**3.2 Solvent effects**

10 Will-be-set-by-IN-TECH

is visible from the increased intensity of the reflection ring in the SAED pattern (inset of Fig. 6 a)). Larger and darker PCBM rich areas can be observed suggesting an increased phase demixing between P3HT and PCBM. It was concluded that the crystallinity of P3HT is improved upon annealing and the demixing between the two components is increased, but large-scale phase separation does not occur. The resulting interpenetrating networks composed of P3HT crystals with a high aspect ratio and aggregated nanocrystalline PCBM domains provide continuous pathways in the entire photoactive layer for efficient hole and

In order to further understand the extent of thermal annealing, 2-D X-ray scattering in a grazing incidence geometry (GIWAXS) was used to study the development of the crystalline structure of P3HT and PCBM during the interdiffusion process at various temperatures. 2D GIWAXS patterns of as-prepared P3HT/PCBM bilayers and annealed samples are shown in Fig. 7.(Treat et al., 2011) The diffraction patterns for P3HT shows that the *a*-axis of the P3HT crystals is predominantly oriented perpendicular to the substrate and the *b*-axis ( pi-stacking)

Fig. 7. Two-dimensional GIWAXS of a P3HT/PCBM bilayer on Si a) as-cast and annealed at b) 70 C, c) 110 C, d) 150 C, and e) 170 C for 5 min. f) The Scherrer equation was used to extract the P3HT crystallite thickness along the a- axis from the full-width-at-half-maximum of the (100) reflection. PCBM incorporation from the DSIMS measurements was plotted for comparison at various annealing temperatures. g) Growth in the crystal thickness with time using in-situ heating 2D GIWAXS of a P3HT/PCBM bilayer on Si at 110 C (orange) and 170 C

(blue). The Scherrer equation was used to determine crystal thickness from the (100) reflection corresponding to P3HT. The dotted line corresponds to a neat P3HT/Si sample heated for 5 min at 110 C (orange) and 170 C (blue). Copyright 2011 Wiley. Used with

permission from (Treat et al., 2011).

electron transport.

Postproduction treatment requires a rather well controlled environment, it adds an additional fabrication costs to the solar cell manufacturing process, which might not be attractive for large-scale industrial production. Furthermore, some material systems, like the low band gap organic semiconductor poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) blended with [6,6]-phenyl C71-butyric acid methyl ester (C71-PCBM), do not shown any improvement upon thermal annealing.

Phase separation and molecular self-organization can be influenced by solvent evaporation since the solvent establishes the film evolution environment. Slow drying or solvent annealing techniques have also been used to control the morphology of the blends by changing the rate of solvent removal.(Li et al., 2005; Li, Yao, Yang, Shrotriya, Yang & Yang, 2007; Sivula et al., 2006) The use of different solvents and their effect on the film nano-structure of BHSC has been studied in detail in the past.(Li, Shrotriya, Yao, Huang & Yang, 2007) High boiling point solvents were used with the device placed in an enclosed container, in which the atmosphere rapidly saturates with the solvent.

Grazing-incidence x-ray diffraction (GIXRD) studies provided evidence that the solvent evaporation rate directly influences the polymer chain arrangement in the film.(Chu et al., 2008) It was shown that the use of higher boiling point solvent strongly improves the PCE of MDMO-PPV and PCBM blends.(Shaheen et al., 2001) Higher PCE values due to improved film morphology and crystallinity have been reached by substituting chloroform with chlorobenzene for P3HT/PCBM BHSC.(Ma et al., 2005) The difference between chlorobenzene and 1,2-dichloro benzene for use as a solvent was shown in the novel low bandgap polymer PFco-DTB and C71-PCBM blend systems, where chlorobenzene resulted in films with higher

electrical conductivity and in situ photodoping.(Chen et al., 2004) A copolymer including thieno-thiophene units (DHPT3) has been used as a nucleating agent for crystallization in the active layer of P3HT and PCBM BHSC.(Bechara et al., 2008) It was demonstrated that the addition of DHPT3 in P3HT/PCBM thin films induces a structural ordering of the polythiophene phase, leading to improved charge carrier transport properties and stronger active layer absorption. High-performance P3HT/PCBM blends were fabricated using quick drying process and 1-dodecanethiol as an additive.(Ouyang & Xia, 2009) Ternary blends of P3HT, PCBM and poly(9,9-dioctylfluorene-co-benzothiadiazode) (F8BT) showed enhanced optical absorption and partly improved charge collection.(Kim, Cook, Choulis, Nelson, Durrant & Bradley, 2005) A few volume percent of 1,8-diiodooctane in o-xylene was used to dissolve poly(9,9-di-n-octylfluorene) PFO allowing the control of film morphology.(Peet et al., 2008) Block-copolymers and diblock copolymers with functionalized blocks have also shown to be able to influence the film morphology.(Sivula et al., 2006; Sun et al., 2007; Zhang, Choi,

Relation Between Nanomorphology and Performance of Polymer-Based Solar Cells 133

The incorporation of other solvents into the host solvent is capable of controlling the film morphology of BHSC.(Chen et al., 2008; Wienk et al., 2008; Xin et al., 2008; Zhang, Jespersen, Björström, Svensson, Andersson, Sundstr"om, Magnusson, Moons, Yartsev & Ingan"as, 2006) In some cases, changes in the solvent composition lead to interchain order that cannot be obtained by any other method.(Campbell et al., 2008; Moulee et al., 2008; Peet et al., 2007) The use of nitrobenzene as an additive has been shown to improve the phase-separation between the donor and acceptor (P3HT/PCBM blend), where P3HT was shown to be present in both

Fig. 9. Schematic depiction of the role of the processing additive in the self-assembly of bulk heterojunction blend materials (a) and structures of PCPDTBT, C71-PCBM, and additives (b). Reprinted with permission from (Lee et al., 2008). Copyright 2008 American Chemical

The concept of mixing a host solvent with a "bad" solvent has been explored resulting in solvent-selection rules for desired film morphology.(Alargova et al., 2001) Solvents, distinctly dissolving one component of the blend, induce the aggregation of nanofibers and nanoparticles in the solvent prior to film deposition.(Yao et al., 2008) It was shown

amorphous and crystalline phase.(Moule & Meerholz, 2008; van Duren et al., 2004)

Haliburton, Cleveland, Li, Sun, Ledbetter & Bonner, 2006)

3.3.0.1 "Bad" solvent effect

Society.

roughness.(Yao et al., 2006) Non-aromatic solvents have shown to be able to affect the photovoltaic performance of MEH-PPV and PCBM blends.(Yang et al., 2003)

An interesting method to study the morphology of BHSC optically by recording exciton lifetime images within the photoactive layer of P3HT and PCBM has been demonstrated by Huan et al.(Huang et al., 2010) Using a confocal optical microscopy combined with a fluorescence module they were able to image the spacial distrubution of exciton lifetime for both slow and fast dried films, as shown in Fig. 8.

Fig. 8. (a, c) Transmitted images and (b, d) exciton lifetime images of the BHJ film prepared from rapidly and slowly grown methods, respectively, measured after excitation at 470 nm using a picosecond laser microscope (512 × 512 pixels). Scale bars: 2 *μ*m. Reprinted with permission from (Huang et al., 2010). Copyright 2010 American Chemical Society.

The transmitted image of the rapidly grown film (Fig. 8 (a)) shows a uniform and featureless characteristics throughout the structure, indicating that P3HT and PCBM were mixed well within the films. This monotonous transmitted image corresponds to a uniform exciton lifetime distribution. Fig. 8 (c)-(d) shows transmitted and exciton lifetime images for the slowly dried films. The bright spots are emissions from many polymer chains that have stacked or aggregated into a bulk cluster leading to a reduced PL quenching. The red regions (P3HT-rich domains Fig. 8 (d)) correspond to the bright spot of the transmitted image (Fig. 8 (c)). In agreement with previous studies, the images showed that the active layers during slow solvent evaporation provide a 3D pathways for charge transport reflecting better cell performance.

### **3.3 Processing additives**

This method is based on the usage of a third non-reacting chemical compound, a processing additive, to the donor and acceptor solution. Improvement of the performance of polymer/fullerene photovoltaic cells doped with triplephenylamine has been reported.(Peet et al., 2009) The ionic solid electrolyte (LiCF3SO3) used as a dopant also resulted in enhanced PCE of MEH-PPV/PCBM blends due to an optimized polymer morphology, improved electrical conductivity and in situ photodoping.(Chen et al., 2004) A copolymer including thieno-thiophene units (DHPT3) has been used as a nucleating agent for crystallization in the active layer of P3HT and PCBM BHSC.(Bechara et al., 2008) It was demonstrated that the addition of DHPT3 in P3HT/PCBM thin films induces a structural ordering of the polythiophene phase, leading to improved charge carrier transport properties and stronger active layer absorption. High-performance P3HT/PCBM blends were fabricated using quick drying process and 1-dodecanethiol as an additive.(Ouyang & Xia, 2009) Ternary blends of P3HT, PCBM and poly(9,9-dioctylfluorene-co-benzothiadiazode) (F8BT) showed enhanced optical absorption and partly improved charge collection.(Kim, Cook, Choulis, Nelson, Durrant & Bradley, 2005) A few volume percent of 1,8-diiodooctane in o-xylene was used to dissolve poly(9,9-di-n-octylfluorene) PFO allowing the control of film morphology.(Peet et al., 2008) Block-copolymers and diblock copolymers with functionalized blocks have also shown to be able to influence the film morphology.(Sivula et al., 2006; Sun et al., 2007; Zhang, Choi, Haliburton, Cleveland, Li, Sun, Ledbetter & Bonner, 2006)

### 3.3.0.1 "Bad" solvent effect

12 Will-be-set-by-IN-TECH

roughness.(Yao et al., 2006) Non-aromatic solvents have shown to be able to affect the

An interesting method to study the morphology of BHSC optically by recording exciton lifetime images within the photoactive layer of P3HT and PCBM has been demonstrated by Huan et al.(Huang et al., 2010) Using a confocal optical microscopy combined with a fluorescence module they were able to image the spacial distrubution of exciton lifetime for

Fig. 8. (a, c) Transmitted images and (b, d) exciton lifetime images of the BHJ film prepared from rapidly and slowly grown methods, respectively, measured after excitation at 470 nm using a picosecond laser microscope (512 × 512 pixels). Scale bars: 2 *μ*m. Reprinted with permission from (Huang et al., 2010). Copyright 2010 American Chemical Society.

The transmitted image of the rapidly grown film (Fig. 8 (a)) shows a uniform and featureless characteristics throughout the structure, indicating that P3HT and PCBM were mixed well within the films. This monotonous transmitted image corresponds to a uniform exciton lifetime distribution. Fig. 8 (c)-(d) shows transmitted and exciton lifetime images for the slowly dried films. The bright spots are emissions from many polymer chains that have stacked or aggregated into a bulk cluster leading to a reduced PL quenching. The red regions (P3HT-rich domains Fig. 8 (d)) correspond to the bright spot of the transmitted image (Fig. 8 (c)). In agreement with previous studies, the images showed that the active layers during slow solvent evaporation provide a 3D pathways for charge transport reflecting better cell

This method is based on the usage of a third non-reacting chemical compound, a processing additive, to the donor and acceptor solution. Improvement of the performance of polymer/fullerene photovoltaic cells doped with triplephenylamine has been reported.(Peet et al., 2009) The ionic solid electrolyte (LiCF3SO3) used as a dopant also resulted in enhanced PCE of MEH-PPV/PCBM blends due to an optimized polymer morphology, improved

photovoltaic performance of MEH-PPV and PCBM blends.(Yang et al., 2003)

both slow and fast dried films, as shown in Fig. 8.

performance.

**3.3 Processing additives**

The incorporation of other solvents into the host solvent is capable of controlling the film morphology of BHSC.(Chen et al., 2008; Wienk et al., 2008; Xin et al., 2008; Zhang, Jespersen, Björström, Svensson, Andersson, Sundstr"om, Magnusson, Moons, Yartsev & Ingan"as, 2006) In some cases, changes in the solvent composition lead to interchain order that cannot be obtained by any other method.(Campbell et al., 2008; Moulee et al., 2008; Peet et al., 2007) The use of nitrobenzene as an additive has been shown to improve the phase-separation between the donor and acceptor (P3HT/PCBM blend), where P3HT was shown to be present in both amorphous and crystalline phase.(Moule & Meerholz, 2008; van Duren et al., 2004)

Fig. 9. Schematic depiction of the role of the processing additive in the self-assembly of bulk heterojunction blend materials (a) and structures of PCPDTBT, C71-PCBM, and additives (b). Reprinted with permission from (Lee et al., 2008). Copyright 2008 American Chemical Society.

The concept of mixing a host solvent with a "bad" solvent has been explored resulting in solvent-selection rules for desired film morphology.(Alargova et al., 2001) Solvents, distinctly dissolving one component of the blend, induce the aggregation of nanofibers and nanoparticles in the solvent prior to film deposition.(Yao et al., 2008) It was shown

The concentration of the processing additive allows the amount of phase-separation between

Relation Between Nanomorphology and Performance of Polymer-Based Solar Cells 135

1,8-di(R)octanes with various functional groups (R) allow control of the film morphology.(Peet et al., 2007) The best results were obtained with 1,8-diiodooctane. Progressively longer alkyl chains, namely 1,4-butanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol or 1,9-nonanedithiol were used to manipulate the morphology of solution processed films. It was concluded that approximately six methylene units are required for the alkanedithiol to have an appreciable

Fig. 11. AFM topography of films cast from PCPCTBT/C71-PCBM with additives: (a) 1,8-octanedithiol, (b) 1,8-cicholorooctane, (c) 1,8-dibromooctane, (d) 1,8-diiodooctane, (e) 1,8-dicyanooctane, and (f) 1,8-octanediacetate. Reprinted with permission from (Chen, Yang,

Yang, Sista, Zadoyan, Li & Yang, 2009). Copyright 2009 American Chemical Society.

Fig. 11 shows a Atomic Force Microscopy (AFM) surface topography of films cast from PCPCTBT/C71-PCBM with the various processing additives.(Lee et al., 2008) The 1,8-octanedithiol (a), 1,8-dibromooctane (c), and 1,8-diiodooctane (d) resulted in phase-segregated morphologies with finer domain sizes than those obtained with 1,8-dichlorooctane (b), 1,8-dicyanooctane (e), and 1,8-octanediacetate (f). The morphology of films processed with 1,8-diiodooctane showed more elongated domains than those processed with 1,8-octanedithiol and 1,8-dibromooctane. The 1,8-di(R)octanes with *SH*, *Br*, and *I*, gave finer domain sizes and exhibited more efficient device performances than those with *R* = *Cl*, *CN*, and *CO*2*CH*3. The AFM images of the BHJ films processed using 1,8-di(R)octanes with

the donor and the acceptor to be controlled.

3.3.0.2 Different processing additives

effect on the morphology.

that (independent of the concentration of the additive) fullerene molecules crystallized into distributed aggregates in the presence of a "bad" solvent in the host solvent. Well aligned P3HT aggregates resulting in high degree of crystallinity due to the interchain *π* − *π* stacking were observed upon addition of hexane.(Li et al., 2008; Rughooputh et al., 1987) The addition of 1-chloronaphthalene (a high boiling point solvent) into dichlorobenzene has also resulted in similar self-organization of polymer chains.(Chen et al., 2008) It was shown that in the blends of poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(40,70-di-2-thienyl-20,10,3-benzothiadiazole)) and PCBM dissolved in chloroform with a small addition of chlorobenzene, a uniform domain distribution was attained, whereas the addition of xylene or toluene into the chloroform host solvent resulted in larger domains, stronger carrier recombination and a smaller photocurrent. Alkane-thiol based compounds were extensively used as processing additives in the past.(Lee et al., 2008) The photoconductivity response was shown to increase strongly in polymer/fullerene composites by adding a small amount of alkane-thiol based compound to the solution prior to the film deposition.(Coates et al., 2008; Peet et al., 2006) By incorporating a few volume percent of alkanethiols into the PCPDTBT/C71-PCBM BHSC (Fig. 9) it was shown that the PCE improves almost by a factor of two.(Alargova et al., 2001; Peet et al., 2007)

Fig. 10. UV-visible absorption spectra of PCPDTBT/C71-PCBM films processed with 1,8-octanedithiol: before removal of C71-PCBM with alkanedithiol (black); after removal of C71-PCBM with alkanedithiol (red) compared to the absorption spectrum of pristine PCPDTBT film (green). Reprinted with permission from (Lee et al., 2008). Copyright 2008 American Chemical Society.

The alkanedithiol effect was explained by the ability of alkanedithiols to selectively dissolve the fullerene component, where the polymer is less soluble, Fig. 9 The effect has been proven by removing the fullerene domains by dipping the BHJ film into an alkanedithiol solution and measuring light absorption before and after dipping.(Lee et al., 2008) The normalized absorption spectra (shown in Fig. 10) demonstrate that after dipping the film the absorption matches that of the pristine polymer.

As a consequence, "bad" solvent addition provides a means to select solvent-additives in order to control the phase-separation in BHSC. It was shown that during film processing the fullerene stays longer in its dissolved form, due to the rather high boiling point of alkanedithiol (> 160 C), allowing for self-aligning and phase-separation between the polymer and fullerene as suggested in Fig. 7 b). Two effects control the morphology of the blends:

a) selective solubility of one of the components;

b) a high boiling of the additive compared to the host solvent.

The concentration of the processing additive allows the amount of phase-separation between the donor and the acceptor to be controlled.

3.3.0.2 Different processing additives

14 Will-be-set-by-IN-TECH

that (independent of the concentration of the additive) fullerene molecules crystallized into distributed aggregates in the presence of a "bad" solvent in the host solvent. Well aligned P3HT aggregates resulting in high degree of crystallinity due to the interchain *π* − *π* stacking were observed upon addition of hexane.(Li et al., 2008; Rughooputh et al., 1987) The addition of 1-chloronaphthalene (a high boiling point solvent) into dichlorobenzene has also resulted in similar self-organization of polymer chains.(Chen et al., 2008) It was shown that in the blends of poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(40,70-di-2-thienyl-20,10,3-benzothiadiazole)) and PCBM dissolved in chloroform with a small addition of chlorobenzene, a uniform domain distribution was attained, whereas the addition of xylene or toluene into the chloroform host solvent resulted in larger domains, stronger carrier recombination and a smaller photocurrent. Alkane-thiol based compounds were extensively used as processing additives in the past.(Lee et al., 2008) The photoconductivity response was shown to increase strongly in polymer/fullerene composites by adding a small amount of alkane-thiol based compound to the solution prior to the film deposition.(Coates et al., 2008; Peet et al., 2006) By incorporating a few volume percent of alkanethiols into the PCPDTBT/C71-PCBM BHSC (Fig. 9) it was shown that the PCE improves almost by a factor of two.(Alargova et al., 2001; Peet et al., 2007)

Fig. 10. UV-visible absorption spectra of PCPDTBT/C71-PCBM films processed with 1,8-octanedithiol: before removal of C71-PCBM with alkanedithiol (black); after removal of C71-PCBM with alkanedithiol (red) compared to the absorption spectrum of pristine PCPDTBT film (green). Reprinted with permission from (Lee et al., 2008). Copyright 2008

The alkanedithiol effect was explained by the ability of alkanedithiols to selectively dissolve the fullerene component, where the polymer is less soluble, Fig. 9 The effect has been proven by removing the fullerene domains by dipping the BHJ film into an alkanedithiol solution and measuring light absorption before and after dipping.(Lee et al., 2008) The normalized absorption spectra (shown in Fig. 10) demonstrate that after dipping the film the absorption

As a consequence, "bad" solvent addition provides a means to select solvent-additives in order to control the phase-separation in BHSC. It was shown that during film processing the fullerene stays longer in its dissolved form, due to the rather high boiling point of alkanedithiol (> 160 C), allowing for self-aligning and phase-separation between the polymer and fullerene as suggested in Fig. 7 b). Two effects control the morphology of the blends:

American Chemical Society.

matches that of the pristine polymer.

a) selective solubility of one of the components;

b) a high boiling of the additive compared to the host solvent.

1,8-di(R)octanes with various functional groups (R) allow control of the film morphology.(Peet et al., 2007) The best results were obtained with 1,8-diiodooctane. Progressively longer alkyl chains, namely 1,4-butanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol or 1,9-nonanedithiol were used to manipulate the morphology of solution processed films. It was concluded that approximately six methylene units are required for the alkanedithiol to have an appreciable effect on the morphology.

Fig. 11. AFM topography of films cast from PCPCTBT/C71-PCBM with additives: (a) 1,8-octanedithiol, (b) 1,8-cicholorooctane, (c) 1,8-dibromooctane, (d) 1,8-diiodooctane, (e) 1,8-dicyanooctane, and (f) 1,8-octanediacetate. Reprinted with permission from (Chen, Yang, Yang, Sista, Zadoyan, Li & Yang, 2009). Copyright 2009 American Chemical Society.

Fig. 11 shows a Atomic Force Microscopy (AFM) surface topography of films cast from PCPCTBT/C71-PCBM with the various processing additives.(Lee et al., 2008) The 1,8-octanedithiol (a), 1,8-dibromooctane (c), and 1,8-diiodooctane (d) resulted in phase-segregated morphologies with finer domain sizes than those obtained with 1,8-dichlorooctane (b), 1,8-dicyanooctane (e), and 1,8-octanediacetate (f). The morphology of films processed with 1,8-diiodooctane showed more elongated domains than those processed with 1,8-octanedithiol and 1,8-dibromooctane. The 1,8-di(R)octanes with *SH*, *Br*, and *I*, gave finer domain sizes and exhibited more efficient device performances than those with *R* = *Cl*, *CN*, and *CO*2*CH*3. The AFM images of the BHJ films processed using 1,8-di(R)octanes with

1,8-octanedithiol added. The AFM results were consistent with PL spectra showing higher PL

Relation Between Nanomorphology and Performance of Polymer-Based Solar Cells 137

AFM provides information about the film surface only, the bulk of the film has been studied using synchrotron-based grazing incidence X-ray diffraction (GIXD) in P3HT:PCBM blends.(Chen, Yang, Yang, Sista, Zadoyan, Li & Yang, 2009) Fig. 13 (a) represents 2-D GIXD

Fig. 13. (a) 2D GIXD patterns of films with different amounts of 1,8-octanedithiol. (b) 1D out-of-plane X-ray and (c) azimuthal scan (at q(100)) profiles extracted from (a). Inset of b: calculated interlayer spacing in the (100) direction with various amounts of 1,8-octanedithiol.

Reprinted with permission from (Chen, Yang, Yang, Sista, Zadoyan, Li & Yang, 2009).

broader with increasing amount of 1,8-octanedithiol, as shown in Fig. 13 (c).

patterns of the as-spun P3HT:PCBM films with different concentrations of 1,8-octanedithiol. It was found that the hexyl side chains and backbone of P3HT are oriented perpendicular and parallel to the surface, respectively regardless of 1,8-octanedithiol concentration. However, the crystallinity of P3HT in the films significantly increases in the presence of 1,8-octanedithiol and tends to keep steady above 5 *μ*L 1,8-octanedithiol, as seen from in 1-D out of-plane X-ray profiles normalized by film thicknesses (see Fig. 13 (b). The average interlayer spacing was observed to change significantly in the presence of 1,8-octanedithiol. It was concluded that the interaction between P3HT is stronger in the presence of 1,8-octanedithiol with the P3HT crystallinity improved due to stacking. The size distribution of P3HT crystals was found to be

Copyright 2009 American Chemical Society.

intensity with increased 1,8-octanedithiol concentration.

*R* = *Cl*, *CN*, and *CO*2*CH*<sup>3</sup> showed large scale phase separation with round-shape domains and no indication of a bicontinuous network.

3.3.0.3 Concentration of processing additives

Once the most effective thiol functional group has been indentified, it is interesting to find how the concentration of the processing additive in solution affects the film morphology. The effect of additive concentration in the solution was clearly observed in surface topography images in AFM.(Chen, Yang, Yang, Sista, Zadoyan, Li & Yang, 2009)

Fig. 12. Tapping mode AFM images of films with different amounts of 1,8-octanedithiol in 500 nm × 500 nm. Left: topography. Right: phase images. (a) 0 *μ*L, (b) 7.5 *μ*L, (c) 20 *μ*L, and (d) 40 *μ*L of 1,8-octanedithiol. The scale bars are 10.0 nm in the height images and 10.0 ◦ in the phase images. Reprinted with permission from from (Chen, Yang, Yang, Sista, Zadoyan, Li & Yang, 2009). Copyright 2009 American Chemical Society.

AFM images (a), (b), (c), and (d) of Fig. 12 show the height (left) and phase (right) images of polymer films with 0, 7.5, 20, and 40 *μ*L of 1,8-octanedithiol, respectively, showing an increasing trend in roughness with increasing amount of 1,8-octanedithiol. The domain sizes were found to be consistent with the higher crystallization observed with increasing amount of 1,8-octanedithiol. Finely dispersed structures were observed when there was no 16 Will-be-set-by-IN-TECH

*R* = *Cl*, *CN*, and *CO*2*CH*<sup>3</sup> showed large scale phase separation with round-shape domains

Once the most effective thiol functional group has been indentified, it is interesting to find how the concentration of the processing additive in solution affects the film morphology. The effect of additive concentration in the solution was clearly observed in surface topography

Fig. 12. Tapping mode AFM images of films with different amounts of 1,8-octanedithiol in 500 nm × 500 nm. Left: topography. Right: phase images. (a) 0 *μ*L, (b) 7.5 *μ*L, (c) 20 *μ*L, and (d) 40 *μ*L of 1,8-octanedithiol. The scale bars are 10.0 nm in the height images and 10.0 ◦ in the phase images. Reprinted with permission from from (Chen, Yang, Yang, Sista, Zadoyan,

AFM images (a), (b), (c), and (d) of Fig. 12 show the height (left) and phase (right) images of polymer films with 0, 7.5, 20, and 40 *μ*L of 1,8-octanedithiol, respectively, showing an increasing trend in roughness with increasing amount of 1,8-octanedithiol. The domain sizes were found to be consistent with the higher crystallization observed with increasing amount of 1,8-octanedithiol. Finely dispersed structures were observed when there was no

Li & Yang, 2009). Copyright 2009 American Chemical Society.

images in AFM.(Chen, Yang, Yang, Sista, Zadoyan, Li & Yang, 2009)

and no indication of a bicontinuous network. 3.3.0.3 Concentration of processing additives

1,8-octanedithiol added. The AFM results were consistent with PL spectra showing higher PL intensity with increased 1,8-octanedithiol concentration.

AFM provides information about the film surface only, the bulk of the film has been studied using synchrotron-based grazing incidence X-ray diffraction (GIXD) in P3HT:PCBM blends.(Chen, Yang, Yang, Sista, Zadoyan, Li & Yang, 2009) Fig. 13 (a) represents 2-D GIXD

Fig. 13. (a) 2D GIXD patterns of films with different amounts of 1,8-octanedithiol. (b) 1D out-of-plane X-ray and (c) azimuthal scan (at q(100)) profiles extracted from (a). Inset of b: calculated interlayer spacing in the (100) direction with various amounts of 1,8-octanedithiol. Reprinted with permission from (Chen, Yang, Yang, Sista, Zadoyan, Li & Yang, 2009). Copyright 2009 American Chemical Society.

patterns of the as-spun P3HT:PCBM films with different concentrations of 1,8-octanedithiol. It was found that the hexyl side chains and backbone of P3HT are oriented perpendicular and parallel to the surface, respectively regardless of 1,8-octanedithiol concentration. However, the crystallinity of P3HT in the films significantly increases in the presence of 1,8-octanedithiol and tends to keep steady above 5 *μ*L 1,8-octanedithiol, as seen from in 1-D out of-plane X-ray profiles normalized by film thicknesses (see Fig. 13 (b). The average interlayer spacing was observed to change significantly in the presence of 1,8-octanedithiol. It was concluded that the interaction between P3HT is stronger in the presence of 1,8-octanedithiol with the P3HT crystallinity improved due to stacking. The size distribution of P3HT crystals was found to be broader with increasing amount of 1,8-octanedithiol, as shown in Fig. 13 (c).

**5.1 Power conversion efficiency and current-voltage dependence**

(2) thermally annealed films (refereed to as treated in text, no alkyl thiol);

(3) as produced films with alkyl thiol (refereed to as treated in text, with alkyl thiol); (4) thermally annealed films with alkyl thiol (refereed to as treated in text, with alkyl thiol). The fabrication procedures were kept the same for all four types of cells. The details on device

fabricated in four different ways:

(1) as produced films (untreated, no alkyl thiol);

preparation can be found elsewhere.(Pivrikas et al., 2008)

In order to clarify the effect of chemical additives on the photophysical properties and photovoltaic performance, regioregular P3HT and PCBM bulk-heterojunction solar cells were

Relation Between Nanomorphology and Performance of Polymer-Based Solar Cells 139

Current-voltage (I-V) characteristics under illumination of devices are shown in Fig. 15. Untreated solar cells gave the worst performance with the least short circuit current and low fill factor. However, these cells demonstrate a relatively higher open circuit voltage, but, due to a low short circuit current and a low fill factor, their power conversion efficiency was low, around 1 %. The difference in photocurrents between annealed cells and these with alkyl thiol

Fig. 14. Schematic structures of the film nanomorphology of bulk-heterojunction blends - all emphasizing the importance of the interpenetrating network in polymer-based solar cells. Top figures: (a) chlorobenzene and (b) toluene cast MDMO-PPV and PCBM blend layers. Center figures: vertical phase morphology of (a) rapidly and (b) slowly grown P3HT and

morphology of a mixed-layer photovoltaic cell. The interface between donor and acceptor is shown as a green surface. Donor is shown in red and acceptor is transparent. Top figures reprinted with permission from (Hoppe et al., 2006), copyright 2006, with permission from Elsevier. Middle figures reprinted with permission from (Huang et al., 2010), copyright 2010

PCBM blends. Bottom figures: the simulated effects of annealing on the interface

American Chemical Society. Bottom figures adapted by permission from Macmillan

Publishers Ltd: (Peumans et al., 2003), copyright 2003.

Improved crystallization of P3HT and broader crystal size distribution at higher 1,8-octanedithiol concentrations was explained by solvent volume ratios. During the film fabrication, the main solvent evaporates faster than the additive solvent resulting in a sudden increase of the volume ratio of the additive solvent to the main solvent. Polymer molecules lower their internal energy by aggregating when the additive solvent volume ratio reaches a critical point. At higher additive concentrations, the time required to reach this point is reduced and aggregation is stronger. As a result, polymer molecules aggregate with larger average domain sizes due to the stronger driving force and broader size distributions arises due to the shorter aggregation time.
