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Quasi‐2D perovskite solar cells (PSCs), with impressive stability and tunable optoelectronic properties, have become a promising alternative to 3D PSCs. However, the lack of understanding about rationally promoting the film quality of 2D perovskites has significantly undermined their power conversion efficiency (PCE). Herein, a thermally induced crystallization strategy to process the Dion–Jacobson (DJ) 2D perovskite films based on (BDA)(MA)4Pb5I16 (n = 5) afforded by thermal treatments is reported, realizing high‐quality perovskite films with increased crystallinity and dense structure. The photovoltaic performance of PSCs improved by thermal treatments is found to be mainly attributed to the significantly suppressed nonradiative recombination, more efficient charge generation, and restricted interfacial charge accumulation. As a result, the optimal device processed with thermal treatments produces a high open‐circuit voltage of 1.21 V and a PCE of over 17.6% with suppressed energy loss down to 0.42 eV. Moreover, the treated devices without encapsulation show a satisfactory stability with <20% PCE degradation after 1000 h under maximum power point tracking. The demonstrated strategy in this work offers a promising route for the performance enhancement of 2D DJ PSCs toward realistic energy conversion applications. [ABSTRACT FROM AUTHOR]

Although the photovoltaic performance of perovskite solar cells (PSCs) has reached the commercial standards, the unsatisfactory stability limits their further application. Hydrophobic interface and encapsulation can block the damage of water and oxygen, while the instability induced by intrinsic residual strain remains inevitable. Here, the residual strain in a two‐dimensional (2D) Ruddlesden–Popper (RP) perovskite film is investigated by X‐ray diffraction and atomic force microscopy. It's found that the spacer cations contribute to the residual strain even though they are not in the inorganic cages. Benefited from strain relaxation, the film quality is improved, leading to suppressed recombination, promoted charge transport and enhanced efficiency. More significantly, the strain‐released devices maintain 86 % of the initial efficiency after being kept in air with 85 % relative humidity (RH) for 1080 h, 82 % under maximum power point (MPP) tracking at 50 °C for 804 h and 86 % after continuous heating at 85 °C for 1080 h. [ABSTRACT FROM AUTHOR]

Molecular doping is an effective means to tune the optoelectronic properties of organic semiconductors. Despite its versatility, its application in bulk heterojunction (BHJ) organic solar cells (OSCs) is still limited, especially blend‐cast doping. Difficulties in desolving molecular dopants in weakly polar solvents have led to the formation of undesired structures, with relatively high concentrations of dopants affecting the bicontinuous BHJ morphology. Bilayer OSCs are stacked structures with sequential deposition of donor and acceptor films, which make important contributions to the fine tuning of morphology and balanced charge transport. The sequential deposition of thin films in bilayer devices organically facilitates the sequential doping of pure donor polymers by dopants. The sequential doping of the dopant at the interface between the donor and the acceptor can efficiently improve the carrier mobility and the charge transfer between the donor and the acceptor without damaging the quality of the film, thus improving the device performance efficiently. With three different dopants F4TCNQ, F6TCNNQ, and BCF, sequential interfacial doping in bilayer devices is found to be a versatile and effective method to improve device performance. Making this simple doping strategy promising for high‐performance bilayer OSCs, it is evaluated as a promising alternative to BHJ OSCs. [ABSTRACT FROM AUTHOR]

• A green ionic liquid was introduced as solvent to prepare BDA-based DJ quasi-2D PSCs. • Defect/interface-induced recombination loss in DJ PSCs was significantly suppressed. • The optimal device showed an impressive PCE of 18.76 % with a remarkable V oc of 1.24 V. • Loss was quantified by photovoltaic parameters relative to DB limit and QFLS values. Dion-Jacobson (DJ) quasi-two-dimensional (quasi-2D) perovskites have emerged as a potential photovoltaic material due to their versatile n-layer variation and superior stable structure. However, compared to their three-dimensional counterparts, the power conversion efficiency (PCE) of quasi-2D perovskite solar cells (PSCs) reported to date still lags behind the detailed balance (DB) limit for single-junction PSCs, which derives from the presence of non-radiative recombination loss. Herein, we demonstrated that the defect/interface-induced recombination loss in DJ quasi-2D PSCs could be significantly suppressed by introducing a green ionic liquid methylammonium acetate (MAAc) into (BDA)(MA) 4 Pb 5 I 16 (n = 5) (BDA, 1,4-butanediamine) perovskite precursor. Both the film quality of 2D perovskites and the interfacial property of perovskite/charge transporting layer (CTL) were improved after introducing MAAc in precursor solution. Correspondingly, a champion PCE of 18.76 % with a remarkable open-circuit voltage (V oc) of 1.24 V was achieved. Through the quantitative loss analysis, which involved the calculation of photovoltaic parameters relative to DB limit and quasi-Fermi level splitting values, the improvement of PCE and V oc by introducing MAAc was mainly attributed to the reduction of defects in perovskite films and at perovskite/CTL interfaces, and the optimization of interfacial energy-level alignment. The present work provides a green solvent strategy for improving the performance of quasi-2D PSCs, and offers an insight into the fundamental physics of non-radiative recombination loss tailored by ionic liquid. [ABSTRACT FROM AUTHOR]

In this work, antimony selenide (Sb2Se3) thin films were prepared using electron-beam (e-beam) evaporation followed by selenization process (two-step method) for the first time. The effect of selenization temperature on the properties of Sb2Se3 thin films and solar cells was investigated systematically. Formation of pure polycrystalline Sb2Se3 was confirmed and the intensity of (hk1) and (hk2) patterns was found to decrease while that of (hk0) to increase with temperature elevating. However, when the temperature became higher than 360 °C, the shape of Sb2Se3 grains gradually changed to rod-like shape from round shape and some craters and cracks in the films can be observed. Sb2Se3 films prepared at 360 °C showed an average grain size of 450 nm, a roughness of 42 nm, an optical bandgap of 1.24 eV and a carrier concentration of 4.99 × 1012 cm−3. The corresponding Sb2Se3 solar cells exhibited an optimal power conversion efficiency of 1.15%. Our results demonstrated that the process of e-beam evaporation of Sb followed by selenization at appropriate temperature is a good way to prepare high quality Sb2Se3 thin films for solar cells. [ABSTRACT FROM AUTHOR]

Flexible Cu(InGa)Se2 (CIGS) solar cells on metallic substrates are highly desirable for versatile energy applications. However, related researches reported on CIGS devices usually used a diffusion barrier, which involves an issue of thermal expansion coefficient matching and complicates the fabrication process. In this study, CIGS solar cells were fabricated on Ni and Ti foils through selenization of metal stacks deposited by e-beam evaporation, without the use of diffusion barrier. The films on Ti foils showed (112) preferred orientation while those on Ni foils exhibited (220) preferred orientation, due to the different crystal orientation of substrates. In–Ni and Ga–Ni phase were observed in films prepared on Ni foils, which led to the existence of Cu2–xSe. As a result, the CIGS solar cells fabricated on Ni foils showed inferior photoelectric properties, whereas those on 50 and 100 μm Ti foils presented conversion efficiency of 4.9% and 6.6%. The discrepancy on efficiency between two cells on Ti foils was ascribed to the different structural properties induced by the thermal stress. After 100 bending cycles, efficiency of cells on 50 and 100 μm thick Ti foils decreased by 1.0% and 2.6% respectively, indicating that CIGS solar cell on 50 μm Ti foil is more promising as flexible device applied on harsh circumstance. [ABSTRACT FROM AUTHOR]

Abstract It is considered that alkaline elements (such as Na, K, Rb) introduced through post-deposition treatment are able to modify the near-surface region of Cu(InGa)Se 2 thin films. However, research on the effects of Ca element on properties of Cu(InGa)Se 2 thin films is hardly reported. In this paper, Cu(InGa)Se 2 thin films were prepared on quartz glass by e-beam evaporation via two-step process. Study of structural property showed that a certain amount of CaF 2 through post-deposition treatment was beneficial to the uniform composition distribution of Cu(InGa)Se 2 thin films. Near-surface region of Cu(InGa)Se 2 thin films was more Cu-depleted by increasing introduced amount of Ca element. It was inferred that Ca diffused into the Cu(InGa)Se 2 thin films substituted the In sites as Ca In so as to increase the carrier concentration. By calculation, the spike barrier of CdS/Cu(InGa)Se 2 heterojunction was shown to decrease with increased thickness of CaF 2 layers. As a result, Cu(InGa)Se 2 solar cell subjected to 20 nm CaF 2 post-deposition treatment, which possessed higher fill factor, open circuit voltage and short circuit current density, achieved the optimal power conversion efficiency. An efficiency of Cu(InGa)Se 2 solar cell was enhanced by 2.9% after 20 nm CaF 2 post-deposition treatment. Highlights • Well crystallized CIGS thin films were prepared on quartz glass via two-step process. • Ga graded distribution in the films was alleviated after CaF 2 PDT. • Near-surface region of CIGS thin films was more Cu-poor by increasing amount of Ca. • An efficiency of CIGS solar cell was enhanced by 2.9% through 20 nm CaF 2 PDT. [ABSTRACT FROM AUTHOR]

To investigate the effects of Sb doping on the structural and electrical properties of Cu(InGa)Se 2 (CIGS) thin films and solar cells, CIGS thin films, prepared by e-beam evaporation on soda-lime glass, were doped with lower and upper Sb layers in the precursor stacks respectively. Change of structure and introduction of stress were observed in the CIGS thin films with upper Sb layer in stack through XRD and Raman measurement. Both crystalline quality and compactness of CIGS thin films were improved by the doping of upper Sb layer in stack and the CIGS thin film showed an optimal structural property with 20 nm Sb layer. Movement of Fermi level of the surface of CIGS thin film after doping of upper Sb layer in stack and electrons transfer between Cu/Cu + redox couple and CIGS thin films, which provided probability for the substitution of Sb for Cu sites at the surface of CIGS thin films, were proposed to explain the migration of Cu from the surface to the bulk of CIGS thin films. The larger barrier at the CIGS/CdS interface after doping of upper Sb layer in stack made contribution to the increase of V OC of CIGS solar cells. The efficiency of CIGS solar cell was improved from 3.3% to 7.2% after doping with 20 nm upper Sb. Compared to the CIGS solar cell with lower Sb layer in stack, in which an additional Cu 2-x Se phase was found, the CIGS solar cell with upper Sb layer in stack possessed a higher efficiency. [ABSTRACT FROM AUTHOR]

• CIGS solar cells were fabricated on ultrathin glass. • Potassium was doped by a simple method during the post-selenization process. • The efficiency of solar cell got enhanced by 2.5% after potassium incorporation. • It was first revealed that K doping retarded the property degradation of solar cell. Flexible Cu(InGa)Se 2 solar cells fabricated on metal foils and plastics have achieved high conversion efficiency so far. However, metal impurities and low temperature tolerance hinder them from further considerable development. Here, 150 μm-thick ultrathin glass with desirable advantages was applied as substrate for Cu(InGa)Se 2 solar cell. Potassium element was doped by a simple method during the post-selenization process. The results showed that the KInSe 2 phase existed in K-doped Cu(InGa)Se 2 films. The K-doped CIGS films were more compact than the pristine one due to the fact that K incorporated in films could form quasi-liquid alkali-metal-Se compounds and improve the compactness of films. K-incorporated CIGS films exhibited enhanced p-type conductivity and different surface energy level. Consequently, K-doped Cu(InGa)Se 2 solar cell achieved an optimal efficiency of 8.3%, which was relatively 43% higher than that of pristine solar cell. Investigation of performance and stability of solar cells manifested that the K incorporation retarded the performance degradation of device during cyclic bending. [ABSTRACT FROM AUTHOR]

• Cd-free CIGS solar cells with a-Si buffer layers are fabricated for the first time. • The conduction band alignment of CIGS/a-Si is a favorable spike. • SCAPS predicts enhanced efficiency by optimization of bandgap and E CB of CIGS films. • The efficiency of cell with modified CIGS film is relatively improved by over 30%. CdS, In 2 S 3 and ZnO-based materials are typically adopted as buffer layers in high efficiency CIGS solar cells. However, the drawbacks involved toxicity, complicated synthesis process and environmental damage of aforementioned materials would someday limit the development themselves. Here, for the first time, eco-friendly and low-cost a-Si thin films prepared by e-beam evaporation were used as buffer layers in CIGS solar cells. Thickness of a-Si films was changed to optimize the performance of CIGS solar cells through investigating their optical, structural and electrical properties. It revealed that the conduction band alignment of CIGS and a-Si is a favorable spike and a 60 nm a-Si film is optimal for CIGS/a-Si interface properties. Following the SCAPS simulation, bandgaps and conduction band energy level of CIGS films were facilely modulated through altering the Ga/(Ga + In) ratios. The conversion efficiency of solar cell with modified CIGS film was relatively improved by over 30% compared to the pristine one. The present work provides a new and eco-friendly strategy for fabrication of CIGS solar cells and expands the application of a-Si thin films as buffer layers for solar cells. [ABSTRACT FROM AUTHOR]

CZTSSe thin films based on three-layer precursors with Cu-poor and Zn-rich in top and bottom layer while stoichiometric in middle layer were prepared. Compared with CZTSSe thin film obtained from a stoichiometric single layer precursor, the crystallization of thin film from a three-layer precursor got enhanced. Element distribution and growth nuclei position in precursor played an important role on CZTSSe thin film formation and solar cell performance. The conversion efficiency of CZTSSe solar cell from a three-layer precursor attained an increase of 3.54% over that from a single layer precursor. [ABSTRACT FROM AUTHOR]