Enhanced photovoltage for inverted planar heterojunction perovskite solar cells.

Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells

Organic-inorganic hybrid perovskite solar cells are considered as one of the most promising next-generation solar cells due to their advantages of low-cost precursors, high power conversion efficiency (PCE) and easy of processing. In the past few years, the PCEs have climbed from a few to over 20% for perovskite solar cells. Recent developments demonstrate that perovskite exhibits ambipolar semiconducting characteristics, which allows for the construction of planar heterojunction (PHJ) perovskite solar cells. PHJ perovskite solar cells can avoid the use of high-temperature sintered mesoporous metal oxides, enabling simple processing and the fabrication of flexible and tandem perovskite solar cells. In planar heterojunction materials, hole/electron transport layers are introduced between a perovskite film and the anode/cathode. The hole and electron transporting layers are expected to enhance exciton separation, charge transportation and collection. Further, the supporting layer for the perovskite film not only plays an important role in energy-level alignment, but also affects perovskite film morphology, which have a great effect on device performance. In addition, interfacial layers also affect device stability. In this review, recent progress in interfacial engineering for PHJ perovskite solar cells will be reviewed, especially with the molecular interfacial materials. The supporting interfacial layers for the optimization of perovskite films will be systematically reviewed. Finally, the challenges remaining in perovskite solar cells research will be discussed.

In this study, we report the utilization of solution-processed high electrical conductive [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) combined with solution-processed TiO2 as the electron transport layer (ETL) to overcome extremely low electrical conductivity of solution-processed TiO2 ETL in planar heterojunction (PHJ) perovskite hybrid solar cells (pero-HSCs). Due to the much more preferable electron extraction and transportation of PC61BM at the cathode side, a tremendously boosted short-circuit current density (JSC), fill factor (FF) and enhanced power conversion efficiency (PCE) are observed. To further address the wettability issues of perovskite materials on the top of PC61BM, water-soluble fullerene derivative is applied to modulate the surface of PC61BM. Consequently, further advanced FF with slightly enlarged JSC and open-circuit voltage (VOC) are observed. The resulted PCE is comparable with the meso-superstructured solar cells in which high PCEs can be produced. Our studies certainly provide a simple approach to boost the efficiency of PHJ pero-HSCs.

In recent years, perovskite solar cell (PSC) has achieved power conversion efficiency as high as over 20 %, making it competitive with high-efficiency thin film solar cells such as CuInGaSe and CdTe solar cells. However, the critical issue of reliability and stability for PSC should be addressed since a significant degradation of photovoltaic (PV) performance at low temperature has been found regardless of planar mesoporous PSC. To reveal the degradation of PV performance in PSC, the temperature-dependent PV performance of the planar PSC is investigated in detail. A PSC sample is loaded into a cryostat chamber connected to a compressor and illuminated by a halogen lamp. The operating temperature varies from 200 K to 325 K and the current-voltage (J-V) characteristic of planar PSC is measured at different scan rates from 10 V/s to 0.0017 V/s. At a fast scan rate of 10 V/s, the PSC shows a low PV performance at either low temperature or high temperature. The short-circuit current (JSC), open-circuit voltage (VOC) and maximum power point (PMPP) are found to decline with the temperature decrteasing. Moreover, the J-V curve also shows the S-shape characteristic. This suggests that the inefficient transport of photo-generated carriers occurs in the PSC. Ions such as Pb2+, CH3NH3+ and I-vacancies cause the screening effect of built-in field and the photo-generated carriers cannot be separated nor collected efficiently. As a result, JSC and VOC show small values in J-V curves measured at a fast scan rate. However, the degradation in PV performance is temporary. The PV performance gradually reaches a steady state at different operating temperatures with scan rate going down to 0.0017 V/s. The PMPP and VOC increase with temperature decreasing. These results indicate that a long illumination time is necessary for PSC to reach a steady state. After long-time illumination under biased condition (i.e., J-V curves measured at slow scan rate), the built-in field is compensated for by the external bias and the ions piling in the interface regions have enough time to diffuse towards the opposite direction. Thus, the screening effect of built-in field is reduced and the PV performance of PSC reaches a steady state. According to the result of device simulation, the increasing VOC at low temperature is attributed to the enhanced built-in potential difference and the reduced recombination rate of carriers. The temperature-dependent external quantum efficiency measurements of planar PSC before and after light illuminationis are performed to investigate the mechanism of carrier transport. It reveals that the separation and collection efficiencies of photo-generated carriers can be improved significantly after light illumination due to the fact that the screening effect of built-in field is reduced. These findings help understand the carrier transport mechanism in planar PSC.

Inorganic metal oxide, especially TiO2, has been commonly used as an electron transport layer (ETL) in regular-structure (n-i-p) planar heterojunction perovskite solar cells (PHJ-PSCs) but generally suffers from high electron recombination rate and incompatibility with low-temperature solution processability. Herein, by applying an ionic liquid (IL, 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM]PF6)) as either a TiO2-modifying interlayer or an independent ETL, we investigated systematically IL interface engineering for PHJ-PSCs. Upon spin-coating [EMIM]PF6-IL onto TiO2 ETL as a modification layer, the average power conversion efficiency (PCE) of CH3NH3PbI3 PHJ-PSC devices reaches 18.42 ± 0.65%, which dramatically surpasses that based on commonly used TiO2 ETL (14.20 ± 0.43%), and the highest PCE (19.59%) is almost identical to that of the record PCE for planar CH3NH3PbI3 PSCs (19.62%) reported very recently. On the other hand, by applying [EMIM]PF6-IL as an independent ETL, we achieved an average PCE of 13.25 ± 0.55%, and the highest PCE (14.39%) approaches that obtained for PHJ-PSCs based on independent TiO2 ETL (14.96%). Both IL interface engineering methods reveal the effective electron transport of [EMIM]PF6-IL. The effects of [EMIM]PF6-IL on the surface morphology, crystallinity, and optical absorption of the perovskite film and the interface between the perovskite layer and substrate were investigated and compared with the case of independent TiO2 ETL, revealing the role of [EMIM]PF6-IL in efficient electron transport.

High-performance methylammonium-free ideal-band-gap perovskite solar cells

Electrode Design to Overcome Substrate Transparency Limitations for Highly Efficient 1 cm2 Mesoscopic Perovskite Solar Cells

Efficient and stable planar heterojunction perovskite solar cells fabricated under ambient conditions with high humidity

Carrier Dynamics Engineering for High-Performance Electron-Transport-Layer-free Perovskite Photovoltaics

Dual-Resistance of Ion Migration and Moisture Erosion via Hydrolytic Crosslinking of Siloxane Functionalized Poly(Ionic Liquids) for Efficient and Stable Perovskite Solar Cells

Ni-doped α-Fe2O3 as electron transporting material for planar heterojunction perovskite solar cells with improved efficiency, reduced hysteresis and ultraviolet stability

Boosting interfacial charge transfer by constructing rare earth–doped WOx nanorods/SnO2 hybrid electron transport layer for efficient perovskite solar cells

Highly efficient air-stable/hysteresis-free flexible inverted-type planar perovskite and organic solar cells employing a small molecular organic hole transporting material

Efficient lead acetate sourced planar heterojunction perovskite solar cells with enhanced substrate coverage via one-step spin-coating

Two cyclohexanofullerenes used as electron transport materials in perovskite solar cells