Sodium doping of solution‐processed amine‐thiol based CIGS solar cells by thermal evaporation of NaCl

Poor crystallinity, high degree of porosity and rough surfaces are the main drawbacks of solution‐processed CIGS absorbers resulting in lower power conversion efficiencies when compared to vacuum‐based CIGS solar cells. Therefore, promoting absorber grain growth is key to further improve solution‐based solar cell performance. The effect of alkali elements such as Na in CIGS absorbers is generally recognised to have beneficial effects not only on the absorber opto‐electronic properties but also on the grain growth. In this work, thermal evaporation of a thin layer of NaCl prior to selenisation resulted in absorbers with significantly larger CIGS grains than previously seen with Na diffusing directly from the from soda‐lime glass substrate. NaCl is non‐toxic, abundant and readily available compound that has not been typically used as an evaporation source, but rather as an additive into CIGS precursor solution. The effect of Na on these solution‐processed CIGS devices was primarily observed in the spectacular morphological changes leading to improved carrier collection and minority carrier lifetimes, but less on the absorber doping. Transmission electron microscopy (TEM) revealed voids forming around large CIGS grains upon NaCl addition and these had a negative effect on inter‐grain carrier transport. Nonetheless, the resulting device performance doubled from 5% to 10% with addition of Na using this doping approach; however, a compromise between the optimum grain growth and optimum electronic properties had to be made. This study demonstrates a novel, simple and effective Na‐doping strategy for CIGS absorbers and reveals the current limitations of the Na‐doping in solution‐processed atmospherically deposited cells.


| INTRODUCTION
Cu (In,Ga)Se 2 (CIGS) solar cells are currently the best performing commercially deployed thin film photovoltaic (PV) technology. 1,2 To further reduce manufacturing costs, development of a large-scale atmospheric processing method of CIGS solar cells and modules is highly desirable. In recent years a wide range of solution-based approaches have been investigated, including a variety of precursor materials, solvents, deposition techniques, and annealing methods. 3,4 However none of the vacuum-free methods resulted in CIGS absorbers with comparable quality to the vacuum-based ones, with device efficiencies still lagging behind. 5 Inferior quality was often attributed to the presence of voids, impurities, and large number of grain boundaries (GBs). 6 Carbon and oxygen from the solvent or additives can impede the grain growth and remain in the final film causing performance deterioration. 7 Using nanoparticles dispersed in a solution can leave rough films with stoichiometry deviations due to nanoparticle agglomeration. 8 The most efficient solution-based method to date uses a highly toxic and explosive solvent hydrazine, and therefore has limited potential for large scale industrial application. 9 The solution-processing method presented in our as well as other groups' previous work is a very promising approach which takes the benefits of the hydrazine method but using a safer and more environmentally friendly solvent system. [10][11][12][13] This approach consists of dissolving metal chalcogenides in 1,2-ethylenediamine/1,2-ethanedithiol (EDA/EDT) and was shown to result in devices with 12.05% power conversion efficiency (PCE) prepared by a scalable spray-coating technique in ambient conditions. 14 This binary amine-thiol solvent mixture discovered in 2013 by Brutchey and Webber was found to have the ability to dissolve over 65 bulk inorganic materials. These were utilised in a range of applications such as PV devices, electrocatalysts, photodetectors, thermoelectrics and nanocrystal ligand exchange. 15,16 However, whilst high PCE was obtained, complete absorber crystallisation was not achieved when devices were prepared on Mo-coated soda-lime glass (SLG) substrates. Moreover, the repeatability of the process was difficult, and the variability in absorber morphologies and consequently device performances were suspected to be attributed to non-homogeneous, and uncontrolled supply of sodium from the SLG substrate.
Sodium is widely accepted as a beneficial dopant in CIGS absorbers, affecting device fill factor (FF), open-circuit voltage (V OC ), p-type conductivity and also crystal size and orientation in some cases. [17][18][19] However the quantity of Na beneficial to the device performance is very small, in the order of 0.1 at% and higher than optimum quantity leads to performance losses. 20,21 Na is typically supplied to the absorber during its growth from SLG substrates. The control over the amount of Na diffusing is difficult however: the substrates are never identical, the diffusion rate is strongly dependent on the Mo properties, and it would require exact reproducibility of the substrate temperature during the selenisation step which is difficult within most experimental setups. [22][23][24] Therefore alternative approaches allowing for precise control over the amount of Na introduced have been developed.
The most common approach is the NaF post-deposition treatment (PDT) where a thin layer of NaF (20-40 nm) is typically evaporated after the 3-stage CIGS growth. 25 As this treatment is applied after the CIGS growth, it does not modify significantly the absorber microstructural properties. 26 Sutter-Fella et al. reported that when NaF was evaporated onto solution-processed copper zinc tin sulphide (CZTS) devices prior to selenisation, the beneficial effects of Na were observed on electronic as well as morphological properties of the devices. 27 In this work, NaCl was chosen as the Na source because of its low cost, benign nature, and ease of thermal evaporation with lower thermal budget, as compared to NaF. A thin layer of NaCl was evaporated onto the as-deposited CIGS absorbers and the effects of different NaCl layer thicknesses on the device electronic and morphological properties were studied.

| CIGS absorber and solar cell preparation
Individual metal chalcogenide precursor solutions of 0.2 M concentration were prepared by dissolving indium sulphide (In 2 S 3 ), copper sulphide (Cu 2 S) and gallium together with selenium powder (Ga + Se) in EDA/EDT solvent mixture of 10/1 v/v ratio. After the full precursor dissolution, the separate solutions were combined in specific ratios targeting final composition of Cu 0.9 In 0.7 Ga 0.3 Se 2 . This solution was subsequently sprayed, in ambient atmosphere, onto Mo-coated substrates in a total of six layers corresponding to approximately 2-2.5 μm of precursor thickness. Further details on the precursor solution preparation and deposition can be found elsewhere. 11 Eagle XG (Corning) glass substrates were used to ensure no additional Na was added to the precursor from the substrate. Mo/Mo-N/Mo multilayers were deposited onto the substrates by DC magnetron sputtering at a base pressure lower than 3 × 10 −6 Torr. The Mo-N served as a diffusion barrier against excessive MoSe 2 formation and was shown to be crucial in improving performance in our solutionprocessed CIGS solar cells. 14 The final Mo multilayer thickness was approximately 1 μm with sheet resistance of 0.4 Ω/sq. NaCl layers were thermally evaporated onto the as-deposited CIGS absorber inside a homemade thermal evaporation system. The evaporation was performed at a base pressure below 3 × 10 −6 Torr, applying current of 60 A to a tungsten boat containing NaCl. The substrates were placed directly above the source at distance of 15 cm. NaCl layers of different thicknesses ranging between 15 and 150 nm were evaporated onto the CIGS substrate, as shown in Figure 1.
The layer thickness during the evaporation was controlled using a quartz crystal microbalance. The absorbers were subsequently selenised in a closed SiC-coated graphite box at 550 C and 200 Torr for 70 min, using 900 mg of Se in a rapid thermal processing (RTP) system. To minimise any 'accidental' Na-doping, the RTP oven and the graphite box were cleaned by high temperature and low pressure annealing under flowing nitrogen and mechanical scrubbing of the quartz annealing tube to remove residual selenium between each run.  some grains extend to the full absorber thickness, which had never been observed for these absorbers when the Na source was SLG.
When thicker NaCl layers were evaporated (100 and 150 nm), the absorber grain size started to decrease. The NaCl layer was still present at the end of the selenisation for these two samples and is clearly visible on the surface of the 150 nm NaCl absorber.
To identify absorber composition and residual NaCl present, EDX surface mapping was performed and confirms the presence of 9.2 at% and 19.4 at% of Cl for 100 nm and 150 nm NaCl-coated absorbers respectively, whereas no Cl was detected for any other sample. Absorber composition of the reference sample (0 nm NaCl) is very close to the targeted CGI = 0.9 and GGI = 0.3. However absorbers with NaCl layers show reduced CGI and GGI ratios. These compositional changes are attributed to the effects of Na, which is known to cause absorber grading by hindering In/Ga interdifusion. 19,30,31 The morphological observations from the SEM images are in agreement with the XRD patterns of the films presented in Figure 3A.
The peak found at 2θ 31.7 in 100 and 150 nm NaCl samples

| PV performance
To study the effect of the NaCl layer on CIGS solar cell performance, the above presented absorber films were made into CIGS devices.
The PV performance indicators are plotted in Figure 4. From Figure 4 it is apparent that variations in J SC with increasing NaCl content follow the same trend as the changes in grain sizes.
However FF and V OC follow more-or-less an opposite trend causing performance degradation for devices with NaCl layers thicker than 15 nm. In addition, samples with the largest grains (15 and 30 nm NaCl) suffered from partial delamination, which might be related to the grain size or the high amount of Na incorporated. Sudden improvement in device performance was observed for the sample with 150 nm NaCl. Here, a visible NaCl layer remained after selenisation on the absorber surface. It is possible that the remaining NaCl layer served as a chemical barrier to protect the surface from oxidation that leads to the formation of detrimental anion Se-Cu divacancies. 36 In addition, the more compact absorber morphology apparent from the SEM micrograph could have been able to accommodate more Na at GBs which manifested by increased carrier concentration ( Figure 6). Increased p-type doping led to an increase in V OC and FF of this device. The remaining highly water soluble NaCl was washed out during the CdS CBD process and therefore did not affect the final device structure.

| Carrier density and lifetime
To better understand the device performance and the role of Na doping, CV and DLCP measurements at room temperature were performed on these devices. The net acceptor concentration (N A ) was obtained from the minima of the doping profiles shown in Figure 6.
The CV measurement and the resulting doping profiles can depend on the amount of Na present in the absorber, however also affected by the morphology. The N A is clearly the lowest for 30 nm of NaCl, 1 × 10 15 cm −3 . The highest doping density was measured for the 150 nm NaCl sample, approximately one order of magnitude higher.
The width of the depletion region (W) at zero bias was estimated from the CV measurement and the values are summarised in Table 3 together with the values for N A extracted from CV and DLCP measurements.
Depletion width at 0 V bias is significantly larger for the 30 nm NaCl sample than for any other studied sample. A larger depletion width improves the long wavelength charge carrier collection; however, lower doping density results in a weaker electric field across the space charge region (SCR). As opposed to CV, the DLCP method is mostly insensitive to the response from interface states. The net acceptor concentrations extracted from these two methods are compared in Table 3. DLCP doping profiles are shown in Figure S1 in the Supplementary Information. For the undoped and 150 nm NaCl To explain the presence of interface states and unexpectedly low carrier concentrations of all the other Na-doped samples, some assumptions and speculations must be made due to the differences in their morphology. It is suggested that high concentration of Na accumulates along CIGS GBs. 19 Na can also be found in grain interior; however, the concentration is in order of magnitude lower than at GBs due to slower volume diffusion. 32 Table 3. Carrier lifetime was obtained from the TRPL spectra by fitting a biexponential decay curve. These values are also shown in Table 3.    Figure 7A. PL signals for 15-50 nm NaCl can be fitted with a single peak, but PL signals corresponding to 100-and 150-nm NaCl are both deformed, fitting two distinct peaks. higher Ga diffusivities resulting in changes in the bandgap across the absorber depth. 19,31,32 In the 100 nm NaCl, sample, the less intense low bandgap peak at 1.14 eV corresponds to a Ga-poor phase whilst the more pronounced high bandgap peak at 1.19 eV to a Ga-rich phase. Alternative hypothesis could be that during selenisation, Na could have replaced some Cu atoms in the CIGS lattice and form a stable Na(In,Ga)Se 2 compound having larger bandgap. Na(In,Ga)Se 2 and Cu(In,Ga)Se 2 having limited mutual solubility will result in phase separation and the precipitation of a secondary phase on surfaces or GBs. 44 It was observed that the bandgaps measured by PL were smaller than those measured using EQE spectra with a particularly big dis-

| Compositional analysis using TEM
To understand the V OC and FF loss in the device with the largest grains, the composition and structure of the device was studied using TEM in combination with EDX elemental mapping. A TEM bright field cross-section through the best performing cell of the 30 nm NaCl sample is shown in Figure 8.

| CONCLUSIONS
The evaporation of NaCl onto CIGS absorbers in order to enhance the morphological and electrical properties of the devices induced by absorber Na doping was successfully achieved. Depositing NaCl by thermal evaporation is a cheap, non-toxic and previously unexplored method of CIGS doping, with lower thermal budget, compared to NaF. NaCl was also employed as a Na precursor for CIGS, however mostly by direct introduction into the precursor solution in solutionbased absorbers. It was shown that NaCl was relatively easy and fast to evaporate under vacuum and its effects on the absorber morphology and performance were similar to those described in the literature for NaF evaporation. 27 It was found that a compromise between the optimum NaCl thickness for CIGS grain growth and the optimum NaCl Similar doping density for most of the evaporated NaCl thicknesses suggests that addition of Na primarily promoted the grain growth of the absorbers consequently improving the CIGS material quality rather than affected the absorber doping. Finally, bandgap changes and segregation of Ga likely forming gallium oxides in the inter-grain voids were also observed in the presence of sodium.
All these observations have to be taken into account for the further optimisation of this approach of sodium doping of solutionprocessed CIGS prepared in ambient conditions. In the future work, methods to minimize absorber porosity will be investigated for more efficient Na doping. These will include deposition by ultrasonic spraying allowing for variation in droplet size and varying the precursor solution dilution. In addition, one method suggested to reduce film porosity of CIGS absorbers is by soaking the absorber in a cation solution in order to fill the voids seen in the film. 51 Overall, this method resulted in a device performance of twice that of the Na-free CIGS solar cells, from PCE of 5% to 10% with evaporation of only 15 nm of NaCl. More NaCl produced higher quality CIGS material with greatly improved lifetime. This is a very promising step towards increasing efficiency of sprayed amine-thiol based CIGS solar cells in the future.