Device performance of bulk hetero-junction cells

Fig. 4-1-a shows the statistical results of the seven series of devices. Among the series based on toluene solution (series A, B, C, D, and G), the series of devices by blade coating have the higher efficiencies except the series with bladed the PEDOT:PSS layer. Because the V_ocare about the same, the high performances of series B, C, and D result from the high short circuit current density Jsc and fill factor. Among the series by blade and spin coating (series D, E, and F), the series of devices from toluene solution and dichlorobenzene have the higher performances. While the Jsc of series E and series F are about the same, the high performance of series F results from the relatively high fill factor. For further discussion, the best devices in each series are chosen to show the advanced device properties.

Fig. 4-1-a Statistical results of the seven series of devices, (a) the PCE, (b) the Jsc, (c) the Voc, and (d) the fill factor. The horizontal lines in the box denote the 25th, 50th, and 75th percentile values. The error bars denote the 5th and 95 percentile values. The open square inside the box denotes the mean value.

Fig. 4-1-b (a) shows the current density-voltage (J-V) curves of five devices in toluene solution made by different active layer coating processes. The short circuit currents J_scmade by blade coating (device B, C and D) are larger than that of the device made by conventional spin coating (device A). Using blade coating the J_sc increases from 9.3 mA/cm² with spin coating to 11.5 mA/cm². The fill factor rises from 47% to 55% and the V_oc remains the same. The efficiency, which is proportional to J_sc, V_oc, and fill factor as a whole, is improved from 2.6% (device A) by spin coating to 3.8% (device C) by blade coating on a hot plate.

(c) (d)

Fig. 4-1-b Current density-voltage (J-V) relations of the devices. (a) Devices made by spin coating (device A, solid square), blade coating (device B, empty square), blade coating at 60 ℃

(device C, solid circle), and blade and spin coating (device D, empty circle) in toluene solution, blade and spin coating with bladed PEDOT:PSS (device G, solid star) (b) Devices made by blade and spin coating in toluene solution (device D, empty circle), chlorobenzene solution (device E, solid triangle), and dichlorobenzene solution (device F, empty triangle)

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(b)

It is believed that in order to get a high efficiency in bulk hetero-junction polymer solar cell the microscopic morphology of the active layer needs to be well controlled to achieve an ordered structure by certain annealing processes such as slow solvent evaporation1 and postproduction heat treatment [22]. Such annealing promotes molecular self-organization and makes the polymer chains more ordered in its domains. In spin coating for low boiling point solvents such as toluene (110 ℃), high volatility leads to the fast drying of the active layer and may limit the self-assembly as well as the PCE. However, the polymer films made by blade coating could be more ordered than those by spin coating due to the fact that the polymer chains are relatively free to move in the absence of centrifugal force. Therefore even without the slow drying process the donors and acceptors quickly self-assemble into the desired ordered and interpenetrating morphology during the blade coating process. We speculate that the quick assembly occurs at the beginning of the blade coating because the device made by blade and spin coating (device D) shows high efficiency as well.

Interestingly, device B, C, and D have different film thickness but similar performances.

High efficiency is maintained for device B with 412 nm thickness probably because pure blade coating with neither heating nor spinning give the highest freedom to chain motions and the most ordered morphology. In short, the blade coating method allows ordered polymer morphology in a fast drying solution like toluene. In addition, the result of the device with the bladed PEDOT:PSS and blade and spin coated P3HT:PCBM layer is also shown in Fig. 4-1-b (a). The efficiency of device G is 2.5% with J_scof 8.35 mA/cm², V_ocof 0.59 V, and fill factor of 52%. The relative low performance of device G results from the thick PEDOT:PSS film (120 nm), which causes high series resistance. It is difficult for us to reduce the PEDOT:PSS film thickness now because we use the PEDOT:PSS solution from H. C. Stark without any further dilution. The concentration needs to be lowered without sacrificing the conductivity to get the normal film thickness (40 nm). More experiments need to be done to optimize the PEDOT:PSS layers by blade coating. Nevertheless, such

device shows the feasibility of all blade coated devices with very low cost and high throughput in mass production.

Now we turn to different solvent systems. Two devices are made by conventional high boiling points solvents chlorobenzene (device E) and dichlorobenzene (device F). Fig. 4-1-b (b) shows the results of the devices by the fast-drying blade and spin process with different solvents. There is no slow solvent evaporation in the fabrication process. The power conversion efficiencies are 2.5% in the device from chlorobenzene solution (device E) and 3.5% in that from dichlorobenzene solution (device F). The J_sc are both about 8.8 mA/cm², which is significantly smaller than Jsc of 11.4 mA/cm² in device D from toluene.

Interestingly high boiling point solvents give higher J_sc for spin coating but smaller J_sc for blade coating. There are probably more P3HT/PCBM interfaces in toluene solution than those in chlorobenzene and dichlorobenzene solutions, resulting in more efficient exciton dissociation. It is remarkable that the fill factor of dichlorobenzene solution is 66 %, much higher than 55 % for toluene and 49 % for chlorobenzene. Dichlorobenzene has the highest boiling point among the three solvents. The highest fill factor in dichlorobenzene solution may result from the highest carrier mobility due to the enhanced self-assembly of P3HT taking place during the relatively slow drying in spinning. The device performances are summarized in Table 1.

Table 4-1 Performance of bulk hetero-junction solar cells in this work.

A. (Toluene)Spin 9.25 0.59 47 2.6 223

B. (Toluene)Blade 11.07 0.59 53 3.5 412

C. (Toluene)Blade at 60℃ 11.49 0.59 55 3.8 304

D. (Toluene)Blade and spin 11.36 0.58 55 3.7 245

E. (Chlorobenzene)Blade and spin 8.87 0.57 49 2.5 345 F. (Dichlorobenzene)Blade and spin 8.79 0.60 66 3.5 242 G. (Toluene)Blade and spin

with bladed PEDOT:PSS (120 nm) 8.35 0.59 52 2.5 245

The AFM images of the devices are shown in Fig. 4-1-c. The root-mean-square (RMS) roughness are 2.3 nm for device A, 15.1 nm for device B, 4.9 nm for device C, 2.8 nm for device D, 1.9 nm for device E, and 5.8 nm for device F. The different root-mean-square values are attributed to the different fabrication processes with different solvents. We think that the devices with high efficiency (device B, C, D, and F) have stronger self-organization than those with low efficiency (device A and E). Higher surface roughness corresponds to higher degree of self-organization [34]. However, there is no clear evidence for the correlation between self-organization and device efficiency. Nevertheless, we speculate that device B by blade coating shows clear self-organization, implying ordered structures within each component. Therefore, the carrier mobility is high and the power conversion efficiency is high as well even with a thick film of 412 nm. The clear self-organizations in device C and device F also could be seen due to their relative high roughness. Interestingly, the roughness of device D by blade and spin coating is just slightly higher than that of device A only by spin coating, but the efficiency of device D is still much higher. This may be due to the ordered structure occurred at the beginning of blade coating, combined with thicker film thickness by blade coating. As for the low efficiency devices such as device A and device E, the films are relatively smooth. The images show that the self-organization could be

achieved within the short drying time by blade coating.

Fig. 4-1-c AFM images of the devices in this work. P3HT:PCBM thin film made by (a)spin coating, (b)blade coating, (c)blade coating at 60⁰C, (d)blade and spin coating, (e) blade and spin coating in chlorobenzene solution, (f) blade and spin coating in dichlorobenzene solution. The P3HT:PCBM films in (a) to (d) are made in toluene solution.

Fig. 4-1-d (a) shows the absorption spectra of the P3HT and PCBM blend films deposited by different methods and solutions. The IPCE for all devices are shown in Fig.

4-1-d (b). Despite of the different morphologies there is no significant variations among the absorption spectra. So the differences in the device performances must come from the exciton dissociation and carrier transport processes. The IPCE values appear similar in all the devices except that of device G and show slightly difference at about 600 nm. However, the J_scvalues show much difference among the devices. In principle the measured J_scshould

Blade + spin

be proportional to the product of the IPCE and the illuminating spectrum, integrated over all wavelengths. We may attribute the inconsistency between IPCE and J_sc values to the different spectral-mismatch factors of the different light sources [26].

Fig. 4-1-d (a) The absorption spectra of devices. (b) The incident photon to current efficiency (IPCE) in this work

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Device C

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