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]. The exact position of this peak was determined by small angle X-ray scattering measurements, and it corresponds to the repeating distance between the polymer backbones separated by the side chains (~28 A).

Since the preparation of devices for mobility measurements requires very soluble materials, the conjugated polymer resulting from the polymerization of the 70:30 weight % mixture of monomer A and monomer B (referred to as ‘70:30 MDMO-PPV’) was selected, and its charge transport and photovoltaic properties were compared to regiorandom MDMO-PPV (referred to as ‘RRa-MDMO-PPV’).

Figure 10.11 shows the room temperature mobility values determined by the ToF technique for both polymers with various film thicknesses. The mobility of 70:30 MDMO-PPV is approx. 3.5 times larger at all measured electric fields for all film thicknesses as compared to RRa – MDMO-PPV. The electric field dependence of the mobility has been determined for 70:30 MDMO-PPV and RRa-MDMO-PPV at various temperatures, and analyzed in the framework of disorder formalism developed by Bassler and coworkers to describe the temperature and electric field dependence of mobility in a charge transport medium with superimposed energetic and positional disorder [64]:

Подпись:image259"

image260,image261 Подпись: OR image454

(10.7)

image263

Figure 10.11 Charge carrier mobility determined by the ToF technique for various thicknesses of 70:30 MDMO-PPV (solid symbols) and RRa-MDMO-PPV (empty symbols) at room temperature. Reprinted from Figure 2 with permission from A. J. Mozer et al, J. Phys. Chem. B, 108, 5235, Novel regiospecific MDMO-PPV copolymer with improved charge transport for bulk heterojunction solar cells. Copyright (2004) by the American Chemical Society.

where d [eV] is the width of the Gaussian density of states, £ is a parameter characterizing positional disorder, ц0 [ cm2 V-1 s-1] is a prefactor mobility in the energetically disorder free system, E [V cm-1] is the electric field, and C is a fit parameter.

The logarithm of the mobility extrapolated to zero electric field (log ц(E = 0)) is plotted versus inverse temperature squared in Figure 10.12. The ordinate intercepts of the lines de­termine the value of the prefactor mobility (ц0), while the slope is related to the width of the Gaussian distribution of density of states (a). The parameters щ,, a, and C of Equation (10.7) have been calculated, and are summarized in Table 10.1. The fitting constant C was calculated from the slope of the field dependence of mobility versus (a/kT )2. The prefactor mobility is one order of magnitude higher for 70:30 MDMO-PPV and a also increases slightly. Interestingly, the zero field mobility of 70:30 MDMO-PPV in Figure 10.12 is only higher above ~ 230 K, but decreases faster due to the larger energetic disorder, and eventually becomes lower than that of RRa-MDMO-PPV below 230 K. Furthermore, the field dependence of mobility characterized by parameter C is slightly larger for 70:30 MDMO-PPV. The determined value of C agrees well within a factor of only two with the theoretically calculated value (C = 2.9 x 10-4 [(cm V-1)1/2]).

The prefactor mobility is primarily governed by the amount of electronic coupling between neighboring transport sites, which is a sensitive function (exponential) of the intersite distance. The slight increase in the energetic disorder of 70:30 MDMO-PPV may originate from lower lying energy states of ordered nanoaggregates as compared to the amorphous matrix acting as energetic deep traps for overall charge transport. The above model is schematically illustrated in

image264

Figure 10.12 Logarithm of the zero field mobility versus (1000/Г)2 for 70:30 MDMO-PPV (solid symbols) and RRA-MDMO-PPV (empty symbols). The lines represent linear fit of the data. Reprinted from Figure 5 with permission from A. J. Mozer et al, J. Phys. Chem. B, 108, 5235, Novel regiospecific MDMO-PPV copolymer with improved charge transport for bulk heterojunction solar cells. Copyright (2004) by the American Chemical Society.

Figure 10.13. The first diagram illustrates charge motion in the amorphous regions of the films, in which the energy of the transport sites experience a statistically varying environment. The second diagram, on the other hand, depicts regions where the conjugated chains are partially aligned with better interchain interactions. These regions are expected to create preferential paths (‘highways’) for the charge carriers, therefore increasing the mobility. Since the more extended electronic wavefunctions of the ordered regions are thought to be more polarizable, the extent of the dipole induced interactions are expected to be higher in the ordered regions of the films, which lowers the site energy. Since the ordered regions in our model are embedded in an amorphous matrix, these lower lying energy states may act as traps and broaden the distribution of the density of states. Charge hopping at the end of these ‘highways’ (indicated by the large arrow in Figure 10.13) requires sufficient thermal energy and/or tilting of the barrier by an external electric field. Therefore, at high temperatures and high electric fields,

Table 10.1 Determined values of p. RT (room temperature mobility), and parameters ц0 (prefactor mobility), о (energetic disorder) and C of the disorder formalism (Equation 10.7)

Sample

URT[cm2V-1s-1]

|r0[cm2V-1s-1]

a [meV]

C [(cmV-2)1/2]

70:30 MDMO-PPV

2.8 x 10-5

2.6 x 10-3

115

1.54 x 10-4

RRa-MDMO-PPV

0.85 x 10-5

0.22 x 10-3

105

1.35 x 10-4

image265

Figure 10.13 Schematic illustration of charge motion and the energy of hopping sites in cases where the conjugated chains are randomly oriented (top) and where local ordering of the conjugated chains takes place (bottom). Reprinted from Scheme 2 with permission from A. J. Mozer et al, J. Phys. Chem. B, 108, 5235, Novel regiospecific MDMO-PPV copolymer with improved charge transport for bulk heterojunction solar cells. Copyright (2004) by the American Chemical Society.

films of 70:30 MDMO-PPV with better interchain packing show higher charge carrier mobility. At low temperatures and low electric fields, however, charges might be trapped at the lower lying energy sites of the aligned regions hence causing stronger temperature and electric field dependence of mobility.

The proposed increased interchain interactions between the conjugated chains of the re – gioregular MDMO-PPV is supported by the increased tendency to aggregate as the regioregu – larity increases. Further indication is given by recent thermally stimulated luminescence (TSL) studies [63].

image458 Подпись: (10.8)

Bulk heterojunction solar cells based on the 1:4 weight ratio mixture of 70:30 MDMO – PPV:PCBM have been fabricated and compared to RRa-MDMO-PPV in Figure 10.14. Al­though the short-circuit current and open-circuit voltage are quite similar for the two devices, 70:30 MDMO-PPV shows a slightly increased power conversion efficiency due to the very high (0.7) FF. The current density versus voltage curves have been analyzed using a simple one diode model according to Equation (10.8). The equivalent circuit of the one diode model is displayed in Figure 10.15.

Table 10.2 Photovoltaic performance of bulk heterojunction solar cells and the parameters of Equation (10.8) obtained by numerical calculation

Jsc

mA cm-2

Voc

V

FF

nAM1.5

%

J0

mA cm-2

Rs

^ cm-2

Rp

^ cm-2

n

70:30 MDMO-PPV

5.0

0.8

0.71

2.65

6 x 10-7

1.3

2150

1.9

RRa-MDMO-PPV

5.25

0.82

0.61

2.5

6 x 10-7

3

950

2

where j [A cm-2] and V [V] are the current density and voltage values, j0 [A cm-2] is the reverse bias dark current, Rs [^ cm-2] is the series resistance, Rp [^ cm-2] is the parallel resistance and jsc [A cm-2] is the short circuit current density under illumination. The equation

10.8 has been numerically solved, and the parameters obtained by the best fit are summarized in Table 10.2. The series resistance of the bulk heterojunction photovoltaic devices based on 70:30 MDMO-PPV is reduced by a factor of 2.3 as compared to RRa-MDMO-PPV, which may be attributed to the improved charge carrier mobility of the photogenerated charge carriers as is suggested by the ToF mobility studies.

Leave a reply

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>