Measurement of Surface Photovoltage by Atomic Force Microscopy under Pulsed Illumination Zeno Schumacher, * Yoichi Miyahara, Andreas Spielhofer, and Peter Grutter McGill University, Montreal H3A 2T8, Canada (Received 13 January 2016; revised manuscript received 24 February 2016; published 28 April 2016) Measuring the structure-function relation in photovoltaic materials has been a major drive for atomic force microscopy (AFM) and Kelvin-probe force microscopy (KPFM). The local surface photovoltage (SPV) is measured as the change in contact potential difference (CPD) between the tip and sample upon illumination. The quantities of interest that one will like to correlate with the structure are the decay times of SPV and/or its wavelength dependence. KPFM depends on the tip and sample potential; therefore, SPV is prone to tip changes, rendering an accurate measurement of SPV challenging. We present a measurement technique which allows us to directly measure the difference in the CPD between illuminated and dark states and, thus, SPV as well as the capacitance derivative by using pulsed illumination. The variation of the measured SPV can be minimized due to the time-domain measurement, allowing accurate measurements of the SPV. The increased accuracy enables the systematic comparison of SPV across different measurement setups and excitation conditions (e.g., wavelength dependence and decay time of SPV). DOI: 10.1103/PhysRevApplied.5.044018 I. INTRODUCTION The structure-function relationship of photoactive mate- rials is of great interest and can be studied with atomic force microscopy (AFM) [1–6]. Two main modes of measure- ment can be used: photocurrent and photovoltage mea- surements. Photoconductive atomic force microscopy (PC AFM) was first presented by Sakaguchi et al. [7] in 1999 using a conductive AFM to measure the photocurrent of organic thin films. Photoconductive AFM was later used by Coffey et al. [8] to map the photocurrent distribution in an organic photovoltaic blend with approximately 20-nm resolution. In 2013, Beinik et al. [9] used PC AFM to investigate the response of ZnO nanorods in a time range greater than seconds. In contrast to photoconductive AFM, photovoltage measurements can be performed without contact between the sample and the AFM tip. Kelvin-probe force micros- copy (KPFM) has become a widely used technique to study not only inorganic but also organic photovoltaic materials [1,3,5,10]. In particular, surface photovoltage (SPV) meas- urement is commonly used to measure minority-carrier diffusion length [11] and lifetime under illumination in semiconductors [12–14]. The spatially resolved measure- ment of carrier lifetime and local recombination rates are of great interest to understand the fundamental charge gen- eration process in photovoltaic materials. SPV values are the result of two measurements typically performed con- secutively: SPV ¼ CPD illuminated - CPD dark where CPD is the contact potential difference between the AFM tip and sample. The CPD is measured using KPFM in which a dc-bias voltage (KPFM signal) minimizing the electrostatic force is determined with a feedback circuit. Note that the CPD is the difference between the tip potential and the sample potential—an intrinsic assumption of CPD measurements using KPFM is, thus, that the tip potential remains constant. This assumption is often violated due to the time it takes to measure the spatial dependence of SPV, its wavelength dependence, or decay times. Note that even under an ultra-high-vacuum condition, tip contamina- tion by adsorption of rest gas atoms can change the tip potential (i.e., the work function) in less than a minute [15]. The accuracy of a SPV measurement is, thus, ultimately limited by the stability of the tip surface potential during the time it takes to measure CPD illuminated and CPD dark . Measurement times can extend over multiple hours or days when the SPV is investigated under various illumination conditions, such as light intensity, wavelength, or polariza- tion, or changes in the sample environment such as the temperature or electric field. Therefore, a measurement method which is insensitive to such tip surface potential changes is needed. Takihara et al. [16] presented a method to measure the local carrier lifetime with SPV and AFM by using pulsed illumi- nation. By varying the repetition rate of the pulse illumination and measuring the average KPFM signal, a higher time resolution than the KPFM feedback time constant can be achieved. This method has also been used on silicon nano- crystal solar cells [17] and organic photovoltaic materials to study local carrier-recombination rates [10]. Note that the accuracy of the SPV measurement determines the temporal resolution achievable with this approach. A further limiting factor of conventional SPV measure- ments based on KPFM is that they are only sensitive to a * zenos@physics.mcgill.ca PHYSICAL REVIEW APPLIED 5, 044018 (2016) 2331-7019=16=5(4)=044018(6) 044018-1 © 2016 American Physical Society