**3.1 Experimental**

12 Solar Cells – New Aspects and Solutions

These defects increase the exciton-to-free-carrier probability. More important, the transport process becomes the dominant limiting factor for *JSC*, and the exciton-to-free-carrier process becomes relatively unimportant. Then it seems that the assumption of exciton-to-free-carrier probability as unity can satisfy the need of the prediction. In Fig. 7, we can see that there are two regions in the fitting curve. The left region is determined by equation (20). In this region, the lifetimes of both carriers are longer than their transient time. The solid line in the right region is determined by equation (22). In this region, hole lifetime is shorter than its

If it is assumed that the drift length ratio of hole and electron is very small, then the equation (23) can be used to predict *JSC*. As shown in Fig. 7 (dash line), it can predict *JSC* very

In this part, the exciton generation rate was calculated by taking the optical interference effect into account. Based on the calculated exciton generation rate, the dependence of *JSC* on the active layer thickness was analyzed and compared with experimental data. Because of the optical interference effect, the total exciton generation rate does not monotonously increase with the increase of the active layer thickness, but behaves wavelike which induces the corresponding variation of *JSC*. The carrier lifetimes also influence *JSC* greatly. When the lifetimes of both electrons and holes are long enough, dissociation probability plays an important role for the thick active layer. *JSC* behaves wavelike with the variation of the active layer thickness. When the hole lifetime is too short (drift length is smaller than device thickness), accumulation of charges appears near the electrode and *JSC* increases at the initial stage and then decreases rapidly with the increase of the active layer thickness. The accordance between the predictions and the experimental results confirms the validity of the

The detail of the interpenetrating network, or to say, the morphology is essentially important for the performance of polymer solar cells. In order to achieve an optimal morphology, a thermal treatment is usually utilized in the device fabrication. The thermal treatment can be carried out after and before the electrode deposition. Both the methods can greatly improve the device performance. The functions of the thermal treatment have been extensively investigated, and it has been shown that the morphology will be rearranged through the nanoscale phase separation between donor and acceptor components during the thermal treatment. By carefully optimizing the thermal treatment condition, an optimal interpenetrating network can be formed, which greatly improves the charge transport property. Besides, the thermal treatment can also effectively enhance the crystallization of P3HT, which will increase the hole mobility and the optical absorption capability. Due to the importance of the thermal treatment for P3HT:PCBM solar devices, great efforts have been devoted into the study of the thermal annealing process in the past few years. How the thermal annealing ambient, thermal annealing temperature and thermal annealing time affect the device performance has been well studied. However, only very few studies paid attention to the role of cathode in the thermal treatment. As is known, the thermal treatment can be done before and after the cathode deposition and both methods can greatly improve the device performance. The unique difference between them is whether there is cathode confinement in the thermal treatment or not. Although most of the previous studies have

transit time and electron lifetime is longer than its transit time.

proposed model. These results give a guideline to optimize *JSC*.

**3. Effects of annealing sequence on** *JSC*

well, which means *c<<1*.

**2.3 Summary** 

Fig. 8 shows the layer structure of our polymer solar cells and the chemical structures of P3HT and PCBM. All the devices were fabricated on the ITO-coated glass substrates. Briefly, after being cleaned sequentially with detergent, de-ionized water, acetone, and isopropanol in an ultrasonic bath for about 15 mins, the dried ITO glass substrates were treated with oxygen plasma for about 3 mins. Then the filtered PEDOT:PSS (Baytron P VP AI 4083) suspension (through 0.45 *m* filter) was spun coated on top of the ITO surface to form a ~50 nm layer under ambient condition, and dried at 120oC in an oven for about one hour.

Fig. 8. Layer structure of the polymer solar cells investigated in this work.

P3HT:PCBM solution dissolved in 1,2-dichlorobenzene with a weight ratio of 1:0.8 was then spun coated on the PEDOT:PSS layer in the glove box to form a 100 nm blend layer. A 100 nm Al cathode was further thermally evaporated through a shadow mask giving an active device area of 20 mm2. In order to investigate the effects of the cathode confinement on the device performance in the thermal treatment, two different types of devices were investigated: the devices without the cathode confinement in the thermal treatment (anneal the devices before the cathode deposition, pre-anneal) and the devices with the cathode confinement in the thermal treatment (anneal the devices after the cathode deposition, post-

Effects of Optical Interference and Annealing on the

Performance of Polymer/Fullerene Bulk Heterojunction Solar Cells 15

Fig. 9. *J-V* characteristics of the solar cells under the AM 1.5 illumination with the light intensity of 100 mW/cm2. *T*he devices without (circle) and with (triangle) the cathode confinement in the thermal treatment and the device without any thermal treatment (squire)

are shown in the graph. Solid lines are the fitting curves according to equation (24).

 <sup>0</sup> 1 *S B qV RJ*

can be described approximated by the Shockley equation

were addressed by the XPS measurement.

In order to understand the functions of the cathode confinement in the thermal treatment, the electrical parameters need to be extracted. The *J-V* characteristics of organic solar cells

*nk T S*

*V RJ JJe <sup>J</sup> <sup>R</sup>* 

Where *J0*, *Jph*, *Rs*, *Rsh*, *q*, *n*, *kB* and *T* are the saturation current density, the photocurrent density, the series resistance, the shunt resistance, the electron charge, the ideality factor, the Boltzmann constant and the temperature, respectively. By fitting the Shockley equation (Fig. 9), the estimated parameters are extracted and listed in Table 1. It is shown that *Rs* of the device with the cathode confinement in the thermal treatment is greatly reduced compared to the device without the cathode confinement in the thermal treatment (from 9.34 cm2 to 4.43 cm2). *Rs* can significantly affect the device performance and reducing the value of *Rs* is an efficient method to increase *PCE*. The reduced *Rs* by using the cathode confinement plays one main role for the significant performance improvement of polymer solar cells. *Rs* is directly related to the contacts between the cathode and the active layer. Thus, these contacts

The interfacial analysis results obtained by XPS measurement are shown in Fig. 10. Each top curve and bottom curve in the Al 2p, C 1s, O 1s and S 2p core level spectra graphs are corresponding to the samples with and without cathode confinement in the thermal treatment. As shown in Fig. 10, both samples show the Al 2p spectrum peaks located at the binding energy (BE) of 74.95 eV and 74.6 eV, which are corresponding to the Al oxide and Al-O-C bond, respectively, by referring to Table 2. The Al-O-C bond is also confirmed by the peaks located at the BE of 286.2 eV in the C 1s spectrum and 531 eV in the O 1s spectrum as

*ph*

(24)

*Sh*

anneal). The thermal treatment was carried out by annealing the devices in the glove box at the optimized temperature of 160 °C for about 10 mins as our previous report [Zhang et al., 2008]. For reference, the devices without any thermal treatment were also fabricated.

The current-voltage (*J-V*) characteristics were measured by a Keithley 2400 source-measure unit under AM 1.5 solar illumination at intensity of 100 mW/cm2 calibrated by a Thorlabs optical power meter. The XPS samples were consisted of an identical sandwiched structure: ITO coated glass/P3HT:PCBM(100 nm)/Al (3 nm). Because XPS is a surface chemical analysis technique (top 1-10 nm usually), here only a very thin metal layer is used as others [39]. The XPS spectra were measured by transferring the samples to the chamber of a Kratos AXIS HSi spectrometer at once. The operating pressure of the analysis chamber was maintained at 8x10-9 Torr. A 1486.71 eV monochromatic Al K x-ray gun source was used to achieve the Al 2p, O 1s, C 1s and S 2p spectra. Tapping mode AFM measurements were taken with a Nanoscope III A (Digital Instruments) scanning probe microscope. The samples were prepared in the same sequence as the XPS samples. The phase images and the line scanning profiles of the samples were then recorded under air operation. For both the optical absorption study and x-ray diffraction measurement, the thin films of P3HT:PCBM in the same thickness of 100 nm were spun cast on the microscope slides. The optical absorption study was recorded by a Shimadzu UV-3101 PC UV-VIS-NIR scanning spectrophotometer. The XRD measurement was carried out by the θ-2θ scan method with CuKα radiation ( = 0.1542 nm) using a Shimadu X-Ray diffractometer.

### **3.2 Results and discussion**

Fig. 9 shows the *J-V* characteristics of the devices with the same configuration of ITO/PEDOT:PSS/P3HT:PCBM/Al. For the device without any thermal treatment, it shows the solar response with *JSC* of 5.12 mA/cm2, *VOC* of 0.58 V, *FF* of 47.63% and *PCE* of 1.41%. The device performance is greatly improved by the thermal treatment. However, there are obvious differences for the devices with and without the cathode confinement in the thermal treatment as shown in Fig. 9 and Table 1. For the device without the cathode confinement in the thermal treatment, it shows the performance of *JSC*=7.50 mA/cm2, *VOC*=0.58 V, *FF*=57.13% and *PCE*=2.49%. However, a further performance improvement is observed for the device with the cathode confinement in the thermal treatment, which shows a better performance of *JSC*=8.34 mA/cm2, *VOC*=0.60 V, *FF*=62.57% and *PCE*=3.12%. It can be seen that the cathode confinement in the thermal treatment effectively increases *JSC* and *FF*, which makes the overall *PCE* improved by 25%. This trend was found for a series of cells. Similar results are reported recently [Kim et al, 2009] where they also observed that the device with thermal treatment after cathode deposition could show a better performance. This further confirms our experimental results.


Units of parameters, VOC: V; JSC, J0 and Jph: mA/cm2; FF and PCE: %; Rshand Rs: cm2.

Table 1. Summary of the Parameters Extracted from the J-V Curves Shown in Fig. 9

anneal). The thermal treatment was carried out by annealing the devices in the glove box at the optimized temperature of 160 °C for about 10 mins as our previous report [Zhang et al.,

The current-voltage (*J-V*) characteristics were measured by a Keithley 2400 source-measure unit under AM 1.5 solar illumination at intensity of 100 mW/cm2 calibrated by a Thorlabs optical power meter. The XPS samples were consisted of an identical sandwiched structure: ITO coated glass/P3HT:PCBM(100 nm)/Al (3 nm). Because XPS is a surface chemical analysis technique (top 1-10 nm usually), here only a very thin metal layer is used as others [39]. The XPS spectra were measured by transferring the samples to the chamber of a Kratos AXIS HSi spectrometer at once. The operating pressure of the analysis chamber was maintained at 8x10-9 Torr. A 1486.71 eV monochromatic Al K x-ray gun source was used to achieve the Al 2p, O 1s, C 1s and S 2p spectra. Tapping mode AFM measurements were taken with a Nanoscope III A (Digital Instruments) scanning probe microscope. The samples were prepared in the same sequence as the XPS samples. The phase images and the line scanning profiles of the samples were then recorded under air operation. For both the optical absorption study and x-ray diffraction measurement, the thin films of P3HT:PCBM in the same thickness of 100 nm were spun cast on the microscope slides. The optical absorption study was recorded by a Shimadzu UV-3101 PC UV-VIS-NIR scanning spectrophotometer. The XRD measurement was carried out by the θ-2θ scan method with

Fig. 9 shows the *J-V* characteristics of the devices with the same configuration of ITO/PEDOT:PSS/P3HT:PCBM/Al. For the device without any thermal treatment, it shows the solar response with *JSC* of 5.12 mA/cm2, *VOC* of 0.58 V, *FF* of 47.63% and *PCE* of 1.41%. The device performance is greatly improved by the thermal treatment. However, there are obvious differences for the devices with and without the cathode confinement in the thermal treatment as shown in Fig. 9 and Table 1. For the device without the cathode confinement in the thermal treatment, it shows the performance of *JSC*=7.50 mA/cm2, *VOC*=0.58 V, *FF*=57.13% and *PCE*=2.49%. However, a further performance improvement is observed for the device with the cathode confinement in the thermal treatment, which shows a better performance of *JSC*=8.34 mA/cm2, *VOC*=0.60 V, *FF*=62.57% and *PCE*=3.12%. It can be seen that the cathode confinement in the thermal treatment effectively increases *JSC* and *FF*, which makes the overall *PCE* improved by 25%. This trend was found for a series of cells. Similar results are reported recently [Kim et al, 2009] where they also observed that the device with thermal treatment after cathode deposition could show a better performance. This further

Samples VOC JSC FF PCE J0 Jph n Rsh Rs

Without thermal treatment 0.58 5.13 47.64 1.42 2.75e-4 5.32 2.32 778.25 29.00 Without cathode confinement 0.58 7.50 57.13 2.49 4.80e-5 7.62 1.89 575.19 9.34 With cathode confinement 0.60 8.34 62.25 3.12 3.03e-5 8.40 1.88 617.28 4.43

Units of parameters, VOC: V; JSC, J0 and Jph: mA/cm2; FF and PCE: %; Rshand Rs: cm2.

Table 1. Summary of the Parameters Extracted from the J-V Curves Shown in Fig. 9

2008]. For reference, the devices without any thermal treatment were also fabricated.

CuKα radiation ( = 0.1542 nm) using a Shimadu X-Ray diffractometer.

**3.2 Results and discussion** 

confirms our experimental results.

Fig. 9. *J-V* characteristics of the solar cells under the AM 1.5 illumination with the light intensity of 100 mW/cm2. *T*he devices without (circle) and with (triangle) the cathode confinement in the thermal treatment and the device without any thermal treatment (squire) are shown in the graph. Solid lines are the fitting curves according to equation (24).

In order to understand the functions of the cathode confinement in the thermal treatment, the electrical parameters need to be extracted. The *J-V* characteristics of organic solar cells can be described approximated by the Shockley equation

$$J = J\_0 \left( e^{\frac{q\left(V - R\_S I\right)}{mk\_B T}} - 1 \right) + \frac{V - R\_S I}{R\_{Sh}} - J\_{ph} \tag{24}$$

Where *J0*, *Jph*, *Rs*, *Rsh*, *q*, *n*, *kB* and *T* are the saturation current density, the photocurrent density, the series resistance, the shunt resistance, the electron charge, the ideality factor, the Boltzmann constant and the temperature, respectively. By fitting the Shockley equation (Fig. 9), the estimated parameters are extracted and listed in Table 1. It is shown that *Rs* of the device with the cathode confinement in the thermal treatment is greatly reduced compared to the device without the cathode confinement in the thermal treatment (from 9.34 cm2 to 4.43 cm2). *Rs* can significantly affect the device performance and reducing the value of *Rs* is an efficient method to increase *PCE*. The reduced *Rs* by using the cathode confinement plays one main role for the significant performance improvement of polymer solar cells. *Rs* is directly related to the contacts between the cathode and the active layer. Thus, these contacts were addressed by the XPS measurement.

The interfacial analysis results obtained by XPS measurement are shown in Fig. 10. Each top curve and bottom curve in the Al 2p, C 1s, O 1s and S 2p core level spectra graphs are corresponding to the samples with and without cathode confinement in the thermal treatment. As shown in Fig. 10, both samples show the Al 2p spectrum peaks located at the binding energy (BE) of 74.95 eV and 74.6 eV, which are corresponding to the Al oxide and Al-O-C bond, respectively, by referring to Table 2. The Al-O-C bond is also confirmed by the peaks located at the BE of 286.2 eV in the C 1s spectrum and 531 eV in the O 1s spectrum as

Effects of Optical Interference and Annealing on the

needs to be ascertained by further experiments.

Performance of Polymer/Fullerene Bulk Heterojunction Solar Cells 17

Bonding states Al 2p (eV) C 1s (eV) O 1s (eV) S 2p (eV)

Al-S 76 162.4

C-S 285.7 164.1, 165.3

Al-O-C 74.6 286.2 531 Al2O3 74.95 532.3

Table 2. Summary of the XPS Binding Energies of Different Bonding States

Fig. 11. The proposed molecular structure transits from P3HT to P3HT-Al complex.

Since the direct reaction between the Al atoms and the sulfur atoms is unlikely to occur because of the inherently high electron density on these sites, it is suggested that the Al atoms form bonds with the carbon atoms on the thiophene ring in the positions adjacent to the sulfur atom and form the P3HT-Al complex. One possible structure of the P3HT-Al complex is proposed in Fig. 11. The formation of the P3HT-Al complex will change the electron density of the sulfur atoms. In the P3HT-Al complex, the overall charge density of the sulfur atoms is smaller than that of the pristine P3HT. Thus the S 2p peaks located at the BE of 164.1 eV and 165.3 eV are shifted to the higher BE side at 164.3 eV and 165.5 eV, respectively, for the sample with the cathode confinement in the thermal treatment. Although the P3HT-Al complex is formed, there is only a slight energy difference (~0.1 eV shift in BE) in the C 1s spectrum for both samples as shown in Fig. 10. This is because the C 1s peak is dominated by the aliphatic carbon atoms while the Al atoms preferentially react with the carbon atoms in the conjugated system (thiophene ring of P3HT in this case). The signal arose from the interaction between P3HT and Al is too weak to affect the C 1s spectrum of the sample with cathode confinement. This explains why only very small energy difference in the C 1s spectrum is observed. The exact structure of P3HT-Al complex

It has been reported that Al metal can effectively transfer the electron to the conjugated polymer with the sulfide species and this feature makes it as a potential cathode for polymer electronics [Ling et al., 2002]. Another study [Reeja-Jayan et al., 2010] also has reported that Cu can react with P3HT and form sulfide-like species. The formed sulfide-like species can improve the solar cell performance. It is believed that the formation of the P3HT-Al complexes will play the same role. With the help of P3HT-Al complexes and the Al-O-C bonds, there is a better contact between the electrode and the active layer. This improved contact effectively reduces Rs and results in the improvement of the device performance. How *Rs* affects the device performance is clearly shown in Fig. 12. It is shown that a large *Rs* will induce the decrease of *FF* and *JSC*. By reducing *Rs*, *FF* and *JSC* are increased and thus the

COOH 289.5 C-C 285.1

shown in Fig. 10. It has indicated that the Al-O-C bond is formed by the reaction of Al atoms and the carbonyl groups in PCBM and its existence will improve the contact between the polymer and the metal for both samples. However, by using the cathode confinement in thermal treatment, there is an additional shoulder peak at the BE of 76 eV in the Al 2p spectrum, which means that there forms an additional chemical bond. The additional chemical bond signal can also be seen from the S 2p spectrum. Although the typical peaks of P3HT appeared at the BE of 164.1 eV (2p3/2) and 165.3 eV (2p1/2) due to the spin-orbit coupling are observed for both samples in S 2p spectrum, there is an extra shoulder peak at the BE of 162.4 eV for the sample by using the cathode confinement. Considering the donation of electron density from the Al metal to the thiophene ring of P3HT, these additional peaks suggest that the interaction between P3HT and the Al metal occurs by using the cathode confinement in the thermal treatment.

Fig. 10. High-resolution Al 2p, C 1s, O 1s and S 2p XPS spectra of the samples without (bottom curve) and with (top curve) the cathode confinement in the thermal treatment. The samples have the configuration of ITO/P3HT:PCBM (100 nm)/ Al(3 nm). By using the cathode confinement, there is an additional shoulder peak at the BE of 76 eV in the Al 2p spectrum and an additional shoulder peak at the BE of 162.4 eV in the S 2p spectrum.

shown in Fig. 10. It has indicated that the Al-O-C bond is formed by the reaction of Al atoms and the carbonyl groups in PCBM and its existence will improve the contact between the polymer and the metal for both samples. However, by using the cathode confinement in thermal treatment, there is an additional shoulder peak at the BE of 76 eV in the Al 2p spectrum, which means that there forms an additional chemical bond. The additional chemical bond signal can also be seen from the S 2p spectrum. Although the typical peaks of P3HT appeared at the BE of 164.1 eV (2p3/2) and 165.3 eV (2p1/2) due to the spin-orbit coupling are observed for both samples in S 2p spectrum, there is an extra shoulder peak at the BE of 162.4 eV for the sample by using the cathode confinement. Considering the donation of electron density from the Al metal to the thiophene ring of P3HT, these additional peaks suggest that the interaction between P3HT and the Al metal occurs by

Fig. 10. High-resolution Al 2p, C 1s, O 1s and S 2p XPS spectra of the samples without (bottom curve) and with (top curve) the cathode confinement in the thermal treatment. The samples have the configuration of ITO/P3HT:PCBM (100 nm)/ Al(3 nm). By using the cathode confinement, there is an additional shoulder peak at the BE of 76 eV in the Al 2p spectrum and an additional shoulder peak at the BE of 162.4 eV in the S 2p spectrum.

using the cathode confinement in the thermal treatment.



Fig. 11. The proposed molecular structure transits from P3HT to P3HT-Al complex.

Since the direct reaction between the Al atoms and the sulfur atoms is unlikely to occur because of the inherently high electron density on these sites, it is suggested that the Al atoms form bonds with the carbon atoms on the thiophene ring in the positions adjacent to the sulfur atom and form the P3HT-Al complex. One possible structure of the P3HT-Al complex is proposed in Fig. 11. The formation of the P3HT-Al complex will change the electron density of the sulfur atoms. In the P3HT-Al complex, the overall charge density of the sulfur atoms is smaller than that of the pristine P3HT. Thus the S 2p peaks located at the BE of 164.1 eV and 165.3 eV are shifted to the higher BE side at 164.3 eV and 165.5 eV, respectively, for the sample with the cathode confinement in the thermal treatment. Although the P3HT-Al complex is formed, there is only a slight energy difference (~0.1 eV shift in BE) in the C 1s spectrum for both samples as shown in Fig. 10. This is because the C 1s peak is dominated by the aliphatic carbon atoms while the Al atoms preferentially react with the carbon atoms in the conjugated system (thiophene ring of P3HT in this case). The signal arose from the interaction between P3HT and Al is too weak to affect the C 1s spectrum of the sample with cathode confinement. This explains why only very small energy difference in the C 1s spectrum is observed. The exact structure of P3HT-Al complex needs to be ascertained by further experiments.

It has been reported that Al metal can effectively transfer the electron to the conjugated polymer with the sulfide species and this feature makes it as a potential cathode for polymer electronics [Ling et al., 2002]. Another study [Reeja-Jayan et al., 2010] also has reported that Cu can react with P3HT and form sulfide-like species. The formed sulfide-like species can improve the solar cell performance. It is believed that the formation of the P3HT-Al complexes will play the same role. With the help of P3HT-Al complexes and the Al-O-C bonds, there is a better contact between the electrode and the active layer. This improved contact effectively reduces Rs and results in the improvement of the device performance.

How *Rs* affects the device performance is clearly shown in Fig. 12. It is shown that a large *Rs* will induce the decrease of *FF* and *JSC*. By reducing *Rs*, *FF* and *JSC* are increased and thus the

Effects of Optical Interference and Annealing on the

Performance of Polymer/Fullerene Bulk Heterojunction Solar Cells 19

by the aggregates of PCBM, the smoother surface morphology means that the cathode

Fig. 13. Tapping-mode AFM phase images of Al covered P3HT:PCBM blend film: (a) sample

It is well known that the main roles of annealing process are to induce the redistribution of PCBM and increase the crystallization of P3HT, so that the bicontinuous interpenetrating networks is achieved and meanwhile the optical absorption capability is enhanced. However, a too fast PCBM diffusion will lead to the formation of very large PCBM aggregates and thus destroy the optimal bicontinuous interpenetrating network. Besides, too large PCBM domains also reduce the interfacial contact area between P3HT and PCBM and lead to the inefficient exciton dissociation. In order to achieve a high performance, it is required to well control the PCBM domain size. It is shown here that the overgrowth of the PCBM domains in the thermal treatment is effectively inhibited by using the cathode confinement. Thus a better nanoscale morphology control is achieved. Similar metal confinement effect was also demonstrated on the organic surface by using silver cap [Peumans et al., 2003]. The improved morphology will

*JSC* is also directly related to the optical absorption of the active layer. In order to investigate the effects of cathode confinement on the optical absorption capability, the UV-Vis absorption spectra of the active layer capped with the Al electrode were measured. Because the annealed metal results in a slight variation of the light absorption, the optical spectra were obtained by subtracting the pure metal spectra. The results are shown in Fig. 14. All the samples show the typical absorption spectrum of P3HT:PCBM blend film with the absorption peak at the wavelength of 515 nm and shoulders at 550 nm and 604 nm. The thermal treatment obviously increases the optical absorption of the P3HT:PCBM film. However, there is a better optical absorption capability for the sample with the cathode confinement (e in Fig. 14) compared to the sample without the cathode confinement (d in Fig. 14). It is well known that the absorption capability of P3HT:PCBM system is directly

without any thermal treatment and samples without (b) and with (c) the cathode confinement in the thermal treatment. Their corresponding cross sectional profiles are shown in (d) to (f) with root mean square roughness 5.5, 6.3 and 5.9 nm respectively.

decrease the exciton loss, facilitate the charger transport and thus increase *JSC*.

related to the P3HT crystallites. The crystallization of P3HT was measured by XRD.

confinement can prevent the formation of too large underlying PCBM domains.

device performance is improved. At the same time, it is also noted that although both *FF* and *JSC* can be affected by *Rs*, their dependences on *Rs* are different. From Fig. 12, it can be seen that *FF* can be greatly adjusted by *Rs* when the value of *Rs* is just larger than 1.0 cm2. The decrease of *Rs* from 9.34 cm2 to 4.43 cm2 (Table 1) should be the main reason for the increase of *FF* from 57.13% to 62.25% (Table 1) for the sample by using the cathode confinement. However, there is no obvious change of *JSC* observed until *Rs* is larger than 25 cm2 (Fig. 12). Since *Rs* of the two devices are relative low (9.34 cm2 and 4.43 cm2 respectively, Table 1), it seems that the decrease of *Rs* is not the main reason for the obvious increase of *JSC* (from 7.50 mA/cm2 to 8.34 mA/cm2, Table 1) by using the cathode confinement. This conclusion is also confirmed by the extracted parameter of *Jph*. *Jph* is mainly determined by the properties of the active layer and only slightly depends on *Rs* (independent parameters in equation (24)). If the cathode confinement in the thermal treatment is only to improve the contact and reduce *Rs*, there should be no such obvious change of *Jph* (from 7.62 mA/cm2 to 8.40 mA/cm2, Tabel I). Thus, there must be other more important factors besides *Rs* which lead to the obvious increase of *Jph*. It is well known that *Jph* is very sensitive to the device morphology and material absorption, and thus these aspects should be well addressed.

Fig. 12. Effect of *Rs* variation on *J–V* characteristics of the P3HT:PCBM solar cells according to (1). Only the value of *Rs* is changed while keep *J0*=3.03e-5 mA/cm2, *Jph* =8.40 mA/cm2, *Rsh* =617.28 cm2 and *n*= 1.88. *Rs* greatly affects *FF* and thus the overall *PCE*. *J0*, *Rsh* and *n* only slightly affect *JSC* in the value range shown in Table 1.

The effects of cathode confinement on the device morphology are firstly investigated by the AFM measurement. Because the interface between the active layer and the cathode is mainly enriched by PCBM upon thermal treatment, the evolution of the surface morphology directly reflects the change of the PCBM domains. As shown in Fig. 13, it is shown that the thermal treatment effectively leads to the growth of the PCBM domains and thus increases the root mean square roughness. However, comparing to the device without the cathode confinement, there is a smoother surface morphology for the device with the cathode confinement. As shown in the AFM phase images (Fig. 13 b and c), there is a smaller island size for the sample with the cathode confinement. The profile measurements (Fig. 13 e and f) also show that the average peak-to-peak height and the width of the surface morphology are reduced by 20% and 33% by using the cathode confinement. Since surface morphology change is mainly induced

device performance is improved. At the same time, it is also noted that although both *FF* and *JSC* can be affected by *Rs*, their dependences on *Rs* are different. From Fig. 12, it can be seen that *FF* can be greatly adjusted by *Rs* when the value of *Rs* is just larger than 1.0 cm2. The decrease of *Rs* from 9.34 cm2 to 4.43 cm2 (Table 1) should be the main reason for the increase of *FF* from 57.13% to 62.25% (Table 1) for the sample by using the cathode confinement. However, there is no obvious change of *JSC* observed until *Rs* is larger than 25 cm2 (Fig. 12). Since *Rs* of the two devices are relative low (9.34 cm2 and 4.43 cm2 respectively, Table 1), it seems that the decrease of *Rs* is not the main reason for the obvious increase of *JSC* (from 7.50 mA/cm2 to 8.34 mA/cm2, Table 1) by using the cathode confinement. This conclusion is also confirmed by the extracted parameter of *Jph*. *Jph* is mainly determined by the properties of the active layer and only slightly depends on *Rs* (independent parameters in equation (24)). If the cathode confinement in the thermal treatment is only to improve the contact and reduce *Rs*, there should be no such obvious change of *Jph* (from 7.62 mA/cm2 to 8.40 mA/cm2, Tabel I). Thus, there must be other more important factors besides *Rs* which lead to the obvious increase of *Jph*. It is well known that *Jph* is very sensitive to the device morphology and material absorption,

Fig. 12. Effect of *Rs* variation on *J–V* characteristics of the P3HT:PCBM solar cells according to (1). Only the value of *Rs* is changed while keep *J0*=3.03e-5 mA/cm2, *Jph* =8.40 mA/cm2, *Rsh* =617.28 cm2 and *n*= 1.88. *Rs* greatly affects *FF* and thus the overall *PCE*. *J0*, *Rsh* and *n*

The effects of cathode confinement on the device morphology are firstly investigated by the AFM measurement. Because the interface between the active layer and the cathode is mainly enriched by PCBM upon thermal treatment, the evolution of the surface morphology directly reflects the change of the PCBM domains. As shown in Fig. 13, it is shown that the thermal treatment effectively leads to the growth of the PCBM domains and thus increases the root mean square roughness. However, comparing to the device without the cathode confinement, there is a smoother surface morphology for the device with the cathode confinement. As shown in the AFM phase images (Fig. 13 b and c), there is a smaller island size for the sample with the cathode confinement. The profile measurements (Fig. 13 e and f) also show that the average peak-to-peak height and the width of the surface morphology are reduced by 20% and 33% by using the cathode confinement. Since surface morphology change is mainly induced

and thus these aspects should be well addressed.

only slightly affect *JSC* in the value range shown in Table 1.

by the aggregates of PCBM, the smoother surface morphology means that the cathode confinement can prevent the formation of too large underlying PCBM domains.

Fig. 13. Tapping-mode AFM phase images of Al covered P3HT:PCBM blend film: (a) sample without any thermal treatment and samples without (b) and with (c) the cathode confinement in the thermal treatment. Their corresponding cross sectional profiles are shown in (d) to (f) with root mean square roughness 5.5, 6.3 and 5.9 nm respectively.

It is well known that the main roles of annealing process are to induce the redistribution of PCBM and increase the crystallization of P3HT, so that the bicontinuous interpenetrating networks is achieved and meanwhile the optical absorption capability is enhanced. However, a too fast PCBM diffusion will lead to the formation of very large PCBM aggregates and thus destroy the optimal bicontinuous interpenetrating network. Besides, too large PCBM domains also reduce the interfacial contact area between P3HT and PCBM and lead to the inefficient exciton dissociation. In order to achieve a high performance, it is required to well control the PCBM domain size. It is shown here that the overgrowth of the PCBM domains in the thermal treatment is effectively inhibited by using the cathode confinement. Thus a better nanoscale morphology control is achieved. Similar metal confinement effect was also demonstrated on the organic surface by using silver cap [Peumans et al., 2003]. The improved morphology will decrease the exciton loss, facilitate the charger transport and thus increase *JSC*.

*JSC* is also directly related to the optical absorption of the active layer. In order to investigate the effects of cathode confinement on the optical absorption capability, the UV-Vis absorption spectra of the active layer capped with the Al electrode were measured. Because the annealed metal results in a slight variation of the light absorption, the optical spectra were obtained by subtracting the pure metal spectra. The results are shown in Fig. 14. All the samples show the typical absorption spectrum of P3HT:PCBM blend film with the absorption peak at the wavelength of 515 nm and shoulders at 550 nm and 604 nm. The thermal treatment obviously increases the optical absorption of the P3HT:PCBM film. However, there is a better optical absorption capability for the sample with the cathode confinement (e in Fig. 14) compared to the sample without the cathode confinement (d in Fig. 14). It is well known that the absorption capability of P3HT:PCBM system is directly related to the P3HT crystallites. The crystallization of P3HT was measured by XRD.

Effects of Optical Interference and Annealing on the

optical absorption capability and increase *JSC*.

thermal treatment done after cathode deposition.

Samples 2

**3.3 Summary** 

Performance of Polymer/Fullerene Bulk Heterojunction Solar Cells 21

the cathode in the thermal treatment, the PCBM diffusion is slowed down. Thus it is easier for P3HT to be crystallized. The increased P3HT crystallites will enhance the active layer

Fig. 15. X-ray diffraction spectra of various samples: (a) Al covered P3HT:PCBM blend film without the thermal treatment (b) Al covered P3HT:PCBM blend film with the thermal treatment done before cathode deposition (c) Al covered P3HT:PCBM blend film with the

Without thermal treatment 5.49 0.83 318 9.6 1.61 Without cathode confinement 5.44 0.61 596 13 1.625 With cathode confinement 5.44 0.45 617 17.7 1.625

P3HT:PCBM solar cells with the cathode confinement in the thermal treatment show better performance than the solar cells without the cathode confinement in the thermal treatment. The effects of the cathode confinement on the device performance have been investigated in this work. According to the XPS results, it is found that the Al-O-C bonds and P3HT-Al complexes are formed at the interface between the active layer and the cathode by using the cathode confinement. These chemical structures effectively reduce the contact resistance and improve the device performance. More importantly, the cathode confinement effectively improves the active layer morphology. According to the AFM, UV-Vis absorption spectra and XRD measurement results, it is found that the cathode confinement in the thermal treatment not only prevents the overgrowth of the PCBM domains, but also increases the crystallization of P3HT. With the help of cathode confinement in the thermal treatment, a better optical absorption and a more ideal bicontinuous interpenetrating networks can be obtained at the same time. This will effectively reduce the exciton loss and improve the

[] <sup>2</sup> [] *h* [counts/s] *L* [nm] *d* [nm]

Table 3. Summary of X-Ray Diffraction Peaks of P3HT:PCBM from Fig.15

charge transport capability. Thus an improved device performance is achieved.

Fig. 14. Optical absorption spectra of various samples: (a) bare P3HT:PCBM blend film without thermal treatment (b) bare P3HT:PCBM blend film with thermal treatment (c) Al covered P3HT:PCBM blend film without the thermal treatment (d) Al covered P3HT:PCBM blend film with the thermal treatment done before cathode deposition (e) Al covered P3HT:PCBM blend film with the thermal treatment done after cathode deposition.

Fig. 15 shows the obtained XRD measurement results. A characteristic peak around 2 = 5.4 is observed for all the samples, which is associated with the lamella structure of thiophene rings in P3HT. Based on Bragg's law and Scherrer relation, the lattice constant (d) and the size of the polymer crystallites (L) can be determined:

$$n\mathcal{X} = \mathcal{Z}d\sin\theta\tag{25}$$

$$L = \frac{0.9\,\text{\AA}}{\Delta\_{2\theta}\cos\theta} \tag{26}$$

where is the wavelength of the x-ray, the Bragg's angle, Δ<sup>2</sup> the smallest full width at half maximum of the peak. The extracted *d* and *L* are listed in Table 3. It is shown that all the samples show the lattice constant of 1.62 0.01 nm that represents the P3HT crystallites in aaxis orientation. Thermal treatment increases the crystallization of P3HT. However, the increased magnitudes are different for the devices with and without the cathode confinement. The sample with the help of the cathode confinement in the thermal treatment shows the highest peak. As listed in Table 3, the size of the P3HT crystallites (*L* value of 17.7 nm) is increased by 36% by using the cathode confinement compared to without the cathode confinement (*L* value of 13 nm). The increased crystallite size may come from the effective inhibition of the strong PCBM diffusion by the cathode confinement. It has been shown [Swinnen et al., 2006] that a too strong diffusion of PCBM from the P3HT matrix would reduce the P3HT crystallization and optical absorption property. Because of the presence of the cathode in the thermal treatment, the PCBM diffusion is slowed down. Thus it is easier for P3HT to be crystallized. The increased P3HT crystallites will enhance the active layer optical absorption capability and increase *JSC*.

Fig. 15. X-ray diffraction spectra of various samples: (a) Al covered P3HT:PCBM blend film without the thermal treatment (b) Al covered P3HT:PCBM blend film with the thermal treatment done before cathode deposition (c) Al covered P3HT:PCBM blend film with the thermal treatment done after cathode deposition.


Table 3. Summary of X-Ray Diffraction Peaks of P3HT:PCBM from Fig.15

### **3.3 Summary**

20 Solar Cells – New Aspects and Solutions

Fig. 14. Optical absorption spectra of various samples: (a) bare P3HT:PCBM blend film without thermal treatment (b) bare P3HT:PCBM blend film with thermal treatment (c) Al covered P3HT:PCBM blend film without the thermal treatment (d) Al covered P3HT:PCBM blend film with the thermal treatment done before cathode deposition (e) Al covered P3HT:PCBM blend film with the thermal treatment done after cathode deposition.

Fig. 15 shows the obtained XRD measurement results. A characteristic peak around 2 = 5.4 is observed for all the samples, which is associated with the lamella structure of thiophene rings in P3HT. Based on Bragg's law and Scherrer relation, the lattice constant (d) and the

(25)

(26)

*L*

2 0.9 cos

where is the wavelength of the x-ray, the Bragg's angle, Δ<sup>2</sup> the smallest full width at half maximum of the peak. The extracted *d* and *L* are listed in Table 3. It is shown that all the samples show the lattice constant of 1.62 0.01 nm that represents the P3HT crystallites in aaxis orientation. Thermal treatment increases the crystallization of P3HT. However, the increased magnitudes are different for the devices with and without the cathode confinement. The sample with the help of the cathode confinement in the thermal treatment shows the highest peak. As listed in Table 3, the size of the P3HT crystallites (*L* value of 17.7 nm) is increased by 36% by using the cathode confinement compared to without the cathode confinement (*L* value of 13 nm). The increased crystallite size may come from the effective inhibition of the strong PCBM diffusion by the cathode confinement. It has been shown [Swinnen et al., 2006] that a too strong diffusion of PCBM from the P3HT matrix would reduce the P3HT crystallization and optical absorption property. Because of the presence of

size of the polymer crystallites (L) can be determined:

2 sin *n d*

P3HT:PCBM solar cells with the cathode confinement in the thermal treatment show better performance than the solar cells without the cathode confinement in the thermal treatment. The effects of the cathode confinement on the device performance have been investigated in this work. According to the XPS results, it is found that the Al-O-C bonds and P3HT-Al complexes are formed at the interface between the active layer and the cathode by using the cathode confinement. These chemical structures effectively reduce the contact resistance and improve the device performance. More importantly, the cathode confinement effectively improves the active layer morphology. According to the AFM, UV-Vis absorption spectra and XRD measurement results, it is found that the cathode confinement in the thermal treatment not only prevents the overgrowth of the PCBM domains, but also increases the crystallization of P3HT. With the help of cathode confinement in the thermal treatment, a better optical absorption and a more ideal bicontinuous interpenetrating networks can be obtained at the same time. This will effectively reduce the exciton loss and improve the charge transport capability. Thus an improved device performance is achieved.

Effects of Optical Interference and Annealing on the

structure of ITO/PEDOT:PSS/P3HT:PCBM/Al.

160oC for 10 mins.

Performance of Polymer/Fullerene Bulk Heterojunction Solar Cells 23

Fig. 16. *JSC* versus P3HT:PCBM thickness, P3HT:PCBM with weight ratio of 1:0.8 and device

(a) (b)

Fig. 17. Optimization of active layer thickness. (a) around the first optical interference peak, and (b) around the second optical interference peak. All devices were post-annealed at

Fig. 18. UV-visible absorption spectra of P3HT:PCBM (about 80 nm thick and 208 nm thick).
