Pathway Towards High Performance Organic Photovoltaics: Contact Doping and Morphology Control

Open Access
Author:
Le, Thinh Phuc
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 02, 2016
Committee Members:
  • Enrique Daniel Gomez, Dissertation Advisor
  • Enrique Daniel Gomez, Committee Chair
  • Janna Kay Maranas, Committee Member
  • Michael John Janik, Committee Member
  • Michael Anthony Hickner, Outside Member
Keywords:
  • organic photovoltaics
  • organic semiconductors
  • polarized resonant soft X-ray scattering
  • PSOXS
  • Morphology control
  • contact doping
  • crystallization control
  • polymer crystallization
Abstract:
State-of-the-art organic solar cells rely on kinetically trapped, partially phase separated structures of electron donor and acceptor blend mixtures. However, blend systems suffer from morphological instability and disorder near metal contacts which hamper device performance and lifetime. We demonstrate potential strategies to improve performance of organic photovoltaics via contact doping with polymer electrolytes and controlling the active layer morphology using fully conjugated block copolymers. We demonstrated that polymer acids can act as p-type dopants near electrode interfaces for active layers containing poly(3-hexylthiophene-2,5-diyl) (P3HT). By varying the pendant acidic groups across different backbones, we find the effectiveness of doping the conjugated polymer at the interface depends on the strength of the pendant acid group with stronger acid moieties being capable of creating more carriers in the doped system. Nevertheless, strong pendant acid groups also cause phase separation between dopants and conjugated polymers, thus hindering the doping effectiveness. The overall doping efficacy near electrodes therefore depends on the interplay between the strength of pendant acid groups and miscibility between polymeric dopants and conjugated polymers. To better control the active layer morphology and address morphological instability problem, we employed fully conjugated block copolymer composed of donor and acceptor blocks as active layer material. Previously, we have demonstrated that poly(3-hexylthiophene)−block−poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (P3HT-b-PFTBT) can self-assemble into 10 nm lamellae with alternating electron donor and acceptor domains. The resulting solar cell performance is approximately 3% which clearly outperforms blends of the same component obtained due to the self-assembly of the block copolymers. Nevertheless, one of the challenges in controlling the self-assembly of fully conjugated block copolymers is controlling the interplay between crystallization of the P3HT block and microphase separation between the donor and acceptor. To this end, we have examined the kinetics of the morphological evolution during two processes: solution casting and thermal annealing. We find that during film drying, P3HT crystallization happens on a much faster time scale than phase separation of the two blocks but the crystallization is significantly suppressed with respect to neat materials, enabling the microphase separation to proceed at time scales after crystallization of P3HT takes place. This enables the mesoscale structure to develop during processes such as thermal annealing, because self-assembly of the lamellar structure takes place before the crystallization of P3HT is complete. We also discover there is competitive crystallization between P3HT and PFTBT. In P3HT-b-PFTBT, P3HT crystallization dominates while PFTBT crystallization is either delayed or completely subdue, depending on the volume fraction of P3HT. The overall device performance strongly depends on the interplay between order phase formation in both P3HT and PFTBT. To further understand morphology evolution in block copolymer, we employed polarized resonant soft X-ray scattering (PSOXS) to study the interfacial molecular alignment in P3HT-b-PFTBT block copolymer. Using two different batches of block copolymer with different degree of P3HT crystallization, we found that in block copolymer where P3HT strongly crystallize, it’s harder to achieve lamellar morphology due to the formation of P3HT crystallites within a kinetically trapped P3HT and PFTBT amorphous matrix after solvent casting. High thermal annealing is required to provide enough energy to promote nanoscale phase separation and lamellae formation. However, the lamellar domains that formed at high temperature are highly-ordered with P3HT crystallites embedded inside. Here, P3HT chains aligned parallel with respect to lamellae domain interface. PSOXS also reveals the molecular orientation of P3HT crystallites within the P3HT lamellar domain. In block copolymer where P3HT crystallization is suppressed, weak P3HT crystallization allows for easier lamellae formation at low annealing temperature but at the cost of well-ordered phase separated domains.