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.
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Extrinsic Doping of Ink‐Based Cu(In,Ga)(S,Se) 2 ‐Absorbers for Photovoltaic Applications
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.
more » « less- Award ID(s):
- 1735282
- NSF-PAR ID:
- 10367022
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Energy Materials
- Volume:
- 12
- Issue:
- 18
- ISSN:
- 1614-6832
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Earth‐abundant and air‐stable Cu2BaSnS4−
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In recent years, oxide electronics has emerged as one of the most promising new technologies for a variety of electrical and optoelectronic applications, including next-generation displays, solar cells, batteries, and photodetectors. Oxide electronics have a lot of potential because of their high carrier mobilities and ability to be manufactured at low temperatures. However, the preponderance of oxide semiconductors is n-type oxides, limiting present applications to unipolar devices and stifling the development of oxide-based bipolar devices like p-n diodes and complementary metal-oxide–semiconductors. We have contributed to oxide electronics, particularly on transition metal oxide semiconductors of which the cations include In, Zn, Sn and Ga. We have integrated these oxide semiconductors into thin film transistors (TFTs) as active channel layer in light of the unique combination of electronic and optical properties such as high carrier mobility (5-10 cm2/Vs), optical transparency in the visible regime (>~90%) and mild thermal budget processing (200-400°C). In this study, we achieved four different results. The first result is that unlike several previous reports on oxide p-n junctions fabricated exploiting a thin film epitaxial growth technique (known as molecular beam epitaxy, MBE) or a high-powered laser beam process (known as pulsed laser deposition, PLD) that requires ultra-high vacuum conditions, a large amount of power, and is limited for large-area processing, we demonstrate oxide-based heterojunction p-n diodes that consist of sputter-synthesized p-SnOx and n-IGZO of which the manufacturing routes are in-line with current manufacturing requirements. The second result is that the synthesized p-SnOx films are devoid of metallic Sn phases (i.e., Sn0 state) with carrier density tuneability and high carrier mobility (> 2 cm2/Vs). The third result is that the charge blocking performance of the metallurgical junction is significantly enhanced by the engineering of trap/defect density of n-IGZO, which is identified using photoelectron microscopy and valence band measurements. The last result is that the resulting oxide-based p-n heterojunction exhibits a high rectification ratio greater than 103 at ±3 V (highest among the sputter-processed oxide junctions), a low saturation current of ~2×10-10 A, and a small turn-on voltage of ~0.5 V. The outcomes of the current study are expected to contribute to the development of p-type oxides and their industrial device applications such as p-n diodes of which the manufacturing routes are in-line with the current processing requirements.more » « less
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Abstract 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.
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