Role of Localized States on Carrier Transport in Bulk Heterojunction Materials Comprised of Organic Small Molecule Donors

Abstract

Solution-processable conjugated small-molecule donor materials have proven to be an attractive alternative to the more widely studied conjugated polymer donor counterparts in the fi eld of organic photovoltaics (OPVs) comprised of bulk-heterojunction (BHJ) materials. [1][2][3] Recently, it has been demonstrated that solution processed small molecule OPV devices can reach high power conversion effi ciency of ∼ 9%, [ 4 ] close to state-of the-art effi ciency of narrow band gap polymer-based OPVs. [ 5,6 ] Potential advantages of the small molecule donor materials are the monodispersity, ease of purifi cation, ease of structural characterization, and the possibility of modifying the molecular structures in order to fi ne-tune the electronic and optical properties without signifi cantly altering the molecular weight and polydispersity. [ 7 ] Generally, one of the key factors that limits the power conversion effi ciency of OPV devices is the presence of disorderinduced localized states in the BHJ composite active layer. Structural disorder within domains of small molecule donor and fullerene acceptor phases and at the donor/acceptor heterojunction interface plays a major role in introducing trap states. [8][9][10] The present studies are aimed at investigating the photoexcited carrier relaxation through the manifold of localized (trap) states in two BHJ systems comprising state-of-the-art high effi ciency small molecule donor materials: p-DTS(PTTh 2 ) 2 :PC 71 BM [ 1 ] and p-DTS(FBTTh 2 ) 2 :PC 71 BM. Various aspects of the results will be compared to those obtained from polymerbased PCDTBT:PC 71 BM BHJ system.This work follows previous studies that have initiated a new approach for elucidating the photoexcited carrier relaxation process in polymer-based BHJ systems such as P3HT:PC 60 BM [ 11,12 ] and PCDTBT:PC 71 BM [13][14][15] using transient photoconductivity (TPC) measurements. These earlier studies indicated thermally activated carrier transport in the nanosecond time regime, and identifi ed how the activation energy ( E A ) evolves in time. In particular, the time evolution of Δ E was investigated in the PCDTBT:PC 60 BM BHJ system into which electron traps were deliberately introduced at various concentrations. [ 15 ] Briefl y, our present studies reveal that localized states limit the carrier transport in solution-processed small molecule polycrystalline BHJ fi lms less than in polymer-based BHJs; this is demonstrated by the distinct time evolution of the activated energy exhibited by the small molecule based BHJ systems.The molecular structures of the two small molecule donors, the polymer donor, and the fullerene molecule used in these studies are displayed in Figure 1.The photoexcited charge carrier dynamics were investigated using 50 fs pulsed laser tuned at photon wavelength of 400 nm, with variable incident intensity in the range of 1-100 μ J/cm 2 per pulse, which would generate initial charge carrier densities equivalent to that generated by continuous illumination intensity equivalent to 1-100 suns.Opti...

Organic Optoelectronic Materials: Mechanisms and Applications

“…552,942 The local mobility may also exhibit considerably different temperature dependence than the "bulk" one, 27,201,347 having been governed by a different mechanism. In some cases (e.g., PPV, 730 PCDTBT:PCBM 943 ), the charge carrier mobility became more thermally activated when it was probed at longer timescales, for example due to the multiple-trapping carrier transport as the carriers relax into the deeper states. 549 In other systems, such as p-DTS:PCBM, 943 the activation energy did not change appreciably with time, at least through nanosecond time-scales, and charge transport proceeded via localized sites confined to a narrow energy range.…”

Balancing the H- and J-aggregation in DTS(PTTh2)2/PC70BM to yield a high photovoltaic efficiency

“…[7] The domain size of DTS(PTTh 2 ) 2 in active layer of BHJ device based on DTS(PTTh 2 ) 2 /PC 70 BM (7/3, w/w) blend is 20-30 nm without any treatments. Most recent studies concerned with the DTS(PTTh 2 ) 2 /PC 70 BM system focused on tuning the film morphology including crystalinity and crystal size and the influence of morphology on device performance including photoconductivity, PCE, J sc , open-circuit voltage (V oc ), fill factor (FF) and incident photon conversion efficiency (IPCE) et al [7,[9][10][11] Attractive items in SM BHJ solar cells, including photocurrent response, [12] charge generation [13] and carrier transport, [14] were also explored in depth. Most recent studies concerned with the DTS(PTTh 2 ) 2 /PC 70 BM system focused on tuning the film morphology including crystalinity and crystal size and the influence of morphology on device performance including photoconductivity, PCE, J sc , open-circuit voltage (V oc ), fill factor (FF) and incident photon conversion efficiency (IPCE) et al [7,[9][10][11] Attractive items in SM BHJ solar cells, including photocurrent response, [12] charge generation [13] and carrier transport, [14] were also explored in depth.…”

“…Most recent studies concerned with the DTS(PTTh 2 ) 2 /PC 70 BM system focused on tuning the film morphology including the crystallinity and crystal size and the influence of morphology on the device performance including the photoconductivity, PCE, J sc , open-circuit voltage (V oc ), fill factor (FF) and incident photon conversion efficiency (IPCE) etc. 7,[9][10][11] Attractive items in SM BHJ solar cells, including the photocurrent response, 12 charge generation 13 and carrier transport, 14 were also explored in depth. Adding solvent additives, i.e., 1,8-diiodooctane (DIO), was a powerful strategy to enhance the device performance and yielded excellent results.…”

Ultrabroadband photosensitivity from visible to terahertz at room temperature

“…We employed a pulse femtosecond laser using a regenerate amplifier and optical parametric amplifier (OPA) with a repetition rate of 1 KHz to investigate the photoresponse of 1T-TaS2 devices at RT. The temporal 'time-of-flight' response (27,28) by the photo-excitations under applied bias voltage is recorded by an oscilloscope with a bandwidth of 350 MHz, for which a channel in 50 nm thickness and 30 μm length between the electrodes is used for measurement. The result is shown in Fig.…”