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Influence of External Pressure on the Performance of Quantum Dot Solar Cells Jaehoon Kim, Byeong Guk Jeong, Heebeom Roh, Jiyun Song, Myeongjin Park, Doh C. Lee, Wan Ki Bae, and Changhee Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07771 • Publication Date (Web): 23 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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ACS Applied Materials & Interfaces

Influence of External Pressure on the Performance of Quantum Dot Solar Cells Jaehoon Kim,a Byeong Guk Jeong,b Heebeom Roh,a Jiyun Song,a Myeongjin Park,a Doh C. Lee,b Wan Ki Bae*,c and Changhee Lee*,a a

Department of Electrical and Computer Engineering, Global Frontier for Multiscale Energy

Systems, Seoul National University, Seoul 08826, Republic of Korea b

Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST Institute for

the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea c

Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST),

Seoul 02792, Republic of Korea KEYWORDS solar cells, quantum dots, pressure, densification, compression

ACS Paragon Plus Environment


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ABSTRACT

We report the influence of the post-treatment via the external pressure on the device performance of quantum dot (QD) solar cells. The structural analysis in chorus with optical and electrical characterization on QD solids reveal that the external pressure compacts QD active layers by removing the mesoscopic voids and enhances the charge carrier transport along QD solids, leading to significant increase in JSC of QD solar cells. Increasing the external pressure, by contrast, accompanies reduction in FF and VOC, yielding the trade-off relationship between JSC and FF and VOC in PCE of devices. Optimization at the external pressure, in present study, 1.41.6 MPa enables to achieve over 10 % increase in PCE of QD solar cells. The approach and results show that the control over the organization of QDs is the key for the charge transport properties in ensemble and also offer simple yet effective mean to enhance the electrical performance of transistors and solar cells using QDs.

INTRODUCTION The use of nanomaterials in photovoltaics has received great attention because they hold numerous advantages that cannot be achieved from their bulk counterparts, such as high photonto-exciton conversion efficiency and cost-efficient processability based on solution processing methods1-3. Among potential candidates, colloidal quantum dots (QDs) have been extensively assessed as a photo-active material owing to their optoelectrical properties suitable for photovoltaics. Specifically, lead chalcogenide QDs (PbX, X = S, Se or Te) possess the ease of bandgap (Eg) tunability covering the ideal bandgap (1.34 eV) where the Schockley-Queisser


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efficiency limit is the highest as well as the wide absorption range over UV, visible and near IR, motivating their use in the photo-active layer in photovoltaic applications4-6. Besides, the ease of control over the electrical characteristics of QDs (e.g., the band positions and the carrier densities)7 boosts the improvement in the photovoltaic performance. Moreover, the strong correlation of charge carriers in QDs promotes the carrier multiplication process, in which two or more electron and hole pairs are created by one high energy photon, promising the realization of high efficiency photovoltaics exceeding the theoretical Schockley-Queisser limit8. Multilateral efforts, such as energy level alignment through surface ligand modification3, 9-10

, increase in light absorption through structural alteration11 and enhanced charge extraction by

interfacial engineering12-15, have led substantial improvement in the device performance of QD solar cells. Among the variables associated with the device characteristics, the mean QD-to-QD distance in the photo-active layer, in particular, is known to be the governing factor for the electrical property of devices16. Reduction in the mean inter-QD distance strengthens the electronic coupling between QDs and reduces the effective distance for charge carrier transport along the QD active layers toward the electrodes, leading to the enhancement both in the short circuit current and the fill factor in QD solar cells. Driven by the physical guideline, extensive studies have been conducted to develop processing methods that compact QD active layers. One representative example is the exchange of relatively long native ligands from the synthesis (e.g., oleic acid) with ions5 or short ligands7 during the fabrication of QD active layers. Given that the morphology of QD active layers determines the charge carrier transport property and thereby the device performance, post-treatments that can further pack QD active layers are expected to have a synergistic effect on the device performance with the ligandexchange method. Although a number of articles have reported the relationship between



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nanocrystal array and external force,17-18 the systematic study that relates the post-densification on the performance of QD solar cells14, 19 has been lacking. In line of this reasoning, in the present study, we explore the influence of the post-densification of QD active layers on the performance of QD solar cells. As a mean of the post-treatment, we compress QD solar cells with varying external pressures ranging from 0 to 1.8 MPa. The changes in device characteristics (i.e., short-circuit current (JSC), open circuit voltage (VOC), fill factor (FF) and power conversion efficiency (PCE)) are understood in relationship with the structural variation upon the external pressure.

RESULTS AND DISCUSSION As a model system to assess the influence of the external pressure on the device performance, we chose an inverted QD solar cell that is widely adopted in previous studies and reported high efficiency and stability3, 11 (Figure 1a). The energy levels of each device elements are given in Figure 1b. The onset of light absorption of the QDs is found to be 1,100 nm, indicating the optical bandgap of 1.12 eV (Supporting Information). 25 nm thick ZnO layer composed of randomly oriented ZnO nanoparticles (5 nm in diameter) is adopted as an electron transport layer (ETL) due to its transparency (T ~ 95%) over visible and near IR regime20 and suitable charge extraction mobility21-22. A thin layer (10 nm) of thermally evaporated MoO3 is chosen as a hole transport layer (HTL) due to its excellent hole mobility as well as its transparency. The Au top electrode and the ITO bottom electrode have been used after considering its work functions, which enable charge extraction at the interfaces without energetic barrier.


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Figure 1. Schematic illustrations of (a) the QD solar cell and (b) its energy band diagram at open circuit voltage (VOC). Representative current (J)-voltage (V) characteristics of pristine (empty circle) versus pressed (0.2 MPa) (filled square) QD solar cells (c) under AM 1.5 G illumination and (d) in the dark condition.

The external pressure was applied on the perpendicular direction of QD solar cells (Figure S1). A clean substrate modified with self-assembled molecules of low surface energy was placed on the top surface of QD solar cell, so we could avoid unexpected contamination of



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the surface or detachment of QDs when removing the substrate. We characterized the device performance of QD solar cells upon the external pressure as well as the optical and electrical properties of QD solids. Figure 1(c-d) display the representative current (J)-voltage (V) characteristics of pristine versus pressed (0.2 MPa) QD solar cells. Two notable changes are observed in electrical characteristics of devices after applying the external pressure. QD solar cell employing the external pressure shows over 13 % increase in the short-circuit current (JSC) compared to pristine QD solar cell. The post-treatment with the external pressure accompanies slight but apparent decrease (at most 2.3 %) in the open circuit voltage (VOC). Overall, solar cell shows significant improvement in power conversion efficiency (PCE) from 3.66% to 4.11% that is over 12 % increase after the post-treatment with the external pressure. To validate the effect and gain deeper insight, we varied the external pressures imposed on QD solar cells and correlated them with the changes in device performance (Figure 2 and Table 1). As increasing the external pressure up to 1.6 MPa, JSC increases continuously from 12.37 mA/cm2 to 15.79 mA/cm2, whereas VOC reduces from 0.44 V to 0.42 V. The trends observed in electrical properties of devices are fairly consistent upon increasing external pressures for multiple runs. In chosen variation of the applied external pressure, the increase in JSC outpaces the reduction observed in VOC and fill factor (FF), leading to the enhancement of PCE. However, the external pressure over 1.8 MPa causes a sudden drop in FF that yields the decrease in PCE of QD solar cells down to the level of pristine QD solar cells.


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Figure 2. (a) Short circuit current (JSC), (b) open circuit voltage (VOC), (c) fill factor (FF) and (d) power conversion efficiency (PCE) of QD solar cells after compression with varying external pressures. Table 1. Device characteristicsa) after compression with varying external pressures. Pressure (MPa)

JSC (mA/cm2)

VOC (V)

FF (%)

PCE (%)

0

12.37 ± 0.38

0.44 ± 0.00

52.08 ± 2.74

2.85 ± 0.21

0.4

13.13 ± 0.33

0.43 ± 0.01

50.97 ± 2.93

2.93 ± 0.21

0.6

13.74 ± 0.46

0.43 ± 0.00

50.34 ± 3.18

3.01 ± 0.23

0.8

14.10 ± 0.47

0.43 ± 0.00

48.76 ± 3.67

2.97 ± 0.25



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1.0

14.75 ± 0.35

0.43 ± 0.00

48.95 ± 3.27

3.11 ± 0.26

1.2

14.44 ± 0.63

0.42 ± 0.00

48.62 ± 3.53

2.99 ± 0.13

1.4

15.42 ± 0.42

0.42 ± 0.00

47.93 ± 3.40

3.14 ± 0.28

1.6

15.79 ± 0.37

0.42 ± 0.00

47.17 ± 3.75

3.14 ± 0.32

1.8

15.93 ± 0.34

0.42 ± 0.00

42.33 ± 4.04

2.84 ± 0.33

a)

Extrapolated values from 5 independent devices.

In order to identify the reason for the reduction of VOC and FF, we have conducted electrical characterization of QD solar cells upon external pressure (Table S1). It is known that the increase in the leakage current is related with the decrease in VOC and FF.23-24 Upon increasing the external pressure from 0 MPa to 1.8 MPa, the shunt resistance (RSH) of devices decreases gradually from 257 Ω ·cm2 to 164 Ω ·cm2, signifying the development of shunt paths across QD solar cells. The series resistance (RS) rises significantly from 11.7 Ω ·cm2 to 16.9 Ω ·cm2 upon the external pressure, suggesting the abrupt increase in the contact resistance either at the interface between the QD solid and the MoO3\Au electrode or at the interface between the ITO electrode and the ZnO solid. To isolate the influence of the external pressure on QD and MoO3\Au interface, we conducted a controlled experiment, applying the external pressure onto ITO\ZnO\QD prior to MoO3\Au deposition, for comparison (Figure 3). In contrast to the pressure test onto the whole device that accompanies the decrease in VOC and FF (Figure 1 and Figure 2), the pressure onto ITO\ZnO\QD leads to the increase in JSC without notable change in VOC and FF. The comparative experiments clearly show that the external pressure leads to the increase in the


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contact resistance at the interface between the QD film and the MoO3\Au top electrode. The external pressure gives rise to structural deformation (e.g., crack formation) at the interface leading to the gradual development of shunt paths and the sudden rise of the contact resistance, yielding the decrease in VOC and FF of devices.

Figure 3. Current (J)-voltage (V) characteristics of QD solar cells without external pressure (empty circle) and with pressure on ITO/ZnO/QD prior to MoO3/Au deposition (0.2 MPa, filled square). VOC and FF are 0.45 V and 56.51% for pristine and 0.45 V, 55.49% for pressed QD solar cell, respectively.



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Figure 4. Cross sectional TEM images of (a) pristine and (b) pressed (0.2 MPa) QD solar cells (inset: magnified TEM images of circled area). The changed electrical properties of QD solar cells remain constant over time after the compression, suggesting that the external pressure leads to an irreversible structural change of devices and thereby the electrical properties. To verify the influence of the external pressure on the devices, we have conducted cross-sectional TEM analysis on pristine versus pressed QD solar cells (Figure 4). The overall thickness of QD solar cell does not alter upon the external pressure, but notable morphological changes are perceived in QD active layers. Specifically, mesoscopic cracks that are observed in the pristine QD active layer disappear upon the external pressure. We consider these voids are created during the deposition of QD active layers. The exchange of long native ligands to short ones inevitably accompanies volume contraction of the deposited QD layers that leaves mesoscopic voids. These cracks in QD active layers likely hinder charge carrier transport across the active layers toward charge transport layer/electrodes. Subsequent deposition of QDs, charge transport layer (MoO3) or metal (Al) does not seem to fill


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the voids, but the external pressure applied upon QD solar cells compacts QD active layers, leading to the enhancement of charge carrier transport.

Figure 5. Transfer curves of bottom gate thin film transistors using pristine (empty circle) and pressed (0.8 MPa (filled square) and 1.6 MPa (filled triangle)) QD solids. Table 2. Field effect mobilitya) with respect to increasing pressure.

Top gate Bottom gate

Pristine

0.8 MPa

1.62 × 10-4 3.53 × 10-5

1.90 × 10-4 5.01 × 10-5

1.6 MPa 1.96 × 10-4 5.72 × 10-5 a) unit: cm2/V·s

To assess the influence of the structural change observed in QD active layers to their electrical properties, we fabricated thin film transistors (TFTs) with QD solids as the semiconducting channel (Figure 5). The field effect mobility (ߤ୊୉୘ ) can be obtained from the characteristics of TFT in the saturation region. ௐ

‫ܫ‬ୈୗ ൌ ଶ௅ ‫୓ܥ‬ଡ଼ ߤሺܸୋୗ െ ܸ୘ ሻଶ

(1)



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where IDS is the drain to source (S/D) current, W is the width of the channel, L is the length of the channel, COX is the capacitance per unit area of the gate insulator, VGS is the gate to source voltage and VT is the threshold voltage. In the top gate TFTs, ߤ increases from 1.62×10-4cm2/V·s to 1.90×10-4cm2/V·s and 1.96×10-4cm2/V·s for as applying 0.8 MPa and 1.6 MPa, respectively. Same trends are also observed in the bottom gate TFTs (Table 2). These results indicate that the external pressure removes the mesoscopic voids within QD solids that impedes the charge carrier transport, and thus increase JSC in QD solar cells. There is also possibility that the external pressure activates the rearrangement of QDs in solids and reduces the inter-QD distance, leading to the enhancement in the electrical conductivity throughout the QD solids. However, any noticeable change in the refractive indices (݊஛ ) or the 1S transition in absorbance spectra of QD solids was not monitored upon the external pressure (Supporting Information). This suggests that the enhancement in the electrical conductivity of QD solids is attributed to the removal of mesoscopic voids upon the external pressure rather than either the substantial change in the order of QD solids or the inter-QD distances. We note that QD solar cells exemplified in the present study will need improvement in terms of device performances compared to the state-of-the-art QD solar cells.3, 25 However, we conducted a systematic investigation on the relationship between external pressure and electrical characteristics of QD solar cells which showed over 10% increase in PCE. We believe that our simple yet effective approach will provide an implication on development of efficient and stable QD solar cells. CONCLUSION


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We demonstrated that the post-treatment via the external pressure can improve the device performance of QD solar cells. The structural analysis in chorus with optical and electrical characterization on QD solids reveal that the external pressure compacts QD active layers by removing the mesoscopic voids and enhances the charge carrier transport, leading to significant increase in JSC of QD solar cells. Increasing the external pressure, by contrast, accompanies reduction in FF and VOC, yielding the trade-off relationship between JSC and FF and VOC in PCE of devices. Optimization at the external pressure enables to achieve over 10 % increase in PCE of QD solar cells. The approach and results evince that the control over the organization of QDs is the key for the charge transport properties in ensemble and also offer simple yet effective mean to enhance the electrical performance of transistors and solar cells using QDs.

EXPERIMENTAL SECTION Synthesis of PbS QD. Lead(II) oxide (1.006g) and oleic acid (OA) (3.4 ml) dissolved in 1octadecene (ODE) (40 ml) was degassed at 110 °C under vacuum for 2 hours and filled with Ar. At the elevated temperature, Bis(trimethylsilyl) sulfide ((TMS)2S) (0.402 ml) diluted with ODE (10 ml) was rapidly injected into the reaction flask. The reaction flask was naturally cooled to room temperature to cease the reaction. The products were purified twice via the precipitation and redispersion method using acetone and hexane, respectively. Final products were dispersed in octane at a concentration of 90 mg/ml. Synthesis of ZnO NP. Mixture of zinc acetate dehydrate (Zn(CH3COO)2·2H2O) (2 g) and methanol (80 ml) was prepared and heated up to 60 °C. At the elevated temperature, potassium hydroxide (KOH) solution (0.96 g in 40 ml in methanol) was added into the reaction flask for



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400 sec. After 2 hr of reaction, the flask was cooled down to room temperature to cease the reaction. ZnO NPs formed in the mixture were isolated by centrifugation at 4000 rpm for 10 min, and re-dispersed in 1-butanol at a concentration of 20 mg/ml. Solar cell fabrication. 25 nm thick ZnO NP layers were prepared by spin-casting ZnO NP solution (20 mg/ml) onto cleaned ITO/Glass substrates at 2,000 rpm for 40 sec, followed by thermal annealing in N2 oven at 100 °C for 30 min. QD active layers were coated through the layer-by-layer (LBL) deposition method3, 26. Briefly, QDs stabilized with OA (50 mg/ml) were formed through spin-casting on ZnO/ITO/Glass at 2500 rpm for 20 sec. Then tetrabutyl ammonium iodide (TBAI) (10mg/ml) dissolved in methanol was dropped on the QD layers with following 30 sec of waiting for sufficient reaction and spin casting at 2500 rpm for 20 sec. Then methanol was spin-coated for three times at same condition for sufficient removal of residues. These LBL steps were repeated sequentially with respect to the number of QD layers. For the QD solar cell in Figure 1 and Figure 4, 8 layers of QD (260 nm) were stacked through LBL method while 6 layers of QD were coated in Figure 2 and Figure 3. Then, QD layers were dried overnight in glove box filled with Ar in prior to MoO3 (10 nm)/ Al (100 nm) thermal evaporation. TFT fabrication. TFT was fabricated in order to analyze the carrier transport property of QD solids. Top gate TFT was fabricated as following steps. Cleansed glass substrate was prepared. On top of the substrate, S/D with channel of 50 µm length and 1mm width were patterned by evaporating Au. Above the patterned S/D, 55 nm thick QD solids were deposited via LBL method. Dielectric layer CYTOP27 was spin-coated on top of QD layers. Finally Au gate was thermally evaporated. For bottom gate TFT, Si substrate covered with 100 nm thick SiO2


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dielectric layer was used. On the substrate, 80 nm thick QD solids were stacked via LBL method. Au S/D with channel of 150 µm length and 1mm width were thermally evaporated. Pressure treatment. External pressure was applied onto QD solar cells using micro-stamping machine (ModuSystems) for 1 min. Octadecyltrichlorosilane (OTS) treated glass substrates were mounted on the prepared QD solar cells. To briefly explain, bare glass is dipped into solution containing OTS (0.1 ml) and toluene (80 ml) for two hours. The substrates were then cleaned by sonicating with toluene and isopropyl alcohol for 20 min. During gradually increasing compression, almost half of the devices showed shortened current-voltage characteristics due to high pressure while remaining half devices showed gradual trends. The results of latter devices were chosen and summarized in Figure 2. Characterizations. The current density-voltage (J-V) characteristics were measured using Keithley 237 source measurement unit with AM 1.5G solar simulator (Newport, 91160A). The cross-sectional scanning transmission electron microscopy (STEM) images of devices were obtained using JEOL USA JEM-2100F. Infra-red (IR) absorbance of QD samples was measured using Cary 5000 UV-Vis-NIR spectrophotometer (Agilent Technologies). ASSOCIATED CONTENT Supporting Information. Shunt and series resistance, J-V characteristics, IR absorbance, refractive index, extinction coefficient and AFM images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors



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*E-mail: [email protected] *E-mail: [email protected] ACKNOWLEDGMENT This work was financially supported by the Korea Ministry of Science, ICT & Future through the Global Frontier R&D Program on Center for Multiscale Energy System (2011-0031567) and the Human Resources Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20124010203170). This research was also supported by the MOTIE (Ministry of Trade, Industry & Energy (project number 2MR3600)) and KDRC (Korea Display Research Consortium) support program for the development of future devices technology for display industry. REFERENCES 1.

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