Lucio Cinà

Graphene–Perovskite Solar Cells Exceed 18 % Efficiency: A Stability Study

Interface engineering is performed by the addition of graphene and related 2 D materials (GRMs) into perovskite solar cells (PSCs), leading to improvements in the power conversion efficiency (PCE). By doping the mesoporous TiO2 layer with graphene flakes (mTiO2 +G), produced by liquid-phase exfoliation of pristine graphite, and by inserting graphene oxide (GO) as an interlayer between the perovskite and hole-transport layers, using a two-step deposition procedure in air, we achieved a PCE of 18.2 %. The obtained PCE value mainly results from improved charge-carrier injection/collection with respect to conventional PSCs. Although the addition of GRMs does not influence the shelf life, it is beneficial for the stability of PSCs under several aging conditions. In particular, mTiO2 +G PSCs retain more than 88 % of the initial PCE after 16 h of prolonged 1 sun illumination at the maximum power point. Moreover, when subjected to prolonged heating at 60 °C, the GO-based structures show enhanced stability with respect to mTiO2 +G PSCs, as a result of thermally induced modification at the mTiO2 +G/perovskite interface. The exploitation of GRMs in the form of dispersions and inks opens the way for scalable large-area production, advancing the possible commercialization of PSCs.

Fully-Sprayed and Flexible Organic Photodiodes with Transparent Carbon Nanotube Electrodes

In this study, we demonstrate the feasibility of TCO-free, fully sprayed organic photodiodes on flexible polyethylene terephthalate (PET) substrates. Transparent conducting films of single-wall carbon nanotubes are spray deposited from aqueous solutions. Low roughness is achieved, and films with sheet resistance values of 160 Ω/sq at 84% in transmittance are fabricated. Process issues related to the wetting of CNTs are then examined and solved, enabling successive spray depositions of a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer and a blend of regioregular poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). The active layer is then optimized, achieving a process yield above 90% and dark currents as low as 10(-4) mA/cm(2). An external quantum efficiency of 65% and high reproducibility in the performance of the devices are obtained. Finally, the impact of the characteristics of the transparent electrode (transmittance and sheet resistance) on the performances of the device are investigated and validated through a theoretical model and experimental data.

Global MPPT method for partially shaded photovoltaic modules

Among Renewable Energy Sources, the solar energy represents an attractive alternative solution to traditional sources because is available everywhere; moreover, photovoltaic (PV) modules are nowadays becoming more and more attractive as the price per watt of the photovoltaic modules is decreasing. The output voltage furnished by a PV module is quite low; therefore, a series connection of different modules has to be employed in order to obtain a sufficient output dc voltage. In non-uniform irradiation conditions of a PV array, traditional Maximum Power Point Tracking (MPPT) algorithms can converge to a relative maximum point, thus output power becomes lower than the absolute maximum. Since the conversion efficiency of commercial photovoltaic modules is still rather low, to increase the power produced, also under partially shaded conditions, is necessary to employ an efficiently, fast and simple MPPT. The paper proposes, for a Switched Capacitor-Boost converter structure, a non-conventional MPPT method able to track the absolute Maximum Power Point (MPP).

High-Efficiency Perovskite Solar Cell Based on Poly(3-Hexylthiophene): Influence of Molecular Weight and Mesoscopic Scaffold Layer

Here, we investigated the effect of the molecular weight (MW) of poly 3-hexylthiophene (P3HT) hole-transport material on the performance of perovskite solar cells (PSCs). We found that by increasing the MW the photovoltaic performances of the cells are enhanced leading to an improvement of the overall efficiency. P3HT-based PSCs with a MW of 124 kDa can achieve an overall average efficiency of 16.2 %, double with respect to the ones with a MW of 44 kDa. Opposite to spiro-OMeTAD-based PSCs, the photovoltaic parameters of the P3HT-based devices are enhanced by increasing the mesoporous TiO2 layer thickness from 250 to 500 nm. Moreover, for a titania scaffold layer thickness of 500 nm, the efficiency of P3HT-based PSCs with high MW is larger than the spiro-OMeTAD based PSCs with the same scaffold layer thickness. Recombination reactions of the devices were also investigated by voltage decay and electrochemical impedance spectroscopy. We found that the relationship between P3HT MW and cell performance is related to the reduction of charge recombination and to the increase of the P3HT light absorption by increasing the MW.

Mesoscopic Perovskite Light-Emitting Diodes

Solution-processed hybrid bromide perovskite light-emitting-diodes (PLEDs) represent an attractive alternative technology that would allow overcoming the well-known severe efficiency drop in the green spectrum related to conventional LEDs technologies. In this work, we report on the development and characterization of PLEDs fabricated using, for the first time, a mesostructured layout. Stability of PLEDs is a critical issue; remarkably, mesostructured PLEDs devices tested in ambient conditions and without encapsulation showed a lifetime well-above what previously reported with a planar heterojunction layout. Moreover, mesostructured PLEDs measured under full operative conditions showed a remarkably narrow emission spectrum, even lower than what is typically obtained by nitride- or phosphide-based green LEDs. A dynamic analysis has shown fast rise and fall times, demonstrating the suitability of PLEDs for display applications. Combined electrical and advanced structural analyses (Raman, XPS depth profiling, and ToF-SIMS 3D analysis) have been performed to elucidate the degradation mechanism, the results of which are mainly related to the degradation of the hole-transporting material (HTM) and to the perovskite-HTM interface.

Fabrication and Characterization of Mesoscopic Perovskite Photodiodes

Mesoscopic photodiodes were fabricated with hybrid organic/inorganic perovskite as absorber layer and Spiro-OMeTAD as hole transport Layer. The perovskite layer was grown using a two-step deposition technique. Our photodiode in addition to a good rectification behavior (three orders of magnitude, range -1 to 1 V) shows a small noise current (<; 1 pA/(Hz)1/2), a high responsivity value (0.35 A/W) at 500 nanometers, and a good spectral response in the entire visible range. The Bode analysis shows a bandwidth of 108 KHz.

Efficient fully laser-patterned flexible perovskite modules and solar cells based on low-temperature solution-processed SnO 2 /mesoporous-TiO 2 electron transport layers

Efficient flexible perovskite solar cells and modules were developed using a combination of SnO2 and mesoporous-TiO2 as a fully solution-processed electron transport layer (ETL). Cells using such ETLs delivered a maximum power conversion efficiency (PCE) of 14.8%, which was 30% higher than the PCE of cells with only SnO2 as the ETL. The presence of a mesoporous TiO2 scaffold layer over SnO2 led to higher rectification ratios, lower series resistances, and higher shunt resistances. The cells were also evaluated under 200 and 400 lx artificial indoor illumination and found to deliver maximum power densities of 9.77 μW/cm2 (estimated PCE of 12.8%) and 19.2 μW/cm2 (estimated PCE of 13.3%), respectively, representing the highest values among flexible photovoltaic technologies reported so far. Furthermore, for the first time, a fully laser-patterned flexible perovskite module was fabricated using a complete three-step laser scribing procedure (P1, P2, P3) with a PCE of 8.8% over an active area of 12 cm2 under an illumination of 1 sun.

A crystal engineering approach for scalable perovskite solar cells and module fabrication: a full out of glove box procedure

In the present work we used some crystallization trends which could be classified as a Crystal Engineering (CE) approach, for deposition of a pure cubic-phase thin film of CH3NH3PbI3 (MAPbI3) on the surface of a mesoporous TiO2 layer. Accordingly, by using the CE approach, we fabricated high efficiency perovskite solar cells (PSCs) and perovskite solar modules (PSMs) utilizing several Hole Transport Layers (HTLs). We optimized the sequential deposition method, developing the entire realization procedure in air. The results show that the CE approach remarkably improved the device performance reaching a power conversion efficiency of 17%, 16.8% and 7% for spiro-OMeTAD, P3HT and HTL free (direct contact of the perovskite layer with the gold layer) PSCs, respectively. Furthermore, perovskite solar modules (active area of 10.1 cm2), which are fabricated by the CE approach, could reach an overall efficiency of 13% and 12.1% by using spiro-OMeTAD and P3HT as HTLs, respectively. The sealed modules showed promising results in terms of stability maintaining 70% of the initial efficiency after 350 hours of light soaking at the maximum power point.

Spray deposition of exfoliated MoS2 flakes as hole transport layer in perovskite-based photovoltaics

We propose the use of solution-processed molybdenum disulfide (MoS₂) flakes as hole transport layer (HTL) for metal-organic perovskite solar cells. MoS₂ bulk crystals are exfoliated in 2-propanol and deposited on perovskite layers by spray coating. We fabricated cells with glass/FTO/compact-TiO₂/mesoporous-TiO₂/CH₃NH₃PbI₃/spiro-OMeTAD/Au structure and cells with the same structure but with MoS₂ flakes as HTL instead of spiro-OMeTAD, the most widely used HTL. The electrical characterization of the cells with MoS₂ as HTL show promising power conversion efficiency -η- of 3.9% with respect to cells with pristine spiro-OMeTAD (η=3.1%). Endurance test on 800-hour shelf life has shown higher stability for the MoS₂-based cells (ΔPCE/PCE=-17%) with respect to the doped spiro-OMeTAD-based one (ΔPCE/PCE =-45%). Further improvements are expected with the optimization of the MoS₂ deposition process

Fabrication and characterization of printed perovskite-based photodiodes

We fabricated the first perovskite-based photodiodes using one-step and two-step deposition with Spiro-OMeTAD as Hole Transport Material (HTM). Our photodiode show excellent rectification behavior, low dark currents under reverse bias and good dynamic properties evaluated with Bode and eye diagram. A frequency resolved noise analysis was performed in order to extract detectivity and optimal bias potential values.