Triple-cation perovskite solar cells for visible light communications Download: 734次
1. INTRODUCTION
Over recent years, hybrid perovskite solar cells have become an increasingly competitive technology for energy harvesting due to their high solar power conversion efficiencies reaching up to 25% [1], stability reaching several thousands of hours [2], and better performance than inorganic solar cells in low-intensity indoor lighting conditions [35" target="_self" style="display: inline;">–
Here, we explore the potential of hybrid organic–inorganic perovskites for visible light communication (VLC). VLC is a fast and secure method for wireless communication while avoiding increasing congestion in the radio-frequency spectrum. For this technology, ordinary light emitting diode (LED) lighting can be retrofitted with circuitry to act as a fast data transmitter, while photodetectors, typically in the form of avalanche or photodiodes, collect the light to allow for data decoding. Solar cells can also be used as a VLC receiver with appropriate circuitry that detects the alternating current (AC) data input separately from the direct current (DC) lighting background. This allows for simultaneous data communication and energy harvesting and it has been explored with inorganic and organic photovoltaics [8–
While much of the attention for receivers has been on silicon photodetectors, perovskite devices have also recently been studied by various groups [12
In this paper, we demonstrate data transfer of 56 Mbps to perovskite solar cells used in a configuration that allows for communication and power generation concurrently. The cells are fabricated using triple-cation perovskites and optimized for VLC application by varying the active layer thickness over a large range. These devices are then put into a circuit to allow for the measurement of their bandwidth, data rate, and bit error ratio (BER). This circuitry allows for the devices to be held under zero bias, with a small load resistor to allow for the exploitation of their ability to collect data and energy simultaneously [19]. By varying the perovskite thickness over such a large range, we are able to identify that the time constant is controlling the bandwidth, which is proved using transient photovoltage and internal resistance measurements. Within this paper, we also study the performance of these devices under white LED illumination that could be used within a real indoor VLC setup as well as a red laser used for testing their communication capability.
2. Results
2.1 A. Solar Cell Performance
In this study, the triple-cation perovskite was used due to its high power conversion efficiency reported elsewhere and higher bandwidth compared with or [17,20]. To identify the factors controlling the device time constant and to maximize the device bandwidth and data rate, a wide range of active layer thicknesses were fabricated and tested. These range from 60 to 965 nm and were made using a single spin-coating method, with antisolvent washing, by varying spin speeds and solution concentration. Figure
Fig. 1. SEM images of triple-cation perovskite films for all thicknesses. The red bar corresponds to a length of 2 μm.
The indoor energy harvesting performance of these devices measured under illumination from a typical white LED with incident power is reported in Table
Table 1. Cell Performance of Triple-Cation Devices with Varied Active Layer Thickness Using a White LED with an Incident Optical Power of a, b
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Fig. 2. Triple-cation perovskite devices with their (a) best J-V curves under indoor white LED illumination, (b) EQE, and (c) I-V curves under 50 mW red laser (660 nm) illumination.
Fig. 3. Low-magnification SEM images of the three thickest triple-cation perovskite films. The blue scale bar represents a length of 10 μm.
The external quantum efficiency (EQE) spectra for all perovskite devices with different thicknesses are shown in Fig.
To assess the capability of these devices for simultaneous energy and data harvesting, the solar cell performance was tested under the lighting conditions subsequently used for VLC experiments. A red (660 nm) laser diode with an output power of 50 mW was incident on the solar cell with a variable resistor across the solar cell to measure the I-V curve. As seen in Table
Table 2. External Quantum Efficiency of Triple-Cation Solar Cells under 660 nm Low Intensity and Laser Illumination, and Power Conversion Efficiency and Power Generated under 50 mW Laser Power
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2.2 B. VLC Results
To characterize the capability of these triple-cation devices for VLC, all solar cells were mounted 40 cm from a red (660 nm) laser diode with an incident power of 50 mW. This laser was focused onto the active area of the cell using two lenses at the transmitter and receiver sides. A sequence of random data was transmitted by the laser using the DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) digital modulation technique. The signal is detected by a single perovskite cell, and the output current of the cell is split into its DC and AC components, respectively, for energy harvesting and data detection. The AC component is captured by an oscilloscope and sent to a computer for analysis. A training sequence is used for estimating the system frequency response and signal-to-noise ratio (SNR) at various frequency bands. The bandwidth of the system is determined by the perovskite solar cells, as other components (laser, oscilloscope, etc.) are significantly faster (i.e., bandwidth higher than several hundreds of MHz). Based on the system frequency response and SNR, random data are transmitted, and the data rates achieved and associated bit error ratios (BERs) are measured [27,28]. As DCO-OFDM with adaptive bit loading is used, the achievable data rate will depend on the SNR at each subcarrier across the whole used frequency range but not on the bandwidth of the device. Examples for the measured frequency response and bit loading for each perovskite thickness device are shown in Appendix
The resulting bandwidth and data rate are presented in Fig.
Table 3. Average and Standard Deviation of Bandwidth, Data Rate, BER, and Number of Measured Samples of Triple-Cation Photodetectors with Varied Active Layer Thickness
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Fig. 4. (a) Box and whisker distributions of the bandwidth and (b) achieved data rate for perovskite devices with varied active layer thickness. Here, the mean of the data is represented as a square, the median a solid line, and the ends of the box represent the 25%–75% range.
3. DISCUSSION
3.1 A. Limitation of Bandwidth
Figure
Another explanation could be due to the intrinsic properties of the materials used in these devices: the low mobility of the organic hole transporting layer of Spiro-OMeTAD [32]. While organic semiconductors have charge mobilities orders of magnitude lower than perovskites, OLEDs and organic solar cells utilized in previous VLC studies have achieved bandwidths in the MHz region [8]. Because of this, it seems unlikely that the presence of an organic semiconductor limits the bandwidths reported here. As well, if the transit time of charge carriers within the Spiro-OMeTAD did limit the device bandwidth, then we would not expect to change the frequency response by changing the perovskite layer thickness. This is not what we observe in Fig.
In general, the lifetime of charge carriers within solar cells is sought to be maximized to allow for better collection efficiency and reduced charge recombination [33,34]. However, the charge lifetime being very long is thought to perhaps limit how quickly the device can respond to rapid changes in light level, thus limiting the photodetector bandwidth. To determine if this lifetime could be a limiting factor for these solar cells, time-resolved photoluminescence (TRPL) measurements were performed on devices to estimate the charge carrier lifetime. The extracted lifetimes (using a double-exponential decay fit) are listed in Table
Table 4. Coefficients and Lifetimes Extracted from the Two-Exponential Fit of TRPL Data for All Perovskite Devices of Varied Active Layer Thicknessa
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Instead, these devices could be described as a first-order low-pass filter and limited by their time constant. As the active layer thickness increases, the geometric capacitance of the device is expected to decrease, leading to a lower time constant and a faster bandwidth. To test this, the time constant of these perovskite solar cells is explored in detail in the following section using multiple methods for measuring and estimating the capacitance.
3.2 B. RC Time Constant Estimation
An initial estimation of the solar cell’s time constant can be made by using its bandwidth and measuring the resistance. For an low-pass filter, the bandwidth, , is given by , such that is the device resistance and is its capacitance. The cell capacitance can be subsequently calculated with the knowledge of the resistance. This resistance is calculated by measuring the cell voltage under open-circuit conditions, then putting a load resistor in series, and measuring the voltage again. Calculating this can be done using the following approximation:
Here, is the device resistance, is the load resistance, and is the output voltage of the solar cell when the load resistor is connected. Table
Table 5. Measured Device Resistance, Calculated Capacitance, and RC Time Constant for Triple-Cation Devices of Varied Active Layer Thickness Using Two Methods: Bandwidth Estimation and Transient Photovoltage
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To assess if the time constant is what limits the bandwidth, transient photovoltage tests were performed. In this measurement, the solar cell is illuminated by a square-wave light signal of frequency 200 kHz, and its output voltage as a function of time is measured. Fitting the rise of this curve with an exponential allows for the extraction of an time constant. Example data from this measurement and the extracted time constants as a function of perovskite thickness are shown in Fig.
Fig. 5. (a) Transient photovoltage measurements for triple-cation perovskite solar cells with varied thickness. (b) Fitted RC time constant from this measurement.
Using the time constant from transient photovoltage measurements, the corresponding bandwidth can be calculated and is shown in the final column of Table
4. CONCLUSIONS
In this study, triple-cation perovskite solar cells are optimized for VLC applications by varying the active layer thickness over an order of magnitude. This study is the first of its kind to show these devices in a configuration that allows for simultaneous energy and data harvesting. Here, it is observed that the device bandwidth increases with perovskite thickness, and the highest data rate is achieved for the 250 nm thick perovskite solar cells using an OFDM signal. To the best of the authors’ knowledge, this average data rate of 49 Mbps and maximum of 56 Mbps are the highest reported for perovskite photodetectors thus far. Using several measurements, it is shown here that these solar cells have bandwidth limited by their time constant. Our results suggest that hybrid perovskite solar cell technology is promising for VLC due to its high-power conversion efficiency and competitive data transmission rates.
5. EXPERIMENTAL METHODS
5.1 A. Solar Cell Fabrication
Perovskite precursor solutions were prepared in a glovebox approximately 4 h before use. Triple-cation stoichiometry was achieved by combining with DMF (800 μL) and DMSO (200 μL). Finished solar cells had an structure: (60–80 nm). ITO glass substrates (Xin Yan Tech. Ltd.) were cleaned in an ultrasonic bath with a detergent solution (Hellmanex III), acetone, and then isopropyl alcohol for 10 min each. These substrates were then dried with and then plasma ashed (GaLa Instrumente GmbH) with gas for 3 min. A solution was prepared by diluting a stock nanoparticle solution (Alfa Aesar) in a 1∶6.5 volume ratio with DI water, vortex mixed to ensure mixing. This solution was then spun onto the cleaned substrates at 3000 r/min for 30 s, and the film was annealed at 150°C for 30 min. The -coated substrates were then transferred into a glovebox for perovskite spin coating. To form the large range of perovskite thicknesses, the precursor concentration and spin speed were adjusted. All conditions to form the films used in this study are included in Table
Table 6. Spin Coating Conditions for the Triple-Cation Perovskite Film Formationa
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All films underwent an antisolvent washing step using chlorobenzene 10 s before the end of the spinning time, and then were annealed at 100°C for 1 h. The Spiro-OMeTAD solution was prepared at a 100 mg/mL concentration in chlorobenzene and doped with tBP (36 μL, Sigma Aldrich) + Li-TFSI (20 μL, 517 mg/mL in acetonitrile) + FK209 (8 μL, 376 mg/mL in acetonitrile). All solutions for this film were prepared just before use with a vortex mixer, and the final solution was spun at 4000 r/min for 30 s. This film was then allowed to oxygen dope for more than 12 h before being put into an evaporator for gold electrode deposition. The evaporator was pumped down to a pressure of and 60 nm of gold was thermally evaporated at a rate of 0.05 nm/s. The ITO electrode in these devices was 115 nm thick, the layer was 30 nm thick, and the Spiro-OMeTAD film was approximately 250 nm thick.
5.2 B. Solar Cell Performance
Performance was measured using an AM 1.5 solar simulator and a Keithley 2400 source meter to supply the bias voltage and measure current. EQE was calibrated using a photodiode, and the measurement was made using a Bentham lamp (Model 605) to send light through a monochromator (Bentham TMc300) connected to a fiber-couple. This fiber was set at the calibrated height, and a Keithley 6517A captured the output current from the solar cell. Performance for indoor lighting was tested using a white LED (UltraFire, CREEXML-T6) controlled by a function generator (Keithley 3390) with variable output voltage to adjust the output light intensity. The irradiance level was measured using an RK5710 power radiometer and optometer P9710. The same Keithley 2400 source meter for solar simulator performance was used to measure device performance under white LED illumination.
To calculate the power conversion efficiency of the I-V curves produced under red laser illumination, the curves were fitted to a modified ideal diode equation: where is the charge of an electron, is Boltzmann’s constant, is temperature, and is a fitting parameter to correct for the nonideal nature of the devices. This equation for each perovskite solar cell was multiplied by to find the maximum power point, which was then used to calculate the power conversion efficiency for the device.
5.3 C. Bandwidth and Data Rate Measurements
A DCO-OFDM scheme is used for measuring the system bandwidth and estimating the achievable data rate. Due to the low-pass frequency response of the system, adaptive bit and energy loading is used for estimating the maximum achievable data rate. Details of the communication technique can be found in Refs. [27,28] and references therein. A communication system is set up for the measurement in the lab. On the transmitter side, a red laser (Thorlabs, HL6544FM) with a central wavelength of 660 nm is used as the light source. The DC bias value supplied by a bench power supply (TENMA 72-10505) and the AC signal generated by an arbitrary waveform generator (AWG, Keysight 81180A) are combined in a bias-Tee (Mini-Circuit, ZFBT-4R2GW+) and sent to the laser. On the receiver end, a single perovskite cell is used for capturing the optical signal. The output is connected to a custom-designed receiver circuit, which contains two branches: one branch with an inductor for energy harvesting and another branch with a capacitor for data detection. The received AC signal is captured by an oscilloscope (Keysight, MSO7104B) and sent to a computer, where the data are processed offline in MATLAB software.
5.4 D. TRPL Measurement
Time-resolved PL was captured using an Edinburgh Instruments fluorimeter with excitation using a 640 nm laser. For this measurement, devices with the complete interlayer structure were used to determine the charge lifetime within these solar cells. All fitting was done using the software for the Edinburgh Instrument control and measurement.
5.5 E. Transient Photovoltage
A similar setup to the one described for the bandwidth and data rate measurements was used. The output of each perovskite cell is directly captured by the oscilloscope without any additional circuit at the receiver. The estimation of RC constant is performed in MATLAB software. Square waves with different frequencies and peak-to-peak values have been used for the measurements, and all the results are consistent.
6 Acknowledgment
Acknowledgment. We are grateful to J. A. Smith and D. G. Lidzey for useful discussions on interlayers. H.H. also acknowledges support from the Wolfson Foundation, and the Royal Society.
Article Outline
Natalie A. Mica, Rui Bian, Pavlos Manousiadis, Lethy K. Jagadamma, Iman Tavakkolnia, Harald Haas, Graham A. Turnbull, Ifor D. W. Samuel. Triple-cation perovskite solar cells for visible light communications[J]. Photonics Research, 2020, 8(8): 08000A16.