Gytis Juška

Two dimensional Langevin recombination in regioregular poly(3-hexylthiophene)

In this work, it is shown that recombination in regioregular poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (RRP3HT:PCBM) bulk-heterojunction solar cells is caused by the two dimensional (2D) Langevin recombination in the lamellar structures of RRP3HT, which are formed after annealing process. Due to 2D Langevin process, bimolecular recombination coefficient is reduced in comparison with three dimensional Langevin case, and bimolecular recombination coefficient depends on the density of charge carriers n1/2. Data obtained from the different experimental techniques (charge extraction with linearly increasing voltage, integral time of flight, double injection current transients and transient absorption spectroscopy) confirms 2D Langevin recombination in RR3PHT.

Photocarrier drift distance in organic solar cells and photodetectors

Light harvesting systems based upon disordered materials are not only widespread in nature, but are also increasingly prevalent in solar cells and photodetectors. Examples include organic semiconductors, which typically possess low charge carrier mobilities and Langevin-type recombination dynamics – both of which negatively impact the device performance. It is accepted wisdom that the “drift distance” (i.e., the distance a photocarrier drifts before recombination) is defined by the mobility-lifetime product in solar cells. We demonstrate that this traditional figure of merit is inadequate for describing the charge transport physics of organic light harvesting systems. It is experimentally shown that the onset of the photocarrier recombination is determined by the electrode charge and we propose the mobility-recombination coefficient product as an alternative figure of merit. The implications of these findings are relevant to a wide range of light harvesting systems and will necessitate a rethink of the critical parameters of charge transport.

Double injection as a technique to study charge carrier transport and recombination in bulk-heterojunction solar cells

Ambipolar charge carrier mobility and recombination in bulk-heterojunction solar cells based on the mixture of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (PCBM) has been studied using injection current transients. The experimental results demonstrate double injection with bimolecular recombination limiting the injection current. We found that charge carrier bimolecular recombination is significantly reduced compared to Langevin recombination. We have measured the temperature and electric field dependence of the reduced bimolecular recombination coefficient and the results suggest that the electron and hole pathways are different and the recombination is controlled by the probability of the carriers to meet at the polymer/PCBM interface.

Avalanche multiplication phenomenon in amorphous semiconductors : Amorphous selenium versus hydrogenated amorphous silicon

Although the effect of the impact ionization and the consequent avalanche multiplication in amorphous selenium (a-Se) was established long ago and has led to the development and commercialization of ultrasensitive video tubes, the underlying physics of these phenomena in amorphous semiconductors has not yet been fully understood. In particular, it is puzzling why this effect has been evidenced at practical electric fields only in a-Se among all amorphous materials. For instance, impact ionization seems much more feasible in hydrogenated amorphous silicon (a-Si:H) since the charge carrier mobility in a-Si:H is much higher than that in a-Se and also the amount of energy needed for ionization of secondary carriers in a-Si:H is lower than that in a-Se. Using the description of the avalanche effect based on the lucky-drift model recently developed for amorphous semiconductors we show how this intriguing question can be answered. It is the higher phonon energy in a-Si:H than that in a-Se, which is responsible f...

The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells

A typical signature of charge extraction in disordered organic systems is dispersive transport, which implies a distribution of charge carrier mobilities that negatively impact on device performance. Dispersive transport has been commonly understood to originate from a time-dependent mobility of hot charge carriers that reduces as excess energy is lost during relaxation in the density of states. In contrast, we show via photon energy, electric field and film thickness independence of carrier mobilities that the dispersive photocurrent in organic solar cells originates not from the loss of excess energy during hot carrier thermalization, but rather from the loss of carrier density to trap states during transport. Our results emphasize that further efforts should be directed to minimizing the density of trap states, rather than controlling energetic relaxation of hot carriers within the density of states.

Double-injection current transients as a way of measuring transport in insulating organic films

We propose a double-injection current transient technique for the study of charge-carrier transport in thin insulating films of low-mobility materials with reduced carrier bimolecular recombination compared to the Langevin type. This experimentally simple technique, allows us to estimate the sum of the faster carrier and the slower carrier mobility’s (μf+μs) and the slower carrier mobility (μs). Furthermore, in thin films when the RC current overlaps the injection current transients we propose to estimate these transport parameters using the extracted charge as a function of injection pulse duration. The method is applied on bulk-heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene. We have experimentally verified the technique by measuring the charge carrier mobility’s and compared them with results obtained using standard time-of-flight and carrier extraction using linearly increasing voltage techniques.

Electric field redistribution and electroluminescence response time in polymeric light-emitting diodes

The electric field redistribution due to injected and trapped charge carriers in Langmuir–Blodgett (LB) films of poly(3-hexylthiophene) (P3HT) sandwiched between indium tin oxide and aluminum (Al) electrodes as function of applied voltage has been studied using charge collection measurements by the time-of-flight technique. For μτE<d (the drift distance shorter than the interelectrode distance) the amount of the collected photocharge is a function of electric field near the Al electrode and has been used to probe the time evolution of it. The response time for the field to redistribute inside the P3HT LB film was found to be of the order of 5–200 μs, in good agreement with the delay time observed in time-resolved electroluminescence measurements in light-emitting diodes (LED) of similar LB films. We suggest a model for the response times in organic LEDs based on these results.The electric field redistribution due to injected and trapped charge carriers in Langmuir–Blodgett (LB) films of poly(3-hexylthiophene) (P3HT) sandwiched between indium tin oxide and aluminum (Al) electrodes as function of applied voltage has been studied using charge collection measurements by the time-of-flight technique. For μτE<d (the drift distance shorter than the interelectrode distance) the amount of the collected photocharge is a function of electric field near the Al electrode and has been used to probe the time evolution of it. The response time for the field to redistribute inside the P3HT LB film was found to be of the order of 5–200 μs, in good agreement with the delay time observed in time-resolved electroluminescence measurements in light-emitting diodes (LED) of similar LB films. We suggest a model for the response times in organic LEDs based on these results.

Electron drift mobility in a-Si : H under extremely high electric field

Abstract Electron drift mobility in a -Si : H is studied in a broad range of temperatures (140–295 K) as a function of electric field up to its extremely high values ( F ≈4 × 10 5 V cm -1 ) on a -Si : H samples with the thickness ( d ) ranging from 1.7 to 18 μm. The observed clear increase of electron drift mobility (independent of sample thickness) and accompanying decrease of mobility activation energy for electric field F ⩾ 1 × 10 5 V cm -1 cannot be explained only on the basis of the model of dispersive transport in this region of electric fields.

Effect of 2-D Delocalization on Charge Transport and Recombination in Bulk-Heterojunction Solar Cells

Charge-carrier transport and recombination in thermally treated and untreated films of poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-l-phenyl-[6,6]-methanofullerene (PCBM) bulk-heterojunction solar cells (BHSCs) have been measured using various electrooptical techniques. The formation of lamellar structure in P3HT has a large effect on the efficiency, carrier transport, and recombination of photogenerated charge carriers. Treated P3HT/PCBM solar cells show greatly reduced carrier recombination compared to what is typically expected in low-mobility materials and electric-field-independent carrier generation. In untreated films, the recombination is close to Langevin-type with electric-field-dependent quantum efficiency, consistent with the typically observed Onsager-type generation. Furthermore, we observe an increased effective capacitance in treated films, consistent with increased charge screening. The importance of the interface between the lamellar structured P3HT and PCBM is evident from optical spectroscopies showing that 2-D polarons are directly generated using sub-gap excitation. We conclude that the formation of lamellar structures in the polymer donor, and subsequent, derealization of the charges is favorable for making efficient BHSCs.

Charge carrier transport and recombination in bulk-heterojunction solar-cells

We have measured charge carrier transport and recombination in bulk-heterojunction solar-cells. Time-of-flight, carrier extraction by linearly increasing voltage and double injection techniques which are complementary to each other have been used to study the solar-cells of different thicknesses and conductivities. We show the importance of carrier mobility and bimolecular recombination coefficient for testing the suitability of materials in bulk-heterojunction solar-cells. The reduced bimolecular recombination coefficient at zero electric field and its electric field dependence are measured directly. The bimolecular recombination coefficient and Langevin-type coefficient values β/βL ≈ 10-4 are in the good agreement when measured with presented techniques.