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Decoupling Charge Transfer and Transport at Polymeric Hole Transport Layer in Perovskite Solar Cells Dong Hun Sin, Hyomin Ko, Sae Byeok Jo, Min Kim, Geun Yeol Bae, and Kilwon Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12023 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 26, 2016

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ACS Applied Materials & Interfaces

Decoupling Charge Transfer and Transport at Polymeric Hole Transport Layer in Perovskite Solar Cells

Dong Hun Sin, Hyomin Ko, Sae Byeok Jo, Min Kim, Geun Yeol Bae, and Kilwon Cho* Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea

D. H. Sin, H. Ko, Dr. S. B. Jo, Dr. M. Kim, G. Y. Bae, and Prof. K. Cho Department of Chemical Engineering Pohang University of Science and Technology (POSTECH) Pohang, 790−784, Korea E-mail: [email protected]

ACS Paragon Plus Environment


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Abstract Tailoring charge extraction interfaces in perovskite solar cells (PeSCs) critically determines the photovoltaic performance of PeSCs. Here, we investigated the decoupling of two major determinants of the efficient charge extraction, the charge transport and interfacial charge transfer properties at hole transport layers (HTLs). A simple physical tuning of a representative polymeric HTL, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate), provided a wide range of charge conductivities from 10-4 to 103 S cm-1 without significant modulations in their energy levels, thereby enabling the decoupling of charge transport and transfer properties at HTLs. The transient photovoltaic response measurement revealed that the facilitation of hole transport through the highly conductive HTL promoted the elongation of charge carrier lifetimes within the PeSCs up to 3 times, leading to enhanced photocurrent extraction and finally 25% higher power conversion efficiency.

Keywords conductivity, hole transport layer, charge transport, perovskite solar cells, charge carrier lifetime


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1. Introduction Methylammonium lead halide (CH3NH3PbX3, X=Cl, Br, and I) is a promising material for next generation solar cells because its power conversion efficiency is superior to that of organic materials in organic and dye-sensitized solar cells.1-6 Recently, TiO2 mesostructurebased PeSCs were found to exhibit a power conversion efficiency (PCE) of up to 20.1%.7 Furthermore, planar heterojunction (PHJ) PeSCs to achieve processing simplicity and device flexibility have been fabricated without TiO2 mesostructure that exhibit a PCE of 19.3%.8 In order to enhance the photovoltaic performance of the PeSCs, many researchers have attempted to control the morphology of perovskite film2-4, 9-11 and improve the interfaces in the PeSCs.8, 12-17 Interfacial engineering of the electrode interface in PHJ PeSCs is essential because this interface critically decides charge collection and the built-in potential.13, 14, 18-22 Among the interfacial materials, organic materials are good candidates because of their solution processability and mechanical flexibility. Jeng et al. first reported a PeSC with poly(3,4ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the HTL and [6,6]-phenylC61-butyric acid methyl ester (PCBM) as the electron transport layer (ETL).23 The work function of PEDOT:PSS, 4.9 ~ 5.2 eV, produces the ohmic contact with the valence band of CH3NH3PbI3 (5.3 eV).23-25 Furthermore, its conductivity is easily modulated by changing the ratio of insulating PSS to conductive PEDOT and further treatment.26-30 The alignment of the energy levels of the charge transport layer and the photoactive layer is generally the first aim of interfacial engineering for efficient charge transfer.18-20,

31

However, conductivity is another key factor in an efficient charge transport layer because charge carriers transferred from the photoactive layer should transport to the electrode without charge carrier recombination loss. The importance of the conductivity of the charge transport layer has been emphasized by several researchers, but the effects of conductivity have not previously been elucidated in isolation because the change in energy level is typically accompanied by the change in conductivity.32, 33 Therefore, the study to decouple charge transfer and transport by revealing the conductivity effects of the charge transport layer without energy level change is required to facilitate the design of efficient charge transport layers for high performance PeSCs.



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When charge transport in the PeSCs is modified with the interfacial engineering, the charge carrier recombination dynamics is affected, which determines the PCE parameters.8, 34 Total photo-generated charge carriers, thus photocurrents, are decided by germinate recombination, and free charge carriers dissipate through bimolecular recombination and trap-assisted recombination before charge collection.35 If there are energy barriers for charge carriers or unbalanced charge transports in the device, charge carriers can accumulate at certain positions, which results in severe charge carrier recombination.36-38 In addition, defects such as atomic vacancies or grain boundaries act as trap sites where charge carriers recombine.24, 39 When charge transport in the PeSCs is enhanced, charge carriers transport to the electrode quickly to minimize the possibilities of meeting counter charge carriers to recombine, which significantly affects the charge carrier recombination dynamics. Because the charge carrier recombination dynamics governs the overall photovoltaic parameters of PeSCs,35,

40, 41

the relationship between the charge transport and the charge carrier

recombination dynamics inside a PeSC should be characterized to optimize its performance. Here, we fabricated CH3NH3PbI3-based PHJ PeSCs with PEDOT:PSS HTLs with various conductivity levels, and investigated the effects of HTL conductivity on the photovoltaic performance of the PeSCs. PEDOT:PSS was chosen because the conductivity of its films can be tuned with only weak effects on the Fermi energy level.27 We systematically changed the conductivity of the PEDOT:PSS films from 10-4 to 103 S cm-1 without significant changes in Fermi energy level. As a result, we were able to decouple charge transfer and transport at the PEDOT:PSS HTL, and determine charge transport properties of the HTLs independently of charge transfer. The photovoltaic performance of the PeSCs with highly conductive PEDOT:PSS HTLs were found to be enhanced, especially with increases in the short circuit current density (JSC) and the fill factor (FF), because the high conductivity of such HTLs facilitates charge transport. The charge carrier recombination dynamics of the PeSCs were investigated to explain the relationship between facilitated charge transport and enhanced PCE parameters by using incident light power (ILP) dependent photocurrent density (JPh), transient photovoltage (TPV), and transient photocurrent (TPC) analysis. Slow charge carrier recombination without charge accumulation was observed for the device with the highly conductive PEDOT:PSS HTL, which results in enhanced JSC and FF. Therefore, enhancing the conductivity of the HTL should be considered to design new HTLs for high performance PeSCs.


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2. Results and discussion 2.1. PEDOT:PSS Conductivity Control and Characterization. The properties of the PEDOT:PSS films used in this study are shown in Figure 1 and Table 1. The PEDOT:PSS conductivity was controlled by changing the ratio of PSS to PEDOT and adding small amounts of dimethyl sulfoxide (DMSO), which resulted in three PEDOT:PSS films with different conductivities: low conductivity, 7.24 × 10-4 S cm-1 (PEDOT-L), medium conductivity, 253 S cm-1 (PEDOT-M), and high conductivity, 882 S cm-1 (PEDOT-H). The hole mobilities were calculated by assigning the hole density using a geometrical consideration. The hole mobility calculation was explained in the supplementary information. The tendency of hole mobility followed the tendency of conductivity: low mobility, 2.53 × 10-5 cm2 V-1 s-1 with PEDOT-L, medium mobility, 5.18 cm2 V-1 s-1 with PEDOT-M, and high mobility, 17.4 cm2 V-1 s-1 with PEDOT-H (Table 1). The ratios of PSS to PEDOT were verified with X-ray photoelectron spectroscopy (XPS). Figure 1b shows S 2p XPS spectra of the three PEDOT:PSS films. The XPS peaks at the binding energy of 164.6 and 168.6 eV are sulfur (S) signals from PEDOT and PSS respectively.26-28 The ratios of PSS to PEDOT were calculated by integrating and comparing the peak areas. The conductivity of the PEDOT:PSS films increases as the proportion of PSS decreases. Decreasing the PSS content improves the conductivity and long-term stability of PEDOT:PSS films because PSS is insulating and hygroscopic.27 Figure 1a shows atomic force microscopy (AFM) images of the PEDOT:PSS films. Small grains with a size of 20 nm are evident in PEDOT-L. Larger grains with a size of 30 nm and with a size of 40 nm can be seen in PEDOT-M and PEDOT-H respectively with fibril structures. The PEDOT:PSS coils move closer and form larger clusters, which improves conductivity as the ratio of PSS to PEDOT decreases.27, 29 The root-mean-square roughnesses (Rrms) of the PEDOT:PSS films are 0.86, 1.05, and 1.49 nm for PEDOT-L, PEDOT-M, and PEDOT-H respectively. Figure S1 shows the transmittance of the PEDOT:PSS films, which do not disrupt the transmission of visible light of solar spectrum. To use these films as HTLs, their energy levels should be aligned with the valence band of the CH3NH3PbI3 film. Figure 1c show ultraviolet photoelectron spectroscopy (UPS)



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secondary and valence spectra of the PEDOT:PSS films. Onset points in UPS valence spectra are same with aligned vacuum levels of each film. The work functions (WFs) were calculated from UPS secondary spectra with the ionization potential of gold as a reference. There is no variation in the WFs of the PEDOT:PSS films with the WF of 5.23 eV. The WFs of the PEDOT:PSS films are thus well matched with the valence band of the CH3NH3PbI3 film (5.3 eV), which leads to ohmic contact for hole transfer from CH3NH3PbI3 to PEDOT:PSS.23-25 Therefore, the charge transport can be decoupled from the charge transfer by investigating the effects of varying the conductivity of HTLs with identical ohmic contact and similar quasi hole Fermi levels inside the PeSCs. The energy levels of the layers inside the PeSCs are represented in Figure 2b. 2.2. Device Structure and Photovoltaic Performance. The CH3NH3PbI3-based PHJ PeSCs with PEDOT-L, PEDOT-M, and PEDOT-H were fabricated with the device architecture ITO (140 nm)/PEDOT:PSS (40 nm)/CH3NH3PbI3(250 nm)/PCBM (100 nm)/Al (120 nm) with an actual device area of 0.0555 cm2 (Figure 2a). The 250 nm-thick CH3NH3PbI3 film is sufficiently thick to absorb most of the light spectrum due to its high absorption coefficient above 104 cm-1.14, 15 We deposited the PbI2 film from a PbI2 solution in N,N-dimethylformamide (DMF) first, and then dipped the dried PbI2 film into CH3NH3I solution in 2-propanol (isopropanol, IPA) at 60 oC for 3 minutes. PCBM was deposited as an ETL to fill and smooth the perovskite film surface and prevent direct contact between the cathode and anode.24 The current density-voltage (J-V) characteristics of the PeSCs were measured under AM 1.5 with an illumination intensity of 100 mW cm-2. The J-V curves and relevant PCE parameters are shown in Figure 3 and Table 2. We fabricated PeSCs with various film thicknesses, reaction conditions, thermal treatment conditions, and fabrication environments to optimize the PCE parameters. We kept the O2 concentration below 50 ppm and the H2O concentration below 1 ppm inside the glove-box. Figure S2 shows AFM and scanning electron microscope (SEM) images of CH3NH3PbI3 films on PEDOT-L, PEDOT-M, and PEDOT-H. The CH3NH3PbI3 films have a grain size of 200 ~ 300 nm, which are comparable to the CH3NH3PbI3 film thickness. There are no significant morphological differences between the CH3NH3PbI3 films on the three PEDOT:PSS HTLs. Furthermore, X-ray diffraction (XRD) (Figure S3) and ultraviolet-visible (UV-Vis) absorption spectra (Figure S5) of CH3NH3PbI3 films on the three PEDOT:PSS


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films are identical as well as the morphological similarity, which means that the resulting photovoltaic parameters are affected solely by the PEDOT:PSS HTL properties. Figure S4 shows the two-dimensional wide angle X-ray diffraction (2D-WAXD). After CH3NH3PbI3 conversion, the strong (001) peak at a qz value of 0.886 Å-1 due to PbI2 disappears, and strong (110), (220), and (310) peaks due to CH3NH3PbI3 are observed at 0.987, 1.992, and 2.210 Å-1 respectively.2 The reaction between the PbI2 film and CH3NH3I solution at 60oC for 3 minutes results in the complete conversion from PbI2 to CH3NH3PbI3. The tetragonal crystal structure of CH3NH3PbI3 is verified by the 2D-WAXD pattern in Figure S4b. The UV-Vis absorption spectra of the PbI2, CH3NH3PbI3, and PCBM films are shown in Figure S6. The absorption onset of the CH3NH3PbI3 film at 800 nm is in agreement with the band gap of 1.55 eV, which results in the broad absorption in the light spectrum.25 The UV-Vis absorption spectra of the PbI2 and CH3NH3PbI3 films are completely different, which confirms the complete conversion of PbI2 to CH3NH3PbI3. The different conductivities of the three PEDOT:PSS HTLs directly affect the photovoltaic performances of the PeSCs. In the absence of a PEDOT:PSS HTL, the PeSC has the following properties: JSC of 13.92 mA cm-2, open circuit voltage (VOC) of 0.63 V, FF of 51.9%, and PCE of 4.55%, which are inferior to those of the devices with PEDOT:PSS HTLs. With PEDOT:PSS HTLs, JSC and FF increase gradually, but VOC does not change significantly as the conductivity of the PEDOT:PSS films increases. The best properties are those of the device with PEDOT-H: JSC of 16.44 mA cm-2, VOC of 0.83 V, FF of 75.0%, and PCE of 10.23%. The hysteresis is not shown up in J-V curves with different scan directions for all the devices (Figure S7 and Table S2). JSC increases gradually from 14.25 to 16.44 mA cm-2 as hole collection at the ITO anode becomes more efficient with the increases in the PEDOT:PSS HTL conductivity.42 The series resistance (RS) was reduced from 8.9 to 6.2 Ω cm-2 and the shunt resistance (RSh) increases from 2026.7 to 2705.1 Ω cm-2 as the PEDOT:PSS HTL conductivity is increased. Figure 3b shows semi-logarithmic plot of dark J-V curves. For a conventional p-n diode, the current density follows the diode equation, =

+ + − 1 + , 1



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where J0 is the reverse saturation current density, q is the elementary charge, V is the applied voltage, ni is the ideality factor, k is Boltzmann’s constant, and T is the temperature. In the intermediate voltage region (0.2 V < V < 0.6 V), J is proportional to the exponential term. The lower current density of the PEDOT-H-based PeSC in the intermediate voltage region indicates smaller J0, which means that charge carrier recombination inside the device is reduced.43 The photoluminescence (PL) quenching results for the CH3NH3PbI3 films with PEDOT:PSS HTLs show that the extent of hole transfer from the CH3NH3PbI3 film to the PEDOT:PSS HTL increases with the conductivity of the PEDOT:PSS HTL (Figure 3d). Although ohmic contact is present in all the PeSCs, additional holes can be transferred with the highly conductive HTL as transferred holes leave the interface rapidly. As a result, hole collection at the ITO anode is improved with highly conductive HTLs leading to increase in the external quantum efficiency (EQE) over the whole absorbed light spectrum range (Figure 3c). In addition, improved hole collection at the ITO anode decreases hole accumulation and increases FF from 67.7 to 75.0%.37 The PEDOT:PSS conductivity effects on the photovoltaic performance are shown distinctly when the thickness of PEDOT:PSS HTL increases. (Figure S8 and Table S3) The PEDOT:PSS thicknesses were controlled varying the spin speeds and the number of depositions. As the PEDOT:PSS thicknesses are increased, the photovoltaic parameters of the PeSCs based on PEDOT-H are comparable while those of the PeSCs based on PEDOT-L become worse because of increased RS. Although there are increments in JSC and FF with the highly conductive HTLs, there is not significant change in VOC. The UPS spectra of the PEDOT:PSS films show that their WFs are same (Figure 1c) and well-aligned with the valence band of the CH3NH3PbI3 film, so holes overcome a negligible barrier. The quasi hole Fermi levels inside the PeSCs with the three different HTLs are similar with a VOC of around 0.83 V. However, VOC of PEDOT-L-based PeSC is little smaller than those of PEDOT-M and PEDOT-H-based devices due to higher charge carrier recombination in the device, which will be discussed with the following section. Electrochemical impedance spectroscopy (EIS) was conducted to measure the internal electrical properties including inner resistances and capacitances of the PeSCs based on the PEDOT:PSS HTLs with different conductivity.44-47 Figure S9 shows the Nyquist plots of the PeSCs measured in the range from 10 Hz to 2 MHz in dark at open circuit condition and the equivalent circuit model for the PeSCs. Because the Nyquist plot consists with two arcs, two resistor-capacitor elements are introduced in the equivalent circuit to account for the HTL


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and the bulk: Ri and Ci are resistance and capacitance for the PEDOT:PSS HTL, and Rb and Cb1 are resistance and capacitance for the CH3NH3PbI3 bulk.45-47 RL is a lead resistance to account for contact resistances, and Cb2, and CPEb are capacitance and constant phase element considered to account for the dispersive transport in the bulk. CPE-Tb is a capacitive contribution of non-ideal capacitor and CPE-Pb is a quality factor.46, 47 Extracted equivalent circuit parameters from simulated EIS data are listed in Table S4. All the PeSCs have contact resistances in common and show similar RLs. Ris at the charge transferring interfaces are 490.7, 196.6, and 125.9 Ω for the PeSCs based on PEDOT-L, PEDOT-M, and PEDOT-H, respectively. Difference in Ri is originated from the PEDOT:PSS HTLs with different conductivities and mainly governed by the interfaces between PEDOT:PSS and CH3NH3PbI3 film. Smaller Ri with the highly conductive PEDOT:PSS HTL means that holes are effectively transferred and transported, which enhances JSC and FF. In addition, Cb1 and Cb2 decrease as the HTL conductivity increases indicating that capacitive property of the bulk decreases, which presumably results from reduced charge accumulation in the bulk with efficient charge extraction by the use of a highly conductive HTL. Rb also decreases gradually as the HTL conductivity increases due to reduced charge accumulation in the bulk even though the CH3NH3PbI3 films are almost identical. Charge accumulation and recombination dynamics will be discussed in detail with the following section. 2.3. Charge Carrier Recombination Characterization The ILP-dependent JPh values of the PeSCs were measured for various ILPs from 20 to 100 mW cm-2 inside a glove-box. When there is no recombination inside the PeSCs, JPh is proportional to the charge generation rate, thus ILP. = , 2 q is the elementary charge, G is the charge generation rate under light, and L is specimen thickness. However, when the electron and hole collections are unbalanced and certain charge carriers accumulate inside the device, space charge arises. Then, the dependence of JPh on the charge generation rate decreases with an exponent of 0.75. 9!" !# $ '/) */+ = & , 3 8



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εoεr is the dielectric permittivity, µ is the carrier mobility, and V is the applied voltage. JPh depends on ILP with the exponent S that approaches a unity when the PeSC is ideally fabricated without charge accumulation inside the device and approaches 0.75 as charge accumulation increases.36-38 Therefore, by investigating the dependence of JPh on ILP, the charge collection balance or charge accumulation inside the PeSCs can be detected. Logarithmic plots of JPh versus ILP are shown in Figure 4a-b, and Figure S10. When the effective applied voltage, V0-V, is 3 V which was far from open circuit condition with minimum charge carrier recombination, the S parameter for the PEDOT-H-based PeSC is 0.969, higher than 0.904 for the PEDOT-L-based PeSC, which represents that hole accumulation is reduced by the use of a highly conductive PEDOT:PSS HTL. Even for a low effective applied voltage of 0.4 V, which is near open circuit condition with vigorous charge carrier recombination, the S parameter of the PEDOT-H-based device is 0.953, much higher than 0.893 for the PEDOT-L-based device, which means that non-geminate recombination near open circuit condition is significantly reduced by the increases in the conductivity of the PEDOT:PSS HTL.37, 38 As a result, JSC and FF increase due to reduced charge accumulation and recombination loss with highly conductive PEDOT:PSS HTL. In order to investigate further relationship between the conductivities of the PEDOT:PSS HTLs and charge carrier recombination, TPV and TPC measurements were conducted. A N2 dye laser with a wavelength of 536 nm was used for pulsed excitation under various white light intensities. An oscilloscope was used to trace the transients and record them on the voltage scale with an amplifier. For the TPV measurement, the resistance of the oscilloscope was kept at 100 MΩ to create open circuit condition. The charge carrier lifetime, τn, can be extracted from the TPV measurement by fitting the TPV transient because all photogenerated charges recombine under open circuit condition.35 For the TPC measurement, the resistance of the oscilloscope was maintained at 50 Ω to produce short circuit condition. The total charge carrier density, n, can be extracted by integrating the TPC transient with fitting parameters from the TPC transient.35 Figure 4c-d show the TPV and TPC transients, and Figure 4e shows the relationship between the τn and n extracted from the TPV and TPC transients. Under AM1.5 illumination, the τn of the PEDOT-H-based PeSC was found to be 3.09 µs with slightly increased n, which is 1.26 times longer than that of the PEDOT-M-based PeSC


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(2.45 µs) and 3.15 times longer than that of the PEDOT-L-based PeSC (0.98 µs). It means that the photo-generated charges in the PEDOT-H-based PeSC travel to electrode for three times longer before meeting opposite charge carriers due to facilitated charge transport than the charges in the PEDOT-L-based PeSC resulting in efficient charge collection with reduced recombination loss. The charge carrier recombination rate, Jrec, is #-. = − /

0 = #-. 2 , 4 01

where l is the thickness of the photo-active layer, t is the time, krec is the recombination rate constant, and Ф is the overall reaction order.35 When bimolecular recombination inside the PeSCs is excessive, Jrec is dependent on the square of n with Ф’s approaching 2. The calculated Фs were 1.25, 1.15, and 1.13 for the PEDOT-L, PEDOT-M, and PEDOT-H-based PeSCs respectively. The Jrec of the PEDOT-H-based PeSC is less dependent on the n than those of the PEDOT-L and PEDOT-M-based PeSCs. It means that bimolecular recombination inside the PeSCs decreases as the conductivity of PEDOT:PSS HTL increases. As a result, the holes in the PEDOT-H-based PeSC transport to the anode electrode more efficiently with prolonged charge carrier lifetime and reduced bimolecular recombination loss comparing with holes in the PEDOT-L and PEDOT-M-based PeSCs. Figure 4f shows an integrated TPC transient, which indicates the number of collected charge carriers under short circuit condition. More charge carriers are collected in the PEDOT-H-based PeSC due to reduced charge carrier recombination loss with long lifetime coinciding with a trend of JSC value. In conclusion, the model experiment that separates charge transport from charge transfer at the HTL and concentrates on charge transport ability of the HTLs was carried out by modulating the HTL conductivity and stabilizing interfacial energetics. JSC and FF gradually increase with high reproducibility as the conductivity of the HTL increases because the hole transport is facilitated with a prolonged charge carrier lifetime, so hole collection is more efficient with reduced bimolecular recombination (Figure 5a). The schematic diagram of enhanced hole collection with the highly conductive HTL is represented in Figure 5b. In order to generalize the effect of the highly conductive HTL, PeSCs based on Cl-containing perovskite, CH3NH3PbI3-xClx, were also fabricated. Following the previous results in this study, JSC and FF were enhanced with the highly conductive HTL (Figure S11 and Table S5).



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3. Conclusions In summary, our study successfully decoupled charge transfer and transport at the HTL by revealing the effects of the HTL conductivity on the PCE parameters of the PeSCs in isolation from energy level difference and by investigating the relationship between charge transport and charge carrier recombination dynamics. The conductivity of the PEDOT:PSS HTL is systematically controlled from 10-4 to 103 S cm-1 while the energy levels of the HTLs are unaffected, as is the morphology of the perovskite layer, so the overall photovoltaic performance of the PeSCs is only affected by the conductivity of the PEDOT:PSS HTL. As the conductivity of the PEDOT:PSS HTL is increased, the series resistance of the PeSC is diminished with three times prolonged charge carrier lifetime and reduced bimolecular recombination, which leads to efficient hole transport. As a result, JSC and FF are gradually increased from 14.25 to 16.44 mA cm-2 and from 67.7 to 75.0% respectively with VOC unaffected, and 25% increase of PCE is achieved. These results indicate that increasing the conductivity of the HTL is critical key issue to enhance the photovoltaic performances of the PeSCs as well as aligning energy levels of layers.

Supporting Information Mobility calculations, experimental procedures, transmittance of PEDOT:PSS films, AFM, SEM, XRD, absorption characterizations of perovskite films on different PEDOT:PSS films, hysteresis, thickness-dependent HTL conductivity effects, EIS, ILP dependent photocurrent characterizations of the PeSCs, J-V characteristics of CH3NH3PbI3-xClx-based PeSCs.

Acknowledgements This work was supported by a grant (Code No. 2011-0031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea. The authors thank the Pohang Accelerator Laboratory for providing the synchrotron radiation sources at 4D, 8A2, 3C and 9A beamlines used in this study


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Figure Captions Figure 1. AFM topography of (a) PEDOT-L (left), PEDOT-M (middle), and PEDOT-H (right) films. Scale bar is 500 nm. (b) S (2p) XPS spectra and (c) UPS secondary (left) and valence spectra (right) of the PEDOT:PSS films. Figure 2. (a) Cross-sectional SEM image of the optimized device configuration. (b) Schematic diagram of the energy levels of the PeSC layers. Figure 3. Photovoltaic performance characteristics: (a) Current density-voltage curves of PeSCs under AM1.5 illumination. (b) Semi-logarithmic plot of the dark current densityvoltage curves of the PeSCs (c) EQE spectra of the optimized devices. (d) PL quenching of the perovskite films on the PEDOT:PSS HTLs. Figure 4. Incident light power dependent photocurrent of the PeSCs with (a) PEDOT-L and (b) PEDOT-H. (c) Transient photovoltage decay and (d) transient photocurrent decay. (e) Charge carrier lifetime-carrier density relation and (f) integrated transient photocurrent spectra. Figure 5. (a) Short circuit current and fill factor versus conductivity and mobility of the PEDOT:PSS HTLs. (b) Schematic diagram of the differences between the HTLs. Table 1. Physical properties of the PEDOT:PSS films. *with DMSO v/v 5%. Table 2. Photovoltaic parameters of the PeSCs. *Measured at operating condition under AM1.5 illumination.



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Figure 1.


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Figure 2.



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Figure 3.


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Figure 4.



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Figure 5.


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PEDOT: PSS ratio

Conductivity [S cm-1]

Mobility [cm2 V-1 s-1]

PEDOT-L

1:5.38

7.28×10-4

2.53×10-5

PEDOT-M

1:2.85

253*

5.18*

PEDOT-H

1:2.72

882*

17.4*

PEDOT:PSS

Table 1.



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S parameter

JSC [mA cm-2]

VOC [V]

FF [%]

PCE [%]

RS [Ω cm2]

RSh [Ω cm2]

w/o PEDOT: PSS

13.92

0.63

51.9

4.55

13.5

PEDOT-L

14.25

0.80

67.7

7.72

PEDOT-M

14.74

0.83

71.4

PEDOT-H

16.44

0.83

75.0

V0-V = 0.4 V

V0-V = 3V

1737.9

0.808

0.897

8.9

2026.7

0.893

8.74

7.4

2576.7

10.23

6.2

2705.1

Table 2.


Φ

n* 15

-3

4n*

[10 cm ]

[µs]

-

-

-

0.904

1.25

8.25

0.98

0.910

0.961

1.15

9.64

2.45

0.953

0.969

1.13

13.7

3.09

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Graphical abstract


Decoupling Charge Transfer and Transport at ... - ACS Publications

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