구동 안정성 향상을 위한 유기 전자 소자용 고성능 기체 차단 봉지막에 관한 연구

Abstract

최근 bendable, foldable, rollable organic electronic devices가 주목 받으면서, 유기전자소자에 관한 연구가 산업현장이나 학계에서 활발히 진행되고 있다. 유기태양전지의 경우, 무기재료 기반의 태양전지만큼의 효율이 보고가 되고 있는 상황이며, 곡률반경이 10mm 이하의 값을 갖는 유기발광다이오드, 이동도 증가, 히스테리시스(hysteresis) 최소화, endurance time를 증가하기 위한 연구 등 여러 방향으로 연구중인 유기박막트랜지스터, 이러한 모든 것들이 bendable, foldable, rollable organic electronic devices를 구현 및 상업화에 성공하고자 하는 노력들이다. 그 중에서 수분 투과 봉지막(封止膜)에 관한 연구 또한 필수적 이다. 수분 투과 방지 봉지막은 약학, 식품, 반도체 분야 등 여러 분야에서 중요한 역할을 하고 있다. 특히, 유기반도체 분야의 경우, 유기반도체 재료 특성상, 선천적으로 공기 중 수분과 산소에 취약하기 때문에 유기반도체 소자의 안정적인 구동과 충분한 구동 수명의 확보를 위해 치밀한 봉지막 연구는 필수불가결하다. 상온, 상압 조건에서, 유기박막트랜지스터의 경우, ~10^-4 g/m^2/day, 유기태양전지의 경우, ~10^-5 g/m^2/day, 유기발광다이오드의 경우, ~10^-6 g/m^2/day의 수분 투과 조건을 만족하는 봉지막이 제작되어야 한다. 또한, 높은 수분 투과 활성화 에너지, ITO 전극의 임계 곡률반경 보다 충분히 낮은 곡률반경, 가시광선 영역 내의 고투명성을 갖는 요구조건도 갖추어야 한다. 본 연구에서는 플라즈마 기반의 원자층 증착방식(ALD, atomic layer deposition)을 이용한 알루미늄 옥사이드 (Al2O3) 박막과 유-무기(organic-inorganic) 나노 하이브리드(nanohybrid) 졸겔(sol-gel) 물질을 교대 적층하여 우수한 봉지막을 제작 및 연구를 하였다. 이 봉지막은 4 pair에서 7.83x10^-5 g/m^2/day (가속조건: 60⁰C, 90%)의 수분 투과도, 103.10 kJ/mol의 수분 투과 활성화 에너지, 7~9mm의 임계 곡률반경, 96% 이상의 가시광선 투과도를 갖는다. 따라서, 본 연구에서 보고한 수분 투과 방지 봉지막 연구는 앞으로 bendable, foldable, rollable organic electronic devices의 구현 및 상업화 성공을 앞 당길 수 있는 중요한 연구이다.

Gas barrier films have an important role in many applications such as food, pharmaceutical, flat-panel and organic electronic devices packaging to protect the water vapor and oxygen originating from the air. Among these applications, gas barrier films for organic electronic devices have been focused because the gas barrier films for organic electronic devices require very high degree of protection from water vapor and oxygen. In particular, OLEDs require the highest degree of protection from water vapor and oxygen in the ambient condition; the degree of WVTR and OTR (oxygen transmission rate) for gas barrier films of OLEDs is ~10^-6 g/m^2/day and ~10^-3 cm^3/m^2/day, respectively. In general, there are two types of critical degradations of organic electronic devices induced by water vapor and oxygen originating from the air; 1) oxidization of aluminum layer as cathode and organic materials as organic semiconductor in the OTFTs, OPVs and OLEDs, 2) hydrogen gas generated by electrochemical reduction of water vapor penetrated through pinhole of the cathodic electrode induces delamination of cathodic electrode at the interface between aluminum cathode and organic layer (2H2O + 2e− → H2 + 2OH−). These degradations cause the short lifetimes and instabilities of organic electronic devices. In order to prevent these degradations, the rigorous thin films encapsulation (TFE), which is adequate to flexible organic electronic devices, is indispensable. Therefore, many groups have investigated various TFE for organic electronic devices. Extensive investigations regarding the TFE have been developed by utilizing sputtering, chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD). However, these type of vacuum deposition processes manufacturing the TFE are unsuitable for achieving excellent gas barrier films for organic electronic devices because these techniques produce the relatively low step coverage, uniformity and loose packed structure at low temperature (below 100°C); high temperature process is generally not compatible with polymer materials. The optimal solution to overcoming the hurdles is to use plasma enhanced atomic layer deposition (PEALD) which is well recently known for producing the very high step coverage, uniformity, packed structure and pinhole-free gas barrier films due to its self-limiting surface reactions at low temperature. Moreover, the PEALD requires the short purging time to eliminate excess precursors and side-products on the surface, compared to the thermal H2O-based ALD, because the thermal H2O-based ALD is based on a large dipole moment of H2O instead of high reactive O2 plasma. Unless the purging time is enough, the residual H2O can cause undesired reactions leading to defects, increasing surface roughness and loose packed structure in the thermal H2O-based ALD system. The PEALD-based Al2O3 single films have successfully been demonstrated as an effective gas barrier films. Al2O3 films, which is based on trimethylaluminum (TMA) as a high reactive precursor, are most commonly used as an exceptional gas barrier due to its a lot of advantages such as high conformality, thermal stability, amorphous structure, low temperature process and high optical transmittance. As a workaround, many groups have used and introduced the organic/inorganic hybrid gas barrier films, which has a configuration of inorganic/organic alternating structure, to achieve lower WVTR than that of inorganic single gas barrier films by interposing Al2O3 between organic layers. The gas barrier films based on alternating structure are superior to single inorganic gas barrier films. There are mainly two reasons; 1) prolonging the diffusion path length of water vapor permeation, 2) decoupling defects formed by the limitation of the vacuum process. Recently, molecular layer deposition (MLD) based on sequential self-limiting surface reaction for depositing the organic layers in gas barrier films based on alternating structure by using organometallic and organic precursor has been highlighted. There are several types of MLD films which are well known as “metalcone”. A representative organic/inorganic hybrid gas barrier film is that alucone/Al2O3 gas barrier films. However, alucone has demerits including the low vapor pressure of ethylene glycol (EG) which is precursor source of alucone, decrease of thickness of alucone in the air, cracking in the absence of strain and necessity of long purging time to eliminate the EG on the surface at low temperature so it is incompatible with excellent gas barrier films for organic electronic devices. On the other hand, inorganic nanolaminate structures such as Al2O3/TiO2, Al2O3/ZrO2, and Al2O3/HfO2 have been introduced and investigated by many groups to improve the resistance to the water vapor-induced corrosion of Al2O3 single films. Despite the developed researches, issues such as relatively low optical transmittance, formation of crystalline8 and limited available precursors to deposit inorganic films at low temperature remain to be resolved; if limited precursors are used to deposit inorganic films, island or/and random growth of inorganic films are occurred and carbon impurities are increased in the inorganic films because of low reactive precursors at low temperature. In this article, I reported advanced thin gas barrier films incorporating alternating structures of PEALD-based Al2O3/inorganic-organic nanohybrid (ION) sol-gel. The ION sol-gel includes synthesized amphiphilic polymer which can contribute to improving long-term storage stability of sol-gel. Furthermore, the amphiphilic polymer containing ethoxysilane group is able to inhibit the corrosion of PEALD-based Al2O3 films and provide good adhesion by forming thermodynamically stable Al-O-Si bonds originating from sol-gel reaction route at the interface between PEALD-based Al2O3 film and ION sol-gel film. The WVTR value of the PEALD-based Al2O3/ION sol-gel 4 pair gas barrier film was measured using a calcium (Ca) test; the result exhibited 7.83 x 10^-5 g/m^2/day at high temperature and relative humidity (60oC, 90%). The WVTR value of the 4 pair gas barrier film was around twice that of glass encapsulation in our laboratory. The actual thickness of the PEALD-based Al2O3/ION sol-gel 4 pair gas barrier film was 150 nm on PEN (poly ethylene naphtalate); the result was measured using transmission electron microscopy (TEM). I also discussed performances of the PEALD-based Al2O3/ION sol-gel 4 pair gas barrier film in terms of activation energy for permeation, optical transmittance and flexibility. On the other hand, in order to emphasize the importance of anticorrosion of PEALD-based Al2O3 films and good adhesion, I conducted comparative investigations replacing ION sol-gel film with PMMA film as a comparison group by comparing WVTR values, AFM surface analysis and cross-sectional TEM analysis. Such comparative investigation between polymer and ION sol-gel as protective film is still lacking. I successfully demonstrated that inhibiting the water vapor-induced corrosion of PEALD-based Al2O3 film and providing good adhesion are very important in gas barrier films based on PEALD-based Al2O3 films. Consequently, I conjecture that our PEALD-based Al2O3/ION sol-gel gas barrier films are advantageous to apply to organic electronic devices as an encapsulation.