4.1. Inorganic Solar Cells
To allow power generation power from both sunlight and rain precipitation energy, a harvesting structure combining a triboelectric nanogenerator (TENG) device and a solar cell was manufactured. An electrode made of a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) film was introduced to a heterojunction silicon (Si) solar cell integrated with a TENG. PEDOT:PSS was used to increase short current density. Imprinted-polydimethylsiloxane (PDMS) was used as a triboelectric material, whereas a PEDOT:PSS layer served as an electrode. The output of the TENG was significantly improved due to a bigger area of contact between the raindrop and the imprinted PDMS. The values obtained were ca. 33.0 nA for short-circuit current and ca. 2.14 V for open-circuit voltage. To make use of renewable energy the TENG harvests solar energy during bright days and on cloudy days and when it rains raindrop energy is harvested. However, the structure of a hybrid energy harvesting system structure needs to be improved in order to reduce losses of the solar cell with reduced influence on performance of the TENG device. Thus, an electron hole-blocking layer, antireflection layer, structural modification of PEDOT:PSS, texturing the Si nanostructure, and surface passivation were applied and the Si/PEDOT:PSS solar cells obtained. In the paper, Liu et.al. [
36] proposed a digital video disk pattern; a heterojunction Si solar cell was integrated with a single-electrode mode TENG. A PEDOT:PSS layer served as the common element of the two devices. In the case of the planar Si imprinted with PEDOT:PSS (called Si/imprint PEDOT:PSS) device the values obtained were the following (PCE, V
OC, J
SC, and FF, respectively): 13.6%, 0.628 V, 29.1 mA/cm², and 0.745. When textured Si/PEDOT:PSS was fabricated, the values differed and were as follows: PCE of over 12.6%, V
OC of 0.612 V, J
SC of 29.4 mA/cm², and FF of 0.702. The values for flat PEDOT:PSS on planar Si (Si/planar PEDOT:PSS) were as follows (PCE, V
OC, J
SC, and FF, respectively): 12.0%, 0.625 V, and J
SC of 25.8 mA/cm², and FF of 0.746. J
SC of 29.4 mA/cm² was due to light trapping. Textured Si’s inferior diode property was due to a large Si surface/volume ratio. Furthermore the authors [
36] studied the effective minority carrier lifetimes (τ
ff) mapping measurement to evaluate the surface recombination velocity for different Si substrates, whose value for textured Si equals 10 μs, for textured Si/PEDOT:PSS equals 24 μs, for planar it was 37 μs, for planar Si/PEDOT:PSS the value was 62 μs, and that of the planar Si/imprinted PEDOT:PSS was 60 μs. To harvest raindrop energy, the TENG was built on the heterojunction Si solar cell. Vital information on imprinted PDMS is that film is hydrophobic, which is an advantage for a raindrop TENG device, as the rainwater will not wet the substrate. The vital characteristics of the TENG PDMS films were measured; the imprinted PDMS TENG device displayed superior electric performance as a consequence of a larger surface area. The influence of the angle (α) and the angle (β), angles between water dripping direction and device surface or imprinted structure, were studied for α and ranged from 15° to 75°. Average power first increased slightly and then decreased with the increase of α. As far as β is concerned, it was found that an increased angle leads to better output performance. The designed hybrid power system included TENG-generated current being transferred from AC via a bridge rectifier to DC. After rectification, current output of the TENG was ~24.2 nA. The high voltage TENG could compensate the shortcomings of the solar cell. The system underwent five recycling charging processes and indicated good stability and repeatability, with 1.74 mW m
−2 for the average value of power density. The system combines the possibility of high current output (solar cell) and high voltage output (TENG) (see
Figure 10) [
36].
The best performance was obtained for the cell with the imprinted layer of PEDOT:PSS without negative effect on the performance of the silicon solar cell during sunny days. The process of the texturized form of the polymeric layer had another beneficial input, meaning, its plaid role of antireflective coating. The best harvesting performance for the TENG module was observed for the incident angle of water set as 45°.
4.2. Dye-Sensitized Solar Cells
Since electricity generation from the sun is zero at night and rainy weather, the authors decided to create an all-weather solar cell which would take advantage from both sun and rain. By hot-pressing graphene onto the rear side of the indium tin oxide/polyethylene terephthalate plastic substrate, a modified solar cell was built on an ITO layer. Due to the salt content in raindrops, when dropping onto a graphene surface, they reach the periphery forming an electric double-layer (EDL) pseudocapacitor at the interface of the raindrop and graphene. The shrinking drop releases electrons to the graphene, thus, charging the pseudocapacitor. It is not applicable for the fluorinated tin oxide (FTO) glass substrate to be used in solar cells due to the frangibility of glass, as well as its divergent nature; however, in all-weather solar cells it can be useful.
Zhang et al. [
37] manufactured an all-weather solar cell on double-sided conductive glass. It was done via coating an ITO film on the rear surface (nonconducting side) of the commercially available FTO glass and then depositing a graphene film onto the solar cell. The deposition was carried out using the electrophoretic deposition method. In this type of solar cell, di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) (N719 dye), as photodye absorbs photons, release electrons to the conduction band of the TiO
2 nanocrystallite layer of the photocathode when the solar cell is irradiated from the anode by incident light. As far as electrodes are concerned, the authors [
37] developed a CoNi alloy electrode in order to be more cost-effective in comparison to the Pt-based option. The efficiency (PCE) obtained equalled 8.16%. However, the increased triiodine monoanion to iodine redox process accelerates the recovery of N719 dye and, therefore, cell performance was improved to 9.14% when catalytic activity of ternary PtCoNi was used. To generate electricity on rainy days, the graphene monolayer needed to be placed on the top surface of the solar cell. In the visible-light region, the graphene monolayer was characterised by high optical transparency equalling ca. 92%. The efficiencies were 5.63% for solar cells with Pt, 6.96% in the case of the CoNi counter electrode, and 7.69% for the PtCoNi. Other characteristics for the CoNi were the following: PCE was 8.16%, V
oc was 0.725, J
sc was 17.68 m Acm
−2, and FF was 63.7%, whereas for PtCoNi the results were the following: PCE was 9.14%, V
oc was 0.739, J
sc was 18.48 mA cm
−2, and FF was 66.9%. Good results were also obtained for the RuCoSe alloy electrode. PtCoNi and RuCoSe alloy electrodes exhibited maximal catalytic activities toward the redox electrolyte, hence, their maximal V
oc and J
sc. Furthermore, an experiment has been conducted demonstrating that deionized water does not generate electrical signals when contacting the graphene surface. Since graphene is considered a Lewis base due to its cations, the Na
+ ion in raindrops behave as Lewis acids, absorbing the π-electrons from the graphene structure in terms of a Lewis acid–base interaction, thus, forming EDL pseudocapacitance at the interface of Na
+/graphene [
38]. Other materials which are electron-enriched and can be applied to fabricate all-weather solar cells include polyaniline, polypyrrole, and alloys. The time interval between two droplets is called rain intensity (controlled by regulating injection velocity) and, apart from raindrop concentration, highly impact the signal output. Examples of electrical parameter dependence on simulated raindrop intensity is shown in
Table 4.
When the case of acid rain was studied the authors hypothesised that a bi-triggering solar cell (see
Figure 11) should yield higher current and voltage signals due to the greater ion concentration in acid rain. Despite promising results, the electricity generated from rain itself is much lower than required in practical applications, thus, a need for energy storage devices (i.e., all-weather solar cells) may be the optimal solution [
37].
To achieve the objective of a combination of photovoltaics (conversion of sunlight energy) with other energy conversion working during night time or under dark conditions, Tang and his group [
10] manufactured an all-weather solar cell by introducing long-persistence phosphors (LPPs) into m-TiO
2 photoanodes for the afterglow effect. LPPs are characterised by their ability to collect energy from ultraviolet and/or visible light and then produce afterglow light in a visible range at room temperature without the need for irradiation. A dye-sensitized solar cell (DSSC) is a third-generation solar cell consisting of dye-sensitized m-TiO
2 photoanode, a I
−/I
3− redox electrolyte, and a platinum counter electrode. It provided photovoltaic characteristics with PCE of 8.08%. The final
m-TiO
2/LPP photoanodes were created by applying a coating with purple, blue, cyan, green, red or white-emitting LPP layer and these could be built into all-weather cells. Solar cells with LPPs and without LPPs were compared and the efficiencies were significantly greater and equalled 10.08% as a result of simulated light and afterglow effect. Moreover, it was found that the charge-transfer processes were improved because of the addition of an LPP layer. Furthermore, PCE was also increased and equalled 26.69%, 22.62%, 20.87%, 19.78%, 15.35%, and 3.02% for all-weather solar cells characterised by green, cyan, blue, purple, red, and white luminescence, respectively. Electricity could be generated in all of the above wavelengths. The incorporation of the above mentioned LPPs, the maximal incident photon to current efficiency (IPCE) values obtained equalled 84%, 78%, 71%, 56%, 47%, and 63%, respectively. However,
Voc, J
sc, and FF obtained under dark conditions were lower than those during daytime and the authors speculate that the reduced
Voc and FF is due to the increased electron recombination reactions in the dark conditions, since the lower fluorescent light yields low electron density at TiO
2. It was found that the light ranging from 625 to 720 nm has low impact on excitation and, therefore, electricity output. However, the photoanode irradiated with white luminescence achieved broad emission in a range from ~415 to ~632 nm, with a maximum at 474 nm and other smaller maxima at 450, 468, 495, 511, 537, 586, 616, and 625 nm, in comparison with the double emission peaks for the red-luminescence anode (594 and 625 nm). This points to the possibility of emitting monochromatic light by the
m-TiO
2/LPP photoanodes simply by absorbing sunlight. The authors [
10] also studied the durability factor under dark and light conditions and found that the only darkening
m-TiO
2/LPP photoanode is the one for purple and red luminescence, as a massive number of trapped electrons was released in the LPPs purchased by the authors. For the same TiO
2/LPP photoanodes with these above listed luminescence (in the order: green, cyan, blue, white, purple, and red), the following maximal emission intensities were the following: 171, 176, 160, 140, 136, and 134 mW cm
−2, respectively. As far as decay time (dark characteristics) is concerned, the authors [
10] noted that all the relevant parameters (
Voc, J
sc, FF, and P
max) exhibited sudden reductions in the first 5 min and were almost constants following 55 min.
To summarize, the studied LPPs can capture and release incident light with wavelength >550 nm when illuminated by sunlight and subsequently afterglow at dark conditions. The six all-weather solar cells with different colours of luminescence and characterised by good long-term stability were manufactured and the maximized conversion efficiency of up to 26.69% was obtained in the total darkness (see
Table 5).
Figure 12 presents some experimental data of dark J–V characteristics [
10].
Tang et al. [
6,
39] presented a preliminary study on all-weather solar cells capable of harvesting both the solar and raindrop energy via the incorporation of a graphene-based layer with a solar cell. The mechanism behind graphene adsorption from liquids to form an electrical double layer (EDL) (separation of π-electron and cation) at the interface of the graphene and ionic liquid was as follows:
- (i)
When they spread to the periphery and form this EDL pseudocapacitance they drag the electron transfer, and the front of the raindrops are charged;
- (ii)
Afterward, the raindrops shrink and release electrons to the graphene and discharge the pseudocapacitance, and the repeated charging and discharging processes yield current and voltage.
This capacitive response during the shrinking and spreading was investigated using cyclic voltammogram characterisation. Graphene (conducting electrons) and carbon black with a polytetrafluorethylene (PTFE) insulator to construct a graphene–carbon black conducting composite ((G-CB)/PTFE) was fabricated in order to modify graphene dosage, improving film-forming ability. Carbon black serves as the compatibility improver for the graphene/ PTFE mixture. In this construction, graphene is responsible for electron migration by the π-electron system. By combining a solar cell with a G-CB/PTFE film (characterised using Raman, XRD, FTIR, TGA, SEM, and TEM), an all-weather solar cell was created to harvest energy from sunlight and rain. Photosensitive N719 dye, which absorbs photons, releasing electrons to the FTO layer along porous TiO
2 pathways, was used. Although the solar cell with a Pt electrode can yield a PCE of 7.23%, it is a costly option for mass production and that is why a PtNi alloy counter electrode was fabricated, and an efficiency of 9.80% was obtained (see
Figure 13). The voltage and current response to simulated rain are presented in
Figure 14.
The J–V curves were recorded when simulated raindrops (NaCl aqueous solution) were being dropped onto G-CB/PTFE conducting films, whereas pure, deionized water produced no current. To describe the adsorption and transportation of the charges (see
Figure 15), electrochemical impedance spectroscopy (EIS) was performed.
Furthermore, the Na
+ concentration influence on electric signals was studied: NaCl aqueous solution concentration increased from 0.2 to 2 mol L
−1. The current and voltage values for 2 M NaCl solution were 1.14 μA and 100.10 μV, which is a significant increase from the values for the 0.2 mol L
−1 solution (0.35 μA and 25.65 μV, respectively, see
Figure 14). The greater the cation concentrations (such as in the case of acid rain) involved more π-electrons involved in improving pseudocapacitance and, thus, electrical response. The authors [
39] also wanted to cross-check the G-CB conducting pathway formation, so they used a polyacrylate/sericite composite because of the sericite distinctive polarized light behaviour and the formation of interconnected networks within the composite film. By studying the electrical percolation, the authors [
39] found that the beneficial wt% of G-CB is 95, as the 95 wt% G-CB/PTFE composite electrode showed a lot of promise, as far as rain energy generation was concerned. Rainfall intensity (injection intensity) is another factor impacting the current and voltage signals, as they decrease with increased rain intensity. This is because, low injection velocity causes an electron recombination with the Na
+ in the previous droplet when the next raindrop falls at high velocity. This reduces charging pseudocapacitance. Furthermore, since positively charged species and charge number exert vital impact on the current and voltage signals, other solutions have been studied. The cell characteristics increased to 1.14 μA and 89.31 μV for 0.6 M LiCl solution; however, changing a lithium cation for potassium causes a reduction to 0.61 μA and 33.89 μV. The results obtained for magnesium chloride and calcium chloride aqueous solutions gave better results than that for sodium and potassium chloride, respectively. Lastly, pH and temperature impacting current and voltage were studied, and the findings were that current and voltage outputs increase with increased pH value and temperature. That conclusion is key for manufacturing all-weather solar cells for those regions of the globe with high temperatures and acid rain.
It is well known that solar cells can only be excited by sunlight on sunny days and do not function (or function very weakly) on rainy days. To address this issue, professor Tang’s group [
5] developed a new solar cell composed of an electron-enriched electrode (rain energy harvesting) with a DSSC for photoelectric conversion. Electricity generation was conducted by raindrops falling on graphene film, as rain contains both the positively (Na
+, Ca
2+, and NH
4+) and negatively charged ions. When raindrops drop onto the graphene surface, the positively charged ions are adsorbed onto this surface by Lewis acid–base interactions to drive electron migration. Thus, electrical π-electron/cation double-layer pseudocapacitors are created (see
Figure 16).
The authors [
5] fabricated a bi-triggered DSSC with PtNi
3 alloy, which provided photoelectric conversion efficiency equalling 6.53% and noted that the use of Pt-Ni alloy can remarkably increase electrolyte contact and enhance the catalytic activity. This cell can also generate energy from the rear, counter electrode, and side by the application of Co
0.85, Se, and Ni
0.85 Se with high optical transparency [
40,
41]. Efficiencies of 4.26% and 4.09%, respectively, were obtained. During rain periods, proposed new solar cells could be covered with a reduced graphene oxide (rGO) film, allowing for current and voltage production under simulated rain (0.6, 1 and 2 M NaCl aqueous solution). The reduction of the lateral distance between the falling point and electrode to 7.58 and 4.52 mm increases the current intensity to 0.33 and 0.54 mA, respectively. Another factor influencing the electrical signals is the time interval between two droplets (controlled by injection velocity). The results obtained by the authors were the following: 20 mLh
−1 (ca. 0.49 mA, ca. 109.26 mV, ca. 54.19 pW), >50 mLh
−1 (ca. 0.37 mA, ca. 44.48 mV, ca. 20.63 pW), >80 mLh
−1 (ca. 0.17 mA, ca. 31.89 mV, ca. 5.12 pW). The third factor is the high dependence of current and voltage on the ion concentration, the higher the Na
+ concentration (2 M vs. 1 M), the more enhanced the current and voltage. The authors [
5] demonstrated that those electrons which had not formed the π-electron/cation pseudocapacitance could further interact with Na
+ ions when the concentration is high. The authors [
5] also focused on the long-term stability and they highlighted their supremacy over the actual value of the signal. Following the repeated operation of this new type of solar cell, 88.8% of initial current and 52.3% of voltage remained after the repeated dropping of 0.6 M NaCl solution on the rGO during the 1000 s period with 4 s intervals. The deterioration is most likely due to rGO susceptibility to NaCl solution, namely the increased surface wettability or increasing the interfacial resistance. Focusing on the future design improvement and stability, the authors recommend discarding rGO in favour of a highly hydrophobic graphene film with compact stacking in order to improve this particular characteristic.
Tang et al. [
5] explained the process of generating electricity from graphene when ions (from raindrops) interact to form electron/cation EDL pseudocapacitors based on Yin et al. [
38]. During rain, current and voltage signals persistently yield charging/discharging cycles, and when sunlight illuminates the device, the photoelectric conversion processes follow the DSSCs principles. When capacitance was studied (using cyclic voltammetry) as a relationship between the rainwater contact area and graphene surface, it was found that during the spreading process it increased from zero to a maximum, returning to zero at the shrinking stage. Different methods of all-weather solar cells were presented, including inkjet printing. This method is characterized by simplicity, low cost, and capability of being produced on a mass scale, and it consists of a solute dissolved or otherwise dispersed in a solvent to form inks. The example provided is a conducting coating, a graphene-carbon black/polytetrafluorethylene (G-CB/PTFE), where the graphene allows for electron migration and the carbon black compatibility improves the graphene and PTFE matrix. Apart from graphene, some alloys (such as platinum with transition metals) can be used in all-weather solar cells, and the results obtained in the form of electric signals are at the same level as for graphene. Another approach includes long-persistence phosphors (LPPs) being coated onto a
m-TiO
2 layer to produce electricity in dark conditions without reducing photo efficiency. LPP (either purple, blue, cyan, green, red, or white) is a phenomenon in which ultraviolet, visible, NIR, or infrared spectral regions are emitted for a specific period of time after irradiation has ceased. In the absence of light, the photoactive dyes are irradiated by the afterglow from LPP phosphors, yielding as 26.69% efficiency for the green-emitting LPPs devices. The N719 dye can be substituted with converted carbon quantum dots (CQDs) [
42]; however, the efficiencies yield much lower values than those of the high-efficiency solar cells due to weak affinity of the CQDs and TiO
2 surface, hence, the need for modifications of the band energy structure.
Basic DSSC modules obtain PCE up to 11% and is similar for all bi-triggered setups. The biggest difference lies in the rain harvesting system and system with LPP. The biggest disadvantage of the rain harvesting system are rain-dependent factors such as rain direction and force of the impact, hence, the obtained energy is rather punctual and gives small values of current and voltage under a load. Furthermore, the systems with a photon storage layer gives a continuous response in dark conditions with values comparable to those obtained under illumination; however, the storage capacity is very limited and the produced current drops with time. For comparison reasons, in
Table 6 are presented photovoltaic parameters of all-weather solar cells.