Quantifying the optimal thickness in polymer:fullerene solar cells from the analysis of charge transport dynamics and photoabsorption

Abstract

We provide a semi-empirical equation that quantifies the optimal layer thickness in bulk heterojunction organic solar cells, which is based on time-of-flight and time-resolved microwave conductivity measurements and photoabsorption of a film.

Combined Charge Extraction by Linearly Increasing Voltage and Time-Resolved Microwave Conductivity to Reveal the Dynamic Charge Carrier Mobilities in Thin-Film Organic Solar Cells

“…The decrease in mobility can be attributed to mainly three reasons (more details in Supporting Note S3 ): (i) mobility relaxation due to charge transport via carrier hopping into deeper trap states, as revealed by Melianas et al; 18 (ii) sequential charge extraction, where the faster carriers with higher mobility are extracted prior to the slower carriers; (iii) mobility difference between injected carriers (uneven distribution, Figure S19a ) and dark equilibrium carriers (even distribution), where the latter shows ∼4 times larger initial mobility compared to the former ( Figure S19b ). The lower μ h of P3HT:PCBM than that of PffBT4T:PCBM also match with our previous TOF mobility results (1.8 × 10 –3 cm 2 V –1 s –1 for P3HT:PCBM and 1.4 × 10 –2 cm 2 V –1 s –1 for PffBT4T:PCBM), 31 , 33 whereas the order difference was mainly due to the difference in the measurement time scale 18 and carrier injection method. 60 For the NFA BHJ samples, PM6:ITIC exhibited a very stable μ h of ∼22.2 × 10 –6 cm 2 V –1 s –1 during the first 30 μs, which is close to its μ CELIV (16.8 × 10 –6 cm 2 V –1 s –1 ), indicating that the hole mobility in PM6:ITIC is very stable throughout the transport process.…”

“…We previously reported a combination of TRMC and TOF measurements to determine the time-dependent electron/hole mobilities in various BHJ OSCs . Very distinct mobility relaxation characteristics in the fullerene-based OSCs (PCPDTBT:PC 71 BM and PffBT4T:PC 61 BM) , and NFA OSCs (PBDB-T:PC 61 BM and PBDB-T:ITIC) were revealed, with the mobility relaxation lifetimes differing by as much as 2 orders of magnitude. However, the TOF-TRMC measurement exhibits drawbacks, namely, (i) only the normalized mobility is derived; (ii) the mobility determination includes a diffusion-based simulation, which may result in a deviation in the analyzed mobility; and (iii) most importantly, the photoactive layer thickness of a TOF-TRMC device is 1–2 μm (5–10-fold thicker than real OSCs) owing to the TOF limitation, which may lead to morphological differences.…”

“…Conventionally, charge mobility is regarded as a space- and time-independent constant during charge transport in OSCs and is usually measured using techniques such as SCLC, time-of-flight (TOF), field-effect transistors, and charge extraction by linearly increasing voltage (CELIV). − These measurements are direct and easy to implement, but they provide only a long-range and averaged mobility value. Empowered by time-resolved techniques, such as electric field-induced second harmonic, , time-resolved terahertz spectroscopy, and time-resolved microwave conductivity (TRMC), ,− the mobility of charge carriers has recently been observed to decay during charge transport (termed mobility relaxation) after photo injection. Melianas et al reported that the charge mobility in OSCs exhibits a very large initial mobility (>1 cm 2 V –1 s –1 ), which gradually decreases (by as large as 6 orders of magnitude) because the charge carrier transports to the electrode via hopping between the tail states in the density of states .…”

Current Progress of Efficient Active Layers for Organic, Chalcogenide and Perovskite-Based Solar Cells: A Perspective

“…Nevertheless, the discovery of photoinduced electronic transference between optically excited conjugated polymers and the fullerene molecule (C 60 ) [67,68], along with the elevated photoconductivities shown by the addition of C 60 to the conjugated polymers [69], allowed the fabrication of devices based on polymer-fullerene bilayer heterojunction [70,71] or bulk heterojunction (BHJ) [72][73][74]. This advance significantly increased the values of PCEs of the corresponding devices for almost two decades (Table 1) [75][76][77][78][79][80][81][82]. The photoinduced electron mobility favorably happens in the polymer-fullerene system when the electron in the excited state of the polymer is transferred to the fullerene, which is quite more electronegative since the electron is injected from a p-type hole belonging to the conducting polymer, donor (D), to the n-type electron from the conducting C 60 molecule, acceptor (A) [83].…”