cleaner for 5 min. Then, the FTO substrates were transferred into a glove box. The Cs-FAMAPbIBr perovskite precursor was prepared inside a nitrogen-filled glove box with oxygen and moisture levels <1 ppm by mixing 0.0986 g of FAI, 0.2776 g of PbI2, 0.0119 g of MABr (Dyesol), and 0.039 g of PbBr2 in a mixed solvent of DMF: DMSO = 4: 1 (volume ratio). Another solution of CsI was also prepared in DMSO. Then, these solutions were mixed as 5 vol% ratio. Cs-FAMAPbIBr was then spin-coated onto the FTO substrate at 2000 rpm for 10 s and at 5000 rpm for 30 s, and the antisolvent trifluorotoluene (TFT) was injected for 20 s in the second spin step. The spin-coated perovskite film was dried at 100°C for 1 h to remove the TFT. The ion beam sputtering system was used to deposit the Cu/Cu2O films (4-nm-thick) onto perovskite films. Perovskite/Au is also prepared as a reference sample, where Au was deposited by thermal evaporation using a shadow mask.
5.2. Characterization
The crystallographic properties of the films were determined by grazing incidence X-ray diffraction using Cu Kα radiation (λ = 1.5418 Å, D8, Bruker, Germany) at room temperature with a scanning step size of 0.005°. A microscopic PL system (MRI, Protrustech Co., Ltd., Taiwan) was used to determine the band gap of the films with a pumping wavelength of 532 nm. Optical absorbance and transmission spectra were obtained using a UV/VIS/NIR Spectrophotometer (HITACHI U4100) at a fixed incidence angle perpendicular to the film surface in the range of 300-900 nm. The FTO glass was used as a reference for absorbance and transmission measurements. The UPS experiment was performed at beamline 24A of the Taiwan Light Source at the National Synchrotron Radiation Research Center (NSRRC). The I–V characteristics were acquired using a Keithley 2400 source meter in the dark and under illumination. A 632.25-nm laser with a neutral density filter to adjust optical power was utilized as the light source, and the output power was measured using a laser power meter. A beam expander expands the laser spot size to a circle of 1 cm in diameter.
Appendix
Before spin-coating perovskite, cleaned fluorine-doped tin oxide (FTO) substrates were treated with UV-ozone to increase hydrophilicity and facilitate perovskite film formation. Annealing the perovskite film at 100°C for 1 hour was done to remove the antisolvent of residual trifluorotoluene during spin-coating of the perovskite. Next, the perovskite samples were transferred to a sputtering vacuum chamber, and a Cu/Cu2O HTM was deposited on the surface of the perovskite. Then, an Au electrode was deposited on the Cu/Cu2O layer by thermal evaporation using a shadow mask (Fig. 10). The reference photodetector is the one without the Cu/Cu2O film.
Funding
Ministry of Science and Technology (MOST) of Taiwan (103-2923-M-006-001-MY3, 104-2119-M-006-018-MY3, 103-2221-E-006-029-MY3, 105-2623-E-006-002-ET, 104-2119-M-006-004, 107-2221-E-006-190-MY3, 107-2119-M-006-002, 106-2119-M-006-027, 106-2119-M-006-017).
Acknowledgments
Prof. J. C. A. Huang and Prof. Chen are grateful for the research grant from the Ministry of Science and Technology (MOST) of Taiwan. This work was financially supported by the Hierarchical Green-Energy Materials (Hi-GEM) Research Center, from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. The authors acknowledge financial support from the Top-Notch Project under the Headquarter of University Advancement at National Cheng Kung University, which is sponsored by the Ministry of Education, Taiwan. Funding from the Advanced Optoelectronic Technology Center (AOTC), Research Center for Energy Technology and Strategy (RCETS), and National Cheng Kung University is also acknowledged. The authors would also like to thank Dr. Y. W. Yang of the National Synchrotron Radiation Research Center (NSRRC) for valuable technical support.
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