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Influence of Environmentally Affected Hole Transport Layers on Spatial Homogeneity and Charge Transport Dynamics of Organic Solar Cells

After the efficiency of organic photovoltaic (OPV) cells achieved more than 10%, the control of stability and degradation mechanisms of solar cells became a prominent task. The improvement of device efficiency due to incorporation of a hole-transport layer (HTL) in bulk-heterojunction solar cells has been extensively reported. However, the most widely used HTL material, PEDOT:PSS is frequently suspected to be the dominating source for devices instability under environmental conditions. Thereby effects like photooxidation and electrode corrosion are often reported to shorten device lifetime. However, often in environmental device studies, the source of degradation, whether being from the HTL, the active layer or the metal cathode are rather difficult to distinguish, because the external diffusion of oxygen and water affects all components. In this study, different HTLs, namely prepared from traditional PEDOT:PSS and also two types of molybdenum trioxide (MoO3), are exposed to different environments such as oxygen, light or humidity, prior to device finalization under inert conditions. This allows investigating any effects within the HTL and from reactions at its interface to the indium-tin-oxide electrode or the active layer. The surface and bulk chemistry of the exposed HTL has been monitored and discussed in context to the observed device physics, dynamic charge transport and spatial performance homogeneity of the according OPV device. The results show that merely humidity-exposure of the HTL leads to decreased device performance for PEDOT:PSS, but also for one type of the tested MoO3. The losses are related to the amount of absorbed water in the HTL, inducing loss of active area in terms of interfacial contact. The device with PEDOT:PSS HTL after humid air exposure showed seriously decreased photocurrent by micro-delamination of swelling/shrinkage of the hygroscopic layer.


INTRODUCTION
The research on organic photovoltaics (OPV) reached remarkable growth due to their potential for realization of flexible, large-area and light-weight devices, categorized by environmental friendliness, low thermal budget and low-cost production e.g. via roll-to-roll printing. [1][2][3][4]. The power conversion efficiency (PCE) of OPVs has been greatly improved by introducing the bulkheterojunction (BHJ) concept by the combination of organic donor and acceptor on the nanoscale. [1,5] The most prominent system here is the donor-acceptor couple of the conjugated polymer poly(3-hexylthiophene) (P3HT) and the fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM). [6][7] Best systems currently reach up to 12% efficiency. [8][9] Typical BHJ OPVs require a selective hole-transport layer (HTL) to block electrons and enhance hole-transfer to the anode, further to stabilize the anode's work function and reducing its surface roughness. [10][11] The most commonly used material for the anode interlayer is poly (3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) due to its benefits such as high conductivity, high transparency and good solution processibility [12][13]. However, beside high efficiencies and cost-effective processing, also long device lifetimes are desirable for OPVs. [14] Plenty of research has been focusing on environmentally induced mechanisms leading to device degradation, based on various approaches and techniques. [15][16][17][18][19][20] This is not exclusively caused by environments like air, light and humidity, even solvent vapors which are present during fabrication or even testing, can affect the device performance considerably.
Spatially resolved methods such as conductive atomic force microscopy [28] ,near-field scanning optical microscopy [29], lock-in thermography [30], photo-and electroluminescence [31] and light-beam induced current (LBIC) [27,31] have been applied as tools to characterize surface and device performance of OPVs. In particular, laser beam induced current (LBIC) has been widely used to investigate degradation and efficiency losses by spatial homogeneity across the entire photoactive pixel area. [20,[32][33][34] Further, transient photocurrents (TPC) have been used to gain knowledge about the dynamics of charge separation, determine mobilities, to describe the buildup of trapped charges and the dynamics of recombination of OPVs. [35][36][37][38][39][40][41] Aim of this work is to distinguish the environmental effects solely induced by exposition of the HTL, without the impact of a total device exposure. Therefore three different types of solution The as-prepared HTLs on indium-tin-oxide (ITO) glass substrates are exposed to dry air, humid air in the dark or with illumination, before further processing of the ITO/HTL/P3HT:PCBM/LiF/Al device stack in inert atmosphere. Surface and local bulk chemistry of the exposed HTLs have been investigated by X-ray photoelectron spectroscopy (XPS) and Raman microscopy, respectively. Finalized devices are characterized regarding their general device performance and scanned regarding their photocurrent homogeneity and local device characteristics by LBIC. The dynamic behavior of charge transport in the devices was 5 studied by TPC. Altogether, these results allow conclusions on device changes/degradation solely connected to chemical/photochemical reactions of the HTL itself or at the closest interfaces in consequence. briefly before further processing.

Preparation of the MoO3 and PEDOT:PSS HTLs
Two different formulations were used to generate the MoO3 films. The quasi-continuous films, consisting of a nanosized network with particle mean diameter of 6 nm, [27] are referred to in the following as MoO3-1. For their preparation, following a procedure reported by K. Zilberberg et al., [25] MoO2(acac)2 was dissolved in isopropanol to form a 0.5% (w/v) precursor solution.
The colloidal films, comprising a layer of separated particles (10-30 nm), [27] are referred to as 6 MoO3-2 in this study and were prepared as reported by Liu et al. [26]. Here, (NH4)2MoO4 was dissolved in distilled water to form a 0.005 mol/L solution. Then 2 mol/L aqueous HCl solution as catalyst was added drop-wise under stirring until the pH value of the solution was between 1 and 1.5. Particles of MoO3 start forming already in solution by hydrolysis and condensation, thus generating a suspension. All three formulations, PEDOT:PSS, MoO3-1 and MoO3-2, were filtered by 0.22 μm PVDF membrane filters (Sigma Aldrich) prior to deposition and spin-coated onto ITO substrates at 4000 rpm for PEDOT: PSS and 3000 rpm for MoO3 for 40s, respectively.
The layers of MoO3-1 and MoO3-2 were kept in ambient air at room temperature for 1 h for hydrolysis of the precursor. Finally, films were annealed at 160 °C for 20 min to dry and promote oxide formation. The PEDOT: PSS layers were merely heated to 160 °C for 20 min under Argon (Ar) flow for removal of water. The film thickness of PEDOT:PSS is around 40 nm, MoO3-1 and MoO3-2 are both around 10 nm. The as-prepared HTLs were either used immediately for device fabrication (referred to as */Fresh) or underwent selected exposure before. For this purpose films were submitted to dry air with <20% relative humidity (*/Air) or humid air with >80% relative humidity (*/H2O) without any illumination, or to dry air under white light illumination (tungsten halogen) at 10% equivalent of AM1.5G (*/Air & light), respectively. For each condition, the temperature was 25°C and the duration of exposure was 18 hours. Devices were fabricated immediately after exposure. Samples for XPS investigation were prepared with the same procedure on unpatterned ITO substrates. Different to the standard process, samples for Raman spectroscopy were prepared on ITO substrates for PEDOT:PSS and on Si substrates for MoO3-1 and MoO3-2 by drop casting method to obtain thicker layers (~80nm) to increase the weak signal intensity. 7 Standard solar cells with P3HT:PCBM photoactive layer according to the architecture shown in Figure 1 were assembled in inert gas atmosphere, using named HTLs of MoO3-1, MoO3-2 and PEDOT: PSS directly after different environments exposures. The active layer was applied on the according ITO/HTL substrate by spin-coating from a solution of P3HT and PCBM (1:1 weight ratio, 18 mg/mL each) in 70°C chlorobenzene at 1500 rpm for 60 s, followed by annealing at 120° C for 5 min. The film thickness obtained is around 150 nm. The cathode was thermally evaporated as a bilayer of LiF (2nm) and Al (100 nm). The nominal active area of the devices is 6 mm 2 (pixel size 1.5 mm x 4.0 mm).

Characterization
The surface chemistry of HTLs was characterized by XPS and local bulk chemistry by Raman spectroscopy. The XPS (Thermo Fischer Scientific, Waltham, MA USA) is equipped with a monochromatic Al Kα X-ray source (1486.6 eV). All scans were performed at room temperature.
The peaks were fitted using a Gaussian/Lorentzian mixed function employing Shirley background correction (Software XPSPEAK41). Raman measurements were performed with a Horiba Jobin Yvon LabRam 800 HR spectrometer equipped with a 1024 x 256 CCD (Peltiercooled) and an Olympus BX41 microscope. All scans were done using a laser wavelength of 532 nm (50 mW), an x50 Olympus LMPlanFLN (N.A. 0.5) objective and a 300 l/mm grating (spectral resolution of 3 cm -1 ). Spatial Raman spectroscopy characteristics for point-to-point analysis were taken in line-scans along 100 μm scanning area with 2 μm step size for PEDOT:PSS, and with 4 μm for MoO3-1 and MoO3-2, respectively. In order to avoid damage to the sample, the laser intensity was reduced to 10 % for the measurements of PEDOT:PSS and MoO3-2 and to 1 % for the measurements of MoO3-1. The photocurrent density-voltage (J-V) characteristics of the OPV cells were recorded using a computer-controlled Keithley 2636A source meter and AM1.5G illumination (100 mW/cm 2 ) from a solar simulator (Model 10500, 8 ABET Technologies, rated ABB). Spatial photocurrent distribution was determined by laser beam induced current (LBIC) method, scanning with a computer-controlled nano-manipulatordriven xy-stage (Kleindiek Nanotechnik, NanoControl NC-2-3) and excitation with a 532 nm laser (<5mW) with a focused spot-size of ≈2 µm. Spatial photocurrent mappings were scanned with 40 μm step size across the entire photoactive pixel area of 4.0 mm x 1.5 mm (Fig. 1). Local photocurrent-voltage characteristics for point-to-point analysis were taken in line-scans along the long side of each pixel, scanning from the left (0 mm) to the right (4 mm) with 160 μm step size.

RESULTS AND DISCUSSION
To investigate how the environmental conditions such as air, illumination and humidity affect the HTLs and in consequence lead to degradation of the OPV devices, solar photocurrent density-voltage (J-V) characteristics were recorded, shown in Figure 2. In every case the "fresh" condition acts as blind sample in this study, to account for environment-independent relaxation, aging or measurement-related changes of the materials. Figure 2 the by far lowest performance is shown by PEDOT:PSS/H2O, which exhibits lower rectangularity with FF of 0.39 and additionally greatly reduced photocurrent of JSC of -6.4 mA/cm 2 , but stable VOC of 0.55. The behaviour of the fresh sample compared to the air-and light-exposed ones can be explained by reduced interfacial charge transport efficiency between PEDOT:PSS and neighboring layers and an associated higher series resistance of the device, indicated by low FF and the change in slope around the VOC, respectively. The reason for the lack in interfacial transfer is the non-equilibrium condition the PEDOT:PSS layer is in when it is processed into a device right away. In that case, a certain degree of aging is necessary for relaxation of the polymer and its surface groups to make better contact to the ITO electrode and the active layer. The low FF accompanied by reduced photocurrent in PEDOT:PSS/H2O on the other hand, is a sign for loss of active area, in the sense of ITO corrosion producing insulating islands or local (photo)degradation of the adjacent active area facilitated by the absorbed water diffusing from the exposed PEDOT:PSS layer. [42][43][44] Figure 2 mA/cm 2 , while neither the FF nor the VOC are affected. The fact that FF and series resistance (indicated by the slope near VOC) do not change compared to the other samples, but only the current, indicates that the loss of photocurrent is caused by loss of photoactive area contributions, either by local insulating areas due to corrosion of the ITO electrode interface or by degradation of the adjacent organic semiconductor. Though the current slopes in VOC look quite parallel for MoO3-2/H2O compared to the other exposure conditions, there is a difference in series resistance with an increase of around 9 %, implying that the bulk or interfacial charge transport is slightly affected.
To differentiate any effects caused by the devices' charge transport ability and potential trapping phenomena, charge transport dynamics were studied via TPC. Complete sets of biased photocurrent transients following 300 µs square laser pulses for devices with exposed HTLs, for all conditions are shown in the Supporting Information with relative ( Figure S1) and normalized currents ( Figure S2). Selected normalized transient photocurrents are displayed in Figure 3, for devices of exposed PEDOT:PSS HTL (all conditions) and for the humidity-exposed MoO3 HTLs.  Figure S2) with rise and fall times of around t90 unhindered, giving an idea about the strength/depth of the charge traps in these devices. The longer rise and fall times also imply a lower charge transport efficiency of solar cell Thereby the rising relative magnitude of the overshoot transient peaks to the steady state photocurrent with increasing applied voltage, correlates with trap-facilitated recombination [37].
In this case their origin could be migrated indium or oxidation sites in the organic active layer, or at surface defects (e.g. dangling bonds) of the corroded inorganic ITO. [36]. The build-up of trapped charges in the device may hereby lead to increased recombination rates either directly through trap-assisted recombination routes such as Shockley-Read-Hall recombination, or through space charge mediating increases in bimolecular recombination and/or decreases in charge separation efficiency. [37] When the overshoot transient peaks are not observed in the reversed bias region this indicates that the larger internal electric field in the device inhibits trapmediated recombination. [46] XPS and Raman spectroscopy were investigated to understand changes in surface chemistry and local bulk chemistry of HTL films after different environmental exposure, which might explain Raman spectroscopy provides more detailed information related to the chemical properties of the bulk and is not recorded under vacuum, allowing the exposed films to maintain their chemical 14 surface composition during the measurement. In the present experiment, spatially resolved Raman measurements were performed which allow a point-to-point analysis of the film's chemistry, completing the highly accurate surface sensitive but area-integrated information of XPS. Figure 5 a-c present the averaged spectra of the area scans for all exposition scenarios for PEDOT:PSS, MoO3-1 and MoO3-2, respectively. The complete sets of the corresponding spatially resolved Raman spectra sequences obtained by Raman microscopy scans for all exposure conditions can be found in the Supplemental Information in Figure S3. As the humidity-exposed films seem to exhibit the most relevant changes, these are shown exemplarily for each of the materials in Figure 5 d-f. Figure 5 demonstrates spatially resolved Raman spectra from the PEDOT: PSS HTL after humid air exposure. The spectra present equal chemical composition across scanning area, merely varying by intensities with film thickness, which corresponds to the bubble feature displayed in the corresponding optical microscopy picture (Fig. 5d inset). These bubbles were exclusively found in humidity-exposed PEDOT:PSS films. It is suggested that the bubbles form as a consequence of film swelling upon water uptake by the hygroscopic polymer PSS. This means that the water exposure does not induce irreversible chemical changes in the PEDOT:PSS layer, but merely reversible water uptake, with eventual effect on morphology.
The average spectra of MoO3-1 HTLs for freshly prepared, air, air with illumination and humid air exposures are shown Figure 5  To investigate how altered surface and bulk properties of environmentally exposed HTLs and their local homogeneities affect OPV devices they have been integrated in, microscale laserbeam-induced photocurrent mapping was conducted.  Figure S4 of the Supporting Information. For better visibility of any performance variance between local J-V characteristics across a device area, the curves have been color-coded (analogous to the relative photocurrent of the maps in Figure 6) in order of their achieved shortcircuit current.  This is in good agreement with their integral device performance and the high homogeneity 20 shown in the photocurrent maps. The minor deviation of photocurrent (around 5% compared to maximum) across the device area is suspected to originate from regular morphological inhomogeneities of the devices [27] and not related to the treatment.  corrosion or humidity diffusion into the active layer with associated degradation, but instead merely signs for partial interfacial contact loss were found. These are caused by the consecutive swelling and shrinking of the layer on top of the ITO substrate and after incorporation within the device stack. Also fresh PEDOT:PSS layers which are immediately incorporated into devices present interfacial resistance, but, in that case because the layer is in a completely dry rigid nonequilibrium state at use. Any form of relaxation time leads to improvement. The two investigated solution processed molybdenum oxide films show extremely stable unchanging electrical behavior and homogeneity for any condition, except for humidity exposure of the particle layer.
In that case humidity treatment leads to swelling of the particles, similar to PEDOT:PSS, but due to the thin non-continuous layer, the effect leads to generally higher series resistance and thus lower current by either detachment of particles from the other layers or increased resistance of the particles themselves by the insulating hydrate shell. In no condition and for no material any material degradation was found for the HTL itself or the device they were incorporated in. The 25 observation of decreased device performance merely by the loss of interfacial contact upon water-uptake induced swelling is a completely new view on the matter.

Supporting Information
The Supporting Information contains complete sets of relative and normalized transient photocurrents, spatially resolved sequences of Raman spectra, statistical data on the solar cell performance and sets of local photo-J-V curves across the device for different holetransportation layer materials after various environmental exposures.

Notes
The authors declare no competing financial interest.