Device Physics Of Efficient Non-Fullerene Organic Solar Cells

Non-fullerene organic solar cells（NF-OSCs）are gaining increasingly attention for its distinct advantages such as facile synthesis,strong optical absorption and minimized energy loss.They are becoming promising candidates for organic solar cells.Recently,the PCE of NF-OSCs have increased dramatically and outperformed fullerene-based OSCs.However,most research have been focused on the development of new acceptors,lacking of in-depth understanding of device physics in NF-OSCs.Studying NF-OSCs from device standpoint provides useful guideline for materials design and device optimization.The energy consuming synthesis route of fullerene derivative limit the large-scale application of fullerene-based OSCs.In the second chapter,we explored using pristine mixture of PC₆₁BM and PC₇₁BM（mix-PCBM）as acceptor.It saves the energy and time consuming procedure of purification,thus can greatly reduce the cost.The PCE of mix-PCBM devices are better than PC₆₁BM devices and close to PC₇₁BM devices.It manifest in the difference of short circuit current(J_SC),of which mix-PCBM devices larger than PC₆₁BM devices and lower than PC₇₁BM devices.Study of photoelectric conversion process indicates that the discrepancy results from different short wavelength absorption and charge carrier collection efficiency in the three systems.By utilizing inverted structure and employing electron transporting material PFN,we achieved a PCE of 8.01%in PTB7:Mix-PCBM devices.Mix-PCBM shows better performance and lower cost than PC₆₁BM,proved to be a promising acceptor for large-scale application.One of the key advantages leads to the breakthrough of PCE in NF-OSCs is the low energy loss,which manifest high open-circuit voltage（Voc）in devices.Moreover,the energy loss not only dependents on acceptor itself,but also the donor in the blend.In the third chapter,we comparatively studied the fullerene and non-fullerene devices based on P3HT and PTB7-Th donor.charge transfer energy(E_CT)was analyzed in two different type of OSCs through investigating the sub-bandgap external quantum efficiency（EQE）and electroluminescence（EL）.Two different mechanisms behind the increase of Voc were revealed,raising E_CT without altering the chemical structure/environment（singlet energy）can effectively improve V_OC without sacrificing photocurrent.In addition,the impact of energetic and DOS on energy loss was assessed,implying reduced disorder in acceptor LUMO can further reduce the loss in V_OC.Efficient charge generation involves absorption complementation for maximizing photons harvest as well as photo-induced electron transfer and hole transfer between donor/acceptor interface upon photoexcitation,a high yield of charge from both processes usually requires sufficient energy offset between donor and acceptor for charge separation.From energy loss point of view,high energy charge-transfer excitons and reduced non-radiative recombination loss afford high V_OC.In the fourth chapter,we studied a wide-bandgap polymer donor blended with two narrow bandgap non-fullerene acceptors IDIC and i-IEICO-4F.Both systems demonstrate balanced and efficient charge generation and high V_OC（as a result of reduced non-radiative recombination loss）,as well as high fill factor due to excellent charge transport properties.Thus lead to a PCE of 13%（certified as 12.5%）in PMOT40:i-IEICO-4F with single-junction device architecture.