Photo-induced Charge Carrier Extraction Research Articles

Dual-Functional Enantiomeric Compounds as Hole-Transporting Materials and Interfacial Layers in Perovskite Solar Cells.

In this paper, we describe the application of the enantiomeric compounds YLC-1-YLC-4, each featuring a bulky spiro[fluorene-9,9'-phenanthren]-10'-one moiety, as both hole-transporting materials (HTMs) and interfacial layers in both n-i-p and p-i-n perovskite solar cells (PSCs). These HTMs contain an enantiomeric mixture and a variety of core units linked to triarylamine donors to extend the degree of π-conjugation. The n-i-p PSCs incorporating YLC-1(a) exhibited a power conversion efficiency (PCE) of 19.15% under AM 1.5G conditions (100 mW cm-2); this value was comparable with that obtained using spiro-OMeTAD as the HTM (18.25%). We obtained efficient and stable p-i-n PSCs having the dopant-free structure indium tin oxide (ITO)/NiOx/interfacial layer (YLC)/perovskite/PC61BM/BCP/Ag. The presence of the spiro-based compounds YLC-1 and YLC-2 efficiently passivated the interfacial and grain boundary defects of the perovskite and enhanced the sizes of its grains, more so than did YLC-3 and YLC-4. These spiro-based YLC derivatives packed densely and functioned as Lewis bases to coordinate Pb and Ni ions in the perovskite and NiOx layers, respectively. Together, the effects of smaller grain boundaries and defect passivation of the perovskite enhanced the optoelectronic properties of the PSCs. The photoinduced charge carrier extraction in the linearly increasing voltage (photo-CELIV) curves of NiOx/YLC-1(a) showed the faster carrier transport 3.3 × 10-3 cm2 V-1 s-1, which improved the carrier mobility, supporting the notion of defect passivation of the perovskite. The best-performing NiOx/YLC-1(a) device provided a short-circuit current density (JSC) of 22.88 mA cm-2, an open-circuit voltage (VOC) of 1.10 V, and a fill factor (FF) of 80.93%, corresponding to an overall PCE of 20.37%. In addition, the PCEs of the NiOx/YLC-1(a) and NiOx/YLC-4(b) PSC devices underwent decays of only 98.1 and 97.0% of their original values after 41 days under an Ar atmosphere. Thus, these YLC derivatives passivated the NiOx surface and optimized the film quality of perovskites, thereby leading to superior PCEs of their respective PSCs.

Binary Additive–Induced Performance Improvement of PM7:PC71BM Organic Solar Cells with High Open‐Circuit Voltage and Enhanced Current Intensity

Adding a solvent additive into the active layer is a simple and effective strategy to improve active layer morphology and performance of bulk heterojunction organic solar cells (OSCs). Herein, poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl‐3‐chloro)thiophen‐2‐yl)‐benzo[1,2‐b:4,5‐b′]dithiophene))‐alt‐(5,5‐(1′,3′‐di‐2‐thienyl‐5′,7′‐bis(2‐ethylhexyl)benzo[1′,2′‐c:4′,5′‐c′]dithiophene‐4,8‐dione)] (PM7) is used as donor and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) as acceptor, and 1,8‐diiodooctane (DIO) and 1‐chloronthalene (CN) are utilized as solvent additives to prepare OSCs. The impact of binary additives on the photoelectric performance of OSCs is studied. The binary additives restrict charge recombination and improve carrier mobility effectively, which is proved by the transient photovoltage/transient photocurrent measurement and photoinduced charge carrier extraction by linearly increasing the voltage technology. With the synergetic effect of DIO and CN, the binary additive devices exhibit a high open‐circuit voltage of 0.96 V. Apart from the improved short‐circuit current density and fill factor, the power conversion efficiency reaches to 9.85%, much higher than that of the pristine device without additive (3.79%).

Charge transport and extraction of PTB7:PC71BM organic solar cells: effect of film thickness and thermal-annealing.

Charge carrier transport in the active layer and charge extraction at the electrode have significant impact on the performance of solar cells. In this study, the effect of active layer thickness and thermal-annealing treatment on the charge transport and extraction performance of PTB7:PC71BM organic solar cells was studied comprehensively. Thin films of active layer couldn't utilize enough sunlight, while thick films could bring about large bulk resistance and deteriorate carrier transport. There is a trade-off between active layer thickness and carrier transport. The optimized active layer thickness is about 100 nm for the PTB7:PC71BM bulk heterojunction organic solar cells. Thermal-annealing could improve the morphology of the active layer, and facilitate charge transport in the active layer and charge collection at the electrode. The improved carrier transport and extraction were verified by the transient photocurrent/transient photovoltage and photo-induced charge carrier extraction by linearly increasing voltage measurements. The optimal power conversion efficiency was obtained as 8.28% for the device with an active layer thickness of 100 nm and treated with 90 °C thermal-annealing.

Additive assisted morphological optimization of photoactive layer in polymer solar cells

Herein, we clearly demonstrate that the inclusion of the organic solid additive (2,3-dihydroxypyridine, DOH) into the poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) and phenyl-C61-butyric acid methyl ester (PCBM) (PDPP3T:PCBM) photoactive blend system alters the nanoscale morphology and enhances the photovoltaic (PV) performance of the inverted bulk heterojunction (BHJ) based polymer solar cells (PSCs). The influence of DOH additive casted PDPP3T: PCBM thin films on the optical, morphology and structural features is evaluated and correlated with PV characteristics. Topography images through atomic force microscopy reveal that the incorporation of DOH into PDPP3T: PCBM induces a finer nanoscale phase segregation between polymer and fullerene domains with fibrillar morphology. The PV performance of the DOH processed PDPP3T: PCBM devices is evaluated by current-voltage (J-V) characteristics and compared with pristine and 1, 8-diiodooctane (DIO) modified PDPP3T: PCBM devices. Interestingly, the incorporation of DOH (0.5 wt%) into PDPP3T: PCBM device witnessed the best PCE of 6.36%, which is significantly higher (69.1%) than that of the reference (3.76%). This significant enhancement in the performance of the device is mainly attributed to a dramatic increase in the short-circuit current density and fill-factor due to the improved bicontinuous interpenetrated phase separation and balanced charge transport. In addition, the measurements on photo-induced charge carrier extraction by linearly increasing voltage reveal that DOH incorporated devices exhibit higher mobility and charge carrier density as compared to those of pristine modified BHJ PSCs. The presence of vicinal functional groups in DOH contributes to the possible molecular level interactions with the blend components and accounts for the morphology and device performance enhancements.

Efficiency enhancement in perovskite solar cell utilizing solution-processable phthalocyanine hole transport layer with thermal annealing

We demonstrate efficiency enhancement in perovskite solar cells (PSCs) utilizing a free-dopant hole transporting material (HTM), non-peripherally substituted octapentyl phthalocyanine (C5PcH2) with thermal annealing. Particularly, by using thermal annealing approach, the external quantum efficiency at around 480 nm increase from 78 to 84%. Hence, the fill factor and short-circuit current density are markedly improved from 0.35 ± 0.02 to 0.55 ± 0.05 and from 18 ± 1 to 18.8 ± 0.3 mA cm-2, respectively. Finally, the best device is achieved with power conversion efficiencies of 12.2% by annealing at 130 °C for 10 min. The photoluminescence and photo-induced charge carrier extraction in linearly increasing voltages measurements indicate that the charge carrier mobility in C5PcH2 increases, and thereby the hole extraction and transportation from the perovskite layer to the Au anode as well the photovoltaic performance of PCS is improved by using thermal annealing processing.

Charge carrier dynamics in PDPP-F/PCBM heterojunction solar cells

The morphology dependent of migration and recombination kinetics of charge carriers in the polymer solar cell based on poly {2,7′-9,9-dioctylfluorene-alt-5-diethylhexyl-3,6-bis(5-bromothiophen-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione}(PDPP-F) was investigated with photo-induced charge carrier extraction by linearly increasing voltage technique. The recombination coefficient of charge carriers decreased and the mobility of charge carriers increased, after the mass ratio of [6,6]-phenyl-C61-butyric acid methyl ester(PCBM) increased from 3:2 to 2:1. Meanwhile, both of them were sensitive to the applied electric field and could be together responsible for the improvement of voltage performance of polymer solar cell.

Charge transport and its characterization using photo-CELIV in bulk heterojunction solar cells

This mini-review gives a simple overview of the workings of organic photovoltaic (OPV) devices, the way in which charge transfer occurs through the active layers, and then introduces how photo-induced charge carrier extraction by linearly increasing voltage (photo-CELIV) and time of flight (TOF) techniques can be employed to give comprehensive indications of charge carrier mobility, density and recombination in OPVs. It is shown how photo-CELIV and TOF characterizations, using extraction current transients, can give an understanding of degradation mechanisms through observation of the trapping of charge carriers and bimolecular recombination. Examples of deployment and the interpretation of the results are given. It is hoped that this brief introduction will serve as a stepping stone into more in-depth papers and books and encourage wider use of photo-CELIV and TOF technologies which can be employed with whole devices. (c) 2016 Society of Chemical Industry

The effect of polymer solar cell degradation on charge carrier dynamics in benzodithiophene-diketopyrrolopyrrole polymers

Degradation in polymer based organic solar cells still remains poorly understood despite prominent research efforts in recent years. In this work, the impact of photostability and documented morphological evolution of a couple of poly(benzodithiophene-diketopyrrolopyrrole) polymers in their blends with phenyl-C61-butyric acid methyl ester (PCBM) are investigated by a combination of charge extraction techniques. Photo-induced charge carrier extraction by linearly increasing voltage (photo-CELIV) and integral-time of flight (TOF) methods are employed to determine charge carrier mobility and recombination at various stages of photo-degradation in the absence of air. We report the progressive formation of deeper trap states as crucial outcome of photo-degradation within these blends as deduced from our measurements.

Investigating the Influence of Alkyl Chain Length in Poly(3-alkylthiophene)s Over the Thin Film Morphology by Optical and Electrical Characterization.

This paper studies the influence of alkyl-chain length in poly(3-alkylthiophene)s over the morphology of thin films and electrical parameters of the devices based on it. Regioregular poly(3-hexylthiophene) and poly(3-octylthiophene) were chosen as the semiconducting materials for the study. The morphological variations were studied by absorption spectroscopy, photoluminescence spectroscopy and X-ray diffraction study. The absorption and photoluminescence showed decreased coplanarity of main chain in poly(3-octylthiophene) over poly(3-hexylthiophene) and which was later confirmed using X-ray diffraction studies which clearly showed increased interchain spacing in case of poly(3-octylthiophene). The schottky diodes fabricated using these materials showed decreased mobility in poly(3-octylthiophene) based diodes as measured by space-charge limiting current method and photo-induced charge carrier extraction by linearly increasing voltage technique. Moreover, we observed a negative field dependence of mobility at room temperature in both the devices and attributed this to the presence of dominant positional disorder in poly(3-alkylthiophene)s. Furthermore, the photocurrent dependence on electric field too showed inferior mobility of poly(3-octylthiophene) based diodes.

Low-Temperature Solution-Processed Kesterite Solar Cell Based on in Situ Deposition of Ultrathin Absorber Layer.

The production of high-performance, solution-processed kesterite Cu2ZnSn(Sx,Se1-x)4 (CZTSSe) solar cells typically relies on high-temperature crystallization processes in chalcogen-containing atmosphere and often on the use of environmentally harmful solvents, which could hinder the widespread adoption of this technology. We report a method for processing selenium free Cu2ZnSnS4 (CZTS) solar cells based on a short annealing step at temperatures as low as 350 °C using a molecular based precursor, fully avoiding highly toxic solvents and high-temperature sulfurization. We show that a simple device structure consisting of ITO/CZTS/CdS/Al and comprising an extremely thin absorber layer (∼110 nm) achieves a current density of 8.6 mA/cm(2). Over the course of 400 days under ambient conditions encapsulated devices retain close to 100% of their original efficiency. Using impedance spectroscopy and photoinduced charge carrier extraction by linearly increasing voltage (photo-CELIV), we demonstrate that reduced charge carrier mobility is one limiting parameter of low-temperature CZTS photovoltaics. These results may inform less energy demanding strategies for the production of CZTS optoelectronic layers compatible with large-scale processing techniques.

Photophysics of Molecular‐Weight‐Induced Losses in Indacenodithienothiophene‐Based Solar Cells

The photovoltaic performance and optoelectronic properties of a donor–acceptor copolymer are reported based on indacenodithienothiophene (IDTT) and 2,3‐bis(3‐(octyloxy)phenyl)quinoxaline moieties (PIDTTQ) as a function of the number‐average molecular weight (Mn). Current–voltage measurements and photoinduced charge carrier extraction by linear increasing voltage (photo‐CELIV) reveal improved charge generation and charge transport properties in these high band gap systems with increasing Mn, while polymers with low molecular weight suffer from diminished charge carrier extraction because of low mobility–lifetime (μτ) product. By combining Fourier‐transform photocurrent spectroscopy (FTPS) with electroluminscence spectroscopy, it is demonstrate that increasing Mn reduces the nonradiative recombination losses. Solar cells based on PIDTTQ with Mn = 58 kD feature a power conversion efficiency of 6.0% and a charge carrier mobility of 2.1 × 10−4 cm2 V−1 s−1 when doctor bladed in air, without the need for thermal treatment. This study exhibits the strong correlations between polymer fractionation and its optoelectronics characteristics, which informs the polymer design rules toward highly efficient organic solar cells.

Direct determination of trap density function based on the photoinduced charge carrier extraction technique

Photoinduced charge carrier extraction experiments using a linearly increasing voltage (photo-CELIV) are reported for an organic thin film device from 1.8 to 150K. This device is composed of an active layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Photo-CELIV data for a zigzag shape voltage sweep provide critical evidence that the CELIV signal reflects the evacuation of charged carriers captured by traps under a high electric field. The data are analyzed using the Poole–Frenkel model. The trap density as a function of escape energy is obtained as ρ(ε)=Dexp{-(ε-ε0)2/σ2}, with D=1.0×1024statesm−3eV−1, ε0=0.087eV, and σ=0.029eV. The carrier drift mobility is estimated to be 2.3×10−6cm2V−1s−1 at 1.8K. As inferred from the light intensity dependence of the photo-CELIV data, geminate pairs are proposed as the origin of traps. This study demonstrates that carrier evacuation from a Coulomb potential effectively plays an important role in the electrical conduction of organic thin films.

Determination of Trap Density Function Based on Measurements of Photoinduced Charge Carrier Extraction Using Linearly Increasing Voltage at Low Temperatures

Experiments on photoinduced charge carrier extraction using linearly increasing voltage (photo-CELIV) are reported for an organic thin-film device having an active layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) below 90 K down to 1.6 K. CELIV peaks for different rates of voltage increase appear at almost the same electric fields at a temperature below 30 K. From this result, it is concluded that the CELIV signal does not reflect mobility at low temperatures. An analytical model that relates low-temperature CELIV signals to the trap density function is proposed.

Charge Transporting Properties and Output Characteristics in Polythiophene:Fullerene Derivative Solar Cells

The carrier mobility (µ) and lifetime (τ) of polymer solar cells based on the active layer with various compositions of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been investigated through photoinduced charge carrier extraction by linearly increasing voltage measurement. Increase in the PCBM content resulted in the increase in the mobility up to one order of magnitude and the highest performance was attained at the composition of P3HT:PCBM (3:2), which corresponds to the bending point of mobility and lifetime. Furthermore, the mobility-lifetime product µτ was almost constant with various weight ratios of P3HT and PCBM. This result implies that the composition does not affect the charge correction efficiency before recombination. A semiconducting thin-film optics simulator was applied for prediction of the photovoltaic performance. Theoretical and experimental cell performance results were in good agreement with each other.

Trap distribution and the impact of oxygen-induced traps on the charge transport in poly(3-hexylthiophene)

The trap distribution in the conjugated polymer poly(3-hexylthiophene) was investigated by fractional thermally stimulated current measurements. Two defect states with activation energies of about 50 and 105 meV and Gaussian energy distributions were revealed. The first is assigned to the tail of the intrinsic density of states, whereas the concentration of the second trap is directly related to oxygen exposure. The impact of the oxygen induced traps on the charge transport was examined by performing photo-induced charge carrier extraction by linearly increasing voltage measurements that exhibited a strong decrease in the mobility with air exposure time.

Charge carrier mobility and lifetime versus composition of conjugated polymer/fullerene bulk-heterojunction solar cells

Abstract Charge carrier mobility (μ), recombination kinetics, and lifetime (τ) have been investigated with the photo-induced charge carrier extraction by linearly increasing voltage technique (photo-CELIV) in blends of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61(PCBM). Different MDMO-PPV/PCBM ratios have been studied showing that increasing the PCBM content induces an increase of the photo-CELIV mobility up to two orders of magnitude. Simultaneously, the lifetime of the charge carriers decreases in such a way that the product μ × τ appears almost constant independently of the blend composition. Recombination kinetics close to the Langevin one is observed for all PCBM concentrations studied.

Time-dependent mobility and recombination of the photoinduced charge carriers in conjugated polymer/fullerene bulk heterojunction solar cells

Time-dependent mobility and recombination in the blend of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-${\mathrm{C}}_{61}$(PCBM) is studied simultaneously using the photoinduced charge carrier extraction by linearly increasing voltage technique. The charge carriers are photogenerated by a strongly absorbed, 3 ns laser flash, and extracted by the application of a reverse bias voltage pulse after an adjustable delay time $({t}_{\mathrm{del}})$. It is found that the mobility of the extracted charge carriers decreases with increasing delay time, especially shortly after photoexcitation. The time-dependent mobility $\ensuremath{\mu}(t)$ is attributed to the energy relaxation of the charge carriers towards the tail states of the density of states distribution. A model based on a dispersive bimolecular recombination is formulated, which properly describes the concentration decay of the extracted charge carriers at all measured temperatures and concentrations. The calculated bimolecular recombination coefficient $\ensuremath{\beta}(t)$ is also found to be time-dependent exhibiting a power law dependence as $\ensuremath{\beta}(t)={\ensuremath{\beta}}_{0}{t}^{\ensuremath{-}(1\ensuremath{-}\ensuremath{\gamma})}$ with increasing slope $(1\ensuremath{-}\ensuremath{\gamma})$ with decreasing temperatures. The temperature dependence study reveals that both the mobility and recombination of the photogenerated charge carriers are thermally activated processes with activation energy in the range of 0.1 eV. Finally, the direct comparison of $\ensuremath{\mu}(t)$ and $\ensuremath{\beta}(t)$ shows that the recombination of the long-lived charge carriers is controlled by diffusion.

Charge transport and recombination in bulk heterojunction solar cells studied by the photoinduced charge extraction in linearly increasing voltage technique

Charge carrier mobility and recombination in a bulk heterojunction solar cell based on the mixture of poly[2-methoxy-5-(3,7-dimethyloctyloxy)-phenylene vinylene] (MDMO-PPV) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)-C61 (PCBM) has been studied using the novel technique of photoinduced charge carrier extraction in a linearly increasing voltage (Photo-CELIV). In this technique, charge carriers are photogenerated by a short laser flash, and extracted under a reverse bias voltage ramp after an adjustable delay time (tdel). The Photo-CELIV mobility at room temperature is found to be μ=2×10−4cm2V−1s−1, which is almost independent on charge carrier density, but slightly dependent on tdel. Furthermore, determination of charge carrier lifetime and demonstration of an electric field dependent mobility is presented.