Femtosecond Transient Absorption Spectroscopy (FTAS) allows to investigate the temporal evolution at different wavelengths of some processes such as exciton formation, exciton diffusion, energy transfer, charge generation and recombination. The rise or decay times of these processes can give information on their relative efficiency .
J. Dagar et al.  have developed an organic polymer-based photovoltaic (OPV) cell with “inverted” architecture where a DNA layer (thickness 1-6 nm) has been used as an electron extraction layer (EEL). They have demonstrated that the DNA layer improves the PCE of the device with respect to that of the cell with the same architecture but with only ZnO as the EEL. In this study we perform FTAS experiments to study the role of DNA in different OPV cell configurations.
The transient absorption measurements were performed by exciting the solar cells with the second harmonic of a 35 fs 800 nm Ti:Sapph laser.
The experiments were performed in a pseudo-reflectivity mode in which the focused pump beam
passes through the cell from the glass side, reflects from the Ag electrode and exits again through the glass with an incidence angle of 4deg. The probe beam is generated by focusing ∼3 μJ of 800nm light into a CaF2crystal thus generating a white light continuum in the range from 350 - 800 nm.
The FTAS spectra can be divided in two zones: 1) photoinduced absorption (PIA) (positive ∆A) in the region between 620 and 750 nm which can be further divided into two features (∼720 nm, ∼650nm) 2) photoinduced bleaching/ground state bleaching (PB) (negative ∆A) in the wavelength region from 470 - 620 nm which can be further divided into three regions (∼600nm, ∼550nm, ∼500nm).
We observed that regions of PIA do not appear to be significantly sensitive to the changes in the EEL; also PB@600nm region is not affected to the changes in the EEL.
The region of the spectra containing the 550nm and 500nm features, exhibits significant variations both in the peak wavelengths and temporal behaviors of the signals with varying composition of the EEL. We discuss these results in order to explain the photo-physical processes of the devices and the role of DNA in the solar cells.
 J. Dagar, M. Scarselli, M. De Crescenzi, and T. M. Brown, ACS Energy Lett. 2016, 1, 510−515