Deliberate and Accidental Gas-Phase Alkali Doping of Chalcogenide Semiconductors: Cu(In,Ga)Se2

Above 15% Efficient Directly Sputtered CIGS Solar Cells Enabled by a Modified Back-Contact Interface

The Schottky back-contact barrier at the Mo/Cu(In,Ga)Se₂ (CIGS) interface is one of the critical issues that restrict the photovoltaic performance of CIGS solar cells. The formation of a MoSe₂ intermediate layer can effectively reduce this back-contact barrier leading to efficient hole transport. However, the selenium-free atmosphere is unfavorable for the formation of the desired MoSe₂ intermediate layer if the CIGS films are prepared by the commonly used direct sputtering process. In this work, high-efficiency CIGS solar cells with a MoSe₂ intermediate layer were fabricated by the direct sputtering process without a selenium atmosphere. This is enabled by an intermediate CIGS layer deposited on the Mo substrate at room temperature before being ramped to a high temperature (600 °C). The room-temperature-deposited amorphous CIGS intermediate layer is Se rich, which reacts with the Mo substrate and forms very thin MoSe₂ at the interface during the high-temperature process. The formed MoSe₂ decreased the CIGS/Mo barrier height for better hole transport. Consequently, the CIGS solar cell with an 80 nm intermediate layer achieved a power conversion efficiency of up to 15.8%, which is a benchmark efficiency for the direct sputtering process without Se supply. This work provides the industry a new approach for commercialization of directly sputtered CIGS solar cells.

Aluminum Precursor Interactions with Alkali Compounds in Thermal Atomic Layer Etching and Deposition Processes

Surface fluorination and volatilization using hydrogen fluoride and trimethyaluminum (TMA) is a useful approach to the thermal atomic layer etching of Al₂O₃. We have previously shown that significant enhancement of the TMA etching effect occurs when performed in the presence of lithium fluoride chamber-conditioning films. Now, we extend this enhanced approach to other alkali halide compounds including NaCl, KBr, and CsI. These materials are shown to have varying capacities for the efficient removal of AlF₃ and ultimately lead to larger effective Al₂O₃ etch rates at a given substrate temperature. The most effective compounds allow for continuous etching of Al₂O₃ at substrate temperatures lower than 150 °C, which can be a valuable route for processing temperature-sensitive substrates and for improving the selectivity of the etch over other materials. The strong interaction between TMA and alkali halide materials also results in material-selective thin-film deposition at these reduced substrate temperatures. We discuss possible mechanisms of this etching enhancement and prospects for extending this approach to other material systems. The consequences of using TMA as an ALD and ALE precursor are discussed in the context of interface engineering for alkali-containing substrates such as lithium battery materials.

Micro-sized thin-film solar cells via area-selective electrochemical deposition for concentrator photovoltaics application

Micro-concentrator solar cells enable higher power conversion efficiencies and material savings when compared to large-area non-concentrated solar cells. In this study, we use materials-efficient area-selective electrodeposition of the metallic elements, coupled with selenium reactive annealing, to form Cu(In,Ga)Se₂ semiconductor absorber layers in patterned microelectrode arrays. This process achieves significant material savings of the low-abundance elements. The resulting copper-poor micro-absorber layers’ composition and homogeneity depend on the deposition charge, where higher charge leads to greater inhomogeneity in the Cu/In ratio and to a patchy presence of a CuIn₅Se₈ OVC phase. Photovoltaic devices show open-circuit voltages of up to 525 mV under a concentration factor of 18 ×, which is larger than other reported Cu(In,Ga)Se₂ micro-solar cells fabricated by materials-efficient methods. Furthermore, a single micro-solar cell device, measured under light concentration, displayed a power conversion efficiency of 5% under a concentration factor of 33 ×. These results show the potential of the presented method to assemble micro-concentrator photovoltaic devices, which operate at higher efficiencies while using light concentration.

Combinatorial Nitrogen Gradients in Sputtered Thin Films

High-throughput synthesis and characterization methods can significantly accelerate the rate of experimental research. For physical vapor deposition (PVD), these methods include combinatorial sputtering with intentional gradients of metal/metalloid composition, temperature, and thickness across the substrate. However, many other synthesis parameters still remain out of reach for combinatorial methods. Here, we extend combinatorial sputtering parameters to include gradients of gaseous elements in thin films. Specifically, a nitrogen gradient was generated in a thin film sample library by placing two MnTe sputtering sources with different gas flows (Ar and Ar/N₂) opposite of one another during the synthesis. The nitrogen content gradient was measured along the sample surface, correlating with the distance from the nitrogen source. The phase, composition, and optoelectronic properties of the resulting thin films change as a function of the nitrogen content. This work shows that gradients of gaseous elements can be generated in thin films synthesized by sputtering, expanding the boundaries of combinatorial science.

Reducing series resistance in Cu2ZnSn(S,Se)4 nanoparticle ink solar cells on flexible molybdenum foil substrates

Earth abundant Cu₂ZnSnS₄ nanoparticle inks were deposited on molybdenum foil substrates and subsequently converted to high quality thin film Cu₂ZnSn(S,Se)₄ photovoltaic absorbers. Integration of these absorbers within a thin film solar cell device structure yields a solar energy conversion efficiency which is comparable to identical devices processed on rigid glass substrates. Importantly, this is only achieved when a thin layer of molybdenum is first applied directly to the foil. The layer limits the formation of a thick Mo(S,Se)_x layer resulting in a substantially reduced series resistance.

The flexible CZTSSe solar cells on Mo foil achieved efficiency of 3.8%.