The addition of cesium into Cu(In,Ga)(S,Se)2‐short CIGSSe‐absorber layers (fabricated via vacuum deposition methods), has most recently culminated in devices with record power conversion efficiencies up to 23.4%. However, research is increasingly being devoted to the development of ink deposition routes to prepare high‐quality CIGSSe thin films while requiring only a fraction of the processing costs. Such non‐vacuum deposition routes must compete with efficiencies of incumbent technologies to find adoption on a wide scale. At present, the performance of ink‐based devices still fall short of their vacuum counterparts with certified champion cell efficiencies up to 17.7%. The recent performance progression for vacuum‐processed CIGSSe exemplifies the importance of controlling the concentration of extrinsic impurities and serves as an inspiration for gains (e.g., morphological, optoelectronic) for devices with ink‐based absorber layers. This article reviews extrinsic doping concepts for CIGSSe‐type absorbers fabricated by ink‐based deposition routes (both nanoparticle dispersions and molecular inks), provides a performance comparison of select high‐efficiency ink‐based devices, and offers an outlook for future process development in general. It is suggested that the mechanisms by which dopant atoms diffuse, interact, and alter the properties of an ink‐based absorber are fundamentally different than those fabricated from vacuum‐based processes, and require further investigation.
Large-scale deployment of photovoltaic modules is required to power our renewable energy future. Kesterite, Cu2ZnSn(S, Se)4, is a p-type semiconductor absorber layer with a tunable bandgap consisting of earth abundant elements, and is seen as a potential ‘drop-in’ replacement to Cu(In,Ga)Se2in thin film solar cells. Currently, the record light-to-electrical power conversion efficiency (PCE) of kesterite-based devices is 12.6%, for which the absorber layer has been solution-processed. This efficiency must be increased if kesterite technology is to help power the future. Therefore two questions arise: what is the best way to synthesize the film? And how to improve the device efficiency? Here, we focus on the first question from a solution-based synthesis perspective. The main strategy is to mix all the elements together initially and coat them on a surface, followed by annealing in a reactive chalcogen atmosphere to react, grow grains and sinter the film. The main difference between the methods presented here is how easily the solvent, ligands, and anions are removed. Impurities impair the ability to achieve high performance (>∼10% PCE) in kesterite devices. Hydrazine routes offer the least impurities, but have environmental and safety concerns associated with hydrazine. Aprotic and protic based molecular inks are environmentally friendlier and less toxic, but they require the removal of organic and halogen species associated with the solvent and precursors, which is challenging but possible. Nanoparticle routes consisting of kesterite (or binary chalcogenides) particles require the removal of stabilizing ligands from their surfaces. Electrodeposited layers contain few impurities but are sometimes difficult to make compositionally uniform over large areas, and for metal deposited layers, they have to go through several solid-state reaction steps to form kesterite. Hence, each method has distinct advantages and disadvantages. We review the state-of-the art of each and provide perspective on the different strategies.
more » « less- NSF-PAR ID:
- 10302757
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Journal of Physics: Energy
- Volume:
- 2
- Issue:
- 1
- ISSN:
- 2515-7655
- Page Range / eLocation ID:
- Article No. 012003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Earth‐abundant and air‐stable Cu2BaSnS4−
x Sex (CBTSSe) and related thin‐film absorbers are regarded as prospective options to meet the increasing demand for low‐cost solar cell deployment. Devices based on vacuum‐deposited CBTSSe absorbers have achieved record power conversion efficiency (PCE) of 5.2% based on a conventional device structure using CdS buffer and i‐ZnO/indium tin oxide (ITO) window layers, with open‐circuit voltage (V OC) posing the major bottleneck for improving solar cell performance. The current study demonstrates a >20% improvement inV OC(from 0.62 to 0.75 V) and corresponding enhancement in PCE (from 5.1% to 6.2% without antireflection coating; to 6.5% with MgF2antireflection coating) for solution‐deposited CBTSSe solar cells. This performance improvement is realized by introducing an alternative successive ionic layer adsorption and reaction‐deposited Zn1−x Cdx S buffer combined with sputtered Zn1−x Mgx O/Al‐doped ZnO window/top contact layer, which offers lower electron affinities relative to the conventional CdS/i‐ZnO/ITO stack and better matching with the low electron affinity of CBTSSe. A combined experimental (temperature‐ and light intensity‐dependentV OCmeasurements) and device simulation (SCAPS‐1D) evaluation points to the importance of addressing relative band offsets for both the buffer and window layers relative to the absorber in mitigating interfacial recombination and optimizing CBTSSe solar cell performance. -
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In recent years, Cu2ZnSn(S,Se)4(CZTSSe) materials have enabled important progress in associated thin‐film photovoltaic (PV) technology, while avoiding scarce and/or toxic metals; however, cationic disorder and associated band tailing fundamentally limit device performance. Cu2BaSnS4(CBTS) has recently been proposed as a prospective alternative large bandgap (~2 eV), environmentally friendly PV material, with ~2% power conversion efficiency (PCE) already demonstrated in corresponding devices. In this study, a two‐step process (i.e., precursor sputter deposition followed by successive sulfurization/selenization) yields high‐quality nominally pinhole‐free films with large (>1 µm) grains of selenium‐incorporated (
x = 3) Cu2BaSnS4−x Sex (CBTSSe) for high‐efficiency PV devices. By incorporating Se in the sulfide film, absorber layers with 1.55 eV bandgap, ideal for single‐junction PV, have been achieved within the CBTSSe trigonal structural family. The abrupt transition in quantum efficiency data for wavelengths above the absorption edge, coupled with a strong sharp photoluminescence feature, confirms the relative absence of band tailing in CBTSSe compared to CZTSSe. For the first time, by combining bandgap tuning with an air‐annealing step, a CBTSSe‐based PV device with 5.2% PCE (total area 0.425 cm2) is reported, >2.5× better than the previous champion pure sulfide device. These results suggest substantial promise for the emerging Se‐rich Cu2BaSnS4–x Sex family for high‐efficiency and earth‐abundant PV. -
Solution processing of CuInSe 2 /CuInGaSe 2 (CISe/CIGSe) photovoltaic devices via non-hydrazine based routes has been studied for the past few years and a significant improvement in the device performance has been achieved for multiple solvent routes. However, none of these routes have ever reported the fabrication of absorbers with a thickness of above 1.2–1.3 microns which is almost half of what has been traditionally used in vacuum based high efficiency CIGSe devices. The main reason for this limitation is associated with the formation of a fine-grain layer in solution based systems. Here we manipulate the formation of such a fine-grain layer in an amine–thiol based solution route through surface modifications at the bottom Mo interface and achieve an active area efficiency of up to 14.1% for CIGSe devices. Furthermore, with a detailed analysis of the fine-grain layer, not just in the amine–thiol based film, but also in the film fabricated via the dimethylformamide-thiourea route, we identify the reason for the formation of such a fine-grain layer as the presence of the sulfide material and carbon impurity (if any) in the precursor film. We utilize the amine–thiol solvent system's ability for selenium and metal selenide dissolution to manipulate the ink formulations and demonstrate the reduction in the formation of sulfide materials as well as the extent of trapped carbon in the precursor film. With modified precursor films, we then successfully grow CISe/CIGSe thin films of 2-micron thickness with the complete absence of a fine-grain layer through a high temperature, thickness independent bulk growth mechanism making the film morphology similar to the one fabricated using a high efficiency hydrazine based route.more » « less
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A solid‐state thermoelectric device is attractive for diverse technological areas such as cooling, power generation and waste heat recovery with unique advantages of quiet operation, zero hazardous emissions, and long lifetime. With the rapid growth of flexible electronics and miniature sensors, the low‐cost flexible thermoelectric energy harvester is highly desired as a potential power supply. Herein, a flexible thermoelectric copper selenide (Cu2Se) thin film, consisting of earth‐abundant elements, is reported. The thin film is fabricated by a low‐cost and scalable spin coating process using ink solution with a truly soluble precursor. The Cu2Se thin film exhibits a power factor of 0.62 mW/(m K2) at 684 K on rigid Al2O3substrate and 0.46 mW/(m K2) at 664 K on flexible polyimide substrate, which is much higher than the values obtained from other solution processed Cu2Se thin films (<0.1 mW/(m K2)) and among the highest values reported in all flexible thermoelectric films to date (≈0.5 mW/(m K2)). Additionally, the fabricated thin film shows great promise to be integrated with the flexible electronic devices, with negligible performance change after 1000 bending cycles. Together, the study demonstrates a low‐cost and scalable pathway to high‐performance flexible thin film thermoelectric devices from relatively earth‐abundant elements.
