Crystallinity Effects in Sequentially Processed and Blend-Cast Bulk-Heterojunction Polymer/Fullerene Photovoltaics

Abstract

Although most polymer/fullerene-based solar cells are cast from a blend of the components in solution, it is also possible to sequentially process the polymer and fullerene layers from quasi-orthogonal solvents. Sequential processing (SqP) not only produces photovoltaic devices with efficiencies comparable to the more traditional bulk heterojunction (BHJ) solar cells produced by blend casting (BC) but also offers the advantage that the polymer and fullerene layers can be optimized separately. In this paper, we explore the morphology produced when sequentially processing polymer/ fullerene solar cells and compare it to the BC morphology. We find that increasing polymer regioregularity leads to the opposite effect in SqP and BC BHJ solar cells. We start by constructing a series of SqP and BC solar cells using different types of poly(3-hexylthiophene) (P3HT) that vary in regioregulary and polydispersity combined with [6,6]-phenyl-C 61 -butyric-acid-methyl-ester (PCBM). We use grazing incidence wideangle X-ray scattering to demonstrate how strongly changes in the P3HT and PCBM crystallinity upon thermal annealing of SqP and BC BHJ films depend on polymer regioregularity. For SqP devices, low regioregularity P3HT films that possess more amorphous regions allow for more PCBM crystallite growth and thus show better photovoltaic device efficiency. On the other hand, highly regioregular P3HT leads to a more favorable morphology and better device efficiency for BC BHJ films. Comparing the photovoltaic performance and structural characterization indicates that the mechanisms controlling morphology in the active layers are fundamentally different for BHJs formed via SqP and BC. Most importantly, we find that nanoscale morphology in both SqP and BC BHJs can be systematically controlled by tuning the amorphous fraction of polymer in the active layer.

Sequential Processing for Organic Photovoltaics: Design Rules for Morphology Control by Tailored Semi‐Orthogonal Solvent Blends

“…We also explore how our design rules apply to highly crystalline polymers such as P3HT when the degree of fi lm crystallinity is controllably varied. [ 57 ] Thus, the SqP design rules we establish here open up an entire new avenue of morphology control in processing polymer:fullerene BHJ photovoltaics.…”

“…[ 57 ] In particular, we found that as we decreased the P3HT crystallinity, traditional blend-cast devices had their performance suffer, but the performance of sequentially-processed devices improved because there was more swelling of the polymer underlayer leading to better fullerene intercalation and photocurrent. [ 57 ] We therefore conclude that porosimetry-ellipsometry measurements can be used to understand-and thus ultimately manipulate-the extent of fullerene intercalation into polymer/fullerene active layers produced by SqP. Hence, by controlling the degree of underlayer swelling, [ 57 ] SqP offers a more rational route to polymer:fullerene BHJ formation.…”

“…[ 37,[53][54][55][56] This results in a P3HT/PCBM quasibilayer structure that must be thermally annealed to drive the remaining fullerene into the amorphous regions of the polymer fi lm to form the requisite BHJ. [ 48,57 ] Fortunately, the hierarchical semi-crystalline structure of P3HT can tolerate such annealinga property that many other photovoltaic conjugated polymers lack. [ 58 ] Thus, to this point, SqP research has been mostly focused on polymers that either have a low DCM solubility and a nm-scale structure stable enough for thermal annealing (e.g., P3HT) [ 59,60 ] or polymers that happen to swell an ideal amount when using DCM as the fullerene casting solvent.…”

“…[69][70][71][72][73][74] In these measurements the changes in a conjugated polymer fi lm's refractive index and thickness are monitored in a non-absorbing spectral region while the sample is exposed to controlled amounts of the co-solvent vapor (for experimental details, see the Supporting Information (SI)). Since most of the previous work on SqP is focused on P3HT, [ 20,22,37,38,48,56,57 ] we begin by examining the volume changes of swollen thin fi lms of P3HT that have varying degrees of crystallinity.…”

Roughening Conjugated Polymer Surface for Enhancing the Charge Collection Efficiency of Sequentially Deposited Polymer/Fullerene Photovoltaics

“…All of these features lead to the excellent electrical properties of these materials [6,7]. Recently, many interesting reports on SqSD-processed polymer/fullerene OPVs based on P3HT and fullerene derivatives have been reported [4,5,[8][9][10][11][12][13][14][15]. However, similar to a BSD-processed P3HT/fullerene OPVs, SqSD-processed P3HT/fullerene OPVs require a thermal annealing step to obtain a satisfactory heterojunction area.…”