A Shockley‐Type Polymer: Fullerene Solar Cell

Abstract

Charge extraction rate in solar cells made of blends of electron donating/accepting organic semiconductors is typically slow due to their low charge carrier mobility. This sets a limit on the active layer thickness and has hindered the industrialization of organic solar cells (OSCs). Herein, charge transport and recombination properties of an efficient polymer (NT812):fullerene blend are investigated. This system delivers power conversion efficiency of >9% even when the junction thickness is as large as 800 nm. Experimental results indicate that this material system exhibits exceptionally low bimolecular recombination constant, 800 times smaller than the diffusion‐controlled electron and hole encounter rate. Comparing theoretical results based on a recently introduced modified Shockley model for fill factor, and experiments, clarifies that charge collection is nearly ideal in these solar cells even when the thickness is several hundreds of nanometer. This is the first realization of high‐efficiency Shockley‐type organic solar cells with junction thicknesses suitable for scaling up.

Impact of Bimolecular Recombination on the Fill Factor of Fullerene and Nonfullerene-Based Solar Cells: A Comparative Study of Charge Generation and Extraction

“…In the SI we present a comparison between the measured data with that of the literature, on the competition factor between charge extraction and second-order bimolecular recombination as defined by Bartesaghi et al 4 (see Figure S4). Previous report has suggested 800 times reduced recombination for NT812:PCBM[70], 15 however we note that NT812 has a different molecular weight in this study and also yields lower efficiency compared to the previous work. Concerning charge recombination, from pre-bias dependent TDCF measurements at different delay times between excitation and extraction, we observe that whilst both devices exhibit a dependence of extraction on pre-bias, the PCBM device however, shows an almost invariant behaviour with the delay time, whilst the ITIC system shows a strong dependence of extracted carriers on the delay time.…”

“…33 In NT812:PCBM[70], the suppressed recombination is assigned to the slower loss decay and back electron transfer rate of the CT states compared to their dissociation rate which results in equilibrium between the CTS and free charges. 15 We note that the this kinetic completion is very sensitive to the active layer's morphology and even for the same material system, one can observe different kinetics by changing the processing conditions or the molecular weight of the polymer.…”

“…21,22 We had previously measured hole and electron mobilities of 4 × 10 -4 and 2 × 10 -3 cm 2 V -1 s -1 respectively, in NT812:PCBM[70] devices. 15 There are differing views on the importance of mobility balance from the theoretical perspective; most simulations have predicted photogeneration to suffer strongly from imbalanced mobilities. 23 In addition, the preferential extraction of only one carrier type is well-known to cause space charge of the slower species to build up.…”

“…14 In previous work we presented charge transport and recombination properties of NT812:PCBM[70] and found significantly suppressed bimolecular recombination in this system. 15 In this study, we employ both steady state and transient electro-optical measurements to disentangle geminate and non-geminate loss processes in NT812 when blended with PCBM[70] and ITIC devices (see scheme 1 and Figure S1 for chemical structure and energy levels). We investigate possible reasons for the lower FF in ITIC blends.…”

Fully Organic Photocontrolled Deformable Mirror

“…It is relatively difficult to embed all these three features in a single organic material. While a low dielectric constant (ε r ≅ 3) is a common feature of organic semiconductors, relatively high carrier mobilities (of the order of 10 −3 cm 2 V −1 s −1 ) are recorded in photovoltaics and photodetectors, but only when materials are processed in the form of thin films (typically from 10 nm to 1 µm) . Higher values (1–10 cm 2 V −1 s −1 ) have been measured in organic crystals and in the latest generation high‐mobility polymers (but only at high carrier densities reached in the accumulation channel of transistors).…”