Mesoscopic Perovskite Solar Cells and Modules

Transcription

1 Proceedings of the 14th IEEE International Conference on Nanotechnology Toronto, Canada, August 18-1, 14 Mesoscopic Perovskite Solar Cells and Modules A. Di Carlo, Member, IEEE, F. Matteocci, S. Razza, M. Mincuzzi, F. Di Giacomo, S. Casaluci, D. Gentilini, T. M. Brown, A. Reale, F. Brunetti, A. D Epifanio, S. Licoccia Abstract In this work we exploit the use of a new promising class of light harvesting materials, namely the hybrid organic halide perovskites (CH NH PbI -x Cl x ), for the fabrication of mesoscopic perovskite solar cells and seriesconnected monolithic perovskite module. To achieve this goal, important innovative procedures were implemented in order to define a reproducible fabrication path applicable also to large area devices. Small area solar cells were fabricated with both Spiro-OMeTAD and the PHT polymer as Hole Transporting Material (HTM) both showing a Power Conversion Efficiency (PCE) up to 1.5%. First attempts to scale up the size of the cell to a module size shown a PCE of 5.1% on an active area of 1.44cm. In order to improve the efficiency of the module, we developed a new Laser assisted patterning of the perovskite/compact layers together with an optimized perovskite deposition in controlled atmosphere. This allowed us to improve the module PCE up to 7.% which represent the state of art efficiency for a perovskite module. A promising long-term stability was obtained for the module with Spiro-OMeTAD as HTM. Supporting simulations of Mesoscopic Perovskite Solar Cells were obtained by using the multiscale device simulator TiberCAD. I. INTRODUCTION Solid State Dye Solar Cells (SDSCs) have been investigated in the last years in order to solve typical problems of liquid Dye Solar Cells, namely the insufficient electrochemical stability of the electrolyte under reverse bias conditions and thermal stress[1] and corrosion of metal fingers induced by Iodine (a typical component of the electrolyte). In SDSC, liquid electrolyte is replaced with a hole transport material (HTM) for the dye regeneration process. Although small area devices have been widely studied[], there are very few reports showing the upscaling of this device. Recently, we have reported the first attempt to fabricate a SDSC module using an organic dye (D5) as light harvester and poly(-hexylthiophene-,5-diyl) (PHT) polymer as HTM. A power conversion efficiency (PCE) of % was obtained for the series-connected SDSC module with an active area of 1.44cm []. Unfortunately, the use of this technology is very limited due to the small efficiency achieved even for small area SDSC which is still below 8%. In the last few years, however, a new promising class of light harvesting materials, namely the hybrid organic halide based perovskites, have been employed to realize high efficiency photovoltaic solar cells [7] which mimic very closely the SDSC. This kind of crystalline material shows good properties in terms of light harvesting (high absorption in a broad region of the visible spectrum) and of electron and hole mobility. A maximum PCE of 15% was published using CH NH PbI -sensitized TiO together with the Spiro- OMeTAD as HTM for small area devices[8]. Moreover, a certified PCE of 17.9% has been reported on the National Renewable Energy Laboratory table. Alternative HTMs have also been studied to replace the Spiro-OMeTAD in order to reduce the fabrication cost for scaled-up devices. Perovskite solar cells made with polymeric HTMs such as polytriarylamine (PTAA) and PHT have shown a PCE of 1% and 9.%, respectively[9,1]. II. RESULTS A. Small area Perovskite Solar cells We fabricate perovskite based solar cells using CH NH PbI -x Cl x with different hole-transporting materials. The most used HTL, Spiro-OMeTAD, has been compared to a regio-regular, high molecular weight PHT. PHT was blended with LiN(CF SO ) N (Lithium TFSI) salt and tertbutylpiridine. These additives are used in SDSCs to improve device efficiency by doping the HTM. In Fig. 1, a comparison of the photovoltaic parameters (PCE, V OC, J SC, FF) using different mesoporous TiO scaffold thicknesses (nm and nm) for both PHT and Spiro-OMeTAD are reported. To obtain a similar perovskite structure for the two scaffold thicknesses (in terms of pore filling fraction and capping layer), two different perovskite concentration are used, namely 4% w/w for the nm TiO thickness and % w/w for the nm. When a transparent Spiro-OMeTAD layer is used, the average IV results show that the best performance are realized using a nm-thick TiO scaffold especially in terms of the J SC. In fact, an higher J SC is related to an higher reflection effect from the Au back-contact. Therefore, due to a lower transparency of the PHT samples, the back-reflection effect is minimized as compared to Spiro-OMeTAD based devices. *Research supported by Polo Solare Organico Regione Lazio and Italian Ministry of Education, University and Research (MIUR) with the PRIN DSSCX project. Authors are with Center for Hybrid and Organic Solar Energy (CHOSE), University of Rome Tor Vergata, 1 Rome (Italy) (corresponding author: A. Di Carlo phone: ; fax: ; aldo.dicarlo@uniroma.it) /$1. 14 IEEE 7

2 Current Density (ma/cm ) J [ma/cm ] Energy (ev) TiO scaffold (with the possibility of vary the perovskite concentration). This is supported by the fact that the mesoporous TiO scaffold has an important role in the formation of the perovskite crystal, but its function in the transport of charges is not completely understood. The difference in band-gap among the perovskite and HTM materials induces a band discontinuity at the interface which enhances injection of holes from the perovskite region into the HTM and suppress the back injection of electrons. An example of the energy alignment is presented in Fig. Fig.1 Electrical characteristics of fabricated Perovskite Solar cells with different thickness of TiO scaffold ad two different HTM, PHT and Spiro-OMeTAD By tuning the scaffold thickness and optimizing the device s fabrication, however, we were able to reach a PCE of 1.5% also for PHT based Perosvkite Solar Cells. Fig reports the JV characteristic of the best PHT-based cell obtained by using nm-thick TiO scaffold. This is the highest reported efficiency for a solar cell using this hole transporting material, outperforming the PCE obtained in our previous work [1] V OC =.81V J SC = -18.1mA/cm^ FF= 71.% PCE= 1.5% Area=.1cm^ SC Conduction Band Valence Band E Fn 4 Thickness (nm) Fig. Band profile for a compact TiO/TiO scaffold with perovskite /Spiro-OMeTAD at short circuit current. Results are obtained with the Drift-Diffusion simulator TiberCAD. Simulations were performed considering a high and low loading of perovskite into the mesoporous TiO. Here the absorption coefficient of the cell is strongly influenced by the concentration of the perovskite. However, a complex interplay between thickness and porosity of mesoporous TiO exist. The effect of a high loaded perovskite into the mesoporous TiO is clear for both the short circuit current and the open circuit voltage. E Fp - Fig. Current density as a function of voltage for the most efficient PHT based perovskite solar cell. B. Perovskite Solar cells simulations The optimization procedure is also based on a detailed physical simulation of the Perovskite Solar Cell. We use a Finite Element Method for solving drift diffusion equations in steady state conditions, within TiberCAD software tool which has been successfully used for other SDSC [11]. In this simulation we idealize the porous titania /perovskite material considering that the active layer has the transport properties (band-gap, effective mass, mobility) of pure perovskite and optical properties of a perovskite-loaded -1 - Low Loading High Loading Voltage (mv) Fig.4 Simulated current density as a function of voltage for two loading of perovskite into the mesoporous TiO. 71

3 I [ma] C. Perovskite Photovoltaic Modules Based on the results obtained on small area solar cells, monolithic-integrated, series-connected, perovskite modules were fabricated. Our first attempts showed a PCE of 5.1% on an active area of 1.8 cm (five monolithic seriesinterconnected cells) [1]. To accomplish the purpose of achieving a higher efficient perovskite module, important innovative procedures were studied and optimized: i) the patterned deposition procedure of the TiO under-layer, ii) the optimization of the monolithic interconnection between constituent cells, iii) the uniform deposition of the thin titania scaffold by screen-printing, iv) new Laser assisted patterning of the perovskite and compact layer, and v) optimization of perovskite deposition performed in controlled atmosphere. These fabrication processes were here used for the first time to define a reproducible fabrication procedure applicable to large area. The fabrication process of the perovskite module starts with the FTO/glass substrates which is etched with a raster scanning laser (Nd:YVO 4 ) to form the desired electrode patterns consisting of five of four FTO strips (1mm x 57mm) each separated by 1mm wide etched areas. By using the Spray Pyrolysis Deposition (SPD) technique, a compact c-tio (1nm) film was deposited onto the FTO surface (heated at 45 C) after screen-printing of a metallic mask in order to obtain a patterned c-tio deposition. over the crystalline perovskite layer with a final thickness 15nm. Both perovskite and PHT layers were successively cleaned by using a CO Laser. A patterned thermal evaporation of Au was used as back contact and for interconnections. The SEM image of the layers is shown in Fig. 5. Fabricated modules (Fig. a) permit an easy access to the electrical characteristic of the single cell forming the module. The I-V characteristic of a cell of the module is shown in Fig. b and has the following photovoltaic parameters: Voc =.841V, I SC = ma (J SC = mA/cm ), FF = 7.5% and a PCE = 7.%. Figure 4b also shows the I-V characteristic of the entire module. Here Voc =.4V, I SC = -. ma (J SC = -1.7mA/cm ), FF =.% and finally a PCE = 7.% a) b) Module Single Cell Fig. 5 Cross-sectional FE-SEM images of a perovskite based solar module without the gold electrode. A nanocristalline (nc) mesoporous TiO layer was screen-printed onto a compact TiO (c-tio ) and successively sintered at 48 C for min. The final thickness of the nc-tio film was 7 nm. A perovskite solution was spin-coated over the nc-tio film in air and successively heated at 1 C for 45 minutes obtaining the final crystalline structure. Afterwards, a doped HTM solution (PHT or Spiro-OMETAD) was in turn spin coated Fig. (a) Three perovskite modules fabricated with the procedure described in the text. (b) I-V characteristic of series-connected Perovskite based module of Fig. (a) at 1 SUN AM1.5 G illumination. The I-V characteristic of the single cell forming the module is also shown. The results show that the PCE drops by about 7% scaling the cell from.1cm to a module size cell of.5cm. This drop could be due to different factors such as the nonuniform perovskite deposition on the large area substrates induced by the use of the spin coating technique, and to the 7

4 I [ma] PCE (%),V OC [V] resistivity of the FTO electrode. Passing from large-area cell to the module, the PCE drops by about 4% only. The various losses of the scaling-up process are depicted in Fig. 7. Power Conversion Efficiency (%) % 7.% 1.5% Small Area Device (.1cm) Large Area Device (.5cm) PCE Voc 9 1 Shelf Life Time (Hours) FF/1 Jsc JSC [ma/cm ], FF/1 (%) Module (1.1cm) Fig. 7 Variation of the Power Conversion Efficiency moving from the small area device up to the module size. D. Long-term Stability Figure shows a shelf-life test of about 1 hours for the PHT-based module. Between successive IV measurement the device is stored into a dry box in dark. In the first part of the shelf life test (until 144 hours), the device remains without encapsulation showing a rapid increase of the J SC and V OC due to the oxygen p-doping of the PHT facilitating the regeneration process of the perovskite. A PCE increase of about 5% is showed respect to the its initial IV measurement (passing from.45% to 5.5%). Thus, after 144 hours, the device was sealed with a thermoplastic sealant deposited over the whole scaffold area at 9 C and with a secondary sealing on the edge of the protective glass using an cyanoacrylate glue. After the sealing procedure, the PCE drops from 5.5% to 4.9%. From 144h to h, a rapid J SC decrease is showed passing from 9.8mA/cm to 8.9 ma/cm. This decrease could be ascribed to the partial de-doping of the PHT. From h to 17h, the PCE remains almost stable passing from 4.4% to 4.%. Hysteresis effects were also investigated with the same PHT-based module used in the shelf life test. Measurements of the IV characteristic from V OC to the I SC and vice versa, using a delay time of 5ms and scan rate equal to 8 mv/s, do not show appreciable hysteresis effects as reported in Fig.9. Finally, light soaking tests under 1 Sun (AM1.5G) were also performed. As shown in Fig. 1, PHT based perovskite module shows a significant PCE decrease due to intrinsic photo-instability of the PHT. In fact, in the first h of the continuous light stress the PCE drops of about 4% respect to its initial value Fig.8 Shelf-life test for a Perovskite module made with PHT as HTM Voc Jsc Jsc --- Voc V Max I Max P Max Voc Isc Jsc Fill Factor PCE V ma mw V ma ma/cm^ % % Voc --->Jsc Jsc --->Voc Fig.9 Forward and backward measurement of the Perovskite moduled with PHT HTM. Hysteresis efects are quite limited. To reach a promising long-term stability the PHT was replaced with Spiro-OMeTAD. In fact, after 5 hours under AM 1.5 illumination at 1 Sun the Spiro-based sample retains more than % of its initial PCE as showed in Fig.1. Fig.1 Light soaking stress for Spiro-OMeTAD and PHT based perovskite modules 7

5 III. REFERENCES [1] S. Mastroianni, A. Lanuti, S. Penna, A. Reale, T.M. Brown, A. Di Carlo, F. Decker, Chemphyschem : a European journal of chemical physics and physical chemistry, 1, 1, 95. [] U. Bach, et al. Nature, 1998, 95, 58. [] H.J. Snaith, A. Petrozza, S. Ito, H. Miura, M. Grätzel, Advanced Functional Materials, 9, 19, 181. [4] R. Zhu, C.-Y. Jiang, B. Liu, S. Ramakrishna, Advanced Materials, 9, 1, 994. [5] F. Matteocci, G. Mincuzzi, F. Giordano, A. Capasso, E. Artuso, C. Barolo, G. Viscardi, T.M. Brown, A. Reale, A. Di Carlo, Organic Electronics, 1, 14, 188. [] F. Matteocci, S. Casaluci, S. Razza, A. Guidobaldi, T. Brown, A. Reale, A.D. Carlo, Journal of Power Sources, 14, 4, 1.. [7] M.M. Lee, et al., Science, 1, 8, 4. [8] J. Burschka, et al. Nature, 1, 499, 1. [9] J.H.Im, C.R. Lee, J.W. Lee, S.W. Park, N.G. Park, Nanoscale, 11,, 488. [1] F. Di Giacomo, S. Razza, F. Matteocci, A. D'Epifanio, S. Licoccia, A. Reale, T. Brown, A. Di Carlo, Journal of Power Sources, Volume 51 (14) - pages [11] D. Gentilini et al. J. Phys. Chem. C, 1, 11 (45), pp [1] F. Matteocci, S. Razza, F. Di Giacomo, S. Casaluci, G. Mincuzzi, T. M. Brown, A. D Epifanio, S. Licoccia and A. Di Carlo, Phys. Chem. Chem. Phys., 1 (14) - pages