Tongfa Liu

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.

Full Printable Processed Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells with Carbon Counter Electrode

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.

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%.

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.

Mesoporous nitrogen-doped TiO2 sphere applied for quasi-solid-state dye-sensitized solar cell

A mesoscopic nitrogen-doped TiO2 sphere has been developed for a quasi-solid-state dye-sensitized solar cell [DSSC]. Compared with the undoped TiO2 sphere, the quasi-solid-state DSSC based on the nitrogen-doped TiO2 sphere shows more excellent photovoltaic performance. The photoelectrochemistry of electrodes based on nitrogen-doped and undoped TiO2 spheres was characterized with Mott-Schottky analysis, intensity modulated photocurrent spectroscopy, and electrochemical impedance spectroscopy, which indicated that both the quasi-Fermi level and the charge transport of the photoelectrode were improved after being doped with nitrogen. As a result, a photoelectric conversion efficiency of 6.01% was obtained for the quasi-solid-state DSSC.

Hole-conductor-free perovskite solar cells with carbon counter electrodes based on ZnO nanorod arrays

A one dimensional nanostructure array has been considered as a successful charge transport material for perovskite solar cells (PSCs), because of its large internal surface area, superior charge collection efficiency and fast charge transport. Herein we demonstrate a ZnO nanorod (NR) array as the electron collector in a hole-conductor-free PSC with a carbon counter electrode (CE). A relatively low initial power conversion efficiency (PCE) of 5.6% was achieved using a 1 μm long ZnO NR array as an electron collector. However, by introduction of a thin TiO2 coating layer on the surface of ZnO via TiCl4 treatment, the PCE of the cell has been improved to the highest value of 8.24%. It is revealed that the performance enhancement of the ZnO/TiO2 NR based PSCs is largely attributed to the larger surface area, reduced electron combination, and superior electron transport properties.