Improvement of Charge Collection and Performance Reproducibility in

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Improvement of charge collection and performance reproducibility in inverted organic solar cells by suppression of ZnO sub-gap states Bo Wu, Zhenghui Wu, Qingyi Yang, Furong Zhu, TszWai Ng, Chun-Sing Lee, Sin-Hang Cheung, and Shu Kong So ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03619 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 26, 2016

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ACS Applied Materials & Interfaces

Improvement of charge collection and performance reproducibility in inverted organic solar cells by suppression of ZnO sub-gap states Bo Wu,† Zhenghui Wu,† Qingyi Yang,† Furong Zhu,*† Tsz-Wai Ng,‡ Chun-Sing Lee,*‡ Sin-Hang Cheung,† and Shu-Kong So*† †

Department of Physics, Institute of Advanced Materials, and Institute of Research and Continuing Education (Shenzhen), Hong Kong Baptist University, 224 Waterloo Road, Kowloon Tong, NT, Hong Kong ‡

Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Ave, Kowloon Tong, Hong Kong KEYWORDS: organic solar cells, oxide interlayer, organic/cathode interface, sub-gap states, charge collection

ABSTRACT: Organic solar cells (OSCs) with inverted structure usually exhibit higher power conversion efficiency (PCE) and are more stable than corresponding devices with regular configuration. Indium tin oxide (ITO) surface is often modified with solution-processed low work function metal oxides, such as ZnO, serving as the transparent cathode. However, the defect-induced sub-gap states in the ZnO interlayer hamper the efficient charge collection and the performance reproducibility of the OSCs. In this work, we demonstrate that suppression of the ZnO sub-gap states by modification of its surface with an ultrathin Al layer significantly improves the charge extraction and performance reproducibility, achieving PCE of 8.0%, which is ~15% higher than that of a structurally identical control cell made with a pristine ZnO interlayer. Light intensity-dependent current density-voltage characteristic, photothermal deflection spectroscopy and X-ray photoelectron spectroscopy measurements point out the enhancement of charge collection efficiency at the organic/cathode interface, due to the suppression of the sub-gap states in the ZnO interlayer.

1. Introduction Organic solar cells (OSCs) are a promising alternative photovoltaic technology to the inorganic solar cells due to their cost effectiveness and broad range of application.1,2 Significant progresses have been made in new material development,3-5 device design and optimization7-9 over the past decade. The regular configuration OSCs have a donor/acceptor blend layer sandwiched between a front transparent indium tin oxide (ITO) anode and a rear opaque metal cathode. In the regular OSCs, a solutionprocessed poly(3,4-ethylene dioxythiophene): (polystyrene sulfuric acid) (PEDOT:PSS) interlayer is often coated on ITO surface prior to the subsequent deposition of the organic function layer, for improving hole transport and hole extraction. The use of acidic PEDOT:PSS holetransporting layer is not the optimal choice for an efficient operation of OSCs over a long period of time. This is due to the deterioration in the contact property at the ITO/PEDOT:PSS interface.10 An inevitable initial oxidation at the organic/cathode interface, due to the presence of residual moisture and oxygen, is one of the degradation pathways in the regular OSCs.11 In a recent work, it is found that the interfacial exciton dissociation at the cathode/organic interface of regular OSCs hampers the electron collection, caused by the compensation of drifted photo-generated electrons at the cathode/organic interface.12 The unfavorable interfacial exciton dissociation can

be eliminated, e.g., by inserting a thin ZnO cathode interlayer between the cathode contact and organic layer. OSCs with inverted structure are more favorable than corresponding devices with regular configuration. The reverse configuration improves the absorbance of the cells, and therefore the power conversion efficiency (PCE).13 The properly surface-modified ITO can also serve as a front transparent cathode in the inverted OSCs.14,15 For example, the high work function of ITO can be decreased with an interlayer of zinc oxide (ZnO),15,16 titanium oxide,17 cesium carbonate18 or n-type organic layers.14,19 Among these, ZnO is commonly used for its low cost and simple solution-based preparation process. The morphology of the electron extraction, e.g., solution-processed oxide interlayer, plays an important role determining the contact quality at the cathode/active layer interface in the cells. The correlation between the surface morphology of the oxide interlayer and the morphology evolution of the organic functional active layer in the inverted OSCs was analyzed. It is demonstrated that the OSCs with a reverse geometry are favorable for forming smaller domains in the active layer as compared to that in the active layer of the cells having a regular configuration. The improvement in the performance of inverted devices is attributed to the reduced domain size, which could then be closer to the exciton diﬀusion length in the polymer blend, favoring efficient operation of the OSCs.20

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In addition to the surface morphology, the electronic properties of the ultra-thin ZnO cathode interlayer are essential for the development of high performance solution-processed inverted OSCs.21 A thin film ZnO cathode interlayer, prepared by the sol-gel process at a sintering temperature of 200 OC, has been used for application in inverted OSCs. Surface engineering of a sol-gel processed ZnO with a thin conjugated polyelectrolyte interfacial layer has a significant impact on enhancing the cell performance, achieved due to an improved charge transport via suppressed bimolecular recombination.22 A crosslinkable water/alcohol soluble conjugated polymer interlayer was also adopted for tuning the surface work function of ZnO interlayer in high eﬃciency ZnO-based inverted OSCs.23 It has been reported that the associated fluctuation in morphological and surface electronic properties of ZnO-modified ITO cathode, e.g., changes due to the experimental conditions of ZnO formulation,24 film fabrication25 and post-treatment,16,26 also place a practical challenge limiting the reproducibility of the performance of OSCs. However, the report on the effect of the sub-gap states, induced by the defects in the ZnO interlayer, on charge collection behavior and thereby the performance reproducibility of the solution-processed OSCs is rather rare. For commercialization of OSCs, there is a need to develop a robust and an effective ZnO modification approach that can be easily repeatable for application in making high performance OSCs via a solutionprocessable route. In this work, the ZnO electron-transporting layer was prepared by the solution process at the room temperature. The effect of the suppression of ZnO sub-gape states on charge extraction and performance reproducibility of inverted OSCs, based on the blend system of poly [[4,8- bis [(2-ethylhexyl) oxy]benzo [1,2-b:4,5-b'] dithiophene-2,6diyl] (PTB7): 3'H-Cyclopropa [8,25][5,6] fullerene-C70D5h(6) -3'- butanoicacid, 3'-phenyl-,methyl ester (PC70BM), was analyzed. It shows that inverted OSCs can benefit from the modification of ZnO surface with an ultrathin Al (~1.2 nm) layer in two ways, (1) to reduce the density of the sub-gap states in ZnO buffer enhancing the charge collection efficiency thereby PCE, and (2) to improve significantly the performance reproducibility of the cells. Light intensity-dependent current density−voltage (J–V) characteristics suggest an enhanced charge collection efficiency at organic/electrode interface in inverted OSCs having an Al-modified ZnO interlayer. The surface morphology of the pristine ZnO and Al-modified ZnO cathode interlayers was characterized using Scanning Electron Microscopy (SEM). There is no observable change in the surface morphology, revealing the enhancement in the PCE of the cells is originated from the improved charge collection at the cathode/organic interface. X-ray Photoelectron Spectroscopy (XPS) analysis indicates clearly that the thermally evaporated Al layer is partially oxidized to AlOx and interacts with PC70BM.

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Photothermal Deflection Spectroscopy (PDS) results reveal an obvious decrease in the density of the sub-gap states in Al-modified ZnO compared to the pristine ZnO interlayer. It is clear that suppression of the defectinduced sub-gap states in the ZnO interlayer favors the efficient operation of the cells. This leads to the increase in the open circuit voltage (VOC), the short-circuit density (JSC), the fill factor (FF), and thereby the improvement of PCE of 8.0%, which is ~15% higher than a structurally identical control inverted OSC with a pristine ZnO interlayer. 2. Experimental section The inverted OSCs made with Al-modified ZnO interlayer have a device configuration of ITO/ZnO (10 nm)/Al(1.2 nm)/PTB7:PC70BM (90 nm)/MoO3(2.0 nm)/Ag. The structurally identical control OSCs with a single cathode interlayer of ZnO (10 nm) were also prepared for comparison. Pre-patterned ITO coated glass substrates, with a sheet resistance of ~15 Ω/square, were cleaned by ultrasonication sequentially with acetone, ethanol, deionized water, and isopropanol for 10 min each. The ZnO interlayer was solution-prepared following the processes described in a previous work.15 For OSCs with Al-modified ZnO interlayer, ~1.2 nm thick Al was deposited on ZnO/ITO surface by thermal evaporation in a vacuum chamber with a base pressure of 3.37eV, caused by the band-to-band absorption in the ZnO layers. The absorption of both samples drops rapidly when the photon energy is less than 3.4eV. The Urbach Energy, Eu, can be extracted by the linear fitting of the cut-off edge in the PDS spectra, an important parameter that provides the information of sub-band gap states in the materials. When the photon energy, E, is less than

band gap Eg, the relationship between the absorption coefficient A and Eu can be expressed by:31 A = A0 exp ((E - Eg) / Eu).

(3)

According to Equation (3), the corresponding Eu values calculated for the pristine ZnO and Al-modified ZnO films are 59 meV and 53 meV, implying that the sub-band states in the solution-processed ZnO layer can be suppressed effectively by modification of its surface with an ultrathin Al layer.

Figure 5. PDS spectra measured for the pristine ZnO and Almodified ZnO layers.

As shown in Figure 5, there is an obvious reduction in the absorption in the Al-modified ZnO layer as compared to the pristine ZnO layer over the sub-gap energy region from 2.7 eV and 3.4 eV. The change in the PDS spectra suggests that the sub-gap states can be suppressed effectively in Al-modified ZnO interlayer. The suppression of the sub-gap states in the ZnO interlayer benefits the charge collection at the organic/electrode interface, which otherwise they would act as the shallow electron traps to give rise to the bimolecular charge recombination, and hamper the charge transport and charge extraction.

Figure 6. Dark J−V characteristics measured for the inverted

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OSCs with the Al-modified ZnO and pristine ZnO interlayers.

The relevance of PDS analyses can be seen by measuring the dark J−V characteristics of the inverted OSCs with the Al-modified ZnO and pristine ZnO interlayers. As shown in Figure 6, the decrease in the leakage current is clearly observed in the inverted OSCs with an Al-modified ZnO interlayer, demonstrating the effect of the suppression of the sub-gap states in the ZnO buffer layer on organic/electrode contact quality. The sub-band states in the ZnO interlayer provide additional sites to assist in charge injection, thereby an increase of the leakage current as seen in Figure 6. The dark J−V characteristics clearly illustrate the change in the contact properties at the organic/ZnO and organic/Al-modified ZnO interfaces in the inverted OSCs. The leakage current can be reduced by suppressing the sub-gap states in ZnO interlayer in the OSCs, as shown in Figure 5. In a related work, the vertical phase separation in the polymer bulk heterojunction, based on the PTB7:PC70BM blend system, was studied by the XPS measurement, and its effect on the performance of the regular and inverted OSCs was examined.33 The XPS results indicate that the segregation of donor/acceptor forming a PC70BM rich region at the bottom of the blend films can be moderated using solvent additive. The quantitative analyses show that the size of PC70BM domains in the bulk heterojunction decreases from 67 nm to 38 nm with the addition of the solvent additive. The use of the solvent additive helps to improve the homogeneous vertical phase separation in the PTB7:PC70BM bulk heterojunction, and thereby favoring an enhanced exciton dissociation process for attaining high performing OSCs. The hole collection and electron extraction in the cells are closely correlated to the interfacial electronic properties at the electron donor (PTB7)/anode and cathode/ acceptor (PC70BM) interfaces. As the OSCs with different cathodes of ZnO/ITO and Al-modified ZnO/ITO have the identical anode of MoO3/Ag, the variation in charge collection of the cells can be associated with the modification in the contact property at the cathode/PC70BM interface. A better understanding on electron collection behavior with respect to the interfacial electronic properties at the cathode/PC70BM interface is important for achieving high performance OSCs. Following our previous work of analyzing the interfacial exciton dissociation at the interface between the cathode and the surface of PC70BM-rich PTB7:PC70BM blend layer in the OSCs,12,33 the interfacial electronic properties at the ZnO/PC70BM and Al-modified ZnO/PC70BM contacts were investigated by XPS measurements in situ. In this work, a thin layer of PC70BM was deposited using sublimation process stepwise onto the Al(1 nm)/ZnO(10 nm)/ITO sample. For XPS characterizations, a ZnO(10 nm)/ITO substrate was loaded into an ultra-high vacuum (UHV) deposition chamber. A 1.2 nm Al film was deposited onto the sub-

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strate. The sample was then transferred to the UHV analysis chamber for XPS measurements. The bottom dashed line in Figure 7(a) shows an XPS Al 2p core level spectrum of the Al(1 nm)/ZnO(10 nm)/ITO sample. The sharp Al 2p peak at 72.8 eV can be attributed to metallic Al. Meanwhile, a broad peak emerges at 75.2 eV indicating the evaporated Al was partially oxidized forming AlOX. Al 2p and C 1s XPS core level spectra of PC70BM (x nm)/Al(1 nm)/ZnO(10 nm)/ITO, with x of 0.2, 0.5, 1, 2, 5 and 10 nm, are shown as solid lines in Figure 7(a) and 7(b) respectively. No obvious change in the Al 2p peak is observed as the PC70BM coverage increases. On the other hand, a strong C 1s peak can be found at ~284.8 eV corresponding to the CC bond signal from PC70BM. It is interesting to note that a small C 1s peak emerges at 287.1 eV during the initial coverage of PC70BM. This C 1s peak later disappears when the coverage of PC70BM reaches 5.0 nm. The XPS results reveal an interfacial interaction between AlOX and PC70BM during the initial deposition of PC70BM (0.5 - 2.0 nm), which provides a better contact and energy alignment of PC70BM and the interlayer, favoring the charge transport and collection at the cathode interface.

Figure 7. The high resolution XPS core level spectra (a) Al 2p and (b) C 1s measured for the surface of Al(1 nm)/ZnO(10 nm)/ITO/glass substrate with different PC70BM thickness coverages from 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 nm.

For device fabrication, the yield of high performance OSCs is affected by fluctuation in morphology and surface electronic characteristics of the ZnO interlayer, that vary due to many processing variables. There is thus a need to develop a robust and an effective interfacial modification approach that does not require any post treatment for application in making high performing OSCs via solutionprocessable route. The results of this work reveal clearly that the use of an Al-modified ZnO interlayer improves the performance reproducibility of PTB7:PC70BM-based inverted OSCs fabricated in different batches as shown in Figure 8. Performance parameters of OSCs made with the pristine ZnO interlayer and Al-modified ZnO interlayer are summarized in Table 2. OSCs with different cathode interlayers of Al(~1.2 nm)/ZnO(10 nm) and ZnO(10 nm),

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and different surface treatment conditions, e.g., with and made for comparison. It shows that pre-UV exposure of without UV exposure prior to the cell fabrication, were the ZnO(10 nm)/ITO cathode has a substantial influence Table 2. The performance of inverted OSCs made with the Al-modified ZnO and pristine ZnO interlayers, and different surface treatment conditions, e.g., with and without UV exposure, prior to the cell fabrication. OSCs Al-modified ZnO Pristine ZnO

2

Voc (V)

Jsc (mA/cm )

FF (%)

PCE (%)

without UV

0.73 ± 0.01

15.7 ± 0.2

69 ± 1

7.8 ± 0.2

with UV

0.73 ± 0.01

15.6 ± 0.3

68 ± 2

7.8 ± 0.2

without UV

0.71 ± 0.01

15.1 ± 0.3

64 ± 2

6.8 ± 0.2

with UV

0.68 ± 0.02

14.7 ± 0.3

59 ± 3

5.9 ± 0.3

on the performance of the cells, resulting in a much reduced PCE of 5.9% for cells made with UV treated ZnO/ITO cathode, as compared to 6.8% for the structurally identical OSCs fabricated with non-UV treated ZnO/ITO cathode. However, there is almost no change in the performance of the set of OSCs made with Almodified ZnO/ITO cathode with and without UV exposure.

Figure 8. J-V characteristics of inverted OSCs with an Almodified ZnO interlayer (circle) and a control cell with a pristine ZnO interlayer (square), and the cells prepared with (solid symbol and line) and without (open symbol and dashed line) UV exposure.

The deviation in the performance of both type OSCs is attributed to suppress of the sub-gap states in the ZnO interlayer, arising from the significant decrease in the interfacial defects at the organic/electrode interface. For example, the reaction between Al and loosely bound oxygen atoms in ZnO passivating the defects generated upon UV irradiation, which act as electron traps due to their deep energy level if they were not removed.34 The density of such interfacial traps in the solution-processed ZnO interlayer may vary according to the different film preparation conditions and post deposition treatments, e.g., with different UV exposures and post-annealing temperatures etc., causing an undesired increase in the charge recombination at the cathode/organic interface.

The surface condition of the ultrathin Al-modified ZnO is very stable, due to the formation of AlOX even in the UHV condition, as illustrated by the Al 2p high-resolution XPS core level spectra in Figure 7(a). It is found that the cells made with a freshly prepared Al (~1.2 nm)-modified ZnO/ITO cathode and the ones that were prepared with the same condition but kept overnight in the glovebox, with O2 and H2O levels below 0.1 ppm, remain essentially no observable change in the device performance. The Almodified solution-processed ZnO interlayer assists in creation of a more stable cathode/organic interface for attaining high performance OSCs. The above analyses, based on the PDS, XPS and light-intensity dependent J−V characteristic measurements, provide an insight on understanding the effect of suppression of ZnO sub-gap states on charge collection and performance reproducibility in inverted OSCs. This suggests that ultra-thin Al (~1.2 nm)-modified ZnO is a simple and effective cathode modification approach, revealing the robustness and the process condition independent characteristics of the cathode that is suitable for high performance OSC fabrication in a consistent and repeatable way, a process that can be easily adopted for mass production. 4. Conclusions In summary, we have found that the modification of solution-processed ZnO with an ultrathin Al layer creates a more stable cathode/organic contact by suppressing the sub-gap states in the ZnO electron extraction layer. The use of Al-modified ZnO interlayer favors the efficient charge collection, thereby improving the efficiency and performance reproducibility of PTB7:PC70BM-based inverted OSCs.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Furong Zhu: Email: [email protected]. *Chun-Sing Lee: Email: [email protected] *Shu-Kong So: Email: [email protected]

Author Contributions

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All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported by Research Grants Council of Hong Kong Special Administrative Region, China, Project No. T23-713/11, GRF12303114 and National Natural Science Foundation of China (No. 61275037). One of us (SK. So) would like to acknowledge the Research Grants Council of Hong Kong for the support of the PDS project through Project No. 211913.

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