Jean Manca

On the origin of the open-circuit voltage of polymer–fullerene solar cells

The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.

Perovskite‐Based Hybrid Solar Cells Exceeding 10% Efficiency with High Reproducibility Using a Thin Film Sandwich Approach

Organometal halide perovskites have tremendous potential as light absorbers for photovoltaic applications. In this work we demonstrate hybrid solar cells based on the mixed perovskite CH3 NH3 PbI2 Cl in a thin film sandwich structure, with unprecedented reproducibility and generating efficiencies up to 10.8%. The successfulness of our approach is corroborated by the experimental electronic structure determination of this perovskite.

Phase diagram of P3HT/PCBM blends and its implication for the stability of morphology.

In this work, the phase diagram of poly(3-hexyl thiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blends is measured by means of standard and modulated temperature differential scanning calorimetry. Blends were made by solvent-casting from chlorobenzene, as blends cast from toluene or 1,2-dichlorobenzene prove to retain effects of phase segregation during casting, hindering the determination of the phase diagram. The film morphology of P3HT/PCBM blends cast from chlorobenzene results from a dual crystallization behavior, in which the crystallization of each component is hindered by the other component. A single glass transition is observed for all compositions. The glass transition temperature (Tg) increases with increasing concentration of PCBM: from 12.1 degrees C for pure P3HT to 131.2 degrees C for pure PCBM. The observed Tg defines the operating window for the thermal annealing and explains the long-term instability of both the morphology and the photovoltaic performance of the P3HT/PCBM solar cells.

Electroluminescence from Charge Transfer States in Polymer Solar Cells

In this article we report the weak but omnipresent electroluminescence (EL) from several types of organic polymer:fullerene bulk heterojunction solar cells biased in the forward direction. The light emitted from blends of some commonly used polymers and the fullerene molecule is significantly different from that of any of the pure materials comprising the blend. The lower energy of the blend EL is found to correlate with both the voltage onset of emission and the open-circuit voltage of the photovoltaic cell under solar illumination. We accordingly interpret the emission to originate from interfacial charge transfer state recombination and emphasize EL as a very valuable tool to characterize the charge transfer state present in donor/acceptor organic photovoltaic (OPV) cells.

Charge Transfer State Versus Hot Exciton Dissociation in Polymer−Fullerene Blended Solar Cells

We examine the significance of hot exciton dissociation in two archetypical polymer-fullerene blend solar cells. Rather than evolving through a bound charge transfer state, hot processes are proposed to convert excitons directly into free charges. But we find that the internal quantum yields of carrier photogeneration are similar for both excitons and direct excitation of charge transfer states. The internal quantum yield, together with the temperature dependence of the current-voltage characteristics, is consistent with negligible impact from hot exciton dissociation.

Disclosure of the nanostructure of MDMO-PPV:PCBM bulk hetero-junction organic solar cells by a combination of SPM and TEM

Abstract The microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells is studied by atomic force microscopy (AFM) to image the surface morphology and by means of transmission electron microscopy (TEM) to disclose the bulk nanostructure of the film. Typical thin films, as used for state-of-the-art organic bulk hetero-junction solar cells consist of a 1:4 ratio by weight of MDMO-PPV as electron donating polymer and PCBM, a soluble electron accepting C 60 derivative. For these films it is found, using both TEM an AFM, that phase separation occurs. A two-phase system is observed that consists of PCBM-rich domains that are embedded in a matrix consisting of a mixture of MDMO-PPV and PCBM. By combining planar and cross-sectional views, three-dimensional information is obtained on the phase separated PCBM-rich regions, formed during spincoating. Changing the solvent is found to influence the size of the phase separated PCBM-rich domains. But not only the dimensions of the phase separated regions are affected by changing the solvent. Also the composition of the matrix is found to be determined by the choice of solvent. This was studied by changing the ratio of PCBM compared to MDMO-PPV. Since it is commonly believed that the morphology of the active layer influences electrical properties and photovoltaic performance, the nanostructural information obtained with the presented analytical techniques will attribute to a better understanding and improvement of present organic photovoltaic devices.

Observation of the subgap optical absorption in polymer-fullerene blend solar cells

This letter reports on highly sensitive optical absorption measurements on organic donor-acceptor solar cells, using Fourier-transform photocurrent spectroscopy (FTPS). The spectra cover an unprecedented dynamic range of eight to nine orders of magnitude making it possible to detect defect and disorder related sub-band gap transitions. Direct measurements on fully encapsulated solar cells with an active layer of poly[2-methoxy-5-(3′,7′-dimethyl-octyloxy)]-p-phenylene-vinylene:(6,6)-phenyl-C61-butyric-acid (1:4 weight ratio) enabled a study of the intrinsic defect generation due to UV illumination. Solar cell temperature annealing effects in poly(3-hexylthiophene):PCBM (1:2 weight ratio) cells and the induced morphological changes are related to the changes in the absorption spectrum, as determined with FTPS.

PEDOT/PSS: synthesis, characterization, properties and applications

AGFA Gevaert NV, R&D Mat, B-2640 Mortsel, Belgium. IMOMEC, B-3590 Diepenbeek, Belgium.Louwet, F, AGFA Gevaert NV, R&D Mat, Septestr 27, B-2640 Mortsel, Belgium.

Towards Efficient Hybrid Solar Cells Based on Fully Polymer Infiltrated ZnO Nanorod Arrays

L.B. and B.C. contributed equally to this work. The authors thank R. Rieke from Rieke Metals for useful suggestions. This work was financially supported by BOF, UHasselt, the Flemish Odysseus program, and the Interreg project Organext. A. H. is a postdoctoral research fellow of the Research Foundation-Flanders (FWO Vlaanderen).

Assessing the toxicity of Pb- and Sn-based perovskite solar cells in model organism Danio rerio.

Intensive development of organometal halide perovskite solar cells has lead to a dramatic surge in power conversion efficiency up to 20%. Unfortunately, the most efficient perovskite solar cells all contain lead (Pb), which is an unsettling flaw that leads to severe environmental concerns and is therefore a stumbling block envisioning their large-scale application. Aiming for the retention of favorable electro-optical properties, tin (Sn) has been considered the most likely substitute. Preliminary studies have however shown that Sn-based perovskites are highly unstable and, moreover, Sn is also enlisted as a harmful chemical, with similar concerns regarding environment and health. To bring more clarity into the appropriateness of both metals in perovskite solar cells, we provide a case study with systematic comparison regarding the environmental impact of Pb- and Sn-based perovskites, using zebrafish (Danio Rerio) as model organism. Uncovering an unexpected route of intoxication in the form of acidification, it is shown that Sn based perovskite may not be the ideal Pb surrogate.