Use of lateral structures to monitor and evaluate degradation of key photovoltaic parameters in an organic bulk heterojunction material

Polymer fullerene bulk heterojunction (BHJ) systems are an important class of active material for organic photovoltaic (OPV) cells, and have attracted extensive research on their charge transport and recombination properties with the goal of improving device efficiency. A variety of techniques have been employed to study these materials in the vertical transport direction, such as transient photocurrents,¹ photo-generated charge extraction in a linearly increasing voltage (photo-CELIV),² time-of-flight,^3,4 impedance spectroscopy,^5,6 time resolved terahertz spectroscopy,⁷ time-resolved microwave conductivity,⁸ and dark-injection space charge limited current (DI-SCLC).⁹ We have developed a set of complementary methods based on lateral BHJ devices to measure charge transport along the in-plane axis, or lateral direction.^10–14 In a lateral BHJ device, both electrodes lie on the same plane and the photoactive region is defined by the channel between the two electrodes (Fig. 1(a)).¹⁵ This device geometry is a poor configuration for enhanced OPV efficiency, but is a useful materials diagnostic platform. Lateral devices are of interest because they allow for measurements over a wide range of transport length scales, are more amenable to device modeling due to their flat photogeneration profile, and most importantly, they enable direct access to the active region for spatially resolved measurements. The open geometry of lateral devices also allows us to monitor the effect of environmental conditions on important charge transport parameters in BHJ materials. Although carrier mobilities can be isotropic in BHJ materials due to their morphology or the presence of surface modifiers, other parameters such as carrier density and the recombination coefficient are scalar quantities and can be accurately evaluated using these lateral techniques.

Theoretical modeling and previous studies of lateral BHJ devices under large reverse voltage biases have shown that they exhibit space charge limited (SCL) transport behavior.^10,12–14 We use this to analyze charge transport properties within the bulk of the material. Although the bias voltages we employ are large, the value of the electric field in these structures is similar to that present in a typical vertical solar cell device. Due to these large biases, regions of space charge form adjacent to the electrodes even when the carrier mobilities are similar. These space charge regions (SCRs) extend 2–5 μm into the device channel and contribute the majority of the photocurrent. We utilize the potential drops in the SCRs (which can be easily measured) to calculate the carrier mobility ratio. The channel of a small (≤1 μm channel length) lateral BHJ device or one with a severe mobility imbalance in favor of one carrier will be composed almost entirely of one SCR dominated by the slower mobility carrier. Goodman and Rose first proposed a theoretical treatment of SCL transport,¹⁶ and Mihailetchi et al. initially observed SCL photocurrent in a BHJ solar cell.¹⁷ Adapting these results for a small lateral BHJ device, the current vs. voltage characteristics can be described by¹⁵ 

where χ is a constant coefficient related to the carrier mobility ratio, μ is the mobility of the slower carrier, ε is the dielectric constant, q is the fundamental charge, G is the generation rate, V is the applied voltage bias, and J_photo is the lateral photocurrent density. This equation is an adaptation of Eq. (15) reported in Ref. 15, for the SCL photocurrent in a unipolar lateral device. The modification here arises from the extension to the ambipolar case. The parameter χ ranges from 1 in the completely unipolar case to 1.4 for the case of equal carrier mobilities. The relationship between the mobility ratio and the parameter χ will be explored in more detail in an upcoming manuscript.¹⁸ 

The charge transport behavior of larger lateral BHJ devices and in materials with more equal mobilities is more complicated. Theoretical modeling predicts the formation of three distinct transport zones within the device channel: two SCRs adjacent to the electrodes, separated by a region where recombination is dominant (recombination zone).^15,18 This has also been confirmed in previous experimental studies.^13,14,19 The SCR adjacent to the cathode is electron dominated, while the SCR adjacent to the anode is hole dominated. Due to enhanced extraction near the electrodes, the electric field in the SCRs will be greater than in the recombination zone, and the majority of the applied voltage will be across the SCRs.¹⁵ The ratio of the SCR voltage drops is related to the carrier mobility ratio via

where μ_n and μ_p are the electron and hole mobilities, and ΔV_a and ΔV_c are the voltages dropped across the anode and cathode SCRs, respectively. Equation (2) was derived from a large number of simulations that related the potential drop across the SCRs to the ratio of the mobilities over a wide range of typical properties of organic BHJ films such as photogeneration rates, recombination coefficients, and relative permittivities. These simulations also allow us to estimate χ, the ambipolarity factor in Eq. (1), due to the dependence of χ on the mobility ratio.²⁰ 

Lateral BHJ devices with in situ potentiometric probes were fabricated on a glass substrate (Fig. 1). Interdigitated electrodes and voltage probes were defined using a JEOL JBX-6000 electron beam lithography tool. The potentiometry device channel lengths were 50 μm with a W/L of 500, while the smaller devices had channel lengths of 300 nm to 1 μm with a W/L of 1000. Five Ni probes extended 150 μm into the channel of the larger devices and were 200 nm wide. These voltage probes were placed at least 5 μm from the electrodes, inside the recombination zone of the large channel device. Aluminum (Al) was used as the cathode, gold (Au) as the anode, and nickel (Ni) for the voltage probes and device pads, due to its mechanical robustness. All metal layers were deposited via thermal evaporation to a thickness of 500 Å.

Before the deposition of the BHJ layer, the substrate was dipped into phosphoric acid solution for 10 s to remove any oxide from the surface of the Al electrodes. This was followed by a solvent rinse procedure of acetone, methanol, and isopropyl alcohol. BHJ absorber layers were deposited from a 20 mg/ml solution of P3HT:PCBM or P3HT:PC₇₁BM (1:1 by weight) in chloroform that had been heated to 50 °C and stirred for over 24 h. The BHJ was spun-cast at 1200 rpm for 60 s, followed by annealing at 140 °C for 15 min in a nitrogen atmosphere.

The sample was placed under vacuum in a Desert Cryogenics probe station for measurement. Current vs. voltage characteristics were measured in the dark and under illumination at 300 K, while in situ potentiometry was performed simultaneously using the Ni probes in the channel. The electric field generated due to work function differences between the electrodes and Ni probes was negligible compared to the overall field acting on the charge carriers within the lateral BHJ structure.

These simplified structures greatly reduce the measurement time, in contrast to previous work done using lateral BHJ devices with up to 24 in situ voltage probes for a higher resolution channel potential profile.¹⁴ This technique can therefore be used to actively monitor the degradation of BHJ materials, and determine the effect of degradation on parameters such as the carrier mobilities and recombination coefficient. The primary degradation mechanisms of P3HT:PCBM are known to be photo-oxidation processes that lead to a breaking of the macromolecular backbone.^21–23 To investigate processes that operate over shorter time scales, the P3HT:PCBM lateral devices were measured under high background pressure conditions (≥5 × 10⁻³Torr) in the cryogenic probe station at 300 K.

The voltage probe measurements yield linear voltage sweeps throughout the applied reverse bias voltage range. Reverse bias data were used as these conditions best simulate the internal electric field during OPV operation and minimize carrier injection.¹⁰ The presence of SCR behavior and the location of the voltage probes within the central recombination zone was confirmed using voltage exponent analysis; the calculation methods can be found in a previous manuscript.¹⁴ In Figure 2 inset, the probe voltages measured at different applied biases in which SCR behavior was observed are plotted spatially, constructing a simplified potential profile of the device channel. By measuring the slope of the potential profile, we determine the strength of the electric field in the recombination zone, E_r. In Fig. 2, the black projected line indicates E_r with a total applied reverse bias of 75 V, at the start of the measurement (t = 0 min). As the total applied voltage is greater than the voltage drop due to E_r, the remainder is dropped across the two SCRs. The cathode and anode SCR voltage drops are −19.42 V and −11.18 V, respectively, as shown by the intersection of the line at 0 μm and 50 μm. The unequal SCR voltage drops are due to an asymmetry in the carrier mobilities, as Eq. (2) shows. The larger cathode SCR voltage drop indicates that electrons are the lower mobility carrier. The carrier mobility ratio is approximately 0.76. This calculation was repeated for the different applied biases in Figure 2 inset and the mobility ratio is consistent over this range. The change in slope of the potential profile indicates a change in the mobility ratio of the BHJ film. Over the course of the measurement, a proportionally smaller amount of voltage is dropped across the hole dominated SCR at the anode. The electron mobility therefore decreases relative to the hole mobility over the measurement time. The higher slope can also imply a higher recombination rate, due to the increase in voltage across the recombination zone.

The actual carrier mobilities are calculated from the photocurrent measurements and Eq. (1). To determine when the devices are operating in the SCL regime, the functional dependence of J_photo on the applied voltage was calculated over the entire applied bias range, as described in previous studies.^10,12 Outside of the SCL regime, contact effects or carrier injection determine the photocurrent-voltage relationship.^10,19 Within this range, the lateral photocurrent can be described as J ∝ αV^1/2, where α is dependent on the slower carrier mobility. The calculated slope is used to find the slower (electron) mobility via Eq. (1). The faster (hole) mobility is calculated using the previously determined mobility ratio, and the carrier concentration and the bimolecular recombination coefficient are calculated from the recombination zone conductivity of the larger device.¹⁴ Table I lists charge transport parameters of the P3HT:PCBM film at multiple times, relative to the beginning of the measurement. These values are consistent with previous reports using photo-CELIV and SCL based methods.^12,24–26

Although a decrease in the recombination zone conductivity was observed over time, we attribute this to a significant decrease in mobility rather than a change in the recombination characteristics. The carrier concentration and recombination coefficient remain relatively constant over the course of the experiment, while both carrier mobilities decrease significantly. The mobilities also become more imbalanced over time, with the electron mobility more severely affected. This observed imbalance is consistent with EFISH measurements of lateral P3HT:PCBM devices over similar time scales.²⁷ A rapid increase in the voltage across the electron dominated SCR was noted and attributed to photo-oxidation introducing negatively charge electron traps, resulting from moisture within the device.^27,28

This procedure was repeated with the same P3HT:PCBM solution, but with 5% by volume of the solvent additive DIO (1,8-diiodooctane). DIO is known to produce better phase separation and structural order within the BHJ film, resulting in higher performance of OPV cells.^29,30 We repeated this experiment with the solvent additive in an attempt to mitigate the mobility imbalance in the material. Table II lists the charge transport parameters with respect to time of this modified P3HT:PCBM solution.

Addition of DIO does not eliminate the mobility imbalance in favor of the hole mobility or the decreasing trend of the mobility ratio. The initial measured mobilities are lower but more stable over time. The mobility ratio gradually reaches a stable point of approximately 0.4 over several hours. Although it does not correct the mobility asymmetry, the addition of DIO does improve the other charge transport parameters of the P3HT:PCBM film. The carrier concentration within the bulk P3HT:PCBM increases and the bimolecular recombination coefficient decreases with respect to the normal film.

Previously, we have shown using light intensity dependent measurements that bimolecular recombination is the dominant mechanism for nongeminate recombination loss in lateral P3HT:PCBM devices.¹⁴ The dominant recombination mechanism, however, can vary between organic BHJ material systems and change due to environmental conditions. In addition, multiple mechanisms may be significant during device operation.^1,31 Lateral devices offer a unique characterization method that enables us to alter the recombination mechanism in the device and measure the resulting change in charge transport parameters of the BHJ material. When an organic semiconductor is exposed to a polar molecule such as ethanol, there is enhanced self-trapping of charges, which reduce the photocurrent.³² The polar molecule is absorbed on to the surface of the semiconductor, where its dipole induces a local field through polarization of the semiconductor. The dipole in the polar molecule may also shift in response to charge carriers at material interfaces in the BHJ material. This effect has been previously utilized in sensing applications for organic FETs,^32,33 and we use a similar procedure to measure the effect of the increased charge trapping on lateral BHJ devices.

Lateral P3HT:PC₇₁BM devices were measured initially under vacuum better than 5 × 10⁻⁵Torr at 300 K. The probe station was then filled with nitrogen containing 2000 ppm of ethanol vapor (dipole moment = 1.69 D) to approximately 100 Torr and the lateral BHJ devices were measured again. Finally, the cryo station was pumped back down to vacuum better than 5 × 10⁻⁵Torr and the devices measured again. The sample was kept at 300 K for the entire procedure. Figure 3 illustrates the change in recombination zone conductivity² behavior with respect to incident light intensity, for these three conditions. We expect a linear dependence of conductivity² vs. light intensity for the case of bimolecular recombination.¹⁴ 

The conductivity² data within the recombination zone are fitted to a function of the form: $σR2=aPb$⁠, where P is the incident light intensity. At the highest light intensity, the measured conductivity² is consistently lower than expected from the fit, due to the carrier concentration dependence of the recombination coefficient.³⁴ Initially, the fitted exponent b is at a value of 1.71, indicating the presence of both unimolecular and bimolecular recombination mechanisms. A purely unimolecular recombination process would show a quadratic dependence of the recombination zone conductivity on the light intensity, as the electron and hole concentrations in the recombination zone are equal. After the sample is exposed to the ethanol vapor, the exponent increases, indicating a shift to more unimolecular dominated recombination as traps are introduced into the BHJ system. This shift is not reversible by pumping to low vacuum, and the increase in unimolecular recombination increases even after the removal of the ethanol vapor. It is likely that the introduced trap states remain in the system without additional treatment, such as annealing, to remove them. The effect of the introduced dipole can also been seen in the measured SCL mobility of the BHJ material, calculated from small lateral device photocurrent measurements as described previously. The recombination coefficients were not calculated due to the apparent change in recombination mechanism over the course of the experiment.

Initially, the measured SCL mobilities are consistently high (∼3.5 × 10⁻³ cm²/Vs) for all measured incident light intensities (Fig. 4). The mobility ratio μ_n/μ_p is close to 1.0, determined from potentiometry measurements on large devices. Upon introduction of the ethanol vapor, the SCL mobilities at low intensities begin to decrease, and the mobility ratio diverges from 1.0 at these low intensities as well, with electrons as the slower carrier. At higher intensities, there is little change. The increased number of traps would have the most noticeable effect on the SCL mobilities at low intensities, where the low photogenerated carrier concentrations fill a smaller proportion of the trap states. After the ethanol vapor is removed and the probe station has been pumped back down to low pressures, the material has deteriorated further. Much of the absorbed ethanol remains in the film and increases the self-trapping of charges. The SCL mobility at high intensities decreases, and there is a more significant drop in mobility and the mobility ratio at low intensities. The larger effect on the electron mobility may indicate the higher number of electron trap states introduced upon exposure to ethanol vapor.

A new technique utilizing lateral device structures to determine important charge transport parameters in organic BHJ systems has been demonstrated. Small channel length lateral BHJ devices are dominated by a single space charge region, with a high carrier density of the slower type. Larger channel length lateral BHJ devices, in contrast, are divided into 3 separate charge transport zones. Space charge regions exist adjacent to the electrodes, separated by a large region in which recombination is dominant. Simple voltage probe configurations can be used to measure the potential profile of these recombination zone dominated devices, detecting carrier mobility asymmetries quickly and easily. We combine these methods with lateral mobility measurement techniques from the SCL photocurrent of small devices. This allows us to study the time dependence of the carrier mobilities, concentrations, and recombination coefficient within a BHJ material under certain conditions. Spun cast P3HT:PCBM films from chloroform solutions can be observed to decrease in mobility ratio over a time scale of an hour under high background pressure conditions. This is consistent with previous observations of degradation due to photo-oxidation within lateral BHJ devices. The addition of DIO does not eliminate the mobility asymmetry, and the mobility ratio continues to decrease over several hours until stabilizing at 0.4. We are also able to induce traps within a P3HT:PC₇₁BM film and shift the recombination mechanism to a more unimolecular nature when exposed to ethanol vapor. The calculated SCL mobilities are observed to decrease and become asymmetric upon exposure to the ethanol vapor. Large decreases in the SCL mobility at low incident light intensities are consistent with the appearance of dipole-induced traps. This technique offers a novel method to monitor degradation within BHJ films and couple degradation to changes in the charge transport parameters of the film.

The authors would like to thank the facilities staff at the Microelectronics Research Center at The University of Texas at Austin. In addition, the authors acknowledge the ONR under STTR Project No. N00014-10-M-0317. Z.-E. Ooi acknowledges support from A*STAR Singapore under the VIP program.