Open Readings 2021 • O7-5
CHARGE CARRIER TRANSPORT IN CLOSE TO REAL WORKING CONDITIONS OF ORGANIC SOLAR CELLS
Rokas Jasiūnas1, Vidmantas Jašinskas1, Huotian Zhang2, Feng Gao2, Vidmantas Gulbinas1
1 Department of Molecular Compound Physics, Center for Physical Sciences and Technology, Saulėtekio av. 3, LT-10257 Vilnius, Lithuania
2 Department of Physics Chemistry and Biology (IFM), Linköping University, Linköping SE-58183, Sweden
[email protected]
Ever since organic photovoltaic devices have been introduced to scientific community decades ago, debates about underlaying physics of charge transport ignited and continue to flourish to this day. Charge carrier mobility is one of the main parameters determining carrier extraction and recombination. Thus, not surprising that complex and still poorly understood it becomes the center of debates.
It has been demonstrated in several publications that charge carrier mobility in an active layer of the organic solar cell decreases with time after photogeneration by several orders of magnitude [1-5]. On the other hand, there are also works showing that steady-state mobility analysis can be successfully applied to organic photovoltaic devices[6-9]. Recently, it was argued that carrier mobility in operating solar cells is indeed constant and that transient methods showing time-dependent carrier mobility, such as transient photocurrent, where the sample is excited by a short laser pulse, are inadequate because carrier traps remain almost empty [10]. While under constant Solar illumination carrier traps become populated, therefore mobility does not decrease while the carrier is drifting.
In this work, we have experimentally investigated carrier extraction dynamics in TQ1:PC71BM, one of a standard model of polymer-fullerene solar cells, and highly effective non-fullerene acceptor based PBDB-T:Y1 organic photovoltaic devices. We employed several transient methods, covering the wide temporal window from ps to μs, and numerical modelling to address carrier transport in close to real working conditions of a solar cell. To evaluate the role of trapped carriers, we compared carrier extraction dynamics under excitation of samples by weak laser pulses when trap states were nonpopulated and with additional 1 Sun illumination populating the trap states.
We showed that mobility decay during initial carrier drift is the same when carriers are either continuously generated by steady-state illumination or by a short laser pulse. Nevertheless, steady-state illumination alters carrier transport in μs timescale differently, depending on active layer materials. For fullerene-based device, continuous illumination repeals mobility decay present in case of periodic excitation; however, in highly effective non-fullerene devices due to extremely low trap population mobility decay in μs timescale is marginal in both cases.
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