Evidence for “Slow” Electron Injection in Commercially Relevant Dye-Sensitized Solar Cells by vis–NIR and IR Pump–Probe Spectroscopy

We present femtosecond to nanosecond transient absorption (TA) data on electron injection in dye-sensitized solar cells (DSSCs) fabricated with low volatility, commercially relevant electrolytes, with and without added lithium. Results are shown over an extended time range (300 fs–6.3 ns) and extended wavelength range (800–1400 nm) for both N719 and C106 dyes. Kinetics were measured on both TiO₂ and noninjecting ZrO₂. Using the latter, we have determined the spectra and absorption coefficient of N719* across the wavelength range. We find an isosbestic point in the TA spectra on TiO₂ near 900 nm for all cells, existing from <1 ps to >1 ns. We show how measurements near this isosbestic point can give a false impression of uniformly femtosecond injection dynamics in DSSCs. Comparison of dynamics measured at 1200 nm with mid-IR transient absorption measured at 5100 nm confirms a majority proportion of slow (>10 ps) electron injection in these commercially relevant cells. We also comment on a recent publication which appears to directly contradict the results we present

Charge Separation in Intermixed Polymer:PC₇₀BM Photovoltaic Blends: Correlating Structural and Photophysical Length Scales as a Function of Blend Composition

A key challenge in achieving control over photocurrent generation by bulk-heterojunction organic solar cells is understanding how the morphology of the active layer impacts charge separation and in particular the separation dynamics <i>within</i> molecularly intermixed donor–acceptor domains versus the dynamics <i>between</i> phase-segregated domains. This paper addresses this issue by studying blends and devices of the amorphous silicon–indacenodithiophene polymer SiIDT-DTBT and the acceptor PC₇₀BM. By changing the blend composition, we modulate the size and density of the pure and intermixed domains on the nanometer length scale. Laser spectroscopic studies show that these changes in morphology correlate quantitatively with the changes in charge separation dynamics on the nanosecond time scale and with device photocurrent densities. At low fullerene compositions, where only a single, molecularly intermixed polymer–fullerene phase is observed, photoexcitation results in a ∼ 30% charge loss from geminate polaron pair recombination, which is further studied via light intensity experiments showing that the radius of the polaron pairs in the intermixed phase is 3–5 nm. At high fullerene compositions (≥67%), where the intermixed domains are 1–3 nm and the pure fullerene phases reach ∼4 nm, the geminate recombination is suppressed by the reduction of the intermixed phase, making the fullerene domains accessible for electron escape

Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells

We present the synthesis and characterization of two novel thiazole-containing conjugated polymers (<b>PTTTz</b> and <b>PTTz</b>) that are isostructural to poly­(3-hexylthiophene) (P3HT). The novel materials demonstrate optical and morphological properties almost identical to those of P3HT but with HOMO and LUMO levels that are up to 0.45 eV deeper. An intramolecular planarizing nitrogen–sulfur nonbonding interaction is observed, and its magnitude and origin are discussed. Both materials demonstrate significantly greater open circuit voltages than P3HT in bulk heterojunction solar cells. <b>PTTTz</b> is shown to be an extremely versatile donor polymer that can be used with a wide variety of fullerene acceptors with device efficiencies of up to 4.5%. It is anticipated that this material could be used as a high-open circuit voltage alternative to P3HT in organic solar cells