Efficiency Improvement of Inverted Organic Solar Cells via Introducing

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The Efficiency Improvement of Inverted Organic Solar Cells via Introducing a Series of Polyfluorene Dots in Electron Transport Layer Chunyu Liu, Xinyuan Zhang, Zhiqi Li, Yeyuan He, Jinfeng Li, Liang Shen, Zhihui Zhang, Wenbin Guo, and Shengping Ruan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04079 • Publication Date (Web): 26 Jun 2015 Downloaded from http://pubs.acs.org on June 30, 2015

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The Journal of Physical Chemistry

The Efficiency Improvement of Inverted Organic Solar Cells via Introducing a Series of Polyfluorene Dots in Electron Transport Layer Chunyu Liu, a Xinyuan Zhang,a

Zhiqi Li, a Yeyuan He, a Jinfeng Li, a Liang Shen,a,b

Zhihui Zhang,a Wenbin Guo, a,*, and Shengping Ruan a a

State Key Laboratory on Integrated Optoelectronics, Jilin University, 2699 Qianjin Street, Changchun 130012, China b

Department of Mechanical and Materials Engineering and Nebraska Center for Materials and Nanoscience, University of Nebraska–Lincoln, Lincoln, Nebraska 68588-0656, USA

ABSTRACT This article describes a positive effect on improving the performance of organic solar cells (OSCs) by introducing a series of water-soluble polyfluorene (PF) dots. When poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo1-thiadiazole)] (PF1), poly (9,9-dioctylfluorenyl-2,7-diyl) (PF2) and poly(9,9-dioctylfluorene)-co-(4,7-di-2-thienyl -2,1,3-benzothiadiazole) (PF3) dots are mixed into the polyethylenimine (PEI) cathode buffer layer, active layer and PEI films show an enhancement on light absorption in comparison with control film, which leads to the realization of poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) solar cells with power conversion efficiency (PCE) of 4.33% for PF1 doping, 4.49% for PF2 doping, and 4.72% for PF3 doping, accounting for 23.0%, 27.6% and 34.1% enhancement, respectively. Simultaneously, doping of PF dots also contributes to the improvement of exciton dissociation, charge transport, and charge collection. PF dots could be used as a dual functional additive to enhance optical and electrical properties for organic solar cells.

1．．INTRODUCTION Solar cells, an important way to solve energy and environment problems, have drawn much attention and made great achievements.1,2 Organic solar cells (OSCs) possess more advantages than inorganic solar cells, such as light-weight, high flexibility and process ability, having been widely studied in recent years. 3-7 To date, the best single junction OSCs have achieved a power conversion efficiency (PCE) approaching 10%,

8,9

and the highest PCEs for

cascade and triple-junction OSCs are 11.3% and 11.83%, respectively,10,11 which are threshold values to compensate fabrication costs. The major bottlenecks that hamper the PCE promotion for OSCs are attributable to the unsatisfactory exciton generation,12,13 exction dissociation,14,15 charge transport,16,17 and charge recombination, which generally results in a relatively lower photocurrent for organic solar cells. Decreasing the optical band gap of the photoactive layer and enlarging the absorption coefficient of active materials by incorporating nanocomposites and nanostructures

18-23

are

widely adopted strategy to increase the photons trapping and resultant photocurrent. However, increasing the photons harvesting by the active layer does not necessarily assure an increase of free charge carriers. The ultimate yield of the 1

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photocurrent heavily relies on the fate of charge carriers after exciton splitting,24 which is primarily dictated by the competitive charge sweep-out and charge recombination not only in active layer but also in the buffer layer.

25,26

Therefore, buffer layers including cathode buffer layer and anode buffer layer play a crucial role on device efficiency, and the choice of materials is worth exploring. PEDOT:PSS and different p-type oxides like MoO3 or V2O5 have been applied as hole transport layers.27 Also, TiOx, ZnO, and CsCO3 were employed as efficient electron transport layers.28-31 Amine-rich polymer polyethylenimine (PEI) possesses potential to act as an electron transport layer in OSCs, and it has been used as hole-blocking layer between the photoactive layer and an electrode material. 32 Moreover, many methods have been adopted to improve charge transport and extraction capacity in OSCs, such as active layer doping

33,34

and

buffer layer doping.35,36 In this contribution, a series of

polyfluorene (PF) dots were prepared and incorporated into PEI cathode buffer

layer to enhance the efficiency of inverted OSCs. The cells were based on poly (3-hexylthiophene) (P3HT) donor blending with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) acceptor. The motivation of this work was to unravel the role of PF dots on device performance improvement. The influence of PF dots on light absorption, exciton generation, exciton dissociation, electron transport, charge recombination, and electron collection were investigated in detail.

2. EXPERIMENTAL PEI material was purchased from Sigma-Aldrich (St. Louis, MO, USA), and dissolved in deionized water (2 mg/ml). Poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo- 1-thiadiazole)] (PF1),

Poly(9,9-dioctylfluorenyl-2,7-diyl)

(PF2) and Poly(9,9-dioctylfluorene)-co-(4,7-di-2-thienyl-2,1,3-benzothiadiazole) (PF3) were purchased from American

Dye source (Quebec, Canada). The molecular structure of PF1, PF2, and PF3 are shown in Figure 1a. PF1 was dissolved in tetrahydrofuran (THF) to obtain stock solutions of 1.0 mg/mL. A solution (3 mL) of PF1 (80 ppm) was injected quickly into deionized water (10 mL) under sonication. The THF was removed by nitrogen (N2) flowing under heating, followed by filtration through a 0.2 micrometer filter to remove larger aggregates, then concentrated by heating to get PF1 dots solution. The concentration of finally prepared PF1 dots solution was 1000 PPM. The preparation of PF2 and PF3 dots aqueous solution was as same as PF1. Various amounts of PF1, PF2 and PF3 dots were blended into water-soluble PEI. PEI layer was spin-cast at 4000 rpm then thermally annealed at 100 °C for 10 min in glove box. Adding the Pdots do not brought thickness change for the PEI film. The cells were fabricated with structure of indium tin oxide (ITO)/PEI: PF dots/P3HT: PCBM/molybdenum oxide (MoO3)/silver (Ag) and the energy level is indicated in Figure 1b, and the detailed processes of devices were described in our previous papers. 37, 38

2


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3. RESULT AND DISCUSSION Figure 1c shows typical current density versus voltage (J-V) characteristics of devices under AM 1.5 G illumination. J-V curve in Figure 1c for PF1 was the optimal devices performance which was selected from three kinds of doping concentration as well as the choice of the PF2 and PF3. The weight of PEI and PF dots are PEI/PF1 (2 mg/0.050 mg), PEI/PF2 (2 mg/0.035 mg), and PEI/PF3 (2 mg/0.020 mg), and corresponding weight ratios (wt) are 2.5%wt%, 1.75wt%, and 1.0wt%. Devices with pristine PEI and doping with above three kinds of PF derivative are named as Device A, Device B, Device C and Device D, respectively. The detailed data of all fabricated devices are summarized in Table 1 and all values are typically average of 50 devices. With addition of these three kinds of PF dots, we observe a noticeable enhancement of the Jsc and FF irrespective of the difference of ligands for the PF additive, Jsc raised from 11.26 mA/cm2 to 12.08 mA/cm2, 12.32 mA/cm2 and 12.68 mA/cm2 for the devices doped with PF1, PF2 and PF3, and FF raised from 52.1% to 59.8%, 59.7% and 61.0% respectively. But the increase of Voc is insensitive to the materials doping, thus considerable PCE enhancement from 3.52% up to 4.33% for PF1, 4.49% for PF2, and 4.72% for PF3 are achieved, corresponding to the enhancement of 23.0%, 27.6% and 34.1%, respectively. The improvement of Jsc mainly arises from the enhancement of the light absorption due to the introducing of PF dots into PEI cathode buffer layer. Furthermore, PEI layer doping possesses an advantage to improve electron transport and reduce charge recombination which reduces the space charge buildup affecting the internal field in the case of undoped device, leading to an increase of FF. To understand the nature of the Jsc enhancement, the external quantum efficiency (EQE) spectra and enhancement ratios of three different doped devices were measured and shown in Figure 2a for intuitive comparison. The devices show the maximum EQE for PF1, PF2, and PF3 doping of about 63%, 62% and 63% at 530 nm respectively, while the EQE of control device is 57% at 520 nm. We can find that three doped devices demonstrate a consistent increase for EQE spectra from 350 nm to 700 nm, but the spectra of enhancement ratios show some height difference under the different light wavelength. For device with PF1, EQE increased over a broad wavelength range from 350 to 550 nm. While for device with PF3, EQE mainly increased from 550 to 700 nm. For device with PF2 dots, EQE has a very steady growth from 300nm to 700 nm compared to PF1 and PF3 doping devices. Some inferences about the difference in EQE enhancement ratios were associated to the three different emission spectra of PF derivative which were shown in Figure 2b. Pdots have an ability of absorbing ultraviolet photons and emitting visible photons, which is a smart down-conversion (DC) process. For the PF3 dots, it mainly emitting the photons in the range of 550 nm to 750 nm, which would be absorbed by active layer and generate free carriers, leading to an enhancement of EQE range from 550 nm to 750 nm. The mechanism of PF1 and PF2 is the same as the PF3. 3



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Figure 3 shows the dark J-V characteristic of inverted solar cells without and with three kinds of PF dots. Upon doping PF dots, the diode rectification ratios are significantly changed, indicating that PF dots doping greatly decreases the shunt resistance (Rsh) for doped devices.39 It can be seen that all doped devices show smaller leakage current at negative voltages. In addition, Jsc of doped devices increase gradually with the increase of the applied voltage and overtop to the control devices eventually at positive voltages, suggesting that the electron-dominant charge transport in PEI cathode buffer layer is drastically altered by adding PF dots. The enhancement of dark J-V characteristics is reflected in the FF improvement. The effect of PF dots doping on the electron transport in PEI cathode buffer layer was further investigated with electron-only devices, and measurements was employed the device structure of ITO/PEI:PF/P3HT:PC60BM/BCP/Ag, where the BCP is hole blocking and electron transport layer. J-V characteristics of single charge carrier cells were shown in Figure 4a, tested ranging from 0 V to 8 V and fitted SCLC model including field-dependent mobility. It displays that doped devices demonstrate higher Jsc than that of the undoped device at the same driving voltage no matter what kind of PF derivative doping in the PEI cathode buffer layer. This indicates that the electron transport property is greatly enhanced due to the introduction of PF dots. In order to make a realistic evaluation on the enhancement of electron transport in devices, electron mobility was calculated from SCLC model.40 At a typical applied voltage of 1.0 V, corresponding to an electric field of 105 Vcm-1 across the bulk of a 100 nm device, apparent electron mobilities of 6.87×10-5 cm2V-1s-1, 1.75×10-4 cm2V-1s-1, 8.51×10-5 cm2V-1s-1 and 1.31×10-4 cm2V-1s-1 have been determined for the devices without doping and with PF1, PF2 and PF3 dots, respectively. It can be concluded that an apparent increase of electron mobilities leads to more quickly charge carrier transporting to the electrode, thus reducing current losses via recombination. Meanwhile, charge carriers transfer becomes more balance, thus reducing the effects of space charge formation, 41 beneficial for mitigating the charge recombination. 42-45 To explore the effect of PF derivative on the optical absorption of OSCs, dependence of the photocurrent density (Jph) on the effective voltage (Veff) on a double-logarithmic scale was investigated, which was shown in Figure 4b. The maximum photoinduced carrier generation rate (Gmax) in devices with and without Pdots according to that curves. Jph is determined as Jph= JL-JD, where JL and JD are the current density under illumination and in the dark, respectively. Veff is determined as Veff= V0-V, where V0 is the voltage at which Jph= 0 and V is the applied bias voltage,46 It can seen from Figure 4b that Jph increases linearly at low Veff range and it will tend to saturate at a sufficiently high value of Veff. From Figure 4b, we therefore determined the values of the saturation photocurrent density (Jsat), assuming that all the photogenerated excitons are dissociated into free charge carriers and collected by electrodes afterward at a high Veff region,47,48 Jsat is then only limited by total amount of absorbed incident photons. Then Gmax could be calculated from Jsat= qGmaxL, where q is the electronic charge and L is the thickness of active layer (100 nm). The values of Gmax for 4


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the control and doped devices were 7.90 × 1027 m-3 s-1 (Jsat=131 A m-2), 8.57 × 1027 m-3 s-1 (Jsat= 142 A m-2), 8.69 × 1027 m-3 s-1 (Jsat= 144 A m-2) and 8.45 × 1027 m-3 s-1 (Jsat= 140 A m-2), respectively. Thus, a noticeable enhancement in Gmax occurred after incorporating the Pdots into the active layer. Since the value of Gmax is related to maximum absorption of incident photons, such an increase suggests enhanced light absorption in the active layer of the doped device. Therefore, UV−vis absorption measurements of active layer films and PEI doped without and with Pdots were measured, shown in Figure 4c. We can see that doped films exhibit higher absorption than that of pristine PEI range from 300 nm to 600 nm. Due to the PEI films are close to all transparent, light are barely absorbed by them. So the enhancement of light is mainly arising from the increased absorption of active layer. The introducing of the Pdots could play a role of the scattering center, which will increase the optical path length in the active layer leading to an increased light absorption. At the same time, the DC originating from Pdots could also contribute to the light absorption of active layer. The Pdots demonstrate a positive effect on the light absorption and contribute to generation of the photogenerated excitons. Next, we compared the exciton dissociation probabilities [P(E,T)] (or charge collection probability (PC)), which is an importance parameter for the achievement of high efficiency OSCs, relating to the electric field (E)and temperature (T), for our devices. For OSCs, only a portion of photogenerated excitions can be dissociated into free carriers. As a result, Jph can be expressed using the equation, Jph=qGmax P(E, T)L So, the value of P(E,T) at any bias can be obtained from the plot of the normalized photocurrent density (Jph/Jsat) with respect to Veff. Figure 4d shows that the value of P(E,T) under the short-circuit conditions (Va= 0 V) increased from 87% for the control device to 88%, 89% and 90% for the doped device respectively, indicating that the incorporation of Pdots has a positive influence on the charge collection by electrodes. According to the result of carriers mobility, electron can transport fastly in doped PEI films, resulting in a higher electron extraction rate from active layer, which contributes to the exciton dissociation. Thus, the incorporation of Pdots increased both the exciton generation rate and the dissociation probability, thereby enhancing the photocurrent of the OSCs. In order to investigate the effect of introducing Pdots in active layer on resistance of the devices, the impedance spectra of OSCs with and without Pdots were measured with frequency range from 20 Hz to 1 MHz and shown in Figure 5 and insert is the equivalent circuit model. The shunt pair with R1 and C1 corresponds to the active layer. C1 is the capacitance of the active layer, which is also named as “diffusion capacitance” or “chemical capacitance” when cells operate at under forward bias. The shunt pair of R2 and C2 exists at the electrical contacts interfaces between PEI/PCDTBT:PCBM and PCDTBT:PCBM/MoO3. The R3 represents the electrodes including the resistance of ITO, PEI, MoO3 and Ag. Impedance spectroscopy was tested and three columns of data were got, including frequency (Hz), 5



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impedance and angle. The values of x-axis was calculated with the equation “impedance × cos (angle/180π)”, which indicates real part of the complex impedance, and the values of y-axis was got by the equation “−1 × impedance × sin (angle/180π)”, which exhibits imaginary part of the complex impedance. These four semicircles can help us to understand the internal resistance of different doped devices. From the impedance spectra, it clearly depicts that the semicircle’s diameter of device without doping is much larger than that of device with Pdots doping. It demostrates the resistance of the doped devices are apparently decreased by introducing Pdots, which contributes to the increase of Jsc. And this also makes clear that charge transport property is enhanced for the doped devices.

4. CONCLUSIONS To conclude, we improved the PCEs of OSCs by incorporating PF dots into PEI cathode buffer layer. The improvement of performance not only attribute to increased light absorption of active layer but also benefits electron transport and reduce charge recombination after doping that three kinds of dots. A highest PCE of 4.72% was achieved for the device of doping the PF3 dots. Our study demonstrates that PF derivatives with different ligands could play a general effect on the improved P3HT:PCBM cell device performance. We believe that the results of our research offer an effective approach to enhance the efficiency of OSCs.

AUTHOR INFROMATION Corresponding Author *Tel: +86 431 85168241-8221. Fax: +86 431 85168270. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to the Natural Science Foundation of China (61275035, 61274068, 61077046, 51303061), Chinese National Programs for High Technology Research and Development (2013AA030902), Project of Science and Technology Development Plan of Jilin Province (20130206075SF), the Open Fund of the State Key Laboratory on Integrated Optoelectronics (No. IOSKL2012KF03), Project of Graduate Innovation Fund of Jilin University (2015098) for providing support to the study.

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(29) White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. S. Inverted Bulk-heterojunction Organic Photovoltaic Device Using a Solution-derived ZnO Underlayer. Appl. Phys. Lett. 2006, 89, 143517. (30) Li, X. C.; Xie, F. X.; Zhang, S. Q.; Hou, J. H.; Choy, W. C. MoOx and V2Ox as Hole and Electron Transport Layers Through Functionalized Intercalation in Normal and Inverted Organic Optoelectronic Devices. Light: Sci. Appl. 2015, 4, e273. (31) Huang, J.; Watanabe, T.; Ueno, K.; Yang, Y. Highly Efficient Red-Emission Polymer Phosphorescent Light-Emitting Diodes Based on Two Novel Tris(1-phenylisoquinolinato-C2,N)iridium(III) Derivatives. Adv. Mater. 2007, 19, 739-743. (32) Zhou, Y.; Fuentes-Hernandez, C.; Shim, J.; Meyer, J.; Giordano, A. J.; Li, H.; Winget, P.; Papadopoulos, T.; Cheun, H.; Kim, J.; Fenoll, M.; et al. A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics. Science. 2012, 336, 327-332. (33) Wang, D. H.; Kim, D. Y.; Choi, K. W.; Seo, J. H.; Im, S. H.; Park, J. H.; Park, O. O.; Heeger, A. J. Enhancement of Donor-Acceptor Polymer Bulk Heterojunction Solar Cell Power Conversion Efficiencies by Addition of Au Nanoparticles. Angew. Chem. Int. Ed. 2011, 50, 5519–5523; (34) Zhang, X. Y.; Liu, C. Y.; Li, J. F.; He, Y. Y.; Li, Z. Q.; Li, H.; Guo, W. B.;Shen, L.; Ruan, S. P. Unraveling the Effect of Polymer Dots Doping in Inverted Low Bandgap Organic Solar Cells. Phys. Chem. Chem. Phys. 2015, 17, 16086-16091. (35) Wu, J. L.; Chen, F. C.; Hsiao, Y. S.; Chien, F. C.; Chen, P.; Kuo, C. H.; Huang, M. H.; Hsu, C. S. Surface Plasmonic Effects of Metallic Nanoparticles on the Performance of Polymer Bulk Heterojunction Solar Cells. ACS Nano. 2011, 5, 959–967; (36) He, Y. Y.; Li, Z. Q.; Li, J. F.; Zhang, X. Y.; Liu, C. Y.; Li, H.; Shen, L.; Guo, W. B.; Ruan, S. P. The Role of Au Nanorods in Highly Efficient Inverted Low Bandgap Polymer Solar Cells. Appl. Phys. Lett. 2014, 105, 223305. (37) Liu, C. Y.; Guo, W. B.; Jiang, H. M.; Shen, L.; Ruan, S. P.; Yan, D.W. Efficiency Enhancement of Inverted Organic Solar Cells by Introducing PFDTBT Quantum Dots into PCDTBT:PC71BM Active Layer. Org. Electron. 2014, 15, 2632-2638. (38) Meng, F. X.; Liu, S.; Wang, Y. F.; C.; Tao, P. Xu,; Guo, W. B.; Shen, L.; Zhang, X. D.; Ruan, S. P. Open-circuit Voltage Enhancement of Inverted Polymer Bulk Heterojunction Solar Cells by Doping NaYF4 Nanoparticles/PVP Composites. J. Mater. Chem. 2012, 22, 22382-22386. (39) Liu, X.; Wen, W.; Bazan, G. C. Post-Deposition Treatment of an Arylated-Carbazole Conjugated Polymer for Solar Cell Fabrication. Adv. Mater. 2012, 24, 4505-4510. (40) Goh, C.; Kline, R. J.; Mcgehee, M. D.; Kadnikova, E. N.; Frechet, J. M. J. Molecular-weight-dependent Mobilities 9



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in Regioregular Poly(3-hexyl-thiophene) Diodes. Appl. Phys. Lett. 2005, 86, 122110. (41) Mihailetchi, V. D.; Wildeman, J.; Blom, P. W. M. Space-Charge Limited Photocurrent. Phys. Rev. Lett. 2005, 94, 126602. (42) Zhou, H.; Zhang, Y.; Seifter, J.; Collins, S. D.; Luo, C.; Bazan, G. C. ; Nguyen, T. Q.; Heeger,A. J. High-Efficiency Polymer Solar Cells Enhanced by Solvent Treatment. Adv. Mater. 2013, 25, 1646-1652. (43) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science. 1995, 270,1789-1791. (44) Roman, L. S.; Andersson, M. R.; Yohannes, T.; Inganas, O. Photodiode Performance and Nanostructure of Polythiophene/C60 Blends, Adv. Mater. 1997, 9, 1164-1168. (45) Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. Electron Trapping in Dye/Polymer Blend Photovoltaic Cells. Adv. Mater. 2000, 12, 1270-1274. (46) Mihailetchi, V. D.; Wildeman, J.; Blom, P. W. M. Space-Charge Limited Photocurrent. Phys. Rev. Lett. 2005, 94, 126602. (47) Mihailetchi, V. D.; Koster, L. J. A.; Hummelen, J. C.; Blom, P. W. M. Photocurrent Generation in Polymer-Fullerene Bulk Heterojunctions. Phys. Rev. Lett. 2010, 93, 216601. (48) Shuttle, C. G.; Hamilton, R.; O’Regan, B. C.; Nelson, J.; Durrant, J. R. Charge-density-based Analysis of the Current–voltage Response of Polythiophene/fullerene Photovoltaic Devices. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 16448-16452.

Table 1 Device performance, including open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (PCE), dependent on the different PF dots. Doping ratios (wt%)

0.60±0.02

Pristine PEI PF1 doping

PF2 doping

PF3 doping

Voc (V)

Jsc (mA/cm2) 11.26±0.04

FF (%)

PCE (%)

52.1±0.11

3.52±0.13

2.0

0.59±0.01

12.00±0.05

57.3±0.08

4.06±0.09

2.5

0.60±0.02

12.08±0.04

59.8±0.16

4.33±0.12

2.75

0.59±0.01

11.91±0.03

56.5±0.13

3.97±0.10

1.5

0.60±0.02

12.48±0.06

57.1±0.12

4.28±0.16

1.75

0.61±0.01

12.32±0.04

59.7±0.09

4.49±0.11

2.0

0.60±0.01

11.44±0.03

58.3±0.15

4.00±0.09

0.5

0.59±0.02

11.77±0.05

58.7±0.11

4.08±0.16

1.0 1.5

0.61±0.01

12.68±0.03

61.0±0.15

4.72±0.10

0.60±0.02

12.18±0.02

59.0±0.08

4.31±0.15

10


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Figure 1. (a) The chemical structures of PEI and three kinds of PF dots, (b) the energy level of inverted solar cells, and (c) J-V characteristics of devices doping without and with different PF dots.

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Figure 2. (a) EQE spectra of the corresponding devices with and without PF dots and EQE enhancement obtained by dividing doped devices by that of the undoped device, (b) light emission of pristine PF1, PF2 and PF3 solution.

Figure 3. The J-V characteristics of devices doping without and with different PF dots in dark.

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Figure 4. (a) J-V characteristics of electron-only device in dark, (b) photocurrent density (Jph) as a function of the effective voltage (Veff) for control and doped devices under constant incident light intensity, (c) the absorption spectra of films including active layer and PEI layer doped with different PF dots, (d) exciton dissociation probability [P(E,T)] plotted with respect to e ective bias (Veff) for these OSCs devices.

Figure 5. The impedance spectra of OSCs devices without and with different Pdots in dark, and inset is the equivalent circuit model.

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For Table of Contents Only

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Efficiency Improvement of Inverted Organic Solar Cells via Introducing

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