Polymer Stabilization of Lead(II) Perovskite Cubic Nanocrystals for Semitransparent Solar Cells

DOI: 10.1002/aenm.201502317 We fi rst describe the preparation of a fi lm of PVP–PV nanocrystals. A 4:1:1 molar mixture of dry crystalline methylammonium iodide (MAI), PbI 2 , and PbCl 2 , and x wt% of PVP ( x = 0, 3, 6; M w = 40 k) were dissolved in N , N -dimethylformamide (DMF) at 60 °C for 12 h and used as a precursor solution. While the value x was varied, the concentration of the precursors excluding PVP was kept at 25 wt%. We then spincoated the precursor solution on either glass/ITO or glass/ITO/ PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate)), followed by heating at 100 °C for 25 min and rapid cooling to room temperature under nitrogen. In accordance with a recent report, [ 26 ] the PV fi lm thus prepared contains 97.1% iodine atoms and only 2.9% Cl atoms, indicating that the chloride anion was largely extruded as MA + Cl − (see Figure S1, Supporting Information, Energy-dispersive X-ray spectroscopic (EDS) data). Thus, the composition of the active layer discussed below is denoted CH 3 NH 3 PbI 3– x Cl x (PV), where x is a very small number. PVP ( M w = 40 k) is widely used for the stabilization of inorganic nanoparticles [ 27 ] and was found to be the best among several different types of polymers (e.g., polymethylmethacrylate, polyvinylpyridine). Next, we describe differential scanning calorimetry (DSC) analysis of PVP–PV fi lms prepared on glass/ITO. A thin fi lm for analysis was carefully scraped off from the glass/ITO/PVP– PV with a surgical blade (note that mechanical stress causes a phase change, as discussed below). The DSC data are shown in Figure S2 (Supporting Information), providing evidence for the persistence of a cubic lattice over 5–100 °C. The 0 wt% PVP– PV samples, upon cooling to 5 °C or to –50 °C as shown in Figure S2a,b (Supporting Information), underwent a CTT transition at 52 °C, and a reverse transition at 55 °C upon heating to 100 °C and cooling, respectively. These data conform to the standard behavior of lead PV. [ 28 ] By contrast, the 3 wt% PVP– PV sample in Figure S2c (Supporting Information) cooled from 100 to –50 °C showed no discernable peak, but showed a broad peak at 42 °C upon heating from –50 °C. When we performed the cooling/heating cycle between 100 and 5 °C, we did not observe any phase transition, indicating that the cubic phase persists between 5 and 100 °C, as supported by X-ray diffraction (XRD) analysis described below. We fi rst describe the surface morphology of glass/ITO/ PEDOT:PSS/PVP–PV. The atomic force microscopy (AFM) analysis ( Figure 2 a–c) gave us the surface roughness parameter (Rq, root mean square). The Rq value of the surface of 0 wt% PVP–PV was 14.55 nm, the value for 3.0 wt% PVP (Figure 2 b) was 3.11 nm, i.e., much smoother, and the value for 6 wt% was 2.91 nm (Figure 2 c). The scanning electron microscopic (SEM) images are consistent with the AFM roughness data (Figure 2 d–f). With their power conversion effi ciency (PCE) having rapidly increased from 3.8% to ≈20% within six years, [ 1–6 ] organic– inorganic hybrid solar cells (SCs) based on lead perovskite (PV) of CH 3 NH 3 PbI 3– x Cl x have become an important part of solution-processable thin-fi lm photovoltaic devices. [ 7 ] Such progress notwithstanding, the instability of a PV fi lm under heat and moisture is seen presently as a bottleneck in their development. [ 8 ] It is particularly problematic in the fabrication of semitransparent SCs, key devices for window applications in zero-energy building technology, [ 9–14 ] because the small crystals required for a semitransparent PV fi lm (50–100 nm) [ 15,16 ] are less stable than the larger crystals in a thick-fi lm device (e.g., 300 nm). [ 17–23 ] We report here that an ≈90-nm-thick fi lm of PV prepared in the presence of a polar polymer, 3 wt% polyvinylpyrrolidone (PVP), shows an increase in the thermal stability against decomposition into PbI 2 , shows stable crystal size and morphology, and allows us reproducibly to fabricate, under ambient air, semitransparent SCs with increased open circuit voltage ( V oc ), fi ll factor (FF), and PCE ( Figure 1 ). Thus, an SC device made of an ≈90-nm-thick PVP-doped PV (PVP– PV) showed a PCE value of 7.34% (with a 150 nm Ag electrode) at 53% average transmittance (AVT, at 400–800 nm for organic layer only). [ 15 ] The same SC device using a semitransparent 6-nm-thick Ag or Au electrode showed 5.36% PCE and 34% AVT (including the metal electrode with glass/Indium tin oxide (ITO) as reference)—respectable values among those reported for semitransparent PV devices. [ 15,22,24,25 ]