• Energy Research

AbstractAn easily accessible DPP‐based small molecule (DMPA‐DTDPP) has been synthesized by a simple and efficient route. The resulting molecule, when incorporated into a P3HT:PCBM‐based BHJ solar cell, is found to significantly improve the efficiency. The utility of DMPA‐DTDPP as an additive yields an increase in the short circuit current density (Jsc) because DMPA‐DTDPP serves as an energy funnel for P3HT excitons at the P3HT:PCBM interfaces, resulting in an improved overall power conversion efficiency, compared to the P3HT:PCBM control device. Considering the trouble‐free and cost effective synthesis of DMPA‐DTDPP, it may prove very useful in high‐performance solar cells.

AbstractTandem device architectures offer a route to greatly increase the maximum possible power conversion efficiencies (PCEs) of polymer solar cells, however, the complexity of tandem cell device fabrication (such as selecting bandgaps of the front and back cells, current matching, thickness, and recombination layer optimization) often result in lower PCEs than are observed in single‐junction devices. In this study, we analyze the influence of front cell and back cell bandgaps and use transfer matrix modeling to rationally design and optimize effective tandem solar cell structures before actual device fabrication. Our approach allows us to estimate tandem device parameters based on known absorption coefficients and open‐circuit voltages of different active layer materials and design devices without wasting valuable time and materials. Using this approach, we have investigated a series of wide bandgap, high voltage photovoltaic polymers as front cells in tandem devices with PTB7‐Th as a back cell. In this way, we have been able to demonstrate tandem devices with PCE of up to 12.8% with minimal consumption of valuable photoactive materials in tandem device optimization. This value represents one of the highest PCE values to date for fullerene‐based tandem solar cells.

Transient absorption spectroscopy reveals that the superior performance of three-versus two-phase polymer : fullerene blends is associated with hole migration from intermixed to pure polymer phases.

High performance flexible polymer solar cells are realized by using the 3D printer-based slot die coating method.

Highly efficient ITO-free polymeric electronic devices were successfully demonstrated by replacement of the ITO electrode with a solution-processed PEDOT:PSS electrode containing Ag nanoparticles (NPs). Polymer solar cells (PSCs) and light emitting diodes (PLEDs) were fabricated based on poly(5,6-bis(octyloxy)-4-(thiophen-2-yl)benzo[c][1,2,5]thiadiazole) (PTBT):PC61BM and Super Yellow as a photoactive layer, respectively. The surface plasmon resonance (SPR) effect and improved electrical conductivity by the Ag NPs clearly contributed to increments in light absorption/emission in the active layer as well as the conductivity of the PEDOT:PSS electrode in PSCs and PLEDs. The ITO-free bulk heterojunction PSCs showed a 1% absolute enhancement in the power conversion efficiency (3.27 to 4.31%), and the power efficiency of the PLEDs was improved by 124% (3.75 to 8.4 lm W−1) compared to the reference devices without Ag NPs. The solution-processable conducting polymer, PEDOT:PSS with Ag NPs, can be a promising electrode for large area and flexible optoelectronic devices with a low-cost fabrication process.

Power conversion efficiency up to 8.6% is achieved for a solution-processed tandem solar cell based on a diketopyrrolopyrrole-containing polymer as the low-bandgap material after using a thin polyelectrolyte layer to modify the electron-transport ZnO layers, indicating that interfacial engineering is a useful approach to further enhancing the efficiency of tandem organic solar cells.

Abstract A new accepter unit, pyrrolo[3,2-b]pyrrole-2,5-dione, deserves much attention as the electron-deficient unit for the generation of electron donor material for organic photovoltaic cells (OPVs). Pyrrolo[3,2-b]pyrrole-2,5-dione unit, regioisomer of the known pyrrolo[3,4-c]pyrrole-1,4-dione, is originated from the structure of stable synthetic pigment. The organic low band gap molecules with pyrrolo[3,2-b]pyrrole-2,5-dione, thiophene and triphenyl amine units were synthesized using Suzuki polymerization to generate SM-B, SM-M and SM-H. The spectrum of SM-B as the solid thin film shows absorption band with maximum peaks at 356 and 517 nm, and the absorption onset at 667 nm, corresponding to band gap of 1.86 eV. The device comprising SM-B and PC71BM (1:4) showed a VOC of 0.79 V, a JSC of 6.04 mA/cm2, and a FF of 0.33, giving a power conversion efficiency of 1.56%.

Abstract A new accepter unit, 4H-dithieno[3,2-c:2′,3′-e]azepine-4,6(5H)-dione, was prepared and utilized for the synthesis of the conjugated polymers containing electron donor–acceptor pair for OPVs. The dithienoazepinedione, bithiophene imide (BTI) group, is an attractive electron deficient unit since it demonstrates strong electron-withdrawing character, a planar architecture, and good solubility. The polymer with linear dodecyl-BTI unit and benzodithiophene unit substituted with bulky 2-octyldodecyl side chain was synthesized using Stille polymerization to generate P1 . The new polymer, P2 , was also synthesized by Suzuki polymerization of carbazole, thiophene and linear octyl BTI unit to increase conjugation length. The spectra of P1 and P2 solid films show absorption bands with maximum peaks at 395, 538 and 380, 491 nm, and the absorption onsets at 652 and 634 nm, corresponding to band gaps of 1.90 and 1.96 eV, respectively. The best device with P1 : PC71BM in ODCB with 2% DIO showed a VOC of 0.97 V, a JSC of 3.06 mA/cm 2 , and a FF of 0.49, which yielded PCE of 1.49%. The highest PCE of P2 :PC71PM (1:2) solar cells reaches 0.76% with a VOC of 0.84 V, a JSC of 2.48 mA/cm 2 , and a FF of 0.36.

Novel conjugated copolymers consisting of bis(2-octyldodecyloxy)benzo[1,2-b:3,4-b]dithiophene (BDT) and N-alkyl-2,2′-bithiophene-3,3′-dicarboximide (BTI) with thiophene and bithiophene linkages have been synthesized and evaluated in bulk heterojunction solar cell. BDT, BTI and thiophene (or bithiophene) units were incorporated using Stille polymerization to generate poly(5-dodecyl-3-(2-thienyl)-4H-dithieno[3,2-c:2,3-e]azepine-4,6(5H)-dione-co-4,8-di(2-octyldodecyloxy)benzo[1,2-b;3,4-b]dithiophene) (P1, P2 and P3) and poly(5-dodecyl-3-di(thien-2-yl)-4H-dithieno[3,2-c:2,3-e]azepine-4,6(5H)-dione-co-4,8-di(2-octyldodecyloxy)benzo[1,2-b:3,4-b]dithiophene) (P4, P5 and P6). The BTI unit, as the electron deficient moeity, and the BDT–thiophene (or bithiophene) unit, as the electron rich moeity, have been applied for the efficient intramolecular charge transfer. The introduction of thiophene or bithiophene unit improved power conversion efficiency (PCE) of the OPVs. The best device with P2:PC61BM (1:1) showed a open circuit voltage (VOC) of 0.88 V, a short circuit current (JSC) of 7.36 mA/cm2, and fill factor (FF) of 0.63, which yield PCE of 4.17%.