High-performance inverted solar cells with a controlled ZnO buffer layer

Abstract

ZnO is a versatile cathode buffer layer for organic photovoltaics (OPV) due to its appealing optical and electronic properties. Using the sol–gel method, we find that the processing temperature of ZnO cathode buffer layers significantly influences the device performance of inverted polymer OPVs composed of blended films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). In particular, ZnO processed at relatively low temperatures results in better device performance than those processed at higher temperatures despite the improved crystallinity and electron mobility of the latter. We attribute this finding to the tuning of the ZnO work function with the annealing temperature, which determines the interface energetics at the cathode and thus influences the open circuit voltage, series resistance and fill factor.

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1. Introduction

Organic photovoltaics (OPV) have attracted a significant amount of attention as an alternative clean energy solution, which also offer potential benefits of low cost and mechanical flexibility.^1,2 It is known that in OPV devices the energy level alignment at the interfaces between metal electrodes and photoactive layers is important for determining short circuit current (J_sc), open circuit voltage (V_oc), and fill factor (FF).^3–10 At weakly interacting interfaces such as with spin coated polymers on non-reactive substrates, vacuum level alignment is often assumed in OPV design.¹¹ However, Fermi level pinning to the integer charge-transfer states of organic semiconductors has been observed in a number of systems including P3HT and PCBM.^5,12

Interfacial buffer layers therefore play a critical role in adjusting the contact properties between active layers and electrodes. For instance, in the presence of non-ohmic contacts in bulk heterojunction (BHJ) devices, injection barriers may give rise to a reduction of the internal electric field and a decrease in V_oc,^13,14 while interfacial buffer layers can be readily utilized to optimize these contacts.^15–17 Additionally, interfacial buffer layers may contribute to enhancement of the charge collection and reduction of the interfacial contact resistance and charge recombination, leading to smaller series resistance (R_series), larger shunt resistance (R_shunt), and hence improved performance.^3,6,7,18,19

Among the n-type buffers used in inverted structures,^3,20–24 ZnO offers the advantages of high conductivity, excellent optical transparency and environmental stability.^17,25,26 Various preparation methods have been utilized to fabricate high-quality ZnO films in planar and nanowire configurations for solar cell applications.^27–34 Among them, spin coating with the sol–gel method is considered to be cost effective and compatible with solution processing of organic solar cells. The sol–gel method also allows for ZnO-based nanostructuring,^35,36 elemental doping,^19,37 and surface modification,^7,38 to improve the ZnO functionality. Here we focus on inverted OPVs with the planar ZnO cathode buffer layer fabricated by the sol–gel method from zinc acetate decomposition. Several parameters can be tuned in the sol–gel process to improve the ZnO thin film morphology, as well as its optical and electrical properties. However there have been conflicting reports of this optimization in the literature.^18,39 Moreover, optimal annealing temperatures reported in the literature range widely from 150 to 450 °C,^{4,18,33,34,39–46} although it is recognized that low temperature annealing is more compatible with the processing of plastic organic solar cells. Therefore, thorough investigation of the characteristics of ZnO buffer layers is necessary.

In this article, we report the effects of ZnO processing temperature on the photovoltaic properties of inverted solar cells with the structure of ITO/ZnO/P3HT:PCBM/MoO₃/Ag. We find that the formation of the ZnO cathode buffer layer at 300 °C results in the best device performance with an average power conversion efficiency of ∼4% despite the improvement in thin film crystallinity and electron mobility for samples annealed at even higher temperatures. We attribute this phenomenon to the tuning of the ZnO work function, monitored with Kelvin Probe force microscopy, which determines the interface energetics and leads to an improved ohmic contact at the low-temperature processed ITO/ZnO cathode. With the enhanced charge collection and electronic coupling at the contact, the open circuit voltage (V_oc), sheet resistance (R_s) and filled factors (FF) are all improved.

2. Experimental

Zinc acetate solutions (0.1 M) were prepared in 2-methoxyethanol (MXL) and monoethanolamine (MEA) with a molar ratio of 1 : 1 between zinc acetate dihydrate and MEA.⁴⁷ All materials were purchased from Sigma-Aldrich. The solution was spin-coated at 4000 RPM for 40 s onto patterned ITO/glass substrates (Xinyan Technology Ltd.) which were pre-cleaned with sonication in acetone and isopropanol. The samples were then annealed for 10 min at different temperatures to decompose the acetate precursor and crystallize the ZnO film. These films were subsequently rinsed in DI water and ethanol, followed by annealing at 150 °C for 10 min to dry the sample.³⁴ With the same procedure, separate ZnO films were prepared on fully-covered ITO substrates for optical transmittance (PerkinElmer, Lambda 800) measurements and on glasses for X-ray diffraction pattern (XRD) 2θ–ω measurements (Bruker, D8 X-ray Diffractometer) to avoid the overlapping crystalline peaks from the ITO substrate. Surface morphology and Kelvin probe force microscopy (KPFM) experiments were conducted on an Asylum MFP-3D-Bio AFM using AC240TM tips (Asylum Research, k = 2 N m⁻¹, f = 70 kHz). The samples were sealed in a fluid cell with a dry N₂ gas flow to avoid surface contamination during AFM measurements and highly oriented pyrolytic graphite (HOPG) was used as a work function reference.⁴⁸ All film thicknesses were calibrated using AFM.

ZnO film preparation was followed by spin-coating a poly(3-hexylthiophene) (P3HT) and PCBM blend (1 : 0.8 in weight) in chlorobenzene (27 mg ml⁻¹ in total) at 800 RPM for 30 s. P3HT and PCBM were purchased from Rieke Metal and American Dye Sources Inc., respectively. The samples were then heated to 105 °C at a rate of ∼20 °C min⁻¹ and annealed at this temperature for 20 min, followed by cooling at a rate of ∼10 °C min⁻¹ to room temperature. The entire process was carried out in a glove box where the oxygen and water impurity levels are below 0.1 ppm.

The resulting samples were transferred to a vacuum deposition chamber (Angstrom Engineering), where 10 nm MoO₃ and 100 nm Ag were thermally deposited with shadow masks under a deposition pressure of ∼3.5 × 10⁻⁶ Torr. Solar cell efficiencies were characterized in air using a Keithley 2420 source-meter and a Newport solar simulator under 0.98 ± 0.01 kW m⁻² illumination measured with an NREL calibrated mc-Si detector with KG5 filter. The mismatch factor of the solar simulator to the P3HT:PCBM based inverted cells was determined to be 1.07 ± 0.02 from the external quantum efficiency measurements.

3. Results and discussion

3.1 Impact of ZnO preparation temperature on device performance

Fig. 1(a) illustrates the electronic structure of the inverted solar cells.⁴⁹ In this electrically inverted structure, electrons are transported in PCBM to the ITO/ZnO cathode and holes in P3HT toward the MoO₃/Ag anode. The concentration of zinc acetate (0.1 M), the thickness of ZnO (8 nm) and MoO₃ (10 nm) films, and the annealing temperature of the P3HT/PCBM active layer (105 °C) were all optimized. The J–V characteristic curves for devices with a single layer of ZnO buffer prepared at 300 and 450 °C are shown in Fig. 1(b). The average power conversion efficiency (PCEs), and the corresponding J_sc, V_oc, and FF are included in Table 1. The low-temperature annealing results in significant improvements of V_oc, FF, and device performance. Fig. 1(b) also suggests that R_series increases with the temperature, leading to a smaller slope of the J–V curve at the open-circuit condition. Indeed, R_series which is extracted by fitting the photocurrent J–V curves to the Shockley diode equation⁵⁰ is 10 Ω cm² for the 300 °C case and 16 Ω cm² for the 450 °C case.

Fig. 1 (a) Layout of the inverted organic solar cell with ZnO as the cathode buffer layer and the schematics of energy levels before reaching the equilibrium (vacuum level alignment). (b) J–V characteristics for devices based on a single layer ZnO cathode buffer prepared at 300 °C and 450 °C, respectively. (c) J–V characteristics for two devices based on a single layer ZnO cathode buffer prepared at 300 °C but one piece of the ITO substrate was preheated at 450 °C.

Table 1 Summary of the average solar cell performance parameters J_sc, V_oc, FF, and PCE for devices made of 0 layer (0L), 1 layer (1L), 2 layers (2L) and 3 layers (3L) ZnO buffer prepared at 300 and 450 °C, respectively. The performance parameters for 1L–3L samples are averaged over more than 10 devices at each condition and the parameters for 0L samples are averaged over 4 devices (the device area is 4.84 mm²), with the standard deviations included

ZnO buffer layer	Jsc (mA cm^−2)	Voc (V)	FF	PCE (%)
0L at 300 °C	9.7 ± 0.2	0.33 ± 0.01	0.43 ± 0.01	1.4 ± 0.1
1L at 300 °C	10.5 ± 0.8	0.62 ± 0.01	0.60 ± 0.02	3.9 ± 0.3
2L at 300 °C	10.4 ± 1.0	0.61 ± 0.02	0.58 ± 0.04	3.7 ± 0.6
3L at 300 °C	10.2 ± 1.1	0.61 ± 0.02	0.59 ± 0.06	3.8 ± 0.8
0L at 450 °C	9.6 ± 0.2	0.32 ± 0.01	0.41 ± 0.01	1.3 ± 0.1
1L at 450 °C	9.7 ± 1.0	0.53 ± 0.04	0.52 ± 0.03	2.7 ± 0.5
2L at 450 °C	9.7 ± 0.3	0.53 ± 0.03	0.52 ± 0.01	2.7 ± 0.2
3L at 450 °C	9.6 ± 0.4	0.51 ± 0.04	0.51 ± 0.02	2.5 ± 0.3
ITO at 450 °C/1L at 300 °C	10.1 ± 0.7	0.60 ± 0.02	0.58 ± 0.03	3.6 ± 0.3

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It is well known that the optical and electrical properties of the ITO substrate can be sensitive to high temperature annealing. Accordingly, to eliminate the impact of ITO at higher annealing temperatures we performed control experiments with zinc acetate spun on pre-annealed (450 °C) and pristine ITO substrates, followed by 300 °C annealing of both to create ZnO thin films. The J–V curves of the two devices are plotted in Fig. 1(c), which show nearly identical performance, suggesting that any changes in the optical and electronic properties of ITO from the higher temperature annealing do not play a significant role here, distinct from an earlier report.⁴⁶

To further distinguish the contributions of the interface quality and the ZnO thin film conductivity, we fabricated inverted devices with the ZnO buffer layer of various thickness via multiple spin-coatings followed by thermal annealing at each step. As presented in Fig. 2 and Table 1, devices without ZnO buffer layer show diminished performance, likely due to enhanced recombination at the cathode which decreases the shunt resistance, leading to reduced V_oc and FF. Additionally, devices with the ZnO buffer layer processed at 300 °C consistently outperform those processed at 450 °C at each corresponding buffer layer thickness, where the different ZnO film thicknesses under a given annealing condition show similar performance in V_oc and FF. This limited thickness dependence suggests that interface properties at the cathode are markedly influenced by the ZnO processing temperature and the resistance of the ZnO thin film plays a lesser role in determining the performance of the inverted devices.

Fig. 2 J–V characteristics for devices based on 0 layer, 1 layer, 2 layers and 3 layers of ZnO buffer prepared at 300 and 450 °C, respectively.

3.2 Origin of the temperature dependence

To clarify the underlying mechanisms of the improved device performance in inverted solar cells with the ZnO buffer annealed at 300 °C, we perform comprehensive experiments and discuss the potential effects of thermal annealing on the properties of ZnO thin films including optical transmittance, thin film crystallinity, surface morphology, and work function. After examining all these factors, we find that interface energetics at the cathode plays the most dominating factor in determining the device performance.

The optical transmittance of ITO/ZnO substrates is depicted in Fig. 3(a) where the low-temperature annealed substrate yields a higher transmittance than the high-temperature annealed one in the visible range. However, such a difference mainly originates from the ITO substrate, as illustrated by the similar transmittance between the bare ITO and the ITO/ZnO substrates annealed at 450 °C, respectively. This observation implies a limited impact of the ZnO optical properties on the OPV device performance as the films are very thin.

Fig. 3 (a) Optical transmittance for bare ITO annealed at 450 °C and ZnO/ITO prepared at 300 and 450 °C, respectively. The ZnO layer is about 8 nm. (b) XRD 2θ–ω spectra for ZnO films prepared at 300 and 450 °C on glass. The peak near 34.4 degree is characteristic for the diffraction of ZnO wurtzite phase. Inset: TGA curve of the zinc acetate gel. The significant mass loss near 300 °C corresponds to the complete thermal decomposition of zinc acetate precursor.

It has been well established that higher annealing temperatures lead to improved thin film crystallinity and enhanced electron mobility.^41,47,51 As presented in the XRD data of ZnO thin films in Fig. 3(b), samples annealed at 300 °C show amorphous structures, whereas a pronounced ZnO (002) peak is observed in thin films processed at 450 °C. One may expect to obtain better electron transport, reduced R_series and therefore enhanced fill factors in devices with high-temperature processed ZnO films. However, the devices processed at 300 °C show better performance, including fill factor which suggests that there are other dominant factors compensating the crystallinity and mobility effect. It is worth noting that annealing the ZnO buffer layer below 300 °C results in significantly reduced device performance, which likely stems from the incorporation of residue zinc acetate in the ZnO film as indicated by the thermogravimetric analysis (TGA) of zinc acetate (inset of Fig. 3(b)).

We have also investigated the roughness and homogeneity of ZnO thin films with AFM. Fig. 4(a) and (b) are AFM images of the single layer ZnO film prepared at 300 °C and 450 °C, respectively. The film prepared at 450 °C is rougher with a root-mean-square (rms) roughness of 2.2 nm as compared to the film prepared at 300 °C (rms of 1.7 nm), which might result in a higher leakage current (smaller R_shunt) and enhanced recombination between injected holes and photo-generated electrons at the cathode. However, this difference in roughness is unlikely to be the driving force for the performance difference we observed between various annealing conditions since the OPV device performance is not strongly dependent on the ZnO film thickness, as suggested in Fig. 2, even though the multiple layer coating is expected to improve the compactness of the ZnO film and thus reduces the leakage paths.

Fig. 4 (a) and (b) are tapping-mode AFM morphology images of single layer ZnO thin film deposited on the ITO substrate and subsequently annealed at 300 °C and at 450 °C, respectively. The image size is 1 × 1 μm².

3.3 Tuning of the ZnO work function and interface energetics at the cathode

In the inverted bulk heterojunctions where the exciton dissociation occurs predominantly at the P3HT–PCBM interface, we focus on the capability of ZnO as the cathode buffer layer to collect photo-generated electrons from the PCBM. Thus the energy alignment (or rather, collection barriers) at the ITO/ZnO and ZnO/PCBM interfaces is crucial to the device performance.

KPFM is a useful tool to measure the work function of electrodes and the interface energetics in solar cells.^5,52,53 It is worth noting that oxygen molecules adsorbed on ZnO grain boundaries can trap free electrons and cause a depletion layer near the surface.^54–56 Accordingly, the interface energetics between ZnO and photoactive materials and the resultant transport properties (in dark) in solar cell devices can be impacted. We found that light soaking using the solar illuminator is effective at removing these surface states, after which the dark current and the photocurrent merge together in the forward bias. The work functions obtained at such conditions are summarized in Table 2.

Table 2 Work functions (in eV) measured for ITO, ITO/ZnO, and ITO/ZnO/PCBM by KPFM. The ITO and ITO/ZnO were annealed at 300 and 450 °C, respectively. A representative error bar is ±0.04 eV

ZnO preparation temperature	ITO	ITO/ZnO	ITO/ZnO/PCBM
300 °C	4.77	4.36	4.34
450 °C	4.76	4.53	4.38

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The energy level alignments between ITO, ZnO and PCBM from KPFM are shown in Fig. 5 where the depletion widths and band bending are inferred. One can see that the ZnO work function has been tuned by thermal treatment where the Fermi level in the 300 °C annealed film is positioned in closer proximity to the conduction band edge, as compared to the one annealed at 450 °C. This implies that the former sample is more heavily n-doped by native defects, including Zn interstitials and oxygen vacancies, that have been partially annihilated by annealing at higher temperatures due to the improved thin film crystallinity.^51,57 Consequently, the width of the Schottky barrier formed at the ITO/ZnO interface may be significantly reduced in the 300 °C annealed sample, as shown in Fig. 5.

Fig. 5 (a) and (b) are schematic band diagrams to illustrate the energy level alignment at various interfaces for devices based on the 300 and 450 °C processed ZnO films, respectively, as deduced from KPFM measurements.

In addition, we find that the Fermi level of the ITO/ZnO cathode is pinned at the negative integer charge transfer state (E_ICT−) of PCBM for both samples within the experimental error. As shown in Fig. 5(a), in the 300 °C case, a neutral contact is formed at the ZnO/PCBM interface due to the alignment of energy levels between the cathode and PCBM. While for the ZnO film annealed at 450 °C, although its work function falls well within the transport gap of PCBM so that one may expect the vacuum level alignment at the interface,¹² our results suggest that the Fermi level is still pinned at the E_ICT− of PCBM, leading to the formation of an interface dipole. Here we propose two possibilities to reconcile the discrepancy. First, the charge transfer behavior between ZnO and PCBM may be disturbed by the existence of interface gap states.¹¹ Second, if the PCBM film is unintentionally doped by impurities, the imbalance in work functions upon contact with the ITO/ZnO cathode (450 °C) can be compensated by the electron flow from PCBM to the cathode.^52,53

Finally, we discuss how the interface energetics affect the solar cell device performance. As presented in Table 1, the devices made of the 300 °C annealed ZnO cathode buffer display a higher FF, a larger PCE and an optimal V_oc as obtained in the P3HT:PCBM systems with ohmic contacts.^16,58 On the contrary, V_oc, FF, and PCE are reduced by about 15%, 13%, and 31%, respectively in the devices composed of the 450 °C annealed ZnO film. Our studies suggest that there are several factors contributing to the enhanced performance in devices composed of the ZnO buffer layer processed at 300 °C. (1) Charge collection at the ITO/ZnO interface may be improved by electron tunneling through the Schottky barrier of reduced width; (2) the Fermi level of the ITO/ZnO cathode lines up with the E_ICT− of PCBM which enhances the electronic coupling at the interface and minimizes the V_oc loss. In contrast, the extraction barrier at the ZnO (450 °C)/PCBM interface may result in a significant charge accumulation and the consequent recombination loss at the interface; (3) these two effects also contribute to the low contact resistance, thereby a smaller R_series and a larger FF in the 300 °C case.

4. Conclusions

We report inverted solar cells with controlled ZnO cathode buffer layers in the ITO/ZnO/P3HT:PCBM/MoO₃/Ag structure comparable to the best conventional cells. Through comprehensive characterization of the surface morphology, thin film crystallinity and optical and electrical properties, we determine that the tuning of the ITO/ZnO work function and the interface energetics play a dominant role in determining the device performance for sol–gel processed ZnO. These findings could aid in the design and interface engineering of high quality OPVs incorporating ZnO buffer layers on low temperature, flexible substrates.

Acknowledgements

This work is supported by startup funds from Michigan State University. We acknowledge helpful discussions with J. Sun. R. R. Lunt acknowledges the support from the National Science Foundation (CAREER award, CBET-1254662).

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