Comparing Matched Polymer:Fullerene Solar Cells Made by Solution-Sequential Processing and Traditional Blend Casting: Nanoscale Structure and Device Performance

Abstract

Polymer:fullerene bulk heterojunction (BHJ) solar cell active layers can be created by traditional blend casting (BC), where the components are mixed together in solution before deposition, or by sequential processing (SqP), where the pure polymer and fullerene materials are cast sequentially from different solutions. Presently, however, the relative merits of SqP as compared to BC are not fully understood because there has yet to be an equivalent (composition-and thickness-matched layer) comparison between the two processing techniques. The main reason why matched SqP and BC devices have not been compared is because the composition of SqP active layers has not been accurately known. In this paper, we present a novel technique for accurately measuring the polymer:fullerene film composition in SqP active layers, which allows us to make the first comparisons between rigorously composition-and thickness-matched BHJ organic solar cells made by SqP and traditional BC. We discover that, in optimal photovoltaic devices, SqP active layers have a very similar composition as their optimized BC counterparts (≈44−50 mass % PCBM). We then present a thorough investigation of the morphological and device properties of thickness-and composition-matched P3HT:PCBM SqP and BC active layers in order to better understand the advantages and drawbacks of both processing approaches. For our matched devices, we find that small-area SqP cells perform better than BC cells due to both superior film quality and enhanced optical absorption from more crystalline P3HT. The enhanced film quality of SqP active layers also results in higher performance and significantly better reproducibility in larger-area devices, indicating that SqP is more amenable to scaling than the traditional BC approach. X-ray diffraction, UV−vis absorption, and energy-filtered transmission electron tomography collectively show that annealed SqP active layers have a finer-scale blend morphology and more crystalline polymer and fullerene domains when compared to equivalently processed BC active layers. Charge extraction by linearly increasing voltage (CELIV) measurements, combined with X-ray photoelectron spectroscopy, also show that the top (nonsubstrate) interface for SqP films is slightly richer in PCBM compared to matched BC active layers. Despite these clear differences in bulk and vertical morphology, transient photovoltage, transient photocurrent, and subgap external quantum efficiency measurements all indicate that the interfacial electronic processes occurring at P3HT:PCBM heterojunctions are essentially identical in matched-annealed SqP and BC active layers, suggesting that device physics are surprisingly robust with respect to the details of the BHJ morphology.

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

“…[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.…”

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

“…55 Moreover, nominally matched sequentially processed and BC photovoltaic devices show different behaviors upon thermal annealing. 42,48,49,56 Thus, the most important issues concerning solar cells fabricated via SqP are precisely how the nm-scale morphology is different from that in BC films and what factors control the extent and distribution of fullerene interpenetration into the polymer underlayer.…”

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

“…UV-VIS absorption spectra of the bilayer films confirm that the PC 61 BM was successfully deposed on the P3HT layer. Considering the reports on the SqSD-processed P3HT/PCBM film, the P3HT/PCBM is expected to form an inter-diffused bilayer instead of forming a bilayer with clear interface [5,11,12]. Interestingly, the P3HT/PC 61 BM bilayer film shows vibronic peaks from the P3HT at longer wavelengths, which is not observed in the P3HT:PC 61 BM BHJ film prepared from the blended solution of P3HT and PC 61 BM (triangle of Figure 2c).…”

“…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.…”