Abstract :
[en] The demand for photovoltaics (PV) is increasing to meet the requirement of the Paris agreement. \ch{Cu(In,Ga)(S,Se)2} (CIGSSe) technology is gaining some parts of the market which is currently dominated by silicon cells. This thesis focuses on the electron transfer layer (buffer layer) of CIGSSe technology. An increase in the demand of PV means an increase in the consumption of raw materials. The current deposition techniques of the buffer produce significant waste of materials because of their low utilization efficiency. In this thesis, inkjet printing, a high material utilization deposition technique, is proposed as an alternative for the buffer layer.\\
Inkjet printing requires the synthesis of an ink that is deposited on a substrate to form a continuous film with the appropriate properties to form a pn junction with CIGSSe. In this work, \ch{In2S3}, CdS and Zn(O,S) layers are synthesized through combustion reaction. Their morphology, chemical composition, crystallinity and optical properties are investigated and their efficiency as buffer layers is tested in CIGSSe devices.
In order to form a desirable film morphology, different solvent mixtures were tested and the ternary water, ethanol and propylene glycol mixture showed the highest power conversion efficiency when applied to the solar cell, which was attributed to a smoother macro-morphology. \\
Solution processed \ch{In2S3} has already been studied in the past and it was argued that a pre-synthesized S-In bond (called tristhioureaindium(III) chloride Complex) in the ink was essential for an oxygen free \ch{In2S3}. It is shown in this work that the Complex is spontaneously created in solution and no pre-formation (powder) is required. The formation of oxide is avoided in both cases and the efficiency of completed devices was comparable to a reference with a Zn(O,S) buffer. \\
In this Complex, the ratio of sulfur to indium ([S]/[In]) is 3. Inks with [S]/[In] ratios of 2, 1.5 and 1 (without pre-synthesis of a complex) were studied and it was shown that a [S]/[In]=2 is optimal in terms of efficiency. The record \ch{In2S3} based device has an efficiency of 16.5\%, which is 0.5\% higher than the reference with a Zn(O,S). In terms of morphology, a high porosity was found for [S]/[In]$\geq$2, while the layers were dense for [S]/[In]$\leq$1.5. This porosity is found to be related to solid state dewetting and is influenced by the sulfur content as well as the substrate nature. The film based on [S]/[In]$\geq$2 did not show the presence of oxide, while the [S]/[In]$\leq$1.5 incorporated oxygen, proving the importance of an excess of thiourea in the ink. \\
An inkjet printable CdS layer was synthesized to replace the wasteful chemical bath deposition. Due to the instability of the ink containing both the cadmium and sulfur sources, two inks were printed successively. The layer ordering is important and the highest efficiency and smoothest morphology was obtained by printing the cadmium source, followed by the thiourea. The efficiency of this inkjet printed buffer in a CIGSSe device was similar to the conventional chemical bath deposited CdS reference.\\
Zn(O,S) is a semiconductor that has attracted interest thanks to its wide band gap, increasing light absorption in the absorber layer. In this work, the band gap of \ch{In2S3}, CdS and Zn(O,S) buffer layers were found wider than their bulk counterparts. This was associated to the small crystallite size producing quantum confinement effects. The Brus equation gave a trend in the variation of the band gap with the crystallite size which can be controlled via the annealing temperature of the material.\\
Inkjet printing is a versatile technique that enables the near zero waste deposition of buffer layers. The efficiency of the devices with inkjet printed buffers are similar to the common deposition techniques that have been used for decades. Many more materials can be synthesized using this technique and the future development is nearly infinite.
