Origin of the High Donor–Acceptor Composition Tolerance in Device Performance and Mechanical Robustness of All-Polymer Solar Cells

Abstract

High tolerance regarding photovoltaic performance in terms of donor:acceptor (D:A) composition ratio is reported for all-polymer solar cells (all-PSCs), which is a crucial advantage in producing large-scale devices with high reproducibility. To understand the origin of high D:A ratio tolerance in all-PSCs, we investigate the molecular weight (MW) effects of the P­(NDI2OD-T2) polymer acceptor (P A) on photovoltaic and mechanical robustness of PBDB-T:P­(NDI2OD-T2) all-PSCs. Also, we compare the all-PSCs with other types of PSCs consisting of the same polymer donor but using small molecule acceptors (SMAs) including ITIC and PC71BM. We observe that the D:A ratio tolerances of both the photovoltaic and mechanical properties are highly dependent on the P A MW and the acceptor material types. For example, at a high D:A ratio of 15:1, all-PSCs using high MW P A (number-average molecular weight (M n) = 97 kg mol–1) exhibit 13 times higher normalized power conversion efficiency (PCE) than all-PSCs using low MW P A (M n = 11 kg mol–1), and 20 times higher than ITIC-based PSCs. In addition, the electron mobilities in all-PSCs based on high MW P A are well-maintained even at very high D:A ratio, whereas the electron mobilities in low MW P A all-PSCs and SMA-based PSCs decrease by 3- and 4-orders of magnitude, respectively, when the D:A ratio increases from 1:1 to 15:1. Thus, we suggest that the formation of tie molecules and chain entanglements by long polymer chains bridging adjacent crystalline domains is the main origin of excellent D:A tolerance in both mechanical robustness and photovoltaic performance. This work provides an important material design guideline for the reproducible production of flexible and stretchable all-PSCs.

A Terpolymer Acceptor Enabling All‐Polymer Solar Cells with a Broad Donor:Acceptor Composition Tolerance and Enhanced Stability

“…The best combination (PTB7‐Th:PBTI2(30HD)‐FT blend) is then used as the candidate to investigate the D:A ratio tolerance in all‐PSCs. [ 36–38 ] Under the optimized conditions, PTB7‐Th:PBTI2(30HD)‐FT‐based all‐PSCs were fabricated as a function of D:A weight ratio from 1:30 to 10:1. The current density−voltage ( J−V ) characteristics of the all‐PSCs with distinct D:A ratio are shown in Figure a, and the corresponding device parameters, including V oc , J sc , FF, and PCE, are shown in Figure 3c and Table 1 .…”

“…The V oc s for all devices basically remained unchanged over time and previous works have reported the similar phenomena. [ 37,38 ] As we all know, the V oc is mainly related to the energetics of solar cell components, which shows minimum change with time unless the components are chemically decomposed, the film morphology changes substantially, or carrier transporting layer is damaged. In our case, the solar cells still function properly without catastrophic degradation; therefore, the V oc is quite stable over time.…”

“…For example, Yang and Kim groups independently demonstrated that the well‐studied PBDB‐T:N2200‐based all‐PSCs show impressive PCEs of 6.4–8.6% or 4.5–7.0% with high D:A ratio tolerance, e.g., ranging from 1:1.5 to 9:1 or from 1:1 to 15:1, respectively ( Figure ). [ 36,37 ] In addition, Ma et al also reported that polymer:polymer combination (PBDB‐T:N2200) is much better than polymer:small molecule combination with a high D:A ratio. [ 38 ] Nevertheless, processing additive, such as 1,8‐diiodioctane (DIO), is usually required to maintain PCE at high D:A ratio in the aforementioned systems, which complicates device fabrication and is detrimental to film morphology over time, downgrading the stability of all‐PSCs.…”

“…This is one of the highest values in all‐PSCs for D:A ratio tolerance studies. [ 36–38 ] The best combination is adopted to probe the effect of D:A ratio on film morphology and photovoltaic performance. Specifically, the all‐PSCs can retain over 40% of its original champion PCE with ultrabroad D:A composition tolerance from 1:30 to 10:1.…”

Recent Progress in Flexible and Stretchable Organic Solar Cells

“…Later, Kim and co‐workers investigated the molecular weight effects of the poly[[ N,N ′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐ alt ‐5,5′‐(2,2′‐bithiophene)] (P(NDI2OD‐T2)) polymer acceptor on the electronic property and mechanical robustness of PBDB‐T:P(NDI2OD‐T2) all‐PSCs to understand the origination of high D:A ratio tolerance. [ 146 ] At the same D:A ratio, the PCE of the all‐PSCs increased with the higher acceptor molecular weight. The optimal PCE of PBDB‐T:P L , PBDB‐T:P M , and PBDB‐T:P H was 5.13%, 6.52%, and 6.89%, respectively.…”