M. Westerling

Charge transport and recombination in bulk heterojunction solar cells studied by the photoinduced charge extraction in linearly increasing voltage technique

Charge carrier mobility and recombination in a bulk heterojunction solar cell based on the mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61 (PCBM) has been studied using the novel technique of photoinduced charge carrier extraction in a linearly increasing voltage (Photo-CELIV). In this technique, charge carriers are photogenerated by a short laser flash, and extracted under a reverse bias voltage ramp after an adjustable delay time (tdel). The Photo-CELIV mobility at room temperature is found to be μ=2×10−4cm2V−1s−1, which is almost independent on charge carrier density, but slightly dependent on tdel. Furthermore, determination of charge carrier lifetime and demonstration of an electric field dependent mobility is presented.

Bimolecular recombination in regiorandom poly(3-hexylthiophene)

Using cw and transient photoinduced absorption (PA) techniques we study the photoexcitation dynamics in the ms time regime, in thin films of regiorandom poly(3-hexylthiophene). The PA spectrum shows a high energy band (HE) peaked at 1.46 eV and a low energy band (LE) at 0.5 eV. The PA dependence on pump intensity and modulation frequency is modeled by analytically solving the full bimolecular rate equation using harmonic analysis. As an important result of these calculations we found that in the steady-state limit (i.e., xsB � 1), the out-of-phase PA signal saturates and becomes independent of pump intensity. This gives rise to a new possibility of directly estimating the bimolecular recombination coefficient b from the saturation value of the out-of-phase PA signal at high pump intensities. The transient PA decay of the HE band shows two components having different relaxation times. The PA decay of the HE band is mainly dominated by the fast component, which is attributed to a bimolecular recombination process in good agreement with the result from the cw measurements, together yielding a bimolecular recombination constant b ¼ 1:6 � 1:4 � 10 � 14 cm 3 =s. 2002 Elsevier Science B.V. All rights reserved.

Recombination of electronic excitations in regioregular poly(3-dodecylthiophene)

We use optical spectroscopy to study the recombination kinetics of long lived electronic excitations in a thin film of regioregular poly(3-dodecylthiophene). We have measured the steady-state photo-induced absorption spectrum, as well as the time-resolved decay of the transient photo-induced absorption in the ms time domain as a function of temperature. The transient decay of these absorptions seems to follow two hyperbolic decay components. We discuss one possible model that could give rise to such decay components.

Recombination of electronic excitations in regioregular poly(3-hexylthiophene)

We use optical spectroscopy to study the recombination dynamics of long-lived polarons in thin films of regioregular poly(3-hexylthiophene) (RRPHT). The modulation-frequency dependence and transient decay of the photoinduced polarons in RRPHT has been measured and modeled using a bimolecular recombination mechanism. In both cases, the data can be explained with two relaxation times, a slow component in the ms time regime and a faster component in the μs time regime.

Charge recombination studies in polyfluorene:[6,6]-phenyl c61-butyric acid methyl ester blend photovoltaic cells

We present a study of charge recombination in polyfluorene:[6,6]-Phenyl C61-butyric acid methyl ester (PCBM) blend photovoltaic cells. The recombination kinetics of photogenerated charge carriers are investigated at room temperature by frequency and time domain photoinduced absorption. Monomolecular and bimolecular recombination processes are discussed in detail. Finally, a discussion on what kind of recombination can be dominant in our photovoltaic cells is addressed.

Recombination dynamics in regiorandom poly(3-hexylthiophene)

Using cw photoinduced absorption (PA) techniques we study the photoexcitation dynamics in the ms time regime, in thin films of regiorandom poly(3-hexylthiophene). The PA spectrum shows a high energy band (HE) peaked at 1.46eV and a low energy band (LE) at 0.5eV. The PA dependence on pump intensity and modulation frequency is modeled by analytically solving the full bimolecular rate equation using harmonic analysis. As an important result of these calculations we found that in the steady-state limit (i.e. ωτ B «1), the out-of-phase PA signal saturates and becomes independent of pump intensity. This gives rise to a new possibility of directly estimating the bimolecular recombination coefficient β from the saturation value of the out-of-phase PA signal at high pump intensities. The decay of the HE band is attributed to a bimolecular recombination process with β ≃ 0.6s * 10 -14 cm 3 /s.