Wolfgang Tress - Academia.edu (original) (raw)

Papers by Wolfgang Tress

npj flexible electronics, Aug 14, 2018

Metal halide perovskites have achieved great success in photovoltaic applications during the last... more Metal halide perovskites have achieved great success in photovoltaic applications during the last few years. The solar to electrical power conversion efficiency (PCE) of perovskite solar cells has been rapidly improved from 3.9% to certified 22.7% due to the extensive efforts on film deposition methods, composition and device engineering. Further investigation on eliminating the defect states in perovskite absorbers is necessary to push forward the PCE of perovskite solar cells approaching the Shockley-Queisser limit. In this review, we summarize the defect properties in perovskite films and present methodologies to control the defects density, including the growth of large size crystals, photo-curing method, grain boundary and surface passivation, and modification of the substrates. We also discuss the defects-related stability and hysteresis issues and highlight the current challenges and opportunities in defects control of perovskite films.

Nature Energy, 2021

Large datasets are now ubiquitous as technology enables higher-throughput experiments, but rarely... more Large datasets are now ubiquitous as technology enables higher-throughput experiments, but rarely can a research field truly benefit from the research data generated due to inconsistent formatting, undocumented storage or improper dissemination. Here we extract all the meaningful device data from peer-reviewed papers on metal-halide perovskite solar cells published so far and make them available in a database. We collect data from over 42,400 photovoltaic devices with up to 100 parameters per device. We then develop open-source and accessible procedures to analyse the data, providing examples of insights that can be gleaned from the analysis of a large dataset. The database, graphics and analysis tools are made available to the community and will continue to evolve as an open-source initiative. This approach of extensively capturing the progress of an entire field, including sorting, interactive exploration and graphical representation of the data, will be applicable to many fields ...

Advanced Functional Materials, 2020

Layered hybrid perovskites have emerged as a promising alternative to stabilizing hybrid organic–... more Layered hybrid perovskites have emerged as a promising alternative to stabilizing hybrid organic–inorganic perovskite materials, which are predominantly based on Ruddlesden‐Popper structures. Formamidinium (FA)‐based Dion‐Jacobson perovskite analogs are developed that feature bifunctional organic spacers separating the hybrid perovskite slabs by introducing 1,4‐phenylenedimethanammonium (PDMA) organic moieties. While these materials demonstrate competitive performances as compared to other FA‐based low‐dimensional perovskite solar cells, the underlying mechanisms for this behavior remain elusive. Here, the structural complexity and optoelectronic properties of materials featuring (PDMA)FAn–1PbnI3n+1 (n = 1–3) formulations are unraveled using a combination of techniques, including X‐ray scattering measurements in conjunction with molecular dynamics simulations and density functional theory calculations. While theoretical calculations suggest that layered Dion‐Jacobson perovskite stru...

Nature Energy, 2020

Improving the long-term stability of perovskite solar cells is critical to the deployment of this... more Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failu...

Nature Communications, 2019

All-inorganic metal halide perovskites are showing promising development towards efficient long-t... more All-inorganic metal halide perovskites are showing promising development towards efficient long-term stable materials and solar cells. Element doping, especially on the lead site, has been proved to be a useful strategy to obtain the desired film quality and material phase for high efficient and stable inorganic perovskite solar cells. Here we demonstrate a function by adding barium in CsPbI2Br. We find that barium is not incorporated into the perovskite lattice but induces phase segregation, resulting in a change in the iodide/bromide ratio compared with the precursor stoichiometry and consequently a reduction in the band gap energy of the perovskite phase. The device with 20 mol% barium shows a high power conversion efficiency of 14.0% and a great suppression of non-radiative recombination within the inorganic perovskite, yielding a high open-circuit voltage of 1.33 V and an external quantum efficiency of electroluminescence of 10−4.

Research, 2019

High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be real... more High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be realized by controlling the undesired nonradiative charge carrier recombination. Here, we introduce a judicious amount of guanidinium iodide into mixed-cation and mixed-halide perovskite films to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. By introducing guanidinium iodide into the perovskite precursor solution, the bandgap of the resulting absorber material changed minimally; however, the nonradiative recombination diminished considerably as revealed by time-resolved photoluminescence and electroluminescence studies. Furthermore, using capacitance-frequency measurements, we were able to correlate the hysteresis features exhibited by the PSCs with interfacial charge accumulation. This study opens up a path to realize new record efficiencies for PSCs ...

Advanced Materials Interfaces, 2018

Oxides employed in halide perovskite solar cells (PSCs) have already demonstrated to deliver enha... more Oxides employed in halide perovskite solar cells (PSCs) have already demonstrated to deliver enhanced stability, low cost and the ease of fabrication required for the commercialisation of the technology. The most stable PSCs configuration, the carbon-based hole transport layerfree PSC (HTL-free PSC), has also demonstrated a stability of more than one year of continuous operation partially due to the dual presence of insulating oxide scaffolds and conductive oxides. Despite these advances, the stability of PSCs is still a concern and a strong limiting factor for their industrial implementation. The engineering of oxide interfaces functionalized with molecules (like self-assembly monolayers, or SAMs) or polymers results in the passivation of defects (traps) at the interface, providing numerous advantages such as the elimination of hysteresis and the enhancement of solar cell efficiency. But most important is the beneficial effect of interfacial engineering on the lifetime and stability of PSCs. In this work, we provide insight into the recent developments reported on the surface Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

Energy & Environmental Science, 2018

Planar perovskite solar cells yield efficiency of over 20%.

Nature materials, Jan 16, 2018

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inor... more The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

Energy & Environmental Science, 2017

Perovskite cells benefit from a good night's sleep.

ACS Energy Letters, 2017

Several aspects on the photophysical characterization of lead halide hybrid organicinorganic pero... more Several aspects on the photophysical characterization of lead halide hybrid organicinorganic perovskite solar cells still remain unsolved. It has been observed that ionic transport and polarization of the interfaces cause very slow changes that interfere transient measurements with effects that cannot be separated from recombination kinetics. Here we establish a protocol of initial measurement of the solar cell that provides information on recombination characteristics prior to applying any voltage cycling. The photovoltaic device is measured by several methods: photovoltage vs. light intensity, open circuit voltage decay (OCVD) and impedance spectroscopy (IS), whilst minimizing the exposure to external voltage stimulus to avoid ionic migration to the contacts. Results are independently confirmed by the analysis of samples with interdigitated electrodes. We show that the high efficiency perovskite solar cells behave very closely to a bulk recombination ideal photovoltaic model. However, when voltage is scanned to determine current-density voltage curves and Impedance Spectroscopy at fixed illumination intensity, the cell undergoes a significant change, which we attribute to a dominance of recombination at contacts that have been modified by ionic polarization. Our method provides an effective approach to determine quantitatively the rather significant changes that occur to perovskite solar cells during standard measurements such as current-voltage curves.

Advanced Energy Materials, 2016

The Journal of Physical Chemistry C, 2016

Terrestrial applications of solar cells during day–night cycling as well as operation in winter a... more Terrestrial applications of solar cells during day–night cycling as well as operation in winter and summer involve substantial temperature variations, which influence the photophysics as well as the charge separation and transport properties in the various materials employed in a device. In this study, the optical absorption of methylammonium lead iodide (MAPbI3) and the device performance of MAPbI3 solar cells have been investigated in an extended temperature range between −190 and 80 °C. The optical properties were found to change by only a small amount in that temperature range. The device performance did, however, show more dramatic changes and decreased in a reversible manner for temperatures both higher and lower than room temperature. For temperatures up to 80 °C and down to −80 °C, the drop in performance was up to 25% compared to the room temperature value. Given thermal stability and reversible device performance, this is probably not a showstopper for terrestrial applications of perovskite sola...

ACS nano, Jun 20, 2016

Perovskite solar cells (PSCs) have now achieved efficiencies in excess of 22%, but very little is... more Perovskite solar cells (PSCs) have now achieved efficiencies in excess of 22%, but very little is known about their long-term stability under thermal stress. So far, stability reports have hinted at the importance of substituting the organic components, but little attention has been given to the metal contact. We investigated the stability of state-of-the-art PSCs with efficiencies exceeding 20%. Remarkably, we found that exposing PSCs to a temperature of 70 °C is enough to induce gold migration through the hole-transporting layer (HTL), spiro-MeOTAD, and into the perovskite material, which in turn severely affects the device performance metrics under working conditions. Importantly, we found that the main cause of irreversible degradation is not due to decomposition of the organic and hybrid perovskite layers. By introducing a Cr metal interlayer between the HTL and gold electrode, high-temperature-induced irreversible long-term losses are avoided. This key finding is essential in ...

Advanced Energy Materials, 2022

Since their introduction in 2017, the efficiency of lead‐free halide perovskite solar cells based... more Since their introduction in 2017, the efficiency of lead‐free halide perovskite solar cells based on Cs2AgBiBr6 has not exceeded 3%. The limiting bottlenecks are attributed to a low electron diffusion length, self‐trapping events and poor selectivity of the contacts, leading to large non‐radiative VOC losses. Here, 2D/3D hybrid double perovskites are introduced for the first time, using phenethyl ammonium as the constituting cation. The resulting solar cells show an increased efficiency of up to 2.5% for the champion cells and 2.03% on average, marking an improvement by 10% compared to the 3D reference on mesoporous TiO2. The effect is mainly due to a VOC improvement by up to 70 mV on average, yielding a maximum VOC of 1.18 V using different concentrations of phenethylammonium bromide. While these are among the highest reported VOC values for Cs2AgBiBr6 solar cells, the effect is attributed to a change in recombination behavior within the full device and a better selectivity at the ...

Nature Communications, 2019

So-called negative capacitance seems to remain an obscure feature in the analysis of the frequenc... more So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities arising from the mixed ionic-electronic conductivity of this class of semiconductor. Here we show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative ‘capacitance’ dependent on the sign of its gradient. The ‘capacitance’ appears so large because the associated resistance, when thinking of a resistor-capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phenomenon rel...

Nature Communications

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the... more Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm−1) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at...

Journal of Materials Chemistry A

Changes in Voc for the mixed halide composition are not correlated with the emergence of the low-... more Changes in Voc for the mixed halide composition are not correlated with the emergence of the low-gap phase, confirming that this phase is not the sole culprit for a low and unstable Voc.

npj flexible electronics, Aug 14, 2018

Metal halide perovskites have achieved great success in photovoltaic applications during the last... more Metal halide perovskites have achieved great success in photovoltaic applications during the last few years. The solar to electrical power conversion efficiency (PCE) of perovskite solar cells has been rapidly improved from 3.9% to certified 22.7% due to the extensive efforts on film deposition methods, composition and device engineering. Further investigation on eliminating the defect states in perovskite absorbers is necessary to push forward the PCE of perovskite solar cells approaching the Shockley-Queisser limit. In this review, we summarize the defect properties in perovskite films and present methodologies to control the defects density, including the growth of large size crystals, photo-curing method, grain boundary and surface passivation, and modification of the substrates. We also discuss the defects-related stability and hysteresis issues and highlight the current challenges and opportunities in defects control of perovskite films.

Nature Energy, 2021

Large datasets are now ubiquitous as technology enables higher-throughput experiments, but rarely... more Large datasets are now ubiquitous as technology enables higher-throughput experiments, but rarely can a research field truly benefit from the research data generated due to inconsistent formatting, undocumented storage or improper dissemination. Here we extract all the meaningful device data from peer-reviewed papers on metal-halide perovskite solar cells published so far and make them available in a database. We collect data from over 42,400 photovoltaic devices with up to 100 parameters per device. We then develop open-source and accessible procedures to analyse the data, providing examples of insights that can be gleaned from the analysis of a large dataset. The database, graphics and analysis tools are made available to the community and will continue to evolve as an open-source initiative. This approach of extensively capturing the progress of an entire field, including sorting, interactive exploration and graphical representation of the data, will be applicable to many fields ...

Advanced Functional Materials, 2020

Layered hybrid perovskites have emerged as a promising alternative to stabilizing hybrid organic–... more Layered hybrid perovskites have emerged as a promising alternative to stabilizing hybrid organic–inorganic perovskite materials, which are predominantly based on Ruddlesden‐Popper structures. Formamidinium (FA)‐based Dion‐Jacobson perovskite analogs are developed that feature bifunctional organic spacers separating the hybrid perovskite slabs by introducing 1,4‐phenylenedimethanammonium (PDMA) organic moieties. While these materials demonstrate competitive performances as compared to other FA‐based low‐dimensional perovskite solar cells, the underlying mechanisms for this behavior remain elusive. Here, the structural complexity and optoelectronic properties of materials featuring (PDMA)FAn–1PbnI3n+1 (n = 1–3) formulations are unraveled using a combination of techniques, including X‐ray scattering measurements in conjunction with molecular dynamics simulations and density functional theory calculations. While theoretical calculations suggest that layered Dion‐Jacobson perovskite stru...

Nature Energy, 2020

Improving the long-term stability of perovskite solar cells is critical to the deployment of this... more Improving the long-term stability of perovskite solar cells is critical to the deployment of this technology. Despite the great emphasis laid on stability-related investigations, publications lack consistency in experimental procedures and parameters reported. It is therefore challenging to reproduce and compare results and thereby develop a deep understanding of degradation mechanisms. Here, we report a consensus between researchers in the field on procedures for testing perovskite solar cell stability, which are based on the International Summit on Organic Photovoltaic Stability (ISOS) protocols. We propose additional procedures to account for properties specific to PSCs such as ion redistribution under electric fields, reversible degradation and to distinguish ambient-induced degradation from other stress factors. These protocols are not intended as a replacement of the existing qualification standards, but rather they aim to unify the stability assessment and to understand failu...

Nature Communications, 2019

All-inorganic metal halide perovskites are showing promising development towards efficient long-t... more All-inorganic metal halide perovskites are showing promising development towards efficient long-term stable materials and solar cells. Element doping, especially on the lead site, has been proved to be a useful strategy to obtain the desired film quality and material phase for high efficient and stable inorganic perovskite solar cells. Here we demonstrate a function by adding barium in CsPbI2Br. We find that barium is not incorporated into the perovskite lattice but induces phase segregation, resulting in a change in the iodide/bromide ratio compared with the precursor stoichiometry and consequently a reduction in the band gap energy of the perovskite phase. The device with 20 mol% barium shows a high power conversion efficiency of 14.0% and a great suppression of non-radiative recombination within the inorganic perovskite, yielding a high open-circuit voltage of 1.33 V and an external quantum efficiency of electroluminescence of 10−4.

Research, 2019

High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be real... more High photovoltages and power conversion efficiencies of perovskite solar cells (PSCs) can be realized by controlling the undesired nonradiative charge carrier recombination. Here, we introduce a judicious amount of guanidinium iodide into mixed-cation and mixed-halide perovskite films to suppress the parasitic charge carrier recombination, which enabled the fabrication of >20% efficient and operationally stable PSCs yielding reproducible photovoltage as high as 1.20 V. By introducing guanidinium iodide into the perovskite precursor solution, the bandgap of the resulting absorber material changed minimally; however, the nonradiative recombination diminished considerably as revealed by time-resolved photoluminescence and electroluminescence studies. Furthermore, using capacitance-frequency measurements, we were able to correlate the hysteresis features exhibited by the PSCs with interfacial charge accumulation. This study opens up a path to realize new record efficiencies for PSCs ...

Advanced Materials Interfaces, 2018

Oxides employed in halide perovskite solar cells (PSCs) have already demonstrated to deliver enha... more Oxides employed in halide perovskite solar cells (PSCs) have already demonstrated to deliver enhanced stability, low cost and the ease of fabrication required for the commercialisation of the technology. The most stable PSCs configuration, the carbon-based hole transport layerfree PSC (HTL-free PSC), has also demonstrated a stability of more than one year of continuous operation partially due to the dual presence of insulating oxide scaffolds and conductive oxides. Despite these advances, the stability of PSCs is still a concern and a strong limiting factor for their industrial implementation. The engineering of oxide interfaces functionalized with molecules (like self-assembly monolayers, or SAMs) or polymers results in the passivation of defects (traps) at the interface, providing numerous advantages such as the elimination of hysteresis and the enhancement of solar cell efficiency. But most important is the beneficial effect of interfacial engineering on the lifetime and stability of PSCs. In this work, we provide insight into the recent developments reported on the surface Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff))

Energy & Environmental Science, 2018

Planar perovskite solar cells yield efficiency of over 20%.

Nature materials, Jan 16, 2018

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inor... more The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor-acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor-acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

Energy & Environmental Science, 2017

Perovskite cells benefit from a good night's sleep.

ACS Energy Letters, 2017

Several aspects on the photophysical characterization of lead halide hybrid organicinorganic pero... more Several aspects on the photophysical characterization of lead halide hybrid organicinorganic perovskite solar cells still remain unsolved. It has been observed that ionic transport and polarization of the interfaces cause very slow changes that interfere transient measurements with effects that cannot be separated from recombination kinetics. Here we establish a protocol of initial measurement of the solar cell that provides information on recombination characteristics prior to applying any voltage cycling. The photovoltaic device is measured by several methods: photovoltage vs. light intensity, open circuit voltage decay (OCVD) and impedance spectroscopy (IS), whilst minimizing the exposure to external voltage stimulus to avoid ionic migration to the contacts. Results are independently confirmed by the analysis of samples with interdigitated electrodes. We show that the high efficiency perovskite solar cells behave very closely to a bulk recombination ideal photovoltaic model. However, when voltage is scanned to determine current-density voltage curves and Impedance Spectroscopy at fixed illumination intensity, the cell undergoes a significant change, which we attribute to a dominance of recombination at contacts that have been modified by ionic polarization. Our method provides an effective approach to determine quantitatively the rather significant changes that occur to perovskite solar cells during standard measurements such as current-voltage curves.

Advanced Energy Materials, 2016

The Journal of Physical Chemistry C, 2016

Terrestrial applications of solar cells during day–night cycling as well as operation in winter a... more Terrestrial applications of solar cells during day–night cycling as well as operation in winter and summer involve substantial temperature variations, which influence the photophysics as well as the charge separation and transport properties in the various materials employed in a device. In this study, the optical absorption of methylammonium lead iodide (MAPbI3) and the device performance of MAPbI3 solar cells have been investigated in an extended temperature range between −190 and 80 °C. The optical properties were found to change by only a small amount in that temperature range. The device performance did, however, show more dramatic changes and decreased in a reversible manner for temperatures both higher and lower than room temperature. For temperatures up to 80 °C and down to −80 °C, the drop in performance was up to 25% compared to the room temperature value. Given thermal stability and reversible device performance, this is probably not a showstopper for terrestrial applications of perovskite sola...

ACS nano, Jun 20, 2016

Perovskite solar cells (PSCs) have now achieved efficiencies in excess of 22%, but very little is... more Perovskite solar cells (PSCs) have now achieved efficiencies in excess of 22%, but very little is known about their long-term stability under thermal stress. So far, stability reports have hinted at the importance of substituting the organic components, but little attention has been given to the metal contact. We investigated the stability of state-of-the-art PSCs with efficiencies exceeding 20%. Remarkably, we found that exposing PSCs to a temperature of 70 °C is enough to induce gold migration through the hole-transporting layer (HTL), spiro-MeOTAD, and into the perovskite material, which in turn severely affects the device performance metrics under working conditions. Importantly, we found that the main cause of irreversible degradation is not due to decomposition of the organic and hybrid perovskite layers. By introducing a Cr metal interlayer between the HTL and gold electrode, high-temperature-induced irreversible long-term losses are avoided. This key finding is essential in ...

Advanced Energy Materials, 2022

Since their introduction in 2017, the efficiency of lead‐free halide perovskite solar cells based... more Since their introduction in 2017, the efficiency of lead‐free halide perovskite solar cells based on Cs2AgBiBr6 has not exceeded 3%. The limiting bottlenecks are attributed to a low electron diffusion length, self‐trapping events and poor selectivity of the contacts, leading to large non‐radiative VOC losses. Here, 2D/3D hybrid double perovskites are introduced for the first time, using phenethyl ammonium as the constituting cation. The resulting solar cells show an increased efficiency of up to 2.5% for the champion cells and 2.03% on average, marking an improvement by 10% compared to the 3D reference on mesoporous TiO2. The effect is mainly due to a VOC improvement by up to 70 mV on average, yielding a maximum VOC of 1.18 V using different concentrations of phenethylammonium bromide. While these are among the highest reported VOC values for Cs2AgBiBr6 solar cells, the effect is attributed to a change in recombination behavior within the full device and a better selectivity at the ...

Nature Communications, 2019

So-called negative capacitance seems to remain an obscure feature in the analysis of the frequenc... more So-called negative capacitance seems to remain an obscure feature in the analysis of the frequency-dependent impedance of perovskite solar cells. It belongs to one of the puzzling peculiarities arising from the mixed ionic-electronic conductivity of this class of semiconductor. Here we show that apparently high capacitances in general (positive and negative) are not related to any capacitive feature in the sense of a corresponding charge accumulation. Instead, they are a natural consequence of slow transients mainly in forward current of the diode upon ion displacement when changing voltage. The transient current leads to a positive or negative ‘capacitance’ dependent on the sign of its gradient. The ‘capacitance’ appears so large because the associated resistance, when thinking of a resistor-capacitor element, results from another physical process, namely modified electronic charge injection and transport. Observable for a variety of devices, it is a rather universal phenomenon rel...

Nature Communications

Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the... more Achieving high-efficiency indium tin oxide (ITO)-free organic optoelectronic devices requires the development of high-conductivity and high-transparency materials for being used as the front electrode. Herein, sol-gel-grown zinc oxide (ZnO) films with high conductivity (460 S cm−1) and low optical absorption losses in both visible and near-infrared (NIR) spectral regions are realized utilizing the persistent photoinduced doping effect. The origin of the increased conductivity after photo-doping is ascribed to selective trapping of photogenerated holes by oxygen vacancies at the surface of the ZnO film. Then, the conductivity of the sol-gel-grown ZnO is further increased by stacking the ZnO using a newly developed sequential deposition strategy. Finally, the stacked ZnO is used as the cathode to construct ITO-free organic solar cells, photodetectors, and light emitting diodes: The devices based on ZnO outperform those based on ITO, owing to the reduced surface recombination losses at...

Journal of Materials Chemistry A

Changes in Voc for the mixed halide composition are not correlated with the emergence of the low-... more Changes in Voc for the mixed halide composition are not correlated with the emergence of the low-gap phase, confirming that this phase is not the sole culprit for a low and unstable Voc.