Oskar J. Sandberg

Effect of a large hole reservoir on the charge transport in TiO2/organic hybrid devices.

We have fabricated hybrid devices in the form of indium tin oxide/titanium dioxide/poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester/copper (ITO/TiO(2)/P3HT:PCBM/Cu) to clarify the impact of the TiO(2)/P3HT:PCBM interface on the charge transport using the charge extraction by linearly increasing voltage (CELIV) technique. We found that a large equilibrium charge reservoir is accumulated at negative offsets at the TiO(2)/P3HT:PCBM interface leading to space charge limited extraction current (SCLC) transients. We show analytically the SCLC transient response and compare the experimental data to calculated SCLC at a linearly increasing voltage. The theoretical calculations indicate that the large charge reservoir at negative offset voltages is due to thermally generated charges combined with poor hole extraction at the ITO/TiO(2) contact, due to the hole blocking character of TiO(2).

Voltage dependent displacement current as a tool to measure the vacuum level shift caused by self-assembled monolayers on aluminum oxide

We present charge extraction by a linearly increasing voltage measurements on diodes based on an n-channel naphthalenetetracarboxylic acid diimide semiconductor and an aluminum oxide blocking layer. Results show a large displacement current (roughly 15 times that expected from the geometrical capacitance), which we associate with trap filling in the oxide. The trap density is calculated to be on the order of 1019 cm−3, in agreement with preceding work. We present a way of using the displacement current as a tool for probing the vacuum level shift caused by modifying the oxide surface with self-assembled monolayers in operating devices.

On the validity of MIS-CELIV for mobility determination in organic thin-film devices

The charge extraction (of injected carriers) by linearly increasing voltage in metal-insulator-semiconductor structures, or MIS-CELIV, is based on the theory of space-charge-limited currents. In this work, the validity of MIS-CELIV for mobility determination in organic thin-film devices has been critically examined and clarified by means of drift-diffusion simulations. It is found that depending on the applied transient voltage, the mobility might be overestimated by several orders of magnitude in the case of an ohmic injecting contact. The shortcomings of the MIS-CELIV theory can be traced back to the underlying assumption of a drift-dominated transport. However, the effect of diffusion can be taken into account by introducing a correction factor. In the case of non-ohmic injecting contacts, the extracted mobility becomes strongly dependent on device parameters, possibly leading to large deviations from the actual mobility.

Characterization of the dominating bulk recombination in bulk-heterojunction blends using photoinduced absorption

In this work we show how to clarify the dominating bulk recombination in organic solar cells by using photoinduced absorption. We show how to use the intensity and frequency dependence of the in-phase and quadrature signals to obtain the effective reaction order. For trap-assisted recombination, we can show using a multiple trapping and retrapping model with an exponential tail-state distribution that a temperature dependent reaction order is obtained which allows for determination of the characteristic energy of the exponential distribution of trap-states. In the model system pBTTT:PC60BM, we show that trap-assisted recombination is the dominating bulk recombination in 1:1 blends with a characteristic energy of the exponential trap distribution Ech=44±5 meV. The 1:4 blend, on the other hand, shows temperature independent behavior in good agreement with a dominating 2D Langevin bulk recombination.

Determination of Surface Recombination Velocities at Contacts in Organic Semiconductor Devices Using Injected Carrier Reservoirs

A method to determine surface recombination velocities at collecting contacts in interface-limited organic semiconductor devices, based on the extraction of injected carrier reservoirs in a single-carrier sandwich-type structure, is presented. The analytical framework is derived and verified with drift-diffusion simulations. The method is demonstrated on solution-processed organic semiconductor devices with hole-blocking TiO_{2}/organic and SiO_{2}/organic interfaces, relevant for solar cell and transistor applications, respectively.

Method for characterizing bulk recombination using photoinduced absorption

The influence of reaction order and trap-assisted recombination on continuous-wave photoinduced absorption measurements is clarified through analytical calculations and numerical simulations. The results reveal the characteristic influence of different trap distributions and enable distinguishing between shallow exponential and Gaussian distributions and systems dominated by direct recombination by analyzing the temperature dependence of the in-phase and quadrature signals. The identifying features are the intensity dependence of the in-phase at high intensity, P A I ∝ I γ HI, and the frequency dependence of the quadrature at low frequency, P A Q ∝ ω γ LF. For direct recombination, γHI and γLF are temperature independent, and for an exponential distribution, they depend on the characteristic energy Ech as γ HI = 1 / ( 1 + E ch / k T ) and γ LF = k T / E ch, while a Gaussian distribution shows γHI and γLF as functions of I and ω, respectively.

Quantifying loss-mechanisms related to charge carrier collection in thin-film solar cells (Conference Presentation)

Processes taking place at contacts are of particular importance in organic and perovskite solar cells where selective contacts that are able to efficiently collect majority carriers, simultaneously blocking minority carriers are desired. The surface recombination velocity S_R, describing the quality of the contact interface, is a key parameter in obtaining an increased understanding of the kinetics taking place at contacts in thin-film devices [1]. We have extended the analytical framework of the charge extraction by linearly increasing voltage (CELIV) theory taking the effect of built-in voltage, diffusion and band-bending into account [2] and show how we can experimentally quantify loss mechanisms in charge collection [3-4]. We have derived analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish various (loss) mechanisms for contact related effects in thin film solar cells [2-4]. References [1] O. Sandberg, M. Nyman, R. Osterbacka, Physical Review Applied 1, 024003 (2014) [2] O. Sandberg, M. Nyman, R. Osterbacka, Organic Electronics 15, 3413-3420 (2015) [3] A. Sundqvist, M. Nyman, O. Sandberg, S. Sanden, J.-H. Smatt, and R. Osterbacka, Advanced Energy Materials, 1502265 (2016) [4] O.J. Sandberg, et. al, Physical Review Letters, 118, 076601 (2017).

Doping-induced carrier profiles in organic semiconductors determined from capacitive extraction-current transients

A method to determine the doping induced charge carrier profiles in lightly and moderately doped organic semiconductor thin films is presented. The theory of the method of Charge Extraction by a Linearly Increasing Voltage technique in the doping-induced capacitive regime (doping-CELIV) is extended to the case with non-uniform doping profiles and the analytical description is verified with drift-diffusion simulations. The method is demonstrated experimentally on evaporated organic small-molecule thin films with a controlled doping profile, and solution-processed thin films where the non-uniform doping profile is unintentional, probably induced during the deposition process, and a priori unknown. Furthermore, the method offers a possibility of directly probing charge-density distributions at interfaces between highly doped and lightly doped or undoped layers.

Estimating the surface recombination velocity at contacts in organic devices using charge extraction by linearly increasing voltage (Conference Presentation)

The kinetics at contacts plays a crucial role in sandwich-type thin-film devices based on organic semiconductors. This is of particular importance in organic and perovskite solar cells where selective contacts that are able to efficiently collect majority carriers, simultaneously blocking minority carriers, are desired. Despite the vast progress made, a comprehensive understanding, needed for developing new electrode materials to improve and optimize device performance is still lacking. A key parameter for obtaining information about processes taking place at the contacts is the effective surface recombination velocity.[1] However, means to quantitatively measure surface recombination velocities at contact interfaces in sandwich-type thin-film devices based on organic semiconductors are lacking. The Charge Extraction by a Linearly Increasing Voltage (CELIV) technique is one of the most common methods to measure the charge transport properties in organic semiconductor devices. In this work, we show how CELIV can be used to determine surface recombination velocities at selective and/or blocking contacts in thin-film devices. The analytical framework behind the method is presented, and confirmed by numerical drift-diffusion simulations. We furthermore demonstrate the method on organic semiconductor devices, employing TiO2 and SiO2 as cathode buffer layers. The method allows for an increased understanding of contact properties in sandwich-type thin-film devices based on organic semiconductors. [1] O. J. Sandberg, A. Sundqvist, M. Nyman, and R. Osterbacka, Phys. Rev. Appl. 5, 044005 (2016).

Effect of two-dimensional-Langevin and trap-assisted recombination on the device performance of organic solar cells

Abstract. Using drift-diffusion simulations, we have clarified the effect of two-dimensional lamellar ordering on the device performance and, in particular, the open circuit voltage in donor–acceptor type organic solar cells. The simulations are performed both in systems where direct (band-to-band) recombination dominates and in systems where trap-assisted recombination dominates. Results show that lamellar ordering reduces both the amount of direct and trap-assisted recombination, which is beneficial for device performance. The effect is particularly prominent for small lamellar thicknesses (∼1  nm). It is furthermore shown that in the case of s-shaped current–voltage characteristics due to electrostatic injection barriers the s-shape becomes less prominent for thinner lamellar thicknesses.