We show that solar cells, widely used in portable devices for power generation, can simultaneously extract a high-speed data signal in an optical wireless communication link. This Letter reports, to the best of our knowledge, the first use of an organic solar cell as an energy-harvesting receiver for visible light communications (VLCs). While generating maximum power in the cell, the communication link can deliver a data rate of 34.2 Mbps with a bit error rate of using an implementation of orthogonal frequency-division multiplexing. This approach could lead to printed optical data receivers in future eco-friendly VLC systems. Simultaneous functions of data communication and energy harvesting have great implications for the connectivity of future smart devices, many of which could become self-powered units as part of the “Internet of Things.”
© 2015 Optical Society of America
The looming radio-frequency (RF) spectrum crunch has prompted research into alternative regions of the electromagnetic (EM) spectrum for future wireless communication systems. The infrared and visible light regions of the EM spectrum are of particular interest since the vast amount of available bandwidth is unregulated and free of EM interference. The growing infrastructure of LED lighting offers a particular opportunity for visible light communications (VLCs) with power-efficient dual-use sources, which provide simultaneous illumination and very-high-rate wireless data transmission [1–5]. While VLC data links normally use photodiode (PD) receivers  [either positive-intrinsic-negative PDs or avalanche PDs], there is a parallel opportunity for duality by using instead a solar cell for simultaneous energy harvesting and data detection. Wang et al. [6,7] recently showed that a standard commercial multicrystalline silicon solar panel could be used for this dual purpose, to receive a data rate of 11.84 Mbps while generating approximately 2 mW of electrical power. This advance implies that it could be possible for any piece of electronic equipment with an integrated solar cell to engage in high-speed data communication while utilizing the energy from the light signal or ambient lighting to power the receiver electronics.
However, to unlock the full potential of this approach, it would be very attractive to use an alternative solar cell technology that could be more easily integrated on different devices/substrates, or even be mechanically flexible. The field of organic electronics offers these possibilities, and here we report the first use of an organic semiconductor solar cell as the energy-harvesting receiver in an optical wireless data link. When the cell generates maximum power, the communication link is capable of delivering a data rate of 34.2 Mbps with a bit error rate (BER) of using an implementation of orthogonal frequency-division multiplexing (OFDM). Such organic solar cells could serve as a future platform for ubiquitous VLC receivers. They can be mass manufactured by roll-to-roll printing on a flexible foil , giving the possibility of very inexpensive flexible/conformal panels that could be integrated with an extremely wide range of devices. They offer relatively easy bandgap tuning through the visible spectrum that could allow a selective response to wavelengths relevant for VLC. Such receivers could therefore have great implications for the connectivity of future smart devices, from sensor networks to smart clothing, many of which could become self-powered units as part of the “Internet of Things.”
Organic semiconductor materials have recently attracted much attention in the field of VLC [9–11]. They offer tremendous scope for simple processing and integration on different substrates, and have been used as color converters to generate white light with a blue LED backlight. A white light data link was recently demonstrated, taking advantage of the organic material’s fast radiative decay rate compared with commercial phosphor color converters . The same class of materials can also be used as signal detectors for VLC as organic PDs. Haigh et al.  have demonstrated a communication link with a data rate of 0.75 Mbps by using a poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester () organic PD. The data rate was further improved to 3.8 Mbps by Ghassemlooy et al.  by improving the bandwidth. However, these organic PD receivers operated under an external bias and so in this mode of operation cannot be used for energy harvesting.
In this work, we use state-of-the-art organic solar cells based on material blends [Fig. 1(a)] [14–16] to achieve high power conversion efficiency (PCE). We show that data rates of over 34 Mbps are possible while the solar cell maintains its full power generation capabilities. The achieved data rates are an order of magnitude higher than those reported using organic PDs.
The device structure of the organic solar cell is shown in Fig. 1(a). Patterned commercial indium tin oxide (ITO)-coated glass substrates were cleaned by ultrasonication in acetone and isopropanol successively for 10 min each, followed by an oxygen plasma ashing for 3 min. A 40 nm hole injecting layer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was spin-coated on the ITO substrates and annealed on a hotplate at 140°C for 10 min to remove the residual water. The 85 nm active layer made of PTB7 and in a weight ratio of was spin-coated on top of the PEDOT:PSS layer from chlorobenzene mixed with 3% (v/v) 1,8-diiodooctane in a nitrogen-purged glove box. Detailed descriptions of the blend preparation can be found in the works of Liang et al.  and He et al. . A 15 nm calcium cathode and a 100 nm aluminum cathode were then thermally deposited on top in a vacuum chamber at a pressure of . After the electrode deposition, the devices were encapsulated with a UV optical adhesive and a glass coverslip. The active areas of the devices were . The current–voltage characteristics were determined under an illumination intensity of in air using an air mass 1.5 global (AM 1.5G) Sciencetech solar simulator and a Keithley 2400 source-measure unit. The illumination intensity was verified with a calibrated monosilicon detector and a KG-5 filter.
Figure 1(b) shows the experimental setup of the VLC data link. The conditioned digital signal was passed onto an arbitrary waveform generator, which outputted an analog signal. The analog signal was used to drive a red laser diode (LD) with a narrow emission centered at 658 nm. The light emission of the LD was collimated with an aspheric lens and directed onto the solar cell. The output of the solar cell was terminated on a custom circuit designed for simultaneous energy harvesting and communication, and the output of the circuit was captured with a digital oscilloscope. The VLC data link used an implementation of OFDM (see Supplement 1). For the digital signal, a pseudorandom bit sequence was generated and encoded into -ary quadrature amplitude modulation (-QAM) symbols. The -QAM symbols were modulated onto different frequency subcarriers. This was implemented by applying an inverse fast Fourier transform operation on a block of -QAM symbols. The procedure generated a discrete time-domain signal, which was conditioned for transmission by oversampling, pulse shaping, and clipping any values outside a predetermined range in order to reduce the peak-to-average power ratio of the modulation signal. All digital processing was implemented in MATLAB.
The incident photon conversion efficiency (IPCE) spectrum of the solar cell was previously reported in Ref. . has a broad absorption spectral range. IPCE values can reach over 50% in the range of 400–700 nm, and at the wavelength of the LD, the IPCE is around 70%. Figure 2 shows the curve of the solar cell in the dark and under illumination. The blue curve represents the dark current and the red curve represents the curve under the illumination of the AM 1.5G solar simulator. Device parameters such as short-circuit current (), open-circuit voltage (), fill factor (FF), and PCE can all be obtained from Fig. 2. The fresh solar cells show a of , a of 0.75 V, and a FF of 63.8%, leading to a solar PCE of 7.0%. The values of series resistance () and shunt resistance () can also be obtained from the curve. The solar cells exhibit an and an . The small value of indicates good contact between the metal contact and the active layer. The high value of indicates that most of the light-generated current contributes to the power generation of the solar cell.
The curve of the solar cell was also measured under the illumination of the LD transmitter (black dots in Fig. 2). The LD was biased at 2.5 V and was modulated with a peak-to-peak signal of 0.4 V. The output power of the LD varies from 50 μW at a bias of 2.3 V, to 8.5 mW at a bias of 2.5 V, and 14.3 mW at a bias of 2.7 V. For these measurements, the solar cell was connected to a variable load resistor, and by varying the value of the load resistor, the curve in the fourth quadrant was obtained. The branch of the circuit for the communication signal was open during this measurement. A similar value of and a slightly smaller value of were obtained under the LD illumination.
Table 1 summarizes the data rates and power generated when both the energy-harvesting circuit branch and the communication circuit branch were connected. It is worth noting that although no external bias is directly applied to the solar cell receiver, different choices of will set different biases across the receiver. The data rates strongly depend on the load resistor. With a load resistance of 5 kΩ, the data rate is 21.3 Mbps at a BER of . As the resistance decreases, the data rate increases. The data rate reaches 42.3 Mbps at a BER of when the load resistance drops to 200 Ω. In terms of energy harvesting, the circuit branch can generate a maximum power of 0.43 mW () when the load resistance is 600 Ω. At the same time, the communication circuit branch can deliver a data rate of 34.2 Mbps with a BER of . Compared with the power of 0.44 mW generated by the same transmitter without the communication circuit branch, the negligible difference in power generation indicates that the communication circuit branch has no negative impact on the energy-harvesting circuit branch.
The organic solar cell receiver system can be described by an equivalent circuit as shown in Fig. 3(a). The capacitor in parallel with captures the internal capacitance of the solar cell and the inductor models the inductance of any wire connections to the solar cell [17,18]. The output of the solar cell contains both DC and AC components. The DC component of the electrical signal, used for energy harvesting, and the AC component, used for communication, are decoupled with a capacitor () and an inductor (RF choke) (). The DC current is terminated through a load resistor and the AC current is terminated through the resistor , which is matched to the inductor–resistor–capacitor equivalent circuit of the solar cell in order to obtain a desired bandwidth and a required signal gain in the receiver system. The frequency response of the receiver system depicted in Fig. 3(a) can be expressed as3(b). The frequency profile of the communication system follows a low-pass filter response caused by the capacitance of the solar cell. A small portion of the low frequencies in the channel gain curve in Fig. 3(b) are also attenuated due to the AC-coupling capacitor in the receiver circuit. The 3 dB attenuation frequency of the system is 1.3 MHz. It is interesting to note that the gain of the receiver rolls off gradually, which suggests that a significant amount of bandwidth beyond the 3 dB point may be used for communication. This observation is confirmed by the SNR estimated for a bandwidth of up to 10 MHz using OFDM and error-vector magnitude estimation. The achievable capacity for and is shown in Fig. 3(c) (red curve). From the values presented, it is clear that the entire frequency range up to 10 MHz can be loaded with a minimum 4-QAM constellation. The actual bit and energy loading used for the 42.3 Mbps result is presented in black lines in the same figure. The optimal constellation size and energy for each subcarrier is determined using an adaptive bit and energy loading algorithm based on the work of Campello . The received constellations presented in Fig. 4 clearly illustrate that the system does not experience any unanticipated distortion effects.
There are two main factors that may limit the data rates of the organic solar cells. These are the low drift mobility of the organic materials and the time required to discharge the solar cell through the load resistor. In order to further improve the data rates, it is important to understand which factor is dominant in this case. The transit time for the charges to travel across the active layer is given by , where is the thickness of the active layer, is the hole mobility, is the electrical field, and is the cell voltage. For a load resistance of 600 Ω, the measured cell voltage was 0.51 V. Taking the reported value of of  gives a transit time of 83 ns. The time required to discharge the solar cell through the load resistor, or the so-called RC time constant, is given by . is the load resistance and is the vacuum permittivity. The capacitance of the solar cell is determined by the thickness of the active layer, the relative permittivity of the active layer, and the area of the device pixel. Taking for a wavelength of 658 nm  gives an RC time constant of 2.4 μs. This is much longer than the transit time, so the receiver performance is limited by the RC time constant.
In summary, we have demonstrated a VLC link using an organic solar cell as the receiver. Such a receiver operates without an external bias and is able to simultaneously communicate data and harvest power. To the best of our knowledge, this is the first reported work using an organic solar cell to achieve energy harvesting and data communication at the same time. The VLC link was tested using adaptive OFDM. The load resistor in the energy-harvesting circuit branch can strongly affect the data rates of the VLC link. The maximum data rate achieved was 42.3 Mbps with a BER of . The load resistor also affects the amount of power generated. However, it was found that the communication circuit branch has no negative effect on the power generation. The maximum power output from the solar cell with active area was 0.43 mW and the corresponding data rate was 34.2 Mbps with a BER of . The RC time constant of the solar cell was found to limit the data rate. This work demonstrates the great potential of organic solar cells for optical wireless communication.
Engineering and Physical Sciences Research Council (EPSRC) (EP/I00243X, EP/K00042X/1, EP/K008757/1).
See Supplement 1 for supporting content.
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