Effect of ZnO:Cs2CO3 on the performance of organic photovoltaics
© Kim et al.; licensee Springer. 2014
Received: 20 May 2014
Accepted: 20 June 2014
Published: 27 June 2014
We demonstrate a new solution-processed electron transport layer (ETL), zinc oxide doped with cesium carbonate (ZnO:Cs2CO3), for achieving organic photovoltaics (OPVs) with good operational stability at ambient air. An OPV employing the ZnO:Cs2CO3 ETL exhibits a fill factor of 62%, an open circuit voltage of 0.90 V, and a short circuit current density of −6.14 mA/cm2 along with 3.43% power conversion efficiency. The device demonstrated air stability for a period over 4 weeks. In addition, we also studied the device structure dependence on the performance of organic photovoltaics. Thus, we conclude that ZnO:Cs2CO3 ETL could be employed in a suitable architecture to achieve high-performance OPV.
The performance of organic solar cells significantly improved during the last few years. Both industrial and academic sectors have focused on the enhancement of their performance, developed new materials, and also improved the stability of the devices. Organic solar cells have attracted a huge interest, given that they are easy to make on flexible substrates, using roll-to-roll technology [1–4], which significantly reduces the manufacturing costs [5, 6].
Although we have seen a significant improvement in the performance of organic solar cells, the efficiency of organic solar cells is still far behind their counterparts, inorganic solar cells. Organic solar cells are basically fabricated by sandwiching a photoactive layer between two electrodes. Normally, in the conventional device architecture, a poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) layer is employed as an anode buffer layer [7–9]. However, one major drawback of using PEDOT:PSS is its poor stability.
Therefore, another alternative to avoid the use of PEDOT:PSS is to make use of an inverted structure [10–22], where the anode and cathode positions are reversed, and n-type metal-oxide-semiconductors, namely, ZnO, TiO x , AZO, and NiO x , are used [2–5], instead of the PEDOT:PSS. Despite device architecture, there is another factor which one can consider in order to enhance the performance of optoelectronic devices, which is the energy barrier between layers. One may find that by decreasing this energy barrier, charge carrier injection at the interface can be significantly improved and therefore, device performance can be improved [23–26]. To date, various methods have been introduced to tune the work functions between semiconductors and metals such as plasma treatment, absorption of atoms, and also the introduction of additional thin-films [27–31].
Zinc oxide (ZnO) has attracted considerable interest for its optical, electrical, and mechanical properties. Experimental and theoretical studies on ZnO crystals have revealed the presence of a permanent dipole moment, which yields a significant piezoelectric effect for a variety of mircomechanical devices. ZnO has been shown to be a good electron selective and hole blocking contact in inverted solar cells. The conduction band (CB) and valence band (VB) of ZnO have been reported to be −4.4 and −7.8 eV, respectively . This allows ZnO to function as a good interfacial layer between ITO and the bulk-heterojunction blend for inverted solar cell devices. ZnO also has large exciton binding energy of about 60 meV, which has been shown to be valuable for optoelectronic devices such as light-emitting diodes and lasers. Nevertheless, ZnO has one major drawback, which is the lack of stable and reproducible p-type ZnO with low resistivity, high carrier concentration, and high carrier mobility.
Doping with the first group elements like Li, Na, K, and Cs in ZnO would substitute Zn2+ by the monovalent cations, thus making it possible to realize n-type conduction. The realization of n-type conduction is very important for ZnO applications in optoelectronic devices, and there are reports on the electrical property of the first group element-doped ZnO thin-films [32–36]. Various techniques such as pulsed laser deposition [37, 38], magnetron sputtering [39, 40], and molecular beam epitaxy  have been used to deposit thin-films of ZnO. The sol-gel method  has been receiving increased attention because of its many advantages such as low cost, simple deposition procedure, easier composition control, low processing temperature, and easier fabrication of large area films. Therefore, here, we demonstrate the improved performance of P3HT:PCBM and P3HT:ICBA-based inverted bulk-heterojunction solar cells through the appropriate interface modification by Cs2CO3-doped ZnO on the electron collecting ITO interface. Recently, Yang et al. has reported that a solution-processed Cs2CO3 is able to make interface dipoles layer on ITO. One may say that these two entities (ZnO and Cs2CO3) are completely different but the most important thing is that these entities do improve the performance of the device. Moreover, we have seen a number of works on tuning the work function of ITO by adding an electron transport layer such as ZnO , TiO2[44–46], Cs2CO3[44–46], and poly(ethylene oxide) (PEO) . The created dipole moment helps to reduce the work function of ITO, allowing ITO to serve as the cathode. The improved device performance is due to the reduction of series resistance, improved shunt performance, and enhanced open-circuit voltage of the cell which can be attributed to the improvement of the following aspects: (1) reduction of the contact resistance between the ZnO:Cs2CO3 and active organic layer, (2) enhancement of the electronic coupling between inorganic ZnO:Cs2CO3 and active organic layer to mediate better forward charge transfer and reduce back charge recombination at the interface, and (3) affect the upper organic layer growth mode and morphology.
ZnO solution preparation
ZnO solution was prepared using similar procedures to the one reported by Jang et al. . Cs2CO3 solution was prepared by dissolving in ethanol in the ratio of 1.25 wt%.
Organic solar cell fabrication
The solution for electron selective layer was prepared by mixing ZnO and Cs2CO3 with different blend ratios, namely, 1:1, 1:2, 1:3, 2:1, and 3:1. The solution-processed ZnO or ZnO:Cs2CO3 was spin-coated at 1,000 rpm for 25 s onto the cleaned substrates and later annealed at 300°C for 10 min. The photoactive layer either P3HT:PCBM or P3HT:ICBA dissolved in 1,2-dichlorobenzene was spin-coated at 700 rpm for 25 s and subsequently annealed at 130°C for 30 min or 150°C for 10 min, respectively. Later, PEDOT:PSS was spin-coated at 4,000 rpm for 25 s onto the photoactive layer and annealed at 120°C for 20 min. To complete the device, 100-nm thick of Al was thermally evaporated at rates 4 A/s through a shadow mask at a base pressure of 10−7 Torr. The active area of the complete devices is 0.04 cm2. To ensure the reproducibility of our results, we have fabricated 83 devices throughout this work.
The following are the fabricated devices based on different photoactive materials.
Thin film and device characterizations
The J-V characteristics of the conventional solar cells were measured using the Keithley 2400 source meter under a solar simulator (AM1.5) with an irradiation intensity of 100 mW/cm2.
The EQE measurements were performed using an EQE system (Model 74000) obtained from Newport Oriel Instruments, Irvine, CA, USA, and the HAMAMATSU calibrated silicon cell photodiode (HAMAMATSU, Shizuoka, Japan) was used as the reference diode. The wavelength was controlled with a monochromator to range from 200 to 1,600 nm.
AFM imaging was achieved in air using a Digital Instrument Multimode that is equipped with a nanoscope IIIa controller.
XPS measurements were performed in a PHI 5000 VersaProbe (Ulvac-PHI, Chigasaki, Kanagawa, Japan) with background pressure of 6.7 × 10−8 Pa, using a monochromatized Al Kα (hv = 1,486.6 eV) anode (25 W, 15 kV).
Ultraviolet photoemission spectroscopy (UPS) measurements were carried out using the He 1 photon line (hv = 21.22 eV) of a He discharge lamp under UHV conditions (4 × 10−10 mbar).
The transmittances of ZnO, and ZnO:Cs2CO3 coated on ITO-glass substrates were recorded at room temperature with a SCINCO S4100 (SCINCO, Seoul, South Korea) spectrophotometer.
XRD measurements were carried out using X'PERT PRO of PANalytical Diffractometer (PANalytical, Seongnam City, South Korea) with a Cu Kα source (wavelength of 1.5405 Å) at 40 kV and 100 mA and at a speed of 1°/min.
Raman scattering experiments were performed at room temperature using a Ramanor T-64000 microscopy system (Jobin Yvon, Longjumean, France).
Photoluminescence (PL) spectra were recorded using a lock-in technique with JASCO FP-6500 (JASCO, Easton, MD, USA)composed of two monochromators for excitation and emission, a 150-Watt Xe lamp with shielded lamp house and a photomultiplier as light detector.
Results and discussion
ii-UPS and contact angle
In order to clarify the advantage of the ZnO:Cs2CO3 as the interfacial layer, the effect of ZnO:Cs2CO3 on interfacial layer properties is investigated by UPS. As shown in UPS spectra (Figure 1a), the work function of ITO is determined to be 4.7 eV, and upon the interface modification, the work function of ITO decreased to 3.8 eV. We interpret this decrease in work function as arising from the interfacial dipoles from the modified ZnO:Cs2CO3 layer, which reduces the vacuum level, resulting in a lower electron injection barrier, thus facilitating electron injection . Therefore, the establishment of the interfacial dipole or interface modification induces lower work function of ITO, which may reduce the electron-injection barrier height compared to the case without interface modification. The detailed values extracted from the UPS spectra are shown in Figure 1a. As depicted from the energy diagram shown in Figure 1b, the electron injection barrier from ITO to ZnO:Cs2CO3 is reduced from 2.1 to 1.2 eV. Obviously, this cathode interface modification greatly reduces the electron injection barrier, which should be beneficial for the improvement of PCE. The complete structure of our inverted organic solar cells is shown in Figure 1b. The interface modification was also carried out by taking multiple contact angle measurements from few locations on the substrates, with and without interface modification. Contact angle measurements were performed to confirm that interface modification was present on the ITO film. Six separate contact angle determinations were performed on each sample. Without interface modification, the surface of ZnO after oxygen plasma had a low wetting angle to DI water (~26°) - showing a hydrophilic (oleophobic) surface. It is worth noting that such a low contact angle indicates a higher surface energy, which is characteristic for polar surfaces. The creation of the interface modification layer was confirmed from the data, which demonstrates the enhancement in contact angle (hydrophobic/oleophilic surface) after surface modification (~68°).
iv-Transmittance, Raman, XRD, and PL
Figure 4b presents the room-temperature (RT) Raman spectra of the ZnO and ZnO:Cs2CO3 in the spectral range 200 to 1,500 cm−1. Raman active modes of around 322 cm−1 can be assigned to the multiphonon process E2 (high) to E2 (low). The second order E2 (low) at around 208 cm−1 is detected due to the substitution of the Cs atom on the Zn site in the lattice. The strong shoulder peak at about 443 cm−1 corresponds to the E2 (high) mode of ZnO, which E2 (high) is a Raman active mode in the wurtzite crystal structure. The strong shoulder peak of E2 (high) mode indicates very good crystallinity . For the ZnO:Cs2CO3 layer, one additional and disappearance peaks has been detected in the Raman spectra. The additional peak could be assigned to the combination modes such as A1(2 TO + 1 LO), while this diminished peak can be assigned to A1(2 TO) as shown in Figure 4b . From these observations, we conclude that doping can be considered to be the main factor that would cause the lattice distortion of the crystals, for it is usually different from the atomic radius of different elements. As the ZnO is doped with Cs2CO3, the shoulder peak position (the E2 (high) mode) shifts to 435 cm−1 from 433 cm−1 as shown in Figure 4b.
Figure 4c shows the XRD patterns of the ZnO and ZnO:Cs2CO3 thin films deposited on ITO substrates. It is found that the ZnO and ZnO:Cs2CO3 thin layers show peaks corresponding to (100), (002), and (101) planes. All detected peaks match the reported values of the hexagonal ZnO structure with lattice constants a = 3. 2374 Å and c = 5. 1823 Å; the ratio c/a ~1.60 and this value is indeed in agreement with the ideal value for a hexagonal cell (1.633). The intensity of the peak corresponding to the (002) plane is much stronger than that of the (100) and (101) plane in the pure ZnO as well as ZnO:Cs2CO3 layers. This suggests that the c axis of the grains become uniformly perpendicular to the substrate surface. The XRD pattern of ZnO:Cs2CO3 layer is dominated by the (002) plane, with very high intensity. The highest intensity of the XRD peaks obtained from ZnO:Cs2CO3 film indicates a better crystal quality. One possible reason for such a high intensity is probably the possibility of heterogeneous nucleation, which is facilitated with the presence of Cs ions in the ZnO structure. It is evident that as the Cs2CO3 doping concentration increases, the lattice parameters ‘a’ and ‘c’ slightly increase (data not shown).
Figure 4d shows the PL spectra of the ZnO and ZnO:Cs2CO3 films excited by 325-nm Xe light at room temperature. The PL spectra of ZnO contain a strong UV band peak at 326 nm and a weak and broad green band located from 400 to 450 nm. The UV emission peak is originated from excitonic recombination, which is related to the near-band-edge emission of ZnO. Additional weak broad green peak located from 400 to 450 nm refers to a deep-level or trap state emission. The green transition is designated to the singly ionized oxygen vacancy in ZnO and the emission results from the radiative recombination of electron occupying the oxygen vacancy with the photo-generated hole . The strong UV and weak broad green bands imply good crystal surface. The blue shift of the UV emission peak position of ZnO:Cs2CO3 (330 nm) thin film with respect to the ZnO layer is probably caused by the band-filling effect of free carriers. A strong quenching of the UV emissions also indicates that the crystalline ZnO:Cs2CO3 layer contains a large numbers of defects that can trap photogenerated free electron and/or holes.
Lattice parameters, FWHM, and grain size of ZnO and ZnO:Cs 2 CO 3
Grain size (nm)
Resistivity (ohm cm)
2.2 × 10−3
5.7 × 10−2
v-J-V, EQE, and stability characteristics
An important issue is to check whether the work function shifts are also reflected in the performance of devices when other active materials are used. As demonstrated by several authors, if one employs interface modification into any device, it should, in principle, be possible to see some positive improvements as a result of the interface modification, regardless of active materials used. For this purpose, we made another set of inverted solar cells based on P3HT:ICBA with the configuration ITO/ZnO:Cs2CO3/P3HT:ICBA/PEDOT:PSS/Al. As a reference, solar cells without interface modification were also fabricated.
Figure 5b shows the J-V characteristics of P3HT:ICBA-based devices with two different structures. As mentioned in the ‘Experimental’ Section, these two different types of structures are ITO/ZnO/P3HT:ICBA/PEDOT:PSS/Al-device C and ITO/ZnO:Cs2CO3/P3HT:ICBA/PEDOT:PSS/Al-device D. As expected, the effective dipole moment created by interface modification shifted the work function of the ITO electrode by nearly 1 eV, thereby reducing the electron injection and improving the ohmic contact for electron injection with P3HT:ICBA. The inverted solar cells (device A) exhibited a contact energy barrier of typically 2.1 eV due to the work function of ITO (4.8 eV), resulting in Jsc that was slightly lower. As observed in Figure 5b, the reference device exhibits Jsc, Voc, FF, and PCE of about 6.28 mA/cm2, 0.89 V, 60.7%, and 3.40%, respectively. The calculated Rs for this device is around 6,666 ohm. For device D, the PCE increases from 3.40 to 3.43%. This 0.88% increment in the PCE is attributed to the improvement in FF, where the FF increases from 57.7 to 59.3%. A similar trend in Rs can also be seen in P3HT:ICBA-based device, where the Rs decreases together with the increment of FF.In addition, the performance of inverted solar cells in terms of external quantum efficiency (P3HT:PCBM-based devices) is shown in Figure 5c. Basically, the EQE is defined as the ratio between the generated charge carriers and the incident photons. Device A shows a maximum EQE value of ~51.80% at the absorption wavelength of ~520 nm. However, the EQE of device B has outperformed the EQEs of device A, in which it exhibits a maximum of about 55% at ~520 nm of absorption wavelength.
The external quantum efficiency of the P3HT:ICBA-based devices with the inverted device geometries are shown in Figure 5d. For inverted reference solar cells (device C), the maximum EQE is 51.51% at 500 nm, where the EQE of device D is 53.05%. These results (device B and device D) further shed the light that the improvement in devices performances is related to interface modification which has modified the work function of the ITO electrode. As mentioned above, the presence of Cs2CO3 have improved the surface area of ZnO:Cs2CO3 and PEDOT:PSS through the good interfacial contact between ZnO:Cs2CO3 layer and ITO layer, and PEDOT:PSS and Al layer, leading to the considerably high EQE.
Environmental degradation parameters of P3HT:PCBM-based devices (ZnO and PEDOT:PSS-device A)
Environmental degradation parameters of P3HT:PCBM-based devices (ZnO:Cs 2 CO 3 and PEDOT:PSS-device B)
Environmental degradation parameters of P3HT:ICBA-based devices (ZnO and PEDOT:PSS-device C)
Environmental degradation parameters of P3HT:ICBA-based devices (ZnO:Cs 2 CO 3 and PEDOT:PSS-device D)
In conclusion, we have found that modification of the interface between the inorganic ITO and photoactive layer can improve the performance of inverted solar cells. The modification of ITO leads to 8% improvement over unmodified ITO inverted devices. This interface modification serves multiple functions that affect the photoinduced charge transfer at the interface, which include the reduction the recombination of charges, passivation of inorganic surface trap states, and improvement of the exciton dissociation efficiency at the polymer/ZnO interface. Moreover, the stability of these modified devices is slightly better compared with unmodified ones.
This work was supported by the Industrial Strategic Technology Development (10045269, Development of Soluble TFT and Pixel Formation Materials/Process Technologies for AMOLED TV) funded by MOTIE/KEIT.
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