Donor polymer design for organic solar cells with high open-circuit voltage
by Jing Liu
xv, 130 pages : illustrations ; 30 cm
Organic solar cells (OSCs) have achieved tremendous progress during the past few years due to their prevailing advantages of low cost, light weight and mechanical flexibility. To date, the highest achieved performance for fullerene OSCs is 11.7% by structural optimization of donor polymers. Recently, small molecule acceptors (SMAs) have outperformed fullerene derivatives in the bulk heterojunction (BHJ) blend of OSCs due to their intrinsic advantages of broad and strong absorption, tunable energy levels, and stable morphology. Encouragingly, the impressive power conversion efficiency (PCE) for the small molecule based OSCs are over 14%, which is a vital milestone for the commercialization of OSCs. However, even in these champion solar cell systems, the voltage losses are still relativel...[ Read more ]
Organic solar cells (OSCs) have achieved tremendous progress during the past few years due to their prevailing advantages of low cost, light weight and mechanical flexibility. To date, the highest achieved performance for fullerene OSCs is 11.7% by structural optimization of donor polymers. Recently, small molecule acceptors (SMAs) have outperformed fullerene derivatives in the bulk heterojunction (BHJ) blend of OSCs due to their intrinsic advantages of broad and strong absorption, tunable energy levels, and stable morphology. Encouragingly, the impressive power conversion efficiency (PCE) for the small molecule based OSCs are over 14%, which is a vital milestone for the commercialization of OSCs. However, even in these champion solar cell systems, the voltage losses are still relatively large, normally 0.6-0.8 V, indicating further improvement of solar cell performance could be realized by increasing open-circuit voltage (Voc) and then minimizing the voltage loss of OSC devices.
One effective way to reduce the voltage loss in OSCs is designing donor polymers with low-lying highest occupied molecular orbital (HOMO) levels to match SMAs. Meanwhile, sufficient driving force need to be guaranteed between the donor and the acceptor for efficient charge generation. To achieve low-lying HOMO level of donor polymers and thus high Voc of polymer-based OSCs, one effective design strategy is adding electron-withdrawing substitution to the polymer backbone, such as carboxylate substitution, carbonyl substitution, fluorination, and chlorination. Previously, it is reported that there is generally a trade-off between Voc and the short-circuit current (Jsc) and the elevation of Voc usually leads to the significant decrease
of Jsc. As a result, it is very tough and challenging to obtain a high Voc for the solar cells while simultaneously maintaining reasonable high device performance.
My major research during the postgraduate study is designing promising donor polymers with deep HOMO levels for non-fullerene OSCs. Accordingly, carboxylate group and fluorination are employed to rationally tune the energy levels of polymers for the target solar cell systems. The results show that the energy levels of donor polymers are certainly down-shifted, which are characterized as good matches for SMAs. By optimizing the acceptor properties and the blend morphology at the same time, we can achieve highly efficient non-fullerene OSCs with high Voc values, which unlocks the trade-off between Voc and Jsc to a great extent.
In chapter I, a brief introduction on the history of OSCs, working mechanisms of OSCs, and the development of donors and acceptors for OSCs, are given to provide basic information and concept of the OSC field.
In chapter II, a novel polymer P3TEA with a deep HOMO level was synthesized for the SMA SF-PDI2. Relative characterizations demonstrate that P3TEA:SF-PDI2-based OSCs exhibit ultrafast and efficient charge separation despite a negligible driving force, as the charge transfer (CT) state is nearly identical to bandgap. Moreover, the small driving force is found to have minimal detrimental effects on charge transfer dynamics of the OSCs. We achieve a non-fullerene OSC with 1.11 V Voc, nearly 90% internal quantum efficiency, and 9.5% efficiency, despite a low voltage loss of 0.61 V, which reveals important implications for the development of more efficient OSCs.
In chapter III, a novel donor polymer P3TAE was constructed by exchanging the position of the carboxylate substitutions and the alkyl side chains along the backbone of P3TEA. Compared to P3TEA, the resulting polymer P3TAE exhibits a 0.18 eV deeper HOMO level. As a result, when P3TAE is blended with a SMA FTTB-PDI4, the blend shows a high PCE of 8.10% with a Voc up to 1.20 V, indicating a small voltage loss of merely 0.51 V. Importantly, the 1.20 V Voc of the solar cell is one of the highest achieved Voc for single-junction OSCs with device
performance higher than 8.10%. The results validate our design strategy is promising to push the energy level limit of donor polymers to achieve high Voc for non-fullerene OSCs.
In chapter IV, we reported a random donor polymer PTFB-M to tune the device morphology and thus enhance the performance of OSCs. The asymmetric polymer backbone introduces some randomness to PTFB-M, yielding several beneficial effects. PTFB-M exhibits smaller and more favorable domain sizes in the blend, leading to high Jsc. Also, PTFB-M can, to our surprise, maintain its crystallinity perfectly when blended with the SMA called ITIC-Th, leading to relatively high hole mobility and fill factor (FF) of the device. As a result, PTFB-M-based OSCs yield a PCE of 10.4%, which provides a useful approach to tune the morphology of donor polymers for high-performing OSCs.
In chapter V, a tetrafluorobenzene-based donor polymer PT4FB was designed and synthesized to achieve high Voc for SMA-based OSCs. Although the polymer-based device can achieve a Voc of 0.97 V, the device performance is only 9.81% because PT4FB exhibits excessively strong aggregation and it tends to form large fiber-like domains in the blend. In order to reduce the polymer crystallinity and thus tune the morphology of the blend film, alkylthiophene side chains were utilized to replace alkyl side chains, yielding a new donor polymer PTT4FB with more twisted backbone and weaker aggregation. As a result, PTT4FB-based blend exhibits reduced domain sizes and more efficient charge generation, leading to an elevated PCE of 10.6% for PTT4FB-based OSCs. Our work provides an effective design strategy to depress the excessive aggregation of donor polymers to achieve a favorable morphology for efficient OSCs.