Sin Hang Cheung

Efficiency enhancement by defect engineering in perovskite photovoltaic cells prepared using evaporated PbI2/CH3NH3I multilayers

We report, for the first time, on the synthesis of perovskite films by thermal annealing of evaporated lead(II) iodide (PbI2)/methylammonium iodide (CH3NH3I) multilayers. Detailed characterization of the resulting films is presented. Our work demonstrates that compact, high quality and uniform perovskite films can be grown using this technique. Optimization of the device structure was achieved by careful design of the layer thickness and the number of PbI2/CH3NH3I pairs used in the formation of the absorber layer. Utilizing additional annealing steps in a controlled atmosphere was shown to result in significant improvement in the device performance. Our experimental data indicate that O2 treatments may result in substantial reduction in the trap density of the device and thereby significant improvement in the lifetimes of the carriers. A high power conversion efficiency (PCE) of 12.5% was recorded for the champion device.

The detrimental effect of excess mobile ions in planar CH3NH3PbI3 perovskite solar cells

The origin of the impact of mobile ions in perovskite solar cells (PVSCs) has recently become a hot topic of debate. Here, we investigate systematically the structural effect and various recombination pathways in PVSCs with different ion concentrations. By probing the transient ionic current in PVSCs, we extract mobile ion concentrations in a range of 1016 cm−3 to 1017 cm−3 depending on the processing conditions during a two-step process. The PVSC with the lowest ion concentration has both the highest efficiency over 15% and shelf-life over 1300 hours. Interestingly, in contrast to the commonly adopted models in the literature, we find that the crystal size and the bimolecular and trap-assisted recombination are not responsible for the large difference in photovoltaic performance. Instead, by using transient photocurrent and steady-state photoluminescence approaches, we find that the large reduction of short-circuit current (Jsc) in mobile ion populated devices is ascribed to the slow decay in photocurrent and the increasing amount of non-radiative recombination. In addition, we also find that the excess mobile ions trigger the deformation of perovskite to PbI2, which severely reduces the device lifetime. The results provide valuable information on the understanding of the role of excess mobile ions in the degradation mechanism of PVSCs.

Investigation of high performance TiO2 nanorod array perovskite solar cells

In this paper, systematic investigations on the fabrication and characterization of high performance TiO2 nanorod array perovskite solar cells (NAPSCs) are reported. The TiO2 nanorods, of length around 350–400 nm, were grown by solvothermal technique directly on glass/FTO substrates. From the scanning transmission electron microscopy (STEM) we demonstrate that excellent crystallinity for the TiO2 nanorods can be produced using the solvothermal technique. Precursor consisting of a mixture of PbI2, CH3NH3I (MAI) and CH3NH3Cl (MACl) was used for the growth of perovskite thin films on the glass/FTO/TiO2 nanorod array (TiO2-NA) substrates. It is found that the morphology and quality of the perovskite layer depend strongly on the concentration of MACl in the precursor. Experimental studies on femtosecond transient absorption (fs-TA) indicate that the incorporation of TiO2-NA greatly enhances the collection efficiency of the photo-generated carriers due to substantial increase of interfacial area between the perovskite and TiO2-NA, leading to a reduction in carrier diffusion distance. It is shown to be the key factor that the proposed technique facilitates the use of a thicker perovskite absorber layer (∼500 nm) without compromising on the series resistance. Detailed J–V characterization shows that the NAPSCs exhibit negligible hysteresis with a power conversion efficiency (PCE) >19% for the champion device.

Pinning Down the Anomalous Light Soaking Effect toward High-Performance and Fast-Response Perovskite Solar Cells: The Ion-Migration-Induced Charge Accumulation

The light soaking effect (LSE) is widely known in perovskite solar cells (PVSCs), but its origin is still elusive. In this study, we show that in common with hysteresis, the LSE is owed to the ion migration in PVSCs. Driven by the photovoltage, the mobile ions in the perovskite materials (MA+/I-) migrate to the selective contacts, forming a boosted P-i-N junction resulting in enhanced charge separation. Besides, the mobile ions (MA+) can soften and suture the PCBM/perovskite interface and thus reduce the trap density, in keeping with a higher open-circuit voltage. Finally, almost LSE-free PVSCs can be prepared by using 0.1 wt % MAI-doped PCBM as the electron transport material, whereas overdoping (1 wt % MAI doping) makes the LSE even more pronounced due to excess mobile ions that need time to migrate to reach a new quasi-static state.

Boosting the photovoltaic thermal stability of fullerene bulk heterojunction solar cells through charge transfer interactions

Fullerene-based bulk heterojunction organic solar cells (BHJ-OSCs) represent one of the current state-of-the-art organic solar cells. Nonetheless, most of these devices still suffer from adverse performance degradation due to thermally induced morphology changes of active layers. We herein demonstrate that the photovoltaic performance stability of BHJ-OSCs can be profoundly enhanced with an appositely functionalized 9-fluorenylidene malononitrile. The latter, through charge transfer (CT) interactions with a donor polymer, enables the formation of a “frozen” 3-dimensional mesh-like donor polymer matrix, which effectively restrains free movement of embedded fullerene molecules and suppresses their otherwise uncontrolled aggregation. 9-Fluorenylidene malononitrile derivatives with multiple CT interaction sites are particularly effective as preservation of a power conversion efficiency of over 90% under severe thermal stress has been accomplished. The generality of this novel strategy has been affirmed with several common donor polymers, manifesting it to be hitherto the most efficient approach to stabilized fullerene-based BHJ-OSCs.

On the understanding of energetic disorder, charge recombination and voltage losses in all-polymer solar cells

All polymer solar cells (all-PSCs), compared to fullerene or non-fullerene small molecule based solar cells possess the merits of high morphological stability, good film forming properties and superior mechanical flexibility. However, in contrast to the rapid progress in molecule-design and device engineering in this field, our fundamental insights into key photo-physical parameters that govern the device characteristics still lag behind. Here, based on state of the art PTzBI:N2200 all-PSCs with efficiencies of ∼9% compared with PBDB-T:N2200 all-PSCs, we investigate properties of charge transport, bimolecular recombination, energetic disorder and voltage losses. We show that expedited charge extraction with weaker bias-dependence in the PTzBI:N2200 cell results in the reduction of bimolecular recombination, which explains the increases in photocurrent and fill factor. Further mitigation of the voltage losses in this high performance all-polymer system may be allowed by decreasing the energetic disorder in PTzBI:N2200 bulk heterojunctions through delicate morphological optimization.

Porphyrin-Based Thick-Film Bulk-Heterojunction Solar Cells for Indoor Light Harvesting

A porphyrin-based donor material P1 is used to fabricate bulk-heterojunction solar cells harvesting room light. P1 consists of a porphyrin ring linked by two ethylrhodanine end caps via phenylene ethynylene bridges. Organic photovoltaic (OPV) cells using P1 as the donor exhibit significantly improved performances under indoor illumination compared to 1 sun conditions. For P1:PC71BM cells, an optimized-PCE of 19.2% was achieved under a 300 lux illumination of a 3000 K LED tube. Besides high PCEs, the porphyrin-based devices also exhibit enhanced OPV performances in thick film regions (∼200 nm) compared to a control BHJ cell using PCDTBT:PC71BM. The thick-film P1-based device shows an overall PCE of 18.2%. The origins of the thickness independent OPV performances were examined, and they can be attributed to the low electronic disorders of these materials when compared to their polymer counterparts.

High performance low-bandgap perovskite solar cells based on a high-quality mixed Sn–Pb perovskite film prepared by vacuum-assisted thermal annealing

Tandem perovskite solar cells are an effective concept to overcome the Shockley–Queisser limit of a single-junction perovskite solar cell. For a high-performance tandem cell, besides a wide-bandgap perovskite top cell, a high-quality low-bandgap perovskite bottom cell with an optimum bandgap of ∼1.2 eV is urgently needed. Moreover, a simple process technique needs to be developed for a high-quality perovskite film with good reproducibility, in order to further simplify the whole tandem-cell fabrication. Accordingly, we develop a simple one-step process (vacuum-assisted thermal annealing) for a high-quality low-bandgap CH3NH3Sn0.5Pb0.5IxCl3−x film, where the absorption edge can exceed 1000 nm. After comparing CH3NH3Sn0.5Pb0.5IxCl3−x films annealed in a vacuum and in a nitrogen environment, we find that vacuum-assisted thermal annealing can result in CH3NH3Sn0.5Pb0.5IxCl3−x films with better film coverage and crystallinity. This process can also accelerate the sublimation of methylammonium chloride and reduce the trap density in the CH3NH3Sn0.5Pb0.5IxCl3−x film. With this process, we successfully fabricated an efficient low-bandgap perovskite solar cell with a power conversion efficiency of more than 12% and good device reproducibility as well as long-term stability.

A Cryogenic Process for Antisolvent‐Free High‐Performance Perovskite Solar Cells

A cryogenic process is introduced to control the crystallization of perovskite layers, eliminating the need for the use of environmentally harmful antisolvents. This process enables decoupling of the nucleation and the crystallization phases by inhibiting chemical reactions in as-cast precursor films rapidly cooled down by immersion in liquid nitrogen. The cooling is followed by blow-drying with nitrogen gas, which induces uniform precipitation of precursors due to the supersaturation of precursors in the residual solvents at very low temperature, while at the same time enhancing the evaporation of the residual solvents and preventing the ordered precursors/perovskite from redissolving into the residual solvents. Using the proposed techniques, the crystallization process can be initiated after the formation of a uniform precursor seed layer. The process is generally applicable to improve the performance of solar cells using perovskite films with different compositions, as demonstrated on three different types of mixed halide perovskites. A champion power conversion efficiency (PCE) of 21.4% with open-circuit voltage (VOC ) = 1.14 V, short-circuit current density ( JSC ) = 23.5 mA cm-2 , and fill factor (FF) = 0.80 is achieved using the proposed cryogenic process.