Chapter 3: Origin of Strong Cathodoluminescence and Fast Photoresponse from Embedded
3.3. Results and Discussion
3.3.8. Performance of a Per NP/Si NW Heterojunction Photodetector
The ultrahigh surface area and enhanced optical absorption by the Per NP-decorated NW array make this heterostructure also promising in other optoelectronic applications along with different light-emitting applications. Here, we have fabricated photodetectors using the CH3NH3PbBr3 NPs grown on mesoporous Si NWs, and the device architecture is schematically shown in Fig. 3.11(a).
In the device structure, gold is used as the bottom electrode at the back side of the Si wafer where NWs were grown, and a thin layer of Au serves as the semitransparent top electrode deposited on top of the Per NP-decorated NW (N1P2). Note that the starting Si wafer is heavily doped, and it makes a reasonably good Ohmic contact with the Au layer on the back side. Fig. 3.11(b) shows time-dependent photocurrent response curves with the light on/off intervals of 5 s under 405 nm incident light at an intensity of 8 mW/cm2 and an applied bias of 2 V. The dynamic photoresponse of the CH3NH3PbBr3/Si NW heterojunction PD indicates that the devices possess a good photoswitching behavior. To quantitatively understand the response speed, the rise and decay edges of the time-dependent photocurrent were fitted using a single exponential function given by28
1 exp (3.4) and exp , (3.5)
75 | S t r o n g C L , P L & P h o t o d e t e c t i o n o f E m b e d d e d P e r o v s k i t e N P s
where I0r, I0d, A1, and A2 are constants, while τg and τd are time constants for the photocurrent growth and decay, respectively.
Fig. 3.11: (a) An schematic illustration of Per NPs/Si NW heterojunction photodetector. (b) Time-dependent photoresponse of CH3NH3PbBr3 NPs/Si NW photodetector under 405 nm laser with intensity 8 mW/cm2 at 2V bias.
(c) An enlarged view of growth and decay of time-dependent photocurrent fitted with single exponential function in each case. (d) The wavelength-dependent responsivity of the photodetector; the inset shows light intensity-dependent photocurrents under 405 nm laser irradiation with linear fitting. (e) Wavelength dependent detectivity of the photodetector. (f) Time-dependent photoresponse of the photodetector after 20 days of storage in ambient air with 70% humidity.
The photocurrent growth time constant (τg) was obtained to be ∼0.32 s, whereas photocurrent decay time constant (τd) was obtained to be ∼0.28 s from the fitting, as shown in Fig. 3.11(c). The spectral responsivity (Rλ) is an important parameter for the estimation of the performance of the
PD, which shows how efficiently the detector responds to an optical signal. The responsivity (Rλ) of the PD was calculated using the equation,
∗ (3.6)
where IPh refers to the photocurrent, P is the incident light power density, and S is the effective illuminated area. Fig. 3.11(d) depicts the wavelength-dependent responsivity of the heterojunction PD at 2 V reverse bias, which shows a maximum responsivity of 223 mA/W at 440 nm incident light. The inset of Fig. 3.11(d) shows the dependence of photocurrent on the incident light intensity, which reveals that with the increase in incident light intensity, the photocurrent increases linearly. On increasing the incident light intensity, more photogenerated carriers are produced, and hence the photocurrent of the device is increased linearly at a low-intensity level. The detectivity (D*) of the PD signifies the capability of detecting low-level light signals, taking into account the contributions of the photocurrent and dark current. D* is expressed as
∗ (3.7)
where Jd is the dark current density and q is the charge of the electron. The maximum detectivity of the PD was found as 7.22 × 1010 Jones at 440 nm, as shown in Fig. 3.11(e). To check the stability of the PD, we measured the time-dependent photocurrent under the same measurement conditions keeping the device for 20 days in ambient air with 70% humidity. Fig. 3.11(f) shows time- dependent photocurrent response curves with on/off intervals of 5 s under 405 nm incident light with an illumination intensity of 8 mW/cm2 under a 2 V reverse bias. After 20 days of storage of the device in ambient air, the photocurrent decreased from 66 to 46 μA i.e., by ~30%, which shows the reasonable stability of the device under humid air.
Fig. 3.12 shows the schematic representation of the band diagram of the Si NW/CH3NH3PbBr3 NP heterostructure without and with reverse bias condition.29 It is well known that the MACE-grown Si NW array is covered with a thin SiO2 layer due to the wet etching process. This insulating SiO2 layer plays a crucial role in enhancing the PL emission from the Per NPs confined in the mesopores of the NW surface. The oxide layer acts as an energy barrier between Si NW and CH3NH3PbBr3 NPs. As shown in Fig. 3.12(a), due to the photoexcitation, electron-hole pairs are generated inside Per NPs. These electrons cannot overcome the energy barrier at the oxide layer between Si NW and Per NPs and hence they recombine radiatively
77 | S t r o n g C L , P L & P h o t o d e t e c t i o n o f E m b e d d e d P e r o v s k i t e N P s
causing high-yield PL emission. Therefore, any possible quenching effect due to charge transfer to the underlying Si layer is less probable owing to the presence of the oxide layer.
Fig. 3.12: (a, b) Energy band diagram of Per NPs and Si NW without bias and with reverse bias conditions, respectively.
In the case of the perovskite/Si NW PD, when a reverse bias is applied, the photogenerated electrons-holes acquire the energy to partly overcome the oxide barrier level or tunnel through the thin layer to contribute to the photocurrent, as shown in Fig. 3.12(b). As a result, the photocurrent is enhanced in the external circuit.