Yusong Sheng

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.

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.

Organic–Inorganic Copper(II)-Based Material: A Low-Toxic, Highly Stable Light Absorber for Photovoltaic Application

Lead halide perovskite solar cells have recently emerged as a very promising photovoltaic technology due to their excellent power conversion efficiencies; however, the toxicity of lead and the poor stability of perovskite materials remain two main challenges that need to be addressed. Here, for the first time, we report a lead-free, highly stable C6H4NH2CuBr2I compound. The C6H4NH2CuBr2I films exhibit extraordinary hydrophobic behavior with a contact angle of ∼90°, and their X-ray diffraction patterns remain unchanged even after 4 h of water immersion. UV/vis absorption spectrum shows that C6H4NH2CuBr2I compound has an excellent optical absorption over the entire visible spectrum. We applied this copper-based light absorber in printable mesoscopic solar cell for the initial trial and achieved a power conversion efficiency of ∼0.5%. Our study represents an alternative pathway to develop low-toxic and highly stable organic-inorganic hybrid materials for photovoltaic application.

Boron-Doped Graphite for High Work Function Carbon Electrode in Printable Hole-Conductor-Free Mesoscopic Perovskite Solar Cells

Work function of carbon electrodes is critical in obtaining high open-circuit voltage as well as high device performance for carbon-based perovskite solar cells. Herein, we propose a novel strategy to upshift work function of carbon electrode by incorporating boron atom into graphite lattice and employ it in printable hole-conductor-free mesoscopic perovskite solar cells. The high-work-function boron-doped carbon electrode facilitates hole extraction from perovskite as verified by photoluminescence. Meanwhile, the carbon electrode is endowed with an improved conductivity because of a higher graphitization carbon of boron-doped graphite. These advantages of the boron-doped carbon electrode result in a low charge transfer resistance at carbon/perovskite interface and an extended carrier recombination lifetime. Together with the merit of both high work function and conductivity, the power conversion efficiency of hole-conductor-free mesoscopic perovskite solar cells is increased from 12.4% for the pristine graphite electrode-based cells to 13.6% for the boron-doped graphite electrode-based cells with an enhanced open-circuit voltage and fill factor.

Push–pull porphyrins with different anchoring group orientations for fully printable monolithic dye-sensitized solar cells with mesoscopic carbon counter electrodes

Here, four D–π–A porphyrin dye molecules with carboxylic acid at the para-position or meta-position of the benzene ring (coded as WH-C4, WH-C1, WH-C6 and WH-C7, respectively) were designed and synthesized for monolithic dye-sensitized solar cells with mesoscopic carbon counter electrodes. Significant optical and electrochemical differences were found for WH-C6 and WH-C7 with carboxylic acid at the meta-position of the benzene ring in comparison with WH-C4 and WH-C1 with carboxylic acid at the para-position of the benzene ring. The influence of anchoring group positions on the photovoltaic performance of corresponding devices was systematically investigated. The devices sensitized by WH-C6 and WH-C7 show inferior open circuit voltage (Voc) and short circuit photocurrent (Jsc) compared with those sensitized by WH-C4 and WH-C1. The above performance differences were confirmed and explained by electrochemical impedance spectroscopy (EIS), UV-visible absorption spectra and dye loading measurement, respectively.

The effect of different alkyl chains on the photovoltaic performance of D–π–A porphyrin-sensitized solar cells

In this study, a series of novel D–π–A porphyrin sensitizers with different alkyl chains were designed and synthesized for dye-sensitized solar cells. The effects of different alkyl chains (methyl, methoxyl and hexyloxy groups) on the photophysical, electrochemical and photovoltaic performance were investigated systematically. The results indicate that the molar extinction coefficients of three dyes normally increase as the alkyl chain length increases. Furthermore, it is also found that the incident photo-to-current conversion efficiencies (IPCEs), short circuit current (Jsc) and open circuit voltage (Voc) of the dye-sensitized solar cells (DSSCs) based on WH-C1, YD20 and WH-C2 increase with the elongation of alkyl chains in the order of WH-C1 < YD20 < WH-C2. Accordingly, under standard global air mass 1.5 solar conditions, the optimized WH-C2-sensitized cell could produce a high conversion efficiency (η) of 7.77%, with a Jsc of 13.10 mA cm−2, a Voc of 831.10 mV, and a fill factor (FF) of 0.70.

Spacer improvement for efficient and fully printable mesoscopic perovskite solar cells

Highly dispersible TiO2@ZrO2 nanoparticles are synthesized to prepare an ultra-flat and crack-free spacer film, leading to an enhanced insulating ability compared to a conventional spacer. The average power conversion efficiency of fully printable mesoscopic perovskite solar cells is improved from 10.2% to 12.5%, and the highest steady output power conversion efficiency is 13.8%.

The effect of porphyrins suspended with different electronegative moieties on the photovoltaic performance of monolithic porphyrin-sensitized solar cells with carbon counter electrodes

Three D–π–A porphyrin sensitizers with different electronegative moieties (2,4,6-triphenyl-1,3,5-triazine, carbazole and triphenylamine) attached at the meso-position were designed and synthesized for monolithic dye-sensitized solar cells based on mesoscopic carbon counter electrodes. The effects of these different electronegative moieties on the photophysical, electrochemical and accordingly photovoltaic performance of the corresponding devices were investigated systematically. Electrochemical measurements indicate that the HOMO and LUMO energy levels could be tuned through the introduction of different electronegative groups onto the backbone of D–π–A porphyrin molecules. Current–voltage characteristics indicate that the Jsc and Voc of the DSSCs based on WH-C3, WH-C4 and WH-C5 increase as the electron-donating ability of their donors enhance in the order of WH-C3 < WH-C4 < WH-C5 and WH-C5-sensitized cells showed the best photovoltaic performance: a short circuit photocurrent density (Jsc) of 11.43 mA cm−2, an open circuit voltage (Voc) of 633.84 mV, and a fill factor (FF) of 0.69, corresponding to an overall conversion efficiency (η) of 5.00%.

Mixed (5-AVA)xMA1−xPbI3−y(BF4)y perovskites enhance the photovoltaic performance of hole-conductor-free printable mesoscopic solar cells

We report on a simple one-step solution processing strategy to fabricate new stable mixed cation/mixed halide (5-AVA)xMA1−xPbI3−y(BF4)y perovskite solar cells. The results showed that the power conversion efficiency (PCE) of the optimized mixed (5-AVA)0.034MA0.966PbI2.95(BF4)0.05 perovskite solar cells was substantially increased to 15.5% with a remarkably high VOC of 0.97 V. We have analyzed different mixed perovskites focusing on the characterization of the charge recombination by means of intensity-modulated photovoltage spectroscopy. In addition, our study discloses that the incorporation of MABF4 into the (5-AVA)0.034MA0.966PbI3 perovskite helps to reduce the charge recombination in the mixed perovskite cells, thus improving the open circuit voltage of the device. Furthermore, the new mixed perovskite was demonstrated to produce high-quality perovskite crystals and enhance the conductivity of the mixed perovskite.