Photoexcitation dynamics in polythiophene/fullerene blends for photovoltaic applications

We used the transient and steady state photomodulation spectroscopies for studying the photoexcitations dynamics in blends of regio-regular poly(3-hexylthiophene) (RR-P3HT) and fullerene in a broad spectral range from 0.13 to 2.25 eV. We found that both localized polarons and singlet excitons are instantaneously photogenerated in the blends. However the photogeneration process of delocalized polarons which contribute to the photocurrent proceeds in two steps: first, within a couple of ps the excitons generated in the polymer domains populate the charge transfer complex states at the RR-P3HT/fullerene interfaces; this is followed by the charge transfer ionization into delocalized charge polarons in the polymer and fullerene constituents within ～20 ps. In contrast, the localized polaron dynamics are unrelated with the excitons and delocalized polarons dynamics. We also report on the occurrence of ultrafast quantum interference anti-resonances between photoinduced infrared-active vibrations and the delocalized polaron band in the blends, which shows the delocalization character of the photogenerated charges that contribute to the photocurrent.     

低维聚合物纳米线中的激子限域和能量传递效应研究

在共混的两种低维聚合物量子线中,采用分子自组装技术制备了嵌段低维聚合物纳米线的异质结构,通过对室温下吸收光谱、发光光谱和激发光谱的研究,证实了在这种嵌段低维聚合物纳米线的异质结构中存在很强的激子限域效应以及链内和链间的能量传递效应。     

Ultrafast Spectroscopy of Photoexcitations in Organometal Trihalide Perovskites

Studying the room temperature broadband ultrafast transient response of photoexcitations in three perovskite films, namely MAPbI₃, MAPbI_1.1Br_1.9, and MAPbI_3?xCl_x (MA = CH₃NH₃), allowed unravelling the branching ratio between photogenerated carriers and excitons, a key factor for optoelectronic applications of perovskites. An instantaneously generated mid‐IR photoinduced absorption (PA) band, PA₁ is observed in all three perovskites, as well as a strong derivative‐like band of photoinduced bleaching (PB) and PA (PA₂) close to the corresponding absorption band edge. From the distinguished different decay dynamics of the PA bands in MAPbI₃, PA₁ is interpreted as due to the exciton transition, whereas PA₂ and PB are due to band‐filling effect caused by the photocarriers. In contrast, all bands in MAPbI_1.1Br_1.9 and MAPbI_3?xCl_x share the same dynamics and are therefore due to the same species, namely photogenerated excitons. The transient photoinduced polarization memory (POM) for both excitons and photocarriers as well as the steady‐state photoluminescence (PL) emission are observed in MAPbI₃, but not in MAPbI_1.1Br_1.9 and MAPbI_3?xCl_x because they possess cubic symmetry at room temperature. The estimated long excitons diffusion length (≈150 nm) in MAPbI₃ opens up the possibility of photocarriers generation at interfaces and grain boundaries even when the exciton binding energy is large compare to k_BT.     

Electron correlation effects on polarons recombination in coupled polymer chains

The recombination dynamics of singlet and triplet oppositely charged polarons under the influence of electron–electron (e–e) interactions in coupled polymer chains are investigated using a multi-configurational time-dependent Hartree–Fock (MCTDHF) method. During recombination processes, singlet and triplet intrachain excitons are important products. By calculating the yields of the singlet and triplet intrachain excitons as a function of the on-site and long-range e–e interactions, it is found that the yields of the singlet and triplet intrachain excitons both decrease with increasing on-site e–e interactions. On the other hand, as the long-range e–e interactions increase, the yields of singlet intrachain excitons initially increase and then maintain a constant value, while the yields of the triplet intrachain excitons decrease. Our results show that the long-range e–e interaction is of fundamental importance and improves the luminescence efficiency in coupled polymer chains. Finally, the influence of the polymer chain length on the yields of singlet and triplet intrachain excitons is discussed.     

The Impact of Polymer Regioregularity on Charge Transport and Efficiency of P3HT:PCBM Photovoltaic Devices

The charge transport in pristine poly(3‐hexylthiophene) (P3HT) films and in photovoltaic blends of P3HT with [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) is investigated to study the influence of charge‐carrier transport on photovoltaic efficiency. The field‐ and temperature dependence of the charge‐carrier mobility in P3HT of three different regioregularities, namely, regiorandom, regioregular with medium regioregularity, and regioregular with very high regioregularity are investigated by the time‐of‐flight technique. While medium and very high regioregularity polymers show the typical absorption features of ordered lamellar structures of P3HT in the solid state even without previous annealing, films of regiorandom P3HT are very disordered as indicated by their broad and featureless absorption. This structural difference in the solid state coincides with partially non‐dispersive transport and hole mobilities µ_h of around 10^?4 and 10^?5 cm² V^?1 s^?1 for the high and medium regioregularity P3HT, respectively, and a slow and dispersive charge transport for the regiorandom P3HT. Upon blending the regioregular polymers with PCBM, the hole mobilities are typically reduced by one order of magnitude, but they do not significantly change upon additional post‐spincasting annealing. Only in the case of P3HT with high regioregularity are the electron mobilities similar to the hole mobilities and the charge transport is, thus, balanced. Nonetheless, devices prepared from both materials exhibit similar power conversion efficiencies of 2.5%, indicating that very high regioregularity may not substantially improve order and charge‐carrier transport in P3HT:PCBM and does not lead to significant improvements in the power‐conversion efficiency of photovoltaic devices.     

Photoinduced Hole Transfer Becomes Suppressed with Diminished Driving Force in Polymer‐Fullerene Solar Cells While Electron Transfer Remains Active

Device performance and photoinduced charge transfer are studied in donor/acceptor blends of the oxidation‐resistant conjugated polymer poly[(4,8‐bis(2‐hexyldecyl)oxy)benzo[1,2‐b:4,5‐b′]dithiophene)‐2,6‐diyl‐alt‐(2,5‐bis(3‐dodecylthiophen‐2‐yl)benzo[1,2‐d;4,5‐d′]bisthiazole)] (PBTHDDT) with the following fullerene acceptors: [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM); [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PC₆₁BM); and the indene‐C₆₀ bis‐adduct IC₆₀BA). Power conversion efficiency improves from 1.52% in IC₆₀BA‐based solar cells to 3.75% in PC₇₁BM‐based devices. Photoinduced absorption (PIA) of the PBTHDDT:fullerene blends suggests that exciting the donor polymer leads to long‐lived positive polarons on the polymer and negative polarons on the fullerene in all three polymer fullerene blends. Selective excitation of the fullerene in PC₇₁BM or PC₆₁BM blends also generates long‐lived polarons. In contrast, no discernible PIA features are observed when selectively exciting the fullerene in a PBTHDDT/IC₆₀BA blend. A relatively small driving force of ca. 70 meV appears to sustain charge separation via photoinduced hole transfer from photoexcited PC₆₁BM to the polymer. The decreased driving force for photoinduced hole transfer in the IC₆₀BA blend effectively turns off hole transfer from IC₆₀BA excitons to the host polymer, even while electron transfer from the polymer to the IC₆₀BA remains active. Suppressed hole transfer from fullerene excitons is a potentially important consideration for materials design and device engineering of organic solar cells.     

Ultrafast Spectroscopic Study of Photoinduced Electron Transfer in an Oligo(thienylenevinylene):Fullerene Composite

Photoinduced electron transfer and competing processes have been studied in composites of an oligo(thienylenevinylene) (OTV), comprised of ten dibuthoxyl‐thiophene units separated by vinylene units, and a C₆₀ derivative, [6,6]‐phenyl‐C₆₁ butyric acid methyl ester (PCBM), by using femtosecond transient absorption spectroscopy and sub‐nanosecond transient photoconductivity. We find that in OTV:PCBM the photoexcitations decay primarily via intrachain relaxation rather than photoinduced electron transfer from OTV to PCBM. The electron‐transfer process requires ca. 14 ps; larger by more than two orders of magnitude than the required time observed in conjugated‐polymer:C₆₀ composites, and also larger than the 0.6 ps singlet‐state lifetime in OTV. These observations indicate that the quantum efficiency for photoinduced electron transfer in OTV:PCBM is less than 5 %.     

Long lived photoexcitation dynamics in π-conjugated polymer and fullerene blended films

We used continuous wave photoinduced absorption (PIA) spectroscopy to investigate long lived polarons in blend of [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) and regio-regular poly (3-hexylthiophene) (RR-P3HT), and in blend of PCBM and 2-methoxy-5-(2-ethylhexyloxy) poly(para-phenylenevinylene) (MEH-PPV). In millisecond time regime, delocalized polarons (DP) and localized polarons (LP) in RR-P3HT/PCBM as well as polarons in MEH-PPV/PCBM all exhibit dispersive bimolecular recombination process which was limited by the trap states, with the average lifetimes of those polarons inverse proportional to the square root of pump intensity (I). The recombination in RR-P3HT/PCBM was weak temperature dependence with small thermal activation energy, Δ for DPs and LPs of 25 meV and 13 meV, respectively; in contrast, Δ for polarons in amorphous MEH-PPV/PCBM was ～160 meV. Furthermore, we proved that the values of Δ for both of LP and DP increase, as well as the relatively intensity ratio of DP and LP decreases, in an intentionally degraded RR-P3HT/PCBM film. Overall, it is demonstrated that steady state photomodulation technique with thermal-activated-recombination analysis can be applied to evaluate polymer (dis)order in bulk heterojunction films.     

Correlation between ultrafast transient photomodulation spectroscopy and organic photovoltaic solar cell efficiency based on RR-P3HT/PCBM blends

Ultrafast transient spectroscopy was applied to various films of regio-regular polythiophene (RR-P3HT, donor-D) and C₆₀ derivative (PCBM, acceptor-A) blends, in conjunction with organic photovoltaic (OPV) solar cell fabrication and evaluation based on the same blends, for investigating the existence of a correlation between the device efficiency and the transient photophysics characteristics. For our transient spectroscopy measurements we used the ps pump–probe transient photomodulation (PM) technique having a unique probe spectral range in the mid-IR (0.25–1.05 eV). We found that the transient PM spectra contain photoinduced absorption bands of excitons in the donor polymer, charge transfer excitons (CTE) at the D–A interfaces, and free polarons. We compared the relative density of photogenerated CTE in D–A blends having various D–A weight ratio with the photocurrent density of fabricated solar cells based on the same blends. We found that the dissociation of CTE into free charges correlates well with the optoelectronic measurements of the corresponding solar cell. The more efficient CTE dissociation occurs in films having the optimum D–A weight ratio (which is 1.2:1 for the P3HT/PCBM system) that shows the highest OPV power conversion efficiency; this is due to the lowest CTE binding energy for this blend that results from the most suitable D- and A- grain sizes. We also show that the exciton lifetime is the shortest for the optimum blend, and this helps boosting the device efficiency by reducing energy loss.     

Dynamics of photogenerated solitons in trans-polyacetylene

Time-resolved absorption using pole wavelengths of 2.5-5.5 μm has been used to study photoexcitations in the quasi-one-dimensional semiconductor, trans-polyacetylene. Present apparatus has 0.5-ps resolution, which is sufficient to observe the decay of the photogenerated charged soliton pairs which form after intrachain excitation of electron-hole pairs. The tunability of the apparatus permits the unambiguous identification of solitons and observation of spectral relaxation during their formation     

Optical,Electrical, and Magnetic Studies of Organic Solar Cells Based on Low Bandgap Copolymer with Spin ½ Radical Additives

The charge photogeneration and recombination processes in organic photovoltaic solar cells based on blend of the low bandgap copolymer, PTB7 (fluorinated poly‐thienothiophene‐benzodithiophene) with C60‐PCBM using optical, electrical, and magnetic measurements in thin films and devices is studied. A variety of steady state optical and magneto‐optical techniques were employed, such as photoinduced absorption (PA), magneto‐PA, doping‐induced absorption, and PA‐detected magnetic resonance (PADMR); as well as picosecond time‐resolved PA. The charge polarons and triplet exciton dynamics in films of pristine PTB7, PTB7/fullerene donor–acceptor (D–A) blend is followed. It is found that a major loss mechanism that limits the power conversion efficiency (PCE) of PTB7‐based solar cell devices is the “back reaction” that leads to triplet exciton formation in the polymer donor from the photogenerated charge‐transfer excitons at the D–A interfaces. A method of suppressing this “back reaction” by adding spin½ radicals Galvinoxyl to the D–A blend is presented; this enhances the cell PCE by ≈30%. The same method is not effective for cells based on PTB7/C70‐PCBM blend, where high PCE is reached even without Galvinoxyl radical additives.     

Charge Transfer Excitons in Polymer/Fullerene Blends: The Role of Morphology and Polymer Chain Conformation

Here, it is shown how carrier recombination through charge transfer excitons between conjugated polymers and fullerene molecules is mainly controlled by the intrachain conformation of the polymer, and to a limited extent by the mesoscopic morphology of the blend. This experimental result is obtained by combining near‐infrared photoluminescence spectroscopy and transmission electron microscopy, which are sensitive to charge transfer exciton emission and morphology, respectively. The photoluminescence intensity of the charge transfer exciton is correlated to the degree of intrachain order of the polymer, highlighting an important aspect for understanding and limiting carrier recombination in organic photovoltaics.     

Hybrid Solar Cells from Regioregular Polythiophene and ZnO Nanoparticles

Blends of nanocrystalline zinc oxide nanoparticles (nc‐ZnO) and regioregular poly(3‐hexylthiophene) (P3HT) processed from solution have been used to construct hybrid polymer–metal oxide bulk‐heterojunction solar cells. Thermal annealing of the spin‐cast films significantly improves the solar‐energy conversion efficiency of these hybrid solar cells to ～ 0.9 %. Photoluminescence and photoinduced absorption spectroscopy demonstrate that charge‐carrier generation is not quantitative, because a fraction of P3HT appears not to be in contact with or in close proximity to ZnO. The coarse morphology of the films, also identified by tapping‐mode atomic force microscopy, likely limits the device performance.     

The Effect of Ageing on Exciton Dynamics,Charge Separation,and Recombination in P3HT/PCBM Photovoltaic Blends

A study of how light‐induced degradation influences the fundamental photophysical processes in the active layer of poly(3‐hexylthiophene)/[6,6]‐phenyl C₆₁‐butyric acid methyl ester (P3HT/PCBM) solar cells is presented. Non‐encapsulated samples are systematically aged by exposure to AM 1.5 illumination in the presence of dry air for different periods of time. The extent of degradation is quantified by the relative loss in the absorption maximum of the P3HT, which is varied in the range 0% to 20%. For degraded samples an increasing loss in the number of excitons within the P3HT domains is observed with longer ageing periods. This loss occurs rapidly, within the first 15 ps after photoexcitation. A more pronounced decrease in the population of polarons than excitons is observed, which also occurs on a timescale of a few picoseconds. These observations, complemented by a quantitative analysis of the polaron and exciton population dynamics, unravel two primary loss mechanisms for the performances of aged P3HT/PCBM solar cells. One is an initial ultrafast decrease in the polaron generation, apparently not related to the exciton diffusion to the polymer/fullerene interface; the second, less significant, is a loss in the exciton population within the photoexcited P3HT domains. The steady‐state photoinduced absorption spectra of degraded samples exhibits the appearance of a signal ascribed to triplet excitons, which is absent for non‐degraded samples. This latter observation is interpreted considering the formation of degraded sites where intersystem crossing and triplet exciton formation is more effective. The photovoltaic characteristics of same blends are also studied and discussed by comparing the decrease in the overall power conversion efficiency of solar cells.     

Amplified Spontaneous Emission of Poly(ladder‐type phenylene)s – The Influence of Photophysical Properties on ASE Thresholds

Amplified spontaneous emission (ASE) of a series of blue‐emitting poly(ladder‐type phenylene)s (LPPP)s has been studied in thin film polymer waveguide structures. The chemically well‐defined step‐ladder polymers consist of an increasing number of bridged phenylene rings per monomer unit starting from fully arylated poly(ladder‐type indenofluorene) up to poly(ladder‐type pentaphenylene). The ASE characteristics of the polymers including the onset threshold values for ASE, the gain and loss coefficients as well as the photoluminescence (PL) properties, i.e., the solid state fluorescence lifetimes, decay kinetics and solid state quantum efficiencies have been studied by time‐resolved PL spectroscopy. A fully arylated polyfluorene has been synthesized and its photophysical properties were compared to the step‐ladder polymers. Steady‐state photoinduced absorption and ultrafast transient absorption spectroscopy have been used to study excited state absorption of singlet and triplet states and polarons present in the solid state. The results demonstrate a minimum regarding the onset threshold value of ASE for a fully arylated poly(ladder‐type indenofluorene) and a successive increase of the ASE threshold for the step‐ladder polymers with more bridged phenylene rings. In particular, carbazole‐containing step‐ladder LPPPs exhibit significantly increased ASE threshold values as compared to their carbazole‐free analogues due to a pronounced overlap of stimulated emission (SE) and photoinduced absorption (PA).     

Soluble Phenanthrenyl‐Imidazole‐Presenting Regioregular Poly(3‐octylthiophene) Copolymers Having Tunable Bandgaps for Solar Cell Applications

We have used Grignard metathesis polymerization to successfully synthesize a series of regioregular polythiophene copolymers that contain electron‐withdrawing and conjugated phenanthrenyl‐imidazole moieties as side chains. The introduction of the phenanthrenyl‐imidazole moieties onto the side chains of the regioregular polythiophenes increased their conjugation lengths and thermal stabilities and altered their bandgap structures. The bandgap energies, determined from the onset of optical absorption, could be tuned from 1.89 eV to 1.77 eV by controlling the number of phenanthrenyl‐imidazole moieties in the copolymers. Moreover, the observed quenching in the photoluminescence of these copolymers increases with the number of phenanthrenyl‐imidazole moieties in the copolymers, owing to the fast deactivation of the excited state by the electron‐transfer reaction. Both the lowered bandgap and fast charge transfer contribute to the much higher external quantum efficiency of the poly(3‐octylthiophene)‐side‐chain‐tethered phenanthrenyl‐imidazole than that of pure poly(3‐octylthiophene), leading to much higher short circuit current density. In particular, the short circuit current densities of the device containing the copolymer having 80 mol % phenanthrenyl‐imidazole, P82 , improved to 14.2 mA cm^–2 from 8.7 mA cm^–2 for the device of pure poly(3‐octylthiophene), P00 , an increase of 62 %. In addition, the maximum power conversion efficiency improves to 2.80 % for P82 from 1.22 % for P00 (pure P3OT ). Therefore, these results indicate that our polymers are promising polymer photovoltaic materials.     

Conjugated Block Copolymers as Model Systems to Examine Mechanisms of Charge Generation in Donor–Acceptor Materials

Fully conjugated donor–acceptor block copolymers are established as model systems to elucidate fundamental mechanisms of photocurrent generation in organic photovoltaics. Using analysis of steady‐state photoluminescence quenching, exciton dissociation to a charge transfer state within individual block copolymer chains is quantified. By making a small adjustment to the conjugated backbone, the electronic properties are altered enough to disrupt charge transfer almost entirely. Strong intermolecular coupling of the electron donor is introduced by synthesizing block copolymer nanoparticles. Transient absorption spectroscopy is used to monitor charge generation in block copolymer isolated chains and nanoparticles. While efficient charge transfer is observed in isolated chains, there is no indication of complete charge separation. In the nanoparticles, long‐lived polarons are observed as early as ≈15 ns. Thus, aggregation of electron donors can facilitate efficient charge generation.     

Spectroscopic Evaluation of Mixing and Crystallinity of Fullerenes in Bulk Heterojunctions

The microstructure of blend films of conjugated polymer and fullerene, especially the degree of mixing and crystallization, impacts the performance of organic photovoltaic devices considerably. Mixing and crystallization affect device performance in different ways. These phenomena are not easy to screen using traditional methods such as imaging. In this paper, the amorphous regiorandom poly(3‐hexylthiophene) is blended with the potentially crystalline fullerene [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester PCBM and the amorphous bis‐adduct. First, the degree of mixing of polymer: fullerene blends is evaluated using UV–Vis absorption, steady‐state and ultra‐fast photoluminescence spectroscopy. The blue‐shift of the polymer emission and absorption onset are used in combination with the saturation of the polymer emission decay time upon fullerene addition in order to infer the onset of aggregation of the blends. Second, the crystallinity of the fullerene is probed using variable angle spectroscopic ellipsometry (VASE), electroluminescence and photoluminescence spectroscopy. It is shown that the red‐shift of charge transfer emission in the case of PCBM based blends cannot be explained solely by a variation of optical dielectric constant as probed by VASE. A combination of optical spectroscopy techniques, therefore, allows to probe the degree of mixing and can also distinguish between aggregation and crystallization of fullerenes.     

Band‐Edge Exciton Fine Structure and Exciton Recombination Dynamics in Single Crystals of Layered Hybrid Perovskites

2D perovskite materials have recently reattracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. Here, the band‐edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)₂PbI₄ are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2 × 10³ cm s^?1 and an intrinsic lifetime of 185 ns. Biexciton emission is evidenced at room temperature, with a binding energy of ≈45 meV and a lifetime of 80 ps. At low temperature, exciton state splitting is observed, which is caused by the electron–hole exchange interaction. Transient photoluminescence resolves the low‐lying dark exciton state, with a bright/dark splitting energy estimated to be 10 meV. This work contributes to the understanding of the complex scenario of the elementary photoexcitations in 2D perovskites.