Contrasting Effects of Energy Transfer in Determining Efficiency Improvements in Ternary Polymer Solar Cells

Abstract

Crystallizable, high-mobility conjugated polymers have been employed as secondary donor materials in ternary polymer solar cells in order to improve device efficiency by broadening their spectral response range and enhancing charge dissociation and transport. Here, contrasting effects of two crystallizable polymers, namely, PffBT4T-2OD and PDPP2TBT, in determining the efficiency improvements in PTB7-Th:PC 71 BM host blends are demonstrated. A notable power conversion efficiency of 11% can be obtained by introducing 10% PffBT4T-2OD (relative to PTB7-Th), while the efficiency of PDPP2TBT-incorporated ternary devices decreases dramatically despite an enhancement in hole mobility and light absorption. Blend morphology studies suggest that both PffBT4T-2OD and PDPP2TBT are well dissolved within the host PTB7-Th phase and facilitate an increased degree of phase separation between polymer and fullerene domains. While negligible charge transfer is determined in binary blends of each polymer mixture, effective energy transfer is identified from PffBT4T-2OD to PTB7-Th that contributes to an improvement in ternary blend device efficiency. In contrast, energy transfer from PTB7-Th to PDPP2TBT worsens the efficiency of the ternary device due to inefficient charge dissociation between PDPP2TBT and PC 71 BM. and acceptors, [1,2] controlling and optimizing the nanoscale morphology, [3][4][5] and via interfacial engineering of the device architectures. [6,7] Power conversation efficiency (PCE) metrics for this technology now stand at 13% for lab-scale single junction and tandem devices. [8,9] Ternary photovoltaic blends, [10][11][12][13][14][15] prepared by incorporating a third component into the donor:acceptor active layer, have emerged as a promising strategy for realizing further improvements in PCE by enhancing device spectral response and charge collection efficiency. This method is favorable as it removes the time-consuming and expensive process of synthesizing new conjugated polymers, in addition to the complicated manufacturing steps that are associated with tandem solar cell fabrication. [16,17] Recent work has shown that semicrystalline conjugated macromolecules or small molecules are effective third components when preparing efficient ternary solar cells. [18][19][20] For example, both the crystallinity and face-on preferential polymer orientation in PTB7-Th:PC 71 BM binary blends can be simultaneously enhanced via the addition of a highly crystalline small molecule p-DTS(FBTTH 2 ) 2 , resulting in a high PCE of 10.5% (a relative improvement of 14%). [21] Elsewhere, the incorporation of Si-PCDTBT into the PTB7:PC 71 BM system can result in high device fill factors (FF; up to 77%) through a significant reduction in charge recombination within the active layer. [22] Although these crystallizable additives can be highly ordered in relatively simple pure and binary systems, their ability to undergo ordering in ternary blends is not always realized. [23] Their exact location within the ternary blend morphology-and the cor...

Molecular engineering of conjugated polymers for efficient hole transport and defect passivation in perovskite solar cells

“…This weak PCE dependence on the size of the active area of perovskite solar cells is significantly different with that of polymer solar cells which show strong size-dependence. [42] We proceed to illustrate how the molecular structure of the three conjugated polymers used in this work could affect the charge transportation and recombination within devices that consequently determine the efficiency via their impact on V oc and FF. We first explored the charge extraction efficiency of perovskite devices at various interfaces when incorporating different HTMs using steady-state and time-resolved photoluminescence (PL) measurements.…”

The role of the third component in ternary organic solar cells

“…1), has yielded record PCEs of >10% for binary devices. For many years, blends based on PTB7 (or PTB7-Th):PC70BM were used as reference cells for TSCs owing to their low charge-carrier mobility and trapassisted recombination 65,76,78,[104][105][106][107] . For example, the polymer PID2 can be introduced as a D2 component that forms an energy cascade alignment between the IP and EA levels of PTB7 and PC70BM, respectively 85 .…”

Molecular Order Control of Non-fullerene Acceptors for High-Efficiency Polymer Solar Cells

“…Although these large crystals have the possibility to facilitate charge transport, the pronounced phase separation will also reduce the efficiency of free charge generation due to the reduced donor:acceptor interfacial area. 38 On the contrary, when the blend film was cast on 100 C HS, by kinetically inhibiting their growth during the solution-casting process the lamellar-like crystals almost completely disappeared. Fine phase separation can be observed in Figures 4C and 4F, and a low surface RMS roughness of 2.2 nm was observed.…”