Role of Cl on Diffusion of Cu in in 2 S 3 Layers Prepared by Ion Layer Gas Reaction Method

Ion layer gas reaction (ILGAR) method allows for deposition of Cl-containing and Cl-free In2S3 layers from InCl3 and In(OCCH3CHOCCH3)3 precursor salts, respectively. A comparative study was performed to investigate the role of Cl on the diffusion of Cu from CuSCN source layer into ILGAR deposited In2S3 layers. The Cl concentration was varied between 7 and 14 at.% by varying deposition parameters. The activation energies and exponential pre-factors for Cu diffusion in Cl-containing samples were between 0.70 to 0.78 eV and between 6.0 × 10 −6 and 3.2 × 10 −5 cm 2 /s. The activation energy in Cl-free ILGAR In2S3 layers was about three times less compared to the Cl-containing In2S3, and the pre-exponential constant six orders of magnitude lower. These values were comparable to those obtained from thermally evaporated In2S3 layers. The residual Cl-occupies S sites in the In2S3 structure leading to non-stoichiometry and hence different diffusion mechanism for Cu compared to stoichiometric Cl-free layers.


Introduction
In2S3 has found increased attention in photovoltaic's as a replacement for toxic CdS [1][2][3][4] because of its suitable properties.It has been effectively used as a buffer layer for both chalcopyrite [1][2][3] and nanocomposites [5,6] solar cells and as an extremely thin absorber (eta) in eta cells [7,8] and nanocomposites [9] solar cells with ZnO nanorods or nanoporous-TiO2 electrodes, respectively.Diffusion of Cu from Cu(In,Ga)(S,Se)2 absorber [3,10,11] or CuSCN hole conductor [12] into In2S3 layers has been a major drawback to effective performance and stability of these solar cells.
In2S3 exists in three phases: β-phase which is stable at room temperature up to 420° [13,14], the α-phase which is stable above 420 °C [15] up to 754 °C and the γ-phase which is stable between 754 °C and its melting point 1090 °C [16].The most commonly used phase is the β-In2S3.β-In2S3 has a defect spinel superstructure in which In occupies all octahedral sites and a third of the tetrahedral sites [16,17].A third of the tetrahedral cationic sites remain vacant but ordered along 4I screw with alignment of three spinel blocks in c-direction [17].β-In2S3 can accommodate foreign atoms, for example Fe [18], Ag [19], Cl [20], Cu and Na [10].
The presence of foreign impurities in host In2S3 affects occupation and migration of new foreign atoms.Diffusion of Cu in In2S3 is inhibited by the presence of Na because both Na and Cu compete for the same cationic vacancies and substitution sites in In2S3 structure [21].Different authors have reported diffusion of Cu in In2S3 to be governed by vacancy [22] or insertion/substitution [21] mechanism.A better understanding and hence control of Cu diffusion in In2S3 layers is crucial for photovoltaic applications [23].
ILGAR is a sequential and cyclic method of depositing highly conformal In2S3 layers on different substrates [24,25].In2S3 layers can be deposited from InCl3 or In(OCCH3CHOCCH3)3 to obtain Cl-free and Cl-containing layers [26].The presence of Cl in In2S3 increased the optical band up to 2.4 eV compared to 2.0 eV for Cl-free layers.Higher conversion efficiency was achieved for Cl-free In2S3 layers compared to Cl-containing layers both prepared by ILGAR [26].The stoichiometry as well as opto-electronic properties of In2S3 layers also changes with inclusion of Cl.Crystallinity and photosensitivity increased with increasing Cl content [20].It is therefore important to investigate the effect of Cl on diffusion of Cu in ILGAR deposited In2S3 layers.
Rutherford backscattering spectroscopy (RBS) is an absolute method for determination of elemental composition and depth profiling of various materials with high accuracy [27,28].The diffusion coefficient of Cu in In2S3 can be obtained from Cu concentration profiles by solving the diffusion equation analytically or numerically with appropriate boundary conditions and comparing or fitting the resulting profiles with the experimental data.Activation energy and diffusion prefactor of 0.3 eV and 9 × 10 −11 cm 2 •s −1 determined from RBS depth profiling have been reported for Cu diffusion in thermally evaporated In2S3 layers [23].
In this work, we report a comparative study of Cu diffusion in Cl-free and Cl-containing In2S3 layers.In2S3 layers were prepared by ILGAR method, which was modified to obtain layers with varying Cl content.The Cu source was CuSCN deposited on top of In2S3.The distribution of diffused Cu in In2S3 was profiled by RBS from which the diffusion coefficients were determined as a function of annealing temperature.

Experimental Section
In2S3 layers were deposited by ILGAR [24,25] onto c-Si wafers without additional treatment of the c-Si substrates.InCl3 or In(acac)3 precursor salt was dissolved into ethanol solvent to obtain a solution of 25 mM.In(acac)3 precursor was used to deposit Cl-free layers while InCl3 was used for Cl-containing layers.The deposition temperature for deposition of all samples was 200 °C.The standard ILGAR cycles were adjusted to produce In2S3 layers with varying Cl content.The details of the deposition parameters for the different samples are given in Table 1.CuSCN was deposited onto each of the c-Si/In2S3 samples by spray-spin coating method [23] from a solution of 50 mM CuSCN dissolved in propyl sulfide [29].For each sample, ten spray-spin coating cycles were deposited, which corresponds to about 500 nm thickness as determined by Dektak profilometer before heat treatment, while the thickness of the samples after heat treatment varied as shown in Table 1.The c-Si/In2S3/CuSCN samples were then cut into smaller sizes of about 7 mm × 7 mm.The smaller pieces were then annealed for 5 min at temperatures between 150 °C and 250 °C.A set of small samples annealed at these temperatures and one not annealed was obtained for each large c-Si/In2S3/CuSCN sample.
CuSCN layer was then etched away in pyridine solution [23].The sample was first dipped into pyridine solution for 2 s then rinsed in deionized water to wash away the pyridine.This was repeated until a shiny surface similar to the as-deposited In2S3 was obtained.RBS measurements were performed with 1.4 MeV He + ion beam and the backscattered ions detected at a scattering angle of 168° [23].

Results and Discussion
The composition of In2S3(acac) and In2S3(Cl) was determined from RBS measurements on asdeposited samples.The integral of the respective peaks were used to calculate the areal densities, from which the layer thicknesses were obtained.The number of atoms (Nit) of a given element in a layer is given by [27], where t is the layer thickness, A the integral of the peak, Q the number of ions incident on the sample, Ω the detector solid angle and σ(E0) the differential scattering cross-section.The concentration of each element was obtained by dividing the atomic density (Ni) of each element by that of In2S3 (4.53 × 10 22 cm −3 ).The values of the atomic concentrations and layer thicknesses of the samples are given in Table 1.Sample S0 In2S3(acac) while samples S8, S9, S11 and S12 were In2S3(Cl).Deposition parameters were adjusted to obtain In2S3(Cl) with different Cl content.The difference in layer thicknesses was due to a difference in deposition rate due to the changes in the deposition steps.The presence of large amounts of Cl in In2S3(Cl) layers led to a strong deviation from stoichiometry while In2S3(acac) layers were more stoichiometric with S/In ration of 1.56, which is very close to the expected value of 1.5.The amount of In was almost constant for all In2S3(Cl) samples while S content dependent on the residual Cl.This means that the InCl3 precursor layers were not completely sulfurized, leaving behind Cl residues.
RBS spectra for In2S3(acac) and In2S3(Cl) containing diffused Cu after annealing for 5 min at different temperatures are shown in Figure 1.The various peaks corresponding to the elements detected in the samples are shown.The Cu peak increases in height with increasing diffusion (annealing) temperature.The shape of the peaks depicts a concentration gradient of Cu in In2S3 with higher concentration at the surface.Inclusion of Cu into In2S3 matrix results in an increase in the volume of the In2S3 that can be seen by increase in the width of the In and S peaks with increasing diffusion temperature.The depth profiles of Cu were extracted from the Cu peaks of the RBS spectra for the different samples using WiNDF software [30].A recursion equation was developed from the Fick's second law of diffusion and used to numerically calculate the Cu concentration profiles and compare the results to the measured data.Details can be found in reference [23].The depth profiles were fitted numerically with minimum deviation and values for the diffusion coefficients were obtained.The measured and simulated depth profiles for In2S3(acac):Cu and In2S3(Cl):Cu are shown in Figure 3.The depth profiles show a thin surface layer with high Cu concentration independent of diffusion temperature.This means an interfacial layer with a large amount of Cu was formed upon deposition of CuSCN and annealing.A thin interfacial layer due to diffusion of Cu into In2S3 layer was also observed for Cu(In,Ga)Se2/In2S3 layer system for Chalcopyrite solar cells [3].This thin surface layer was therefore not used in fitting the profiles to obtain diffusion coefficient values.Diffusion values were calculated for all the samples with different Cl contents and plotted against inverse temperature in an Arrhenius plot as shown in Figure 4.The activation energy of Cu diffusion in the different In2S3 layers and exponential prefactors were obtained from the Arrhenius plots.The results for Cu diffusion in thermally evaporated In2S3 layers [23] are included for comparison.The values of the activation energies and exponential prefactor for all the samples with different Cl concentrations are shown in Table 2.The activation energy and prefactor for the diffusion of Cu in Cl-free In2S3 layers was of the same order for In2S3 deposited by ILGAR and PVD but much lower than in Cl-containing layers.Residual Cl in In2S3 is known to occupy S places in the In2S3 matrix resulting in an increase in the optical band gap, photosensitivity and a decrease in work function [4,20,31].The presence of Cl changes the stoichiometry and local bond configuration of In2S3 and this can limit Cu migration in In2S3:Cl compared to migration in stoichiometric In2S3.The In-Cl bond energy is much stronger than that of In-S and this could be the reason for the higher activation energy needed for Cu to diffuse in Cl-containing In2S3.A higher amount of energy is therefore required to overcome the barrier for Cu migration.The prefactor in In2S3:Cl was five to six orders of magnitude higher than in In2S3:acac and In2S3:PVD.An increase in the prefactor could be concomitant with an increase in the entropy.)

Conclusions
In2S3 layers were deposited by Ion layer gas reaction onto c-Si wafers and CuSCN deposited onto the samples by spray-spin coating, deposition parameters were varied to obtain varying chlorine contents in the sample that ranged from 7.8 at.% to 13.8 at.%.The distribution of copper in the samples with chlorine was strongly influenced by chlorine for annealing temperatures above 200 °C.This shows that chlorine plays a crucial role in copper diffusion in In2S3.The activation energies and exponential prefactors for Cu diffusion in Chlorine containing samples were between 0.7 to 0.78 eV and between 6 × 10 −6 and 3.2 × 10 −5 cm 2 /s.The activation energy in Cl free In2S3 layers was about three times less and the pre-exponential constant six orders of magnitude lower.

Figure 2
Figure 2 shows the Cu peaks from Figure 1.The distribution of Cu in the In2S3 bulk is not similar for In2S3(acac):Cu and In2S3(Cl):Cu layers.The distribution in the other In2S3(Cl):Cu layers with varying Cl content was similar to Figure 2b.The Cu peaks increased systematically with diffusion temperature for In2S3(acac):Cu similar to the case for thermally evaporated In2S3 layers reported in reference [23].The Cu peaks from In2S3(Cl):Cu increased slowly with diffusing temperature for temperatures below 200 °C and then increased strongly above 200 °C.The distribution of Cu tends to be more constant in the In2S3(Cl):Cu bulk than in In2S3(acac):Cu.The difference in the nature of the concentration gradients can be attributed to the presence of Cl.Cl therefore plays a crucial role in the diffusion of Cu in In2S3 layers.The depth profiles of Cu were extracted from the Cu peaks of the RBS spectra for the different samples using WiNDF software[30].A recursion equation was developed from the Fick's second law of diffusion and used to numerically calculate the Cu concentration profiles and compare the results to

Figure 2 .
Figure 2. Cu peaks for (a) In2S3(acac):Cu and (b) In2S3(Cl):Cu for different diffusion temperatures.The scatters represent measured data while the solid lines are from simulation.

Figure 3 .
Figure 3. Simulated and measured depth profiles for Cu in (a) In2S3(acac):Cu and (b) In2S3 (Cl):Cu from which diffusion coefficients were determined.

Figure 4 .
Figure 4. Arrhenius plot of diffusion coefficient against inverse temperature for different In2S3 layers.

Table 1 .
Thickness and layer composition of In2S3 films.

Table 2 .
Activation energy and exponential prefactor of In2S3 films with varying chlorine percentages.