Category Thin Film Solar Cells Fabrication, Characterization and Applications

Thickness dependence of MDMO-PPV/PCBM solar cells

It is expected that the short circuit current of bulk heterojunction solar cells will rise with increasing active layer thickness due to increased absorption. According to d > ld = p x т x E, on the other hand, electrical losses due to recombination are expected when the thickness of the active layer exceeds the drift distance of the charge carriers, where E = Voc/d is the electric field. Based on these arguments, the thickness dependence of the short circuit current of bulk heterojunction solar cells should give an estimate of the drift distance of the charge carriers, which can then be compared to the рт product determined by the photo-CELIV technique.

As has been emphasized, the performance of bulk heterojunction solar cells is morphology dependent; therefore the proposed comparative...

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Charge transport and recombination in bulk heterojunction solar cells

10.1.2.1 Photo-CELIV technique

Charge carrier mobility in bulk heterojunction solar cells has been studied using a ToF technique [79], or calculated from the transfer characteristics of an FET [80]. These experiments showed that the electron mobility (PCBM phase) and the hole mobility (conjugated polymer phase) in the photoactive blend is fairly balanced, which is counterintuitive to experiments performed on the pristine materials. Space charge limited current measurements showed that the mobility of injected holes in the pure MDMO-PPV thin films is several orders of magnitude lower than injected electrons in PCBM thin films [81]...

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Regioregular poly(3-hexylthiophene)

A more drastic improvement of the performance of bulk heterojunction solar cells was recently achieved using regioregular poly(3-hexylthiophene) (P3HT) as an electron donor [65, 66]. These devices show incident photon to converted electron efficiency (IPCE) close to 75 % at the absorption maximum, which indicates nearly 100 % collection of the photogenerated charges at the electrodes in a thin 100-200 nm device. Charge carrier mobility in P3HT is often investigated in an FET structure; however detailed temperature and electric field dependence of mobility studies by ToF was not available...

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Regioregular MDMO-PPVs

A series of MDMO-PPV copolymers has been synthesized by mixing two stereoisomers of the same monomer (monomer A) and (monomer B) via the sulfinyl precursor route as illustrated Scheme 10.2. The solubility of the MDMO-PPV copolymers is greatly reduced as the ratio of either of the isomers is increased above 80 %, which is attributed to aggregation of the conjugated chains in solution. The improved tendency for aggregation is further supported by X-ray power diffraction (XRD) measurements, which showed a clearly distinguished reflection peak in the MDMO-PPV powders at around 3 ° as the regioregularity increases [63]...

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Charge transport in conjugated polymers

Conjugated polymers can be viewed as one-dimensional semiconductors, in which the semi­conducting properties are attributed to the extended n electron systems formed by the nz elec­trons of the c-conjugated backbone. Although microwave conductivity techniques [54, 55], showed that the on-chain mobility of conjugated polymers can reach very high values, the macroscopic transport properties are controlled by orders of magnitude slower interchain hop­ping processes. The solubility and processability of conjugated polymers are achieved by attaching side chains to the conjugated backbone...

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Transient conductivity techniques: ToF, CELIV and photo-CELIV

The measurement principles and the schematic responses of the ToF, CELIV and photo-CELIV techniques are illustrated in Figure 10.10. In the time of flight technique, the transit time (ttr) of a two-dimensional sheet of photogenerated charge carriers drifting through a sample of known thickness (d) is determined under an applied external electric field (E = U/d). The ToF mo­bility is then calculated as ц = d2/(U x ttr). The condition of surface photogeneration in a ToF technique requires large film thicknesses with high optical density (OD > 10). Photocurrent transients can be characterized as nondispersive exhibiting a well developed plateau, in which case the transit time is defined as the intersection of the plateau with the tail of the photocurrent transient as shown in Figure 10.10...

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CHARGE CARRIER MOBILITY AND RECOMBINATION

10.1.2 Measurement techniques

The drift distance of charge carriers photogenerated anywhere within the active layer of the solar cell is given by ld = ц x т x E, where ц is the mobility, т is charge carrier lifetime and E is the electric field. This equation assumes that the charge carriers are electric field driven [50]. Due to the rather low mobility of organic materials, high concentrations of photogener­ated charge carriers are required to reach a short circuit current density of ~ 10 mAcm-2, e. g, n ~ 1016cm-3 if ц = 10-4 cm2 V-1s-1. High charge carrier concentration generally leads to increased bimolecular recombination resulting in short lifetimes (т(t) = [вn(t)]-1 andconse – quently, short drift and diffusion distances.

Charge carrier mobility and the lifetime of the charge...

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Improving the photon harvesting

The absorption of the MDMO-PPV:PCBM blend is compared to the terrestrial total photon flux at AM1.5 conditions in Figure 10.9. The absorption of the MDMO-PPV:PCBM blend is limited in spectral regions around <1.8 eV [45]. To improve the spectral sensitivity of bulk heterojunction solar cells, conjugated polymers with lower n-n* bandgaps are required (i. e. low bandgap materials). The need to tune the emission color of conjugated polymer based light emitting diodes (LEDs) has triggered the development of several synthetic strategies to influence the band – gap of conjugated polymers. Today a detailed understanding of the molecular structure-bandgap correlation has been reached (‘bandgap engineering’) [46].

An alternating electron rich N-dodecyl-2,5-bis(2’-thienyl)pyrrole and an electron-...

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