David B. Mitzi

High-Efficiency Solar Cell with Earth-Abundant Liquid-Processed Absorber

2010 WILEY-VCH Verlag Gmb Chalcogenide-based solar cells provide a critical pathway to cost parity between photovoltaic (PV) and conventional energy sources. Currently, only Cu(In,Ga)(S,Se)2 (CIGS) and CdTe technologies have reached commercial module production with stable power conversion efficiencies of over 9 percent. Despite the promise of these technologies, restrictions on heavy metal usage for Cd and limitations in supply for In and Te are projected to restrict the production capacity of the existing chalcogen-based technologies to <100GWp per year, a small fraction of our growing energy needs, which are expected to double to 27 TW by 2050. Earth-abundant copper-zinc-tin-chalcogenide kesterites, Cu2ZnSnS4 and Cu2ZnSnSe4, have been examined as potential alternatives for the two leading technologies, reaching promising but not yet marketable efficiencies of 6.7% and 3.2%, respectively, by multilayer vacuum deposition. Here we show a non-vacuum, slurry-based coating method that combines advantages of both solution processing and particlebased deposition, enabling fabrication of Cu2ZnSn(Se,S)4 devices with over 9.6% efficiency—a factor of five performance improvement relative to previous attempts to use highthroughput ink-based approaches and >40% higher than previous record devices prepared using vacuum-based methods. To address the issue of cost, non-vacuum ‘‘ink’’-based approaches—both from solutions and suspensions—are being developed for chalcogenide-based absorber layer deposition to replace potentially more expensive vacuum-based techniques. True solutions allow intermixing of the constituents at a molecular level and the formation of smooth homogeneous films, as demonstrated with spin-coated CIGS absorber layers from hydrazine (N2H4) solutions. [11–13] The chemically reducing character of hydrazine stabilizes solutions of anions with direct metal-chalcogen bonding for select elements (e.g. Cu, In, Ga, Sn), without the necessity to introduce typical impurities (e.g., C, O, Cl). Suspension approaches employ solid particles designed to be deposited on a substrate and reacted or fused with each other, to form a desired crystalline phase and grain structure. Normally insoluble components can be deposited by this approach using typical liquid-based deposition (e.g., printing, spin coating, slit casting, spraying). Although high-quality large-grained absorber layers can be formed for selected systems using either solutionor particlebased deposition, numerous challenges confront each approach for more general deposition needs. Solution processing is limited by the solubility of many materials of interest (e.g., ZnSe1–xSx in hydrazine solvents—relevant for the deposition of Cu2ZnSnS4 or Cu2ZnSnSe4). In addition, volume contraction upon drying of solution-deposited layers creates stress in the film that may cause crack formation in thicker films. In suspension approaches, a common difficulty is achieving single-phase crystallization among the solid particles. Particle-based approaches (as well as some solution methods) typically require the addition of organic agents to improve wetting and particle dispersion, and to avoid film cracks and delamination. Most of these non-volatile organic additives introduce carbon contamination in the final layer. Because of these challenges, vacuum-based techniques have historically shown superior performance to liquid coating. In the case of the earth-abundant Cu2ZnSn(S,Se)4 materials, ink-based approaches have to date yielded at most <1.6% efficiency devices. Here we demonstrate an hybrid solution-particle approach, using the earth-abundant Cu2ZnSn(S,Se)4 system as an example, which enables fabrication of PV devices with over 9.6% power conversion efficiency. The slurry (or ink) employed for deposition comprises a Cu–Sn chalcogenide (S or S–Se) solution in hydrazine (see Experimental section), with the in situ formation of readily dispersible particle-based Zn-chalcogenide precursors, ZnSe(N2H4) (Figure 1a,d) or ZnS(N2H4) (Figure 1b). Thermogravimetric analysis (TGA) of the isolated selenide particle precursor shows decomposition at approximately 200 8C, with mass loss of about 20%, close to the theoretical value expected upon transition to pure ZnSe (Figure 1c,d). Deposition using this hybrid slurry successfully combines the advantages of solution and suspension deposition routes by use of solutions containing solid particles, wherein both components (i.e., solution and particle) contain metal and chalcogen elements that integrate into the final film. Using the hybrid slurry method (i) solubility limitations are resolved, as virtually any materials system can be constituted by a combination of solid and dissolved components; (ii) the dissolved components can be engineered as an efficient binding media for the particles, eliminating the need of separate organic binders; (iii) solid particles act as stress-relief and crack-deflection centers allowing the deposition of thicker layers than pure solution processes; and (iv) the intimate contact between the two phases allows rapid reaction and homogeneous phase formation. Complete conversion of all constituents of the spin-coated hybrid precursor films into a single-phase, highly crystalline Cu2ZnSn(S,Se)4 is achieved by annealing at 540 8C on a hot plate. Three main types of samples were targeted – high selenium content (A), intermediate sulfoselenide (B) and pure sulfide (C) –

Materials interface engineering for solution-processed photovoltaics

Advances in solar photovoltaics are urgently needed to increase the performance and reduce the cost of harvesting solar power. Solution-processed photovoltaics are cost-effective to manufacture and offer the potential for physical flexibility. Rapid progress in their development has increased their solar-power conversion efficiencies. The nanometre (electron) and micrometre (photon) scale interfaces between the crystalline domains that make up solution-processed solar cells are crucial for efficient charge transport. These interfaces include large surface area junctions between photoelectron donors and acceptors, the intralayer grain boundaries within the absorber, and the interfaces between photoactive layers and the top and bottom contacts. Controlling the collection and minimizing the trapping of charge carriers at these boundaries is crucial to efficiency.

Thermally evaporated Cu2ZnSnS4 solar cells

High efficiency Cu2ZnSnS4 solar cells have been fabricated on glass substrates by thermal evaporation of Cu, Zn, Sn, and S. Solar cells with up to 6.8% efficiency were obtained with absorber layer thicknesses less than 1 μm and annealing times in the minutes. Detailed electrical analysis of the devices indicate that the performance of the devices is limited by high series resistance, a “double diode” behavior of the current voltage characteristics, and an open circuit voltage that is limited by a carrier recombination process with an activation energy below the band gap of the material.

Organic–Inorganic Perovskites: Structural Versatility for Functional Materials Design

Although known since the late 19th century, organic-inorganic perovskites have recently received extraordinary research community attention because of their unique physical properties, which make them promising candidates for application in photovoltaic (PV) and related optoelectronic devices. This review will explore beyond the current focus on three-dimensional (3-D) lead(II) halide perovskites, to highlight the great chemical flexibility and outstanding potential of the broader class of 3-D and lower dimensional organic-based perovskite family for electronic, optical, and energy-based applications as well as fundamental research. The concept of a multifunctional organic-inorganic hybrid, in which the organic and inorganic structural components provide intentional, unique, and hopefully synergistic features to the compound, represents an important contemporary target.

Conducting Layered Organic-inorganic Halides Containing -Oriented Perovskite Sheets

Single crystals of the layered organic-inorganic perovskites, [NH2C(I=NH2]2(CH3NH3)m SnmI3m+2, were prepared by an aqueous solution growth technique. In contrast to the recently discovered family, (C4H9NH3)2(CH3NH3)n-1SnnI3n+1, which consists of (100)-terminated perovskite layers, structure determination reveals an unusual structural class with sets of m <110>-oriented CH3NH3SnI3 perovskite sheets separated by iodoformamidinium cations. Whereas the m = 2 compound is semiconducting with a band gap of 0.33 � 0.05 electron volt, increasing m leads to more metallic character. The ability to control perovskite sheet orientation through the choice of organic cation demonstrates the flexibility provided by organic-inorganic perovskites and adds an important handle for tailoring and understanding lower dimensional transport in layered perovskites.

High-mobility ultrathin semiconducting films prepared by spin coating.

The ability to deposit and tailor reliable semiconducting films (with a particular recent emphasis on ultrathin systems) is indispensable for contemporary solid-state electronics. The search for thin-film semiconductors that provide simultaneously high carrier mobility and convenient solution-based deposition is also an important research direction, with the resulting expectations of new technologies (such as flexible or wearable computers, large-area high-resolution displays and electronic paper) and lower-cost device fabrication. Here we demonstrate a technique for spin coating ultrathin (∼50 Å), crystalline and continuous metal chalcogenide films, based on the low-temperature decomposition of highly soluble hydrazinium precursors. We fabricate thin-film field-effect transistors (TFTs) based on semiconducting SnS2-xSex films, which exhibit n-type transport, large current densities (>105 A cm-2) and mobilities greater than 10 cm2 V-1 s-1—an order of magnitude higher than previously reported values for spin-coated semiconductors. The spin-coating technique is expected to be applicable to a range of metal chalcogenides, particularly those based on main group metals, as well as for the fabrication of a variety of thin-film-based devices (for example, solar cells, thermoelectrics and memory devices).

Band tailing and efficiency limitation in kesterite solar cells

We demonstrate that a fundamental performance bottleneck for hydrazine processed kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with efficiencies reaching above 11% can be the formation of band-edge tail states, which quantum efficiency and photoluminescence data indicate is roughly twice as severe as in higher-performing Cu(In,Ga)(S,Se)2 devices. Low temperature time-resolved photoluminescence data suggest that the enhanced tailing arises primarily from electrostatic potential fluctuations induced by strong compensation and facilitated by a lower CZTSSe dielectric constant. We discuss the implications of the band tails for the voltage deficit in these devices.

High Efficiency Cu2ZnSn(S,Se)4 Solar Cells by Applying a Double In2S3/CdS Emitter

High-efficiency Cu2ZnSn(S,Se)4 solar cells are reported by applying In2S3/CdS double emitters. This new structure offers a high doping concentration within the Cu2ZnSn(S,Se)4 solar cells, resulting in a substantial enhancement in open-circuit voltage. The 12.4% device is obtained with a record open-circuit voltage deficit of 593 mV.

Loss mechanisms in hydrazine-processed Cu2ZnSn(Se,S)4 solar cells

We present a device characterization study for hydrazine-processed kesterite Cu2ZnSn(Se,S)4 (CZTSSe) solar cells with a focus on pinpointing the main loss mechanisms limiting device efficiency. Temperature-dependent study and time-resolved photoluminescence spectroscopy on these cells, in comparison to analogous studies on a reference Cu(In,Ga)(Se,S)2 (CIGS) cell, reveal strong recombination loss at the CZTSSe/CdS interface, very low minority-carrier lifetimes, and high series resistance that diverges at low temperature. These findings help identify the key areas for improvement of these CZTSSe cells in the quest for a high-performance indium- and tellurium-free solar cell.

Solution-processed inorganic semiconductors

The search for semiconductors that can be solution-processed into thin-film form at low temperature, while simultaneously providing quality device characteristics, represents a significant challenge for materials chemists. Continuous thin films with field-effect mobilities of 10 cm2 V−1 s−1 or greater are particularly desirable for high-speed microelectronic applications. Attainment of this goal should provide important opportunities for electronic devices, including potentially low-cost, large-area and flexible computing devices, displays, sensors and solar cells. While the majority of work toward this goal has focused on organic semiconductors (both molecular and polymeric), with highest reported mobilties in the range of 1 cm2 V−1 s−1, this review will address recent developments in the search for semiconductors with extended inorganic frameworks (offering the potential for higher mobility) that can be processed from solution using high-throughput, low-cost and low-temperature techniques such as spin-coating, printing or stamping. Two areas of recent interest will be highlighted – tin(II) iodide based organic–inorganic hybrids and soluble chalcogenide semiconductors. Application of the resulting thin films in devices will primarily be discussed in terms of thin-film field-effect transistors (TFTs), although other device applications can be envisioned.