TY  - JOUR
A1  - Yazmaciyan, Aren
A1  - Stolterfoht, Martin
A1  - Burn, Paul L.
A1  - Lin, Qianqian
A1  - Meredith, Paul
A1  - Armin, Ardalan
T1  - Recombination losses above and below the transport percolation threshold in bulk heterojunction organic solar cells
JF  - Advanced energy materials
N2  - Achieving the highest power conversion efficiencies in bulk heterojunction organic solar cells requires a morphology that delivers electron and hole percolation pathways for optimized transport, plus sufficient donor:acceptor contact area for near unity charge transfer state formation. This is a significant structural challenge, particularly in semiconducting polymer:fullerene systems. This balancing act in the model high efficiency PTB7:PC70BM blend is studied by tuning the donor:acceptor ratio, with a view to understanding the recombination loss mechanisms above and below the fullerene transport percolation threshold. The internal quantum efficiency is found to be strongly correlated to the slower carrier mobility in agreement with other recent studies. Furthermore, second-order recombination losses dominate the shape of the current density-voltage curve in efficient blend combinations, where the fullerene phase is percolated. However, below the charge transport percolation threshold, there is an electric-field dependence of first-order losses, which includes electric-field-dependent photogeneration. In the intermediate regime, the fill factor appears to be limited by both first- and second-order losses. These findings provide additional basic understanding of the interplay between the bulk heterojunction morphology and the order of recombination in organic solar cells. They also shed light on the limitations of widely used transport models below the percolation threshold.
KW  - bulk heterojunctions
KW  - charge transport
KW  - organic solar cells
KW  - percolation threshold
KW  - recombination losses
Y1  - 2018
U6  - https://doi.org/10.1002/aenm.201703339
SN  - 1614-6832
SN  - 1614-6840
VL  - 8
IS  - 18
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Lang, Felix
A1  - Köhnen, Eike
A1  - Warby, Jonathan
A1  - Xu, Ke
A1  - Grischek, Max
A1  - Wagner, Philipp
A1  - Neher, Dieter
A1  - Korte, Lars
A1  - Albrecht, Steve
A1  - Stolterfoht, Martin
T1  - Revealing fundamental efficiency limits of monolithic perovskite/silicon tandem photovoltaics through subcell characterization
JF  - ACS Energy Letters
N2  - Perovskite/silicon tandem photovoltaics (PVs) promise to accelerate the decarbonization of our energy systems. Here, we present a thorough subcell diagnosis methodology to reveal deep insights into the practical efficiency limitations of state-of-the-art perovskite/silicon tandem PVs. Our subcell selective intensity-dependent photoluminescence (PL) and injection-dependent electroluminescence (EL) measurements allow independent assessment of pseudo-V-OC and power conversion efficiencies (PCEs) for both subcells. We reveal identical metrics from PL and EL, which implies well-aligned energy levels throughout the entire cell. Relatively large ideality factors and insufficient charge extraction, however, cause each a fill factor penalty of about 6% (absolute). Using partial device stacks, we then identify significant losses in standard perovskite subcells due to bulk and interfacial recombination. Lastly, we present strategies to minimize these losses using triple halide (CsFAPb(IBrCI)(3)) based perovskites. Our results give helpful feedback for device development and lay the foundation toward advanced perovskite/silicon tandem PVs capable of exceeding 33% PCE.
Y1  - 2021
U6  - https://doi.org/10.1021/acsenergylett.1c01783
SN  - 2380-8195
VL  - 6
IS  - 11
SP  - 3982
EP  - 3991
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Pena-Camargo, Francisco
A1  - Thiesbrummel, Jarla
A1  - Hempel, Hannes
A1  - Musiienko, Artem
A1  - Le Corre, Vincent M.
A1  - Diekmann, Jonas
A1  - Warby, Jonathan
A1  - Unold, Thomas
A1  - Lang, Felix
A1  - Neher, Dieter
A1  - Stolterfoht, Martin
T1  - Revealing the doping density in perovskite solar cells and its impact on device performance
JF  - Applied physics reviews
N2  - Traditional inorganic semiconductors can be electronically doped with high precision. Conversely, there is still conjecture regarding the assessment of the electronic doping density in metal-halide perovskites, not to mention of a control thereof. This paper presents a multifaceted approach to determine the electronic doping density for a range of different lead-halide perovskite systems. Optical and electrical characterization techniques, comprising intensity-dependent and transient photoluminescence, AC Hall effect, transfer-length-methods, and charge extraction measurements were instrumental in quantifying an upper limit for the doping density. The obtained values are subsequently compared to the electrode charge per cell volume under short-circuit conditions ( CUbi/eV), which amounts to roughly 10(16) cm(-3). This figure of merit represents the critical limit below which doping-induced charges do not influence the device performance. The experimental results consistently demonstrate that the doping density is below this critical threshold 10(12) cm(-3), which means << CUbi / e V) for all common lead-based metal-halide perovskites. Nevertheless, although the density of doping-induced charges is too low to redistribute the built-in voltage in the perovskite active layer, mobile ions are present in sufficient quantities to create space-charge-regions in the active layer, reminiscent of doped pn-junctions. These results are well supported by drift-diffusion simulations, which confirm that the device performance is not affected by such low doping densities.
Y1  - 2022
U6  - https://doi.org/10.1063/5.0085286
SN  - 1931-9401
VL  - 9
IS  - 2
PB  - AIP Publishing
CY  - Melville
ER  - 
TY  - JOUR
A1  - Zeiske, Stefan
A1  - Sandberg, Oskar J.
A1  - Zarrabi, Nasim
A1  - Wolff, Christian Michael
A1  - Raoufi, Meysam
A1  - Peña-Camargo, Francisco
A1  - Gutierrez-Partida, Emilio
A1  - Meredith, Paul
A1  - Stolterfoht, Martin
A1  - Armin, Ardalan
T1  - Static disorder in lead halide perovskites
JF  - The journal of physical chemistry letters
N2  - In crystalline and amorphous semiconductors, the temperature-dependent Urbach energy can be determined from the inverse slope of the logarithm of the absorption spectrum and reflects the static and dynamic energetic disorder. Using recent advances in the sensitivity of photocurrent spectroscopy methods, we elucidate the temperature-dependent Urbach energy in lead halide perovskites containing different numbers of cation components. We find Urbach energies at room temperature to be 13.0 +/- 1.0, 13.2 +/- 1.0, and 13.5 +/- 1.0 meV for single, double, and triple cation perovskite. Static, temperature-independent contributions to the Urbach energy are found to be as low as 5.1 ?+/- 0.5, 4.7 +/- 0.3, and 3.3 +/- 0.9 meV for the same systems. Our results suggest that, at a low temperature, the dominant static disorder in perovskites is derived from zero-point phonon energy rather than structural disorder. This is unusual for solution-processed semiconductors but broadens the potential application of perovskites further to quantum electronics and devices.
KW  - Cations
KW  - External quantum efficiency
KW  - Perovskites
KW  - Solar cells
KW  - Solar energy
Y1  - 2022
U6  - https://doi.org/10.1021/acs.jpclett.2c01652
SN  - 1948-7185
VL  - 13
IS  - 31
SP  - 7280
EP  - 7285
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Stolterfoht, Martin
A1  - Caprioglio, Pietro
A1  - Wolff, Christian Michael
A1  - Marquez, Jose A.
A1  - Nordmann, Joleik
A1  - Zhang, Shanshan
A1  - Rothhardt, Daniel
A1  - Hörmann, Ulrich
A1  - Amir, Yohai
A1  - Redinger, Alex
A1  - Kegelmann, Lukas
A1  - Zu, Fengshuo
A1  - Albrecht, Steve
A1  - Koch, Norbert
A1  - Kirchartz, Thomas
A1  - Saliba, Michael
A1  - Unold, Thomas
A1  - Neher, Dieter
T1  - The impact of energy alignment and interfacial recombination on the internal and external open-circuit voltage of perovskite solar cells
JF  - Energy & environmental science
N2  - 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.
Y1  - 2019
U6  - https://doi.org/10.1039/c9ee02020a
SN  - 1754-5692
SN  - 1754-5706
VL  - 12
IS  - 9
SP  - 2778
EP  - 2788
PB  - Royal Society of Chemistry
CY  - Cambridge
ER  - 
TY  - JOUR
A1  - Zhang, Shanshan
A1  - Hosseini, Seyed Mehrdad
A1  - Gunder, Rene
A1  - Petsiuk, Andrei
A1  - Caprioglio, Pietro
A1  - Wolff, Christian Michael
A1  - Shoaee, Safa
A1  - Meredith, Paul
A1  - Schorr, Susan
A1  - Unold, Thomas
A1  - Burn, Paul L.
A1  - Neher, Dieter
A1  - Stolterfoht, Martin
T1  - The Role of Bulk and Interface Recombination in High-Efficiency Low-Dimensional Perovskite Solar Cells
JF  - Advanced materials
N2  - 2D Ruddlesden-Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite-based cells. Herein, 2D (CH3(CH2)(3)NH3)(2)(CH3NH3)(n-1)PbnI3n+1 perovskite cells with different numbers of [PbI6](4-) sheets (n = 2-4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open-circuit voltage (V-OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C-60 interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi-Fermi level splitting matches the device V-OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements.
KW  - 2D perovskites
KW  - interface recombination
KW  - perovskite solar cells
KW  - photoluminescence
KW  - V-OC loss
Y1  - 2019
U6  - https://doi.org/10.1002/adma.201901090
SN  - 0935-9648
SN  - 1521-4095
VL  - 31
IS  - 30
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Shoaee, Safa
A1  - Stolterfoht, Martin
A1  - Neher, Dieter
T1  - The Role of Mobility on Charge Generation, Recombination, and Extraction in Polymer-Based Solar Cells
JF  - dvanced energy materials
N2  - Organic semiconductors are of great interest for a broad range of optoelectronic applications due to their solution processability, chemical tunability, highly scalable fabrication, and mechanical flexibility. In contrast to traditional inorganic semiconductors, organic semiconductors are intrinsically disordered systems and therefore exhibit much lower charge carrier mobilities-the Achilles heel of organic photovoltaic cells. In this progress review, the authors discuss recent important developments on the impact of charge carrier mobility on the charge transfer state dissociation, and the interplay of free charge extraction and recombination. By comparing the mobilities on different timescales obtained by different techniques, the authors highlight the dispersive nature of these materials and how this reflects on the key processes defining the efficiency of organic photovoltaics.
KW  - charge generation
KW  - charge recombination
KW  - extraction
KW  - mobility
KW  - organic solar cells
KW  - polymer:fullerene bulk heterojunction
Y1  - 2018
U6  - https://doi.org/10.1002/aenm.201703355
SN  - 1614-6832
SN  - 1614-6840
VL  - 8
IS  - 28
PB  - Wiley-VCH
CY  - Weinheim
ER  - 
TY  - JOUR
A1  - Stolterfoht, Martin
A1  - Armin, Ardalan
A1  - Philippa, Bronson
A1  - Neher, Dieter
T1  - The Role of Space Charge Effects on the Competition between Recombination and Extraction in Solar Cells with Low-Mobility Photoactive Layers
JF  - The journal of physical chemistry letters
N2  - The competition between charge extraction and nongeminate recombination critically determines the current-voltage characteristics of organic solar cells (OSCs) and their fill factor. As a measure of this competition, several figures of merit (FOMs) have been put forward; however, the impact of space charge effects has been either neglected, or not specifically addressed. Here we revisit recently reported FOMs and discuss the role of space charge effects on the interplay between recombination and extraction. We find that space charge effects are the primary cause for the onset of recombination in so-called non-Langevin systems, which also depends on the slower carrier mobility and recombination coefficient. The conclusions are supported with numerical calculations and experimental results of 25 different donor/acceptor OSCs with different charge transport parameters, active layer thicknesses or composition ratios. The findings represent a conclusive understanding of bimolecular recombination for drift dominated photocurrents and allow one to minimize these losses for given device parameters.
Y1  - 2016
U6  - https://doi.org/10.1021/acs.jpclett.6b02106
SN  - 1948-7185
VL  - 7
SP  - 4716
EP  - 4721
PB  - American Chemical Society
CY  - Washington
ER  - 
TY  - JOUR
A1  - Stolterfoht, Martin
A1  - Wolff, Christian Michael
A1  - Marquez, Jose A.
A1  - Zhang, Shanshan
A1  - Hages, Charles J.
A1  - Rothhardt, Daniel
A1  - Albrecht, Steve
A1  - Burn, Paul L.
A1  - Meredith, Paul
A1  - Unold, Thomas
A1  - Neher, Dieter
T1  - Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells
JF  - Nature Energy
N2  - 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 pintype 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 (V-oc) of the complete cell to similar 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 cm(2) perovskite solar cells surpassing 20% efficiency (19.83% certified) with stabilized power output, a high V-oc (1.17 V) and record fill factor (>81%).
KW  - Energy science and technology
KW  - Solar cells
Y1  - 2018
U6  - https://doi.org/10.1038/s41560-018-0219-8
SN  - 2058-7546
VL  - 3
IS  - 10
SP  - 847
EP  - 854
PB  - Nature Publ. Group
CY  - London
ER  - 
TY  - JOUR
A1  - Stolterfoht, Martin
A1  - Le Corre, Vincent M.
A1  - Feuerstein, Markus
A1  - Caprioglio, Pietro
A1  - Koster, Lambert Jan Anton
A1  - Neher, Dieter
T1  - Voltage-Dependent Photoluminescence and How It Correlates with the Fill Factor and Open-Circuit Voltage in Perovskite Solar Cells
JF  - Acs energy letters
N2  - Optimizing the photoluminescence (PL) yield of a solar cell has long been recognized as a key principle to maximize the power conversion efficiency. While PL measurements are routinely applied to perovskite films and solar cells under open circuit conditions (V-OC), it remains unclear how the emission depends on the applied voltage. Here, we performed PL(V) measurements on perovskite cells with different hole transport layer thicknesses and doping concentrations, resulting in remarkably different fill factors (FFs). The results reveal that PL(V) mirrors the current-voltage (JV) characteristics in the power-generating regime, which highlights an interesting correlation between radiative and nonradiative recombination losses. In particular, high FF devices show a rapid quenching of PL(V) from open-circuit to the maximum power point. We conclude that, while the PL has to be maximized at V-OC at lower biases < V-OC the PL must be rapidly quenched as charges need to be extracted prior to recombination.
Y1  - 2019
U6  - https://doi.org/10.1021/acsenergylett.9b02262
SN  - 2380-8195
VL  - 4
IS  - 12
SP  - 2887
EP  - 2892
PB  - American Chemical Society
CY  - Washington
ER  -