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Photo detector based on graded band gap perovskite crystal
Solar Energy 194 (2019) 563–568
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Photo detector based on graded band gap perovskite crystal Priyabrata Sadhukhan, Sachindranath Das
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T
Department of Instrumentation Science, Jadavpur University, Kolkata 700032, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite Graded band gap Photo diode Single crystal
In this work, millimeter sized graded band gap mixed halide hybrid perovskite crystal have been synthesized by utilizing reversible halide exchange property of hybrid perovskites. Optoelectronic performance of photo diodes fabricated using the graded band gap crystal has been studied in details. Band gap engineering has formed a smoother path for carrier transport by aligning HOMO and LUMO levels with the charge collection layers. This is reflected in good diode quality and fast photo response has been achieved. On the other hand, effect from both bulk and surface recombination of photo generated carriers are observed in the plateau type spectral response curve with a peak near band edge.
1. Introduction Photo detectors are the integral part of light sensing, imaging, automation, optical communication and many other fields. A good photo detector should have high detectivity and low response time. For imaging applications, high spectral selectivity is also desirable. Spectrally selective photo detectors are used in bio medical applications, scientific research instruments, imaging, defense etc. Light sensing organic inorganic hybrid perovskite materials have recently gained huge attention as a potential replacement of silicon in the future photovoltaic technology. High dielectric constant, high light absorption coefficient, low excitonic binding energy, high carrier mobility and long carrier diffusion length are the key features which has escalated the material to this stage. However, apart from solar cell applications, hybrid perovskite materials find its way in photo diode also. Relatively easy way to fabricate millimeter size crystals backed up with astonishingly long 175 µm (Dong et al., 2015) carrier diffusion length is another advantage. Parvez et al. reported self-biased photo detector using CH3NH3PbBr3 crystal/Pt junction as Schottky contact (Shaikh et al., 2016) Fang et al used surface charge recombination to achieve charge collection narrowing mechanism to realize spectral selectivity in perovskite single crystal photo diode (Fang et al., 2015). Lian and his group has fabricated a perovskite planer type photo detector on the (1 0 0) facet of CH3NH3PbI3 crystal (Lian et al., 2015). All these reports are made with pure halide hybrid perovskite crystals. CH3NH3PbI3 is already well established as a very good photo voltaic material, but this is not the only thing to determine the performance of a photovoltaic cell. Very good charge collection efficiency is required to
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efficiently collect the photo generated carriers and drive them to the external circuitry. Two of the most dominant things to affect charge collection efficiency are the quality of the hole selective layer and the alignment of energy band between the hole selective layer and the perovskite. To match the need of perfect sync of energy bands of hybrid perovskite layer, many organic and inorganic hole transport materials (HTM) like spiro-MeOTAD (Tian et al., 2019), PeDOT:PSS (Montoya et al., 2019), CuI (Christians et al., 2014), crystalline selenium (Di et al., 2018), etc. have been investigated. Organic HTMs outperforms the inorganic HTM in most studies (Christians et al., 2014; Di et al., 2018). However spiro-MeOTAD is very costly. A less expensive and efficient alternative is PeDOT:PSS, a polymer type HTM widely used in inverted structure perovskite devices. In the perovskite part, several strategies have been taken by researchers in order to reduce defect assisted recombination of charged carriers and to improve charge collection. Sun et al. demonstrated that defect density in the perovskite layer can be reduced by introducing I3− ions into organic cation dripping solution (Sun et al., 2018). The charge collection efiiciency can be improved by carefully tuning perovskite the energy band near the perovskite/HTM interface. Wang et al. introduced a iodine concentration gradient near the perovskite/HTM interface tunes the band gap of the perovskite to effectively improve hole extraction (Wang et al., 2018). Another effective approach for carefully tuning the band gap of hybrid perovskite is to use mixed halide hybrid perovskite. These hybrid Perovskites have an interesting property that they participate in halogen exchange reaction. It means that if CH3NH3PbBr3 is made in touch with CH3NH3PbI3 solution, the former will react with the later in which Br−1 ions will be replaced by the I−1 ions of the solution and the final product will be a mixed halide hybrid
Corresponding author. E-mail address: [email protected] (S. Das).
https://doi.org/10.1016/j.solener.2019.11.001 Received 23 November 2018; Received in revised form 5 October 2019; Accepted 1 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
Solar Energy 194 (2019) 563–568
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perovskite (CH3NH3PbBr3−xIx). The final percentage of Br−1 and I−1 will depend on time of contact and the molarity of the solution. As we already know, the band gap of the pure CH3NH3PbBr3 is 2.3 eV while CH3NH3PbI3 has 1.5 eV. Thus the mixed halide perovskite (CH3NH3PbBr3−xIx) will pick up a band gap in between these two limits depending on the composition(x). The band gap can be tuned by carefully controlling the Iodine/bromine ratio. We have exploited this property to form a graded band gap perovskite crystal where the band gap varies from 1.5 eV in one side to 2.3 eV on the other side of the crystal. Here in this work, we have successfully grown graded band gap mixed halide hybrid perovskite crystal (CH3NH3PbBr3−xIx) and fabricated crystal based photo diode absorber layer using carbon paste (C) as electrodes and PeDOT:PSS is used as carrier selective layer. Optoelectronic properties of the device is studied in details with different light intensity and different wavelength.
2.2. Fabrication of photo diode The device was fabricated with a 1 mm thick graded band gap MAPIB crystals using carbon paste as the outer electrodes and PeDOT:PSS as hole transport layer. The crystal was coated with conducting carbon paste at one end and PeDOT:PSS followed by conducting carbon paste at another end to make C/MAPIB/PeDOT/C architecture. To deposit PeDOT:PSS layer, a piece of tissue paper was firstly soaked with aqueous solution of PeDOT:PSS. Then the bromine rich face of the MAPIB crystal was pressed against the wet tissue paper for 3 s and then immediately dried with hot air gun at 50 °C to form the PeDOT: PSS layer. The outer electrodes were deposited by coating conducting carbon paste on either sides of the crystal afterwards. Separation between two electrode layers was kept to 1 mm in each cases. Cross sectional area of the device was measured to be 1 mm × 1 mm.
2. Experimental
2.3. Material and device characterization
2.1. Formation of the crystal
X-Ray diffraction pattern was recorded with a RIGAKU miniflex-600 bench top Diffractometer. Current–voltage (I-V) data with different light intensity was recorded with a Keithley 2602B multichannel source measure unit and a variable intensity solar simulator (Royal enterprise, India). UV–Vis spectra of the material was performed with a Optizen POP (Korea) UV–Vis-NIR spectrometer. Spectral photo response measurement was done using a 150 W XENON lamp and the monochromator of a PTI QuantaMaster 400 spectroflurometer, an optical power meter and the Keithley 2602B source meter. For transient photo response measurement we used an NVis 105CT digital storage oscilloscope and three different LEDs of wavelengths 462 nm, 520 nm and 630 nm, blinking at 25 Hz chopping frequency. Characterization of the crystal and the device were performed in ambient environment with 60% relative humidity at room temperature.
Methylammonium lead bromide (CH3NH3PbBr3) crystals were synthesized following inverse temperature crystallization method described by Saidaminov et al. (2015). Firstly Methylammonium bromide (CH3NH3Br) and Methylammonium iodide (CH3NH3I) was prepared by following the procedure as described in our earlier work (Sadhukhan et al., 2019, 2018). This CH3NH3Br and lead iodide (PbBr2) were mixed at equimolar ratio in dimethyl formamide (DMF) solvent at 60 °C and kept on stirring for 6–7 h followed by filtering to get a clear pale yellow solution of CH3NH3PbBr3. This solution was kept undisturbed at 80 °C in an oil bath. Orange coloured CH3NH3PbBr3 crystals (MAPB) started to grow after few hours which became larger over time. CH3NH3PbI3 crystal (MAPI) was also grown in a similar way from γ-butyrolactone solution of CH3NH3PbI3. The graded junction crystal (CH3NH3PbBr3−xIx) was formed by immersing a MAPB crystal partially in 1 M CH3NH3PbI3 solution for 5 min to allow the halide exchange reaction to occur. The dipping actuator of a SILAR dip coater (Apex Instrument, India) was used to precisely immerse one side of a 1 mm × 1 mm × 1 mm MAPB crystal into the CH3NH3PbI3 solution so that MAPI can grow only on that part of the MAPB crystal. Length of the submerged portion of the crystal was about 0.3 mm. We name this crystal MAPIB. All the steps of the synthesis part were carried out in ambient condition with 60% relative humidity. Synthesis method of the graded band gap mixed halide hybrid perovskite single crystal and fabrication process of the photo diode is outlined in Fig. 1(a).
3. Results and discussion Crystalline quality of the synthesized MAPB and MAPI crystals was checked with powder X-Ray diffraction measurement. Sharp XRD peaks shown in Fig. 1(b) indicate formation of good quality crystal. Using Rietveld refinement the crystal structure of MAPB is found to be cubic with pm3m space group and lattice parameter a = b = c = 5.91417 Å while MAPI possesses tetragonal symmetry with I4/mcm space group and a = b = 8.8560 Å, c = 12.6525 Å. Hybrid perovskites degrades to lead halides when exposed to moisture or UV light. The degradation leaves a mark by easily detectable change of colour of the sample.
Fig. 1. (a) Schematic diagram of the fabrication of the graded band mixed halide hybrid perovskite crystal and photo diode and (b) Powder XRD pattern of the MAPB ad MAPI. 564
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a linear dependency on voltage in both dark and illuminated (0.5S) state which indicated ohmic nature of interface [Not shown here]. Hence the rectifying nature of the photo diode comes from the perovskite – PeDOT:PSS junction. PeDOT:PSS is a popular material for hole transporting layer used in organic solar cell. All hybrid perovskites are very sensitive to water. CH3NH3PbBr3 degrades to methylamine (CH3NH2), hydrobromic acid (HBr) and lead bromide (PbBr2) upon contact with water (Manser et al., 2016; Sultana et al., 2018). Among these three by products, methylamine and hydro bromic acid escape as gas, leaving behind lead bromide. This lead bromide has much lower electrical conductivity. So when an aqueous solution of PeDOT:PSS was coated over the perovskite crystal, the bromine rich surface of the crystal gets dissociated by the water present in the solution and degrades to PbBr2. PbBr2 is poorly soluble in water and the PeDOT:PSS coating is quickly dried. As perovskites single crystals are much more stable than thin films or powder sample, the crystal can save itself for that time from getting heavily dissociated by the moisture of the PeDOT:PSS solution. These two mechanisms together makes a thin layer of insulating PbBr2 in between the perovskite and the PeDOT: PSS layer. The high cut in voltage may be due to the formation of very thin insulating barrier in between the perovskite and PeDOT:PSS layer. The device performed well as photo detector while illuminated with white light of different intensities which can be seen in increase in photo current with illumination intensity. The energy band diagram is shown in Fig. 2(b). The graded band of MAPIB aligns between PeDOT and C layer in a much linear way. This alignment makes an easy path for the charge carriers to move to the outer electrode without getting hindered at the perovskite/PeDOT interface. The device shows a good rectification behavior, both in light and dark condition. Now looking close into the I-V curves, we see that there are two different regions. The region-I is the linear part with Ohmic behavior at the low voltage area. In region II, photo current starts to increase at a much higher rate following power law dependence relation I ∝ V n . Now we refer to the DC equivalent circuit (Fig. 2c) of the photo diode in photovoltaic mode, The equivalent circuit consists of a diode in parallel with a shunt resistance (Rshunt) and a photo generated current source. This whole system is accompanied by a series resistance (Rseries). Our device is based on a mixed halide perovskite single crystal. The rectifying junction is developed at one end of the crystal while rest of the crystal works likes a photo sensitive resistance. This resistance is contributing a major part in the Rseries which is quite high for the 1 mm thick crystal. Rshunt is contributed by recombination centres formed at the rectifying junction by crystal defects and impurities. Another major contribution in shunt resistance comes from the pin holes in the PeDOT:PSS layer which may allow carbon electrodes to directly touch the perovskite crystal. This would provide an easy route to bypass the current when the diode is in cut off region, i.e. in the low bias region. Thus, below the cut in voltage, the current is bypassed through the shunt resistance instead of the external circuitry. This is reflected through the linear current –voltage relationship in region I. As the bias voltage goes above the diode cut in voltage, the diode starts to conduct and dominates over the shunt resistance. The non-linear profile of the I-V curve in region II portrays the activation of the diode. Sharp increase in current at higher voltage indicates the transition to the trap filled limit (TFL) of the I-V curve where all the trap states get filled with charge carriers (Maculan et al., 2015). Transition point between the Ohmic and TFL region is denoted by VTFL. This voltage can be used to calculate trap density in the crystal exploiting the following relation,
Fig. 2. (a) Current–voltage curve under various light illumination of device. (b) Corresponding band position of different layers. (All energy values are in eV.) (c) DC equivalent circuit of the device in photovoltaic mode.
Bright orange colour of MAPB becomes white and shiny black colour of MAPI gets yellow marks when they degrade. It has already been reported that the hybrid perovskite single crystals are much more stable than their thin film counterpart (Fang et al., 2015; Shaikh et al., 2016). Sample crystals of MAPB and MAPI were left in open air at 80% humidity. No visible change of colours of the crystals has been observed even after 12 h which indicates that the single crystals are significantly stable in presence of moisture. I-V curve of the device with have been plotted in Fig. 2(a) with different light intensities measured in sun unit (S). One sun corresponds to 100 mW/cm2. Under light illumination, the current increases with increasing light intensity till 0.5S. After that, the device goes into saturation region, so no further increase in current is observed. In dark condition, I-V curve (inset of Fig. 2(a)) shows rectifying nature with a cut-in voltage of 2.01 V and reverse saturation current of 9.78 nA. As carbon electrode comes in direct contact with the iodine rich part of MAPIB crystal, we have fabricated a C/MAPI/C device and studied its IV curve to check the origin of this rectifying nature. The current showed
ntrap =
2εε0 VTFL , eL2
where ε, ε0 and L are the dielectric constant, permittivity and thickness of the crystal respectively. Above relation yields trap density value of 9.96 × 107/cm3 with ohmic to TFL transition voltage at 3.53 V. The lower trap density contributes to the superior performance of single crystal based photo detector by lowering the chance of charge trapping 565
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detectivity and external quantum efficiency (EQE) all have sudden drop after the peak in corresponding spectra. This sharp fall agrees with the absorption spectra (Fig. 3(d)) recorded using UV–Vis spectrophotometer. Being a medium band gap material CH3NH3PbBr3 cannot absorb the whole visible spectra. Using Tauc plot on the UV–Visible spectra, the band gap is found to be 2.3 eV or 539 nm in terms of wavelength. This is confirmed from the spectral response behaviors. Studying all these three spectral responses, we observe the photo response is primarily flat with variation of wavelength with a peak response near band edge. The flat portion comes from the contribution of the bulk crystal while the narrow peak at the band edge is the result of surface charge recombination assisted charge collection narrowing (CCN) mechanism (Fang et al., 2015). It has been seen in the UV–Vis spectra (Fig. 3d) that light with shorter wavelengths have higher absorbance which makes them absorbed near the surface region of the crystal. The photo carriers generated from these wavelengths suffer intense surface charge recombination and thus quench. Lights with near band edge wavelength have much higher penetration depth due to lower absorption coefficient and they can travel deep inside the crystal. Photo carriers produced from these wavelengths are less prone to surface recombination and they constitute photo current. This CCN mechanism plays crucial role behind the appearance of the photo response peak near band edge. This type of photo response curve can find different device application like colour detection and imaging as well as radiometric optical power meters, light sensing application, colour blind photo detector utilizing its broad plateau like spectral response. Another important parameter required to characterize a photo diode is its transient photo response time from which charge extraction efficiency can be determined. The rise and fall times are the times required by the device to switch from 10% to 90% and from 90% down to 10% of maximum photocurrent respectively when incident light was turned on and off. Photodynamic responses of the device has been illustrated in Fig. 4. Using chopping frequency of 25 Hz, we found the rise time of 431 µs and fall time 5.823 ms respectively. Rise and fall time of a photo detector depend on the absorption coefficient and the junction capacitance. Considerably high fall time indicates the presence of interfacial defects which work as charge accumulation center. This accumulation centers slow down the device with the diffusion current while discharging. Moreover, long crystal length adds some extra path in charge flow route. Response time can be reduced by thinning the crystals.
and recombination rate. The I–V characteristics of a rectifying diode at forward bias can be expressed by:
qV ⎞ − 1⎤, I = I0 ⎡exp ⎛ ⎢ ⎥ nkT ⎝ ⎠ ⎣ ⎦ where Io is the reverse saturation current which can be calculated from the straight-line fitting of the reverse bias current–voltage profile. Fitting this I-V curve to Schottky diode model unveils different diode parameters like ideality factor, series resistance, etc. Here we have used modified Norde method as proposed by Bohlin (1986). Using this model, ideality factor was calculated to be 4.74. The deviation of ideality factor from unity may be originating from high series resistance between the contact and recombination sites, severe edge recombination (Mcintosh and Honsberg, 2000), junction inhomogeneity (Hadj Belgacem and El-Amine, 2018; Shaikh et al., 2016). The exact reason behind the ideality factor > 2 is still under discussion. In our case, high series resistance, severe edge recombination and junction inhomogeneity may be playing the role behind the high value of ideality factor. The calculated value of series resistance (Rs) of the diode is 7.41 × 106 Ω. This value is quite high. During PeDOT: PSS deposition, water from the solvent degrades the surface of the perovskite crystal to form a thin layer of less conducting Lead bromide (PbBr2) thus increasing the series resistance of the whole device. The computed barrier height gives 0.23 eV. We can see from Fig. 2c, the expected value of barrier height formed in between CH3NH3PbBr3 crystal and PeDOT:PSS layer is 0.36 eV. This kind of discrepancy is also observed for metalsemiconductor interface (Das et al., 2010). This mismatch between the measured and calculated values of barrier height is coming from image force lowering, barrier height inhomogeneity, surface defects and Fermi level pinning. Spectral response of the device has been characterized by measuring Responsivity, detectivity and external quantum efficiency of the photo diode. Responsivitry (R(λ)) of a photo detector is defined as the ratio of the photo current density (Jph) and the incident optical power (Pop).
R (λ ) =
Jph Pop
;
The responsivity spectrum has been plotted in Fig. 3(a). The device shows two peaks in the responsivity spectra. The highest one is at 539 nm with a spectral responsivity of 73.21 mA/W while the smaller one shows up at 554 nm with a responsitivity of 60.01 mA/W. The 2nd peak in responsitivity spectra comes from the contribution of lower band gap part of the MAPIB. Detectivity of a photo diode is calculated by
D (λ ) =
4. Conclusion In summary we have successfully synthesized a graded band gap mixed halide hybrid perovskite crystal (MAPIB). Allowing one side a MAPbBr3 crystal to MAPbI3 solution, replaces the bromine anions of that portion with heavier iodine by using reversible halide exchange reaction mechanism. A photo diode has been fabricated using the MAPIB crystal as light absorber and PeDOT:PSS as polymer hole selective layer with carbon paste as outer electrodes. Graded band gap with much improved band alignment allows smoother carrier transport which has contributed to this diode quality improvement. On the spectral response side, the fabricated device contains effect from both bulk and surface recombination of photo carriers which is reflected in the plateau type nature of detectivity with a peak detection area near band edge absorption.
R (λ ) , 2qJD
where ‘R’ is responsinvity, ‘q’ is electronic charge and ‘JD’ id dark current density. Fig. 3(b) shows the detectivity spectrum. The detectivity is calculated using the dark current value at 5 V bias voltage. 5 V chosen as it is a very common and standard biasing voltage for modern digital systems. Flat nature in photo response is observed in the detectivity spectrum with a peak detectivity 5.51 × 1010 Jones at 539 nm. This values of detectivity are comparable with already reported data (Fang et al., 2015). From the above equation, it is clear that for higher detectivity, dark current density must be very low. Hence perovskite single crystals will be more suitable than thin film for its reduced of dark current. External quantum efficiency (EQE) is calculated using the following relation
EQE (λ ) =
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
1240R (λ ) ∗ 100%; λ
Acknowledgement
EQE spectrum has been pictured in Fig. 3(c). Mixed band gap device shows 18% peak EQE at 350 nm. It is observed that responsitivity,
This work has been supported by Science and Engineering Research 566
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Fig. 3. (a) Responsivity, (b) Detectivity and (c) EQE spectra of the device. (d) UV–Vis absorption spectra of the perovskite crystals. iodide. J. Am. Chem. Soc. 136, 758–764. https://doi.org/10.1021/ja411014k. Das, S.N., Choi, J.H., Kar, J.P., Moon, K.J., Lee, T. Il, Myoung, J.M., 2010. Junction properties of Au/ZnO single nanowire Schottky diode. Appl. Phys. Lett. 96, 092111–92112. https://doi.org/10.1063/1.3339883. Di, Y., Zeng, Q., Huang, C., Tang, D., Sun, K., Yan, C., Wang, Y., Ke, S., Jiang, L., Hao, X., Lai, Y., Liu, F., 2018. Thermal-evaporated selenium as a hole-transporting material for planar perovskite solar cells. Sol. Energy Mater. Sol. Cells 185, 130–135. https:// doi.org/10.1016/j.solmat.2018.05.022. Dong, Q., Fang, Y., Shao, Y., Mulligan, P., Qiu, J., Cao, L., Huang, J., 2015. Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals. Science (80-.) 347, 967–970. https://doi.org/10.1126/science.aaa5760. Fang, Y., Dong, Q., Shao, Y., Yuan, Y., Huang, J., 2015. Highly narrowband perovskite single-crystal photodetectors enabled by surface-charge recombination. Nat. Photon. 9, 679–686. https://doi.org/10.1038/nphoton.2015.156. Hadj Belgacem, C., El-Amine, A.A., 2018. Theoretical models for anomalously high ideality factor in a Au/SnO2-Si(n)/Al solar cell. Silicon 10, 1063–1066. https://doi.org/ 10.1007/s12633-017-9572-7. Lian, Z., Yan, Q., Lv, Q., Wang, Y., Liu, L., Zhang, L., Pan, S., Li, Q., Wang, L., Sun, J.L., 2015. High-performance planar-type photodetector on (100) facet of MAPbI3 single crystal. Sci. Rep. 5, 1–10. https://doi.org/10.1038/srep16563. Maculan, G., Sheikh, A.D., Abdelhady, A.L., Saidaminov, M.I., Haque, M.A., Murali, B., Alarousu, E., Mohammed, O.F., Wu, T., Bakr, O.M., 2015. CH3NH3PbCl3 single crystals: inverse temperature crystallization and visible-blind UV-photodetector. J. Phys. Chem. Lett. 6, 3781–3786. https://doi.org/10.1021/acs.jpclett.5b01666. Manser, J.S., Saidaminov, M.I., Christians, J.A., Bakr, O.M., Kamat, P.V., 2016. Making and breaking of lead halide perovskites. Acc. Chem. Res. 49, 330–338. https://doi. org/10.1021/acs.accounts.5b00455. Mcintosh, K.R., Honsberg, C.B., 2000. The influence of edge recombination on a solar cells I-V curve. In: 16th PVSEC, pp. 1651–1654. Montoya, M.P., Sidhik, S., Esparza, D., López-Luke, T., Zarazua, I., Rivas, J.M., De la Rosa, E., 2019. Study of inverted planar CH3NH3PbI3 perovskite solar cells fabricated under environmental conditions. Sol. Energy 180, 594–600. https://doi.org/10.1016/j. solener.2019.01.061. Sadhukhan, P., Kundu, S., Roy, A., Ray, A., Maji, P., Dutta, H., Pradhan, S.K., Das, S., 2018. Solvent-free solid-state synthesis of high yield mixed halide perovskites for easily tunable composition and band gap. Cryst. Growth Des. 18, 3428–3432. https:// doi.org/10.1021/acs.cgd.8b00137. Sadhukhan, P., Pradhan, A., Mukherjee, S., Sengupta, P., Roy, A., Bhunia, S., Das, S., 2019. Low temperature excitonic spectroscopy study of mechano-synthesized hybrid perovskite. Appl. Phys. Lett. 114, 131102. https://doi.org/10.1063/1.5084145.
Fig. 4. Switching plot of photo detectors under blinking light for calculating response time the device.
Board (SERB) funded project (PI- Sachindranath Das, Grant no. YSS/ 2015/000109), Government of India. P Sadhukhan acknowledges INSPIRE (DST), Govt. of India for the financial support through INSPIRE fellowship programme (Fellowship no. IF160132). References Bohlin, K.E., 1986. Generalized Norde plot including determination of the ideality factor. J. Appl. Phys. 60, 1223–1224. https://doi.org/10.1063/1.337372. Christians, J.A., Fung, R.C.M., Kamat, P.V., 2014. An inorganic hole conductor for organo-lead halide perovskite solar cells. Improved hole conductivity with copper
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Photo detector based on graded band gap perovskite crystal Priyabrata Sadhukhan, Sachindranath Das
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T
Department of Instrumentation Science, Jadavpur University, Kolkata 700032, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Perovskite Graded band gap Photo diode Single crystal
In this work, millimeter sized graded band gap mixed halide hybrid perovskite crystal have been synthesized by utilizing reversible halide exchange property of hybrid perovskites. Optoelectronic performance of photo diodes fabricated using the graded band gap crystal has been studied in details. Band gap engineering has formed a smoother path for carrier transport by aligning HOMO and LUMO levels with the charge collection layers. This is reflected in good diode quality and fast photo response has been achieved. On the other hand, effect from both bulk and surface recombination of photo generated carriers are observed in the plateau type spectral response curve with a peak near band edge.
1. Introduction Photo detectors are the integral part of light sensing, imaging, automation, optical communication and many other fields. A good photo detector should have high detectivity and low response time. For imaging applications, high spectral selectivity is also desirable. Spectrally selective photo detectors are used in bio medical applications, scientific research instruments, imaging, defense etc. Light sensing organic inorganic hybrid perovskite materials have recently gained huge attention as a potential replacement of silicon in the future photovoltaic technology. High dielectric constant, high light absorption coefficient, low excitonic binding energy, high carrier mobility and long carrier diffusion length are the key features which has escalated the material to this stage. However, apart from solar cell applications, hybrid perovskite materials find its way in photo diode also. Relatively easy way to fabricate millimeter size crystals backed up with astonishingly long 175 µm (Dong et al., 2015) carrier diffusion length is another advantage. Parvez et al. reported self-biased photo detector using CH3NH3PbBr3 crystal/Pt junction as Schottky contact (Shaikh et al., 2016) Fang et al used surface charge recombination to achieve charge collection narrowing mechanism to realize spectral selectivity in perovskite single crystal photo diode (Fang et al., 2015). Lian and his group has fabricated a perovskite planer type photo detector on the (1 0 0) facet of CH3NH3PbI3 crystal (Lian et al., 2015). All these reports are made with pure halide hybrid perovskite crystals. CH3NH3PbI3 is already well established as a very good photo voltaic material, but this is not the only thing to determine the performance of a photovoltaic cell. Very good charge collection efficiency is required to
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efficiently collect the photo generated carriers and drive them to the external circuitry. Two of the most dominant things to affect charge collection efficiency are the quality of the hole selective layer and the alignment of energy band between the hole selective layer and the perovskite. To match the need of perfect sync of energy bands of hybrid perovskite layer, many organic and inorganic hole transport materials (HTM) like spiro-MeOTAD (Tian et al., 2019), PeDOT:PSS (Montoya et al., 2019), CuI (Christians et al., 2014), crystalline selenium (Di et al., 2018), etc. have been investigated. Organic HTMs outperforms the inorganic HTM in most studies (Christians et al., 2014; Di et al., 2018). However spiro-MeOTAD is very costly. A less expensive and efficient alternative is PeDOT:PSS, a polymer type HTM widely used in inverted structure perovskite devices. In the perovskite part, several strategies have been taken by researchers in order to reduce defect assisted recombination of charged carriers and to improve charge collection. Sun et al. demonstrated that defect density in the perovskite layer can be reduced by introducing I3− ions into organic cation dripping solution (Sun et al., 2018). The charge collection efiiciency can be improved by carefully tuning perovskite the energy band near the perovskite/HTM interface. Wang et al. introduced a iodine concentration gradient near the perovskite/HTM interface tunes the band gap of the perovskite to effectively improve hole extraction (Wang et al., 2018). Another effective approach for carefully tuning the band gap of hybrid perovskite is to use mixed halide hybrid perovskite. These hybrid Perovskites have an interesting property that they participate in halogen exchange reaction. It means that if CH3NH3PbBr3 is made in touch with CH3NH3PbI3 solution, the former will react with the later in which Br−1 ions will be replaced by the I−1 ions of the solution and the final product will be a mixed halide hybrid
Corresponding author. E-mail address: [email protected] (S. Das).
https://doi.org/10.1016/j.solener.2019.11.001 Received 23 November 2018; Received in revised form 5 October 2019; Accepted 1 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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perovskite (CH3NH3PbBr3−xIx). The final percentage of Br−1 and I−1 will depend on time of contact and the molarity of the solution. As we already know, the band gap of the pure CH3NH3PbBr3 is 2.3 eV while CH3NH3PbI3 has 1.5 eV. Thus the mixed halide perovskite (CH3NH3PbBr3−xIx) will pick up a band gap in between these two limits depending on the composition(x). The band gap can be tuned by carefully controlling the Iodine/bromine ratio. We have exploited this property to form a graded band gap perovskite crystal where the band gap varies from 1.5 eV in one side to 2.3 eV on the other side of the crystal. Here in this work, we have successfully grown graded band gap mixed halide hybrid perovskite crystal (CH3NH3PbBr3−xIx) and fabricated crystal based photo diode absorber layer using carbon paste (C) as electrodes and PeDOT:PSS is used as carrier selective layer. Optoelectronic properties of the device is studied in details with different light intensity and different wavelength.
2.2. Fabrication of photo diode The device was fabricated with a 1 mm thick graded band gap MAPIB crystals using carbon paste as the outer electrodes and PeDOT:PSS as hole transport layer. The crystal was coated with conducting carbon paste at one end and PeDOT:PSS followed by conducting carbon paste at another end to make C/MAPIB/PeDOT/C architecture. To deposit PeDOT:PSS layer, a piece of tissue paper was firstly soaked with aqueous solution of PeDOT:PSS. Then the bromine rich face of the MAPIB crystal was pressed against the wet tissue paper for 3 s and then immediately dried with hot air gun at 50 °C to form the PeDOT: PSS layer. The outer electrodes were deposited by coating conducting carbon paste on either sides of the crystal afterwards. Separation between two electrode layers was kept to 1 mm in each cases. Cross sectional area of the device was measured to be 1 mm × 1 mm.
2. Experimental
2.3. Material and device characterization
2.1. Formation of the crystal
X-Ray diffraction pattern was recorded with a RIGAKU miniflex-600 bench top Diffractometer. Current–voltage (I-V) data with different light intensity was recorded with a Keithley 2602B multichannel source measure unit and a variable intensity solar simulator (Royal enterprise, India). UV–Vis spectra of the material was performed with a Optizen POP (Korea) UV–Vis-NIR spectrometer. Spectral photo response measurement was done using a 150 W XENON lamp and the monochromator of a PTI QuantaMaster 400 spectroflurometer, an optical power meter and the Keithley 2602B source meter. For transient photo response measurement we used an NVis 105CT digital storage oscilloscope and three different LEDs of wavelengths 462 nm, 520 nm and 630 nm, blinking at 25 Hz chopping frequency. Characterization of the crystal and the device were performed in ambient environment with 60% relative humidity at room temperature.
Methylammonium lead bromide (CH3NH3PbBr3) crystals were synthesized following inverse temperature crystallization method described by Saidaminov et al. (2015). Firstly Methylammonium bromide (CH3NH3Br) and Methylammonium iodide (CH3NH3I) was prepared by following the procedure as described in our earlier work (Sadhukhan et al., 2019, 2018). This CH3NH3Br and lead iodide (PbBr2) were mixed at equimolar ratio in dimethyl formamide (DMF) solvent at 60 °C and kept on stirring for 6–7 h followed by filtering to get a clear pale yellow solution of CH3NH3PbBr3. This solution was kept undisturbed at 80 °C in an oil bath. Orange coloured CH3NH3PbBr3 crystals (MAPB) started to grow after few hours which became larger over time. CH3NH3PbI3 crystal (MAPI) was also grown in a similar way from γ-butyrolactone solution of CH3NH3PbI3. The graded junction crystal (CH3NH3PbBr3−xIx) was formed by immersing a MAPB crystal partially in 1 M CH3NH3PbI3 solution for 5 min to allow the halide exchange reaction to occur. The dipping actuator of a SILAR dip coater (Apex Instrument, India) was used to precisely immerse one side of a 1 mm × 1 mm × 1 mm MAPB crystal into the CH3NH3PbI3 solution so that MAPI can grow only on that part of the MAPB crystal. Length of the submerged portion of the crystal was about 0.3 mm. We name this crystal MAPIB. All the steps of the synthesis part were carried out in ambient condition with 60% relative humidity. Synthesis method of the graded band gap mixed halide hybrid perovskite single crystal and fabrication process of the photo diode is outlined in Fig. 1(a).
3. Results and discussion Crystalline quality of the synthesized MAPB and MAPI crystals was checked with powder X-Ray diffraction measurement. Sharp XRD peaks shown in Fig. 1(b) indicate formation of good quality crystal. Using Rietveld refinement the crystal structure of MAPB is found to be cubic with pm3m space group and lattice parameter a = b = c = 5.91417 Å while MAPI possesses tetragonal symmetry with I4/mcm space group and a = b = 8.8560 Å, c = 12.6525 Å. Hybrid perovskites degrades to lead halides when exposed to moisture or UV light. The degradation leaves a mark by easily detectable change of colour of the sample.
Fig. 1. (a) Schematic diagram of the fabrication of the graded band mixed halide hybrid perovskite crystal and photo diode and (b) Powder XRD pattern of the MAPB ad MAPI. 564
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a linear dependency on voltage in both dark and illuminated (0.5S) state which indicated ohmic nature of interface [Not shown here]. Hence the rectifying nature of the photo diode comes from the perovskite – PeDOT:PSS junction. PeDOT:PSS is a popular material for hole transporting layer used in organic solar cell. All hybrid perovskites are very sensitive to water. CH3NH3PbBr3 degrades to methylamine (CH3NH2), hydrobromic acid (HBr) and lead bromide (PbBr2) upon contact with water (Manser et al., 2016; Sultana et al., 2018). Among these three by products, methylamine and hydro bromic acid escape as gas, leaving behind lead bromide. This lead bromide has much lower electrical conductivity. So when an aqueous solution of PeDOT:PSS was coated over the perovskite crystal, the bromine rich surface of the crystal gets dissociated by the water present in the solution and degrades to PbBr2. PbBr2 is poorly soluble in water and the PeDOT:PSS coating is quickly dried. As perovskites single crystals are much more stable than thin films or powder sample, the crystal can save itself for that time from getting heavily dissociated by the moisture of the PeDOT:PSS solution. These two mechanisms together makes a thin layer of insulating PbBr2 in between the perovskite and the PeDOT: PSS layer. The high cut in voltage may be due to the formation of very thin insulating barrier in between the perovskite and PeDOT:PSS layer. The device performed well as photo detector while illuminated with white light of different intensities which can be seen in increase in photo current with illumination intensity. The energy band diagram is shown in Fig. 2(b). The graded band of MAPIB aligns between PeDOT and C layer in a much linear way. This alignment makes an easy path for the charge carriers to move to the outer electrode without getting hindered at the perovskite/PeDOT interface. The device shows a good rectification behavior, both in light and dark condition. Now looking close into the I-V curves, we see that there are two different regions. The region-I is the linear part with Ohmic behavior at the low voltage area. In region II, photo current starts to increase at a much higher rate following power law dependence relation I ∝ V n . Now we refer to the DC equivalent circuit (Fig. 2c) of the photo diode in photovoltaic mode, The equivalent circuit consists of a diode in parallel with a shunt resistance (Rshunt) and a photo generated current source. This whole system is accompanied by a series resistance (Rseries). Our device is based on a mixed halide perovskite single crystal. The rectifying junction is developed at one end of the crystal while rest of the crystal works likes a photo sensitive resistance. This resistance is contributing a major part in the Rseries which is quite high for the 1 mm thick crystal. Rshunt is contributed by recombination centres formed at the rectifying junction by crystal defects and impurities. Another major contribution in shunt resistance comes from the pin holes in the PeDOT:PSS layer which may allow carbon electrodes to directly touch the perovskite crystal. This would provide an easy route to bypass the current when the diode is in cut off region, i.e. in the low bias region. Thus, below the cut in voltage, the current is bypassed through the shunt resistance instead of the external circuitry. This is reflected through the linear current –voltage relationship in region I. As the bias voltage goes above the diode cut in voltage, the diode starts to conduct and dominates over the shunt resistance. The non-linear profile of the I-V curve in region II portrays the activation of the diode. Sharp increase in current at higher voltage indicates the transition to the trap filled limit (TFL) of the I-V curve where all the trap states get filled with charge carriers (Maculan et al., 2015). Transition point between the Ohmic and TFL region is denoted by VTFL. This voltage can be used to calculate trap density in the crystal exploiting the following relation,
Fig. 2. (a) Current–voltage curve under various light illumination of device. (b) Corresponding band position of different layers. (All energy values are in eV.) (c) DC equivalent circuit of the device in photovoltaic mode.
Bright orange colour of MAPB becomes white and shiny black colour of MAPI gets yellow marks when they degrade. It has already been reported that the hybrid perovskite single crystals are much more stable than their thin film counterpart (Fang et al., 2015; Shaikh et al., 2016). Sample crystals of MAPB and MAPI were left in open air at 80% humidity. No visible change of colours of the crystals has been observed even after 12 h which indicates that the single crystals are significantly stable in presence of moisture. I-V curve of the device with have been plotted in Fig. 2(a) with different light intensities measured in sun unit (S). One sun corresponds to 100 mW/cm2. Under light illumination, the current increases with increasing light intensity till 0.5S. After that, the device goes into saturation region, so no further increase in current is observed. In dark condition, I-V curve (inset of Fig. 2(a)) shows rectifying nature with a cut-in voltage of 2.01 V and reverse saturation current of 9.78 nA. As carbon electrode comes in direct contact with the iodine rich part of MAPIB crystal, we have fabricated a C/MAPI/C device and studied its IV curve to check the origin of this rectifying nature. The current showed
ntrap =
2εε0 VTFL , eL2
where ε, ε0 and L are the dielectric constant, permittivity and thickness of the crystal respectively. Above relation yields trap density value of 9.96 × 107/cm3 with ohmic to TFL transition voltage at 3.53 V. The lower trap density contributes to the superior performance of single crystal based photo detector by lowering the chance of charge trapping 565
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detectivity and external quantum efficiency (EQE) all have sudden drop after the peak in corresponding spectra. This sharp fall agrees with the absorption spectra (Fig. 3(d)) recorded using UV–Vis spectrophotometer. Being a medium band gap material CH3NH3PbBr3 cannot absorb the whole visible spectra. Using Tauc plot on the UV–Visible spectra, the band gap is found to be 2.3 eV or 539 nm in terms of wavelength. This is confirmed from the spectral response behaviors. Studying all these three spectral responses, we observe the photo response is primarily flat with variation of wavelength with a peak response near band edge. The flat portion comes from the contribution of the bulk crystal while the narrow peak at the band edge is the result of surface charge recombination assisted charge collection narrowing (CCN) mechanism (Fang et al., 2015). It has been seen in the UV–Vis spectra (Fig. 3d) that light with shorter wavelengths have higher absorbance which makes them absorbed near the surface region of the crystal. The photo carriers generated from these wavelengths suffer intense surface charge recombination and thus quench. Lights with near band edge wavelength have much higher penetration depth due to lower absorption coefficient and they can travel deep inside the crystal. Photo carriers produced from these wavelengths are less prone to surface recombination and they constitute photo current. This CCN mechanism plays crucial role behind the appearance of the photo response peak near band edge. This type of photo response curve can find different device application like colour detection and imaging as well as radiometric optical power meters, light sensing application, colour blind photo detector utilizing its broad plateau like spectral response. Another important parameter required to characterize a photo diode is its transient photo response time from which charge extraction efficiency can be determined. The rise and fall times are the times required by the device to switch from 10% to 90% and from 90% down to 10% of maximum photocurrent respectively when incident light was turned on and off. Photodynamic responses of the device has been illustrated in Fig. 4. Using chopping frequency of 25 Hz, we found the rise time of 431 µs and fall time 5.823 ms respectively. Rise and fall time of a photo detector depend on the absorption coefficient and the junction capacitance. Considerably high fall time indicates the presence of interfacial defects which work as charge accumulation center. This accumulation centers slow down the device with the diffusion current while discharging. Moreover, long crystal length adds some extra path in charge flow route. Response time can be reduced by thinning the crystals.
and recombination rate. The I–V characteristics of a rectifying diode at forward bias can be expressed by:
qV ⎞ − 1⎤, I = I0 ⎡exp ⎛ ⎢ ⎥ nkT ⎝ ⎠ ⎣ ⎦ where Io is the reverse saturation current which can be calculated from the straight-line fitting of the reverse bias current–voltage profile. Fitting this I-V curve to Schottky diode model unveils different diode parameters like ideality factor, series resistance, etc. Here we have used modified Norde method as proposed by Bohlin (1986). Using this model, ideality factor was calculated to be 4.74. The deviation of ideality factor from unity may be originating from high series resistance between the contact and recombination sites, severe edge recombination (Mcintosh and Honsberg, 2000), junction inhomogeneity (Hadj Belgacem and El-Amine, 2018; Shaikh et al., 2016). The exact reason behind the ideality factor > 2 is still under discussion. In our case, high series resistance, severe edge recombination and junction inhomogeneity may be playing the role behind the high value of ideality factor. The calculated value of series resistance (Rs) of the diode is 7.41 × 106 Ω. This value is quite high. During PeDOT: PSS deposition, water from the solvent degrades the surface of the perovskite crystal to form a thin layer of less conducting Lead bromide (PbBr2) thus increasing the series resistance of the whole device. The computed barrier height gives 0.23 eV. We can see from Fig. 2c, the expected value of barrier height formed in between CH3NH3PbBr3 crystal and PeDOT:PSS layer is 0.36 eV. This kind of discrepancy is also observed for metalsemiconductor interface (Das et al., 2010). This mismatch between the measured and calculated values of barrier height is coming from image force lowering, barrier height inhomogeneity, surface defects and Fermi level pinning. Spectral response of the device has been characterized by measuring Responsivity, detectivity and external quantum efficiency of the photo diode. Responsivitry (R(λ)) of a photo detector is defined as the ratio of the photo current density (Jph) and the incident optical power (Pop).
R (λ ) =
Jph Pop
;
The responsivity spectrum has been plotted in Fig. 3(a). The device shows two peaks in the responsivity spectra. The highest one is at 539 nm with a spectral responsivity of 73.21 mA/W while the smaller one shows up at 554 nm with a responsitivity of 60.01 mA/W. The 2nd peak in responsitivity spectra comes from the contribution of lower band gap part of the MAPIB. Detectivity of a photo diode is calculated by
D (λ ) =
4. Conclusion In summary we have successfully synthesized a graded band gap mixed halide hybrid perovskite crystal (MAPIB). Allowing one side a MAPbBr3 crystal to MAPbI3 solution, replaces the bromine anions of that portion with heavier iodine by using reversible halide exchange reaction mechanism. A photo diode has been fabricated using the MAPIB crystal as light absorber and PeDOT:PSS as polymer hole selective layer with carbon paste as outer electrodes. Graded band gap with much improved band alignment allows smoother carrier transport which has contributed to this diode quality improvement. On the spectral response side, the fabricated device contains effect from both bulk and surface recombination of photo carriers which is reflected in the plateau type nature of detectivity with a peak detection area near band edge absorption.
R (λ ) , 2qJD
where ‘R’ is responsinvity, ‘q’ is electronic charge and ‘JD’ id dark current density. Fig. 3(b) shows the detectivity spectrum. The detectivity is calculated using the dark current value at 5 V bias voltage. 5 V chosen as it is a very common and standard biasing voltage for modern digital systems. Flat nature in photo response is observed in the detectivity spectrum with a peak detectivity 5.51 × 1010 Jones at 539 nm. This values of detectivity are comparable with already reported data (Fang et al., 2015). From the above equation, it is clear that for higher detectivity, dark current density must be very low. Hence perovskite single crystals will be more suitable than thin film for its reduced of dark current. External quantum efficiency (EQE) is calculated using the following relation
EQE (λ ) =
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
1240R (λ ) ∗ 100%; λ
Acknowledgement
EQE spectrum has been pictured in Fig. 3(c). Mixed band gap device shows 18% peak EQE at 350 nm. It is observed that responsitivity,
This work has been supported by Science and Engineering Research 566
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Fig. 4. Switching plot of photo detectors under blinking light for calculating response time the device.
Board (SERB) funded project (PI- Sachindranath Das, Grant no. YSS/ 2015/000109), Government of India. P Sadhukhan acknowledges INSPIRE (DST), Govt. of India for the financial support through INSPIRE fellowship programme (Fellowship no. IF160132). References Bohlin, K.E., 1986. Generalized Norde plot including determination of the ideality factor. J. Appl. Phys. 60, 1223–1224. https://doi.org/10.1063/1.337372. Christians, J.A., Fung, R.C.M., Kamat, P.V., 2014. An inorganic hole conductor for organo-lead halide perovskite solar cells. Improved hole conductivity with copper
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