Cu ( In , Ga ) Se 2 Thin Films Codoped with Sodium and Bismuth Ions for the Use in the Solar Cells

The codoping effects of sodium and bismuth ions on the characteristics of Cu(In,Ga)Se 2 films prepared via the solution process were investigated in this study. When sodium and bismuth ions were incorporated into Cu(In,Ga)Se 2 , the ratio of the intensity of (112) diffraction peak to that of (220/204) diffraction peak was greatly increased. The codoping process not only enlarged the sizes of the grains in the films but also resulted in densification of the films. The carrier concentration of Cu(In,Ga)Se 2 was found to be effectively increased to cause a reduction in the resistivity of the films. The above phenomena were attributed to the densified microstructures of the films and a decrease in the amount of the donor-type defects.The leakage current of the solar cells was found to be also decreased via the codoping process. Owing to the improved electrical properties of Cu(In,Ga)Se 2 films, the conversion efficiency of the fabricated solar cells was significantly enhanced.


Introduction
Solar energy has played an important role in energy production due to the increasing demand for renewable energy.Cu(In,Ga)Se 2 solar cells have reportedly reached the highest efficiency with the value of 20.3% among all thin-film solar cells [1].Traditionally, Cu(In,Ga)Se 2 films are prepared via the vacuum processes including a multistage coevaporation route [2,3] and the sputtering route [4,5].These methods have several shortcomings such as high cost and sophisticated processes; therefore, these problems cause the difficulty of large-scale manufacturing [6].For overcoming the drawbacks of the vacuum processes, several kinds of nonvacuum processes such as the printing method [7], the electrodeposition process [8], and the spray pyrolysis method [9] have been developed to synthesize Cu(In,Ga)Se 2 films.
Although Cu(In,Ga)Se 2 films have been successfully obtained via these nonvacuum methods, further controlling the grain sizes of the films as well as the morphologies is required for improving the electrical properties of the solar cells.Doping sodium ions [10] and antimony ions into [11] Cu(In,Ga)Se 2 has been demonstrated to be effective way for enhancing the conversion efficiency of the fabricated solar cells.The incorporation of sodium ions is reported to increase the open-circuit voltage and the fill factor of the solar cells [12].The growth of the (112)-oriented Cu(In,Ga)Se 2 films can be also enhanced [13].Sodium ions are considered to substitute on a copper site in the form of Na Cu and result in the annihilation of the amount of the donor-type defects in the form of In Cu [14].On the other hand, the authors have examined the doping effects of bismuth ions into Cu(In,Ga)Se 2 on the characteristics of the fabricated solar cells [15].Doping bismuth ions has been found to result in densification of films and the enlargement of the grain sizes.The open-circuit voltage of the bismuth-doped Cu(In,Ga)Se 2 solar cells has been significantly enhanced.Although the doping effects of sodium ions and bismuth ions have been confirmed, the codoping effects of sodium and bismuth ions for Cu(In,Ga)Se 2 films have not been investigated in detail.
For exploring the codoping influence of sodium and bismuth ions, Cu(In,Ga)Se 2 films doped with the above two ions were prepared via the solution process on the sodium-free substrates.The doping effects of sodium and bismuth ions on the properties of the films were investigated.The relation between the electrical properties of the prepared films and the dopants was investigated.The electrical properties of the films doped with a single kind of ions were compared with those of the codoped films.

Experimental
Analytical Cu(NO 3 ) 2 (99.99%),In(NO 3 ) 3 (99.99%),and Ga(NO 3 ) 3 (99.99%)were used as the starting materials and dissolved in ethanol.The molar ratios of copper ions to indium ions to gallium ions were set to be 0.9 : 0.7 : 0.3.Bismuth nitrate and sodium nitrate were added to the above solutions as the bismuth source and the sodium source, respectively.The molar ratio of sodium ions to IIIA ions (indium and gallium ions) and that of bismuth ions to IIIA ions were set at 1.0 mol%, respectively.The solutions were mixed and stirred, followed by adding ethylcellulose that was used as a binder.Subsequently, the solution was spincoated on the sodium-free glass and dried to 200 ∘ C on a hot plate for 0.5 h.The precursor films were placed in a tubular reactor.Selenium powders (99.99%) were used as the selenium source.The mixed gas (5% H 2 /95% N 2 ) was used as the carrier gas to supply the heated selenium source.The Se partial pressure was about 2 Torr in the reactor.The precursor films were selenized at around 500 ∘ C for 30 min.
The temperature was controlled via a PID controller.The prepared film without adding dopants was named as sample A, and the films doped with sodium ions, bismuth ions, and sodium ions as well as bismuth ions are named as samples B, C, and D, respectively.The kinds of the dopants added and the corresponding sample names are as listed in Table 1.Cu(In,Ga)Se 2 films formed on the sodium-free glass were identified via X-ray diffraction diffractometer (XRD, Philips X'Pert) operated at 40 kV and 40 mA with Cu K radiation.The microstructures of the selenized specimens were observed using a scanning electronic microscope (SEM, Hitachi S800).The effects of the added ions on the electrical properties of Cu(In,Ga)Se 2 films were investigated via a Hall effect measurement (HMS-3000) at room temperature.The low-temperature photoluminescence (PL) spectra of the prepared films were recorded under the excitation of a laser diode (808 nm, 50 W) using an InGaAs array spectrometer (512 elements, 900-1650 nm).In order to examine the electrical properties of the prepared films, the solar cells with a structure of sodium-free glass/Mo/CIGS/CdS/i-ZnO/ITO were fabricated.The current-voltage (I-V) measurement under AM1.5G conditions at 25 ∘ C was used to investigate the fabricated Cu(In,Ga)Se 2 solar cells.

Results and Discussion
The X-ray diffraction patterns of 500 ∘ C selenized Cu(In,Ga)Se 2 films without adding dopants (sample A) are shown in Figure 1(a), and that of the films doped with sodium ) was greatly increased to 2.28.The above results indicated that the doping of sodium and bismuth ions might vary the preferred orientation of the prepared films.Moreover, Cu(In,Ga)Se 2 films with the preferred (112) orientation are reportedly suitable for lattice matching with the window layers of solar cells [16,17].Figure 2 illustrates the microstructural evolution of the obtained Cu(In,Ga)Se 2 films doped with various kinds of ions.The size of the grains of Cu(In,Ga)Se 2 without dopants (sample A) was around 0.3 m (Figure 2(a)).When sodium ions were incorporated (sample B), the particle size of the prepared films was significantly enlarged to 1 m (Figure 2(b)).On the other hand, the size of the grains in the bismuthdoped films (sample C) was around 0.5 m (see Figure 2(c)).Upon codoping sodium ions and bismuth ions (sample D), the microstructures became effectively densified, and the particle size was enlarged to around 2 m as seen in Figure 2(d).Figure 3 shows the surface morphology of the corresponding films via the atomic force micrographs (AFM).The values of root-mean-square (RMS) roughness for samples A, B, C, and D were 86 nm, 68 nm, 60 nm, and 56 nm, respectively.It implies that the roughness of the films was greatly reduced as sodium and bismuth ions were codoped.When sodium ions are added to Cu(In,Ga)Se 2 films, sodium selenides are reported to be formed and act as a flux agent to facilitate the grain growth during the selenization process [18].As bismuth ions are incorporated into Cu(In,Ga)Se 2 films, copper bismuth selenide compound is considered to be formed and plays a fluxing role according to our previous study [15].Hence, the codoping of sodium and bismuth ions into Cu(In,Ga)Se 2 can effectively facilitate the densification process of the films and enlarge the grain sizes.
The J-V characteristics of the fabricated Cu(In,Ga)Se 2 solar cells are depicted in Figure 4, while the corresponding parameters of the devices are summarized in Table 2.The value of short-circuit current density ( sc ) was around 32-34 mA/cm 2 for four different samples.The values of opencircuit voltage ( oc ) and the values of fill factor (FF) were found to increase when sodium and bismuth ions were codoped.The conversion efficiency of the solar cells without any kinds of dopants was merely 4.8%.The values of the efficiency increased to 6.5% and 6.3% for samples separately doped with sodium ions and bismuth ions, respectively.Codoping both ions greatly increased the conversion efficiency to 6.9%.Based on the data shown in Table 2, codoping both ions also effectively increased  oc and the FF of the solar cells.The current for solar cells under illumination can be expressed as where  is the diode current density,  is voltage,  0 is the saturation current density,  is the electron charge,  is the diode ideality factor,  is the Boltzmann constant,  is the temperature in Kelvin,   is the series resistance,  is the shunt conductance, and   denotes the photocurrent density.Diode parameters determined from the J-V curve in Figure 4 are depicted in Figures 5 and 6, and the obtained data are summarized in Table 2.The value of the shunt conductance () can be extracted from Figure 5 (/ versus ).When the value of  is small, the internal leakage of the current through the cell will be decreased.The values of  of samples A, B, C, and D were calculated to be 21.0 mS/cm 2 , 9.6 mS/cm 2 , 14.8 mS/cm 2 , and 9.8 mS/cm 2 , respectively.The value of  was significantly reduced after the incorporation of sodium and As shown in (2), a reduction in  results in an increase in  oc .Hence, codoping sodium and bismuth ions caused a decrease in  and an increase in  oc .From the figure shown in Figure 6, the value of   can be obtained from / versus ( +  sc ) −1 .It can be seen that sample D has the lowest value of   compared with other devices as shown in Table 2.
The carrier concentration and the resistivity of Cu(In,Ga)Se 2 films were also examined via the Hall effect measurement and the results were listed in Table 3.The carrier concentration was 2.47 * 10 16 cm −3 for sample A and increased for films incorporated with dopants.The films codoped with both ions (sample D) had a maximum value of the carrier concentration (8.56 * 10 16 cm −3 ).On the other hand, the resistivity of the codoped samples was greatly decreased to 5.04 * 10 −2 Ω cm due to the densified microstructures.Hence, the value of   of the films which codoped both ions was effectively reduced (Table 2).The incorporation of sodium ions into Cu(In,Ga)Se 2 is verified to result in a decrease in the amount of the donortype defects in the form of In •• Cu [14].The formation of defects can be expressed as When sodium ions are doped into Cu(In,Ga)Se 2 , sodium ions could substitute the site of In •• Cu .Hence, the amount of donor-type defects in the form of In •• Cu is decreased.Besides, the amount of the donor-type defects in the form of V •• Se is also found to be reduced for bismuth-doped films [15].Therefore, the codoping of sodium as well as bismuth ions can significantly decrease the amount of the donor-type defects (In •• Cu and V •• Se ), thereby causing an increase in the relative amount of holes that acted as the major carriers in Cu(In,Ga)Se 2 .Hence, the carrier concentration of holes was considerably raised as sodium and bismuth ions were codoped.If the influence of the tunneling mechanism and the surface recombination can be ignored, the variation of the  oc and the carrier concentration (  ) is expressed as [20] where  is the Boltzmann constant,  denotes the temperature in Kelvin,  is the charge of an electron, and  1 and  2 represent the carrier concentration of the films with absence and presence of dopants, respectively.From (4), an increase in  2 / 1 results in a positive value of Δ oc .The carrier concentration in Cu(In,Ga)Se 2 films was substantially increased via the codoping process, thereby leading to a significant improvement of the  oc .
Intensity (a.u.) Photon energy (eV) 1.00 0.95 0.90 For investigating the codoping effects on Cu(In,Ga)Se 2 films, the low-temperature photoluminescence (PL) spectra were measured.Figure 7(a) shows temperature dependence of the PL spectra for Cu(In,Ga)Se 2 films codoped with sodium and bismuth ions.A peak at 0.94 eV was observed, which is attributed to the donor-acceptor pair (DAP) recombination [21,22].As the temperature was increased, the intensity of the DAP PL peak became reduced.It is due to the thermal quenching behavior.The thermal quenching equation in PL spectra can be described as [23] where  0 ,  1 , and  2 are the three fitting parameters and   is the thermal quenching energy.It is clearly demonstrated that the intensity of the PL peak decreases as the temperature elevates.In Figure 7(b), the low-temperature PL spectra measured at 10 K showed that the intensity of the DAP peak for Cu(In,Ga)Se 2 codoped with sodium and bismuth ions was lower than that of the films without doped ions, indicating that the amount of donor-type defects was decreased via the codoping process.Hence, it is verified that the carrier concentration of holes was increased as shown in Table 3.The above results confirmed that codoping sodium and bismuth ions effectively improved the microstructures and enhanced the electrical properties of Cu(In,Ga)Se 2 , therefore increasing the conversion efficiency of the fabricated solar cells.

Conclusions
The influences of codoping sodium and bismuth ions in the electrical properties of Cu(In,Ga)Se 2 films prepared via a solution route were examined.An increase in the ratio of the intensity of (112) diffraction peak to that of (220/204) diffraction peak was found in the films codoped with sodium and bismuth ions.Codoping these ions led to the densification of the films and the enlargement of the particle sizes.The Hall effect characteristics indicated that the incorporation of sodium and bismuth dopants effectively increased the carrier concentration of Cu(In,Ga)Se 2 films and enhanced the opencircuit voltage of the fabricated solar cells.The resistivity of the prepared films was substantially decreased; therefore, the series resistance of the fabricated solar cells was effectively reduced.The codoping of sodium and bismuth ions was also found to reduce the amount of leakage current through the solar cells and substantially increased the conversion efficiency of Cu(In,Ga)Se 2 solar cells.

Figure 1 :
Figure 1: X-ray diffraction patterns of Cu(In,Ga)Se 2 films doped with (a) absence of dopants, (b) sodium ions, (c) bismuth ions, and (d) sodium ions and bismuth ions upon heating at 500 ∘ C for 30 min.Inset refers to relation between the ratio of I (112) /I (220/204) and the sample name.

Figure 2 :
Figure 2: Scanning electron micrographs of Cu(In,Ga)Se 2 films doped with (a) absence of dopants, (b) sodium ions, (c) bismuth ions, and (d) sodium ions and bismuth ions upon heating at 500 ∘ C for 30 min.

Figure 3 :
Figure 3: Atomic force micrographs of Cu(In,Ga)Se 2 films doped with (a) absence of dopants, (b) sodium ions, (c) bismuth ions, and (d) sodium ions and bismuth ions upon heating at 500 ∘ C for 30 min.

Table 1 :
Sample names for Cu(In,Ga)Se 2 films doped with varied ions.

Table 2 :
Solar cell parameters of Cu(In,Ga)Se 2 films.

Table 3 :
Electrical properties of Cu(In,Ga)Se 2 films.