Properties of interlayer for organic photovoltaics

Materials Today  Volume 16, Number 11  November 2013

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Properties of interlayer for organic photovoltaics Tzung-Han Lai, Sai-Wing Tsang, Jesse R. Manders, Song Chen and Franky So* Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA

Interfacial materials play an important role in determining the efficiency of an organic photovoltaic (OPV) cell. They are not only responsible for establishing ohmic contact, but also determining different device parameters such as the internal electric field, the film morphology, and the carrier recombination rate which are important to the device performance. Here, we will present the material properties and requirements for these interlayers used in high efficiency OPV cells. This paper aims to reveal the different roles of interlayers, introduce techniques for characterizing their properties, and provide an insight into the future development of novel interlayers for high efficiency organic photovoltaic cells.

1. Introduction Organic photovoltaic (OPV) cells have been a topic of research focus in recent years as they are a low-cost renewable energy source due to their compatibility with large-scale, flexible, and high throughput roll-to-roll production [1]. Over 10% power conversion efficiency (PCE) has been reported in both small molecular and polymeric solar cells. These results demonstrate the potential of OPVs to reduce the cost per kWh and make it more favorable for practical applications. Recent progress in OPVs has been driven by the development of new donor–acceptor photoactive materials and novel device architectures. The PCE of single junction OPV cells has reached 8% for both small molecular and polymeric solar cells [2–7]. Recently, He et al. reported that with the inclusion of an interlayer, poly [(9,9-bis(30 -(N,N-dimethylamino)propyl)-2,7fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN), the efficiency of single junction polymeric cells reached 9.2% [8]. Utilizing both low and high bandgap photoactive materials to improve the light harvesting efficiency, the PCEs of multi-junction cells have been reported to reach 10.6% for polymer cells [9,10] and 12% for small molecule cells [11]. Interface engineering has been a useful approach to facilitate carrier extraction to enhance OPV performance. Review articles by Steim et al. and by Chen et al. on interface engineering discuss different types and functions of interfacial materials [12,13]. However, understanding the physical mechanism as well as choosing *Corresponding author:. So, F. ([email protected])

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the proper techniques to characterize the interlayer properties have not been addressed in previous reviews. Therefore, in this article, we aim to discuss the key properties of the interlayers which have a direct impact on device performance along with the characterization techniques used to probe these properties. In inorganic semiconductor devices, differential doping is a common approach to make ohmic contacts. However, it is difficult to implement this approach in polymeric solar cells and a different strategy is used to extract charge carriers from the photoactive layer. A common approach is to insert a charge extracting interfacial layer between the BHJ layer and the electrode to facilitate efficient charge extraction. Specifically, an electron extracting layer (ETL) and a hole extracting layer (HTL) are adopted at the cathode and the anode, respectively, as shown in Fig. 1, for both conventional and inverted device architectures. The selection of interlayers has a significant effect on the performance of OPVs. Table 1 gives a summary of recent data on inverted poly(3-hexylthiophene) (P3HT) cells using various interlayers. Using the same photoactive layer with a blend of P3HT:[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) as an active layer, the PCE varies from 2.3% to 4.2% depending on the interlayers used. A number of interlayer materials have been studied extensively and they can be categorized as follows.

1.1. Low work function metals Low work function alkali earth metals (e.g. Ca, Ba, Mg) and alkali metal compounds such as cesium carbonate (Cs2CO3) and lithium

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FIGURE 1

Schematic diagram for conventional (a) and inverted (b) OPV device structure. Electron transporting layer (ETL) and hole extracting layer (HTL) is adopted to form ohmic contact and extract charges at cathode and anode respectively. The arrows indicate the direction of electron and hole transport.

fluoride (LiF) have been used as the cathode interlayer to reduce the electrode work function for more efficient electron extraction. Such interlayers also serve as a buffer layer to prevent cathode materials, such as aluminum or silver, from penetrating into the organic layer during the thermal evaporation process. This material system has been extensively studied and used in the field of organic light emitting diodes (OLEDs) [14–17] and has become a standard cathode used in OPVs [18–20].

1.2. Metal oxides Due to their air stability, optical transparency, and ease of synthesis, various transition metal oxides have been utilized extensively for interlayers used in OPVs. A comprehensive review regarding the metal oxide interlayers in polymeric solar cells was reported by Chen et al. [7] Proper selection of electron-extracting and holeextracting metal oxides is determined by considering the energy level of their conduction band and valence band as an electron or a hole extraction layer, respectively. Hole extracting materials such as V2O5 [21–24], MoO3 [22,25,26], WO3 [27–29] and NiO [30–35] have been demonstrated to have the potential to replace PEDOT (poly(3,4-ethylenedioxythiophene)) doped with PSS (poly(styrene sulfonate)) (PEDOT:PSS) as an anode interlayer [22]. Electron extracting semiconducting oxides such as TiOx [36–41] and ZnO [6–8,42–45] serve as excellent cathode contacts for both conventional and inverted devices.

1.3. Organic interlayers PEDOT:PSS has been used extensively as a solution-processed hole extracting interlayer or transparent electrode for its good conductivity and transparency [46–55]. PEDOT:PSS has a high conductivity,

which reduces losses due to the series resistance effects. However, one major disadvantage is the acidic nature of PEDOT:PSS which corrodes the underlying indium tin oxide (ITO) electrodes and causes cell degradation [56]. Alternative p-type interfacial layers such as sulfonated poly(diphenylamine) [52,53], polyaniline (PANI) [49,55] and polyaniline–poly(styrene sulfonate) (PANI–PSS) [50] have also been reported to give comparable performance as PEDOT:PSS. Small molecular materials such as bathophenanthroline (BPhen) [57] and bathocuproine (BCP) [58–61] are also used for transporting electrons and blocking holes and thus are good candidates for electron extracting organic interlayers.

1.4. Self-assembled monolayers (SAMs) High open-circuit voltage in OPVs requires an anode with a large work function to match the highest occupied molecular orbital (HOMO) of the donor polymer. In many cases, the work function of PEDOT:PSS is not sufficient and modification of the electrode work function is required to facilitate efficient hole extraction. This can be realized by introducing a SAM interlayer with an intrinsic dipole at the electrode interface to control the interface energies. SAMs with electron withdrawing groups lead to protonation of the ITO surface which facilitates the formation of an interfacial dipole pointing away from the electrode and increases the work function of the electrode. On the other hand, it should be noted that SAMs with electron donating groups form dipoles in the opposite direction and therefore decrease the electrode work function to be used for cathode contacts without using alkaline metal compounds [62,63]. SAMs are also used in combination with functional metal oxides such as TiO2 and ZnO. ZnO/metal interface modification has been

TABLE 1

Summary of relevant improvements in device performance for the inverted device structure. The data from Ref. [19] was measured at 130 mW/cm2 and all others at 100 mW/cm2. Adapted from Ref. [97]. Copyright 2009, Wiley-VCH. Device structure

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

Reference

ITO/Cs2CO3/P3HT:PCBM/V2O5/Al ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag ITO/TiOx/P3HT:PCBM/PEDOT:PSS/Au ITO/PTE/TiOx/P3HT:PCBM/PEDOT:PSS/Ag ITO/ZnO(NPs)/P3HT:PCBM/PEDOT:PSS/Ag FTO/TiO2/P3HT:PCBM/PEDOT:PSS/Au ITO/annealed-Cs2CO3/P3HT:PCBM/V2O5/Al

8.42 11.22 9.0 10.2 11.17 12.40 11.13

0.56 0.556 0.56 0.56 0.623 0.641 0.59

62.1 47.5 62 64 54.3 51.1 63

2.25 2.58 3.1 3.6 3.3 4.07 4.19

[19] [44] [38] [41] [45] [40] [21] 425

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FIGURE 2

Energy alignment at the (a) ITO/AlPcCl and (b) ITO/MoO3/AlPcCl interfaces. Reprinted with permission from Ref. [26]. Copyright 2006, American Institute of Physics.

reported by adopting series of SAMs containing benzoic acid (BA); it was shown that SAMs with a negative dipole moment decrease the barrier at the Al/ZnO interface, resulting in enhanced performance [64]. In addition to interface energies, SAMs can also change the wettability of the substrate surface. Changes in wettability can affect the film morphology which subsequently results

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in changes in the charge separation efficiency and the charge transport properties [65,66]. To further understand how interlayers affect the OPV performance, the PCE, which is the product of short circuit current (JSC), open circuit voltage (VOC) and fill factor (FF), needs to be examined carefully. VOC is determined by the energy level alignment between the photoactive donor materials and fullerenes. JSC is determined by the light harvesting efficiency and the charge separation efficiency under a high extraction field near the short circuit condition. FF is determined by the device series resistance, dark current and the charge recombination rate and extraction efficiency at low internal fields close to the open-circuit condition. The interlayer at the electrode/organic interface not only modifies the built-in potentials but affects the extraction efficiency, leading to a reduction in recombination rate and hence a higher charge collection efficiency. In this paper, we will discuss the impact of interlayers on injection barriers, built-in field, surface energy and surface charge recombination in OPV cells. Examples of experimental techniques such as photoelectron spectroscopy, electro-absorption spectroscopy (EA), water contact angle and transient photocurrent measurements to characterize the interface properties will be presented.

FIGURE 3

(a) Schematic setup for electroabsorption (EA) spectroscopy. First harmonic EA signal is measured with a lock-in amplifier referenced at an AC modulation frequency and then measured as a function of DC bias (VDC) with fixed small ac modulation (VAC). Monochromatic light is constantly incident on the deivce, (b) photo J–V curves with inverted DTG-TPD:PC71BM with UV-ozone treated ZnO–PVP nanocomposite films as ETL for various treatment time (as-prepared, 5, 10, 20, 30 min) under initial AM 1.5 G solar illumination at 100 mW cm2. Reprinted with permission from Ref. [6]. Copyright 2012, Nature publishing group, and (c) normalized first harmonic electroabsorption spectroscopy signal variation with respect to DC bias (VDC) for device with structure ITO/ZnO–PVP/P3HT/ Al with ZnO–PVP, ‘‘as annealed’’, and ‘‘UVO treated for 10 min’’. ZnO–PVP energy evolution is illustrated in the inset. 426

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2. Interlayers to control the electrode-polymer energy alignment The selection of interfacial layers to form ohmic contacts is important for OPVs as it is difficult to form ohmic contacts via gradient doping, as is done in inorganic solar cells. Ohmic contact not only reduces the energy barrier height for efficient charge extraction at the metal/organic interface, but also maximizes the VOC with an optimum built-in potential in the resulting OPV cells. For cells with ohmic contacts, VOC is governed by the difference between the lowest unoccupied molecular orbital (LUMO) of the fullerene acceptor and the highest occupied molecular orbital (HOMO) of the polymer donor [67]. Reduction of built-in potential will reduce the electric field strength, leading to an increase in carrier recombination and a reductions in VOC as well as JSC and FF, and hence an overall reduction of PCE. Ohmic contact at the organic/metal interface can be achieved by interface engineering. Typical approaches are to insert an interface layer to match the interface energy alignment or insert a dipole layer at the metal–organic interface. Low work function metals and its related components are the most common materials to form favorable energy alignment [18]. Metal oxide and transition metals are also favorable materials for energy alignment at the interface especially in inverted devices [68,69]. PEDOT:PSS is the most commonly used HTL to form ohmic contacts at the anode [46–55]. Modification of the electrode work function is also used to adjust the barrier height by introducing interfacial dipoles using SAMs [64,70]. To study the energy band structure evolution modified by the interfacial layer, ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) have often been used [63]. For example, small molecule solar cells with two different interlayers, aluminum phthalocyanine chloride (AlPcCl) and molybdenum oxide (MoO3) were investigated [26]. When adopting MoO3 instead of AlPcCl for OPV with the structure ITO/AlPcCl or MoO3/AlPcCl:C60/C60/LiF/Al, VOC increases from 0.79 V to 0.83 V and FF increases from 40% to 47.7% which leads to an increase in PCE from 1.78% to 2.24%. To understand the origin of the enhancement, the energy level evolution between different interlayers was studied by UPS (Fig. 2). For devices with AlPcCl as an interlayer, a vacuum level shift of 0.2 eV in the device was observed upon deposition of the first monolayer (0.2 nm thick) of AlPcCl and then it remained constant with further deposition. As for MoO3 as an interlayer, the valence band maximum of the MoO3 layer is 2.53 eV below the Fermi level along with a 2 eV shift in the vacuum level. Inclusion of MoO3 leads to a shift in the HOMO level of the AlPcCl layer, where the band bending extends to about 10 nm into the AlPcCl layer and leading to formation of a built-in field at the MoO3/AlPcCl interface. Enhanced hole extraction due to formation of this built-in field results in a reduction of the cell series resistance and carrier recombination, leading to an increase in both VOC and FF [26].

3. Interlayers to control the built-in electric field The built-in electric field of OPV cells affect the charge extraction efficiency and can be optimized by modifying the electrode work function using interlayers. Reduction of the built-in potential will reduce the electric field strength, leading to an increase in carrier recombination and a reductions in VOC as well as JSC and FF, and

hence an overall reduction of PCE. Common materials such as SAMs as well as low work function metals have been reported to increase the built-in potential and VOC [18,62]. Recently, PFN has been reported by He et al. as a polymer interlayer to modify the ITO work function and increase the built-in potential OPV cell [8]. More recently, insertion of a ferroelectric interlayer by Langmuir– Blodgett technique has also been reported to control the built-in electric field of an OPV cell [71,72]. To measure the built-in potential of a device, electroabsorption (EA) spectroscopy is one of the most commonly used techniques because of its non-invasiveness to probe the energies in OLEDs [73–78] as well as OPV cells [79–84]. Chemical reactions at the metal/organic interfaces could lead to the formation of interface dipoles which may lead to a difference in the built-in electric field as estimated by the difference in the electrode work function measured by photoelectron spectroscopy. On the other hand,

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FIGURE 4

(a) Illuminated J–V characteristics of solar cells comprising of NiO or PEDOT:PSS with device structure of ITO/NiO or PEDOT:PSS/DTGTPD:PC71BM/LiF/Al. The solar cells with NiO outperform those with PEDOT:PSS due to a higher fill factor and short circuit current. Reprinted with permission from Ref. [35]. Copyright 2013, Wiley-VCH, (b) external quantum efficiency for device with NiO or PEDOT:PSS as HTL. Reprinted with permission from Ref. [35]. Copyright 2013, Wiley-VCH, (c) and (d) images of water contact angle measurement on top of glass substrate coated with (c) 5 nm solution-processed NiO and (d) PEDOT:PSS. The result shows that the average contact angles are 29.3  2.88 for NiO and 12.5  1.48 for PEDOT:PSS. Reprinted with permission from Ref. [35]. Copyright 2013, Wiley-VCH. 427

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EA spectroscopy directly probes the built-in potential in solar cell devices by measuring the signal coming from the Stark effect. Due to the perturbation of the electronic wave function under an applied electric field in OPV devices, a shift in the spectrum of the absorption layer occurs and leads to a change in absorption coefficient (Da) according to the Stark effect [85]. For most organic materials, Da is proportional to the imaginary part of the third order susceptibility Imx(3)(hy) and the square of the electric field (E). RESEARCH: Review

DaðhyÞa 

DT ðhyÞ / Imxð3Þ ðhyÞE2 T

(1)

where a is the absorption coefficient, hy is the photon energy, T is the optical transmission and E is the electric field. Given the applied electric field is the sum of the DC and AC bias (V) plus the presence of an internal built-in potential (VBI), the first harmonic EA signal is: 

DT ðhyÞ /  ðV DC  V BI Þ  V AC  sinðvtÞ T

(2)

where VDC is the dc bias voltage and VAC is the with a fixed small ac modulation voltage applied to the device. According to Eq. (2), the first harmonic EA signal vanishes when the term (VDC  VBI) is zero at which the applied DC bias cancels out the built-in potential of the cell. Using this method, VBI can be determined as VDC cancels out the first harmonic signals. The schematic setup of an EA measurement setup is shown in Fig. 3a where the first harmonic EA signal is measured with a lock-in amplifier as a function of VDC.

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To understand the built-in potential evolution by interfacial layer treatment, EA spectroscopy is performed. It has been reported that with UV-ozone (UVO) treatment of the ZnO–poly(vinyl pyrrolidone) (PVP) composite, which acts as an ETL in an inverted OPV cell, the performance of the solar cell improved substantially. The report shows that 10 min exposure to UVO resulted in an optimum performance, as shown in Fig. 3b. In order to determine the origin of the effect, EA measurements were performed to investigate the effect of UVO treatment on the built-in field of solar cells. The device was exposed to 635 nm monochromatic light which corresponds to the first excitonic peak. The results are shown in Fig. 3c where the EA signal decreases with decreasing reverse bias, indicating a reduction of the internal field. It should be noticed that carriers started to be injected from both electrodes when VDC is close to the built-in potential, the EA signal deviates the linearity as described in Eq. (2). To determine the built-in potential, a linear extrapolation of the EA signal of a 10 min UVO treated device gives a VBI value of 0.5 V as shown in Fig. 3c. On the other hand, the built-in potential for the as-annealed device shows a built-in potential of 0.22 V. Since the photoactive layer and top contact remain the same, we conclude that the change in built-in potential comes from the change in work function of the interlayer as a result of UVO treatment. The as-annealed ZnO–PVP is capped with PVP on the surface, therefore such an insulating layer inhibits the electronic coupling between ZnO and the polymer. On the other hand, as illustrated in the inset of Fig. 3c, once the ZnO has been brought into contact with the active layer by removing the PVP on the surface with UVO treatment, the energy

FIGURE 5

AFM roughness images of 10 nm thick DTG-TPD/PC71BM BHJ films on (a) NiO and (b) PEDOT:PSS, showing the contrast in physical formation of the different interlayers. The active layer films deposited on NiO are smoother than those deposited on PEDOT:PSS. AFM phase image of thin polymer/fullerene blend layers on (c) NiO and (d) PEDOT:PSS, showing a drastic change in material distribution near the HTL/active layer interface, caused by energy differences of NiO and PEDOT:PSS. Reprinted with permission from Ref. [35]. Copyright 2013, Wiley-VCH. 428

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levels at the interface re-align, as evident in a change in built-in potential as measured by EA.

In BHJ OPVs, the photo-generated excitons must first diffuse to the donor/acceptor interface and get separated. The separated electrons and holes then travel through the domains of the donor and acceptor to the corresponding electrodes. Since the diffusion length of excitons is typically in the range of 1–10 nm [86], the phase morphology therefore plays an important role in determining the exciton separation and charge collection processes for OPVs. The film morphology of the donor–acceptor blend is controlled by the thermodynamic and kinetic parameters which depend on the interactions between the polymer-fullerence blend and the solvent as well as the properties of the substrate surface [87]. The surface energy of the interlayer affects the wettability of solvents and therefore affects the film morphology and subsequently the OPV cell performance. Hau et al. reported that modification of the interface by SAMs changes the surface of TiO2 from hydrophilic to hydrophobic [88]. Kim et al. reported that the hydrophilic SAMs-treated surface results in an unfavorable PCBM segregation after annealing [62]. Khodabakhsh et al. also reported that the surface energy can be altered by adopting SAMs which result in a different morphology of evaporated organic films [65]. Also, different interlayers used in inverted cells have been reported to affect the vertical phase composition profile [89,90]. Solution-processed NiO has been reported to be an ideal candidate to replace PEDOT:PSS as HTL due to its favorable energy alignment, easy synthesis route and good chemical stability [34,91,92]. Moreover, the hydrophobic nature of the NiO surfaces induces a favorable BHJ morphology to enhance the OPV performance [35]. In order to understand the interfacial surface property effects on the morphology, a comparison is made between solar cells based on polydithienogermole-thienopyrro-lodione (DTGTPD) using PETDOT:PSS and solution processed NiO as an HTL and the device data is shown in Fig. 4a where it shows that the NiO device has a 9.4% enhancement in JSC and a 6.5% enhancement in FF compared to the PEDOT:PSS device. Although the VOC of NiO devices showed an average 10 mV lower than that in the PEDOT:PSS devices, the NiO devices have an overall 14.7% improvement in PCE. A closer look into the external quantum efficiency (EQE) (Fig. 4b) of the device shows that while the transmission of NiO is slightly lower than that of PEDOT:PSS devices at wavelength between 425 and 590 nm, the EQE of the NiO devices is actually higher. The increase in EQE is due to the difference in the refractive indices of the HTLs and optical resonances, leading to a higher JSC. Also, the results of EA spectra show that the difference in the builtin potential of the two devices is 10 mV which is the same as the difference in VOC. While the origin of the differences in VOC and JSC can be attributed to the changes in interface energies and optical effects respectively, the reason for the enhancement in FF is not understood. To account for the difference in FF, the surface properties of NiO need to be examined in detail and water contact angle measurements were used to understand the difference in surface energy between NiO and PEDOT:PSS. The results show that the average contact angles are 29.3  2.88 for NiO and 12.5  1.48 for PEDOT:PSS

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FIGURE 6

(a) The steady-state photoluminescence spectra of colloidal ZnO NPs films. The ‘‘thermally annealed only’’ film was tested directly after 15 min annealing at 80 8C, shown as circles. The PL of a ‘‘10 min UVO treated’’ ZnO film, shown as triangles. The conduction band edge emission locates around 372 nm, while the broad emission from defects ranges from 450 nm to 600 nm. Reprinted with permission from Ref. [7]. Copyright 2012, WileyVCH, (b) J–V characteristics of DTG-TPD/PC71BM cells. The ZnO film devices ‘‘as annealed’’ and ‘‘UVO-treated’’ are shown as circles and triangles respectively. Reprinted with permission from Ref. [7]. Copyright 2012, WileyVCH, and (c) EQE spectra of DTG-TPD/PC71BM cells, showing a maximum EQE of 72% at 675 nm for UVO treated ZnO. Reprinted with permission from Ref. [7]. Copyright 2012, Wiley-VCH.

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respectively as shown in Fig. 4c and d. These data indicate that the NiO surface is more hydrophobic than PEDOT:PSS, resulting in better wetting by the nonpolar solvents used in the active layer leading to an improved interface with an optimum donor/acceptor phase morphology. AFM roughness and phase scans of 10 nm thick active layers deposited on the PEDOT:PSS and NiO surfaces were performed and the results are shown in Fig. 5. The RMS roughness of the thin active layer film on NiO is 2.3 nm while that of PEDOT:PSS is 3.6 nm. More importantly, the AFM phase scan showed a more optimized donor/acceptor phase morphology at the interface between the HTL and the active layer for the NiO film. The improved morphology leads to more uniform electrical contact to NiO. In fact, a reduction of 40% series resistance (RS) and an increment of 31% shunt resistance (RSH) facilitate more efficient hole extraction and less interfacial recombination.

5. Interlayers to control surface recombination Interfacial layers can prevent the detrimental effect of metal ions from diffusion into the organic layer which results in a shunting of the organic device [93,94]. Chemical reactions at the metal/ organic interface can also form interfacial dipole barriers affecting the built-in potential. The use of interlayers circumvents the direct contact between the photoactive material and the electrode where a high density of traps can lead to carrier recombination. Various interlayers have been reported to reduce the surface recombination [12,68,69,95]. Surface trap passivation via surface treatment or passivation layer is also a common approach to reduce surface recombination. Loiudice et al. reported that UV treatment of TiO2

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resulted in reduced surface recombination [96]. Hau et al. reported that different SAMs have been used to passivate the trap states on the TiO2 surface [88]. ZnO colloidal nanoparticles (NPs) have been used extensively as an ETL in polymeric solar cells due to its easy synthetic route and its low temperature processing capability. However, 30% of the atomic bonds on the ZnO NPs surface are dangling bonds, which act as recombination centers, resulting in a low power conversion efficiency in these cells. These defect states on the NP surface are sensitive to light and it has been reported that brief exposure to UV light helps partially fill the defect states and results in enhanced device performance [7]. Specifically, ZnO NP UVO treatment was reported by Chen et al., by exposing the ZnO NPs film to 245 nm UV light. Photoluminescence (PL) measurements were used to study its interface properties. As shown in Fig. 6a, the PL emission from the as-deposited ZnO NP thin films has a strong emission band at 519 nm which is associated with the trap states on the ZnO NPs surface. Upon UVO treatment, the broadband defect emission is significantly reduced, indicating an effective reduction of the defect states. The band-to-band emission peak at 372 nm increases in intensity due to a reduction of the defect emission. The enhancement in PL is an evidence of surface passivation of ZnO NPs resulting from a reduction of oxygen vacancies in the NPs film due to UVO treatment. The current density-voltage characteristics of PV cell with an active layer of DTG-TPD:PC71BM are shown in Fig. 6b. After UVO treatment of ZnO NPs, the devices show an enhancement in JSC from 12.9 mA/cm2 to 14.1 mA/cm2. The external quantum efficiency spectra show a maximum of 72%

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FIGURE 7

(a) Transient photo-current arrangement. A pulsed laser with an emission power attenuated to 102 nJ pulse1 cm2 was used to photo-excite the active layer while the samples were under constant illumination from a solar simulator. The transient photo-current was measured by probing the voltage signal across a 30 V resistor connected in series with the solar cell, simulating the short circuit current condition, (b) transient photo-current decay for the inverted DTG-TPD:PC71BM devices with ‘‘as-annealed’’ and ‘‘UVO-treated’’ ZnO NPs films. The perturbation photo-current is excited by 527 nm pulse laser with a width of 8 ns. Reprinted with permission from Ref. [7]. Copyright 2012, Wiley-VCH. 430

at 675 nm which is especially high for inverted polymer cells. In addition, there is a slight increase in FF resulting in an increase of PCE from 7.4% to 8.1%. Transient photo-current (TPC) measurements were performed to study the photo-carrier decay dynamics under an extraction field for the cells with UVO treated ZnO NPs. The device is under constant illumination of the solar simulator while a small perturbation is given by a pulse laser to introduce a small photo-current. The single exponential decay of the transient photo-current perturbation for the inverted cells with as-annealed and UVO treated ZnO NPs films are due to photo-generated carriers recombining either in the bulk or at the ZnO/photo-active layer interface. Since these devices have the same hole extraction contact and the same photo-active layer, the difference in carrier lifetime must be due to the difference in the ZnO NP surfaces. The setup for transient photo-current is shown in Fig. 7a. A pulsed laser with an emission power attenuated to 102 nJ pulse1 cm2 was used to photo-excite the active layer with a perturbation current <0.1 mA/cm2 while the sample was under a constant illumination from the solar simulator. The photo-generated carriers only come from the polymer:fullerene blend since the pulsed laser source which emits at 527 nm does not excite the ZnO NPs films. The transient photo-current was measured by probing the voltage signal across a 30 V resistors connected in series with the solar cell, simulating the short circuit current condition. The result is shown in Fig. 7b, as-annealed and UVO-treated devices show decay time constants of 130 ns and 210 ns, respectively. The results indicate that due to the presence of defect states in the ZnO NPs, the photo-generated carriers recombine at a faster rate via the mid-gap trap states. Upon defect passivation in ZnO NPs film by UVO treatment, carrier recombination at the ZnO/photo-active layer interface was reduced resulting in a longer carrier lifetime and better device performance.

6. Summary and outlook In summary, different properties of interfacial layers and their impact on the performance of OPVs have been discussed. Interlayers not only modify the electrode work function but also alter the BHJ film morphology and carrier recombination dynamics. Post-deposition treatments on interlayers are often used to reduce interfacial charge recombination. Different surface characterization techniques such as photoelectron spectroscopy, electroabsorption spectroscopy, water contact angle measurements, and transient photo-current measurements were presented to reveal the properties of these interlayers and how they relate to the device performance. For the future development of OPV devices, the careful selection of interlayers through a better understanding of the role of interlayers will be essential to further improve the efficiency. Through the development of interface engineering, a promising future for the commercialization of high efficiency OPV modules can be realized. References [1] [2] [3] [4] [5] [6] [7]

R.F. Service, Science 332 (2011) 293. A.K.K. Kyaw, et al. Adv. Mater. 25 (2013) 2397–2402. T.Y. Chu, et al. J. Am. Chem. Soc. 133 (2011) 4250–4253. Z.C. He, et al. Adv. Mater. 23 (2011) 4636. Y.Y. Liang, et al. Adv. Mater. 22 (2010) E135. C.E. Small, et al. Nat. Photon. 6 (2012) 115–120. S. Chen, et al. Adv. Energy Mater. 2 (2012) 1333–1337.

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