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High Efficiency Tandem Thin-Perovskite/Polymer Solar Cells with a Graded Recombination Layer Yao Liu, Lawrence A. Renna, Monojit Bag, Zachariah A. Page, Paul Y. Kim, Jaewon Choi, Todd Emrick, Dhandapani Venkataraman, and Thomas P. Russell ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12740 • Publication Date (Web): 26 Feb 2016 Downloaded from http://pubs.acs.org on March 3, 2016

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ACS Applied Materials & Interfaces

High Efficiency Tandem Thin-Perovskite/Polymer Solar Cells with a Graded Recombination Layer

Yao Liu,† Lawrence A. Renna,‡ Monojit Bag, ‡, $ Zachariah A. Page,† Paul Kim,† Jaewon Choi,† Todd Emrick,†,* Dhandapani Venkataraman, ‡, * and Thomas P. Russell†, *

†

Department of Polymer Science & Engineering, Conte Center for Polymer Research 120 Governors Drive, University of Massachusetts, Amherst, MA 01003, USA

‡

Department of Chemistry, 710 North Pleasant Street, University of Massachusetts, Amherst, MA 01003-9303, USA

$

Department of Physics, Roorkee 247667, Indian Institute of Technology, Roorkee, Uttarakhand, India

1 ACS Paragon Plus Environment


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Abstract: Perovskite-containing tandem solar cells are attracting attention for their potential to achieve high efficiencies. We demonstrate a series connection of a ~90 nm thick perovskite front sub-cell and a ~100 nm thick polymer:fullerene blend back sub-cell that benefits from an efficient graded recombination layer containing a zwitterionic fullerene, silver (Ag), and molybdenum trioxide (MoO3). This methodology eliminates the adverse effects of thermal annealing or chemical treatment that occurs during perovskite fabrication on polymer-based front sub-cells. The record tandem perovskite/polymer solar cell efficiency of 16.0%, with low hysteresis, is 75% greater than the corresponding ~90 nm thick perovskite single junction device and 65% greater than the polymer single junction device. The high efficiency of this hybrid tandem device, achieved using only a ~90 nm thick perovskite layer, provides an opportunity to substantially reduce lead in the device, while maintaining the high performance derived from perovskites.

Keywords: Interface engineering, Tandem solar cells, Perovskite solar cells, Inverted structure, Polymer solar cells, Fullerene interlayers, High open-circuit voltage, High efficiency


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INTRODUCTION Perovskite solar cells have recently emerged as a promising photovoltaic technology1-18 with reported power conversion efficiencies (PCEs) approaching crystalline silicon solar cells. 19, 20

In pursuit of optimal efficiency, perovskite-containing tandem solar cells are especially

attractive. 21-28 However, the overwhelming majority of research has been focused on combining perovskite solar cells with copper indium gallium diselenide (CIGS) or silicon based devices. 2224, 26, 27

Polymer-based solar cells, which share similar processing and architecture characteristics,

are promising candidates for integration with perovskite solar cells to form hybrid tandem devices. 25 Such devices would have enhanced mechanical flexibility, while maintaining solution processability.

25,

29

Moreover, recently developed perovskite solar cells with planar

heterojunction structures are compatible with well-established solution-based, low temperature, roll-to-roll fabrication procedures used for the production of polymer-based solar cells. 12, 30-38 The one reported polymer/perovskite hybrid tandem solar cell gave a maximum PCE of 10.2%, consisting of a polymer front sub-cell and a perovskite back sub-cell. 25 The preparation of perovskite/polymer tandem solar cells faces two primary limitations: 1) if the perovskite was prepared as the back sub-cell on top of the polymer front sub-cell in the layer-by-layer deposition procedure, the thermal/chemical treatment typically used during perovskite fabrication would damage a polymer-based sub-cell, and 2) if the perovskite was prepared as the front sub-cell, the thick perovskite active layer (hundreds of nanometers) generally used to capture incident light would prevent light from reaching the back sub-cell. Recently, ultrathin39 or semitransparent14, 40 perovskite films were utilized in devices showing high efficiency and a perovskite thickness of 80 nm can afford a PCE of 10.1%, 41 which represents an avenue towards tandem solar cells with a thin perovskite active layer comprising the front sub-cell. While there is currently no



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environmentally friendly alternative for lead in perovskite devices, 42 thinner active layer can reduce the amount of lead within a perovskite-containing solar cell. However, reducing the thickness of the perovskite active layer comes at the cost of reduced light absorption, resulting in an overall lower efficiency than their thicker counterparts. We mitigate this issue by implementing a new design strategy that combines a thin perovskite layer with a low band gap conjugated polymer showing a similar optical absorption to supplement light absorption and boost efficiency. We specifically demonstrate the facile solution-based fabrication of high performance tandem perovskite/polymer solar cells containing a ~90 nm thick perovskite front sub-cell and a ~100 nm thick polymer-based back sub-cell connected with a graded recombination layer. A record maximum PCE of 16.0% was achieved for these tandem perovskite/polymer solar cells with low hysteresis, demonstrating excellent synergy between the perovskite and polymer components. These hybrid tandem devices show impressive device metrics, including a maximum open circuit voltage (VOC) of 1.80 V and a maximum fill factor (FF) of 77%.

EXPERIMENTAL SECTION Solar-cell fabrication: The indium tin oxide (ITO)-coated glass substrates (20 ± 5 ohms/square) were obtained from Thin Film Devices Inc., and were cleaned through ultrasonic treatment in detergent, deionized water, acetone, and isopropyl alcohol and then dried in an oven (100 °C) for 6 hours. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole transport layer was spin coated on pre-cleaned ITO substrates at 2500 rpm for 40 s and annealed at 150 for 30 min. The perovskite layer was formed by spin-coating a solution of lead acetate (Pb(OAc)2) and methylammonium iodide (MAI) (1:3 molar ratio) in N,N-dimethylformamide


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(DMF) onto the hot PEDOT: PSS/ ITO substrates (~ 100 °C) at a spin-speed of 6000 rpm for 60 s inside a glove box (N2 atmosphere, < 1 ppm O2,

Polymer Solar Cells with a

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