Organic Photovoltaic Cells Based on a Medium-Bandgap

Feb 23, 2012 - Recent progress in organic photovoltaic (OPV) cells has led to dramatic progress with a power conversion efficiency as high as 8 to 9%...
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Organic Photovoltaic Cells Based on a Medium-Bandgap Phosphorescent Material and C60 Nana Wang, Junsheng Yu,* Yifan Zheng, Zhiqiang Guan, and Yadong Jiang State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China ABSTRACT: Organic photovoltaic (OPV) cells using a mediumbandgap phosphorescent material of bis(1,2-dipheny1-1H-benzoimidazole) iridium (acetylacetonate) [(pbi)2Ir(acac)] as an electron-donating layer were fabricated. An S-shaped kink was observed in the current−voltage curves with the increase in (pbi)2Ir(acac) thickness. Two ways of using different hole transport layers to reduce interfacial energy step and doping to improve charge carrier mobility were used to investigate the origin of the kink. The results showed that this anomalous feature is due to the presence of large interfacial energy step between the anode/donorinterface and the low hole mobility of (pbi)2Ir(acac). An improved power conversion efficiency of 2.23% under AM 1.5 solar illumination at an intensity of 100 mW/cm2 was obtained using N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine to minimize the energy barrier for charge injection.

1. INTRODUCTION Recent progress in organic photovoltaic (OPV) cells has led to dramatic progress with a power conversion efficiency as high as 8 to 9%.1−3 Because of the potential for low-cost solar energy conversion, light weight, and flexibility, various innovative materials and device concepts have been introduced to improve the device performance.4−6 Among these, utilizing low-bandgap materials to enhance the short circuit current (JSC) has received great attention.7,8 However, to achieve a narrow band gap, the higher highest occupied molecular orbital (HOMO) level often results in a low open-circuit voltage (VOC).9,10 An alternative promising way is to stack two or more single cells absorbing in a complementary wavelength range and use medium band-gap donor material with a low HOMO energy level to achieve a high VOC.11,12 While using a low HOMO energy level material as an electron-donating material (donor) to achieve a high VOC, the device current−voltage characteristic sometimes shows an Sshaped deformation, which is detrimental to the device parameters. Therefore, to illuminate the origin of the kink will be beneficial to increase the device performance significantly. To date, the explanation of the undesirable Skinks is still under debate. The effects are proven by theoretical studies and experimentally include: (i) large interfacial energy step,13,14 (ii) strong interface dipoles,15 and (iii) a strong imbalance of charge carrier mobilities.16 Furthermore, some numeric simulations show that local space charges in multilayer devices are responsible for the S-shaped generation.17 However, because of the lack of systematic investigation of metal/organic interface with a medium-bandgap and low HOMO energy level organic layer, the reason of anomalous S-kinks requires further in-depth investigation. © 2012 American Chemical Society

In this work, we present a phosphorescent material of bis(1,2-dipheny1-1H-benzoimidazole) iridium (acetylacetonate) [(pbi)2Ir(acac)] in OPV cells, which was adopted in phosphorescent organic light-emitting diodes (OLEDs).18 It possesses an optical gap of 2.0 eV and longer exciton lifetime,19 and Shao et al. reported that introducing appropriate organic materials with long exciton lifetime is a very promising way to improve photovoltaic performance.20 The effect of the thickness of (pbi)2Ir(acac) layer on the electrical characteristics of the OPV cells was studied. The origin of the S-kinks was systematically investigated by using different hole-transport layers between the anode and the donor layer and doping method to vary charge carrier mobility. Moreover, a qualitative model was applied to explain such anomalous curves.

2. EXPERIMENTAL SECTION The OPV cells were fabricated on patterned indium−tin oxide (ITO)-coated glass substrates with a sheet resistance of 10 Ω/ sq, which had been cleaned consecutively in ultrasonic baths containing detergent, acetone, ethanol, and deionized water for 10 min each step and finally dried by high-purity nitrogen blow. Prior to loading into a vacuum chamber, the substrates were treated by O2 plasma for 5 min. A thin layer of commercially available aqueous poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) film (Clevios P AI4083) was spin-coated onto the ITO glass with a speed of 3000 rpm for 1 min and then baked at 150 °C for 10 min in ambient. The Received: October 25, 2011 Revised: February 8, 2012 Published: February 23, 2012 5887

dx.doi.org/10.1021/jp210245a | J. Phys. Chem. C 2012, 116, 5887−5891

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synthesis and characterization of (pbi)2Ir(acac) used in the device fabrication were as reported elsewhere,19 and 4,4′,4″tris(3-methylphenylphenylamino) triphenylamine (m-MTDATA) (99.9%, Lumtec), copper phthalocyanine (CuPc) (99.9%, Aldrich), N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl4,4′-diamine (NPB) (99%, Aldrich), 1,1-bis((di-4-tolylamino)phenyl) cyclohexane (TAPC) (99%, Aldrich), pentacene (99.9%, Lumtec), C60 (99.9%, Aldrich), and bathophenanthroline (Bphen) (99%, Fluka) were commercially available. All materials were used without further purification. Organic materials were deposited onto the ITO/PEDOT:PSS substrates successively at a rate of 1 to 2 Å/s at a pressure of 3 × 10−4 Pa, followed by the deposition of Ag cathode at a rate of ∼10 Å/s under a pressure of 3 × 10−3 Pa. The deposition rate and film thickness were in situ monitored using a quartz crystal oscillator mounted to the substrate holder. The typical area of OPV cells is 12 mm2, with 4 mm-wide ITO overlapped by 3 mm-wide Ag film. All electrical measurements were performed in air at ambient circumstance. A light source integrated with a xenon lamp (CHF-XM35, Beijing Trusttech) with an illumination power of 100 mW/cm2 was used as a solar simulator. The current−voltage curves in dark and under simulated AM 1.5G solar illumination were measured with a Keithley 4200 programmable voltage−current source.

Figure 2. J−V characteristics of devices with different (pbi)2Ir(acac) thicknesses under illumination with an intensity of 100 mW/cm2.

Table 1. Short Circuit Current, JSC, Open Circuit Voltage, VOC, Fill Factor, FF, and Power Conversion Efficiency, ηp, of Devices with Different Donor Thicknesses As Determined from the J−V Characterization

3. RESULTS AND DISCUSSION Figure 1 shows the chemical structures of organic materials, the configuration of the device structure, and the schematic energy

donor thickness (nm)

VOC (V)

JSC (mA/cm2)

FF

ηp (%)

5 10 15 20 30

0.79 0.81 0.79 0.80 0.77

3.82 4.64 2.98 2.80 1.87

0.40 0.44 0.40 0.40 0.16

1.20 1.65 0.94 0.91 0.23

of ITO/PEDOT:PSS (30 nm)/(pbi)2Ir(acac) (10 nm)/C60 (40 nm)/Bphen (5 nm)/Ag (150 nm) shows a maximum ηp of 1.65% with JSC = 4.64 mA/cm2, VOC = 0.81 V, and FF = 0.44. However, it is obviously exhibited that this device had a relatively low fill factor and there is a slight S-shaped kink, which shows a local saturation and a later again increasing current around a certain applied voltage region. When the thickness of (pbi)2Ir(acac) layer was further improved, the strength of the S-kink increases, and the JSC and FF decrease significantly. However, the J−V curves under dark display typical diode characteristics, which do not show such kind of Skink feature, and the increase in donor thickness just decreases the injection current in forward bias and changes the device conductivity. One possibility to the S-kink could be the large interfacial energy step between the ITO/PEDOT:PSS anode and the donor material. To scrutinize the reason of this S-shaped kink, several devices with different hole transport materials were fabricated as listed in Table 2. The hole-transport layers are mMTDATA, CuPc, NPB, and TAPC in cells A, B, C, and D, respectively. As shown in Figure 3, it can be seen that after introducing m-MTDATA or CuPc as a hole-transport layer there also exists the S-shape kink. However, when NPB and TAPC were inserted between PEDOT:PSS and (pbi)2Ir(acac), the S-shape of J−V curves vanished. This excludes the interfacial dipoles at the contacts and indicates that the Sshape results from the carrier transport problem because of the hole-injection barrier from the anode to donor layer, as shown in Figure 4. Nelson et al.13 observed that the kink can be removed either by replacing the donor with another of lower HOMO or by replacing the anode with higher work function. Tress et al.14 shows that as soon as the HOMO mismatch does not exceed a value of 0.2 eV, the curve does not show S-kinks. In this work, the hole injection problems at the interface have been solved by using hole transport layer with proper HOMO

Figure 1. Chemical structures of organic materials, configuration of the device structure, and schematic energy level diagram.

level diagram. The ionization energy (HOMO) of (pbi)2Ir(acac) has been determined to be 5.6 eV by cyclic-voltammetry, which shows a potential high VOC for OPV cell. The current density−voltage (J−V) characteristics of the cells with various (pbi)2Ir(acac) thicknesses under illumination with an intensity of 100 mW/cm2 are shown in Figure 2. Short circuit current (JSC), open circuit voltage (VOC), fill factor (FF), and power conversion efficiency (ηp), which derived from the J−V characteristics are listed in Table 1. It can be seen that the device with different donor thicknesses has a VOC up to 0.8 V, which is much higher than that of the CuPc/C60 heterojunction of 0.44 V.21 This shows that the OPV cell using a low HOMO energy level and medium band gap phosphorescent material as donor layer can achieve a high VOC. The device with a structure 5888

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Table 3. Short Circuit Current, JSC, Open Circuit Voltage, VOC, Fill Factor, FF, and Power Conversion Efficiency, ηp of Devices with Different Hole Transport Layers As Determined from the J−V Characterization

Table 2. Structures for the Investigation of S-Kink Devices device

structure

A

ITO/PEDOT:PSS (30 nm)/m-MTDATA (5 nm)/(pbi)2Ir(acac) (20 nm)/C60 (40 nm)/Bphen (5 nm)/Ag (150 nm) ITO/PEDOT:PSS (30 nm)/CuPc (5 nm)/(pbi)2Ir(acac) (20 nm)/ C60 (40 nm)/Bphen (5 nm)/Ag (150 nm) ITO/PEDOT:PSS (30 nm)/NPB (5 nm)/(pbi)2Ir(acac) (20 nm)/ C60 (40 nm)/Bphen (5 nm)/Ag (150 nm) ITO/PEDOT:PSS (30 nm)/TAPC (5 nm)/(pbi)2Ir(acac) (20 nm)/ C60 (40 nm)/Bphen (5 nm)/Ag (150 nm) ITO/PEDOT:PSS (30 nm)/NPB (5 nm)/(pbi)2Ir(acac) (40 nm)/ C60 (40 nm)/Bphen (5 nm)/Ag (150 nm) ITO/PEDOT:PSS (30 nm)/10 wt % pentacene:(pbi)2Ir(acac) (20 nm)/C60 (40 nm)/Bphen (5 nm)/Ag (150 nm) ITO/PEDOT:PSS (30 nm)/(pbi)2Ir(acac):PCBM (100 nm; 1:4) /Bphen (5 nm)/Ag (150 nm)

B C D E F G

hole transport layer

VOC (V)

JSC (mA/cm2)

FF

ηp (%)

m-MTDATA CuPc NPB TAPC

0.78 0.78 0.77 0.77

3.00 2.80 3.12 3.10

0.40 0.37 0.51 0.50

0.92 0.81 1.23 1.12

Figure 5. J−V characteristics of devices in dark and under illumination with an intensity of 100 mW/cm2.

Even though a 5 nm NPB hole transport layer can remove the S-kink of cell C and further improve the optimized device performance, when further increasing the donor thickness of cell C with NPB transport layer, an S-shaped kink is observed again in the structure of ITO/PEDOT:PSS (30 nm)/NPB (5 nm)/(pbi)2Ir(acac) (40 nm)/C60 (40 nm)/Bphen (5 nm)/Ag (150 nm) (cell E), as shown in Figure 6, which should not have

Figure 3. J−V characteristics of devices with different hole transport layers under illumination with an intensity of 100 mW/cm2. Inset: variation around VOC and enlarged abscissa.

Figure 4. Energy level diagram of the photovoltaic device with different hole transport layers. (The work functions of electrodes as well as HOMO and LUMO levels of organic materials are taken from refs 22−26.)

level in cells C and D, and this also favors for the fill factor, which increases from 0.40 to 0.50. The characteristic parameters of these cells are listed in Table 3. The VOC is independent of the choice of hole transport layers because of a spatial separation of electrons and holes.14 It is worth noting that after introducing a 2.5 nm NPB transport layer, the device with 10 nm (pbi)2Ir(acac) shows much better performance in Figure 5. Here the JSC is as high as 4.82 mA/cm2. In particular, a fill factor of 0.60 has been achieved, and the power conversion efficiency increases from 1.65 to 2.23%. This enhancement can be ascribed to the minimization of the energy step between anode and donor, which improves the charge transport and collection. Moreover, this shows that phosphorescent material is another good choice to achieve high performance of OPV cells.

Figure 6. J−V characteristics under illumination with an intensity of 100 mW/cm2 for devices. F:ITO/PEDOT:PSS (30 nm)/10 wt % pentacene:(pbi)2Ir(acac) (20 nm)/C60 (40 nm)/Bphen (5 nm)/Ag (150 nm). G:ITO/PEDOT:PSS (30 nm)/(pbi)2Ir(acac):PCBM (100 nm; 1:4)/Bphen (5 nm)/Ag (150 nm). Inset: J−V characteristics of devices E in dark and under illumination with an intensity of 100 mW/ cm2.

the hole injection problem. This shows that maybe it is not only the large interfacial energy step inducing this S-shaped deformation but also other possible problems arising due to the charge carrier transport in the active layer. The possible candidate for the kink was the low hole mobility of (pbi)2Ir(acac). We measured the mobility of hole-only diodes with the configuration of ITO/PEDOT:PSS (30 nm)/(pbi)2Ir5889

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(acac) (80 nm)/Au (100 nm) using the space-charge limited current theory26

J=

9 V2 εε0μ 3 8 d

(1)

where μ is the mobility, ε ≈ 3 is the relative dielectric constant of the organic film, ε0 is the vacuum dielectric constant 8.85 × 10−12 F/m, and d is the film thickness. Figure 7 presents the J−

Figure 8. Schematic of the S-shape curve, and energy levels of device under short circuit condition and an applied voltage close to VOC, respectively.

diffusion. After this point, injection can take place. Accordingly, in the voltage range between 0.7 and 0.8 V, those holes of cell with large interfacial energy step can pile up at the anode/ (pbi)2Ir(acac)-interface because they are blocked by an energetic barrier of about 0.6 eV. This will reduce the number of injected holes at the anode interface. Likewise, in the case of cell E, due to the low hole mobility of (pbi)2Ir(acac), the electrons start to pile up at the (pbi)2Ir(acac)/C60 interface, and the J−V characteristic approaches the low mobility curve. Therefore, under the condition of both large interfacial energy step and lower carrier mobility, the J−V characteristic is Sshaped. At higher forward bias, the J−V characteristic is dominated by the injected carrier concentration, and the typical exponential increase in diode forward current is re-established.

Figure 7. Current voltage characteristics of hole-only devices, (ITO/ PEDOT:PSS (30 nm)/(pbi)2Ir(acac) (80 nm)/Au (100 nm), ITO/ PEDOT:PSS (30 nm)/(10% pentacene:(pbi)2Ir(acac) (80 nm)/Au (100 nm)) and fits to the space-charge limited expression.

V characteristics of hole-only device. The hole mobility of (pbi)2Ir(acac) film is calculated to be 6.49 × 10−6 cm2/(V s). However, the mobility of C60 is reported to be in the range of 10−2 cm2/(V s).27 This imbalance of charge carrier mobilities can cause S-kink.16 Improving the charge mobility of donor layer is chosen to prove this point. Pentacene is one of a few organic semiconductors that has high carrier mobility, and the reported field-effect carrier mobility for pentacene is 0.35 cm2/(V s).28 Doping a high carrier mobility material is widely used to improve the carrier transport. In this work, by doping pentacene (10 wt %) into (pbi)2Ir(acac), the device F also shows no S-kink. This leads to a fill factor of 0.54, which is even higher than that of cells C and D with reduced the interfacial energy step, and as shown in Figure 7, the hole mobility of pentacene (10 wt %)/(pbi)2Ir(acac) film is observed to be 1.84 × 10−4 cm2 V−1 S−1. This suggests that the low carrier mobility is another reason causing the S-kink of (pbi)2Ir(acac)/C60 device. In the future, the device efficiency can be further improved by phosphorescent materials with better carrier mobility. Moreover, in the bulk heterojunction of cell G, there is also no S-kink. However, the VOC is only 0.56 V, which is not much high for bulk heterojunction cells because it is difficult to avoid that the components interconnect partially both electrodes, leading to a loss of voltage.29 This is consistent with the report that the imbalance carrier mobilities cannot induce an Skink in a bulk heterojunction,16 and it further excludes interface dipoles as the factor for the S-kink. To verify the origin of the S-shape curve, the drift-diffusion model was analyzed in Figure 8. Under the short circuit situation, the charge carriers generated at the interface of (pbi)2Ir(acac)/C60 can be extracted by the internal field assisted by diffusion. The charge carrier transport to the contacts is barrier-free. While increasing the applied voltage around the built-in potential (Vbi), the charge carriers are extracted only by

4. CONCLUSIONS In summary, we have investigated the influence of a mediumbandgap phosphorescent material of (pbi)2Ir(acac) on the performance of OPV cells. It was found that an S-shaped kink occurred in the current-density versus voltage curve close to VOC at the optimized thickness of (pbi)2Ir(acac) layer. We investigated the reasons for the anomalous S-kink by inserting hole-transport layers and improving the charge transport. Because of the large interfacial energy step between the anode/ donor-interface and the low hole mobility of donor material, charges will pile up, creating a space charge, which is able to create an S-shape under extraction and transport conditions and lead to loss of fill factor significantly. The results showed that minimizing injection barrier and increasing hole mobility are critical ways to improve device performance of OPV cells based on medium-bandgap donor material with low HOMO level. Moreover, further designing of phosphorescent material with higher mobility could result in better device performance.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-28-83207157. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (NSFC) (grant no. 61177032), the Foundation for 5890

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Innovative Research Groups of the NSFC (grant no. 61021061), the Fundamental Research Funds for the Central Universities (grant no. ZYGX2010Z004), SRF for ROCS, SEM (grant no. GGRYJJ08-05), and Doctoral Fund of Ministry of China (grant no. 20090185110020).

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REFERENCES

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