Yongzhen Wu

Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers

Perovskites go large Solar cells made of planar organic-inorganic perovskites now have reported efficiencies exceeding 20%. However, these values have been determined from small illuminated areas. Chen et al. used highly doped inorganic charge extraction layers to make solar cells on the 1 cm2 scale (see the Perspective by Sessolo and Bolink). The layers helped to protect the active layer from degradation by air. The cells achieved governmentlab–certified efficiencies of >15%. Furthermore, 90% of the efficiency was maintained after 1000 hours of operation. Science, this issue p. 944; see also p. 917 Highly doped inorganic layers both improve charge extraction and help protect the active layer from humid air. [Also see Perspective by Sessolo and Bolink] The recent dramatic rise in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has triggered intense research worldwide. However, high PCE values have often been reached with poor stability at an illuminated area of typically less than 0.1 square centimeter. We used heavily doped inorganic charge extraction layers in planar PSCs to achieve very rapid carrier extraction, even with 10- to 20-nanometer-thick layers, avoiding pinholes and eliminating local structural defects over large areas. The robust inorganic nature of the layers allowed for the fabrication of PSCs with an aperture area >1 square centimeter that have a PCE >15%, as certified by an accredited photovoltaic calibration laboratory. Hysteresis in the current-voltage characteristics was eliminated; the PSCs were stable, with >90% of the initial PCE remaining after 1000 hours of light soaking.

Retarding the crystallization of PbI2 for highly reproducible planar-structured perovskite solar cells via sequential deposition

On a planar substrate the sequential deposition of CH3NH3PbI3 perovskite is optimized by retarding the crystallization of PbI2. This strategy overcomes the problem of incomplete conversion and uncontrolled particle sizes of perovskite in the absence of mesoporous scaffolds, greatly increasing the film reproducibility. Highly efficient and reproducible planar-structured perovskite solar cells were obtained with the best efficiency of 13.5%, average efficiency of 12.5% and a small standard deviation of 0.57 from a total of 120 cells.

A dopant-free hole-transporting material for efficient and stable perovskite solar cells

An efficient pristine hole-transporting material (HTM), tetrathiafulvalene derivative (TTF-1), was introduced into perovskite solar cells, without the use of p-type dopants. As compared to cells based on well-known p-type doping with spiro-OMeTAD, perovskite solar cells based on dopant-free TTF-1 performed with a comparable efficiency of 11.03%; moreover, the stability of the dopant-free TTF-1 based cell was greatly improved two fold in air at a relative humidity of ∼40%. To the best of our knowledge, this is the first case of perovskite solar cells employing a dopant-free HTM based on a tetrathiafulvalene derivative yielding an efficiency over 11%. The present finding paves the way for the development of efficient dopant-free HTMs for perovskite solar cells, which promotes the advancement of cost-effective and practical perovskite solar cells.

High-conversion-efficiency organic dye-sensitized solar cells: molecular engineering on D–A–π-A featured organic indoline dyes

This paper reports a new D–A–π-A organic dye WS-9, which is derived from the known dye WS-2 by incorporating an n-hexylthiophene unit into the π-conjugation. Due to the presence of a strong electron-withdrawing benzothiadiazole unit in the π-bridge, the specific D–A–π-A organic dyes show more complicated electronic transition absorption bands than traditional D–π-A dyes. The origins of the absorption bands in D–A–π-A organic dyes are analysed by density functional theory (DFT). The calculated results in combination with the deprotonation experiments suggest that the spectral response range of D–A–π-A organic dyes is superior to that of D–π-A ones. When employed in dye-sensitized solar cells (DSSCs), the two dyes show a large difference in aggregation behaviour. It was found that WS-2 forms the unfavourable aggregates more easily. High performance of WS-2 strongly depends on the coadsorbent and suitable dye bath solvent. In contrast, WS-9 shows strong anti-aggregation ability, and always exhibits high performance regardless of the coadsorbent and dye bath solvent. Transient photovoltage and photocurrent decay experiments as well as electrochemical impedance spectroscopy indicate that the injected electron lifetime and charge recombination resistance are largely increased due to the introduction of a hexylthiophene unit, resulting in the high photovoltage based on WS-9. The optimized power conversion efficiency of WS-9 reaches 9.04% with high photocurrent (18.00 mA cm−2) and photovoltage (696 mV). The accelerating dye photo-stability was tested upon light irradiation of a dye-adsorbed TiO2 film in the absence of redox electrolyte, and a WS-9-based DSSC device with ionic liquid redox electrolyte. These results suggest that the structural engineering of organic dyes is important for highly efficient photovoltaic performance of solar cells, and our research will pave a novel way to design new efficient D–A–π-A organic dye sensitizers.

Hybrid interfacial layer leads to solid performance improvement of inverted perovskite solar cells

Despite the sky-rocketing efficiencies being reported for perovskite solar cells (PSSCs) with several different configurations recently, it is as yet unclear which configuration will prove beneficial over others. In this work, we report a novel, inverted PSSC with the configuration of FTO/NiO/meso-Al2O3/CH3NH3PbI3/PCBM/BCP/Ag. The first implementation of the hybrid interfacial layer of an ultrathin NiO compact layer (10–20 nm) plus an inert mesoporous Al2O3 (meso-Al2O3) scaffold, featuring high optical transparency and specific dual blocking effect, leads to minimal light absorption loss and interfacial recombination loss. The device performance has been significantly improved with respect to the control PSSCs without the meso-Al2O3 layer. Synchronized improvements in photovoltage, photocurrent and fill factor lead to a high efficiency of >13%, which is the highest reported so far for NiO based PSSCs. Small hysteresis and stable power output under working conditions have been demonstrated for this type of solar cells. The results also highlight the general and critical importance of interfacial control in PSSCs, and their effects on device performance.

Constructing organic D-A-π-A-featured sensitizers with a quinoxaline unit for high-efficiency solar cells: the effect of an auxiliary acceptor on the absorption and the energy level alignment.

Four organic D-A-π-A-featured sensitizers (TQ1, TQ2, IQ1, and IQ2) have been studied for high-efficiency dye-sensitized solar cells (DSSCs). We employed an indoline or a triphenylamine unit as the donor, cyanoacetic acid as the acceptor/anchor, and a thiophene moiety as the conjugation bridge. Additionally, an electron-withdrawing quinoxaline unit was incorporated between the donor and the π-conjugation unit. These sensitizers show an additional absorption band covering the broad visible range in solution. The contribution from the incorporated quinoxaline was investigated theoretically by using DFT and time-dependent DFT. The incorporated low-band-gap quinoxaline unit as an auxiliary acceptor has several merits, such as decreasing the band gap, optimizing the energy levels, and realizing a facile structural modification on several positions in the quinoxaline unit. As demonstrated, the observed additional absorption band is favorable to the photon-to-electron conversion because it corresponds to the efficient electron transitions to the LUMO orbital. Electrochemical impedance spectroscopy (EIS) Bode plots reveal that the replacement of a methoxy group with an octyloxy group can increase the injection electron lifetime by a factor of 2.4. IQ2 and TQ2 can perform well without any co-adsorbent, successfully suppress the charge recombination from TiO(2) conduction band to I(3)(-) in the electrolyte, and enhance the electron lifetime, resulting in a decreased dark current and enhanced open circuit voltage (V(oc)) values. By using a liquid electrolyte, DSSCs based on dye IQ2 exhibited a broad incident photon-to-current conversion efficiency (IPCE) action spectrum and high efficiency (η=8.50 %) with a short circuit current density (J(sc)) of 15.65 mA cm(-2), a V(oc) value of 776 mV, a fill factor (FF) of 0.70 under AM 1.5 illumination (100 mW cm(-2)). Moreover, the overall efficiency remained at 97% of the initial value after 1000 h of visible-light soaking.

Insight into D–A−π–A Structured Sensitizers: A Promising Route to Highly Efficient and Stable Dye-Sensitized Solar Cells

The dye-sensitized solar cell (DSSC) is one of the most promising photovoltaic technologies with potential of low cost, light weight, and good flexibility. The practical application of DSSCs requires further improvement in power conversion efficiency and long-term stability. Recently, significant progress has been witnessed in DSSC research owing to the novel concept of the D-A-π-A motif for the molecular engineering of organic photosensitizers. New organic and porphyrin dyes based on the D-A-π-A motif can not only enhance photovoltaic performance, but also improve durability in DSSC applications. This Spotlight on Applications highlights recent advances in the D-A-π-A-based photosensitizers, specifically focusing on the mechanism of efficiency and stability enhancements. Also, we find insight into the additional acceptor as well as the trade-off of long wavelength response. The basic principles are involved in molecular engineering of efficient D-A-π-A sensitizers, providing a clear road map showing how to modulate the energy bands, rationally extending the response wavelength, and optimizing photovoltaic efficiency step by step.

Constructing high-efficiency D-A-π-A-featured solar cell sensitizers: a promising building block of 2,3-diphenylquinoxaline for antiaggregation and photostability.

Controlling the sensitizer morphology on a nanocrystalline TiO2 surface is beneficial to facilitating electron injection and suppressing charge recombination. Given that the grafted alkyl chain on a π-bridge thiophene segment for preventing π aggregation can deteriorate its intrinsic photostability, we incorporate a promising building block of 2,3-diphenylquinoxaline as the additional acceptor to construct a novel D-A-π-A-featured dye IQ4, which exhibits several characteristics: (i) efficiently decreasing the molecular HOMO-LUMO energy gap by extending its absorption bands; (ii) showing a moderate electron-withdrawing capability for an ideal balance in both promising photocurrent and photovoltage; (iii) endowing an ideal morphology control with strong capability of restraining the intermolecular aggregation and facilitating the formation of a compact sensitizer layer via two twisted phenyl groups grafted onto the quinoxaline unit. The coadsorbent-free dye-sensitized solar cell (DSSC) based on dye IQ4 exhibits very promising conversion efficiency as high as 9.24 ± 0.05%, with a short-circuit current density (Jsc) of 17.55 mA cm(-2), an open-circuit voltage (Voc) of 0.74 V, and a fill factor (FF) of 0.71 under AM 1.5 illumination (100 mW cm(-2)). IQ4-based DSSC devices with an ionic liquid electrolyte can keep constant performance during a 1000 h aging test under 1 sun at 60 °C. Because of spatial restriction, the two phenyl groups grafted onto the additional electron-withdrawing quinoxaline are demonstrated as efficient building blocks, not only improving its photostability and thermal stability but also allowing it to be a successful antiaggregation functional unit. As a consequence, the incorporated 2,3-diphenylquinoxaline unit can realize a facile structural modification for constructing organic coadsorbent-free D-A-π-A-featured sensitizers, thus paving a way to replace the common, stability-deleterious grafted alkyl chain on the thienyl bridge.

Highly compact TiO2 layer for efficient hole-blocking in perovskite solar cells

A uniform and pinhole-free hole-blocking layer is necessary for high-performance perovskite-based thin-film solar cells. In this study, we investigated the effect of nanoscale pinholes in compact TiO2 layers on the device performance. Surface morphology and film resistance studies show that TiO2 compact layers fabricated using atomic layer deposition (ALD) contain a much lower density of nanoscale pinholes than layers obtained by spin coating and spray pyrolysis methods. The ALD-based TiO2 layer acts as an efficient hole-blocking layer in perovskite solar cells; it offers a large shunt resistance and enables a high power conversion efficiency of 12.56%.

Thermally Stable MAPbI3 Perovskite Solar Cells with Efficiency of 19.19% and Area over 1 cm2 achieved by Additive Engineering

Solution-processed perovskite (PSC) solar cells have achieved extremely high power conversion efficiencies (PCEs) over 20%, but practical application of this photovoltaic technology requires further advancements on both long-term stability and large-area device demonstration. Here, an additive-engineering strategy is developed to realize a facile and convenient fabrication method of large-area uniform perovskite films composed of large crystal size and low density of defects. The high crystalline quality of the perovskite is found to simultaneously enhance the PCE and the durability of PSCs. By using the simple and widely used methylammonium lead iodide (MAPbI3 ), a certified PCE of 19.19% is achieved for devices with an aperture area of 1.025 cm2 , and the high-performing devices can sustain over 80% of the initial PCE after 500 h of thermal aging at 85 °C, which are among the best results of MAPbI3 -based PSCs so far.