Alexandre M. Nardes

Solid-State Mesostructured Perovskite CH3NH3PbI3 Solar Cells: Charge Transport, Recombination, and Diffusion Length

We report on the effect of TiO2 film thickness on charge transport and recombination in solid-state mesostructured perovskite CH3NH3PbI3 (via one-step coating) solar cells using spiro-MeOTAD as the hole conductor. Intensity-modulated photocurrent/photovoltage spectroscopies show that the transport and recombination properties of solid-state mesostructured perovskite solar cells are similar to those of solid-state dye-sensitized solar cells. Charge transport in perovskite cells is dominated by electron conduction within the mesoporous TiO2 network rather than from the perovskite layer. Although no significant film-thickness dependence is found for transport and recombination, the efficiency of perovskite cells increases with TiO2 film thickness from 240 nm to about 650-850 nm owing primarily to the enhanced light harvesting. Further increasing film thickness reduces cell efficiency associated with decreased fill factor or photocurrent density. The electron diffusion length in mesostructured perovskite cells is longer than 1 μm for over four orders of magnitude of light intensity.

Effective hole extraction using MoOx-Al contact in perovskite CH3NH3PbI3 solar cells

We report an 11.4%-efficient perovskite CH3NH3PbI3 solar cell using low-cost molybdenum oxide/aluminum (i.e., MoOx/Al) as an alternative top contact to replace noble/precious metals (e.g., Au or Ag) for extracting photogenerated holes. The device performance of perovskite solar cells using a MoOx/Al top contact is comparable to that of cells using the standard Ag top contact. Analysis of impedance spectroscopy measurements suggests that using 10-nm-thick MoOx and Al does not affect charge-recombination properties of perovskite solar cells. Using a thicker (20-nm) MoOx layer leads to a lower cell performance caused mainly by a reduced fill factor. Our results suggest that MoOx/Al is promising as a low-cost and effective hole-extraction contact for perovskite solar cells.

Free Carrier Generation in Organic Photovoltaic Bulk Heterojunctions of Conjugated Polymers with Molecular Acceptors: Planar versus Spherical Acceptors

A comparative study of the photophysical performance of the prototypical fullerene derivative PC61BM with a planar small-molecule acceptor in an organic photovoltaic device is presented. The small-molecule planar acceptor is 2-[{7-(9,9-di-n-propyl-9H-fluoren-2-yl)benzo[c][1,2,5]thiadiazol-4-yl}methylene]malononitrile, termed K12. We discuss photoinduced free charge-carrier generation and transport in blends of PC61BM or K12 with poly(3-n-hexylthiophene) (P3HT), surveying literature results for P3HT:PC61BM and presenting new results on P3HT:K12. For both systems we also review previous work on film structure and correlate the structural and photophysical results. In both cases, a disordered mixed phase is formed between P3HT and the acceptor, although the photophysical properties of this mixed phase differ markedly for PC61BM and K12. In the case of PC61BM the mixed phase acts as a free carrier generation region that can efficiently shuttle carriers to the pure polymer and fullerene domains. As a result, the vast majority of excitons quenched in P3HT:PC61BM blends yield free carriers detected by the contactless time-resolved microwave conductivity (TRMC) method. In contrast, approximately 85% of the excitons quenched in P3HT:K12 do not result in free carriers over the nanosecond timescale of the TRMC experiment. We attribute this to poor electron-transport properties in the mixed P3HT:K12 phase. We propose that the observed differences can be traced to the respective shapes of PC61BM and K12: the three-dimensional nature of the fullerene cage facilitates coupling between PC61BM molecules irrespective of their relative orientation, whereas for K12 strong electronic coupling is only expected for molecules oriented with their π systems parallel to each other. Comparison between the eutectic compositions of the P3HT:PC61BM and P3HT:K12 shows that the former contains enough fullerene to form a percolation pathway for electrons, whereas the latter contains a sub-percolating volume fraction of the planar acceptor. Furthermore, the planar K12 co-assembles with P3HT into a disordered, glassy phase that partly accounts for the poor electron-transport properties, and may also enhance recombination due to the strong intermolecular interactions between the donor and the acceptor. The implication for the performance of organic photovoltaic devices with the two acceptors is also discussed.

Impact of the Crystallite Orientation Distribution on Exciton Transport in Donor–Acceptor Conjugated Polymers

Conjugated polymers are widely used materials in organic photovoltaic devices. Owing to their extended electronic wave functions, they often form semicrystalline thin films. In this work, we aim to understand whether distribution of crystallographic orientations affects exciton diffusion using a low-band-gap polymer backbone motif that is representative of the donor/acceptor copolymer class. Using the fact that the polymer side chain can tune the dominant crystallographic orientation in the thin film, we have measured the quenching of polymer photoluminescence, and thus the extent of exciton dissociation, as a function of crystal orientation with respect to a quenching substrate. We find that the crystallite orientation distribution has little effect on the average exciton diffusion length. We suggest several possibilities for the lack of correlation between crystallographic texture and exciton transport in semicrystalline conjugated polymer films.

Integrated optical and electrical modeling of plasmon-enhanced thin film photovoltaics: A case-study on organic devices

The nanoscale light control for absorption enhancement of organic photovoltaic (OPV) devices inevitably produces strongly non-uniform optical fields. These non-uniformities due to the localized optical modes are a primary route toward absorption enhancement in OPV devices. Therefore, a rigorous modeling tool taking into account the spatial distribution of optical field and carrier generation is necessary. Presented here is a comprehensive numerical model to describe the coupled optical and electrical behavior of plasmon-enhanced polymer:fullerene bulk heterojunction (BHJ) solar cells. In this model, a position-dependent electron-hole pair generation rate that could become highly non-uniform due to photonic nanostructures is directly calculated from the optical simulations. By considering the absorption and plasmonic properties of nanophotonic gratings included in two different popular device architectures, and applying the Poisson, current continuity, and drift/diffusion equations, the model predicts quantum efficiency, short-circuit current density, and desired carrier mobility ratios for bulk heterojunction devices incorporating nanostructures for light management. In particular, the model predicts a significant degradation of device performance when the carrier species with lower mobility are generated far from the collecting electrode. Consequently, an inverted device architecture is preferred for materials with low hole mobility. This is especially true for devices that include plasmonic nanostructures. Additionally, due to the incorporation of a plasmonic nanostructure, we use simulations to theoretically predict absorption band broadening of a BHJ into energies below the band gap, resulting in a 4.8% increase in generated photocurrent.

Applications of admittance spectroscopy in photovoltaic devices beyond majority-carrier trapping defects

Admittance spectroscopy is commonly used to characterize majority-carrier trapping defects. In todays practical photovoltaic devices, however, a number of other physical mechanisms may contribute to the admittance measurement and interfere with the data interpretation. Such challenges arise due to the violation of basic assumptions of conventional admittance spectroscopy such as single-junction, ohmic contact, highly conductive absorbers, and measurement in reverse bias. We exploit such violations to devise admittance spectroscopy-based methods for studying the respective origins of “interference”: majority-carrier mobility, non-ohmic contact potential barrier, minority-carrier inversion at heterointerface, and minority-carrier lifetime in a device environment. These methods are applied to a variety of photovoltaic technologies: CdTe, Cu(In, Ga)Se2, Si HIT cells, and organic photovoltaic materials.

Surface plasmon enhanced infrared absorption in P3HT-based organic solar cells: the effect of infrared sensitizer (Presentation Recording)

We have theoretically and experimentally investigated the effects of Ag-grating electrode on the performance of polymer:fullerene based bulk heterojunction organic solar cells. First, an integrated numerical model has been developed, which is capable of describing both the optical and the electrical properties simultaneously. The Ag-grating patterned back electrode was then designed to enhance the absorption in sub-bandgap region of P3HT:PCBM binary devices. Laser interference lithography and metal lift-off technique were adopted to realize highly-uniform and large-area nanograting patterns. We measured almost 5 times enhancement of the external quantum efficiency at the surface plasmon resonance wavelength. However, the overall improvement in power conversion efficiency was not significant due to the low intrinsic absorption of active layer in this sub-bandgap region. We, then, investigated about the effect of surface plasmon on the ternary device of P3HT:Si-PCPDTBT:ICBA. It was demonstrated that the infrared absorption by the Si-PCPDTBT sensitizer can be substantially enhanced by matching the surface plasmon resonance to the sensitizer absorption band. Besides, we also observed an additional enhancement in the visible range which is due to the scattering effect of the gratings. An overall short-circuit current enhancement of up to 40% was predicted numerically. We have then fabricated the device by the lamination technique and observed a 30% increase in the short circuit current. Plasmon enhancement of sensitized organic solar cell presents a promising pathway to high-efficiency, broadband-absorbing polymer:fullerene bulk heterojunction organic solar cells.

Surface plasmon enhanced infrared absorption in the sensitized polymer solar cell

We have theoretically demonstrated an enhanced infrared absorption of the sensitizer in ternary polymer solar cell by introducing silver gratings at the back metal electrode. A combined model which incorporates the complex optical absorption profile and the electrical transport of the generated charge carriers was successfully developed. Using this model, we considered Si-PCPDTBT as an infrared sensitizer for P3HT:ICBA bulk heterojunction solar cells. A silver grating feature was optimized to produce a highly localized optical field inside the active polymer layer and enhance the infrared absorption of the sensitizer. Finally, an overall short-circuit current enhancement of about 40% is obtained theoretically.

Thermal annealing affects vertical morphology, doping and defect density in BHJ OPV devices

We demonstrate that a post-annealing step results in enhanced open-circuit voltage (Voc) and fill factor (FF) and lower reverse saturation current (Js) that consequently increases the power conversion efficiency (PCE) of organic bulk-heterojunction (BHJ) devices by about 40 % as a result of better contact formation, as typically assumed. Although true, we show that additional device properties are affected as well. We found that annealing induces vertical phase segregation and consequently the enrichment of donor and acceptor materials at the correct electrical contact. In addition, a de-doping process and a decrease in defect density also take place and are the major causes for device improvement after post-annealing the OPV devices. Implications for OPV basic research and manufacturing are discussed.

Triphenylamine-based star-shaped absorbers with tunable energy levels for organic photovoltaics

A new family of soluble low-bandgap star-shaped molecules based upon triphenylamine (TPA) and containing benzothiadiazole (BTD) acceptor moieties have been designed and synthesized for organic photovoltaic (OPV) applications. The design results in the lowest unoccupied molecular orbital (LUMO) being spatially distributed on the periphery of the molecule, allowing facile photo-induced electron transfer to the fullerene phenyl C61 butyric acid methyl ester (PCBM). Photoluminescence quenching studies indicate efficient quenching of excitons, while time-resolved microwave conductivity experiments demonstrate effective separation of charges. Adjunct electron-withdrawing moieties allow tuning of the LUMO level. Theoretical calculations indicate three derivatives with LUMO levels in varying proximity to that of PCBM, which allows empirical testing of the theorized need for a 0.3 eV LUMO offset to ensure efficient charge transfer to PCBM. Design, characterization and bulk heterojunction device results for the new materials will be presented.