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Studies of Voltage Losses in Organic Solar Cells
Doctoral thesis, comprehensive summary
Deping Qian
Publication Year
<p>Organic photovoltaic (OPV) devices based on semiconducting polymers and small molecules are potential alternatives to inorganic solar cells, owing to their advantages of being inexpensive, lightweight, flexible and suitable for roll-to-roll production. The state of art organic solar cells (OSCs) performed power conversion efficiencies (PCEs) over 13%.</p><p>The quantum efficiency losses in OSCs have been significantly reduced within the charge generation and extraction processes, resulting in high <em>EQE</em><sub>PV</sub> (70-90%) and high FF (70-80%). Whereas, large voltage losses (Δ𝑉 = 𝐸<sub>𝑔</sub>/<sub>𝑞</sub> − 𝑉<sub>𝑂𝐶</sub>) were observed in conventional fullerene based solar cells, and it has been the main limiting factor for further OPV advancement. Therefore, strategies to reduce the voltage losses are required.</p><p>In this thesis, newly designed non-fullerene (NF) acceptors are used to construct novel material systems for high efficiency solar cells. In particular, we studied the hole transfer in these fullerene free systems. We also reported a NF system that exhibit ultrafast and efficient charge separation despite a negligible driving force, as <em>E</em><sub>CT</sub> is nearly identical to 𝐸<sub>𝑔</sub>. Moreover, the small driving force is found to have minimal detrimental effects on charge transfer dynamics of the OSCs. We demonstrate a NF based OSC with efficiency of 9.5% and internal quantum efficiency nearly 90% despite a low voltage loss of 0.61 V. This creates a path towards highly efficient OSCs with a low voltage loss.</p><p>CT states in OSCs are also investigated, since VOC is governed by the CT energy (<em>E</em><sub>CT</sub>), which is found as 𝑞𝑉<sub>𝑂𝐶</sub> = 𝐸<sub>𝐶𝑇</sub> − 0.6 in a large set of fullerene based solar cells. In order to reduce these recombination losses from CT states, we explored polymer-diPDI systems which exhibited weakened D-A coupling strength, due to the steric hindrance effect. The radiative recombination losses at D/A interface in these NF devices are all reduced to less than 0.18 eV. In particular, in some cases, the additional emission from pure material is favorable for suppressing the non-radiative CT states decay. Consequently, the recombination losses in these NF systems are reduced to 0.5 eV, while the charge generation is still efficient as confirmed by PL quenching and<em> EQE</em><sub>PV</sub>.</p><p>Novel material systems based on non-fullerene acceptors are investigated. The systems performed energy offsets (ΔHOMO or ΔLUMO) less than 0.15eV, resulting in the same energy of CT states and bulk excitons. In this regard, the charge transfer energy loss is minimized. We also found that the EL spectra as well as the <em>EQE</em><sub>EL</sub> of the blend solar cells are similar with that of lower gap components in blends. Thus the non-radiative voltage losses are reduced to &lt; 0.3V and small voltage loss of 0.5-0.7V are obtained. Meanwhile, the charge generation in systems are still efficient and high <em>EQE</em><sub>PV</sub> of 50-70% can be achieved. It confirms that there is no intrinsic limit for the <em>V</em><sub>OC</sub> and efficiency of OPVs as compared with other photovoltaic technologies.</p>