Leveraging InGaN solar cells for visible light communication reception

Solar cells are increasingly being utilised for both energy harvesting and reception in free ‐ space optical (FSO) communication networks. The authors focus on the implementation of a mid ‐ band p ‐ In 0.01 Ga 0.99 N/p ‐ In 0.5 Ga 0.5 N/n ‐ In 0.5 Ga 0.5 N (PPN) solar cell, boasting an impressive 26.36% conversion efficiency (under 1.5AM conditions) as a receiver within an indoor FSO communication network. Employing a solar cell with dimensions of 1 mm in length and width, the FSO system underwent simulation using Optisystm software, while the solar cell's behaviour was simulated using SCAPS ‐ 1D. The received power from the solar cell was then compared to that of four commercially available avalanche photodiode (APD) receivers. Exploring incident wavelengths spanning 400–700 nm within the visible spectrum, across transmission distances of 5, 10, 15, and 20 m, the study presented current ‐ voltage (IV) and power ‐ voltage curves. Notably, the InGaN solar cell exhibited superior electrical power output compared to all commercial APDs. In conclusion, the findings underscore that augmenting received power has the potential to enhance FSO network quality and support extended transmission distances.


| INTRODUCTION
5 G technology is poised to revolutionise modern communication networks, boasting the potential to enhance capacity by a remarkable factor of 1000 [1,2].However, the expansion of radio frequency communication's capacity has been hindered by limited spectrum resources [3,4].In the near future, downlink data rates could soar to 20 Gb/s, while uplink rates could reach 10 Gb/s [5].To address escalating demand, researchers are increasingly investigating light as a communication medium [6][7][8][9].
Free-space optical (FSO) communications, also known as optical wireless communication (OWC), employ light as a wireless communication channel.Recent advancements in communication networks necessitate substantial bandwidth and superior network quality.While radio-frequency networks offer restricted speed, FSO communication networks provide expansive bandwidth without the confines of frequency regulations [10,11].Moreover, FSO communication offers several advantages over conventional systems, including impressive bandwidth (up to 2000 THz), narrow beamwidth, unrestricted spectrum use, cost-effective installation, and flexible development opportunities [12].Originally used in military applications [13] and inter-satellite links [14], FSO systems have evolved, with companies introducing innovative equipment for FSO communication systems [15][16][17][18], opening up new avenues for communication technology.
Five distinct categories of OWC systems exist based on transmission distance: (i) Ultra-short range, such as chip-to-chip communication [19].(ii) Short-range applications, including wireless body area networks, wireless personal area networks, and underwater communication [20,21].(iii) Medium-range scenarios, encompassing indoor infrared and visible light communication (VLC) for wireless local area networks [22].(iv) Long-range OWC, such as inter-building connections.(v) Ultra-long-range setups, such as inter-satellite links and deep space links [23,24].
Remarkably, an OWC network capable of delivering 100 Gbps was successfully established for indoor illuminations [25].Optical communication networks also align with the growing trend towards energy-efficient communication systems [26,27], presenting a greener alternative that requires minimal hardware resources, thus aligning with cost-effective and environmentally friendly agendas [28].Notably, due to light's inability to penetrate walls, OWC systems offer heightened security [29].
In contemporary light communication systems, either lasers or LEDs transmit light [11], while positive-intrinsicnegative diodes or avalanche photodiodes (APDs) serve as receivers [30].A notable breakthrough occurred in 2013 when Kim and Won proposed the use of solar cells as receivers in FSO communication systems, simultaneously harnessing solar energy to generate electricity [31].Subsequent studies explored the integration of polycrystalline Si solar cells [32], GaAs solar cells [33], organic solar cells [34], and perovskite solar cells [35] for light reception and energy harvesting.Intriguingly, the potential of InGaN solar cells for FSO communication remains untapped.
Recent investigations have spotlighted third-generation solar cells like InGaN solar cells, driven by their potential for high conversion efficiency [36][37][38][39][40].The InGaN alloy boasts versatile applications in photodetectors, electronic devices [41], and laser diodes [42].A distinguishing feature of InGaN lies in its adjustable direct wide bandgap, spanning from 0.7 eV (InN) to 3.42 eV (GaN) [43].With excellent thermal stability and mobility, InGaN has the potential to replace Si in highfrequency optoelectronic and electronic applications, even in demanding environments [44].
This paper delves into the utilisation of InGaN-based solar cells as receivers in FSO communication networks for indoor settings.Specifically, a single-channel 100 Gbps FSO channel is evaluated.The study compares the FSO communication system's performance using a mid-band p-In 0.01 Ga 0.99 N/p-In 0.5 Ga 0.5 N/n-In 0.5 Ga 0.5 N solar cell as an optical receiver to that with a conventional optical receiver.Leveraging Optisystem software for FSO communication system creation and SCAPS-1D software for solar cell evaluation, the research explores incident light varying from 400 to 700 nm, assessing FSO communication system performance based on received electrical power.

| SIMULATION SETUP
A 100 Gbps single-channel employing on-off keying (OOK) modulation was simulated using Optisystem software based on the previous work [45].Developed by Optiwave System Inc., Optisystem offers comprehensive capabilities for simulating various optical communication systems.The OOK FSO network under consideration comprises three main components: the transmitter, FSO channel, and receiver.
The downlink transmitter comprises a non-return-to-zero pulse generator with a 0.5-bit duty cycle, driven by a pseudorandom bit sequence generator to achieve a 100 Gbps data rate.This signal is modulated using a continuous wavelength (CW) laser through a Mach-Zehnder modulator.The CW laser operates at a power of −20 dBm, covering wavelengths (λ) ranging from 400 to 700 nm.The optical signal produced by the Mach-Zehnder modulator is propagated through the FSO channel, varying in transmission lengths of 5, 10, 15, and 20 m.The FSO channels exhibit an attenuation of 25 dB/km and a beam divergence of 2 mrad.
At the receiver end, an APD with a responsivity of 1 A/ W and a dark current of 10 nA is employed.Additionally, a low-pass filter with a cutoff frequency of 0.75 times the bit rate is utilised to convert the optical signals into electrical signals, as depicted in Figure 1.To ensure the proper functioning of FSO communication, eye diagram visualisers are utilised to obtain eye diagrams for all FSO channels at the receiving end, as illustrated in Figure 2.This comprehensive simulation and analysis framework provide insights into the performance and efficacy of the considered OOK FSO network.and 2.91 � 10^−4 a.u. at 5 m.The received optical power was measured using an optical power meter at the FSO channel's end, while an electrical power meter gauged received electrical power at the receiver.Various commercially available APDs suitable for the visible light spectrum are detailed in Table 1.
- and Thor labs, respectively.Table 2 summarises the results of received optical and electrical powers.A notable trend in Table 2 indicates that APDs tend to perform more effectively with longer wavelengths, aligning with the influence of responsivity on optical receiver outputs [46].Responsivity (R), a vital factor in APD performance, is approximately given by the equation [47]: This Equation (1) underscores that responsivity increases proportionally with the applied wavelength of light.

| InGaN-BASED SOLAR CELL
In this study, the InGaN-based solar cell is composed of a threelayered PPN InGaN structure as depicted in Figure 3 [48].The uppermost layer comprises p-In0.01GaN0.99 with a thickness of 0.1 μm, while the middle and bottom layers consist of p-In0.5GaN0.5 and n-In0.01GaN0.99 layers, each with a thickness of 1.0 μm.These layers possess a carrier density of 1019 cm -3 .The material parameters for all InGaN components utilised in the simulation are detailed in Table 3.The performance evaluation of this solar cell was conducted using the SCAPS-1D software, employing a 1.5 AM solar spectrum and operating at a temperature of 300 K.   p-In 0.01 Ga 0.99 N/p-In 0.5 Ga 0.5 N/n-In 0.5 Ga 0.5 N solar cell as the receiver.The solar cell, with dimensions of 1 mm � 1 mm, was subjected to a series of simulations using the SCAPS-1D software.To emulate the optical received power from Optisystem software, as detailed in Table 2, the incident light's power was adjusted.These simulations were conducted across varying incident wavelengths within the visible spectrum, ranging from λ = 400 to 700 nm.The summarised results can be found in Table 4 and are graphically represented in Figure 6.Notably, the highest power generation occurred at λ = 700 nm, attributed to an increased responsivity at this wavelength.As λ decreases, the received electrical power proportionally decreases, although it remains superior to commercially available APDs.A comparison between Tables 2 and 4 suggests that the solar cell's Bandgap, E g [43] x.

| Solar cell as optical receiver
InN is 0.7 eV and E g GaN is 3.42 eV.The b is known as the bowing factor, which is equal to 1.43 eV.

2
Electron affinity, 4 Effective density state of the conduction band N c [51] ( Effective density state of the valance band N v [51] ( enhanced spectral response positions it favourably as a receiver for FSO communication networks, outperforming conventional APDs.The responsivity of the pIn 0.01 Ga 0.99 N/p-In 0.5 Ga 0.5 N/n-In 0.5 Ga 0.5 N solar cell can be evaluated using Equation ( 2 The Specral Response of the p-In 0.01 Ga 0.99 N/p-In 0.5 Ga 0.5 N/n-In 0.5 Ga 0.5 N solar cell is depicted in Figure 6.Analysis indicates that the responsivity of this solar cell varies from 54 A/W to 34 A/W as the incident wavelength decreases from 700 to 400 nm.A comparison with Table 1 suggests that the p-In 0.01 Ga 0.99 N/p-In 0.5 Ga 0.5 N/n-In 0.5 Ga 0.5 N solar cell delivers a more robust spectral response compared to APD15-8-150-T5H, APD230-LCC, MTAPD-06-001, and ADP120A2, consequently yielding higher output power.Spectral response data for APD15-8150-T5H, APD230-LCC, MTAPD-06-001, and ADP120A2 were obtained from their datasheets and are outlined in Table 1.
Leveraging solar cells with energy-efficient devices can pave the way for self-powered networks [53], thereby reducing overall system costs by eliminating the need for batteries and minimising human intervention [54].In such networks, the solar cell's energy can power devices like body-mounted sensors [55], pollution monitoring equipment [56], and medical devices [57].With the discovery of new solar materials [58], solar cells will have more efficacy and will be able to perform better in these applications.Utilising solar cells as receivers in optical communication holds importance by enabling energy-efficient data reception, harnessing the power of ambient light to support sustainable and self-powered communication systems.This approach aligns with the growing emphasis on energy conservation and innovative integration of renewable resources within modern communication technologies.In this paper, we utilise a threelayered p-In 0.01 Ga 0.99 N/pIn 0.5 Ga 0.5 N/n-In 0.5 Ga 0.5 N solar cell, boasting a remarkable 26.36% conversion efficiency under 1.5AM conditions, as an optical receiver in indoor FSO applications employing visible light.To evaluate its feasibility, we conducted simulations for a 100 Gbps downlink channel utilising the OOK modulation format within an FSO communication network.We employed four commercially available APDs alongside a PPN InGaN-based solar cell as the receiver.Remarkably, our p-In 0.01 Ga 0.99 N/p-In 0.5 Ga 0.5 N/nIn 0.5 Ga 0.5 N solar cell exhibited superior output power compared to its commercial counterparts, including APD15-8-150-T5H, APD230-LCC, MTAPD-06-001, and ADP120A2.This heightened output power is poised to enhance the performance of the received signal, encompassing crucial metrics such as bit error rate, Q factor, and height of the eye diagram.Furthermore, the amplified output power holds the potential to extend transmission distances, contributing to the overall robustness of the system.

Figures
Figures 2a,b,c,d depict eye diagrams captured at transmission distances of 20, 15, 10, and 5 m, respectively, employing a wavelength (λ) of 700 nm.Notably, the clarity of the eye diagrams improves as the transmission distance decreases, with the 5 m distance exhibiting greater clarity due to diminished attenuation losses.The heights of the received eye diagrams correspond to 1.01 � 10^−4 arbitrary units (a.u.) at

T A B L E 1
Abbreviation: APD, avalanche photodiode.

F I G U R E 2
Eye diagram at the receiving end for a transmission distance of (a) 20 m, (b) 15 m, (c) 10 m, and (d) 5 m, at an incident wavelength of λ = 700 nm.

Figure 4
illustrates the I-V and power-voltage (P-V) characteristics of the PPN InGaN solar cell.The simulation outcomes indicate that the cell can achieve an open circuit voltage (Voc) of 1.32 V, a current density (Jsc) of 21.95 mA/ cm2, a fill factor of 90.37%, and a conversion efficiency (η) of 26.36%.Additionally, this cell demonstrates a power density of 26.4 mW/cm2.

Figure 5
Figure 5 depicts the schematic diagram of a singlechannel OOK FSO communication system utilising the

T A B L E 4 T A B L E 3
Received optical and electrical power of a single channel on-off keying frees pace optical communication system with the PPN InGaN solar cell as the receivers.U R E 4 I-V and power-voltage curves of the PPN InGaN-based solar cell under 1.5 AM and temperature of 300 K. Parameters for the InGaN solar cell.

− 3 F I G U R E 6
Specral Response of the p-In 0.01 Ga 0.99 N/p-In 0.5 Ga 0.5 N/ n-In 0.5 Ga 0.5 N solar cell.F I G U R E 5 Schematic diagram of a single channel on-off keying frees pace optical communication system with the PPN InGaN solar cell as the receivers.

F
I G U R E 7 I-V and power-voltage curves at different transmission lengths and different incident wavelengths: λ = (a) 700 nm, (b) 600 nm, (c) 500 nm, and (d) 400 nm.