Numerical simulation: Toward the design of high

Numerical simulation: Toward the design of high-efficiency planar perovskite solar cells Feng Liu, Jun Zhu, Junfeng Wei, Yi Li, Mei Lv, Shangfeng Yang, Bing Zhang, Jianxi Yao, and Songyuan Dai Citation: Applied Physics Letters 104, 253508 (2014); doi: 10.1063/1.4885367 View online: http://dx.doi.org/10.1063/1.4885367 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/25?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High-performance hybrid organic-inorganic solar cell based on planar n-type silicon Appl. Phys. Lett. 104, 193903 (2014); 10.1063/1.4875913 A futuristic approach towards interface layer modifications for improved efficiency in inverted organic solar cells Appl. Phys. Lett. 104, 041114 (2014); 10.1063/1.4863434 Highly efficient inverted polymer solar cells with a solution-processable dendrimer as the electron-collection interlayer Appl. Phys. Lett. 102, 083302 (2013); 10.1063/1.4794065 High efficiency and high photo-stability zinc-phthalocyanine based planar heterojunction solar cells with a double interfacial layer Appl. Phys. Lett. 101, 113301 (2012); 10.1063/1.4748123 Highly efficient crystalline silicon/Zonyl fluorosurfactant-treated organic heterojunction solar cells Appl. Phys. Lett. 100, 183901 (2012); 10.1063/1.4709615

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APPLIED PHYSICS LETTERS 104, 253508 (2014)

Numerical simulation: Toward the design of high-efficiency planar perovskite solar cells Feng Liu,1 Jun Zhu,1,a) Junfeng Wei,1 Yi Li,1 Mei Lv,1 Shangfeng Yang,2 Bing Zhang,3 Jianxi Yao,3 and Songyuan Dai1,3,a)

1 Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China 2 Hefei National Laboratory for Physical Sciences at Microscale, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, People’s Republic of China 3 State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, People’s Republic of China

(Received 9 May 2014; accepted 15 June 2014; published online 26 June 2014) Organo-metal halide perovskite solar cells based on planar architecture have been reported to achieve remarkably high power conversion efficiency (PCE, >16%), rendering them highly competitive to the conventional silicon based solar cells. A thorough understanding of the role of each component in solar cells and their effects as a whole is still required for further improvement in PCE. In this work, the planar heterojunction-based perovskite solar cells were simulated with the program AMPS (analysis of microelectronic and photonic structures)-1D. Simulation results revealed a great dependence of PCE on the thickness and defect density of the perovskite layer. Meanwhile, parameters including the work function of the back contact as well as the hole mobility and acceptor density in hole transport materials were identified to significantly influence the performance of the device. Strikingly, an efficiency over 20% was obtained under the moderate C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4885367] simulation conditions. V

Solid-state organo-metal halide perovskite solar cells have emerged as a kind of promising alternatives to existing photovoltaic technologies with both solution-processability and superb photovoltaic performance. Significantly, a maximum power conversion efficiency (PCE) over 16% has been achieved after the past several years of vigorous work.1–3 Those achieved high efficiencies were believed to result from (i) overwhelming characteristics such as high absorption coefficient and long charge-carrier diffusion lengths in perovskites,4 (ii) careful design of device configuration to fully exploit the merits of materials,2 (iii) techniques developed to achieve high quality interfacial contact, together with the crystallinity and concentration of perovskites within networks.5 However, the achieved PCEs still lag behind of those established photovoltaics, cadmium telluride thin film solar cells (CdTe, 19.6%), copper indium gallium diselenide (CIGS, 19.8%), crystalline silicon (c-Si, 25%), and gallium arsenide (GaAs, 28.8%).6 Further improvement in perovskite solar cells requires a thorough understanding of the role of each component and their effects as a whole. For planar heterojunction-based solar cells, recent efforts have revealed that increasing conductivity of the hole transport materials (HTMs) by doping and optimizing charge collection by adjusting the absorber thickness could bring a positive impact on PCEs.1,2,7 Yet up to now, there has been no report concerning the numerical simulation of the perovskite-based solar cells. Simulation methods describe the basic phenomena present in photovoltaic devices, allowing intuitive examination of each parameter in solar cells and thus identifying the optimal conditions for operating. Our works here aimed a)

Authors to whom correspondence should be addressed. Electronic addresses: [email protected] and [email protected]

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toward the design of high-efficiency perovskite solar cells through investigating the effects of material parameters on device behavior. Analysis of microelectronic and photonic structures-1D (AMPS-1D) is a general solar cell simulation program that is designed based on three basic semiconductor equations (see Ref. 8 for more details)8 and it is well adapted to modeling various hetero- and homo-junctions, multi-junction, and Schottky barrier devices.9,10 The structure of the employed device here is depicted in Fig. S1 (see Ref. 8). It is a p-i-n solar cell with low p-type doped CH3NH3PbI3 sandwiched between a n-type TiO2 and p-type 2,20 ,7,70 -tet-rakis(N,N0 -di-pmethoxyphenylamine)–9,90 -spirobifluorene (spiro-OMeTAD) layer. Material parameters set for device simulation are summarized in Table S1 (in Ref. 8). Note that the main material parameters are carefully selected from those reported experimental work (cited in supplementary material). Surprisingly, an efficiency of 22.7% was obtained under those moderate conditions (J-V curves were shown in Ref. 8, Fig. S2). Since AMPS explicitly does not take into account interface recombination, only bulk and contact loss, the open-circuit voltage (Voc) simulated would be higher than it actually is.10 We notice that if the work function of the back contact is reduced to 4.4 eV, the efficiency would decrease to 17% (Voc ¼ 1.27 eV; Jsc ¼ 21.58 mA/cm2; Fill factor, FF ¼ 0.62), which is close to the experimental value for Ag cathode-based planar heterojunction CH3NH3PbI3 solar cells with efficiency of 15.4% (Voc ¼ 1.07 eV, Jsc ¼ 21.5 mA/cm2, FF ¼ 0.67).2 In addition, an efficiency of 12.2% was obtained for the hole conductor-free CH3NH3PbI3/TiO2 heterojunction solar cells (data not shown), similar to the results reported by Etgar’s group,11 evidencing to an extent the validity of the values set for characterizing the layers involved.

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As a key factor that determines all the properties of the device, spectral response of the solar cells is highly dependent on the thickness of the absorber layer. We first examine the J-V characteristics of the device as a function of the CH3NH3PbI3 thickness. As plotted in Fig. 1, the short-circuit current density (Jsc) increases with the thickness of the perovskite layer and saturates to a plateau (23.6 mA/cm2) when the thickness reaches 1 lm. The Voc, however, decreases slightly, indicating an increased charge recombination in thicker films. The high photocurrent (19 mA/cm2) obtained with a considerably thin thickness (about 200 nm) is primarily attributed to the high absorption coefficient of the perovskite films. Fig. S3 (in Ref. 8) presents the effective absorption coefficient of the CH3NH3PbI3 layer. In addition, benefiting from the long charge-carrier diffusion lengths,4,12 perovskite semiconductors possess a long charge-collection length and thus a high charge-collection efficiency. In fact, CH3NH3PbI3 has been shown with superior hole-transport property, ensuring the long-distance charge transport in perovskite films.11 Therefore, this active layer can be designed much thicker and thus absorb more light than most other solution-processed photovoltaic materials such as quantum dots and organic conjugated materials.4 The benefits of employing thicker CH3NH3PbI3 layer in improving the device performance indicated in the simulation results can be supported by the fact that through further increasing the thickness of perovskite layer to about 300 nm in CH3NH3PbI3/TiO2 heterojunction solar cells, Etgar et al. achieved a higher efficiency than a thinner one (200 nm).11,13 Likewise, after succeeded in obtaining a thicker perovskite film, Lam et al. achieved a record efficiency for [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-based perovskite junction solar cells.14 However, we have noticed a discrepancy in optimal thickness of the perovskite layer between the simulation results and the experimental results, which we consider to result primarily from large-scale inhomogeneity in film uniformity and layer thickness in actual devices, as has been discussed in the report of Sum and Mathews.15 The properties and the amount of defects, especially, in CH3NH3PbI3 bulk films play a critical role in determining the performance of the device, since the photoelectrons are mainly generated in this light-absorber layer and the charge recombination behaviors there should become dominant in determining the Voc of the device. Therefore, for brevity, we

FIG. 1. J-V curves of the device simulated with varying values of the CH3NH3PbI3 thickness.

Appl. Phys. Lett. 104, 253508 (2014)

FIG. 2. J-V curves of the device obtained with varying defect densities in CH3NH3PbI3 films.

did neither include the effects of the defects in TiO2 compact films nor in HTMs. J-V curves plotted with varying defect densities in CH3NH3PbI3 films are shown in Fig. 2. It is pretty clear that the presence of the defect states in perovskite films cause a considerable drop in Voc, yet almost no influence on Jsc value. This is consistent with the report that the high crystallinity of the perovskite achieved by Liu and Kelly and Snaith et al. helps to minimize the trap density within perovskites, reducing charge recombination and hence allow high Voc obtained.1,2 High Voc values have been widely achieved in perovskite solar cells and attributed to the rather low deep defect densities, which are responsible for the non-radiative recombination in perovskite films.16 A further investigation on photoelectric behaviors of the device was shown in Fig. 3, whereby the recombination rate was unambiguously illustrated to increase with the trap density. In fact, a pretty low trap-assisted recombination rate has been demonstrated in CH3NH3PbI3 films using transient THz spectroscopy by Herz et al.12 The inset in Fig. 3 gives the simulated electron lifetime (se) corresponding to those defect densities. Case with the lowest density gives the longest lifetime, as expected, consistent with the trend in recombination rate. Interestingly, the simulated se with the defect density of 1015 cm3 is comparable with the one measured from transient Voc decay experiment and intensity-modulated photovoltage spectroscopy (IMVS) method.17 As mentioned above that AMPS does not take into account the interface recombination loss (mainly at the TiO2/perovskite interface), rendering the simulated Voc

FIG. 3. Total recombination distribution under open-circuit condition in the cell thickness direction for varying defect densities in perovskite films. Inset is the corresponding electron lifetime vs position.

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FIG. 4. Simulation results for Voc and PCE as a function of interface trap density.

higher than it actually is. A typical method to simulate a more realistic device is to artificially insert a very thin layer in perovskite region with a large number of defect states distributed evenly within the bandgap. So that we are allowed to evaluate the role of interface defects in determining the performance of the device. Parameters defined for this thin layer are shown in Table S2 (in Ref. 8). Fig. 4 gives the simulation results for Voc and PCE as a function of interface trap density. Both values decreased largely with the increase in trap density, highlighting the significance of interface modification in achieving high-efficiency perovskite solar cells. Interestingly, through SCAPS model, Sargent et al. have also revealed the great importance of interface modification in heterojunction-based quantum dots photovoltaics, which should lead to a considerable reduction in carrier recombination and hence a great improvement in Voc of the device.18 The performance of the simulated solar cells reduced with decreasing the work function (u) of the counter electrode (CE) and a dependence of the FF value on u was found as depicted in Fig. 5. 1D distribution of the electric field in HTM layer was inspected to reveal the mechanism underlying this trend. Inset in Fig. 5 displays that the direction of the electric field close to HTM/CE interface becomes negative when the u value below 5.0 eV, which means the electric field direction is directed from CE to HTM, rendering it energetically unfavorable for hole transport to electrode. Therefore, FF decreased with the increase in series resistance at the HTM/CE interface. We attribute the cause to the possible formation of the Schottky junction at the HTM/CE

FIG. 5. FF of the device vs the back contact work function. Inset shows the distribution of the electric field at the HTM/CE interface in the film thickness direction.

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interface, which may arise from Fermi level pinning at this interface.19 On contrary, an Ohmic contact has been verified between the Au (u ¼ 5.1 eV) and the spiro-MeOTAD.20 However, Au cathode could dissipate the majority of the incident light in actual uses due to its poor reflectivity in the visible region of the spectrum.21 Therefore, although being plagued by the relatively low u, Ag cathode is widely employed to assemble high-efficiency solar cells owing to its excellent property of reflectivity. A possible way to combine both advantages is incorporating pure Ag into Au electrodes, as has been suggested by Snaith et al.21 Spiro-MeOTAD as a superior HTM material is widely used in various high-performance solid-state cells. Here, through numerical simulation, both the effects of the hole mobility and acceptor density in Spiro-MeOTAD on the performance of the device were examined. Same trend was observed that J-V curves improved with the increase in each value (see Fig. 6). However, the pristine Spiro-MeOTAD has a low hole mobility and a low acceptor concentration; the series resistance at open-circuit is thus limited by insufficient hole-conduction through Spiro-MeOTAD.22 Experimental methods, such as doping with additional additives or p-type dopants in HTMs, have been shown to significantly improve device performance through enhancing its conductivity, as they mainly functioned to increase the hole mobility and charge density of HTMs, respectively, or simutaneously.20,22,23 In this paper, the planar heterojunction CH3NH3PbI3 solar cells were simulated with AMPS program. Several parameters that are of paramount relevance to the performance of the solar cells were carefully investigated. Simulation results indicate that (i) owing to the low defect density, combined with superior absorption coefficient, perovskite solar cells can exhibit both high Voc and Jsc values, (ii) cathode

FIG. 6. J-V curves of the device simulated under (a) different hole mobilities with constant density of 3  1016 cm3, (b) varying acceptor densities with constant mobility of 1  104 cm2/V/s.

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material with high-work function is required to avoid the possible formation of the Schottky junction at the HTM/CE interface, and (iii) series resistance related hole mobility and acceptor concentration in HTM materials greatly influence the performance of the device. Significantly, under moderate conditions, an inspiring efficiency over 20% was obtained, further highlighting the great potential of perovskites in achieving high PCEs. The results simulated here should provide both the insight and understanding of the role of each component in solar cells and help to establish device parameters that allowing for further analysis and design optimization. This work was supported by the National Basic Research Program of China under Grant No. 2011CBA00700, the National High Technology Research and Development Program of China under Grant No. 2011AA050527, the Program of Hefei Center for Physical Science and Technology (No. 2012FXZY006). 1

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