Anyi Mei

A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability

Improved perovskite photovoltaic performance A recent entry in the solar cell race is perovskite cells, named for the structure adopted by salt made from metal halides and organic cations that absorb the light and generate charges. The charges generated have to be transferred to a metal oxide (typically titanium oxide), and some of these charge carriers are lost in the transfer. Mei et al. made this process more efficient by growing a more crystalline perovskite with fewer defects inside porous versions of titanium and zirconium oxide. They added a second organic cation that stuck to the pore walls and directed the growth of the perovskite crystals. The improved solar cells operated for more than 1000 hours under full sunlight. Science, this issue p. 295 A mixed organic phase perovskite grown in mesoporous templates boosts solar cell stability. We fabricated a perovskite solar cell that uses a double layer of mesoporous TiO2 and ZrO2 as a scaffold infiltrated with perovskite and does not require a hole-conducting layer. The perovskite was produced by drop-casting a solution of PbI2, methylammonium (MA) iodide, and 5-ammoniumvaleric acid (5-AVA) iodide through a porous carbon film. The 5-AVA templating created mixed-cation perovskite (5-AVA)x(MA)1-xPbI3 crystals with lower defect concentration and better pore filling as well as more complete contact with the TiO2 scaffold, resulting in a longer exciton lifetime and a higher quantum yield for photoinduced charge separation as compared to MAPbI3. The cell achieved a certified power conversion efficiency of 12.8% and was stable for >1000 hours in ambient air under full sunlight.

Fully Printable Mesoscopic Perovskite Solar Cells with Organic Silane Self-Assembled Monolayer

By the introduction of an organic silane self-assembled monolayer, an interface-engineering approach is demonstrated for hole-conductor-free, fully printable mesoscopic perovskite solar cells based on a carbon counter electrode. The self-assembled silane monolayer is incorporated between the TiO2 and CH3NH3PbI3, resulting in optimized interface band alignments and enhanced charge lifetime. The average power conversion efficiency is improved from 9.6% to 11.7%, with a highest efficiency of 12.7%, for this low-cost perovskite solar cell.

Hole-Conductor-Free Mesoscopic TiO2/CH3NH3PbI3 Heterojunction Solar Cells Based on Anatase Nanosheets and Carbon Counter Electrodes.

A hole-conductor-free fully printable mesoscopic TiO2/CH3NH3PbI3 heterojunction solar cell was developed with TiO2 nanosheets containing high levels of exposed (001) facets. The solar cell embodiment employed a double layer of mesoporous TiO2 and ZrO2 as a scaffold infiltrated by perovskite as a light harvester. No hole conductor or Au reflector was employed. Instead, the back contact was simply a printable carbon layer. The perovskite was infiltrated from solution through the porous carbon layer. The high reactivity of (001) facets in TiO2 nanosheets improved the interfacial properties between the perovskite and the electron collector. As a result, photoelectric conversion efficiency of up to 10.64% was obtained with the hole-conductor-free fully printable mesoscopic TiO2/CH3NH3PbI3 heterojunction solar cell. The advantages of fully printable technology and the use of low-cost carbon-materials-based counter electrode and hole-conductor-free structure provide this design a promising prospect to approach low-cost photovoltaic devices.

Efficient hole-conductor-free, fully printable mesoscopic perovskite solar cells with a broad light harvester NH2CHNH2PbI3

Formamidinium lead trihalide perovskite (FAPbI3) was successfully introduced into hole-conductor-free, fully printable mesoscopic perovskite solar cells with a carbon counter electrode. With the sequential deposition method, a FAPbI3 based solar cell yielded an efficiency of 11.9%, superior to the methylammonium lead trihalide perovskite (MAPbI3) solar cell efficiency of 11.4%, which is due to broadening of the light to 840 nm. By optimizing the mixing ratio of the formamidimium and methylammonium cations to 3 : 2, a power conversion efficiency of 12.9% was achieved with this low-cost, fully printable mesoscopic solar cell, which indicated a promising prospect for low-cost photovoltaic technology.

The effect of carbon counter electrodes on fully printable mesoscopic perovskite solar cells

Mesoporous graphite/carbon black counter electrodes (CEs) using flaky graphite with different sizes were applied in hole-conductor-free mesoscopic perovskite solar cells by a screen-printing technique. Conductivity measurements, current–voltage characteristics, and impedance spectroscopy measurements were carried out to study the influence of CEs on the photovoltaic performance of devices. The results indicated that graphite, which acted as the conductor in carbon counter electrodes (CCEs), could significantly affect the square resistance of CCEs, thus resulting in differences in fill factor and power conversion efficiency (PCE) of the devices. Based on the optimized CCEs with a thickness of 9 μm, PCEs exceeding 11% could be achieved for the fully printable hole-conductor-free mesoscopic perovskite solar cells due to the low square resistance and large pore size of graphite based CCEs. The abundant availability, low cost and excellent properties of such carbon material based CEs offer a wide prospect for their further applications in perovskite solar cells.

The size effect of TiO2 nanoparticles on a printable mesoscopic perovskite solar cell

The size effect of the TiO2 photoanode has been investigated on the hole-conductor-free fully printable mesoscopic perovskite solar cells based on the carbon counter electrode and (5-AVA)x(MA)1−xPbI3 perovskite. With TiO2 nanoparticles with an optimized diameter of 25 nm, a champion device exhibits an efficiency of 13.41%.

Synergy of ammonium chloride and moisture on perovskite crystallization for efficient printable mesoscopic solar cells

Organometal lead halide perovskites have been widely used as the light harvester for high-performance solar cells. However, typical perovskites of methylammonium lead halides (CH3NH3PbX3, X=Cl, Br, I) are usually sensitive to moisture in ambient air, and thus require an inert atmosphere to process. Here we demonstrate a moisture-induced transformation of perovskite crystals in a triple-layer scaffold of TiO2/ZrO2/Carbon to fabricate printable mesoscopic solar cells. An additive of ammonium chloride (NH4Cl) is employed to assist the crystallization of perovskite, wherein the formation and transition of intermediate CH3NH3X·NH4PbX3(H2O)2 (X=I or Cl) enables high-quality perovskite CH3NH3PbI3 crystals with preferential growth orientation. Correspondingly, the intrinsic perovskite devices based on CH3NH3PbI3 achieve an efficiency of 15.6% and a lifetime of over 130 days in ambient condition with 30% relative humidity. This ambient-processed printable perovskite solar cell provides a promising prospect for mass production, and will promote the development of perovskite-based photovoltaics.

Highly ordered mesoporous carbon for mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cell

Highly ordered mesoporous carbon (OMC) with well-connected frameworks was applied in mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells as counter electrode. The OMC were synthesized by a template method and mixed with flaky graphite to prepare the carbon paste, which was used to fabricate the counter electrode by screen-printing technology. The OMC based solar cell presented a fill factor (FF) of 0.63 and a power conversion efficiency (η) of 7.02%, which was a remarkable improvement compared with the carbon black based device. The electrochemical impedance spectrum measurement demonstrated that the uniform mesopores and interconnected structures in the carbon counter electrode promoted the decrease of charge transfer resistance at the interface and thereby the higher FF and η was obtained.

Enhanced electronic properties in CH3NH3PbI3via LiCl mixing for hole-conductor-free printable perovskite solar cells

By mixing perovskite MAPbI3 (MA = CH3NH3+) with LiCl, an effective one-step drop-coating approach was developed to improve the performance of hole-conductor-free printable perovskite solar cells. The LiCl-mixed perovskite exhibited superior electronic properties because of the improved conductivity of the perovskite layer enabling faster electron transport. LiCl-mixing also improved the crystallinity and morphology of the perovskite layer. As a consequence, perovskite solar cells prepared using the LiCl-mixed perovskite as the light harvester produced higher performances compared with the unmixed perovskite, improving the power conversion efficiency from 10.0% to 14.5%.

Tunable hysteresis effect for perovskite solar cells

Perovskite solar cells (PSCs) usually suffer from a hysteresis effect in current–voltage measurements, which leads to an inaccurate estimation of the device efficiency. Although ion migration, charge trapping/detrapping, and accumulation have been proposed as a basis for the hysteresis, the origin of the hysteresis has not been apparently unraveled. Herein we reported a tunable hysteresis effect based uniquely on open-circuit voltage variations in printable mesoscopic PSCs with a simplified triple-layer TiO2/ZrO2/carbon architecture. The electrons are collected by the compact TiO2/mesoporous TiO2 (c-TiO2/mp-TiO2) bilayer, and the holes are collected by the carbon layer. By adjusting the spray deposition cycles for the c-TiO2 layer and UV-ozone treatment, we achieved hysteresis-normal, hysteresis-free, and hysteresis-inverted PSCs. Such unique trends of tunable hysteresis are analyzed by considering the polarization of the TiO2/perovskite interface, which can accumulate positive charges reversibly. Successfully tuning of the hysteresis effect clarifies the critical importance of the c-TiO2/perovskite interface in controlling the hysteretic trends observed, providing important insights towards the understanding of this rapidly developing photovoltaic technology.