An investigation into the effects of band gap and doping concentration on Cu(In,Ga)Se2 solar cell efficiency
© The Author(s). 2016
Received: 16 December 2015
Accepted: 29 April 2016
Published: 10 May 2016
A simulation study of a Cu(In1 − xGax)Se2 (CIGS) thin film solar cell has been carried out with maximum efficiency of 24.27 % (Voc = 0.856 V, Jsc = 33.09 mA/cm2 and FF = 85.73 %). This optimized efficiency is obtained by determining the optimum band gap of the absorber and varying the doping concentration of constituent layers. The Ga content denoted by x = Ga/(In + Ga) is selected as 0.35 which provides the optimum band gap of absorber layer as 1.21 eV. Theoretically, the effects of Ga fraction “x” on CIGS absorber band gap are investigated and to avoid the lattice mismatch effect, the efficiency measurements due to the CIGS band gaps >1.21 eV have not come to the consideration. A one-dimensional simulator ADEPT/F 2.1 has been used to analyze the fabricated device parameters and hence to calculate open circuit voltage, short circuit current, fill factor and efficiency.
The CIGS thin film hetero-junction solar cell based on the chalcopyrite p-type absorber layer Cu(In1-xGax)Se2 is a promising option in industrial productivity due to its lower manufacturing cost and higher efficiency (Rampino et al. 2015; Powalla and Dimmler 2001; Minemoto et al. 2003). Although the CIGS solar cell is recorded as a highly efficient (~21.7 %) thin film solar cell (Jackson et al. 2015) either there must still need to enhance efficiency and reduce cost for mass productivity. The inline co-evaporated CIGS absorber (Lindahl et al. 2013) has band gap range from 1.04 (CIS) to 1.67 eV (CGS) depending on x (from 0 to 1) (Tverjanovich et al. 2006; Gloeckler and Sites 2005; Gabor et al. 1996). The mismatch effect of CIGS layer (Lee et al. 2011) with adjacent CdS buffer layer and Mo back contact is avoided and the absorber band gap is adjusted with its corresponding electron affinity. Furthermore, the doping concentration of different layers is also an important factor to maximize the efficiency and minimize the fabrication cost of any solar cell (Haque and Galib 2013). The influence of the CIGS absorber band gap and the doping concentration of each layer on the performance of the solar cell have been investigated in this study. The radio frequency (RF) sputtered ZnO (deposition of Al doped ZnO and intrinsic ZnO) with its wider band gap of 3.3 eV and the chemical bath deposited (CBD) CdS with its direct band gap of 2.42 eV have been used as the window and the buffer layer respectively (Lindahl et al. 2013; Jung et al. 2010). All the efficiency measurements and comparisons are done under a solar spectrum AM1.5G for which the solar irradiance on earth is 0.1 W/cm2 (Haque et al. 2013). The shadowing factor used in the simulation is of 5 %.
The default values of device parameters
Band gap (eV)
Electron affinity (eV)
Electron mobility (cm2/Vs)
Hole mobility (cm2/Vs)
Conduction band effective density of states (cm−3)
2.2 × 1018
1.7 × 1019
2 × 1018
Valence band effective density of states (cm−3)
1.8 × 1019
2.4 × 1018
1.6 × 1019
Donor concentration (cm−3)
1 × 1018
1 × 1018
Acceptor concentration (cm−3)
2 × 1016
Electron lifetime (s)
5 × 10−8
2 × 10−8
1 × 10−8
Hole lifetime (s)
5 × 10−9
6 × 10−8
5 × 10−8
Contact parameters for device simulation
Recombination velocity for holes
Recombination velocity for electrons
Band gap and electron affinity of Cu(In1−xGax)Se2 alloy composition
Ga/(In + Ga) ratio, x
Band gap, Eg
Electron affinity, χe
It can be remarked that the band gap increases and the electron affinity decreases with the raising of “x”. Initially, the effect of absorber layer band gap was observed to determine the optimum result. Then, the energy band profile with optimum band gap and the efficiency graph due to the variation in energy gap were plotted. Afterwards, the doping concentration of each layer was varied and the optimal level of doping was determined by analyzing the corresponding efficiencies. Finally, the efficiency was calculated by using the optimized values and hence the highest performance was obtained.
Results and discussion
Simulation outcome with default values
Effect of absorber layer band gap on cell efficiency
Performance variation due to absorber band gap
Effect of doping concentration on cell efficiency
The doping density of CdS buffer on n-p junction (constituted between the buffer and the absorber) highly affects the output current. Analyzing the effects of drift velocity and holes recombination rate (Lee et al. 2009), the optimum doping concentration of CdS buffer layer was obtained as 5 × 1018 cm−3.
The determined higher doping level of the absorber, 1 × 1019 cm−3, is satisfactory for the electron affinity of the CIGS absorber, 4.21 eV.
As discussed earlier, the absorber layer optimal band gap of 1.21 eV and the optimized doping concentration of all layers are determined through device simulation which in turn provides the highest performance. In Fig. 5a, describes the doping concentrations of different layers of designed CIGS solar cell, Fig. 5b shows the spatially resolved current, Fig. 5c denotes the electric field corresponding to the thickness of the layers and finally Fig. 5d represents the J-V characteristic curve from which the optimum efficiency has been calculated. The simulation result presents the J-V characteristic curve with short-circuit current density of 33.09 mA/cm2 and open circuit voltage of 0.856 V. Finally, the maximum efficiency of CIGS thin film was calculated from the simulation outcomes as 24.27 %.
The numerical simulation of CIGS hetero-structure thin film solar cell was conducted by the ADEPT/F 2.1 one-dimensional online simulator. From various reliable sources the default values for simulation were collected and tabulated to obtain the default outcome. The mathematical equations of energy band gap and electron affinity for CIGS absorber as a function of “x” were developed by plotting some known results. At different Ga fraction the absorber band gap and electron affinity were calculated. The simulation of the cell with the Cu(In0.65Ga0.35)Se2 absorber layer results in higher efficiency rather than other compositions. Afterwards, the doping concentrations of the component layers were optimized in terms with drift velocity of the majority carrier and recombination rate of the minority carrier. At last, the cell performance was investigated by simulating with optimized values.
MA and MH conducted the design of study and led the simulation. ANB supervised the study and helped to draft the manuscript. All authors read and approved the final manuscript.
The authors would like to attribute their extreme gratitude to the researchers working in Purdue University, USA who contributed to develop the ADEPT/F simulator sponsored by Network for Photovoltaic Technology (NPT).
The authors declare that they have no competing interests.
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