Transient Absorption Studies of Bimolecular Recombination Dynamics in Polythiophene/Fullerene Blend Films

Abstract

Bimolecular recombination, an important loss mechanism in organic solar cells, has been investigated using transient absorption spectroscopy for poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) films and analogues of these components. Data are analyzed as a function of blend composition, postdeposition thermal annealing, and excitation density. Comparison of transient spectra for P3HT:PCBM films with analogous films employing P3HS and PC70BM allows the assignment of the photoinduced absorption features. These decay dynamics are analyzed on the nanosecond to millisecond time scales and are shown to be in excellent agreement with a bimolecular recombination model in the presence of an exponential distribution of localized (trap) states. Thermal annealing results in an acceleration of these decay dynamics, which is assigned to a reduction in the depth of the trap states and correlated with an increase in film crystallinity. The decay dynamics are analyzed to obtain an effective recombination coefficient that is charge density dependent at low polaron densities, but becomes independent of charge density at high charge carrier densities (>1018 cm−3). This transition is assigned to trap filling, with the recombination coefficient measured at high charge density (k = 3 × 10−12 cm3 s−1) corresponding to the trap-free limit. From transient spectroscopic behavior we estimate the density of intraband trap states in unannealed P3HT:PCBM blend films to be ∼7 × 1017 cm−3.

Photodegradation in Encapsulated Silole‐Based Polymer: PCBM Solar Cells Investigated using Transient Absorption Spectroscopy and Charge Extraction Measurements

“…Such behaviour has only been observed in a few systems. It has also been observed in P3HS:PCBM, [26] a selenophene analogue of P3HT, where thermal annealing causes the growth of a new transient feature at 820 nm in addition to the pre-existing 1150 nm localised polaron band. This new 820 nm feature was assigned to delocalised polarons that increase in population upon annealing, which is known to enhance the crystallinity of the blend film.…”

“…This method is widely-known and has previously been applied to numerous polymer:blend systems. [12,[17][18][19][20][21][22][23][24][25] In general, the charge carrier decay kinetics exhibit a power law decay (∆OD ∝ t -α ); this has been observed in P3HT, [26] polyselenophenes, [27] and MDMO-PPV, [28,29] all blended with various fullerene derivatives. This power law behaviour is consistent with models describing bimolecular recombination of dissociated charge carriers in the presence of an exponential distribution of localised (trapped) states.…”

“…At a probe wavelength of 1000 nm ( Figure 5b) the power law α value of 0.43 is similar to what has been observed previously for P3HT:PCBM at probe wavelengths of 980 -1000 nm. [20,26] The kinetics at 1000 nm thus have a significant contribution from localised, trapped polarons. Thermal activation of these polarons out of the trap states is necessary before recombination can occur, thus causing a relatively slow recombination rate.…”

Universality in the intensity-modulated photocurrent in bulk-heterojunction polymer solar cells

“…The absence of contributions from possible frequency dependence of the light source output and the associated measurement-electronics in the set up was rigorously verified [see S2]. (PCPDTBT) donor polymer [22] blended with acceptor either [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) macromolecule [23] or poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2), Polyera ActivInk N2200) polymer [24]. The typical value of I SC (responsivity) and V OC were in the range of 20-30 mA/W and 0.55-0.6 V respectively in all the solar cells studied.…”

Photochemistry of P3HT and PC 60 BM in toluene solution: Evidence of T–T energy transfer

“…Photochemical processes involving these materials are widely studied mostly in solid polymer/fullerene composites, which are used as active layer of organic solar cells [6]. The majority of transient absorption studies of such composites are devoted to charge formation [7,8] and recombination [9][10][11][12][13]. In some works the triplet state [14] formation is studied [7,[15][16][17][18].…”