N-Doping Donor-Dilute Semitransparent Organic Solar Cells to Weaken Donor: Acceptor Miscibility and Consolidate Donor-Phase Continuity.

Lightweight and semi-transparent organic solar cells (ST-OSCs) offer bright promise for applications such as building integrated photovoltaics. Diluting donor content in bulk-heterojunction active layers to allow greater visible light transmittance (AVT) effectively enhances device transparency, yet the ineluctable compromise of the donor-phase continuity is challenging for efficient charge transport. Herein, a trace amount of n-type N-DMBI dopant is incorporated, which facilitates the donor:acceptor (D:A) de-mixing by strengthening both acceptor polarity and D/A crystallization. With the diminution of component inter-mixing, the limited number of donors increasingly self-aggregate to establish the more continuous phases. For the benchmark PM6:Y6-based ST-OSCs, when the donor content is reduced from regular 45 to optimal 30 wt.%, the device AVT is remarkably raised by more than a quarter, accompanied by a marginal drop in power conversion efficiency from 13.89% to 13.03%. This study reveals that by decreasing the donor content to <30 wt%, acceptor excitons induced by Förster resonance energy transfer are prone to severe radiative recombination. This is nonetheless mitigated by dopant inclusion within the acceptor phase by providing extra energy offset and prolonging charge transfer state lifetime to assist exciton dissociation.

Semitransparent Organic Photovoltaics Utilizing Intrinsic Charge Generation in Non-Fullerene Acceptors.

In organic semiconductors, a donor/acceptor heterojunction is typically required for efficient dissociation of excitons. Using transient absorption spectroscopy to study the dynamics of excited states in non-fullerene acceptors (NFAs), it is shown that NFAs can generate charges without a donor/acceptor interface. This is due to the fact that dielectric solvation provides a driving force sufficient to dissociate the excited state and form the charge-transfer (CT) state. The CT state is further dissociated into free charges at interfaces between polycrystalline regions in neat NFAs. For IEICO-4F, incorporating just 9 wt% donor polymer PTB7-Th in neat films greatly boosts charge generation, enhancing efficient exciton separation into free charges. This property is utilized to fabricate donor-dilute organic photovoltaics (OPV) delivering a power conversion efficiency of 8.3% in the case of opaque devices with a metal top-electrode and an active layer average visible transmittance (AVT) of 75%. It is shown that the intrinsic charge generation in low-bandgap NFAs contributes to the overall photocurrent generation. IEICO-4F-based OPVs with limited PTB7-Th content have high thermal resilience demonstrating little drop in performance over 700 h. PTB7-Th:IEICO-4F semitransparent OPVs are leveraged to fabricate an 8-series connected semitransparent module, demonstrating light-utilization efficiency of 2.2% alongside an AVT of 63%.