Quantitative Analyses of Competing Photocurrent Generation Mechanisms in Fullerene-Based Organic Photovoltaics

Abstract

The performance of fullerene-based organic photovoltaic devices (OPVs) with low donor concentrations is not limited by the tradeoff between short-circuit current density (J sc ) and open-circuit voltage (V oc ), unlike bulk heterojunction OPVs. While the high V oc in this novel type of OPVs has been studied, here we investigate the mechanisms that govern J sc , which are not well understood. Three mechanisms, diffusion limited exciton relaxation, geminate recombination during exciton dissociation, and nongeminate recombination during charge transport, are studied analytically by combining various experimental techniques and transfer matrix simulation. We find that exciton dissociation at donor/acceptor interfaces is the dominant factor to produce high J sc , and at low P3HT concentrations exciton relaxation limits photocurrent generation. With more P3HT inclusion, the creation of interfaces promotes exciton dissocation but also reduces fullerene crystallinity, weakening the driving force for charge separation, and introduces nongeminate recombination sites. Quantitative analyses show that the magnitude of measured J sc and the donor concentration dependence are well accounted for by these three competing mechanisms.

Transport Effects on Capacitance-Frequency Analysis for Defect Characterization in Organic Photovoltaic Devices

“…Given that all devices have the same materials, P3HT and PC 71 BM, the defect is expected to have the same energy level in all devices. Indeed, our previous work shows that the energetics measured from the low energy quantum efficiency spectroscopy is the same in all devices with varying P3HT concentration [54]. The experimental studies together with the simulation results indicate that, even in fully depleted thin devices, the low mobility can introduce artifacts and spurious measurements of defect energy position.…”

“…Recent high performance OPVs mostly use relatively thin (~100 nm), and therefore fully depleted, active layers to ensure good charge collections [52,53]. To further assess the effect of hole mobility in fully depleted devices, which doesn't manifest the fictitious shallow defect due to the C d -to-C g transition (Figure 2), we investigate the PC 71 BMbased OPVs with small amounts of P3HT (see Figure S6 and Table S2 for J-V characteristics) [54]. As shown in Figure 4(a), in all P3HT concentrations, capacitance decreases as frequency increases; additionally, with decreasing P3HT concentration, the capacitance decrease shifts to lower frequency.…”

Tunable Twisting Motion of Organic Linkers via Concentration and Hydrogen-Bond Formation

“…Organic molecules have an enormous range of applications in various fields such as fluorescent probes, 1 photovoltaics [2][3] , transistors 4 , supramolecular electronics 5 , white-light generation [6][7] , and metal-organic frameworks (MOFs). [8][9][10] Many fundamental photophysical and photochemical properties of organic molecules including charge transfer, charge separation and charge recombination are well understood in several chemical systems using time-resolved laser spectroscopy.…”

n‐Type Doping Induced by Electron Transport Layer in Organic Photovoltaic Devices

“…[38] The total absorption spectra were then calculated as "absorption = 1-reflection. [38] The total absorption spectra were then calculated as "absorption = 1-reflection.…”

“…In the absorption efficiency measurement, the incident and reflected light spectra were measured using an Ocean Optics USB 4000 spectrometer, an ISP‐R‐GT integrating sphere with gloss‐trap setup, and an HL‐2000 light source . The total absorption spectra were then calculated as “absorption = 1‐reflection.”…”