Perovskite solar cells have shown unprecedent performance increase up to 22% efficiency. However, their photovoltaic performance has shown fast deterioration under light illumination in the presence of humid air even with encapulation. The stability of perovskite materials has been unsolved and its mechanism has been elusive. Here we uncover a mechanism for irreversible degradation of perovskite materials in which trapped charges, regardless of the polarity, play a decisive role. An experimental setup using different polarity ions revealed that the moisture-induced irreversible dissociation of perovskite materials is triggered by charges trapped along grain boundaries. We also identified the synergetic effect of oxygen on the process of moisture-induced degradation. The deprotonation of organic cations by trapped charge-induced local electric field would be attributed to the initiation of irreversible decomposition.

Hysteresis-free and highly efficient CH3NH3PbI3 perovskite solar cells employing a compact C60 material as an electron transport layer have been developed for the first time using both rigid glass and plastic substrates.

have already been a lot of studies on bendable conducting electrodes in the fi eld of organic photovoltaics (OPVs) to replace brittle transparent conductive oxides (TCOs) for fl exible solar cell applications, such as graphene, [17][18][19][20] carbon nanotubes, [21][22][23] metal grids, [24][25][26] and conductive polymers. [ 27,28 ] Among them, graphene, a single-layer 2D carbon material, would be the most promising candidate because it is optically highly transparent (about 97% in visible range), mechanically robust, fl exible, and stretchable. TCO-free OPV devices with graphene anode have already been successfully demonstrated showing a PCE of 8.48%, the highest effi ciency for the TCO-free tandem polymer solar cells, [ 20 ] although still lower than 11.0% PCE of the TCOfree perovskite solar cells. [ 29 ] Graphene electrodes also have been recently employed in perovskite devices; [ 30,31 ] however, in these studies, graphene was not used for replacing the conventional TCO electrode but for a top electrode.Here we report highly effi cient TCO-free inverted perovskite solar cells consisting of graphene/molybdenum trioxide (MoO 3 )/ PEDOT:PSS/MAPbI 3 /fullerene (C 60 )/bathocuproine (BCP)/ lithium fl uoride (LiF)/aluminum (Al). A few nanometer thick MoO 3 layers are employed between the graphene and PEDOT:PSS layers, similar to the OPVs adopting graphene as an anode, [ 18 ] which provides hydrophilicity to the graphene surface and elevates its lower work function (4.23 eV) to a higher level (4.71 eV) by hole doping of graphene. The wettability of PEDOT:PSS and the device properties are affected by the thickness of the MoO 3 layer, and, as a result, best PCE of 17.1% is achieved with the graphene-based devices incorporating a 2 nm thick MoO 3 interfacial layer. For comparison, ITO-based perovskite solar cells employing MoO 3 interfacial layers have been also fabricated. Their PCEs also vary with the thickness of the MoO 3 layer, showing the best PCE of 18.8% with a 1 nm thick MoO 3 layer. The effects of the MoO 3 thickness on PCEs of the graphene-and the ITO-based devices are thoroughly investigated by analyzing hydrophilicity of electrode surfaces, electrode work functions, surface morphologies of constitutive fi lms, and device properties.The structure of the devices is schematically illustrated in Figure 1 . We adopted an inverted MAPbI 3 perovskite solar cell structure using PEDOT:PSS and C 60 /BCP as the HTL and the ETL, respectively, because the structure is low-temperature processable and thus suitable for future application on fl exible plastic substrates. A single layer graphene, grown by chemical vapor deposition (CVD), was utilized as a transparent anode rather than a cathode because increasing its work function (≈4.3 eV) by p-doping induced not only an enhanced conductivity but also a desirable energy level alignment with the highest occupied molecular orbital level of HTLs (≈5.2 eV for PEDOT:PSS, for example). Between the graphene and the Organic/inorganic hybrid perovskites are promising materials f...

In article number 1501873, Mansoo Choi and co‐workers demonstrate highly efficient transparent conductive oxide (TCO)‐free inverted perovskite (CH3NH3PbI3) solar cells by using a graphene transparent electrode. Careful engineering of the interface between the graphene electrode and the hole transport layer enables the highest energy conversion efficiency of 17.1% among TCO‐free solar cells to be obtained.

Reliable doping and carrier concentration control in graphene have been realized by depositing aerosol-derived metal nanoparticles (NPs) with consistent size and configuration on the channel of a graphene field-effect transistor. Here, the spherically shaped Ag or Pt NPs with a fairly narrow size distribution of 7.5 AE 1.5 or 6.4 AE 1.4 nm, respectively, have been produced through the spark discharge process. The transfer characteristics show that Ag NPs deposited on graphene induce n-type conduction with an electron concentration of 0.3-1.9 Â 10 12 cm À2 at a NP surface coverage of 12-21%, while Pt NPs lead p-type doping with a hole concentration of 3.9-5.2 Â 10 12 cm À2 at 24-32% surface coverage. The observed electrical transport properties are interpreted as the metal NP doping-induced Fermi level shift of initially p-type doped graphene by the adsorbed oxygen molecules under ambient conditions. Also, the minimum conductance at the Dirac point before and after the deposition of metal NPs shows no appreciable change, implying the absence of noticeable sp 3 hybridization of the graphene surface due to aerosol-derived NPs. Fig. 7 Schematic energy band diagrams of graphenes before and after (a) Ag NPs doping with different surface coverage of 12, 16, 18, and 21%, and (b) Pt NPs doping with 24, 26, 29 and 32%. Journal of Materials Chemistry C Communication