Impact of work function of back contact of perovskite solar cells without hole transport material analyzed by device simulation

Accepted Manuscript Impact of work function of back contact of perovskite solar cells without hole transport material analyzed by device simulation Takashi Minemoto, Masashi Murata PII:

S1567-1739(14)00243-0

DOI:

10.1016/j.cap.2014.08.002

Reference:

CAP 3701

To appear in:

Current Applied Physics

Received Date: 25 June 2014 Revised Date:

25 July 2014

Accepted Date: 7 August 2014

Please cite this article as: T. Minemoto, M. Murata, Impact of work function of back contact of perovskite solar cells without hole transport material analyzed by device simulation, Current Applied Physics (2014), doi: 10.1016/j.cap.2014.08.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Impact of Work Function of Back Contact of Perovskite Solar Cells without Hole Transport Material Analyzed by Device Simulation

Takashi Minemotoa,* and Masashi Murataa Department of Electrical and Electronic Engineering, Ritsumeikan University, 1-1-1

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Nojihigashi, Kusatsu, Shiga 525-8577, Japan.

Takashi Minemoto, Tel/Fax: +81-77-561-3065,

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E-mail: [email protected] (T. Minemoto)

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Corresponding author:

Co-authors:

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Masashi Murata, E-mail: [email protected]

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ACCEPTED MANUSCRIPT ABSTRACT

The impact of the work function of a metal back contact on lead methylammonium tri-iodide based perovskite solar cells without hole transport material (HTM) was analyzed using device

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simulation. The elimination of the HTM is attractive in terms of the simplification of device structure and fabrication process. In the solar cell, a back junction is formed by the perovskite absorber and metal back contact. The device simulation revealed that the elimination of the

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HTM did not change the built-in voltage (Vbi) of the device when the work function of the metal back contact (φM) was similar to the valence band maximum of the absorber (Ev_absorber).

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In the HTM-free structure, Vbi showed a high value if φM was equal to or deeper than Ev_absorber. In contrast, when φM was shallower than Ev_absorber, Vbi monotonically decreased, resulting in the decrease in open-circuit voltage of the device. The results showed the importance of the φM matching to maintain Vbi, which is useful guideline for the design of the

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HTM-free perovskite solar cells.

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transport material.

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Keywords: Perovskite solar cells; Device simulation; Back contact; Work function; Hole

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ACCEPTED MANUSCRIPT 1. Introduction The first application of organometal halide perovskite, i.e., CH3NH3PbI3 and CH3NH3PbBr3, to photo sensitizer was reported in 2009 [1]. The perovskite absorber was in the form of nanocrystalline particles in dye-sensitized solar cell configuration. After the first report of

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all-solid state thin film mesoscopic solar cells based on CH3NH3PbI3 in 2012 [2], the perovskite absorbers, such as CH3NH3PbI3 and CH3NH3PbI3-xClx, gathers much attention as excellent novel absorber materials for high efficiency and cost-effective solar cells [3-11].

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Initially, the perovskite solar cells employed mesoporous structure [2-7] using scaffold layers such as porous TiO2. However, the elimination of the mesoporous layers, i.e. planar junction

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architecture, also enables to achieve high efficiency [8-11]. The flexibility of the structures was realized by high absorption coefficient and excellent carrier transport property of the absorbers [4,12]. Interestingly, one more elimination, i.e. the elimination of a hole transport material (HTM), also worked well [13-18]. The HTM-free perovskite solar cells employed

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back contacts with deep work function such as Au (-5.1 eV) [13-16] and carbon (-5.0 eV) [17,18]. The highest efficiency reported so far is 12.8% [18] using carbon as a back contact. These eliminations are attractive from the view points of structure and process simplifications,

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which should be useful for cost reduction. Also, the elimination of the HTM may lead to

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improve the stability of the devices because the HTM can be one of the origins of degradation. Nevertheless, the reason why the HTM can be eliminated is not understood completely. The understanding of device operation mechanism is essential for the optimization of the device structure, which leads to the efficiency improvement. From the analogy of the early date of developments in crystalline silicon and thin-film compound solar cells using Schottky junction [19-21], the work function of a metal back contact should be an important parameter to obtain a reasonable built-in voltage (Vbi) of the devices. For the understanding of the

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ACCEPTED MANUSCRIPT device operation, device simulation is a powerful tool, which was widely applied to inorganic semiconductor solar cells [22-28]. In contrast, device simulation studies on the perovskite solar cells are not enough. The exciton type of the perovskite materials is Wannier type [29], and the device structure of the perovskite and inorganic semiconductor solar cells, such as

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Cu(In,Ga)Se2 (CIGS), are similar. Therefore, device simulators used in CIGS solar cells can be applicable to the perovskite solar cells. In this study, the impact of the elimination of the HTM was analyzed using the Solar Cell Capacitance Simulator (SCAPS) developed by

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University of Gent [30] together with the optimum design of the work function of the back

2. Device simulation parameters

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contact.

The device simulator SCAPS ver. 3.2.01 was used as simulation platform. In the simulation, the perovskite solar cells employed planar junction architecture with layer configuration of

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transparent conductive oxide (TCO)/blocking layer (BL)/absorber/HTM/metal back contact. In the case of the HTM-free structure, the HTM was simply removed from the above structure. Table 1 summarizes the base parameter set of each layer in the simulation. Here,

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NA and ND denote acceptor and donor densities, εr is relative permittivity, χ is electron

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affinity, Eg is band gap energy, µn and µp are mobility of electron and hole, and Nt is defect density. The physics parameters of TCO, BL, absorber and HTM are based on those of SnO2:F, TiO2, and CH3NH3PbI3-xClx, 2,2’,7,7’-tetrakis(N,N-p-dimethoxy-phenylamino)-9,9’spirobifluorene (Spiro-OMeTAD), respectively. The thicknesses of each layer were taken from an experimental report on high efficiency perovskite solar cells with planar junction architecture [9]. The interface defect layers of IDL1 and IDL 2 were inserted between BL/absorber and absorber/HTM (or absorber/metal) interfaces to consider interface

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ACCEPTED MANUSCRIPT recombination. The parameters of IDL1 and IDL2 were set to be identical to those of the absorber except high defect density. One of the most important parameters to determine the absolute value of the efficiency (Eff) is Nt of the absorber. In this study, Nt of the absorber was set to be 2.5 x 1013 cm-3 to adjust electron and hole diffusion length (Ln and Lp) to be 1

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µm, which is an experimentally reported value for high quality perovskite material [4]. The other parameters not included in Table 1 were set to be identical in each layer; effective density of states of conduction band (NC) and valence band (NV) were set to be 2.2 x 1018 and

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1.8 x 1019 cm-3, respectively. Thermal velocity of electron and hole was 107 cm/s. Defect energy level was located at the center of Eg and defect type was neutral. Energetic

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distribution was Gaussian with characteristic energy of 0.1 eV. Capture cross section of electron and hole was 2 x 10-14 cm2. Pre-factor Aα was 105 to obtain absorption coefficient (α) calculated by α=Aα(hv-Eg)1/2. In this study, the conduction band offset of BL/absorber and the valence band offset of absorber/HTM, which are important parameters to determine

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interface recombination, were set to be zero because the consideration of the band offsets makes this analysis too complicated and we should focus on the effect of the elimination of HTM in this study. The effect of the band offsets will be reported elsewhere.

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The band diagram of the perovskite solar cell calculated with the parameter set showed that

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the absorber was fully depleted and the solar cell had an n-i-p structure. An experimental report using electron beam induced current measurement also pointed out the n-i-p structure of the perovskite solar cells [33]. The current density – voltage (J-V) characteristics calculated with above assumption gave the solar cell parameters of short-circuit current density (Jsc) of 22.2 mA/cm2, open-circuit voltage (Voc) of 1.03 V, fill factor (FF) of 80.0% and efficiency of 17.9%. FF was higher than experimental values [9,10] because no additional series resistances such as sheet resistance of the TCO and contact resistances were

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ACCEPTED MANUSCRIPT considered in the simulation. Beside this, reasonable Jsc and Voc consistent with experimental reports [9,10] were obtained, demonstrating that this simulation can be used and the parameter set is not far from that in the real devices. In this calculation, the work function of the metal back contact (φM) was set to be identical to the Fermi level of the HTM of -5.39 eV

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(relative to vacuum level), which is almost similar to the valence band maximum of the absorber (Ev_absorber) of -5.45 eV, to adjust a majority carrier barrier height at the HTM/metal interface to be zero, which is an ideal case. In the HTM-free structure, a back junction is

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formed by the absorber and the metal back contact. Thus, the energy difference between Ev_absorber and φM should affect the solar cell performance. In the next section, the effects of

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the elimination of the HTM and the variation of φM were discussed by simply removing the HTM from the structure and varying φM.

3. Results and discussion

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Figure 1 exhibits the band diagrams of the perovskite solar cells (a) with HTM and (b) without HTM with different Ev_absorber - φM. Here, Ev_absorber and φM indicate energy position relative to vacuum level and Ev_absorber is -5.45 eV from Table 1. Thus, positive and negative

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values of Ev_absorber - φM indicate that the work function of the metal back contact is deeper and

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shallower (relative to vacuum level) than the valence band maximum of the absorber, respectively. As clearly shown in the figures, the absorbers are completely depleted, which is consistent with the experimental report [33]. In the HTM-free structure, when φM is equal to and deeper than Ev_absorber, i.e. Ev_absorber - φM = 0.0 and 0.3 eV in Fig. 1 (b), a high Vbi identical to the case with the HTM can be obtained. Here in Fig. 1(b), the two band diagrams are overlapped. However, if the φM is shallower than Ev_absorber, i.e. Ev_absorber - φM = -0.3 and -0.6

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ACCEPTED MANUSCRIPT eV in the figure, Vbi monotonically decreases, which leads to the reduction in the electric field across the absorber. Figure 2 shows the J-V curves of the HTM-free perovskite solar cells with different Ev_absorber - φM. A J-V curve for the perovskite solar cell with the HTM and with the identical φM to the

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Fermi level of the HTM is also shown by a dashed line in the figure. When φM is in the adequate range to maintain high Vbi, the similar J-V curves are obtained regardless of the presence of the HTM, which is well understood from the band diagrams shown in Fig. 1.

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Here, we should note that the direct deposition of the metal on the perovskite absorber in the real devices may lead to an increase in the defect density at the interface. However, the

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excess carrier densities by light irradiation at the back junction (absorber/metal) are significantly smaller than that at the front junction (BL/absorber), and thus the effect of the defect density at the back junction should be limited.

In Fig. 2, the J-V curves for Ev_absorber - φM = 0.0, 0.2 and 0.4 eV are overlapped, which is

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consistent with the band diagrams of Fig. 1. On the other hand, the J-V curves for Ev_absorber -

φM < 0.0 eV, i.e. shallower φM than Ev_absorber, monotonically shift to lower voltage with moving φM upward to vacuum level. The shift resulted in the decrease in Voc, which has a

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clear relation with Vbi as shown in Fig. 3. Therefore, the control of φM is essential to maintain

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a high Vbi, which leads to a high Voc. In the above calculation, we discussed the importance of the φM matching in the case of the excellent quality of the absorber (Ln=1.0 µm). Next, we discuss the effect of φM matching in the case of the poor quality of the absorber. Figure 4 shows the J-V curves of the HTM-free perovskite solar cells with poor quality of the absorber (Ln=0.05 µm) with different Ev_absorber - φM. A J-V curve for the perovskite solar cell with the HTM and with the identical φM to the Fermi level of the HTM is also shown by a

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ACCEPTED MANUSCRIPT dashed line in the figure. The poor Ln was adjusted by increasing Nt of the absorber to 1.05 x 1016 cm-3. Similar to Fig. 2, the similar J-V curves are obtained regardless of the presence of HTM. With moving φM upward to vacuum level, Voc monotonically decreases and FF also decreases. Figure 5 displays the solar cell parameters of the HTM-free perovskite solar cells

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as a function of Ev_absorber - φM and the absorber quality as a parameter. The impact of the absorber quality is rather small on Voc. However, the inadequate range of φM, i.e. φM > Ev_absorber, leads to the reduction of Vbi and the electric field across the absorber, inducing the

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poor collection of photo-generated carriers. Thus, the severe reduction in Jsc and FF are observed for poor quality absorber. Therefore, both the improvement of the absorber quality

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and the matching of φM are important to achieve high efficiency.

Another interesting topic on φM adjustment is to understand the effect of the HTM insertion when φM is in the inadequate range (φM > Ev_absorber). If the HTM can work as a buffer for φM mismatching, the material choice for new absorbers and back contacts will expand. Figure 6

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shows the solar cell parameters of the perovskite solar cells with and without HTM as a function of Ev_absorber - φM. Note that the conditions for Ev_absorber - φM < -0.8 eV with HTM

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could not be calculated for convergence failure of the program. Even at φM > Ev_absorber, a relatively high Voc is maintained by the insertion of the HTM because of the increase in Vbi by

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the presence of the HTM as shown in Fig. 7. Thus, the HTM is useful to maintain a high Voc; however, FF drastically decreases at the same time to counterbalance the high Voc, resulting in the similar or slightly higher efficiency. Figure 8 shows the comparison of the band diagrams of the perovskite solar cells with and without HTM at the φM mismatching of Ev_absorber - φM = -0.3 eV. As clearly shown by the figure, the presence of the HTM helps to maintain a high Vbi; however, a Schottky barrier for hole, indicated by the dashed circle in the

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ACCEPTED MANUSCRIPT figure, is formed at the HTM/metal interface, which impedes a hole current flow, thus decreasing FF. Consequently, the presence of the HTM is slightly beneficial to the efficiency but that can not be the solution for the φM mismatching. Thus, regardless of the presence of HTM, the choice of the back contact material is essential to realize high efficiency. The

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results well explained the device operation of the HTM-free perovskite solar cells and indicated the possibility of efficiency improvement on simplified structure.

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4. Conclusions

The impacts of the elimination of the HTM and φM on the perovskite solar cells were

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analyzed by device simulator SCAPS. If φM is similar in the solar cells with and without HTM, the similar band diagrams were obtained regardless of the presence of the HTM. By controlling φM to be equal to or deeper than Ev_absorber, high Vbi and Voc were maintained. In contrast, when φM is shallower than Ev_absorber, Vbi monotonically decreased with moving φM

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upward to vacuum level which leads to the decrease in Voc. The results indicate that the φM matching is a requirement to achieve high efficiency and is one of the important guidelines

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for device design of the perovskite solar cells.

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Acknowledgements

The authors would like to thank Professor Marc Burgelman, Department of Electronics and Information Systems, University of Gent for the development of the SCAPS software package and allowing its use. Also, the authors would like to thank Dr J. Chantana and Dr. Z. Tang of Ritsumeikan University for useful discussion.

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ACCEPTED MANUSCRIPT References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050-6051. [2] H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. E. Moser, M. Gratzel, N. G. Park, Lead iodide perovskite

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Humphry-Baker, J. H. Yum, J.

sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%, Scientific Reports 591 (2012) 1-7.

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[3] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites, Science 338 (2012)

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643–647.

[4] S. D. Stranks, G. E. Eperson, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Electron-hole diffusion lengths exceeding 1 micrometer in organometal trihalide perovskite absorber, Science 342 (2013) 341-344.

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[5] J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal, S. I. Seok, Chemical management for colorful, efficient, and stable inorganic-organic hybrid nanostructured solar cells, Nano Lett. 13 (2013) 1764-1769.

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[6] J. Burschka, N. Pellet, S.-J. Moon, R. H.-Baker, P. Gao, M. K. Nazeeruddin, M. Gratzel,

AC C

Sequential deposition as a route to high-performance perovskite-sensitized solar cells, Nature 499 (2013) 316-319.

[7] J. M. Ball, M. M. Lee, A. Hey, H. J. Snaith, Low-temperature processed meso-superstructured to thin-film perovskite solar cells, Energy Environ. Sci. 6 (2013) 1739-1743. [8] J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T. B. Song, C. C. Chen, S. Lu, Y. Liu, H. Zhou, Y. Yang, Low-temperature solution-processed perovskite solar cells with high

10

ACCEPTED MANUSCRIPT efficiency and flexibility, ACS Nano 8 (2014) 1674-1680. [9] M. Liu, M. B. Johnston, H. J. Snaith, Efficient planar heterojunction perovskite solar cells by vapour deposition, Nature 501 (2013) 395-398. [10] D. Liu, T. L. Kelly, Perovskite solar cells with a planar heterojunction structure prepared

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using room –temperature solution processing techniques, Nature Photonics 8 (2013) 133-138. [11] G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely, H. J. Snaith, Morphological control for high performance, solution-processed planar heterojunction perovskite solar cells,

SC

Adv. Funct. Mater. 24 (2014) 151–157.

[12] G. C. Xing, N. Mathews, S. Y. Sun, S. S. Lim, Y. M. Lam, M. Gratzel, S. Mhaisalkar, T.

M AN U

C. Sum, Long-range balanced electron- and hole- transport lengths in organic-inorganic CH3NH3PbI3, Science 342 (2013) 344–347.

[13] L. Etgar, P. Gao, Z. Xue, Q. Peng, A. K. Chandiran, B. Liu, Md. K. Nazeeruddin, M. Grätzel, Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells, J. Am. Chem. Soc. 134

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(2012) 17396−17399.

[14] P. Qin, S. Tanaka, S. Ito, N. Tetreault, K. Manabe, H. Nishino, M. K. Nazeeruddin, M. Grätzel, Inorganic hole conductor-based lead halide perovskite solar cells with 12.4%

EP

conversion efficiency, Nat. Commun. published online, DOI: 10.1038/ncomms4834.

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[15] J. Shi, J. Dong, S. Lv, Y. Xu, L. Zhu, J. Xiao, X. Xu, H. Wu, D. Li, Y. Luo, Q. Meng, Hole-conductor-free perovskite organic lead iodide heterojunction thin-film solar cells: High efficiency and junction property, Appl. Phys Lett. 104 (2014) 063901-1-4. [16] S. Aharon, S. Gamliel, B. E. Cohen, L. Etgar, Depletion region effect of highly efficient hole conductor free CH3NH3PbI3 perovskite solar cells, Phys. Chem. Chem. Phys. 16 (2014) 10512-10518. [17] Y. Rong, Z. Ku, A. Mei, T. Liu, M. Xu, S. Ko, X. Li, H. Han, Hole-conductor-free

11

ACCEPTED MANUSCRIPT mesoscopic TiO2/CH3NH3PbI3 heterojunction solar cells based on anatase nanosheets and carbon counter electrodes, J. Phys. Chem. Lett. 5 (2014) 2160−2164. [18] A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, J. Chen, Y. Yang, M.

high stability, Science 345 (2014) 295-298.

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Gratzel, H. Han, A hole-conductor–free, fully printable mesoscopic perovskite solar cell with

[19] W. A. Anderson, A. E. Delahoy, Schottky barrier diodes for solar energy conversion, Proceedings of the IEEE (1972) pp. 1457-1458.

solar cell, J. Appl. Phys. 45 (1974) 3913-3915.

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[20] W. A. Anderson, A. E. Delahoy, R. A. Milano, An 8% efficient layered Schottky‐barrier

M AN U

[21] M. Bhushan, A. Catalano, Polycrystalline Zn3P2 Schottky barrier solar cells, Appl. Phys. Lett. 38 (1981) 39-41.

[22] P. Basore, Numerical modeling of textured silicon solar cells using PC-1D, IEEE Transactions on Electron Devices 37(2) (1990) 337-343.

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[23] P. Nollet, M. Kontges, M. Burgelman, S. Degrave, R. R.- Koch, Indications for presence and importance of interface states in CdTe/CdS solar cells, Thin Solid Films 431-432 (2003) 414–420.

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[24] R. Klenk, Characterization and modeling of chalcopyrite solar cells, Thin Solid Films

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387 (2001) 135–140.

[25] T. Dullweber, O. Lundberg, J. Malmstrom, M. Bodegard, L. Stolt, U. Rau, H. W. Schock, J. H. Werner, Back surface band gap gradings in Cu(In,Ga)Se2 solar cells, Thin Solid Films 387 (2001) 11–13.

[26] T. Minemoto, T. Matsui, H. Takakura ,Y. Hamakawa,T. Negami, Y. Hashimoto, T. Uenoyama, M. Kitagawa, Theoretical analysis of the effect of conduction band offset of window/CIS layers on performance of CIS solar cells using device simulation, Sol. Energy

12

ACCEPTED MANUSCRIPT Mater. Sol. Cells 67 (2001) 83-88. [27] M. Murata, D. Hironiwa, N. Ashida, J. Chantana, K. Aoyagi, N. Kataoka, T. Minemoto, Optimum bandgap profile analysis of Cu(In,Ga)Se2 solar cells with various defect densities by SCAPS, Jpn. J. Appl. Phys. 53 (2014) 04ER14-1-4.

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[28] D. Hironiwa, M. Murata, N. Ashida, T. Zeguo, T. Minemoto, Simulation of optimum band-gap profile of Cu2ZnSn(S,Se)4 solar cells with different optical and defect properties, Jpn. J. Appl. Phys. 53 (2014) 071201-1-9.

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[29] M. Hirasawa, T. Ishihara, T. Goto, K. Uchida, N. Miura, Magnetoabsorption of the lowest exciton in perovskite-type compound (CH3NH3)PbI3, Physica B 201 (1994) 427-430.

cells,

Thin

Solid

Films

M AN U

[30] M. Burgelman, P. Nollet, S. Degrave, Modeling polycrystalline semiconductor solar 361-362

(2000)

527–532.

Also,

see

http://www.elis.ugent.be/ELISgroups/solar/projects/scaps.html

[31] H. J. Snaith, M. Grätzel, Electron and hole transport through mesoporous TiO2 infiltrated

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with Spiro-MeOTAD, Adv. Mater. 19 (2007) 3643-3647.

[32] D. Poplavskyy, J. Nelson, Nondispersive hole transport in amorphous films of methoxy-spirofluorene-arylamine organic compound, J. Appl. Phys. 93 (2003) 341-346.

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[33] E. Edri, S. Kirmayer, S. Mukhopadhyay, K. Gartsman, G. Hodes, D. Cahen, Elucidating

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the charge carrier separation and working mechanism of CH3NH3PbI3-xClx perovskite solar cells, Nat. Commun. published online, DOI: 10.1038/ncomms4461.

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Table 1 Base parameter set in simulation. For HTM-free structure, HTM was simply removed

TCO (SnO2 :F)

BL (TiO2 )

IDL1 (defect layer)

Absorber (CH3 NH3 PbI3-xClx)

IDL2 (defect layer)

HTM (Spiro-OMeTAD)

Thickness (nm)

500

50

10

330

10

350

NA (cm-3 )

-

-

-

-

-

2 x 1018 [4]

ND (cm-3 )

2 x 1019

1016

1013

1013

1013

-

εr

9.0

9.0

6.5

6.5 [9]

6.5

3.0 [31]

χ (eV)

4.00

3.90

Eg (eV)

3.50

3.20

µn / µp (cm2/Vs)

20/10

20/10

Nt (cm-3 )

1015

1015

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Parameter

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from the structure.

3.90 [29]

3.90

2.45 [32]

1.55

1.55 [5]

1.55

3.00 [32]

2.0/2.0

2.0/2.0 [3]

2.0/2.0

2 x 10-4 /2 x 10-4 [32]

1017

2.5x1013

1017

1015

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3.90

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ACCEPTED MANUSCRIPT Figure captions

Fig. 1. Band diagrams of perovskite solar cells (a) with HTM and (b) without HTM with

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different Ev_absorber - φM.

Fig. 2. J-V curves of perovskite solar cells without HTM with different Ev_absorber - φM. A J-V

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of the HTM is also shown by a dashed line in the figure.

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curve for the perovskite solar cell with the HTM and with the identical φM to the Fermi level

Fig. 3. Built-in voltage and open-circuit voltage of perovskite solar cells without HTM as a function of Ev_absorber - φM.

Fig. 4. J-V curves of perovskite solar cells with poor absorber quality without HTM with

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different Ev_absorber - φM. A J-V curve for the perovskite solar cell with the HTM and with the

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identical φM to the Fermi level of the HTM is also shown by a dashed line in the figure.

Fig. 5. Solar cell parameters of perovskite solar cells without HTM as a function of Ev_absorber

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- φM and absorber quality as a parameter.

Fig. 6. Solar cell parameters of perovskite solar cells with and without HTM as a function of Ev_absorber - φM.

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ACCEPTED MANUSCRIPT Fig. 7. Built-in voltages of perovskite solar cells with and without HTM as a function of Ev_absorber - φM.

Fig. 8. Band diagrams of perovskite solar cells with and without HTM at Ev_absorber - φM = -0.3

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eV.

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3

Energy (eV)

HTM

qVbi Ec

-1

TCO

-2

absorber BL

-3

1

0.0, 0.3 -0.3 -0.6

0

Ec

-1

TCO

-4

absorber

BL

-3

Ev

Ev

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Depth from surface (µm)

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Depth from surface (µm)

qVbi

-2

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Fig. 1

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Energy (eV)

1

-4

Ev_absorber - φM (eV)

2

2

0

(b)

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3

4

(a)

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4

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ACCEPTED MANUSCRIPT

20

Ln=1.0 µm Ev_absorber - φM (eV)= 0, 0.2, 0.4 (overlap) with HTM

15 10 5

0.2

−0.6

−0.4 −0.2

0.4 0.6 0.8 Voltage (V)

1.0

M AN U

0 0.0

−0.8

SC

−1.0

RI PT

2

Current Currentdensity density(mA/cm (mA/cm2))

25

AC C

EP

TE D

Fig. 2

18

1.2

ACCEPTED MANUSCRIPT

1.5

Vbi

0.9

RI PT

Voc , Vbi (V)

1.2

Voc

0.6

0.0 -1.2

-0.8

-0.4

SC

0.3 0.0

M AN U

Ev_absorber - φM (eV)

AC C

EP

TE D

Fig. 3

19

0.4

ACCEPTED MANUSCRIPT

Ev_absorber - φM (eV) = 0, 0.2, 0.4 (overlap)

2

10

−0.2

5 −1.0

0.2

−0.6

−0.4

0.4 0.6 Voltage (V)

0.8

M AN U

0 0.0

−0.8

RI PT

15

SC

Current density (mA/cm )

Ln=0.05 µm

with HTM

20

AC C

EP

TE D

Fig. 4

20

1.0

ACCEPTED MANUSCRIPT

20

1.2

1.0

0.1

10

0.8

0.1

0.6

Ln=0.05 µm

0.4

Ln=0.05 µm

0 -1.2

-0.8

-0.4

0.0

0.2 0.0 -1.2

0.4

Ev_absorber - φM (eV) 25

60 50

Ln=0.05 µm

40 30 -1.2

-0.8

-0.4

0.0

2

20

0.4

0.1

15

EP

Fig. 5

21

Ln=0.05 µm

10

5 -1.2

0.4

TE D

Ev_absorber - φM (eV)

0.0

1.0 & 0.5 (overlap)

M AN U

0.1

AC C

FF (%)

0.5

JJscsc (mA/cm (mA/cm2))

1.0

70

-0.4

Ev_absorber - φM (eV)

90 80

-0.8

SC

5

RI PT

Eff (%)

Voc (V)

0.5

15

1.0 & 0.5 (overlap)

1.0

-0.8

-0.4

0.0

Ev_absorber - φM (eV)

0.4

ACCEPTED MANUSCRIPT

20

1.2

with

1.0

10

without

0.8

without

0.6 0.4

5 0.2 0 -1.2

-0.8

-0.4

0.0

0.0 -1.2

0.4

-0.8

Ev_absorber - φM (eV)

without 2 (mA/cm2 ) JJscsc (mA/cm

50 40 30 -1.2

-0.8

-0.4

0.0

0.4

20

with

M AN U

with

without

15 10

5 -1.2

0.4

TE D

Ev_absorber - φM (eV)

EP

Fig. 6

AC C

FF (%)

60

0.0

SC

25

70

-0.4

Ev_absorber - φM (eV)

90 80

RI PT

with Voc (V)

Eff (%)

15

22

-0.8

-0.4

0.0

Ev_absorber - φM (eV)

0.4

ACCEPTED MANUSCRIPT

1.5

with

without

0.9 0.6 0.3 -0.8

-0.4

0.0

0.4

SC

0.0 -1.2

RI PT

Vbi (V)

1.2

M AN U

Ev_absorber - φM (eV)

AC C

EP

TE D

Fig. 7

23

ACCEPTED MANUSCRIPT

Ev_absorber - φM = -0.3 eV with HTM

1

absorber

without HTM

0

Ev TCO

-1

barrier for hole BL

-2 0.0

0.2

0.4

0.6

0.8

SC

Energy (eV)

2

Ec

RI PT

3

1.0

M AN U

Depth from surface (µm)

1.2

AC C

EP

TE D

Fig. 8

24

ACCEPTED MANUSCRIPT

Absorber

IDL2

HTM

(SnO2:F)

(TiO2)

(defect layer)

(CH3NH3PbI3-xClx)

(defect layer)

(Spiro-OMeTAD)

500

50

10

330

10

350

-

-

-

-

-3

ND (cm )

19

2 x 10

16

10

6.5

9.0

9.0

χ (eV)

4.00

3.90

Eg (eV)

3.50

3.20

20/10

20/10

2

(cm /Vs) Nt

-3

(cm )

15

10

3.90

1.55

AC C

εr

µn / µp

13

10

15

10

2.0/2.0

17

10

SC

RI PT

IDL1

M AN U

-3

NA (cm )

BL

TE D

Thickness (nm)

TCO

EP

Parameter

13

10

6.5

[9]

3.90

1.55

18 [4]

-

2 x 10

13

-

10

[31]

6.5

3.0

[29]

3.90

2.45

[5]

1.55

3.00

2.0/2.0

2 x 10 /2 x 10

[3]

2.0/2.0

13

2.5x10

17

10

[32]

[32]

-4

-4 [32]

15

10

ACCEPTED MANUSCRIPT

Hole transport material (HTM)-free perovskite solar cells was simulated. Deeper work function than valence band of absorber maintained high built-in voltage. Shallower work function than valence band of absorber decreased built-in voltage. Adequate work function gave similar high efficiency without HTM.

AC C

EP

TE D

M AN U

SC

RI PT

This study showed useful guideline for design of HTM-free perovskite solar cells.