Imaging Local Trap Formation in Conjugated Polymer Solar Cells: A Comparison of Time-Resolved Electrostatic Force Microscopy and Scanning Kelvin Probe Imaging

Abstract

We study local photooxidation and trap formation in all-polymer bulk-heterojunction organic photovoltaics (OPVs) using both time-resolved electrostatic force microscopy (trEFM) and conventional scanning Kelvin probe microscopy (SKPM). We create electron-trapping defects at known locations by locally photooxidizing blends of poly[(9,9′-dioctylfluorene-alt-(bis(N,N′-(4-butylphenyl))-bis(N,N′-phenyl-1,4-phenylenediamine)] and poly[9,9′-dioctylfluorene-alt-1,4-benzothiadiazole]. We then compare the local surface photovoltage shifts measured via SKPM and the changes in local photoinduced charging rates measured via trEFM with changes in the performance of macroscopic photodiodes that have been exposed to similar photooxidation. We find that the trEFM charging rate images can identify local photooxidation and trap formation with much better sensitivity than conventional SKPM images. In addition, the changes in the trEFM charging rates correlate well with the external quantum efficiencies of the macroscopic photodiodes. In contrast, the SKPM images not only are less sensitive to trap formation but also show a more complicated response. We conclude that trEFM is well suited to studying local trap formation in organic solar cells and caution that SKPM data by itself can be difficult to interpret on OPV films, especially when materials have been exposed to photooxidation.

Water Ingress in Encapsulated Inverted Organic Solar Cells: Correlating Infrared Imaging and Photovoltaic Performance

“…Multiple degradation routes have been identifi ed in operating devices featuring P3HT:PCBM, which are related to the infl uence of ambient conditions (temperature, moisture, oxygen) in combination with light but also to material diffusion, and physical stress. [ 9,11,13,[15][16][17][18][19][20][21][22][23][24] More recently, the focus has been shifted toward correlating photochemical degradation reactions, such as photobleaching of the polymer, [ 9,25,26 ] fullerene dimerization, [ 27 ] cross-linking of photoactive components, [ 28 ] and interfacial reactions, with the observation of trap states [ 18,29,30 ] and charge carrier accumulation in the bulk and at the electrode interfaces of photovoltaic devices. [ 31,32 ] While elucidating the relationship between observed performance loss and cause is often diffi cult, the underlying degradation mechanisms depend critically on the active materials and the device geometry.…”

A multiscale modeling study of loss processes in block-copolymer-based solar cell nanodevices

“…This leads to a lower charge carrier mobility and ultimately to a lower device performance. Such a phenomenon has been investigated, e.g., by time-resolved electrostatic force microscopy and scanning Kelvin probe imaging by Reid et al 18 Due to the complex relationship of these loss mechanisms within the photovoltaic process and their strong dependence on the structural-dynamical characteristics of the material, their impact on the photovoltaic properties has not been minimized in a satisfactory way for the polymer solar-cell materials currently available. 5,19 Therefore, it is not surprising that their power conversion efficiency is still relatively low, compared to their inorganic counterparts.…”

Computation of full polymer-based photovoltaic nanodevices using a parametrized field-based multiscale solar-cell approach

“…70 The causes for the low power conversion efficiency of PSCs have 71 primary been associated with loss phenomena of the elementary 72 particles, occurring during the photovoltaic process, such as pho- 73 ton loss, exciton loss and charge carrier loss [25]. Their complex 74 relationship and strong dependence on the structural-dynamical 75 characteristics of the photoactive material have been investigated 76 in various recent experimental [26][27][28][29] and theoretical studies 77 [17,[30][31][32][33][34]. For example, photon losses can be caused by the 78 [17,39].…”

“…The graphs (a) and (b) correspond to the results of the conventional field-based approach, the graphs (c) and (d) to the results of the parametrized field-based approach and the graphs (e) and (f) to the results of the atomistic particle-based approach. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article [73,[75][76][77]…”