Si-Solar cell and Nanogenerator Hybrid Cells

Since the report on the invention of the TENG in 2012 [15], Yang et al. [47] were the first group to report the successful fabrication of a HC consisting of micro-pyramidal Si SC and a wind-driven CS-TENG made of thin films of PDMS nanowire array and ITO. The transparent CS-TENG replaced the conventional glass cover of the SC and though the PCE of the SC was reduced from 16% to 14% the net output of the HC was greater compared to that of only the SC. The CS-TENG could operate simultaneously as the SC and the HC generated enough power to achieve 98% self-powered electro-degradation of rhodamine B in 10 minutes.

The HC, shown in Fig. 8 (a) was also capable of driving a couple of LEDs and in conjunction with an energy storage device (Li-ion cell) could charge it enough to drive a laser diode or power a cellular phone.

Zheng et al. [17] proposed a similar HC structure that integrated a single electrode TENG and SC for the simultaneously harvesting solar and raindrop electrostatic energy. The

protective glass cover of a micro-pyramidal Si-based solar cell was replaced with a transparent TENG. The TENG consisted of a superhydrophobic nanowire array of polytetrafluoroethylene (PTFE) and a thin film of PET with an ITO electrode layer in the middle. The TENG was used to harvest the electrostatic energy of falling rain drops while also maintaining the transparency necessary for the SC. The SC was reported to have a 2% drop in performance which was comparable to that due to conventional glass covers. The TENG could supplement the power output of SC during rainy, overcast days and serve to keep the SC surface clean due to its hydrophobic property. The HC was projected to be able to power a 10 W bulb with a surface area of 614 m².

Fig. 8. Schematics of Rigid Si-Solar Cell and Nanogenerator Hybrid cells. (a) The first instance of a PV-TENG HC [47]. (b) Dual-mode TENG and PV HC: (I) Overview of the HC assembly; (II) magnified view showing the various layers [19]. (c) HC with a Micro-bowl PDMS array structure [48].

(d) HC with super-hydrophobic Moth’s eye Mimicking (MM) TENG, inset shows a photograph of the HC [49]. (e) HC consisting of a TENG with a nano-wrinkled top surface [50].

In 2015 [19], the same research group presented an improved version of their previous prototype which was a HC capable of harvesting energy from solar, wind and raindrop electrostatic energies. The water TENG in the previous work was replaced by a dual-mode

TENG consisting of a sandwich of the same single electrode TENG and a contact separation TENG consisting of PTFE and Nylon thin films separated by a PET spacer. This HC as seen in Fig. 8 (b) was reported to generate a maximum output of 86 mW/m² from raindrop electrostatic energy and 8 mW/m² from wind without having any deleterious effects on the SC performance.

Jeon et al. [48] also proposed a similarly structured HC of Si-based conventional SC and a water TENG as seen in Fig. 8 (c). In their report polydimethylsiloxane (PDMS) and polyethylene naphthalate (PEN) layers along with an ITO electrode were used to develop the water TENG. The PDMS was nanotextured into a micro-bowl array lending it superhydrophobic properties which was demonstrated to improve the long-term efficiency of the HC. Maximum output power of 0.27 µW was achieved at a load resistance of 25 MΩ.

Yoo et al. [49] proposed replacing the conventional glass cover of SC with a high transmittivity moth’s eye mimicking TENG (MM-TENG) as in Fig. 8 (d). This MM-TENG has a 0.01% greater Solar Weighted Transmittance (SWT) compared to conventional single electrode water TENGs which in turn improved the fill factor and PCE of SC by 0.5 and 0.17%

respectively. The self-cleaning property exhibited by the MM-TENG helped maintain the long- term operational efficiency of the SC. A novel switching circuit coupled with a rectifier was developed to convert and store the output of the HC. The Moth’s eye structure consisted of an array of nano-pillars which generated a gradual transition region between regions of varying refractive indexes thus minimizing reflections and maximizing transmittivity.

Working along the same vein as Yoo et al.[49], Liu et al. [50] also reported the development of a transparent TENG to harvest energy from raindrops integrated with a Si SC as seen in Fig. 8 (e). A nano wrinkled PDMS was deposited onto the SC in lieu of a conventional protective glass cover or water drop (WD)-TENG to serve the dual function of

anti-reflective coating along with generating power via the single electrode mode of TENG operation. The PCE of the SC was observed to have increased by 1.02%. The TENG component displayed a significant improvement in Voc and Isc of 385.5% and 299.1%

respectively over a conventional WD-TENG with planar PDMS film. The considerable increase in output was attributed to the strong hydrophobicity, surface fluorination and high aspect ratio of the nano wrinkled surface texture compared to a normal planar surface.

In the subsequent year, Wang et al. [51] developed a rigid HC to harvest both solar and wind energies. The suitability of the HC application at large scale and the potential of replacing conventional rooftop solar panels were also discussed in detail. The HC consisted of a Si-based solar cell with a contact separation mode TENG made of FEP and Kapton films with copper (Cu) electrodes between the two layers. The entire device was encapsulated in acrylic apart from the air flow channel. For a device area of 120 mm x 22 mm the maximum power output from the TENG was reported as 26 mW while that of the SC was 8 mW. A transformer was used for the TENG output for impedance matching with the SC output by reducing the impedance of the TENG.

Liu et al. [52] fabricated a HC with a common electrode made of a PEDOT:PSS film.

A hetero junction Si SC and a PDMS based single electrode water TENG (S TENG) were connected to a common PEDOT:PSS film which served as the electrode while maintaining a low degree of reflectivity for incident solar radiation. The SC exhibited a PCE of 13.6% higher compared with a textured Si structure while the TENG generated a 33 nA and 2.14 V Isc and Voc respectively from raindrops. The HC provided the combined advantages of high current from the SC and high voltage from the TENG.

Shao et al. [53] designed a HC made up of a commercial water proof Si-based SC, a contact separation mode TENG (CS TENG) and a freestanding sliding mode electromagnetic

generator (FS-EMG) to harness blue energy (solar + wave motion energies) from the oceans.

The inclusion of both TENG and FS-EMG was justified by the utilization of TENG to harvest low-frequency wave energy and FS-EMG to harvest high-frequency energy. The whole assembly of the TENG and the FS-EMG was sealed within acrylic while a pair of non- contacting magnets were used to transfer the external wave motion to the electrodes. A water- proof SC was integrated at the top of the assembly to complete the setup. The individual components could function simultaneously or separately to yield enough power to be utilized by LED or charge energy storage devices.

Roh et al [18] fabricated a HC made of stacked transparent S-TENG, Si SC and CS- TENG. This device was capable of harnessing energy from the wind, the rain and the sun. The application of the HC as an IoT based self-powered active weather sensor was demonstrated.

Fig. 9. Schematics of α-Si Solar Cell and Nanogenerator Hybrid Cells. (a) Flexible self-powered HC consisting of a CS-TENG and α-Si SC: (I) Overall structure of the HC assembly; (II) magnified view showing the polyamide spacer used in the CS-TENG [54]. (b) Rotary mode wind and solar powered HC: (I) overall assembly; (II) Internal view showing the radial layout of the components; (III) Magnified view of a single TENG unit; (IV) Photograph of the HC [20].

Ma et al. [54] described a flexible self-charged power panel which harvested solar and mechanical energies as in Fig. 9 (a). The SC consisted of an amorphous Si SC along with an ETFE film-based contact separation TENG with ITO electrodes and a polyamide spacer. The ETFE also acted as protective transparent covering for the SC. A Li-ion battery to store the HC output formed the bottom of the HC. The TENG was observed to extend the power collection period of the HC while having no ill effects on the SC. The TENG output also served to further raise the voltage of the Li-ion cell after the SC has finished charging it.

Qian and Jing [20] developed a HC of a TENG, EMG and water-proof conventional Si SC to harness wind and solar energies as in Fig. 9 (b). The individual components were arranged coaxially with a wind-driven rotor and a stator which helped convert the momentum of the wind into rotational energy which in turn was utilized by the EMG and TENG to produce power. The development of a self-sustaining Wireless Sensor Network (WSN) made of the proposed HCs to serve as disaster monitoring system was discussed and the capacity of the HC to power small electronics and sensors was demonstrated.

Ahmed et al. [55] reported the design of a tree-shaped hybrid nanogenerator (TSHG).

This HC was the combination of a thin-film solar cell, an FEP leaf and a PVDF LDT4-028K/L piezo film based TENG stacked together to form individual leaves. The leaves were then arranged in the topology of a tree and demonstrated the capability to generate power from solar and wind and/or rain drop energies. The power output of each TENG was sent to a full-wave rectifier before the conjoined output was utilized to charge a capacitor. The TSHG produced a maximum power of 3.42 mW at a load resistance of 5 kΩ.

Si-solar cells are the most mature among the solar cell technologies with an established pedigree of good, stable long-term performance. As such, in HCs consisting of crystalline Si- solar cells, the nanogenerators were mostly relegated to a supporting role. Most of these types

of hybrid cells involve the integration of a transparent S-TENG, CS-TENG or a combination of both to harness raindrop electrostatic and wind energies respectively. The nanogenerators were surface engineered to exhibit minimal scattering losses and improve transmissivity to enhance solar cell performance. The most common method involved the use of micro/nano textured surface such a micro-bowl texture[48] or nano-pillar texture[49] which improved the optical performance of the hybrid device by reducing scattering of light while simultaneously imparting hydrophobic and self-cleaning properties for stable long-term performance. There have also been instances of flexible hybrid devices involving thin-film α-Si solar cells[54,55].

These devices have shown a similar performance as flexible HCs using other types of solar cells. Si-solar cells have also been combined with nanogenerators and EMGs simultaneously to produce portable, encapsulated devices which were proposed for applications such as blue energy harvesting [53].