Keywords

1 Introduction

Nano-structure that is being used for the infrared solar cell is advantageous regarding cost and size, yielding better efficiency for the solar cell [1,2,3,4,5,6,7,8,9]. These nanowires and nanorods are able to accumulate charge carriers more efficiently for various solar cells [4,5,6]. Absorption of photons for converting it to charge carriers effectively is used in third-generation solar cells [7]. Since we are dealing with infrared solar cells, solar cells should be able to operate in various temperatures [3,4,5,6, 10]. Further investigation has shown that nanorod arrays have that advantages for the optimal light absorption and photon conversion as compared to the conventional solar cell with single-crystal bulk semiconductors when the same materials is used [8, 9]. It is to be noted that a combination of nanosturctures and single crystal bulk semiconductor-based solar cells leads to decrease in efficiency and poor stability [1].

In this paper, we are implementing a 3D coaxial infrared solar cell having three layers, namely positive type, intrinsic type and negative type layers. The three layers mentioned above will be forming the (p-i-n) layout. Recently, Si-nanowires-based solar cells are of major concern in the research area, and our paper is compared to that of the experimental data obtained for the Si-nanowires-based solar cells [11]. Si is vastly used for making solar cells owing to its properties such as abundant availability and developed manufacturing method. However, its indirect band gap gives rise to certain limitation as it is unable to harness maximum solar energy due to phonon emission. The power conversion efficiency has increased within these last five years, and in this article, we will be modeling a nanowire-based solar cell which will work mostly in the infrared region. An efficiency of 5.6% can be obtained for GaSb nanowire solar cell under 1 sun. With the use of nanopillars, we are able to obtain large interface area and cascading of nanopillars will increase the overall efficiency of the solar cell. It can act as an alternative renewable energy fuel for exploring outer space as GaSb works in the infrared region. GaSb solar cell can be used further for understating the working of electron and holes generation optically for energy conversion and artificial photosynthesis [12]. Coaxial GaSb nanowire solar cell can be used for detecting low light level photons [13]. Nanowire technology is used in various photonic application [14, 15].

From Fig. 1, we are able to observe the arrangement of the positive type, intrinsic and negative type which make up the 3D coaxial solar cell [16, 17]. N-type AlGaSb crowned the n-type GaSb and p-type GaSb. AlGaSb reduces the band gap with rise in temperature and short-circuit current decrease in a nonlinear trend [18]. GaSb has some desirable properties when working in higher temperature as photo-generated current increases with the decrement in band gap [19]. This type of structure made with GaSb and AlGaSb is beneficial as charge separation takes place radially instead of axial direction which in turn gives advantages to the photon-induced carriers to reach the p-i-n junction easily as compared to the bulk crystal structure [8]. When the 3D co-axial infrared solar cell is illuminated, regular charge distribution occurs radially as the voltage across the shell is reduced due to highly conductive n-shell [8]. Nanowire is advantageous over thin-film solar cells regarding reduced reflection, more light absorption, band gap flexibility, cost of manufacturing and least problem with defects [20].

Fig. 1
figure 1

Simulation scheme of a coaxial nanowire

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2 Photovoltaic Simulation

2.1 Nanowire Modeling

The device structure depicted in Fig. 1 is simulated using ATLAS in cylindrical coordinate system with the outer n-type radial size of 190 nm. The intrinsic type has a maximum radius of 160 nm and that of p-type having a maximum of 80 nm. Thus, the overall thickness of the nanowire is 380 nm, and it is similar to the nanowire give in structure of the paper with smaller length [2].

Drift diffusion model is being used for calculating the carrier movement, considering the smaller length of the nanowire [13, 14]. It takes less amount of time for simulation if the nanowire is of smaller length. We use Shockley–Read–Hall recombination and concentration-dependent mobility for evaluating the recombination and mobility. We need to check whether quantum effect is present during our simulation. Poisson-Schrodinger equations self-consistently are used for quantum effect check, and later, we found it to be very small. Self-consistent calculation is confirmed with the first bound state having a position of 1 meV just above the conduction band when infinite potential well approximation is implemented. The dark current–voltage relation was attained initially, after which the structure is illuminated with AM 1.5 Global solar irradiance. Coaxial nanowires parameters such as potential and the band energy were obtained self-consistently. The spectrum of the solar illumination using multi-spectral photogeneration in the simulator is given in Fig. 2.

Fig. 2
figure 2

AM1.5G spectrum used to illuminate the model

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Isc is set to pico Ampere range by scaling the power of the solar spectrum by 1:5 × 10–3. The generation rate is calculated from the equation as below:

$$G={\eta }_{o}\frac{P\lambda }{hc}{\alpha e}^{-\alpha y}$$

where G is the photogeneration rate, P is the cumulative effects, transmissions and loss due to absorption over the ray path, y is the relative distance for the given ray, h is the Planck’s constant, λ is the wavelength, c is the speed of light and α is the absorption coefficient calculated for each set of (n, k) value. Absorption coefficient is given by the total number of photons absorbed by a specific wavelength (λ) falling on it. This particular property is constant for the given material. The absorption coefficient in terms of the extinction coefficient is given by:

$$\mathrm{\alpha }\left(\uplambda \right)=\frac{4\pi k}{\lambda }{10}^{7}{\mathrm{cm}}^{-1}$$

where λ is in Nano meter.

The simulated nanowire structure is being illuminated with an air mass of 1.5 global and various profiles can be obtained as shown in Figs. 3, 4, 5 and 6. Photogeneration rates can be obtained by considering the different legends expressed in the log of the electron–hole pair generation rate with respect to the color-coded display. From the legions, it is evident that more photo generation rates are occurring at upper solar layer as it has the advantages to absorb photons earlier than the inside layer.

Fig. 3
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Electric field profile

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Fig. 4
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Potential-generated profile

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Fig. 5
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Net doping profile

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Fig. 6
figure 6

Recombination rate profile

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2.2 TCAD Simulation

Charges are produced in a nanowire when photons fall on it. The charges are pairs of electrons and holes which has different movement paths radially. Electrons flow outward from the center, whereas holes flow towards the center of the nanowire. With the movement of the carriers, it can produce power in the range of a few nano Watt. In order for the output power to drive external circuit, we need to either increase the light intensity or cascade various nanowires. Practically, the illumination is one directional, and the photogeneration rate along the radial direction has been plotted. For the nanowire to absorb the photon in all the direction, simulation is performed with all round illumination using three dimension simulation (Table 1).

Table 1 Standard major parameters used in our model
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3 Result and Discussion

The nanowire is being illuminated with air mass 1.5 G and various photovoltaic properties of p-i-n coaxial GaSb are obtained as shown in Fig. 7. Even though the solar cell is mainly focused on the infrared spectrum, we were able to obtain an open-circuit voltage, Voc of 0.34 V under 100 suns, a short-circuit current (Isc) of 0.31 nA under 1 sun and a fill factor of 71.0%. A maximum efficiency of 5.6% can be obtained as the output of the nanowire under 1 sun.

Fig. 7
figure 7

I–V characteristic under AM1.5G illumination and dark current

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Voc remains independent on the length of the p-i-n Si nanowire but Isc has linear scaling along the Si nanowire length. This short-circuit current scaling indicates that induced charge carriers are uniformly collected along the radial structures and metal contact’s light scattering does not contribute to the Isc (Table 2).

Table 2 Output parameters obtained from the simulated model
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The simulated model can be further improved so that the upper and lower bound efficiency can be increased with the usage of vertical integration or multi-layer stacking [8]. Photo-generated carriers have systematic increment when the light is incident on the nanowire with linear increment of open-circuit voltages and logarithmic increment of short-circuit current [21]. When the model is illuminated with multiple suns, efficiency increases considerably.

The infrared region of the spectrum can be absorbed more effectively with the introduction of intrinsic layer in GaSb-based solar cell. When the GaSb is based on p-i-n structure, more charge carriers can be generated as compared to normal diode-based solar cell, as it minimizes surface recombination [16]. GaSb-based solar cell can convert heat generated by the infrared spectrum to electrical power in an efficient way [10]. We can note that most of the energy from the solar radiation is found within the infrared region at wavelengths longer than 1 micron, but silicon-based solar cell responds to wavelengths shorter than 1.1 micron [10]. This gives the advantages for GaSb nanowire over Si nanowire in infrared region.

In Fig. 7, we are able to observe the i–v characteristics under illumination and dark current. Cathode current of the simulated model has 0.04 nA in Fig. 7 as compared to the 0.02 nA of the Si nanowire model when same voltages are applied. Cathode current increase when the model is illuminated with 100 suns.

4 Conclusions

GaSb-based infrared solar cell has been modeled using TCAD, and results are verified after running the simulation. With this work, we are able to simulate and model infrared solar cell, without actual product which gives a better option for testing different model. The details of the model can be analyzed with the 3D structure. When the number of suns illuminating increases, GaSb nanowire performs better than Si nanowire. The output power can be increased in future work by connecting co-axial nanowires together to form nanopillar array PV system. In the nano photovoltaic study, the methodology used in this work and the precise details that can be obtained from the 3D structure can be beneficial for further research and application. It is found from other literature that such a model is able to provide power in the range of nano Watt for nanoelectronics devices.