View Article Online View Journal

Nanoscale Accepted Manuscript

This article can be cited before page numbers have been issued, to do this please use: T. Liu, D. Kim, H. Han, A. R. B. Mohd Yusoff and J. Jang, Nanoscale, 2015, DOI: 10.1039/C5NR01433F.

This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

www.rsc.org/nanoscale

Page 1 of 10

Nanoscale

ARTICLE

View Article Online

DOI: 10.1039/C5NR01433F

Journal Name

Fine-tuning Optical and Electronic Properties of Graphene Oxide for Highly Efficient Perovskite Solar Cells

Simplifying the process of fine-tuning the electronic and optical properties of graphene oxide (GO) is of importance in order to fully utilize it as the hole interfacial layer (HIL). We introduced silver trifluoromethanesulfonate (AgOTf) inorganic chemical dopant that tune and control the properties of single-layer GO films synthesized by chemical vapor deposition. The morphology, work function, mobility, sheet resistance, and transmittance of the GO film were systematically tuned by various doping concentrations. We further developed solution-processable low-temperature hole interfacial layer (HIL) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS):AgOTf-doped GO HIL in highly efficient perovskite solar cells. The PEDOT:PSS:AgOTf-doped GO HIL grants desirable charge-collection in the HIL allowing the entire device to be prepared at temperatures less than 120 ˚C. The fabricated perovskite solar cells utilize rigid substrate demonstrate compelling photovoltaic performance with a power conversion efficiency (PCE) of 11.90%. Moreover, flexible devices prepared using a polyethylene terephthalate (PET)/ITO demonstrate a PCE of 9.67%, while ITO-free flexible devices adopting PET/aluminum doped zinc oxide (AZO)/silver (Ag)/AZO demonstrate a PCE of 7.97%. This study shows that PEDOT:PSS:AgOTf-doped GO HIL has significant potential to contribute to the development of lowcost solar cells

Introduction Today, the perovskite solar cells, that employ organolead halide, has attracted huge attention due its outstanding photovoltaic performance and inexpensive processing technology. Huge improvement has been made compared to the first attempt which used CH3NH3PbX3 (X=Br or I) in dye-sensitized liquid type solar cells. During the first trial in 2009, the power conversion efficiency (PCE) of 3.1-3.8% was attained by Kojima and co-workers.1 Two years later, Im and co-workers fully optimized the concentration of the CH3NH3PbI3 coating solution, thermal annealing temperature, and electrolyte compositions of their perovskite solar cell, leading to a PCE to 6.5%.2 Nonetheless, one major shortcoming in the liquidtype dye sensitized solar cell is that it easily dissolves in the polar liquid electrolyte. This issue has been overcome by substituting the liquid electrolyte with a solid hole conductor, and recently, all-solid state solar cells utilizing CH3NH3PbI3 which is coated on the compact titanium dioxide (TiO2) layer as demonstrated by the PCE of 9.7%.3 This highly efficient perovskite solar cell is also accompanied by good lifetime.3 Previous reports established that perovskite not only performed well using TiO2 film, but also using aluminum oxide (Al2O3), in which it demonstrated a 10.9% PCE.4 Nowadays, the PCE of the perovskite solar cell has surpassed 20%, 5 thanks to its high open-circuit voltage VOC of well exceeding 1V, and fill factor FF above 70%. Such rapid progress since 2009 shows that the organolead halide is a very promising material for solar cells and have a predicted PCE of 20%.6,7 In one of the reported articles, the authors introduced a new material, yttrium-doped TiO2 as the

This journal is © The Royal Society of Chemistry 2013

electron transport layer to improve electron extraction and transport. In their study, they reported a significantly high open-circuit voltage (VOC) of 1.13 V along with the short-circuit current density (JSC) of 22.75, the fill factor (FF) of 75.01%.6 To date, although many groups have reported high performance devices, unfortunately, high performance perovskite solar cell requires high quality condensed TiO2 layer which involves nearly 500 ºC treatment. This will definitely hamper our future prospect of flexible perovskite solar cell. Alternatively, a solution-processable low-temperature approach provides an ample choice of probable substrates and electrode materials that could be utilized in such devices, including polymerbased flexible substrates, and solution processed interfacial materials that could thereby be integrated into perovskite solar cells. A few research groups have demonstrated all low-temperature processed perovskite solar cells using different transport layer processing. 8,9 Compared to that of the high-temperature compact TiO2, the performance of their devices is slightly worse. The poor performance in this low-temperature processed perovskite solar cell can be attributed to a charger combination at inadequate interfaces and structural or chemical defects in the perovskite films. In the meantime, advancements in graphene research have led to various fundamental breakthroughs. Graphene is a two-dimensional single atomic material which is constructed with sp2 hybridized carbon atoms in hexagonal form.10 It has exceptional electronic, optical, thermal and mechanical properties.11-14 Its derivative, graphene oxide (GO) on the other hand has oxygen functional

J. Name., 2013, 00, 1-3 | 1

Nanoscale Accepted Manuscript

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

Tongfa Liu, a Dongcheon Kim, b Hongwei Han, a Abd. Rashid bin Mohd Yusoff,* b and Jin Jangb

View Article Online

Nanoscale

Page 2 of 10

DOI: 10.1039/C5NR01433F

groups on the graphene plane. Therefore, it has aroused interest in applications for graphene-based HIL including battery,15 supercapacitor,16 photochemical water splitting,17 photocatalysis,18 hydrogen storage,19 fuel cell,20 bio-fuel cell,21 photovoltaic,22 sensor,23 high frequency devices,24 electrochemical systems,25 spintronics26 and photonics and terahertz devices.27 Nowadays, various dopants have been introduced as main strategies to use graphene as a transparent electrode as well as HIL to reduce the sheet resistance without sacrificing transmittance. Hence, it is of importance to search for a suitable dopant for improving conductivity without deteriorating the transmittance. In particular we have proposed that, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) mixed GO HIL exploit the benefits of high conductivity and better electrical contacts of GO and GO-like materials to electron transport.28-31 To date, PEDOT:PSS:GO HIL has been widely used in many organic electronic devices including organic photovoltaic,32 and organic light emitting diode.33 The objective of this study are: i) to propose an alternative material to condensed TiO 2 layer which does not need high temperature treatment, ii) to propose a new dopant for graphene oxide and subsequently improve its optical and electrical properties, and iii) to propose new flexible substrate for our flexible perovskite solar cell. Thus, in this study, we demonstrated the first attempt to chemically dope GO with silver trifluoromethanesulfonate (AgOTf) as an inorganic dopant that uses a chemical approach to control the optical and electronic properties of single-layer GO, and demonstrates the effects of this approach in perovskite solar cell. The morphology, work function (WF), mobility, sheet resistance, and transmittance were successfully controlled by AgOTf inorganic chemical dopants with various concentrations. By using a WF tuned GO as HIL, we demonstrated that device performance can be modulated based on the perovskite solar

2 | J. Name., 2012, 00, 1-3

Journal Name cells. We observe a decrease in series resistance (Rs) as well as a reduction in recombination losses with a low temperature HIL. More importantly, we achieved a PCE of 11.90% based on a glass/ITO substrate, which is comparable to what is obtained with high-temperature approaches. To access the applicability to flexible, and ITO-free flexible cells, the polyethylene terephthalate (PET)/ITO and PET/aluminum were doped with zinc oxide (AZO)/silver (Ag)/AZO and respectively demonstrated 9.67% and 7.97% PCEs.

Results and Discussion Firstly, we carried out comprehensive thin film characterizations of PEDOT:PSS:AgOTf-doped GO in order to access details about the proposed PEDOT:PSS:AgOTf-doped GO HIL. The GO films studied here were prepared using a modified Hummers method.34 2 g of graphite powder (Alfa Aesar, natural, universal grade, -200 mesh, 99.9995%) was stirred for 1 day in an ice water bath with 2 g of sodium nitrate (NaNO3) and 100 mL of concentrated sulfuric acid (H2SO4). Afterwards, 12 g of KMnO4 were gradually added. The solution was further stirred at 35˚C until a highly viscous liquid was obtained. Later, 100 mL of pure water was added and the viscous liquid was heated in a water bath for 15 min at 90˚C. Then, it was further treated with warm water and H2O2 in sequence. The mixture was centrifuged at 6000 rpm and washed with hydrochloric acid (HCl) and water. The centrifuging and washing processes were repeated five times. Finally, GO was dried in a vacuum oven for 24 h at 50˚C. To evaluate the quality our newly synthesized GO, we used atomic force microscopy (AFM) imaging and profiling, and Raman spectroscopy characterizations. The GO layers were prepared on silicon dioxide/silicon (SiO2/Si) substrates. The GO aqueous solutions were spin-coated and then soft baked on a hot plate for 5 min at 100 °C.

This journal is © The Royal Society of Chemistry 2012

Nanoscale Accepted Manuscript

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

ARTICLE

Page 3 of 10

View Article Online

Nanoscale

DOI: 10.1039/C5NR01433F

Journal Name

ARTICLE

3

3

2

2

1

1

0

0 5 10 15 20 25 AgOTf concentration (mM)

0

240 200 160 120 0 5 10 15 20 25 AgOTf concentration (mM)

Figure 1. AFM images and height profiles of AgOTf-doped GO layer for various doping concentrations: (a) as-prepared, (b) 5 mM, (c) 10 mM, (d) 15 mM, (e) 20 mM, and (f) 25 mM. (g) Roughness and thickness of AgOTf-doped GO layers as a function of doping concentration. (h) The Rs as a function of doping concentration. PEDOT:PSS:AgOTf-doped GO downward, which is in agreement with previously reported works.39,40 This corresponds to the charge In order to dope GO, the dopant solution with various doping concentrations was dropped on the GO film. Figure 1 shows the effects of doping concentration on the surface topographies of transfer from the AgOTf dopant to GO, which resulted in phonon PEDOT:PSS:AgOTf-doped GO film. As shown in Figure 1a by the softening.40 The Raman intensity ratios of the G to 2D peaks, and D AFM height profile, the as-prepared GO film is a single layer. In to G peaks were calculated and plotted as a function of the doping addition, the roughness (Rq) and thickness (t) of concentration in Figure 2c. The I(G/2D) of as-prepared GO is 0.47, PEDOT:PSS:AgOTf-doped GO were found to be almost identical. which is signature of the single-layer GO.41 The I(G/2D) is initially As shown in Figure 1b-f, white spots of Ag nanoparticles (NPs) proportional to doping concentrations and then saturated with a (~10-30 nm) were distributed on the surface of the GO films. As the higher doping concentration. As shown in Figure 2c, the increase in doping concentration increases, the GO surface becomes rougher. the I(D/G) with increased doping concentration can be attributed to This can be clearly seen from the Rq value that is proportional to the increased molecular bonds with impurities from AgOTf dopants. doping concentrations (Figure 1g). With the increase of dopant Figure 2d demonstrates the work function (WF) of AgOTf-doped concentrations from 0 to 25 mM, the Rq and t increased to ~2.72 and GO as a function of doping concentrations. The WF monotonically ~3.2 nm respectively. The Rs was measured as a function of doping decreases to ~4.68 eV with the increase of doping concentrations to concentration for the PEDOT:PSS:AgOTf-doped GO films 25 mM, originating from the variations in the potential of the ionic transferred onto 300 nm of SiO2/Si. The Rs of the as-prepared GO is charges and the host lattice. Since the work function of Ag (4.26 eV) around 228 Ω/sq. Figure 1h demonstrates the Rs as a function of the is smaller than that of the as-prepared GO (~4.9 eV), the hole doping concentration. As the doping concentration increases up to transfer from the Ag NPs to the GO occurs during the doping 5 mM, the Rs sharply decreased to ~140 Ω/sq, but once above 5 mM process, resulting in a shift of the Fermi level above the Dirac point it began to saturate, and finally reached 118 Ω/sq at a doping of GO. It has been verified that Ag0/Ag-based nanoparticles and GO concentration of 25 mM. These results suggest that by controlling might serve as donors and acceptors respectively; thereby inducing the doping concentration the Rs can be adjusted to the values the p-type doping.42 Figure 2e shows the transmittance spectra of required. AgOTf-doped GO. At the highest AgOTf concentration, the Raman spectroscopy is sensitive to electronic structure, and has transmittance with a wavelength of 550 nm was reduced by only been established as a powerful tool for characterizing of GO about 5%, which is small compared to other kinds of dopants.43 providing vital information on defects (D band), in-plane vibration To further understand the doping concentration effect, we of sp2 carbon atoms (G band), and the stacking orders (2D band).35 fabricated a GO transistor on n+ Si/SiO2 substrates. The as prepared Figure 2a presents the effect of the doping concentration on the GO and doped GO were transferred onto the substrates followed by Raman spectra, which illustrates two dominant peaks corresponding the deposition of Au as source/drain electrodes, while n+ Si was to well documented G and 2D peaks at ~1580 and ~2700 cm−1, chosen to be the gate electrode. Figure 3a exhibits the transfer respectively involving the in-plane optical vibration (degenerate characteristics (IDS versus VGS, VGS= 0 V) of the fabricated zone center E2g mode) and second-order zone boundary phonons transistors at different conditions, where IDS is the channel current, (structural defects and disorders that can break the symmetry and VDS is the drain voltage, and VG is the gate voltage. We observed a selection rule). A relatively small Raman D peak at ~1350 cm−1 can p-type behavior of the GO due to the absorbed water since the Dirac be assigned as a disorder induced band. The shifted G- and 2D-peaks point is located at ~ 65.47 V VG,with the as-prepared GO transistors. due to the phonon stiffening is evidence that doping induced the In contrast, the AgOTf-doped GO transistor demonstrates an charge transfer.35 In general, other effects namely temperature, increase in the current IDS at zero VG by a factor of 3. This is surface charge, and strain can also be responsible for the shifting of consistent with the decrease in Rs of about 48% (Figure 1d) along the G peak.36-38 However, we excluded these effects since with a clear p-type doping effect and the shift of the Dirac point to measurements (before and after the doping process) were conducted higher VG compared to that of the as-prepared GO transistor. It is under identical conditions. Figure 2b summarizes the shifting of the generally accepted that PEDOT:PSS is a p-doped conducting G and 2D peaks. As illustrated in Figure 2b, the increase in the polymer with a WF of ~ 5 eV, which is a little bit higher than that of doping concentration shifted both the G and 2D peaks of the graphene (4.9 eV) (Figure 2d); electrons will transfer from GO to

This journal is © The Royal Society of Chemistry 2012

J. Name., 2012, 00, 1-3 | 3

Nanoscale Accepted Manuscript

4

Roughness (nm)

(h) 4

Sheet resistance (/sq.)

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

Thickness (nm)

(g)

View Article Online

Nanoscale

Page 4 of 10

DOI: 10.1039/C5NR01433F

ARTICLE

Journal Name

PEDOT:PSS after the coating and thus the GO will be doped with holes. The decrease of sheet resistance is mainly due to the p-type doping by the AgOTf solution. The distribution of carrier concentration and mobility for the as-prepared and AgOTf-doped

(b)

(a)

1600

Wavenumber (cm-1)

Intensity (a.u.)

1595 2690 2D 2685

2800

0 5 10 15 20 25 AgOTf concentration (mM)

(c) 1.0

0.5

0.2 0 5 10 15 20 25 AgOTf concentration (mM)

1590 1585 1580

5.0 4.8 4.6 4.4 4.2 4.0

0 5 10 15 20 25 AgOTf concentration (mM)

100

Transmittance (%)

(e)

0.0

I(G/2D)

0.6

Work Function (eV)

(d)

0.8

0.4

G

80 60 40 20

0 mM 5 mM 10 mM 15 mM 20 mM 25 mM

0 300 400 500 600 700 800 900 Wavelength (nm) Figure 2. (a) Raman spectra of AgOTf-doped GO layers for various doping concentrations. (b) The 2D and G peaks shift as a function of doping concentration. (c) The intensity ratios of the D to G and G to 2D peaks as a function of doping concentration.(d) Work function of AgOTf-doped GO layers as a function of doping concentration. (e) Transmittance spectra of AgOTf-doped GO layers for various doping concentrations.

4 | J. Name., 2012, 00, 1-3

This journal is © The Royal Society of Chemistry 2012

Nanoscale Accepted Manuscript

1600 2000 2400 Raman Shift (cm-1)

Wavenumber (cm-1)

2695

25 mM 20 mM 15 mM 10 mM 5 mM as prepared

1200

I(D/G)

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

GO (at zero bias) is shown in Figure 3B. The average mobility decreases from 1145 cm2 V-1 s-1 to 898 cm2 V-1 s-1 after a doping of 25 mM AgOTf due to the presence of additional charge impurity scattering.

Page 5 of 10

View Article Online

Nanoscale

DOI: 10.1039/C5NR01433F

ARTICLE

Journal Name

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

IDS (mA)

4 3

AgOTf-doped GO

2 1 0

As-prepared GO 0 20 40 60 80 VGS (V)

100

1400

As-prepared GO AgOTf-doped GO

1200 1000 800 0

2

4 6 8 N (x1013 cm-2)

10

Figure 3. (a) Transfer characteristics of GO transistors with as-prepared GO and 25 mM AgOTf-doped GO. (b) Carrier mobility and sheet carrier concentration distribution for as-prepared GO and 25 mM AgOTf-doped GO.

Low temperatures for the PEDOT:PSS:AgOTf-doped GO layer at ~ 120 ˚C can be applied for flexible perovskite solar cells. The high quality GO layer may significantly contribute to Rs, as it has been previously observed in bulk heterojunction (BHJ) organic solar cells.44,45 In order to investigate the influence of the AgOTf concentration, we prepared and characterized a series of perovskite solar cells with varying its concentration. Figure 4a depicts the device structure integrating the PEDOT:PSS:AgOTf-doped GO HIL, and the photovoltaic properties of the PEDOT:PSS:AgOTf-doped GO were investigated in conventional single-junction solar cells with a configuration of ITO/PEDOT:PSS:AgOTf-doped GO/CH3NH3PbI3-xClx/PCBM/Au. Figure 4b shows the energy level of all materials used in this study. The current density-voltage (J-V) curves of the solar cells measured under 1 sun, AM 1.5G illumination (25 °C, 100 mW/cm2). The perovskite solar cell with the 40 nm PEDOT:PSS:AgOTf-doped GO HIL was deposited onto the ITO substrate from the spin-coated AgOTf on to the PEDOT:PSS:GO mixture at a spin-speed of 2500 rpm for 60 s, and then annealed for 15 min at 120 ˚C. Later, the organo metal trihalide perovskite absorber CH3NH3PbI3-xClx was deposited on top of the HIL by spin-casting at a spin-speed 5000 rpm for 60 s, and later annealed for 15 min at 95˚C. We then deposit a 2% PCBM in a chlorobenzene solution onto the CH3NH3PbI3-xClx layer at a spinspeed of 1000 rpm for 60 s as the electron interfacial layer (EIL). This was done without any further heat treatment. Finally, devices were completed using thermal evaporation of the Au contacts. Glass/ITO substrates were used for rigid devices while PET/ITO, and PET/AZO/Ag/AZO substrates were used for flexible devices. The device area is 0.1 cm2. It is worth noting that another type of perovskite solar cells without the presence of any HIL would also

This journal is © The Royal Society of Chemistry 2013

fabricate according to the above mentioned procedures. We varied the doping concentrations of AgOTf in the PEDOT:PSS:GO HIL to find a suitable composition. From Figure 4c, the presence of an efficient HIL is essential to obtain highly efficient devices. The perovskite solar cells fabricated with PEDOT:PSS:AgOTf-doped GO presents the JSC and FF with around 14.76 mA/cm2 and 64.71%, respectively. The JSC and FF increase with the increased doping concentration of up to 15 mM to 19.18 mA/cm2 and 70.51%, respectively. However, further increases in the doping concentration to 25 mM led to a decrease in both the JSC and FF. It is worth noting that the average open circuit voltage (VOC) changed slightly with the realization of AgOTf in the PEDOT:PSS:GO with the V OC of around 0.88 V up to 15 mM. VOC plunges as the doping concentration further increases over 15 mM, probably due to bare Ag NPs contacting the perovskite directly, resulting in a less selective electrode. Recombination took place directly between the electrons in the PEDOT:PSS:GO and holes in the perovskite. Accordingly, the values of the device Rs extracted from the inverse slopes of the dark J-V curves near the VOC decreased after the AgOTf-doped GO was incorporated into the PEDOT:PSS (not shown), indicating that the improved performance of the PEDOT:PSS:AgOTf-doped GO perovskite solar cells resulted from a reduction of Rs in the device resistance. One should note that the increase of JSC probably related with plasmonic effects since the Ag NPs are distributed on the surface of GO films. In general, the presence of Ag NPs trigger the local enhancement of incident electromagnetic irradiation field in the vicinity of the small-sized Ag NPs as well as to multiple scattering by the Ag NPs.46 However, in this study we do not relate the increase in JSC due to the presence of plasmonic effect rather than trying to understand the doping behavior of the newly proposed HIL.

J. Name., 2013, 00, 1-3 | 5

Nanoscale Accepted Manuscript

(b)

5

Mobility (cm2/Vs)

(a)

View Article Online

Nanoscale

Page 6 of 10

DOI: 10.1039/C5NR01433F

ARTICLE

Journal Name (c)

(d)

14

20 15

2 JSC (mA/cm )

PCE (%)

12 10 8

10

6

5

2

(e)

0

0 5 10 15 20 25 AgOTf concentration (mM) (f)

75

0 5 10 15 20 25 AgOTf concentration (mM)

1.0 0.8

VOC (V)

70

65

0.6 0.4 0.2

60

0 5 10 15 20 25 AgOTf concentration (mM)

0.0

0 5 10 15 20 25 AgOTf concentration (mM)

Figure 4. (a) Device structure. (b) Energy level of perovskite solar cells employing PEDOT:PSS:AgOTf-doped GO as HIL. (c) AgOTf concentrations dependence in the PEDOT:PSS:AgOTf-doped GO measured under 1 sun, AM 1.5G illumination (25 °C, 100 mW/cm 2).

Figure 5a displays the J-V characteristics of the perovskite solar cells measured under 1 sun, AM 1.5G illumination (25 °C, 100 mW/cm2), namely PEDOT:PSS:AgOTf-doped GO (Device I), PEDOT:PSS (Device II), and GO (Device III), and no HIL (Device IV). As seen in Table 1, as expected, the perovskite solar cell without HIL exhibits the worst performance. The major drawbacks for the perovskite solar cells with ITO are both the JSC, and FF. This is most likely due to an improper interface contact at the ITO/perovskite. Low FF and V OC in the perovskite solar cell without any HIL can be attributed to increased recombination at the ITO surface and a correspondingly low shunt resistance (Rsh). The Rsh is due to the direct contact of ITO and perovskite material and no Ohmic contact forms, and this will eventually increase the carrier traps or unfavourable interface dipoles and thereby decrease the FF of the device. The perovskite solar cells with a presence of

either GO or PEDOT:PSS experience the enhancement of the JSC and FF compared to that of perovskite solar cells without any HIL. The most prominent feature comes from the perovskite solar cells with pristine PEDOT:PSS HIL, where the JSC and FF increased from 11.00 to 18.68 mA/cm 2, and 48.93 to 65.12%, respectively. Moreover, a distinctive improvement in the photovoltaic parameters can be observed when PEDOT:PSS:AgOTf-doped GO HIL is sandwiched between the CH3NH3PbI3-xClx and ITO substrate. The highly efficient solution-processable low-temperature perovskite solar cells utilizing a PEDOT:PSS:AgOTf-doped GO HIL measured at 100 mW/cm2 illumination demonstrates a J SC of 19.18 mA/cm2, VOC of 0.88 V, and FF of 70.51% leading to 11.90% PCE. As seen from Table 1, it is clear that the presence of HIL is practically important to acquire a high J SC and FF.

Table 1. Summary of performance parameters of perovskite solar cells fabricated with different hole transport layers. HTL JSC (mA/cm2) VOC (V) FF (%) PCE (%) RS (Ω cm2) PEDOT:PSS:AgOTf-doped GO 19.18 0.88 70.51 11.90 12.11 PEDOT:PSS 18.68 0.87 65.12 10.70 16.44 GO 11.29 0.80 56.42 5.10 21.35 No HIL 11.00 0.82 48.93 4..43 27.84

The improvement in efficiency can be attributed to the improved JSC, VOC, and FF. It may rise from beneficial factors resulting from synergetic effects of PEDOT:PSS:AgOTf-doped

This journal is © The Royal Society of Chemistry 2013

RSh (kΩ cm2) 765.41 648.05 589.13 315.11

GO. This is supported by the reduction in the WF of PEDOT:PSS:AgOTf (Figure 2d). The WF of PEDOT:PSS, GO, and PEDOT:PSS:AgOTf-doped GO were measured with X-ray

J. Name., 2013, 00, 1-3 | 6

Nanoscale Accepted Manuscript

0

FF (%)

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

4

Page 7 of 10

View Article Online

Nanoscale

DOI: 10.1039/C5NR01433F

well as in light emitting devices. 47-53 Second, the higher mobility of PEDOT:PSS:AgOTf-doped GO may yield improved electrical conductivity of the layer (not shown). From the high FF, it is clear that the PEDOT:PSS:AgOTfdoped GO composite reduces the Rs compared to the PEDOT:PSS or even the GO solar cells. We have achieved high device performance based on low-temperature (less than 120 ˚C) solution processed perovskite solar cells, suggesting that it is also possible to attain high device performance flexible solar cells based on which processing techniques are utilized. Here, we replaced the rigid glass/ITO substrate with a flexible PET/ITO substrate and fabricated the devices via the same procedures. A photograph of the flexible device is shown in inset Figure 5c. The flexible device (PET/ITO) shows a V OC of 0.88 V, JSC of 15.58 mA/cm2, and FF of 70.50% (Figure 5c, Table 2).

Table 2. Summary of performance parameters of perovskite solar cells fabricated with different substrates. Device JSC (mA/cm2) VOC (V) FF (%) Glass/ITO 19.18 0.88 70.51 PET/ITO 15.58 0.88 70.50 PET/AZO/Ag/AZO 12.85 0.88 70.37

Furthermore, a PCE of 9.67% is achieved, keeping 80% of the rigid devices' performance (Table 3). In comparison with the rigid devices, the loss in PCE arises from the decreased J SC and FF, which could be due to the higher Rs of the flexible devices, issues that can be resolved in the near future. For the ITO-free perovskite solar cell (PET/AZO/Ag/AZO), we

demonstrated exceptional mechanical stability, in which the PCE degraded by 1.26% after 2000 bending cycles (Figure 5d, Table 4). Results show that the device maintains its performance by mechanically bending up to 2000 times, indicating that both devices tolerate repeated mechanical deformation.

0 -4 -8

Device I Device II Device III Device IV

-12 -16 -20

0.0

0.2

0.4 0.6 Bias (V)

0.8

(c)

Current Density (mA/cm2)

(b)

(a) Current Density (mA/cm2)

PCE (%) 11.90 9.67 7.97

0 -4

Glass/ITO PET/ITO PET/AZO/Ag/AZO

-8 -12 -16 -20

0.0

0.2

0.4 0.6 Bias (V)

0.8

0

Current Density (mA/cm2)

(d) Initial 100 times 500 times 1000 times

-4 -8 -12 -16 -20

0.0

0.2

0.4 0.6 Bias (V)

This journal is © The Royal Society of Chemistry 2012

0.8

0

Initial 100 times 500 times 1000 times

-4 -8 -12 -16 -20

0.0

0.2

0.4 0.6 Bias (V)

0.8

J. Name., 2012, 00, 1-3 | 7

Nanoscale Accepted Manuscript

ARTICLE

photoemission spectroscopy (XPS). The as-prepared GO has a work function of ~ 4.9 eV, while the work function of PEDOT:PSS was calculated to be 5.0 eV, which means that it is suitable to use as the HIL. After depositing the AgOTf (25 mM)-doped GO onto the PEDOT:PSS film, its work function decreases to 4.68 eV, indicating a lower energy barrier for hole transport at the PEDOT:PSS:AgOTf-doped GO/perovskite interface than the GO/perovskite or PEDOT:PSS/perovskite interfaces. As PEDOT:PSS:AgOTf-doped GO has a WF between that of ITO and the CH 3NH3PbI3-xClx conduction band, it may reduce the energy barriers at the interfaces. Therefore, it behaves as a better hole transport than if the PEDOT:PSS or GO were alone. It is worth noting that many other groups have also reported high work function hole extraction materials and used it to align their energy level with the HOMO level of active layers in perovskite solar cells as

Current Density (mA/cm2)

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

Journal Name

Nanoscale

View Article Online

Page 8 of 10

DOI: 10.1039/C5NR01433F

ARTICLE

Journal Name

Table 4. Summary of performance parameters of flexible perovskite solar cells (PET/AZO/Ag/AZO) fabricated with different bending cycles. Device JSC (mA/cm2) VOC (V) FF (%) PCE (%) Initial 12.85 0.88 70.37 7.97 100 12.85 0.88 70.34 7.95 500 12.81 0.88 70.30 7.92 1000 12.75 0.88 70.21 7.87

To systematically characterize the flexibility of the present devices, a series of controlled bending tests were performed. Figure 6a shows the dependence of the PET/ITO and PET/AZO/Ag/AZO sheet resistances at various bending radius. Sheet resistances were measured by standard four-probe techniques after relaxing the PET/ITO and PET/AZO/Ag/AZO substrates. The sheet resistances before bending for both PET/ITO and PET/AZO/Ag/AZO are about ~16 and ~15.4 Ω/sq., respectively. During the flexibility measurements, the conductivities of these two substrates did not illustrate any notable discrepancy with decreasing bending radius (not shown). The sheet resistances experienced slight changes after the substrates were bent at ~2.5 mm. Usually, changes in RS were accompanied by microstructural changes resulting from the bending measurements, therefore, we carried out optical microscopic, and cross-sectional SEM measurements for both substrates after bending at radius ~ 5 mm and ~ 1.5 mm (Figure 6b). As seen from the optical microscopic images, one observes crack formation (Figure 6b),

(a)

whereas PET/AZO/Ag/AZO retains a continuous, and uniform morphology. To evaluate the effects of smaller radius, SEM images were obtained for substrates with a bending radius ~0.5 mm. Note that the PET/ITO substrate shows higher crack densities, leading to incredible increments in sheet resistance from 17.5 to 3 M Ω/sq (data not shown) compared to that of PET/AZO/Ag/AZO substrate. It is also worth noting that under such extreme bending conditions, the PET/ITO image shows unique continuous “arc” features, implying that the PET/ITO has a tough structural continuity without detectable crack formations. At various bending radiuses from 100 to 1 mm, there were excellent perovskite solar cell (PET/AZO/Ag/AZO) performances with only modest PCE degradation from 7.97% to 7.65%, mainly due to reduced FFs (Figures 6c). In contrast, the behaviour of perovskite solar cell (PET/ITO) exhibits significant mechanical degradation since the PCE dropped from 9.69% to 8.87% at various bending radii from 100 to 1 nm. Results illustrate that the PEDOT:PSS:AgOTf-doped GO is considered to be an excellent HIL for developing high performance flexible perovskite solar cells. (b)

30 24 PET/AZO/Ag/AZO

18 12

r

α

6 0

0

PET/ITO

20 40 60 80 100 Bending radius (mm)

This journal is © The Royal Society of Chemistry 2013

J. Name., 2013, 00, 1-3 | 8

Nanoscale Accepted Manuscript

Table 3. Summary of performance parameters of perovskite solar cells (PET/ITO) fabricated with different bending cycles. Bending times JSC (mA/cm2) VOC (V) FF (%) PCE (%) Initial 15.58 0.88 70.50 9.67 100 15.43 0.86 70.31 9.33 500 15.21 0.86 70.15 9.17 1000 15.15 0.86 70.11 9.13

Sheet resistance (/sq.)

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

Figure 5. (a,b) Current density-voltage curves of different hole transport layers and different substrates, respectively measured under 1 sun, AM 1.5G illumination (25 °C, 100 mW/cm 2). Bending characteristics of flexible perovskites solar cells (c, and d) PET/ITO/PEDOT:PSS:AgOTf-doped GO/CH3NH3PbI3-xClx/PCBM/Au perovskite solar cell, and PET/AZO/Ag/AZO/PEDOT:PSS:AgOTf-doped GO/CH3NH3PbI3-xClx/PCBM/Au perovskite solar cell with various bending cycles measured under 1 sun, AM 1.5G illumination, respectively (25 °C, 100 mW/cm2).

Page 9 of 10

Nanoscale

Journal Name

View Article Online

DOI: 10.1039/C5NR01433F

ARTICLE

(c)

10

8 PET/AZO/Ag/AZO 7

0

20 40 60 80 100 Bending radius (mm)

Figure 6. Mechanical flexibility of PET/ITO and PET/AZO/Ag/AZO substrates and perovskite devices. (a)Dependence of sheet resistance on bending radius (measured after fully relaxing) of PET/ITO and PET/AZO/Ag/AZO electrodes. (b) Optical microscopic and SEM cross-sectional images of PET/ITO and PET/AZO/Ag/AZO substrates, collected after bending at a radius of 5 mm and 1.5 mm, respectively. (c) Dependence of perovskite solar cell performance on bending radius.

Conclusions In conclusion, we have successfully demonstrated the potential dopant for graphene oxide and also successfully integrated in a highly efficient perovskite solar cell. We have obtained 11.90% and 7.97% efficiency in a rigid and flexible format of devices, respectively. This is among the best recorded efficiency for perovskite solar cell employing graphene oxide hole transport layer and the best recorded efficiency for low temperature flexible perovskite solar cell to date. The present work demonstrates that a suitable dopant can be fully utilized in achieving high device performance especially in terms of flexible solar cell, where a high processing temperature should be avoided.

Experimental Single layer graphene oxide sheets were grown on Cu foils by CVD and subsequently transferred onto SiO2/Si and quartz substrates. A layer of PMMA was spin coated onto the graphene oxide/Cu layer. The Cu foil substrate was removed by electrochemical reactions with aqueous 0.1 M ammonium persulfate solution NH4/2S2O8. After placing the PMMA/graphene oxide stack on the target substrate, the PMMA/graphene oxide/substrate stack was heated at 180 °C for 2 h to ensure close bonding of the graphene onto the substrate. The PMMA was then removed using acetone and deionized (DI) water, and the graphene oxide/substrate stack was annealed at 400 °C for 1 h under a vacuum to remove the residual PMMA. Devices were all fabricated on indium tin oxide (ITO) coated glass substrates (14 Ω/sq). Firstly, the substrates were cleaned in acetone and then isopropanol. The low-temperature processed hole collection layer of PEDOT:PSS:GO was spun at 1400 rpm for 60 s followed by a soft bake at 120 °C for 10 min. 200 L of mixed AgOTf (Sigma Aldrich) in nitromethane (Sigma Aldrich) with different AgOTf concentrations was dropped on the PEDOT:PSS:GO film and spun at 2500 rpm for 60 s and annealed at 120 °C for 15 min. For the perovskite layer, a CH3NH3PbI3−xClx was deposited by spin-coating the precursor solution (40 wt%) on the PEDOT:PSS:AgOTF-doped GO at 5000 rpm for 60 s. After drying at room temperature for 10 min, the perovskite films were annealed on a hot plate at 95 °C for 15 min. After that, 2% of PCBM in chlorobenzene solutions was

This journal is © The Royal Society of Chemistry 2012

coated onto the perovskite layer at 1000 rpm. Finally, devices were completed by evaporating 1000 nm Au contact electrodes through a shadow mask. The performance of the perovskite solar cells was obtained from J-V characteristics measured using a Keithley 2400 LV source meter. Solar cell performance was measured using a solar simulator, with an Air Mass 1.5 Global (AM 1.5 G) and had an irradiation intensity of 100 mW/cm2. All measurements were carried out at room temperature, under a relative humidity of 60%. The EQE measurements were performed using the EQE system (Model 74000) obtained from Newport Oriel Instruments USA and HAMAMATSU calibrated silicon cell photodiodes as a reference diode. The wavelength was controlled with a monochromator of 200-1600 nm.

Acknowledgements This work was supported by Human Resources Development program (No. 20134010200490) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. 1 A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050. 2 J. –H. Im, C. –R. Lee, J.–W. Park, S. –W. Park, N.–G. Park, Nanoscale 2011, 3, 4088. 3 H. –S. Kim, C. –R. Lee, J. –H. Im, K.–B. Lee, T. Moehl, A. Marchioro, S. –J. Moon, R. Humphry-Baker, J. –H. Yum, J. E. Moser, Sci. Rep. 2013, 2, 591. 4 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643. 5 http://www.nrel.gov/ncpv/images/efficiency_chart.jpg 6 H. Zhou, Q. Chen, G. Li, S. Luo, T-B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 2014, 345, 542. 7 H. J. Snaith, J. Phys. Chem. Lett. 2013, 4, 3623. 8 M. H. Kumar, N. Yantara, S. Dharani, M. Gratzel, S. Mhaisalkar, P. P. Boix, N. Mathews, N. Chem. Commun. 2013, 49, 11089. 9 S. Y. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T. C. Sum, Y. M. Lam, Energy Environ. Sci. 2014, 7, 399. 10 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I.V. Grigorieva, A. A. Firsov, Science 2004, 306, 666. 11 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, M. I. Katsnelson, I. V. Grigorieva, S. V. Dubonos, A. A. Firsov, Nature 2005, 438, 197. 12 A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183.

J. Name., 2012, 00, 1-3 | 9

Nanoscale Accepted Manuscript

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

PCE (%)

PET/ITO 9

Nanoscale

13 A. K. Geim, Science 2009, 19, 1530. 14 S. Park, R. S. Ruoff, Nat. Nanotechnol. 2009, 4, 217. 15 N. Li, Z. Chen, W. Ren, F. Li, H. –M. Cheng, Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 17360. 16 Z. –S. Wu, K. Parvez, X. Feng, K. Mullen, Nat. Commun. 2013, 4, 2487. 17 D. W. Boukhalov, Y.W. Son, R. S. Ruoff, ACS Catal. 2014, 4, 2016. 18 Q. Xiang, J. Yu, J. Phys. Chem. Lett. 2013, 4, 753. 19 V. Tozzini, V. Pellegrini, Phys. Chem. Chem. Phys. 2013, 15, 80. 20 H. Tateishi, K. Hatakeyama, C. Ogata, K. Gezuhara, J. Kuroda, A. Funatsu, M. Koinuma, T. Taniguchi, S. Hayami, Y. Matsumoto, J. Electrochem. Soc. 2013, 160, F1175. 21 C. Liu, S. Alwarappan, Z. Chen, X. Kong, C. –Z. Li, Biosens. Bioelectron. 2010, 25, 1829. 22 A. R. B. M. Yusoff, S. J. Lee, F. K. Shneider, W. J. da Silva, J. Jang, Adv. Energy Mater. 2014, 4, DOI: 10.1002/aenm.201301989. 23 F. Yavari, N. Koratkar, J. Phys. Chem. Lett. 2012, 3, 1746. 24 Y. Wu, K. A. Jenkins, A. V. Garcia, D. B. Farmer, Y. Zhu, A. A. Bol, C. Dimitrakopoulos, W. Zhu, F. Xia, P. Avouris, Y. –M. Lin, Nano Lett. 2012, 12, 3062. 25 F. Liu, C. W. Lee, J. S. Im, J. Nanomaterials, 2013, 2013, 642915. 26 M. Zeng, L. Shen, M. Zhou, C. Zhang, Y. Feng, Phys. Rev. B 2011, 83, 115427. 27 A. Tredicucci, M. S. Vitiello, J. Select. Topics Quantum Electron. 2013, 20, 8500109. 28 Y. Zhang, Z. R. Tang, X. Fu, Y. J. Xu, ACS Nano 2011, 5, 7426. 29 C. Zhu, S. Guo, P. Wang, L. Xing, Y. Fang, Y. Zhai, S. Dong, Chem. Commun. 2010, 46, 7148. 30 Y. Yang, H. Wang, H. S. Casalongue, Z. Chen, H. Dai, Nano Res. 2010, 3, 701. 31 N. Yang, J. Zhai, D. Wang, Y. Chen, L. Jiang, ACS Nano 2010, 4, 887. 32 A. R. B. M. Yusoff, F. K. Shneider, W. J. da Silva, J. Jang, Submitted to Journal of Materials Chemistry A 33 W. J. da Silva, A. R. B. M. Yusoff, J. Jang, IEEE Electr. Dev. Lett. 2013, 34, 1566. 34 W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339. 35 A. Das, S. Pisana, B. Chakraborty, S. Piscanec, S. K. Saha, U. V. Waghmare, K. S. Novoselov, H. R. Krishnamurthy, A. K. Geim, A. C. Ferrari, A. K. Sood, Nat. Nanotechnol. 2008, 3, 210. 36 Z. H. Ni, T. Yu, Y. H. Lu, Y. Y. Wang, Y. P. Feng, Z. X. Shen, ACS Nano 2008, 2, 2301. 37 I. Calizo, W. Bao, F. Miao, C. N. Lau, A. A. Balandin, Appl. Phys. Lett. 2007, 91, 201904. 38 I. Calizo, A. A. Balandin, W. Bao, F. Miao, C. N. Lau, NanoLett. 2007, 7, 2645. 39 A. K. Singh, M. W. Iqbal, V. K. Singh, M. Z. Iqbal, J. H. Lee, S. –H. Chun, K. Shin, J. Eom, J. Mater. Chem. 2012, 22, 15168. 40 H. –J. Shin, W. M. Choi, D. Choi, G. H. Han, S. –M. Yoon, H.–K. Park, S. –W. Kim, Y. W. Jin, S. Y. Lee, J. M. Kim, J. –Y. Choi, Y. H. Lee, J. Am. Chem. Soc. 2010, 132, 15603. 41 S. Chen, W. Cai, R. D. Piner, J. W. Suk, Y. Wu, Y. Ren, J. Kang, R. S. Ruoff, Nano Lett. 2011, 11, 3519 42 T. Wu, H. Shen, L. Sun, B. Cheng, B. Liu, J. Shen, ACS Appl. Mater. Interfaces 2012, 4, 2041. 43 S. Kim, D. H. Shin, C. O. Kim, S. S. Kang, J. M. Kim, C. W. Jang, S. S. Joo, J.S. Lee, J. H. Kim, S.H.Choi, E. Hwang, ACS Nano 2013, 7, 5168. 44 A. R. B. M. Yusoff, H. P. Kim, J. Jang, Sol. Energy Mater. Sol. Cells 2013, 109, 63. 45 J. Nelson, The Physics of Solar Cells, Imperial College Press:London 2003. 46 M. Pelton, J. Aizpurua, G. Bryant, G. Laser Photonics Rev. 2008, 2, 136. 47 S. Ryu, J. H. Noh, N. J. Jeon, Y. C. Kim, W. S. Yang, J. Seo, S. I. Seok, Energy Environ. Science 2014, 7, 2614. 48 A. S. Subbiah, A. Halder, S. Ghosh, N. Mahuli, G. Hodes, S. K. Sarkar, J. Phys. Chem. Lett. 2014, 5, 1748. 49 J. A. Christians, R. C. M. Fung, P. V. Kamat, J. Am. Chem. Soc. 2013, 136, 758 50 K.-G. Lim, H.-B. Kim, J. Jeong, H. Kim, J. Y. Kim, T.-W. Lee, Adv. Mater. 2014, 26, 6461. 51 Y.-H. Kim, H. Cho, J. H. Heo, T.-S. Kim, N. Myoung, C.-L. Lee, S. H. Im, T.-W. Lee, Adv. Mater. 2015, 27, 7, 1248. 52 M.-R. Choi, T.-H. Han, K.-G. Lim, S.-H. Woo, D. H. Huh, T.-W. Lee, Angew. Chem. Int. Ed. 2011, 50, 6274.

10 | J. Name., 2012, 00, 1-3

Page 10 of 10

Journal Name 53 T.-H. Han, Y. Lee, M.-R. Choi, S.-H. Woo, S.-H. Bae, B. H. Hong, J.H. Ahn T.-W. Lee, Nat. Photon. 2012, 6, 105.

Nanoscale Accepted Manuscript

Published on 14 May 2015. Downloaded by Freie Universitaet Berlin on 15/05/2015 04:49:09.

ARTICLE

View Article Online

DOI: 10.1039/C5NR01433F

This journal is © The Royal Society of Chemistry 2012

Fine-tuning optical and electronic properties of graphene oxide for highly efficient perovskite solar cells.

Simplifying the process of fine-tuning the electronic and optical properties of graphene oxide (GO) is of importance in order to fully utilize it as t...
1MB Sizes 3 Downloads 87 Views