Table of Contents Author Guidelines Submit a Manuscript
International Journal of Photoenergy
Volume 2015 (2015), Article ID 982064, 8 pages
http://dx.doi.org/10.1155/2015/982064
Research Article

Enhanced Power Conversion Efficiency of P3HT : PC71BM Bulk Heterojunction Polymer Solar Cells by Doping a High-Mobility Small Organic Molecule

1State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China
2Co-Innovation Center for Micro/Nano Optoelectronic Materials and Devices, Research Institute for New Materials and Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China

Received 28 July 2015; Revised 9 September 2015; Accepted 9 September 2015

Academic Editor: Maria Vasilopoulou

Copyright © 2015 Hanyu Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The effect of molecular doping with TIPS-pentacene on the photovoltaic performance of polymer solar cells (PSCs) with a structure of ITO/ZnO/poly(3-hexylthiophene-2,5-diyl) (P3HT) : [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) : TIPS-pentacene/MoOx/Ag was systematically investigated by adjusting TIPS-pentacene doping ratios ranged from 0.3 to 1.2 wt%. The device with 0.6 wt% TIPS-pentacene exhibited the enhanced short-circuit current and fill factor by 1.23 mA/cm2 and 7.8%, respectively, resulting in a maximum power conversion efficiency of 4.13%, which is one-third higher than that of the undoped one. The photovoltaic performance improvement was mainly due to the balanced charge carrier mobility, enhanced crystallinity, and matched cascade energy level alignment in TIPS-pentacene doped active layer, resulting in the efficient charge separation, transport, and collection.

1. Introduction

Polymer solar cells (PSCs), as one of the most promising energy conversion technologies, have attracted much attention in last decades due to their unique properties of low cost, being easily manufactured, large scale, and being flexible [15]. The PSCs have many excellent applications, such as incorporation with wearable products, decoration of buildings, and space application for optimizing the design of space solar power. Recently, the power conversion efficiencies (PCEs) have reached 10% and 11% for the PSCs using single-junction and multijunction structures, respectively [6, 7]. However, the PCEs are still not high enough for commercialization. Therefore, great effort is devoted to further improve the photovoltaic performance of PSCs. The major drawbacks of PSCs are often attributed to low light absorption, limited exciton migration, and low hole transport ability [8]. Particularly, the low hole transport ability could increase the carrier recombination in the active layer and suppress the charge carrier collection [9]. Hence, a relatively low short-circuit current () and fill factor (FF) are often observed in such PSCs.

In order to increase the hole transport ability of organic active layers, numerous approaches have been applied, such as modifying metal/semiconductor interface [10], introducing multisolvents [11], processing solvent additives [12], and doping a small amount of high-mobility materials [13]. Among them, molecular doping is an effective method to enhance hole transport ability of PSCs. For example, Liu et al. enhanced the PCE of PSCs by adding a high-mobility conjugated polymer with suitable energy band structure [13]. P-type molecular doping of F4-TCNQ improved the hole density and hole mobility in the polymer: fullerene derivative blends [14]. Pentacene, a high hole mobility small molecule used in organic thin-film transistors (OFETs), was successfully added in the poly(3-hexylthiophene-2,5-diyl) (P3HT) : 6,6-phenyl C61-butyric acid methyl ester (PC61BM) blends to balance hole and electron mobility and improve the photovoltaic performance of relative PSCs [1517].

In this work, a high hole-mobility pentacene derivative of TIPS-pentacene (0.8 cm2V−1s−1, which is nearly 4 folders as high as that of P3HT) [18, 19], with high solubility in organic solvents and deeper highest occupied molecular orbital (HOMO) of 5.20 eV compared with 5.00 eV of P3HT [20], was introduced in the P3HT : 6,6-phenyl C71-butyric acid methyl ester (PC71BM) blends. By adjusting the doping ratios of TIPS-pentacene from 0.3 to 1.2 wt%, the optimized PSC with 33% PCE improvement was obtained. The mechanism of TIPS-pentacene doping was elucidated through characterizing the morphology of active layers by X-ray diffraction (XRD) and atomic force microscopy (AFM). Furthermore, the variation of charge carrier mobility was investigated in the active layers from the hole-only and electron-only devices by using the space-charge-limited current (SCLC) method.

2. Experiment

The chemical structures of organic materials are shown in Figure 1(a), and the structure of PSCs is indium tin oxide (ITO)/ZnO (30 nm)/P3HT : PC71BM : TIPS-pentacene (180 nm)/ (15 nm)/Ag (100 nm) as depicted in Figure 1(b). The ITO-coated glass substrates with a sheet resistance of 10 Ω/sq were consecutively in an ultrasonic bath containing detergent, acetone, deionized water, and isopropyl alcohol for 10 min each step and finally dried in an oven for 30 mins [21]. The ZnO precursor solution was spin-coated on the ITO-glass substrates. After baking at 200°C for 60 min in atmosphere, the substrates were transferred to a glove box (1 ppm O2 and H2O). P3HT (99.9%, Rieke Metals) and PC71BM (99.9%, Solarmer) were dissolved in 1,2-dichlorobenzene (DCB) and mixed in the glove box to obtain blend solutions (30 mg/mL) with a weight ratio of 1 : 1. TIPS-pentacene (99.9%, Rieke Metals) solution was separately prepared in DCB at a concentration of 2 mg/mL and then mixed with the blend solutions of P3HT : PC71BM. TIPS-pentacene doping ratios in P3HT : PC71BM blends were adjusted from 0.3, 0.6, and 0.9 to 1.2 wt%. Then, P3HT : PC71BM : TIPS-pentacene blend solutions were spin-coated on ZnO thin layer. After that, the substrates were solvent-annealed in a covered Petri dish for 20 mins and then were thermal-annealed at 120°C for 10 mins [22]. (99.98%, Aldrich) layer was deposited onto the active layers at a rate of 1 to 3 Å/s at a pressure of 3.0 × 10−3 Pa in vacuum, followed by the deposition of Ag anode at a rate of 10 Å/s under a pressure of 3.0 × 10−3 Pa. The typical area of PSCs was 0.02 cm2. All measurements were performed under ambient condition without encapsulation.

Figure 1: (a) Chemical structures of P3HT, PC71BM, and TIPS-pentacene. (b) Schematic structure of PSCs in this work. (c) Energy band diagram of materials used in PSCs.

A light source integrated with a xenon lamp (CHF-XM35, Beijing Trust Tech) with an illumination power of 100 mW/cm2 was used as a solar simulator. The curves under illumination and in the dark were measured with a Keithley 4200 programmable current-voltage source, and the external quantum efficiency (EQE) spectra were measured under the lump light passing through a monochromator. The ultraviolet-visible (UV-Vis) absorption spectra of the active layer on quartz substrates were measured using a Shimadzu UV1700 system. The film preparation condition for XRD (D1-HR XRD, Bede, Inc.) and AFM (MFP-3D-BIO, Asylum Research) measurement was kept the same as the device fabrication for comparison.

3. Results and Discussion

The absorption spectra of thin films of P3HT, PC71BM, and TIPS-pentacene are shown in Figure 2(a). It can be seen that P3HT shows strong light absorption from 450 to 650 nm, while PC71BM has compensatory absorption from 350 to 550 nm. TIPS-pentacene exhibits a wide absorption in the visible region from 400 to 750 nm [20]. Figure 2(b) shows the absorption spectra of P3HT : PC71BM blend films with various TIPS-pentacene doping ratios. It is found that the absorption in the wavelength from 400 nm to 700 nm does not change significantly with the increase of TIPS-pentacene doping ratio. For the small amount of TIPS-pentacene, the absorption contribution of TIPS-pentacene is negligible. This phenomenon also indicates that the active layer thicknesses are unchanged with the doping of TIPS-pentacene.

Figure 2: (a) Absorption spectra for thin films of P3HT, PC71BM, and TIPS-pentacene. (b) Absorption spectra of P3HT : PC71BM active layers blended with various TIPS-pentacene doping ratios.

The current density-voltage (-) characteristics of P3HT : PC71BM PSCs with various TIPS-pentacene doping ratios are displayed in Figure 3(a). The detailed parameters with error statistics including open circuit voltage (), , FF, and PCE are listed in Table 1. For the precise comparison of photovoltaic performance of PSCs with different TIPS-pentacene doping ratios, the relative change of the device parameters with increasing TIPS-pentacene doping ratios is summarized in Figure 3(b). The performance of pristine P3HT : PC71BM devices was chosen as the reference and set to 100%. All other devices data were normalized to that reference.

Table 1: Comparison of device characteristics of P3HT : PC71BM PSCs with various TIPS-pentacene doping ratios.
Figure 3: (a) - characteristics of P3HT : PC71BM PSCs with various TIPS-pentacene doping ratios. The inset shows the equivalent circuit model of PSC. (b) Relative change of the device parameters with increasing TIPS-pentacene doping ratios. The performance of pristine P3HT : PC71BM PSCs was chosen as reference and set to 100%. All other devices data were normalized to that reference.

is 0.59 V for undoped devices and 0.3 wt% TIPS-pentacene doped devices and slightly increased to 0.61 V for the relative high TIPS-pentacene doping ratios devices. The increased is due to the multicharge separation phenomenon [23]. As shown in Figure 1(c), the HOMO of TIPS-pentacene is deeper than that of P3HT. After doping TIPS-pentacene, donor : donor : accepter blends were formed, and the additional TIPS-pentacene : PC71BM interface could facilitate the exciton separation in the bulk heterojunction. Even though little exciton can be formed on TIPS-pentacene for its negligible light absorption, the exciton can be formed on PC71BM for the obvious light absorption of PC71BM. The exciton on PC71BM can be directly dissociated into free charge carrier by P3HT or TIPS-pentacene. of TIPS-pentacene doped PSCs is dependent on the composition of P3HT : PC71BM and TIPS-pentacene : PC71BM interfaces as reported by the previous works [24]. Thus, the additional TIPS-pentacene : PC71BM interface can affect the overall heterojunction energetics, resulting in the increased . With doping of TIPS-pentacene, a remarkable enhancement of and FF is observed. In the device doped with 0.3 wt% TIPS-pentacene, is increased to 10.05 mA/cm2, and FF is increased to 58.4%. When doping 0.6% TIPS-pentacene, the device reaches its optimized performance with of 10.86 mA/cm2, a FF of 62.4%, and a PCE of 4.13%. However, when the doping ratios are increased to 0.9% and 1.2%, the devices show decreased photovoltaic performance.

To further investigate the diode characteristics of PSCs, we modeled - characteristics with the Shockley diode equation as [25]where is the photocurrent, is the reverse saturation current density, and is the ideal factor. is Boltzmann’s constant, and is the temperature. is the electron charge, and is the device area. The relevant diode parameters, including series resistance () and shunt resistance (), are summarized in Table 1. The decreased and increased of PSC with 0.6 wt% TIPS-pentacene made a contribution to the enhanced and FF for efficient charge carrier transport and collection.

To illustrate the effect of molecular doping of TIPS-pentacene on of P3HT : PC71BM PSCs, EQE was employed. Figure 4 shows EQE spectra of P3HT : PC71BM PSCs with various TIPS-pentacene doping ratios in the wavelength ranging from 400 to 700 nm. The shape of EQE spectra resembles that of UV-Vis absorption spectra of the blend films. EQE values for devices exceed 50% in the region of 400–570 nm. It is found that the largest EQE of 68.9% is obtained for PSCs with 0.6 wt% TIPS-pentacene doped. However, when the doping ratios of TIPS-pentacene in P3HT : PC71BM blends were further increased, EQE of PSCs was decreased.

Figure 4: EQE spectra of P3HT : PC71BM PSCs with various TIPS-pentacene doping ratios. The inset shows the integral of PSCs with various TIPS-pentacene doping ratios.

To further investigate EQE variation, the theoretical values () are obtained via integrating EQE. The formula of integration is presented as where is AM1.5 solar spectral density.

The inset of Figure 4 shows of PSCs with various TIPS-pentacene doping ratios. All are about 1.31 mA/cm2 less than the measured values in - curves, as the offered EQE spectra lack the part less than 400 nm. is increased from 7.76 mA/cm2 for undoped devices to 9.23 mA/cm2 for 0.6 wt% TIPS-pentacene doped devices and then decreased to 7.61 mA/cm2 for 1.2 wt% TIPS-pentacene doped devices. This situation is similar to the characteristics of as listed in Table 1.

The effect of different TIPS-pentacene doping ratios in P3HT : PC71BM blends on the charge carrier transport properties was further investigated by using SCLC model. The hole-only devices with a configuration of ITO/ (15 nm)/P3HT : PC71BM : TIPS-pentacene (180 nm)/ (15 nm)/Ag (100 nm) and electron-only devices with a configuration of ITO/ZnO (30 nm)/P3HT : PC71BM : TIPS-pentacene (180 nm)/Bphen (5 nm)/Ag (100 nm) were fabricated, respectively. The hole mobility in the hole-only devices and the electron mobility in the electron-only devices can be calculated using Mott-Gurney law as where is the charge carrier mobility. is the relative permittivity of polymer assumed to be 3, and is the vacuum dielectric constant of 8.85 × 10−12 F/m. is the voltage, and is the thickness of the layer.

- characteristics of the hole-only and electron-only devices with various TIPS-pentacene doping ratios in P3HT : PC71BM blends are presented in Figures 5(a) and 5(b), respectively. The hole mobility and the electron mobility of corresponding devices are listed in Table 2. In the hole-only devices, the hole mobility of undoped P3HT : PC71BM device was 1.3 × 10−4 cm2V−1s−1. After doping TIPS-pentacene, hole mobilities of the devices were calculated to be 1.7 × 10−4, 2.6 × 10−4, 2.0 × 10−4, and 9.5 × 10−5 cm2V−1s−1 for 0.3 wt%, 0.6 wt%, 0.9 wt%, and 1.2 wt% doping ratios. On the other hand, in the electron-only devices, the electron mobility is slightly decreased with the increase of TIPS-pentacene doping ratio in P3HT : PC71BM : TIPS-pentacene blends. In particular, balanced charge carrier mobility of 1.00 was obtained in the active layer with 0.6 wt% TIPS-pentacene, which ensures efficient charge transport and collection in P3HT : PC71BM : TIPS-pentacene blends, resulting in the enhancement of both and FF. However, when TIPS-pentacene doping ratios are high, PCE is decreased. The degraded performances are due to the decreased hole mobilities, leading to an unbalance charge transport in P3HT : PC71BM : TIPS-pentacene blends. In this condition, hole with lower mobility induced cumulative charge carriers in the active layer due to the SCLC effect [26]. Therefore, PCE of PSCs with high TIPS-pentacene doping ratios is decreased by the reduction of and FF.

Table 2: The hole mobility and electron mobility of the hole-only and electron-only devices with various TIPS-pentacene doping ratios.
Figure 5: - characteristics of (a) hole-only and (b) electron-only devices.

To demonstrate TIPS-pentacene doping effect on the crystallinity of P3HT, P3HT : PC71BM blends without and with 0.6 wt% TIPS-pentacene were characterized by XRD as shown in Figure 6. XRD spectra show obvious α-axis orientation of P3HT crystallite at a peak of 2θ near 5.4°, corresponding to the (100) crystal plane [21]. Compared with P3HT : PC71BM blends, the 0.6 wt% TIPS-pentacene doped P3HT : PC71BM exhibits higher (100) peak. This result indicated that the enhancement of hole mobility for TIPS-pentacene doped blends could result from the crystallinity enhancement of P3HT.

Figure 6: XRD spectra of P3HT:PC71BM blends without and with 0.6 wt% TIPS-pentacene.

To further investigate the effect of molecular doping of TIPS-pentacene on the morphology of P3HT : PC71BM blends, the surface morphology of P3HT :  PC71BM blends and 0.6 wt% TIPS-pentacene doped P3HT :  PC71BM blends was examined by using AFM as shown in Figure 7. For P3HT : PC71BM blends, the root-mean-square (RMS) roughness is 6.14 nm. The rough texture suggests that the enhancement of ordered structure is due to the self-organization of thick P3HT : PC71BM blends during slow growing process. After doping 0.6 wt% TIPS-pentacene, RMS increases slightly to 6.28 nm. It has been reported that the increased RMS of P3HT : PC71BM blends demonstrated that P3HT had enhanced crystallinity [22]. Based on this phenomenon, it could also be deducted that the crystallinity of P3HT in the active layer was improved with the molecular doping of TIPS-pentacene, which is consistent with the results of XRD measurement mentioned above. In addition, the rough surface could shorten the charge-transport distance from P3HT : PC71BM blends to Ag anode, resulting in an efficient hole collection. AFM phase images of P3HT : PC71BM blends and 0.6 wt% TIPS-pentacene doped P3HT : PC71BM blends are presented in Figures 7(c) and 7(d). It can be seen from the two phase images that no obvious ternary TIPS-pentacene domain was observed, indicating that small doping ratio would not change P3HT : PC71BM interpenetrating network.

Figure 7: AFM morphology images of (a) P3HT:PC71BM blend film and (b) P3HT:PC71BM with a 0.6 wt% TIPS-pentacene doping ratio blend film. Corresponding AFM phase images ((c) and (d)) are shown below each respective morphology image. Image size is 5 μm × 5 μm.

4. Conclusion

In summary, the solution-processed P3HT : PC71BM : TIPS-pentacene PSCs with 0, 0.3, 0.6, 0.9, and 1.2 wt% TIPS-pentacene doping ratios were fabricated. 0.6 wt% TIPS-pentacene doped PSC exhibited an enhanced and FF, resulting in a maximum PCE of 4.13%, which is 33% higher compared with the undoped PSC. The improved photovoltaic performance was originated from the balanced charge carrier mobility, enhanced crystallinity, and matched cascade energy level alignment in P3HT : PC71BM : TIPS-pentacene blends, resulting in efficient charge separation, transport, and collection. This work can significantly enhance our understanding of the mechanism of molecular doping with high-mobility small organic molecules on the photovoltaic performance of PSCs.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This research was funded by the Foundation of the National Natural Science Foundation of China (NSFC) (Grant no. 61177032) and the National Science Funds for Creative Research Groups of China (Grant no. 61421002).

References

  1. A. J. Heeger, “25th anniversary article: bulk heterojunction solar cells: understanding the mechanism of operation,” Advanced Materials, vol. 26, no. 1, pp. 10–28, 2014. View at Publisher · View at Google Scholar · View at Scopus
  2. V. Vohra, K. Kawashima, T. Kakara et al., “Efficient inverted polymer solar cells employing favourable molecular orientation,” Nature Photonics, vol. 9, no. 6, pp. 403–408, 2015. View at Publisher · View at Google Scholar
  3. J. Yu, Y. Zheng, and J. Huang, “Towards high performance organic photovoltaic cells: a review of recent development in organic photovoltaics,” Polymers, vol. 6, no. 9, pp. 2473–2509, 2014. View at Publisher · View at Google Scholar · View at Scopus
  4. P.-T. Huang, Y.-H. Chen, B.-Y. Lin, and W.-P. Chuang, “Homogenized poly(3-hexylthiophene)/methanofullerene film by addition of end-functionalized compatibilizer and its application to polymer solar cells,” International Journal of Photoenergy, vol. 2015, Article ID 762532, 7 pages, 2015. View at Publisher · View at Google Scholar
  5. A. Iwan, B. Boharewicz, I. Tazbir, A. Sikora, and B. Zboromirska-Wnukiewicz, “Silver nanoparticles in PEDOT:PSS layer for polymer solar cell application,” International Journal of Photoenergy, vol. 2015, Article ID 764938, 9 pages, 2015. View at Publisher · View at Google Scholar
  6. J.-D. Chen, C. Cui, Y.-Q. Li et al., “Single-junction polymer solar cells exceeding 10% power conversion efficiency,” Advanced Materials, vol. 27, no. 6, pp. 1035–1041, 2015. View at Publisher · View at Google Scholar · View at Scopus
  7. C.-C. Chen, W.-H. Chang, K. Yoshimura et al., “An efficient triple-junction polymer solar cell having a power conversion efficiency exceeding 11%,” Advanced Materials, vol. 26, no. 32, pp. 5670–5677, 2014. View at Publisher · View at Google Scholar · View at Scopus
  8. L. Dou, J. You, Z. Hong et al., “25th anniversary article: a decade of organic/polymeric photovoltaic research,” Advanced Materials, vol. 25, no. 46, pp. 6642–6671, 2013. View at Publisher · View at Google Scholar · View at Scopus
  9. X. G. Guo, N. J. Zhou, S. J. Lou et al., “Polymer solar cells with enhanced fill factors,” Nature Photonics, vol. 7, no. 10, pp. 825–833, 2013. View at Publisher · View at Google Scholar · View at Scopus
  10. N. Wang, J. Yu, Y. Zheng, Z. Guan, and Y. Jiang, “Organic photovoltaic cells based on a medium-bandgap phosphorescent material and C60,” The Journal of Physical Chemistry C, vol. 116, no. 9, pp. 5887–5891, 2012. View at Publisher · View at Google Scholar · View at Scopus
  11. L. Ye, S. Zhang, W. Ma et al., “From binary to ternary solvent: morphology fine-tuning of D/A blends in PDPP3T-based polymer solar cells,” Advanced Materials, vol. 24, no. 47, pp. 6335–6341, 2012. View at Publisher · View at Google Scholar · View at Scopus
  12. Y. Liang, Z. Xu, J. Xia et al., “For the bright future-bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%,” Advanced Materials, vol. 22, no. 20, pp. E135–E138, 2010. View at Publisher · View at Google Scholar · View at Scopus
  13. S. Liu, P. You, J. Li et al., “Enhanced efficiency of polymer solar cells by adding a high-mobility conjugated polymer,” Energy & Environmental Science, vol. 8, no. 5, pp. 1463–1470, 2015. View at Publisher · View at Google Scholar
  14. Y. Zhang, H. Zhou, J. Seifter et al., “Molecular doping enhances photoconductivity in polymer bulk heterojunction solar cells,” Advanced Materials, vol. 25, no. 48, pp. 7038–7044, 2013. View at Publisher · View at Google Scholar · View at Scopus
  15. C.-T. Lee and C.-H. Lee, “Conversion efficiency improvement mechanisms of polymer solar cells by balance electron-hole mobility using blended P3HT:PCBM:pentacene active layer,” Organic Electronics, vol. 14, no. 8, pp. 2046–2050, 2013. View at Google Scholar
  16. H.-Y. Lee and H.-L. Huang, “Investigation performance and mechanisms of inverted polymer solar cells by pentacene doped P3HT: PCBM,” International Journal of Photoenergy, vol. 2014, Article ID 812643, 9 pages, 2014. View at Publisher · View at Google Scholar · View at Scopus
  17. H. Du, Z. Deng, J. Lun et al., “Pentacene doping for the efficiency improvement of polymer solar cells,” Synthetic Metals, vol. 161, no. 19-20, pp. 2083–2086, 2011. View at Publisher · View at Google Scholar · View at Scopus
  18. G. Giri, E. Verploegen, S. C. B. Mannsfeld et al., “Tuning charge transport in solution-sheared organic semiconductors using lattice strain,” Nature, vol. 480, no. 7378, pp. 504–508, 2011. View at Publisher · View at Google Scholar · View at Scopus
  19. R. Mauer, M. Kastler, and F. Laquai, “The impact of polymer regioregularity on charge transport and efficiency of P3HT:PCBM photovoltaic devices,” Advanced Functional Materials, vol. 20, no. 13, pp. 2085–2092, 2010. View at Publisher · View at Google Scholar · View at Scopus
  20. R. J. Davis, M. T. Lloyd, S. R. Ferreira et al., “Determination of energy level alignment at interfaces of hybrid and organic solar cells under ambient environment,” Journal of Materials Chemistry, vol. 21, no. 6, pp. 1721–1729, 2011. View at Publisher · View at Google Scholar · View at Scopus
  21. W. Ma, C. Yang, X. Gong, K. Lee, and A. J. Heeger, “Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology,” Advanced Functional Materials, vol. 15, no. 10, pp. 1617–1622, 2005. View at Publisher · View at Google Scholar · View at Scopus
  22. M. Reyes-Reyes, K. Kim, and D. L. Carroll, “High-efficiency photovoltaic devices based on annealed poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C61 blends,” Applied Physics Letters, vol. 87, no. 8, Article ID 083506, 2005. View at Publisher · View at Google Scholar · View at Scopus
  23. J. Huang, J. Yu, Z. Guan, and Y. Jiang, “Improvement in open circuit voltage of organic solar cells by inserting a thin phosphorescent iridium complex layer,” Applied Physics Letters, vol. 97, no. 14, Article ID 143301, 3 pages, 2010. View at Publisher · View at Google Scholar · View at Scopus
  24. P. P. Khlyabich, B. Burkhart, and B. C. Thompson, “Compositional dependence of the open-circuit voltage in ternary blend bulk heterojunction solar cells based on two donor polymers,” Journal of the American Chemical Society, vol. 134, no. 22, pp. 9074–9077, 2012. View at Publisher · View at Google Scholar · View at Scopus
  25. K. Bouzidi, M. Chegaar, and A. Bouhemadou, “Solar cells parameters evaluation considering the series and shunt resistance,” Solar Energy Materials and Solar Cells, vol. 91, no. 18, pp. 1647–1651, 2007. View at Publisher · View at Google Scholar · View at Scopus
  26. C. Melzer, E. J. Koop, V. D. Mihailetchi, and P. W. M. Blom, “Hole transport in poly(phenylene vinylene)/methanofullerene bulk-heterojunction solar cells,” Advanced Functional Materials, vol. 14, no. 9, pp. 865–870, 2004. View at Publisher · View at Google Scholar