Polymer Hybrid Photovoltaic Devices

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Improved Performance of ZnO/Polymer Hybrid Photovoltaic Devices by Combining Metal Oxide Doping and Interfacial Modification Olympia Pachoumi,† Artem A. Bakulin,‡ Aditya Sadhanala,† Henning Sirringhaus,† Richard H. Friend,† and Yana Vaynzof*,†,§ †

Cavendish Laboratory, University of Cambridge, JJ Thomson Ave, Cambridge CB3 0HE,U.K. FOM Institute AMOLF, Science Park 104, Amsterdam, The Netherlands § Centre for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 227, Heidelberg, Germany ‡

S Supporting Information *

ABSTRACT: Photoinduced charge separation at hybrid organic−inorganic interfaces is poorly understood and challenging to control. We investigate charge separation at a model system of ZnO/poly(3-hexylthiophene) (P3HT) and employ Sr doping of ZnO and phenyl-C61-butyric acid (PCBA) self-assembled modification to study and enhance the charge separation efficiency. We find that doping alone lowers the efficiency of charge separation due to the introduction of defect states at the oxide surface. However, with the combination of doping and molecular modification, charge separation efficiency is significantly enhanced due to the passivation of interfacial traps and improved modifier coverage. This demonstrates a complex noncumulative effect of doping and surface modification and shows that with the correct choice of metal oxide dopant and organic modifier, a poorly performing hybrid interface can be turned into an efficient one. devices with ZnO as an electron transport layer,20 and use a PCBA self-assembled monolayer as the interfacial modifier. The device architecture and materials’ chemical structures are shown in Figure 1. First, we examine the effect of Sr doping on the charge separation at the interface with P3HT and correlate it to the photovoltaic performance of the solar cells. Next, we demonstrate that by modifying the doped oxide films we can significantly enhance the efficiency of charge separation. The effect of both doping and surface modification demostrated a complex noncumulative character resulting in a promising power conversion efficiency of 0.4% for a simple bilayer device architecture.

1. INTRODUCTION Hybrid organic−inorganic photovoltaic devices have drawn significant interest from the scientific community due to the possibility to utilize the advantages of both organic and inorganic materials.1,2 Typically, these devices consist of a conjugated polymer as a donor material and a metal oxide as the electron acceptor. The metal oxide acceptor can be easily processed and forms a variety of nano structures.3−7 The organic counterpart can then be introduced to backfill the oxide nano structure or to form a bilayer with a planar oxide layer. Upon photoexcitation, excitons created on the donor polymer chains diffuse to the organic−inorganic interface, where charge separation can occur. Holes remain on the donor polymer forming a polaron, and electrons are transferred to the acceptor metal oxide. Recently, we have reported that, in the model P3HT/ZnO system, over half of the excitations result in bound charge pairs.8 These electron−hole pairs arise from trapping of the electron in a surface state of the inorganic material, which in turn coulombically attracts the hole polaron on the polymer chain.9,10 We and others11−16 demonstrated that interfacial modifiers can be used to functionalize the oxide surface in order to enhance charge separation at the interface. Alternatively, doping of the metal oxide has also been reported as a possible means to improve photovoltaic device performance.17−19 Here, we combine these approaches and investigate the effects of ZnO doping and surface modification on charge separation and device performance. We study the influence of Sr as a dopant in ZnO, as it was recently reported that it has beneficial effects on device stability of bulk heterojunction inverted photovoltaic © 2014 American Chemical Society

2. EFFECT OF SR DOPING CONCENTRATION ON THE PHOTOVOLTAIC PERFORMANCE OF ZNO/P3HT SOLAR CELLS The photovoltaic performance of ZnO/P3HT solar cells with varying degrees of Sr doping (0% up to 8%) is summarized in Figure 2. Figure 2a shows that the open circuit voltage (Voc) of the devices increases with increasing doping % from an average of 0.36 V for undoped ZnO/P3HT solar cell to 0.58 V for 8% doping. Figure 2b shows that the Sr doping lowers the short circuit current (Jsc) of the devices by approximately 20%, suggesting that the long-range charge separation at the ZnSrO/ P3HT interface is inferior to that at the ZnO/P3HT interface. Received: June 24, 2014 Revised: July 16, 2014 Published: July 24, 2014 18945

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in an increased energetic difference between the highest occupied orbital (HOMO) of the donor and the conduction band of the oxide, and thus, increased Voc. This approach was demonstrated for Mg-doped ZnO by Olson et al., where the authors achieved a 0.4 V increase in the Voc through the engineering of the conduction band offset.21 Another possible cause for enhanced Voc is a decrease in the dark current of the device22 which may arise from a decrease in conductivity of the metal oxide with doping. To investigate if the improvement of Voc in the case of Sr doping arises from shifts in the conduction band of the metal oxide, we performed ultraviolet photoemission spectroscopy (UPS) measurements, which in conjunction with optical absorption measurements of the optical gap of the doped and undoped metal oxides, can be used to reconstruct the energy level alignment between the metal oxide and the P3HT polymer (Figure 3a). We find that the work function of both

Figure 1. Chemical structure of zinc bis(methoxyethoxide), strontium isopropoxide, poly[3-hexylthiophene] (P3HT), and phenyl-C61butyric acid (PCBA). Bottom: photovoltaic diode architecture.

Figure 3. (a) Ultraviolet photoemission spectrum of the valence band of ZnO and ZnSrO (energy level diagrams are shown in the inset), (b) two-point probe in-plane conductivity measurements on ZnO and ZnSrO (inset depicts measurement architecture), and (c) dark current I−V measurements of P3HT/ZnO and P3HT/ZnSrO photovoltaic devices.

the undoped and Sr-doped ZnO films is equal to 3.6 eV. The valence band position is found to be 3.2 eV below the Fermi level for both films. UV−vis measurements reveal that the optical gaps of ZnO and ZnSrO are 3.25 and 3.3 eV, respectively (see Supporting Information, Figure S1). These optical gap values are a good approximation for the transport gap (exciton binding energy in these materials is ∼60 meV)23 and can be used to estimate the position of the conduction band. In the case of ZnO the conduction band coincides (within the experimental error, 0.1 eV) with the position of its Fermi level. Such behavior is consistent with the degenerate ntype nature of ZnO. The conduction band of ZnSrO is estimated to be only 0.05 eV above that of ZnO and 0.1 eV above the Fermi level. However, since the measured increase in Voc for 6% Sr doping of ZnO is 0.2 V, we find that the small shift in the position of the conduction band cannot be the main reason for the increased Voc. Next, we investigated if the increased Voc is a result of reduced conductivity of ZnSrO. Figure 3b shows the semilog current−voltage two-point probe conductivity measurements

Figure 2. (a) Open circuit voltage (Voc), (b) short circuit current (Jsc), (c) fill factor (FF), and (d) power conversion efficiency (PCE) of photovoltaic devices with varying % of Sr doping.

Increasing the doping % further resulted in a drastic decrease of the Jsc due to the low conductivity of ZnSrO at these high doping concentrations.20 Figure 2c shows no specific trend in the fill factor (FF) of the devices as their FF remains largely comparable to that of undoped ZnO/P3HT devices, with only a small increase. The power conversion efficiency (PCE) is presented in Figure 2d and shows that the overall efficiency can be increased by nearly 50% and its maximum value is achieved for 6% Sr doping. The remainder of the study is performed using this optimal doping concentration. 2.1. Origin of the Increased Open Circuit Voltage. An enhancement of the open-circuit voltage in doped ZnO/ polymer solar cells can arise from several reasons. For example, an upward shift of the conduction band upon doping will result 18946

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(measured between two planar Al electrodes; W = 3 mm, L = 100 μm) on ZnO and ZnSrO, with an almost 2 orders of magnitude decreased current for the doped films. We interpret this to be a result of the reduced intrinsic charge carrier concentration in ZnSrO, similarly to what has been observed for Mg-doped ZnO.24 This results in a significant reduction of the dark current, as shown in Figure 3c. Thus, we conclude that the origin of the improved Voc is a reduction in the device dark current due to decreased metal oxide conductivity. 2.2. Origin of the Decrease in Short Circuit Current. In order to investigate the origin of the reduction in Jsc, we performed pump−push photocurrent (PPP) measurements25,26 and compared the relative amount and recombination dynamics of bound charge pair (BCP) states at the ZnSrO/P3HT interface to that of ZnO/P3HT (Figure 4a). This technique

act as trap sites for electrons and hamper charge separation at the interface of both oxide systems.27 To conclude, the increased amount of surface local lowenergy states at the ZnSrO surface results in an increased amount of bound charge pairs at the interface and a decrease in short circuit current.

3. INTERFACIAL MODIFICATION Recently, we and others demonstrated that self-assembled monolayers (SAMs) can be used to modify the oxide surface and significantly improve charge separation at the interface.11−13,28 This is achieved through the multiple beneficial effects of the SAM, such as control of energy level alignment, trap passivation, increased electron−hole separation, and other factors. We utilize PCBA as a surface modifier, which we previously demonstrated can halve the amount of bound charge pairs at the organic−inorganic interface. This significantly increases the Jsc of the ZnSrO/P3HT devices. 3.1. Photovoltaic Performance of PCBA-Modified Devices. Figure 5 compares the photovoltaic performance of

Figure 4. (a) Pump−push photocurrent spectroscopy measurements on P3HT/ZnO and P3HT/ZnSrO photovoltaic devices, (b) photothermal deflection spectroscopy measurements on ZnO and ZnSrO, and X-ray photoemission O 1s spectra of (c) ZnO and (d) ZnSrO.

optically activates and probes in real time the formation and recombination of BCPs which, as we proposed previously, are related to the existence of defect states on the surface of the metal oxide. We find that the lifetime of the BCP states is almost the same for ZnSrO and ZnO acceptors. This indicates the molecular origin of the trapping sites is the same and Sr does not introduce qualitativily a new type of interfacial defects. At the same time, the relative amount of BCP states over free charges is higher at the ZnSrO/P3HT interface. This indicates that the density of interfacial trap states increases, resulting in less efficient charge separation at that interface. To compare the amount of subgap states in ZnO and ZnSrO, we employ photothermal deflection spectroscopy (PDS) which is an ultrasensitive absorption measurement technique. PDS measurements reveal that the subgap absorption of ZnSrO is higher than that of ZnO (Figure 4b). The density of subgap states measured by UPS is also higher for ZnSrO (Figure 3a), consistent with PDS measurements. We further investigated the surfaces of ZnO and ZnSrO by means of X-ray photoemission spectroscopy (XPS) which is able to reveal compositional changes at the surface of the oxide. Figure 4, c and d shows the O 1s spectra on ZnO and ZnSrO, respectively. The spectra show that the high binding energy peak at 532.4 eV, assigned to hydroxyl (OH) groups, is significantly higher for ZnSrO than for ZnO. These groups can

Figure 5. (a) Open circuit voltage (Voc), (b) short circuit current (Jsc), (c) fill factor (FF), and (d) power conversion efficiency (PCE) of photovoltaic devices with and without PCBA modifier.

unmodified and PCBA-modified devices. Although the Voc of the devices is slightly reduced upon modification, the value for P3HT/PCBA/ZnSrO devices is still significantly improved as compared to P3HT/PCBA/ZnO. This further confirms that this improvement originates from the reduced conductivity of ZnSrO and not from the interfacial energetics, which in this case is largely determined by the PCBA monolayer. The short circuit current of the PCBA-modified devices is significantly improved (Figure 5b), consistent with previous reports. Surprisingly, the Jsc of the P3HT/PCBA/ZnSrO is 30% higher than that of P3HT/PCBA/ZnO devices. This suggests that the charge separation at the modified interface is superior to that of the modified P3HT/ZnO interface. The overall power conversion efficiency is tripled for the ZnO devices.7 On the other hand, the power conversion efficiency of the P3HT/ PCBA/ZnSrO devices has increased nearly 6 times, with the best device showing PCE of 0.4%. 3.2. Efficiency of Interfacial Charge Separation of PCBA-Modified Devices. From these results, it appears that upon modification the efficiency of charge separation at the 18947

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the absorption features of N719 when formed on ZnSrO, consistent with our PDS results for PCBA. It is interesting that the increased amount of OH groups at the oxide surface which hampered the charge separation at the P3HT/ZnSrO is likely to be responsible for the improved PCBA surface coverage of the oxide, which makes its charge separation efficiency to be superior to that of ZnO. This shows that interfacial modifiers can be used to control the electronic structure of disordered interfaces and turn their unfavorable properties to an advantage.

ZnSrO surface was improved more than for ZnO. We performed pump−push measurements on PCBA- modified devices, which are shown in Figure 6a together with the

4. CONCLUSION To summarize, we demonstrate that, with a combined approach of metal oxide doping and surface modification, it is possible to control and enhance the photoinduced charge separation properties of a metal oxide/conjugated polymer interface. We report a promising power conversion efficiency of 0.4% for a bilayer configuration of ZnO/poly(3-hexylthiophene) (P3HT). This approach could be extended to nanostructured metal oxide systems and used to further increase the efficiency of hybrid photovoltaic devices.

Figure 6. (a) Pump−push photocurrent spectroscopy measurements on P3HT/ZnO and P3HT/ZnSrO photovoltaic devices with and without PCBA modifier, (b) photothermal deflection spectroscopy measurements on ZnO and ZnSrO with and without PCBA modifier, and (c) ultraviolet photoemission spectra of ZnO/PCBA and ZnSrO/ PCBA.

5. EXPERIMENTAL SECTION Photovoltaic Device Fabrication and Characterization. ITO-coated glass substrates were cleaned by successive sonication in acetone and isopropanol, followed by oxygen plasma treatment. Zinc bis(methoxyethoxide), Zn(OC2HOCH3)2, was used for ZnO deposition and strontium isopropoxide, Sr(OC3H7)2, for doping (provided by Multivalen Ltd.). This was done by spin-coating of a precursor solution or a mixture of two different precursor solutions onto the substrates under nitrogen atmosphere, followed by heat annealing at 120 °C for 5 min. The samples were then annealed in air at 300 °C for additional 60 min. The substrates were modified with a SAM of PCBA (Solenne) by immersing the samples for 30 min into an anhydrous cholorobenzene (CB) solution 0.1 g/L of the SAM. The samples were then flushed three times with CB to remove excess PCBA. P3HT (Rieke Metals) polymer was dissolved in anhydrous CB at 70 °C overnight and spin-coated onto the oxide substrates. The samples were then transferred to a thermal evaporation chamber for WO3 (10 nm) and Ag (80 nm) deposition under high vacuum (1 × 10−6 mbar). Finally, the samples were postannealed at 130 °C for 15 min. To measure the I−V curves of the devices under AM1.5 conditions, an Oriel 81160-1000 solar cell simulator was used. In order to obtain reliable data, a spectral mismatch correction was carried out using a calibrated and certified inorganic solar cell. Ultraviolet and X-ray Photoemission Spectroscopy (XPS and UPS). Samples for photoemission spectroscopy were fabricated on Si substrates and transferred into an ultrahigh vacuum chamber (ESCALAB 250Xi). XPS measurements were carried out using a XR6 monochromated Al Kα source (1486.6 eV) and a pass energy of 20 eV. UPS measurements were performed using a double-differentially pumped He gas discharge lamp emitting He I radiation (hv = 21.22 eV) with a pass energy of 2 eV. Atomic Force Microscopy (AFM). Films for AFM were prepared in identical fashion to those prepared for device fabrication. AFM was performed in tapping mode using a Digital Instruments Nanoscope IIIa microscope.

previously measured unmodified devices for comparison. In the case of ZnO, the relative amount of BCP states is significantly reduced by the introduction of the PCBA monolayer, consistent with previous results. In the case of ZnSrO, the amount of BCP states is almost negligible, confirming that the charge separation at this interface is very efficient. Improvements in Jsc may arise from increased surface roughness of the ZnSrO surface which would effectively increase the surface area for charge separation. Another reason might be improved coverage of the PCBA monolayer on ZnSrO. We performed atomic force microscopy (AFM) measurements to characterize both surfaces (see Supporting Information, Figure S2) and found that the surface roughness of ZnSrO is 0.9 nm, lower than that measured for ZnO (1.2 nm). This means that the surface area for charge separation is very similar for both oxides and the improved Jsc cannot be attributed to the oxide surface roughness. Additionally, increased surface area in ZnSrO would have resulted in increased Jsc for unmodified devices, which as discussed above is actually descreased. On the other hand, PDS measurements revealed increased abroption from PCBA/ZnSrO as compared to PCBA/ZnO (Figure 6b) as the PCBA monolayer weakly absorbs in this range. UPS measurements also show a slight increase in the PCBA valence band features (Figure 6c). This suggests that PCBA forms a more complete monolayer on ZnSrO than on ZnO. It is possible that this is achieved due to the increased amount of OH groups on the oxide surface.29 To confirm that increased coverage of another monolayer can be achieved on ZnSrO, we chose to investigate N719 dye, which is similar in size to PCBA and binds to the oxide surface with the same functional group (COOH). As the absorption of N719 is significantly stronger than that of PCBA, we measured UV−vis spectra to compare the coverage of ZnO versus ZnSrO (see Supporting Information, Figure S3). We find a slight increase in 18948

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UV−Vis Absorption. UV−vis absorption was measured using a Hewlett-Packard 8453 UV−vis spectrometer with a 280−1100 nm spectral range. Photothermal Deflection Spectroscopy (PDS). Samples prepared in an identical fashion to the PV device preparation were spun onto spectrosil quartz slides (which were cleaned with acetone, isopropanol, and water followed by a 10 min oxygen plasma etch). The samples were then immersed into a cuvette (with optically accessible windows) containing an inert liquid Fluorinert FC-72. The PDS measurements were performed using a chopped monochromatic light illuminating the sample and CW laser beam (670 nm) passing through the refractive index gradient near the sample surface producing a deflection proportional to the absorbed light at that particular wavelength. The deflection is measured using a position sensing detector and read by a lock-in detector and is proportional to the absorption in the sample. Conductivity Measurements. ZnO and ZnSrO films were deposited on glass and transferred to a thermal evaporation chamber for Al (50 nm) deposition of T-bar terminals (W/L = 3 mm/100 μm) under high vacuum (1 × 10−6 mbar). The I−V characteristics were measured using a Keithley 2635 Source Measure Unit (SMU). Pump−Push Photocurrent Spectroscopy. A regenerative 1 kHz Ti:sapphire amplifier system (Coherent, Legend Elite Duo) was used to pump a broadband noncollinear optical amplifier (Clark) and a 3-stage home-built optical parametric amplifier (OPA) to generate visible pump pulses (∼540 nm central wavelength, ±20 nm bandwidth) and infrared push pulses (2000 ± 100 nm), respectively. ∼1 nJ pump and ∼1 μJ push pulses were focused onto a ∼1 mm2 spot on the device. The reference photocurrent from a photodiode was detected at a pump repetition frequency of 1 kHz by a lock-in amplifier. The push beam was mechanically chopped at ∼370 Hz, and its effect on the photocurrent was also detected by a lock-in amplifier. Push pulse by itself also induced minor current in the device due to absorption from subgap states and long-lived trapped charges. This current component was delay independent and was subtracted from the presented measurements.

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Engineering and Physical Sciences Research Council and the A.G. Leventis Foundation for financial support.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis absorption spectra of ZnO and ZnSrO (Figure S1); atomic force microscopy images of ZnO and ZnSrO surface (Figure S2); UV−vis absorption spectra of ZnO/N719 and ZnSrO/N719 (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.A.B. acknowledges a VENI grant from The Netherlands Organization for Scientific Research (NWO). O.P. thanks the 18949

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