Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of...
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Efficiency Enhancement of Perovskite Solar Cells through Fast Electron Extraction: The Role of Graphene Quantum Dots Zonglong Zhu, † ,‡ Jiani Ma, ⊥ Zilong Wang, ‡ Cheng Mu, ‡ Zetan Fan, § Lili Du, ⊥ Yang Bai, ‡ Louzhen Fan, * ,§ He Yan, ‡ David Lee Phillips, ⊥ and Shihe Yang * ,† ,‡ † Nano Science and Technology Program and ‡ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ⊥ Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S.A.R., P. R. China § Department of Chemistry, Beijing Normal University, Beijing 100875, China * S Supporting Information ABSTRACT: We repo rt on a si gn icant pow er con- version efficiency improvement of perovskite solar cells from 8.81% to 10.15% due to insertion of an ultrathin graphene quantum dots (GQDs) layer between perovskite and Ti O 2 . A strong quenching of pe ro vs ki te ph oto- luminescence was observed at ∼760 nm upon the addition of the GQDs, which is pronouncedly correlated with the increase of the IPCE and the APCE of the respective cells. From the transient absorption measurements, the improved cell efficiency can be attributed to the much faster electron extraction with the presence of GQDs (90− 106 ps) than without their presence (260−307 ps). This work highlights that GQDs can act as a superfast electron tunnel for optoelectronic devices. H igh-efficiency phot ovol taic s that uses sola r radia tion is attractiv e in meeti ng societal needs for energy , as i t is clean and renewa ble. 1 Perovs kit e solar cells based on the CH 3 NH 3 PbI 3 sensitizer family have attracted great attention in terms of both mesoscopic 2,3 and plan ar heter ojunc tions , 4 with their powe r conversion efficiencies (PCEs) reaching >15%. 5,6 One prom- inent featu re of the perov skites is that the elect ron/h ole di ff usion length has reached 100 nm in the t r ii odi de absorber (CH 3 NH 3 PbI 3 ) 7 and >1 μm in the mixed hali de absorb er (CH 3 NH 3 PbI 3‑x Cl x ), 8 all much long er than in cur rent low - temperat ure solution-proc essed photovoltaic materials (typically ∼10 nm) . 9 This, tog eth er wit h a bro ad spe ctr al abs orp tio n, a hi gh open circuit voltage, etc., has aroused enthusiastic optimism among researchers over the potential of perovskite solar cells to achieve PCEs of >20%. 10 ,11 However, the injection times of electrons and holes in perovskite solar cells have been measured to be 0.4 and 0.6 ns, respectively, which are still orders of magnitude longer than the hot carrier cooling (or thermal- iza tion) time (∼0.4 ps). 7 Clear ly, a lar ge amount of theconve rte d photon energy is wasted due to thermalization, and some of the energy is also lost to carrier trapping. A variety of routes have been proposed to facilitate electron/hole extractions by taking advantage of some special mat eri als such a s fullerene, 12 graphene, 13 or core/ shell meta l nanoparticles, 14 but direct evidence is yet to be obtained regarding their role as a fast electron funnel rather than just an electron trapping center. Together, to improve PCE of the perovskite solar cells, faster electron injection is always desirable in competing with carrier trapping and even thermalization/cooling. In the work reported here, we examine the eff ects of graphene quantum dots (GQDs), 15 inserted as an ultrathin layer between perovskite and TiO 2 , on the solar cell performance. GQDs are single- or few-layer graphene but have a tiny size of only several nanometers with special quantum-con nement eff ects and edge eff ects, making them distinct from both conventional quantum dots and graphene. 16 Some unique optoelectronic properties of GQDs have alrea dy been uncovered , 17 such as hot-electron lifetimes as long as hundreds of picoseconds 18 and ultrafast hot- electron extraction (time constant <15 fs) through a GQD to TiO 2 interface. 19 In fact , GQDs were utilized as a sensitizer in a dye-sensitized solar cell, 20 but the low PCE (<0.1%) limits the further application as hot carrier chromopho re, perhaps because of some not-yet-optimized materials issues. In another device congurationorganic photovoltaics GQDs hav e bee n widely use d aselectr on acc eptorsanalogoustoful ler enes w ithresp ect to the active layer of poly(3-hexylthio phene) (P3HT), 21,22 but they are still far inferior to fullerenes in this role, due perhaps to their lower electron affinity and less appropriate assembly morphol- og y. To ou r su rprise, however, th e GQDs in our de vi ce conguration could serve as a superfast bridge to facilitate the ele ctr on inj ection fro m a hal ide per ovs kit e into TiO 2 , leading to a signicantl y enhan ced photocurrent and PCE of the correspond- ing solar cells. Indeed, from the transient absorption measure- ments, we directly observed a faster electron extraction time (90 ps) after inserting the ultrathin GQDs layer than that (280 ps) obtained without the presence of the GQDs. We synthesized the single-/few-layer GQDs by a fa ci le electrochemical method reported previously. 15 The structure of GQDs is schematically illustrated in Figure 1C. In particular, the water-soluble GQDs have a modi ed edge structure with an average diameter of 5−10 nm (Figure S1), a broad absorption band from 200 to 400 nm (Figu re S2a), and a strong photol uminesc ence peak at ∼540 nm (Fig ure S2b). Th e perovskite solar cell devices were fabricated following the layout in Figure 1 A, and the relative energy levels of the diff erent layers are shown in Figure 1B (see Supporting Information for full Received: Janua ry 5, 2014 Published: February 24, 2014 Communication pubs.acs.org/JACS © 2014 American Chemical Society 3760 dx.doi.org/10.1021/ja4132246 | J. Am. Chem. Soc. 2014, 136, 3760−3763
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7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
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Efficiency Enhancement of Perovskite Solar Cells through FastElectron Extraction The Role of Graphene Quantum DotsZonglong Zhudagger Dagger Jiani Maperp Zilong WangDagger Cheng MuDagger Zetan Fansect Lili Duperp Yang BaiDagger Louzhen Fan sect
He YanDagger David Lee Phillipsperp and Shihe Yang dagger Dagger
daggerNano Science and Technology Program and DaggerDepartment of Chemistry The Hong Kong University of Science and TechnologyClear Water Bay Kowloon Hong KongperpDepartment of Chemistry The University of Hong Kong Pokfulam Road Hong Kong SAR P R ChinasectDepartment of Chemistry Beijing Normal University Beijing 100875 China
S Supporting Information
ABSTRACT We report on a signi1047297cant power con- version efficiency improvement of perovskite solar cells
from 881 to 1015 due to insertion of an ultrathingraphene quantum dots (GQDs) layer between perovskiteand TiO2 A strong quenching of perovskite photo-luminescence was observed at sim760 nm upon the additionof the GQDs which is pronouncedly correlated with theincrease of the IPCE and the APCE of the respective cellsFrom the transient absorption measurements theimproved cell efficiency can be attributed to the muchfaster electron extraction with the presence of GQDs (90minus106 ps) than without their presence (260minus307 ps) This
work highlights that GQDs can act as a superfast electrontunnel for optoelectronic devices
H igh-efficiency photovoltaics that uses solar radiation isattractiv e in meeting societal needs for energy as it is clean
and renewable1 Perovskite solar cells based on the CH3NH3PbI3
sensitizer family have attracted great attention in terms of bothmesoscopic23 and planar heterojunctions4 with their powerconversion efficiencies (PCEs) reaching gt1556 One prom-inent feature of the perovskites is that the electronhole diff usionlength has reached 100 nm in the triiodide absorber(CH3NH3PbI3)7 and gt1 μm in the mixed halide absorber(CH3NH3PbI3‑xClx)8 all much longer than in current low-temperature solution-processed photovoltaic materials (typically sim10 nm)9 This together with a broad spectral absorption a highopen circuit voltage etc has aroused enthusiastic optimismamong researchers over the potential of perovskite solar cells to
achieve PCEs of gt2010 11 However the injection times of electrons and holes in perovskite solar cells have been measuredto be 04 and 06 ns respectively which are still orders of magnitude longer than the hot carrier cooling (or thermal-ization) time (sim04 ps)7 Clearly a large amount of theconvertedphoton energy is wasted due to thermalization and some of theenergy is also lost to carrier trapping A variety of routes have
been proposed to facilitate electronhole extractions by takingadvantage of some special materials such as fullerene12
graphene13 or coreshell metal nanoparticles14 but directevidence is yet to be obtained regarding their role as a fastelectron funnel rather than just an electron trapping center
Together to improve PCE of the perovskite solar cells fasterelectron injection is always desirable in competing with carriertrapping and even thermalizationcooling
In the work reported here we examine the eff ects of graphenequantum dots (GQDs)15 inserted as an ultrathin layer betweenperovskite and TiO2 on the solar cell performance GQDs aresingle- or few-layer graphene but have a tiny size of only severalnanometers with special quantum-con1047297nement eff ects and edgeeff ects making them distinct from both conventional quantumdots and graphene16 Some unique optoelectronic properties of GQDs have already been uncovered17 such as hot-electronlifetimes as long as hundreds of picoseconds18 and ultrafast hot-electron extraction (time constant lt15 fs) through a GQD toTiO2 interface19 In fact GQDs were utilized as a sensitizer in adye-sensitized solar cell20 but the low PCE (lt01) limits thefurther application as hot carrier chromophore perhaps becauseof some not-yet-optimized materials issues In another device
con1047297guration
organic photovoltaics
GQDs have been widely used as electron acceptors analogous to fullerenes w ithrespect tothe active layer of poly(3-hexylthiophene) (P3HT)2122 but they are still far inferior to fullerenes in this role due perhaps to theirlower electron affinity and less appropriate assembly morphol-ogy To our surprise however the GQDs in our devicecon1047297guration could serve as a superfast bridge to facilitate theelectron injection from a halide perovskite into TiO2 leading to asigni1047297cantly enhanced photocurrent and PCE of the correspond-ing solar cells Indeed from the transient absorption measure-ments we directly observed a faster electron extraction time (90ps) after inserting the ultrathin GQDs layer than that (280 ps)obtained without the presence of the GQDs
We synthesized the single-few-layer GQDs by a facile
electrochemical method reported previously15 The structureof GQDs is schematically illustrated in Figure 1C In particularthe water-soluble GQDs have a modi1047297ed edge structure with anaverage diameter of 5minus10 nm (Figure S1) a broad absorption
band from 200 to 400 nm (Figure S2a) and a strongphotoluminescence peak at sim540 nm (Figure S2b) Theperovskite solar cell devices were fabricated following the layoutin Figure 1 A and the relative energy levels of the diff erent layersare shown in Figure 1B (see Supporting Information for full
Received January 5 2014Published February 24 2014
Communication
pubsacsorgJACS
copy 2014 American Chemical Society 3760 dxdoiorg101021ja4132246 | J Am Chem Soc 2014 136 3760minus3763
7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
httpslidepdfcomreaderfullefficiency-enhancement-of-perovskite-solar-cells-through-fast-electron-extraction 24
experimental details) The cross-sectional scanning electronmicroscope (SEM) image of a typical such device is shown inFigure 1D The distribution of GQDs within the TiO2
mesoporous 1047297lm appears to be uniform according to theenergy-dispersive X-ray spectroscopy (EDS) elemental mapping(Figure S3) X-ray powder diff raction patterns in Figure S4 show peaks ascr i ba bl e t o G QD s T iO2 and tetragon alCH3NH3PbI323 24
We measured current ( J )minus voltage (V ) characteristics of thesolar cells under simulated air mass 15 global (AM15G) solarirradiation which are designated as the perovskiteCH3NH3PbI3GQDsTiO2 (or PGT for short) and the standardperovskite CH3NH3PbI3TiO2 (or PT for short) solar cellsrespectively Figure S5 shows a schematic and a photograph of arrayssuch solid-state solar cells each with an active area of 0126cm2 (18 mm times 70 mm) Figure 2 shows J minusV curves of the solarcells The most important observation from the J minusV curves is the
bene1047297cial role of the ultrathin GQDs layer between perovskite
and TiO2 the cell with the GDQs layer achieved a powerconversion efficiency (PCE) of 1015 whereas that without itonly achieved a PCE of 881 The short-circuit current density ( J sc) of the QD loading device of PGT has a signi1047297cant increasefrom 1543 to 1706 mAcm2 compared with the non-GQDloading PT device whereas the 1047297ll factor (FF) and open-circuit
voltages (V oc) are similar 0939 V and 0612 FF for the PTdevice and 0937 V and 0635 FF for the PGT device The smallV oc change is consistent with the fact that the main role of theultrathin GQDs layer is to facilitate the electron extraction Toinvestigate the eff ects of the loading amount of GQDs on the cellperformance we fabricated a few tens of the cells with diff erentamounts of GQDs loaded in and measured their performanceThe performance testing results of these cells are collected in theSupporting Information (see Figure S6) We found that the cellsloaded with 05 mgmL GQDs concentration have the bestphotocurrent improvement among all of the cells we havestudied Speci1047297cally the cells with 05 mgmL GQDs attained anaverage PCE of 98 whereas those without GQDs had anaverage PCE of only 86
From the J minusV curves of various cells described above theimproved photocurrent is almost the sole contribution to the
enhanced perovskite solar cell performance achieved by theinsertion of the ultrathin GQD layer To further understand theorigin of this phenomenon a series of optical absorption andphotocurrent spectroscopic studies have been carried out Theabsorption spectra in Figure 3 A show that there is little diff erence
for the 1047297lms with or without the GQDs loading This means thatthe small amount of GQDs in the 1047297lms had little in1047298uence on thetotal absorbance which is dominated by the much more strongly absorbing and thicker perovskite layer To con1047297rm theenhancement of the photocurrent we recorded incident-photon-to-current conversion efficiency (IPCE) spectra andthis result is shown in Figure 3B The integrated photocurrentdensities (Figure S7) of the IPCE spectra read 1526 mAcm2
(PT) and 1694 mAcm2 (PGT) respectively which are in fairly
Figure 1 Schematic representation of the typical full device structure(A) where themesoporous oxide is eitherloaded with GQDs or not theenergy band alignment relative to vacuum (B) and the e dge-modi1047297edGQD structure determined by theoretical calculation (C)15 (D) Cross-
sectional SEM image of a complete photovoltaic device based on theCH3NH3PbI3GQDsTiO2 structure Scale bar 200 nm
Figure 2 Current density minus voltage curves for the best-performing solarcells CH3NH3PbI3GQDsTiO2 (red line) and CH3NH3PbI3TiO2
(black line) cells were measured under AM15G illumination at 100mWcm2 (solid line) Dashed lines depict the data obtained at the dark condition
Figure 3 (A) UVvis absorbance spectra of PGT (red line) and PT(black line) (B) IPCE spectra (the left ordinate) The right ordinateindicates the improved IPCE (ΔIPCE) due to GQDs (C) APCEspectra derived from the IPCE and UVvis absorbance spectra andΔ APCE derived from the APCE change due to GQDs Designation of the IPCE and APCE curves PGT (red circles) PT (black squares)ΔIPCE and Δ APCE (blue triangles) (D) PL spectra of the PGT (redline) and PT (black line) 1047297lms excited at 5145 nm (20 mW)
Journal of the American Chemical Society Communication
dxdoiorg101021ja4132246 | J Am Chem Soc 2014 136 3760minus37633761
7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
httpslidepdfcomreaderfullefficiency-enhancement-of-perovskite-solar-cells-through-fast-electron-extraction 34
7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
httpslidepdfcomreaderfullefficiency-enhancement-of-perovskite-solar-cells-through-fast-electron-extraction 44
perovskite layer For the sake of comparison below the timeevolution pro1047297les of the PB peak at 760 nm and the PA peak at520 nm are shown in Figure 4CD respectively
To simplify the kinetic analysis of the TA data we only consider the time evolution of the excited perovskite from 10 to2000 pssince the decay ofexcited TiO2 andGQD is relatively fast(within 10 ps see Figure S8) The decay traces of the PA and PB
features (Figure 4 AB) have been well 1047297tted with a doubleexponential function (Figure 4CD) and the relevant decay timeconstants are collected in Table S3 For both the PT and PGT1047297lms at the probe wavelengths of both the PB and PA featuresthe slower time constants are allwithin 2000minus2300 ps which can
be ascribed to the CH3NH3PbI3 excited-state decay time orrecombination dynamics of CH3NH3PbI3 absorber as reportedpreviously78 However the faster time constants of the two 1047297lmsare quite diff erent Speci1047297cally for the PT 1047297lm the PB bleachingpeakat 760 nm(Figure 4C) shows a faster decay time constant of 260 ps while the PA absorption peak at 530 nm (Figure 4D)decays with a faster time constant of 310 ps The two similarfaster time constants are plausibly related to the electron
extraction from the photoexcited CH3NH3PbI3 to the TiO2conduction band Most signi1047297cantly when the ultrathin GQDslayer is inserted between the perovskite and TiO2 the fasterdecay time constant of the PB bleaching peak is dramatically reduced to 90 ps while that of the PA absorption peak experiences a similar decrease to 106 ps As a result the fasterdecay time constants for the PAand PB peaks of the PGT 1047297lmarenearly 3 times shorter than those of the PT 1047297lm Thus we havedemonstrated that the electron extraction from perovskite toTiO2 is signi1047297cantly enhanced by the interlayer of GQDs whichact as an ultrathin glue for enhancing the electronic coupling andas a superfast electron funnel for efficiently mediating theelectron transfer from the absorber to acceptor Althoughultrafast hot electron extraction from GQDs to TiO
2 has been
reported previously this is the1047297rst time such GQD-mediated fastelectron extraction from perovskite to TiO2 was directly observed Future theoretical studies should help to clarify themechanism involved
In conclusion we have designed a high-efficiency (gt10)perovskite solar cell by inserting an ultrathin layer of GQDs
between the perovskite and the mesoporous titanium dioxideand this efficiency is signi1047297cantly higher than the 881 efficiency
without GQDs The efficiency enhancement is shown to bemostly ascribable to the boosting of the photocurrent The IPCEand APCE spectra show a signi1047297cantly enhanced absorbedphoton-to-current conversion in the visible to the near-infraredregions resulting from the GQDs loading By virtue of the
ultrafast transient absorption spectroscopy measurements wefound a considerably faster electron extraction time at 90minus106 psin the PGT 1047297lm than in the PT 1047297lm (260minus307 ps) which couldeff ectively compete with carrier trapping and thus provide areasonable explanation to the GQDs-induced cell performanceenhancement This 1047297nding highlights the bene1047297cial role of GQDs in facilitating the electron transfer from the perovskieabsorber to the current collector other than its conventional roleas electron acceptor in OPV cells Further work on GQDsaccelerated electron injection may allow hot carrier extraction
before cooling down to the band edges opening up new pathways for future developments of new generation solar cells
ASSOCIATED CONTENT
S Supporting InformationExperimental details and additional tables and 1047297gures Thismaterial is available free of charge via the Internet at httppubsacsorg
AUTHOR INFORMATION
Corresponding Authorslzfanbnueducnchsyangusthk
NotesThe authors declare no competing 1047297nancial interest
ACKNOWLEDGMENTS
This work was supported by the HK-RGC General ResearchFunds (HKUST 606511 and 605710) and the MajorResearchPlan of NSFC (21233003) Partial support from
AoEP-0308 and SEG HKU07 is also acknowledged
REFERENCES
(1) Chu S Majumdar A Nature 2012 488 294
(2) Kojima A Teshima K Shirai Y Miyasaka T J Am Chem Soc2009 131 6050
(3) Etgar L Gao P Xue Z Peng Q Chandiran A K Liu BNazeeruddin M K Graetzel M J Am Chem Soc 2012 134 17396
(4) Chen Q Zhou H ZiruoH LuoS DuanH Wang H LiuYLi G Yang Y J Am Chem Soc 2014 136 622
(5) Burschka J Pellet N Moon S J Humphry-Baker R Gao PNazeeruddin M K Gratzel M Nature 2013 499 316
(6) Liu M Johnston M B Snaith H J Nature 2013 501 395(7) Xing GC Mathews N Sun SY LimS S LamY M Gratzel
M Mhaisalkar S Sum T C Science 2013 342 344(8) Stranks S Eperon G Grancini G Menelaou C Alcocer M P
Leijtens T Herz L Petrozza A Snaith H Science 2013 342 341(9) Janssen R A J Nelson J Adv Mater 2013 25 1847(10) McGehee M D Nature 2013 501 323(11) Park N G J Phys Chem Lett 2013 4 2423
(12) Abrusci A Stranks S D Docampo P Yip H L Jen A K YSnaith H Nano Lett 2013 13 3124
(13) Wang J Ball J Barea E Abate A Alexander-Webber JHuang J Saliba M Mora-Sero I Bisquert J Snaith H Nicolas R
Nano Lett 2014 14 724(14)Zhang WSaliba M Stranks S D Sun YShi XWiesner U
Snaith H Nano Lett 2013 13 4505(15) Zhang M Bai L Shang W Xie W Ma H Fu Y Fang D
Sun H Fan L Han M Liu C Yang S H J Mater Chem 2012 22 7461
(16) Yan X Li B S Li L S Acc Chem Res 2013 46 2254(17) Zhang Z Zhang J Chen N Qu L Energy Environ Sci2012 5
8869(18)MuellerM LYan XDragneaB LiL Nano Lett 2011 11 56(19) Williams K Nelson C Yan X Li L Zhu X ACS Nano 2013
7 1388(20) Yan X Cui X Li B Li L Nano Lett 2010 10 1869(21) Li Y Hu Y Zhao Y Shi G Deng L Hou Y Qu L Adv
Mater 2011 23 776(22) Gupta V Chaudhary N Srivastava R Sharma G D
Bhardwaj R Chand S J Am Chem Soc 2011 133 9960(23) Liang K N Mitzi D B Prikas M Chem Mater 1998 10 403(24) Qiu J H Qiu Y C Yan K Y Zhong M Mu C Yan H
Yang S H Nanoscale 2013 5 3245(25) Ma J Su T Li M-D Du W Huang J Guan X Phillips D
L J Am Chem Soc 2012 134 14858
Journal of the American Chemical Society Communication
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7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
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experimental details) The cross-sectional scanning electronmicroscope (SEM) image of a typical such device is shown inFigure 1D The distribution of GQDs within the TiO2
mesoporous 1047297lm appears to be uniform according to theenergy-dispersive X-ray spectroscopy (EDS) elemental mapping(Figure S3) X-ray powder diff raction patterns in Figure S4 show peaks ascr i ba bl e t o G QD s T iO2 and tetragon alCH3NH3PbI323 24
We measured current ( J )minus voltage (V ) characteristics of thesolar cells under simulated air mass 15 global (AM15G) solarirradiation which are designated as the perovskiteCH3NH3PbI3GQDsTiO2 (or PGT for short) and the standardperovskite CH3NH3PbI3TiO2 (or PT for short) solar cellsrespectively Figure S5 shows a schematic and a photograph of arrayssuch solid-state solar cells each with an active area of 0126cm2 (18 mm times 70 mm) Figure 2 shows J minusV curves of the solarcells The most important observation from the J minusV curves is the
bene1047297cial role of the ultrathin GQDs layer between perovskite
and TiO2 the cell with the GDQs layer achieved a powerconversion efficiency (PCE) of 1015 whereas that without itonly achieved a PCE of 881 The short-circuit current density ( J sc) of the QD loading device of PGT has a signi1047297cant increasefrom 1543 to 1706 mAcm2 compared with the non-GQDloading PT device whereas the 1047297ll factor (FF) and open-circuit
voltages (V oc) are similar 0939 V and 0612 FF for the PTdevice and 0937 V and 0635 FF for the PGT device The smallV oc change is consistent with the fact that the main role of theultrathin GQDs layer is to facilitate the electron extraction Toinvestigate the eff ects of the loading amount of GQDs on the cellperformance we fabricated a few tens of the cells with diff erentamounts of GQDs loaded in and measured their performanceThe performance testing results of these cells are collected in theSupporting Information (see Figure S6) We found that the cellsloaded with 05 mgmL GQDs concentration have the bestphotocurrent improvement among all of the cells we havestudied Speci1047297cally the cells with 05 mgmL GQDs attained anaverage PCE of 98 whereas those without GQDs had anaverage PCE of only 86
From the J minusV curves of various cells described above theimproved photocurrent is almost the sole contribution to the
enhanced perovskite solar cell performance achieved by theinsertion of the ultrathin GQD layer To further understand theorigin of this phenomenon a series of optical absorption andphotocurrent spectroscopic studies have been carried out Theabsorption spectra in Figure 3 A show that there is little diff erence
for the 1047297lms with or without the GQDs loading This means thatthe small amount of GQDs in the 1047297lms had little in1047298uence on thetotal absorbance which is dominated by the much more strongly absorbing and thicker perovskite layer To con1047297rm theenhancement of the photocurrent we recorded incident-photon-to-current conversion efficiency (IPCE) spectra andthis result is shown in Figure 3B The integrated photocurrentdensities (Figure S7) of the IPCE spectra read 1526 mAcm2
(PT) and 1694 mAcm2 (PGT) respectively which are in fairly
Figure 1 Schematic representation of the typical full device structure(A) where themesoporous oxide is eitherloaded with GQDs or not theenergy band alignment relative to vacuum (B) and the e dge-modi1047297edGQD structure determined by theoretical calculation (C)15 (D) Cross-
sectional SEM image of a complete photovoltaic device based on theCH3NH3PbI3GQDsTiO2 structure Scale bar 200 nm
Figure 2 Current density minus voltage curves for the best-performing solarcells CH3NH3PbI3GQDsTiO2 (red line) and CH3NH3PbI3TiO2
(black line) cells were measured under AM15G illumination at 100mWcm2 (solid line) Dashed lines depict the data obtained at the dark condition
Figure 3 (A) UVvis absorbance spectra of PGT (red line) and PT(black line) (B) IPCE spectra (the left ordinate) The right ordinateindicates the improved IPCE (ΔIPCE) due to GQDs (C) APCEspectra derived from the IPCE and UVvis absorbance spectra andΔ APCE derived from the APCE change due to GQDs Designation of the IPCE and APCE curves PGT (red circles) PT (black squares)ΔIPCE and Δ APCE (blue triangles) (D) PL spectra of the PGT (redline) and PT (black line) 1047297lms excited at 5145 nm (20 mW)
Journal of the American Chemical Society Communication
dxdoiorg101021ja4132246 | J Am Chem Soc 2014 136 3760minus37633761
7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
httpslidepdfcomreaderfullefficiency-enhancement-of-perovskite-solar-cells-through-fast-electron-extraction 34
7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
httpslidepdfcomreaderfullefficiency-enhancement-of-perovskite-solar-cells-through-fast-electron-extraction 44
perovskite layer For the sake of comparison below the timeevolution pro1047297les of the PB peak at 760 nm and the PA peak at520 nm are shown in Figure 4CD respectively
To simplify the kinetic analysis of the TA data we only consider the time evolution of the excited perovskite from 10 to2000 pssince the decay ofexcited TiO2 andGQD is relatively fast(within 10 ps see Figure S8) The decay traces of the PA and PB
features (Figure 4 AB) have been well 1047297tted with a doubleexponential function (Figure 4CD) and the relevant decay timeconstants are collected in Table S3 For both the PT and PGT1047297lms at the probe wavelengths of both the PB and PA featuresthe slower time constants are allwithin 2000minus2300 ps which can
be ascribed to the CH3NH3PbI3 excited-state decay time orrecombination dynamics of CH3NH3PbI3 absorber as reportedpreviously78 However the faster time constants of the two 1047297lmsare quite diff erent Speci1047297cally for the PT 1047297lm the PB bleachingpeakat 760 nm(Figure 4C) shows a faster decay time constant of 260 ps while the PA absorption peak at 530 nm (Figure 4D)decays with a faster time constant of 310 ps The two similarfaster time constants are plausibly related to the electron
extraction from the photoexcited CH3NH3PbI3 to the TiO2conduction band Most signi1047297cantly when the ultrathin GQDslayer is inserted between the perovskite and TiO2 the fasterdecay time constant of the PB bleaching peak is dramatically reduced to 90 ps while that of the PA absorption peak experiences a similar decrease to 106 ps As a result the fasterdecay time constants for the PAand PB peaks of the PGT 1047297lmarenearly 3 times shorter than those of the PT 1047297lm Thus we havedemonstrated that the electron extraction from perovskite toTiO2 is signi1047297cantly enhanced by the interlayer of GQDs whichact as an ultrathin glue for enhancing the electronic coupling andas a superfast electron funnel for efficiently mediating theelectron transfer from the absorber to acceptor Althoughultrafast hot electron extraction from GQDs to TiO
2 has been
reported previously this is the1047297rst time such GQD-mediated fastelectron extraction from perovskite to TiO2 was directly observed Future theoretical studies should help to clarify themechanism involved
In conclusion we have designed a high-efficiency (gt10)perovskite solar cell by inserting an ultrathin layer of GQDs
between the perovskite and the mesoporous titanium dioxideand this efficiency is signi1047297cantly higher than the 881 efficiency
without GQDs The efficiency enhancement is shown to bemostly ascribable to the boosting of the photocurrent The IPCEand APCE spectra show a signi1047297cantly enhanced absorbedphoton-to-current conversion in the visible to the near-infraredregions resulting from the GQDs loading By virtue of the
ultrafast transient absorption spectroscopy measurements wefound a considerably faster electron extraction time at 90minus106 psin the PGT 1047297lm than in the PT 1047297lm (260minus307 ps) which couldeff ectively compete with carrier trapping and thus provide areasonable explanation to the GQDs-induced cell performanceenhancement This 1047297nding highlights the bene1047297cial role of GQDs in facilitating the electron transfer from the perovskieabsorber to the current collector other than its conventional roleas electron acceptor in OPV cells Further work on GQDsaccelerated electron injection may allow hot carrier extraction
before cooling down to the band edges opening up new pathways for future developments of new generation solar cells
ASSOCIATED CONTENT
S Supporting InformationExperimental details and additional tables and 1047297gures Thismaterial is available free of charge via the Internet at httppubsacsorg
AUTHOR INFORMATION
Corresponding Authorslzfanbnueducnchsyangusthk
NotesThe authors declare no competing 1047297nancial interest
ACKNOWLEDGMENTS
This work was supported by the HK-RGC General ResearchFunds (HKUST 606511 and 605710) and the MajorResearchPlan of NSFC (21233003) Partial support from
AoEP-0308 and SEG HKU07 is also acknowledged
REFERENCES
(1) Chu S Majumdar A Nature 2012 488 294
(2) Kojima A Teshima K Shirai Y Miyasaka T J Am Chem Soc2009 131 6050
(3) Etgar L Gao P Xue Z Peng Q Chandiran A K Liu BNazeeruddin M K Graetzel M J Am Chem Soc 2012 134 17396
(4) Chen Q Zhou H ZiruoH LuoS DuanH Wang H LiuYLi G Yang Y J Am Chem Soc 2014 136 622
(5) Burschka J Pellet N Moon S J Humphry-Baker R Gao PNazeeruddin M K Gratzel M Nature 2013 499 316
(6) Liu M Johnston M B Snaith H J Nature 2013 501 395(7) Xing GC Mathews N Sun SY LimS S LamY M Gratzel
M Mhaisalkar S Sum T C Science 2013 342 344(8) Stranks S Eperon G Grancini G Menelaou C Alcocer M P
Leijtens T Herz L Petrozza A Snaith H Science 2013 342 341(9) Janssen R A J Nelson J Adv Mater 2013 25 1847(10) McGehee M D Nature 2013 501 323(11) Park N G J Phys Chem Lett 2013 4 2423
(12) Abrusci A Stranks S D Docampo P Yip H L Jen A K YSnaith H Nano Lett 2013 13 3124
(13) Wang J Ball J Barea E Abate A Alexander-Webber JHuang J Saliba M Mora-Sero I Bisquert J Snaith H Nicolas R
Nano Lett 2014 14 724(14)Zhang WSaliba M Stranks S D Sun YShi XWiesner U
Snaith H Nano Lett 2013 13 4505(15) Zhang M Bai L Shang W Xie W Ma H Fu Y Fang D
Sun H Fan L Han M Liu C Yang S H J Mater Chem 2012 22 7461
(16) Yan X Li B S Li L S Acc Chem Res 2013 46 2254(17) Zhang Z Zhang J Chen N Qu L Energy Environ Sci2012 5
8869(18)MuellerM LYan XDragneaB LiL Nano Lett 2011 11 56(19) Williams K Nelson C Yan X Li L Zhu X ACS Nano 2013
7 1388(20) Yan X Cui X Li B Li L Nano Lett 2010 10 1869(21) Li Y Hu Y Zhao Y Shi G Deng L Hou Y Qu L Adv
Mater 2011 23 776(22) Gupta V Chaudhary N Srivastava R Sharma G D
Bhardwaj R Chand S J Am Chem Soc 2011 133 9960(23) Liang K N Mitzi D B Prikas M Chem Mater 1998 10 403(24) Qiu J H Qiu Y C Yan K Y Zhong M Mu C Yan H
Yang S H Nanoscale 2013 5 3245(25) Ma J Su T Li M-D Du W Huang J Guan X Phillips D
L J Am Chem Soc 2012 134 14858
Journal of the American Chemical Society Communication
dxdoiorg101021ja4132246 | J Am Chem Soc 2014 136 3760minus37633763
7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
httpslidepdfcomreaderfullefficiency-enhancement-of-perovskite-solar-cells-through-fast-electron-extraction 34
7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
httpslidepdfcomreaderfullefficiency-enhancement-of-perovskite-solar-cells-through-fast-electron-extraction 44
perovskite layer For the sake of comparison below the timeevolution pro1047297les of the PB peak at 760 nm and the PA peak at520 nm are shown in Figure 4CD respectively
To simplify the kinetic analysis of the TA data we only consider the time evolution of the excited perovskite from 10 to2000 pssince the decay ofexcited TiO2 andGQD is relatively fast(within 10 ps see Figure S8) The decay traces of the PA and PB
features (Figure 4 AB) have been well 1047297tted with a doubleexponential function (Figure 4CD) and the relevant decay timeconstants are collected in Table S3 For both the PT and PGT1047297lms at the probe wavelengths of both the PB and PA featuresthe slower time constants are allwithin 2000minus2300 ps which can
be ascribed to the CH3NH3PbI3 excited-state decay time orrecombination dynamics of CH3NH3PbI3 absorber as reportedpreviously78 However the faster time constants of the two 1047297lmsare quite diff erent Speci1047297cally for the PT 1047297lm the PB bleachingpeakat 760 nm(Figure 4C) shows a faster decay time constant of 260 ps while the PA absorption peak at 530 nm (Figure 4D)decays with a faster time constant of 310 ps The two similarfaster time constants are plausibly related to the electron
extraction from the photoexcited CH3NH3PbI3 to the TiO2conduction band Most signi1047297cantly when the ultrathin GQDslayer is inserted between the perovskite and TiO2 the fasterdecay time constant of the PB bleaching peak is dramatically reduced to 90 ps while that of the PA absorption peak experiences a similar decrease to 106 ps As a result the fasterdecay time constants for the PAand PB peaks of the PGT 1047297lmarenearly 3 times shorter than those of the PT 1047297lm Thus we havedemonstrated that the electron extraction from perovskite toTiO2 is signi1047297cantly enhanced by the interlayer of GQDs whichact as an ultrathin glue for enhancing the electronic coupling andas a superfast electron funnel for efficiently mediating theelectron transfer from the absorber to acceptor Althoughultrafast hot electron extraction from GQDs to TiO
2 has been
reported previously this is the1047297rst time such GQD-mediated fastelectron extraction from perovskite to TiO2 was directly observed Future theoretical studies should help to clarify themechanism involved
In conclusion we have designed a high-efficiency (gt10)perovskite solar cell by inserting an ultrathin layer of GQDs
between the perovskite and the mesoporous titanium dioxideand this efficiency is signi1047297cantly higher than the 881 efficiency
without GQDs The efficiency enhancement is shown to bemostly ascribable to the boosting of the photocurrent The IPCEand APCE spectra show a signi1047297cantly enhanced absorbedphoton-to-current conversion in the visible to the near-infraredregions resulting from the GQDs loading By virtue of the
ultrafast transient absorption spectroscopy measurements wefound a considerably faster electron extraction time at 90minus106 psin the PGT 1047297lm than in the PT 1047297lm (260minus307 ps) which couldeff ectively compete with carrier trapping and thus provide areasonable explanation to the GQDs-induced cell performanceenhancement This 1047297nding highlights the bene1047297cial role of GQDs in facilitating the electron transfer from the perovskieabsorber to the current collector other than its conventional roleas electron acceptor in OPV cells Further work on GQDsaccelerated electron injection may allow hot carrier extraction
before cooling down to the band edges opening up new pathways for future developments of new generation solar cells
ASSOCIATED CONTENT
S Supporting InformationExperimental details and additional tables and 1047297gures Thismaterial is available free of charge via the Internet at httppubsacsorg
AUTHOR INFORMATION
Corresponding Authorslzfanbnueducnchsyangusthk
NotesThe authors declare no competing 1047297nancial interest
ACKNOWLEDGMENTS
This work was supported by the HK-RGC General ResearchFunds (HKUST 606511 and 605710) and the MajorResearchPlan of NSFC (21233003) Partial support from
AoEP-0308 and SEG HKU07 is also acknowledged
REFERENCES
(1) Chu S Majumdar A Nature 2012 488 294
(2) Kojima A Teshima K Shirai Y Miyasaka T J Am Chem Soc2009 131 6050
(3) Etgar L Gao P Xue Z Peng Q Chandiran A K Liu BNazeeruddin M K Graetzel M J Am Chem Soc 2012 134 17396
(4) Chen Q Zhou H ZiruoH LuoS DuanH Wang H LiuYLi G Yang Y J Am Chem Soc 2014 136 622
(5) Burschka J Pellet N Moon S J Humphry-Baker R Gao PNazeeruddin M K Gratzel M Nature 2013 499 316
(6) Liu M Johnston M B Snaith H J Nature 2013 501 395(7) Xing GC Mathews N Sun SY LimS S LamY M Gratzel
M Mhaisalkar S Sum T C Science 2013 342 344(8) Stranks S Eperon G Grancini G Menelaou C Alcocer M P
Leijtens T Herz L Petrozza A Snaith H Science 2013 342 341(9) Janssen R A J Nelson J Adv Mater 2013 25 1847(10) McGehee M D Nature 2013 501 323(11) Park N G J Phys Chem Lett 2013 4 2423
(12) Abrusci A Stranks S D Docampo P Yip H L Jen A K YSnaith H Nano Lett 2013 13 3124
(13) Wang J Ball J Barea E Abate A Alexander-Webber JHuang J Saliba M Mora-Sero I Bisquert J Snaith H Nicolas R
Nano Lett 2014 14 724(14)Zhang WSaliba M Stranks S D Sun YShi XWiesner U
Snaith H Nano Lett 2013 13 4505(15) Zhang M Bai L Shang W Xie W Ma H Fu Y Fang D
Sun H Fan L Han M Liu C Yang S H J Mater Chem 2012 22 7461
(16) Yan X Li B S Li L S Acc Chem Res 2013 46 2254(17) Zhang Z Zhang J Chen N Qu L Energy Environ Sci2012 5
8869(18)MuellerM LYan XDragneaB LiL Nano Lett 2011 11 56(19) Williams K Nelson C Yan X Li L Zhu X ACS Nano 2013
7 1388(20) Yan X Cui X Li B Li L Nano Lett 2010 10 1869(21) Li Y Hu Y Zhao Y Shi G Deng L Hou Y Qu L Adv
Mater 2011 23 776(22) Gupta V Chaudhary N Srivastava R Sharma G D
Bhardwaj R Chand S J Am Chem Soc 2011 133 9960(23) Liang K N Mitzi D B Prikas M Chem Mater 1998 10 403(24) Qiu J H Qiu Y C Yan K Y Zhong M Mu C Yan H
Yang S H Nanoscale 2013 5 3245(25) Ma J Su T Li M-D Du W Huang J Guan X Phillips D
L J Am Chem Soc 2012 134 14858
Journal of the American Chemical Society Communication
dxdoiorg101021ja4132246 | J Am Chem Soc 2014 136 3760minus37633763
7212019 Efficiency Enhancement of Perovskite Solar Cells Through Fast Electron Extraction the Role of Graphene Quantumhellip
httpslidepdfcomreaderfullefficiency-enhancement-of-perovskite-solar-cells-through-fast-electron-extraction 44
perovskite layer For the sake of comparison below the timeevolution pro1047297les of the PB peak at 760 nm and the PA peak at520 nm are shown in Figure 4CD respectively
To simplify the kinetic analysis of the TA data we only consider the time evolution of the excited perovskite from 10 to2000 pssince the decay ofexcited TiO2 andGQD is relatively fast(within 10 ps see Figure S8) The decay traces of the PA and PB
features (Figure 4 AB) have been well 1047297tted with a doubleexponential function (Figure 4CD) and the relevant decay timeconstants are collected in Table S3 For both the PT and PGT1047297lms at the probe wavelengths of both the PB and PA featuresthe slower time constants are allwithin 2000minus2300 ps which can
be ascribed to the CH3NH3PbI3 excited-state decay time orrecombination dynamics of CH3NH3PbI3 absorber as reportedpreviously78 However the faster time constants of the two 1047297lmsare quite diff erent Speci1047297cally for the PT 1047297lm the PB bleachingpeakat 760 nm(Figure 4C) shows a faster decay time constant of 260 ps while the PA absorption peak at 530 nm (Figure 4D)decays with a faster time constant of 310 ps The two similarfaster time constants are plausibly related to the electron
extraction from the photoexcited CH3NH3PbI3 to the TiO2conduction band Most signi1047297cantly when the ultrathin GQDslayer is inserted between the perovskite and TiO2 the fasterdecay time constant of the PB bleaching peak is dramatically reduced to 90 ps while that of the PA absorption peak experiences a similar decrease to 106 ps As a result the fasterdecay time constants for the PAand PB peaks of the PGT 1047297lmarenearly 3 times shorter than those of the PT 1047297lm Thus we havedemonstrated that the electron extraction from perovskite toTiO2 is signi1047297cantly enhanced by the interlayer of GQDs whichact as an ultrathin glue for enhancing the electronic coupling andas a superfast electron funnel for efficiently mediating theelectron transfer from the absorber to acceptor Althoughultrafast hot electron extraction from GQDs to TiO
2 has been
reported previously this is the1047297rst time such GQD-mediated fastelectron extraction from perovskite to TiO2 was directly observed Future theoretical studies should help to clarify themechanism involved
In conclusion we have designed a high-efficiency (gt10)perovskite solar cell by inserting an ultrathin layer of GQDs
between the perovskite and the mesoporous titanium dioxideand this efficiency is signi1047297cantly higher than the 881 efficiency
without GQDs The efficiency enhancement is shown to bemostly ascribable to the boosting of the photocurrent The IPCEand APCE spectra show a signi1047297cantly enhanced absorbedphoton-to-current conversion in the visible to the near-infraredregions resulting from the GQDs loading By virtue of the
ultrafast transient absorption spectroscopy measurements wefound a considerably faster electron extraction time at 90minus106 psin the PGT 1047297lm than in the PT 1047297lm (260minus307 ps) which couldeff ectively compete with carrier trapping and thus provide areasonable explanation to the GQDs-induced cell performanceenhancement This 1047297nding highlights the bene1047297cial role of GQDs in facilitating the electron transfer from the perovskieabsorber to the current collector other than its conventional roleas electron acceptor in OPV cells Further work on GQDsaccelerated electron injection may allow hot carrier extraction
before cooling down to the band edges opening up new pathways for future developments of new generation solar cells
ASSOCIATED CONTENT
S Supporting InformationExperimental details and additional tables and 1047297gures Thismaterial is available free of charge via the Internet at httppubsacsorg
AUTHOR INFORMATION
Corresponding Authorslzfanbnueducnchsyangusthk
NotesThe authors declare no competing 1047297nancial interest
ACKNOWLEDGMENTS
This work was supported by the HK-RGC General ResearchFunds (HKUST 606511 and 605710) and the MajorResearchPlan of NSFC (21233003) Partial support from
AoEP-0308 and SEG HKU07 is also acknowledged
REFERENCES
(1) Chu S Majumdar A Nature 2012 488 294
(2) Kojima A Teshima K Shirai Y Miyasaka T J Am Chem Soc2009 131 6050
(3) Etgar L Gao P Xue Z Peng Q Chandiran A K Liu BNazeeruddin M K Graetzel M J Am Chem Soc 2012 134 17396
(4) Chen Q Zhou H ZiruoH LuoS DuanH Wang H LiuYLi G Yang Y J Am Chem Soc 2014 136 622
(5) Burschka J Pellet N Moon S J Humphry-Baker R Gao PNazeeruddin M K Gratzel M Nature 2013 499 316
(6) Liu M Johnston M B Snaith H J Nature 2013 501 395(7) Xing GC Mathews N Sun SY LimS S LamY M Gratzel
M Mhaisalkar S Sum T C Science 2013 342 344(8) Stranks S Eperon G Grancini G Menelaou C Alcocer M P
Leijtens T Herz L Petrozza A Snaith H Science 2013 342 341(9) Janssen R A J Nelson J Adv Mater 2013 25 1847(10) McGehee M D Nature 2013 501 323(11) Park N G J Phys Chem Lett 2013 4 2423
(12) Abrusci A Stranks S D Docampo P Yip H L Jen A K YSnaith H Nano Lett 2013 13 3124
(13) Wang J Ball J Barea E Abate A Alexander-Webber JHuang J Saliba M Mora-Sero I Bisquert J Snaith H Nicolas R
Nano Lett 2014 14 724(14)Zhang WSaliba M Stranks S D Sun YShi XWiesner U
Snaith H Nano Lett 2013 13 4505(15) Zhang M Bai L Shang W Xie W Ma H Fu Y Fang D
Sun H Fan L Han M Liu C Yang S H J Mater Chem 2012 22 7461
(16) Yan X Li B S Li L S Acc Chem Res 2013 46 2254(17) Zhang Z Zhang J Chen N Qu L Energy Environ Sci2012 5
8869(18)MuellerM LYan XDragneaB LiL Nano Lett 2011 11 56(19) Williams K Nelson C Yan X Li L Zhu X ACS Nano 2013
7 1388(20) Yan X Cui X Li B Li L Nano Lett 2010 10 1869(21) Li Y Hu Y Zhao Y Shi G Deng L Hou Y Qu L Adv
Mater 2011 23 776(22) Gupta V Chaudhary N Srivastava R Sharma G D
Bhardwaj R Chand S J Am Chem Soc 2011 133 9960(23) Liang K N Mitzi D B Prikas M Chem Mater 1998 10 403(24) Qiu J H Qiu Y C Yan K Y Zhong M Mu C Yan H
Yang S H Nanoscale 2013 5 3245(25) Ma J Su T Li M-D Du W Huang J Guan X Phillips D
L J Am Chem Soc 2012 134 14858
Journal of the American Chemical Society Communication
dxdoiorg101021ja4132246 | J Am Chem Soc 2014 136 3760minus37633763
