Perovskite-polymer hybrid solar cells with near-infrared external quantum efficiency over 40%

ARTICLES SCIENCE CHINA Materials mater.scichina.com link.springer.com Published online 11 December 2015 | doi: 10.1007/s40843-015-0102-x Sci China Mater 2015, 58: 953–960 Perovskite-polymer hybrid solar cells with near-infrared external quantum efficiency over 40% Long Ye, Benhu Fan, Shaoqing Zhang, Sunsun Li, Bei Yang, Yunpeng Qin, Hao Zhang and Jianhui Hou* In the past several years, conjugated polymers and organometal halide perovskites have become regarded as promising light-absorbing materials for next-generation photovoltaic devices and have attracted a great deal of interest. As the main part of this contribution, we describe the enhancement of near-infrared (NIR) photoresponse of well-known CH3NH3PbI3−xClx-based solar cells by the integration of bulk heterojunction (BHJ) small band gap polymer:fullerene absorbers. Particularly, the integration of a commercially available polymer PDPP3T and PCBM-based BHJ boosts the peak external quantum efficiency (EQE) by up to 46% in the NIR region (800−1000 nm), which is outside of the photoresponsive region (300−800 nm) of conventional perovskite solar cells. This substantial improvement in the EQE over the NIR region offers an additional current density of ~5 mA cm−2 for the control perovskite solar cell, and a high power conversion efficiency (PCE) of over 12% was obtained in the perovskite/BHJ-based solar cells. In addition, the insertion of the BHJ absorber consisting of a small band gap polymer PDTP-DFBT and PCBM also results in nearly 40% EQE for the perovskite/BHJ solar cell. The results also reveal that controlling over the polymer/PCBM weight ratio for a BHJ absorber is the key to achieving the optimal efficiency for this type of perovskite-polymer hybrid solar cell. INTRODUCTION The urgent need for high-performance, large-scale, and low-cost thin film photovoltaics is driving extensive research on new light absorber materials [1]. Owing to the outstanding properties of high absorption coefficients, bipolar charge transport characteristics, low exciton binding energy, and long exciton diffusion lengths, organolead trihalide perovskites are attractive light absorbers for realizing high performance thin-film solar cells [2−5]. Rapid advances in perovskite/fullerene planar heterojunction solar cells have been achieved over the past three years. In particular, the power conversion efficiency (PCE) has experienced a dramatic increase in excess of 10% by utilizing an inverted structure (Fig. 1a) of indium tin oxide (ITO)/ poly(3,4-ethylenedioxythiophene) PEDOT: poly(styrenesulfonate) (PSS)/peroskite/ [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/Al(or Ag) [6−19]. For this type of peroskite/PCBM solar cell, generally only photons in the ultraviolet-visible range (300−800 nm) are harvested in the devices due to the medium optical band gap (~1.55 eV) of CH3NH3PbI3-xClx or CH3NH3PbI3. Solution-processed polymer solar cells utilizing conjugated polymer and PCBM bulk-heterojunction (BHJ) blend as a light absorber are also a promising photovoltaic technology due to tunable absorption features and energy levels, as well as excellent solution processibility of the diverse polymers [20−29]. Therefore, integrating the advantages of both photovoltaic technologies into a simple photovoltaic device will be of great importance to achieve cost-effective and low temperature processed photovoltaic technologies. It is well-established that nanoscale bicontinuous interpenetrating networks in polymer:PCBM BHJ solar cells are beneficial for realizing efficient exciton dissociation and charge transport [30,31]. Very recently, researchers from polymer solar cell community have introduced feasible strategies to push forward planar junction perovskite/ fullerene solar cells [32−36]. For insance, Liu et al. [33] integrated polymer:PCBM BHJ into a conventional device containing ITO/TiO2/CH3NH3PbI3-xClx/MoO3/Ag and found that the perovskite/BHJ solar cell yielded an improved photocurrent and PCE. Nevertheless, the utilization of TiO2 may not meet the requirements for low temperature processing. Alternatively, Gong᾽s group [34,35] introduced perovskite:fullerene BHJ and found that solar cells based on perovskite:PCBM BHJ function better than conventional perovskite/PCBM planar junction solar cells, and large fill factors (FF) of up to ~80% can be achieved in this type of photovoltaic device. Zuo and Ding [36] also State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), Beijing 100190, China * Corresponding author (email: ) 953 December 2015 | Vol.58 No.12 © Science China Press and Springer-Verlag Berlin Heidelberg 2015 ARTICLES SCIENCE CHINA Materials a Al b Al Al Al PCBM BHJ Perovskite Perovskite PEDOT:PSS PEDOT:PSS ITO ITO Perovskite solar cell BHJ absorber SBG polymer Perovskite -polymer solar cell (PPSC) c PDPP3T Eg = 1.30 eV PDTP-DFBT Eg = 1.38 eV Figure 1 (a) Schematic illustration of the perovskite/PCBM solar cells, (b) schematic illustration of the perovskite/BHJ solar cells (PPSCs) and (c) chemical structures of the involved SBG polymers: PDPP3T and PDTP-DFBT. integrated PDPP3T:PCBM BHJ into inverted CH3NH3PbI3based perovskite solar cells and found that the near-infrared (NIR) photoresponse can be enhanced; however, the efficiency did not improve. Until now, the spectral responses in the NIR region of these integrated perovskite solar cells have limited the external quantum efficiency (EQE) to below 25%. Boosting the NIR photoresponse without sacrificing the excellent efficiency of perovskite solar cells is still a great challenge in the field of hybrid solar cells. Accordingly, if the drawbacks associated with ineffective photon harvesting in the long wavelength region and improvements in the NIR EQE of the BHJ absorber can be overcome without sacrificing the primary EQE of CH3NH3PbI3-xClx absorber, we expect that the photovoltaic properties of these types of perovskite-polymer hybrid solar cells will be improved. In this study, we successfully presented high-efficiency and broad photoresponse perovskite-polymer hybrid solar cells (PPSCs) by simply incorporating the small band gap (SBG) polymer:fullerene BHJ as an absorber (Fig. 1b). Compared with the control CH3NH3PbI3-xClx/PCBM solar cells, the novel perovskite/BHJ solar cells exhibited a ~20% enhancement to short-circuit current density (Jsc) and PCE. When the SBG polymer poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) and PCBM blend was selected as the BHJ light absorber, a high photoresponse with an EQE of up to ~46% over the NIR region (800−1000 nm) was observed. This resulted in a ~20% enhancement to the Jsc and PCE for perovskite/BHJ solar cells. Moreover, the BHJ absorber based on the SBG polymer is also applicable to this novel perovskite/BHJ device configuration. Our study identifies a new direction in which to integrate polymer-b (...truncated)