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 photoemi...

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

With rapid progress in a power conversion efficiency (PCE) to reach 25%, metal halide perovskite-based solar cells became a game-changer in a photovoltaic performance race. Triggered by the development of the solid-state perovskite solar cell in 2012, intense follow-up research works on structure design, materials chemistry, process engineering, and device physics have contributed to the revolutionary evolution of the solid-state perovskite solar cell to be a strong candidate for a next-generation solar energy harvester. The high efficiency in combination with the low cost of materials and processes are the selling points of this cell over commercial silicon or other organic and inorganic solar cells. The characteristic features of perovskite materials may enable further advancement of the PCE beyond those afforded by the silicon solar cells, toward the Shockley-Queisser limit. This review summarizes the fundamentals behind the optoelectronic properties of perovskite materials, as well as the important approaches to fabricating high-efficiency perovskite solar cells. Furthermore, possible next-generation strategies for enhancing the PCE over the Shockley-Queisser limit are discussed.

High-Efficiency Perovskite Solar Cells.

Defects play an important role in the degradation processes of hybrid halide perovskite absorbers, impeding their application for solar cells. Among all defects, halide anion and organic cation vacancies are ubiquitous, promoting ion diffusion and leading to thin-film decomposition at surfaces and grain boundaries. Here, we employ fluoride to simultaneously passivate both anion and cation vacancies, by taking advantage of the extremely high electronegativity of fluoride. We obtain a power conversion efficiency of 21.46% (and a certified 21.3%-efficient cell) in a device based on the caesium, methylammonium (MA) and formamidinium (FA) triple-cation perovskite (Cs0.05FA0.54MA0.41)Pb(I0.98Br0.02)3 treated with sodium fluoride. The device retains 90% of its original power conversion efficiency after 1,000 h of operation at the maximum power point. With the help of first-principles density functional theory calculations, we argue that the fluoride ions suppress the formation of halide anion and organic cation vacancies, through a unique strengthening of the chemical bonds with the surrounding lead and organic cations. Defects and defect migration are detrimental for perovskite solar cell efficiency and long-term stability. Li et al. show that fluoride is able to suppress the formation of halide anion and organic cation vacancy defects by restraining the relative ions via ionic and hydrogen bonds.

Cation and anion immobilization through chemical bonding enhancement with fluorides for stable halide perovskite solar cells

Solar cells based on inorganic absorbers, such as Si, GaAs, CdTe and Cu(In,Ga)Se2, permit a high device efficiency and stability. The crystals’ three-dimensional structure means that dangling bonds inevitably exist at the grain boundaries (GBs), which significantly degrades the device performance via recombination losses. Thus, the growth of single-crystalline materials or the passivation of defects at the GBs is required to address this problem, which introduces an added processing complexity and cost. Here we report that antimony selenide (Sb2Se3)—a simple, non-toxic and low-cost material with an optimal solar bandgap of ∼1.1 eV—exhibits intrinsically benign GBs because of its one-dimensional crystal structure. Using a simple and fast (∼1 μm min–1) rapid thermal evaporation process, we oriented crystal growth perpendicular to the substrate, and produced Sb2Se3 thin-film solar cells with a certified device efficiency of 5.6%. Our results suggest that the family of one-dimensional crystals, including Sb2Se3, SbSeI and Bi2S3, show promise in photovoltaic applications. Materials with a one-dimensional crystal structure, such as antimony selenide, show considerable potential for making efficient thin-film solar cells.

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Thin-film Sb2Se3 photovoltaics with oriented one-dimensional ribbons and benign grain boundaries

A mesoscopic methylammonium lead iodide (CH3NH3PbI3) perovskite/TiO2 heterojunction solar cell is developed with low-cost carbon counter electrode (CE) and full printable process. With carbon black/spheroidal graphite CE, this mesoscopic heterojunction solar cell presents high stability and power conversion efficiency of 6.64%, which is higher than that of the flaky graphite based device and comparable to the conventional Au version.

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Full Printable Processed Mesoscopic CH 3 NH 3 PbI 3 /TiO 2 Heterojunction Solar Cells with Carbon Counter Electrode

The holy grail of solar cell technology would be a low-cost, high efficiency solar cell composed of earth-abundant, non-toxic elements. A thin film Cu2ZnSnS4 (CZTS) solar cell is proposed as the technology to achieve this holy grail and thus it has received much attention in recent years. CZTS’s earth-abundant and non-toxic elemental composition makes it an ideal candidate to replace Cu(In,Ga)Se2 (CIGS) and CdTe solar cells which face material scarcity and toxicity issues. CZTS is reported to have a bandgap of 1.32–1.85 eV and a band edge absorption coefficient of above 104 cm−1 [1–3] which makes it highly attractive as a single junction solar cell material. Efficiencies of up to 8.4% and 10.1% have been achieved for CZTS[4] and Cu2ZnSn(S,Se)4 (CZTSSe)[5] solar cells, respectively. The question that remains is whether these solar cells are able to reach higher efficiencies that are comparable to or higher than CIGS and CdTe solar cells. To answer this question, we need to understand what enables CIGS and CdTe solar cells to achieve their high efficiencies and whether CZTS and CZTSSe have the same beneficial property. This is the aim of this paper. Polycrystalline CIGS and CdTe solar cells have achieved 20.3% and 16.7% efficiencies, respectively,[6] while their singlecrystal counterparts have surprisingly only achieved 12% and 13.5%, respectively.[7,8] Typically, semiconductor materials for optoelectronic or transistor applications perform worse in their polycrystalline form because of carrier recombination at the grain boundaries (GBs). However, for polycrystalline CIGS and CdTe solar cells, this is not the case. This begs the question of why these materials perform better in the polycrystalline form rather than their single-crystal form. Studies have identified GBs as the source of high efficiency for polycrystalline CIGS and CdTe solar cells.[9–13] These GBs enhance minority carrier collection and provide a current pathway for minority carriers to reach the n-type CdS and ZnO layers and be collected.[10] Minority carrier collection is enhanced by the presence of an electric field in the vicinity of the GBs, which increases charge separation. This electric field

Investigating the role of grain boundaries in CZTS and CZTSSe thin film solar cells with scanning probe microscopy.

The greatest challenge for improving the power conversion efficiency of Cu2ZnSn(S,Se)4 (CZTSSe)/CdS/ZnO thin film solar cells is increasing the open circuit voltage (VOC). Probable leading causes of the VOC deficit in state-of-the-art CZTSSe devices have been identified as bulk recombination, band tails, and the intertwined effects of CZTSSe/CdS band offset, interface defects, and interface recombination. In this work, we demonstrate the modification of the CZTSSe absorber/CdS buffer interface following the deposition of 1 nm-thick Al2O3 layers by atomic layer deposition (ALD) near room temperature. Capacitance-voltage profiling and quantum efficiency measurements reveal that ALD-Al2O3 interface modification reduces the density of acceptor-like states at the heterojunction resulting in reduced interface recombination and wider depletion width. Indications of increased VOC resulting from the modification of the heterojunction interface as a result of ALD-Al2O3 treatment are presented. These results, while ...

Reduced defect density at the CZTSSe/CdS interface by atomic layer deposition of Al2O3

Recent work on Cu2ZnSn(S,Se)4 (CZTSSe) has shown that it is an excellent candidate for use in thin film solar cells due to its earth abundant, inexpensive, non-toxic constituents and optimal material properties. However, the composition window for growth of CZTSSe capable of high efficiency is still unknown. The focus of this work is to use sputter deposition to synthesize this material as a thin film and determine a composition window for growth of high efficiency CZTSSe devices. CZTSSe thin films of various compositions were grown using co-sputtering of compound targets followed by an anneal at high temperature. Efficiency of devices was mapped as function of copper, zinc, tin, selenium and sulfur content in the films. Extraneous phases that are relatively benign (ZnS, SnS2) and those that are detrimental (Cu2S) to device performance were identified using x-ray diffraction. The range of efficiencies mapped in this work was 0–8% and the best efficiency achieved using this fabrication process was 9.3%.

Effect of composition on high efficiency CZTSSe devices fabricated using co-sputtering of compound targets

Interplay between surface preparation and device performance in CZTSSe solar cells: Effects of KCN and NH4OH etching

The success of CIGS and CdTe solar cells has shown that thin films have the potential to greatly reduce costs while matching the efficiencies of the best silicon devices. As such, a number of new thin film materials that have optimal properties are being explored for solar applications. One of these materials is Cu 2 ZnSnS 4 (CZTS), which is composed of entirely abundant and nontoxic elements. Currently, all the processes used to grow this material involve the deposition of metallic/sulfide precursors and a subsequent anneal at high temperature in a H2S environment to incorporate sulfur into the film. Here we present a novel one-step deposition process where sulfur is incorporated into the film during deposition. Films grown by this process are textured in the (112) direction and show large columnar grains with diameters ranging from 50–500nm. The best of these films were incorporated into devices and efficiencies as high as 1.5% have been achieved.

Vardaan Chawla

Papers

Investigating the role of grain boundaries in CZTS and CZTSSe thin film solar cells with scanning probe microscopy.

Reduced defect density at the CZTSSe/CdS interface by atomic layer deposition of Al2O3

Effect of composition on high efficiency CZTSSe devices fabricated using co-sputtering of compound targets

Interplay between surface preparation and device performance in CZTSSe solar cells: Effects of KCN and NH4OH etching

Inexpensive, abundant, non-toxic thin films for solar cell applications grown by reactive sputtering