Efficient Inverted ITO-Free Organic Solar Cells Based on Transparent Silver Electrode with Aqueous Solution-Processed ZnO Interlayer

China Electronic Product Reliability and Environmental Testing Research Institute, No. 110 Dongguanzhuang Road, Guangzhou 510610, China State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, School of Microelectronics, Xidian University, 2 South Taibai Road, Xi’an 710071, China School of Physics and Optoeletronic Engineering, Xidian University, 2 South Taibai Road, Xi’an 710071, China


Introduction
With the advantages of low cost, solution process, flexibility, and the potential of roll-to-roll mass fabrication, organic solar cells (OSCs) have attracted much more attention as one kind of the third generation solar cells [1][2][3][4][5].In the past two decades, with the rapid development of the research, considerable improvement has been achieved in the photovoltaic performance of OSCs, particularly in the power conversion efficiency (PCE) and stability.Recently, it has been reported that the PCE of single junction and tandem organic solar cells [6][7][8] have already exceeded 10%, which is regarded to be the limit for the large-scale commercialization of efficient OSCs [8][9][10][11].
Due to the high optical transparency in the visible light region and good conductivity, indium tin oxide (ITO) transparent electrode has found wide applications in light-emitting diodes and OSCs.However, the cheap and flexible electrode material is the prerequisite for rollto-roll mass production of OSCs, which ITO cannot meet.For instance, both the high-temperature process (usually sputter) and intrinsic brittleness of ITO make it a nonideal transparent electrode candidate for flexible devices [12].What is more, indium is a scarce and toxic material, and thus, the highest share in material cost [13,14] and energy input [15,16] of ITO is inevitable in the fabrication of ITO-based OSCs.Therefore, more and more efforts are dedicated to the ITO-free or indium-free transparent electrodes, such as poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) [17,18], various transparent conducting oxides (Al-doped ZnO (AZO) or Ga-doped ZnO (GZO)) [19][20][21], Ag nanowires [22,23], carbon nanotubes [24], graphene [25], and so on.However, although the PEDOT:PSS electrode can be prepared by simple solution process, its conductivity needs to be further improved [26,27].Although the cost of AZO or GZO is lower than ITO, their similar brittleness and high-temperature sputter process are also undesirable.Meanwhile, the relatively large surface roughness of Ag nanowire, graphene, and carbon nanotube electrodes limits the carrier transport in devices [22,24,28].Additionally, the complex preparation of these film electrodes is unsuitable for large-scale fabrication of OSCs.On the contrary, a metal thin film (for example Ag) can be easily fabricated by thermal evaporation.Moreover, due to their low absorption in the visible spectrum, high conductivity, smooth surface, and ductile nature, metal thin film electrodes are promising to be used for roll-to-roll mass production of OSCs [28].Thus, more and more works are devoted to the research interest of transparent metal electrodes [28][29][30][31].
It has been reported in our previous works that the introduction of 2 nm thick unclosed MoO 3 layer may create preferred nucleation sites on the substrate to enhance the lateral growth of Ag film, which improves the electrical, optical, and morphological properties of MoO 3 /Ag electrodes [32][33][34].Based on the optimal MoO 3 (2 nm)/Ag (9 nm) anode with high transparency and good conductivity, ITOfree OSCs with a conventional structure are fabricated on glass substrates, and the resulted device shows the comparable performance compared to the ITO-based reference OSCs [32,33].Furthermore, flexible ITO-free OSCs fabricated on poly(ethylene terephthalate) (PET) substrates exhibit a good performance and excellent mechanical flexibility [34], which reveals the potential of Ag electrode for the roll-to-roll mass fabrication of OSCs.
However, the conventional OSCs in above works show the fast performance degradation and the relatively poor stability even stored in the N 2 -filled glove box [33].Fortunately, compared to conventional OSCs, the stability could be obviously improved by employing an inverted structure and more stable high work function metal anodes [35,36].To develop ITO-free OSCs with high efficiency and better stability, inverted ITO-free OSCs are fabricated on the MoO 3 (2 nm)/Ag (12 nm) electrode with an aqueousprocessed ZnO electron extraction layer.The blend of low bandgap poly[ [4,8-bis[(2-ethylhexyl) [3,4-b]thiophenediyl]] (PTB7) and [6,6]phenyl-C 71 -butyricacidmethylester (PC 71 BM) is used as the photoactive layer.By choosing a thicker Ag electrode to completely cover the underlying MoO 3 layer, the erosion of ZnO solution to MoO 3 is effectively prevented.The resulted ITO-free OSCs with an inverted structure obtains a PCE as high as 5.55% under AM 1.5G simulated illumination, which is comparable to the PCE (6.11%) of ITO-based reference OSCs.Moreover, these unencapsulated OSCs also show a good stability.Their PCE still remains above 85% of the original values after 30 days, which is slightly superior to ITO-based reference OSCs where the 16% degradation in PCE is observed after 30 days.This further confirms the potential of our electrode for the future commercialization of OSCs.It is also instructive for further research of metal thin film electrodes.

Experimental
The inverted OSCs based on solution-processed ZnO interlayer and MoO 3 /Ag cathode were fabricated as following.Firstly, the ZnO pecursor of 0.125 M (Zn(NH 3 ) 4

2+
) solution was obtained by dissolving ZnO powder (particle size <5 μm, Sigma-Aldrich) into ammonium hydroxide.The preparation details can be found in our previous work [3,36].The blend of PTB7 (1 material) and PC 71 BM (American Dye Source) was dissolved in chlorobenzene containing 3% 1,8-diiodooctane (Aldrich) and stirred at room temperature overnight.Then the PTB7:PC 71 BM blend has a concentration of 25 mg/mL with a weight ratio of 1 : 1.5.Glass substrates were ultrasonically cleaned with detergent (Decon 90, UK), deionized water, acetone, ethanol, and deionized water for about 15 min.All the materials were used without any further purification.
Secondly, the dried glass substrates were transferred into an ultrahigh vacuum chamber with a pressure below 5 × 10 −4 Pa.To obtain the MoO 3 /Ag electrodes, MoO 3 and Ag films were thermally evaporated on glass substrates sequentially with the evaporation rate of 0.02 nm/s and 0.1 nm/s, and the details of electrode deposition can be found in our previous works [32][33][34].The thickness of 2 nm given here has to be a nominal value obtained by the quartz crystal monitor calibrated with a step profiler, representing the amount of MoO 3 on the sample.The optical transmittance and surface morphology of MoO 3 /Ag electrode were characterized by using an UV-vis spectrophotometer (Lambda 950, Perkin Elmer) from 200 nm to 1000 nm and the tapping mode atomic force microscopy (AFM, Agilent 5500).
Thirdly, after the electrode deposition, the device fabrication began with the deposition of ZnO cathode buffer layer.The ZnO aqueous precursor solution was spin-coated onto the MoO 3 /Ag surface at 3000 rpm for 40 s and annealed in the oven at 150 °C for 30 min in air.Then PTB7:PC 71 BM solution was spin-coated onto the ZnO surface at 1200 rpm for 55 s in a N 2 -fulfilled glove box.Next, the samples were transferred into the vacuum chamber again, a 10 nm thick MoO 3 layer and a 70 nm thick Ag layer were further evaporated upon the PTB7:PC 71 BM through a shadow mask to form the device anode.There are eight devices in one substrate, and each device has an active area of 12.5 mm 2 .Meanwhile, ITO-based OSCs were fabricated as the reference devices in the same process conditions.The geometry structure of fabricated devices based on the ITO and MoO 3 /Ag electrodes is illustrated in Figure 1.The current densityvoltage (J-V) characteristics of unencapsulated devices were measured in air under the AM 1.5G solar simulator (1 sun, San-Ei Electric) with a Keithley 2400 source unit.The devices were illuminated from the substrates side, and the illumination intensity was kept at 1000 W/m 2 by using a silicon

Results and Discussion
The measured typical J-V characteristics and corresponding photovoltaic parameters of ITO-free OSCs and ITO-based reference OSCs with an inverted structure are shown in Figure 2. Eight devices are measured for each kind of OSCs, and the values in the parentheses of Figure 2 represent standard deviations of corresponding parameters.It can be seen that a PCE of 6.11% is achieved for ITO-based reference OSCs, with an open-circuit voltage (V OC ) of 0.72 V, a shortcircuit current density (J SC ) of 13.60 mA•cm −2 , and a fill factor (FF) of 63%.Notably, the photovoltaic performance of ITOfree OSCs based on MoO 3 /Ag cathodes is comparable to that of ITO-based reference OSCs, with a comparable PCE of 5.55%, a nearly identical V OC of 0.71 V, a slightly higher FF of 66%, and a relatively smaller J SC of 11.90 mA•cm −2 .Here, the identical V OC for ITO-free devices demonstrates that the aqueous solution-processed ZnO interfacial modifying layer could efficiently block the hole and provide a suitable contact for electron extraction from PTB7:PC 71 BM to MoO 3 /Ag cathode.
To study the effect of different electrodes on the performance of OSCs, the wavelength-dependent transmittance spectra of glass/ITO and glass/MoO 3 /Ag electrodes are characterized as well as that of glass/ITO/ZnO and glass/MoO 3 / Ag/ZnO reference samples.As shown in Figure 3, a high transparency is achieved for the ITO electrode, with the first V oc (V)  3 International Journal of Photoenergy maximum of 84% at 367 nm and the second maximum of 90% at 600 nm, respectively.Particularly, the average transparency of the electrode over 85% is achieved in the visible region.Compared with that of the ITO electrode, the transparency of the MoO 3 (2 nm)/Ag (12 nm) electrode is relatively lower in the visible and near-infrared spectral range, with the maximum of 73% at 358 nm.Thus, the lower transparency of MoO 3 (2 nm)/Ag (12 nm) electrode leads to a relatively smaller J SC of 11.9 mA•cm −2 than the J SC (13.6 mA•cm −2 ) of ITO-based reference OSCs.When coated with the ZnO layer, the first transmittance maximum of 89% and the second maximum of 87% for the stack glass/ ITO/ZnO sample are shifted to 403 nm and 656 nm, respectively.Meanwhile, the transparency of the stack glass/ITO/ ZnO has been decreased to some extent in the region over 472 nm, compared with that of the ITO electrode.More interestingly, it can be observed from Figure 3 that the transparency of the stack has been enhanced when depositing a ZnO buffer layer onto the MoO 3 (2 nm)/Ag (12 nm) electrode.Due to the optical interference effect, compared with that of the MoO 3 (2 nm)/Ag (12 nm) electrode, the transparency of the glass/MoO 3 /Ag/ZnO in the wavelength range from 350 nm to 705 nm has been improved with a maximum of 77% at 369 nm while it has been slightly decreased in other regions.Similar results can also be found in our previous work [34].Since the light harvesting of photoactive materials is most efficient in the visible light region, this provides us an optical strategy to further enhance the performance of our inverted ITO-free OSCs by improving the transparency of the ZnO/Ag/MoO 3 electrode with the optimized ZnO buffer layer thickness.It will be discussed in our future work.
To investigate the influence of different electrodes on the surface morphology and film formation of the ZnO layer, AFM tests are carried out for the glass/ITO/ZnO and glass/ MoO 3 /Ag/ZnO samples.Figure 4 shows the tapping mode AFM images of the ZnO layer on the ITO electrode and the MoO 3 /Ag electrode, respectively.The value of root-meansquare (RMS) surface roughness is 1.11 nm for the ZnO layer deposited on the MoO 3 /Ag electrode, lower than that of the ZnO layer fabricated on the ITO electrode (RMS of 1.71 nm).And the top to valley values are 11.1 nm and 8.6 nm for the ZnO films on ITO and MoO 3 /Ag electrodes, respectively.Both of them indicate the formation of the smoother ZnO layer on the MoO 3 /Ag electrode.Commonly, a smoother ZnO layer will lead to a lower trap concentration at the interface between the ZnO buffer layer and active layer and, thus, the improved electron extraction efficiency at the cathode [38].This maybe one reason that a smoother ZnO layer fabricated on the MoO 3 /Ag electrode can achieve a higher FF for corresponding inverted ITO-free OSCs, compared with that of ITO-based reference OSCs.
Besides, the stability of photovoltaic parameters has also been investigated for inverted ITO-free OSCs.The unencapsulated devices are stored in the N 2 -filled glove box and measured under air ambient conditions.The normalized V OC , J SC , FF, and PCE as a function of the time are shown in Figure 5

Conclusion
In this paper, based on the MoO 3 (2 nm)/Ag (12 nm) cathode, efficient inverted ITO-free OSCs are fabricated with an aqueous solution-processed ZnO electron extraction layer.The blend of PTB7:PC 71 BM is employed as the photoactive layer here.A PCE of 5.55% is achieved for such ITO-free OSCs under AM 1.5G simulated illumination, which is comparable to that of ITO-based reference OSCs (PCE of 6.11%).Moreover, ITO-free OSCs also show a good stability.The PCE of the devices still remains above 85% of the original efficiency after 30 days, which is slightly superior to ITObased reference OSCs where the 16% degradation in PCE is observed after 30 days.It is instructive for further research of OSCs based on metal thin film electrodes.

Figure 1 :
Figure 1: Schematic diagrams of inverted OSCs based on (a) the MoO 3 /Ag electrode and (b) the ITO electrode.

Figure 2 :Figure 3 :
Figure 2: J-V characteristics for inverted ITO-free OSCs based on MoO 3 /Ag electrode and ITO-based reference OSCs.The corresponding photovoltaic parameters have also been shown here as an inset for comparison.

Figure 5 :
Figure 5: Variation of the normalized photovoltaic performance parameters with the time for ITO-free OSCs and ITO-based reference OSCs, respectively.

4
International Journal of Photoenergy and FF for both devices decrease slightly, and the degradation in PCE is mainly attributed to the decrease in J SC .For ITObased reference OSCs, V OC decreases by 5%, FF decreases by 1%, J SC decreases by 11%, and thus PCE decreases by 16% after 30 days.Meanwhile, for ITO-free OSCs, V OC decreases by 3%, FF decreases by 3%, J SC decreases by 9%, and thus PCE decreases by 15% after 30 days.Obviously, our ITO-free OSCs show a good stability, slightly superior to ITO-based reference OSCs.