The value and temperature dependence of the ideality factor provides essential information about the dominant recombination route in solar cells. This study presents experimental results of accurate ...ideality factor determination for representative organic photovoltaic cells (OPV) evaluated at different temperatures over a large current density regime. It is noted that standard dark I–V curves strongly deviate from those obtained by evaluations based on short circuit current density (J
SC)–open circuit voltage (V
OC) pairs. This is attributed to the applied external voltage in a dark I–V measurement not being representative of internal chemical potential, particularly at lower temperatures. Complementary electroluminescence measurements attest that the current density dependence of the ability of the solar cell to emit light is better correlated to the series resistance free ideality factor. For the studied set of OPV devices it is observed that the ideality factors are quite low, and with very weak temperature dependence. The J
SC–V
OC method to determine ideality factors further allows good estimates of activation energies as well as recombination current prefactors J
00. The findings imply that the principal OPV non‐radiative recombination mechanism is not recombination of free carriers with trapped carriers in an exponential density of tail states as previously reported.
Improved determination of ideality factors is performed by evaluating open circuit voltage–short circuit current pairs as opposed to traditional dark I–V curves. The true temperature dependence of the ideality factor can then be obtained for organic solar cells otherwise substantially limited by series resistance effects. The relation to radiative efficiency, activation energy, and dark saturation current is clarified.
The lifetime of photogenerated charge carriers is one of the most important parameters in solar cells, as it rules the recombination rate that defines the open circuit voltage and the required ...minimum extraction time. It is therefore also one of the most discussed factors in all photovoltaic research fields. Lifetime evaluation of solar cells is frequently conducted via both optical and electrical means with the purpose of obtaining a deeper understanding of the dominant performance limiting recombination mechanisms. In many earlier recombination designations, performed via transient electrical means in novel thin film solar cells, the lifetime has been observed to be a decaying exponential function of the open circuit voltage. In this work we re-evaluate these previously assigned lifetimes as often being severely influenced by capacitive decay rates of spatially separated charge carriers. These “lifetimes” have thus very little in common with lifetimes relevant under steady state operational conditions of the solar cell. We show that the problem of lifetime determination via electrical means arises from that the relaxation of such charges, being associated with quasi-static capacitances of geometric type or from space-charge regions in the device, is also a decaying exponential function of the instantaneous open circuit voltage. This misconception hence also explains the often observed large discrepancy between optically and electrically determined lifetimes. We finally provide a simple expression outlining under what conditions relevant bulk recombination lifetimes are electrically accessible in thin film solar cells.
The charge carrier lifetime is an important parameter in solar cells as it defines, together with the mobility, the diffusion length of the charge carriers, thus directly determining the optimal ...active layer thickness of a device. Herein, we report on charge carrier lifetime values in bromine doped planar methylammonium lead iodide (MAPbI
) solar cells determined by transient photovoltage. The corresponding charge carrier density has been derived from charge carrier extraction. We found increased lifetime values in solar cells incorporating bromine compared to pure MAPbI
by a factor of ~2.75 at an illumination intensity corresponding to 1 sun. In the bromine containing solar cells we additionally observe an anomalously high value of extracted charge, which we deduce to originate from mobile ions.
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, TPV measurements are often performed in conjunction with charge extraction (CE) measurements, to determine the photogenerated 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.
The increasing amount of research on solution-processable, organic donor-acceptor bulk heterojunction photovoltaic systems, based on blends of conjugated polymers and fullerenes has resulted in ...devices with an overall power-conversion efficiency of 6%. For the best devices, absorbed photon-to-electron quantum efficiencies approaching 100% have been shown. Besides the produced current, the overall efficiency depends critically on the generated photovoltage. Therefore, understanding and optimization of the open-circuit voltage (Voc) of organic solar cells is of high importance. Here, we demonstrate that charge-transfer absorption and emission are shown to be related to each other and Voc in accordance with the assumptions of the detailed balance and quasi-equilibrium theory. We underline the importance of the weak ground-state interaction between the polymer and the fullerene and we confirm that Voc is determined by the formation of these states. Our work further suggests alternative pathways to improve Voc of donor-acceptor devices.
Intermolecular charge-transfer states at the interface between electron donating (D) and accepting (A) materials are crucial for the operation of organic solar cells but can also be exploited for ...organic light-emitting diodes
. Non-radiative charge-transfer state decay is dominant in state-of-the-art D-A-based organic solar cells and is responsible for large voltage losses and relatively low power-conversion efficiencies as well as electroluminescence external quantum yields in the 0.01-0.0001% range
. In contrast, the electroluminescence external quantum yield reaches up to 16% in D-A-based organic light-emitting diodes
. Here, we show that proper control of charge-transfer state properties allows simultaneous occurrence of a high photovoltaic and emission quantum yield within a single, visible-light-emitting D-A system. This leads to ultralow-emission turn-on voltages as well as significantly reduced voltage losses upon solar illumination. These results unify the description of the electro-optical properties of charge-transfer states in organic optoelectronic devices and foster the use of organic D-A blends in energy conversion applications involving visible and ultraviolet photons
.
The maximum efficiency of any solar cell can be evaluated in terms of its corresponding ability to emit light. We herein determine the important figure of merit of radiative efficiency for ...Methylammonium Lead Iodide perovskite solar cells and, to put in context, relate it to an organic photovoltaic (OPV) model device. We evaluate the reciprocity relation between electroluminescence and photovoltaic quantum efficiency and conclude that the emission from the perovskite devices is dominated by a sharp band-to-band transition that has a radiative efficiency much higher than that of an average OPV device. As a consequence, the perovskite have the benefit of retaining an open circuit voltage ~0.14 V closer to its radiative limit than the OPV cell. Additionally, and in contrast to OPVs, we show that the photoluminescence of the perovskite solar cell is substantially quenched under short circuit conditions in accordance with how an ideal photovoltaic cell should operate.
Thermally stimulated current (TSC) measurements are used to characterize electronic trap states in methylammonium lead iodide perovsite solar cells. Several TSC peaks were observed over the ...temperature range from 20 K to room temperature. To elucidate the origins of these peaks, devices with various organic charge transport layers and devices without transport layers were tested. Two peaks appear at very low temperatures, indicating shallow trap states that are mainly attributed to the PCBM/C60 electron transport bilayer. However, two additional peaks appear at higher temperatures, that is, they are deeper in energy, and are assigned to the perovskite layer. At around T = 163 K, a sharp peak, also present in the dark TSC measurements, is assigned to the orthorhombic–tetragonal phase transition in the perovskite. However, a peak at around T = 191 K is assigned to trap states with activation energies of around 500 meV but with a rather low concentration of 1 × 1021 m–3.
Adding cesium (Cs) and rubidium (Rb) cations to FA0.83MA0.17Pb(I0.83Br0.17)3 hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the ...enhancement has not been fully understood yet. In this work, time‐of‐flight, time‐resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs‐incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open‐circuit voltage and fill factor of Cs‐containing devices. By comparison, Rb‐addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs‐Rb‐FA‐MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple‐cation perovskite solar cells, which are essential for the design of high‐performance devices.
Time‐resolved microwave conductivity, time‐of‐flight, and thermally stimulated current measurements reveal that Cs reduces the trap density in hybrid lead halide perovskites. Rb additives enhance the charge carrier mobility, but show minor effects on the trap landscape. The increase in open‐circuit voltage in multiple‐cation perovskite solar cells can be related to a reduced trap density through Cs‐incorporation.
The field dependence of photocurrent found in many organic solar cells is a significant and detrimental setback for internal quantum efficiency. In this work we study the important contribution to ...this field dependence due to the dissociation efficiency of the weakly bound interfacial charge transfer (CT) state, crucial for organic bulk heterojunction solar cells. Three different donor polymers and two different acceptors are examined, and their respective dissociation characteristics are evaluated by photoluminescence (PL) quenching, both for Frenkel excitons and for the intermolecular charge transfer excitons. We observe that while the field-dependent photocurrent for pure polymers does correlate well with quenching efficiency, the CT exciton quenching from the blend generally displays a less pronounced correlation with extracted photocurrent. We further note that while the electroluminescence and photoluminescence of the pure polymer are identical, we observe a red shift for the blend electroluminescence. This indicates that lower energetic states, not visible in PL, are available in the blend. The emissive state of the blends probed by PL is therefore proposed to originate from sites that are involved in photocurrent generation to a lesser extent.
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