Match the Interfacial Energy Levels between Hole Transport Layer and Donor Polymer To Achieve High Solar Cell Performance

Abstract

The interfacial energy level alignment is shown to play an important role in determining solar cell performance. Replacing hole transport layer poly(3,4-ethylene dioxythiophene)−(polystyrene sulfonic acid) (PEDOT:PSS) with vanadium pentoxide (V 2 O 5 ) leads to a simultaneous improvement in short-circuit current density (J sc ), open-circuit voltage (V oc ) and fill factor (FF) for two donor polymers with deep HOMO energy levels, resulting in a power conversion efficiency (PCE) of 7.03% and 4.14%. This is 18% and 106% increase in PCE over the 5.97% and 2.01% achieved with PEDOT:PSS. V 2 O 5 is shown to increase J sc and FF by enhancing hole mobility, reducing bimolecular recombination, and facilitating charge collection and to maximize V oc by providing a better ohmic contact. We also demonstrate that PEDOT:PSS still works better for donor polymers with a HOMO energy level around 5.1 eV, such as PTB7.

Simultaneous enhancement of the efficiency and stability of organic solar cells using PEDOT:PSS grafted with a PEGME buffer layer

“…Next, we measured the work function of PEDOT:PSS films modified with different PSS:PEGME weight ratio to confirm the more relevant cause of the enhancement of the PCE. The interfacial energy level alignment between HTL and active layer is important for increasing the hole extraction efficiency of the photo-generated hole, which leads to improvements in the PCE of OSCs [45,51]. As shown in Fig.…”

“…The increased J sc and FF for the PCE enhancement can be attributed to increased hole extraction and reduced bimolecular recombination by matching the interfacial energy level between PEDOT:PSS and active layer [51]. To further investigate the cause of the improved FF, R s and R sh were estimated from the J-V curves of the organic solar cells by using a simple equivalent circuit model because FF is determined by both R s and R sh [56].…”

“…However, even though the conductivity of PEDOT:PSS still increased after the 1:0.50 ratio, R s also increased, which may be attributed to the increased interfacial resistance by the increased energy level barrier or the presence of thin insulating layer of PEGME on the surface of modified PEDOT:PSS film. Up to the 1:0.50 ratio, the increased R sh may result from the decreased charge carrier recombination by the reduced energy level barrier at the interface between HTL and active layer [51]. After the 1:0.50 ratio, R sh decreased as the energy level barrier increased.…”

“…After the 1:0.50 ratio, R sh decreased as the energy level barrier increased. Thus the better energy level alignment between HTL and active layer can improve FF by decreasing R s and increasing R sh [51].…”

Status and prospects for ternary organic photovoltaics

“…Additionally, device engineering has played a key role in improving the performance of a given donor material. For example, interfacial layers between the active layer (for example, the BHJ blend) and the electrodes can be tuned to achieve ohmic contacts and promote charge collection [24][25][26] . The device polarity (conventional or inverted) can be controlled by using interfacial transport materials with different work functions.…”