Search

Photovoltaic technologies offer a sustainable solution to the challenge of meeting increasing energy demands. Chalcopyrite Cu(In,Ga)(S,Se)2, short CIGS—thin‐film solar cells—having intrinsically p‐type absorbers with a tunable direct bandgap—exhibits one of the highest stabilized power conversion efficiencies of 23.35%, utilizing absorbers typically fabricated via vacuum deposition methods. Research is increasingly devoted to absorbers deposited by solution processing techniques, which may inherently improve material usage, increase throughput, and lower financial barriers to commercialization. However, the performance of current devices with solution‐processed absorbers is still falling short of their vacuum‐processed counterparts with record power conversion efficiencies up to 18.7% reported to date. While hydrazine solvent‐based routes offer reduced residual impurities, their toxicity poses hindrances to widespread adoption. Alternatively, less toxic and environmentally friendly routes based on protic and aprotic solvents are being researched and are showing promising device efficiencies well above 14%. This review describes the current status of CIGS solar cell absorber layers fabricated by pure solution‐based deposition methods, provides a comparison of champion solution‐processed devices (with and without hydrazine), and offers an outlook for future improvements. [ABSTRACT FROM AUTHOR]

Abstract: A novel molecular‐ink deposition route based on thiourea and N,N‐dimethylformamide (DMF) that results in a certified solar cell efficiency world record for non‐vacuum deposited CuIn(S,Se)2 (CIS) absorbers and non‐vacuum deposited absorbers with a bandgap of 1.0 eV, is presented. It is found that by substituting the widely employed solvent dimethyl sulfoxide with DMF, the coordination chemistry of InCl3 could be altered, dramatically improving ink stability, enabling up to tenfold increased concentrations, omitting the necessity for elevated ink temperatures, and radically accelerating the deposition process. Furthermore, it is shown that by introducing compositionally graded precursor films, film porosity, compositional gradients, and the surface roughness of the absorbers are effectively reduced and device conversion efficiencies are increased up to 13.8% (13.1% certified, active area). The reduced roughness is also seen as crucial to realize monolithically interconnected CIS‐perovskite tandem devices, where semitransparent MAPbI3 devices are directly deposited on the CIS bottom cell. Confirming the feasibility of this approach, monolithic devices with near perfect voltage addition between subcells of up to 1.40 V are presented. [ABSTRACT FROM AUTHOR]

Cu2ZnSn(S,Se)4 absorbers deposited from a nontoxic, DMSO-based molecular ink have yielded the most efficient, hydrazine-free Cu2ZnSn(S,Se)4 photovoltaic (PV) device made through solution-processing. Although this chemistry has been widely adopted, absorber morphologies with a device-limiting fine-grained bottom layer are often reported. Here we demonstrate how the annealing profile of coatings from this ink critically affect absorber morphologies. Calibrated glow discharge optical emission spectroscopy (GDOES) is used to determine depth-dependent elemental ratios and atomic concentrations of impurities before and after selenization. An annealing temperature of 400 °C is shown to exhibit the most pronounced fine-grained bottom layer, which contains 10 atom % carbon (C) and 5 atom % nitrogen (N). Raman analysis of the mechanically exfoliated, fine-grained layer reveals that it contains amorphous carbon nitride, which is attributed to the polymerization of thiourea decomposition products during annealing. An annealing temperature of 300 °C avoids this polymerization and allows C and N to escape during selenization, while an annealing temperature of 500 °C vaporizes C and N compounds before selenization. The annealing profile of 500 C 1.5 min/layer removes nearly all but ~0.25 atom % of C and N, which impedes the formation of a fine-grained layer and allows for a PV device efficiency of 10.7%. It is also shown how lithium-doping enhances sodium transport from the soda-lime glass substrate. [ABSTRACT FROM AUTHOR]

Cu2ZnSnS4 (CZTS) layers are deposited by a direct solution coating of metal salts dissolved in dimethyl sulfoxide (DMSO) and thiourea as sulphur source. An intentional Sn-rich precursor is used and the influence of reactive and inert atmosphere on the layer morphology is investigated during high temperature sintering. Larger grain growth is observed when annealing the precursor in inert N2 atmosphere as compared to reactive sulphur atmosphere. An initial Sn-rich precursor composition with Zn/Sn < 1 promotes the grain growth while Sn leaves the sample during crystallization ending up with a final Zn-rich (Zn/Sn > 1) layer composition. [ABSTRACT FROM AUTHOR]

Highlights: [•] Cu2ZnSnSe4 formation during the selenization of metal salt precursor is studied. [•] Growth is kinetically driven and starts with fast formation of Cu–Se phases. [•] Ternary Cu2SnSe3 phase co-exists with Cu2ZnSnSe4 between 340°C and 370°C. [•] Cu2ZnSnSe4 is the dominant phase above >420°C. [ABSTRACT FROM AUTHOR]

Cu2ZnSnSe4 (CZTSe) thin film solar cells are fabricated by a simple, non-vacuum deposition of metal salts dissolved in non-hazardous solvents followed by selenization in Se atmosphere. Despite a residual carbon-rich layer between the back contact and the CZTSe absorber layer, solar cells with up to 4.28% conversion efficiency are obtained for Cu-poor and Zn-rich CZTSe absorbers. A frequently reported problem, the loss of tin, is investigated with respect to the influence of the selenization conditions such as substrate temperature and selenium partial pressure. EDX point measurements directly confirm that the remaining decomposed layer consists of a mixture of binary ZnSe and Cu2− xSe phases if the substrate temperature is too high and not sufficient Se is supplied. [ABSTRACT FROM AUTHOR]

Abstract: A non-vacuum, solution-based method is investigated to deposit thin layers of Cu(In,Ga)Se2, which is the core component of CIGS thin film solar cells. The concept is based on paste coating of Cu, In, and Ga salt solution with an organic binder and a subsequent annealing of the paste in selenium atmosphere. Previous experiments with ethylcellulose as the binder resulted in photo conversion efficiencies up to 6.7%, although further improvements were hindered by a limited thickness of the CIGS layer and a residual carbon layer between the CIGS and metal back contact. In the present work, polymethyl methacrylate (PMMA) is tested as an alternative binder material, which theoretically should leave no char residues upon thermal degradation at temperatures higher than 350°C. A series of pastes with different solvents are prepared and the resulting CIGS layers are investigated systematically by microscopy, SEM, EDX, XRF, and XRD. Absorber layers are processed into Mo/CIGS/CdS/ZnO solar cells and their I–V characteristics are measured. PMMA can be used as the organic binder alternative to cellulose and more complete evaporation of the organic matrix is achieved as compared to the reference cellulose-containing recipe. However, maximum solar cell efficiencies are limited to 3% because the obtained CIGS layers are porous and contain traces of parasitic oxide phases when heated above 330°C in ambient atmosphere. [Copyright &y& Elsevier]

A N, N‐dimethylformamide and thiourea‐based route is developed to fabricate submicron (0.55 and 0.75 µm) thick CuIn(S,Se)2 (CISSe) thin films for photovoltaic applications, addressing challenges of material usage, throughput, and manufacturing costs. However, reducing the absorber film thickness below 1 µm in a regular CISSe solar cell decreases the device efficiency due to losses at the highly‐recombinative, and mediocre‐reflective Mo/CISSe rear interface. For the first time, to mitigate the rear recombination losses, a novel rear contacting structure involving a surface passivation layer and point contact openings is developed for solution processed CISSe films and demonstrated in tangible devices. An atomic layer deposited Al2O3 film is employed to passivate the Mo/CISSe rear surface while precipitates formed via chemical bath deposition of CdS are used to generate nanosized point openings. Consequently, Al2O3 passivated CISSe solar cells show an increase in the open‐circuit voltage (VOC) and short‐circuit current density when compared to reference cells with equivalent absorber thicknesses. Notably, a VOC increase of 59 mV contributes to active area efficiencies of 14.2% for rear passivated devices with 0.75 µm thick absorber layers, the highest reported value for submicron‐based solution processed, low bandgap CISSe solar cells. [ABSTRACT FROM AUTHOR]

Impedance spectroscopy (IS) is an effective characterization technique used to probe and distinguish charge dynamics occurring at different timescales in optoelectronic and electric devices. With the rapid rise of research being conducted on perovskite solar cells (PSCs), IS has significantly contributed to the understanding of their device performance and degradation mechanisms, including metastable effects such as current–voltage hysteresis. The ionic–electronic behavior of PSCs and the presence of a wide variety of perovskite compositions and cell architectures add complexity to the accurate interpretation of the physical processes occurring in these devices. In this review, the most common IS protocols are explained to help perform accurate impedance measurements on PSC devices. It critically reviews the most commonly used equivalent circuits alongside drift‐diffusion modeling as a complementary technique to analyze the impedance response of PSCs. As an emerging method for characterizing the interfacial recombination between the perovskite layer and selective contacts, light intensity modulated impedance spectroscopy technique is further discussed. Lastly, important works on the application of IS measurement protocols for PSCs are summarized followed by a detailed discussion, providing a critical perspective and outlook on the growing topic of IS on PSCs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article discusses a two-dimensional perovskite layer using pentafluorophenylethylammonium (FEA) as a fluoroarene cation inserted between the 3D light-harvesting perovskite film and the hole-transporting material (HTM). Preventing the degradation of metal perovskite solar cells (PSCs) by humid air poses a challenge for their future deployment. The 2D layer enhances interfacial hole extraction, suppressing nonradiative carrier recombination and enabling a power conversion efficiency (PCE).

Solution processing of inorganic thin films has become an important thrust in material research community because it offers low-cost and high-throughput deposition of various functional coatings and devices. Especially inorganic thin film solar cells - macroelectronic devices that rely on consecutive deposition of layers on large-area rigid and flexible substrates - could benefit from solution approaches in order to realize their low-cost nature. This article critically reviews existing deposition approaches of functional layers for chalcogenide solar cells with an extension to other thin film technologies. Only true solutions of readily available metal salts in appropriate solvents are considered without the need of pre-fabricated nanoparticles. By combining three promising approaches, an air-stable Cu(In,Ga)Se2 thin film solar cell with efficiency of 13.8% is demonstrated where all constituent layers (except the metal back contact) are processed from solutions. Notably, water is employed as the solvent in all steps, highlighting the potential for safe manufacturing with high utilization rates. [ABSTRACT FROM AUTHOR]

Cu2ZnSn(S,Se)4thin layers processed fromsolution-deposited earth-abundant precursors emerge as absorber materialsfor low-cost thin film solar cells. A frequently observed drawbackof the chemical solution processingpoor crystallinity of thechalcogenide absorbercan be overcome by employing a sodium-containingreactive agent. We demonstrate a massive improvement in grain growthin the presence of sodium. It enhances the surface chemisorption ofselenium molecules and can promote the formation of liquid Na2Sexphases during reactive annealingof the precursor. The sodium is also incorporated into the semiconductorabsorber and significantly modifies its electronic properties. Byadjusting the sodium precursor quantity, it is possible to tune dopinglevels and gradients to maximize the collection of photogeneratedcarriers in thin film Cu2ZnSn(S,Se)4solar cells.The presented approach can be extended to other solution-processedmetal chalcogenides to enhance their structural and electronic properties,which are critical for applications such as thin film solar cellsand transistors. [ABSTRACT FROM AUTHOR]

Thin-film photovoltaic devices based on chalcopyrite Cu(In,Ga)Se2 (CIGS) absorber layers show excellent light-to-power conversion efficiencies exceeding 20% (refs , ). This high performance level requires a small amount of alkaline metals incorporated into the CIGS layer, naturally provided by soda lime glass substrates used for processing of champion devices. The use of flexible substrates requires distinct incorporation of the alkaline metals, and so far mainly Na was believed to be the most favourable element, whereas other alkaline metals have resulted in significantly inferior device performance. Here we present a new sequential post-deposition treatment of the CIGS layer with sodium and potassium fluoride that enables fabrication of flexible photovoltaic devices with a remarkable conversion efficiency due to modified interface properties and mitigation of optical losses in the CdS buffer layer. The described treatment leads to a significant depletion of Cu and Ga concentrations in the CIGS near-surface region and enables a significant thickness reduction of the CdS buffer layer without the commonly observed losses in photovoltaic parameters. Ion exchange processes, well known in other research areas, are proposed as underlying mechanisms responsible for the changes in chemical composition of the deposited CIGS layer and interface properties of the heterojunction. [ABSTRACT FROM AUTHOR]

Abstract: A non-vacuum, solution-based method to deposit thin layers of (CZTSe) as absorber layer for kesterite-based thin film solar cells was investigated. The concept is based on multiple spin-coated layers of ether solutions containing Cu, Zn, and Sn salts as precursors and monoethanolamine (MEA) as stabilizer, which are subsequently converted into CZTSe layers by annealing in selenium atmosphere. The influence of initial metal ratios and the number of spin-coated layers on the composition and homogeneity of the CZTSe layers was studied. Both zinc and, to a smaller extent, tin contents tend to decrease in selenized CZTSe layers, because of the partial evaporation of their volatile compounds during the selenization process. Using optimized metal ratios in the precursor solution, single phase kesterite layers of thickness were obtained. Increasing the number of spin-coating cycles caused the formation of “bubbles” between the CZTSe and the Mo layer, as a result of the compressive stress originating from the volume expansion during the selenization step. SEM micrographs and EDX compositional measurements of cross-sections also revealed a thin carbon-rich layer at the Mo back contact/CZTSe interface, which nevertheless provided a sufficient electrical contact between the metal contact and the absorber. The optical energy gap of the kesterite absorber determined from a quantum efficiency measurement is about 0.9eV. Solar cells with efficiencies of up to 2.76%, with of 381mV, of 15.8mA/cm2 and fill factor of 42.1% were achieved for copper poor and zinc rich CZTSe absorbers. [Copyright &y& Elsevier]

Solar cells based on polycrystalline Cu(In,Ga)Se2 absorber layers have yielded the highest conversion efficiency among all thin-film technologies, and the use of flexible polymer films as substrates offers several advantages in lowering manufacturing costs. However, given that conversion efficiency is crucial for cost-competitiveness, it is necessary to develop devices on flexible substrates that perform as well as those obtained on rigid substrates. Such comparable performance has not previously been achieved, primarily because polymer films require much lower substrate temperatures during absorber deposition, generally resulting in much lower efficiencies. Here we identify a strong composition gradient in the absorber layer as the main reason for inferior performance and show that, by adjusting it appropriately, very high efficiencies can be obtained. This implies that future manufacturing of highly efficient flexible solar cells could lower the cost of solar electricity and thus become a significant branch of the photovoltaic industry. [ABSTRACT FROM AUTHOR]