Perovskites, the rising star for energy harvesting

Perovskites are promising candidates for photovoltaic cells, having reached an energy harvesting of more than 20% while it took silicon three decades to reach an equivalent. Scientists from all over the world are exploring these materials at the ESRF.

Photovoltaic (PV) panels exist in our society since several years now. The photovoltaic market is currently dominated by wafer-based photovoltaics or first generation PVs, namely the traditional crystalline silicon cells, which take a 90% of the market share.

Although silicon (Si) is an abundant material and the price of Si-PV has dropped in the past years, their manufacturing require costly facilities. In addition, their fabrication typically takes place in countries that rely on carbon-intensive forms of electricity generation (high carbon footprint).

But there is room for hope. There is a third generation of PV: those based on thin-film cells. These absorb light more efficiently and they currently take 10% of the market share.

>Read more on the European Synchrotron website

Image: The CEA-CNRS team on ID01. From left to right: Peter Reiss, from CEA-Grenoble/INAC, Tobias Schulli from ID01, Tao Zhou from ID01, Asma Aicha Medjahed, Stephanie Pouget (both from CEA-Grenoble/INAC) and David Djurado, from the CNRS. 
Credits: C. Argoud.

Stable solvent for solution-based electrical doping…

… of semiconducting polymer films and its application to organic solar cells.

Controlled and stable electrical doping of organic semiconductors is desirable for the realization of efficient organic photovoltaic (OPV) devices. Thus, progress has been made to understand the fundamental doping mechanisms.1-3 In 2016, Aizawa et al. reported the use of 12-molybdophosphoric acid hydrate (PMA) to induce p-type doping and crosslinking of neat films of poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole)](PCDTBT).4 Later on, a more general approach of sequential solution-based doping was presented, by post-process immersion of donor-like polymer films in PMA-nitromethane solutions.5 However, critical to the method is the use of nitromethane, a highly unstable solvent, to dissolve PMA and thus limited the applicability to large-scale fabrication of organic solar cells.

A collaboration between a team of researchers from the Kippelen Research Group at Georgia Tech and the Toney Research Group at SSRL developed a solution-based doping method using the highly stable solvent, acetonitrile. Figure 1a displays the chemicals used in this work. In Figure 1b, the optical properties of poly(3-hexylthiophene-2,5-diyl)(P3HT) films immersed for 30 min in a 0.5 M solution of PMA in acetonitrile (PMA-im-P3HT) were studied by comparing their transmittance spectra against pristine P3HT and P3HT immersed similarly in a 0.5 M solution of PMA in nitromethane. The normalized change of transmittance ΔT T-1 as a function of wavelength (inset of Fig.1b) reveals the same spectral signatures reported for PMA-im-P3HT films when PMA was dissolved in nitromethane. That is, changes in the region where ΔT T-1< 0 correlate with the P3HT polaron bands, and deviations in the region where ΔT T-1> 0 correlate to the bleaching of the main π-π* absorption bands.6 The data suggests electrical p-doping into the depth of the organic film. Figure 1c shows that the performance of PMA-doped OPV devices using PMA in acetonitrile is comparable to that of OPVs made using PMA in nitromethane or MoO3, under simulated AM 1.5G solar illumination. Furthermore, if the light soaking mechanism is used before each measurement, OPVs made using PMA in nitromethane or acetonitrile remain stable for up to 524 h in the air, retaining 80% of their initial power conversion efficiency (PCE).

>Read more on the Standfort Synchrotron Radiation Lightsource website

Figure: (extract) of GIWAXS data as measured on pristine and PMA doped P3HT, when using various solvents to dissolve the PMA. a, Two-dimensional GIWAXS data converted to q-space for pristine P3HT and P3HT immersed in PMA solutions in nitromethane, acetonitrile or ethanol for 60 seconds. b, One-dimensional scattering profiles (out-of-plane and in-plane profiles), obtained from the two-dimensional GIWAXS data.