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High-Efficiency Solution-Processed Planar Perovskite SolarCells with a Polymer Hole Transport Layer

Dewei Zhao, Michael Sexton, Hye-Yun Park, George Baure, Juan C. Nino, and Franky So*

Dr. D. Zhao, M. Sexton, Dr. H.-Y. Park, G. Baure,Prof. J. C. Nino, Prof. F. SoDepartment of Materials Science and EngineeringUniversity of Florida100 Rhines Hall, Gainesville, FL 32611, USAE-mail: [email protected]

DOI: 10.1002/aenm.201401855

In this work we demonstrate a high-efficiency solution-processed inverted CH3 NH3 PbI3 perovskite solar cell, whichis free of PEDOT:PSS and high-temperature processed metaloxides (Figure 1 a). We use poly[N ,N ′-bis(4-butylphenyl)-N ,N ′-bis(phenyl)benzidine] (poly-TPD) as the HTL and electronblocking layer for the perovskite cells. In previous reports, poly-TPD was used as an HTL in vacuum deposited perovskite solarcells.[14] Here, the perovskite film was formed by sequentialdeposition of lead iodide (PbI2 ) and methyl ammonium iodide(CH3 NH3 I). We found that the resulting film consisted of large

crystallites with a complete coverage on the poly-TPD surface,and the average efficiency of the final devices reach a value of13.8% and a maximum value as high as 15.3%.

To deposit the perovskite film on the poly-TPD surface, aconcentrated solution of PbI2 was first spin-coated and thenheated to partially evaporate the solvent and crystallize PbI2 .Subsequently, a dilute solution of CH3 NH3 I is spin-coated ontop of the PbI2 layer and CH3 NH3 PbI3 is formed by interdif-fusion of the precursors. As shown in Figure 1b, a compositelayer of spin-coated [6,6]-phenyl-C61 -butyric acid methyl ester(PC60 BM), and thermally evaporated C60 and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is deposited on top ofthe CH3 NH3 PbI3 layer to planarize the surface of the perovskitelayer, and to facilitate electron extraction and hole blocking.[17] More details on device fabrication can be found in the Experi-mental Section. To better understand the device characteristics,devices were also fabricated with PEDOT:PSS as the HTL forcomparison.

The average current density–voltage ( J–V ) characteristics ofthe devices with poly-TPD or PEDOT:PSS as the HTL under100 mW cm–2 illumination (AM1.5G) are shown in Figure 2 a.As shown in the figure, the poly-TPD devices perform sig-nificantly better than the PEDOT:PSS devices. The poly-TPDdevices have an average PCE of 13.8% with a short-circuit cur-rent density ( J sc ) of 20.01 mA cm–2 , a V oc of 0.99 V, and a fillfactor (FF) of 69.55% (Table 1 ). As shown in the histogram ofthe poly-TPD device data in Figure S1a (Supporting Informa-

tion), the highest PCE of the poly-TPD device is 15.3%. Thedependence of perovskite solar cell performance on the poly-TPD thickness is also plotted in Figure S1b,c (SupportingInformation). The results show that both J sc and V oc are notdependent on the poly-TPD thickness, while the FF is signifi-cantly reduced with increasing the poly-TPD thickness up to100 nm due to an increase in series resistance. An optimumthickness of 40 nm was used for the devices in this study. How-ever, the PEDOT:PSS devices produce a significantly lower PCEof 4.63% with a J sc of 9.41 mA cm–2 , a V oc of 0.80 V, and a FFof 61.8%. The external quantum efficiency (EQE) spectra meas-ured with and without white light bias (WLB) are shown in

Organometallic halide perovskite solar cells are rapidlybecoming a promising technology for solar energy conver-sion. Organic/inorganic hybrid perovskite materials have sev-eral unique properties for photovoltaic applications, such asstrong absorption across the visible spectrum,[1] long carrierdiffusion length (100–1000 nm),[2,3] solution processability,and insensitivity to defect formation.[4–6] In most perovskitecells, compact or mesoporous metal oxides are used as theelectron transport layers (ETLs).[7] These ETLs usually requirehigh-temperature processing to achieve efficient carrier trans-

port and the resulting devices are not stable with hysteresis inthe current–voltage characteristics.[8–11] On the other hand, themost commonly used hole transport layer (HTL) for perovskitecells is 2,2′,7,7′-Tetrakis (N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) which requires a complex-doping mechanism to promote oxidation of spiro-OMeTAD anddegrades the device stability and repeatability.[12]

An alternative to this architecture is to place the HTL onthe transparent electrode in the so-called “inverted” struc-ture.[13] Most inverted devices employ either poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) or solu-tion-processed nickel oxide (NiOx ) as the HTL, which presenttheir own issues for perovskite solar cells. [14–19] PEDOT:PSScorrodes the indium-doped tin oxide (ITO) electrode, andcauses migration of indium into PEDOT:PSS. [20] The hygro-scopic nature of PEDOT:PSS is prone to degrade the resultingorganic devices due to the water uptake.[20] This is specificallyproblematic for perovskite cells because the perovskite mate-rial methyl ammonium lead iodide (CH3 NH3 PbI3 ) is vulner-able to decomposition upon water exposure. [21,22] While theefficiency of inverted devices with NiOx has reached a powerconversion efficiency (PCE) value as high as 11.6%, NiOx

requires high-temperature or high-vacuum processing. Poorwetting of the perovskite film on NiOx leads to formation ofcrystallite islands resulting in a rough surface with shuntingpaths and hence a lower open-circuit voltage (V oc ).

[19] Addi-tionally, NiOx also forms trap states at the perovskite inter-

face leading to significant carrier recombination affectingthe device performance.[23,24] Therefore, it is highly desired todevelop alternative low-temperature solution-processed HTLmaterials for perovskite solar cell applications.

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Figure 2b. The EQE for the poly-TPD devices decreases by only3% under WLB while the PEDOT:PSS device exhibits a 50%decrease in EQE across the entire spectrum. This strong lightbias dependence in the PEDOT:PSS devices indicates signifi-cant carrier recombination under normal operating conditions.The detailed recombination mechanism will be discussed later.

Figure 3 a,b show the SEM images of the perovskite films

deposited on poly-TPD and PEDOT:PSS, respectively. Theaverage grain size of the perovskite film on poly-TPD is signifi-cantly larger than that on PEDOT:PSS. The smaller grain sizeresults in a higher grain boundary density. It has been shownby thermal admittance spectroscopy and X-ray photoelectron

spectroscopy that gap states form at the grainboundaries affecting the quasi-Fermi levelsplitting,[17,25] and defects at grain boundariesreduce V oc due to trap filling of the photo-generated electrons and an accumulation ofholes, and lead to the formation of barriers toextract carriers. From the cross-section SEMimages of the perovskite films on differentHTLs (Figure S2, Supporting Information),the grain size along the normal of the perovs-kite film on poly-TPD is larger than that ofthe perovskite film on PEDOT:PSS, resultingin the presence of grain boundaries along

the direction of carrier transport in the devices fabricated onPEDOT:PSS. Hence, we conclude that large grains on poly-TPDfacilitates a more efficient charge extraction.

To determine the charge generation efficiency, we meas-ured the absorption coefficients of the perovskite films onPEDOT:PSS and poly-TPD and a pure PbI2 film, and the dataare shown in Figure 3c. Both perovskite films have similar

absorption coefficients at wavelengths beyond 550 nm. How-ever, at shorter wavelengths, the perovskite film on PEDOT:PSSexhibits higher absorption coefficients than the film on poly-TPD, which is attributed to a greater PbI2 content in the filmdeposited on PEDOT:PSS. This indicates that charge genera-tion alone does not account for the significant difference in J sc in both devices.

The X-ray diffraction (XRD) patterns in Figure 3d show thatthe perovskite films form a tetragonal phase with randomlyoriented crystals as previously reported.[14] The peak at 12.7° is associated with PbI2 and it is less intense for the perovskitefilm on poly-TPD compared with the film on PEDOT:PSS,suggesting a greater amount of PbI2 present in the film onPEDOT:PSS. It should be noted that the device performanceand reproducibility were found better with a small amountof decomposed PbI2 in the devices fabricated on poly-TPD(Figure S3, Supporting Information). These results are con-sistent with the experimental and theoretical evidences that asmall amount of PbI2 generates a passivating layer at the grainboundaries resulting in a reduction of recombination and anenhanced device performance.[26–29]

To understand the recombination mechanism in thesedevices, the current–voltage characteristics were measuredunder light intensities ranging from 0.5 to 100 mW cm–2 . Tostudy the effect of photogenerated carrier density on carrierextraction, the J–V characteristics of the two devices are nor-malized by the reverse saturation current density ( J sat ) and the

results are shown in Figure 4 a,b. There is very little differencein the normalized current density from reverse bias to the

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Figure 1. a) Device structure and b) energy band diagram of studied solar cells.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

-20

-15

-10

-5

0

Voltage (V)

C u r r e n t d e

n s i t y ( m A / c m

2 ) PEDOT:PSS

Poly-TPD (a)

400 500 600 700 8000

10

20

30

40

50

60

70

80

E Q E

( % )

Wavelength (nm)

PEDOT:PSS PEDOT:PSS under WLB Poly-TPD Poly-TPD under WLB

(b)

Figure 2. a) Average J–V curves and b) EQE spectra with and without aWLB (WLB) for perovskite solar cells with PEDOT:PSS and poly-TPD asthe HTL.

Table 1. Figures of merit for PEDOT:PSS and poly-TPD based devices.

PCE

[%]

V oc

[V]

J sc

[mA cm–2 ]

FF

[%]

Poly-TPD 13.78[±0.81] 0.99[±0.02] 20.01[±1.04] 69.55[±3.42]

Best 15.3 1.10 22.0 69.70

PEDOT:PSS 4.63[±0.67] 0.80[±0.01] 9.41[±1.35] 61.80[±1.10]

Best 5.58 0.79 11.30 62.70

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