Numerical Investigation of Cu 2 O as Hole transport layer for High-Efficiency CIGS solar cell

: Copper indium gallium selenide (CIGS) is an inexpensive material that has the potential to dominate the next-generation photovoltaic (PV) industry. Here we detail computational investigation of CIGS solar cell with encouragement of adopting cuprous dioxide (Cu 2 O) as a Hole Transport Layer (HTL) for efficient fabricated CIGS solar cells. Although Cu 2 O as a HTL has been studied earlier for perovskite and other organic/inorganic solar cell yet no study has been detailed on potential application of Cu 2 O for CIGS solar cells. With the proposed architecture, recombination losses are fairly reduced at the back contact and contribute to enhanced photo-current generation. With the introduction of Cu 2 O, the overall cell efficiency is increased to 26.63%. The wide-band of Cu 2 O pulls holes from the CIGS absorber which allows smoother extraction of holes with experiencing lesser resistance. Further, it was also inferred that, HTL also improves the quantum efficiency (QE) for photons with large wavelengths thus increases the cell operating spectrum.


Introduction
Thin-film copper indium gallium di-selenide [Cu(In,Ga)Se2] or CIGSe technology is considered the most promising nextgeneration photovoltaic (PV) technology due to its direct bandgap (~1.0-1.14 eV), high absorption coefficient (~10 5 cm -1 ), and lower material losses [1][2][3][4][5][6]. CIGS cells have the highest power conversion efficiency as compared to other thin-film technologies like Cuchalcopyrite, cadmium telluride (CdTe), and amorphous silicon (a-Si), with a certified record efficiency of 23.40% for lab-scale cells [7]. CIGS material is inexpensive and can deposit on any rigid or flexible substrate which increases its applications in commercial and wearables devices as well [8][9][10]. CIGS solar cells exhibit outstanding stability, high efficiencies and radiation tolerance hence can also be used for space application [11].
CIGS is a direct bandgap p-type semiconductor that minimizes the requirement of large diffusion length and also decreases losses. Such semiconductor with high absorption co-efficient is ideal for thin-film PV technology [12]. The tunable bandgap of ptype CuIn1-xGaxSe2 can be adjusted by varying the concentration of gallium and indium, to match the solar spectrum [13]. A thicker CIGS layer means more material which results in higher cost, particularly indium which is a costly material and its availability will be an issue in the future [14]. Aluminum (Al)/Nickle (Ni) and molybdenum (Mo) act as front and rear contact for the cell while Magnesium fluoride (MgF2) acts as an anti-reflecting coating. Mo deposited on substrates offers lower electrical resistance (< 0.5 Ω/sq.), higher thermal stability, and also less resistance to p-CIGSe by forming a thin layer of molybdenum di-selenide (MoSe2) during the absorber formation [15].
The optimum band gap for CIGS is ~1.14 eV due to material properties [16,17]. In general, for efficient absorption, a bandgap of ~1.4eV is required to match the solar spectrum. In CIGS crystal Indium can replaced by Gallium to achieve the required optical bandgap, hence by the introduction of Indium the bandgap of CIGS can be varied from 1.04 eV to 1.7 eV. Other elements can also be used in chalcopyrite Cu(In,Ga,Al)(S,Se)2 , this can vary bandgap from 1.04 eV (CuInSe2) to about 3.5 eV (CuAlS2) covering most of the visible spectrum. However, for CIGS material a higher bandgap causes complications. The efficiency of the cell decreases because of defect states and dangling bonds. For a wider bandgap, the defect level is in the middle of the bandgap thus increasing the bandgap also increases defect levels [18]. Fig. 1 [19] depicts a scanning electron microscope image (SEM) of a CIGS cell. In this substrate configuration, the light enters through a transparent conductive oxide (TCO) i.e., zinc oxide (ZnO) layer. The working of CIGS cell is as follows: n-type cadmium sulfide (CdS) buffer layer (bandgap, E g~2 . 4 ) absorbs the higher energy photons ≥ 2.4 , while photons of energy less than 2.4 are transmitted to the p-type CIGS absorber layer where electronhole pairs are mainly generated. Due to in-built junction voltage of n-CdS/p-CIGS, the electrons within the diffusion length are carried away from the p-CIGS layer to the n-CdS layer and are collected at the n-type electrode. Similarly, holes are carried away from the n-type buffer layer to the p-type absorber layer and are collected at the p-type electrode. A highly transparent, large bandgap (>3.3 eV) n-type TCO layer i.e., ZnO act as front contact over CdS called window layer. Typically, ZnO (E g~3 . 2 ) the layer consists of a double layer of a thick (400nm) layer of n-doped ZnO layer and a thin (50nm) intrinsic ZnO. The intrinsic-zinc oxide (i-ZnO) provides isolation between CdS and n-doped ZnO (ZnO: Al) so that i-ZnO prevents the diffusion of Al into the CdS/CIGS absorber layer.
The buffer layer acts as an intermediate layer between the window layer and absorber layer which provides stability [20,21] . The buffer layer is highly-resistive and it prevents shunting between the absorber layer and TCO i.e., ZnO. The buffer layer affects the band offset and electric field in the junction, and thus improves the charge transport [22]. CdS is the most common material used as a buffer layer. The narrow bandgap of CdS causes current losses due to parasitic absorption. Additionally, cadmium (Cd) is a toxic element and is classified as a human carcinogen. For these reasons, extensive research is being done on the alternative buffer layer, such as Zinc sulfides, oxides, and selenides [23][24][25][26]. Like CdS, zinc selenide (ZnSe) is an n-type semiconductor but with a wide bandgap and the most promising material to replace CdS [27][28][29][30]. The bandgap of ZnSe is E g~2 . 9 which is greater than the CdS E g~2 . 4 , that means more light is passed to the CIGS absorber layer and the larger number of photocarriers will be generated increasing efficiency.
This research effort proposes a theoretical approach to enhance the power conversion efficiency (PCE) of the CIGS solar cells with structure Mo/Cu2O/CIGS/buffer layer/intrinsic-i-ZnO/n-doped ZnO. The CIGS modeling and analyses are done by using a solar cell capacitance simulator (SCAPS-1D) that gives simulation results close to fabrication results [21,[31][32][33][34][35]. Here CdS is replaced with is replaced by a double buffer layer CdS/ZnSe. In addition to the double buffer layer, the Cu2O is implemented as the hole transport layer (HTL). Cu2O is an earth-abundant, non-toxic material and is easy to fabricate. The thickness of the CIGS absorber layer is reduced to 850nm which is usually about 3000nm. Under standard illumination (AM1.5 G, 100 mW/cm 2 , 300 K) [36], the photovoltaic parameters, short circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and efficiency (η) of different hole transport layers and buffer layers under varied thicknesses are measured.
Cu2O is a p-type semiconductor and has an optical bandgap of 2.17 eV. Its application as photovoltaic material lies in the fact that the constituent materials are nontoxic and abundantly available on the earth, and that the Cu2O has a high absorption coefficient in visible regions and low-cost producibility. Doping concentration in Cu2O can be achieve up to 10 17 cm 3 [37].
Cuprite Cu2O is known to have a low value of thermal expansion coefficient around room temperature, and to exhibit a negative thermal-expansion behavior at low temperatures. The thermal decomposition temperature of Cu2O exceeds 900 O C. So, for the high temperature processing of CIGS material, may not have any serious effect of the Cu2O layer [38]. Moreover, there is possibility of Cu diffusion in CIGS material at the rear end which can change the CIGS bandgap and its performance. This can be address by the grading process of CIGS structure and the exact bandgap of CIGS can be achieved from In/Ga ratio to compensate for Cu diffusion.
As compared to conventional CIGS cells the device with Cu2O as HTL gives greater output voltage. To optimize the light absorption and output current we put forward Cu2O as HTL for further enhancement.

Novelty
Theoretical studies show the significant increase in the efficiency of chalcopyrite based solar cells with the introduction of Ga2O3, MoO3, MoSe2 and MoS2 as back interface. MoO3 gives maximum efficiency of 24% among all due to its highest bandgap [39][40][41]. Bandgap of Cu2O make it suitable to work as back interface for CIGS solar cells. Although for perovskite and copper zinc tin sulfide (CZTS) extensive studies are present on Cu2O as HTL but for CIGS, it's a novel approach. Here different parameters of CIGS cells are observed using SCAPS. It is observed by numerical calculations that Cu2O increases the flow of charge carriers which greatly enhances the efficiency of CIGS cells. As compared to conventional CIGS cells the device with Cu2O as HTL gives greater output voltage. By using Cu2O recombination losses were reduced which results in high efficiency and stability. As Cu2O is a transparent and inexpensive inorganic material it does not increase the cost and complexity of cell fabrication.

Fig. 2 depicts the proposed CIGS solar cell with a p-type CIGS absorber layer and a thin layer of Mo deposited on a glass
substrate. An n-type ZnSe material acts as a buffer layer and n-ZnO acts as a window layer. The proposed structure is simulated on SCAPS-1D software which solves semiconductor equations, the equation of continuity, and the Poisson equation for electrons and holes. These equations are as follows [42]: Where is electrostatic potential, is electric charge, 0 , is the electric permittivity of vacuum and relative electric permittivity, ( ) and ( ) are the concentration of free holes and electrons, , ⅆ are ionized acceptor and donor concentration, ( ), ( ) are the concentration of trapped holes and electrons.
Where , is change in holes and electron concentration, is generation rate of holes and electrons, , is hole and electron lifetime, is diffusion coefficient, , is the mobility of holes and electrons and is the electric field. SCAPS numerically calculate a steady-state solution of these equations with appropriate boundary conditions [43].

Material Parameters
The basic parameters used for simulation for the proposed CIGS solar cell are shown in the Table 1. The thickness of the CIGS layer is 850 nm while the thickness of each buffer layer is varied from 20 nm to 100 nm at a time to observe the behavior and optimize thickness for maximum efficiency.

Effect of ZnSe layer thickness
As illustrated in Fig.3, the thickness of ZnSe was varied from 20nm to 100nm and its effect on basic photovoltaic parameters , FF, Voc, Jsc was observed. With the increase in the thickness of ZnSe, the output voltage Voc decreases gradually from 0.85003 to 0.84994 V, a decrease of 0.10% is observed while current density Jsc changes from 37.05 to 36.7 mA/cm 2 . From efficiency formula (4) and fill factor (5), efficiency decreases from 26.71 to 26.62% an overall decrease of about 1%, and FF increases from 84.84 to 85.35% an increase by .51%, As the thickness of the ZnSe layer increases the efficiency decreases which can be attributed to the carrier recombination. As thickness increases the recombination rate within the CIGS absorber layer due to the increase of conduction band edge which opposes the charge separation. This has little effect on overall device performance because it has a larger bandgap while a lesser number of carriers have little effect on the depletion region.

Fig.5 depicts the response of proposed absorption of incident photons and is represented as a quantum efficiency (QE) graph.
Here with the lower thickness of the CdS layer, QE is higher as compared to higher thickness at the same wavelength. Fig. 6 shows QE at different ZnSe thickness, which shows a similar response but here at a lower wavelength, the thickness of ZnSe has very little effect in QE. Fig.11. shows an increase in the efficiency of CIGS cells with the introduction of a thin layer of Cu2O as an HTL. Cu2O decreases the recombination of photo-generated carriers at the CIGS absorber layer thus increase the number of carriers that contribute to photo-current [46,47]. The wide-band of Cu2O reduces recombination losses and pulls holes from the CIGS absorber. This allows smoother extraction of holes with experiencing lower resistance. The proposed CIGS cell with HTL shows an increase in output, Voc, and Jsc as compared to cell without HTL and η increases from 21.04 to 26.63%, a total increase of 6.12% is recorded. This increase in efficiency can also be attributed to the p-type semiconductor nature of Cu2O. The high absorption coefficient of Cu2O allows to absorb photons and contribute in photocurrent as recombination in this region is minimum. The absorption in Cu2O can also be observed if the cell is simulated in inverted manner.

Optical Losses
For CIGS, the absorption coefficient, in the photon energy range ℎ < is = 4 / = (4-5) ×10 3 cm -1 [48]. For these values of α should result in a high value of quantum efficiency but this is not observed in this case. For > = ℎ ∕ the quantum efficiency decreases drastically to zero within a range of 100 nm above . This rapid decrease of quantum efficiency can be attributed to the 'band tails' of the density of states which is due to disordered crystal structures and strong doping in the semiconductor. The force field of impurity atom and wave function of electrons overlap and transformed into an impurity band. At critical concentration, the conduction band joins with the impurity band which results in tails of the density of states. The absorption coefficient depends exponentially on incident photon energy.
Where is Urbach energy, these empirical dependencies are represented by the Urbach rule [49]. For an electron to take part in photogenerated current it must absorb energy greater than or equal to . In the case of tail absorption at ℎ < , the Fig.5. Effect of ZnSe buffer layer on Quantum Efficiency of the solar cell electron gets energy equal to ℎ from the incident photon and the rest is covered by phonons. The Urbach rule represents that as hv decreases, the probability of the multiphonon process also decreases. The electron absorbed a photon having energy hv < Eg takes part in photocurrent just as that of ℎ ≥ . The part of spectrum hv < Eg makes a small contribution to short circuit current 0.40 mA/cm 2 (1.10%). For spectral range ℎ ≥ absorption coefficient follows direct-bandgap law for semiconductors.
For CIGS material with a bandgap = 1.16 eV the wavelength is given by, = ℎ / =1068 nm and can be observed from Fig.7

Absorption and Reflective losses
Optical losses can be calculated quantitively by calculating short circuit current density . If hv is the photon energy and is the spectral radiation power density, can be written as follows.
Where is the charge on the electron, is the quantum efficiency here assumes = 1, ( ) is the transmission of photons from one layer to the other, the summation in the above equation should range from = 300 to = ℎ / = 1068 for bandgap = 1.16 eV. For such CIGS cells, the total reflection losses are about 8.1% and total absorption losses are 9.4% which adds up to 17.5% of total optical losses.

Recombination Losses
Charges created from the absorption of incident photons in the space charge region (SCR) move due to the electric field across the junction, electrons towards the front ZnO layer, and holes towards the neutral CIGS layer to further Cu2O and back contact of Mo contact. However not all photogenerated charges contribute to the photocurrent, some of these carriers recombine inside the SCR before reaching the electrodes. Recombination can also occur at front surface, outside space charge region, and back surface CIGS/metal interface. Recombination mainly occurs at the interfaces of ZnSe/CIGS is due to discontinuity in the conduction band. The spikes in the recombination curve at interfaces can be observed in Fig.8.
The lifetime of minority carrier determines the diffusion length, = √ and has an impact on efficiency [50]. Recombination losses mainly depend upon carrier lifetime and width of SCR which is calculated from the concentration of uncompensated acceptors − ⅆ . For a CIGS absorber layer, the width of SCR is determined from following relation [51].
Where is barrier height, equals = , and is the diffusion potential, for CIGS relative permittivity equals ( = 13.60) and − ⅆ is uncompensated acceptor concentration.
From Fig 8,at higher − ⅆ width of SCR will decrease and with the decrease in absorption area the cell's efficiency decreases. With the decrease in − ⅆ , the width of SCR increases which gives higher quantum efficiency as more incident photons are absorbed in wider SCR and photocarriers reach the boundaries without recombination. However, expansion in SCR from above some extant reduces the electric field inside SCR which in result cause recombination at the surface.

Recombination Losses inside SCR
Recombination of carriers inside SCR can be calculated by using the Hecht equation [52].
Where and are drift length of electrons and holes inside SCR, is coordinate at which electron-hole pair is generated measured from ZnSe/CIGS interface. If electric field strength is , mobility is and carrier lifetime is , the drift lengths for electron-holes in SCR can be expressed by the following relations.
From the above equation, the charge collection efficiency for an interval equals to ( ) − , hence the quantum efficiency for an incident photon with a wavelength is calculated by the following expression

Energy Band Diagram
In Fig.9 the energy band diagram of the CIGS cell shows the band alignment of buffer layer ZnSe and Cu2O hole transport layer. The buffer layer is efficient due to the band alignment of the CIGS/ZnSe interface. The Cu2O reduces recombination losses and attracts photogenerated holes inside CIGS absorber. This allows smoother extraction of holes with experiencing lower resistance. Table 2 and Fig. (10&11) illustrate a comparison of basic photovoltaic parameters among conventional buffer layer of CdS, ZnSe, and ZnSe along with Cu2O as hole transport layer. The PCE of CIGS cells with HTL is much larger than conventional CIGS cells. As compared to the solar cells with only CdS as buffer layer 20.77% and with ZnSe as double buffer layer it increases to 21.04%. with the introduction of Cu2O as HTL, the overall efficiency increases up to 26.63%.

Conclusions
In this study, CIGS cell with Cu2O as the hole transport layer and ZnSe as buffer layers is investigated. CIGS cells give about 20% efficiency with CdS as the buffer layer. With the introduction of the hole transport layer photovoltaic parameters, Voc and Jsc increase further and overall efficiency increases up to 26.63%. Here Cu2O has a key role in the transmission of photo-carriers and reduces the carrier recombination which enhances the overall performance of the solar cell. The wide-band of Cu2O pulls holes from the CIGS absorber. This allows smoother extraction of holes with experiencing lower resistance. The results presented here give parameters for the fabrication of high-efficiency CIGS solar cells.
The results presented here are obtained from the theoretical calculation which represents the effectiveness of HTL which enhances the efficiency by reducing recombination losses in the absorber layer. CIGS solar cells have lower lab-scale efficiency of about 25% than Si-cells. The numerical calculations represent the increase in efficiency by the addition of HTL. Based on this study it is suggested that CIGS cells with HTL can be a major part of the Solar industry due to higher efficiency and stability.
Funding: This research received no external funding.