Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Abstract

This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span from photoexcitation to charge separation, recombination, and sweep‐out to the electrodes. Exciton formation and relaxation in poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and poly‐3(hexylthiophene) (P3HT) are discussed based on a fluorescence up‐conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re‐examined in light of these femtosecond time‐resolved measurements. Transient photoconductivity, time‐delayed collection field, and time‐delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep‐out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias‐ and charge‐density‐dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.

Uncovering loss mechanisms in silver nanoparticle-blended plasmonic organic solar cells

“…To account for the measurements being performed at high fluence, we included a carrier concentration-dependent mobility mpN a -as a consequence of energetic disorder on the hopping of thermally activated charge carriers [25][26][27][28][29] , where a is the concentration-dependent exponent 23 . Hence, the expressions are:…”

Series resistance in organic bulk-heterojunction solar devices: Modulating carrier transport with fullerene electron traps

“…Finally charge carrier transport within the active layer itself could also be a source for incrementing the series resistance. The overall impact of electronic transport mechanisms is recognized to have dramatic effects when relatively thicker active layer films are used to enhance light harvesting [2]. While thinner films are able to exhibit almost 100% conversion of absorbed photons into collected carriers [3], thus indicating that transport mechanisms do not limit the achievable photocurrent [4], thicker active layer devices suffer from an incomplete collection of photogenerated charges.…”

Effect of leakage current and shunt resistance on the light intensity dependence of organic solar cells

“…As shown in Figure 2(d), the J sc for all four devices here exhibits an identical dependence on light intensity which can be described with a power law fit to J sc / I a with a ¼ 0.99 which is a typical value for organic solar cells such as p-DTS(FBTTh 2 ) 2 :PC 71 BM that have balanced carrier mobilities and only modest bimolecular recombination losses at short circuit. 1,15,19 Comparing Figure 2(c) with the light dependence of the solar cell power conversion efficiency (PCE) shown in Figure 3(a), it is evident that the PCE light dependence is largely set by the trend in FF with the V oc dependence also playing a role. At intensities close to one sun, the PCE of each device is similar as are the FFs however the difference is pronounced at lower light intensities where the leakage current competes favorably with the photocurrent.…”