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Controlling the perovskite morphology and defects at the buried perovskite-substrate interface is challenging for inverted perovskite solar cells. In this work, we report an amphiphilic molecular hole transporter, (2-(4-(bis(4-methoxyphenyl)amino)phenyl)-1-cyanovinyl)phosphonic acid, that features a multifunctional cyanovinyl phosphonic acid group and forms a superwetting underlayer for perovskite deposition, which enables high-quality perovskite films with minimized defects at the buried interface. The resulting perovskite film has a photoluminescence quantum yield of 17% and a Shockley-Read-Hall lifetime of nearly 7 microseconds and achieved a certified power conversion efficiency (PCE) of 25.4% with an open-circuit voltage of 1.21 volts and a fill factor of 84.7%. In addition, 1–square centimeter cells and 10–square centimeter minimodules show PCEs of 23.4 and 22.0%, respectively. Encapsulated modules exhibited high stability under both operational and damp heat test conditions.

W e would like to start off by thanking the authors of the comment, Wetzelaer and Blom, for their very helpful and constructive analysis. They present an interesting alternative view on an important and timely research topic. Before discussing the mechanism suggested in their comment, from the viewpoint of our experimental results, we would like to summarize our findings. In our work, 1 we experimentally showed that • The contact between donor molecules in the active layer of organic solar cells (OSCs) and a molybdenum oxide (MoO 3) hole extraction layer (HEL) causes an increase in nonradiative recombination losses, proportional to the extent of contacts. • We attributed these losses to surface recombination, and we proved that the losses can be suppressed by inserting a thin interfacial fullerene layer at the anode side. • Analyzing various donor−acceptor mixing ratios, with and without a fullerene-modified HEL, we decoupled and quantified the contribution from surface recombi-nation on the total nonradiative losses occurring in these devices. In the best case, we showed an improvement of 150 meV in V OC , as compared to the reference device. This demonstrates that surface recombination is a considerable contributor to nonradiative voltage losses in these solar cells, which are otherwise commonly occurring through charge-transfer states or energetic trap states because of defects in the bulk. We consider this the main result of our work. • Measurements by a modified charge extraction by linearly increasing voltage (CELIV) technique provided evidence that the improvement in V OC could be attributed to an enhanced built-in potential (V bi), reducing the presence of minority charge carriers at the respective electrodes. Although the authors of the comment in general agree with our experimental findings, they argue that the V bi does not play a direct role in suppressing the surface recombination of minority carriers. The introduction of a C 60 interlayer, they argue, renders the MoO 3 contact ohmic. 2 The reduced anodic injection barrier simultaneously increases the V bi , minimizes nonradiative voltage losses upon the extraction of majority carriers (holes), and suppresses minority-carrier (electron) surface recombination, the latter being the result of hole accumulation and associated band bending near the ohmic hole contact. Therefore, the ohmic contact formation suppresses both majority-and minority-carrier surface recombination losses, whereas the built-in voltage per se, they reason, does not play a major role. It is our opinion that the authors of the comment provide a very reasonable alternative explanation for the reduced surface recombination. Injection barriers at the contacts are well-known to be detrimental for the performance of OSC as being a major cause of a reduced V bi and increased surface recombination. Injection barriers have been shown to reduce not only the V OC but also the fill factor (FF), sometimes even leading to s-shaped JV curves. 3−5 It has been suggested that, in the case of very large injection barriers, the V OC is given by V bi. 6,7 Although V bi is determined by the difference in the work functions of the contacts, because of Fermi level pinning and the associated band bending, the built-in potential across the active layer, the effective V bi , typically cannot exceed the effective gap of the bulk-heterojunction blend. In other words, the anode Fermi level pins to the highest occupied molecular orbital (HOMO) of the donor, whereas the cathode pins to the lowest unoccupied molecular orbital (LUMO) of the acceptor. In addition, because of disorder, Fermi level pinning typically occurs to discrete gap or tail states, causing additional band bending, which further limits the effective V bi. 8,9 If there is an injection barrier at one contact, an increase or decrease in this barrier is directly reflected in the V bi. 5 Sachsische Aufbaubank [100325708]; Academy of FinlandAcademy of Finland

The energetic disorder can be reduced by developing new photovoltaic materials, especially non-fullerene acceptors, and significant progress in the PCEs of OSCs has been made.

A systematic spectroelectrochemical approach is presented to precisely determine frontier orbital energies of PM6:Y6 blends in device-relevant films and results are discussed regarding their impact on solar cell performance.

With power conversion efficiencies of perovskite-on-silicon and all-perovskite tandem solar cells increasing at rapid pace, wide bandgap (> 1.7 eV) metal-halide perovskites (MHPs) are becoming a major focus of academic and industrial photovoltaic research. Compared to their lower bandgap (< 1.6 eV) counterparts, these types of perovskites suffer from higher levels of non-radiative losses in both the bulk material and in device configurations, constraining their efficiencies far below their thermodynamic potential. In this work, we investigate the energy losses in methylammonium (MA) free high-Br-content widegap perovskites by using a combination of THz spectroscopy, steady-state and time-resolved photoluminescence, coupled with drift-diffusion simulations. The investigation of this system allows us to study charge-carrier recombination in these materials and devices in the absence of halide segregation due to the photostabilty of formamidinium-cesium based lead halide perovskites. We find that these perovskites are characterised by large non-radiative recombination losses in the bulk material and that the interfaces with transport layers in solar cell devices strongly limit their open-circuit voltage. In particular, we discover that the interface with the hole transport layer performs particularly poorly, in contrast to 1.6 eV bandgap MHPs which are generally limited by the interface with the electron-transport layer. To overcome these losses, we incorporate and investigate the recombination mechanisms present with perovskites treated with the ionic additive 1-butyl-1-methylpipiderinium tetrafluoroborate. We find that this additive not only improves the radiative efficiency of the bulk perovskite, but also reduces the non-radiative recombination at both the hole and electron transport layer interfaces of full photovoltaic devices. In addition to unravelling the beneficial effect of this specific treatment, we further optimise our solar cells by introducing an additional LiF interface treatment at the electron transport layer interface. Together these treatments enable MA-free 1.79 eV bandgap perovskite solar cells with open-circuit voltages of 1.22 V and power conversion efficiencies approaching 17 %, which is among the highest reported for this material system.

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C60 interlayer on the anode to experimentally reveal that surface recombination is a significant contributor to nonradiative recombination losses in organic solar cells. These losses are shown to proportionally increase with the extent of contact between donor molecules in the photoactive layer and a molybdenum oxide (MoO3) hole extraction layer, proven by calculating voltage losses in low- and high-donor-content bulk heterojunction device architectures. Using a novel in-device determination of the built-in voltage, the suppression of surface recombination, due to the insertion of a thin anodic-C60 interlayer on MoO3, is attributed to an enhanced built-in potential. The increased built-in voltage reduces the presence of minority charge carriers at the electrodes-a new perspective on the principle of selective charge extraction layers. The benefit to device efficiency is limited by a critical interlayer thickness, which depends on the donor material in bilayer devices. Given the high popularity of MoO3 as an efficient hole extraction and injection layer and the increasingly popular discussion on interfacial phenomena in organic optoelectronic devices, these findings are relevant to and address different branches of organic electronics, providing insights for future device design.

This work investigates halide segregation in methylammonium-free wide bandgap perovskites by photoluminescence quantum yield (PLQY) and advanced electron microscopy techniques. Our study reveals ho...

The enhancement and control of the electrical conductivity of organic semiconductors is fundamental for their use in optoelectronic applications and can be achieved by molecular doping, which introduces additional charge carriers through electron transfer between a dopant molecule and the organic semiconductor. Here, we use Electron Paramagnetic Resonance (EPR) spectroscopy to characterise the unpaired spins associated with the charges generated by molecular doping of the prototypical organic semiconductor poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) and tris(pentafluorophenyl)borane (BCF). The EPR results reveal the P3HT radical cation as the only paramagnetic species in BCF-doped P3HT films and show evidence for increased mobility of the detected spins at high doping concentrations as well as formation of antiferromagnetically coupled spin pairs leading to decreased spin concentrations at low temperatures. The EPR signature for F4TCNQ-doped P3HT is found to be determined by spin exchange between P3HT radical cations and F4TCNQ radical anions. Results from continuous-wave and pulse EPR measurements suggest the presence of the unpaired spin on P3HT in a multitude of environments, ranging from free P3HT radical cations with similar properties to those observed in BCF-doped P3HT, to pairs of dipolar and exchange-coupled spins on P3HT and the dopant anion. Characterisation of the proton hyperfine interactions by ENDOR allowed quantification of the extent of spin delocalisation and revealed reduced delocalisation in the F4TCNQ-doped P3HT films.

Perovskites offer exciting opportunities to realize efficient multijunction photovoltaic devices. This requires high-VOC and often Br-rich perovskites, which currently suffer from halide segregatio...

Multication metal-halide perovskites exhibit desirable performance and stability, compared to their monocation counterparts. However, the study of the photophysical properties and the nature of defect states in these materials is still a challenging and ongoing task. Here, we study bulk and interfacial energy loss mechanisms in solution-processed MAPbI3 (MAPI) and (CsPbI3)0.05[(FAPbI3)0.83(MAPbBr3)0.17]0.95 (triple cation) perovskite solar cells using absolute photoluminescence (PL) measurements. In neat MAPI films, we find a significantly smaller quasi-Fermi level splitting than for the triple cation perovskite absorbers, which defines the open-circuit voltage of the MAPI cells. PL measurements at low temperatures (∼20 K) on MAPI films demonstrate that emissive subgap states can be effectively reduced using different passivating agents, which lowers the nonradiative recombination loss at room temperature. We conclude that while triple cation perovskite cells are limited by interfacial recombination, the passivation of surface trap states within the MAPI films is the primary consideration for device optimization.

The mechanism and the nature of the species formed by molecular doping of the model polymer poly(3-hexylthiophene) (P3HT) in its regioregular (rre-) and regiorandom (rra-) forms in solution are investigated for three different dopants: the prototypical π-electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), the strong Lewis acid tris(pentafluorophenyl)borane (BCF), and the strongly oxidizing complex molybdenum tris[1-(methoxycarbonyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd-CO2Me)3). In a combined optical and electron paramagnetic resonance study, we show that the doping of rreP3HT in solution occurs by integer charge transfer, resulting in formation of P3HT radical cations (polarons) for all of the dopants considered here. Remarkably, despite the different chemical nature of the dopants and dopant–polymer interaction, the formed polarons exhibit essentially identical optical absorption spectra. The situation is very different for the doping of rraP3HT, where we observe formation of a charge-transfer complex with F4TCNQ and of a “localized” P3HT polaron on nonaggregated chains upon doping with BCF, while there is no indication of dopant-induced species in the case of Mo(tfd-CO2Me)3. We estimate the ionization efficiency of the respective dopants for the two polymers in solution and report the molar extinction coefficient spectra of the three different species. Finally, we observe increased spin delocalization in regioregular compared to regiorandom P3HT by electron nuclear double resonance, suggesting that the ability of the charge to delocalize on aggregates of planarized polymer backbones plays a significant role in determining the doping mechanism.

Optical absorption spectroscopy is a key method to investigate doped conjugated polymers and to characterize the doping-induced charge carriers, i.e., polarons. For prototypical poly(3-hexylthiophene-2,5-diyl) (P3HT), the absorption intensity of molecular dopant induced polarons is widely used to estimate the carrier density and the doping efficiency, i.e., the number of polarons formed per dopant molecule. However, the dependence of the polaron-related absorption features on the structure of doped P3HT, being either aggregates or separated individual chains, is not comprehensively understood in contrast to the optical absorption features of neutral P3HT. In this work, we unambiguously differentiate the optical signatures of polarons on individual P3HT chains and aggregates in solution, notably the latter exhibiting the same shape as aggregates in solid thin films. This is enabled by employing tris(pentafluorophenyl)borane (BCF) as dopant, as this dopant forms only ion pairs with P3HT and no charge transfer complexes, and BCF and its anion have no absorption in the spectral region of P3HT polarons. Polarons on individual chains exhibit absorption peaks at 1.5 eV and 0.6 eV, whereas in aggregates the high-energy peak is split into a doublet 1.3 eV and 1.65 eV, and the low-energy peak is shifted below 0.5 eV. The dependence of the fraction of solvated individual chains versus aggregates on absolute solution concentration, dopant concentration, and temperature is elucidated, and we find that aggregates predominate in solution under commonly used processing conditions. Aggregates in BCF-doped P3HT solution can be effectively removed upon simple filtering. From varying the filter pore size (down to 200 nm) and thin film morphology characterization with scanning force microscopy we reveal the aggregates' size dependence on solution absolute concentration and dopant concentration. Furthermore, X-ray photoelectron spectroscopy shows that the dopant loading in aggregates is higher than for individual P3HT chains. The results of this study help understanding the impact of solution pre-aggregation on thin film properties of molecularly doped P3HT, and highlight the importance of considering such aggregation for other doped conjugated polymers in general.

Charge extraction in organic solar cells (OSCs) is commonly believed to be limited by bimolecular recombination of photogenerated charges. However, the fill factor of OSCs is usually almost entirely governed by recombination processes that scale with the first order of the light intensity. This linear loss was often interpreted to be a consequence of geminate or trap-assisted recombination. Numerical simulations show that this linear dependence is a direct consequence of the large amount of excess dark charge near the contact. The first-order losses increase with decreasing mobility of minority carriers, and we discuss the impact of several material and device parameters on this loss mechanism. This work highlights that OSCs are especially vulnerable to injected charges as a result of their poor charge transport properties. This implies that dark charges need to be better accounted for when interpreting electro-optical measurements and charge collection based on simple figures of merit.

We employed bias-assisted charge extraction techniques to investigate the transient and steady-state recombination of photogenerated charge carriers in complete devices of a disordered polymer-fullerene blend. Charge recombination is shown to be dispersive, with a significant slowdown of the recombination rate over time, consistent with the results from kinetic Monte Carlo simulations. Surprisingly, our experiments reveal little to no contributions from early time recombination of nonequilibrated charge carriers to the steady-state recombination properties. We conclude that energetic relaxation of photogenerated carriers outpaces any significant nongeminate recombination under application-relevant illumination conditions. With equilibrated charges dominating the steady-state recombination, quasi-equilibrium concepts appear suited for describing the open-circuit voltage of organic solar cells despite pronounced energetic disorder. Funding Agencies|German Ministry of Science and Education (BMBF) within the project UNVEIL; German Science Foundation [INST 336/94-1 FUGG, NE410/15-1]; Shell Global Solutions International B.V.; Knut and Alice Wallenberg Foundation [KAW 2016.0494]

Interfacial layers in conjunction with suitable charge-transport layers can significantly improve the performance of optoelectronic devices by facilitating efficient charge carrier injection and extraction. This work uses a neat C

Perovskite semiconductors have demonstrated outstanding external luminescence quantum yields, enabling high power conversion efficiencies PCEs . However, the precise conditions to advance to an efficiency regime above monocrystalline silicon cells are not well understood. Herein, a simulation model that describes efficient p i n type perovskite solar cells well and a range of different experiments is established. Then, important device and material parameters are studied and it is found that an efficiency regime of 30 can be unlocked by optimizing the built in voltage across the perovskite layer using either highly doped 1019 amp; 8201;cm amp; 8722;3 transport layers TLs , doped interlayers or ultrathin self assembled monolayers. Importantly, only parameters that have been reported in recent literature are considered, that is, a bulk lifetime of 10 amp; 8201; amp; 956;s, interfacial recombination velocities of 10 amp; 8201;cm amp; 8201;s amp; 8722;1, a perovskite bandgap amp; 119864;gap of 1.5 amp; 8201;eV, and an external quantum efficiency EQE of 95 . A maximum efficiency of 31 is predicted for a bandgap of 1.4 amp; 8201;eV. Finally, it is demonstrated that the relatively high mobile ion density does not represent a significant barrier to reach this efficiency regime. The results of this study suggest continuous PCE improvements until perovskites may become the most efficient single junction solar cell technology in the near future

Organic solar cells are composed of electron donating and accepting organic semiconductors. Whilst a significant palette of donors has been developed over three decades, until recently only a small number of acceptors have proven capable of delivering high power conversion efficiencies. In particular the fullerenes have dominated the landscape. In this perspective, the emergence of a family of materials-the non-fullerene acceptors (NFAs) is described. These have delivered a discontinuous advance in cell efficiencies, with the significant milestone of 20% now in sight. Intensive international efforts in synthetic chemistry have established clear design rules for molecular engineering enabling an ever-expanding number of high efficiency candidates. However, these materials challenge the accepted wisdom of how organic solar cells work and force new thinking in areas such as morphology, charge generation and recombination. This perspective provides a historical context for the development of NFAs, and also addresses current thinking in these areas plus considers important manufacturability criteria. There is no doubt that the NFAs have propelled organic solar cell technology to the efficiencies necessary for a viable commercial technology-but how far can they be pushed, and will they also deliver on equally important metrics such as stability? The work was supported by the Ser Cymru II Program through the Welsh Government, European Regional Development Fund, Welsh European Funding Office, and Swansea University strategic initiative in Sustainable Advanced Materials. A.A. is a Ser Cymru II Rising Star Fellow and P.M. is a Ser Cymru II National Research Chair. T.W. acknowledges financial support from the National Natural Science Foundation of China (21774097). Z.X. and L.D. thank National Key Research and Development Program of China (2017YFA0206600) and National Natural Science Foundation of China (51773045, 21772030, 51922032, 21961160720) for financial support. J.N. thanks the European Research Council for support under the European Union's Horizon 2020 research and innovation program (grant agreement No 742708). Armin, A; Meredith, P (corresponding author), Swansea Univ, Dept Phys, Sustainable Adv Mat Ser SAM, Singleton Pk, Swansea SA2 8PP, W Glam, Wales. ardalan.armin@swansea.ac.uk; paul.meredith@swansea.ac.uk

Non-fullerene acceptors (NFAs) are far more emissive than their fullerene-based counterparts. Here, we study the spectral properties of photocurrent generation and recombination of the blend of the...

Power conversion efficiencies of donor/acceptor organic solar cells utilizing nonfullerene acceptors have now increased beyond the record of their fullerene-based counterparts. There remain many fu...

Doped spiro-OMeTAD at present is the most commonly used hole transport material (HTM) in n-i-p-type perovskite solar cells, enabling high efficiencies around 22%. However, the required dopants were shown to induce nonradiative recombination of charge carriers and foster degradation of the solar cell. Here, in a novel approach, highly conductive and inexpensive water-free poly(3,4-ethylenedioxythiophene) (PEDOT) is used to replace these dopants. The resulting spiro-OMeTAD/PEDOT (SpiDOT) mixed films achieve higher lateral conductivities than layers of doped spiro-OMeTAD. Furthermore, combined transient and steady-state photoluminescence studies reveal a passivating effect of PEDOT, suppressing nonradiative recombination losses at the perovskite/HTM interface. This enables excellent quasi-Fermi level splitting values of up to 1.24 eV in perovskite/SpiDOT layer stacks and high open-circuit voltages (V-OC) up to 1.19 V in complete solar cells. Increasing the amount of dopant-free spiro-OMeTAD in SpiDOT layers is shown to enhance hole extraction and thereby improves the fill factor in solar cells. As a consequence, stabilized efficiencies up to 18.7% are realized, exceeding cells with doped spiro-OMeTAD as a HTM in this study. Moreover, to the best of our knowledge, these results mark the lowest nonradiative recombination loss in the V-OC (140 mV with respect to the Shockley-Queisser limit) and highest efficiency reported so far for perovskite solar cells using PEDOT as a HTM.

The involvement of charge-transfer (CT) states in the photogeneration and recombination of charge carriers has been an important focus of study within the organic photovoltaic community. In this work, we investigate the molecular factors determining the mechanism of photocurrent generation in low-donor-content organic solar cells, where the active layer is composed of vacuum-deposited C

The incorporation of even small amounts of strontium (Sr) into lead-base hybrid quadruple cation perovskite solar cells results in a systematic increase of the open circuit voltage (V-oc) in pin-type perovskite solar cells. We demonstrate via absolute and transient photoluminescence (PL) experiments how the incorporation of Sr significantly reduces the non-radiative recombination losses in the neat perovskite layer. We show that Sr segregates at the perovskite surface, where it induces important changes of morphology and energetics. Notably, the Sr-enriched surface exhibits a wider band gap and a more n-type character, accompanied with significantly stronger surface band bending. As a result, we observe a significant increase of the quasi-Fermi level splitting in the neat perovskite by reduced surface recombination and more importantly, a strong reduction of losses attributed to non-radiative recombination at the interface to the C-60 electron-transporting layer. The resulting solar cells exhibited a V-oc of 1.18 V, which could be further improved to nearly 1.23 V through addition of a thin polymer interlayer, reducing the non-radiative voltage loss to only 110 meV. Our work shows that simply adding a small amount of Sr to the precursor solutions induces a beneficial surface modification in the perovskite, without requiring any post treatment, resulting in high efficiency solar cells with power conversion efficiency (PCE) up to 20.3%. Our results demonstrate very high V-oc values and efficiencies in Sr-containing quadruple cation perovskite pin-type solar cells and highlight the imperative importance of addressing and minimizing the recombination losses at the interface between perovskite and charge transporting layer.

Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (V-OC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the photoluminescence yield of perovskite/CTL heterojunctions, we quantify the non-radiative interfacial recombination currents in pin- and nip-type cells including high efficiency devices (21.4%). Our study comprises a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes. We find that all studied CTLs limit the V-OC by inducing an additional non-radiative recombination current that is in most cases substantially larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the V-OC of the device. Importantly, the V-OC equals the internal quasi-Fermi level splitting (QFLS) in the absorber layer only in high efficiency cells, while in poor performing devices, the V-OC is substantially lower than the QFLS. Using ultraviolet photoelectron spectroscopy and differential charging capacitance experiments we show that this is due to an energy level mis-alignment at the p-interface. The findings are corroborated by rigorous device simulations which outline important considerations to maximize the V-OC. This work highlights that the challenge to suppress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in proper energy level alignment and in suppression of defect recombination at the interfaces.

Diketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy of the donor (E-D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V-OC) is therefore determined to be due to rather high nonradiative 418 +/- 13 mV) and unavoidable radiative voltage losses (approximate to 255 +/- 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J(SC)) covers a vast span from 7 to 18 mA cm(-2) for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency. The authors acknowledge support by the German Excellence Initiative via the Cluster of Excellence EXC 1056 "Center for Advancing Electronics Dresden" (cfaed). For the GIWAXS measurements, the authors acknowledge KMC-2 diffraction beamline of the Photon source BESSY-II, Helmholtz Zentrum Berlin. Additionally, for TEM microscopy, the authors acknowledge Dr. Petr Formanek from the Leibniz-Institut fur Polymerforschung Dresden. T.E. acknowledges support by the German Alexander von Humboldt foundation. J.B., S.U., and K.V. acknowledge support from the German Federal Ministry for Education and Research (BMBF) through the InnoProfile Projekt "Organische p-i-n Bauelemente 2.2" (03IPT602X). Furthermore, S.U. acknowledges support by the graduate academy of the TU Dresden, financed by the excellence initiative of the German federal and state governments. E.C.-F. and U.H. and D.N. acknowledge funding by the BMBF (UNVEIL, FKZ 13N13719) and the DFG (SFB 951 "HIOS").

The performance of perovskite solar cells is predominantly limited by non-radiative recombination, either through trap-assisted recombination in the absorber layer or via minority carrier recombination at the perovskite/transport layer interfaces. Here, we use transient and absolute photoluminescence imaging to visualize all non-radiative recombination pathways in planar pin-type perovskite solar cells with undoped organic charge transport layers. We find significant quasi-Fermi-level splitting losses (135 meV) in the perovskite bulk, whereas interfacial recombination results in an additional free energy loss of 80 meV at each individual interface, which limits the open-circuit voltage (VOC) of the complete cell to ~1.12 V. Inserting ultrathin interlayers between the perovskite and transport layers leads to a substantial reduction of these interfacial losses at both the p and n contacts. Using this knowledge and approach, we demonstrate reproducible dopant-free 1 cm2 perovskite solar cells surpassing 20% efficiency (19.83% certified) with stabilized power output, a high VOC (1.17 V) and record fill factor (>81%). Non-radiative recombination is a critical limiting factor for perovskite solar cell performance. Stolterfoht et al. visualize the various recombination pathways in planar pin cells with photoluminescence imaging and use it to design improved solar cells with 1 cm2 areas and ~20% efficiency.

Ideally, the charge carrier lifetime in a solar cell is limited by the radiative free carrier recombination in the absorber which is a second order process. Yet, real life cells suffer from severe nonradiative recombination in the bulk of the absorber, at interfaces, or within other functional layers. Here, the dynamics of photogenerated charge carriers are probed directly in pin type mixed halide perovskite solar cells with an efficiency gt;20 , using time resolved optical absorption spectroscopy and optoelectronic techniques. The charge carrier dynamics is found to be in complete device to be fully consistent with a superposition of first , second , and third order recombination processes, with no admixture by a non integer order recombination pathways Under solar illumination, recombination in the studied high efficiency solar cells proceeds predominantly through nonradiative first order recombination with a lifetime of 250 ns, which competes with second order free charge recombination which is mostly if not entirely radiative. Results from the transient experiments are further employed to successfully explain the steady state solar cells properties over a wide range of illumination intensities. It is concluded that improving carrier lifetimes to gt;3 s will take perovskite devices into the radiative regime, where their performance will benefit from photon recycling

We review the Stille coupling synthesis of P(DPP2OD-T) (Poly[[2,5-di(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-3,6-diyl]-alt-[2,2′:5′,2″-terthiophene-5,5″-diyl]]) and show that high-quality, high molecular weight polymer chains are already obtained after as little as 15 min of reaction time. The results of UV–vis spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and atomic force microscopy show that longer reaction times are unnecessary and do not produce any improvement in film quality. We achieve the best charge transport properties with polymer batches obtained from short reaction times and demonstrate that the catalyst washing step is responsible for the introduction of charge-trapping sites for both holes and electrons. These trap sites decrease the charge injection efficiency, strongly reducing the measured currents. The careful tuning of the synthesis allows us to reduce the reaction time by more than 100 times, achieving a more environmentally friendly, less costly ...

Fluorine-containing donor molecules TFTF, CNTF and PRTF are designed and isomer selectively synthesized for application in vacuum-deposited organic solar cells. These molecules comprise a donor acceptor molecular architecture incorporating thiophene and benzothiadiazole derivatives as the electron-donating and electron-withdrawing moieties, respectively. As opposed to previously reported materials from this class, PRTF can be purified by vacuum sublimation at moderate to high yields because of its higher volatility and better stabilization due to a stronger intramolecular hydrogen bond, as compared to TFTF and CNTF. The UV-vis absorption spectra of the three donors show an intense broadband absorption between 500 nm and 800 nm with, similar positions of their frontier energy levels. The photophysical properties of the three donor molecules are thoroughly tested and optimized in bulk heterojunction solar cells with C-60 as acceptor. PRTF shows the best performance, yielding power conversion efficiencies of up to 3.8%. Moreover, the voltage loss for the PRTF device due to the non radiative recombination of free charge carriers is exceptionally low (0.26 V) as compared to typical values for organic solar cells (>0.34V). (C) 2016 Published by Elsevier B.V.

The crystallization behavior, the surface structure, and the nanomechanical properties of a semiconducting polymer play a crucial role in understanding the charge injection process, the transport of the charge carriers and the processability of the material. Here we study the semiconducting copolymer poly([N,N′-bis(2-octyldodecyl)-11-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-12 bithiophene)) (P(NDI2OD-T2)) and investigate the influence of annealing conditions on its surface structure through intermittent contact mode atomic force microscopy (AFM) and AFM-based measurements of amplitude–phase–distance (APD) curves. For spin-cast thin films as well as for films annealed at temperatures up to 320 °C, we find that the edges of crystalline lamellae are exposed at the surface. A 1.2 nm thick layer of alkyl side chains covers the film surface as indicated by the tip indentation. This suggests that charge injection into P(NDI2OD-T2) films is not hindered by a surface layer of amorphous mater...

Efficient electrical doping of organic semiconductors is a necessary prerequisite for the fabrication of high performance organic electronic devices. In this work, we study p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) spin-cast from two different solvents. Using electron diffraction, we find strong dopant-induced π–π-stacking for films from the solvent chloroform, but not from chlorobenzene. This image is confirmed and expanded by the analysis of vibrational features of P3HT and polaron absorptions using optical spectroscopy. Here, a red-shifted polaron absorption is found in doped films from chloroform, caused by a higher conjugation length of the polymer backbone. These differences result in a higher conductivity of films from chloroform. We use optical spectroscopy on the corresponding blend solutions to shed light on the origin of this effect and propose a model to explain why solutions of doped P3HT reveal more aggregation of charge...

The power conversion efficiency (PCE) of state-of-the-art organic solar cells is still limited by significant open-circuit voltage (V-OC) losses, partly due to the excitonic nature of organic materials and partly due to ill-designed architectures. Thus, quantifying different contributions of the V-OC losses is of importance to enable further improvements in the performance of organic solar cells. Herein, the spectroscopic and semiconductor device physics approaches are combined to identify and quantify losses from surface recombination and bulk recombination. Several state-of-the-art systems that demonstrate different V-OC losses in their performance are presented. By evaluating the quasi-Fermi level splitting (QFLS) and the V-OC as a function of the excitation fluence in nonfullerene-based PM6:Y6, PM6:Y11, and fullerene-based PPDT2FBT:PCBM devices with different architectures, the voltage losses due to different recombination processes occurring in the active layers, the transport layers, and at the interfaces are assessed. It is found that surface recombination at interfaces in the studied solar cells is negligible, and thus, suppressing the non-radiative recombination in the active layers is the key factor to enhance the PCE of these devices. This study provides a universal tool to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

Inorganic perovskites with cesium (Cs+) as the cation have great potential as photovoltaic materials if their phase purity and stability can be addressed. Herein, a series of inorganic perovskites is studied, and it is found that the power conversion efficiency of solar cells with compositions CsPbI1.8Br1.2, CsPbI2.0Br1.0, and CsPbI2.2Br0.8 exhibits a high dependence on the initial annealing step that is found to significantly affect the crystallization and texture behavior of the final perovskite film. At its optimized annealing temperature, CsPbI1.8Br1.2 exhibits a pure orthorhombic phase and only one crystal orientation of the (110) plane. Consequently, this allows for the best efficiency of up to 14.6% and the longest operational lifetime, T-S80, of approximate to 300 h, averaged of over six solar cells, during the maximum power point tracking measurement under continuous light illumination and nitrogen atmosphere. This work provides essential progress on the enhancement of photovoltaic performance and stability of CsPbI3 - xBrx perovskite solar cells., Zweitver��ffentlichungen der Universit��t Potsdam : Mathematisch-Naturwissenschaftliche Reihe; 1210