**4. Overall optimization of polymer solar cells**

Because the active layer is very thin in compared with the incident light wavelength, the optical interference effect influences the absorption and *JSC* as discussed in above. According to the simulated results based on the optical model, the thickness will be optimized around the first and second optical interference peaks in this part. In addition, the annealing process can efficiently improve the performance of P3HT:PCBM polymer solar cells. The performance is related to the annealing sequence, and post-annealing is more favored by the devices. Based on this conclusion, all the polymer solar cells were fabricated and post-annealed in this section. These devices were used to optimize the overall solar cell performance.

### **4.1 Experimental**

The fabrication process is the same as above. The devices were fabricated on the ITO-coated glass substrates. After routine solvent cleaning (treated sequentially with detergent, deionized water, acetone, and isopropanol in an ultrasonic bath for about 15 minutes), the dried ITO glass substrates were treated with oxygen plasma for about three mins. Then the filtered PEDOT:PSS suspension was spin coated on the top of the ITO surface under ambient condition. The P3HT:PCBM solution dissolved in dichlorobenzene with a weight ratio of 1:0.8 was spin coated in the glove box. Finally, Al cathode was deposited by e-beam evaporation through a shadow mask. All the devices have same structure: ITO\PEDOT:PSS\P3HT:PCBM\Al, and only the thicknesses of the P3HT:PCBM active layers are different. The active layer thickness was controlled by changing the spin speed and solution concentration. Then different annealing temperatures are tested for the devices based on post-annealing to find the optimized conditions. The *J-V* characteristics were measured using a Keithley 2400 parameter analyzer in the dark and under a simulated light intensity of 100 mW/cm2 (AM 1.5G) calibrated by an optical power meter.

### **4.2 Experimental results and discussion**

### **4.2.1 Optical interference effects and active layer thickness optimization**

The TMF method as discussed in section 2 is used to predict *JSC* for the active layer thickness in a range from 50 nm to 250 nm for 1:0.8 P3HT:PCBM active layer. The results are plotted in Fig. 16. As predicted, obvious polymer solar oscillatory behavior is observed because of the very thin active layer compared with the light wavelength. When the P3HT:PCBM ratio is 1:0.8, the first and second optical interference peaks are found at the P3HT:PCBM layer thicknesses of around 85 nm and 230 nm. Both the two optical interference peaks should be used to optimize the active layer thickness.

According to the simulated results, the devices were fabricated around the first and the second optical interference peaks. The experimental results for the different active layer thicknesses are shown Fig. 17. As predicted, *JSC* shows a periodic behavior with the variation of the active layer thickness. The *JSC* increases from as low as 6.25 mA/cm2 (for the device with active layer thickness, t=64 nm) to as high as 6.93 mA/cm2 (for t=80 nm), and then decreases around the first interference peak. The same trend is observed around the second optical interference peak at a thickness of 208 nm. *JSC* reaches a value as high as 10.37 mA/cm2 at the second optical interference peak. The higher *JSC* comes from the better absorption ability. It is obviously shown that the second peak can absorb more light than the first peak as shown in Fig. 18. Thus the second optical interference peak is more preferred to achieve a higher *PCE*. Then around this peak, the annealing conditions are investigated.

Because the active layer is very thin in compared with the incident light wavelength, the optical interference effect influences the absorption and *JSC* as discussed in above. According to the simulated results based on the optical model, the thickness will be optimized around the first and second optical interference peaks in this part. In addition, the annealing process can efficiently improve the performance of P3HT:PCBM polymer solar cells. The performance is related to the annealing sequence, and post-annealing is more favored by the devices. Based on this conclusion, all the polymer solar cells were fabricated and post-annealed in this section.

The fabrication process is the same as above. The devices were fabricated on the ITO-coated glass substrates. After routine solvent cleaning (treated sequentially with detergent, deionized water, acetone, and isopropanol in an ultrasonic bath for about 15 minutes), the dried ITO glass substrates were treated with oxygen plasma for about three mins. Then the filtered PEDOT:PSS suspension was spin coated on the top of the ITO surface under ambient condition. The P3HT:PCBM solution dissolved in dichlorobenzene with a weight ratio of 1:0.8 was spin coated in the glove box. Finally, Al cathode was deposited by e-beam evaporation through a shadow mask. All the devices have same structure: ITO\PEDOT:PSS\P3HT:PCBM\Al, and only the thicknesses of the P3HT:PCBM active layers are different. The active layer thickness was controlled by changing the spin speed and solution concentration. Then different annealing temperatures are tested for the devices based on post-annealing to find the optimized conditions. The *J-V* characteristics were measured using a Keithley 2400 parameter analyzer in the dark and under a simulated light

**4. Overall optimization of polymer solar cells** 

**4.1 Experimental** 

These devices were used to optimize the overall solar cell performance.

intensity of 100 mW/cm2 (AM 1.5G) calibrated by an optical power meter.

**4.2.1 Optical interference effects and active layer thickness optimization** 

The TMF method as discussed in section 2 is used to predict *JSC* for the active layer thickness in a range from 50 nm to 250 nm for 1:0.8 P3HT:PCBM active layer. The results are plotted in Fig. 16. As predicted, obvious polymer solar oscillatory behavior is observed because of the very thin active layer compared with the light wavelength. When the P3HT:PCBM ratio is 1:0.8, the first and second optical interference peaks are found at the P3HT:PCBM layer thicknesses of around 85 nm and 230 nm. Both the two optical interference peaks should be

According to the simulated results, the devices were fabricated around the first and the second optical interference peaks. The experimental results for the different active layer thicknesses are shown Fig. 17. As predicted, *JSC* shows a periodic behavior with the variation of the active layer thickness. The *JSC* increases from as low as 6.25 mA/cm2 (for the device with active layer thickness, t=64 nm) to as high as 6.93 mA/cm2 (for t=80 nm), and then decreases around the first interference peak. The same trend is observed around the second optical interference peak at a thickness of 208 nm. *JSC* reaches a value as high as 10.37 mA/cm2 at the second optical interference peak. The higher *JSC* comes from the better absorption ability. It is obviously shown that the second peak can absorb more light than the first peak as shown in Fig. 18. Thus the second optical interference peak is more preferred to achieve a higher *PCE*. Then around this peak, the annealing conditions are investigated.

**4.2 Experimental results and discussion** 

used to optimize the active layer thickness.

Fig. 16. *JSC* versus P3HT:PCBM thickness, P3HT:PCBM with weight ratio of 1:0.8 and device structure of ITO/PEDOT:PSS/P3HT:PCBM/Al.

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 160oC for 10 mins.

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

Effects of Optical Interference and Annealing on the

layer thickness.

**6. References** 

best solar device performance.

1510. ISSN 1566-1199

124901-7. ISSN 0021-8979

1622. ISSN 1616-301X

Performance of Polymer/Fullerene Bulk Heterojunction Solar Cells 25

smaller than device thickness), accumulation of charges will appear near the electrode and *JSC* will increase at the initial stage and then decrease rapidly with the increase of active

The experimental studies were carried out to investigate P3HT:PCBM based HJ polymer solar cells in this chapter. It was found that the strengthened contact due to the bonding reinforcements (Al-O-C bonds and P3HT-Al complex) at the active layer/metal interface for post-annealed device improves the charge collection at the cathode side. Carrier separation can be facilitated via the improved nanoscaled morphology of the post-annealed polymer blend. The Al capping layer promotes efficient formation of the P3HT crystallites and thus enhances the light harvesting property of the polymer blend. Evidence for the latter has been derived from the improved shape of the absorption spectrum. The results underline the importance of applying the most efficient annealing sequence in order to achieve the

Based on above results, the overall performance of P3HT:PCBM bulk polymer solar cells were optimized. As predicted by the TMF method, an obvious polymer solar cell oscillatory behavior of *JSC* was observed in the experiments. The devices were optimized around the first two optical interference peaks. It was found that the optimized thicknesses are 80 nm and 208 nm. Based on the post-annealing, different annealing temperatures have been tested. The optimized annealing condition was found to be 160oC for 10 min in a nitrogen atmosphere.

Cheyns, D.; Poortmans, J.; Heremans, P.; Deibel, C.; Verlaak, S., Rand, B. P. & Genoe J.

Goodman, A. M. & Rose, A. (1971). Double Extraction of uniformly generated electron-hole

Kim, H. J.; Park, J. H.; Lee, H. H.; Lee D. R.; & Kim, J. J. (2009). The effect of Al electrodes

Koster, L. J. A.; Smits, E. C. P.; Mihailetchi, V. D. & Blom, P. W. M. Device model for the

Ma, W.; Yang, C.; Gong, X.; Lee, K. & Heeger, A. J. (2005). Thermally stable, efficient

Li, G.; Shrotriya, V. & Yao Y. (2005). Investigation of annealing effects and film thickness

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(2008). Analytical model for the open-circuit voltage and its associated resistance in organic planar heterojunction solar cells. *Physical Review B*, Vol. 77, No. 16, (April

pairs from insulators with nonjecting contacts. *Journal of Appllied Physics.* Vol. 42,

on the nanostructure of poly(3-hexylthiophene): Fullerene solar cell blends during thermal annealing, organic electronics, Vol. 10, No. 8, (December 2009). pp. 1505-

operation of polymer/fullerene bulk heterojunction solar cells. *Physical Review B*,

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### **4.2.2 Optimization of annealing conditions**

The device performance depends greatly on annealing temperatures as clearly seen from Fig. 19. The reasons for the performance to be improved by the annealing process have been widely investigated and discussed in section 3. It is clear that for an efficient bulk HJ polymer solar cell, *D* and *A* domains must be small enough so that most of the excitons can diffuse into the *D/A* interfaces before they decay. At the same time, the interpenetrating transport network must be formed for the efficient charge transport. Thus, the morphology optimization is of great important. By varying the annealing condition, the morphology can be well controlled.

Fig. 19. (a) and (b): Relations of device performance and annealing conditions. The P3HT:PCBM layer thickness keeps constant of 208 nm.

These results were related to the better morphology as discussed in previous and also related to the increase of the charge carrier mobility. The same reason should also be responsible for our results. The highest *PCE* in our experiments is achieved when the annealing temperature is 160oC which is very close to the annealing temperatures reported by Ma (Ma et al., 2005). The analysis of changes in film morphology has shown that the changes in film crystallinity and aggregation within the film PCBM nanophase lead to the improved solar characteristics at this temperature. When the annealing temperature is increased, a steady enhancement of *VOC* is observed because the e-beam evaporated Al can induce dipoles at the interface between active layer and cathode [Zhang et al., 2009]. As shown in Fig. 19, the device shows the optimized performance when it has been annealed at 160oC for 10 min.

### **5. Conclusion**

In polymer solar cells, because of the optical interference effect, the total exciton generation rate does not increase monotonically with the increase of the active layer thickness, but behaves wave-like, which induces the corresponding variation of *JSC*. The carrier lifetime also inffuence *JSC* greatly. When the carrier lifetime is long enough, dissociation probability will play a very important role for a thicker active layer. *JSC* will behave wave-like with the variation of active layer thickness. When the carrier lifetime is too short (drift length is smaller than device thickness), accumulation of charges will appear near the electrode and *JSC* will increase at the initial stage and then decrease rapidly with the increase of active layer thickness.

The experimental studies were carried out to investigate P3HT:PCBM based HJ polymer solar cells in this chapter. It was found that the strengthened contact due to the bonding reinforcements (Al-O-C bonds and P3HT-Al complex) at the active layer/metal interface for post-annealed device improves the charge collection at the cathode side. Carrier separation can be facilitated via the improved nanoscaled morphology of the post-annealed polymer blend. The Al capping layer promotes efficient formation of the P3HT crystallites and thus enhances the light harvesting property of the polymer blend. Evidence for the latter has been derived from the improved shape of the absorption spectrum. The results underline the importance of applying the most efficient annealing sequence in order to achieve the best solar device performance.

Based on above results, the overall performance of P3HT:PCBM bulk polymer solar cells were optimized. As predicted by the TMF method, an obvious polymer solar cell oscillatory behavior of *JSC* was observed in the experiments. The devices were optimized around the first two optical interference peaks. It was found that the optimized thicknesses are 80 nm and 208 nm. Based on the post-annealing, different annealing temperatures have been tested. The optimized annealing condition was found to be 160oC for 10 min in a nitrogen atmosphere.
