ARTICLE Received 5 Dec 2015 | Accepted 25 Apr 2016 | Published 23 May 2016 DOI: 10.1038/ncomms11735 OPEN Selective dissolution of halide perovskites as a step towards recycling solar cells Byeong Jo Kim1,*, Dong Hoe Kim2,*, Seung Lee Kwon1, So Yeon Park1, Zhen Li2, Kai Zhu2 & Hyun Suk Jung1 Most research on perovskite solar cells has focused on improving power-conversion efficiency and stability. However, if one could refurbish perovskite solar cells, their stability might not be a critical issue. From the perspective of cost effectiveness, if failed, perovskite solar cells could be collected and recycled; reuse of their gold electrodes and transparent conducting glasses could reduce the price per watt of perovskite photovoltaic modules. Herein, we present a simple and effective method for removing the perovskite layer and reusing the mesoporous TiO2-coated transparent conducting glass substrate via selective dissolution. We find that the perovskite layer can be easily decomposed in polar aprotic solvents because of the reaction between polar aprotic solvents and Pb2 þ cations. After 10 cycles of recycling, a mesoporous TiO2-coated transparent conducting glass substrate-based perovskite solar cell still shows a constant power-conversion efficiency, thereby demonstrating the possibility of recycling perovskite solar cells. 1 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Korea. 2 Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, USA. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to H.S.J. (email: [email protected]). NATURE COMMUNICATIONS | 7:11735 | DOI: 10.1038/ncomms11735 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11735 he advent of perovskite solar cells (PSCs) has led to a capable of dissolving PSCs such that the Au electrodes and the change in the paradigm of emerging photovoltaic (PV) mp-TiO2-coated TCG substrates are separated through Ttechnology owing to the significant increase in their the dissolution of the perovskite layers. We find that the recycling power-conversion efficiency within the past 3 years1–3. Thus of PSCs can be realized based on the removal of trihalide far, the best reported efficiency, achieved by Seok and colleagues, perovskite materials and that a small amount of Pb residue has a is 22.1% (ref. 4). This efficiency is comparable to that of negligible effect on the ability to reuse the mp-TiO2-coated TCG other commercialized solar cells, such as multicrystalline Si substrate. The selective dissolution mechanism is attributable to (c-Si, 21.3%), cadmium telluride (CdTe, 22.1%) and copper the breaking down of the PbI6 octahedral frame in the trihalide indium gallium selenide (CIGS, 22.3%) solar cells4. However, to perovskite owing to the reaction between the partially facilitate commercialization of these emerging PVs, various issues positive Pb2 þ ions and a polar aprotic solvent. When the must be addressed, such as long-term stability, the need for regenerated electron-transport-layer-coated FTO glass (referred Pb-free perovskite light-absorbing materials, the economic to as the mp-TiO2-coated TCG substrate in this paper) is feasibility of the manufacturing process and an appropriate reused, the original power-conversion efficiency (15%) is retained ecofriendly disposal process for Pb-contained waste. The price of over 10 regeneration cycles. This study reveals the potential a Si solar module ranges from $0.6 to $1.0 per Wp (ref. 5), which impact of easily exchangeable PSCs, similar to electric batteries, is significantly higher than the target price of PSCs (lower than on the use of PSCs in various applications (for example, as $0.2 per Wp, see ref. 6). The target price for PV modules required portable power sources in wearable devices and internet-of-things to achieve grid parity, as determined by the United States devices). Department of Energy, is $0.5 per Wp; therefore, the target 7 figure ($0.2 per Wp) constitutes a paradigm-changing price . Results The competitive price of PSCs results from the low material Recycling procedure. Figure 1 shows the steps of the recycling cost and non-vacuum fabrication process of perovskite process for PSCs. After the deposition and annealing of the light absorbers. However, the relatively high material costs electron-transport layer, which is composed (in general) of for electron- and hole-transport materials, metallic electrode compact and mp-TiO2 layers, the MALI light absorber is coated materials (including gold or silver) and transparent conducting on the substrate. The hole-transport layer (HTL) and gold glass (TCG) substrate materials render realization of the target electrode are then deposited. During recycling, the MALI price difficult. For example, at the laboratory scale, the unit price perovskite and the HTL dissolve when the fabricated solar cell is of spiro-MeOTAD (2,2’,7,7’-tetrakis[N,N-di(4-methoxyphenyl) immersed in a polar aprotic solvent such as dimethylformamide amino]-9,9’-spirobifluorene), a widely used hole-transport (DMF), g-butyrolactone (GBL) or dimethyl sulfoxide (DMSO). material, is $200 per g (Lumtec). Gold, as a metallic electrode The resultant solution contains the recyclable gold electrode and material, and fluorine-doped tin oxide (FTO) glass cost $40 per g the mp-TiO -coated TCG substrate (Supplementary Fig. 1); the and approximately $10 per m2, respectively. As such, these 2 recycling procedure is shown in Supplementary Movie 1. After expensive materials account for 98% of the costs associated rinsing and drying of the selectively dissolved mp-TiO -coated with small, 1 inch  1 inch square devices. The price of 2 TCG substrate, the MALI layer, HTL and gold electrode are hole-conducting materials is expected to decrease by virtue of re-deposited to yield a recycled PSC. new synthesis technologies and large-scale synthesis methods. The effect of the polar aprotic solvent on the removal of the However, a significant reduction in the prices of gold and FTO MALI perovskite material was determined via field-emission glass seems unlikely; in fact, owing to limited mineral resources, scanning electron microscopy (FESEM). Figure 2a,b show their prices may vary significantly depending on global demand. plane-view images of the mp-TiO2 layer coated on the TCG The recycling of degraded PV modules has been considered as substrate and the MALI layer deposited on top of the mp-TiO an alternative approach to making PVs economically viable. In 2 layer, respectively. After selective dissolution processing of the particular, improvement of the long-term stability of PSCs is still MALI-coated substrate using DMF, the MALI layer is easily needed to compete with other types of solar cells, such as c-Si, removed and only the mp-TiO2 layer is observed (Fig. 2c). We CdTe and CIGS solar cells, which are being used in commercial determined the efficacy of this recycling process by repeatedly PV modules. However, considering the energy payback time recycling (in this work, we performed a total of 10 cycles) the (EPBT), which is the time required for a PV module to produce mp-TiO -coated TCG substrate of the PSC. The photocurrent an amount of energy comparable to that consumed during its 2 density–voltage (J–V) curve of a 10-times-recycled device is production, the EPBT of a perovskite PV module is 0.22 years, similar to that of a PSC based on a new substrate (Fig. 2d and which is the shortest nominal EPBT compared with all other Supplementary Fig. 2). These results indicate that PSCs can be competitors such as c-Si and CdTe (ref. 8). Therefore, if it were easily recycled. possible to recycle degraded perovskite PV modules, the issues inhibiting the commercialization of perovskite PV modules would be solved, such as their long-term stability, the production of Recycling mechanism. The selection of an appropriate solvent is large amounts of Pb-containing waste, the high unit cost of PSCs essential for an effective recycling process. To investigate the and the retrieval and reuse of expensive components such as gold, solvent effect, we immersed PSCs in non-polar and protic/aprotic silver and TCG. A shorter payback period of perovskite-based PV polar solvents. In the case of the non-polar solvents, only the modules compared with that of Si-based PV modules could be Au/HTL was removed from the Au/HTL/MALI/mp-TiO2/TCG realized via this process of recycling degraded PSCs. substrate, indicating that non-polar solvents selectively dissolve Therefore, in this paper, we introduce a recycling or repair the spiro-MeOTAD layer (Table 1 and Fig. 3). The perovskite methodology for PSCs. In this system, trihalide perovskite layer decomposed in both types of polar solvents, albeit with materials, such as methyl-ammonium lead iodide (CH3NH3PbI3, differing dissolution behaviours. This layer can dissolve in abbreviated as MALI) as well as formamidinium lead iodide hydrogen-bonded polar protic solvents, such as ethanol or water. (HC(NH2)2PbI3, abbreviated as FALI) and methyl-ammonium However, the dissolution kinetics is quite sluggish compared with lead bromide (CH3NH3PbBr3, abbreviated as MALBr) mixed the kinetics governing dissolution in a polar aprotic solvent. halides (in this study, (FALI)0.85(MALBr)0.15 was used), are When the PSCs were immersed in the polar protic solvents, the removed through a selective dissolution process. This process is perovskite layers became yellow in colour and, after more than 2 NATURE COMMUNICATIONS | 7:11735 | DOI: 10.1038/ncomms11735 | www.nature.com/naturecommunications NATURE COMMUNICATIONS | DOI: 10.1038/ncomms11735 ARTICLE ab Perovskite Deposition of ETL electron- FTO transport layer High-temp. annealing 100°C Re-formation of Rinsing and drying of perovskite layer selectively dissolved Formation of perovskite layer substrate Selective Polar aprotic Deposition of dissolution of hole-transport solvent failed PSC layer for recycling Peeled- metal electrode Au HTL Deposition of Au electrode Fabrication of PSCs Selective dissolution of PSCs Figure 1 | Recycling process for PSCs.