Photocatalytic effect of ZnO on the stability of nonfullerene acceptors and its mitigation by SnO2 for nonfullerene organic solar cells

Youyu Jiang, Lulu Sun, Fangyuan Jiang, Cong Xie, Lu Hu, Xinyun Dong, Fei Qin, Tiefeng Liu, Lin Hu, Xueshi Jiang and Yinhua Zhou*
Wuhan National Laboratory for Optoelectronics, and School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail: yh_zhou@hust.edu.cn

Received 14th March 2019 , Accepted 1st April 2019

First published on 1st April 2019


ZnO is the dominantly used electron transporting material in high-performance inverted nonfullerene (NF) organic solar cells. Here, we report that nonfullerene acceptors tend to decompose in the presence of ZnO due to its photocatalytic activity under UV illumination. This leads to poor device stability of NF solar cells under solar light illumination. To mitigate this issue, SnO2 is used as an electron-transporting layer that has a wide band gap and is almost irresponsive to the AM1.5 solar spectrum to replace ZnO. NF solar cells with SnO2 display a power conversion efficiency of 14.1% with the PM6:IT-4F active layer and show better device illumination stability than the reference cells with ZnO.



New concepts

The use of nonfullerene (NF) active layer-based organic solar cells has recently been a hot topic because of their low voltage loss which gives rise to an opportunity to push the efficiency higher. Device stability is the critical challenge for the organic solar cells to enter the commercial market. To date, research on the stability of NF cells is still in its infancy. Zinc oxide (ZnO) is the dominantly used material as the electron-transporting layer (ETL) in inverted organic solar cells. Despite the ease of fabrication and high efficiency, the ZnO ETL suffers from photo instability due to its intrinsic ultraviolet (UV)-response that could result in poor stability of the organic solar cells. Herein, we report that ZnO tends to decompose high-performance acceptor–donor–acceptor (A–D–A) nonfullerene acceptors due to its photocatalytic activity under UV illumination. This leads to poor device stability of NF solar cells under solar light illumination. In the second part of this work, a mitigation strategy of the photocatalytic effect is reported by using SnO2 as the electron-transporting layer to replace the ZnO. The solar cells with SnO2 display better stability than the reference cells with the ZnO ETL, and in the meantime exhibit higher photovoltaic performance.

Introduction

Organic solar cells (OSCs), with the advantages of light weight, good mechanical flexibility and easy fabrication, have been attracting a lot of attention.1–8 In the past few years, the emergence of non-fullerene acceptors (NFAs) has significantly increased the efficiency of the OSCs because of reduced voltage loss.9–16 To date, the power conversion efficiency (PCE) of the NF OSCs has been improved to above 14% for single-junction cells and 17% for tandem cells (note: OSCs with nonfullerene active layers are denoted as NF OSCs).17–19 In addition to the high efficiencies, the operational stability of the NF OSCs has become a major concern for the community. To date, research on the stability of the NF OSCs is still in its infancy. Optimization of photoactive layers has been demonstrated to improve the device stability, including molecular tailoring (e.g., end groups and side chains) of both the NFAs and donors for enhanced device stability.20,21

Zinc oxide (ZnO) is the widely used material as the electron-transporting layer (ETL) in the NF OSCs. Despite the ease of fabrication and high efficiency, the ZnO ETL suffers from photo instability due to its intrinsic ultraviolet (UV)-response. It absorbs UV light below 380 nm. The absorbed UV light induces oxygen desorption, which could result in “photoinduced shunts” and increased carrier recombination loss.22,23 Furthermore, ZnO is known as a photocatalyst for organic compounds.24 For example, it can facilitate the decomposition of methylene blue under UV illumination via a photocatalytic reaction. Both the “photoinduced shunts” and photocatalytic activity would be detrimental to the operational stability of the ZnO-based NF OSCs under 100 mW cm−2 air mass (AM) 1.5 illumination that contains ca. 2 mW cm−2 UV light below 380 nm.

SnO2 is less sensitive to UV light due to its wider band gap as compared to ZnO. SnO2 has been very widely used as an ETL for high-performance perovskite solar cells.25–27 So far, only a few reports have demonstrated using SnO2 as an ETL in organic photovoltaic devices.28–33

In this communication, we report that: (1) ZnO induces the photo decomposition of the NF acceptor (IT-4F,34,35 Fig. 1a) due to its photocatalytic activity under UV illumination. This leads to poor device stability under continuous AM1.5 illumination. Besides IT-4F, the photocatalytic effect of ZnO also destroys other nonfullerene acceptors (ITIC and IEICO-4F, Fig. S1a, ESI). (2) When SnO2 (that is less sensitive to UV light due to its wider band gap) is used as the ETL to replace ZnO, the photocatalytic effect is mitigated. Solar cells with an SnO2 ETL show higher efficiency (up to 14.1%) and better illumination stability than the reference cells with ZnO.


image file: c9mh00379g-f1.tif
Fig. 1 (a) Chemical structure of the active layer: the PM6 donor and the IT-4F nonfullerene (NF) acceptor. (b) Pictures of IT-4F thin films on glass, glass/ZnO, and glass/SnO2 substrates under continuous UV illumination (365 nm, 5 mW cm−2) for different times. The UV-vis absorption spectra of (c) ZnO/IT-4F and (d) SnO2/IT-4F films upon AM1.5 illumination in N2; (e) absorbance spectra of ZnO and SnO2 films and the AM1.5 solar irradiation spectrum.

Results and discussion

Fig. 1b shows pictures of IT-4F films (ca. 5 nm) on glass, glass/ZnO and glass/SnO2 substrates, after continuous UV illumination (365 nm, 5 mW cm−2) for different times. IT-4F films on the glass substrate are stable under UV illumination. However, when a layer of ZnO is deposited underneath, the color of the IT-4F film changes within 4 h. Fig. 1c shows the evolution of the absorption spectra of ZnO/IT-4F samples upon illumination with AM1.5 simulated solar light in N2. The pristine IT-4F films exhibit strong absorption in the region of 550 to 800 nm with a peak at around 720 nm, which is assigned to an intramolecular charge transfer (ICT) from the donor moiety (6,12-dihydro-dithienoindeno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5, 6-b′]dithiophene, labeled as IDTT) to the acceptor moiety (2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile, labeled as EG-2F).34 After being exposed to illumination, the absorption band disappears. A new absorption band in the region of 400–500 nm appears. The intramolecular charge transfer between the IDTT moiety and the EG-2F moiety in IT-4F is destroyed by ZnO under UV illumination. This photo decomposition also occurs in other efficient nonfullerene acceptors, ITIC and IEICO-4F (Fig. S1b, ESI). Besides sol–gel ZnO, ZnO nanoparticles are also able to induce similar degradation of IT-4F (Fig. S2, ESI). In contrast, we found that ZnO could not induce photo degradation of the polymer donor material PM6 under illumination (100 mW cm−2 of AM1.5 illumination) as confirmed by the unchanged UV-vis absorption spectra (Fig. S3a, ESI). The evolution of the UV-vis absorption of the PM6:IT-4F blend films has also been studied (Fig. S3b, ESI). Compared with the pristine PM6:IT-4F film, the AM1.5-illuminated ZnO/PM6:IT-4F samples show decreased absorption in the region of 550–800 nm and increased absorption in the region of 400–500 nm, which is consistent with the absorption change of the ZnO/IT-4F samples. This observation suggests that ZnO could induce decomposition of the NF acceptors instead of the donor under AM1.5 illumination.

To gain more insight into the reaction between the ZnO and nonfullerene acceptors, we analyzed the products of photo-degraded IT-4F films by using mass spectrometry (MS) measurements (Fig. 2). Fig. 2a and b show the MS spectra of IT-4F and EG-2F (terminal acceptor moieties) as the references. IT-4F and EG-2F show strong peaks at m/z values of 1499.01 and 229.81, respectively. As for photo-degraded IT-4F films after 100 h illumination (Fig. 2c), two peaks at the m/z values of 229.30 and 1074.90 are observed. The results suggest a possible disruption of the C[double bond, length as m-dash]C linkage that links the donor (IDTT) and acceptor (EG-2F) moieties in IT-4F, leading to the loss of the terminal acceptor moiety EG-2F from IT-4F. This presumed reaction is also supported by the Fourier transform infrared (FT-IR) spectra as shown in Fig. S4 (ESI). Compared to the pristine IT-4F films, the characteristic absorption of C[double bond, length as m-dash]O stretching of the photo-degraded samples shifts toward higher wavenumber, which is associated with the reduced conjugation effect. In addition, when a 500 nm cutoff filter is used, absorption spectra of IT-4F films on ZnO upon AM1.5 illumination are not changed (Fig. S5, ESI). This suggests that the reaction is attributed to the UV spectral region of the AM1.5 spectrum. ZnO is a known photocatalyst under UV illumination. It can be proposed that IT-4F is decomposed by the photocatalytic effect of ZnO. Upon UV illumination, electrons are excited from the valence band (VB) to the conduction band (CB) and holes are formed in the VB. The photo-induced electrons and holes in the ZnO films are reductive and oxidative, which can attack the active C[double bond, length as m-dash]C linkage. The photocatalytic effect of ZnO can also induce decomposition of organic compounds, such as organic dyes24,36 and organic–inorganic perovskites.37 When SnO2 (instead of ZnO) is deposited underneath the IT-4F, the IT-4F film is stable under AM1.5 illumination with unchanged absorption (Fig. 1d). Fig. 1e compares the AM1.5 irradiation spectrum and absorbance of ZnO and SnO2. While ZnO absorbs light below 380 nm, the SnO2 film is almost transparent in the AM1.5 solar irradiation spectral region. Therefore, the SnO2 film is expected to deliver better photo stability for the NF OSCs.


image file: c9mh00379g-f2.tif
Fig. 2 MS spectra of pristine nonfullerene acceptor IT-4F (a), pristine acceptor moiety EG-2F (b), and photo-degraded IT-4F (c), respectively.

SnO2 films are fabricated from nanocrystal (NC) dispersions. The particle size of the SnO2 is 5–10 nm as observed via transmission electron microscopy (TEM) (Fig. S6a, ESI). The high-resolution TEM (HRTEM) image (Fig. S6b, ESI) indicates that the SnO2 particles are highly crystalline, and the outer d spacing of the nanocrystals is measured to be 0.33 nm, which matches with the interplanar spacing of the (110) plane of the rutile phase. Distinctive electron diffraction circles corresponding to (110), (101), and (211) crystal planes were observed from the selected area electron diffraction (SAED) images (inset in Fig. S6b, ESI). Pristine and thermally-annealed SnO2 NC films show similar diffraction structures with several peaks (Fig. S6c, ESI). The structure of the SnO2 nanocrystal films is independent of thermal annealing. The atomic force microscopy (AFM) image (Fig. S7, ESI) shows that the spin-coated SnO2 film on the glass substrate has a uniform surface with a root-mean-square (RMS) of 1.6 nm.

Fig. 3 shows the Kelvin probe measurement results of the work function for the SnO2 film and the ZnO film deposited on the ITO substrates under 100 mW cm−2 of AM1.5 illumination and ambient conditions. The as-prepared ITO/ZnO films display a work function of around 4.22 eV. After 60 min of AM1.5 illumination in N2, the work function of ZnO is reduced to about 3.80 eV. When exposed to air, the work function recovers to the intial value within about 5 min. The recovered work function can again be reduced upon illumination. The work function reduction and recovery are associated with oxygen desorption/adsorption on the surface of ZnO.23 The desorption/adsorption of oxygen also correlates with the photocatalytic property of metal oxides under UV illumination.22,24,38,39 In contrast, ITO/SnO2 films show a minor change (4.11 to 4.06 eV) after 60 min of AM1.5 illumination. The work function results further confirm the better stability of SnO2 as compared to that for ZnO under AM1.5 illumination.


image file: c9mh00379g-f3.tif
Fig. 3 Work function of ZnO and SnO2 films under continuous AM1.5 illumination and an oxygen atmosphere.

Fig. 4a shows the JV characteristics of the NF OSCs with SnO2 and ZnO as ETLs. The NF OSCs are in an inverted configuration: glass/ITO/SnO2 or ZnO/PM6:IT-4F/MoO3/Ag. Their photovoltaic parameters are summarized in Table 1. Under simulated AM1.5 irradiation (100 mW cm−2), cells with 10 nm SnO2 annealed at 150 °C deliver a PCE of up to 14.1%, with an open-circuit voltage (VOC) of 0.85 V, a short-circuit current density (JSC) of 21.30 mA cm−2, and a fill factor (FF) of 0.78. The ZnO-based reference devices show a PCE up to 13.0%, with a VOC value of 0.84 V, a JSC value of 20.7 mA cm−2, and a FF of 0.75. The averaged PCE (over 20 cells) of the cells with SnO2 is 13.8 ± 0.2%, higher than the reference cells with the ZnO ETL (12.8 ± 0.2%). With the SnO2 ETL, the cells show reduced recombination loss and improved charge collection efficiency as observed in the plots of the photocurrent density (Jph) versus the effective voltage (Veff) (Fig. S8a, ESI) and JV characteristics versus light-intensity measurements (Fig. S8b) (details are shown in the ESI),40–42 thus leading to higher JSC and FF values in the SnO2 based NF OSCs. Fig. S9 (ESI) shows the external quantum efficiency (EQE) of the devices. The integrated photocurrent densities based on EQE are 20.5 mA cm−2 and 19.5 mA cm−2 for SnO2 and ZnO based devices, which are close to those obtained from JV measurements. For other NF active layers, the SnO2 ETL also delivers higher performance than the ZnO ETL. Cells with PBDB-T:ITIC active layers show efficiencies of 10.5% (with SnO2) and 9.6% (with ZnO). For the active layer of PTB7-Th:IEICO-4F, the efficiencies are 13.1% (with SnO2) and 12.0% (with ZnO) (Fig. S10 and Table S1, ESI).


image file: c9mh00379g-f4.tif
Fig. 4 (a) Current–voltage (JV) curves of NF OSCs with ZnO and SnO2 ETLs. Evolution of the JV characteristics of cells under continuous AM1.5 illumination: (b) with the ZnO ETL and (c) the SnO2 ETL, respectively.
Table 1 Device characteristics of NF OSCs with ZnO and SnO2 NC ETLs, respectively
ETL VOC (V) JSC (mA cm−2) FF PCE (%)
a Integrated JSC based on EQE.b PCE is calculated based on the integrated JSC.
ZnO 0.84 ± 0.01 20.5 ± 0.2 0.75 ± 0.02 12.8 ± 0.2
0.84 20.7 (19.5)a 0.75 13.0 (12.3)b
 
SnO2 0.85 ± 0.01 21.3 ± 0.2 0.77 ± 0.01 13.8 ± 0.2
0.85 21.3 (20.5)a 0.78 14.1 (13.5)b


Fig. 4b shows the evolution of the JV characteristics of the reference cells with the ZnO ETL under continuous illumination (100 mW cm−2, AM1.5). The performance of the reference cells degrades fast. After 24 h of illumination, the PCE of the cells decreases from 13.0% (VOC = 0.84 V, JSC = 20.7 mA cm−2, and FF = 0.75) to 5.1% (VOC = 0.66 V, JSC = 17.0 mA cm−2, and FF = 0.45). The cells with the SnO2 ETL demonstrated better photo stability with the PCE decreasing from 14.1% to 12.5% after 24 h of illumination. Trost et al. and Manor et al. reported that the UV illumination could induce desorption of oxygen on the ZnO surface which results in the “photoinduced shunt effect” in the cells and leads to a substantial decay of the device performance for fullerene-based cells within minutes.22,23 This reduced efficiency is able to recover by restoring the degraded cells in the dark. The recovery can be accelerated with increasing oxygen pressure or applying reverse bias on the cells. We also tested if the efficiency recovery exists in the NF solar cells with the ZnO ETL after continuous AM1.5 illumination. A NF solar cell with an initial efficiency of 12.8% (VOC = 0.83 V, JSC = 20.5 mA cm−2, and FF = 0.75) is used to perform this study. After 2 h of AM1.5 illumination, the efficiency decreases to 10.1% (VOC = 0.79 V, JSC = 19.5 mA cm−2, and FF = 0.65). After 24 h of storing in the dark in N2, the efficiency increases to 11.2% (VOC = 0.82 V, JSC = 19.7 mA cm−2, and FF = 0.69). When the device is further stored in dry air (relative humidity: 10%) in the dark for an additional 100 h, the efficiency doesn’t further increase/recover (Fig. S11a and Table S2, ESI). The efficiency recovery saturates after 24 h of storing. So, there is indeed partial efficiency recovery for the NF OSCs with ZnO after AM1.5 illumination.

When the cell is further illuminated (100 mW cm−2 AM1.5) for up to 20 h, the cell efficiency further decreases to 6.0% (VOC = 0.69 V, JSC = 17.4 mA cm−2, and FF = 0.50) (Fig. S11b, ESI). After being stored in the dark in N2 for 48 h and an additional 100 h in air, the efficiency increases to 8.6% (VOC = 0.73 V, JSC = 18.7 mA cm−2, and FF = 0.63). These results suggest that both the photocatalytic effect of ZnO and the photoinduced shunt effect contribute to the efficiency degradation of the NF cells with the ZnO ETL (Fig. 4b). The photocatalytic effect leads to non-recoverable degradation of efficiency because it decomposes the chemical structure of the NF acceptor. As a reference, the ZnO based NF cells tested using a 500 nm cutoff filter still degraded to approximately 90% of their initial performance over 20 h of illumination (Fig. S12, ESI). The NF cells could still suffer degradation even without UV illumination, which may be related with the stability of the donor/NF acceptor materials and morphology.20 This may help to explain the observed degradation of the cells with the SnO2 ETL. Overall, SnO2 as the ETL displays better photo stability of the NF cells than ZnO. It mitigates the photocatalytic issue of ZnO for the NF acceptors.

The performance of the SnO2 ETL-based cells shows negligible dependence on their annealing temperature (Fig. 5a and Table S3, ESI) and film thickness (Fig. S13a, ESI). Devices with room-temperature processed SnO2 without additional thermal annealing still have a PCE of 12.1% (VOC = 0.84 V, JSC = 20.3 mA cm−2, and FF = 0.72). When the cells with SnO2 were annealed at 100 °C, the cells show a PCE of 13.9%. This thus demonstrates a promising application of such SnO2 NC for low-temperature processed or printable NF OSCs. To validate the suitability of SnO2 as a printable ETL, we fabricated NF solar cells with slot-die coated SnO2 and also a doctor-blade coated NF active layer (PM6:IT-4F). Fig. 5b shows the JV characteristics of the cells. The NF OSCs show an effciiency of 12.5%, with a VOC value of 0.84 V, a JSC value of 20.3 mA cm−2, and a FF of 0.73. This result indicates good compatibility of this SnO2 NC with the slot-die coating process. We also fabricated 1 cm2 NF OSCs with slot-die coated SnO2 and a doctor-blade coated NF active layer, showing an efficiency of 11.5% (Fig. S13b, ESI). The decrease in JSC and FF of the large-area devices compared to the small-area cells is mainly caused by the large series resistance of ITO.


image file: c9mh00379g-f5.tif
Fig. 5 (a) JV curves of the NF OSCs with the SnO2 ETL annealed at different temperatures. (b) JV curve of NF OSCs with the slot-die coated SnO2 ETL and the doctor-blade coated PM6:IT-4F NF active layer. The inset shows the schematics of the device structure.

Conclusions

In summary, we have reported that ZnO film could induce the photo degradation of the high-performance nonfullerene electron acceptor (IT-4F, ITIC and IEICO-4F) under UV illumination because of the photocatalytic properties of ZnO. The ZnO photocatalytic effect under UV illumination leads to disruption of the C[double bond, length as m-dash]C linkage in donor–acceptor (D–A) type nonfullerene acceptors, resulting in the decomposition of the nonfullerene acceptors with the disappearance of the intramolecular charge transfer absorption bands. This photo degradation of the nonfullerene acceptor results in poor device stability of the NF-OSCs with the ZnO ETL under simulated 100 mW cm−2 AM1.5 illumination.

The SnO2 nanocrystal is introduced as an ETL in nonfullerene solar cells and has shown advantages: (1) cells with SnO2 show better illumination stability than those with ZnO. SnO2 doesn’t absorb photons in the AM1.5 irradiation spectrum because of the wider bandgap, which mitigates the photocatalytic effect. (2) SnO2 delivers higher power conversion efficiency than ZnO. Therefore, SnO2 is a promising alternative of ZnO for efficient and photo-stable printed solar cells, tandem cells and flexible NF organic solar cells.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

Y. Y. Jiang, F. Y. Jiang and Y. H. Zhou conceived the idea of the project and directed this work. Y. Y. Jiang performed the device fabrication and characterization. L. L. Sun, F. Y. Jiang, L. Hu, and X. S. Jiang performed the characterization of the printed NF-OSCs. C. Xie and L. Hu performed the EQE measurements. F. Qin, X. Y. Dong, and T. F. Liu performed the characterization of the work function. Y. Y. Jiang and Y. H. Zhou cowrote the manuscript. The work is supported by the National Natural Science Foundation of China (Grant No. 51773072 and 61804060), the HUST Innovation Research Fund (Grant No. 2016JCTD111 and 2017KFKJXX012), and the Science and Technology Program of Hubei Province (2017AHB040). The authors would also like to thank the Analytical and Testing Center of Huazhong University of Science and Technology for providing the facilities to conduct the characterization.

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9mh00379g
Y. Y. Jiang, L. L. Sun and F. Y. Jiang contributed equally to this work.

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