Behzad Bahrami

Higher efficiency perovskite solar cells using additives of LiI, LiTFSI and BMImI in the PbI2 precursor

The efficiencies of perovskite solar cells have been significantly increased to 18%, 17.01% and 15.6% for the cells containing the additives BMImI, LiI and LiTFSI in the PbI2 precursor solutions, respectively, from 11.3% for the devices without any additives. Incorporation of these additives led to the formation of perovskites with larger grain size and higher crystallinity with reduced PbI2 residue as indicated by X-ray diffraction (XRD) and atomic force microscopy (AFM) results. Kelvin Probe Force Microscopy (KPFM) and current sensing (CS)-AFM results were in good agreement with external quantum efficiency (EQE) measurements and proved the great enhancement in short circuit current density (Jsc) as a result of doping. Transient photovoltage measurement results exhibited longer charge carrier lifetimes for the additive incorporated perovskites than those without additives, thus improving the fill factor (FF) and open circuit voltage (Voc). In addition to the improved efficiency, the incorporation of these additives led to higher stability of the CH3NH3PbI3 perovskite solar cells. The new additive BMImI, LiI and LiTFSI incorporated CH3NH3PbI3 perovskite solar cells exhibited a reduced degradation with a 57%, 60%, and 91% decrease in performance respectively after exposure to air for 70 days compared to a 93% decrease for the pristine cell after only 24 days. The lithium salt additives can serve as desiccants to absorb moisture preventing perovskite degradation. Further, the BMImI additive can prevent the formation of free radicals in perovskites upon exposure to light and heat.

Dithieno[3,2-b:2′,3′-d]pyrrole-based hole transport materials for perovskite solar cells with efficiencies over 18%

Dithieno[3,2-b:2′,3′-d]pyrrole (DTP) derivatives are one of the most important organic photovoltaic materials due to better π-conjugation across fused thiophene rings. In this work, two new hole transport materials (HTMs), H16 and H18, have been obtained through a facile synthetic route by cross linking triarylamine-based donor groups with a 4-(4-methoxyphenyl)-4H-dithieno[3,2-b:2′,3′-d]pyrrole (MPDTP) and N-(4-(4H-dithieno[3,2 b:2′,3′-d]pyrrol-4-yl)phenyl)-4-methoxy-N-(4-methoxyphenyl)aniline (TPDTP) unit, respectively. The H16 HTM outperforms the H18 in terms of conductivity, hole mobility, and hole transport at the interface. This result could be attributed to the enhancement of the conductivity, hole mobility and high quality of the film exerted by the MPDTP core. The optimized device based on H16 exhibits a high power conversion efficiency (PCE) of 18.16%, which is comparable to that obtained with the state-of-the-art-HTM spiro-OMeTAD (18.27%). Furthermore, the long-term aging test shows that the H16 based device has good stability after two months of aging under controlled (20%) humidity in the dark. Importantly, the synthesis cost of H16 is roughly 1/5 of that of spiro-OMeTAD. The present finding highlights the potential of DTP based HTMs for efficient PSCs.

Higher efficiency perovskite solar cells using Au@SiO2 core–shell nanoparticles

In this work, we improved photovoltaic performance by about 27% in planar p-i-n perovskite solar cells (PSCs) using plasmonic Au@SiO2 core–shell nanoparticles (NPs). The devices have an architecture of ITO glass/PEDOT:PSS/perovskite(CH3NH3PbI3)/PCBM/Rhodamine/Ag. Four batches of devices were fabricated with different concentrations of Au@SiO2 NPs ranging from 0.4 to 1.6 wt% with an interval of 0.4 wt%. The Au@SiO2 NPs were integrated at the interface between the PEDOT:PSS layer and the active perovskite layer. At an optimized concentration of 1.2 wt% Au@SiO2 NPs, the PSCs achieved 25.1% of enhancement in photocurrent from 17.45 to 22.35 mA cm−2 and an improvement of 27.3% in power conversion efficiency (PCE) from 11.44 to 14.57%. This significant improvement in device performance is attributed to the localized surface plasmon resonance (LSPR) of Au@SiO2 NPs, which enhanced the light absorption in the active perovskite layer. The transient photocurrent and photovoltage measurements revealed that PSCs with Au@SiO2 NPs have a faster charge transport time and longer recombination lifetime than those without Au@SiO2 NPs. These results demonstrate that plasmonic metal nanoparticles substantially improved the efficiency of PSCs.

Solution-processed all-oxide bulk heterojunction solar cells based on CuO nanaorod array and TiO2 nanocrystals

We present a method to synthesize CuO nanorod array/TiO2 nanocrystals bulk heterojunction (BHJ) on fluorine-tin-oxide (FTO) glass, in which single-crystalline p-type semiconductor of the CuO nanorod array is grown on the FTO glass by hydrothermal reaction and the n-type semiconductor of the TiO2 precursor is filled into the CuO nanorods to form well-organized nano-interpenetrating BHJ after air annealing. The interface charge transfer in CuO nanorod array/TiO2 heterojunction is studied by Kelvin probe force microscopy (KPFM). KPFM results demonstrate that the CuO nanorod array/TiO2 heterojunction can realize the transfer of photo-generated electrons from the CuO nanorod array to TiO2. In this work, a solar cell with the structure FTO/CuO nanoarray/TiO2/Al is successfully fabricated, which exhibits an open-circuit voltage (V oc) of 0.20 V and short-circuit current density (J sc) of 0.026 mA cm-2 under AM 1.5 illumination. KPFM studies indicate that the very low performance is caused by an undesirable interface charge transfer. The interfacial surface potential (SP) shows that the electron concentration in the CuO nanorod array changes considerably after illumination due to increased photo-generated electrons, but the change in the electron concentration in TiO2 is much less than in CuO, which indicates that the injection efficiency of the photo-generated electrons from CuO to TiO2 is not satisfactory, resulting in an undesirable J sc in the solar cell. The interface photovoltage from the KPFM measurement shows that the low V oc results from the small interfacial SP difference between CuO and TiO2 because the low injected electron concentration cannot raise the Fermi level significantly in TiO2. This conclusion agrees with the measured work function results under illumination. Hence, improvement of the interfacial electron injection is primary for the CuO nanorod array/TiO2 heterojunction solar cells.

Bias-Dependent Normal and Inverted J–V Hysteresis in Perovskite Solar Cells

Perovskite solar cells (PSCs) typically exhibit hysteresis in current density-voltage ( J- V) measurements. The most common type of J- V hysteresis in PSCs is normal hysteresis, in which the performance in the reverse scan is better than that in the forward scan. However, inverted hysteresis also exists, in which the reverse scan performance is worse than in the forward scan; this hysteresis, however, is significantly less well studied. In this work, we show that the hysteresis decreases when the sweep rate is decreased only in cases involving a small bias range, and it does not decrease with a large bias range. Under large forward bias and slowing sweep rate, we observe enhanced normal hysteresis or inverted hysteresis in PSCs. Moreover, the degree of normal and inverted hysteresis can be adjusted by varying the bias. Here, we hypothesize that the tunable hysteresis is derived from the different distribution of ionic defects (VI and VMA) at the electron (hole) transport layer/perovskite interface due to ionic movement in the perovskite layer under the different bias scanning conditions. This conclusion is confirmed using Kelvin probe force microscopy with different bias voltages and scanning rates, which shows surface potential hysteresis based on ionic-migration-related Fermi level shifting in perovskite films and agrees with the tunable J- V hysteresis hypothesis. Moreover, the increased time response in the milliseconds region in open-circuit voltage decay after J- V scanning further corroborates the mechanism of ionic migration under bias. Our work provides new insights into the ionic movement hypothesis for the J- V hysteresis in PSCs.