Ultra-thick semi-crystalline photoactive donor polymer for efficient indoor organic photovoltaics

• Published in: Nano energy 2019-04, Vol.58, p.466-475
• Main Authors: Shin, Sang-Chul, Koh, Chang Woo, Vincent, Premkumar, Goo, Ji Soo, Bae, Jin-Hyuk, Lee, Jae-Joon, Shin, Changhwan, Kim, Hyeok, Woo, Han Young, Shim, Jae Won
• Format: Article
• Language: English
• Subjects: Indoor light conditions; Organic photovoltaics; Poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2, 5]thiadiazole)]; Semi-crystalline polymer; Single-diode equivalent circuit model; Ultra-thick photoactive layer
• ISSN: 2211-2855
• DOI: 10.1016/j.nanoen.2019.01.061

An in-depth study on the photovoltaic characteristics under indoor lights, i.e., light-emitting diode (LED), fluorescent lamps, and halogen lamps, was performed with varying the photoactive layer thickness (120–870 nm), by comparing those under 1-sun condition. The semi-crystalline mid-gap photoactive polymer, poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) and a fullerene derivative, [6,6]-phenyl C71 butyric acid methyl ester (PC70BM) were used as a photoactive layer. In the contrary to the measurements under 1-sun condition, the indoor devices show a clearly different behavior, showing the thickness tolerant short-circuit current density (JSC) and fill factor (FF) values with 280–870 nm thick photoactive layers. The retained JSC and FF values of thick indoor devices were discussed in terms of the parasitic resistance effects based on the single-diode equivalent circuit model. The much lower series/shunt resistance (Rs/RP) ratio was measured with thick photoactive layer (≥280 nm), resulting in negligible decreases in the JSC and FF values even with a 870-nm-thick active layer under the LED condition. Under 1000 lx LED light, the PPDT2FBT:PC70BM device showed an optimum power conversion efficiency (PCE) of 16% (max power density, 44.8 μW/cm2) with an open-circuit voltage of 587 mV, a JSC of 117 μA/cm2, and a FF of 65.2. The device with a 870-nm-thick active layer still exhibited an excellent performance with a PCE of 12.5%. These results clearly suggest that the critical parasitic resistance effects on the performance vary depending on the light illumination condition, and the large RP associated with the viable thick photoactive layer and the well-matched absorption (of photoactive layer) with the irradiance spectrum (of indoor light) are essential to realize efficient indoor photovoltaic cells with high JSC and FF. The semi-crystalline polymer (PPDT2FBT) and a fullerene derivative (PC70BM) were used as a photoactive layer of organic photovoltaics. By varying the thickness of the photoactive layer (120–870 nm), we compared the performance under 1-sun and the indoor lights (LED, fluorescent and halogen). Unlike 1-sun condition, short-circuit current density and fill factor remained high under indoor illumination even as the active layer thickness was increased from 280 to 870 nm. [Display omitted] •OPVs with a PPDT2FBT:PC70BM-based photoactive layer were fabricated.•The thickness of th