It is well established that the addition of sodium (Na) to chalcopyrite or kesterite based solar cells markedly increases the solar cell performance. In this work, we explore the effect of Na and other alkali metals like potassium (K), rubidium, caesium and lithium (Li)on pure selenide Cu2ZnSnSe4 (CZTSe) solar cells. We demonstrate the deposition of alkali metals using spin coating on e-beam evaporated metal precursors. The stack of metal precursors with alkali layer was then selenised at high temperatures to obtain a good quality CZTSe absorber. The diffusion of alkali metals into the absorber layer was confirmed using glow discharge optical emission spectroscopy. Samples doped with Na or K have shown improvement in the open circuit voltage. A maximum power conversion efficiency of 8.3 % (without anti-reflection coating) and a top open circuit voltage 430 mV was achieved for combination of K and Na. Amongst the rest of alkali metals, Li looks the most promising dopant as far as optoelectronic properties are concerned.

The fabrication and properties of a Ge-based Kesterite Cu2ZnGeSe4 (CZGSe)solar cell are discussed. The existence of the quaternary compound has been verified by physical methods such as X Ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). The Cu2ZnGeSe4 solar cell has a power conversion efficiency (PCE) of 5.5% under AM1.5G illumination which is among the highest reported for pure Ge substitution. Detailed low temperature current-voltage and time-resolved photoluminescence measurements show that the Cu2ZnGeSe4 absorber has less bulk defects and less band tailing in contrast to the typical characteristics of Cu2ZnSnSe4 devices. These beneficial opto-electronic properties also resulted in a high open circuit voltage (Voc) of 744 mV which is amongst the highest values reported for Kesterite materials.

In this work we developed a method to grow Sb 2 Se 3 thin films with a potential use as absorber layers in solar cell structures. The films were grown on top of soda-lime glass, Mo coated soda-lime glass and Si substrates. The Sb-Se precursor's films were deposited by RF magnetron sputtering and then annealed with an H 2 Se gas flow. Different annealing temperatures were tested and analyzed. Compositional and morphological analyses are performed by Energy Dispersive Spectroscopy and Scanning Electron Microscopy, respectively. Phase identification and structural characterization are done by X-ray Diffraction and Dispersive Raman Spectroscopy showing that Sb 2 Se 3 is the dominant phase with an orthorhombic crystalline structure. Traces of rhombohedral and amorphous Se secondary phases are also observed supported by their Se-rich compositions. Spectrophotometry measurements allowed to extract a direct bandgap with a value close to 1.06 eV. Photoluminescence spectroscopy shows a broad shoulder at 0.85 eV for samples selenized at lower temperatures and an intense peak at 0.75 eV for the sample selenized at higher temperatures. Electrical characterization shows low free hole concentrations and mobilities. At low temperatures, the analysed samples show that nearest neighbour hopping is the dominant mechanism for the electronic transport. A discussion on the properties that need to be improved in order that these films can be used in thin film solar cells is done.

In this study, an ultra‐thin MoO3 layer synthesized by a solution‐based technique is introduced as a promising interfacial layer to improve the performance of kesterite Cu2ZnSnSe4 (CZTSe) solar cell. Solar cells with 10 nm of MoO3 between Mo rear contact and CZTSe had larger minority carrier life time and open‐circuit voltage compared to the reference solar cells. Temperature dependent current density–voltage measurement indicated that the activation energy (EA) of the main recombination is higher (∼ 837 meV) in solar cells with MoO3 layer, as compared to conventional solar cells where EA ∼ 770 meV, indicating reduced interface recombination. A best efficiency of 7.1% was achieved for a SLG/Mo/MoO3/CZTSe/CdS/TCO solar cell compared to the reference solar cell SLG/Mo/CZTSe/CdS/TCO for which 5.9% efficiency was achieved.

A study of the electronic conduction mechanisms and electrically active defects in polycrystalline Sb 2 Se 3 is presented. It is shown that for temperatures above 200 K, the electrical transport is dominated by thermal emission of free holes, ionized from shallow acceptors, over the intergrain potential barriers. In this temperature range, the temperature dependence of the mobility of holes, limited by the intergrain potential barriers, is the main contributor to the observed thermal activation energy of the conductivity of 485 meV. However, at lower temperatures, nearest-neighbor and Mott variable range hopping transport in the bulk of the grains turn into the dominant conduction mechanisms. Important parameters of the electronic structure of the Sb 2 Se 3 thin film such as the average intergrain potential barrier height ϕ = 391 meV, the intergrain trap density N t = 3.4 × 10 11 cm −2 , the shallow acceptor ionization energy E A0 = 124 meV, the acceptor density N A = 1 × 10 17 cm −3 , the net donor density N D = 8.3 × 10 16 cm −3 , and the compensation ratio k = 0, 79 were determined from the analysis of these measurements.

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In this work, we used a solution-processed TiO 2 layer between Cu 2 ZnSnSe 4 and CdS buffer layer to reduce the recombination at the p-n junction. Introducing the TiO 2 layer showed a positive impact on V O C but fill factor and efficiency decreased. Using a KCN treatment, we could create openings in the TiO 2 layer, as confirmed by transmission electron microscopy measurements. Formation of these openings in the TiO 2 layer led to the improvement of the short-circuit current, fill factor, and the efficiency of the modified solar cells. Manuscript

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