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Luminescent Waveguides In-situ Integrated with Organic Solar Cells for Internet of Things

Transparent electronics offer exciting light harvesting solutions for generation of electrical power via demonstration of"see-through"optoelectronic devices. The wireless energy harvesting ability can empower unplugged and battery-free operations for Internet of Things (IoT) devices. In this study, we report a transparent, luminescent, and elastomeric optical waveguide incorporating quantum dots that is in-situ coupled with organic solar cell array made of P3HT:PC61BM bulk heterojunction. CdSe@ZnS QDs have a photoluminescence quantum yield (PLQY) of 91% and are synthetically engineered to match their photoluminescence spectra with the photo-response of P3HT:PC61BM solar cells for efficient energy harvesting. Integrated devices can generate sufficient power for the active radio frequency identification (RFID) tags to send signals in the communication distance of 35 meters at low illumination level of 0.1-sun and ~0.1 km under 1-sun condition, respectively, which is sufficient for indoor and outdoor communications. Advantageously, the combination of organic solar cells with the waveguide during the elastomer curing leads to the elimination of the undesired post-fabrication processes such as alignment of the solar cells with waveguide and gluing the separate parts with curable polymers. This study paves the way toward using luminescence as transparent, efficient, and configurable energy harvesting solutions for IoT applications.


INTRODUCTION
Transparent electronics with both high transparency and excellent performance are emerging technologies for the next generation of 'see-through' functional devices [1]. In the transparent electronics technology, the efforts are focused to simultaneously produce 'invisible' electronic circuity and maintain the sufficient optical transparency in optoelectronic devices [2]. The most recently developed technologies of transparent electronics include organic light-emitting diodes (OLEDs) [3], thin film transistors (TFTs) [4], solar cells [5], supercapacitors [6], memory devices [7], and nanogenerators [8].
Luminescent solar concentrators (LSCs) are one of the most promising sunlight energy harvesting technology that can 'invisibly' be integrated inside the buildings in order to generate the sufficient electrical power for indoor use [9]. As a result, it can be categorized into the recent and innovative class of transparent electronics. The most common architectures of LSCs consist of a large-area transparent slab made of polymer or glass. The transparent slab incorporates fluorescent particles, which absorb the incoming light and re-emit luminescence at longer wavelengths. The emitted light is then waveguided through the slab and absorbed by the PV coupled at the edges. Such transparent slabs can be aesthetically integrated with a wide variety of objects for smart devices (i.e., internet of things (IoT)) with the ability to transfer data over a network. However, in general, light energy harvesting via LSCs are mostly focused on the coupling of luminescence to the conventional silicon solar cells [10][11][12] that operate efficiently at high-intensity radiation [13,14]. Alternatively, organic photovoltaics are promising candidates for the next-generation renewable energy harnessing technologies due to solutionprocessability, large-scale production and flexibility [15][16][17]. In addition, the high-performance operation at low light intensity levels for indoor lighting condition is another advantage of organic solar cells in LSC device architecture [18][19][20][21], which have not been studied for IoT devices under different lighting conditions.
Here, we report a LSC architecture that is in-situ integrated with an array of bulk heterojunction organic photovoltaics. Among a wide variety of available fluorophores used as downconversion materials such as organic dyes [22], lanthanide-based materials [23], and fluorescent proteins [24], we used quantum dots (QDs) due to high photoluminescence quantum yield (PLQY) and fine spectral photoluminescence tuning ability [25][26][27][28][29][30][31][32]. The photoluminescence of alloyed CdSe@ZnS QDs was synthetically engineered to match with the photovoltaic responsivity of P3HT:PC61BM solar cells. Moreover, high PLQY of 91% and 64% in the synthesis batch and polymeric waveguide, respectively, facilitated an optical quantum efficiency of 44.7%. The fabricated organic solar cell-LSC architecture can generate sufficient power for the RFID tags to send signals in the high communication distance of 35 meters and ~0.1 km under the different lighting conditions of 0.1-sun and 1-sun conditions, respectively, which is sufficient for indoor and outdoor communications. LSCs integrated with organic solar cells possess potential to power-up independent IoT devices for data transmission, processing, and reception due to their scalable size, transparency, and configurable operation.
Green-emitting QDs synthesis procedure -We synthesized CdSe@ZnS QDs based on the previous method [33,34] with some modifications. To achieve the green-emitting QDs with photoluminescence peak wavelength at 521 nm, 0.2 mmol CdO (0.026 g), 16 mmol OA (5 ml), and 4 mmol Zn(ac)2. 2H2O (0.878 g) were mixed with each other in a 50 ml flask. The flask was heated to 120 °C in the schlenk line under constant purge of nitrogen gas. At 120 °C, the solution was purged by vacuum and nitrogen repeatedly. After the completion of evacuation process, the temperature was increased to 300 °C. After 1 hour, 0.2 mmol Se (0.016 g) and 3 mmol S (0.128 g) in 2 ml TOP was quickly introduced into the flask. After 10 minutes, 0.5 ml of 1-octanethiol was injected. After the injection, the heating mantle was removed to cool down the reaction. The obtained QDs were washed several times and re-dispersed in hexane solvent. Electro-optical measurements of the bulk heterojunction device -In the electro-optical measurements of the bulk heterojunction device, one edge of the LSC was coupled with the organic solar cell and on the three other uncoupled edges, reflecting mirrors were placed in order to direct back the emitted light to the waveguide [35]. A mate black paper was placed at the back of the LSC in order to block any direct illumination from the solar simulator [36].

RFID Chip Integration and Antenna Power Measurements -For RFID transmitter and receiver
chips, we used a 433 MHz WL101-341 RF receiver and DRA888TX ASK transmitter modules.
We used 6 solar cell pixels to achieve the minimum operating voltage for these chips, ~1.5 V, even under light intensities as low as 0.1-sun. Moreover, to utilize voltage and current supply of each pixel, these 6 individual pixels were connected in series corresponding to a total voltage value of 3.51 V to obtain the required turn-on voltage for the RFID chips. An Arduino Uno R3 with an ATmega328P processor was utilized to code the transmitter and receiver modules to work at 8 Kbps data rate. The data transmission and receiving message were constantly monitored to ensure proper communication without data loss in the transmitted message.
Moreover, the antenna power for transmitter module, which was powered by our solar cellintegrated LSC, was measured using a Rigol DSA815-TG spectrum analyzer that works within 9 kHz-1.5 GHz frequency range. During the experiments, the sun illumination intensity was changed from 0.1-to 1-sun and transmitted message and antenna power levels were recorded for each sun intensity.  To maximize the energy harvesting efficiency, the photoluminescence of the fluorophore inside the waveguide needs to have good spectral overlap with the absorption of the solar cells. For that, the photoluminescence of the synthesized alloyed CdSe@ZnS QDs was tuned by varying the reaction time during the synthesis procedure and the photoluminescence was recorded at each reaction time. As the reaction time increased from 5 to 20 minutes, the peak wavelengths changed from 498 nm to 574 nm, respectively (Figure 2a). The green-emitting QDs with photoluminescence peak position at 521 nm, which has a size of 8.6 nm ± 0.97 nm (Figure 2b), was selected for the integration inside the polymeric matrix that matched with the absorbance peak wavelength of the active layer located at 515 nm (Figure 2a). At the same time, the greenemitting CdSe@ZnS QDs showed high PLQY of 91% and 64% in solution-and solid-state, respectively. Both the Jsc and Voc values were increased by increasing the light intensity from 0.1-to 1-sun [43][44][45] and the slope of the Jsc with respect to the light intensity in the log-log plot is linear, which reveals the single photon-induced processes [45,46]. At the same time, the slope of the Voc with respect to light intensity in semi-log plot showed a linear variation for P3HT:PC61BM solar cells (3e, f). Hence, the PCE of the organic solar cells are 2.8% for 0.1-sun and 3.8% under 1-sun illumination showing efficient operation within the intensity range.  In order to investigate the contribution of the illuminated light and the photoluminescence of the green-emitting QDs on the generated output power, we illuminated the waveguide surface with AM 1.5G light and recorded the output spectrum from the edge of the waveguide by using an optical fiber (Figure 5a). The spectral measurement from the edge of the integrated device revealed the QD photoluminescence peak positioned at 525 nm, which was compatible with the photoluminescence of the QDs inside the PDMS matrix (Figure 4b). At the same time, the spectral profile of the AM 1.5G light was also observed in the recorded edge emission spectrum, which was due to the total internal reflection of the scattered light inside the PDMS matrix. We also analyzed the optical performance of the integrated device under the standard AM 1.5G solar radiation. Due to the high PLQY of the synthesized QDs inside the polymeric matrix (64%), the optical quantum efficiency of the LSC was calculated as high as 44.7% based on Equation (1) as follows:

RESULTS AND DISCUSSIONS
In Equation (1), the optical quantum efficiency (i.e., internal quantum efficiency) is defined as the fraction of the photons that reach the edges of the LSC to the absorbed photons. ηTIR is the total internal reflection efficiency of the PDMS polymer calculated as 0.745 and ηPL is the quantum efficiency of the synthesized QDs inside the polymeric matrix measured by using the integrating sphere as 0.64. L is the side length of the LSC equal to 5 cm and β was considered as 1.4 based on reference [50]. ф PL (λ) and α(λ) are the photoluminescence spectrum and the absorption coefficient of QDs over the entire wavelength region [50][51][52][53].  Five main IoT technologies including radio frequency identification (RFID), wireless sensor networks (WSN), middleware, cloud computing, and IoT application software are widely applied for the implementation of IoT products and services [55]. Among them, we selected RFID, which is an automatic technology that aids machines or computers to identify objects, record metadata and control individual target through radio waves [56,57]. A typical RFID system consists of a transmitter/receiver microchip connected to an antenna, which can be attached to an object as the identifier. Active RFID tags have already started to be used for vehicle positioning (Figure 5d), automobile connections, manufacturing, hospital laboratories, and remote-sensing (e.g., temperature, pressure, and chemical) [55,58]. Active RFID tags have their own battery supply and we replaced this unit with our fabricated LSC (Figure 5d). We tested transmitter performance by changing illumination intensity from 0.1-to 1-sun condition to evaluate the RFID functionality. The experiment was carried out without any additional antenna. For RFID part, a commercial 433 MHz DRA888TX ASK RF transmitter module with optimal operating voltage of 1.5 V and injection current of 2.1 mA was used.
The maximum communication distance, which corresponds to the maximum distance to communicate with 8 Kbps rate without any information loss, was measured for different sun Our demonstration can be directly applied for vehicle-to-vehicle communications that prevents car crashes in short range traffic or smart parking lot purposes, and it showed the potential of LSCs integrated with organic solar cells for future IoT applications. A broad range of low power communication modules can be directly integrated to our system [59]. Moreover, utilizing low power GSM modules with 5G technology may bring the opportunity to create vehicle-to-cloud or even vehicle-to-everything communication and our design idea can be utilized efficiently in large area vehicles like trains or ferries as well. Due to the transparency and flexibility of LSC polymer, LSCs can be further integrated on the windows and other surfaces of the vehicles [60].

CONCLUSIONS
In this study, a single integrated unit of bulk heterojunction solar cell array and quantum dot luminescent solar concentrator was designed, fabricated, and used for data transmission for IoT.
The optical properties of the synthesized CdSe@ZnS QDs were tailored to match with the absorbance peak of the P3HT: PC61BM as the active layer of the organic photovoltaics. The synthesized QDs showed high PLQY of 64% inside the polymer that led to an optical quantum efficiency of 44.7%. The tailoring of optical properties and the direct integration of the organic solar cell inside the QD-LSC architecture further led to the optical efficiency of 4.9% and 5.0% under AM 1.5G and blue LED illumination, respectively. The fabricated QD-LSC integrated with organic photovoltaics was used to empower the RFID chips for connecting cars in IoT applications. The generated power by 0.1-sun illumination led to the communication distance of up to 35 meters. By increasing the illumination intensity up to 1-sun illumination, the communication distance can reach up to ~0.1 km, which is suitable for all types of vehicle-tovehicle communication, traffic and parking lots to avoid crashes and optimized navigation. Therefore, this study reporting a new transparent-electronic approach for empowering IoT holds promise for wireless communication of independent devices.