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Simulation approach to reach the SQ limit in CIGS-based dual-heterojunction solar cell

In this article, we demonstrate the design and simulation of a highly-efficient n-CdS/p-CIGS/p+-CGS dual heterojunction solar cell. The simulation was performed using SCAPS-1D software with reported experimental physical parameters. The simulation performance of our proposed design arises 47% with Voc=0.98 V, Jsc=59.94 mA/cm2 and FF=80.07%, respectively. The high short circuit current and hence the high efficiency is predominantly originated from the longer wavelength absorption of photon through a tail-states-assisted two-step upconversion in dual heterojunction (DH) and thus reaches the SQ detailed balance limit of DH solar cell.

in almost all countries in the world, solar photovoltaics has been found as a prominent, cost effective and eco-friendly way to become an alternative for generation of energy.
Recently, most of the photovoltaic explorations are being focused on obtaining high efficient devices at low production cost to meet up the industrial need. Though different investigations on first generation to recent fourth generation solar cells prioritize in crossing up the highest conversion efficiency, new structures are essential to be proposed to obtain a remarkable value [1]. Such process requires effort on choosing proper materials and introduction of a novel design with proper integration of those materials in corresponding layers [2][3].
According to the Shockley-Queisser detailed-balance theory, the efficiency limit of a dualheterojunction solar cell is 42-46% [4][5][6][7]. Recently, Marti et al. have reported in a theory that a three-terminal dual-heterojunction bipolar transistor solar cell can have an efficiency of about 54.7% [8]. All of these theories consider wider band gap (1.54-1.8 eV) top layer followed by a lower band gap (0.6-1.1 eV) bottom layer. However, there is no report of such high efficiency solar cell in practice or even in simulation with dual heterojunction structure. Furthermore, the SQ efficiency limit for the intermediate band solar cell is about 63% that essentially works in two-step photon upconversion [9][10]. On the other hand, the impurity photovoltaic (IPV) which essentially enhance the photocurrent due to the absorption of low energy sub-band photon by two-step photon upconversion theoretically offers an efficiency of 77.2% [11]. However, a simulation result on Si IPV solar cell shows an efficiency of 30.5% [12].
Herein, we demonstrate the design and simulation of a high-efficiency dual-heterojunction solar cell that embeds a relatively wider band gap (1.66 eV), CuGaSe2 (CGS) in the bottom of low band gap (1.1 eV), Cu(In,Ga)Se2 (CIGS) layer. It has been observed that the efficiency of the proposed dual-heterojunction structure can reach the theoretical efficiency limit, at least in simulation as no practical device has yet been developed on this framework, as the wide band gap with longer wavelength absorption and adequate doping forms almost Type-II heterojunction with comparatively lower band gap absorber layer on its top [13].
In thin film photovoltaic devices, CIGS-based thin film solar cell has appeared as one of the promising absorber layer candidates with suitable bandgap for solar spectrum absorption and significant rise in efficiency. It has been found with the experimental efficiency to fall in the range of 19-23% [14][15][16][17], while its efficiency in numerical analysis lies in between 22-26% [18][19]. The efficiency of the CIGS based cells has been tried to increase with application of SnS, MoSe2, SnSe2, and BaSi2 in BSF layer [19][20][21][22][23]. On the other hand, CGS is a wide band gap (1.5-2.2 eV) p-type semiconductor that has been employed as an absorber layer in CGSbased solar cells [24][25][26] and top cell in CGS/CIGS tandem solar cells [27][28]. However, there is hardly any report of the use of CGS as BSF layer in the CIGS-based thin film technologies for solar power generation.
In this work, we devise CGS as BSF layer for the improvement in the efficiency of the CIGS thin film solar cell. Here, CGS also act as bottom cell in the proposed CIGS-based n-CdS/p-CIGS/p + -CGS dual-heterojunction solar layout. The performance of the designed solar cell has been calculated on the optimized device structure by SCAPS-1D simulator. The schematic structure and energy band diagram of the modeled n-CdS/p-CIGS/p + -CGS dual heterojunction solar cell are presented in Fig. 1(a-b), respectively. In this design, light passes through Indium tin oxide (ITO) coated glass substrate and n-type CdS that acts as a window layer. ITO is a popular material for substrate in heterojunction cells due to outstanding transparency [1]. The lights are absorbed by p-type CIGS and then outstretch at Our deigned highly efficient solar cell was numerically simulated using a one-dimensional solar cell capacitance simulator (SCAPS-1D) [29] under the illumination of one sun of 100 mW cm -2 at global air mass AM 1.5G spectrum and selection of temperature at 300 K. The simulation was accomplished considering ideal values of series and shunt resistances.

Proposed cell design and simulation model
Gaussian energetic distribution was taken into consideration for single acceptor and donor defects in bulk materials as well as at the absorber interfaces with window and BSF layers.
However, radiative recombination coefficient was ignored. To perform the program, absorption coefficient (α) data for p + CGS BSF layer were collected from literature [30].The similar results were also obtained from absorption data measured in other studies [31][32]. Table 1 shows the parameters for different layers used in this simulation.   which is shown in Fig. 2(b). It is observed that the short circuit current shows almost a constant behavior throughout the scale. However, a sudden drop in efficiency from around 47% to 43% is observed between 1 × 10 16  The cell performance has great dependency on bulk defects of CIGS absorber layer which is visualized in Fig. 2

Impression of window layer on PV parameters
The influence of window layer on PV parameter performance of n-CdS/p-CIGS/p + -CGS dual heterojunction solar cell has been explored on this step and results are displayed in Fig. 3

Impact of CGS bottom absorber as well as BSF layer on PV parameters
The impact CGS bottom layer along with the effect of absorption of longer wavelength photons in solar spectrum have been studied and ensured during simulation in this section.  Fig. 4(b), it is noticed that the short circuit current sinks with addition of order of doping concentration which in turn playing role in drop in percentage of efficiency [11][12][13]. With higher doping concentration the parasitic absorption can occur at CGS layer which drains out the cell performance. From Fig.4(c) it can be ensured that the PV parameters are totally independent on bulk defects of thin CGS.
A contour plot for quantum efficiency (QE) as function of thickness and doping concentration at constant photon wavelength of 1100 nm is demonstrated in Fig. 4(d). It is clear from the figure that a climbing in efficiency occurs with increase in thickness up to a certain doping concentration. Here, CGS layer acts as second absorber layer as well as BSF layer, which increases photon absorption with thickness. The maximum QE% has been found up to acceptor concentration level of 10 18 cm -3 . Above this level QE% is seen to decline with addition of acceptor impurities. Thus, an optimized value could be selected below doping concentration of 10 18 cm -3 with CGS layer thickness of 0.4 µm to achieve quantum efficiency above 80%.

Tail-States-Assisted (TSA) two-step photon upconversion
The longer wavelength photon absorption in the second wide band gap absorber layer can be explained considering a two-step photon up-conversion process. The photon absorption and hence contribution to the photocurrent occurs when the materials are adequately doped and it has high absorption co-efficient (α) in the longer wavelength region, i.e. 10 3 cm -1 an wavelength of 1800 nm in this case. If we consider the first and second absorber layers have band gaps of 1 and 2 , respectively, the most of the photon with energy ≥ 1 will be absorb in the first absorber layer. Then few high energy photon as well as photons with energy ≤ 1 will enter the second absorber layer. In convention, these photons will not be absorbed in the second absorber layer as these have energies < 2 , even if they are absorbed they will create one free carrier and consequently contribute to the parasitic current [36]. However, these low energy photons may contribute to as a part of two-step photon upconversion process where absorption of two photons in sequence will generate one electron-hole pair and thus contribute to the photocurrent [36][37]. In fact, up-conversion process has been proposed to further improve the efficiency of the solar cell [38][39][40]. Here, we propose the tail states-assisted (TSA) photon upconversion in dual-heterojunction where upconversion occurs in the wide band gap bottom layer. The band tails states are created close to the conduction or valence band in the heavily doped n or p type semiconductor or when the materials have poor crystallinity [36][37]41]. These states will contribute to the twostep photon upcoversion process. Fig. 5 shows the mechanism involved in the TSA upconversion process. The figures consider three possible cases in p-type semiconductor to explain the mechanism. It is seen in the figures that a number of states at different levels exists in the band gap of the layer. When a photon with energy ≥ 2 enters into the layer it will be immediately absorbed by the layer and create e-h pair. A photon with low energy will be absorbed and generated an electron that will reach the tail state and this electron will absorb a second high energy photon to reach the conduction band, while it has leave a hole in the valance band. Thus an e-h pair is generated.
When a photon with relatively high energy enters the layer it will be absorbed in the layer and an electron will be reached at the top of the tail state leaving a hole in the valance band, where it will absorb a low energy photon to reach the conduction band. This will also generate an e-h pair.
Therefore, by this tail-assisted two-step upconversion process almost all longer wavelength photons that enter the second absorber layer are absorbed. However, the amount of photon absorption will depend on the absorption co-efficient of the layer and the extension of the tail states that may depend on the doping and crystallinity states of the materials.

Optimized photovoltaic parameters and quantum efficiency
The simulated J-V and QE curves, respectively of n-CdS/p-CIGS/p + -CGS dual heterojunction solar cell with and without CGS layer are exhibited in Fig. 6(a-b). It is seen that the JSC is enhanced around two times in the dual-heterojunction than in the singleheterojunction due to the addition of CGS layer. The Voc of the DH structure increases by 0.24 V which is mainly originated due to the high built-in potential generated at the CIGS/CGS interface [35,[42][43]. In Fig. 6(b), the quantum efficiency for dual-heterojunction is found around 80% in longer wavelength region whereas in single heterojunction the cell i.e. in absence of CGS the structure seems not to perform energy conversion of impinged solar spectra. Such phenomena represent the fact behind increment of photo-conversion efficiency in dual-heterojunction to 47%.

Conclusion
CIGS-based dual-heterojunction solar cells have been designed and simulated. Based on simulations in SCAPS-1D software, we tried to optimize thickness and doping concentration along with considering bulk and interface defects to reach the SQ limit in dual-heterojunction.
The high photocurrent and hence the higher efficiency of the solar cell has been tried to explain using Tail-states-assisted (TSA) two-step photon upconversion that describes the absorption of longer wavelength of solar spectra. The device provides a PCE of 47% with a 0.5 µm thick absorber layer having a doping concentration of 10 16 cm -3 , and 0.4 µm thick CGS BSF layer having a doping of 10 18 cm -3 .