Theoretical Perspective on Transient Photovoltage and Charge Extraction Techniques

Transient photovoltage (TPV) is a technique frequently used to determine charge carrier lifetimes in thin-film solar cells such as organic, dye sensitized and perovskite solar cells. As this lifetime is often incident light intensity dependent, its relevance to understanding the intrinsic properties of a photoactive material system as a material or device figure of merit has been questioned. To extract complete information on recombination dynamics, the TPV measurements are often performed in conjunction with charge extraction (CE) measurements, employed to determine the photo-generated charge carrier density and thereby the recombination rate constant and its order. In this communication, the underlying theory of TPV and CE is reviewed and expanded. Our theoretical findings are further solidified by numerical simulations and experiments on organic solar cells. We identify regimes of the open-circuit voltage within which accurate lifetimes and carrier densities can be determined with TPV and CE experiments. A wide range of steady-state light intensities is required in performing these experiments in order to identify their “working dynamic range” from which the recombination kinetics in thin-film solar cells can be determined

Trapping light with micro lenses in thin film organic photovoltaic cells.

We demonstrate a novel light trapping configuration based on an array of micro lenses in conjunction with a self aligned array of micro apertures located in a highly reflecting mirror. When locating the light trapping element, that displays strong directional asymmetric transmission, in front of thin film organic photovoltaic cells, an increase in cell absorption is obtained. By recycling reflected photons that otherwise would be lost, thinner films with more beneficial electrical properties can effectively be deployed. The light trapping element enhances the absorption rate of the solar cell and increases the photocurrent by as much as 25%. (C) 2008 Optical Society of Americ

Особенности работы с иностранными студентами с английским языком обучения на кафедре медицинской и биологической физики

ПРЕПОДАВАТЕЛЬСКИЙ СОСТАВ УЧЕБНОГО УЧРЕЖДЕНИЯОБРАЗОВАНИЕФИЗИКА /ОБУЧИНОСТРАННЫЕ СТУДЕНТЫАНГЛИЙСКИЙ ЯЗЫК /ОБУЧМЕДИЦИНСКАЯ ФИЗИКА /ОБУЧБИОЛОГИЧЕСКАЯ ФИЗИКА /ОБУ

Unravelling steady-state bulk recombination dynamics in thick efficient vacuum-deposited perovskite solar cells by transient methods

Accurately identifying and understanding the dominant charge carrier recombination mechanism in perovskite solar cells is of crucial importance for further improvements of this already promising photovoltaic technology. Both optical and electrical transient methods have previously been employed to strive for this warranted goal. However, electrical techniques can be strongly influenced by the capacitive response of the device which overlays with the steady‐state relevant bulk carrier recombination. To ascertain the identification of bulk charge carrier dynamics, it is beneficial to evaluate thicker films to minimize the impact of device capacitance. Herein, the electrical transient response in very efficient planar co‐evaporated n‐i‐p solar cells is studied by varying the active layer thickness from 500 nm to 820 nm and compared to a solution‐processed perovskite device with an active layer of 350 nm. In case of the n‐i‐p devices, the capacitance for the 500 nm solar cell leads to longer perceived decay times in the lower voltage regime, while in the higher voltage regime quite similar kinetics independent of active layer thickness are observed, allowing us to identify the transition from capacitance‐affected to the sought‐after bulk charge carrier dynamics. We show that increasing the perovskite thickness by more than 50 % does not affect the recombination dynamics significantly, confirming the high quality of the vacuum‐processed perovskite solar cells. Finally, it is demonstrated for the first time for perovskite solar cells that the recombination order in both thicker devices is ranging between 1.6 to 2, pointing towards trap‐assisted and free‐carrier recombination under operating conditions. We emphasize that the observed low recombination orders are in strong contrast to earlier literature as well as to the thinner solution‐processed device, which suffers from both shorter carrier lifetimes and a larger device capacitance

Reduced Recombination Losses in Evaporated Perovskite Solar Cells by Postfabrication Treatment

The photovoltaic perovskite research community has now developed a large set of tools and techniques to improve the power conversion efficiency (PCE). One such arcane trick is to allow the finished devices to dwell in time, and the PCE often improves. Herein, a mild postannealing procedure is implemented on coevaporated perovskite solar cells confirming a substantial PCE improvement, mainly attributed to an increased open-circuit voltage (V OC). From a V OC of around 1.11 V directly after preparation, the voltage improves to more than 1.18 V by temporal and thermal annealing. To clarify the origin of this annealing effect, an in-depth device experimental and simulation characterization is conducted. A simultaneous reduction of the dark saturation current, the ideality factor (n id), and the leakage current is revealed, signifying a substantial impact of the postannealing procedure on recombination losses. To investigate the carrier dynamics in more detail, a set of transient optoelectrical methods is first evaluated, ascertaining that the bulk carrier lifetime is increased with device annealing. Second, a drift-diffusion simulation is used, confirming that the beneficial effect of the annealing has its origin in effective bulk trap passivation that accordingly leads to a reduction of Shockley-Read-Hall recombination rates

Light Trapping and Alternative Electrodes for Organic Photovoltaic Devices

Organic materials, such as conjugated polymers, have emerged as a promising alternative for the production of inexpensive and flexible photovoltaic cells. As conjugated polymers are soluble, liquid based printing techniques enable production on large scale to a price much lower than that for inorganic based solar cells. Present day state of the art conjugated polymer photovoltaic cells are comprised by blends of a semiconducting polymer and a soluble derivative of fullerene molecules. Such bulk heterojunction solar cells now show power conversion efficiencies of up to 4-6%. The quantum efficiency of thin film organic solar cells is however still limited by several processes, of which the most prominent limitations are the comparatively low mobility and the high level of charge recombination. Hence organic cells do not yet perform as well as their more expensive inorganic counterparts. In order to overcome this present drawback of conjugated polymer photovoltaics, efforts are continuously devoted to developing materials or devices with increased absorption or with better charge carrier transporting properties. The latter can be facilitated by increasing the mobility of the pure material or by introducing beneficial morphology to prevent carrier recombination. Minimizing the active layer film thickness is an alternative route to collect more of the generated free charge carriers. However, a minimum film thickness is always required for sufficient photon absorption. A further limitation for low cost large scale production has been the dependence on expensive transparent electrodes such as indium tin oxide. The development of cheaper electrodes compatible with fast processing is therefore of high importance. The primary aim of this work has been to increase the absorption in solar cells made from thin films of organic materials. Device construction, deploying new geometries, and evaluation of different methods to provide for light trapping and photon recycling have been strived for. Different routes to construct and incorporate light trapping structures that enable higher photon absorption in a thinner film are presented. By recycling the reflected photons and enhancing the optical path length within a thinner cell, the absorption rate, as well as the collection of more charge carriers, is provided for. Attempts have been performed by utilizing a range of different structures with feature sizes ranging from nanometers up to centimeters. Surface plasmons, Lambertian scatterers, micro lenses, tandem cells as well as larger folded cell structures have been evaluated. Naturally, some of these methods have turned out to be more successful than others. From this work it can nevertheless be concluded that proper light trapping, in thin films of organic materials for photovoltaic energy conversion, is a technique capable of improving the cell performance. In addition to the study of light trapping, two new alternative electrodes for polymer photovoltaic devices are suggested and evaluated

Polarization anisotropy of charge transfer absorption and emission of aligned polymer: fullerene blend films

An improved understanding of the electronic structure of interfacial charge transfer (CT) states is of importance due to their crucial role in charge carrier generation and recombination in organic donor-acceptor (DA) solar cells. DA combinations with a small difference between the energy of the CT state (E-CT) and energy of the donor exciton (E-D*) are of special interest since energy losses due to electron transfer are minimized, resulting in an optimized open-circuit voltage. In that case, the CT state can be considered as a resonance mixture, containing character of a fully ionic state (D+ A(-)) and of the local polymer excited state (D* A). We show that the D* A contribution to the overall CT state wave function can be determined by measurements of the polarization anisotropy of CT absorption and emission of polymer: fullerene blends with aligned polymer chains. We study two donor polymers, P3HT and TQ1, blended with fullerene acceptors with different ionization potentials, allowing variation of the E-D* -E-CT difference. We find that, upon decreasing E-D* -E-CT, the local excitonic D* A character of the CT state increases, resulting in a decreased fraction of charge transferred and an increased transition dipole moment. For typical polymer: fullerene systems, this effect is expected to become detrimental for device performance if E-D* - E-CT andlt; 0.1 eV. This however, depends on the electronic coupling between D* A and D+ A(-), which we experimentally estimate to be similar to 6 meV for the TQ1: PCBM system.Funding Agencies|Swedish Energy Agency||Swedish Research Council||VINNOVA||Knut and Alice Wallenberg foundation||</p

Effects of Masking on Open-Circuit Voltage and Fill Factor in Solar Cells

Guidelines for the correct measurement protocol of novel photovoltaic technologies such as perovskites are becoming more frequent in literature. This because, as will be confirmed in this perspective, it is not straightforward to correctly measure the efficiency parameters of these and many other novel solar cells. This is particularly the case for small area research devices which are prone to overestimate the short circuit current density, due to edge effects of various types. To reduce the inaccuracy of current density determination, the common recommended practice is to utilize masks with well‐defined apertures, often smaller than the device active area. Herein we show both experimentally and theoretically that this common practice however always leads to the erroneous determination of both open circuit voltage and fill factor, which are equally important figures of merit as the short circuit current density. Although the errors induced in voltage and fill factor by using a mask are generally smaller than what the errors in current can amount to when not using a mask, they are on the other hand omnipresent and can be quite well described