Ligang Yuan

Silver nanowire (AgNW)-based transparent electrodes prepared via an all-solution-process are proposed as bottom electrodes in flexible perovskite solar cells (PVSCs). To enhance the chemical stability of AgNWs, a pinhole-free amorphous aluminum doped zinc oxide (a-AZO) protection layer is deposited on the AgNW network. Compared to its crystalline counterpart (c-AZO), a-AZO substantially improves the chemical stability of the AgNW network. For the first time, it is observed that inadequately protected AgNWs can evanesce via diffusion, whereas a-AZO secures the integrity of AgNWs. When an optimally thick a-AZO layer is used, the a-AZO/AgNW/AZO composite electrode exhibits a transmittance of 88.6% at 550 nm and a sheet resistance of 11.86 Ω sq⁻¹, which is comparable to that of commercial fluorine doped tin oxide. The PVSCs fabricated with a configuration of Au/spiro-OMeTAD/CH₃NH₃PbI₃/ZnO/AZO/AgNW/AZO on rigid and flexible substrates can achieve power conversion efficiencies (PCEs) of 13.93% and 11.23%, respectively. The PVSC with the a-AZO/AgNW/AZO composite electrode retains 94% of its initial PCE after 400 bending iterations with a bending radius of 12.5 mm. The results clearly demonstrate the potential of AgNWs as bottom electrodes in flexible PVSCs, which can facilitate the commercialization and large-scale deployment of PVSCs.

A pinhole-free amorphous Al-doped zinc oxide (AZO) protection layer dramatically enhances the chemical stability of silver nanowires (AgNWs). Using all-solution-processed amorphous AZO/AgNW/AZO transparent electrodes in flexible perovskite solar cells, it is possible to achieve a power conversion efficiency of 11.23%.

Crystal engineering of CH₃NH₃PbI₃ perovskite materials through template-directed nucleation and growth on PbI₂ nuclei dispersed in a polar fullerene (C₆₀ pyrrolidine tris-acid, CPTA) electron transport layer (ETL) (CPTA:PbI₂) is proposed as a route for controlling crystallization kinetics and grain sizes. Chemical analysis of the CPTA:PbI₂ template confirms that CPTA carboxylic acid groups can form a monodentate or bidentate chelate with Pb(II), resulting in a lower nucleation barrier that promotes rapid formation of the tetragonal perovskite phase. Moreover, it is demonstrated that a uniform CH₃NH₃PbI₃ film with highly crystalline and large domain sizes can be realized by increasing the spacing between nuclei to retard perovskite crystal growth via careful control of the preferred nucleation site distribution in the CPTA:PbI₂ layer. The improved perovskite morphology possesses a long photoluminescence lifetime and efficient photocarrier transport/separation properties to eliminate the hysteresis effect. The corresponding planar heterojunction photovoltaic yields a high power conversion efficiency (PCE) of 20.20%, with a high fill factor (FF) of 81.13%. The average PCE and FF values for 30 devices are 19.03% ± 0.57% and 78.67% ± 2.13%, respectively. The results indicate that this ETL template-assisted crystallization strategy can be applied to other organometal halide perovskite-based systems.

A C₆₀ pyrrolidine tris-acid:PbI₂ (CPTA:PbI₂) electron transport layer-assisted fast nucleation/slow growth strategy is used to fabricate uniform CH₃NH₃PbI₃ perovskite films with a high crystalline quality and large grain sizes, resulting in enhanced photocarrier generation. The optimal perovskite solar cell exhibits a power conversion efficiency (PCE) of 20.20% (average PCE of 19.03 ± 0.57%) with an 81.13% fill factor.

Organic solar cell optimization requires careful balancing of current–voltage output of the materials system. Here, such optimization using ultrafast spectroscopy as a tool to optimize the material bandgap without altering ultrafast photophysics is reported. A new acceptor–donor–acceptor (A–D–A)-type small-molecule acceptor NCBDT is designed by modification of the D and A units of NFBDT. Compared to NFBDT, NCBDT exhibits upshifted highest occupied molecular orbital (HOMO) energy level mainly due to the additional octyl on the D unit and downshifted lowest unoccupied molecular orbital (LUMO) energy level due to the fluorination of A units. NCBDT has a low optical bandgap of 1.45 eV which extends the absorption range toward near-IR region, down to ≈860 nm. However, the 60 meV lowered LUMO level of NCBDT hardly changes the V_oc level, and the elevation of the NCBDT HOMO does not have a substantial influence on the photophysics of the materials. Thus, for both NCBDT- and NFBDT-based systems, an unusually slow (≈400 ps) but ultimately efficient charge generation mediated by interfacial charge-pair states is observed, followed by effective charge extraction. As a result, the PBDB-T:NCBDT devices demonstrate an impressive power conversion efficiency over 12%—among the best for solution-processed organic solar cells.

An acceptor-donor-acceptor nonfullerene acceptor NCBDT is reported. NCBDT exhibits a low optical bandgap of 1.45 eV and broadened absorption range. The PBDB-T:NCBDT-based device achieves an impressive PCE of 12.12% and J_sc over 20 mA cm^-2—one of the best results for solution-processed OSCs. Further photophysical study reveals slow (≈400 ps) yet efficient free charge generation.

All-inorganic CsPbI₂Br perovskite is fabricated through a one-step method and a 100–130 °C low temperature annealing process. The facile-deposited CsPbI₂Br film demonstrates long-term phase stability at room temperature for a month and exhibits the thermal stability under 100 °C annealing for more than a week. Consequently, the CsPbI₂Br-based all-inorganic perovskite solar cells (PSCs) exhibit power conversion efficiencies (PCE) of up to a record value of 10.56%.

New triptycene-derived hole transport material (HTM) TET is developed for efficient perovskite solar cells (PVSCs). Planar PVSCs with ≈30 nm TET achieve a maximum power conversion efficiency of 19.1% and a steady-state efficiency of 18.6%, comparable with those using 200 nm spiro-OMeTAD. The use of thinner TET HTM cuts the device cost by 25 times, compared with that using spiro-OMeTAD.

The electronic processes occurring within the perovskite solar cells (PSCs) are strongly influenced by the nature of the organic A cations present within the inorganic framework. In this study, the impact of FA (CH(NH₂)₂⁺) and Cs⁺ cations on the intrinsic and interfacial properties in the FAPbBr₃ and CsPbBr₃ PSCs is investigated. The analysis of current density (J_SC) and photovoltage (V_OC) as a function of illumination intensity establishes that the interfacial charge transport is more rapid in FAPbBr₃ devices. Small perturbation measurements including intensity modulated photocurrent and photovoltage spectroscopy are applied to explore the resistive and capacitive elements. Furthermore, electrochemical impedance spectroscopy measurements are found to correlate well with the photovoltaic characteristics of FAPbBr₃ and CsPbBr₃ PSCs. Overall, the in-depth analysis of various phenomena occurring within the bromide PSCs allows to underline the working principle, which provides a key to optimize the device performance. The present protocol is not only valid for PSCs but can also be extended to devices based on alternative light harvesters.

The effect of cations on the intrinsic and interfacial dynamic processes occurring in the perovskite solar cells is explored, which allow to underline their working principle.

Organometallic halide perovskite films with good surface morphology and large grain size are desirable for obtaining high-performance photovoltaic devices. However, defects and related trap sites are generated inevitably at grain boundaries and on surfaces of solution-processed polycrystalline perovskite films. Seeking facial and efficient methods to passivate the perovskite film for minimizing defect density is necessary for further improving the photovoltaic performance. Here, a convenient strategy is developed to improve perovskite crystallization by incorporating a 2D polymeric material of graphitic carbon nitride (g-C₃N₄) into the perovskite layer. The addition of g-C₃N₄ results in improved crystalline quality of perovskite film with large grain size by retarding the crystallization rate, and reduced intrinsic defect density by passivating charge recombination centers around the grain boundaries. In addition, g-C₃N₄ doping increases the film conductivity of perovskite layer, which is beneficial for charge transport in perovskite light-absorption layer. Consequently, a champion device with a maximum power conversion efficiency of 19.49% is approached owing to a remarkable improvement in fill factor from 0.65 to 0.74. This finding demonstrates a simple method to passivate the perovskite film by controlling the crystallization and reducing the defect density.

Graphitic carbon nitride (g-C₃N₄) is incorporated into the perovskite precursor solution to modify the perovskite film by controlling the perovskite crystallization, reducing the intrinsic defect density, and improving the film conductivity. As a result, a champion device with a maximum power conversion efficiency of 19.49% is approached.

Rapid extraction of photogenerated charge carriers is essential to achieve high efficiencies with perovskite solar cells (PSCs). Here, a new mesoscopic architecture as electron-selective contact for PSCs featuring 40 nm sized TiO₂ beads endowed with mesopores of a few nanometer diameters is introduced. The bimodal pore distribution inherent to these films produces a very large contact area of 200 m² g⁻¹ whose access by the perovskite light absorber is facilitated by the interstitial voids between the particles. Modification of the TiO₂ surface by CsBr further strengthens its interaction with the perovskite. As a result, photogenerated electrons are extracted rapidly producing a very high fill factor of close to 80% a V_OC of 1.14 V and a PCE up to 21% with negligible hysteresis.

Cesium modification of bimodal mesoporous TiO₂ surface for perovskite solar cells enhances electron transfer and reduces recombination at the interface between perovskite and elective-selective layers. As a result, photogenerated electrons are extracted rapidly producing a very high fill factor of close to 80% a V_OC of 1.14 V and a power conversion efficiency of 21% with negligible hysteresis.

Organic–inorganic hybrid halide perovskites are proven to be a promising semiconductor material as the absorber layer of solar cells. However, the perovskite films always suffer from nonuniform coverage or high trap state density due to the polycrystalline characteristics, which degrade the photoelectric properties of thin films. Herein, the alkali metal ions which are stable against oxidation and reduction are used in the perovskite precursor solution to induce the process of crystallization and nucleation, then affect the properties of the perovskite film. It is found that the addition of the alkali metal ions clearly improves the quality of perovskite film: enlarges the grain sizes, reduces the defect state density, passivates the grain boundaries, increases the built-in potential (V _bi), resulting to the enhancement in the power conversion efficiency of perovskite thin film solar cell.

The alkali metal ions Na⁺ and K⁺ which are stable against oxidation and reduction are used in the perovskite precursor solution to improve the quality of perovskite film. It is found that the addition of alkali metal ions reduces the defect states and prolonges the carrier lifetime of perovskite film, resulting to the higher performance of the photovoltaic device.

Recently, lead-free double perovskites have emerged as a promising environmentally friendly photovoltaic material for their intrinsic thermodynamic stability, appropriate bandgaps, small carrier effective masses, and low exciton binding energies. However, currently no solar cell based on these double perovskites has been reported, due to the challenge in film processing. Herein, a first lead-free double perovskite planar heterojunction solar cell with a high quality Cs₂AgBiBr₆ film, fabricated by low-pressure assisted solution processing under ambient conditions, is reported. The device presents a best power conversion efficiency of 1.44%. The preliminary efficiency and the high stability under ambient condition without encapsulation, together with the high film quality with simple processing, demonstrate promise for lead-free perovskite solar cells.

A high-quality double perovskite Cs₂AgBiBr₆ film is successfully demonstrated using a low-pressure assisted solution method. A planar heterojunction solar cell based on this Cs₂AgBiBr₆ film shows high stability with an optimal power conversion efficiency of 1.44%. The preliminary results are promising for the development lead-free perovskite solar cells.

A general methodology is reported to create organic–inorganic hybrid metal halide perovskite films with enlarged and preferred-orientation grains. Simply pressing polyurethane stamps with hexagonal nanodot arrays on partially dried perovskite intermediate films can cause pressure-induced perovskite crystallization. This pressure-induced crystallization allows to prepare highly efficient perovskite solar cells (PSCs) because the preferred-orientation and enlarged grains with low-angle grain boundaries in the perovskite films exhibit suppressed nonradiative recombination. Consequently, the photovoltaic response is dramatically improved by the uniaxial compression in both inverted-planar PSCs and normal PSCs, leading to power conversion efficiencies of 19.16%.

Mechanical crystallization methodology for preferred-orientation and close-packed grains of perovskite materials with uniaxial compression is achieved. Simply pressing polyurethane stamps with hexagonal nanodot arrays on partially dried perovskite intermediate films can cause pressure-induced perovskite crystallization, leading to preferred-orientation and enlarged grains with low-angle grain boundaries in the perovskite films. The photovoltaic response dramatically improves in both inverted-planar perovskite solar cells and normal perovskite solar cells, leading to a power conversion efficiency of 19.16%.

Perovskite solar cells have been heralded as one of the most promising emerging technologies in 2016 because of the very high power conversion efficiency of 22% and the low cost of generating electricity compared to even fossil fuels. These are formed with various dimensionalities and can be fully manipulated once their bulk structure is reduced to a low-dimensional structure. Despite being one of the most attractive materials to date, their instability significantly influences device performance and subsequently prevents the timely commercialization of perovskite solar cell technology. In this review, the recent advances in the synthesis of stable low-dimensional metal-halide perovskites are highlighted.

The recent advances in the synthesis of low-dimensional metal-halide perovskite and the sources of instability including water intercalation, ion migration, and thermal decomposition are shown.

The stability of a perovskite solar cell (PSC) is enhanced significantly by applying a customized thin-film encapsulation (TFE). The TFE is composed of a multilayer stack of organic/inorganic layers deposited by initiated chemical vapor deposition and atomic layer deposition, respectively, whose water vapor transmission rate is on the order of 10⁻⁴ g m⁻² d⁻¹ at an accelerated condition of 38 °C and 90% relative humidity (RH). The TFE is optimized, taking into consideration various aspects of thermosensitive PSCs. Lowering the process temperature is one of the most effective methods for minimizing the thermal damage to the PSC during the monolithic integration of the TFE onto PSC. The direct deposition of TFE onto a PSC causes less than 0.3% degradation (from 18.5% to 18.2%) in the power conversion efficiency, while the long-term stability is substantially improved; the PSC retains 97% of its original efficiency after a 300 h exposure to an accelerated condition of 50 °C and 50% RH, confirming the enhanced stability of the PSC against moisture. This is the first demonstration of a TFE applied directly onto PSCs in a damage-free manner, which will be a powerful tool for the development of highly stable PSCs with high efficiency.

The thin-film encapsulation (TFE) via initiated chemical deposition and low-temperature atomic layer deposition effectively enhances the stability of a high-efficient perovskite solar cell (PSC). The TFE is directly deposited onto the PSC without degradation, and the encapsulated PSC retains 97% of its original efficiency after a 300 h exposure to an accelerated condition of 50 °C and 50% relative humidity.

Achieving efficient bulk-heterojunction (BHJ) solar cells from blends of solution-processable small-molecule (SM) donors and acceptors is proved particularly challenging due to the complexity in obtaining a favorable donor–acceptor morphology. In this report, the BHJ device performance pattern of a set of analogous, well-defined SM donors—DR3TBDTT (DR3), SMPV1, and BTR—used in conjunction with the SM acceptor IDTTBM is examined. Examinations show that the nonfullerene “All-SM” BHJ solar cells made with DR3 and IDTTBM can achieve power conversion efficiencies (PCEs) of up to ≈4.5% (avg. 4.0%) when the solution-processing additive 1,8-diiodooctane (DIO, 0.8% v/v) is used in the blend solutions. The figures of merit of optimized DR3:IDTTBM solar cells contrast with those of “as-cast” BHJ devices from which only modest PCEs <1% can be achieved. Combining electron energy loss spectrum analyses in scanning transmission electron microscopy mode, carrier transport measurements via “metal-insulator-semiconductor carrier extraction” methods, and systematic recombination examinations by light-dependence and transient photocurrent analyses, it is shown that DIO plays a determining role—establishing a favorable lengthscale for the phase-separated SM donor–acceptor network and, in turn, improving the balance in hole/electron mobilities and the carrier collection efficiencies overall.

A set of structurally analogous small-molecule (SM) donors with distinct side-chain manifolds shows significant differences in their performance patterns in bulk-heterojunction (BHJ) devices with the nonfullerene SM acceptor IDTTBM. Reducing the lengthscale of the phase-separated network between donor and acceptor effectively suppresses nongeminate recombination in the BHJ active layers and improves the carrier mobility balance.

In this work, room-temperature-operated ultrasensitive solution-processed perovskite photodetectors (PDs) with near infrared (NIR) photoresponse are reported. In order to enable perovskite PDs possessing extended NIR photoresponse, novel n-type low bandgap conjugated polymer, poly[(N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene diimide-2,6-diyl) (2,5-dioctyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione-5,5′-diyl)] (NDI-DPP), which has strong absorption in the NIR region, is developed and then employed in perovskite PDs. By the formation of type II band alignment between NDI-DPP with single-wall carbon nanotubes (SWCNTs), the NIR absorption of NDI-DPP is exploited, which contributes to the NIR photoresponse for the perovskite PDs, where perovskite is incorporated with NDI-DPP and SWCNTs as well. In addition, SWCNTs incorporated with perovskite active layer can offer the percolation pathways for high charge-carrier mobility, which tremendously boosts the charge transfer in the photoactive layer, and consequently improves the photocurrent in the visible region. As a result, the perovskite PDs exhibit the responsivities of ≈400 and ≈150 mA W⁻¹ and the detectivities of over 6 × 10¹² Jones (1 Jones = 1 cm Hz^1/2 W⁻¹) and over 2 × 10¹² Jones in the visible and NIR regions, respectively. This work reports the development of perovskite PDs with NIR photoresponse, which is terrifically beneficial for the practical applications of perovskite PDs.

Room temperature operated uncooled broadband ultrasensitive photodetectors with the responsivities of 400 and 150 mA W^-1 and the detectivities of over 6 × 10¹² and 2 × 10¹² Jones in the visible and near infrared regions are realized by utilization of perovskite incorporated with novel n-type low-bandgap conjugated polymer and single-wall carbon nanotubes through type II band alignment.