The Influence of Energy Levels on Voltage Losses and Charge Generation in Organic Solar Cells

Abstract: Organic solar cells (OSCs) are a next-generation photovoltaic technology that convert solar energy to electrical energy. They have attracted great attention due to their advantages of low cost, ease of synthesis, light weight, mechanical flexibility, and roll-to-roll processability. In the past decades, owing to the development of the materials, device optimization and the understanding of the working mechanism, the power conversion efficiency (PCE) has been boosted to ~19%. However, the efficiency of the OSCs is still not comparable to the conventional inorganic solar cells and emerging perovskite solar cells due to the large open-circuit voltage loss (Vloss). In addition, it is also important to obtain efficient charge generation while reducing the Vloss. Thus, understanding the loss mechanisms in the OSCs is significant for achieving further improvement.In this thesis, a novel small-molecule donor named ZR1 was used to fabricate all-small-molecule OSCs (SM-OSCs), which shows efficient charge separation and transport with the optimized hierarchical morphologies, obtaining a breakthrough efficiency of 13.34% with a low Vloss (0.54 eV) in SM-OSCs. In this system, the energy offsets between the donor and acceptor (ΔHOMO or ΔLUMO) play an important role in the open-circuit voltage (VOC) of the OSCs. According to the optoelectronic reciprocity introduced in this thesis, the sub-gap absorption and emission by charge transfer (CT) states lead to large radiative and non-radiative recombination losses. The results show that the decreased HOMO offsets between donor and acceptor can effectively reduce both radiative and non-radiative recombination losses from the CT states, resulting in a suppressed Vloss.In addition to the SM-OSCs, we also study the Vloss and charge generation in the all-polymer OSCs (all- PSCs). A series of polymer acceptors were designed and applied in all-PSCs. In this work, all devices with negligible LUMO offsets show high VOCs of 1.02-1.15 V and good short-circuit currents (JSCs) of 8.87-15.16 mA cm−2 as well as small Vlosss. This study reveals that the small Vloss and the effective charge generation can also be realized simultaneously in all-PSCs with small energy offsets.Next, we found that introducing a third component can also reduce Vloss. In this work, we start with the fundamental photophysical processes which determine the VOCs of the devices and provide a universal approach framework well explaining the VOC of ternary OSCs (TOSCs) in different situations. By combining experimental investigations with theoretical simulations, we highlight the significant influence of the thermal population arising from the guest component-related CT states and local excited (LE)states on the non-radiative recombination losses in TOSCs. Firmly based on our new understanding, we provide design rules for enhancing the VOC in TOSCs: 1) high emission yield for the guest binary system; close charge-transfer energies between two binary systems; 2) high miscibility of the guest component with the low-optical-gap component in the host binary blends.In the all-PSCs work we did before, we find the small Vloss and the effective charge generation can be achieved simultaneously with small energy offsets, which can be also observed in other non-fullerene based OSCs. It was found that some of non-fullerene acceptors based OSCs can realize an efficient charge generation and a suppressed charge recombination process with small energy offsets (< 0.3 eV) between the donor and the acceptor, leading to a low Vloss, a high JSC, and a high fill factor (FF) simultaneously. Here, we investigate a series of OSCs blends with different HOMO offsets between donor and in a large range of ~ 0 to 0.50 eV. Along with decreasing HOMO offsets, the blends show reduced Vlosss. For the JSC and the FF, we observe a maximum value at an optimal energetic offset around 0.2-0.3 eV and the optimal energetic offset appears at different values for different non-fullerene acceptors. Through the analysis of the ultrafast transient absorption, we find inefficient charge generation when the HOMO offset is close to zero, which attributed to the back transfer of a hole from the donor to the acceptor. The affected charge generation at the small HOMO offsets is probably the main reason for the deceased JSC and FF. This study demonstrates the existence of optimal energy offsets for achieving high-performance OSCs.

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