Hsin-Sheng Duan

Interface engineering of highly efficient perovskite solar cells

A layered approach improves solar cells Perovskite films received a boost in photovoltaic efficiency through controlled formation of charge-generating films and improved current transfer to the electrodes. Zhou et al. lowered the defect density of the film by controlling humidity while the perovskite film formed from lead chloride and methylammonium iodide. Low-temperature processing steps allowed the use of materials that draw current out of the perovskite layer more efficiently. These and other modifications enabled a maximum cell efficiency of just over 19% and an average of 16.6%. Science, this issue p. 542 Optimizing the growth conditions of the perovskite layer and interlayer carrier transport boosts solar cell efficiency. Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.

Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process

Hybrid organic/inorganic perovskites (e.g., CH3NH3PbI3) as light absorbers are promising players in the field of third-generation photovoltaics. Here we demonstrate a low-temperature vapor-assisted solution process to construct polycrystalline perovskite thin films with full surface coverage, small surface roughness, and grain size up to microscale. Solar cells based on the as-prepared films achieve high power conversion efficiency of 12.1%, so far the highest efficiency based on CH3NH3PbI3 with the planar heterojunction configuration. This method provides a simple approach to perovskite film preparation and paves the way for high reproducibility of films and devices. The underlying kinetic and thermodynamic parameters regarding the perovskite film growth are discussed as well.

Controllable self-induced passivation of hybrid lead iodide perovskites toward high performance solar cells.

To improve the performance of the polycrystalline thin film devices, it requires a delicate control of its grain structures. As one of the most promising candidates among current thin film photovoltaic techniques, the organic/inorganic hybrid perovskites generally inherit polycrystalline nature and exhibit compositional/structural dependence in regard to their optoelectronic properties. Here, we demonstrate a controllable passivation technique for perovskite films, which enables their compositional change, and allows substantial enhancement in corresponding device performance. By releasing the organic species during annealing, PbI2 phase is presented in perovskite grain boundaries and at the relevant interfaces. The consequent passivation effects and underlying mechanisms are investigated with complementary characterizations, including scanning electron microscopy (SEM), X-ray diffraction (XRD), time-resolved photoluminescence decay (TRPL), scanning Kelvin probe microscopy (SKPM), and ultraviolet photoemission spectroscopy (UPS). This controllable self-induced passivation technique represents an important step to understand the polycrystalline nature of hybrid perovskite thin films and contributes to the development of perovskite solar cells judiciously.

Metal Oxide Nanoparticles as an Electron‐Transport Layer in High‐Performance and Stable Inverted Polymer Solar Cells

Polymer solar cells have many advantages, including transparency, aesthetically pleasing, fl exibility, and light weight. They are particularly compatible with high throughput and low-cost fabrication processes, which make them a promising photovoltaic technology. [ 1–5 ] These properties enable a wide range of potential applications, even for outer space. [ 6 , 7 ] In the last few years, many high-performance polymers with high solar-cell effi ciency have been reported. [ 8–16 ] Among those, benzodithiophene (BDT) and thionothiophene (TT)-based polymers were the fi rst polymer family to break the 7% and 8% effi ciency barriers. [ 8–12 ] Poly{2,6 ′ -4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4-b] dithiophenealt -5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl) pyrrolo[3,4-c]pyrrole-1,4-dione} (PBDTT-DPP) with a lower bandgap ( ≈ 1.4 eV) showed superior performance in long wavelength regions, which enabled signifi cant progress in tandem solar cells with effi ciency close to 9%. [ 14 ] For historical reasons, these high-effi ciency low-bandgap polymers were mostly evaluated based on standard structures, typically with poly(3,4-ethy lenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a hole-transport layer (HTL) and a low-work-function metal such as Ca as the electron-transport layer (ETL). Inverted polymer solar cells have been developed and continue to grow particularly due to their potential for superior device stability and manufacturing compatibility. [ 17–24 ] In the inverted architecture with the classical poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) active layer, several successful n -type buffer layers such as cesium carbonate (Cs 2 CO 3 ), [ 17 ] titanium oxide (TiO 2 ), [ 22 ] Cs-doped TiO 2 , [ 25 ] zinc oxide (ZnO), [ 18 ] and a combination of ZnO and self-assembled monolayers [ 19 ] have been shown to be able to alter the carrier selectivity of the indium tin oxide (ITO) electrode and convert it to a cathode contact. On the anode side, the most widely used are transition metal oxides

Novel Solution Processing of High‐Efficiency Earth‐Abundant Cu2ZnSn(S,Se)4 Solar Cells

A novel solution-based approach is presented to process earth-abundant Cu(2)ZnSn(S,Se)(4) absorbers using fully dissolved CZTS precursors in which each of the elemental constituents intermix on a molecular scale. This method enables the low-temperature processing of chemically clean kesterite films with excellent homogeneity. The high performance of resulting optoelectronic devices represents a chance to extend the impact of CZTS into the next chapter of thin-film solar cells.

Nanoscale Joule Heating and Electromigration Enhanced Ripening of Silver Nanowire Contacts

Solution-processed metallic nanowire thin film is a promising candidate to replace traditional indium tin oxide as the next-generation transparent and flexible electrode. To date however, the performance of these electrodes is limited by the high contact resistance between contacting nanowires; so improving the point contacts between these nanowires remains a major challenge. Existing methods for reducing the contact resistance require either a high processing power, long treatment time, or the addition of chemical reagents, which could lead to increased manufacturing cost and damage the underlying substrate or device. Here, a nanoscale point reaction process is introduced as a fast and low-power-consumption way to improve the electrical contact properties between metallic nanowires. This is achieved via current-assisted localized joule heating accompanied by electromigration. Localized joule heating effectively targets the high-resistance contact points between nanowires, leading to the automatic removal of surface ligands, welding of contacting nanowires, and the reshaping of the contact pathway between the nanowires to form a more desirable geometry of low resistance for interwire conduction. This result shows the interplay between thermal and electrical interactions at the highly reactive nanocontacts and highlights the control of the nanoscale reaction as a simple and effective way of turning individual metallic nanowires into a highly conductive interconnected nanowire network. The temperature of the adjacent device layers can be kept close to room temperature during the process, making this method especially suitable for use in devices containing thermally sensitive materials such as polymer solar cells.

CZTS nanocrystals: a promising approach for next generation thin film photovoltaics

Cu2ZnSn(S,Se)4 (CZTSSe) has received considerable attention as a material capable of driving the development of low-cost and high performance photovoltaics. Its high absorption coefficient, optimal band gap, and non-toxic, naturally abundant elemental constituents give it a number of advantages over most thin film absorber materials. In this manuscript, we discuss the current status of CZTSSe photovoltaics, and provide a comprehensive review of Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) nanocrystal (NCs)-based fabrication methods and solar cell characteristics. The focus will be on the relevant synthetic chemistry, film deposition, and the production of high efficiency photovoltaic devices. Various colloidal synthesis routes are currently used to form the highest quality CZTSSe film from the nanocrystals with controllable phase, size, shape, composition, and surface ligands. A variety of recipes are summarized for producing nanocrystal inks that are appropriate for forming CZTSSe absorber materials with a wide range of controllable optoelectronic properties. Deposition and post-processing, such as annealing and selenization treatments, play an important role in defining the phase and structure of the resulting material. Various film treatment strategies are outlined here, and their resulting material quality, device performance, and dominant photovoltaic loss mechanisms are discussed. Suggestions regarding needed improvements and future research directions are provided based on the current field of available literature.

Silver Nanowire Composite Window Layers for Fully Solution‐Deposited Thin‐Film Photovoltaic Devices

A silver nanowire-indium tin oxide nanoparticle composite and its successful application to fully solution processed CuInSe(2) solar cells as a window layer are demonstrated, effectively replacing the traditionally sputtered both intrinsic zinc oxide and indium tin oxide layers. The devices utilizing the nanocomposite window layer demonstrate photovoltaic parameters equal to or even beyond those with sputtered intrinsic zinc oxide and indium tin oxide contacts.

A dopant-free organic hole transport material for efficient planar heterojunction perovskite solar cells

We demonstrate efficient planar perovskite solar cells using a dopant-free donor–acceptor (D–A) conjugated small molecule as a hole transport material. The photovoltaic cell reaches a power conversion efficiency (PCE) of 14.9%, which is comparable to or even better than that of the devices using the traditional doped 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) hole transport material under equivalent conditions. We ascribe the high performance to the excellent charge transporting properties of the D–A conjugated small molecule. Time-resolved photoluminescence (PL), transient photocurrent response, and impedance spectroscopy characterization indicate that this D–A conjugated small molecule plays a key role in hole collection and extraction in perovskite based photovoltaic devices. The dopant-free D–A small molecule hole transport material used here not only improves the efficiency, but also facilitates the fabrication process and thus potentially reduces the fabrication cost of perovskite solar cells.

Rational Defect Passivation of Cu2ZnSn(S,Se)4 Photovoltaics with Solution-Processed Cu2ZnSnS4:Na Nanocrystals

An effective defect passivation route has been demonstrated in the rapidly growing Cu2ZnSn(S,Se)4 (CZTSSe) solar cell device system by using Cu2ZnSnS4:Na (CZTS:Na) nanocrystals precursors. CZTS:Na nanocrystals are obtained by sequentially preparing CZTS nanocrystals and surface decorating of Na species, while retaining the kesterite CZTS phase. The exclusive surface presence of amorphous Na species is proved by X-ray photoluminescence spectrum and transmission electron microscopy. With Na-free glasses as the substrate, CZTS:Na nanocrystal-based solar cell device shows 50% enhancement of device performance (∼6%) than that of unpassivated CZTS nanocrystal-based device (∼4%). The enhanced electrical performance is closely related to the increased carrier concentration and elongated minority carrier lifetime, induced by defect passivation. Solution incorporation of extrinsic additives into the nanocrystals and the corresponding film enables a facile, quantitative, and versatile approach to tune the defect property of materials for future optoelectronic applications.