Free Access
Eur. Phys. J. Appl. Phys.
Volume 88, Number 3, December 2019
Article Number 30101
Number of page(s) 6
Section Semiconductors and Devices
Published online 05 February 2020

© EDP Sciences, 2020

1 Introduction

The excellent properties of bromide perovskite owe towards the optoelectronics devices. Moreover, it has achieved the huge attention in the demands of environmental affability. In resent research, there has been vast attention in perovskite material scheme, mostly in optoelectronic devices, because of exceptional optoelectronic properties of semiconducting hybrid halide perovskites i.e. large diffusion length, long carrier recombination lifetimes, and a high charge carrier mobility, along with minute carrier effective masses, low cost, capability to absorb the wide range in solar spectrum and good charge transportation proficiency [1,2]. Therefore, the band gap could be tuned from 1.1 to 2.3 eV by chemical composition controlling. Recently [36], great achievement in the field of perovskite solar cells has been observed.

The new generation of solar cells basically based on perovskite material has much attention because of its attractive efficiencies which rapidly boosted from 4% to 23.3% and the latest record of 23.7% (reported by the U.S. National Renewable Energy Laboratory (NREL) [712]. Consequently, ETL and HTL play vital role in improving the performance of device parameters by enhancing the charge collection process, charge extraction and reduced trap assisted recombination at the interface of ETL or HTL/active layer [13]. However, the device stability is still a giant challenge by using different ETLs (TiO2, ZnO, PCBM, and C60) and HTLs (PEDOT:PSS, Spiro-OMeTAD, and P3HT:PCBM). In this paper, we have proposed an inverted structure using TiO2 as the main common compact layer for electrons transport and hole blocking in photoactive layer, because of its appropriate alignment the perfect energy levels between TiO2 and CH3NH3PbBr3 and good compatibility with a variety of deposition methods [1419].

We have developed an easy method to fabricate photoactive perovskite based device on a one-step formed FTO/TiO2/perovskite/Spiro-OMeTAD/Al heterojunction. Generally, perovskite material CH3NH3PbBr3 (MAPbBr3) was adopted as the light absorbing layer, and Spiro-OMeTAD has been deposited onto active layer to make the easy transportation of the holes from perovskite to the electrode [20]. In this study, we have investigated the metal contact and charge carrier transport mechanisms of fabricated (CH3NH3PbBr3) device, and also shown the similar mechanism of charge transport properties by charge transport IV curve (0–10 V) and log–log scale (SCLC region). Charge transport study has been done by showing the IV graph (with voltage 0–10). We wish the reported work might be accommodated in the field of research and photovoltaic techniques to reduce the global warming and ease the utilization of optoelectronic device.

2 Experimental details

2.1 Substrate-cleaning

Before depositing the material, FTO coated glass-substrate is needed to clean properly for the device performance. We follow the method to clean the substrate with laboline soap and in Ultrasonic bath, sequent, as mentioned in Figure 1.

thumbnail Fig. 1

Illustration of method to clean FTO coated glass-substrate before depositing the thin films of materials.

thumbnail Fig. 2

(a) FESEM image of CH3NH3PbBr3 perovskites thin film, (b) photo image of thickness (L = 420 × 10−7 cm) of corresponding perovskite thin film.

2.2 Synthesis of methylammonium lead bromide precursor solution

For one-step spin coating, very first CH3NH3Br is synthesized by reaction of CH3NH2 and HBr in the suitable solvent (Ethanol) by magnetic stirrer. The mixture is formed in white precipitate after evaporate the solvent by heating at 60 °C for 40 m, the obtained material (CH3NH3Br) has been washed 3 times in di-ethyl-ether and then dried in vacuum oven for overnight. To prepare the precursor solution of CH3NH3PbBr3 (Methylammonium lead bromide), the synthesized CH3NH3Br is incorporated with PbBr2 at a 3:1 molar ratio in appropriate solution (DMF). Now, the prepared perovskite solution has been deposited onto glass-substrate by one-step spin coating technique to show the surface morphology of the synthesized perovskite nanoparticles by Field Emission Scanning Electron Microscopy (FESEM) shown in Figure 2a.

2.3 Fabrication of the device

After cleaning the FTO-substrate, the Ozone treatment is needed for uniform deposition of material onto substrate, which enhanced the device performance. Firstly, TiO2 precursor solution was deposited onto pre-cleaned FTO coated glass-substrate (displayed in Fig. 1) at 3000 rpm for 40 s by spin-coated technique to make thin film of electron transport layer (ETL). After that the substrate was annealed at 70 °C for 5 min and placed in furnace for sintering at 500 °C for 45 s. Then, CH3NH3PbBr3 perovskite precursor solution was deposited onto TiO2 layer to fabricate the active layer.

Spiro-OMeTAD has been deposited onto active layer for the easy movement of holes from active layer to respective electrode. Finally, Al has been deposited by using thermal evaporation technique at a pressure of 10−5 torr to form the top (metal) electrode of device. The resulting device structure and energy level diagrams of the materials used for fabrication of devices are shown in Figure 3a and b, respectively. The fabricated device performance has been characterized by using electrical techniques such as IV and impedance spectroscopy (IS).

thumbnail Fig. 3

(a) Schematic device heterostructure of the perovskite based device, (b) energy level sketch of materials used in the fabrication of device.

3 Results and discussion

The FESEM image of CH3NH3PbBr3 deposited over FTO coated glass-substrate shown in Figure 2 that revealed uniform perovskite thin-film with entire surface coverage with large grains and negligible pinholes that enhance the device performance [21]. Figure 2b shows the thickness (L) of perovskite thin-film, which was calculated of 420 × 10−7 cm. On this basis, we have calculated mobility of the charge carriers. The measured IV characteristic of the fabricated device under dark and illumination is displayed in Figure 4a. The current increases exponentially in forward bias under dark condition, while in reverse bias current is nearly independent of the applied voltage because of high energy barrier at interface of FTO/ETL (TiO2) blocks hole injection, correspondingly energy barrier at interface of FTO/(HTL) Spiro-OMeTAD blocks electron injection, consequently improve the dark IV characteristics. Under illumination condition, current levels increases as compared to dark current, this indicates the device exhibits the photoconductive behavior. By the excitation of light, excitons created inside active layer (perovskite layer), the electron-hole pairs (charge carriers) generated in the photoactive layer then are drifted towards the respective electrodes through the transported layers under the control of the applied electric field due to the low exciton binding energy of material. Therefore, the photogenerated charge carriers (electrons/holes) are extracted at the interface of perovskite/TiO2 (Spiro-OMeTAD) which prevents the occurring of recombination in device, results into the generation of photocurrent in the device [22].

Now, the diode parameters are extracted from the slope of the linear portion of the forward bias region of the semi-logarithmic IV characteristics as shown in Figure 4b [23]. The fabricated device shows good rectifying behavior owing to the formation of a Schottky barrier at the semiconductor/Al interface. The saturation current (Is) has extracted 9.14 × 10−5 mA by using semilogrithmic scale by intercept at V = 0 of IV curve at room temperature. The Schottky barrier height (ΦB) has evaluated of 0.74 eV by using equation [24,25]:(1)

Here, ϕ B is the Schottkey barrier height, A is the junction area, and A* is the Richardson's constant and these parameters are obtained by [26,27]:(2)

Here, h is the Plank's constant and m* is the effective mass of material (perovskite) that used as an active layer in the device.

To recognize the current conduction mechanism, three charge transport properties at different regions are evident in experimental data [28], the IV characteristics curve is re-plotted in double logarithmic scale and fitted with the power law curve (I αV 2), as shown in Figure 5a. Furthermore, charge transport properties have shown in Figure 5b by applying voltage 0–10 in current–voltage characteristics, which exhibits the Ohmic behavior at lower voltage and SCLC regime at higher voltage. Hence proved, the similar results with log-log plot of the planer fabricated device. As can be apparent in Figure 5a, m represents the slope of the curve and distinguishes the different conduction mechanisms [29]. The slope in the low voltage region is close to 1, implying that the current conduction mechanism at low voltage is of Ohmic nature. The slope turn into 4.3 at mid-voltage region, representing trap controlled space charge limited conduction (TCLC) mechanism. In this voltage range, current is limited by the space charge formed in the active layer in the presence of traps within the forbidden gap of the perovskite.

The injected charge carriers unable to transport as fast as they are injected from the electrode into the perovskite layer, causing accumulation of charge carrier, thus formation of the space charge layer near the injecting electrode, that impede further injection process, limiting the current flow in the device. However, in the presence of traps, the accumulated charge carriers might excited to the traps, which allow the further injection of carriers, resulting in the rapid change of the current with applied voltage, as represented by the slope 4.3 in Figure 5a. At large voltage region, slope changes to 2, this is the symptom of space limited charge conduction (SCLC) mechanism. Since all traps are acquired by the charges, so at high voltages space charge again start forming in the perovskite layer [30], therefore current exhibits the SCLC behavior. The electron mobility in the device is found to be 1.65 × 10−4 cm2 V−1 s−1, by applying Mott-Gurney's law in SCLC region or Child' region [31,32]:(3)

Here, V is the applied voltage, J is the current density, L is the perovskite film thickness, A is the area, μ is the mobility, ε is the relative dielectric constant of MAPbBr3, and ε 0 is the vacuum permittivity.

Furthermore, electrochemical impedance spectroscopy (EIS) was carried out at 0 Volt DC to enumerate the internal circuit parameters of the device, such as series resistance, space charge capacitance and leakage resistance. Normally impedance spectroscopy is used to characteristic forward biased junctions [33]. The measured results are fitted with the simulated results and plotted in Figure 6a. An electrical equivalent circuit element (ECE) is required for good fits at low and intermediate frequencies and useful to acquire the simulated results as shown in Figure 6b. As can be apparent from the figures, experimental results are good matched with the simulated results. As can be seen in Figure 6a, semicircle showed reduction in size of the arc appearing in intermediate frequency range (104–103 Hz). This semicircle in the mid-frequency range essentially corresponds to recombination at TiO2/perovskite or perovskite/Spiro-OMeTAD interfaces. The equivalent circuit comprises of parallel combination of leakage resistance (Rp ) and constant phase element (CPE) in series with the series resistance Rs . The CPE is used as an alternative of pure capacitor to obtain the good match between experimental and simulated data, which implies the non-uniformity of the junction. The extracted parameters; Rs , Rp and Cp have been found to be 148.07 Ω, 860.14 Ω and 2.09 × 10−8 F, respectively. The rise time of the device can be determined from the relation [34]:(4)

The value comes out to be equal to 1.72 × 10−5 s. The time constant of such parallel combination is the lifetime of minority carriers. Hence, we investigated that, the rise-time (τr ) is strongly related to the recombination of charge-carriers in the device. We hope this work helps to correlate the charge transport properties that extracted by log–log curve of IV characteristics applying voltage −4 to +4 V and by IV curve applying voltage 0–10 V. Furthermore, the evaluated charge carrier mobility by SCLC region of log-log curve is same as calculated by charge transport IV curve. This text reported the strongest transportation of charge carriers shown by Figure 5. In addition, all traps are acquired by the charges, so at high voltages the space charges again start forming in the perovskite layer, which evidence the enhancement in photovoltaic properties.

thumbnail Fig. 4

(a) IV curve of the device in dark and light, (b) IV characteristics in semi logarithmic scale of device.

thumbnail Fig. 5

(a) Charge transport mechanism by double-logarithmic plot of the device, (b) charge transport properties in IV curve.

thumbnail Fig. 6

(a) Nyquist plot of the device under study, (b) electrical equivalent circuit element (ECE) of the device.

4 Summary and conclusions

The heterojunction device of the architecture FTO/TiO2/CH3NH3PbBr3/Spiro-OMeTAD/Al has been fabricated using spin-coating technique. The morphological study without pinholes shows uniform deposition of the CH3NH3PbBr3 over the glass substrate. Electrical characterization of this device has been analyzed by IV and impedance measurements. Analysis of forward bias semi-logarithmic IV curves shows that the device exhibits the Schottky barrier height of 0.74 eV and reverses saturation current of 9.14 × 10−5 A. The charge transport properties of the device studied under the circumstance of SCLC depicts three distinct voltage dependent regions and enables us to extract the mobility of charge carrier. This work shows the charge transport graph in IV curve with 0–10 V to extract direct charge transport properties without log–log curve of current-voltage characteristics. Electrochemical impedance spectroscopy study has been done under taken to understand the internal electrical structure of device and concluded that the higher value of current under light illumination compared to dark, it implies that device shows good photo response behavior.


The authors gratefully acknowledge financial support from DST, India under CURIE program (grant No. SR/CURIE-Phase-III/01/2015(G)) and MHRD FAST Programme (grant No.5-5/2014-TS.VII), Govt. of India. Corresponding author (Dr Ajay Singh Verma) is thankful to UGC-DAE Consortium of Scientific Research India, for supporting this research under the scheme of UGC-CRS (letter No. CSR/IC/BL-88/5CRS-205/909).


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Cite this article as: Jyoti Chaudhary, Ruchita Gautam, Shaily Choudhary, Ajay Singh Verma, Inverted-heterostructure based device of CH3NH3PbBr3 for Schottky photodiode, Eur. Phys. J. Appl. Phys. 88, 30101 (2019)

All Figures

thumbnail Fig. 1

Illustration of method to clean FTO coated glass-substrate before depositing the thin films of materials.

In the text
thumbnail Fig. 2

(a) FESEM image of CH3NH3PbBr3 perovskites thin film, (b) photo image of thickness (L = 420 × 10−7 cm) of corresponding perovskite thin film.

In the text
thumbnail Fig. 3

(a) Schematic device heterostructure of the perovskite based device, (b) energy level sketch of materials used in the fabrication of device.

In the text
thumbnail Fig. 4

(a) IV curve of the device in dark and light, (b) IV characteristics in semi logarithmic scale of device.

In the text
thumbnail Fig. 5

(a) Charge transport mechanism by double-logarithmic plot of the device, (b) charge transport properties in IV curve.

In the text
thumbnail Fig. 6

(a) Nyquist plot of the device under study, (b) electrical equivalent circuit element (ECE) of the device.

In the text

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