Trade‐Offs in Thin Film Solar Cells with Layered Chalcostibite Photovoltaic Absorbers

Discovery of novel semiconducting materials is needed for solar energy conversion and other optoelectronic applications. However, emerging low‐dimensional solar absorbers often have unconventional crystal structures and unusual combinations of optical absorption and electrical transport properties, which considerably slows down the research and development progress. Here, the effect of stronger absorption and weaker carrier collection of 2D‐like absorber materials are studied using a high‐throughput combinatorial experimental approach, complemented by advanced characterization and computations. It is found that the photoexcited charge carrier collection in CuSbSe₂ solar cells is enhanced by drift in an electric field, addressing a different absorption/collection balance. The resulting drift solar cells efficiency is <5% due to inherent J _SC/V _OC trade‐off, suggesting that improved carrier diffusion and better contacts are needed to further increase the CuSbSe₂ performance. This study also illustrates the advantages of high‐throughput experimental methods for fast optimization of the optoelectronic devices based on emerging low‐dimensional semiconductor materials.     

Loss Mechanisms in High Efficiency Polymer Solar Cells

Performance losses and aging mechanisms are investigated in state‐of‐the‐art PTB7:PC₇₀BM solar cells. Inverted devices incorporating a vanadium pentoxide (V₂O₅) top contact have efficiencies of 8%. After aging the unencapsulated devices, no changes are observed in the open circuit voltage (V_oc) or short circuit current (J_sc); however, the fill factor (FF) drops from 0.7 to 0.61. An s‐shape initially appears in the J–V curve after aging, which can be reduced by cycling through the J–V curve under illumination. This is discussed in context of the redox properties of V₂O₅. With impedance spectroscopy, it is demonstrated that changes to the contact interfaces are completely reversible and not responsible for the performance loss. Intensity modulated photocurrent spectroscopy combined with device modeling reveals that the loss in FF is due to trap formation in the active layer. Additionally it is observed that the performance of pristine devices is limited by optical absorption in the thin active layer and the build‐up of space charge which hinders carrier extraction.     

Disorder‐Induced Open‐Circuit Voltage Losses in Organic Solar Cells During Photoinduced Burn‐In

The photoinduced open‐circuit voltage (V_oc) loss commonly observed in bulk heterojunction organic solar cells made from amorphous polymers is investigated. It is observed that the total charge carrier density and, importantly, the recombination dynamics are unchanged by photoinduced burn‐in. Charge extraction is used to monitor changes in the density of states (DOS) during degradation of the solar cells, and a broadening over time is observed. It is proposed that the V_oc losses observed during burn‐in are caused by a redistribution of charge carriers in a broader DOS. The temperature and light intensity dependence of the V_oc losses can be described with an analytical model that contains the amount of disorder broadening in a Gaussian DOS as the only fit parameter. Finally, the V_oc loss in solar cells made from amorphous and crystalline polymers is compared and an increased stability observed in crystalline polymer solar cells is investigated. It is found that solar cells made from crystalline materials have a considerably higher charge carrier density than those with amorphous materials. The effects of a DOS broadening upon aging are suppressed in solar cells with crystalline materials due to their higher carrier density, making crystalline materials more stable against V_oc losses during burn‐in.     

Origin of the S‐Shaped JV Curve and the Light‐Soaking Issue in Inverted Organic Solar Cells

Inverted organic solar cells generally exhibit a strong s‐shaped kink in the current–voltage characteristics (JV curve) that may be removed by exposure to UV light (light‐soaking) leading to a drastically improved performance. Using in‐device characterization methods the origin of the light‐soaking issue in inverted solar cells employing titanium dioxide (TiO₂) as an electron selective layer is clarified. An injected hole reservoir accumulated at the TiO₂/organic interface of the pristine device is observed from extraction current transients; the hole reservoir increases the recombination and results in an s‐shape in the JV curve of pristine devices. The hole reservoir and the s‐shape is a result of the energetics at the selective contact in the pristine device; the effect of UV exposure is to decrease the work function of the indium tin oxide/TiO₂‐contact, increasing the built‐in potential. This hinders the build‐up of the hole reservoir and the s‐shape is removed. The proposed model is in excellent agreement with drift‐diffusion simulations.     

Extracting Microscopic Device Parameters from Transient Photocurrent Measurements of P3HT:PCBM Solar Cells

By including trap‐limited recombination in a time‐domain drift‐diffusion model of a P3HT:PCBM solar cell, experimental transient photocurrent (TPC) measurements across the current–voltage (J–V) curve in both the light and the dark can be reproduced. Using the same set of model parameters, the steady‐state current–voltage curves and charge‐extraction data are also reproduced. The model is validated by predicting the recombination rate at open circuit and comparing results with the rate as measured by the transient photovoltage (TPV) measurement technique. It is demonstrated that the model, which incorporates transport, carrier trapping, carrier de‐trapping, and recombination, is able to reproduce device dynamics correctly. The fit of the model to the experimental results is improved by varying the shape of the electron and hole density of state (DoS) functions. By discretizing the DoS in energy space and allowing its shape to vary, the TPC transients can be closely fitted and the form of the tails of the electron and hole DoS extracted. It is found that the DoS of trapped carriers can be represented by a series of Gaussians offset in energy. This is tentatively attributed to electronic disorder introduced by molecular packing of the molecules.     

Rationally Induced Interfacial Dipole in Planar Heterojunction Perovskite Solar Cells for Reduced J–V Hysteresis

With the rapid progress in developing hybrid perovskite solar cells, the allure of current density–voltage ( J–V) hysteresis has attracted quite a lot of interest in the research community. It requires feasible approaches that further deepen the fundamental understanding of device physics in specific device architecture in order to solve this problem eventually. Here, perovskite solar cells configured with different counter electrodes are systematically investigated with the focus on charge accumulation within the devices responsible for J–V hysteresis. The results indicate that J–V hysteresis is affected by charge accumulation which can be modulated by carrier extraction efficiency of the electrodes. Through a rationally induced interfacial dipole, the devices have shown improvement in carrier extraction, which thus reduces J–V hysteresis significantly. It provides solid evidence for the proposition that interface charge plays an important role in J–V hysteresis, and demonstrates an applicable strategy that effectively alleviates J–V hysteresis in perovskite solar cells.     

On the Relation between the Open‐Circuit Voltage and Quasi‐Fermi Level Splitting in Efficient Perovskite Solar Cells

Today's perovskite solar cells (PSCs) are limited mainly by their open‐circuit voltage (V_OC) due to nonradiative recombination. Therefore, a comprehensive understanding of the relevant recombination pathways is needed. Here, intensity‐dependent measurements of the quasi‐Fermi level splitting (QFLS) and of the V_OC on the very same devices, including pin‐type PSCs with efficiencies above 20%, are performed. It is found that the QFLS in the perovskite lies significantly below its radiative limit for all intensities but also that the V_OC is generally lower than the QFLS, violating one main assumption of the Shockley‐Queisser theory. This has far‐reaching implications for the applicability of some well‐established techniques, which use the V_OC as a measure of the carrier densities in the absorber. By performing drift‐diffusion simulations, the intensity dependence of the QFLS, the QFLS‐V_OC offset and the ideality factor are consistently explained by trap‐assisted recombination and energetic misalignment at the interfaces. Additionally, it is found that the saturation of the V_OC at high intensities is caused by insufficient contact selectivity while heating effects are of minor importance. It is concluded that the analysis of the V_OC does not provide reliable conclusions of the recombination pathways and that the knowledge of the QFLS‐V_OC relation is of great importance.     

Improving Perovskite Solar Cells: Insights From a Validated Device Model

To improve the efficiency of existing perovskite solar cells (PSCs), a detailed understanding of the underlying device physics during their operation is essential. Here, a device model has been developed and validated that describes the operation of PSCs and quantitatively explains the role of contacts, the electron and hole transport layers, charge generation, drift and diffusion of charge carriers and recombination. The simulation to the experimental data of vacuum‐deposited CH₃NH₃PbI₃ solar cells over multiple thicknesses has been fit and the device behavior under different operating conditions has been studied to delineate the influence of the external bias, charge‐carrier mobilities, energetic barriers for charge injection/extraction and, different recombination channels on the solar cell performance. By doing so, a unique set of material parameters and physical processes that describe these solar cells is identified. Trap‐assisted recombination at material interfaces is the dominant recombination channel limiting device performance and passivation of traps increases the power conversion efficiency (PCE) of these devices by 40%. Finally, guidelines to increase their performance have been issued and it is shown that a PCE beyond 25% is within reach.     

Lateral Organic Solar Cells with Self‐Assembled Semiconductor Nanowires

Solution‐processable organic semiconductor nanowires (NWs) offer a potentially powerful strategy for producing large‐area printed flexible devices. Here, the fabrication of lateral organic solar cells (LOSC) using solution‐processed organic NW blends on a flexible substrate to produce a power source for use in flexible integrated microelectronics is reported. A high photocarrier generation and an efficient charge sweep out are achieved by incorporating 1D self‐assembled poly(3‐hexylthiophene) NWs into the active layer, and an MoO₃ interfacial layer with high work function is introduced to increase the built‐in potential. These structures significantly increase the carrier diffusion/drift length and overall generated photocurrent in the channel. The utility of the LOSCs for high power source applications is demonstrated by using interdigitated electrode patterns that consist of multiple devices connected in parallel or in series. High photovoltage‐producing LOSC modules on plastic substrates for use in flexible optoelectronic devices are successfully fabricated. The LOSCs described here offer a new device architecture for use in highly flexible photoresponsive energy devices.     

Correlation of Absorption Profile and Fill Factor in Organic Solar Cells: The Role of Mobility Imbalance

We investigate the role of the spatial absorption profile within bulk heterojunction small molecule solar cells comprising a 50 nm ZnPc:C₆₀ active layer. Exploiting interference effects the absorption profile is varied by both the illumination wavelength and the thickness of an optical spacer layer adjacent to the reflecting electrode. The fill factor under 1 sun illumination is observed to change from 43 to 49% depending on the absorption profile which approximately equals the charge‐carrier generation profile. It is shown by varying the mixing ratio between ZnPc and C₆₀ that the importance of the generation profile is correlated with the imbalance of mobilities. Therefore, it is concluded that non‐geminate recombination is the dominating loss mechanism in these devices. Numerical drift‐diffusion simulations reproduce the experimental observations showing that charge carrier extraction is more efficient if charge carriers are generated close to the contact collecting the less mobile charge carrier type. Furthermore, this effect can explain the dependence of the internal quantum efficiency measured at short circuit on wavelength and implies that the spectral mismatch for a given solar simulator and device depends on the applied voltage.     

Recombination in Polymer:Fullerene Solar Cells with Open‐Circuit Voltages Approaching and Exceeding 1.0 V

Polymer:fullerene solar cells are demonstrated with power conversion efficiencies over 7% with blends of PBDTTPD and PC₆₁BM. These devices achieve open‐circuit voltages (V_oc) of 0.945 V and internal quantum efficiencies of 88%, making them an ideal candidate for the large bandgap junction in tandem solar cells. V_oc’s above 1.0 V are obtained when the polymer is blended with multiadduct fullerenes; however, the photocurrent and fill factor are greatly reduced. In PBDTTPD blends with multiadduct fullerene ICBA, fullerene emission is observed in the photoluminescence and electroluminescence spectra, indicating that excitons are recombining on ICBA. Voltage‐dependent, steady state and time‐resolved photoluminescence measurements indicate that energy transfer occurs from PBDTTPD to ICBA and that back hole transfer from ICBA to PBDTTPD is inefficient. By analyzing the absorption and emission spectra from fullerene and charge transfer excitons, we estimate a driving free energy of –0.14 ± 0.06 eV is required for efficient hole transfer. These results suggest that the driving force for hole transfer may be too small for efficient current generation in polymer:fullerene solar cells with V_oc values above 1.0 V and that non‐fullerene acceptor materials with large optical gaps (>1.7 eV) may be required to achieve both near unity internal quantum efficiencies and values of V_oc exceeding 1.0 V.     

On the Impact of Contact Selectivity and Charge Transport on the Open‐Circuit Voltage of Organic Solar Cells

The selectivity of electrodes of solar cells is a critical factor that can limit the overall efficiency. If the selectivity of an electrode is not sufficient both electrons and holes recombine at its surface. In materials with poor transport properties such as in organic solar cells, these surface recombination currents are accompanied by large gradients of the quasi‐Fermi energies as the driving force. Experimental results from current–voltage characteristics, advanced photo‐ and electroluminescence as well as charge extraction of three different photoactive materials are shown and compared to drift‐diffusion simulations. It can be concluded that in cases of electrodes with reduced selectivity the decrease of the open‐circuit voltage can be divided into two distinct contributions, the reduction of the overall steady‐state charge carrier density and the gradients of the quasi‐Fermi energies. The results clearly show that for photoactive layers with poor transport properties, the gradient of the quasi‐Fermi energy in the vicinity of the contact is the main contribution to the loss in open‐circuit voltage. For imbalanced mobilities, this gives rise to the phenomenon that it is more challenging to realize a selective contact for the less mobile charge carrier, i.e., the hole contact in most organic solar cells.     

Reducing Voltage Losses in Cascade Organic Solar Cells while Maintaining High External Quantum Efficiencies

High photon energy losses limit the open‐circuit voltage (V_OC) and power conversion efficiency of organic solar cells (OSCs). In this work, an optimization route is presented which increases the V_OC by reducing the interfacial area between donor (D) and acceptor (A). This optimization route concerns a cascade device architecture in which the introduction of discontinuous interlayers between alpha‐sexithiophene (α‐6T) (D) and chloroboron subnaphthalocyanine (SubNc) (A) increases the V_OC of an α‐6T/SubNc/SubPc fullerene‐free cascade OSC from 0.98 V to 1.16 V. This increase of 0.18 V is attributed solely to the suppression of nonradiative recombination at the D–A interface. By accurately measuring the optical gap (E_opt) and the energy of the charge‐transfer state (E_CT) of the studied OSC, a detailed analysis of the overall voltage losses is performed. E_opt – qV_OC losses of 0.58 eV, which are among the lowest observed for OSCs, are obtained. Most importantly, for the V_OC‐optimized devices, the low‐energy (700 nm) external quantum efficiency (EQE) peak remains high at 79%, despite a minimal driving force for charge separation of less than 10 meV. This work shows that low‐voltage losses can be combined with a high EQE in organic photovoltaic devices.     

Balanced Carrier Mobilities: Not a Necessary Condition for High‐Efficiency Thin Organic Solar Cells as Determined by MIS‐CELIV

A novel technique based upon injection‐charge extraction by linearly increasing voltage (i‐CELIV) in a metal‐insulator‐semiconductor (MIS) diode structure is described for studying charge transport in organic semiconductors. The technique (MIS‐CELIV) allows selective measurement of both electron and hole mobilities of organic solar cells with active layers thicknesses representative of operational devices. The method is used to study the model high efficiency bulk heterojunction combination poly[N‐9′′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole)] (PCDTBT) and [6,6]‐phenyl C70‐butyric acid methyl ester (PC70BM) at various blend ratios. The absence of bipolar transport in PCDTBT‐and‐PC70BM‐only diodes is shown and strongly imbalanced carrier mobility is found in the most efficient “optimized” blend ratios. The mobility measurements are correlated with overall device performance and it is found that balanced and high charge carrier mobility are not necessarily required for high efficiencies in thin film organic solar cells.     

Comparing the Device Physics and Morphology of Polymer Solar Cells Employing Fullerenes and Non‐Fullerene Acceptors

There is a need to find electron acceptors for organic photovoltaics that are not based on fullerene derivatives since fullerenes have a small band gap that limits the open‐circuit voltage (V_OC), do not absorb strongly and are expensive. Here, a phenylimide‐based acceptor molecule, 4,7‐bis(4‐(N‐hexyl‐phthalimide)vinyl)benzo[c]1,2,5‐thiadiazole (HPI‐BT), that can be used to make solar cells with V_OC values up to 1.11 V and power conversion efficiencies up to 3.7% with two thiophene polymers is demonstrated. An internal quantum efficiency of 56%, compared to 75–90% for polymer‐fullerene devices, results from less efficient separation of geminate charge pairs. While favorable energetic offsets in the polymer‐fullerene devices due to the formation of a disordered mixed phase are thought to improve charge separation, the low miscibility (<5 wt%) of HPI‐BT in polymers is hypothesized to prevent the mixed phase and energetic offsets from forming, thus reducing the driving force for charges to separate into the pure donor and acceptor phases where they can be collected.     

Ultra‐High Voltage Multijunction Organic Solar Cells for Low‐Power Electronic Applications

Low power electronics are an ideal application for organic photovoltaics (OPV) where a low‐cost OPV device can be integrated directly with a battery to provide a constant power source. We demonstrate ultra‐high voltage small molecule multijunction devices with open circuit voltage (V_OC) values of up to 7V. Optical modelling is employed to aid the optimisation of the complex multi‐layer stacks and ensure current balancing is achieved between sub‐cells, and optimised multijunction devices show power conversion efficiencies of up to 3.4% which is a modest increase over the single junction devices. Sub‐cell donor/acceptor pairs of boron subphthalocyanine chloride (SubPc)/fullerene (C₆₀) and SubPc/Cl₆‐SubPc were selected both for their high V_OC in order to minimise the required number of junctions, but also for their absorption overlap to reduce the spectral dependence of the device performance. As a result, the devices are shown to directly charge a micro‐energy cell type battery under both low illumination intensity white light and monochromatic illumination.     

Charge Carrier Extraction in Organic Solar Cells Governed by Steady‐State Mobilities

Charge transport in organic photovoltaic (OPV) devices is often characterized by steady‐state mobilities. However, the suitability of steady‐state mobilities to describe charge transport has recently been called into question, and it has been argued that dispersion plays a significant role. In this paper, the importance of the dispersion of charge carrier motion on the performance of organic photovoltaic devices is investigated. An experiment to measure the charge extraction time under realistic operating conditions is set up. This experiment is applied to different blends and shows that extraction time is directly related to the geometrical average of the steady‐state mobilities. This demonstrates that under realistic operating conditions the steady‐state mobilities govern the charge extraction of OPV and gives a valuable insight in device performance.     

Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells

Nongeminate recombination in organic solar cells based on copper phthalocyanine (CuPc) and C₆₀ is investigated. Two device architectures, the planar heterojunction (PHJ) and the bulk heterojunction (BHJ), are directly compared in view of differences in charge carrier decay dynamics. A combination of transient photovoltage (TPV) experiments, yielding the small perturbation charge carrier lifetime, and charge extraction measurements, providing the charge carrier density is applied. In organic solar cells, charge photogeneration and recombination primarily occur at the donor–acceptor heterointerface. Whereas the BHJ can often be approximated by an effective medium due to rather small scale phase separation, the PHJ has a well defined two‐dimensional heterointerface. In order to study nongeminate recombination dynamics in PHJ devices the charge accumulation at this interface is most relavent. As only the spatially averaged carrier concentration can be determined from extraction techniques, the charge carrier density at the interface n_int is derived from the open circuit voltage. Comparing the experimental results with macroscopic device simulation, the differences of recombination and charge carrier densities in CuPc:C₆₀ PHJ and BHJ devices are discussed with respect to the device performance. The open circuit voltage of BHJ is larger than for PHJ at low light intensities, but at 0.3 sun the situation is reversed: here, the PHJ can finally take advantage of its generally longer charge carrier lifetimes, as the active recombination region is smaller.     

Carrier‐Selectivity‐Dependent Charge Recombination Dynamics in Organic Photovoltaic Cells with a Ferroelectric Blend Interlayer

Interfacial energetics determines the performance of organic photovoltaic (OPV) cells based on a thin film of organic semiconductor blends. Here, an approach to modulating the “carrier selectivity” at the charge collecting interfaces and the consequent variations in the nongeminate charge carrier recombination dynamics in OPV devices are demonstrated. A ferroelectric blend interfacial layer composed of a solution‐processable ferroelectric poly­mer and a wide bandgap semiconductor is introduced as a tunable electron selective layer in inverted OPV devices with non‐Ohmic contact electrodes. The direct rendering of dipole alignment within the ferroelectric blend layer is found to increase the carrier selectivity of the charge collecting interfaces up to two orders of magnitude. Transient photovoltaic analyses reveal that the increase of carrier selectivity significantly reduces the diffusion and recombination among minority carriers in the vicinity of the electrodes, giving rise to the 85% increased charge carrier lifetime. Furthermore, the carrier‐selective charge extraction leads to the constitution of the internal potential within the devices, even with energetically identical cathodes and anodes. With these carrier‐selectivity‐controlled interlayers, the devices based on various photoactive materials commonly display significant increments in the device performances, especially with the high fill factor of up to 0.76 under optimized conditions.     

Switching Off Hysteresis in Perovskite Solar Cells by Fine‐Tuning Energy Levels of Extraction Layers

Lead halide perovskites often suffer from a strong hysteretic behavior on their j–V response in photovoltaic devices that has been correlated with slow ion migration. The electron extraction layer has frequently been pointed to as the main culprit for the observed hysteretic behavior. In this work three hole transport layers are studied with well‐defined highest occupied molecular orbital (HOMO) levels and interestingly the hysteretic behavior is markedly different. Here it is shown that an adequate energy level alignment between the HOMO level of the extraction layer and the valence band of the perovskite, not only suppresses the hysteresis, avoiding charge accumulation at the interfaces, but also degradation of the hole transport layer is reduced. Numerical simulation suggests that formation of an injection barrier at the organic/perovskite heterointerface could be one mechanism causing hysteresis. The suppression of such barriers may require novel design rules for interface materials. Overall, this work highlights that both external contacts need to be carefully optimized in order to obtain hysteresis‐free perovskite devices.