Amorphous Silicon Particles/Polyaniline Composites for Hybrid Photovoltaic Solar Cell: an Experimental Feasibility Study

Hybrid heterojunctions of Polyaniline/Amorphous Silicon (PAni / a-Si:H) were synthesized in order to fabricate the active layer of the polymeric solar cells. For this purpose, amorphous silicon nanoparticles were blended with polyaniline which was synthesized through oxidative polymerization. Then the resultant nanocomposite was applied on FTO in the FTO/ZnO/PANI:a-Si/Ag structure using the spin coating method. The effect of amorphous silicon content on light-harvesting efficiency was studied using the UV-VIS spectroscopy data, electrochemical impedance spectroscopy (EIS), and the incident photon to current efficiency (IPCE) analysis. PAni / a-Si:H nanocomposites were characterized structurally and morphologically using Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and X-ray diffraction (XRD) analysis. Current-Voltage measurements of the photovoltaic cells based on PAni / a-Si:H nanocomposites show that increasing the amount of a-Si:H in the active layer up to 40%Wt, the efficiency of the cell increases 2k times compared to the cell made with pure PAni. Also, in general, cell efficiencies increase slightly with decreasing the size of amorphous silicon nanoparticles at a constant PAni / a-Si:H weight ratio. The impact of a-Si:H weight percent is more significant in the case of smaller silicon particles.


Introduction
Organic solar cells (OSCs) are now being studied by energy and photophysical scientists for many practical and strategic reasons.The features include flexibility, high performanceto-weight ratio, semi-transparent nature, easy and low-cost installation, solution process-ability, the possibility of costeffective roll-to-roll production and relatively affordable synthesis, tenability.In addition the consumption of organic raw materials than inorganic semiconductors making them a viable avenue for the future energy supply.
In recent years, many advances have been made to increase the Power Conversion Efficiency (PCE) and increase the thermal and light stability of OSCs [1].
According to our survey these efforts can be broadly categorized as follows: (a) Blending two or more different organic materials (polymers or small molecules) as donors [2] and acceptors (fullerene or nonfullerene molecules) [3][4][5] in layer-by-layer heterojunction (LBL) or bulk heterojunction (BHJ) structures [6], with the aim of achieving a cascade energy alignment.(b) Molecular designing and engineering to tune (narrowing) the band gap of donor or acceptor polymers [7][8][9].(c) Donor, acceptor, or both copolymerization or terpolymerization to adjust the narrow band gap and lower-lying HOMO [10].(d) Hybridization of polymers with inorganic semiconductors in the form of bulk heterojunction or with preferential morphologies, to achieve a broader light spectrum harvesting and generate more carriers, and then effective diffusion and splitting of excitons [11][12][13].E) Controlling the morphology of all-polymer solar cells by deployment additives at the donor-acceptor interface for better charge transport properties [14][15][16][17].F) Stacking organic solar cells with different band gaps to absorb wide range wavelengths of the solar spectrum which is called tandem (multijunction) organic solar cells [18,19].This rough classification only includes methods related to changes in the semiconductor physics of organic active layers.This change in semiconductor physic includes the density, mobility and splitting of excitons, the potential difference and driving force of materials, and the mobility and transport of charge carriers in the donor and acceptor.Besides, typically several research works in other fields is prone to enhance the performance of organic solar cells.These efforts consist research on HTL, ETL, anode and cathode layers, deposition processes, solvents, control of interface layers, and posttreatments such as thermal annealing.
But many of these materials and methods being researched in the above category, even despite the relatively good efficiencies in some of them such as fullerene-acceptor-based (FA-OSCs) and multijunction OSCs, currently do not have enough potential to be scaled up and produce champion commercial modules.Therefore, some scientists are currently making many efforts to improve the PCE of single-junction, non-fullerene organic solar cells (NFA-OSCs) by about 20%.Achieving this PCE range will pave the way for their commercialization when features such as enhanced long-term stability, more light absorption, facile processability, and extremely low-cost production are achieved in them.Among the mentioned methods, organic-inorganic hybrid solar cells can have these conditions, so it is of interest to many researchers to find the optimal conditions between the stronger properties of inorganic semiconductors, such as greater PCE and stability, and the stronger properties of organic semiconductors, such as processability and reduce the price [20,21].
Amorphous silicon (a-Si) is a traditional inorganic semiconductor that, unlike crystalline silicon, is a proper choice like organic semiconductors for fabricating hybrid thin-film cells [22].But the production of amorphous silicon in the form of a thin bulk layer increases the cost and difficulty of production and the number of parameters involved.On the other hand, amorphous silicon nanoparticles cannot be directly deposited as an active layer due to the suppression of charge carrier mobility and strong recombination resulting from nanoparticle detachment.Our strategy was to choose a matrix of a cheap organic semiconductor for it, which would give it proper dispersion and deposition properties and the ability to adhere to the underlying surface and also not disturb its efficiency and stability properties as much as possible.For this purpose, we chose polyaniline, which is one of the conjugated polymers with the mentioned conditions.Electrical conductivity, controllable morphology, processability and simple and economical production routes, relatively high environmental stability compared to other organic semiconductors, and very cheap precursors are among these features.More importantly, the properties of polyaniline, in addition to depending on the oxidation state like other conducting polymers, also depend on the level of protonation and the nature of the dopant [23].
In this work, we synthesized Polyaniline/Amorphous Silicon (PAni / a-Si:H) hybrid nanocomposites with different weight ratios as active layers in a solar cell with the architecture shown in Fig. 1 to find the optimal blending ratio of the two materials in terms of efficiency.Also, the effect of the size of amorphous silicon micro/nanoparticles on the distribution and interaction of two phases in the nanocomposite and as a result the cell PCE was investigated by preparing and using micro/nanoparticles in three size ranges around 200, 500 and 1000 nm.

Materials
Aniline monomer with 99.99% purity, both ammonium peroxydisulfate (APS) as an initiator, and p-toluene sulfonic acid (p-TSA) as a dopant with a purity of ≥ 98% were all purchased from Merck.Moreover, N-methyl-2-pyrrolidone (NMP), zinc acetate (Zn(CH 3 CO 2 ) 2 •2H 2 O), tin chloride (SnCl 2 .2H 2 O) and ammonium fluoride (NH 4 F) all with a purity of more than 99% was prepared from Sigma Chemical Co.Despite the use of fresh aniline, it was distilled each time before polymerization.
Also amorphous silicon micro/nano powder was prepared in sizes of 200, 500 and 1000 nm from Sigma, in order to oxygen passivation, the powder was etched in a sealed container with hydrofluoric acid 48% vapor atmosphere for 24 h at room temperature as described elsewhere by Ding et al. [24], and the resulting hydrogenated amorphous silicon (a-Si:H) until the synthesis of nanocomposites was kept under nitrogen atmosphere.

Synthesis Methods
According to our previous works [25], polyaniline with a preferred tubular (spongy bone tissue) morphology was synthesized by controlling the oxidation polymerization parameters, i.e. nature and the ratio of dopant, kinetic parameters, and reactor atmosphere and temperature.The conductivity of polyaniline depends on the oxidation state of the polymer and the degree of protonation by acids, which is interpreted by the level of doping.p-TSA and HCl are two common dopants in the oxidation polymerization of Pani [26].Undoped PAni due to its strong inter-molecular force originating from high polarization and crystalline structure has very low solubility.With the use of a p-TSA dopant, the separation of the chains increases, and solvent molecules, penetrate appropriately into the polymer clusters and cause more separation of the chains.Considering the results in our previous work, although the conductivity using HCl dopant is more than that of p-TSA, in general, due to the higher solubility of the nanocomposites synthesized with p-TSA dopant and with regard to the ultimate use of nanocomposite (used as an active layer in a solar cell and requires film formation) p-TSA was preferred as dopant.
After synthesis of PAni and preparation of a-Si:H, nanocomposites of PAni/a-Si:H with weight ratios of 5, 10, 20, 40, 60, 80 and 90% of a-Si:H for the three mentioned particle sizes were made through blending method.To do this, 0.1125 g of PAni was dissolved in 15 mL of NMP (N-methyl-2-pyrrolidone) at room temperature using probe sonication for 30 min and then, the necessary amounts of a-Si:H were added to this solution to make the desired ratios with a solution concentration of 0.8 mg/mL, and was sonicated again for 30 min and finally was stirred at room temperature for 24 h to homogenize.

Fabrication of PAni/a-Si:H Hybrid Solar cell
To fabricate solar cells with the architecture shown in Fig. 1.First, the patterned Fluorine-doped Tin Oxide (FTO) coated glass substrate with a sheet resistance of 8 Ω/cm2 was fabricated according to the method described elsewhere by Zhao et al. [27] and then, cleaned using de-ionized water, ethanol, and acetone in an ultrasonic bath.
In the next step, the 80 nm ZnO layer was fabricated on the masked FTO.This process was done using a 200 mM solution of zinc acetate (Zn(CH 3 CO 2 ) 2 •2H 2 O) in water and ethanol with a volume ratio of 1:1, by spray coating method at the plate temperature of 650 °C.And finally, PAni/a-Si:H nanocomposites dispersed in NMP obtained from the synthesis step was deposited by spin coating method to form a bulk heterojunction active layer on the ZnO film.In this way, cells numbered according to Table 1 were fabricated with different conditions of weight percentage and size of a-Si:H particles.Finally, the silver electrode with a thickness of 70 nm was sputter deposited on the composite using other masks to fabricate solar cells with dimensions of 5 × 5 mm.

Characterizations
Pure PAni, a-Si:H particles and PAni / a-Si:H nanocomposites were characterized structurally and morphologically using Perkin Elmer Fourier transform infrared spectroscopy, FTIR, (SPEKOL 2000) for analyzing of functional groups present in the synthesized materials, field emission scanning electron microscopy (FESEM) technique equipped with EDS (ZEISS Sigma 300 HV), and X-ray diffraction (XRD) analysis (STOE STADI P-ESSEN) for crystallographic parameters calculations.Light-harvesting efficiency was studied using the Perkin Elmer UV-VIS spectroscopy data (L1600300), electrochemical impedance spectroscopy (EIS) and the incident photon to current efficiency analysis (SHARIF SOLAR IPCE-020).The photovoltaic parameters of the fabricated solar cells were characterized by J-V tracer (SHARIF SOLAR) under the illumination of AM 1.5, 100 mWcm − 2 .

Synthesis of PAni/a-Si:H Nanocomposites
The FTIR spectrum of a-Si:H, PAni, and a number of a-Si:H nanocomposites mentioned in Table 1 are given in Fig. 2. The medium vibrational mode at ~ 1296 cm − 1 in PAni and nanocomposites represent the stretching frequency of the C-N bond in the polyaniline benzenoid units [28].
In fingerprint region of PAni; the weaker peak at ~ 678 cm − 1 shows out-of-plane bending vibrations mode of C-H (not seen in nanocomposites) [29], the peak at 792 cm − 1 corresponds to wag of secondary aromatic amines, the peak at ~ 1296 cm − 1 is related to stretching of benzenoid C-N, vibrations at ~ 1485 cm − 1 and ~ 1566 cm − 1 are assigned to C = C and C-C stretching of benzenoid and quinoid rings respectively [28].The peak at 1044 cm − 1 actually shows the symmetric and asymmetric state of O = S = O and the presence of SO − 3 functional groups in p-TSA dopant [30].This peak is located in the middle of the wide peak from 1000 to 1140 cm − 1 .Also, peak of 1115 cm − 1 related to stretching S-O falling in the middle of 1000-1140 cm − 1 [30].
Characteristic peaks for a-Si:H also appeared in Fig. 2. The signatures at ~ 1020, 2100, and ~ 3400 cm − 1 are due to Si-O-Si stretching, Si-H stretching and O-H stretching, respectively [31][32][33].An influential factor in the electron fluctuation and dielectric properties of hydrogenated silicon [34], is the method and dilution of hydrogen used for passivation, which also affects the band gap increase [35].Using conventional bulk amorphous silicon hydrogenation methods such as CVD is simply not possible for amorphous silicon nanoparticles due to sintering of nanoparticles at high temperature or agglomeration of particles in wet solution [24].Therefore, by exposing the nanoparticles to concentrated HF vapor for a relatively long time, as shown by the stretching peak of Si-H at 2100 cm − 1 , it has been successful to qualitatively hydrogenate the surface of the amorphous silicon particles.
The peaks that are the result of the interaction of a-Si:H nanoparticles and PAni grains in nanocomposites are: Peaks at 561 cm − 1 and 656 cm − 1 express the stretching vibration of C-S bonds and 745 cm − 1 express the stretching vibration of S-O bond in p-TSA dopant [36] and their intensification in the spectra of nanocomposites confirms the connection of p-TSA from the SO − 3 side to the polyaniline chain and from the CH 3 side to the silicon nanoparticles surface.The weak and broad band from 1000 cm − 1 to 1140 cm − 1 characterizing C-H in-plane bending vibrations in polyaniline Fig. 2 FTIR spectra for PAni, a-Si:H, and some synthesized composites [37], which are actually formed during the protonation of the polyaniline chain and express the non-local state of electric charges, become sharp and intense in nanocomposites at wavenumber 1112 cm − 1 , which It means protonation of more number of molecules in the presence of a-Si:H and as a result bending of more number of C-H bonds [38].The NMP (n-methylpyrrolidin) peaks at 1296 and 1659 cm − 1 as solvent, which represent C-N stretch and C = N stretch, respectively, are also intensified in nanocomposites compared to pure polyaniline [39,40].Peaks at 1484 cm − 1 and 1564 cm − 1 , which in polyaniline show the C-C and C = C stretching vibration for benzenoid and quinoid, respectively, have shifted to smaller wavenumbers and become sharper in nanocomposites, which means that in blending, the C-C and C = C bonds in the polyaniline rings have reacted and vibrated under more energy (less wavelengths), which indicates the increase of unlocalized electrons resulting from closeness to surface of the a-Si:H particles.In fact, this interaction may increase the charge transfer between these compounds, and thus increase the degree of electron localization.That is, with the increase of these links, the conductivity of the sample will increase [41,42].The broad peak centered at 3400 cm − 1 indicated the stretching mode of N-H in the polyaniline chain and O-H on the a-Si:H surface [28,43].
As it is clear in Fig. 2, in nanocomposites, this peak slowly intensified with increasing %Wt. of a-Si:H particles because of poor and localized connection between PAni and a-Si:H particles.Due to the decrease in the total surface area and therefore less number of O-H bonds, this peak is weakened by the coarser a-Si:H particles.

PAni/a-Si:H Nanocomposites Structural Study
XRD patterns of synthesized PAni and a-Si:H which are in good accordance with the results in the literature [44,45], were obtained as shown in Fig. 3.
The tubular PAni presented two broad peak centered at 2θ = 20.425° and 2θ = 25.54° which can be ascribed to the periodicity parallel and perpendicular to the chains of PAni, respectively [46].Also as one can see, a-Si:H nanoparticles spectra demonstrate the broad peak at 2θ = 21.135°-22.485°deals with amorphous silicon.The spectra of PAni/a-Si:H nanocomposites have merged the peaks of both a-Si:H and PAni, which shows that these two components are separately blended in the nanocomposite structure according to Fig. 4. The reason for the reduce in intensity of the XRD dominant peak in composites compared to pure PAni when the amount and size of silicon particles changes, seems to be the increase in the ratio of the completely amorphous structure of a-Si:H to the semi-amorphous structure of PAni in the composites.Figure 4A and B shows the structure of bone tissue (tubular) of polyaniline and amorphous silicon nano/micro particles respectively.Figure 4C shows that by dispersing silicon particles in polyaniline matrix, the tubular structure is still well preserved.EDS elemental mapping of B5 PAni/a-Si:H nanocomposite is given in Fig. 4D which shows the placement of silicon particles (in green) individually in the polyaniline matrix, without being encapsulated by it.
While the ratio of a-Si:H increases (Fig. 3a) and a-Si:H nanoparticle size deacreases (Fig. 3b), the crystallinity index (CI %) of the composites, which was calculated according to Eq. 1 [46,47], decreased due to the decrease in the intensity and sharpness of the peaks, which is due to the gradual removal of the characteristic peaks of PAni and the increase in the amorphousness of the composites structure in accordance with the structure of pure a-Si:H.Also, the d-spacing (d) which was calculated according to Eq. 2 and the inter-chain separation (R) which was calculated according to Eq. 3, did not show a meaningful and noticeable relationship with the changes in the amount and size of a-Si:H nanoparticles in nanocomposites because these parameters are only related to the polyaniline component in the composite structure which before mixing with a-Si:H nanoparticles, its polymerization and crystallization ( 1) Area of most intense peak The total area of the peaks × 100 have been completed.Thus the difference in the obtained values in Table 2, especially with the increase of the amount of a-Si:H, may be due to the chance presence or absence of polyaniline crystalline parts with larger or smaller plane and inter-chain distances.

Influence of a-Si:H %Wt. On Bandgap of PAni/ a-Si:H Nanocomposites
Both PAni and a-Si:H are direct bandgap semiconductors, so Eq. 4 was used with r of 1/2 to calculate the bandgap of the samples using Tauc method [48].
Here, α is the absorption coefficient, h is Planck constant, υ is frequency of photons, β is a constant called the band tailing parameter, and E g is band gap of energy and r is power factor of transition mode equal to 1/2, 2, 3/2 and 3 respectively for allowed direct band gap, allowed indirect band gap, direct forbidden transition, and indirect forbidden transition [49].
Although amorphous silicon with a bandgap of about 1.7 eV has a slightly larger bandgap compared to crystalline silicon and does not have the ability to absorb light photons below 1.7 eV, due to the steeper slope of the absorption coefficient and therefore the average absorption coefficient is higher at shorter wavelengths (photons with higher energy) can absorb well in smaller thicknesses than crystalline silicon [50].This is one of the reasons for using its submicron particles in this research, along with PAni, in making a thin active layer.The measured UV-Vis absorption spectra of PAni, a-Si:H, and a number of synthesized composites along with the band gaps calculated by Tauc method plots are shown in Fig. 5. Hydrogenation of amorphous silicon improves the lifetime of charge carriers by elimination of gap states.Indeed replacement of weak Si-Si bonds forming the top of the valence band by inclusion of hydrogen to form more stable (thus energetically deeper) Si-H bonds widens the bandgap by lowering the valence band [35].Therefore, despite this increase in bandgap, which can be an unfavorable factor for using hydrogenated silicon as an active layer of a solar cell, the lowering of the silicon valence band in the energy level diagram of Fig. 1 improves the cascade energy alignment.
As can be seen from Fig. 5, the absorption spectrum of composites is affected by the spectrum of PAni and a-Si:H, and their band gap is between the band gap of PAni (2.98 eV) and a-Si:H (1.69 eV).This characterization showed that the bandgap decreases steadily with increasing silicon ratio.Also, as expected [51], the bandgap of the samples containing  Another result that can be seen in these graphs is the shift of the polaron peaks of the composites spectrum to the red light regions with increasing a-Si:H ratio, which is the result of the strong absorption effect of a-Si:H in these wavelengths.

Impact of a-Si:H Particles %Wt. on Photovoltaic Cell Performance of PAni/a-Si:H
The current density versus voltage (J-V) of the devices fabricated using PAni / a-Si:H nanocomposites as active layer characterized under 100 mW cm − 2 simulated AM 1.5 G solar simulator.(J-V) curves of some Table 1 samples are shown in Fig. 6. and the corresponding device parameters (open circuit voltage, short circuit current, fill factor, and power conversion efficiency) are summarized for result and discussion in Table 3.
The curves in Fig. 6 and the data in Table 3 showed that by adding only 5%Wt. of a-Si:H particles, the PCE of polyaniline cell increased more than a thousand times from 0.001 to 1.23%.The increase in PCE of solar cells made with PAni / a-Si:H nanocomposites continued up to 2.0% for cells containing 40% by weight of a-Si:H particles.And then the further increase of a-Si:H particles weight percentage, led to a decreasing rate of efficiency.
In fact, up to 40% increase in a-Si:H, with the improvement of light absorption and reduction of the bandgap of the composites according to Fig. 5, which is the result of the presence of a-Si:H particles in the polyaniline matrix, the short circuit current J sc , which is dependent on light absorption (A) according to Eq. 5, increases significantly.This increase in J sc along with the improvement of the FF increases the PCE [52][53][54].
In this equation, int coll is the internal collection efficiency, A is a coefficient related to the light absorption, and J ph is the photon flux [55].
But an increase of more than 40% of a-Si:H particles weight percentage, which means a decrease in the content of polyaniline as a matrix for them, leads to the disconnection of a-Si:H particles.This increases the recombination and thus the loss of charge carriers and decreases the internal charge collection efficiency int coll and thus J sc .To better understand these phenomena, it was necessary to find the quality of light harvesting and charge collecting by performing IPCE and EIS characterizations, the results of which are presented and analyzed in the next section.

Photoelectric Properties of a-Si:H, PAni, and PAni/a-Si:H Nanocomposites
The incident photon to current efficiency (IPCE) measurement shows what percentage of the photons entering the solar cell at each wavelength were firstly absorbed and (5)  generate electrons ( int abs ) and secondly collected and reached the external circuit ( int coll ).Therefore, any structural change of the active layer material, such as point, linear, surface and volume crystallographic defects, stacking fault, interface boundaries, dangling bonds, and trap states, etc., which cause disturbances in these efficiencies, changes the IPCE spectrum [56].As Fig. 7 shows, in the wavelength range of 400-550 nm, the low absorption of polyaniline (according to the graphs in Fig. 5) and therefore the reduction of int abs and in the wavelength range of 550-650 nm, the recombination and loss of electrons due to the presence of trap energy states of amorphous silicon particles and therefore the decrease of int coll , they all cause a decrease in IPCE in wavelengths of 400-650 nm.
According to Fig. 7a. the drop in absorption efficiency ( int abs ) is more severe in samples containing more PAni %Wt., but in composites containing more a-Si:H particles, in general, the drop in the average charge collection efficiency ( int coll ) at all wavelengths due to the increase in trap states causes a decrease in integrated photocurrent density, therefore, Jsc and as a result, PCE decreases.
To confirm this conclusion, it is helpful to pay attention to the result of electrochemical impedance spectroscopy (EIS) characterization in Fig. 8.The surface area under the EIS curve, in a way, expresses the sum of the electrical resistances in the path of the charge carriers in the equivalent circuit of the fabricated solar cell.In devices that have comparable resistances of different layers, several semicircle appear in the EIS curve, but in devices like the cells of this research, where the resistance of other layers is negligible compared to the impedance of the active layer, only one semicircle appears [57].As can be seen from Fig. 8, with the increase in the silicon %Wt. in the composites, the impedance increased because in the equivalent circuit, the number of series resistors (equivalent to the bulk resistance of the silicon particle or polyaniline grain in the carrier path) and double layer capacitors (equivalent to the transfer charge resistance at the boundary of two phases) has increased and as a result the recombination rate has increased.By increasing the silicon %Wt. to more than 40%Wt., the sharp increase in shunt paths resulting from the creation of nonuniform active layer at the junction with ZnO and Ag layers has caused a sharp decrease in parallel resistance [58].This has synergistically increased the resistance of the device.
Although, as mentioned before, changing the size of a-Si:H particles in the range of this research (200-1000 nm) did not have a significant effect on the bandgap of the composites, but according to Fig. 7b.and the data in Table 2, finer a-Si:H particles had little effect on improving device performance.Two phenomena simultaneously conflict in this small change in efficiency as the a-Si:H particles become finer: first, increased recombination due to increased defects and broken bonds resulting from the increased total surface area in finer particles [59].Second, the reduction of the probability of recombination due to the increase in int coll Fig. 7 External quantum efficiency, EQE (IPCE) curves of devices based on synthesized composites a with different %Wt. of a-Si:H and, b with different a-Si:H particle sizes Fig. 8 Nyquist plots of PAni and a number of synthesized composites cell at zero applied biase.inset is the magnification of the selected area at left side of plot resulting from the decrease in the transport length of carriers in finer particles [60].It seems that in this confrontation, the second one was the dominant phenomenon.

The Overall Influence of a-Si:H Weight Percent on Photovoltaic Cells
In summary, the influence of increasing the weight percentage of amorphous silicon particles in solar cells containing the hybrid active layer consisting of polyaniline/amorphous silicon, according to all the topics and characterizations raised in this article, is included in Fig. 9.
In general, the increase of a-Si:H particles up to 40% by weight was simultaneously accompanied by a sharp decrease in bandgap and a weak increase in resistance (in fact, the result of bulk resistance and charge transfer resistance, which are synonymous with recombination), which caused the efficiency of the solar cell composed of this composite active layer has an increasing trend and enhanced from 0.0011 to 2%.Then, by increasing the a-Si:H %Wt. to more than 40%, the effect of strong increase in resistance and recombination, overcame the trend of decreasing bandgap and thus increasing light absorption, and the efficiency started to drop.Also it seems by optimizing the size of the a-Si:H particles in dimensions smaller than the sizes of this research [61] and creating a better interaction between the a-Si:H particles and the PAni matrix [23], better efficiencies were achieved which is a new window for using amorphous silicon in polyaniline matrix.

Conclusion
In this work, polyaniline/amorphous silicon (PAni/a-Si:H) nanocomposites with different weight percent and particle size of silicone were successfully synthesized.We determined that these two materials are generally synergistic in terms of band gap.Then these composites were successfully applied as the active layer in the solar cell.Our approach was to synthesize PAni with tubular morphology and introduce a-Si:H particles in this matrix via blending process, which finally result in tubular morphology.As a result of increasing a-Si:H %Wt. up to the optimum point of 40%, the efficiency of the cell improved about 2 K times compared to the reference cell, and then with the further increase of a-Si:H %Wt., the efficiency of cell decreased.This behavior is due to two mutual phenomena: 1-increasing light absorption (decreasing bandgap) due to increasing the content of a-Si:H, which inherently has a small bandgap, and 2-increasing recombination (resistance) due to increasing the amount of a-Si:H %Wt., which is laden with structural defects.Initially, the first phenomenon is dominant, and then, from 40%Wt.onwards, the second phenomenon gradually dominates.The size of a-Si:H particles in the range of 200-1000 nm did not have a significant effect on the bandgap of the nanocomposites, but finer a-Si:H particles in this range, slightly improved device efficiency.It seems that the hybrid of a-Si:H and PAni can be very useful as an active layer due to the issues of bandgap.This feasibility study showed that control of several parameters is required to open a new gate for the application of amorphous silicon /polyaniline composites in flexible solar cells.Consent to Participate All authors have been personally and actively involved in substantial work leading to the paper, and will take public responsibility for its content.

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All authors whose names appear on the submission.
1) Made substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data; or the creation of new software used in the work; 2) drafted the work or revised it critically for important intellectual content; 3) Approved the version to be published; and.4) Agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Fig. 1
Fig. 1 Structure and energy level of the fabricated hybrid BHJ device based on PolyAniline/a-Si:H as active layer

Fig. 3
Fig. 3 XRD patterns of PAni, a-Si:H and a Composites with different %Wt. of a-Si:H, and b Composites with different a-Si:H particle sizes

Fig. 4
Fig. 4 FESEM images of A PAni, B a-Si:H particles, C PAni/a-Si:H nanocomposite, and D EDS Elemental X-map of B5 PAni/a-Si:H nanocomposite.Inset histogram is DLS particle size distribution of 500 nm a-Si particles

Fig. 5
Fig. 5 UV-Visible absorption spectra of PAni, a-Si:H, and different %Wt. of a-Si:H synthesized nanocomposites.Inset graphs are Tauc method plots of band gap determination

Fig. 6 J
Fig. 6 J-V characteristic diagrams of PAni and PAni/a-Si:H based hybrid solar cells.Inset zoom graph shows the scope of the PAni area

Fig. 9
Fig.9 Influence of a-Si:H %Wt. on the bandgap (as a representative of light absorption parameters and charge carrier generation) and resistance (as a representative of charge carrier loss and recombination parameters) and in general on the power conversion efficiency of the hybrid PAni/a-Si:H solar cell (as the result of these two phenomena).The hatched area shows the optimal combination of a-Si:H and PAni in terms of charge generation and recombination to make a photovoltaic cell containing them as an active layer

Table 1
Summary and coding of cell parameters studied in this work