Ultrafast Dynamics of Localized and Delocalized Polaron Transitions in P3HT/PCBM Blend Materials: The Effects of PCBM Concentration

Nowadays, organic solar cells have the interest of engineers for manufacturing flexible and low cost devices. The considerable progress of this nanotechnology area presents the possibility of investigating new effects from a fundamental science point of view. In this letter we highlight the influence of the concentration of fullerene molecules on the ultrafast transport properties of charged electrons and polarons in P3HT/PCBM blended materials which are crucial for the development of organic solar cells. Especially, we report on the femtosecond dynamics of localized (P2at 1.45 eV) and delocalized (DP2at 1.76 eV) polaron states of P3HT matrix with the addition of fullerene molecules as well as the free-electron relaxation dynamics of PCBM-related states. Our study shows that as PCBM concentration increases, the amplified exciton dissociation at bulk heterojunctions leads to increased polaron lifetimes. However, the increase in PCBM concentration can be directly related to the localization of polarons, creating thus two competing trends within the material. Our methodology shows that the effect of changes in structure and/or composition can be monitored at the fundamental level toward optimization of device efficiency

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

Each year we are bombarded with B.Sc. and Ph.D. applications from students that want to improve the world. They have learned that their future depends on changing the type of fuel we use and that solar energy is our future. The hope and energy of these young people will transform future energy technologies, but it will not happen quickly. Organic photovoltaic devices are easy to sketch, but the materials, processing steps, and ways of measuring the properties of the materials are very complicated. It is not trivial to make a systematic measurement that will change the way other research groups think or practice. In approaching this chapter, we thought about what a new researcher would need to know about organic photovoltaic devices and materials in order to have a good start in the subject. Then, we simplified that to focus on what a new researcher would need to know about poly-3-hexylthiophene:phenyl-C61-butyric acid methyl ester blends (P3HT: PCBM) to make research progress with these materials. This chapter is by no means authoritative or a compendium of all things on P3HT:PCBM. We have selected to explain how the sample fabrication techniques lead to control of morphology and structural features and how these morphological features have specific optical and electronic consequences for organic photovoltaic device applications

New C84 Derivative and Its Application in a Bulk Heterojunction Solar Cell

We report here the synthesis and characteristics of a new C84 adduct ([84]PCBM), realized via a diazoalkane addition reaction. [84]PCBM was obtained as a mixture containing three major isomers. [84]PCBM was tested in a fullerene/poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) bulk heterojunction solar cell as the first C84 derivative to be applied in device fabrication. Spin coating the active layer blend from 1-chloronaphthalene (the very best fullerene solvent) instead of ortho-dichlorobenzene was necessary to obtain the more efficient photovoltaic device. The PV results indicate that the hole mobility of MDMO-PPV may not be increased upon blending with [84]PCBM. This explains the relatively low ISC of the device as due to the buildup of space charge. The VOC of the device is ~500 mV lower than that of the one with [60]PCBM, while [84]PCBM has a 350 mV higher electron affinity than [60]PCBM. This loss surpasses the linear relation between the donorHOMO-acceptorLUMO energy gap and the VOC in this type of device. A maximum power conversion efficiency of 0.25% was reached for the MDMO-PPV:[84]PCBM cells.

Charge transport and photocurrent generation in poly (3-hexylthiophene): Methanofullerene bulk-heterojunction solar cells

The effect of controlled thermal annealing on charge transport and photogeneration in bulk-heterojunction solar cells made from blend films of regioregular poly(3-hexylthiophene) (P3HT) and methanofullerene (PCBM) has been studied. With respect to the charge transport, it is demonstrated that the electron mobility dominates the transport of the cell, varying from 10(-8) m(2) V-1 S-1 in as-cast devices to approximate to 3 x 10(-7) m(2) V-1 s(-1) after thermal annealing. The hole mobility in the P3HT phase of the blend is dramatically affected by thermal annealing. It increases by more than three orders of magnitude, to reach a value of up to approximate to 2 x 10(4) m(2) V-1 s(-1) after the annealing process, as a result of an improved crystallinity of the film. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red-shift, improving the spectral overlap with solar emission, which results in an increase of more than 60% in the rate of charge-carrier generation. Subsequently, the experimental electron and hole mobilities are used to study the photocurrent generation in P3HT:PCBM devices as a function of annealing temperature. The results indicate that the most important factor leading to a strong enhancement of the efficiency, compared with non-annealed devices, is the increase of the hole mobility in the P3HT phase of the blend. Furthermore, numerical simulations indicate that under short-circuit conditions the dissociation efficiency of bound electron-hole pairs at the donor/acceptor interface is close to 90%, which explains the large quantum efficiencies measured in P3HT:PCBM blends

Modeling the photocurrent of poly-phenylene vinylene/fullerene-based solar cells

We have studied the photocurrent data of 20:80 wt% blends of poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. Two cases have been investigated: When only drift of charge carriers is taken into account, a voltage-independent photocurrent is expected, corresponding to the extraction of all generated charges. It is demonstrated that the experimental data are in disagreement with this prediction. However, when both drift and diffusion of charges are taken into account, the predicted photocurrent shows a different behavior: At low electric fields a linear behavior is predicted, which results from the diffusion of charges, followed by saturation at high fields. The agreement between the numerical result and the experimental data obtained from MDMO-PPV:PCBM cells is satisfactory when a charge carrier generation rate of G=1.6 x 10(27) m(-3)s(-1) is used showing the importance of diffusion at low fields, i.e., near the open-circuit voltage

Extraction of photo-generated charge carriers from polymer-fullerene bulk heterojunction solar cells

Two models describing charge extraction from insulators have been used to interpret the experimental photocurrent data of 20:80 wt% blends of poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) and [6,6]phenyl C-61,-butyric acid methyl ester (PCBM) bulk heterojunction solar cells. When only drift of charge carriers is taken into account, a square root dependence on voltage of the photocurrent is expected, governed by the difference between electron and hole mobility. It is demonstrated that both the magnitude and functional dependence of the predicted current are in disagreement with experimental data. However, when both drift and diffusion of charges are taken into account, the predicted photocurrent shows a different behaviour: At low electric fields a linear behaviour is predicted, which results from the diffusion of charges, folllowed by saturation at high fields. The agreement between the theoretical result and the experimental data obtained from MDMO-PPV:PCBM cells is satisfactory when a generation rate of G=1.46 x 10(27) electron-hole pairs/m(3)s is used, showing the importance of diffusion at low fields, i.e., near the open-circuit voltage